capacitive discharges driven by combined dc/rf sourceslieber/liebermandcrfgec07crop.… · •...
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University of California, Berkeley PLASMA
CAPACITIVE DISCHARGES DRIVEN BYCOMBINED DC/RF SOURCES
Emi Kawamura, M.A. Lieberman, and A.J. Lichtenberg
Department of Electrical Engineering and Computer SciencesUniversity of California
Berkeley, CA 94720
E.A. Hudson
Lam Research CorporationFremont, CA 94538
Download this talk:
http://www.eecs.berkeley.edu/∼lieber
LiebermanDublin07 1
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University of California, Berkeley PLASMA
MOTIVATIONS FOR ADDING DC SOURCE
• “Tune” discharge particle and energy balance(⇒ Te ↓, ne ↑, radial uniformity)
• “Tune” secondary electron bombardment of substrate(etch selectivities, charging damage)
OUTLINE
• Structure of DC/RF sheaths — theory
• Equal area diode discharges — theory and 1D PIC simulations
• Asymmetric diode discharges — theory and 1D PIC simulations
• Secondary electrons — timescales and energy deposition
• Triode discharges — theory and 2D PIC simulations
LiebermanDublin07 2
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University of California, Berkeley PLASMA
STRUCTURE OF A DC/RF SHEATH
Vdc
Vrf
–
+
~ ss0 s1
V1V0
V0~
V1~
Bulkplasma
+–+–
ADCpart
RFpart
ni
ne(t)z
n
Ji– DC voltage V = V 0 + V 1
RF voltage Ṽ = Ṽ0 + Ṽ1
• New result for Child law for collisionless ions:
J̄i =4
9ǫ0
(2e
M
)1/2 (V 1/2 − 13V
1/2
1 )(V1/2
+ 23V
1/2
1 )2
s2
• New result for Child law for collisional ion sheath also obtained
LiebermanDublin07 3
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University of California, Berkeley PLASMA
EQUAL AREA DIODE DISCHARGE
• Comparison of theory with 1D particle-in-cell (PIC) simulations
Vdc
Vrf
–
+
~ ss0 s1
V1V0
V0~
V1~
Bulkplasma
++ ––+–
DC/RF sheath RFsheath
A
V1~
V1 A
s1
0 0.5 1 1.5 2
V /V
0
0.1
0.2
0.3
0.4
0.5
V /V
45
100
60 45
10060
45
15
45
60
100
4MHz, γ = 0.24MHz, no sec.
13MHz, no sec.
Collisionless
Collisional, β 0
Collisional, β = 0.1
a1
rf
∼
dc rf
i
(Symbols: PIC with argon pressure in mTorr;
lines: theory; β ∝ λD/λi = collisionality;
γi = secondary emission coefficient)
• Excellent agreement of PIC with collisional Child law
LiebermanDublin07 4
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University of California, Berkeley PLASMA
DENSITY AND VOLTAGE AT CONSTANT Prf(60 mTorr, 6 cm gap, 4 MHz, 0.017 W/cm2, Vdc = 350 V)
0 0.01 0.02 0.03 0.04 0.05 0.06
x (m)
0
2e+15
4e+15
6e+15
n (m
)
..
rf (γ =0)dc + rf (γ = 0)rf (γ =0.2)dc + rf (γ =0.2)
(a)
i
i
rf sheath "b" dc/rf sheath "a"
sb
nb
i
i
e-3
−
0 0.01 0.02 0.03 0.04 0.05 0.06
x (m)
-200
0
200
400
600
V (V
)
rf (γ = 0)dc + rf (γ = 0)rf (γ =0.2)dc + rf (γ =0.2)
i
i
rf sheath "b" dc/rf sheath "a"
i
i
(b)
_p
AddDC
γi Vrf (V) V b (V) Eceff (V)
Rf only 0 1064 436 64Dc+rf 0 1277 398 60Rf only 0.2 805 330 53Dc+rf 0.2 980 283 46
(V b = plasma potential; Eceff =collisional energy loss/electron-ion pair)
• Plasma potential V b and sheath width sb independent of Vdc• Secondary electrons increase discharge efficiency• Vdc reduces bulk plasma thickness
LiebermanDublin07 5
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University of California, Berkeley PLASMA
ASYMMETRIC DIODE RESULTS
• Excellent agreement between1D (cylindrical) PIC andcollisional CL theory
Vdc
Vrf
–
+
~ sasa0 sa1
sb
Va1Va0
Va0~
Va1~
Vb~
Bulkplasma
++ ––+– Vb
DC/RF sheath RFsheath
Aa Ab
• Introduce rf voltage asymmetry ratio αab = Ṽa1/Ṽb
-0.5 0 0.5 1 1.5 2
V /V
0
0.2
0.4
0.6
0.8
1
V /
V
15(0.093)
45(2.2)
45(0.46) 45(0.48)
45(0.48)
45(0.15)
45(0.15)
45(0.1)
45(0.13)
50(0.34)
45(0.14)
50(0.38)
ab
4MHz, γ = 0.232MHz, no sec.
6.78MHz, no sec.
0.1
α =10
0.15
0.3
0.5
1.02.0
brf
dc rf
i
∼
(a)
-0.5 0 0.5 1 1.5 2
V /V
0
0.2
0.4
0.6
0.8
1
s /
s 0.1
α =10
0.15
0.3
0.51.0
2.0
15(0.093)
45(2.2)
45(0.46)
45(0.48)
45(0.48)45(0.15)
45(0.15)
45(0.1)45(0.13)
50(0.34)
45(0.14)
50(0.38)
ab 4MHz, γ = 0.232MHz, no sec.
6.78MHz, no sec.
a1
a
dc rf
i
(b)
Numbers near each symbol: mTorr (αab)
LiebermanDublin07 6
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University of California, Berkeley PLASMA
SECONDARY ELECTRON LOSS PROCESSES
• Surface losses to substrate and walls:
— Transit time across gap τfr = d/vh at low pressures
— Diffusion time τdiff = d2/2Dh at higher pressures (Dh = λhv̄h/3)
— Trapping time τtrap = δ/f (favorable configuration of rf voltagescan trap secondaries for a fraction δ of the rf period 1/f)
τlh = (τ2fr + τ
2diff + τ
2trap)
1/2 = ν−1lh
• Volume losses: secondary electrons lose energy and join the thermalpopulation
τ∗izh =Eh
νizhEch(Eh, νizh are secondary energy and ionization frequency;
Ech ≈ 20 V is secondary collisional energy loss/e-i pair created)
• Total loss frequency is νh = νlh + ν∗
izh
If ν∗izh ≫ νlh, secondary electrons efficiently produce e-i pairsIf νlh ≫ ν
∗
izh, secondary electrons efficiently bombard the substrateLiebermanDublin07 7
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University of California, Berkeley PLASMA
ENERGY DEPOSITION REGIONS
10-2
10-1
100
101
102
d/λ
10-3
10-2
10-1
100
101
fd/v
= 70 V, δ = 0.5
h
h
τ = τ
τ =
τ
τ =
τ
τ =
τ
τ =
τ
fr
τ = τ
trap
trap
izh*
trap
diff
dif
ffr
dif
f
fr
*
*
izh
εh
izh
No energydeposition
Trapped anduntrapped
energydeposition
Trappeddepositionuntrapped
diffusion
Trap
ped
depo
sitio
n
untra
pped
flow
(τ , τ , τ < τ )fr trapdiff izh* (τ > τ )diff
*izh
(τ > τ )trap izh*
τ > τ τ > τ diff frfr diff
τ , τ < τ izhfr diff *
Example
pressure
freq
uenc
y
• Most interesting regions are where trapped and untrapped electronsbehave differently
LiebermanDublin07 8
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University of California, Berkeley PLASMA
TRIODE DC/RF DISCHARGE
• Substrate can have a dielectric layer which cannot draw dc current
A triode configuration is necessary
Vdc Vrf
–
+
~sasa0 sa1
sb
Va1Va0
Va0~
Va1~
Vb~
Bulkplasma
++ ––+– Vb
DC/RF sheathRF
sheath
Aa Ab
sg Vg~
+
–Vg
Ag
Substrate
DCcurrent
• A global model incorporating the collisional dc/rf sheath is used todetermine the voltages, currents, and sheath widths
LiebermanDublin07 9
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University of California, Berkeley PLASMA
TRIODE RF PLASMA POTENTIAL Ṽb VERSUS Vdc
• Collisional theory results for triode
0.01 0.1 1 10
V /V
0
0.2
0.4
0.6
0.8
V /
V
A /A =0.5, A /A =0.5A /A =1.0, A /A =0.5A /A =0.5, A /A =1.0
dc rf
brf
∼
a c b c
ca b c
a c b c
• Example (red solid line):
DC electrode area = ground electrode area = 12× RF electrode area
For Vdc → 0, equal area diode and Ṽb/Vrf = 0.5
For Vdc → ∞, asymmetric diode and Ṽb/Vrf = 0.86.LiebermanDublin07 10
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University of California, Berkeley PLASMA
2D PIC SIMULATION BASE CASE
• p = 30 mTorr, Prf = 2.2 W, γi = 0.2 at all surfaces
• Secondaries in “trapped deposition, untrapped diffusion” regime
1.6
cm
CB
ArPlasma
30 mT
3 cm
9 cm
Vrf13.56 MHz
345 V
14
cm
1.6
cm
CB
+
_
ArPlasma
30 mT
3 cm9
cm
VrfVdc
200 V
13.56 MHz
285 V
14
cm
1.6
cm
1.6
cm
RF only DC/RF triode
x
y
LiebermanDublin07 11
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University of California, Berkeley PLASMA
UNIFORMITY IS MODIFIED BY Vdc
0 0.02
0.04 0.06
0.08 0.1
0.12
y (m) 0
0.01
0.02
0.03
x (m)
0
2e+15
4e+15
6e+15
8e+15
0 0.02
0.04 0.06
0.08 0.1
0.12
y (m) 0
0.01
0.02
0.03
x (m)
0
2e+15
4e+15
6e+15
8e+15
0 0.02
0.04 0.06
0.08 0.1
0.12
y (m) 0
0.01
0.02
0.03
x (m)
0
2e+11
4e+11
6e+11
8e+11
1e+12
1.2e+12
0 0.02
0.04 0.06
0.08 0.1
0.12
y (m) 0
0.01
0.02
0.03
x (m)
0
2e+11
4e+11
6e+11
8e+11
1e+12
1.2e+12
1.4e+12
Dc/rfRf only
Plasma
density
(m-3)
Secondary
density
(m-3)
• Thinner bulk plasma near midplane (y = 0.06 m) for DC/RF case
⇒ center-low plasma density profile
LiebermanDublin07 12
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University of California, Berkeley PLASMA
BALLISTIC SECONDARY ELECTRONS CREATED BY Vdc
0 0.02
0.04 0.06
0.08 0.1
0.12y (m) 100
200 300
400
ε (V)
0
1e+11
2e+11
3e+11
4e+11
0 0.02
0.04 0.06
0.08 0.1
0.12y (m) 100
200 300
400
ε (V)
0
2e+11
4e+11
6e+11
0 0.02
0.04 0.06
0.08 0.1
0.12y (m) 0.4
0.8 1.2
1.6
θ (rad.)
0
1e+11
2e+11
3e+11
4e+11
5e+11
6e+11
0 0.02
0.04 0.06
0.08 0.1
0.12y (m) 0.4
0.8 1.2
1.6
θ (rad.)
0
1e+12
2e+12
3e+12
4e+12
Dc/rf
ballistic
highenergy
Rf only
Secondary
energy
distribution
(arb units)
Secondary
angular
distribution
(arb units)
• Secondary electrons are ballistic and have high energies for DC/RFcase
LiebermanDublin07 13
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University of California, Berkeley PLASMA
PLASMA DENSITY PROFILES VERSUS PRESSURE
0 0.02
0.04 0.06
0.08 0.1
0.12
y (m) 0
0.01
0.02
0.03
x (m)
0
2e+15
4e+15
6e+15
8e+15
0 0.02
0.04 0.06
0.08 0.1
0.12
y (m) 0
0.01
0.02
0.03
x (m)
0
2e+15
4e+15
6e+15
8e+15
Dc/rfRf only
30 mTorr
0 0.02
0.04 0.06
0.08 0.1
0.12
y (m) 0
0.01
0.02
0.03
x (m)
0
2e+15
4e+15
6e+15
8e+15
1e+16
0 0.02
0.04 0.06
0.08 0.1
0.12
y (m) 0
0.01
0.02
0.03
x (m)
0
2e+15
4e+15
6e+15
8e+15
1e+16
1.2e+16
1.4e+16
1.6e+16
45 mTorr
• Effects of Vdc on profile:Thinner bulk plasma ⇒ center-low profileIncreased secondary ionization ⇒ center-high profile
LiebermanDublin07 14
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University of California, Berkeley PLASMA
CONCLUSIONS
• Collisionless and collisional DC/RF Child laws determined
• DC voltage can control the discharge asymmetry
• DC voltage increases secondary electron ionization
• DC voltage reduces bulk plasma thickness
• DC voltage promotes the formation of ballistic electrons
• DC voltage modifies the plasma density profile
Download this talk:
http://www.eecs.berkeley.edu/∼lieber
Ref: E. Kawamura, M.A. Lieberman, and A.J. Lichtenberg, “Capacitive DischargesDriven by Combined DC/RF Sources”, J. Vac. Sci. Technol. A, 25, 1456–74,Sept. 2007
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