CONTROL OF ELECTRON ENERGY DISTRIBUTIONS AND FLUX RATIOS IN PULSED CAPACITIVELY COUPLED PLASMAS *
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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
b)Department of Electrical Engineering and Computer ScienceUniversity of Michigan, Ann Arbor, MI 48109, USA
http://uigelz.eecs.umich.edu
Oct 2010 AVS
* Work supported by DOE Plasma Science Center and Semiconductor Research Corp.
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AGENDA
Motivation for controlling f() Description of the model Typical Ar pulsed plasma properties Typical CF4/O2 pulsed plasma properties
f() and flux ratios with different PRF Duty Cycle Pressure
Concluding Remarks
University of MichiganInstitute for Plasma Science & Engr.
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CONTROL OF ELECTRON KINETICS- f() Controlling the generation of reactive species for technological
devices benefits from customizing the electron energy (velocity) distribution function.
University of MichiganInstitute for Plasma Science & Engr.
, , , , ,, , ,
df v r t qE r t f v r tv f r v f v r t
dt m tx ve c
1 2
0
2, , ,ije
k r t f r t dm
,
,,k
e ij ji j
dN r tn k r t N
dt
e + SiH4 SiH3 + H + ek
LCD Solar Cell Need SiH3 radicals*
* Ref: Tatsuya Ohira, Phys. Rev. B 52 (1995)
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
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
E, Ni, ne, Ti
Fluid Kinetics ModuleFluid equations
(continuity, momentum, energy)Poisson’s equation
Te, S, kElectron Monte Carlo Simulation
University of MichiganInstitute for Plasma Science & Engr.
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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
Time = 1/PRF
Duty Cycle
Power(t)
Pmin
0
1 dttPPave
Pmax
University of MichiganInstitute for Plasma Science & Engr.
Use of pulse power provides a means for controlling f(). 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.
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
University of MichiganInstitute for Plasma Science & Engr.
Pulsing with a PRF and moderate duty cycle produces nominal intra-cycles changes [e] but does modulate f().
LF = 10 MHz, 300 W HF = 40 MHz, pulsed 500 W PRF = 100 kHz, Duty-cycle = 25% [e]
Te
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MIN MAX
f()VHF 226 VVLF 106 V
ANIMATION SLIDE-GIF
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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.
University of MichiganInstitute for Plasma Science & Engr.
Duty cycle = 25% Cycle Average Duty cycle = 50%
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LF 10 MHz, pulsed HF 40 MHz PRF = 100 kHz, Ar 40 mTorr
VHF 128 VVLF 67 V
VHF 226 VVLF 106 V
ANIMATION SLIDE-GIF
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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.
University of MichiganInstitute for Plasma Science & Engr.
10 mTorr Cycle Average 40 mTorr
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LF 10 MHz, pulsed HF 40 MHz PRF = 100 kHz
VHF 226 VVLF 106 V
VHF 274 VVLF 146 V
ANIMATION SLIDE-GIF
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CF4/O2
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ELECTRON DENSITY CW
At 415 kHz, the electron density is not significantly modulated by pulsing, so the plasma is quasi-CW.
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.
University of MichiganInstitute for Plasma Science & Engr.
PRF=415 kHz
PRF=100 kHz
MIN MAX
ANIMATION SLIDE-GIF
40 mTorr, CF4/O2=80/20, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W (CW or pulse)
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ELECTRON SOURCES BY BULK ELECTRONS
University of MichiganInstitute for Plasma Science & Engr.
CW
PRF=415 kHz
PRF=100 kHz
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.
Only at the start of the pulse-on cycle, is there a positive electron source due to the overshoot of E/N.
MIN MAX
40 mTorr, CF4/O2=80/20, 200 sccm LF 300 W, HF 500 W
ANIMATION SLIDE-GIFSHS_MJK_AVS2010_13
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ELECTRON SOURCES BY BEAM ELECTRONS
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CW
PRF=415 kHz
PRF=100 kHz
MIN MAX
40 mTorr, CF4/O2=80/20, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W (CW or pulse)
The beam electrons result from secondary emission from electrodes and acceleration in sheaths.
The electron source by beam electron is always positive.
The electron source by beam electrons compensates the electron losses and sustains the plasma.
ANIMATION SLIDE-GIFSHS_MJK_AVS2010_14
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TYPICAL f(): CF4/O2 vs. Ar
Less Maxwellian f() with CF4/O2 due to lower e-e collisions.
Enhanced sheath heating with CF4/O2 due to lower plasma density.
Tail of f() comes up to compensate for the attachment and recombination that occurs at lower energy.
CF4/O2 Ar
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40 mTorr, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W (25% dc)
VHF 226 VVLF 106 V
VHF 203 VVLF 168 V
ANIMATION SLIDE-GIF
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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
RATIO OF FLUXES: CF4/O2
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Flux Ratios: Poly = (CF3+CF2+CF+C) / Ions O = O / Ions F = F / Ions
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f(): CF4/O2, PRF Average
The time averaged f() for pulsing is similar to CW excitation.
Extension of tail of f() beyond CW excitation during pulsing produces different excitation and ionization rates, and different mix of fluxes to wafer.
University of MichiganInstitute for Plasma Science & Engr.
PRF = 100 kHz
40 mTorr, CF4/O2=80/20, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W (25% dc)
ANIMATION SLIDE-GIF
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VHF 203 VVLF 168 V
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0.0
1.0
2.0
3.0
4.0
5.0
6.0
F O POLY
Ave
rage
Flu
x R
atio
CW100
415 kHz
CW100415
CW
100
415
University of MichiganInstitute for Plasma Science & Engr.
RATIO OF FLUXES: CF4/O2, PRF Ratios of fluxes are tunable using pulsed excitation. Polymer layer thickness may be reduced by pulsed excitation
because poly to ion flux ratio decreases.
F O Poly 40 mTorr, CF4/O2=80/20, 200 sccm, Duty-cycle = 25% LF = 10 MHz, 300 W HF = 40 MHz, 500 W
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f(): CF4/O2, DUTY CYCLE Control of average f() over with changes in duty cycle is limited if
keep power constant. ANIMATION SLIDE-GIF
University of MichiganInstitute for Plasma Science & Engr.
40 mTorr, CF4/O2=80/20, 200 sccm LF 10 MHz, Pulsed HF 40 MHz, PRF = 100 kHz
Duty cycle = 25% Cycle Average Duty cycle = 50%
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VHF 191 VVLF 168 V
VHF 203 VVLF 168 V
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0.0
1.0
2.0
3.0
4.0
5.0
6.0
F O POLY
Ave
rage
Flu
x R
atio
RATIO OF FLUXES: CF4/O2, DUTY CYCLE Flux ratio control is limited if keep power constant. With smaller duty cycle, polymer flux ratio is more reduced
compared to the others.
University of MichiganInstitute for Plasma Science & Engr.
F O Poly
50%25%
50%25%
50%
25%
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LF 10 MHz, Pulsed HF 40 MHz, PRF = 100 kHz 40 mTorr, CF4/O2=80/20, 200 sccm
CW
CW
CW
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f(): CF4/O2, PRESSURE Pulsed systems are sensitive to pressure due to differences in the
rates of thermalization in the afterglow. ANIMATION SLIDE-GIF
University of MichiganInstitute for Plasma Science & Engr.
CF4/O2=80/20, 200 sccm, PRF = 100 kHz LF 10 MHz, Pulsed HF 40 MHz
10 mTorr Cycle Average 40 mTorr
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VHF 191 VVLF 168 V
VHF 233 VVLF 188 V
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0.0
1.0
2.0
3.0
4.0
5.0
6.0
F O POLY
Ave
rage
Flu
x R
atio
RATIO OF FLUXES: CF4/O2, PRESSURE Flux ratios decrease as pressure decreases. Polymer layer thickness may be reduced with lower pressure in
the pulsed CCP.
University of MichiganInstitute for Plasma Science & Engr.
F O Poly
10
40 mTorr
1040
10
40
CF4/O2=80/20, 200 sccm LF = 10 MHz, 300 W HF = 40 MHz, 500 W
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CW
CW
CW CW
CW
CWP
P
P
P
P
PRF = 100 kHz, Duty-cycle = 25%
P
P: Pulsed excitation CW: CW excitation
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CONCLUDING REMARKS
Extension of tail of f() 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() and flux ratios, but it is
limited if keep power constant Pressure provide another freedom for customizing f() and flux
ratios in pulsed CCPs.
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