Modeling of Electron Mobility Degradation for

HfSiON MISFETs

Hiroyoshi Tanimoto*, Masaki Kondo*, Toshiyuki Enda*, Nobutoshi Aoki*, Ryosuke lijimat,Takeshi Watanabe*, Mariko Takayanagi* and Hidemi Ishiuchi*

* Center for Semiconductor Research & Development, Toshiba Corp. Semiconductor Company8, Shinsugita-cho, Isogo-ku, Yokohama 235-8522, Japan

TEL: +81-45-770-3645, FAX: +81-45-770-3571, E-Mail: hiroyoshi.tanimoto@toshiba. co.jpt Advanced LSI Technology Laboratory, Corporate Research & Development Center, Toshiba Corp.

I. INTRODUCTION

High-K insulators are to be utilized as gate dielectric filmsfor advanced CMOS devices. One of main problems ofthe high-K MISFETs is reduced carrier mobility comparedwith that of thermally grown pure SiO2 MOSFETs [1]-[3], since lower carrier mobility degrades the device perfor-mance. We investigate the electron mobility degradation forHfSiON MISFETs and find that there are two sources ofthe mobility degradation; one is Coulomb scattering causedby fixed charges in HfSiON films and the other is phononscattering by interfacial thin oxynitrided (SiON) layer; andHfSiO-related remote phonon scattering is not dominant. Themobility degradation caused by the Coulomb scattering andSiON phonon scattering is separated into two components andwe develop an empirical mobility model for HfSiON devicesthat enables accurate simulation of electrical characteristics ofthe HfSiON devices.

II. MOBILITY DEGRADATION OF HFSION DEVICESCOMPARED WITH SIO2 DEVICES

Effective electron mobility for HfSiON n-MISFETs and forcontrol thermal SiO2 MOSFETs is measured by the conven-tional split C-V method. The results for substrate acceptorconcentration NA = 3 x 1016cm-3 are shown in Fig. 1 [4].As shown in this figure, the effective mobility for HfSiONMISFETs is reduced compared with that for the control SiO2MOSFETs.

The mobility reduction for the HfSiON devices can beextracted using Matthiessen's rule:

1 IHJgSiON = 1/gsio2 + 1/Ag, (1)where gHfsioN is the effective mobility for HfSiON devices,gSiO2 is the effective mobility for SiO2 devices and Ay is themobility reduction caused by the HfSiON gate insulators com-pared with the SiO2 films. The extracted mobility reduction Ayis plotted in Fig. 2 as a function of inversion electron densityN,. As can be seen in this figure, the mobility degradation Ayis proportional to the power of NS; AgJN.X; and the slope astrongly depends on the temperature and the substrate acceptorconcentration.

>

E

0

.7ao

>

0)

0)

a)

0

.7-o

Si02

E

0

a)

rn

102

HfSiON

1o-1

102101 1

Effective Field (MWcm)

Fig. 1. Measured effective electron mobility for HfSiON MISFETs andpure SiO2 MOSFETs as a function of effective field at 300-423K. Substrateconcentration NA is 3 x 1016cm-3.

423K373K300K

(a) NA=3 x10 cm

1021101 1 1012

Inversion Carrier Density (cm 2)

(a)

1013

300K

(b) NA=3 X1017 cm-3

1012Inversion Carrier Density (cm 2)

1013

(b)

Fig. 2. Extracted mobility degradation of HfSiON MISFETs compared withpure SiO2 MOSFETs. (a) NA = 3 x 1016 cm-3 and (b) 3 x 1017 cm-3.

In order to explain the dependence of the mobility degrada-

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103

103

103

SISPAD 2006 47

tion AAy on the electron sheet density Ns and the temperature T,we firstly investigated the remote Coulomb scattering (RCS)and the remote phonon scattering (RPS).

In Figs. 3(a) and (b), RCS-limited mobility pc and RPS-limited mobility gp are schematically illustrated as a functionof Ns and T. As shown in these figures, the limited mobilityfor these scattering mechanisms is proportional to the powerof Ns; gc - N, and gp - Nsp; and the slopes for RCS-and RPS-limited mobility; °k and ap; weakly depend on T.The slopes °k and Up are quite different. The RCS-limitedmobility becomes higher at strong inversion condition andat higher temperature, since screening effect becomes largerat higher carrier density and at higher temperature. On theother hand, the RPS-limited mobility becomes lower at stronginversion condition and higher temperature, since phonon-electron interaction becomes stronger under higher electricfield and at higher temperature.By comparing Fig. 2 with Figs. 3(a) and (b), it is clear

that the temperature dependence of the slope measured forHfSiON devices can be qualitatively explained by neitherRCS nor RPS. Furthermore, a combination of the both RCSand RPS mechanisms leads to a complicated Ns dependenceof the limited mobility pcp as shown in Fig. 4. Thoughthe temperature dependence of the slope can be partiallyexplained, gcp is not proportional to Ns" as shown in Fig. 2.

> 104

C)

.

E_-

O 2

5 1 0

ii.ff_

c) 1 021 (

>1

Lower Temperature(a)

1012Inversion Carrier Density (cm 2)

(a)

2 104E

E0

0x0

inrr 10 1

10

Higher Temperature

/ Lower Temperature

1012Inversion Carrier Density (cm 2)

1013

Fig. 4. Schematic limited mobility caused by RCS and RPS Pcp as a functionof inversion carrier density and temperature. These two scattering mechanismsare combined by using the well-known Matthiessen's rule.

devices was not ascribable to simple RCS and RPS mecha-nisms. Changing our viewpoint, we focused on nitrogen atomsin interfacial thin SiO2 between Si substrate and HfSiONdielectric. The N atoms were introduced by Ar/N2 plasmanitridation process following HfSiO deposition to suppressthe phase separation of HfSiO [4], [5]. Since we thoughtmost N atoms were making bonds with Hf atoms in HfSiONand the interfacial oxide had the same characteristics as theusual thermal oxide, the effective mobility of HfSiON deviceswas compared with that of the control thermal SiO2 devices.Koike et al. showed that N atoms were creating not only Hf-Nbonds but also Si-N bonds and the Si-N bonds were increasedwith increasing N atoms in HfSiON while Si-O bonds weredecreased [6]. This indicates that the interfacial SiO2 betweenHfSiON and Si substrate is likely nitrided and will havesimilar characteristics to SiON as shown in Fig. 5 and themobility degradation of HfSiON devices should be discussedin comparison with the mobility for the SiON devices.

1013

Gate Electrode

HfSiONLower Temperature 0-SiON-like interfacial layer

0 103

.2

n 2G 10

1(

Higher Temperature

(b)

1012Inversion Carrier Density (cm 2)

1013

(b)

Fig. 3. Schematic drawings of (a) RCS-limited mobility ,uc and (b) RPS-limited mobility /Ip as a function of inversion carrier density and temperature.Both RCS and RPS limited mobility is proportional to the power of electrondensity NIV. The slope a weakly depends on temperature but differs greatlybetween RCS and RPS.

III. MOBILITY DEGRADATION OF HFSION DEVICESCOMPARED WITH SION DEVICES

As described in the previous section, the mobility degra-dation for the HfSiON devices compared with the pure SiO2

Si Substrate

Fig. 5. Schematic cross-section of HfSiON gate stack. Interfacial oxidebetween HfSiON and Si substrate is nitrided and has similar characteristicsto SiON.

In Fig. 6, backside SIMS (Secondary Ion Mass Spec-troscopy) results are plotted. As shown in this figure, thereare huge numbers of oxygen atoms at the bottom of HfSiONfilms (the left side of the figure) and this indicates existenceof interfacial oxide layer between HfSiON and Si substrate.Also it is shown that the concentration of nitrogen atoms ishigh at interfacial layer. Bonding state of the interfacial layeris not clear, but it is possible that the interfacial layer hascharacteristics similar to those of SiON.

It is well known that the electron mobility of SiON MIS-FETs is lower at moderate electric field but higher at highelectric filed compared with that for SiO2 MOSFETs [7].

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Lk01 1

L1

SISPAD 2006 48

v);5

,;3 1044

o 102

C)

> 1 03

(n 100

L)10

0 200 400 600 800 1000 1200 1400Sputter Time (s)

Fig. 6. Backside SIMS results are plotted as a function of the position fromthe Si substrate. The left side of the figure corresponds to the top of thesubstrate. This figure shows existence of interfacial oxide between HfSiONand Si substrate and N atoms exist at the interfacial region.

An empirical model of effective mobility for SiON MISFETsgSiON in cm2/Vs is shown in Eq. (2) [8] and is plotted in

Fig. 7.

SiON = 0T + 2.4 x 10-' Tj 82E)0.33 +6.5 x10-4E2](2)

where Eeff is the effective field in MV/cm and To = T/300. Thesecond and the third terms of RHS of Eq. (2) express phononand surface roughness scattering mechanisms, respectively [9].The following model is used as bulk Coulomb scatteringlimited mobility pc [8];

4.3 x 10"PC N{'5 NS, (3)

A

where NA is substrate acceptor concentration in cm-3 and NSis inversion electron density in cm-2. By using Fig. 1 andEqs. (2) and (3), the mobility degradation Ay' of HfSiONdevices compared with the SiON devices is extracted:

l/ nfSiON = I/gSiON + I/gc+ 1 /A *'. (4)

The extracted mobility degradation Ay' is plotted in Figs. 8(a)and (b) for devices with NA = 3 x 1016 cm-3 and 3 x

1017cm-3, respectively. As shown in these figures, the mo-

bility degradation Ay' is proportional to the power of NSand the slope is almost independent of temperature T. Thisis Coulombic scattering behavior as schematically shown inFig. 3(a).

This degradation is caused by the RCS of HfSiON's fixedcharges. In the weak inversion condition, the degradation forhigher substrate acceptor density is worse than that for lowersubstrate acceptor density. It is because electron occupationrate of the lowest subband is higher for higher substrateacceptor density. Electrons in the lowest subband are closerto HfSiON's fixed charges than these in the upper subbandsand the scattering rate becomes higher. So the RCS-limitedmobility becomes lower for higher substrate acceptor density.

The extracted mobility degradation can be expressed by thefollowing equation and is plotted by solid lines in Fig. 8.

A(A 0.30Ay'= 240

3 x 1016)

t'(N >0.40+0.035 log( 3 NA16)T2O Ns 3 106

(5)

The developed mobility degradation model for HfSiONdevices is implemented into in-house device simulatorDSStation. Cgc -Vg and Id -Vg characteristics of HfSiONMISFETs are simulated with the developed mobility modeland are shown in Figs. 9 and 10. For comparison, simulatedresults with the conventional SiON mobility model [8] are

shown in Fig. 11. As shown in these figures, the developedmobility model has good accuracy and enables accurate sim-ulation of HfSiON MISFETs' characteristics.

The mobility degradation for SiON MISFETs is discussed incomparison with SiO2 MOSFETs. In Fig. 12, the effective mo-bility for pure SiO2 MOSFETs and SiON MISFETs is plottedas a function of effective field. The mobility degradation forSiON MISFETs compared with SiO2 MOSFETs is extractedand plotted in Fig. 13. As shown in these figures, at moderateelectric field, the degradation for SiON MISFETs is higher athigher temperature and shows dependence similar to that ofphonon scattering. On the other hand, at high electric field,suppression of surface roughness scattering is observed whichis typical for SiON n-type MISFETs [7].

IV. CONCLUSION

The electron mobility degradation for HfSiON MISFETswas investigated. We found that the degradation had twoorigins: interfacial thin SiON and HfSiON's fixed charges.HfSiO-related phonon scattering is not observed but SiONphonon scattering is observed. The degradation caused by thefixed charges was extracted from experimental data using an

SiON mobility model and a simple form of the degradationwas presented.

ACKNOWLEDGMENTS

We are grateful to T. Ishihara, Y. Nakasaki, M. Nakamuraand Y. Toyoshima for fruitful discussion and encouragement.

REFERENCES

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

M. V. Fischetti, D. A. Neumayer and E. A. Cartier, J. Applied Phys., 90,p. 4587 (2001).S. Saito, D. Hisamoto, S. Kimura and M. Hiratani, IEDM Tech. Dig.,p. 797 (2003).K. Torii, Y Shimamoto, S. Saito, K. Obata, T. Yamauchi, D. Hisamoto,T. Onai and M. Hiratani, Microelectron. Eng. 59, p. 447 (2003).R. lijima, M. Takayanagi, T. Ishihara, T. Yamaguchi and A. Nishiyama,Ext. Abs. SSDM 2004, p. 32 (2004).K. Sekine, S. Inumiya, M. Sato, A. Kaneko, K. Eguchi andY Tsunashima, IEDM Tech. Dig., p. 103 (2003).M. Koike, T. Ino, Y Kamimuta, M. Koyama, Y Kamata, M. Suzuki,Y Mitani, A. Nishiyama and Y. Tsunashima, IEDM Tech, Dig., p. 107(2003).M. Takayanagi-Takagi and Y. Toyoshima, IEDM Tech. Dig., p. 575(1998).M. Kondo and H. Tanimoto, IEEE Trans. Electron Devices, 48, p. 265(2001).S. Takagi, M. Iwase and A. Toriumi, IEDM Tech. Dig., p. 398 (1988).

1-4244-0404-5/06/$20.00 2006 IEEE

HfSiON Gate Poly

,ae-

28Si El180Hf160 A Y

14N A-N C A,

SISPAD 2006 49

10 1 176 3NA=3x10 cm-10' cm-,I - 3x

-F 1012 10o

:~ io-; 10-3410-

.: 108-r) 10 -

0o-78 i;iO [.......

Effective Field (MWcm) -0.5 0 0.5Gate Voltage (V)

Fig. 7. An effective mobility model for SiON MISFETs expressed by Eq. (2).This model is used to extract mobility degradation Ayi' of HfSiON MISFETscompared with SiON MISFETs.

104It

Fig. 10. Measured (symbols) and simulated (lines) Id -Vg characteristicsof HfSiON MISFETs. Simulation is performed with the developed mobilitymodel. Lg = 10,m and W = 100um. T = 300K. NA = 3 x 1016cm-3 and3 x1017cm-3.

10n

Ecx

0

3*o 10

>a)0)

0

1 01 1

423K373K300KModel

(a) NA=3 X10 cm

1 12 1 13

Inversion Carrier Density (cm-2)(a)

.1 16 -3NA=3xl 0 cm1 3x107 cm-3

l-101

2 102; 10-c

4

610''ll

108-0o-7 ;

'0 8 . I...I-0.5 0 0.5

Gate Voltage (V)

0.8E

0.6 E'a)

0.4 C).:

0.2

0O1.5

Fig. 11. Simulated Id- V characteristics with the conventional SiONmobility model are shown for comparison. The conventional mobility modelhas poor accuracy to simulate HfSiON MISFETs' characteristics.

300K

Model -

(b) NA=3 X10 cm

1012 1013Inversion Carrier Density (cm-2)

(b)

n

E

0

a1).u

LL]

Fig. 8. Extracted mobility degradation Ayi' of HfSiON MISFETs comparedwith SiON MISFETs as a function of inversion electron density. The substrateacceptor concentration is (a) NA = 3 x 1016cm-3 and (b) 3 x 1017cm-3.The extraction is performed using measured HfSiON effective mobility andSiON mobility model expressed by Eq. (2). Solid lines are modeled mobilitydegradation expressed by Eq. (5).

cui 1.6E

LL 1.4

a 1.2

1

0.8

c- 0.6

mc 0.4

() 0.2

( (0

NA=3xl 016cm 3 o3 1017cm 3 0

0.5 0 0.5Gate Voltage (V)

1021o-1

Effective Field (MWcm)

Fig. 12. Effective mobility for SiO2 MOSFETs and SiON MISFETs isplotted as a function of effective field. Substrate acceptor concentration NA is3 x 1016cm-3.

105>

E

4

C 100

S 10

.7-o

0)

102

1 1.5 1o-1

300K373K423K

1Effective Field (MWcm)

Fig. 9. Measured (symbols) and simulated (lines) Cgc- V characteristicsof HfSiON MISFETs. Lg = 10,um and W = 100,um. T = 300K. NA = 3 x

1016cm-3 and 3x 1017cm-3.

Fig. 13. Effective mobility degradation for SiON MISFETs compared withSiO2 MOSFETs is plotted as a function of effective field.

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SiON

E

0

a1).1

20

10o1

"0.8 -

E

0.6 -

a)I

0.4 C)

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01.5

E

0

a)0)

>1

101110

SiO2 300KSiO2 373KSiO2 423KSiON 300KSiON 373K,SiON 423K

F r~~~~~~

r T T~~I

1

1

103

SISPAD 2006 50