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No d’ordre: 2010-ISAL-0008 Année 2010
THESE
présentée devant
L'INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON
pour obtenir le grade de
DOCTEUR
ECOLE DOCTORALE : Electronique, Electrotechnique, Automatique
SPECIALITE : Dispositifs de l'Electronique Integrée
par
Ligor Octavian
Reliability of the Scanning Capacitance Microscopy and Spectroscopy for
the nanoscale characterization of semiconductors and dielectrics
Soutenue le 11 février 2010 devant la Commission d'Examen
Rapporteurs: Frédéric HOUZE Chargé de Recherche CNRS, HDR
François BERTIN Ingénieur CEA, HDR
Examinateurs: Daniel ALQUIER Professeur LMP
Christophe GIRARDEAUX Professeur IM2NP
George BREMOND Professeur INL/INSA de Lyon
Directeurs de these: Brice GAUTIER Professeur INL/INSA de Lyon
Invité: Jean-Claude DUPUY Professeur émérite INL/INSA de Lyon
2
INSA Direction de la Recherche - Ecoles Doctorales
SIGLE ECOLE DOCTORALE NOM ET COORDONNEES DU RESPONSABLE
CHIMIE
CHIMIE DE LYON
http://sakura.cpe.fr/ED206
M. Jean Marc LANCELIN
Insa : R. GOURDON
M. Jean Marc LANCELIN Université Claude Bernard Lyon 1
Bât CPE 43 bd du 11 novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72.43 13 95 Fax :
E.E.A.
ELECTRONIQUE, ELECTROTECHNIQUE, AUTOMATIQUE
http://www.insa-lyon.fr/eea
M. Alain NICOLAS
Insa : C. PLOSSU [email protected] Secrétariat : M. LABOUNE AM. 64.43 – Fax : 64.54
M. Alain NICOLAS Ecole Centrale de Lyon Bâtiment H9
36 avenue Guy de Collongue 69134 ECULLY Tél : 04.72.18 60 97 Fax : 04 78 43 37 17 [email protected]
Secrétariat : M.C. HAVGOUDOUKIAN
E2M2
EVOLUTION, ECOSYSTEME, MICROBIOLOGIE, MODELISATION
http://biomserv.univ-lyon1.fr/E2M2
M. Jean-Pierre FLANDROIS
Insa : H. CHARLES
M. Jean-Pierre FLANDROIS CNRS UMR 5558 Université Claude Bernard Lyon 1 Bât G. Mendel
43 bd du 11 novembre 1918 69622 VILLEURBANNE Cédex Tél : 04.26 23 59 50 Fax 04 26 23 59 49 06 07 53 89 13
EDISS
INTERDISCIPLINAIRE SCIENCES-SANTE
Sec : Safia Boudjema M. Didier REVEL Insa : M. LAGARDE
M. Didier REVEL Hôpital Cardiologique de Lyon Bâtiment Central 28 Avenue Doyen Lépine 69500 BRON Tél : 04.72.68 49 09 Fax :04 72 35 49 16
INFOMAT
HS
INFORMATIQUE ET MATHEMATIQUES
http://infomaths.univ-lyon1.fr
M. Alain MILLE
Secrétariat : C. DAYEYAN
M. Alain MILLE Université Claude Bernard Lyon 1
LIRIS - INFOMATHS Bâtiment Nautibus 43 bd du 11 novembre 1918
69622 VILLEURBANNE Cedex Tél : 04.72. 44 82 94 Fax 04 72 43 13 10
[email protected] - [email protected]
Matériaux
MATERIAUX DE LYON
M. Jean Marc PELLETIER
Secrétariat : C. BERNAVON 83.85
M. Jean Marc PELLETIER INSA de Lyon MATEIS Bâtiment Blaise Pascal
7 avenue Jean Capelle 69621 VILLEURBANNE Cédex Tél : 04.72.43 83 18 Fax 04 72 43 85 28 [email protected]
MEGA
MECANIQUE, ENERGETIQUE, GENIE CIVIL, ACOUSTIQUE M. Jean Louis GUYADER
Secrétariat : M. LABOUNE PM : 71.70 –Fax : 87.12
M. Jean Louis GUYADER INSA de Lyon Laboratoire de Vibrations et Acoustique
Bâtiment Antoine de Saint Exupéry 25 bis avenue Jean Capelle 69621 VILLEURBANNE Cedex Tél :04.72.18.71.70 Fax : 04 72 43 72 37
ScSo
ScSo* M. OBADIA Lionel
Insa : J.Y. TOUSSAINT
M. OBADIA Lionel Université Lyon 2 86 rue Pasteur 69365 LYON Cedex 07
Tél : 04.78.69.72.76 Fax : 04.37.28.04.48 [email protected]
*ScSo : Histoire, Geographie, Aménagement, Urbanisme, Archéologie, Science politique, Sociologie, Anthropologie
Résumé du mémoire de thèse
Le développement des nanotechnologies, comme les progrès vers les limites de la micro-
électronique et de la nano-électronique, impose le développement des méthodes de
caractérisation capables de fournir des informations avec une résolution compatible avec la
dimension des objets à caractériser. Ceci est vrai pour la métrologie morphologique, mais
aussi pour autres propriétés des nano-objets comme les propriétés magnétiques ou les
propriétés électriques (charge, potentiel, champ électrique, résistance, capacité...). En
particulier, pour les nano-objets de la nano-électronique, les propriétés de transport, de la
rétention de charge comme des nanocristaux, ou la concentration des dopants sont des
paramétrés cruciaux qui contrôlent les propriétés finales des dispositifs basés sur ses objets.
Des nombreuses méthodes de caractérisation sont apparues, capables d'avoir une
résolution nanometrique ou même atomique, comme par exemple des techniques basées sur
la microscopie électronique ou la microscopie a sonde atomique. Même si certaines d'entre
elles fournissent une vraie résolution atomique (il est possible, par exemple, d'imager des
atomes individuels en utilisant la STM (Scanning Tunneling Microscopy) en ultra-vide, et il est
même possible de détecter la présence des atomes individuels de bore grâce a la modification
de la densité d'états des atomes voisins, ces méthodes de caractérisation nécessitent une
préparation complexe des échantillons ou une instrumentation complexe. Pourtant, pour des
applications industrielles, il faut prendre en compte non seulement la reproductibilité, la
quantitativite et la résolution de ces méthodes de caractérisation, mais aussi leur rapidité et
leur facilite d'utilisation.
C'est pour cette raison que les méthodes de caractérisation basées sur la microscopie a
force atomique (AFM) représentent des candidates très sérieuses, capables a fournir des
informations topographiques et électriques des surfaces, avec une résolution nanometrique.
La microscopie a force atomique, utilisée pour la caractérisation topographique des
surface, a évolué dans un grand nombre des différentes méthodes de caractérisation qui
profitent de la précision du positionnement de la sonde AFM dans les trois directions de
l'espace, et de l'opportunité d'utilisation des sondes AFM conductibles comme électrodes de
grille pour réaliser des mesures inspirées des méthodes de caractérisation macroscopiques.
C'est le cas de la KFM (Kelvin Force Microscopy) pour la mesure du potentiel du surface ou de
la SSRM (Scanning Spreading Resistance Microscopy) pour la caractérisation des profils des
dopants. Pour des mesures capacitives, l'analyseur d'impédance utilise pour des mesures
micrométrique ne peut pas être utilise a l'échelle nanometrique, a la cause d'un rapport
signal/bruit insuffisant et doit être adapte pour être capable a mesurer le signal capacitif
correspondant a la capacité de l'ordre de quelques attofarads de la structure M.O.S formée
par le système sonde-échantillon.
Cette dissertation a pour but de montrer les performances, les limites et le potentiel
d'une méthode de caractérisation appelée Scanning Capacitance Microscopy (SCM), utilisée
Résumé du mémoire de thèse
4
pour la cartographie des dopants a nano-échelle et pour la mesure des propriétés des oxydes
minces.
En début du premier chapitre on présente des méthodes de caractérisation alternatives
pour la caractérisation des dopants, basées sur la microscopie a force atomique, la
microscopie électronique ou la sonde atomique tomographique. Des avantages et des
inconvénients de ces méthodes de caractérisation, en terme de sensibilité, résolution et
quantification sont mis en évidence. Le tableau suivant résume les conclusions de cette
comparaison :
SCM SIMS SSRM Holographie électronique
Sonde tomographique
ESEM
Détection porteurs atomes porteurs atomes atomes porteurs
Type de cartographie
2D 1D 2D 2D 3D 2D
Résolution 25 nm 1.5 nm pour le bore en Si
1-2 nm avec des pointes de diamant purs; 5-10 nm avec des pointes
commerciales
5-10 nm ~1 nm pas connu
Quantification possible
avec difficulté
oui, si des échantillons
étalons existent pour les atomes a étudier, dans
la même matrice
oui non oui non
Facilite d'utilisation
oui oui oui Non : nécessite la fabrication
d'une lamelle très mince
Non : nécessite une préparation très complexe de l'échantillon (une
pointe très aigüe)
non
La suite du premier chapitre expose en détails le fonctionnement de la SCM, d'abord en
en présentant le mode contact de l'AFM qui constitue le support pour les mesures capacitive.
Puis les méthodes de préparation des échantillons sont décrites : le clivage et le biseautage.
Les échantillons utilisés pour les investigations quant à la résolution, la localisation des
jonctions et la quantification sont présentés. Des mesures sur des échantillons de type paliers
de concentration (constitués de plusieurs couches de plusieurs centaines de nanomètre
d'épaisseur et de dopages différents) confirment l'utilité de la SCM pour des mesures
qualitatives (dites : "de défaillance") dans l'industrie de la nano-électronique. Les mesures
qualitatives avec la SCM peuvent confirmer très vite si l'implantation de l'échantillon a été
correctement effectue au bon endroit. Les zones de type n et de type p sont correctement
délimitées car la SCM est une technique capable de discriminer le type de dopants,
contrairement à d'autres techniques comme la SSRM. Des concentrations de dopage de l'ordre
de 1015 at/cm3 à 5.1019 at/cm3 de type n et p peuvent être imagées de manière qualitative
sans inversion de contraste, en choisissant correctement les tensions appliquées à
l'échantillon.
Des mesures évaluant la capacité de la SCM à localiser des jonctions, testant la
résolution géométrique (détection et séparation de puits quantiques), la résolution en termes
de dopage, et la quantitativite des profils des dopants sont aussi présentées dans le premier
Résumé du mémoire de thèse
5
chapitre.
Des premiers problèmes rencontres pendant ces tests, lies a la reproductibilité des
mesures sont mis en évidence. Par exemple, pendant les essais pour la localisation des
jonctions, on peut observer que la forme du signal capacitif a l'endroit des jonctions en
fonction de la tension continue appliquée ne correspond pas du tout a la théorie. L'amplitude
du signal, qui est le paramètre principal pour la quantification des dopage varie de manière
significative d'une expérience a l'autre.
Ces évidences motivent le deuxième chapitre de la dissertation, où les paramètres
expérimentaux qui peuvent perturber le signal SCM, diminuer le rapport signal/bruit ou
conduire a une interprétation erronée seront étudiés. Les échantillons étalons (des oxydes
thermiques minces déposés sur un substrat faiblement dope, de concentration connue, 1015
cm-3) sont analysés en parallèle avec une méthode de caractérisation de référence dans
l'industrie de la microélectronique, l'analyseur d'impédance, afin d'obtenir des courbes
capacité-tension (C-V) dont la dérivée pourra être directement comparée avec le même type
de courbe obtenue par SCM en mode spectroscopique (Scanning Capacitance Spectroscopy,
SCS, dont le résultat théorique est la dérivée de la courbe C-V obtenue sur une structure
MOS).
L'une des sources connue dans la littérature pour son impact négatif sur les mesures
SCM est l'influence de la photo génération de porteurs due à la lumière dans la structure MOS,
laquelle est fournie par le laser du système de détection AFM.
En déplaçant le spot laser sur le levier AFM de quelques centaines de microns de la
zone ou les mesures sont faites, nous avons mis en évidence l'influence négative du laser sur
le rapport signal/bruit du signal capacitif (Figure 1), lequel chute de manière brutale lorsque le
laser pointe vers la zone analysée.
Figure 1 : Comparaison entre deux signaux SCM, dans la présence et dans l'absence du laser AFM
En outre, des échantillons avec des profils des dopants graduels ont été utilisés pour
mesurer le taux de génération des porteurs dans le semiconducteurs par le laser AFM qu'on
peut estimer aux alentours de 1017 cm-3.
Des mesures capacitives avec un analyseur d'impédance, sur des électrodes de grille
minces (quelques nanomètres d'épaisseur) qui ont un coefficient de transmission de la lumière
Résumé du mémoire de thèse
6
significatif (laissant passer la lumière, contrairement à des électrodes plusieurs centaines de
nanomètres d'épaisseur), en présence de la lumière laser, ont été effectuées pour mieux
expliquer la diminution de l'amplitude du signal capacitif (Figure 2). En particulier, la remontée
en inversion due à la création de porteurs minoritaires, ainsi que la création de pics
supplémentaires correspondant à de nouveaux points d'inflexion créés par la lumière dans la
courbe C-V a été analysée, et ses conséquences sur le signal SCM explicitées.
Figure 2 : Des mesures capacitives avec un analyseur d'impédance, dans l'absence et dans la présence de la lumière laser, sur un oxyde
thermique de 5 nm déposé sur un substrat faiblement dope.
Nous avons montre que le laser peut avoir pas seulement des effets négatifs sur les
mesures capacitives, mais que des mesures capacitives a l'échelle nanometrique, en présence
de la lumière, d'une manière contrôlée, peuvent conduire a des nouvelles applications.
Nous avons également montre que des différentes sources de capacité parasite sont
présentes dans la configuration présente de l'AFM, et affectent le rapport signal/bruit. D'autres
sources de distorsion du signal capacitif ont été mises en évidence, parmi lesquelles la
topographie des échantillons, le contact en face-arrière et le contact entre la pointe et
l'échantillon. Parmi tous ces sources de distorsion du signal capacitif, la plus importante
semble être le contact pointe-échantillon.
Des mesures capacitives comparatives effectuées avec une pointe AFM sur la surface
d'un oxyde et avec une pointe AFM en contact avec un nano-électrode gravée a partir d'un
électrode micrométrique, utilisée pour des mesures C-V avec un analyseur d'impédance, ont
montre des différences claires du signal capacitif. Des phénomènes parasites sont présents
dans le signal SCM en comparaison des courbes C-V macroscopiques lorsque la pointe est en
contact direct avec la surface de l'oxyde (et lorsque les mesures sont faites à l'air ambiant), en
particulier :
Une hystérésis du signal qui se traduit par un décalage entre les courbes obtenues lors
de la montée en tension et lors de la descente en tension (trace et retrace sur la Figure
3).
Une remontée en inversion se traduisant par un pic dans la zone d'inversion de la SCS
lequel n'est pas présent dans les courbes C-V macroscopiques.
Résumé du mémoire de thèse
7
Ces phénomènes disparaissent complètement lorsque la mesure est effectuée sur une
électrode propre, assurant un contact parfait entre la pointe et l'échantillon (voir Figure 3).
Parmi les causes de la distorsion du signal se dénombrent :
L'état de propreté de la pointe AFM
L'état de propreté de la surface
La couche d'eau présente sur la surface
La géométrie aiguë de la pointe qui amplifie le champ électrique.
Il faut noter par ailleurs que les mesures sur le nanoelectrode ont démontre que le
maximum du signal capacitif différentiel (SCS) est situe a la même tension (0.8 V) que le
point d'inflexion de la courbe C-V obtenue avec un analyseur d'impédance. Ceci constitue une
preuve que la SCM peut devenir, dans des conditions de travail correctes, une méthode de
mesure reproductible et fiables.
Figure 3 : Des mesures capacitives effectuées sur un nanoelectrode (gauche) et avec une pointe AFM (droit) sur un oxyde thermique de 5 nm déposé sur un substrat de type p dope 1015 cm-3
Les causes de l'instabilité du signal capacitif quand les mesures sont effectuées avec
une pointe directement sur la surface de l'échantillon sont approfondies dans le dernier
chapitre, qui concerne la caractérisation des oxydes, où nous mettons en évidence certains
des problèmes les plus difficiles du contact pointe AFM - échantillon:
Le chargement de l'oxyde pendant l'application des tensions nécessaires au
fonctionnement de la SCM.
La modification de topographie de l'échantillon pendant les mesures électriques par
AFM, qui se traduit par l'apparition sur la surface de bosses pouvant atteindre plusieurs
dizaines de nanomètres de hauteur et dont les conditions d'apparition dépendent de la
polarité de la tension appliquée et de son amplitude.
Figure 4 : Image SCM obtenue après chargement de l'oxyde pendant les rampes des tensions consécutives.
Résumé du mémoire de thèse
8
Il y a quelques facteurs qui apportent leur contribution au phénomène du chargement
de l'oxyde. La minceur de l'oxyde favorise l'apparition des courants tunnel. Des mesures
réalisées sur des oxydes plus épais ont montre que le seuil de tension pour le chargement des
oxydes augmente. Sur des oxydes très épais (10 nm -15 nm) par rapport aux oxydes
normalement utilises avec la SCM (1 nm -2 nm), le phénomène du chargement n'apparait
plus.
Des oxydes de mauvaise qualité permettent plus facilement la capture des charges
dans des oxydes et le passage de courant tunnel assisté par les défauts (trap assisted
tunneling). Le chargement se passe plus facilement pour des tensions plus grandes ou pour
des pointes plus aigües. L'humidité de l'air qui génère une couche d'eau sur l'oxyde hydrophile
a également une contribution importante au chargement des oxydes.
Les oxydes représentent a la fois le diélectrique de la structure M.O.S utilisée pour
mesurer le taux de dopage, et un sujet séparé d'étude. Étant donne que les propriétés du
signal capacitif se modifient non seulement avec la concentration des dopants, mais aussi avec
les propriétés des oxydes, les profils des dopants ne peuvent pas être étudiés sans
caractériser de manière rigoureuse d'abord l'oxyde de la structure M.O.S.
En particulier, les états d'interface modifient la largeur à mi-hauteur du signal SCM.
L'épaisseur de l'oxyde, les charges fixes a l'interface oxyde - semiconducteur ou dans le
volume de l'oxyde déterminent le déplacement du maximum du signal SCM par rapport a sa
position idéale.
Des diélectriques différents ont été étudiés dans le troisième et dernier chapitre comme
des possibles candidats pour le diélectrique de grille nécessaire à la caractérisation des profils
des dopants:
Oxydes de silicium obtenu par oxydation plasma et oxyde de silicium natif ou obtenu
après irradiation sous rayonnement ultra-violet et atmosphère d'ozone (UV/Ozone)
Nitrures de silicium
Oxydes high-k, en particulier LaAlO3 (LAO) obtenu par épitaxie par jet moléculaire à
l'école Centrale de Lyon.
La tableau ci-dessous résume les conclusions obtenues :
Épaisseur Position du maximum du
signal SCS
Facile a croitre
Croissance sur la section des échantillons
Inversion de contraste
Oxyde natif
1.2 nm app. 0 V oui oui non
Oxyde UV-ozone
1.7 nm 3 V - 4 V oui oui non
Oxyde plasma
5 nm - 7 nm -1 V - -0.5 V non oui oui
Nitrure 7 nm - 9 nm app 1 V non oui oui
LaAlO3 5 nm 2 V - 5 V no oui non
Résumé du mémoire de thèse
9
Il apparaît très difficile dans cette étude de trouver un compromis parfait entre les
contraintes inhérentes à la croissance de l'oxyde dans le contexte de la cartographie de
dopants (oxyde basse température pour ne pas redistribuer les dopants, de faible épaisseur
sans toutefois donner lieu à des courants tunnels, facile à faire croître dans le but de ne pas
alourdir les procédures de caractérisation...), et les contraintes inhérentes à la reproductibilité
et à la fiabilité de la SCM qui exigent un oxyde reproductible et de la meilleure qualité
possible. Dans l'état actuel de nos recherches, l'oxyde idéal n'a pas été déterminé.
En conclusion, dans ce mémoire de thèse, nous avons étudié les motifs expérimentaux
qui empêchent la SCM d'être une méthode de caractérisation quantitative. De nombreux
facteurs parasites qui provoquent des variations importantes des paramètres du signal SCM
ont été identifies et des solutions pour régler ces problèmes ont été proposes. Notre travail
s'inscrit donc comme une étape vers des mesures SCM plus fiables et plus reproductibles.
Contents
Contents
11
CONTENTS ................................................................................................................................................................ 10
GENERAL INTRODUCTION ........................................................................................................................................ 15
SCANNING CAPACITANCE MICROSCOPY : PRINCIPLE AND OVERVIEW OF PERFORMANCES ..................................... 18
1.1 INTRODUCTION .................................................................................................................................................. 19
1.1.1. SECONDARY IONS MASS SPECTROMETRY (SIMS) .......................................................................................................... 19
1.1.2. ELECTRON HOLOGRAPHY ........................................................................................................................................... 19
1.1.3. SCANNING SPREADING RESISTANCE MICROSCOPY (SSRM) .............................................................................................. 20
1.1.4. TOMOGRAPHIC ATOM PROBE (TAP) ........................................................................................................................... 20
1.1.5. OTHER TECHNIQUES FOR DOPANT PROFILING................................................................................................................. 21
1.2. ATOMIC FORCE MICROSCOPY ............................................................................................................................ 21
1.3. THE PRINCIPLE OF SCANNING CAPACITANCE MICROSCOPY ............................................................................... 22
1.4. DESCRIPTION OF THE SAMPLES USED IN THIS STUDY ........................................................................................ 25
1.5. SURFACE PREPARATION .................................................................................................................................... 26
1.5.1 CLEAVING ............................................................................................................................................................... 26
1.5.2. POLISHING AND BEVELING ......................................................................................................................................... 27
1.6. QUALITATIVE CHARACTERIZATION OF DOPANT PROFILES ................................................................................. 29
1.7. JUNCTION LOCALIZATION .................................................................................................................................. 30
1.7.1 TERMINOLOGY ......................................................................................................................................................... 30
1.7.2. STATE OF THE ART .................................................................................................................................................... 33
1.7.3. COMMENTS ON JUNCTION CHARACTERIZATION .............................................................................................................. 39
1.8. RESOLUTION ...................................................................................................................................................... 44
1.9. DOPANT PROFILE QUANTIFICATION .................................................................................................................. 47
BIBLIOGRAPHY ......................................................................................................................................................... 49
CHAPTER 2 REPRODUCIBILITY PROBLEMS WITH THE SCM. OPTIMIZATION OF THE EXPERIMENTAL CONDITIONS FOR
SCM OPERATION ...................................................................................................................................................... 51
2.1. INTRODUCTION ................................................................................................................................................. 52
2.2. STATE OF THE ART ............................................................................................................................................. 54
2.2.1. LASER LIGHT ..................................................................................................................................................... 54
2.2.2. STRAY CAPACITANCE ................................................................................................................................................. 56
Contents
12
2.2.3. SURFACE RELATED PHENOMENA ................................................................................................................................. 56
2.2.4. TIP RELATED PHENOMENA ......................................................................................................................................... 57
2.3. C-V MEASUREMENTS .................................................................................................................................... 62
2.3.1. INTRODUCTION ....................................................................................................................................................... 62
2.3.2. THE PRINCIPLE OF THE C-V MEASUREMENTS WITH AN IMPEDANCE ANALYZER ...................................................................... 63
2.3.2.1. Introductive notions concerning impedance analyzer ................................................................................. 63
2.3.2.2. The series model .......................................................................................................................................... 64
2.3.2.3. The parallel model ....................................................................................................................................... 65
2.3.2.4. The choice of the model used with C-V measurements ............................................................................... 66
2.3.3. C-V MEASUREMENTS ON A TEST SAMPLE ...................................................................................................................... 69
2.3.3.1. The test sample ........................................................................................................................................... 69
2.3.3.2. Preliminary measurements .......................................................................................................................... 69
2.3.3.3. Oxide thickness ............................................................................................................................................ 70
2.3.3.4. Dopant concentration .................................................................................................................................. 71
2.3.3.5. Flatband capacitance, flatband voltage ...................................................................................................... 72
2.3.3.6. Interface states, surface potential ............................................................................................................... 73
2.3.3.7. A preliminary comparison between the C-V signal and the SCM signal ...................................................... 75
2.4. INFLUENCE OF THE LASER LIGHT ........................................................................................................................ 77
2.4.1. INTRODUCTION ....................................................................................................................................................... 77
2.4.2. EXPERIMENTAL EVIDENCE .......................................................................................................................................... 78
2.4.3. QUANTITATIVE ASPECTS ............................................................................................................................................ 79
2.4.4. COMPARISON WITH C-V MEASUREMENTS .................................................................................................................... 81
2.4.5. SOLUTIONS FOR THE ELIMINATION OF THE PARASITIC AFM LASER EFFECT ............................................................................ 87
2.4.6. CONCLUSIONS AND PERSPECTIVES ............................................................................................................................... 88
2.5. INFLUENCE OF THE PARASITIC CAPACITANCE OF THE GEOMETRY SETUP .......................................................... 89
2.5.1. INTRODUCTION ....................................................................................................................................................... 89
2.5.2. EXPERIMENTAL EVIDENCE .......................................................................................................................................... 90
2.5.3. COMPARISON WITH C-V MEASUREMENTS .................................................................................................................... 97
2.5.4. CONCLUSIONS AND PERSPECTIVES ............................................................................................................................... 98
2.6. ELECTRICAL CONTACTS ...................................................................................................................................... 99
2.6.1. INTRODUCTION ....................................................................................................................................................... 99
2.6.2. ELECTRICAL CONTACTS ............................................................................................................................................ 101
2.6.3. THE SAMPLE BACKFACE CONTACT ........................................................................................................................ 104
2.6.3.1. Experimental.............................................................................................................................................. 104
2.6.3.2. Comparison with C-V measurements ........................................................................................................ 105
2.6.3.3. Conclusions ................................................................................................................................................ 106
2.6.4. THE TIP-SAMPLE CONTACT ................................................................................................................................. 107
Contents
13
2.6.5. CONCLUSIONS ....................................................................................................................................................... 113
2.7. INFLUENCE OF THE SAMPLE TOPOGRAPHY ON THE CAPACITANCE SIGNAL ..................................................... 113
2.7.1. INTRODUCTION ..................................................................................................................................................... 113
2.7.2. EXPERIMENTAL ...................................................................................................................................................... 115
2.7.3. DIRECTION OF APPROACH OF A TOPOGRAPHICAL FEATURE ....................................................................................... 117
2.7.4. SCAN DIRECTION ............................................................................................................................................. 118
2.7.5. DEFLECTION SETPOINT (DS) .............................................................................................................................. 119
2.7.6. SIZE OF THE TIP ............................................................................................................................................... 120
2.7.7. SOLUTIONS FOR DECREASING THE EFFECT OF STRAY CAPACITANCE ARISING FROM TOPOGRAPHY ............................................. 120
2.7.8. CONCLUSION .................................................................................................................................................. 122
2.8. TIP PROPERTIES ............................................................................................................................................... 122
2.8.1. INTRODUCTION ..................................................................................................................................................... 122
2.8.2. CHARACTERIZATION OF AFM TIPS WITH STANDARD AFM SAMPLE GRATINGS .................................................................... 124
2.8.3. CHARACTERIZATION OF AFM TIPS WITH SEM ............................................................................................................. 124
2.8.5. CONCLUSIONS ....................................................................................................................................................... 129
2.9. THE AFM PIEZOELECTRIC SCANNER ................................................................................................................ 129
2.9.1. INTRODUCTION ..................................................................................................................................................... 129
2.9.2. EXPERIMENTAL ...................................................................................................................................................... 132
2.9.3. CONCLUSIONS ....................................................................................................................................................... 138
2.10. CONCLUSIONS ............................................................................................................................................... 139
BIBLIOGRAPHY ....................................................................................................................................................... 140
CHAPTER 3. OXIDES CHARACTERIZATION WITH THE SCM ...................................................................................... 142
3.1 INTRODUCTION ................................................................................................................................................ 143
3.2. OXIDE DEFECTS AND THEIR INFLUENCE ON THE SCS SIGNAL ........................................................................... 143
3.2.1. MOBILE IONS ....................................................................................................................................................... 144
3.2.1.1. The nature of mobile ions .......................................................................................................................... 144
3.2.1.2 The influence of the mobile ions on the capacitive signal ......................................................................... 144
3.2.2. FIXED CHARGES ................................................................................................................................................... 145
3.2.2.1 The nature of fixed charges ........................................................................................................................ 145
3.2.2.2. The influence of the fixed charges on the capacitive signal ...................................................................... 146
3.2.3. INTERFACIAL TRAPS .............................................................................................................................................. 146
3.2.3.1. The nature of interfacial traps ................................................................................................................... 146
3.2.3.2. The influence of the interfacial traps on the capacitive signal .................................................................. 147
3.2.4. BULK OXIDE DEFECTS ............................................................................................................................................. 148
Contents
14
3.2.4.1. The nature of bulk oxide defects................................................................................................................ 148
3.2.4.2. The influence of the bulk defects on the capacitive signal ........................................................................ 148
3.3. STATE OF THE ART ........................................................................................................................................... 149
3.3.1. REQUIREMENTS FOR THE OXIDES USED WITH SCM MEASUREMENTS ................................................................................ 149
3.3.1.1 Low-temperature fabrication ..................................................................................................................... 150
3.3.1.2 Ease of formation ....................................................................................................................................... 150
3.3.1.3 Oxide reproducibility .................................................................................................................................. 150
3.3.1.4 Oxide thickness ........................................................................................................................................... 150
3.3.1.5 Oxide uniformity ......................................................................................................................................... 152
3.3.2 OXIDE PROPERTIES AND PROPERTIES OF THE SCS SIGNAL ................................................................................................ 153
3.3.2.1 The position of the flatband bias ................................................................................................................ 153
3.3.2.2 Full-width at half-maximum ....................................................................................................................... 155
3.3.2.3 Hysteresis of the signal ............................................................................................................................... 158
3.3.2.4 The amplitude of the SCS signal ................................................................................................................. 158
3.3.3 LOW-TEMPERATURE OXIDES ..................................................................................................................................... 159
3.4 OXIDES CHARACTERIZATION WITH THE SCM .................................................................................................... 161
3.4.1. OXIDE RELATED PARASITIC PHENOMENA AT THE TIP - OXIDE INTERFACE ............................................................................. 161
3.4.1.1. Oxide charging ........................................................................................................................................... 162
3.4.1.2. Anodic oxidation ........................................................................................................................................ 165
3.4.1.3. Oxide engravement ................................................................................................................................... 169
3.4.2. STUDY OF LOW-TEMPERATURE OXIDES FOR DOPANT PROFILING ...................................................................................... 170
3.4.2.1 Guidelines for a complete characterization of oxides with the SCM .......................................................... 170
3.4.2.2 Plasma oxide............................................................................................................................................... 174
3.4.2.3 Nitride ......................................................................................................................................................... 177
3.4.2.4 High-k dielectrics ........................................................................................................................................ 179
3.4.2.5 Native oxide ................................................................................................................................................ 180
3.4.2.6 UV/ozone oxide .......................................................................................................................................... 181
3.4.2.6 Summary of low-temperature oxides ......................................................................................................... 183
3.4.3 CONCLUSIONS AND PERSPECTIVES .............................................................................................................................. 183
BIBLIOGRAPHY ....................................................................................................................................................... 185
GENERAL CONCLUSION .......................................................................................................................................... 187
General introduction
General Introduction
16
The development of nanotechnologies, as well as the progress towards the limits of the
micro-electronics and nano-electronics, imposes the development of characterization methods
capable of providing reliable information with a resolution compatible with the size of the
objects to be probed. This is true for the morphological characterization (dimensional
metrology) but also for many other properties of nano-objects like the magnetic properties
(remnant magnetization) or the electrical properties (charge, potential, electric field,
resistance, capacitance...). In particular, for nano-objects involved in the nano-electronics, the
transport properties, the charge retention in e.g. nanocrystals, or the doping level are crucial
parameters which control the final properties of devices based on these objects.
Numerous characterization techniques have arisen which are indeed able to obtain sub-
micronic resolution or even atomic resolution, e.g. techniques based on the electronic
microscopy or scanning probe microscopies. Although some of them provide a true atomic
resolution (for example, it is possible to image individual silicon atoms using Scanning
Tunneling Microscopy – STM - under ultra-high vacuum, and even to check the presence of a
boron atom in its neighborhood thanks to the modification of the electronic density of states
due to its presence), they may also require complex sample preparation or heavy
instrumental setups. Yet, for a potential industrial application, the figures of merit of a
characterization technique are the reproducibility, the quantitativity, the resolution, but the
ease of use has also to be taken into account.
That's why techniques based on the atomic force microscopes (AFM) are very serious
candidates as nanoscale characterization techniques able to provide information on the
topography and on the electric properties of the surfaces with a nanometric resolution because
of their relative ease of use when implemented in air. Starting from the atomic force
microscope for the mapping of surface topography, they have evolved toward a great number
of different characterization techniques that take advantage of the precision of the positioning
of the AFM tip in all direction of space, and of the opportunity of using the conductive AFM tip
like a top electrode in order to perform techniques inspired from macroscopic setups. This is
the case for Kelvin Force Microscopy (KFM) for the measurement of surface potential or
Scanning Spreading Resistance Microscopy (SSRM) for doping profiling. For capacitance
measurements, the macroscopic setup can not be used because of signal to noise ratio
concerns but has to be adapted to reach the level of signal corresponding to the extremely
small surface of the capacitance formed from the tip/sample system.
This dissertation intends to show the performances, limits and potentialities of a
technique called Scanning Capacitance Microscopy (SCM) for the mapping of dopants at the
nanoscale and for the measurement of the properties of very thin dielectric oxides for
applications at the Metal-Oxide-Semiconductor (MOS) structure. After a short presentation of
alternative techniques based on scanning probe techniques, electronic microscopy or atom
probe tomography, some examples of dopant mapping in test samples will be presented with
the aim of underlining the satisfactory behavior of SCM to obtain qualitative images of dopants
of p-type or n-type in silicon. Samples containing quantum wells will demonstrate the
General Introduction
17
resolution of the technique and a discussion on the quantitativity and reproducibility will be
engaged, showing that the route toward fully reliable, reproducible and quantitative images is
not so easy, and that in the general case, only 'failure analysis' images can be obtained in
order e.g. to verify that the doping steps have been correctly implemented in a given region of
a sample or to distinguish between p-type and n-type regions of the sample.
This will motivate the second chapter of this dissertation, where all the experimental
parameters disturbing the SCM signal or lowering the signal to noise ratio, or leading to a
misinterpretation of the images will be reviewed and evidenced. Among them, the influence of
the laser light provided by the laser used by the AFM apparatus to measure the deflexion, the
parasitic capacitance of the sample – tip – lever – chip system and the influence of the
topography, the role of the tip-sample contact, of the measurement environment (humidity,
controlled atmosphere)... will all be addressed separately. To do so, the comparison of
Scanning Capacitance Spectroscopy (SCS) with macroscopic Capacitance-Voltage (C-V)
measurements will be a precious tool in order to better understand the role of each
parameter. The properties of the tip and the parasitic effects introduced by the piezotube used
in our experimental setup will also be taken into account. The aim of this part of the
dissertation is to provide guidelines which allow to improve the signal to noise ratio, the
reliability and the reproducibility of the measurements, and to point out the directions toward
which progress can be made to improve them significantly.
The third chapter will be an application of the previous chapters to the comparisons of
different dielectric oxides grown at low temperature in order to perform the doping mapping in
the semiconductor structures. Based on the understanding of the Scanning Capacitance
Spectroscopy developed in the second chapter, and given the constraints for doping mapping
exposed in the first, different kinds of silicon oxides or alternative oxides which could be
prepared at the laboratory will be compared and the suitability of each one for doping mapping
will be evaluated. Again, a trade off between all the constraints will have to be made and
directions will be indicated for improving the quality of oxides for such applications.
Finally, this work will be concluded by a summary of important experimental concerns
which should be solved for SCM to progress toward a better reliability, reproducibility and
quantitativity for both the mapping of dopants and the quantitative measurement of charges
in thin oxides.
Chapter 1. Scanning Capacitance Microscopy:
principle and overview of performances
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
19
1.1 Introduction
The measurement of the doping concentration is a key step in the fabrication process of
last generation micro and nanoelectronic devices. With the reduction of the size of the
structures, the performances of the characterization method must be enhanced to meet the
requirements of the International Technology Roadmap for Semiconductors (ITRS).
Several characterization methods, like Scanning Capacitance Microscopy (SCM), are able
to provide a 2D, or even a 3D dopant mapping in semiconductor structures. For now, none of
the specified characterization methods is able to fully achieve all of the ITRS goals in terms of
sensitivity, range, resolution, quantification and ease of characterization. Each of them has its
specific features with advantages and drawbacks.
Although the goal of this dissertation is not to provide details about each of them,
several techniques may be shortly described.
1.1.1. Secondary Ions Mass Spectrometry (SIMS)
This is a one dimensional method which is still a reference technique for doping
profiling because of its high sensitivity for dopants like boron in silicon, and its spatial
resolution which can reach 2 nm in optimal conditions. Two different techniques exist for SIMS
: so-called 'dynamic' SIMS and 'static SIMS', the former being the most often used for doping
profiling in silicon. It is based on the bombardment of the sample with an ion beam of Ar+, and
more often of O2+ or Cs+, with energies ranging from ~350 eV to 15 keV (the most often
encountered energies for an optimal depth resolution being in the 500 eV range for boron in
silicon for example). The ions emitted from the sample because of the bombardment
(secondary ions) are accelerated by an electric field, filtered with an electric and a magnetic
field and finally detected using a Faraday Cup or an electron multiplier. The intensity of a
given secondary ion (more precisely a given mass/charge ratio) is recorded as a function of
time as the surface is sputtered by the ionic bombardment. The depth scale of the resulting
depth profiles is retrieved by measuring the depth of the resulting crater with a mechanical
profilometer.
SIMS sensitivity and the depth resolution are not the same for all species. For example
both are excellent for boron in silicon, whereas a mass interference with the SiH+ ion impinges
seriously the results for Phosphorous.
In all this dissertation, test samples are characterized using SIMS which acts as a
reference technique for the concentrations and thicknesses.
1.1.2. Electron holography
This technique is based on a Transmission Electron Microscope setup. It uses a coherent
electron source divided into two different beams : one crosses the sample to be studied
(constituted of a very thin lamella), the other one serves as a reference. Both beams are
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
20
combined to form a hologram which can be interpreted after a Fourier Transform as a phase
and an amplitude image. The phase image is related to the type (p or n) of dopants. Very thin
samples needed for this method can be prepared using a Focused Ion Beam or by polishing.
1.1.3. Scanning Spreading Resistance Microscopy (SSRM)
SSRM is a technique very close to SCM. As the principle of Atomic Force Microscopy and
SCM will be exposed later, we will only describe the principle of SSRM.
As in SCM, the AFM tip is used as a top electrode. The tip is very stiff (40 N/m) and
coated with highly-doped poly-crystalline diamond. Pure diamond probes are also found in the
literature [1.1]. SSRM is a resistive method which measures the current flowing through the
sample when a continuous low voltage is applied. This current is proportional to the total
resistivity of the portion of sample located under the tip, and of the tip itself, including
therefore contributions from :
The tip
The contact resistance between the tip and the sample
The semiconductor under study which resistivity can be expressed as :
1
n p
ρ=q μ n+ μ p
The back-contact.
Where q is the elementary charge, n and p the mobility of the electrons and the holes
respectively and n, p the concentration of electrons and holes respectively. The resulting
signal is thus sensitive both to the concentration of and the mobility of the carriers.
For SSRM to provide a good signal, a very high pressure must be applied on the sample
surface (which justifies the use of very stiff cantilevers), which is truly scratched by the scan
of the surface. As a result, the number of scan is limited by the deterioration of the surface.
1.1.4. Tomographic Atom Probe (TAP)
One very powerful emerging method is the Tomographic Atom Probe (TAP) which is
based on a fabrication of a tip submitted to a strong electric field which allows the field
evaporation and ionization of the atoms located at the probe apex. The resulting ions are
sorted by time of fly and detected in a position sensitive detector. This allows a reconstruction
of the whole volume of the tip where the time-of-fly analysis has allowed to identify the nature
of all the atoms composing the original probe. This technique leads to very impressive results
when the samples are conductive. For semiconductors, as it is more difficult to obtain the field
evaporation, a laser beam is used to evaporate the atoms (Laser Assisted-TAP : LATAP). Test
samples similar to those used in this dissertation have been analyzed using this technique and
allow to identify strength and weaknesses at the present moment. Although the preparation of
the sample is very difficult and requires a skilled staff, delta-doped samples of boron in silicon
have been analyzed in three dimensions with a resolution comparable to SIMS [1.2]. However,
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
21
the level of noise is still rather high and concentrations lower than 1018 cm-3 can not be
detected yet.
1.1.5. Other techniques for dopant profiling
Some emerging techniques are based on the environmental scanning electron
microscope (ESEM). The detection of secondary electrons enables the technique to be
sensitive to the surface potential, but also to the surface band bending. Although a clear
contrast can be obtained with p-type semiconductors on test samples (one example will be
showed in the next sections), the result seems to be less convincing for n-type doping, and
the contamination of the surface seems to impinge the reproducibility of the technique.
It must also be mentioned that some techniques rely of the difference of the etch rate of
differently doped semiconductors to obtain a topographical image of the doped areas after a
wet etching using various acids.
The table below summarizes some of the features known from the literature for some of
the most often encountered techniques able to map the dopants in the semiconductors :
SCM SIMS SSRM Electron Holography
Tomographic Probe
ESEM
detection carriers atoms carriers carriers atoms carriers
Type of mapping 2D 1D 2D 2D 3D 2D
Resolution range 25 nm 1.5 nm for
boron in Si
1-2 nm with
pure
diamond
probes; 5-
10 nm with
commercial
probes
5-10 nm ~1 nm Not known
Quantification possible
Difficult Yes, if a
calibration
sample exists
of the atom
under study in
the same
matrix
yes no yes no
ease of use yes yes yes No : requires
the fabrication
of a very thin
lamella
No : requires a
very complex
sample
preparation (very
sharp tip)
no
1.2. Atomic Force Microscopy
The Atomic Force Microscope (AFM) belongs to the family of scanning probe
microscopes. It makes use of a very sharp probe to detect superficial properties of the
materials on nanometer and subnanometer scale.
The AFM probe is located at the free end of a cantilever that is attached to a piezo
scanner able to move in vertical direction, and in a horizontal plane with a sub-nanometer
precision. In some other configurations, the tip attached to the cantilever remains static and
the sample is attached to the piezotube controlling the direction in all the directions of space.
The AFM is commonly used for topography characterization in three operating modes:
non-contact, tapping, and contact AFM. As this dissertation deals only with SCM, which is
operated in contact mode, we will only describe this operating mode.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
22
Changes in topography are detected in several manners. The most common way is the
use of a laser beam pointing at the reflecting cantilever rear, and being bounced off to a split
photo detector (Figure 1.2.1).
Figure 1.2.1. The Lennard Jones potential the governs the tip-surface interaction with the AFM (left). The AFM laser detection system (right)
In contact mode, the tip stays in contact with the surface at all time. The interaction
between the tip and the surface is repulsive. While scanning the tip on the surface any change
in the topography is accommodated by the probe via a feedback system. The deflexion
imposed by the topography is kept constant by the feed-back system so that the force applied
to the surface remains constant. The voltage applied on the piezotube in order to modify the
vertical position of the tip so that the deflexion remains constant represents the vertical
coordinate of the AFM image.
The AFM cantilevers used for the contact mode are less rigid (stiffness often in the
0.02-1 N/m range compared to the 40 N/m for classical high frequency tapping mode tips).
Considering that the tips are in permanent contact with the surface, they have to be more
resistant to wear than the AFM tips used with the other operation modes.
1.3. The principle of Scanning Capacitance Microscopy
SCM (Scanning Capacitance Microscopy) measures the differential capacitance dC/dV of
a metal - insulator - semiconductor structure formed by the contact between a conductive
probe and an oxidized semiconductor surface (Figure 1.3.1).
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
23
Figure 1.3.1. The MOS-like structure which describes SCM measurements. Here, the metal
is a representation for the AFM tip
The SCM operates in contact mode. With the SCM, the AFM is coupled to a capacitive
sensor, and, with the wear resistant, conductive tip, capacitive measurements can be
performed.
SCM operation is based on the Metal-Oxide-Semiconductor stack where the AFM tips
acts as the metal part. The Capacitance-Voltage (C-V) characteristic of this structure, as can
be measured by a macroscopic setup using large electrodes, can be found in Figure 1.3.2. But
because of the order of magnitude of the measured capacitance with the SCM because of the
very small size of the top electrode – 10-18 F, which is a million times smaller than the level of
noise (10-12 F), a high frequency resonant circuit coupled with a lock-in amplifier must be used
and only a differential capacitance can be measured.
The differential capacitance is measured as follows.
Figure 1.3.2. The operating principle of the SCM detector (Veeco Manual)
A Vdc point of operation must be chosen in the transition region of the C-V curve, where
the capacitance varies with the applied DC voltage . A Vac signal which determines a variation
of the system capacitance value is superimposed. The difference between the capacitance
values corresponding to Vpp is measured.
Ideally, for obtaining a real differential signal dC/dV, the applied Vac should be in a so-
called small signal range, with a maximal value of around 50 mV, as it is the case for
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
24
macroscopic measurements.
However, for practical reasons related to the noise/signal ratio, which are described in
chapter 2, much larger values have to be used in order to obtain a good SCM signal. Thus, the
measured signal is rather a ΔC/ΔVpp signal.
Further on, due to the specific design of the electronic circuit, the output signal
represents in fact a ΔC signal and it is not further divided by ΔV, which is a discrepancy with
the AFM's manual which states that a ΔC/ΔVpp is obtained as the output signal.
Figure 1.3.3. SCS curves for different Vac measured on a n-type substrate covered with a 2nm thermal oxide (left). The same SCS curves divided by the corresponding Vpp
This point can be verified as follows: first Scanning Capacitance Spectroscopy (SCS)
must be performed by applying a ramp of voltage and measuring the SCM signal for all the
values of the voltage ramp. If the applied Vac is sufficiently high, the change in capacitance ΔC
has to represent the difference between the accumulation and the depletion, that is, from a
certain Vac threshold, ΔC has to remain constant.
If ΔC reaches the difference between the capacitance in accumulation and the
capacitance in depletion, it will remain constant even though Vac increases. This means that for
higher values of Vac, the signal ΔC/ΔV should begin to decrease with the increase of the ΔV
(Vpp)
On the other hand, from the measurements in Figure 1.3.3, it can be seen that the
signal, no matter the magnitude of the Vac, monotonically increases until the saturation of the
detector. This is not a normal behavior if the SCM output signal would represent indeed
ΔC/ΔV.
This was confirmed by the Veeco company in a private communication : unlike the
name suggests for the SCM signal, ‘dC/dV’, these values do not get scaled by the ‘dV’ value,
but are the dC values observed when applying a given AC voltage: the lock-in outputs are not
divided by the dV signal. As a consequence, these signals will increase as the AC value is
increased (increased AC voltage mean that the C-V curve is sampled over a larger voltage
range), until at a certain point where the full depletion and accumulation voltage range is
reached, at which saturation will be obtained.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
25
1.4. Description of the samples used in this study
During this thesis, several doping profile samples have been used for evidentiating the
SCM performances on doping profiles in terms of reproducibility, failure analysis images,
resolution, junction localization and quantification. Some of them have been grown at LETI,
Grenoble, others have been obtained from the company STMicroelectronics, Crolles.
Sample name Sample description Fabrication
Maya p p-type substrate 1015 cm-3, 6 p-type regions 250
nm wide with doping concentrations between 1017
cm-3 and 3.1019 cm-3 (Figure 1.3.5)
ST
Maya n p-type substrate 1015 cm-3, 5 n-type regions of
variable width with doping concentrations
between 2.1017 cm-3 and 2.1019 cm-3 (Figure
1.3.6)
ST
Diodes BP p-type substrate 1015 cm-3, 6 alternating p-type
and n-type regions with doping concentrations
between 1017 cm-3 and 1019 cm-3 (Figure 1.3.7)
ST
Maya 6 p-type substrate 1015 cm-3, 9 p-type regions 100
nm wide with doping concentrations between
5.1017 cm-3 and 5.1019 cm-3 (Figure 1.3.8)
LETI
Quantum wells 1 p-type substrate 1015 cm-3, 4 p-type regions of
500 nm, 50 nm, 10 nm and 3 nm wide with
doping concentrations between 1.1015 cm-3 and
3.1019 cm-3 (Figure 1.3.9)
LETI
Quantum wells 2 p-type substrate 1015 cm-3, 9 p-type regions of
480 nm, 240 nm, 120 nm, 60 nm, 30 nm, 15 nm,
and 7.5 nm wide with doping concentrations
between 1.1016 cm-3 and 3.1019 cm-3 (Figure
1.3.10)
LETI
The samples have been fabricated by RP-CVD.
The doping profile samples have been characterized by SIMS, used as reference for
comparison with the SCM doping profiles (Figures 1.4.1 - 1.4.6)
Figure 1.4.1. SIMS doping profile of Maya p sample (INL)
Figure 1.4.2. SIMS doping profile of Maya n sample (INL)
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
26
Figure 1.4.3. SIMS dopant profile of Diode sample (INL)
Figure 1.4.4. SIMS dopant profile of Maya 6 sample (LETI)
Figure 1.4.5. SIMS dopant profile of Quantum wells 1 (LETI)
Figure 1.4.6. SIMS dopant profile of Quantum wells 2 (LETI)
1.5. Surface preparation
1.5.1 Cleaving
The preparation of cross-sectioned silicon sample for SCM has been done by cleaving
with pliers.
A small scratch is performed with a diamond tip on the surface of the sample to be
cleaved and then the sample is cut with a pair of pliers. The first centimeters of the resulting
cross section (near the original scratch) will show a rough surface, but if the cut is done with
care, the crack will further orient itself along a crystalline plane and the rest of the cross-
section will show almost no roughness.
Several authors (as well as the user's manual from Veeco) use polishing as the next
step in order to retrieve a perfectly flat surface. Polishing allows also to stick two different
samples (including a reference sample) and to prepare them in a rigorously identical way. In
our study, considering that polishing introduces a lot of defects at the oxide-silicon interface,
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
27
we consider that this step is unnecessary to the cross-section sample preparation and is to be
avoided.
The cross-section sample preparation can be used for the SCM as well as for other
characterization methods such as the SSRM (Figure 1.5.1).
Figure 1.5.1. SSRM image obtained on the cross-section of the BP diode sample
For SCM qualitative characterization, the samples are put for several hours in a clean
environment such as a clean room in order that a thin native oxide grow on top on the cleaved
cross-section.
1.5.2. Polishing and beveling
We have used polishing for preparing beveled samples. On beveled samples, the doping
profiles are artificially magnified by geometrical means, which allows the characterization of
very small doping profiles.
For polishing, four different supports with different angles have been used:
Angle (degrees) amplification factor 1 / sin (θ)
5 44' 10
2 52' 20
1 9' 50
34' 100
Figure 1.5.2. Beveled surface after polishing
The most used support for the images presented in this dissertation was the one with
an angle of 5 44'. The other polishing supports usually lead to a beveled surface where the
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
28
doping profiles exceeded the AFM lateral range. For example, a doping profile lying on 4
microns becomes, after polishing with a support of 34' (x100), of 400 microns. The maximum
lateral AFM range is of 100 microns, if offsets are not performed.
For the polishing, a glass plate has been used with the Mecapol P300 polisher. On the
glass plate, colloidal silica (which is a colloidal suspension of a mixture of abrasive particles
dispersed throughout a chemically active liquid carrier) with a neutral pH is poured constantly
during the polishing.
The beveled sample must be oriented in the same direction as the direction of rotation
of the plate. The total time of polishing is around 2-3 minutes.
After the polishing, the sample must be immediately cleaned with a cotton swab under
deionized water (Figure 1.5.3).
Figure 1.5.3. Beveled samples cleaned only with deionized water after polishing (left) and cleaned with a cotton swab in deionized water (right)
As with the cross-sample preparation, the beveled samples can also be used by other
characterization methods, such as SSRM or SEM (Figure 1.5.4)
Figure 1.5.4. ESEM image of BP-diodes sample (Gilbert THOLLET - MATEIS). Beveled sample manufactured at INL
It must be specified that the two preparation methods presented above cannot be used
for samples containing localized microstructures such as transistors. In order to be able to
cleave or polish the samples exactly in the spot where the microstructure is located, a
dedicated industrial cleaving system should be used.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
29
1.6. Qualitative characterization of dopant profiles
As presented in the introduction, failure analysis images constitute the primarily
application for the SCM. This means that the doping implantation of micronic and nanometric
structures can be rapidly verified by capacitance images with the SCM.
On the SCM images taken on the maya p, maya n and Diodes-BP samples, the
properties of the SCM measurements are evidentiated (Figures 1.6.1-1.6.3)
Figure 1.6.1 SCM image (left) and cross-section (right) of Staircase sample: maya n
Figure 1.6.2. SCM image (left) and cross-section (right) of Staircase sample: maya p
Figure 1.6.3. SCM image (left) and cross-section (right) of Diodes-BP
On the SCM images, a good contrast difference between the different doping regions is
obtained. The lightly doped regions lead to a higher capacitance signal than the highly doped
regions, according to the theory. The different doping regions are clearly defined and
correlated with the SIMS profiles in terms of geometry (width).
For the maya p sample, the SCM signal is positive and for maya n sample the SCM
signal is negative. SCM is able to make the difference between p-type and n-type regions due
to the phase of the SCM signal. This distinction is even clearer on the Diodes-BP sample,
where there is an alternance between the positive and the negative signals, in agreement with
the type of the doping regions.
Even if contrast reversal problems may appear because of the loss of the tip coating,
the wrong choice of Vdc point of operation or a high density of density of states at the oxide-
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
30
silicon interface, there will still be a difference in contrast between the different doped regions
and SCM qualitative images (failure analysis images) will still be able to evidentiate if the
geometry of the different doped regions is in agreement with the intended design of the
sample.
1.7. Junction localization
The control of the junction depth is a major issue in the processing of modern
semiconductor devices. The junction depths are predicted to be as low as 10 nm for future
deep sub-micron devices. The electrical characteristics of the transistor are affected by minor
changes of the junction position. Therefore, ITRS considers that an exact delineation of the
junction is critical.
1.7.1 Terminology
In semiconductor physics, the junction can be defined in several ways:
The metallurgical junction MJ can be defined as the position where the acceptor
concentration Na equals the donor concentration Nd. A plot of net doping concentration as a
function of position – Nd-Na = f(x) - is referred to as the doping profile.
The metallurgical junction is typically measured with SIMS, since this technique is only
sensitive to atoms.
The electrical junction EJ is defined as the position within the depletion region where
the active carrier concentrations of both types (holes and electrons) are equal (1010 cm-3).
The electrical junction can be measured with SRP, SSRM, and SCM which are
techniques sensitive to the carrier concentration. Further on, we will discuss how to
determine the location of electrical junctions with SCM.
There is no need to measure the MJ and the EJ simultaneously. One can calculate the
carrier distribution and implicitly the position of the electrical junction from the doping profile
and vice-versa.
Here is the algorithm of calculus for obtaining the doping profile when the carrier
distribution is known
At thermal equilibrium, there are no electron and no hole currents:
The density of electron current can be expressed as follows:
jn is equal to 0 at thermal equilibrium
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
31
Moreover:
Thus, it comes:
The charge density can be expressed as:
Moreover, it can also be defined as:
Thus:
Finally it comes:
The last equation allows us to find out analytically the doping profile from the carrier
distribution.
Here is the algorithm of calculus for obtaining the carrier distribution when the
doping profile is known.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
32
Starting from a doping profile Nd – Na, the equation from above allows the calculus of
the potential and consequently, the carrier distribution n and p.
This equation doesn't generally have an analytical solution and has to be solved
numerically.
As an alternative to a home-made program which solves numerically the equation from
above, one may use a commercially TCAD program such as ISE (Dessis).
By introducing Na, Nd and the position of the metallurgical junction as input variables,
Dessis is able to calculate numerically the position of the electrical junction and the boundaries
of the depletion region, as in example from the Figure 1.7.1.
Figure 1.7.1. Simulation of the position of the electrical junction from the position of the metallurgical junction with TCAD-ISE
Although the concepts of metallurgical junction and electrical junction are very precise,
very often a junction is simply defined in terms of the depletion zone (the junction is within
the limits of the depletion zone).
The depletion region or the space charge region is the near-vicinity of the
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
33
metallurgical junction where there is a significant non-zero charge and the carrier
concentrations in the region are greatly reduced or depleted. It should also be mentioned that
the build-up charge and the associated electric field continues until the diffusion of carriers
across the junction is precisely balanced by the carrier drift. [1.3]
For a non-biased junction, the width of the depletion region can be calculated from the
formula:
where:
For a MOS structure:
1.7.2. State of the art
i) When imaging with the SCM across pn junctions, Williams [1.4] was the first to observe
that the SCM signal undergo a 180° phase shift at the passage between the n-type and p-type
regions. So, it has been concluded that the junction location may be estimated simply by
monitoring the phase of the SCM signal relative to the drive signal.
ii) One of the factors that may affect the measurements of the EJ position is the applied
bias.
Kopanski (NIST) [1.5] and Kleiman (Bell Laboratories) [1.6] were the first to observe
that the position of the pn junction shifted with the applied bias. At different biases, the SCM
signal passes through zero at different positions.
From the 2D SCM images on transistors, it can be seen the change in the position of
the junction with the applied bias.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
34
Figure 1.7.2. image SCM of pn junction [1.3] (left). Image SCM of pn-junction in transistor [1.4]
This effect of the change of junction position with the applied bias has been confirmed
theoretically through simulations by Bell Laboratories [1.6].
For the simulations, Kleiman used a simplified 2D simulation model of a junction, with
the AFM tip as a plane electrode (Figure 1.7.3):
Figure 1.7.3. Simulation of the change of the position of the electrical junction with the applied bias [1.4]
The simulated pn-junction has the following parameters:
• 5.1017 at/cm-3 p-type (boron) substrate
• 3.1018 at/cm-3 n-type (phosphorus)
• a 2 nm oxide layer
• a 10 nm electrode (replacing the AFM tip)
• a bottom electrode
The high frequency capacitance data is calculated using steady-state small signal
analysis.
The top electrode is moved across the sample in 10nm steps and C-V and dC/dV -V
curves are generated for each electrode position across the sample and for different biases.
The graph dC/dV versus the electrode position across the junction and the applied bias
voltage Vb: dC/dV = f(x, Vdc) is plotted.
From the simulation, the following observations can be made:
• for Vdc = -1.5V, the p-type response is pushed roughly at the edge of the n-type depletion
boundary
• for Vdc = 0.5V, the n-type response has been pushed at the edge of the p-type depletion
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
35
boundary.
• the position of the sign changing gradually moves from one side of the physical junction to
the other, as a function of Vdc.
In the junction region, 0.99um < x < 1.05um, the sign of the dC/dV changes as Vb
increases from -2 V to +1 V.
iii) Further on, analyzing the data, it is O'Malley [1.8], [1.9] and Kopanski [1.10] who
proposed the use of the so called quiescent-point of operation.
The quiescent bias was defined as the bias where the apparent junction location would
coincide with the electrical junction. O'Malley [1.8] simulated pn-junction has the following
parameters:
• 1.1017 at/cm-3 p-type (boron) substrate
• 3.1017 at/cm-3 n-type (arsenic)
• a 2 nm oxide layer
• a 10 nm electrode (replacing the AFM tip)
• a bottom electrode
The 10 nm electrode was moved across the sample in 10 nm steps and C-V high
frequency curves and dC/dV-V have been generated for each electrode position across the
sample.
The Figure 1.7.4 shows the simulated data for an electrode location above the built-in
depletion region of the pn junction.
Figure 1.7.4. Junction simulation concerning the quiescent point of operation
The contour plot shows that within the depletion region there is an availability of both
carrier types. The applied bias Vdc alters the electric fields within the depletion region allowing
carriers to flow in from the adjacent n-type or p-type regions. The region under the tip will
appear to be p-type for a negative Vdc or n-type for a positive Vdc. This carrier movement
results in an extension of the p-type or n-type dC/dV-V response into the depletion region.
In conclusion, O'Malley has determined from simulations that the cause of the apparent
movement of the junction position with the applied bias is the flow of carriers from the
adjacent p-type and n-type regions under the influence of the applied bias.
The natural conclusion is that the applied bias must be chosen so that it doesn't attract
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
36
free carriers from neighbor regions.
For Vdc = -0.6V , it can be observed that the pn-junction appears unperturbed. Since in
the depletion region the semiconductor appears intrinsic (n=p=1010cm-3), a Vdc chosen at the
silicon mid-gap will cause no perturbations.
In practice, there are a series of supplementary factors which affect the choice of the
bias, such as the workfunction of the tip, the quality of the oxide. That is why the applied bias
can be chosen as an average between the n-type and the p-type peaks in dC/dV. This average
bias it is called the quiescent bias.
The obvious advantage of calculating the quiescent bias as the average between the n-type
and the p-type dC/dV peaks is that, this way:
• time-consuming simulations can be avoided. In addition
• the inaccuracies introduced by the simulations related to the tip modelization or the quality
of the oxide can be avoided.
Kopanski [1.12] confirms the accuracy of the formula only for symmetrical step
junctions. Only for symmetrical step junctions, the Fermi levels are equally distanced from the
intrinsic Fermi level and the bias voltage is midway between the voltages that produce the
peak SCM response on the p-type side and on the n-type side.
iv) The method of the most symmetrical C-V curve (Hall Edwards)
Edward [1.4] has constructed a model for explaining why the pn-junction position
moves with the applied voltage and how one can determine the real position of a pn-junction.
When the tip is above the depletion region, the applied bias Vdc attracts electrons or
holes in the depletion region from the p-region or n-region (carriers which overcome the
electrostatic field from the depletion region due to the net charge density from the positive
and negative P and B ions). These carriers attracted in the depletion region change the
contrast of the image, the carrier concentration under the tip being altered.
The only situation when the tip does not attract carriers from either side of the junction
is when the tip bias is equal to Vfl (flatband bias) for the depletion region.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
37
Figure 1.7.5. Hal Edwards [1.4] simulations explaining the change of the junction's
position with the applied bias
Also, Edwards observed that the C-V characteristic at the EJ is a symmetrical curve. For
a tip bias higher or lower than Vfl, the system is in accumulation due to the carriers attracted
under the tip from the neighboring regions.
Based on this model, the location of the EJ can be determined by analyzing the
symmetry of the SCS curves in the vicinity of the junction.
The procedure Edwards proposes for finding the true position of the junction is as
follows:
• SCM images at different voltages must be obtained.
It is important to use an alternative sequence of bias voltages, in order not to charge or
deteriorate the oxide and obtain a false signal. Duhayon recommends (reference) that
between each applied bias to go back to Vdc = 0V and verify whether the signal corresponding
to this bias has changed or not. Only if the signal did not change, the previous measure has to
be taken into consideration.
• The SCM images at different biases are turned into dC/dV-V characteristics
•The dC/dV-V curves are numerically integrated to obtain C(V) curves. This results in a series
of C(V) curves, one for each pixel in the image.
•The pixel color is given by the voltage value of the minimum in the C(V) curve. For the p-
type Si, the minimum of C(V) is at the maximum voltage and the pixel value is white.
For the n-type Si, the minimum of C(V) is at the minimum voltage and the pixel value
is black.
The most symmetrical C-V curves should be located at the EJ.
v). The method of node accumulation (M.Stangoni)
Stangoni [1.10] proposes an alternative for the delineation of pn-junctions in which she
combines elements from the two previous methods: the SCS method for finding the most
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
38
symmetrical C-V curve, the dC/dV-x graph method to evidentiate the accumulation of the
nodes in the region of the pn-junction (a node being the location where the SCM signal crosses
the zero value)
The proposed procedure consists of two phases.
•1. The dC/dV-x signal is acquired across the junction at different tip biases.
•2. the position of the nodes P of the dC/dV-x curves are plotted as a function of the applied
bias Vdc.
As soon as the probe approaches the location where the CV curves exhibit the best
symmetry (which is at the electrical junction position in the ideal case), the PV curve (the
node curve versus the applied bias) shows a decrease in the slope (the P points accumulate).
Once the tip leaves the location of the best symmetry, the slope increases again.
Figure 1.7.6. dC/dV-x; P-V method of Stangoni for the determination of the junction's position
Finding the position of the EJ with this procedure means finding the interval in the P-V
space where the nodes accumulate. [1.19]
vi) Measurements on beveled samples
Stangoni [1.17] and Duhayon [1.18] suggested that one way to reduce the error in
junction delineation, is the use of samples beveled at very sharp aperture angles. The
principle behind this technique is that the uncertainty in the delineation of the electrical
junctions is the same in cross-sectioned and in beveled samples. Thus, when the distances
measured in beveled samples are scaled back to the cross-section case, the uncertainty in the
delineation of the electrical junction is divided by the geometrical magnification factor, leading
to an error of just some hundreds of angstroms.
However, this solution has also its drawbacks. Polishing will create more interface
states than usual that will distort the position of the electrical junction. Further more, beveling
of samples results in the carriers spilling effects (a different distribution of the carriers due to
different geometrical parameters), which produces the characteristic distortion of the junction
close to the surface, and thus to a systematic error in the location of the electrical junction.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
39
1.7.3. Comments on junction characterization
The precision of the junction delineation depends of the type of pn-junction to be
analyzed:
• symmetrical or asymmetrical pn-junctions.
• abrupt or linearly-graded pn-junctions
• low-doped or high-doped junctions
as well as different combinations between these types.
The EJ as viewed by SCM will be always found in boundaries of the pn depletion region.
For abrupt junctions, the width of the depletion region varies as follows:
[1.1]
Figure 1.7.7. The width of the depletion region for different concentration of the p-type and n-type regions
For linear graded junctions, the width of the depletion region varies as follows:
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
40
Figure 1.7.8. The width of the depletion region for different doping concentrations
From the figure, it is clear that a better junction delineation will be obtained in the case
of highly doped abrupt junction. In this case, the depletion zone is much smaller and the
apparent position of the electrical junction cannot vary much.
Also, at higher doping concentration, the SCM signal is less sensitive to the
experimental conditions. The laser light, as well as oxide defaults have a smaller influence.
Each technique used for the location of a pn-junction has its advantages and
disadvantages. The latest techniques are more accurate but more complicated and some of
them time consuming. The earlier techniques are not so precise for most cases but are very
simple and quick. Each technique can be useful in certain cases, depending on the type of pn-
junction to be analyzed. In the case of high doped junctions, simpler techniques could be used
with a good precision. In the case of low-doped junctions, with a several microns depletion
region, more sophisticated techniques should be used.
When imaging with the SCM across pn junctions, the SCM signal was found to undergo
a 180° phase shift at the passage between the n-type and p-type regions. One way to
determine the position of the junction is by looking for the spot where the SCM signal passes
from positive to negative (Figure 1.7.9)
Even simpler, the junction location may be estimated simply by monitoring the phase
of the SCM signal relative to the drive signal. The phase signal contains only the information
related to the junction, without the unnecessary information (in this case) of the doping
concentration. The figure below shows line scans extracted from both the n-regions and the p-
regions and the change of sign in the SCM signal cant be clearly seen at the passage between
different regions.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
41
Figure 1.7.9. SCM signal for the BP-diode sample (Vac = 500 mV, Vdc= 0 V).
For gradual junction, the SCM signal is presented in Figure 1.3.28. As expected from
the previous calculus (Figure 1.7.8), the junction localization is much clearer for abrupt, highly
doped junctions (Figure 1.7.10).
Figure 1.7.10. SCM signal for two different gradual doping profiles (very abrupt - black signal; less abrupt
- red signal)
Tests for the junction localization from the phase shift between the p-type and the n-
type regions have also been made on beveled samples (Figure 1.7.11).
Figure 1.7.11. SCM image (left) and cross-section (right) on beveled (x10) samples.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
42
On the SCM images, especially on the image of the phase-1D (Figure 1.7.12), it can be
seen that the phase for the n-type dopants, although it is much smaller, it doesn't become
negative anymore, which can be seen as a consequence of the interface states created by
polishing.
The effect has also been encountered when recording SCS signals on M.O.S structures
with plasma oxides and nitrides as gate oxide (Chapter 3). It seems that a large density of
interface states affects the phase of signal, which can affect the accuracy of junction
localization.
Figure 1.7.12. SCM phase image (left) and cross-section (right) on beveled samples.
The other methods of junction delineation presented in the literature have also
been tested.
Here are, for example, attempts for junction delineation by the method of the most
symmetrical C-V curve (Hall Edwards) on the Diodes BP sample.
First, we have done simulations with DESSIS, that we present here, that confirm
Edward's theory regarding the shape of the C-V characteristic at the EJ (Figure 1.7.13)
Figure 1.7.13. Dessis simulation of the capacitive signal at the electrical junction
Further on, we have tried to obtain experimentally symmetrical SCM curves at a
junction's location
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
43
The work procedure was as follows:
• SCM images have been recorded at different biases: 0V, -0.2V, 0.2V, -0.4V, 0.4V.... -2.8V,
2.8V, -3.0V, 3.0V (Vac = 600 mV; Vdc=-3V;+3V with a 0.2V step)
• a SCS curve is constructed by taking into account all the measurements from a column of
the SCM image. The first SCS curve is formed by taking into consideration all the
measurements from the first column of the SCM image.
• 512 SCS curves have been obtained for each of 512 columns.
• the SCS curves have been analyzed in terms of symmetry. The most symmetrical SCS curve
should correspond to the position of the junction.
Figure 1.7.14. Diagram describing the work procedure for the junction delineation according with Hall Edwards procedure.
A representative SCS signal at the junction, obtained with the above procedure, is
presented in the figure 1.7.15:
Figure 1.7.15. Experimental SCS signal obtained for the junction delineation according with Hall Edwards procedure.
At the EJ, the curves should become more and more symmetrical. However, the
experimental signal is not at all similar to the theory. None of the SCS signal doesn't approach
the symmetry of the theoretical signal.
For understanding why are such important differences between the theoretical and the
experimental signals, we have verified the Hall Edward's method for the reconstruction of the
SCS signal on a substrate.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
44
Figure 1.7.16. Comparison between the an SCS signal measured on a native oxide on top of a low-doped 1015 cm-3 substrate, Vac = 1 V (left) and an SCS signal reconstructed with the Hall Edward's method on the same sample, Vac
= 400 mV (right).
The reconstructed SCS signal doesn't match at all the recorded SCS signal. An
explanation for this mismatch is given in chapter 3, section 3.4.1.
After the beginning of the study of junction delineation par SCM, it soon become
apparent that the current experimental setup is not appropriate for such a high precision task.
The depletion zone is very sensitive to carrier photo-generation by the laser of the AFM
system.
High applied biases can attract carriers from the both sides of junction into the
depletion zone, changing the apparent position of the electrical junction. Also, high bias
measurements with a sharp tip, on a very thin, low-quality oxide, can lead to current
tunneling by hot electrons.
The polarization of the water layer always found on a hydrophilic oxide exposed to the
atmospheric conditions, under bias conditions, change the position of the flatband bias,
rending the measurements irreproducible from one measurement to another. Further more,
such humidity under bias conditions favorize the oxidation of the surface. Very close SCS
signals must be recorded in the junction region in order to be able to localize with precision
the junction's position, but the very first measurement could determine surface oxidation,
distorting the rest of the measurements.
Numerous oxide defaults of a low-quality oxide can also change the apparent position
of an electrical junction.
These observations underline the need for a better control of experimental parameters
that affect the SCS signal. This will be the goal of the next chapters.
1.8. Resolution
The spatial resolution with the SCM describes the ability of the SCM to resolve detail in
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
45
the doping profile that is being imaged.
In the literature [1.17], [1.21], the spatial resolution with the SCM is related to a series
of factors, among which the most important role is played by the size of the tip. The spatial
resolution can be artificially increased by imaging on beveled samples.
By comparison between the SIMS profiles and the SCM profiles, it can be observed
that, for maya p and maya n staircase samples, there is an noticeable difference between the
two. With the SCM profiles, the doping steps are rounded at the edges, which does not
correspond to the reality.
By comparing a maya p doping profile obtained on the cross-section of the sample with
a maya p profile obtained on a beveled sampled and reconstructed, it can be observed the
same difference (Figure 1.8.1).
Figure 1.8.1. Comparison between the maya p doping profile on the cross-section of the sample and on the reconstructed beveled sample
The SCM signal measured on the cross-section of the sample is rounded towards the
edges.
For doping profile samples with doping regions even narrower, the steps cannot be
seen at all (Figure1.8.2)
Figure 1.8.2. SCM signals on the cross-section of maya 6 sample with a diamond tip (left) and with a PtIr tip (right). The corresponding SIMS profile is presented in the Figure 1.3.8)
On the SCM profiles of MAYA 6 sample, which consists of steps of ~100 nm width , the
9 steps of different doping concentrations cannot be seen at all. If the SCM profile recorded
with a larger diamond tip is practically a Gaussian, on the SCM profile recorded with a smaller
PtIr tip, the steps can be barely guessed by small changes in the slope of the signal.
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
46
Comparing the two SCM signals, it can be concluded that the size of the tip influences the
resolution of the SCM.
The same conclusion results from the SCM measurements performed on quantum wells.
Figure 1.8.4. SCM signals on the cross-section of quantum wells sample with a diamond tip (left) and with a PtIr tip (right). The corresponding SIMS profile is presented in the Figure 1.3.10)
On the SCM profile recorded with the larger diamond tip, the seventh quantum well of
7.5 nm cannot be seen. However, this quantum well is clearly seen on the SCM image
recorded with the smaller PtIr tip.
Different solutions have been tested in order to achieve a better resolution: the use of
highly doped silicon tips (without coating) or the decrease of the tip size of diamond coated
tips by FIB engravement (Figure 1.8.5).
Figure 1.8.4. Diamond coated tip engraved by FIB at HELIOS Nanolab by ing. Armel Descamps
For now, such tests didn't give better results. The conductive highly doped silicon tips
are too fragile and they break or change their shape very easily with the SCM, in contact
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
47
mode.
The spiral engravement of the diamond coated tips resulted in a complete loss of
capacitive signal, probably because of a discontinuity of the conductive diamond coating.
We consider that achieving a better resolution with the SCM is related to the
manufacturing of smaller, conductive and resistant to wear AFM tips.
1.9. Dopant profile quantification
With the SCM, the capacitance detector output is in volts. The output value is
proportional to the doping concentration, but it only represents a voltage, not a capacitance.
The detector presents a series of operational amplifiers with unknown amplification constants
that render the output signal qualitative.
All the approaches towards the quantification of the doping profiles are based on a
semi-quantification method: recording the amplitude of the signal for a known doping
concentration (usually the substrate of the sample) and then determining the concentration for
the rest of the doping regions by comparing the amplitudes of the capacitance signal for these
regions with the amplitude of the signal for the reference region.
In the literature several attempts to established a working procedure for the
quantification of the doping profiles have been made.
Given that no home-made simulations have been made and the quantification attempts
did not lead to any results, we will not enter in the theory of the quantification of the doping
profiles.
Extensive work on this subject has been done by authors like J.J.Kopanski [1.19] and
L.Ciampolini [1.20]. Detailed explanations concerning the principles and the models used for
the simulations can be found in the manuals that accompany their software.
The attempts to quantify doping profiles have been done by using the software
developed by NIST - Fastc2d, in order to quantify the doping profiles for the maya p and the
maya n samples.
The working procedure has been as follows:
1. We have recorded an SCM image of the doping profile.
2. We have supposed known the substrate concentration and we have tried to obtain with
Fastc2d the doping concentration for the rest of the doping profile.
3. We have introduced as input values with Fastc2d the required parameters:
• the type of the doping profile;
• the amplitude of the output signal for all the doping regions;
• the biases that establish the point of operation (the ac bias voltage Vac, the dc bias voltage
Vdc and the sensor high-frequency voltage Vhf);
• the estimated oxide thickness (for the native oxides, the estimated thickness is around 1.2
nm - Chapter 3, Section 3.3.1.4 Oxide thickness);
• the estimated radius of the tip, base on the specifications given by the diamond tips
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
48
manufacturers.
4. We have compared the obtained concentrations with the doping concentrations of the SIMS
profiles.
The results have been off with almost a decade. Such a difference between the real
doping concentrations and the calculated doping concentrations may have several reasons:
Many of the input values of the required parameters for the simulation cannot be
known at this moment. Such parameters are:
• the tip radius. For the simulation has been used the value of the tip radius specified by the
manufacturers. However, ulterior SEM images of the tips (Chapter 2, Section 2.8.3
Characterization of AFM tips with SEM) proved that there may be important differences
between the tip radius specified by the manufacturers (approximatevely 50 nm for the
diamond tips) and the actually radius of the tips (up to 250 nm).
• the oxide thickness. The oxide thickness used with Fastc2d constitute only an
approximation based on previous work found in the literature (Chapter 3, Section 3.3.1.4
Oxide thickness). We had no means to measure the actual native oxide thickness. Attempt for
measuring the native oxide thickness have been made with C-V measurements. However,
these attempts have been unsuccessful. It is known that the native oxide is too thin and of too
low a quality so that interpretable C-V curves be obtained and the oxide thickness calculated
from the value of the capacitance in accumulation. The oxide thinness and its low quality lead
to strong tunneling currents which conduct to an unstable signal of the capacitance signal in
accumulation with C-V measurements. C-V and I-V measurements must be performed on the
native oxides and laborious simulations must be done in order to obtain the native oxide
thickness.
In addition, it must not be forget that the actual native oxide used with the doping
profiles is found on the cross-section of the doping profile samples, where it is not trivial to
deposit electrodes and perform C-V measurements.
• the value of the sensor high-frequency voltage Vhf is not known. The SCM electronics is a
black box and finding the value of the high-frequency voltage of the detector is not trivial.
• during the quantification attempts, a high instability of the SCM signal in terms of amplitude,
FWHM, position of the peak of the SCS signal and shape of the SCS signal has been observed
(Figure 1.9.1)
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
49
Figure 1.9.1 SCS signals on a 5 nm thermal oxide recorded with the same diamond coated tip, at Vac = 500 mV
Such variations are met on the same sample, using the same tip and the same
operation point. It became obvious that, if any quantification attempt is to be made, the
reproducibility of the SCM signal must be first insured.
Bibliography
[1.1] M. Fouchier, P. Eyben, G. Jamieson, W. Vandervorst, 'Topside release of atomic
force microscopy probes with molded diamond tips', Microelectronic Engineering vol.78–79,
2005
[1.2] E. Cadel, F. Vurpillot, R. Lardé, S. Duguay, B. Deconihout, 'Depth resolution
function of the laser assisted tomographic atom probe in the investigation of semiconductors',
Journal of Applied Physics vol.106, no.4, 2009
[1.3] R. F. Pierret, 'Semiconductor device fundamentals', Addison Wesley Longman,
1996, pg.200
[1.4] C. C. Williams, J. Slinkman, W. P. Hough, H.K.Wickramasinghe, 'Lateral dopant
profiling with 200nm resolution by scanning capacitance microscopy', Appl.Phys.Lett, vol.55,
no.16, 1989
[1.5] J. J. Kopanski, J. F. Marchiando, and J. R. Lowney, 'Scanning capacitance
microscopy measurements and modeling: Progress towards dopant profiling of silicon', J. Vac.
Sci. Technol. B 14, no.1, 1996
[1.6] R. N. Kleiman, M. L. O'Malley, F. H. Baumann, J. P. Garno, and G. L. Timp,
'Junction Delineation of 0.15μm MOS Devices Using Scanning Capacitance Microscopy', 692-
IEDM, 1997
[1.7] H. Edwards, R. McGlothlin, R. S. Martin, U. Elisa, M. Gribelyuk, R. Mahaffy, C. Ken
Shih, R. S. List, V. A. Ukraintsev, 'Scanning capacitance spectroscopy: An analytical technique
for pn-junction delineation in Si devices', Appl.Phys.Lett, vol.72, no.6, 9 February 1998
[1.8] M. L. O’Malley, G. L. Timp, S. V. Moccio, J. P. Garno, R. N. Kleiman,
'Quantification of scanning capacitance microscopy imaging of the pn junction through
electrical simulation', Appl.Phys.Lett, vol.74, no.2, 1999
[1.9] M. L. O’Malley, G. L. Timp, W. Timp, S. V. Moccio, J. P. Garno, R.N.Kleiman,
Chapter 1 Scanning Capacitance Microscopy: principle and overview of performances
50
'Electrical simulation of scanning capacitance microscopy imaging of the pn junction with
semiconductor probe tips', Appl. Phys.Lett, vol.74, no.24, 1999
[1.10] M. Stangoni, M. Ciappa, W. Fichtner, 'Accuracy of scanning capacitance
microscopy for the delineation of electrical junctions', J. Vac. Sci. Technol. B 22, no.1, Jan/Feb
2004
[1.11] J. J. Kopanski, J. F. Marchiando, D. W. Berning, R. Alvis, H. E. Smith, 'Scanning
capacitance microscopy measurement of two-dimensional dopant profiles across junctions', J.
Vac. Sci. Technol. B 16, no.1, 1998
[1.12] J. J. Kopanski, J. F. Marchiando, B. G. Rennex, 'Carrier concentration
dependence of the scanning capacitance microscopy signal in the vicinity of p–n junctions', J.
Vac. Sci. Technol. B 18, no.1, 2000
[1.13] C. J. Kang, C. K. Kim, J. D. Lera, Y. Kuka, K. M. Mang, J. G. Lee, K. S. Suh, C. C.
Williams, 'Depth dependent carrier density profile by scanning capacitance microscopy', Appl.
Phys. Lett. vo.71, no.11, 1997
[1.14] N. Duhayon, T. Clarysse, P. Eyben, W. Vandervorst, L. Hellemans, 'Detailed
study of scanning capacitance microscopy on cross-sectional and beveled junctions', J. Vac.
Sci. Technol. B 20, no.2, 2002
[1.15] T. Clarysse, P. Eyben, N. Duhayon, M. W. Xu, W. Vandervorst, 'Carrier spilling
revisited: On-bevel junction behavior of different electrical depth profiling techniques', J. Vac.
Sci. Technol. B 21, no.2, 2003
[1.16] Hal Edwards, Vladimir A. Ukraintsev, Richard San Martin, F. Scott Johnson,Philip
Menz, Shawn Walsh, Stan Ashburn, K. Scott Wills, Ken Harvey, Mi-Chang Chang, 'pn-junction
delineation in Si devices using scanning capacitance spectroscopy', Journal of Applied Physics,
vol.87, no.3, 2000
[1.17] N. Duhayon 'Experimental study and optimization of scanning capacitance
microscopy for two-dimensional carrier profiling of submicron semiconductor devices', thesis,
2006
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no.13, 2003
Chapter 2. Reproducibility problems with the SCM.
Optimization of the experimental conditions for
SCM operation
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
52
2.1. Introduction
All the measurable parameters with the SCM have an appreciable variation from a
measurement to another (Figure 2.1.1).
The position of the maximum of the SCS signal varies within several hundreds of
milivolts from one measurement to another, on the same sample. Also, sometimes, there is an
important shift between the trace and the retrace of the SCS curves.
The amplitude of the SCS signal, used for the quantification of the doping
concentration, also shows variations between 10% of the signal until 1000%!
The shape of the SCS signal has a very strange behavior. The SCS signal presents very
rarely only one maximum. In most of the cases, two maxima, one positive and one negative,
are present. Sometimes, the SCS signal presents 3 or more maxima (see for example Figure
2.4.13).
Figure 2.1.1. Consecutive SCS signals recorded with the a same diamond coated tip, on the surface of a 5 nm thermal oxide, on
different regions of the sample
All these facts made us wonder whether the capacitance measurements performed with
the AFM are reproducible or not.
Different suppositions have been proposed to explain these measurements. Among
them, we enumerate:
The uniformity of the samples.
The SCM is a nanometric characterization method. It is quite possible that the oxide
properties vary from one spot to another and, in consequence, the parameters of the SCS
signal vary from one spot to another.
It is possible that the oxide doesn't present the same thickness all over the surface of
the sample. Even a variation of a few angstroms of the oxide thickness along the sample
would determine an important variation of the amplitude of the SCS signal. Also, it is possible
that the oxide defects not to be uniformly distributed, which would determine shifts of various
magnitudes of the SCS curve.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
53
The quality and the thickness of the oxide.
Thin oxides often present lots of defects.
The thinness of the oxide means that tunneling currents are possible, which would have
as consequence an irregular shape of the SCS curve. The oxide defects could further favorize
the presence of tunneling currents of different magnitudes.
The variation of the size of the tip.
The AFM tip represents the gate electrode for the SCM measurements. It is known that
the tip dimensions are slightly different from tip to tip. Also, the tip can deteriorate during the
scans, changing shape and increasing its radius and/or losing its coating. Given that the
capacitance signal is proportional to the dimensions of the gate electrode, it seems reasonable
to consider that tips of different sizes, as well as the wear of the tip, may determine a
fluctuation of the amplitude of the SCS signal.
For verifying the above hypothesis, a 5 nm thermal oxide had been characterized with
an impedance analyzer (section 2.3) and from the C-V curve (Figure 2.1.2) its properties
have been calculated.
Figure 2.1.2 CV measurements performed on a 5nm thermal
oxide, at various frequencies.
From the C-V measurements it can be deduced the good quality of this oxide:
the measurements are reproducible. Measurements have been performed on dozens of
electrodes. Also, measurements have been performed with three different impedance
analyzers. Each and every time, the same results have been obtained. The
reproducibility of the measurements gives an indication about the uniformity of the
oxide properties.
the C-V curves don't present a hysteresis between the trace and the retrace at room
temperature, so there are very few mobile charges in the volume of the oxide.
the value of the capacitance signal in accumulation is in correlation with the oxide
thickness measured by TEM. This means there are no tunneling currents.
there is no increase of the C-V signal in inversion which indicates there are no sources
of minority carriers at the semiconductor-oxide interface.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
54
the shape of the C-V curve varies very little with the frequency in depletion, which
indicates that there are few interface states at the semiconductor-oxide interface.
Although the initial hypothesis - concerning the quality of the oxides, the uniformity of
the oxides in terms of thickness and defects repartition, the different sizes of the tips, the
wear of the tips - maybe correct, it became obvious that there may exist other experimental
problems yet to be discovered.
It is also became obvious that a standard reproducible characterization method is
needed in order to understand the characterization challenges faced with the SCM. The C-V
measurements with an impedance analyzer, which provide very accurate and reproducible
results at micro scale and which represent the microscopic counterpart of the capacitive
measurements at nanoscale, is a good candidate.
This chapter contains the following main topics:
- a summary of the experimental conditions with the SCM that are discussed in the literature.
- a general presentation of C-V measurements with impedance analyzers, with the
evidentiation of the similarities and the differences between C-V measurements and SCM
measurements.
- a discussion concerning the parasitic factors that affect the reproducibility of SCM
measurements from the same perspective of comparison with the C-V measurements.
2.2. State of the art
In the literature, the study of the experimental setup concerning SCM is rather scarce.
We consider this to be the main reason for which SCM, after more than 15 years from its
development, to be still in the research and development stage.
2.2.1. Laser light
One of the main concerns regarding the SCM measurements, mentioned in the
literature is the AFM laser effect on the SCM signal.
The influence of the AFM laser on the SCM signal is studied or mentioned in several
publications.
As far as 1989, Williams [2.1] mentions the effects of the laser light, stating that when
a 1 mW laser beam is focused onto the tip and sample, both the amplitude of the capacitance
signal and the apparent location of the lateral depletion edge of a pn junction are significantly
modified by the light. He is also the first one to propose that the laser light may be seen not
only as a source of distortion of the capacitance signal, but it can be used to measure optical
related phenomena into semiconductors, such as carrier generation and recombination rates.
In [2.2], Kopanski et al. show that a significant percentage of the incident laser light on
the cantilever can spill over the cantilever edges. This would result in a decrease of the SCM
signal by comparison with the signal obtained under normal operating conditions (dark
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
55
conditions), which can lead to misinterpretations of the carrier concentration.
Kopanski also provides a solution for avoiding the laser light which, until today,
represents in our opinion, the only practical solution to avoid the influence of laser light upon
the SCM measurements. Kopanski proposes to back the AFM laser as far as possible away
from the tip, by placing it towards the middle of the cantilever. Thus, on a 200 microns
cantilever, the laser can be placed as far as 100 microns from the tip.
In [2.3], Buh et al. evidentiate the effect of laser light on the SCM signal in the case of
a pn junction sample and on the SCS signal (Figure 2.2.1).
Figure 2.2.1.a Line profiles of the SCM signal on
pn junctions with the laser switched on and the
laser switched off [2.3]
Figure 2.2.1.b. i) SCS signals on a MOS like
structure, with the laser on and the laser
switched off, and ii) the numerical integration
of these curves [2.3]
Besides the optical carrier generation, Buh mentions other effects of the laser light,
such as the decrease of the time constant of the minority-carrier generation in the inversion
layer and a decrease of the surface potential, with a corresponding increase of the capacitance
at strong inversion.
Buh is the first one to try and calculate the optically generated excess carriers into
silicon for the case of a commercial AFM detection system which, typically, has a power of 1
mW and a wavelength of 670 nm. According to him, the excess carrier density is estimated to
be around 1017 cm-3.
As a solution for removing the laser light from SCM and SCS measurements, Buh
proposes the modification of a commercial AFM by installing a switch which can turn off the
laser during SCM measurements.
Buh et al continues the study of the influence of the AFM laser in [2.4]. The light
transmission coefficient through the cantilever is studied. Also, an application of the laser light
with the SCM is proposed. Buh suggests that the laser can be used as an active component for
light pumping, for the SCM to measure optical properties of a semiconductor. The effective
carrier recombination lifetime is calculated from a transient capacitance signal measured as a
function of time, when the laser is switched off (Figure 2.2.2).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
56
Figure 2.2.2 Transient capacitance signal measured
as a function of time. The dashed line represents the
laser power intensity Buh et al [2.4]
The parasitic effect of the laser light of the AFM detection system has been mentioned
in other publications, such as [2.5].
2.2.2. Stray capacitance
Another source of signal distortion mentioned in the literature is the stray
capacitance arisen from the interaction between the sample and the cantilever/cantilever
body assembly. Kopanski et al [2.2] are the first to observe this phenomenon. As a solution to
this problem, they propose such a geometrical arrangement that the chip that supports the
cantilever doesn't find itself above the sample, but in lateral of the sample (Figure 2.2.3). In
this way, the parasitic capacitance between the chip of the cantilever and the sample is
avoided.
Figure 2.2.3 SCM sample geometry that minimizes the
stray capacitance from the unintended coupling of the
cantilever and the supporting chip to the sample.
Kopanski et al [2.2]
2.2.3. Surface related phenomena
Stephenson et al [2.6] and Beyer et al. [2.7] mention the influence of surface
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
57
humidity in SCM measurements. They say that, under the influence of the strong electrical
fields between the tip and the sample, there is a possible decomposition of water from the
surface of the oxide and injection of protons H+ into the oxide. Such a charge injection into
the oxide will lead to signal distortion.
Stephenson et al [2.6] and Duhayon [2.8 - pg.118] also brought into discussion the
problem of surface moisture, but in conjunction with another problem in SCM measurements:
surface oxidation. Anodic oxidation is well known from other fields of the AFM, especially
nanolithography [2.9-2.11]. It is also common in STM (Scanning Tunneling Microscopy) [2.12]
and C-AFM (Conductive AFM) [2.13].
The oxidation depends of several factors: the humidity percentage, the magnitude of
the applied biases and the bias polarization, the tip size, the oxide thickness and the quality of
the oxide.
We have to mention that in the C-AFM field, the origins of the protrusions observed on
the surface of the sample still constitute a subject of dispute. There are three hypothesis
concerning the topographical modification after a C-AFM scan: anodic oxidation, electrostatic
repulsion of the AFM tip upon the charge trapping in the oxide and a mechanical deformation
of the silicon substrate. However, all the evidence suggest that the process responsible for the
apparition of the protrusions on the surface of the samples is the oxidation favorized by the
atmospheric conditions and the high biases used during the scans [2.13].
Another phenomenon related to the surface of the sample is the oxide engravement.
SCM measurements are done by scanning a tip on the surface of the oxide in contact mode. If
the interaction force between the tip and the surface is to great, oxide engravement can take
place [2.14]
Figure 2.2.4 The dielectric is scratched away by the tip (AFM profile – left image) which leads to a
thinner dielectric and an increase of the SCM signal (SCM profile – right image) Brezna et al [2.12]
2.2.4. Tip related phenomena
The tip depletion has been discussed in [2.8 - pg.121]. When scanning highly doped
regions, a stronger bias must be applied. This bias can lead to depletion in silicon based tips,
which can lead to signal distortion.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
58
Another problem discussed in the literature, usually in correlation with the phenomenon
of contrast reversal is the tip wear [2.15]. The contact between the tip and the sample can
degrade not only the oxide, but also the tip. If the tip modifies its shape during scanning, a
higher SCM signal will be measured, not because of the modification of the substrate
concentration, but because of a greater 'gate' surface. In the case of the coated tips, tip wear
also means the removal of the coating, which can lead to the shift of the flatband bias and
thus, to contrast reversal.
One of the most discussed subjects in the literature is the contrast reversal. The SCM
signal should increase monotonically with the decrease of doping concentration. Many times at
the beginning of the SCM, cases have been observed when the SCM signal was increasing with
the increase of doping concentration, phenomenon called contrast reversal. Several
explanations have been proposed.
In theory, there is a monotonic dependence between the doping concentration and the
SCM signal: the higher the doping concentration, the lower the SCM signal (Figure 2.2.5).
Figure 2.2.5 The dependence between the doping concentration and the SCM
signal
The formula which describes this relation between capacitance and doping
concentration is:
The contrast reversal appears when this relationship of inverse proportionality is no
longer respected.
Several papers have been published on this subject and several opinions, that often
diverge, have been expressed. Before presenting them, we shall present a theoretical
background which can help create a clearer context for this matter.
We will consider, as a study subject, a MOS structure with: a p-type silicon substrate, a
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
59
5 nm ideal thermal oxide as dielectric and an aluminum metallic gate (workfunction 4.2 eV).
The bias is applied on the gate. The only variable parameter is the substrate concentration
which varies from 1015cm-3 to 1019cm-3.
As it is known from the MOS theory and C-V measurements on MOS structures, the
shape of the C-V curves modifies from one doping concentration to another of the silicon
substrate in the following way (Figure 2.2.6):
• the value of the capacitance in inversion increases with the increase of the doping
concentration
• the slope of the C-V curves diminishes with the increase of doping concentration
• the position of the inflexion point of the C-V curve shifts (towards the right in this case) with
the increase of doping concentration.
Figure 2.2.6 TCV simulation of C-V curves on a MOS structure with a 5nm
oxide thickness. The position of the inflexion points is indicated on the
figure.
As a result, the derivatives of these curves (the SCM signal) will have the following
shape. (Figure 2.2.7) (Tabulated CV v1.1 is a program developed at INSA de Lyon by the PhD.
student Christophe Busseret for capacitance simulations for M.O.S. structures).
The shape of the derivative curves modifies with the increase of doping concentration in
the following way:
C-V curves C-V derivatives (SCM signal)
the value of the capacitance in inversion
increases with the increase of the doping
concentration
• The amplitude decreases
the slope of the C-V curves diminishes
with the increase of doping concentration
• The width at half maximum
increases
the position of the inflexion point of the C-
V curve shifts (towards the right in this
case) with the increase of doping
concentration.
• The position of the maximum shifts
towards the right
The simulated curves correspond to an ideal case. Oxide defaults can further increase
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
60
the shift between the maximum of the signals.
Given the fact that SCM imaging is performed at a fixed Vdc and the maximum for
different doping concentrations are at different voltages, it can be clearly seen that this
characterization method is imperfect. Any single fixed bias cannot correspond to the maxima
of all the curves.
Figure 2.2.7 TCV simulation on dC/dV curves on a MOS structure
with a 5nm oxide thickness.
Several solutions have been found.
• First, SCM imaging is mainly used for qualitative failure analysis imaging. In such
measurements, it is not important to obtain the maximum signal for each doping
concentration, but to obtain only a contrast between the different doped regions. In order to
do this, the operating Vdc is usually chosen to the bias corresponding to the maximum or
around the maximum of the lightest doped region as in the figure below (Figure 2.2.8). In this
case, even if the signal is not corresponding with the maximum for the most doping regions,
the inverse proportionality relation between the doping concentration and the capacitance
signal is however respected.
Figure 2.2.8 The choice of Vdc operating bias with the SCM
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
61
• for doping quantification, simulations try and take into account the shifts between the
signals corresponding to various doping concentrations. However, instead of SCM imaging,
SCS measurements, first introduced by Hal Edwards et al [2.17], are preferred.
However, if the operating Vdc is not carefully chosen, the phenomenon called 'contrast
reversal' takes place, such as in the Figure 2.2.9. As it can be seen, the curve 3 will give a
higher signal than the curve 4 for the VDC0 operating point, although it corresponds to a higher
doping concentration.
Figure 2.2.9 An example where the chosen operating voltage
Vdc will determine the contrast reversal on the SCM image
Such an explanation has been given in the literature by Stephenson et al. [2.6] and
Smoliner et al [2.16]. They explained theoretically and experimentally the origins of the
contrast reversal as a function of the applied bias.
By using this theory, Smoliner has obtained intentionally contrast inversion on a
staircase sample, as a proof (Figure 2.2.10).
Figure 2.2.10 Doping profile of the epitaxial staircase structure determined by SIMS (left). Sections through SCM images taken at different operating voltages (right). Smoliner et al [2.16]
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
62
2.3. C-V measurements
2.3.1. Introduction
As it can be seen from the bibliography (paragraph 2.2) and from our own
measurements (Figure 2.1.1, Figure 2.1.3), SCM is a characterization method with many
problems concerning its reproducibility.
One area where SCM can be used for now with success is in obtaining qualitative failure
analysis images of doping profiles. These images only require a difference in contrast between
the different doped areas and they don't need a rigorous control of the reproducibility of the
signal.
That is why we considered necessary to use an alternative, calibrated method of
measurement of the properties of the MOS structures, in order to validate the data obtained
with the SCM.
Such a capacitance characterization method, closely related to the MOS structures and
transistors manufacturing, is the C-V measurements with an impedance analyzer.
This characterization method has been used in the microelectronics industry for more
than 50 years. A considerable know-how exists in this field. The measurements are
reproducible and quantifiable.
The C-V measurements with an impedance analyzer are not only reproducible and
quantifiable, but also versatile. They have a multitude of applications, illustrate in the table
below.
Parameter or phenomenon studied Technique used
Substrate Doping concentration (uniform) Values of Chf for accumulation and inversion
Doping profile near the Si-SiO2 interface
Chf(V) curve in depletion
Minority carrier lifetime Deep depletion transient capacitance
Oxide Oxide thickness Values of Chf or Clf for accumulation
Oxide leakage current (when not too important)
Quasi-static method
Image charge ΔVFB between experimentally measured Chf(V) and computed ideally Chf(V) curves.
Density of mobile ions ΔVFB from the hysteresis of the Chf(V) curves
Hot carrier trapping ΔVFB from Chf(V) curves
Oxide breakdown (dielectric strength, wear-out)
Voltage ramping, or time before failure measurements
Interface Energy distribution of the interface
states in the silicon gap
Quasi-static, DLTS, conductance methods
Relaxation time of interface states Conductance method
Standard deviation of surface potential Conductance method
Recombination velocity at the interface Deep depletion transient capacitance
Chart after Barbottin [2.18 - pg.261]
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
63
In the next paragraphs, we are going to describe the basic principles of C-V
measurements. We will demonstrate how to obtain data concerning the properties of a MOS
structure from a C-V signal, on the test sample that we also used for SCM measurements.
2.3.2. The principle of the C-V measurements with an impedance analyzer
2.3.2.1. Introductive notions concerning impedance analyzer
The impedance analyzer used for the measurements is an AGILENT 4284A.
The measurements of the capacitance of a MOS structure are done by adding the
sample in an AC circuit.
The frequency of the AC circuit can be chosen by the user in the interval (100Hz;
1MHz).
In order to keep the harmonics of the signal frequency from giving rise to conductance
and capacitance values with no physical signification, only AC voltages of small amplitude can
be applied. The small signal range is defined as the range of signal amplitude in which the
measured capacitance is independent of the AC gate voltage amplitude [2.19 - pg.585].
Commonly, the maximum AC voltage used with the impedance analyzer is 50 mV.
The DC voltage can be varied in the interval (-10V; +10V).
A C-V voltage ramp is obtained by choosing an AC gate voltage amplitude, a voltage
frequency and by slowly varying the DC gate voltage in a chosen interval, with a
predetermined voltage step. For each DC voltage, the capacitance and the admittance of the
MOS structure are measured.
The MOS capacitor under tests can be modeled as an ideal capacitor which presents a
parallel and a series resistance (Figure 2.3.1).
Figure 2.3.1 Simplified modelisation of the MOS
structure under tests
The parallel resistance takes into consideration the tunneling currents through the gate
oxide. The series resistance takes into consideration the substrate resistivity, the resistance of
the contacts, the resistivity of the coaxial cables and other elements of the instrument's
electronic circuit.
However, the impedance analyzer is not able to measure directly these parameters
(C,Rs,Rp).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
64
For the measurement of the MOS capacitance, the impedance analyzer is able to use
two equivalent models, a series model and a parallel model (Figure 2.3.2).
Figure 2.3.2 The equivalent series model (left) and the parallel model (right) used by the impedance analyzer for a MOS capacitor. Cms series measured capacitance;
Rms series measured resistance; Cmp parallel measured capacitance; Rmp parallel measured resistance. All the electric components from this model are considered
ideal.
The impedance analyzer measures the real and the imaginary components of the total
impedance of either of these models by the use of an admittance bridge. Other impedance
analyzers may use lock-in amplifiers. The output measured variables are (Cms;Rms) for the
series model and (Cmp;Rmp) for the parallel model.
In order to obtain the values of the parameters (C,Rp,Rs) of the M.O.S. structure, the
relations between (Cms;Rms) and (C,Rp,Rs) or between (Cmp;Rmp) and (C,Rp,Rs) must be found
and calculations must be performed.
2.3.2.2. The series model
In the series model, the real capacitance of the M.O.S. structure is approximated with
an ideal capacitance in series with an ideal resistor (Figure 2.3.3).
Figure 2.3.3 The equivalence between the simplified capacitance model and the parallel
capacitance model
The equivalent impedance Z*RpC of the capacitor C in parallel with the resistance Rp is:
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
65
The total equivalent impedance of the (Rs,Rp,C) model is:
The equivalent impedance of the series capacitance model is:
By identifying the real part and the imaginary part of the two equivalent impedances,
we obtain:
(1)
(2)
As it was obvious from the beginning from the comparison of the two diagrams, the
measured capacity Cms is not equal to the capacity C of the M.O.S. structure. However, this
equality may become true in certain conditions. Further details will be given in the next
section, when it will be discussed what model should be chosen for the measurements, series
or parallel.
2.3.2.3. The parallel model
In the parallel model, the capacitance of the MOS structure is approximated with an
ideal capacitance in parallel with an ideal resistor (Figure 2.3.4).
Figure 2.3.4 The equivalence between the simplified capacitance model and the parallel capacitance model
The impedance of the MOS capacitor is the same as in the previous case. However, for
calculus reasons, it will be written under a different form.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
66
The equivalent impedance of the parallel capacitance model is:
The equality between the two impedances gives:
After a calculus similar to the one presented for the series model, which involves the
identification of the real terms and the imaginary terms, we obtain:
(3)
(4)
2.3.2.4. The choice of the model used with C-V measurements
On an unknown sample, measurements must be performed by using both the series
model and the capacitance model, on several range of frequencies.
Following these measurements, several situations can be met:
1. Cmp stays constant with the variation of the frequency.
From the Cmp formula (4), it can be seen that Cmp will remain constant with the
variation of the frequency when Rs is negligible. When Rs->0, Cmp->C and Rmp->Rp (Figure
2.3.5)
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
67
Figure 2.3.5 The equivalence between the simplified capacitance model when Rs is negligible and the parallel
capacitance model
In this situation, the measured parameter Cmp represents the capacitance C of the MOS
structure.
2. Cmp varies with the frequency while Cms stays constant.
When Cmp varies with the frequency, it means that there is a non-negligible series
resistance (4). If Cms stays constant, it means that, this time, Rp is infinite, Cms->C (formula 2)
and the capacitance of the MOS structure can be extracted from the series model (Figure
2.3.6)
Figure 2.3.6 The equivalence between the simplified capacitance model when Rp is infinite and the
series capacitance model.
3. Cmp and Cms vary with the frequency
In this case, both Rp and Rs cannot be ignored. The formulas (1-4) cannot be simplified
anymore.
Yang et al [2.20] propose the extraction of the unknown parameters C, Rs and Rp by
performing measurements within the parallel model (Figure 2.3.5), at two different
frequencies. As a result, four experimental values will be obtained, Cmp1, Cmp2, Rmp1, Rmp2. The
unknown parameters will be written as functions of these four experimental values.
The equivalent impedance of the parallel model is:
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
68
This relation can be written using the expression of the quality factor Q. For a complex
impedance, the Q factor is the ratio of the reactance to the resistance. In the case of a
capacitor:
A high quality factor capacitor is always best than a low quality capacitor. The reason is
that is always easy to add supplementary resistances into a circuit. An ideal capacitor has a
quality factor Q->∞, with zero effective resistance.
The total impedance of the parallel model becomes:
The total impedance of the MOS structure is calculated in a similar way:
By equalizing the imaginary parts and the real parts of the two impedances and writing
the obtained expressions for two different experimental frequencies, we will finally obtain:
and
Thus, in the situation when the Cmp and Cms vary both with the frequency, the
impedance of the MOS structure can be calculated from the experimental values Cmp1, Cmp2,
Rmp1, Rmp2 measured with the parallel model, for two different frequencies.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
69
2.3.3. C-V measurements on a test sample
2.3.3.1. The test sample
For comparisons between C-V and SCM measurements, we have used a sample which
consists in a p-type silicon substrate, 1015 cm-3, covered with a 5 nm thick thermal oxide.
Metallic electrodes (Nickel - Gold) which serve as the gate for the MOS structure have
been fabricated by evaporation with an electron gun (Figure 2.3.7)
Figure 2.3.7 Ni-Au electrodes deposited on a p-type substrate covered with a 5 nm thick thermal oxide.
The size of the electrodes has been measured with an optical microscope. There are
electrodes of six different sizes, 100 μm x 100 μm, 150 μm x 150 μm, 200 μm x 200 μm, 300
μm x 300 μm, 400 μm x 400 μm and 600 μm x 600 μm.
2.3.3.2. Preliminary measurements
As we have presented in 2.3.3.4., preliminary measurements must be performed on
the sample, at different frequencies, using both models, the parallel model and the series
model. From the observation of the behavior of Cms and Cmp with the variation of the
frequency, we can establish which model must be used for the following measurements and
whether any corrections are necessary to the measured capacitance in order to find out the
capacitance C of the M.O.S. structure.
For the C-V curves presented in Figure 2.3.8, we have used the following parameters:
AC voltage Vac=50 mV, DC voltage step 20 mV. The C-V curves have been obtained for each
of the frequencies: 100 Hz, 1 kHz, 10 kHz. The measurements in Figure 2.3.8 have been done
on 600 μm x 600 μm electrodes.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
70
Figure 2.3.8 C-V measurements on 600 μ x 600 μ electrodes, in both series and parallel models, in the frequency interval 100 Hz - 10 kHz, at Vac = 50
mV
Cmp remains constant with the variation of the frequency. This means that the series
resistance is negligible. The measured capacitance in the parallel model Cmp represents the
capacitance of the MOS structure, presented in Figure 2.3.1.
2.3.3.3. Oxide thickness
From the value of the capacitance in accumulation Cox, oxide thickness tox can
be calculated.
In accumulation, the majority carriers are attracted towards the interface. In our case,
the majority carriers are the holes and they are attracted towards the Si-SiO2 interface for
negative voltages applied to the gate. The capacitance of the MOS structure is given by the
capacitance Cox of the dielectric (Figure 2.3.9).
where:
Cox is the capacitance in accumulation Cox=2.26 10-9 F
εo is the permittivity of vacuum εo=8.85 10-12 F/m
εSiO2 is the relative permittivity of SiO2 εSiO2=3.9
S is the surface of the gate S=600μm2
tox is the dielectric thickness -
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
71
Figure 2.3.9 The MOS structure in accumulation for a p-type substrate
From this relation, the oxide thickness can be calculated.
For our test sample, we obtain tox = 5.5 nm
2.3.3.4. Doping concentration
From the values of the capacitance in accumulation Cox and in inversion Cmin,
the doping concentration can be calculated.
In inversion, the majority carriers are repelled away from the interface. In our case,
the majority carriers are the holes and they are repelled away from the Si-SiO2 interface for
positive voltages applied to the gate. The capacitance of the MOS structure is given by the
capacitance Cox of the dielectric in series with the capacitance of the depletion layer (Figure
2.3.10).
The total capacitance in inversion is:
This equation can be solved iteratively and the doping concentration can be found. A
program with an iterative algorithm can be written to facilitate the process.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
72
Figure 2.3.10 The MOS structure in inversion for a p-type substrate
In our case, we can simply verify the substrate doping concentration of 1015cm-3,
measured with a four-point probe, by replacing this value into the equation and seeing
whether the equality is verified.
Cox the capacitance in accumulation Cox=2.26e-9 F
Cmin the capacitance in inversion Cmin = 2.55e-11 F
εo the permittivity of vacuum εo=8.85e-12 F/m
εSi the relative permittivity of silicon εsi=11.7
k Boltzmann constant k = 1.38e-23 J/K
T temperature T=300 K
q the elementary charge q=1.6e-19 C
ni intrinsic concentration 1.10cm-3
Na acceptor doping concentration -
2.3.3.5. Flatband capacitance, flatband voltage
By knowing the doping concentration, the flatband capacitance and the
flatband voltage can be calculated.
At the flat-band condition, variations in the gate potential give rise to the addition /
subtraction of incremental charge in the substrate, at a depth LD. We remind that the Debye
length LD is the distance at which the electric field generated by a perturbing charge falls off
by a factor 1/e.
The capacitance of the MOS structure at the flatband condition is:
From calculus, we obtain CFB=2.6 10-10 F
Knowing CFB, we can find the flatband voltage from the C-V curve. The voltage VFB
corresponding to CFB is VFB=-0.56 V (Figure 2.3.11)
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
73
Figure 2.3.11 CFB and VFB calculus from the C-V curves
For substrates with low doping concentrations, the flatband voltage will always be close
to the inversion region of the C-V curve, while for substrates with high doping concentrations,
the flatband voltage will be very close to the accumulation region of the C-V curve.
2.3.3.6. Interface states, surface potential
From C-V measurements performed at two different frequencies, the interface
state density can be calculated.
In the proximity of Si/SiO2 interface, carriers generated by different sources (intrinsic
energy levels, doping impurities, interface states etc) are characterized by a cut-off frequency
beyond which they cannot follow anymore the variations of the surface potential imposed by
the gate bias.
The C-V measurements can be considered as a filtration procedure, where the
frequency chosen for the measurements represents the cut-off frequency. All the carriers that
have a lower cut-off frequency than the measurement frequency will not bring their
contribution to the capacitance signal.
This allows us to calculate the interface state density for a given MOS structure. The
method used here to calculate the interface state density has been used for the first time by
[2.21].
From the C-V measurements performed with the parallel model, we will consider for our
calculus the two measurements performed at the two extremes of the frequency range: 100
Hz and 10 kHz (Figure 2.3.12). In the figure, it can be clearly seen that the capacitance signal
given by the interface states at the lower frequency is higher than the capacitance signal at
higher frequency, which means that, at higher frequency, less carriers are able to respond to
the change in the gate voltage.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
74
Figure 2.3.12 Calculus of the interface state density from the measured C-V curves at two frequencies
The capacitance of the MOS structure for the high frequency can be written:
HF
1 1 1
C ox SemiconductorHFC C= +
and the capacitance of the MOS structure for the lower frequency:
LF
1 1 1
C ox SemiconductorLFC C= +
where Cit is the capacitance given by the interface states.
From these two equations, we obtain:
The interface state density is:
The above formula is a function of the gate bias. For finding out the the energy
distribution of the interface states, we have to represent the interface state density as a
function of the surface potential:
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
75
Figure 2.3.13 The interface state distribution as a function of the surface potential
2.3.3.7. A preliminary comparison between the C-V signal and the SCM signal
From the C-V curves that we presented, it is obvious that the capacitance
measurements with an impedance analyzer are highly reproducible. The properties of the C-V
curve don't change from a measurement to the next. The measurements done on different
electrodes give exactly the same results. Measurements performed in different days on the
same sample confirm the reproducibility of the technique.
The value of the capacitance in accumulation is always 2.26.10-9 F on the 600μ x 600μ
electrodes. The value of the capacitance in inversion remains the same: 2.55.10-11 F. The
inflexion point of the C-V curve (which represents the maximum of the SCS signal) is always
at -0.8 V. The C-V curves have always exactly the same shape.
On the other hand, the SCM signal is instable and sometimes presents significant
differences even between consecutive measurements. As it can be seen from the example
from Figure 2.1.1, the parameters of the SCS signal vary significatively: the amplitude of the
signal, the position of the maximum, the shape of the signal. Sometimes, the SCS signal
presents a single maximum, other times two or more local maxima.
A direct comparison between the C-V and SCS measurements done on our test sample,
several discrepancies can be observed (Figure 2.3.14).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
76
Figure 2.3.14 C-V measurements (left) and SCS measurements (right) performed on a 5 nm thermal oxide deposited on a 1015 cm-3 p-type silicon substrate
The SCS signal presents hysteresis between the trace and the retrace. With the C-V
signal, the hysteresis is given by the presence of mobile charges in the oxide. However, the C-
V curve doesn't present any hysteresis at all. Even if the structure is stressed (a high DC bias
is applied to the MOS structure for several minutes), the trace and the retrace in the case of
the C-V signal will still coincide. In addition, the mobile charges don't play any role at room
temperature. The sample must be heated in order that the mobile charge be able to migrate
through the oxide under the influence of an electric field.
The SCS signal presents a minimum in the inversion region which usually corresponds
to the presence of minority carriers, which doesn't appear on the C-V curve.
From the integral of the SCS signal, it can also be observed that the capacitance level
in the inversion region is higher than the capacitance level in accumulation (Figure 2.3.15).
Physically, this is not possible. The capacitance in accumulation represents the capacitance of
the dielectric which is the maximum capacitance of the MOS structure. Even if minority
carriers reach the Si/SiO2 interface in inversion, the capacitance signal cannot surpass the
accumulation capacitance value.
Figure 2.3.15 The integral curve of the SCS signal. the bias is applied on the substrate. The curve has been inversed and normalized.
In the next paragraphs, we will investigate the source of these problems, based on the
problems already signaled in the literature and on our experimental observations.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
77
2.4. Influence of the laser light
2.4.1. Introduction
The movements of the tip on the surface of the sample and the deflexion of the
cantilever are detected with most commercial AFMs by a laser detection system (Figure 2.4.1)
Figure 2.4.1 Diagram of the laser detection system (left). CCD camera image of the laser positioning at
the end of the cantilever (right)
From the theory of semiconductors, it is known that when a semiconductor is perturbed
from the equilibrium state, an excess or deficit in the carrier concentrations relative to their
equilibrium values is created inside the semiconductor. Recombination/Generation (R-G)
phenomena become dominant inside the semiconductor.
One of the carrier generation mechanisms inside a semiconductor, besides direct
thermal generation, thermally assisted generation by R-G centers and impact ionization, is
photogeneration. If the incident photon energy is greater than the band gap energy of the
semiconductor, then the light will be absorbed and electron-hole pairs will be created as the
light passes through the semiconductor. The photogeneration process always creates an equal
number of electrons and holes (equal number of majority and minority carriers).
In this context, considering the wavelength of the laser and the positioning of the laser
on the cantilever, it seems pertinent to question whether the laser light of the AFM detection
system perturbs or not the capacitance measurements, which are sensitive to the carrier
concentration.
The laser used on the Veeco system has a wavelength in the red domain (670 nm).
Thus, the photon energy is greater than the band gap energy of the silicon.
Band gap of the silicon 1.12 eV
Photon energy for a wavelength of 670 nm 1.85 eV
The laser beam is positioned at the end of the cantilever, exactly above the spot where
the electrical measurements are performed (Figure 2.4.1)
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
78
As discussed in the section (2.2.1), many research groups have already performed
studies regarding the impact of the laser light upon the SCM measurements [2.1-2.5].
In the next section, we present our measurements which show how the SCM
measurements are distorted by the laser light of the AFM detection system.
2.4.2. Experimental evidence
In order to evidentiate whether the laser has a significant influence on the capacitance
measurements on the Veeco system or not, SCS measurements have been performed on
different oxides deposited on low-concentration substrates, in the presence and in the absence
of the laser. The samples used for the analysis consist of various oxides (5 nm thick thermal
oxides, native oxides, plasma oxides) deposited on p-type substrates with a doping
concentration of 1015cm-3 corresponding to a 8 .cm resistivity
Considering the fact there are no ways to shut down the laser by means of the software
controlling the apparatus, we have chosen the method proposed by Kopanski et al [2.2] : we
tried to minimize the laser influence by displacing the spot along the axis of the cantilever so
that the laser is located far (several tens of microns) from the point where the tip touches the
surface (Figure 2.4.2).
For a 1015 at.cm-3 substrate, the diffusion length of carriers in silicon is in the 5 μm
range [2.22]. The cantilevers used for our measurements are approximatively 200 m in
length. If the laser spot is placed towards the middle of the cantilever, the pairs generated by
the laser should recombinate before reaching the point where the SCS is performed.
Figure 2.4.2 CCD camera image of the laser positionement at the end of the cantilever (left) and
towards the base of the cantilever (right)
The positioning of the laser represents a compromise between the quality of the
topography measurements, which need the laser to be reasonably at the end of the cantilever,
and the SCS measurements. Several comparisons have been made between SCS
measurements recorded with the laser spot placed roughly at the middle of the cantilever and
measurements taken by temporary obstructing the laser beam : no difference between these
measurements has been observed.
However, in any cases, remnant light may still influence the measurements.
In Fig.2.4.3 SCS is operated on low-doped p-type substrate covered by a thin native
oxide with and without the presence of the AFM laser light. The difference between the
resulting SCS measured in both conditions emphasizes the decrease of the signal to noise ratio
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
79
due to the electron-hole generation originating from the impact of the laser light on the
surface.
Figure 2.4.3 SCS on a native oxide grown on a low doped p-
type substrate (1015 at.cm-3 ) with and without laser
This is the first clear evidence why in most cases, no signal at all was previously
obtained with the SCS measurements on native oxides. In the image, it can be seen that the
signal decreases more than 6 times in the presence of the laser. The amplitude of the peak
decreases from more than 1.2 V in the absence of the laser to approximatively 200 mV in the
presence of the laser, close to the level noise of 50 mV on the Veeco system
It must also be pointed out that the generation of electron/holes pairs by the laser light
has also created an additional peak in the negative voltage region corresponding to the
inversion zone.
Figure 2.4.4 also illustrate the influence of the laser on a 5 nm thermal oxide.
The strong decrease of the signal to noise ratio due to the laser light is evident. The
thermal oxide under study has been grown on a low doped substrate (1015 at/cm3), in order
that the SCS signal is as strong as possible which explains this strong influence of the
photogeneration of carriers.
Figure 2.4.4 SCSon a 5 nm thick thermal oxide
grown on 1015 at/cm3 p-type substrate
2.4.3. Quantitative aspects
We have tried and quantify the intensity of the light on the surface of the samples and
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
80
the excess of carriers generated by the laser.
On an intrinsic substrate, a gradual doping profile has been created by thermal
diffusion of phosphorous. The sample was cleaved and the SCM measurements have been
performed on the cross-section. The cross-section of the samples is covered by a thin native
oxide.
SIMS measurements have been used as a reference characterization method, in order
to know the value of the concentration doping in correlation with the distance from the surface
of the sample (Figure 2.4.5)
Figure.2.4.5 SIMS profile of Phosphorous diffused in
a silicon intrinsic substrate.
Figure 2.4.6 SCM profile of Phosphorous diffused
in a silicon intrinsic substrate.The dotted green
line indicates the depth from which the SIMS
profile is too noisy too provide the real
concentration of phosphorous.
The profile measured by SCM (Figure 2.4.6) in the presence of light is strongly altered
from 200 nm and deeper (where the carriers concentration falls under 1017 cm-3 according to
SIMS) and does not correspond to the SIMS profile any more. The absolute value of the SCM
signal decreases strongly and even becomes positive at 600 nm depth. The strong influence of
the applied DC voltage on the measured SCM signal indicates that this result is due to the
presence of minority carriers generated by the laser and attracted by the tip. When the laser
light is placed away from the scanning zone, the influence of the laser in the zone probed by
the tip is removed and a SCM profile corresponding to the SIMS profile can be retrieved.
The doping concentration value at 200 nm, where the SCM signal in the presence of
light becomes different from the SCM signal in the absence of light, is between 1017 cm-3 and
1018 cm-3. In consequence it can be estimated that the Veeco AFM laser generates a
concentration of electron-hole pairs between 1017 cm-3 and 1018 cm-3. This value is in
accordance with the value calculated in the literature for the same system [2.4].
However, it must be pointed out that this is only an estimation. Several causes have
prevented it us to make an accurate measurement.
First of all, not all the impurity phosphorous atoms are activated because of the doping
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
81
method. Given the fact that SIMS measures the concentration of phosphorous atoms,
while SCM is sensitive to carriers, there should be a slight difference between both
profiles.
Another reason is that the doping profile starts immediately at the edge of the sample.
Given the drift of the piezoelectric tube in x-y direction and the danger that the tip falls
off the surface, it is possible that the SCM measurements have not taken into
consideration the first tenths of nanometers of the doping profile. In consequence, a
slight shift in the depth direction may exist between the SIMS profile and the SCM
profile. In order to be sure that the entire doping profile has been measured, a capping
layer should have been deposited on the surface of the sample.
Since one of the SCM strengths is to provide a high signal for low concentrations, future
experimental setups should take care of minimizing if not suppressing the impact of the laser
light on the sample's surface or choose another technique to measure the deflexion. This is an
important condition for SCM to be able to provide reliable information e.g. on a junction
located under a shallow trench isolation (STI) where both the n-side and the p-side are low
doped.
From the Figure 2.4.6, it can be also pointed out that SCM without the laser light is
able to provide information about the carrier distribution at concentrations lower than SIMS,
which is limited by the noise in this particular configuration (low erosion speed, severe
interference between the masses of SiH and Phosphorous, which imposes to use a high mass
resolution and leads to a lower signal).
2.4.4. Comparison with C-V measurements
In the literature, there are two main effects of light that influences the capacitance
measurements [2.3].
One effect is a decrease in the time constant of the minority carrier generation in the
inversion layer resulting in a low frequency like characteristic, even at high frequencies.
Another effect is a decrease of the surface potential, resulting in a reduction of the
width of the depletion layer.
However, we considered that neither of these two phenomena cannot fully explain the
drastic decrease of the SCM signal in the presence of light (Figure 2.4.3)
Comparisons with macroscopic curves have been performed in order to better
understand the role of illumination on SCS measurements. The same thermal oxide as in
Figure 2.4.4 has been investigated using 600 x 600 m NiAu electrodes illuminated by a 20
mV laser, (wavelength 632.8 nm), leading to the macroscopic C-V of Figure 2.4.7. The
electrode consists in a 3 nm thick layer of Ni in contact with the oxide and a 300 nm thick Au
top layer. The creation of minority carriers which modify the capacity in depletion and
inversion is evident in this figure and corresponds to the observations from reference [2.3].
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
82
Figure 2.4.7 Effect of a laser on the macroscopic C-V curve for a 5
nm thick thermal oxide and thick NiAu electrodes
Figure 2.4.7 also shows the influence of white light focused on the surface of the
sample on the C-V curve The increase of the signal in the inversion region is clearly detectable
and will also lead to a drop in the SCS signal. This shows that, since in AFM measurements the
sample surface is not protected by anything else than the cantilever, even the ambient light
can have a rather detectable influence on the results provided by SCS.
However, this still doesn't seem to explain the decrease of the SCM signal in the
presence of light. The calculated derivatives of the C-V curves in Figure 2.4.8 show that the
influence of the light on the amplitude of the peak from the depletion region is minimum.
Figure 2.4.8 Numerical calculated derivatives of the C-V curves from
the Figure 2.38
The amplitude of the peak in the presence of light stays unchanged and the amplitude
of the peak in the presence of the laser decreases very little. In consequence, the decrease of
the surface potential in the presence of light, resulting in a reduction of the width of the
depletion layer, doesn't seem to explain the drastic decrease of the amplitude of the SCS peak
under the influence of the AFM laser.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
83
We are proposing the following two explanations :
The consequences in terms of SCS signal could be understood by the following
amplitude of the SCM oscillator Aosc(t) is extracted from the stray capacitance (1e-12 –
1e-13 F) with a lock-in amplifier. When the C-V curve is monotonous and Vac as low as
possible, the variation of Aosc(t) will be reasonably sinusoidal at the same frequency as
Vac(t), leading to a SCM signal equal to A1 in Fig.2.4.9. But if capacitance increases
again before the period Vac(t) is over, then the corresponding Aosc(t) is distorted. The
higher minimum of the C-V curve due to the increase of the signal in inversion reduces
the maximum variation of capacitance and thus the SCM output drops down to A2 in
Figure 2.40. The spectral component of Aosc(t) at the same frequency as Vac(t) is also
reduced, reducing the SCM output further.
Thus, the decrease of the surface potential in the presence of light, resulting in a
reduction of the width of the depletion layer, could have much more influence on the
SCS signal than on the derivatives of the C-V curves
Figure 2.4.9 Schematics of the SCM oscillator amplitude Aosc
with time when the corresponding C-V curve is monotonous
(dotted line) and when it increases in inversion (plain line).
Another explanation could be that, in the case of the C-V measurements with an
impedance analyzer, the surface is mostly protected by the influence of the light by 300
nm thick electrodes that prevent the light from reaching the semiconductor. The
minority carriers that distort the C-V curves are pumped under the electrodes from the
exposed edges of the electrodes. On the other hand, in the case of the SCS, the laser
beam is obstructed only by a metal layer 3-5 nm thick, that represents the back
coating of the cantilever.
In order to test this hypothesis, we have tried to reproduce with the C-V measurements
the same experimental conditions as with the SCS.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
84
Figure 2.4.10 shows the influence of illumination on the same thermal oxide as
Fig.2.4.7, but covered with 5 nm thick aluminum electrodes that present a transmission
coefficient to light in comparison with previously used 300 nm NiAu electrodes that don't let
the light illuminate directly the area under them.
Figure 2.4.10 Effect of light on the macroscopic C-V curve for a 5 nm thick
thermal oxide and 5 nm thick Al electrodes. The arrow in the figure
indicates the change of slope of the C-V curve measured under illumination
Before discussing the results, it must be pointed out that using Al electrodes, the bad
electrical properties of the interface between the electrode and the oxide leads to the
existence of charges at the metal-oxide interface which contribute to the increase of the
capacitance in depletion, even at high frequencies. This effect is well known by people
involved in the macroscopic C-V characterization and imposes the choice of the electrodes
(like NiAu instead of Al). A post metallization annealing is sometimes used to enhance the
electrical properties of the interface between the Al electrode and the oxide.
Figure 2.4.11 Numerical calculated derivatives of the C-V curves from the
Figure 2.4.10
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
85
Besides the increase of the capacitance in inversion arising from the choice of the metal
for the gate electrodes, several other important changes can be observed by comparison with
the C-V curves from the Figure 2.4.7
The first difference is that the minimum capacitance in the depletion region shows even
higher values, which leads to the modification of the position of the inflexion point of
the curves. In Figure 2.4.11, it can be observed that the shift of the inflexion point
translates into the shift of the peak from the depletion region. This effect can explain
why the maxima of the SCS signals measured in the presence and in the absence of
the laser beam, in Figure 2.4.3 (on the native oxide) and in Figure 2.4.4 (on the
thermal oxide) are shifted.
A second difference arises from the fact that the use of the laser leads to a change in
the slope of the C-V curve in the negative voltages (indicated by the arrow in Figure
2.4.10). This means that in this case, even if a very small amplitude for Vac were used,
the change of the slope due to illumination would lead to a bad interpretation of the
SCS in terms of doping concentration (in the case of doping mapping) or oxide
thickness (in the case of oxide characterization). In any case, this change of slope leads
to a decrease of the SCS signal.
The modification of the slope also explains why the peak of the blue curve (laser) shifts
less than the peak of the red curve (white light), although the increase of the minimum of the
C-V curve in depletion is higher. The explanation is that while the increase of the minimum of
the C-V curve in depletion shifts the peak towards left, the modification of the slope shifts the
peak towards the right.
Another surprising observation is the behavior of the interface states in the presence of
light. In the dark, the presence of the interface states can be observed on a C-V curve, at the
frontier between the depletion region and the inversion region (Figure 2.4.12). The signal
provided by the interface states lowers with the increase of the frequency of the Vac voltage
because the interface states have less time to react.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
86
Figure 2.4.12 The variation of the interface states with the variation of
the frequency
Thus, it can be concluded that the presence of the laser is able to create additional
peaks in the SCS, each corresponding to a different inflexion point of the C-V curve in Figure
2.4.10. This can explain why, sometimes, on the SCS images, multiple peaks can be observed
(Figure 2.4.13).
In summary, the decrease of the signal to noise ratio with illumination can now be
explained by
The change of slope of the C-V curve
The increase of the minimum of the C-V curves in depletion which reduces the dynamic
range of SCS and changes the position of the maximum of the SCS peak
The increase of C-V curves in inversion which may distort the SCS when high values of
Vac are used
Figure 2.4.13 SCS on a 2 nm thermal oxide grown on a low
doped n-type substrate (1015 at.cm-3 ) with and without laser
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
87
2.4.5. Solutions for the elimination of the parasitic AFM laser effect
The only practical method at our disposal in order to eliminate the parasitic laser light
was to displace the laser spot towards the base of the cantilever. This method has been first
proposed by Kopanski et al [2.2] and it is fairly efficient.
The laser-beam-reflexion detection method is the most common method in commercial
AFMs to measure cantilever deflection. However, there are many other detection methods for
measuring the cantilever deflection [2.23]: the tunneling detection system, the heterodyne
detection system, the homodyne detection system, the polarization detection system.
However, this approach is improbable. From the all types of characterization methods
with the AFM, SCM is the only one strongly affected by the parasitic light of the laser beam. It
is unlikely that the detection system will be changed only for the SCM on commercial systems
although it can be envisaged for a dedicated instrument.
A practical modality to eliminate the laser effect, proposed in the literature is by simply
turning off the laser. Buh et al [2.4] proposed a home made solution which consists in an
installation of a switch that can be used to turn the laser on and off. This solution is valid only
for the SCS. SCM imaging cannot be performed with the laser off. However, for the
quantification of the doping profiles, the critical information is obtained with the SCS
A similar solution, by turning off the laser temporarily, called dark lift, has been
recently perfected by Veeco for its systems. With the dark lift, each line is scanned twice as for
the lift mode used for e.g. Electric Force Microscopy (EFM) measurements or KFM
measurements. The goal of the first passage, done with the laser on, is to sense the
topographical details. At the second passage on the same line, the laser is turned off. The
AFM, unable to sense the topographical features anymore, will use the topographical data
recorded at the first passage. The second passage is used to do the capacitance
measurements in the dark. This mode can be used only for SCM imaging, but not for the SCS.
One of the critical practical problems with solutions that turn on/off the laser is the
resulting drastic decrease of the lifetime of the photodiode.
A more viable alternative would be the installation of a hatch in front of the photodiode.
The hatch would block the laser beam and prevent it from reaching the surface of the sample
when closed off.
The most practical solution to prevent the laser beam of reaching the surface would be
to modify the parameters of the cantilevers used with the SCM. Larger cantilevers would block
more effectively the light. A thicker backside metallic layer in combination with thicker silicon
cantilevers would completely reflect/absorb the incident laser beam. Buh et al [2.4] discussed
in detail the silicon and the coating metal layer in function of their thicknesses.
Another range of solutions could concentrate on the properties of the laser beam : a
theoretical possibility would be the use of photodiodes that generate a laser beam with a
wavelength corresponding to a smaller photon energy than the silicon band gap energy (or
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
88
any other semiconductor that might be studied with the SCM). If we take into account the
case of the silicon, it would mean that the photon energy should be smaller than Eg=1.12 eV,
which will mean the use of photodiodes/photodetectors starting with the mid infrared
spectrum.
Other solutions may concern the decrease of the intensity of the incident light on the
surface of the sample. This can be done either by using lasers less powerful or more
divergent.
2.4.6. Conclusions and Perspectives
We have shown that the AFM laser strongly affects the signal/noise ratio of the SCS
signal. Considering that the amplitude of the SCS signal is the main parameter used in the
attempts to quantify the doping profiles, it is vital that this parasitic effect be completely
removed.
In this work, the effect of the laser upon the capacitance signal has been removed by
displacing the laser spot towards the base of the cantilever. Gradual doping profiles have been
used to estimate the carrier generation. Comparisons between SCS and C-V measurements in
the presence of light have been made and the phenomenon of the decrease of the SCS signal
has been better explained. C-V measurements have been performed on usual micrometric
electrodes and on thin electrodes for better reproducing the experimental conditions with the
AFM
We have shown that, in the future, gradual doping profiles as the one in Figure 2.4.6
can be used as reference samples for measuring the intensity of the laser light. Such gradual
profiles can be further ameliorated in the following way:
Profiles with all the impurity ions activated should be fabricated.
Such samples should present a capping for an accurate positioning by reference to the
edge of the sample where the doping profile starts.
The variation of the doping concentration should be monotonous.
The doping profile should be as large as possible, extending on several microns. Thus,
the error of the doping positioning would be decreased..
As Buh showed [2.3], studies of the carrier recombination time from the transient
response curve are possible, given that the appropriate setup is put in place. While C-t ramps
can be easily performed with Nanoscope or by using an external software like LabView (Figure
2.4.14), these signals must be correlated with the precise time when the laser beam doesn't
hit the surface any more. A switch controlling the laser or an electromagnetic switch
controlling an obstructing hatch, commanded by a Labview station could be used.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
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Figure 2.4.14 Transient C-t ramp recorded with Nanoscope (left). Zoom in of the transient C-t ramp in
the transition region (right)
We have shown that the presence of light can lead to the presence of multiple peaks on
the SCS signal, some of them caused by the interface states. Interface states can be studied
in this way, in correlation with C-V measurements performed on thin electrodes.
Quantification attempts with an external high intensity laser are possible, in correlation
with simulations.
2.5. Influence of the parasitic capacitance of the geometry setup
2.5.1. Introduction
SCM characterization of doping profiles is done in most cases on the cross-section of
the samples (Figure 2.5.1).
On the other hand, oxides are usually deposited on the surface of the wafers and SCM
measurements are performed on the surface of the samples (Figure 2.5.2).
Figure 2.5.1 Geometry setup for the
characterization of doping profiles
with the SCM
Figure 2.5.2 Geometry setup for the
characterization of dielectrics with the SCM
During our measurements, a strong decrease of the signal/noise ratio has been
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
90
observed on the SCS measured on the surface by comparison with the SCS signal measured
on the cross-section of the samples, as it can be seen in Figure 2.5.3 : the signal measured
on the plane surface is practically at the SCM noise level, which is 50 mV. This observation
was valid even in the case when the same tip was used, during the same session of
experiments for both measurements (plane surface and cross-section). The problem remained
unsolved even after removing the effects of the parasitic AFM laser light.
In the literature, a clue about this problem may be found in the publication of Kopanski
et al [2.2]. As we have presented in the bibliography section, it is considered that the tip-
cantilever/body assembly and the sample form a capacitance that could affect the SCM signal.
This hypothesis is indeed reasonable.
The cantilever and the sample are a part of the same electrical circuit. Both the
cantilever and the substrate of the sample are conductive. The cantilever is connected to the
mass of the circuit, while a bias is applied on the XY stage which is in contact with the sample.
The cantilever and the substrate are separated by dielectric layers – the air between the
cantilever and the sample and the dielectric layer of the sample. In theory, a capacitance
effect should occur.
Figure 2.5.3 SCS signal on two native oxides, on low-doped 1.5.1015cm-3 p-type substrates, in the cross-section setup (black
curve) and on the plane surface (blue curve)
In addition, it is straightforward to say that the main difference between the two
experimental setups from the Figure 2.5.1 and the Figure 2.5.2 is the different positioning of
the cantilever with respect to the sample.
As a result, we decided to study the effect of the cantilever positioning.
2.5.2. Experimental evidence
In the comparison between the two SCS signals from Figure 2.5.3, several factors that
can influence the amplitude of the SCS signals may have overlapped.
The measurements are made on different samples. Although both substrates have a
low-doped concentration, the doping concentration is probably not exactly the same.
The substrates don't have the same crystalline orientation which can mean different
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
91
concentration of interface states (see Chapter 3 : Oxides). As a result, the oxide
thickness may not be the same.
In order to isolate the possible effect of the cantilever positioning from other factors,
we have decided that the measurements should be done on the very same sample, on its
surface. The measurements on the cross section of the sample would be reproduced by
positioning the tip at the very edge of the sample.
In this way, all the measurements are done on the same substrate covered by the
same oxide. Of course, it is possible that the density of oxide defects be different from one
spot to another, but we considered that such variations of the density of oxide defects cannot
determine a drastic variation of the signal amplitude.
The measurements have been done on a sample consisting in a good quality 2 nm thick
thermal oxide deposited on a low-doped 1015 cm-3, n-type substrate.
In both cases that have been studied, the SCS signal and the output of the SCM sensor
have been recorded.
The SCM sensor includes a high frequency oscillator driven with a frequency that can be
varied in the interval 0.88 GHz – 1.05 GHz, which output amplitude varies because of the shift
of its resonance frequency due to C. The demodulated amplitude variation of the oscillator at
the frequency of VAC as a response to C constitutes the SCM signal. The measurement of
this curve, and particularly its slope which governs the SCM output, is a direct indication of the
sensitivity of the SCM during the measurement.
Figure 2.5.4 shows the output of this sensor as a function of frequency for different
positions of the sample far over or on the sample surface.
Figure 2.5.4 Output of the SCM sensor as a function of the frequency for different positions of the sample over or on the surface.
Figure 2.5.5 presents the SCS signals for the tip positioned at the edge of the sample
and towards the middle of the sample. The high value of 1 V for the VAC value was chosen in
order to be able to have a signal for the SCS curve taken towards the middle of the sample.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
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For the SCS curve at the edge of the sample we have used the same VAC = 1 V in order to
compare the two curves.
From Figure 2.5.4 and Figure 2.5.5, it can be clearly seen the correlation between the
sensitivity of the SCM output sensor and the SCS signal. The maximum value of the sensor
output drops from 5 V when the tip is positioned at the edge of the sample to 200 mV when
the tip is positioned towards the middle of the sample (25 times weaker). The peak of the SCS
signal drops from -1000 mV when the tip is positioned at the edge of the sample to -50 mV
when the tip is positioned towards the middle of the sample (20 times weaker). It must be
reminded that the SCS signals show negative values because of the nature of the substrate
(n-type semiconductor).
Figure 2.5.5 The SCS signals on the same sample which consists
in a 2 nm thermal oxide deposited on a low-doped, 1.5 1015cm-3
n-type substrate. The SCS signals are measured for the tip
positioned at the edge of the sample (the black curve) and
towards the middle of the sample (the blue curve).
The important change in the slope of the resulting curve demonstrates that when the
AFM tip is positioned directly in the middle of the sample, with the cantilever and chip just
over the sample surface, the sensitivity and the dynamic range of the measurement drop
dramatically. The stray capacitance is lower when the measurement is conducted at the edge
of the sample, i.e. when the cantilever is not over the sample any more. This means that the
cantilever position, and thus any topographic feature, influences the quantitative
interpretation of SCS because of the change of sensitivity due to the parasitic capacitance, and
that any attempt to reduce the stray capacitance would be beneficial for the reproducibility
and quantitativity of the measurements.
In particular, when SCM is performed in the middle of a plane sample, the sensitivity is
reduced dramatically in comparison with the case when the experiments are conducted at the
edge of the sample. This is an important cause of nonreproducibility of the SCM signal when
comparisons are attempted between experiments performed e.g. on a cross section and
experiments performed on a plane or beveled sample, which leads to a completely different
situation from the stray capacitance point of view, and thus to completely different
sensitivities and applied voltages during SCM operation. From Figure 2.5.5 it can be seen that
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
93
the sensitivity drops of a factor 25 from one configuration to another.
Another question which arises is what is effectively responsible for the huge drop in the
sensitivity of the sensor and hence, in the amplitude of the SCM signal.
We have noticed that the sensitivity of the sensor doesn't drop too much as the
cantilever advances above the sample. In order to verify how the SCS signal and the
sensitivity of the sensor modify with the advancement of the cantilever above the surface of
the sample, we have made the following experiment.
We have positioned the cantilever exactly at the edge of the sample and we have
recorded the signal of the sensor. Then, we made several shifts with various steps, from 50
m steps to 500 m steps towards the end. For each step the sensor output has been
recorded. The measurements are presented in Figure 2.5.6.
Figure 2.5.6 The signal of the output sensor for different shifts of the cantilever above the sample, from the edge towards the middle of the sample.
From Figure 2.5.6, It can be seen that the sensitivity of the output sensor doesn't drop
suddenly with the advancement of the cantilever above the sample. There is a monotonous
decrease in the sensitivity of the sensor, decrease that continues even after the shift of 200
m – 250 m. Given the fact that the cantilever length specified by the manufacturer is of 225
m ± 10 m, it means that the cantilever is not the sole responsible for the decrease of the
sensor output.
Until the 200 m – 250 m shift, the sensitivity of the detector doesn't decrease to
much. Based on the results, we can conclude that, in fact, the chip of the cantilever is the
main responsible for the drop of the capacitance signal when a measurement is
performed towards the middle of a plane sample.
The parasitic capacitance between the cantilever and the sample, although noticeable,
is not the main factor which determines the drop of the sensitivity of the output sensor.
Besides the two observed parasitic capacitances, chip-sample and cantilever-sample,
there are another few observations to be made.
Even if the measurements are done on the edge of a plane sample, but which is placed
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
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on a metallic holder, the signal will still drop, because of a parasitic capacitance given by the
cantilever/ cantilever chip - sample holder assembly. If the sample is pasted on a metallic
holder in order to insure a good electrical contact, the sample edge where the measurements
are to be done must be positioned at the edge of the holder.
For the SCM, it is not critical to insure an ohmic contact between the sample and the XY
stage of the AFM, as it will be showed in the paragraph 2.6. However, with other
characterization methods, like C-V measurements with an impedance analyzer, SSRM, C-AFM,
an ohmic contact between the sample and the stage is crucial and the samples will be pasted
with silver dye or GaIn on metallic holders. If any comparison is intended to be made
between measurements with any of these characterization methods and SCM measurements,
we have to make sure from the beginning that the sample is placed on the metallic holder in a
correct position for the SCM measurements.
Even if the cantilever/ cantilever chip – sample/ sample holder capacitance is
eliminated, a parasitic capacitance can still exist between the XY stage of the AFM and the
cantilever/ cantilever chip (Figure 2.5.7).
Figure 2.5.7 Output of the SCM sensor as a function of the frequency in relation with the position of the cantilever chip by reference to the
XY stage
Although less important, it still affects the sensitivity of the SCM sensor. In order to
avoid this parasitic capacitance, it is better to position the sample at the very edge of the XY
stage.
Another parasitic capacitance is formed between the XY stage of the AFM and the
SCM electronic module (Figure 2.5.8). When the SCM electronic module is just above the
stage, not only the maximum value, but also the slope which controls the sensitivity of the
SCM sensor signal, drop. In order to avoid this parasitic capacitance, it is better to position the
sample in such a way that the SCM electronic module is not above the XY stage (dotted
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
95
curve).
Figure 2.5.8 Output of the SCM sensor as a function of the frequency in relation with the position of the SCM electronic module by reference to
the XY stage
Following is a summary of parasitic capacitances that can appear and distort the SCM
signal:
Parasitic capacitance Influence on the SCM signal
Cantilever -sample/ sample holder/ XY stage Small
Cantilever chip – sample/sample holder/ XY stage Great
SCM electronic module – XY stage Small
The values of all these parasitic capacitances can vary from one experiment to another.
A different geometrical setup can lead to the increase or the decrease of the distance between
the respective metallic surfaces and implicitly, to an increase or a decrease of the parasitic
capacitances. For example, the distance between the SCM electronic module and the XY stage
will be greater when the measurements are done on the cross section of a sample placed on a
vertical holder than in the case where the measurements are performed on a plane sample.
Regarding the parasitic capacitance between the sample and cantilever, an important
problem can appear if the distance between the sample and the cantilever changes during the
same measurement, because of the topography of the sample (Figure 2.5.9).
In this case, the topography of the sample, which changes the overall geometry and
thus stray capacitance of the system, will always play a role in the final SCM signal, and has to
be taken into account.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
96
Figure 2.5.9 Illustration of the parasitic capacitance modification between the cantilever and the sample, because of the topographical modifications
The topographical features would increase the distance between the cantilever and the
surface of the sample as a function of their height. This is why only perfectly flat samples are
studied in this report (RMS topography < 0.5 nm)
Further, we will try and give an explanation of why the sensor sensitivity drops when a
very high parasitic capacitance by comparison with the tip-sample capacitance, is introduced
in the circuit.
In Figure 2.5.10 we present a general diagram of the circuit. The parasitic capacitances
that might appear have been taken into account (the red capacitor in the Figure 2.5.10)
Figure 2.5.10 Diagram of the SCM sensor [2.28]
As it can been observed, the parasitic capacitance is connected in parallel with the tip-
dielectric-substrate capacitance. As it is well known, in a system of parallel capacitors, the
weight of the resultant capacitance is given by the capacitance with the highest value.
The parasitic capacitances have far greater values than the measured capacitance
between the tip and the sample which is of the order of 10-17 – 10-18 F.
As an example, we have calculated the capacitance between the cantilever and the
sample. The cantilevers that we used for the experiments have the dimensions: length=200
m; width=30 m; tip height (distance cantilever sample) = 10 m. The capacitance is of the
order of 10-15 F, which is a thousand times greater than the capacitance between the tip and
the sample.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
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2.5.3. Comparison with C-V measurements
With an impedance analyzer, the sample is glued on a metallic holder with silver dye in
order to assure a good electrical contact (Figure 2.5.11). The assembly is placed on the XY
stage of the apparatus, from which it is isolated with an insulating material, usually mica.
Two probes make the contact with the microelectrodes from the surface of the sample
and with the substrate, respectively.
Figure 2.5.11 A typical setup for C-V measurements. 1. XY stage; 2. isolating layer, usually mica; 3. metallic holder on which the sample is glued using silver dye; 4.
substrate. 5. dielectric layer; 6. micrometric electrode (gate); 7. Metallic contact to the substrate (back contact); 8. Metallic contact to the gate.
From the Figure 2.5.11, it can be seen that the only capacitance in this setup is the
capacitance to be measured, between the microelectrode and the sample.
On the other hand, in the case of the SCM, besides the capacitance between the
sample and the tip, there are several other capacitances that appear, which distort the SCM
signal (Figure 2.5.12). The origin of these parasitic capacitances is given by the very design of
the apparatus.
Figure 2.5.12 A typical setup for SCM measurements on the surface of a sample. 1. cantilever-sample (or sample holder, or XY stage) parasitic capacitance; 2.
cantilever chip – sample (or sample holder, or XY stage) parasitic capacitance; 3. electrical module – XY stage
From the geometry of the AFM (Figure 2.5.13), it must be also noticed that the piezo-
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
98
electric tube, which has a metallic shell, is located right above the cantilever holder. It is very
possible for this part of the AFM head to represent another source of parasitic capacitance.
However, given its position, we had no possibility to test its influence upon the SCM sensor or
the SCS signal.
Figure 2.5.13 Image of the AFM head with the attached SCM module (Veeco manual)
2.5.4. Conclusions and perspectives
In this paragraph, we have shown that the geometry of the AFM setup generates
several parasitic capacitances that influence the SCM measurements.
Some of them (cantilever chip – sample/ sample holder/ XY stage) are very important
and determine a huge drop of the signal/noise ratio. Other sources (cantilever-sample/ sample
holder/ XY stage, SCM electronic module – XY stage) are less important, determining a small
drop of the sensitivity of the signal/noise ratio.
It must be noticed that a small modification of the detector output by parasitic
capacitances doesn't really affect the measurements. For now, SCM is not an absolute
characterization method. The electronic circuit has unknown amplification factors. The
amplitude of the SCM signal has a real significance only relative to other measurements
performed in exactly experimental conditions. The parasitic capacitances have a critical impact
on the SCM measurements only when they determine a dramatic drop of the sensitivity of the
detector.
In this light, it can be said that the measurements performed on the cross
section of the samples are very little affected.
The main problem which remains is the measurements that have to be performed on a
planar sample. For now, such measurements can be done only on the edge of the sample,
which is unrealistic. Such measurements can also be performed by applying very high Vac
voltages to compensate for the drop in sensitivity of the signal, which is also unrealistic.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
99
In the end, we have to make the following remarks : Considering the absence of this
subject – the parasitic capacitance introduced by the geometry of the experimental setup on
the capacitance measurements – from the instrument manual, we cannot know whether the
manufacturer is aware of this problem or whether this problem has been solved for the new
generations of SCM.
Our results are valid for our instrumental setup (Dimension 3100). SCM performed on
other systems may not be sensitive to the geometry of the experimental setup and such
capacitive couplings may not appear. Feedback from other users may be appreciated.
2.6. Electrical contacts
2.6.1. Introduction
In the previous paragraph, we have seen that several parasitic capacitances, in parallel
with the tip-dielectric-sample capacitance, affect the sensitivity of the detector output. In this
paragraph we continue to investigate whether there aren't other parasitic factors of the SCM
electronic circuit that may influence the output signal.
From the diagram of the electronic circuit (Figure 2.6.1), it can be seen that a part of
the circuit is exposed to a series of factors that may change from one experiment to another
and may alter the results.
Figure 2.6.1 Diagram of the SCM electronic circuit (Veeco Manual)
On the one hand, we have the main electronic - the Nanoscope controller, the
Dimension electronics, the capacitance measurement electronics, the resonant capacitance
sensor module (Figure 2.6.2).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
100
Figure 2.6.2 Nanoscope electronics (Veeco manual) Figure 2.6.3 Sample setup (Veeco manual)
On the other hand, the portion of the circuit from the electronic module to the XY stage
(Figure 2.6.3) is susceptible to changes that may take place from one measurement to
another.
An UHF transmission line connects the resonant capacitance sensor with the cantilever
holder. The contact between the cantilever holder and the cantilever is insured by a metallic
spring clip. The cantilever tip is put in contact with the surface of the sample. The backside of
the sample is put in contact with the XY stage or with a metallic sample holder.
The UHF transmission line may introduce parasitic elements into the circuit.
It is known that the electrical components are not ideal. Each component, resistor,
capacitor, inductor, even wires, present parasitic resistance, capacitance or inductive effects.
The contact between the cantilever holder and the cantilever may be affected by
the presence of contaminants on the cantilever chip or on the spring clip.
The manufacturer, in the instruction manual, advices to insure an ohmic contact
between the semiconductor sample and XY stage by pasting the sample on a metallic
holder with silver dye or InAs paste. Only by pasting the sample on a metallic holder with InGa
eutectic and with silver dye, an ohmic contact can be assured. If not, the contact will be a
Shottky contact and additional series resistance will be present in the circuit.
The total resistance between the metal and the semiconductor can further increase
because of the native oxide from the back face of the sample and possible contaminants.
A small contact resistance between the metal and the semiconductor is obtained for low
values of the barrier height between the metal and the semiconductor and for high
concentrations of the semiconductor. [2.24 - Ch.5 Metal-Semiconductor Contacts, pg.304].
Also, the contact resistance vary with the type of substrate, p-type or n-type, because of the
differences in the barrier heights.
If the contact resistance is important relative to the bulk resistance of the
semiconductor, the RC time constant associated with the contact resistance can affect the
frequency response of the circuit.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
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Problems may appear at the contact between the tip and the surface of the oxide.
Some samples present important topographical features that may affect the tip-sample
contact. With the measurements performed in atmospheric conditions, a water layer is usually
present on top of the hydrophilic oxide. Contaminants may be found on the tip and on the
sample. The conical geometry of the tip may further induce capacitance parasitic effects.
2.6.2. Electrical contacts
The UHF transmission line (Figure 2.6.4) insures the electrical contact between the
resonant capacitance sensor and the cantilever holder. The UHF transmission line is glued on
the holder and it continues with a metallic wire that makes the contact, at the end of the
spring clip, with the back face of the cantilever.
Figure 2.6.4 Image of the chain which insures the electrical contact between the SCM detector and the cantilever (Veeco Manual)
The geometrical parameters of this group of components, as well as the connexion
between them, are important and can influence the outcome of the sensor output and of the
measurements.
We start to exemplify this affirmation by showing the output the SCM sensor for the
following cases: the transmission line is not connected to the detector, the transmission line is
connected but there is no cantilever on the holder, the transmission line is connected to the
detector and there is a cantilever in place, ready for measurements (Figure 2.6.5).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
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Figure 2.6.5 The detector output for different configurations described in the figure
It can be observed that the detector output has no meaning except for the standard
setup: the transmission line connected to the detector and the cantilever in place. The
properties of the transmission line and the cantilever affect the detector output and bring the
detector signal at maximum in the frequency range of the oscillator (0.88 GHz – 1.05 GHz).
The detector sensibility varies with the geometrical properties of these components. For
example, a variation of the length of the transmission line determines a variation of the output
detector and implicitly, of the SCM signal (Figure 2.6.6).
Figure 2.6.6 The detector output for two transmission lines of different lengths
The two signals from the figure have been recorded by using two Veeco holders with
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
103
transmission lines of different lengths. For the shorter transmission line, the detector output is
smaller. The difference of sensitivity is significant and can have a negative impact upon the
capacitance measurements.
A loosened spring clip may also determine a loss of signal during the scans (Figure
2.6.7).
This problem, in appearance a simply mechanical problem easily fixed, can create a lot
of difficulties. The user may easily and incorrectly assume that the bad tip is at the origin of
the poor SCS signal. There are no contact problems between the cantilever and a loosened
spring clip when the tip is not engaged on the surface and the detector output doesn't signal
any anomaly.
Only when the tip is engaged on the surface, the strain exerted upon the cantilever
may determine intermittent lose of contact between the cantilever and the spring clip.
Figure 2.6.7 SCS curves with the spring clip loosened and in correct configuration
This problem may be detected during the measurements by observing small, unusual
variations of the position of the laser spot on the photodiode.
The choice of the spot where the cantilever makes the electrical contact with the spring
clip may also slightly affect the output of the detector (Figure 2.6.8).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
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Figure 2.6.8 Output of the SCM sensor as a function of the contact
cantilever chip -holder
Although the difference in the detector output is not significant, it is obvious that the
position of the cantilever on the holder can bring modifications of the SCM amplitude signal for
measurements performed on the same sample, every time we reposition a cantilever on the
holder.
2.6.3. The sample backface contact
2.6.3.1. Experimental
In order to see what is the influence of the back face contact of the sample upon the
SCM signal, the following experiment has been done.
For the experiment, it has been used the test sample consisting in a n-type substrate
doped 1015 cm-3, covered with a 2 nm thermal oxide.
The sample has been cut in two pieces. One piece has been pasted on a metallic holder
with InAs eutectic and silver dye. SCS measurements have been performed on the two
samples, one pasted on a metallic holder, the other one simply put in contact with the XY
stage. The results are showed in Figure 2.6.9.
Figure 2.6.9 SCS measurements on two pieces cut from the same sample.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
105
A significant series resistance would diminish the value of the capacitance in
accumulation, which would lead to the modification of the slope of the C-V curve. The
amplitude of the SCS signal should therefore diminish. As it can be observed, there is a slight
difference between amplitude of the two measured signals. Also, the shift between the trace
and the retrace for the SCS signal performed on sample with a metallic back contact (not
shown) seems to be slightly smaller.
However, the difference in amplitude between the two signals is very small, in the
range of noise level and normal variation of amplitude of the SCS signal from one
measurement to another for the characterizations done in ambient conditions. Apparently,
although the measurements are done at a frequency of 1 GHz, the evidence shows that the
series resistance given by the back-contact of the sample doesn’t have a major impact on the
capacitance signal.
Because the contact resistance depends on the type of the substrate, the same
experiment has been performed on a sample with a p-type substrate 1015cm-3, covered with a
5 nm thermal oxide.
The results are similar to the previous ones. There is very little difference between the
SCS curve on the sample pasted on a metallic holder and the SCS curve placed directly on the
XY stage.
2.6.3.2. Comparison with C-V measurements
With the C-V measurements, it is known that if the sample is simply put on the metallic
holder, there is no ohmic contact between the back face of the sample and the holder. The
contact will be a Schottky contact.
Even if the sample is pasted with InGa and silver dye on a metallic holder, the series
resistance can still become to great at high frequencies (Figure 2.6.10)
Figure 2.6.10 C-V measurements which shows the influence of the series resistance upon the measured capacitance
On the other hand, we have seen from the Figure 2.6.9 that the amplitude of the SCS
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
106
signal changes very little when there is no ohmic contact between the back face of the sample
and the holder.
The nature of the back contact influences the C-V measurements, but doesn't seem to
influence the SCS measurements in the same amount. In order to understand why, we
calculated the capacitance and the total impedance for an ideal capacitor in series with an
ideal resistor, for different frequencies.
*j
Z RCw
= -
We have taken into consideration that the series resistance of a Shottky contact is
approximatively 1kΩ.
In the case of C-V measurements, the value of the capacitance in accumulation is of
the order of nano-farads. Here is the table with the calculated values of the total impedance
for different frequencies:
Frequency Capacitive impedance ZC Series resistance R Total impedance Z
1 kHz 159 kΩ 1 kΩ 160 kΩ
1 MHz 159 Ω 1 kΩ 1.159 kΩ
1 GHz 159 mΩ 1 kΩ 1 kΩ
As it is well known, the capacitive impedance varies inversely proportional with the
frequency. For small frequencies, the capacitive impedance is high and has the dominant
contribution to the total impedance. For high frequencies, starting with 1 MHz, the series
resistance has a dominant role and influences significatively the results.
With the AFM, the measured capacitance is of the order of femto-farads and the
measurements take place at a frequency of around 1 GHz.
Frequency Capacitive impedance ZC Series resistance R Total impedance Z
1 GHz 159 kΩ 1 kΩ 159 kΩ
Although the SCS measurements are performed at much higher frequencies than the C-
V measurements, the capacitance of the MOS structure is also much smaller because of the
small size of the tip. Thus, the capacitive impedance will be much greater than the series
resistance, which shouldn’t play any role at all.
2.6.3.3. Conclusions
The back face contact slightly influences the SCS measurements. Although the
difference in the amplitude of the signal is very little, from our experience, it seems to always
be slightly higher in the case of the samples with a metallic back-contact.
There are arguments in the favor and against the use of an ohmic contact for the back
face of the sample.
In the favor of the procedure, the main argument is that it is best to have a series
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
107
resistance as small as possible. To insure an ohmic back-contact represents the standard
procedure with all the electric measurements. The amplitude of the signal is slightly higher
and the difference between the trace and the retrace seems to be slightly smaller.
On the other hand, such a procedure is useless with the AFM if only qualitative
measurements are envisaged. Compared with the capacitive impedance, the series resistance
is negligible. In addition, the procedure has several drawbacks.
The creation of an ohmic back contact is time consuming. It may add an unnecessary
step to the overall procedure of the characterization of a MOS structure.
As we have seen in the previous section 2.5, a parasitic capacitance can appear
between the metallic holder and the AFM cantilever/chip, which will distort the signal.
After the use of silver dye and InGa eutectic for gluing the sample on a metallic holder,
it is unlikely to be able to submit that particular sample to further technological procedures
(annealings, HF etching of the gate oxide, cleaning procedures etc).
2.6.4. The tip-sample contact
One of the most common parasitic features with the SCS is the presence of a secondary
peak.
In the literature, some authors consider that this effect may appear as a consequence
of tip depletion [2.8]. Many tips used with the SCM are not metallic tips, nor they have a
metallic coating, such as the diamond coated tips that we have used for the measurements
performed during this thesis or the highly doped semiconductor tips.
In order to verify this hypothesis, we have performed simulations of the tip-oxide-
substrate with DESSIS and we have compared the results with the capacitive signals obtained
with the SCM. (DESSIS - Device Simulation for Smart Integrated Systems - from ISE AG, is a
multidimensional, electrothermal, mixed-mode device and circuit simulator for one, two, and
three-dimensional semiconductor devices. It incorporates physical models and numerical
methods
for the simulation of most types of semiconductor devices. A real semiconductor device, such
as a
transistor, is represented in the simulator as a virtual device whose physical properties are
discretized onto a non-uniform grid of nodes).
Figure 2.6.11 ISE-TCAD simulation of a M.O.S structure with a semiconductor tip of the same type of
doping concentration as the substrate. A secondary peak appears on the dC/dV signal as a result of tip
depletion (left - simulated C-V curve; middle - derivative of the C-V curve; right - modulus of the
derivative of the C-V curve)
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
108
A M.O.S structure has been simulated (Figure 2.6.11): a 2 nm thermal oxide on top of
a n-type semiconductor of different doping concentrations. We represented the tip as a highly
doped n-type semiconductor square electrode. The shape of the AFM tip has not been
introduced into the equation. We considered that the 3D shape of the tip would have not been
relevant for the goal of this simulation and it would have introduced supplementary and
unnecessary complications to the simulation.
If the substrate has the same type of doping concentration as the tip, a secondary peak
appears on the dC/dV signal. The explanation is that when the tip is in accumulation, the
substrate will be in inversion and vice-versa.
In the case when the substrate is of a different type of doping concentration (p-type)
than the tip, a secondary peak doesn't appear anymore (Figure 2.6.12)
Figure 2.6.12 ISE-TCAD simulation of a M.O.S structure
with a semiconductor tip of the same doping
concentration as the substrate. The effect of tip
depletion superimposes with the substrate depletion
In this case, both semiconductors, the substrate and the tip, will be in accumulation or
in inversion regime in the same time and the two effects will superimpose.
However, the results obtained with the simulations don't match the experimental
evidence.
Figure 2.6.13 SCS signal obtained with a full-metal
(tungsten) tip on a 5 nm thermal oxide (tips fabricated
by K. Akiyama at Tohoku University - Japan and
provided by Prof. George Bremond)
On one hand, a secondary peak appears even when the tip and the substrate are doped
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
109
with different type of dopants. On the other hand, a secondary peak appears on the SCS
signals when full-metal tips are used (Figure 2.6.13).
Another explanation has been found for the presence of a secondary peak on many of
the samples we have characterized, based on a comparison between the C-V and SCS
experimental setups.
Although SCM is a derivate technique from macroscopic C-V measurements with
impedance analyzers, there are several important differences between the 2 techniques. One
of the differences is that, in the case of C-V measurements with an impedance analyzer, the
measurements are done on deposited metallic electrodes.
The use of electrodes has several advantages.
The surface of the electrodes can be measured with an optical microscope. As we have
seen in the paragraph 2.3.4, the properties of the gate oxide and the properties of the
substrate can be calculated from the experimental C-V curve (the thickness of the oxide, the
doping concentration of the substrate, the surface potential, the flatband bias, the density of
the interface states, the defects density in the volume of the oxide etc) if we know the
dimension of the gate electrode.
The electrodes protect the MOS structure from most of the parasitic light, as we have
shown in the paragraph 2.4.4. The electrodes have a thickness of several hundred
nanometers and the light doesn't transmit across them. Minority carriers can be generated
only at the edges of the electrodes and migrate under the electrodes.
The electrodes also protect the MOS structure from contaminants. The surface of the
oxide is cleaned before the electrode deposition. The electrodes are deposited with an electron
gun in ultra-vacuum. In consequence, there are no contaminants between the surface of the
oxide and the gate electrodes.
The oxides are hydrophilic. A layer of water forms on the surface of the oxides from the
humidity in the air. The water molecules can be easily polarized under the influence of an
electric field. Further on, they can dissociate and generate mobile charges on the surface of
the oxide. These ions can be attracted to the tip/ repulsed away from the tip under the
influence of an alternative voltage. Under the influence of strong electric fields, oxide charging
or even anodic oxidation may occur. The metallic electrodes prevent the generation of a layer
of water on the surface of the oxide.
The C-V measurements on the thermal oxide don't present any sign of hysteresis or
increase of the signal in inversion (Figure 2.6.14).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
110
Figure 2.6.14 C-V curves operated on 100 μm× 100 μm NiAu electrodes
on a thermal 5 nm thick silicon oxide. The measurements show that
there is no hysteresis and no an increase of the signal in inversion
To test the role of the tip-sample contact in the interpretation of SCS, a comparison
has been made between C-V curves operated on NiAu electrodes and the same measurement
operated using a large, home-made tin tip directly on the oxide.
The size of the tip has been chosen so that the level of the signal is sufficient to record
C-V curves using the same impedance analyzer as used for the C-V curves recorded on the
electrodes.
2.6.15 Example of C-V curves obtained with a macroscopic tin tip directly on the oxide surface (without electrodes). The hysteresis obtained while ramping from negative to positive and then from
positive to negative voltages indicates the presence of charging effects
The results can be found in the Figure 2.6.15 and 2.6.16 : an hysteretic behavior
and/or an increase of the signal in the inversion region is recorded, significative of the bad
electrical contact between the probe and the oxide.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
111
2.6.16 Example of C-V curves obtained with a macroscopic tin tip directly on the oxide surface (without electrodes). The increase of the signal when
positive voltages are applied indicates the presence of minority carriers.
However, these experimental results cannot constitute more than a preliminary proof
that something may be wrong with the tip-sample contact with the AFM. Although the C-V
measurements with a home-made tin tip directly on the surface of the oxide respect in
principle the AFM concept, there are important differences.
The dominant forces between an AFM tip and the surface of a sample are the Van der
Waals forces. The forces between the macroscopic tin probe and the sample are rather
mechanical: the probe is forced pushed against the surface of the sample.
The tin probe presents numerous microscopic fissures which render the probe-sample
contact flawed. It is very likely that between the apex of the tin probe and the surface of the
sample remain lots of air gaps, which is not the case between the apex of the AFM tip and the
surface of the sample.
Therefore, in order to confirm this hypothesis, we recorded SCS curves on the same
sample (thermal silicon oxide, 5 nm thick) and on the same electrodes but with a reduced
size, so that it becomes possible to obtain a signal compatible with the range of the SCM. In
order to be sure that we respect exactly the same experimental conditions as with the C-V
measurements, a nanometric electrode has been cut directly in a micrometric electrode using
the tip of the AFM. Very hard diamond coated tips (stiffness in the 40 N/M range, typically
used for the Scanning Spreading Resistance - SSRM - mode of the AFM) have been used to
engrave the electrode. The deepness of the engraved region is higher than the thickness of
the electrode, so that the region delimited by the grooves is electrically independent of the
rest of the electrode. The resulting electrode is ≈ 700 nm × 700 nm large. SCS recorded on
this submicronic electrode can be found in Figure 2.6.17.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
112
Figure 2.6.17 Topographical image and SCS measurement on a
700nm x 700nm electrode, on a 5nm thermal oxide. SCS doesn't
present a secondary peak, nor hysteresis.
A direct comparison can be made with the measurements from Figure 2.6.18, where
the AFM tip has been placed directly on the thermal oxide, as usual with the AFM.
Figure 2.6.18 SCS operated with the tip directly on the oxide:
hysteresis and minority carriers are present in the signal. Voltage
is applied to the sample.
From the comparison between the macroscopic C-V curves form Figure 2.6.14 and from
the SCS spectroscopy shown in Figure 2.6.17 and Figure 2.6.18, it is clear that the quality of
the tip-sample contact is a major parameter which influences dramatically the interpretation of
SCS.
No double peak is obtained in SCS when the measurement is made on a correct
electrode, with a clean interface between the electrode and the oxide, whereas a hysteresis
and a peak in inversion (which traduces an increase in the signal in the inversion part of the
C-V curve) is recorded directly on the oxide surface, similar to C-V curves obtained from a
metallic tip directly on the oxide surface in Figure 2.6.15 and Figure 2.6.16.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
113
All comparisons put together accreditate the role of the tip-sample contact in the
interpretation of SCS and emphasize the need to enhance it if SCS aims at becoming a
metrological tool at the nanoscale.
It must be noted that the maximum of the SCS curves from Figure 2.6.17 is situated at
+0.8V (the bias applied on the substrate), the same value that the inflexion point of the C-V
curves from Figure 2.6.15. (-0.8 V, the voltage applied on the gate).
Although this observation may be considered trivial, given that the both measurements
have been done on the same sample (the same gate metal with the same workfunction, the
same oxide with the same density of defects and the same thickness, the same substrate), it
represents however the first perfect match that we have observed between the C-V and the
AFM measurements.
This correlation is, in our opinion, a significant and promising sign that SCM can
become a great metrological tool, once the numerous experimental variables are understood
and controlled.
2.6.5. Conclusions
The components (the UHF transmission line, the holder, the cantilever) and the
electrical contact between the holder and the cantilever have a minor impact upon the
sensitivity of the detector and may vary from one measurement to another. The problems may
deepen if the standard components are not used and taken care.
The sample backcontact seems to influence very little the capacitance measurements.
Even at the high frequencies used with the AFM, the series resistance is insignificant by
comparison with the capacitive impedance. The explanation is that the very small values of
the measured capacitance conduct to a very high capacitive impedance, several orders higher
than the series resistance.
The face contact however, between the tip and the sample, is negatively influenced by
a multitude of factors: contaminants on the tip and on the surface, surface humidity,
topography etc.
Each of these factors will be analyzed and their influence evaluated in the next
paragraphs. The oxide charging and the anodic oxidation will be discussed in the Chapter 3:
Oxides.
2.7. Influence of the sample topography on the capacitance signal
2.7.1. Introduction
Some samples studied with the SCM present topographical features, either because
unavoidable defects during the sample preparation, or because of the intrinsic structure of the
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
114
sample to be analyzed.
Many of the SCM measurements must be performed on a cross-section of a sample,
given that the doping profiles constitute multi-layers of different doping concentrations.
Cleaving the samples is not an easy procedure and can result in topographical artifacts.
The topographical artifacts that can result from cleaving can lead to misinterpretations
of the doping profile (Figure 2.7.1).
Figure 2.7.1 Comparison between a SIMS profile and an SCM image. A drop of the SCM signal
can be observed on the left of the cross-section, caused by a topographical artifact.
The other method of preparation of a sample for SCM analysis is by polishing. We
present an example on beveled samples obtained by polishing. Because the
topographical pattern doesn't match the doping pattern, the effect of the surface roughness on
the SCM image is obvious (Figure 2.7.2).
Figure 2.7.2. Beveled surface of staircase like sample. Topographical image
(left). SCM image (right)
Sometimes, the nanodevices that are to be characterized have an inherent topography.
It is the case, for example, of the standard sample from Veeco which represents a RAM
memory (Figure 2.7.3).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
115
Figure 2.7.3. Veeco standard sample of a RAM memory.
Topographical image (left). SCM image (right) (Veeco
Manual)
As it can be seen from the Veeco images, the topography presents features of up to
400 nm high and the topographical features match perfectly the different doping domains that
appear in the SCM image. The question that automatically arises is how can someone know
how much of the SCM signal is due to the doping concentration and how much is due to the
changes in the topography of the sample.
One possible application with the SCM is the study of charge storage effect of individual
nanodots and the amount of charge stored in individual nanodots for the long-term memory
retention.
In this case the nanodots themselves introduce topographical features that cannot be
avoided (Figure 2.7.4). But how much of the SCM signal is due to the doping concentration
and charge retention and how much to the topography variations?
Figure 2.7.4. InAs nanodots. Topographical image (left). SCM image (right) after charging
a 500 nm x 500 nm area (center of the image, dark contrast)
2.7.2. Experimental
In order to be able to study separately the supposed stray capacitance given by the
topography of the sample, this signal should be isolated from all the other sources of
capacitance signal.
We believe that this stray capacitance is given by small hops of the tip which determine
the interposition of an air layer between the tip and the sample for short periods of time,
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
116
which leads to the creation of a capacitance-like structure (Figure 2.7.5).
Figure 2.7.5. Possible parasitic capacitance that may
appear between the tip and the sample because of
topographical features of the sample
In order to verify this hypothesis, metallic samples have been designed. On doped
substrates, different metallic layers have been deposed by evaporation with an electron gun.
On some metallic layers (aluminum, nickel) metallic steps of different heights have
been fabricated by lithography or by AFM engravement. These metallic steps are intended to
simulate the topography of the samples from the previous examples - the topography of the
Veeco standard samples, beveled samples or cross-section samples (Figure 2.7.6).
Figure 2.7.6a Nickel metallic surface obtained
by AFM engravement of a micrometric
electrode used for CV measurements
Figure 2.7.6b SCM signal on the nickel surface.
Vdc = 0 V. Vac = 100 mV
Other metallic layers (titan) present in addition a granular topography, similar to the
case of nanodots or topography features met on thick high-k dielectric layers (Figure 2.7.7).
Figure 2.7.7a. Titan metallic surface obtained by Figure 2.7.7b SCM signal on the titan surface, given
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
117
electron canon deposition and lithography by the step and surface rugosity. Vdc=0V. Vac=100mV
A conductive tip in contact with a metallic sample should not give any capacitance
signal at all. But if the conductive tip leaves the surface for a short period of time, a
capacitance formed by the tip, air and the metallic surface will appear and a capacitance signal
will be observed.
The study was made by analyzing various parameters that could affect the SCM signal
(different topographical features of the samples, size of the tip, deflection setpoint – the
interaction force between the surface and the tip – gains, scan direction).
2.7.3. Direction of approach of a topographical feature
The first measurements were done on a nickel metallic step obtained by AFM
engravement of a micrometric electrode used for C-V measurements.
By scanning this metallic surface, there shouldn’t be any capacitance signal at all,
because there is an ohmic contact between the surface and the conductive tip. However, at
the position of the step, a capacitance signal can be clearly seen (Figure 2.7.8 and Figure
2.7.9).
Figure 2.7.8. SCM signal at the position of the metallic step. Perpendicular scan on the step
A large diamond coated tip, specific to SSRM applications, utilized to previously engrave
the metallic step, has been used. Scans were performed in the directions perpendicular and
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
118
parallel to the step (Figure 2.7.8 - 2.7.9) .
Figure 2.7.9. SCM signal at the position of the metallic step. Parallel scan on the step
From the two SCM images, it can be observed that the capacitance signal is higher
when the scans are performed perpendicular to the metallic step.
Figure 2.7.10. Superimposed average topographical profile and average SCM signal. Left
image - Perpendicular scan. Right image – parallel scan
2.7.4. Scan direction
The possible scan directions using an AFM cantilever are represented in Figure 2.7.11.
Figure 2.7.11. Scanning directions with an AFM tip
By analyzing the capacitance signal between the trace (backward movement) and the
retrace (forward movement), it can be seen that the capacitance signal almost vanishes in the
first case (Figure 2.7.13). From these experiments, we conclude that the best scan direction
for diminuating the stray capacitance is a backward movement of the cantilever.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
119
Figure 2.7.12. Topography of the titan 10nm step surface
Figure 2.7.13 Difference between trace (left) and retrace (right) of the capacitance signal.
DS=0.3V. Vac=100mV
2.7.5. Deflection setpoint (DS)
By variating the deflection setpoint, it can be seen that for a lower DS (Figure 2.7.14),
the stray capacitance disappears for almost all the topographical features, although the
deflection error signal doesn't become more important.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
120
Figure 2.7.14a. DS=0.3V+0.2V. Vac=100mV Figure 2.7.14b. DS=0.3V-0.2V. Vac=100mV
2.7.6. Size of the tip
The parasitic capacitance signal diminishes in the case of smaller tips (Figure 2.7.15).
However, we cannot be sure if this happens because smaller tips follow better the
topographical features or because smaller tips give a smaller capacitance signal.
Figure 2.7.15. SCM images performed with tips of different sizes. Diamond coated tip (left),
PtIr coated tip (right)
2.7.7. Solutions for decreasing the effect of stray capacitance arising from
topography
The topographical features due to the preparation of the sample are random and it is
very hard to avoid stray capacitance, even when optimizing the different scan parameters.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
121
However, the advantage in the cases is that the topographical pattern doesn't match the
doping pattern and the stray capacitance can be identified by comparing the SCM image
with the topographical image . On the other hand, when the topography is an intrinsic part
of the sample, the topographical pattern match the doping pattern and it is hard to decide
whether or not the topography influence the capacitance signal. Even if one can reasonably
deduce that the stray capacitance will be present only where topographical transitions exist,
the situation is even more complicated when analyzing very small structures, several
nanometers in size (like nanodots).
We must notice that, although we evidentiated experimentally the existence of the
stray capacitance given by topography, a explanation has yet to be found for this
phenomenon. SCM measures a difference of capacitance, not an absolute capacitance. When
the tip shortly leaves the surface at the metallic steps, even if a temporary capacitance
appears (tip-air-metallic surface), this capacitance is not variable. Thus, an SCM signal
shouldn't appear.
One probable cause can be the water layer on the surface of the sample. It is possible
that the H2O dipoles follow the frequency of the AC bias applied to the sample and generate a
variation of capacitance. For verifying this hypothesis, SCM measurements in a controlled
environment should be performed.
In the case of the SCM measurements performed in atmospheric conditions, we
propose several solutions for eliminating/diminishing the stray capacitance given by
topography
One solution for diminuating the role of the stray signals from the SCM images and
increasing the signal/noise ratio is to use high Vac. Vac biases of hundreds of milivolts and
even several volts are being used. However, we considered that, although high Vac do not
affect the correctitude of the results in the case of failure analysis images, such biases are
unrealistic. SCM is a differential measurement method. The SCM signal should reflect as
accurately as possible the slope of a C-V curve for the quantification to be possible. Other
issues concerning the depletion under the tip makes us consider that high Vac biases should be
avoided as much as possible.
Using sharper tips when there are topographical issues. We must accentuate that the
choice of the tip varies from application to application. Using sharper tips is better for the case
when dealing with samples with significant topography, as with the any topographical
measurement with the AFM. It is also for the best in the cases when resolution is an issue, like
on samples with very small nanodevices. However, sharper tips may not be the best choice for
other samples. Sharper tips means a smaller signal/noise ratio.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
122
Optimization of the deflection setpoint. As it has been demonstrated, the factor that
influences the most the stray capacitance given by the topography is the deflection setpoint.
In these cases, it is best to try to optimize the deflection setpoint, preferably with prior test
measurements on standard sample as the ones we proposed.
The best solution is, in our opinion, using SCS measurements instead of SCM scans.
Stray capacitance generated by topography is an issue only when imaging. It is best to use
SCM scans only for qualitative failure analysis. We consider that the best capacitance method
with the AFM, for obtaining accurate information about the doping profiles and oxide
properties, is by performing SCS measurements. In addition, SCM images present a major
disadvantage in the fact that the scans are done at a fixed dc bias.
2.7.8. Conclusion
With the metallic samples we proposed, we have isolated the stray capacitance
generated by the surface topography from other various factors. We have shown how this
stray capacitance varies with different parameters, such as the size of the tip, the deflection
setpoint, scan direction.
We have proposed several solutions to counter this stray capacitance, the best of
which, in our opinion, is to perform topography independent SCS measurements at fix point
instead of SCM scans.
2.8. Tip properties
2.8.1. Introduction
We continue the analysis of the tip-sample contact by taking a closer look to some of
the properties of the tip.
In order to see what influence might have the tip on the capacitive measurements, we
have recorded the signal given by 19 different diamond tips, from two different
manufacturers, on a well-known sample. The sample consists in a lightly doped n-type
substrate, 1015cm-3, covered with 2 nm thermal oxides. The results are shown in Figure 2.10.1
and 2.10.2.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
123
Figure 2.10.1 SCS signals recorded with 9 different new tips from Nanosensors, on a 2 nm thermal oxide
The measurements have been performed in the absence of parasitic laser light (Section
2.2) and parasitic capacitances (Section 2.2). The sample doesn't present any topography at
all (Section 2.6). All the electrical contacts have been verified (Section 2.5).
Figure 2.10.2 SCS signals recorded with 10 different new tips from Veeco, on a 2 nm thermal oxide
However, it can be seen that not much has change by comparison with the Figure 2.1.1
from the beginning of this chapter when recording SCS signals with different tips and in
different experimental conditions (unknowingly).
Although now we have a good signal/noise ratio for most of the tips that we use, the
same problems persist. There is a high variation in the amplitude of the signal. The maxima of
the SCS curves are located at different voltages for different tips, although the sample
remains the same and the tips are covered with the same p-doped diamond material.
In the following sections of this paragraph, we will characterize the AFM tips by
different means in order to find out the source of the non-reproducibility of the signal. We will
also try and find out the radius of the tips, required for the calculation of the doping
concentration. We will also discuss some of the properties of the tips required for the
capacitance measurements with the AFM.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
124
2.8.2. Characterization of AFM tips with standard AFM sample gratings
The tip represents one of the 'plates' of the MOS capacitor. Thus, the dimension of the
tip affects the amplitude of the signal. The higher the tip radius, the higher the measured
capacitance. It is important to know the radius of the tips in order to explain the differences
from case to case in the amplitude of the capacitance signal. Also, changes in the amplitude
of the signal during a scan could be explain and by the change of the tip radius during the
scan (tip wear).
Standard calibration samples can be used for learning the tip radius (Figure 2.10.3).
2.10.3 GZT1 standard sample NTMDT for measuring the tip radius. Images from NTMDT company
The calibration sample consists of a topography with very sharp features.
The final AFM image in these places will be a convoluted signal between the
topographical features and the AFM tip. After the AFM image is obtained, a specialized
software, like SPIP from NTMDT, is needed for the deconvolution of the tip.
2.10.4 AFM image on the GZT1 grating sample with a diamond coated tip (left) and the
deconvoluted image of the tip - SPIP software (right). Armel Descamps, Ingenieur INSA
Such a procedure has several important applications for the SCM.
The radius of the tips can be obtained. Also, the tip wear can be evaluated by
comparing the tip size before and after the SCM measurements.
However, this procedure gives us no information about the coating of the tips or about
possible contaminants on the surface of the tips.
2.8.3. Characterization of AFM tips with SEM
The AFM tips can be visualized with a Scanning Electron Microscope. By direct
visualization, it is possible to obtain the radius of the tips. It is also possible to observe
eventual problems concerning the properties of the tips (shape, coating etc)
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
125
The description of the principle of a Scanning Electron Microscope exceeds the
framework of this thesis, so we will move on directly to the characterization of the tips
performed with SEM.
The diamond coated tips, as seen with the SEM, have a pyramidal shape and a granular
structure, given by the diamond coating (Figure 2.10.5)
Figure 2.10.5. SEM images of a diamond coated tip
SEM is a very powerful characterization tool, capable to measure the tip radius. With a
sufficiently high magnification, the end of the tip can be observed and recorded (Figure
2.10.6).
Figure 2.10.6 SEM images of a diamond coated tip at 50.000x magnification (left) and 100.000x magnification
(right)
A comparison between different tips can also be made. (Figure 2.10.6-2.10.7). While
the diamond tips exceed easily 200 nm in diameter, the PtIr tips have a diameter smaller than
50 nm.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
126
Figure 2.10.7 SEM images of a PtIr5 coated tip at 5.000x magnification (left) and 80.000x magnification (right)
The images presented previously have been made with an old JEOL JSM-840. The
images taken with modern SEMs, like ESEM FEI-XL30 are even clearer and allow us to have
better images of the shape of the tip.
Figure 2.10.8 SEM images of a diamond coated tip (left) and a PtIr5 coated tip at 65.000x magnification
The most important results with the MEB by comparing with the previous
characterization method with the AFM, concern the state of the coating of the tips and the
contaminants.
In Figures 2.10.9-2.1011 different problems with the coating can be observed.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
127
Figure 2.10.9 SEM image of a diamond coated tip at 20.000x magnification
Figure 2.10.10 SEM image of a diamond coated tip at 20.000x magnification
The state of coating has an important impact upon the SCS signal.
The tip wear determines a change in the amplitude of the SCS signal over time. The
complete loss of coating at the end of the tip means a shift of the peak of the SCS signal, due
to the change of the gate workfunction, from the one corresponding to the coating to the one
corresponding to the silicium. On an SCM image, the loss of coating translates into contrast
reversal.
Problems with the tip coating, observed with SEM can constitute one of the reasons of
the non-reproducibility of the SCS signal.
Figure 2.10.11 SEM image of a diamond coated tip at 20.000x magnification (left) and 80.000x magnification (right)
Figures 2.10.12-2.10.15 represent a few examples of SEM images of contaminated tips.
More than 50% of the tips examined with SEM presented some form of contamination.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
128
Figure 2.10.12 SEM image of a contaminated diamond coated tip (18.000x)
Figure 2.10.13 SEM image of a contaminated diamond coated tip (20.000x)
The contaminants on the SCM tips can have an unpredictable effect on the capacitance
signal, ranging from the total loss of signal, shifts of the SCS peak, modification of the shape
of the SCS signal etc.
Figure 2.10.14 SEM image of a contaminated (40.000x)
Figure 2.10.15 SEM image of a contaminated (40.000x)
With SEM, the shape and the size of the tip, the state of the coating, possible contaminants,
can be observed.
SEM has also several drawbacks related to the characterization of AFM tips.
SEM is destructive with regard to the cantilever. The cantilever must be pasted on a metallic
holder in order to be able to perform the SEM characterization. We have tried to find a solution
around this problem by manufacturing a cantilever holder which fixes the cantilever by
mechanical means. This method was a partial success. The cantilevers were not destroyed
anymore in the process, but it appeared the risk that the cantilevers fall off the holder and
break during the characterization.
SEM characterization is time consuming, especially if we take into account the high number of
characterizations that must be performed (for each tip). The SEM measurements are done in
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
129
ultra-vacuum, which takes time to put in place. Localizing the tip with SEM and obtaining a
high resolution image is no less time consuming. Also, this method assumes the simultaneous
access to a SEM and to an AFM, which is a requirement hard to meet for any laboratory.
2.8.5. Conclusions
The AFM tip and its properties have a great impact upon the capacitive measurements.
The tip represents the gate of the MOS structure. Its dimensions (its radius) must be
known if any quantification of the doping profiles is to be attempted.
The slightest problem concerning the wear of the tip over time, the loss of coating,
contamination, render the capacitive measurements false.
The tip geometry has an impact upon the intensity of the electrical field. For the same applied
bias, a sharper tips will generate a stronger electrical field which will favorize unwanted
tunneling currents or even the dielectric breakdown.
In the semiconductor industry, there is an entire field of research concerned with finding the
best materials for the gate of transistors. Metal electrodes are replacing polysilicon electrodes
in order to eliminate gate depletion and to be able to adjust the threshold voltage through the
control of the gate workfunction. With the SCM, there exists a similar interest in the
engineering of the workfunction of the tip. In order to minimize the electrical fields across the
thin gate oxide, biases as small as possible should be applied to the M.O.S structure. In order
to be able to do this, the tip workfunction (the choice of the metal coating) should be adjusted
in such a way that the peak of the SCS signal be located as close of possible of zero volts.
The tip workfunction can be measured with Kelvin Force Microscopy. We have done extended
measurements with the KFM, trying to find out the workfunction of various tips: diamond
coated tips, PtIr tips, highly doped silicon tips. Layers of different metals (gold, nickel,
platinum, titan, aluminum, copper) have been deposited on semiconductors or mica and
served as samples for measuring the workfunction of the tips. However, the same non-
reproducibility of the signal observed with the SCM and C-AFM, has also been observed with
the KFM. Variations of the tip workfunction up to 500 mV have been noticed.
Rigorous methodologies for the control of the properties of the tips must be put in place.
2.9. The AFM Piezoelectric Scanner
2.9.1. Introduction
With the AFM, the positioning of the tip relative to a sample with nanometric resolution
is accomplished with piezoelectric transducers which expand and contract proportionally to an
applied voltage [2.25 - pg.420-pg.423].
The commercial piezoelectric transducers installed on most commercial AFMs are
cylindrical tubes (Figure 2.9.1). These tubes are most often fabricated from a lead zirconium
titanate material, ranging in size from 3-20 mm in diameter, and 10-50 mm in length, with a
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
130
wall thickness of 0.5-1 mm.
There are two main electrodes, one on the exterior of the tube and the other on the
interior of the tube. The electrode on the exterior of the tube is further divided into 4
electrodes.
Figure 2.9.1 The structure of a piezoelectric scanner
The application of a voltage between the electrodes on the inner and outer surfaces of
the tube causes the tube to bend or to increase or decrease its length.
A voltage applied between the inner electrode and one of the outer electrodes causes
the tube to bend. The application of complementary biases to two electrodes on opposite sides
of the tube doubles the magnitude of the bending. Biases applied on the other two electrodes
induces bending orthogonal to the previous mode. These two bending modes are used for
lateral displacements of the probe relative to the sample (x-y displacements).
Vertical displacements are imposed by changing the bias of the inner electrode relative
to all four of the outer electrodes.
Ideally, the relative length of the piezoelectric scanner varies linearly with the applied
bias. However, the piezoelectric scanners are very sensitive elements, susceptible to non-
linearities caused mainly by ageing or by working in an improper environment, for example in
excessive temperatures (over 150o C) or excessive humidity.
Among the non-idealities that can occur with an AFM piezoelectric scanner and that can
have a pronounced negative impact upon the electric measurements, we mention: hysteresis,
creep, cross-coupling.
It is said that the piezoelectric transducer manifests a hysteretic behavior when the
paths of extension (trace) and contraction (retrace) of the PZT scanner are different due to a
hysteretic relationship between the applied voltage and the change in its dimensions (Figure
2.9.2)
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
131
Figure 2.9.2 Example of a hysteresys curve [2.26]
As a consequence, the surface of a sample seems to be different in the two AFM images
scanned in the forward direction (trace) and the backward direction (retrace). The
discrepancies are more accentuated when the images are scanned with the piezoelectric
transducer very contracted or very extended.
The differences between the trace and the retrace become even more evident when the
scan is performed on large images, of tens of microns.
The scanner creep is a phenomenon that occurs especially when an abrupt voltage is
applied to the scanner. Such a voltage is usually applied in the common situation when the
user wants to move the tip from one spot to another (to make an offset).
When such a voltage is applied, the scanner doesn't extend or contract all at once.
Instead, the change occurs in two steps. In the first step, the main change in the shape of the
scanner occurs as a response to the applied bias. In the second step, called creep, the scanner
slowly continues to extend or contract, although there isn't any modification in the applied bias
(Figure 2.9.3).
Figure 2.9.3. Example of the creep effect [2.26]
The cross-coupling refers to the fact that the lateral bending of the scanner, meant to
generate a movement of the tip in the x–y plane, can also generate an undesired vertical
movement of the tip.
Because of the fact that the cantilever is in feedback, the scanner is commanded to
retract the tip in order to avoid the approach to the surface. As a consequence, it retracts in
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
132
the z direction and the AFM will register an artificial concave surface (Figure 2.9.4)
Figure 2.9.4. Example of the creep effect [2.25]
Further on, we will give a few examples of how these non-linearities may impact the
electrical measurements with the AFM.
2.9.2. Experimental
1. In the case of the doping profile characterization on the cross-section of the samples,
the doping layers are to be found, in most of the cases, on the very edge of the sample
(Figure 2.9.5)
Figure 2.9.5. Doping profile on the cross-section of a sample. 1-initial engagement of the tip; 2-offset of
the tip to the doping profile.
Engaging the tip directly on the doping layer is a very dangerous and delicate
operation. Such a procedure would suppose to be able to engage the tip only a few hundreds
or tens of nanometers off the edge of the sample. The tip could easily slip off the surface and
break.
Such an operation is also hindered by the fact that the position of the tip on the
backface of the cantilever is not known with precision. Different cantilevers have the tips
located in different regions of the cantilever.
A safe way to bring the tip on the doping profile is to engage the tip a few microns off
the edge of the sample (Figure 2.9.5 – position 1) and to slowly approach the doping profile
by consecutive offsets. The downside of this method is that the consecutive abrupt voltages
applied to the piezoelectric scanner accentuate the creep effect. As a result, the SCM images
may present a drift, which is exemplified in Figure 11.6. Besides the intrinsic problems related
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
133
with analysis of such an image, the creep further endangers the tip of falling off the surface.
Figure 2.9.6. Example of creep with doping profile measurements on the cross-section of a
sample
As a parenthesis, it can be said that the problem is even greater with SSRM
measurements. In the case of the resistive measurements, the force between the tip and the
sample is much greater than in the case of the SCM measurements. If the SSRM tip falls off
the surface, not only the shape of the tip or its coating are endangered, but there is also the
possibility that the tip or even the whole cantilever break.
One other problem met with the SSRM measurements is that no good electric signal is
obtained in the first minutes of scanning, because of the creep effect (Figure 2.9.7). With the
SSRM, a very good electrical contact between the tip and the sample must be achieved in
order to obtain an electrical signal. The creep effect, probably combined with a cross-coupling
effect which determines a scanner movement in the z direction, doesn't let the tip to achieve
the necessary electrical contact with the surface of the sample.
Figure 2.9.7. SSRM signal (left) in the first minutes after the offset on the doping profile. The SIMS profile of the dopant profile (right)
2. The creep effect also hinders the SCS measurements on the oxides, on the surface of
the samples.
With the SCS measurements, the tip must stay still in a spot, while the capacitance-
voltage ramps are performed. When the AFM passes from SCM mode in SCS mode, the tip is
brought from a corner of the scanned area towards the middle of the image, where the SCS
measurements are performed by default. During this operation, an abrupt voltage must be
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
134
applied on the scanner, which triggers the creep effect. During the SCS measurements, the tip
will not stay still in the same spot (Figure 2.9.8)
Figure 2.9.8. Topographical image (left) and SCM image (right) on a high-k dielectric. On both images, it can be observed the elongated shape of the spot where the SCS measurements have been
performed. This shape is an indication of the drift of the tip during the SCS measurement.
A solution to prevent the drift during the SCS measurements is to wait several minutes
after passing from image mode in the ramp mode. During this time, the creep effect will
slowly vanish and the measurements will be more stable.
3. An interesting application with the electric measurements is the study of the charging
and the anodic oxidation of the samples during SCS measurements (paragraph 3.4.1) or C-
AFM measurements .
Such a study requires several stages. In a first stage, a high voltage (10 volts for
example) is applied for several minutes in one spot of the sample and the anodic oxidation
takes place.
This operation affects not only the surface of the oxide, but also the tip, so the tip must
be changed and current measurements are performed with a new tip.
In a third stage, the electronic module and the tip are changed in order to perform
capacitance measurements.
During all these operations, exactly the same spot of nanometric size, where the anodic
oxidation took place, must be localized. This can be done by the use of nano-marks [2.27].
Additional oxidations may be performed in order to be able to find the same spot when
measurements are performed with a new tip. (Figure 2.9.9).
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
135
Figure 2.9.9 Nanomarks created on the surface of a sample by anodic oxidation which helps find the spot of interest (the central spot) for consecutive electrical measurements. Image
performed by Wael Hourani, phd. Student, INSA de Lyon
The problems due to creep appear when the tip must be placed again exactly over the
central spot. Consecutive AFM images show that the tip drifts from one image to another
because of consecutive offsets and zoom in operations (Figure 2.9.10).
Even when the AFM image is finally centered, we cannot be sure that, when switching
from the image mode to the ramp mode, the tip will be placed on the oxidized surface and it
will not drift. In order to avoid this possibility, it is best to zoom in on the region of interest as
much as possible before switching to the ramp mode and to leave the piezoelectric tube for
several minutes to stabilize.
Figure 2.9.10. Effect of creep as a result of consecutive offsets, from one scan to the next
4. A very useful and powerful tool with the AFM is 'point&shoot'. After scanning the zone
of interest, several spots can be selected where SCS ramps are to be performed.
The point&shoot is dangerous to use with samples that present important topographical
features, because the feedback loop is deactivated during this operation. However,
point&shoot is a very useful tool for electrical measurements on doping profiles and oxides,
because such samples don't have any important topographical features.
Thus, consecutive automated SCS measurements can be made on all the regions of a
doping profile or in several regions of an oxide, in order to study its homogeneity.
However, because of the consecutive offsets during the point&shoot (consecutive
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
136
abrupt voltages applied to the scanner), there is the possibility that the tip may not perform
the measurements in the chosen spots (Figure 2.9.11)
Figure 2.9.11. Topographical image (left) which shows the distance between the places where measurements are to be performed (261 nm). SCM image (right) which shows the actual distance between the places where the measurements have been done (143 nm)
This is especially problematic in the case of point&shoot measurements on doping
profiles. There is the danger that, because of the creep, SCS measurements are not done on
all the doping regions. Instead, several consecutive measurements may be done on the same
doping region and none on the other regions.
5. A very serious problem with the AFM is the oxide charging and the anodic oxidation for
the measurements done in air (section 2.6.8). The air humidity, combined with the small size
of the tip and the conical shape which generates strong electric fields, and with high applied
biases, can lead to the oxide charging or even to anodic oxidation. Further, contaminants on
the tip and on the sample can distort the measurements.
One solution in the case of oxide characterization that can solve all these problems
consists in the fabrication of nanoelectrodes. Nanoelectrodes protect the surface from
contaminants and humidity, insure a good electrical contact with the surface, as in the case of
macroscopic C-V measurements, eliminate the stray capacitance problems generated by the
conical shape of the tip. In addition, the size of such nanoelectrodes can be measured with the
AFM.
Nanoelectrodes can also constitute a good solution for C-AFM measurements. The main
impediment in the reproducibility of I-V measurements with the AFM is the anodic oxidation
because of the environment humidity, correlated with strong applied biases used with this
characterization method. The unknown surface of the tip prevents a quantitative
characterization of the oxides. Performing I-V measurements on nanoelectrodes would solve
all these problems.
The procedure that we use to fabricate such nanoelectrodes is the following. A thin
metallic layer (5 nm) is deposited on the top of the oxide to be analyzed, by evaporation with
an electron gun. Nanoelectrodes are engraved in the metal with the AFM, by using diamond
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
137
coated tips, specific for SSRM measurements.
By using a nanolithography program or even the point&shoot tool, arrays of such
nanoelectrodes can be fabricated.
Figure 2.9.12. Fabricated nanoelectrode by engraving a nickel layer on top of a 5 nm
thermal oxide
However, with a faulty piezoelectric scanner, because of the creep effect, instead of
creating arrays of nanoelectrodes, all the surface of interest will be randomly engraved,
without being able to fabricate nanoelectrodes.
For now, we have been able to fabricate only isolated nanoelectrodes, one at a time
(Figure 2.9.12)
6. As we have presented in chapter 1, one way to prepare the doping profile samples is by
bevel polishing. In this way, the sample area can be magnified up to a few hundred times and
the geometrical resolution can be improved (Figure 2.9.13).
Figure 2.9.13 Setup for measurements on a beveled sample
Performing measurements on beveled samples instead of the cross-section of the
samples, has several implications. First of all, the scanned area is much larger. If a doping
profile extended over 4 microns for example, as a result of a magnification by a factor 10, the
doping profile on a bevel sample will be 40 microns large. Second, the height difference
between the start and the end of a scan line will be of several hundreds of nanometers.
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
138
Figure 2.9.14. The AFM scan line on a beveled sample. For an amplification coefficient x10 of the
bevel, the height difference between the start and the end of a scan line is 1.000nm for a 100 x 100 μ
image.
Scanning large surfaces with a faulty piezoelectric scanner, on samples that present
such height differences, can lead to the total loss of the electric signal. Because of the cross-
coupling effect, the tip will not stay in contact with the surface and electrical measurements
cannot be performed.
A way to solve this problem is to use a special holder in such a manner that the
beveled sample to be placed in a horizontal position.
Figure 2.9.15. Metallic holder for beveled samples
2.9.3. Conclusions
The examples above show how the non-linearities of the piezoelectric scanner can have
a negative impact on the electrical measurements with the AFM. These problems are
magnified mainly because numerous offsets are necessary to be made with these type of
measurements, which amplify the effect of creep.
The best way to counter such problems is to make sure that the piezoelectric scanner is
perfectly functional and well calibrated at all times. If it is not possible, several other
precautions can be taken.
● When engaged on the surface of the samples, special care must be taken that the
scanner is not in a retracted or contracted position.
● Immediately after engagement, the system should be left for at least a few minutes to
stabilize before starting any measurement. The same thing is valid after each offset that must
be performed during the characterization of the sample.
● In the case of the need of an offset in order to change the location where the
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
139
measurements are performed, it is better to avoid this operation whenever possible.
Sometimes, offsets can be avoided, especially in the case of oxide characterization on plane
surfaces, by disengaging the tip from the sample, making the offset by moving the sample
with the XY stage positioning system and re-engaging the tip on the sample.
● In the case of the measurements performed on the cross-section of the samples,
special care must be taken in order to avoid the tip falling off the sample. The best way to
insure this is by deposing a capping layer on the surface of the sample.
Finally, it must be noted the difference between all the types of problems presented in
the previous sections of this chapter (parasitic light, parasitic capacitance) and the scanner
problems presented in this paragraph. Parasitic light and parasitic capacitance are problems
that appear because of the intrinsic design of the AFM. Piezoelectric issues may sometimes
appear because of maintenance problems of the AFM.
2.10. Conclusions
SCM is a very sensitive technique, prone to numerous sources of stray signal. The
quality of the topography, of the oxide and of the tip, stray laser and stray capacitance, make
SCM a difficult technique to work with.
We have shown that the AFM laser strongly affects the signal/noise ratio of the SCS
signal. Also, the AFM laser can lead to the creation of the additional peaks on the SCS signal.
Considering that the amplitude of the SCS signal is the main parameter used in the attempts
to quantify the doping profiles, it is vital that this parasitic effect be completely removed.
The geometry of the AFM setup generates several parasitic capacitances that influence
the SCM measurements.
Some of them (cantilever chip – sample/ sample holder/ XY stage) are very important
and determine a huge drop of the signal/noise ratio. Other sources (cantilever-sample/ sample
holder/ XY stage, SCM electronic module – XY stage) are less important, determining a small
drop of the sensitivity of the signal/noise ratio.
The electrical components such as the UHF transmission line, the holder, the cantilever,
the electrical contact between the holder and the cantilever, the back contact of the sample,
have a minor impact upon the sensitivity of the detector and may vary from one measurement
to another if necessary precautions are not taken.
The face contact however, between the tip and the sample, is negatively influenced by
a multitude of factors: the properties of the tip, contaminants on the tip and on the surface,
topography.
Other problems related to the quality of the oxide and to the phenomena affecting the
Chapter 2 Reproducibility problems with the SCM. Optimization of the experimental conditions for SCM operation
140
oxide will be presented in the next chapter.
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[2.21] R.Castagne, A. Vapaille, 'Description of the SiO2 - Si interface properties by
means of very low frequency MOS capacitance measurements', Surface Science vol.28, no.1,
1971
[2.22] R. F. Pierret, 'Semiconductor device fundamentals', Addison Wesley Longman,
1996.
[2.23] Dror Sarid, 'Scanning Force Microscopy with applications to electric, magnetic
and atomic forces', Oxford University Press, 1994.
[2.24] S.M. Sze, 'Physics of Semiconductor Devices', Wiley-Interscience Publication,
1981.
[2.25] Springer Handbook of Experimental Solid Mechanics, Springer, 2008.
[2.26] Babak Mokaberi, Aristides A. G. Requicha, 'Compensation of Scanner Creep and
Hysteresis for AFM Nanomanipulation', IEEE Transactions on Automation Science and
Engineering, Nov. 2005.
[2.27] Werner Frammelsberger, Gunther Benstettea, Richard J Stamp, Janice Kiely,
'Combined AFM Methods to Improve Reliability Investigations of Thin Oxides', 2002 IEEE,
poster presentation.
[2.28] L. Ciampolini, 'SCM imaging and modelling', Swiss Federal Institute of
technology (ETH), Zürich, Suisse, 2001.
Chapter 3. Oxides characterization with the SCM
Chapter 3 Oxides characterization with the SCM
143
3.1 Introduction
The oxide layer represents the dielectric of the MOS structure substrate - oxide - tip
characterized with the SCM.
The properties of the oxide layer between the tip and the substrate affect the SCS
curve, making sometimes hard to discern which features of the SCS signal are due to the
doping profile and which ones to the oxide layer. A good quality oxide is required in order to
be able to study the properties of the doping profile in terms of resolution, quantification or
localization of a p-n junction. Further on, good quality thermal oxides cannot be grown on top
of a doping profile. The high temperatures used for the growth of such oxides (1000o-1200oC)
would determine the diffusion of the impurity atoms in the silicon, destroying the very doping
distribution that is to be characterized.
The oxides can constitute a separate subject of study. Given a known substrate, of
known uniform doping concentration , the properties of the SCS signal would be determined
by the oxide properties. It is better to use a low doping concentration, 1014 cm-3 – 1016 cm-3,
because the SCS signal is higher on such substrates. Thus, the oxide defaults (fixed charges,
mobile charges, interface states) can all be characterized at nanoscale with the SCM.
3.2. Oxide defects and their influence on the SCS signal
The oxides present defects that can be grouped in the following categories (Figure
3.2.1):
- mobile charges (mobile ions)
- fixed charges
- interfaced trapped charges (interface states)
- oxide trapped charges (bulk oxide charge)
Figure 3.2.1 Oxide defects - diagram after Pierret [3.1] pg.651.
Each type of defect has a specific influence on the capacitive signal.
Chapter 3 Oxides characterization with the SCM
144
3.2.1. Mobile Ions
3.2.1.1. The nature of mobile ions
The mobile charges in the oxides are alkali (positive) ions, mainly ions of Na.
The mobile charges in the oxides have been discovered when trying to explain a
phenomenon encountered by measuring the C-V characteristics on thermal oxides. It has been
noticed that the C-V characteristics were:
- always shifted in the direction opposite to the applied polarity on the gate
- the C-V curves were always situated to the negative side of the theoretical curves
The alkali ions have been identified as the cause of these abnormalities, because of
previous knowledge concerning these ions.
It was already known that alkali ions, especially sodium ions, are abundant on the
hands, in the glass apparatus, chemical products used in laboratories.
In addition, even from the 19th century, has been demonstrated that alkali ions can
move through quartz or crystalline silicon at low temperatures.
The hypothesis has been confirmed by two experiments.
In one experiment, called neutron activation technique, the oxides have been
bombarded with neutrons, trying to render the sodium, if it existed, radioactive. The analysis
showed the presence of the sodium in the oxides.
In another experiment, semiconductor structures have been intentionally contaminated
before metallization by rinsing the oxidized silicon wafers in a dilute solution of NaCl. These
intentionally contaminated devices showed severe instabilities under bias-temperature
stressing.
3.2.1.2 The influence of the mobile ions on the capacitive signal
As has been already mentioned, the C-V characteristics are always shifted in the
direction opposite to the applied gate polarity. Also, C-V curves are situated to the negative
side of the theoretical curves
The effect of the mobile charges on the SCS signal is similar to the effect on the C-V
characteristics (Figure 3.2.2).
There is a hysteresis and the SCS curves should be situated to the right of the
theoretical signal (the bias being applied in this case on the substrate).
The current methods for detecting the presence of mobile charges in the oxide are:
1. 'bias temperature stressing'.
2. in extreme cases, the instability can be observed by simply biasing the device at
room temperature.
- trace a C-V characteristic;
Chapter 3 Oxides characterization with the SCM
145
- apply a certain bias, positive or negative, for a period of time;
- redo C-V characteristic.
If there are mobile ions, the curve should be shifted in the directions opposite to the
applied bias.
Figure 3.2.2 The influence of mobile charges on the SCS signal. Green
curve - SCS curve in the absence of mobile charges. Red curve - SCS
curve for an oxide stressed to a negative bias applied to the substrate
and the alkali ions shifted towards the Si-SiO2 interface. Black curve -
SCS curve for an oxide stressed to a positive bias applied to the
substrate and the alkali ions shifted towards the gate-SiO2 interface.
The current methods for reducing the amount of mobile charges are:
1. prevention of the alkali-ion contamination throughout the MOS fabrication process
(which is extremely difficult)
2. Technological procedures developed to minimize the effects of residual alkali-ion
contamination
- phosphorous stabilization (PSG);
- chlorine neutralization.
3.2.2. Fixed Charges
3.2.2.1 The nature of fixed charges
From considerations enumerated below, it has been postulated that the fixed oxide
charge is due to excess ionic silicon that has broken away from the silicon crystal and is
waiting to react in the vicinity of Si-SiO2 interface when the oxidation process is abruptly
terminated.
- the fixed charge is independent of the oxide thickness, the semiconductor doping
concentration and the semiconductor doping type (n or p);
- the fixed charge varies as a function of the Si surface orientation.
QF (the amount of fixed charges) is the largest on {111} surfaces, smallest on {100}
surfaces, and the ratio of the fixed charges on the two surfaces is approximately 3:1.
Chapter 3 Oxides characterization with the SCM
146
3.2.2.2. The influence of the fixed charges on the capacitive signal
The C-V characteristic is translated toward negative biases relative to the theoretical
curve.
In the SCS case, the only difference is that the signal is shifted toward positive biases
relative to the theoretical curve, given the bias applied not to the gate, but to the substrate
(Figure 3.2.3).
Figure 3.2.3 The influence of fixed charges on the SCS signal
The presence of the fixed charges can be determined by calculating the flatband bias of
the structure and compare it with the experimental value. However, it must be taken into
account that the shift caused by the fixed charges can superimpose with the shift given by
mobile charges and bulk charges.
Regardless of the oxidation conditions, the fixed charge can always be reduced to a
minimum by annealing in an inert atmosphere (Ar, N2).
3.2.3. Interfacial Traps
3.2.3.1. The nature of interfacial traps
Interfacial traps – also referred to as surface states or interface states – are allowed
energy states in which electrons are localized in the vicinity of a material's surface.
The interfacial traps introduce energy levels in the forbidden gap at the Si-SiO2
interface. Actually, the interface states can, and normally do introduce levels distributed
throughout the band gap, but such levels are usually obscured by the much larger density of
conduction or valence band states.
The physical origin of the traps has not been totally clarified [3.1].
The weight of experimental evidence, however, supports the view that the interfacial
traps primarily arise from unsatisfied chemical bonds or so called 'dangling bonds' at the
surface of the semiconductor.
When the silicon lattice is abruptly terminated along a given plane to form a surface,
Chapter 3 Oxides characterization with the SCM
147
one of the four surface-atom bonds is left dangling. The thermal formation of the SiO2 layer
ties up some but not all of the Si-surface bonds. It is the remaining dangling bonds that
become the interfacial traps.
The interfacial trap density, like the fixed oxide charge, is the greatest on {111} Si
surfaces, the smallest on {100} surfaces, and the ratio of midgap states on the two surfaces
is approximately 3:1.
3.2.3.2. The influence of the interfacial traps on the capacitive signal
The manifestation of a significant interfacial trap concentration within a MOS structure
is the distorted or spread-out nature of the C-V characteristics.
- in a semiconductor, to a first-order approximation, all energy levels above EF are empty and
all the energy levels below EF are filled.
- the interfacial traps charge and discharge as a function of bias.
- in inversion, Ev/Ec passes above/below the fermi level.
The SCS signal on a sample which presents interfacial traps has a larger value of the
width at half maximum compared to the theoretical curve (Figure 3.2.4)
Figure 3.2.4 The influence of interface states on the SCS signal
The presence of the interfacial traps can be revealed by comparing C-V characteristics
before and after a hydrogen annealing. After the annealing, the transition region of a C-V
curve should be more abrupt. In consequence, the SCS signal should be narrower.
The interfacial traps can be reduced by annealing in the presence of hydrogen at
relatively low temperatures (≤500C), from 1011, 1012 states/(cm2 eV) to values smaller than
1010 states/(cm2 eV). There are two well-known procedures in the semiconductor industry:
• postmetallization annealing
• hydrogen annealing
Burte et al [3.2] conducted a study of the density of interface states with the annealing
temperature. The oxides have been annealed in forming gas at temperatures ranging from
250°C to 620°C. A minimum of the density of interface states has been observed for the
samples annealed at 450° C.
Chapter 3 Oxides characterization with the SCM
148
3.2.4. Bulk oxide defects
3.2.4.1. The nature of bulk oxide defects
The SiO2 network often contains imperfections and impurities [3.3] which can be
electrically active if they introduce energy levels in the SiO2 band gap.
The defects in the bulk of the oxide can come from various sources related to the oxide
growth or to various processes which follow the oxide growth.
During the oxide growth, foreign atoms can be introduced by the oxidizing ambient -
H2O, H2, Na, Cl - or by the substrate - B, P, As. Other types of oxide growth, like plasma
assisted oxidation, or wet-oxidation introduce various defects in the bulk of the oxide.
Metal deposition on top of the oxide is mainly responsible for metallic contamination.
The metal atoms can penetrate the oxide as deeply as 2 nm.
During lithography (e-beam, ion beam, X-rays) the radiation can cause bond rupture
and displacement damage.
Post-oxidation annealings let the hydrogen enter the oxide.
A particular case is constitute by the Na ions. Although they may be considered as
mobile charges, as long as the oxide is not stressed under temperature conditions, the sodium
will not move through the oxide and it will act as a volume defect. This explains why
characterization methods using a physical analysis as SIMS indicate a greater sodium
concentration than the electrical methods. SIMS detects the entire amount of sodium present
in the SiO2, while the electrical characterization methods will measure either the quantity of
mobile sodium, or the electrically active sodium trapping centers.
These defects may act as recombination centers or as trapping centers, donor or
acceptor. A donor trap is neutral when filled with an electron and positively charged when
devoid of the electron. An acceptor trap is negatively charged when filled with an electron and
neutral when devoid of the electron.
The trapping mechanism is related to the phenomenon of charge injection into the
oxide by various mechanisms (electrical, optical, thermal). With the SCM, a particular interest
is represented by the electrical charge injection into the oxide, given the strong electrical fields
generated by the geometry of the tip combined with the water layer present on top of the
hydrophilic oxide.
3.2.4.2. The influence of the bulk defects on the capacitive signal
The introduction of trapped charges in the bulk of the oxide will determine a shift of the
C-V curve along the voltage axis. The sign of the shift of the C-V curve will be opposite to the
sign of the charges injected into the oxide (Figure 3.2.5).
Chapter 3 Oxides characterization with the SCM
149
Figure 3.2.5 The influence of volume charge on the SCS signal. The continuous line represent the SCS signal in the absence of
bulk charges. The dotted signals represent the SCS signals in the presence of dotted charges. The shift with the SCS (the bias
applied on the substrate) has the same sign as the sign of the charges.
In the case of volume charges, the shift of the C-V curve depends not only of the
density of volume charges, but also of their distribution in the volume of the oxide. The
volume charges located near the gate will have less impact upon the C-V curves.
The defects distribution into the oxide can be evidentiated by successively etching the
oxide and recording the shifts of the C-V curve for different oxide thicknesses. The drawback
of this method is that metal depositions must be performed after each etching in order to
create a new gate. These depositions may also modify the remaining SiO2 because of the
metal atoms which penetrate through the oxide creating new trap centers and modifying the
trap distribution.
With the SCM, this problem doesn't exist, given that no face contacts or back contacts
are needed (chapter 2, section 6).
3.3. State of the Art
The bibliography related to different types of oxides, in terms of growth and
characterization, is extensive. In this thesis, we are going to limit our approach to the oxides
used with the SCM and characterized by the SCM.
3.3.1. Requirements for the oxides used with SCM measurements
An oxide used as a dielectric layer with the SCM for the measurements of the doping
profiles, besides the number of defaults as low as possible, has other several basic
requirements that must meet.
- low-temperature fabrication
- ease of formation
- reproducibility of the oxide quality
- reproducibility of the oxide thickness
- uniformity of the oxide layer in terms of thickness and defects.
Chapter 3 Oxides characterization with the SCM
150
3.3.1.1 Low-temperature fabrication
One of the most important and limiting conditions for an oxide that can be used in SCM
measurements is that the oxide must be grown at temperatures smaller than the diffusion
temperature of the semiconductor dopants. This condition eliminates from the start the best
known oxide, the thermal oxide, which is grown at temperatures passing 1000oC.
However, thermal oxides can be used in the analysis of the properties of the oxides as
the standard for the other oxides. Thermal oxides can be used with uniform substrates, thus
eliminating any danger of doping diffusion. The capacitance signal measured on thermal
oxides offers a standard to which we can refer when measurements are done on other oxides.
Any parameter that influences the capacitance signal is needed to be understood and
calibrated with a reference sample. We consider that thermal oxides of the highest quality
constitutes the best reference sample in the study of oxide properties and their influence on
the capacitance signal.
Given the fact the thermal oxides cannot be used for the analysis of the doping
concentration, different other oxides, grown at low temperature, have been proposed in the
literature and their quality analyzed.
- native oxides;
- dry oxidation under UV/ozone;
- dry oxidation;
- wet chemical oxidation.
3.3.1.2 Ease of formation
The ease of formation of the oxide is a criteria necessary for practical reasons. For
mass measurements it is important that the sample preparation be quick.
The easiest low-temperature oxide to grow is the native oxide. All that must be done is
to leave the sample in a proper environment for a few hours, after the previously native oxide
has been removed or after the sample has been cleaved.
3.3.1.3 Oxide reproducibility
The oxide reproducibility is also important for mass measurements.
In order to make comparisons between results on different samples, it is important that
the technological processes insures each time the same quality of the oxide on all the
samples.
3.3.1.4 Oxide thickness
The oxide thickness is an important and sensitive criteria. If the oxide is too thin,
tunneling currents will make impossible any measurement attempt. If the oxide is to thick, the
Chapter 3 Oxides characterization with the SCM
151
sensitivity will drop until, eventually, we will not have any signal at all. The oxide thickness
should be correlated with the oxide dielectric constant. Oxides with higher dielectric constants
will allow the use of thicker oxides.
The thickness of the oxides for the SCM is measured in the literature by different
means, mostly often by TEM and XPS.
Goghero et al [3.4] measured thin wet-chemical oxides and low-temperature thermal
oxides by TEM and they obtained a thickness of 2-3 nm for the chemical oxide and 3-6 nm for
the thermal oxide
Yang et al [3.5] measured the thicknesses of a thermal oxides to be around 3 nm by
ellipsometry.
Duhayon [3.6] measured the thicknesses of different oxides with the XPS.
For some types of oxides, like the UV/ozone and the native oxides, the oxidation
reaches saturation after a few nanometers and their thickness can be estimated.
Duhayon [3.6] presents in her thesis a plot of the UV/ozone oxide thickness with
respect to the oxidation time. Based on her measurements, but also on the data of other
authors, Duhayon showed that the oxidation process stops after the UV/ozone oxide reaches a
thickness of around 1.7 nm-1.8 nm.
Figure 3.3.1 The oxide thickness, as determined with XPS, plotted in function of the oxidation time.
Duhayon [3.6], pg.78
Oxide's thicknesses monitoring study done at Standford shows that the native oxide
reaches a maximum thicknesses of approximately 1.2 nm after several days (Figure 3.3.2).
In conclusion, it may be said that the UV/ozone has a maximum thickness of 1.8 nm
and the native oxide a maximum thicknesses of 1.2 nm.
Chapter 3 Oxides characterization with the SCM
152
Figure 3.3.2 The thickness of native oxide as a function of time. U.Thumser, P.Beck,
D.Stewart, Standford University
3.3.1.5 Oxide uniformity
Another important property of the oxides is the oxide uniformity, in terms of
thickness and defaults.
Thickness variations across a sample and different default densities from one spot to
another will surely influence the capacitive signal (FWHM, flatband voltage and hysteresis)
which will lead to misinterpretations of the doping concentration and to contrast inversion.
In the literature, one way to verify the oxide uniformity at nanoscale is by plotting the
Vdc max versus the number of measurements [3.7]
Figure 3.3.3 Analysis of oxide uniformity O. Bowallius et at [3.7]
We consider that this method is not fully efficient if the measurements are done in air,
on surfaces that present contaminants and surface moisture. The contaminants and the
humidity of the surface may be responsible for the differences in flatband voltage from one
spot to another. This method could be rather used to calculate an average of the flatband bias
for the oxide in question.
Chapter 3 Oxides characterization with the SCM
153
Finally, it must be restated that these conditions must be met only by the oxides used
for measurements aiming at the doping concentration.
If the measurements aim at the analysis of the very oxide, the only condition which
stays valid is the oxide thickness. It must not be forgotten that the SCM is able to perform
measurements on thin oxides only.
Measurements on thicker oxides could be performed with the SCM by using
nanoelectrodes. On thicker oxides, bigger nanoelectrodes could be used for compensating for
the loss of signal because of the thickness of the oxide. Thus, we consider that SCM can be
used to perform measurements even on oxides hundreds of nanometers thick with electrodes
of nanometric scale.
3.3.2 Oxide properties and properties of the SCS signal
As we have seen, the oxide defaults and, implicitly, the oxide quality can be determined
by evaluating the parameters of a SCS curve:
- the position of the flatband bias (shift of the flatband bias);
- the FWHM of the curve;
- the hysteresis of the signal – the difference between the trace and the retrace.
However, other factors may influence these parameters. Here is a list of the factors
which may influence the parameters of a SCS curve
3.3.2.1 The position of the flatband bias
With the SCM, the bias for which the maximum amplitude of the capacitance signal is
obtained, is taken as an accurate indicator of the flatband bias, which is very near.
In the ideal case, the flatband bias is the difference between the workfunctions of the
substrate and the tip. Further on, given the fact that the tips (doped silicon, doped diamond,
PtIr5, chromium) have very close workfunction values of those of the silicon substrate, the
flatband bias should be situated around 0 V.
The near-zero flatband voltage is very important in SCM measurements. The
nanometric dimension of the tip and its sharp shape, combined with strong applied biases, will
lead to tunneling currents and injection of charge into the oxide during the experiments. A
near-zero flatband voltage would allow the use of small biases.
In an ideal case (no defaults in the oxide) the flatband voltage should be around 0 V.
The workfunction of the tips used with the SCM (metal coated tips, heavily doped silicon tips,
doped diamond tips) is very close of the workfunction of silicon. In almost all the cases, the
difference between the two workfunctions is not greater than 1 V.
In order to meet the condition of near-zero flatband voltage in practice, the
concentration of defaults, which cause a shift of the flatband towards higher values, should be
kept as low as possible.
However, in current SCS measurements, the flatband bias is shifted towards higher
Chapter 3 Oxides characterization with the SCM
154
values. The causes that may determine the shift of the flatband bias are:
• fixed, mobile charges, oxide trapped charges
• charging and discharging of the interface states
• the state of the oxide surface (contaminants, surface moisture etc)
• tip degradation
We have already shown that the fixed and mobile charges, which are positive in
most of the cases, cause a shift of the flatband voltage to the right of the ideal flatband
voltage.
Beyer et al. [3.8] have shown that different initial voltages in the case of a dc-bias
ramp can alter the position of the flatband bias.
Figure 3.3.4 Shift of the dC/dV peak with the variation of the onset of the dc-bias sweep. Beyer et al [3.8]
The measurements have been done on a n-type low-doped substrate. The sweep has
been done starting from positive biases towards negative biases. The bias was applied to
the tip.
A shift of 1.6 V in the peak position, to the left, has been observed, when the starting
bias changed from +2V to +8V. The authors estimated that 1.6 V corresponds to a quantity of
trapped charges of 5.1012 cm-2.
The increase of positive charge in the dielectric layer has been explained by two
possibilities:
A field assisted emission of weakly bound electrons, especially from trap states, under
the influence of the high electrical field at the starting point
A proton H+ incorporation in the oxide layer due to the decomposition of water layer
from the surface of the hydrophilic oxide. This proves once again, besides surface oxidation,
the negative effect on the SCM measurements of the water layer present on the surface.
Several authors, like Giannazo et al [3.9] have shown that a tip degradation, especially
in the case of metal-coated tips, will determine a shift in the flatband voltage. Once the metal
coating is washed off during scanning letting the doped silicon at the surface of the tip, the
workfunction of the tip will change.
Chapter 3 Oxides characterization with the SCM
155
3.3.2.2 Full-width at half-maximum
FWHM can vary as a function of the following parameters:
◦ - interface states given by the quality of the oxide
◦ - interface states which resulted from the roughness of the surface (higher for
higher roughness)
◦ - different response time of the interface states (the sweep rate of the dC/dV-V
characteristic)
◦
We have already seen that, according to the theory, the SCS signal on a sample which
presents interfacial traps should have a larger value of the width at half maximum compared
to a theoretical, ideal curve.
Giannazzo et al [3.9] have demonstrated this fact by comparing SCS signals between
theoretical curves that do not take into consideration interface states and SCS experimental
curves.
The theoretical characteristics have been
calculated for a MOS structure formed by a
parabolic shaped tip with an apex of 30 nm
radius, and with the corresponding
experimental parameters: substrate doping
concentration varying from 1015 cm-3 to 1019
cm-3, a 2.6 nm oxide thickness, Vpp = 300 mV.
The author states that a shift in the voltage
axis has been applied to the experimental
results, considering that the theoretical model
didn't take into consideration the fixed charges.
It can be seen that the experimental curves
are broader than the theoretical ones, which
can be explained by the presence of the
interface states. Also, it can be noticed that the
effect of the interface states is more
accentuated for lower concentrations of the
substrate.
Figure 3.3.5 Comparison between
experimental and calculated SCS signals
for p-type and n-type silicon Giannazzo et
al [3.9]
Also, Yang et al [3.5] have shown the difference in the FWHM of the SCS signal by
experimental means.
Chapter 3 Oxides characterization with the SCM
156
Figure 3.3.6 Experimental dC/dV-Vdc data on
uniformly doped p-type silicon for two different
oxides. Yang et al [3.5]
The nitride gate oxide is known from the industry to present no interface traps. The
SiO2 oxide has a high interface states density. The measurements have been done on the
same type of substrate and the oxide had similar thicknesses.
From the two SCS signals, it can be seen that the curve measured on the thermal oxide
is much broader than the curve on the nitride oxide.
Bowallius et al. [3.7] showed that on a cross-section of a sample, as a
consequence of higher roughness than on a planar surface, because of cleaving, the
FWHM is also greater than on the surface of the same sample.
Here are his results on the surface of a sample and on a cross-section for native oxide
and for a wet-chemical oxide:
Geometry Oxide type FWHM
Planar (100) Native 0.9 V
Planar (100) Wet-chemical 0.9 V
Cleaved Native 1.5 V-2 V
Cleaved Wet-chemical 1.6 V
However, we must observe that the generation of interface states due to cleaving has
not been isolated from other effects, such as the crystalline orientation. The surface of the
sample has a (100) orientation, while the cross-section of the same wafer cannot still have a
(100) orientation. Thus, we consider that the enlargement of the SCS signal has, in this case,
multiple causes.
Goghero et al [3.4] showed that the FWHM increases with the roughness of the
surface caused by cleaning the native oxide with higher concentrations of HF.
Yang et et [3.10] also showed that an increased roughness of the surface due to
polishing determines higher values for FWHM.
Chapter 3 Oxides characterization with the SCM
157
Figure 3.3.7 dC/dV-Vtip data on samples
polished using 0.02 um colloidal silica and 0.25
um diamond suspension. Yang et al [3.9]
The sample polished with a suspension containing larger grains determines a higher
roughness on the surface and higher densities of interface states. The difference of densities
states for the two samples is reflected by different FWHM of the two signals
Beyer et al [3.8] observed that the FWHM of the SCS signal increases with the sweep
rate.
Figure 3.3.8 Impact of the dc-bias sweep rate on the dC/dV FWHM on a a) native oxide b) thermal
oxide. Beyer et al [3.7]
Beyer considers that the interface states have different response time constants to the
changes introduced by the electrical fields and that the interface states with longer response
times will remain into a non-equilibrium state during fast sweeps.
Based on the observation that fast sweeps broaden more the peaks in the case of the
thermal oxide, the author concludes the state of equilibrium is achieved harder on the thermal
oxide because of tighter bound carriers, in the case of emission of charges, and more effective
barriers, in the case of capture of charges.
According to these observations, the best choice seems to be performing the sweeps as
slow as possible. Slow scans will allow the system achieve equilibrium and the signal will not
be distorted.
Chapter 3 Oxides characterization with the SCM
158
3.3.2.3 Hysteresis of the signal
Hysteresis can appear as a consequence of several factors:
- the presence of mobile charges in the oxide
- the limits of the sweep interval
- experimental conditions (the state of cleanliness of the surface etc)
As we have seen, the mobile charges which consist, in principal, of alkali ions, can
move through the oxide, from one interface to another, under bias. The reverse of the
direction of the electrical field from trace to the retrace of the SCS curve, determine the
reverse in the movement of these positive ions. When a positive bias has been applied on the
gate, it is very likely that the positive ions be situated to the oxide-silicon interface. When a
negative bias has been applied to the gate, it is likely that most of the positive ions be
localized at the air-oxide interface. Thus, a hysteresis, or a difference between the trace and
the retrace of the SCS signal appears. However, it must be emphasized that, with the SCM,
the hysteresis present in almost any SCS measurement is not caused by mobile charges
(section 3.4.1).
The limits of the sweep interval can influence the dimension of the hysteresis. The
amplitude of the electrical field which determines the movement of mobile ions through the
oxide, depends of the values of the sweep interval. If the limits of the sweep interval are
small, one might have no hysteresis at all. If the limits of the sweep interval are to high, the
movement of the mobile ions may be amplificated by the injection of additional charges into
the oxide.
In chapter 2 has been shown that measurements made on thermal oxide presented no
hysteresis with CV measurements and SCS measurements on a nanoelectrode. However, CV
measurements made with a probe directly on the surface of the oxide and SCS measurements
made with a tip also directly on the surface of the oxide, in air, showed the presence of a
hysteresis. Our conclusion is that hysteresis can appear as a result of an unproper surface,
humidity and contact problems between the tip and the sample.
The position of the flatband bias or more precise, the position of the maximum of
the SCS signal the FWHM and the hysteresis are the parameters of the SCS signal that
can bring information about the oxide defaults. Also, the sweep rate and the limits of the
sweep interval can be related with the defaults in the oxide and can distort the above
mentioned parameters.
3.3.2.4 The amplitude of the SCS signal
With the C-V measurements, the difference between accumulation and inversion is not
Chapter 3 Oxides characterization with the SCM
159
dependent of the density of defaults of the oxide. However, with the SCS, one doesn't
measure the difference between the accumulation and inversion, but the difference between
two values of capacitance corresponding of the extremity values of Vac.
If the slope of the C-V curve is higher
because of the interfacial traps, then the
difference between Vmax and Vmin (the
amplitude of the SCS signal) should be
smaller for the same Vac. In conclusion, the
amplitude of the SCS signal should depend
of the density of state interfaces.
However, Yang et al [3.5] have shown
that this is not the case. They showed that,
for each Vdc, the interface states don't
respond to the ac signal, because of its high
frequency. For each Vdc, the dC/dV signal
can be expected to have the same value as
in the case of a trap-free sample.
Thus, the resulting SCS signal will have a
FWHM corresponding to the respective
amount of interface states from the oxide,
but the amplitude of a trap-free sample.
Figure 3.3.9 a)Effect of interface traps on C-V
characteristics; b) Expected SCS signal when
interface traps don’t respond to the Vac
signal; Yang et al [3.5]
Their experimental results confirmed this hypothesis and have been shown in the
Figure 3.3.6. The amplitude of the two signals is practically the same, although one of the
oxides present interface states.
In conclusion, the amplitude of the SCS signal doesn't depend at all of any
type of oxide defaults.
3.3.3 Low-temperature oxides
The low temperature oxides proposed as a dielectric for SCM measurements in the
literature are:
- native oxides
- dry oxidation under UV/ozone
- dry oxidation
- wet chemical oxidation
Bowallius et al. [3.7] have compared a native oxide and a wet-chemical oxide.
The native oxide was obtained simply by exposure to air and the wet-chemical oxide
was obtained in a solution H2SO4+H2O2.
The quality of the two oxides was evaluated by the authors by comparing the flatband
Chapter 3 Oxides characterization with the SCM
160
bias difference between two different doping concentrations, 1014cm-3 and 1017cm-3.
Figure 3.3.10 SCS curves for a. native oxide and b. wet-oxide. Bowallius et al [3.7]
The author's conclusion is that the difference of almost 2 volts between the
workfunctions of the 1014cm-3 substrate and 1017cm-3 p-type substrate is unrealistic and is
caused by large numbers of defaults.
Based on these measurements, the wet-chemical oxide has a better quality and should
be preferred as a top oxide for SCM measurements, while the native oxide is not the best
choice as a top oxide, especially if quantitative measurements are taken into consideration.
The experiments made by Bowallius represent an example of how the position of the
maximum of the SCS signal can be used for a qualitative general estimation of the oxide
quality.
Goghero et al. [3.4] compared the quality of a wet-chemical oxide and a low-
temperature thermal oxide.
The wet-chemical oxide was obtained by immersing the sample in a H2O2 solution for
20 min, followed by heating the sample in air to a temperature between 150oC and 200oC.
The low-temperature thermal oxide was grown at 200oC for 1h in an oxygen gas flow.
In order to evaluate the relative quality of the two
oxides, Goghero evaluated the hysteresis of the SCS
signal.
He found that the hysteresis obtained in the case of
the wet-oxide is almost two times higher than for the
thermal oxide, for two different concentrations.
He also found that the values of the hysteresis in the
two cases remained unchanged from day to day
measurements (the hysteresis measurements are
reproducible).
On the other hand, the flatband bias were not constant
from day to day.
As a consequence of these observations, Goghero
Figure 3.3.11 SCS curves in forward and
reverse scans for a) wet-oxide b)thermal
oxide. Goghero et al [3.4]
considers that the hysteresis criterion is more reliable than the peak position criterion in
determining the oxide quality. In this particular case, the low-temperature thermal oxide
seems to be a better choice than the wet-oxide, as a top oxide for the SCM.
Chapter 3 Oxides characterization with the SCM
161
However, it should be noticed that the preparation method for the wet-oxide doesn't
coincide with Bowallius preparation method.
Goghero et al [3.11] have also compared a UV/ozone oxide with a wet-chemical
oxide.
The UV/ozone has been grown by exposing the surface sample to UV/ozone illumination
for 10 min.
The wet-chemical oxide was obtained by the immersion of the sample into a 30% H2O2
solution for 20 min.
The properties of the two oxides have not been determined by SCS measurements, but
by assessing the SCM signal. The conclusion was that the UV/ozone oxide has a better quality,
measurements with this oxide showing a better stability and smaller variations of the signal.
In her thesis, Duhayon [3.6] thoroughly analyzed several types of oxides: native oxide,
wet-chemical oxide with H2O2, UV/ozone oxide, UV oxides obtained with a UV-gun and
simultaneously baked at 150oC. Duhayon's conclusion, similar to the conclusion of Stangoni
[3.12] was that the best oxide for the SCM is the UV/ozone oxide.
3.4 Oxides characterization with the SCM
In the present chapter we will definitivate our presentation upon the problems at the
tip - sample interface. We will present how these contact problems affect the properties of the
SCS signal and how the parameters of the voltage ramp should be optimized in order to obtain
results with a physical signification in the context of numerous current setup problems.
We will continue with the study of the properties of the SCS curves and their
interpretation.
In the end, we will present a few trails made on different dielectrics to be used as gate
oxides for the study of doping profiles.
3.4.1. Oxide related parasitic phenomena at the tip - oxide interface
During SCS voltage ramps, the properties of the SCS curves often change without
apparent justification.
In chapter 2, we have seen that for almost all the measurements, there is a shift
between the trace and the retrace of the SCS signal, phenomenon that doesn't happen when
the measurements are performed on nanoelectrodes (chapter 2 Figure 2.6.17).
Very often, there is a difference in amplitude between the trace and the retrace of the
SCS curve.
The position of the peak of the SCS signal may change when several consecutive
measurements are performed in the same spot.
Such phenomena have been further investigated. In order to understand what is
Chapter 3 Oxides characterization with the SCM
162
happening at the tip-oxide interface, AFM and SCM scans have been performed before and
after SCS measurements. The AFM images recorded before the SCS measurements constitute
a proof that the aimed regions have nothing special by comparison with the rest of the
sample. The AFM images recorded after the SCS measurements are meant to verify if the
topography or the electrical properties have changed in the spots where the voltage ramps
have been performed.
3.4.1.1. Oxide charging
The first evidence of the oxide charging during SCS measurements has been observed
on the substrate staircase samples covered with a native oxide.
The change of capacitance contrast in the spots where SCS measurements have been
performed has been observed on the SCM image following the SCS curves (Figure 3.4.1).
Figure 3.4.1. SCM image of a p-type staircase sample covered by native oxide following
several voltage ramps [-2V; 2V]. The image shows modifications of the oxide properties
in the spots where the SCS measurements took place
In order to be sure that the surface of the sample didn't present any contrast before
performing the SCS measurements, an SCM image of the area has been recorded before the
SCS curves, showing an uniform capacitance image (Figure 3.4.2 a).
Figure 3.4.2. SCM images on the same surface before (a) and after (b) SCS
measurements of a type p 1015 cm-3 substrate covered by a native oxide. The ramps
have been taken between Vdc [-3V; 3V]. After the SCS measurements, 2 darker spots
can be noticed in the places where the measurements took place.
The SCM image recorded after SCS measurements have been performed, shows two
darker spots in the spots where the measurements took place (Figure 3.4.2 b).
We didn't find in the literature evidence of capacitance contrast modifications following
SCS measurements. However, Beyer et al [3.7] scanned a surface by modifying the dc bias in
steps of 0.4 V from 0 to 2 V. A subsequent scan of the surface at Vdc = 0 V showed that the
stripes scanned at different voltages present a different contrast which suggests a charge
injection into the oxide.
We believe that the two processes are similar. In our case, the oxide charging takes
Chapter 3 Oxides characterization with the SCM
163
place following a voltage ramp in one spot. In Beyer's case, the oxide charging takes place
following the surface scan at a fixed voltage.
Beyer et al [3.7] reported that reiterated scans of same area with the dc bias
maintained at zero reveal the relaxation of the excess charge. On the other hand, in our case,
multiple scans at Vdc = 0 V performed after the voltage ramps didn't show any relaxation of
the excess charge on the native oxide. However, on 2 nm and 5 nm thermal oxides, the
relaxation of the excess charge takes place. We conclude that the relaxation of the excess
charge doesn't take place on the native oxide probably because of its thinness and low-quality
which cause a permanent breakdown of the oxide.
There are several factors which contribute to the oxide charging.
Thin oxides favorize the existence of tunneling currents. Measurements performed on
thicker oxides showed that the oxide is charging only for higher values of the applied bias or
not at all.
Low quality oxides allow more easily the phenomenon of charge trapping due to the
high number of defects in the volume of the oxide (bulk defects). The oxide charging seems
to occur for higher voltages on thermal oxides than on plasma oxides or nitride dielectrics.
This observation is in correlation with Goghero et al (Figure 3.3.11) who considers that the
value of the hysteresis represents an indicator of the quality of the oxide. Further
investigations, under vacuum conditions, which will remove the water layer from the surface
of the oxide, must be performed.
High biases applied to the MOS structure generate higher electrical fields which allow
the injection of carriers into the oxide. The geometry of the tip further amplifies the intensity
of the electrical field.
The most important factor of all seems to be the water layer present on the surface of
the hydrophilic oxide. Test measurements performed in a nitrogen atmosphere with a home
made enclosure have shown that the oxide charging occurs for higher applied voltages.
The lack of appropriate test oxides and the limited access for installing a home made
setups with the AFM in order to create a stable, humidity free atmosphere, don't allow us to
support the above statements with experimental evidence. Further investigations with the
appropriate equipment and the appropriate test samples should be performed.
We have seen that the oxide charging can be detected by scanning the surface after
performing a voltage ramp. By correlating the information given by the SCS measurements
with the information given by the SCM images, we conclude that the oxide charging can be
detected directly with the voltage ramps (Figure 3.4.3).
Chapter 3 Oxides characterization with the SCM
164
Figure 3.4.3. SCS measurement at a sweep rate of 3 V/sec(left) and SCM signal recorded at Vdc = 0 V
(right) on a type n substrate 1015 covered by a 2 nm thick thermal oxide
The value of the signal on the SCM image recorded at 0 V is of approximatively -300
mV. On the SCS curves, the capacitance signal at 0 volts corresponds to -300 mV on the trace
and to -200 mV on the retrace. From these values, we conclude that the trace is the signal
which give the right values. The retrace represents the characterization of the oxide after the
oxide charging. This conclusion corresponds to the intuitive answer. It is intuitive to consider
that the trace of the signal corresponds to the properties of the oxide before the charging
while, during the retrace, the oxide is already charged.
On the other hand, if the SCS scans are done at a much slower voltage sweep, we can
observe that not even the trace of the signal at 0 V doesn't correspond anymore to the value
of the SCM signal recorded at 0 V.
Figure 3.4.4. SCS measurements performed on a type n substrate 1015
covered by a 2 nm thick thermal oxide, at 0.5 V/sec
In the Figure 3.4.4, the value of the capacitive signal at 0 V for the trace is -400 mV
and the value of the capacitive signal at 0 V for the retrace is - 200 mV. We have seen that,
on the SCM image (Figure 3.4.3 b), the value of the signal on the SCM image recorded at 0 V
is of -300 mV.
We consider that, in order to obtain right values of the SCS curves, it is best to perform
the voltage ramps at a faster rate in order minimize the danger of oxide charging. We
emphasize that this conclusion is valid only for ambient measurements and thin oxides which
allow the presence of tunneling currents, in order to avoid the oxide charging because of the
surface water layer. The sweep rate may influence in other aspects the response time of the
interface states or the recombination centers in the volume of the oxide.
Chapter 3 Oxides characterization with the SCM
165
The limits of the voltage ramp also influence the SCS signal (Figure 3.4.5)
Figure 3.4.5. SCS measurements performed on a type n substrate
1015 covered by a 2 nm thick thermal oxide, between -5V;5V
In this case, we observe that for the retrace of the signal, not only that the value of the
capacitive signal at 0 V is even more distant from the real value of - 300 mV, but also the
amplitude of the peak is not the same as the peak of the trace.
The SCM image recorded immediately after the SCS measurement shows a change not
only on the SCM image, but also a change on the topographical image (Figure 3.4.6), in the
spot where the SCS measurement took place.
Figure 3.4.6. Topographical image (left) and cross section of the topographical image (right)
following a voltage ramp between -5V;5V (Figure 3.4.5), on a type n substrate 1015 covered by
a 2 nm thick thermal oxide.
This phenomenon is addressed in the next paragraph.
3.4.1.2. Anodic oxidation
In the previous paragraph, we have seen that applied biases to the M.O.S structure
lead to the oxide charging.
We have noticed that, when higher voltages are applied, even the topographical image
presents changes after SCS measurements (Figure 3.4.6).
The magnitude of the surface oxidation depends of the value of the applied voltage
(Figure 3.4.7).
Chapter 3 Oxides characterization with the SCM
166
Figure 3.4.7. Topographical image (left) and cross section of the topographical image (right) following
an applied bias of -10 V in the center of the image, for 20 seconds, on a type n substrate 1015 covered
by a 2 nm thick thermal oxide. The air humidity percentage: 33.4%
In the Figure 3.4.8 it can be observed the proportionality between the applied bias and
the height of the oxidized surface for negative biases applied on the substrate.
Figure 3.4.8. Variation of the height of the oxidized features with the
applied bias, for negative biases. The voltage stress has been applied
for 20sec at an air humidity percentage of 33.4%
For positive biases, the measured dependence is not linear and the oxidation threshold
increases (Figure 3.4.9)
Chapter 3 Oxides characterization with the SCM
167
Figure 3.4.9. Variation of the height of the oxidized features with
the applied bias, for positive biases. The voltage stress has been
applied for 20sec at an air humidity percentage of 33.4%
If, for negative biases applied on the substrate, the voltage threshold for the anodic
oxidation was -5 V, for positive biases we didn't observe any modification of the surface
topography until + 8 V, on a 2 nm thermal oxide. Even after this threshold, for higher biases,
the topography changes very little, around 2 nm in height.
These observations stay unchanged for a p-type substrate. The tests made on the 5 nm
thermal oxide grown on top of a 1015 cm-3 p-type substrate showed an important topography
change for negative biases applied on the substrate (Figure 3.4.10) and a very small change
for positive biases applied on the substrate (Figure 3.4.11).
Figure 3.4.10 Surface oxidation. Applied bias: -7V on the substrate, air humidity: 48%
Chapter 3 Oxides characterization with the SCM
168
Figure 3.4.11 Surface oxidation Applied bias:+7V, air humidity: 48%
This data is in correlation with the bibliography from the nanolithography field and the
experimental observations made with TUNA [3.13] and nanolithography.
It must be noticed that, in the literature, the phenomena are clearly explained only for
positive biases applied on the substrate. In this case, OH- and O- species are attracted towards
the oxide - silicon interface and the anodic oxidation takes place.
In the case of negative biases applied on the substrate, there are several possible
explanations for the topographical modifications observed with the AFM. Besides the
hypothesis of the anodic oxidation, one other hypothesis is that the topographical
modifications are given by a substrate deformation under the intense electrical field. Another
possible explanation is that the topographical modifications seen on the AFM images are given
by an electrostatic repulsion of the AFM tip, because of the charge accumulation in the
dielectric [3.13]. However the most probable explanation according to us remains for now the
anodic oxidation.
As in the case of oxide charging, this phenomenon is related to the oxide thickness, the
quality of the oxide, the applied bias (the limits of the ramp interval), the stress time, the
polarity of the applied bias, the air humidity percentage. The tip sharpness and geometry may
further amplify the intensity of the electrical field for the same applied voltages.
For a complete study of anodic oxidation of the surface each of the above mentioned
parameters should be thoroughly studied.
In order to avoid humidity, experiments in vacuum or in a controlled atmosphere
(nitrogen) should be performed. Voltage ramps should be performed at different air humidity
percentages.
Sharp tips (PtIr) and cylindrical blunt tips could be used.
Good quality thermal oxides of different thicknesses as well as low quality oxides of
different thicknesses should be used for each of the above mentioned experimental conditions.
However, there are several conclusions that can be deduced based on the
Chapter 3 Oxides characterization with the SCM
169
measurements and the results shown above.
It is imperative to limit the values of the applied biases. The boundaries of the voltage
ramp must be as low as possible. This will not be possible unless the peak of the SCS curve
will be situated as close of 0 volts as possible. The position of the peak of the SCS curve can
be controlled in several ways: by engineering the workfunction of the tip and by choosing the
right oxide thickness. Also, the voltage ramps should be performed as fast as possible in order
to minimize the stress time.
The oxides should be grown on p-type substrates. The topographical modifications are
far less important for positive biases. For p-type substrates the position of the SCS peak is
located towards positive biases applied on the substrate.
In order to minimize the strength of the electrical field (and given that resolution is not
the primarily concern in the case of the study of the properties of the oxides), less sharp tips
should be used.
The electrical measurements should be performed, if possible, into a humidity-free
environment.
3.4.1.3. Oxide engravement
Another problem that has been observed is the engravement of the oxide by the tip.
In contact AFM, the friction between the surface and the tip determines a deterioration
of the tip. However, because the doped diamond coating is very tough, it seems that the
surface also deteriorates (Figure 3.4.12). The problem is further worsen by the method
chosen for avoiding the laser light with the SCM measurements: shifting the laser position
towards the middle of the cantilever, which diminuates the sensitivity of the AFM detection
system.
Figure 3.4.12 Sample: Thermal oxide, 2 nm. Engravement effect after several scans (10 scans)
The choice of the tip being made (diamond coated tips), the only parameter that could
lower the deterioration of the surface is the elastic constant of the cantilever. Cantilevers with
lower elastic constant determine a softer interaction tip-surface, thus a smaller impact on the
deterioration of the surface.
Chapter 3 Oxides characterization with the SCM
170
3.4.2. Study of low-temperature oxides for doping profiling
3.4.2.1 Guidelines for a complete characterization of oxides with the SCM
Currently, the oxide characterization with the SCM means trying to characterize oxides
of unknown properties with an irreproducible and uncalibrated characterization method.
First, a minimum reproducibility must be insured. In the previous chapter, we have
shown a number of causes that distort the SCS signal and impinge the interpretation. In the
present chapter, it has been shown that, with the ambient measurements, oxide charging and
topographical modifications are commonly met. Before any characterization attempt be made
in terms of oxide or doping profile characterization, all these instrumentation problems should
be solved.
Second, SCM should be first calibrated for oxide characterization with test samples.
Thermal oxides of good quality with low density of defects should be used as test samples.
Thermal thin oxides grown by RTO have also proven to be a very good alternative
(Figure 3.4.13).
Figure 3.4.13 SCS signal on a thermal oxide grown by RTO in an oxygen atmosphere, at 940oC, during 4 minutes
(left). C-V curves performed with an impedance analyzer, in the parallel model, at Vac = 50mV
The C-V curves measured with an impedance analyzer, in the parallel model, at
different frequencies, show no variation of the capacitance level in accumulation. There are no
indication of tunneling currents. The curve modifies with the frequency only in the region
concerning the interface states. The calculated oxide thickness, from the level of the
capacitance in accumulation is of 8.4 nm.
The oxide thickness correlated with the absence of tunneling currents on the C-V curve
can explain why the SCS curves don't present any indication of oxide charging or anodic
oxidation.
The test samples should be intentionally contaminated with a controlled density of each
type of defects (interface states, fixed defects at the silicon-SiO2 interface, bulk defects,
mobile defects).
Test samples characterization should be performed with SCM and with other
characterization methods such as ellipsometry, FTIR, DLTS, SIMS etc. From the properties of
Chapter 3 Oxides characterization with the SCM
171
the SCS signal (its shape, the position of the peak, the FWHM, the amplitude, the hysteresis),
the detection capability and the sensitivity of the SCM for oxide characterization should be
determined.
Determination of oxide thickness with the SCM
Thermal oxides of different thicknesses (between 1 nm and 10 nm), grown in the
exactly the same experimental conditions, on the same low-doped substrate should be
available in order to be able to determine the SCM capability and precision of measuring the
oxide thickness.
Figure 3.4.14 TCV simulation: C-V curves (left) and their derivatives (right) for different oxide
thicknesses (1 nm, 5 nm and 10 nm) on a 1015 cm-3 thermal (ideal) oxide
Other methods of measuring the oxide thickness, such as ellipsometry should be
available. Measurements with the SCM, with the same tip, should be performed on all the test
oxides, of different thicknesses. The oxide thickness is a function of the shift of the SCS peak,
the amplitude of the SCS peak as well as the FWHM of the SCS signal. Comparisons between
the SCS experimental data and simulations can be performed (Figure 3.4.14).
Tests measurements on thermal oxides of two different thicknesses, grown in the same
technological conditions and on the same n-type substrate 1015 cm-3, showed differences of
the SCS signals in terms of position of the SCS peak, FWHM and amplitude (Figure 3.4.15).
The peak of the SCS signal is shifted towards right for the thicker oxide. FWHM of the
SCS signal for the 2 nm oxide is greater than the FWHM of the SCS signal for the 1.5 nm
oxide. The amplitude of the SCS signal is greater for the 1.5 nm oxide. All these modifications,
of the position of the peak, of the FWHM and of the amplitude of the SCS signal with the oxide
thickness are in correlation with the theory.
Chapter 3 Oxides characterization with the SCM
172
Figure 3.4.15 Variation of the position of the peak, the FWHM and the
amplitude of the SCS signal with the oxide thickness
Determination of the presence and the density of mobile charges with the SCM
The presence of mobile charges into an oxide is given by the hysteresis of the C-V
curve, when the sample is heated and the oxide is electrically stressed.
We have seen that, with the SCM, oxide charging or/and anodic oxidation occurs when
the oxide is stressed leading to the modification of the SCS parameters and hysteresis. Also,
in our laboratory, there are no means to heat the sample during the measurements.
In order to be able to detect and quantify the mobile charges with the SCM, the
following experience should be put in place.
A thermal oxide of good quality (without defects), grown on a low-doped 1015 cm-3
substrate should be available. The oxide should intentionally be contaminated with alkali ions,
in different concentrations. The samples should be available for characterization with other
methods of characterization, such as C-V measurements, SIMS etc, in order to determine the
amount of alkali ions in the oxide.
SCM measurements should be performed in a controlled environment (ultra-vacuum).
The sample should be heated around 100oC during the SCM measurements (technological
means are available with the AFMs, such as Peltier cells).
Determination of the presence and the density of bulk defects with the SCM
The presence of bulk defects into an oxide is given by the shift of the SCS curve.
It must be kept in mind that the shift of the peak of the SCS signal has a number of
other causes: variations of the oxide's thickness, fixed charges at the silicon - oxide interface.
The experiences should be conceived in such a manner that the shift of the SCS signal be
given only by the defects in the volume of the oxide.
From the literature, it is known that the alkali ions act, at room temperature, as defects
Chapter 3 Oxides characterization with the SCM
173
in the volume of the oxide and not as mobile charges. In consequence, a similar experience as
described above, for the mobile charges, could be put in place in order to evidentiate the bulk
defects with the SCM.
A thermal oxide of good quality (without defects), grown on a low-doped 1015 cm-3
substrate should be available. The oxide should intentionally be contaminated with alkali ions,
in different concentrations. The samples should be available for characterization with other
methods of characterization, such as C-V measurements, SIMS etc, in order to determine the
amount of alkali ions in the oxide.
SCM measurements should be performed in a controlled environment (ultra-vacuum).
Determination of the presence and the density of fixed charges at the silicon -
oxide interface with the SCM
Fixed charges, at the silicon - oxide interface can be evidentiated by the shift of the
SCS peak.
One modality of evidentiating the fixed charges with the SCM is by taking advantage of
the different density of fixed charges on the substrates with different crystalline orientations
In the literature, it is known that there are 3 times more fixed charges on a (111)
surface than on a (100) surface. As a result, the SCS signal measured on a (111) surface
should be shifted more to the right of the theoretical value than a SCS signal measured on a
(100) surface.
Two samples with different crystalline orientation should be used for this experiment.
Both samples should have the same low-doped substrate, covered by a thermal oxide grown
in the same time. The surface of one sample should have the crystalline orientation (100), the
other one (111).
Another way of evidentiating the fixed charges with the SCM is by using the fact that
the amount of fixed charges can be reduced by annealing in a nitrogen atmosphere.
Two samples should be used for this experience. Both samples should have a p-type
substrate, 1015 cm-3, (111), covered by a thermal oxide.
One of the samples should be annealed in a nitrogen atmosphere. Measurements on
both samples should be performed. The comparison between the SCS signals should
evidentiate the detection capability with the SCM of fixed charges at the silicon - oxide
interface.
Determination of the presence and the density of the interface states with the
SCM
HF oxide cleaning, cleaving, polishing, the crystalline orientation can all introduce
interface states.
In order to evidentiate the interface states with the SCM, the following experience could
Chapter 3 Oxides characterization with the SCM
174
be performed.
The surface of a silicon sample, 1015 cm-3, could be altered by polishing or by etching
with a solution of HF or KOH. On this sample and on an unaltered silicon sample, a thermal
oxide should be grown.
Alternative characterization methods, such as DLTS, C-V, should be used for the
characterization of the density of interface states on these two samples.
SCS measurements performed on the two samples should give a different FWHM of the
SCS signals.
The investigation of the interface states could be further continued by performing
annealings on the two samples in a hydrogen atmosphere
During this thesis, we didn't have access to the fabrication of thermal oxides. It has
also not been possible to perform modifications to the existing AFM according to the solutions
we proposed for the amelioration of the capacitive signal. Given this situation, we evaluated
different low temperature oxides by the proximity of the SCS peak to the value of zero volts.
3.4.2.2 Plasma oxide
The plasma oxide has been obtained with a RIE Nextral NE 1110, made by ALCATEL.
Several tests have been made for finding the optimal growth parameters, the
apparatus and its standard operating conditions being mainly used for the etch of oxides.
The parameters that can be varied are: the pression of the oxygen, the flux of oxygen,
the power of the electrical field in which the oxygen plasma is created and the oxidation time.
The first tests showed with the AFM a variation of the topography of the surface due to
a selective etch of the silicon with respect to the doping concentration (Figure 3.4.16 and
Figure 3.4.17).
Figure 3.4.16 Doping SIMS profile of the maya p sample (left). Topography of the sample
put in the chamber in vertical position (right)
Chapter 3 Oxides characterization with the SCM
175
Figure 3.4.17 Topography of the sample put in horizontal position, the polished face upward
(left). Topography of the sample put in horizontal position, the polished face downward
The highly doped regions have been less etched while the low-doped regions have been
more affected. The etched has been more or less aggressive, depending on the position of the
cross-section.
1. native oxide cleaning: BOE 30sec
2. O2 debit = 10 sccm, power =75 W, pressure = 50mtorr, t=5 min
3. oxide cleaning: BOE 10min + rinsing
4. O2 = 10 sccm, P =75 W, p=50 mtorr, t =10 min
The increase of the oxygen pressure led to a substantial decrease of the plasma attack
of the surface (Figure 3.4.18).
0. Surface cleaning with acetone
1. native oxide cleaning: BOE 30sec + rinsing with deionized water
2. O2 = 10sccm, P = 75 W, p = 100mtorr, t = 5 min
3. Nettoyage BOE 10min + rincage
4. O2 = 10 sccm, P = 75 W, p =100 mtorr, t =10 min
Figure 3.4.18 Topography of the sample put in horizontal position, the polished face downward,
for 100mTorr
Chapter 3 Oxides characterization with the SCM
176
In the final step, optimal values for the oxygen flux, the power and the oxidation time
have been searched in order to obtain a thinner oxide. The oxide thickness has been
measured by ellipsometry.
p=100mtorr
t O2 = 10sccm, P=75W O2 = 10sccm, P=60W O2 = 20sccm, P=75W
30 sec 5.6 5.2 nm 5.6 nm
2 min 4.8 - 6.3 6.2 nm 6.4 nm
5 min 5.6 – 7.2 7.0 nm 7.3 nm
8 min 6.0 – 7.5 7.4 nm
10 min 6.4 – 8.0
From the SCM images, no contrast inversion can be seen (Figure 3.4.19).
Figure 3.4.19. SCM image of the BP-diodes (left). SCM profile of the BP-diodes (right). Vac=400mV; Vdc=0V
However, from the SCS sweep on the lowest-doped region – the substrate, it can be
seen that the phase signal is negative (Figure 3.4.20).
Figure 3.4.20. SCS – amplitude (left). SCS phase (right) of the substrate of the diode sample
The C-V measurements performed with the impedance analyzer (Figure 3.4.21)
suggest strong leakage currents (from the shape of the curves in accumulation) and a great
density of bulk charges (from the shift of the C-V curves).
Chapter 3 Oxides characterization with the SCM
177
Figure 3.4.21. C-V curves performed with an impedance
analyzer, in the parallel model, at 1kHz, Vac = 50mV. The
curves shifts towards left for greater applied biases
We conclude that the assisted plasma oxides are not of the best quality and don't
represent the ideal dielectric for SCM measurements on doping profiles.
3.4.2.3 Nitride
The main reason for trying to use this dielectric for SCM measurements is its dielectric
constant, almost 3 times higher than the dielectric constant of an oxide. This means that
thicker layers could be used without signal loss. Also, with a thicker dielectric, the danger of
tunneling currents should diminish.
For the nitride growth, we took advantage of the know-how that other research teams
had and we used directly the optimal parameters possible for obtaining a dielectric with
minimum defaults and the optimal thickness.
The silicon nitride films have been deposited by direct PECVD with a SEMCO furnace.
The parameters used for the deposition are:
frequency 440 kHz
temperature 370oC
pressure 1500 mtorr
power 1000 W
total debit 800 sccm
The SCS signals on three samples of different thicknesses showed promising results.
The peak of the SCS signals (Figure 3.4.22) are close to zero volts.
Chapter 3 Oxides characterization with the SCM
178
Figure 3.4.22. SCS signals obtained on a low-doped substrate
covered with 7 nm, 8 nm and 9 nm, as measured by ellipsometry
The nitride layer used as a dielectric for doping profile characterization showed that it is
possible to obtain a good capacitive signal (Figure 3.4.23)
Figure 3.4.23. SCM image of a doping profile (left) and cross-section of the doping profile,
with the nitride layer as gate dielectric
However, as in the case of oxide plasma assisted deposition, there are problems with
the phase of the signal. From Figure 3.4.23, it can be seen that the substrate (left) has the
lowest signal. As the SCS signal taken on the substrate of the signal prove (Figure 3.4.24) the
small capacitive signal given by the substrate is not caused by the wrong choice of the applied
bias, but by a phase inversion.
An instrumentation problem made that the phase couldn't be recorded for the most
part of the thesis in the ramp mode, but only in the scanning mode. This is the reason why
many SCS signal along this thesis are presented only in absolute value. The SCS curves
recorded in Figure 3.4.22 represent only the amplitude of the signal, without the possibility of
recording the phase of the signal.
Chapter 3 Oxides characterization with the SCM
179
Figure 3.4.24. SCS signal recorded on the low-
doped substrate of the BP doping profile (Vac =
500 mV)
Further investigations confirmed that, on low doped substrates, the phase of the
capacitive signal is inverted, as in the Figure 3.4.24, as if the substrate would be a n-type
substrate.
We observe that this is the same phenomenon occurred with the oxide deposited by
plasma assisted oxidation (section 3.4.2.20).
At the present time, we do not have a consisting explanation about the change of
phase with the dielectrics deposited by plasma assisted oxidation.
From the data we have - the phase inversion happens only on low-doped substrates,
the nitrides are known for the great density of bulk defects, plasma can affect greatly the
surface of the sample leading to an important topography and interface defects - we speculate
that a great density of defects in the bulk of the dielectric or at the silicon - dielectric interface
can lead to a charge accumulation which could be detected by a phase inversion of the
capacitive signal.
3.4.2.4 High-k dielectrics
In the industry of microelectronics, the high-k dielectrics represent an alternate
solution for the gate dielectrics. The permittivity of high-k dielectrics allows to reduce the
thickness of the gate dielectrics or to increase the dielectric thickness without diminishing the
capacitance of the M.O.S structure.
The LaAlO3 layer studied with the SCM has been deposited by Molecular Beam Epitaxy
(MBE) in a O2 atmosphere, at a pressure of 10-5 torr, on a low-doped p-type substrate 1015
cm-3. The estimated thickness of the dielectric layer, following the deposition, is approximately
4-5 nm.
The peak of the SCS signal is shifted towards high biases (5.5 V) - the black curve in
the Figure 3.4.25. The shift reflects a high density of defects.
Chapter 3 Oxides characterization with the SCM
180
Figure 3.4.25. SCS signals of a high-k LaAlO3 dielectric deposited by
MBE on a p-type low-doped substrate 1015 cm-3, before (black curve)
and after annealing in a N2 atmosphere, at 400oC, by RTA.
After annealing by RTA in a nitrogen atmosphere, at 400oC, the SCS curve shifted
more than 3 V, which shows a great amelioration of the dielectric properties. Such an
improvement following a nitrogen annealing reflects the density of fixed charges at the
dielectric-silicon interface.
In spite of the amelioration of the dielectric's quality brought by the thermal annealing,
the peak of the SCS signal continues to be shifted towards biases that are too high for doping
profile characterization.
However, it must be taken into account that the fabrication of good quality high-k
dielectrics is still at the beginning. High-k dielectrics may represent, in the future, a solution
that should be taken into consideration as gate dielectrics for doping profile characterization.
3.4.2.5 Native oxide
Native oxide is currently used as gate oxide for SCM measurements. It has several
important advantages.
It doesn't require a special preparation. It is enough to leave the samples, after
cleaving, exposed to air, in a proper environment, for a few hours.
It's thickness is known. The maximum thickness that the native oxide can reach is
approximately 1.2 nm (Figure 3.3.2).
The failure analysis images obtained with the native oxide as a gate oxide are of good
quality (chapter 1).
The maximum of the SCS signals obtained on native oxides are located around the
value of zero volts (Figure 3.4.26)
Chapter 3 Oxides characterization with the SCM
181
Figure 3.4.26. SCS signal on a native oxide on top of a low-doped
p-type substrate. The diminished amplitude of the retrace (red
curve) suggests strong tunneling currents
The native oxide has a major drawback. The native oxide is a very thin oxide of poor
quality. Tunneling currents are prone to appear during capacitive measurements. In Figure
3.4.26 for example, the diminished amplitude of the retrace suggests the presence of strong
tunneling currents. Further more, in most of the cases, tunneling currents transforms the
M.O.S structure into a short circuit and no SCS signal is recorded on native oxides. This
phenomenon renders impossible any rigorous attempt of doping profile quantification.
However, it must be emphasized that the tunneling currents appear as a consequence
of the limits of the voltage sweep with the SCS. On SCM images recorded with a continuous
applied bias of approximately zero volts, no contrast inversion or signal loss has been noticed.
3.4.2.6 UV/ozone oxide
In the literature, the UV/ozone oxide represents the best solution for SCM
measurements [3.6], [3.12]. Nakanishi et al [3.14] showed that, during UV/ozone oxidation,
the Si-H and Si-OH bonds are replaced with Si-O bonds and a good SiO2 network is expected.
An UV gun or an UV cleaner is used to grow an UV/ozone thin oxide or to ameliorate
the existing native oxide, under a flux of oxygen. In most cases, during the UV/ozone
oxidation, the samples are heated at around 150oC.
For our experience, we have used a UV/ozone cleaner from Jelight Company (42-220).
The cleaner has a timer which allows us to control with precision the growth time.
We have chosen four samples cut from the same p-type low-doped 1015 cm-3 substrate.
We have cleaned the native oxide in a 1% HF solution during 10 seconds and we have rinsed
the samples with deionized water.
Chapter 3 Oxides characterization with the SCM
182
Figure 3.4.27 UV/ozone oxide growth time, after Duhayon [3.6]
Given the UV/ozone growth time (Figure 3.4.27), we have chosen four oxidation times
for each sample: 10 minutes, 30 minutes, 90 minutes and 270 minutes. We have taken into
account that our UV/cleaner doesn't have the possibility to heat the sample during the
oxidation which may slow down the process.
The SCS signals recorded on all four samples are presented in Figure 3.4.28.
Figure 3.4.28. SCS signals on UV/ozone oxides . Vac = 300 mV.
The signals present a number of irregularities. For the oxides grown for 90 minutes and
270 minutes the amplitude is greater and the FWHM smaller than those corresponding to the
oxides grown for 10 minutes and 30 minutes. The shift of the SCS signals, from one oxide to
another, does not seem to have a physical justification other than the improper experimental
conditions.
However, from this data, a conclusion is clear. While the position of the peak of the
SCS signal for the native oxide is located around zero volts, the position of the peak of the
SCS signal for the UV/ozone oxides shifted towards the value of 3 V - 4 V. The difference of
thickness between the two oxides cannot result in such a magnitude of the shift of the SCS
signal. We conclude that the quality of the UV/ozone oxides is poorer than the quality of the
native oxide.
Chapter 3 Oxides characterization with the SCM
183
This result coincides with the work of the Dr. Didier Goghero done in our laboratory in
2005, on UV/ozone oxides. He concluded that an oxygen line and a mean to heat the samples
must be added in order to reach the results found in the literature.
3.4.2.6 Summary of low-temperature oxides
Thickness Position of the peak
of the SCS signal
Ease of growth Growth on the
cross-section
Contrast
reversal
Native 1.2 nm app. 0 V yes yes no
UV-ozone 1.7 nm 3 V - 4 V yes yes no
plasma 5 nm - 7 nm -1 V - -0.5 V no yes yes
nitride 7 nm - 9 nm app 1 V no yes yes
LaAlO3 5 nm 2 V - 5 V no yes no
3.4.3 Conclusions and perspectives
In our opinion, the best way to characterize oxides with the SCM is as follows:
1. Any information on the studied oxide, acquired by other characterization methods, is
welcome.
Previous knowledge about the oxide thickness and the density of various types of
defaults, can help correlating the SCS experimental data with defaults concentration.
Also, such knowledge could help to the theoretical calculation of the SCS signal, which
will further improve the interpretation of the SCS signal.
2. The first option would be to acquire the SCS data on nanoelectrodes previously
deposited on the surface of the studied oxide. Such nanoelectrodes would assure the
protection of the surface from contaminants and humidity and a good electrical contact. Also,
the known geometrical dimensions of the electrodes would help in quantifying the oxide
properties.
In the ideal case, the same sample, or similar samples, should also have electrodes for
CV measurements. Comparisons between C-V and SCS could be made.
3. In the study of an oxide, multiple samples of the oxide in question, on substrates of
different doping concentrations and of different thicknesses would also help in quantifying the
oxide properties.
4. The SCS measurements should aim a complete evaluation of the oxide by taking into
account all the properties that can be deduced from a SCS curve. From our knowledge, no
author from the literature didn't complete a full study on an oxide.
Chapter 3 Oxides characterization with the SCM
184
5. Before any measurement, one should verify the quality of the tip and the state of the
equipment. In order to do this, a standard sample, preferably a thermal oxide of good quality
deposited on a low-concentration substrate, should be used.
A test SCS should be performed on this standard sample, in standard experimental and
operating conditions. If all is normal, the measured SCS signal should correspond, in terms of
amplitude, FWHM, hysteresis and flatband bias to the previous established values.
6. The SCS measurements should be immediately followed by a topographical scan of the
spot where the measurement was done. In air, during a SCS sweep, there is always the
danger of oxidation of the surface. A topographical scan would verify if the topography has
changed.
The topographical scan should be accompanied by a SCM scan at Vdc=0V and a small
Vac. Thus, it can be seen if a injection of charges took place during the SCS.
From all the dielectrics we have taken into account, for now, there is no ideal oxide that
could play the role of gate oxide for the doping profile characterization.
The native oxide continues to represent the best compromise due to the proximity to
zero volts of the peak of the SCS curve, in spite of its thinness and its low quality.
UV/ozone oxide remains a strong candidate, given the possibility that the necessary
equipment for its growth (heating of the substrate, oxygen line) be available.
The high-k dielectrics remain a strong candidate, especially due to the high permittivity
constant, which would allow thicker dielectric layers to be used. Future developments
concerning the growth technology could impose the high-k dielectrics as the best choice for
SCM measurements.
Another choice that we have taken into account is the growth of thermal oxides at
temperatures below the doping diffusion temperature. It is known that the density of defects
of an oxide increases exponentially with the decrease of growth temperature. We think
possible that, by choosing the growth temperature right below the doping diffusion
temperature, the thermal oxide to be good enough for SCM measurements.
A few attempts have already be made, with promising results (Figure 3.4.29).
Chapter 3 Oxides characterization with the SCM
185
Figure 3.4.29. Thermal oxide grown by dry oxidation at 700oC
The numerous problems with the oxidation oven - especially the humidity proofness of
the oven - made that the dry oxidation be in fact a humid oxidation which lowered the quality
of the oxide and rendered the oxide growth uncontrollable.
However, we consider oxides growth by thermal dry oxidation or by RTO, by using a
growth temperature right below the doping diffusion temperature, to be a very good lead in
obtaining the needed gate oxide for SCM measurements.
Bibliography
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Chapter 3 Oxides characterization with the SCM
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General conclusion
General Conclusion
188
This work was devoted to the experimental study of the scanning capacitance microscopy
(SCM) and spectroscopy (SCS) for the mapping of the dopants in the semiconductor structures
and for the characterization of thin oxides. SCM has appeared to be a very powerful technique
for doping mapping as long as qualitative images are needed, for example in order to check
whether fabrication steps like implantations have been correctly operated during the
fabrication of devices (presence or absence of doping of a given type in a region where it
should be present). When quantitativity is needed, the only way of performing a calibration of
SCM images for doping mapping seems to grow exactly the same oxide on two different
samples, one being a calibration sample from which a semi-calibration curve associating
doping levels and SCM signal levels will be measured and applied to the unknown sample
(semi-calibration). This is a very constraining method, which imposes to stick two samples
together and polish them in exactly the same way so that the resulting oxide (possibly of poor
quality) is the same for both.
In this work, we have chosen to study the experimental reasons which prevent SCM and
SCS from being a fully quantitative technique.
In the first chapter, we have shown the capabilities of SCM for doping mapping using a
series a experimental situations and test samples covering almost all frequently encountered
structures in the industry of silicon microelectronics : doping staircases of p-type and n-type
structures, quantum wells and p-n junctions. Bevels have been realized in order to artificially
enhance the resolution in a single direction. Qualitative images have been obtained for a wide
range of doping levels between 2.1015 at.cm-3 to 5.1019 at.cm-3. SCM is able to detect
quantum wells of ~ 7 nm width. By adjusting the applied voltages (especially DC voltage), it is
possible to avoid the well-known contrast inversion, and recover a SCM profile corresponding
to e.g. a SIMS analysis. Also, it is obvious that SCM is a wonderful characterization method for
the detection of dopants of different type (p-type or n-type) especially compared to Scanning
Spreading Resistance Microscopy (SSRM) which can not distinguish between both types. All
these results confirm the usefulness of SCM as a qualitative imaging technique.
Problems start to arise when the doping profile is not precisely known by means of
another technique like SIMS. On the one hand, contrast inversion can be avoided by adjusting
the Vdc voltage but in the absence of any other information it is difficult to decide whether
contrast inversion takes place or not. On the other hand, even theoretically, the 'ideal' Vdc is
not the same for p-type and n-type regions, which further complicates the quest for ideal
experimental conditions. Moreover, a large number of experimental parameters may further
distort the SCM image and modify the measured signal levels and thus the interpretation.
In the second chapter of this work, we have studied all the parameters playing a role
in the interpretation and reproducibility of SCM signal. To do so, we have chosen to work with
test samples of thermal silicon oxide of known thickness and quality, and make a direct
comparison between Capacitance-Voltage (C-V) curves obtained with a macroscopic setup and
General Conclusion
189
results obtained on the same test samples with SCM and Scanning Capacitance Spectroscopy
(SCS). This approach has allowed to clearly point out the influence of the ambient or laser
light on the SCS signal. The comparison of SCS with or without the light provided by the laser
measuring the deflexion of the AFM evidentiate the drop of the signal to noise ratio due to the
creation of electron-holes pairs in the sample under study. This effect has been explained
using the macroscopic C-V curves on test samples covered with metallic electrodes of different
thicknesses and exposed to laser or ambient light. Experimental examples of the influence of
the laser light on the interpretation of doping profiles have been provided. Solutions have been
proposed to avoid or minimize this effect.
The influence of the parasitic capacitance has also been addressed in a context where
the preparation of the samples for doping mapping may require a polishing process leading to
possible important topographical features. It appears that SCM measurements in the middle
of a planar sample suffer from a drop of the sensitivity of the system compared with
measurements on the sample edge or even on a cross section. Using samples covered with a
metallic layer, which should not lead to any signal at all by SCM measurement, we have shown
that a parasitic signal was very still often present. The influence of experimental conditions
has been studied in order to try and find the optimal conditions for the minimization of this
parasitic capacitance. Although the origin of a parasitic signal at the same frequency as the
applied alternating voltage Vac remains unclear, the presence of a water layer on the surface
along with abrupt variations of capacitance whenever a topographical feature is encountered
could play a role in its presence. We have also thoroughly studied the influence of all the parts
of the experimental setup (cantilever, chip, cantilever holder, electronic modules...) on the
SCM signal and sensitivity.
Finally, we have studied the role of the tip sample contact, which appears also to be of
major importance in the quality of the SCM/SCS signal. By a direct comparison of SCS
obtained with a macroscopic C-V setup and SCS obtained on the very same electrode cut from
the electrode used for macroscopic measurements, we have emphasized that SCS was
indeed able to perform an exact measurement of the position of the inflexion point
of the C-V curve (related to the flatband voltage of the MOS structure) when the
measurement was performed on a clean, metallic, water (or any other kind of contamination)-
free electrode. When the tip is positioned directly on the surface of the oxide, parasitic
features appear on the SCS like a hysteresis (difference between the trace and the retrace), or
a peak in the inversion region of the C-V curve. Again, comparisons with macroscopic C-V
curves have allowed to better understand the role of the tip-sample contact in SCS
measurements.
Contaminations possibly present on the AFM tips, reproducibility of the quality of the tips
and artifacts due to the behavior of the piezotube have also been taken into account as
possible sources of errors on the SCM signal.
In the third and last chapter, the use of the SCS in order to assess the quality of
General Conclusion
190
different top oxides for doping imaging has been addressed. A precise description of what can
be expected from SCS curves has been provided and the problem of anodic oxidation has been
described, leading to the conclusion that voltages applied to the MOS structure during
characterization should be limited in order to limit anodic oxidation, growth of 'hillocks' on the
surface and leakage currents through the oxide (leading to a modification (degradation) of its
properties). With the aim of choosing an oxide for the MOS structure used for doping mapping,
we have compared between different candidates : native SiO2, plasma oxide, UV/Ozone SiO2
oxide, silicon nitride... Also, as an illustration of what SCM can bring to the oxide growers, the
use of SCS in order to assess the role of an annealing process on the quality (especially the
position of the flatband voltage) of a LaAlO3 oxide grown by Molecular Beam epitaxy has also
been shown.
These experiments have shown that the compromise between the ease of growth,
thickness (trade off between leakage currents and signal to noise ratio), position of the SCS
maximum in order to limit the value of the applied voltages and limitation of leakage currents
is not easy to find. For qualitative imaging, native oxide grown on clean conditions (clean
room for example) remains a good compromise. Although it is very thin (1.2 nm) and leads to
important leakage currents, it allows to get a good contrast with no contrast reversal with 0 V
applied on the sample. Oxides grown at higher temperatures, like the RTO oxide used in this
work, represent a promising alternative. Advances on the growth and the control of the
properties of alternative oxides (high-k oxides) could be a solution for the future, as these
kind of oxides are often obtained at low temperature.
All these results put together emphasize the importance of the measurement
environment on the quality of capacitance measurements. SCM under vacuum and controlled
environment (dry air, nitrogen), along with a system eliminating the laser light during SCM
measurements (like the dark lift recently operated by Veeco on its systems), a better quality
of the tips (coating adhesion, absence of contamination, reproducibility, known shape, better
knowledge on the work function of the tip...) would allow to improve significantly the reliability
of SCM technique.
FOLIO ADMINISTRATIF
THESE SOUTENUE DEVANT L'INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON
NOM : LIGOR DATE de SOUTENANCE :11/02/2010
Prénoms : Octavian TITRE : Reliability of the Scanning Capacitance Microscopy and Spectroscopy for the nanoscale characterization
of semiconductors and dielectrics NATURE : Doctorat Numéro d'ordre : 2010-ISAL-0008
Ecole doctorale : Electronique, Electrotechnique, Automatisme
Spécialité : Dispositifs de l'Electronique Intégrée Cote B.I.U. - Lyon : T 50/210/19 / et bis CLASSE : RESUME : This work was devoted to the experimental study of the scanning capacitance microscopy (SCM) and
spectroscopy (SCS) for the mapping of the dopants in the semiconductor structures and for the characterization of thin oxides. SCM has appeared to be a very powerful technique for doping mapping as long as qualitative images are needed, for example in order to check whether fabrication steps like implantations have been correctly operated during the fabrication of devices (presence or absence of doping of a given type in a region where it should be present). When quantitativity is needed, the only way of performing a calibration of SCM images for doping mapping seems to grow exactly the same oxide on two different samples, one being a calibration sample from which a semi-calibration curve
associating doping levels and SCM signal levels will be measured and applied to the unknown sample
(semi-calibration). We have shown the capabilities of SCM for doping mapping using a series of experimental situations and test samples covering almost all frequently encountered structures in the industry of silicon microelectronics : doping staircases of p-type and n-type structures, quantum wells and p-n junctions. Qualitative images have been obtained for a wide range of doping levels between 2.1015 at.cm-3 to 5.1019 at.cm-3. SCM is able to detect quantum wells of ~ 7 nm width. SCM is also able to differentiate between
dopants of different type (p-type or n-type) especially compared to Scanning Spreading Resistance Microscopy (SSRM) which can not distinguish between both types. All these results confirm the usefulness of SCM as a qualitative imaging technique.
We have studied all the parameters playing a role in the interpretation and reproducibility of
SCM signal: stray light, stray capacitance, the tip-sample contact, the influence of strong
electrical fields, the sample’s topography, the quality and the properties of the top oxide. We
have proposed solutions for eliminating all these parasitic factors and to render the SCM
measurements reproducible and quantitative.
MOTS-CLES : SCM, SCS, doping profiles, oxide characterization, AFM, resolution, junction localization, quantification, interface states, fixed charges, volume charges, mobile charges, stray light, stray capacitance, C-V measurements, impedance analyzer, M.O.S structure, nano-electrodes.
Laboratoire (s) de recherche : Institut des Nanotechnologies de Lyon Directeur de thèse: Brice Gautier Président de jury : Composition du jury : Frédéric HOUZE, François BERTIN, Daniel ALQUIER, Christophe GIRARDEAUX,
George BREMOND, Jean-Claude DUPUY