Transcript Document

CONTROL OF ELECTRON ENERGY
DISTRIBUTIONS AND FLUX RATIOS IN PULSED
CAPACITIVELY COUPLED PLASMAS*
Sang-Heon Songa) and Mark J. Kushnerb)
a)Department
of Nuclear Engineering and Radiological Sciences
University of Michigan, Ann Arbor, MI 48109, USA
[email protected]
b)Department
of Electrical Engineering and Computer Science
University of Michigan, Ann Arbor, MI 48109, USA
[email protected]
http://uigelz.eecs.umich.edu
Oct 2010 AVS
*
Work supported by DOE Plasma Science Center and Semiconductor Research Corp.
AGENDA
 Motivation for controlling f(e)
 Description of the model
 Typical Ar pulsed plasma properties
 Typical CF4/O2 pulsed plasma properties
 f(e) and flux ratios with different
 PRF
 Duty Cycle
 Pressure
 Concluding Remarks
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CONTROL OF ELECTRON KINETICS- f(e)
 Controlling the generation of reactive species for technological
devices benefits from customizing the electron energy (velocity)
distribution function.
 Need SiH3 radicals*
 LCD
 Solar Cell
k
SiH3 + H + e
e + SiH4
dN k  r , t 
dt

    nekij  r , t  N j
i, j

12
 2e 
kij  r , t  
f e , r , t  
  e  d e
0
 me 

df  v , r , t 
dt
 v  x f  r , v  
qE  r , t 
me
* Ref: Tatsuya Ohira, Phys. Rev. B 52 (1995)
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 f  v , r , t  
v f  v , r , t   


t

c
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
Electron
Monte Carlo
Simulation
Te, S, k
Fluid Kinetics Module
E, Ni, ne, Ti
Fluid equations
(continuity, momentum, energy)
Poisson’s equation
 Fluid Kinetics Module:
 Heavy particle and electron continuity, momentum,
energy
 Poisson’s equation
 Electron Monte Carlo Simulation:
 Includes secondary electron transport
 Captures anomalous electron heating
 Includes electron-electron collisions
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REACTOR GEOMETRY
 2D, cylindrically symmetric
 Ar, CF4/O2, 10 – 40 mTorr, 200 sccm
 Base conditions
 Lower electrode: LF = 10 MHz, 300 W, CW
 Upper electrode: HF = 40 MHz, 500 W, Pulsed
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PULSE POWER
 Use of pulse power provides a means for controlling f(e).
 Pulsing enables ionization to exceed electron losses during a portion
of the period – ionization only needs to equal electron losses
averaged over the pulse period.
Pmax
Power(t)
Pave 
Duty Cycle

Pt dt


1
0
Pmin
 = 1/PRF
Time
 Pulse power for high frequency.
 Duty-cycle = 25%, PRF = 100 kHz, 415 kHz
 Average Power = 500 W
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Ar
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PULSED CCP: Ar, 40 mTorr
 Pulsing with a PRF and moderate duty cycle produces nominal
intra-cycles changes [e] but does modulate f(e).
ANIMATION SLIDE-GIF
 LF = 10 MHz, 300 W
 HF = 40 MHz, pulsed 500 W
 PRF = 100 kHz, Duty-cycle = 25%
 [e]
f(e)
MIN
VHF 226 V
VLF 106 V
MAX
 Te
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PULSED CCP: Ar, DUTY CYCLE
 Excursions of tail are more extreme with lower duty cycle – more
likely to reach high thresholds.
ANIMATION SLIDE-GIF
 Cycle Average
 Duty cycle = 25%
 Duty cycle = 50%
VHF 226 V
VLF 106 V
VHF 128 V
VLF 67 V
 LF 10 MHz, pulsed HF 40 MHz
 PRF = 100 kHz, Ar 40 mTorr
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PULSED CCP: Ar, PRESSURE
 Pulsed systems are more sensitive to pressure due to differences in
the rates of thermalization in the afterglow.
ANIMATION SLIDE-GIF
 Cycle Average
 10 mTorr
 40 mTorr
VHF 274 V
VLF 146 V
VHF 226 V
VLF 106 V
 LF 10 MHz, pulsed HF 40 MHz
 PRF = 100 kHz
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CF4/O2
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 CW
ELECTRON DENSITY
 At 415 kHz, the electron
density is not significantly
modulated by pulsing, so the
plasma is quasi-CW.
 PRF=415 kHz
 PRF=100 kHz
 At 100 kHz, modulation in [e]
occurs due to electron
losses during the longer
inter-pulse period.
 The lower PRF is less
uniform due to larger bulk
electron losses during
longer pulse-off cycle.
 40 mTorr, CF4/O2=80/20, 200 sccm
 LF = 10 MHz, 300 W
 HF = 40 MHz, 500 W (CW or pulse)
MIN
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ANIMATION SLIDE-GIF
MAX
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 CW
 PRF=415 kHz
ELECTRON SOURCES
BY BULK ELECTRONS
 The electrons have two
groups: bulk low energy
electrons and beam-like
secondary electrons.
 The electron source by bulk
electron is negative due to
electron attachment and
dissociative recombination.
 PRF=100 kHz
 Only at the start of the pulseon cycle, is there a positive
electron source due to the
overshoot of E/N.
 40 mTorr, CF4/O2=80/20, 200 sccm
 LF 300 W, HF 500 W
MIN
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ANIMATION SLIDE-GIF
MAX
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 CW
 PRF=415 kHz
ELECTRON SOURCES
BY BEAM ELECTRONS
 The beam electrons result
from secondary emission
from electrodes and
acceleration in sheaths.
 The electron source by beam
electron is always positive.
 PRF=100 kHz
 The electron source by beam
electrons compensates the
electron losses and sustains
the plasma.
 40 mTorr, CF4/O2=80/20, 200 sccm
 LF = 10 MHz, 300 W
 HF = 40 MHz, 500 W (CW or pulse)
MIN
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ANIMATION SLIDE-GIF
MAX
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TYPICAL f(e): CF4/O2 vs. Ar
 Ar
 CF4/O2
 Less Maxwellian f(e)
with CF4/O2 due to
lower e-e collisions.
VHF 226 V
VLF 106 V
VHF 203 V
VLF 168 V
 Enhanced sheath
heating with CF4/O2
due to lower plasma
density.
 Tail of f(e) comes up
to compensate for
the attachment and
recombination that
occurs at lower
energy.
 40 mTorr, 200 sccm
 LF = 10 MHz, 300 W
 HF = 40 MHz, 500 W (25% dc)
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ANIMATION SLIDE-GIF
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RATIO OF FLUXES: CF4/O2
 In etching of dielectrics in fluorocarbon gas mixtures, the polymer
layer thickness depends on ratio of fluxes.
 Ions – Activation of dielectric etch, sputtering of polymer
 CFx radicals – Formation of polymer
 O – Etching of polymer
 F – Diffusion through polymer, etch of dielectric and polymer
 Investigate flux ratios with varying
 PRF
 Duty cycle
 Pressure
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 Flux Ratios:
 Poly = (CF3+CF2+CF+C) / Ions
 O = O / Ions
 F = F / Ions
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f(e): CF4/O2, PRF
 Average
 PRF = 100 kHz
 The time averaged f(e)
for pulsing is similar to
CW excitation.
VHF 203 V
VLF 168 V
 Extension of tail of f(e)
beyond CW excitation
during pulsing
produces different
excitation and
ionization rates, and
different mix of fluxes
to wafer.
ANIMATION SLIDE-GIF
 40 mTorr, CF4/O2=80/20, 200 sccm
 LF = 10 MHz, 300 W
 HF = 40 MHz, 500 W (25% dc)
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RATIO OF FLUXES: CF4/O2, PRF
 Ratios of fluxes are tunable using pulsed excitation.
Average Flux Ratio
 Polymer layer thickness may be reduced by pulsed excitation
because poly to ion flux ratio decreases.
6.0
CW
5.0
4.0
100
100
CW
3.0
2.0
415
415 kHz
1.0
415
100 CW
0.0
F
F
O
O
Poly
POLY
 40 mTorr, CF4/O2=80/20, 200 sccm, Duty-cycle = 25%
 LF = 10 MHz, 300 W
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 HF = 40 MHz, 500 W
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f(e): CF4/O2, DUTY CYCLE
 Control of average f(e) over with changes in duty cycle is limited if
keep power constant.
ANIMATION SLIDE-GIF
 Cycle Average
 Duty cycle = 25%
 Duty cycle = 50%
VHF 203 V
VLF 168 V
VHF 191 V
VLF 168 V
 40 mTorr, CF4/O2=80/20, 200 sccm
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 LF 10 MHz, Pulsed HF 40 MHz, PRF = 100 kHz Institute for Plasma Science & Engr.
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RATIO OF FLUXES: CF4/O2, DUTY CYCLE
Average Flux Ratio
 Flux ratio control is limited if keep power constant.
 With smaller duty cycle, polymer flux ratio is more reduced
compared to the others.
6.0
CW
5.0
50%
4.0
25%
25%
50% CW
3.0
2.0
25% 50% CW
1.0
0.0
F
F
O
O
Poly
 LF 10 MHz, Pulsed HF 40 MHz, PRF = 100 kHz
 40 mTorr, CF4/O2=80/20, 200 sccm
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POLY
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f(e): CF4/O2, PRESSURE
 Pulsed systems are sensitive to pressure due to differences in the
rates of thermalization in the afterglow.
ANIMATION SLIDE-GIF
 Cycle Average
 10 mTorr
 40 mTorr
VHF 233 V
VLF 188 V
VHF 191 V
VLF 168 V
 CF4/O2=80/20, 200 sccm, PRF = 100 kHz
 LF 10 MHz, Pulsed HF 40 MHz
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RATIO OF FLUXES: CF4/O2, PRESSURE
Average Flux Ratio
 Flux ratios decrease as pressure decreases.
 Polymer layer thickness may be reduced with lower pressure in
the pulsed CCP.
6.0
5.0
4.0
3.0
2.0
CW
40 mTorr
10
P
P
CW
10
P
CW
1.0
P CW
40
10
P CW
P CW
0.0
F
F
 CF4/O2=80/20, 200 sccm
 LF = 10 MHz, 300 W
 HF = 40 MHz, 500 W
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40
P: Pulsed excitation
CW: CW excitation
O
O
Poly
POLY
PRF = 100 kHz, Duty-cycle = 25%
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CONCLUDING REMARKS
 Extension of tail of f(e) beyond CW excitation produces different
mix of fluxes.
 Ratios of fluxes are tunable using pulsed excitation.
 Different PRF provide different flux ratios due to different
relaxation time during pulse-off cycle.
 Duty cycle is another knob to control f(e) and flux ratios, but it is
limited if keep power constant
 Pressure provide another freedom for customizing f(e) and flux
ratios in pulsed CCPs.
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