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Lecture 3:Fundamentals of Sputter Deposition
2
Outline
Sputtering yield• Linear cascade• Correction for threshold effect• Sputtering efficiency• Energy of sputtered atoms• Ion reflection
Sputtering systems• Conventional diode sputtering• Triode sputtering• Magnetron sputtering
•Discharge characteristics•Ion distribution at substrate•Reactive sputtering•Independent control of ion flux and ion energy
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P. Sigmund, “Theory of Sputtering I. Sputtering Yield of Amorphous and Polycrystalline Targets,” Physical Review. 184, 383 (1969).
P.Sigmund, “Sputtering by ion bombardment: theoretical concepts”, in Sputtering by particle bombardment I, edited by R. Behrish, Springer-Verlag, 1981
HH Anderson and HL Bay, “Sputtering yield measurements”, in Sputtering by particle bombardment I, edited by R. Behrish, Springer-Verlag, 1981
G.K. Wehner and G.S. Anderson, “The Nature of Physical Sputtering” in Handbook of Thin Film Technology, edited by L.I. Maissel and R. Glang, McGraw-Hill, NY 1970.
J.L. Vossen and J.J. Cuomo, “Glow Discharge Sputter Deposition, “ in Thin Film Processes, edited by J.L. Vossen and W. Kern, Academic Press, NY 1978.
J.A. Thornton and A.S. Penfold, “Cylindrical Magnetron Sputtering,” in Thin Film Processes, edited by J.L. Vossen and W. Kern, Academic Press, NY 1978.
R.K. Waits, “Planar Magnetron Sputtering,” in Thin Film Processes, edited by J.L. Vossen and W. Kern, Academic Press, New York 1978.
B. Chapman, Glow Discharge Processes, Wiley Interscience, New York 1980.
J.A. Thornton and J.E. Greene, “Plasmas in Deposition Processes,” in Deposition Technologies for Films and Coatings, edited by R.F. Bunshah, Noayes Publications, Park Ridge, New Jersey 1994, p. 29.
S.M. Rossnagel, “Magnetron Plasma Deposition Processes,” in Handbook of Plasma Processing Technology, edited by S.M. Rossnagel, J.J. Cuomo, and W.D. Westwood Noyes Publications, Park Ridge, New Jersey 1990.
Bibliography
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Ion surface interactions
Positive ions are returned by the electric field
Inelastic Effects
Ion beam with an energy Ei+I
Implanted ParticlesI0
UV/visible photons
X-rays
SecondaryElectrons
Acceleratedby the field
Sputtered ParticlesT0, T*, Tn
Reflected ParticlesI0,I*
Elastic Effects
T+, I+
Target
T-, I-
Acceleratedby the field
Negative Ions
Figure after G.M. McCracken, Rep. Prog Phys. 28, 241 (1975).
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Collision Cascade
P.Sigmund, “Sputtering by ion bombardment: theoretical concepts”, in Sputtering by particle bombardment I, edited by R. Behrish, Springer-Verlag, 1981
• Ions striking a surface interact with a number of atoms in a series collisions.• recoiled target atoms in turn collide with atom at rest generating a collision cascade.• The initial ion energy and momentum are distributed to among the target recoil atoms. • When Ei > 1 keV, the cascade is “linear”, i.e. approximated by a series of binary collisions in a stationary matrix.
Threshhold regimerecoils sputtered, butno (limited) cascades
Energy increasing (dependent on Mi/Mt)
Linear cascadea series of binary collisions
Spike regimehigh density of recoils
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Stopping cross section
P.Sigmund, “Sputtering by ion bombardment: theoretical concepts”, in Sputtering by particle bombardment I, edited by R. Behrish, Springer-Verlag, 1981
[ ]( )( )) (en S EdE N N SS E Edx
= − = − +
0
'( ')
E dERangeNS E
= ∫
Sn(E)Se(E)
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SRIM/TRIM simulation
James F. Ziegler, IBM : http://www.srim.org/
1 keV Ar -> Be
Monte-Carlo simulation of ion implantation, reflection, recoil cascades, and sputtering
1 keV Ar -> Ti
1 keV Ar -> W
2 impacts
200 impacts
ion trajectories in redrecoils in green
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Sputtering Yield (Y)
Sputtering begins at an energy threshold and increases rapidly. As the energy increases the curve levels off.
Spu
tterin
g Y
ield
(Ato
ms/
ion)
0
1
2
3
4
Ion Energy (eV)0 100 200 300 400 500 600
Sputteringwith Argon
Ag
Co
TiC
Target
Cu
Data from R.Y. Stuart and G.K. Wehner, J. Appl. Phys. 33, 2351 (1962); D. Rosenberg and G.K. Wehner, J. Appl. Phycs. 33, 1842 (1962); and R. Behrisch, Exakt. Naturw. 35, 295 (1964).
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0.0
0.5
1.0
Sigmund’s linear cascade formula for Y(E)
0.04( )Y EU
= t i nα( /M )SM (E)
85 i t in n2 2 0.53 3 t ii t
Z Z MS (E) = s (ε)M + M(Z + Z )
2 2 0.53 3
0.03255
( )t
t ii t i t
M EM MZ Z Z Z
ε =++
3.441 ln( 2.718)( )1 6.355 (6.882 1.708)ns
ε εεε ε ε
+=
+ + −
α - dimensionless coefficientSn(E), collisional energy at the surface (nuclear energy loss function)U – sublimation energy
sn(ε) – function of the reduced energy which is the same for all ion-target combination
P. Sigmund, Physical Review. 184, 383 (1969)
0.730.1+0.155( )t iM /Mα
Mt/Mi
1 100.1
10
2
2
3( ) ( / )4
4( )
t i
t i
t i
EY E M MU
M MM M
γαπ
γ
=
=+
0.1 < E < 1kV, Sigmund derives a remarkably simple formula
102 103 104 105 1061E-3
0.01
0.1
1
Sigmund, full Sigmund, short experiment
Ar -> W
Yie
ld (a
tom
/ion)
Ion energy (eV)
HH Anderson and HL Bay, “Sputtering yield measurements”, in Sputtering by particle bombardment I, edited by R. Behrish, Springer-Verlag, 1981
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20.50.04( ) ( / ) ( ) 1
1 ( )
THt i n
T
e
EY E M M QS EU E
qQs
α
ε
= −
=+
0.1 1 101
10
THEU
( )
20.8341.5 1+1.38 i tM /Mγ
Y. Yamamura et al, Rad.Eff.,11 65 (1983)I. Petrov et al, Bulg.J.Phys., 18 3 (1991)
Sputtering Threshold (ETH)
Sputtering begins at an energy threshold that depends on the efficiency of momentum transfer to the target. This depends on the mass match. It also depends on the surface binding energy of atoms in the target.
Y. Yamamura et al introduced a correction for ETH
102 103 104 105 1061E-3
0.01
0.1
1
Ar -> W
Sigmund, full Sigmund, short Yamamura experiment
Yie
ld (a
tom
/ion)
Ion energy (eV)
8.5(Mt /Mi )-1/3
Mt /Mi
12
10 100 1000
0.01
0.1
1Ar -> Ni
I N O T
Yiel
d
Ion energy (eV)
100 1000
0.01
0.1
1Kr -> Be
Sigmund Eqn. A experiment Yamamura
Yiel
d
Ion energy (eV)
100 1000
0.01
0.1
1Ar -> W
Sigmund Eqn. A experiment Yamamura
Yiel
dIon energy (eV)
100 1000
0.01
0.1
1Ar -> Al
Sigmund Eqn. A experiment Yamamura
Yiel
d
Ion energy (eV)
( ) ( )( )
20.5 0.5 0.50.5
2 2 0.50.53 3
1.8( ) ( / ) 11 /( )
i t i t THt i
t ii t
Z Z M M EY E M M EU EM MZ Z
α = − ++
2
2
3( ) ( / )4
4( )
t i
t i
t
EY E M MU
M MM M
γαπ
γ
=
=+
“Simple” Y(E) formula for E < 1 keV , Mi > 15 amu
Eqn. A
13
Energy efficiency of sputtering
10 100 1000
100
10-1
10-2
101
Kr
HeXeAr
Ne
Cu targetPower of the ion beam - Pion flux = J*E
Power used for sputtering - Psp=U*J*Y(E)
Sputtering efficiency η = Psp/ Pion flux
η = U*Y(E)/E
Yamamura formula: ηmax at E=7*Eth
For ≤ ≤ ≥TH TH max3E E 10E ,η 0.8η
0.1 1 101
10
THEU
Energy (eV)
Y(E
)/E
target ionM /M
Typical example:
U = 4 eV, ETH = 30 eV, ηmax at E=210 eV0.8 ηmax E = 90 – 300 eV
max0.8η
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η Armax
Target atomic number ZT
Target atomic number ZT
ηη
Ne,Kr,Xemax
Armax
I. Petrov et al, Bulg.J.Phys., 18 3 (1991)
10-3
10-2
10-1
0 20 40 60 80 100
Energy efficiency of sputtering
The maximum sputtering efficiency between 0.5 and 5 %
Ar provides high sputtering efficiency for a large number of metal targets (from Al to La)Ne has ~ 20% advantage for Be and CKr ~ 40-60% advantage for targets heavier than Ta
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Energy of Sputtered Particles
Sputtered atom energy has a maximum at ~ U/2 (several eV) and tail extending to tens and hundreds of eV, depending on the ion energy.
Energy (Thompson) distribution:
12
3 3( ) 1( ) ( )ion
E E U EF EE U E E Uγ
+ ∝ − ≈ + +
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Sputtering Yield: Other Species
Distribution of types of sputtered species:[Example for Ar sputtering of Cu]
Single atoms sputtered 100
Diatoms 1
Resputtered trapped gas 5
Single ions 0.1
Diatomic ions 0.001
Reflected incident species 3
Secondary electrons 10
++
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Reflection of Primary Ions
0 1 2 3 4 50
20
40
60
80
100
120
140
PtW
Au Pb
Mo
Sn
Ti
Alaver
age
refle
cted
ion
ener
gy, e
V
Mtarget/Mion
1 2 3 4 5
1E-3
0.01
0.1
1
PtW
Au Pb
Mo Sn
Ti
Al
refle
ctio
n co
effic
ient
Mtarget/Mion
Incident ions may be reflected from the target surface.Reflection coefficient = #reflected ion/#incident ion
Case study: TRIM simulation of 500 eV Ar ion scattering
Both reflection coefficient and the average energy of the reflected ions increasewhen the target atom is heavier than the ion
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Reflection of Primary Ions, cont.Incident ions may be reflected from the target surface.Reflection coefficient = #reflected ion/#incident ion
Both reflection coefficient and the average energy of the reflected ions for a given target decrease when heavier ions are used.
Case study: TRIM simulation of 500 eV , Xe, Ar, and N ion scattering
10 20 30 40 50 60 70 80 900
20406080
100120140160180200220240
N
Ar
XeAve
rage
ene
rgy,
eV
Atomic number, Z10 20 30 40 50 60 70 80 90
1E-3
0.01
0.1
Xe
ArN
Bac
ksca
ttred
ions
yie
ld, i
ons/
ion
Atomic number, Z
19
Reflection of Primary Ions, cont.
Case study: TRIM simulation of 500 eV , Xe, Ar, and N ion scattering
A significant fraction of the incident ion energy (> 10%) is reflected back when ions are much lighter than the target atoms
1 101E-5
1E-4
1E-3
0.01
0.1
1
10
500 eV N ions 500 eV Ar ions 500 eV Xe ions
% o
f ref
lect
ed io
n en
ergy
Mtarget/Mion
20
Variations of sputtering systemsMagnetron sputtering
Unbalanced magnetrons
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H O2
H O2SUBSTRATE HEATER
SU
BST
RAT
E rf
OR
dc
BIA
S
PUMPINGSYSTEM
DARK SPACE SHIELD
HIG
H V
OLT
AG
E
DC
(RF)
SU
PPLY
TARGET
PR
ES
SU
RE
GA
UG
ES
FLO
WC
ON
TRO
L SPUTTERING GAS
PLASMA
SHUTTER
Diode sputter deposition systemComponents and typical parameters
VT ~ 2-5 kV JT ~ 1 mA/cm2
p ~ 50-80 mTorrλ << dTS
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VT ~ 1 kV JT ~ 5 mA/cm2
p ~ 10-20 mTorrλ ~ dTS
Triode sputter deposition systemComponents and typical parameters
Independent control of ion flux and energyThe presence of a hot filament hampers reactive deposition
23
Triode sputter deposition systemComponents and typical parameters
VT ~ 0.3-0.5 kV JT ~ 10-100 mA/cm2
p ~ 2-20 mTorrλ > dTS,,, λ < dTS
ExB field near target enhances ionization efficiency, thus reducing both VT and p
24
ExB configurations
J.A. Thornton and A.S. Penfold, “Cylindrical Magnetron Sputtering,” in Thin Film Processes, edited by J.L. Vossen and W. Kern, Academic Press, NY 1978.
25
Magnetron discharge characteristics
0 GThornton, cylindrical magnetrons
J.A. Thornton and A.S. Penfold, “Cylindrical Magnetron Sputtering,” in Thin Film Processes, edited by J.L. Vossen and W. Kern, Academic Press, NY 1978.
26
DC magnetron discharge characteristics
I. Petrov, I. Ivanov, V. Orlinov, and J.E. Sundgren, J. Vac. Sci. Technol, 11 2733 (1993)
800
700
600
500
400
300
200
Ar
XeKr
Ne
V (V
) T
0.1 1 10p (Pa)
0 0.5 1 1.5
800
600
400
200
Ar
XeKr
Ne
I (A)T
V (V
) T
p = 1 Pa (7.5 mTorr)IT = 0.3 A
Example: 50 mm Vanadium target, planar magnetron
270.1 1 10
p (Pa)
Ar
XeKr
Ne
0.2
0.4
0.6
0.8
1.0
0
Thermal
Balistic
Frac
tion
Ar
XeKr
Ne0.1
0.01
0.001
R
/R
Dep
spR
/R
D
epsp
0.1
0.01
0.001
Ar
XeKr
Ne
I. Petrov, I. Ivanov, V. Orlinov, and J.E. Sundgren, J. Vac. Sci. Technol, 11 2733 (1993)
Thermalization of sputtered species
In typical pressure range for magnetronsputter deposition, both collisionless anddiffusive transport are effective
28
0.01 0.02 0.05 0.1 0.2 0.5 1
0.01 0.02 0.05 0.1 0.2 0.5 1Nitrogen fraction, fN2
1.4
14
1.2
0.12
12
1.0
0.10
10
0.8
0.08
8
0.6
0.06
6
0.4
0.04
0.2
0.02
24
0.0
0.00
0
N/T
i
(d)
∆P
(mTo
rr)to
tTa
rget
Vol
tage
, V (V
)T
500
450
350
400
300
Target = Tip = 3 mTorrI = 0.2 A
Ti
T
(a)
(b)
(c)
Ti fl
ux, J
(10
cm
s )
Ti14
-2-1
0.01 0.02 0.05 0.1 0.2 0.5 1
0.01 0.02 0.05 0.1 0.2 0.5 1Nitrogen fraction, fN2
5
10
8
7
9
4
8
3
6
7
2
5
6
1
0
4
5
J / J
i
Ti(d)
6
5
3
4
2
Target = Tip = 3 mTorrI = 0.2 A
Ti
T
(a)
(b)
(c)
J (
10 c
m s
)i
14-2
-1
(10
cm
)
n
e11
-3T
(eV)
e
Reactive sputtering
I. Petrov, A. Myers, J.E. Greene, and J.R. Abelson, JVST A 12, 2846 (1994)
29
Ion distribution at the substrate
0.01 0.1 110-6
10-5
10-4
10-3
10-2
10-1
100
TiN+
N2+
N+
Ti+
Ti TargetPT = 3 mTorrIT = 0.2A
Ar2+
Ar+
Rel
ativ
e io
n flu
xes
fN2
I. Petrov, A. Myers, J.E. Greene, and J.R. Abelson, JVST A 12, 2846 (1994)
Ti
Thompson
Ar+
Ti+
Vbias
30
Independent control of ion flux and ion energy
Ei = e(Vplasma - Vbias)Ji = f(Bext)
I. Petrov, F. Adibi, J.E. Greene, W.D. Sproul, and W.-D. Münz, JVST A10, 3283 (1992).
