Transcript PPT
CONTROLLING ION AND UV/VUV PHOTON
FLUXES IN PULSED LOW PRESSURE
PLASMAS FOR MATERIALS PROCESSING*
Peng Tian and Mark J. Kushner
University of Michigan, Ann Arbor, MI 48109 USA
[email protected], [email protected]
65th Gaseous Electronics Conference 2012, Austin, Texas, USA
* Work supported by Semiconductor Research Corporation, DOE Office of Fusion
Energy Science and National Science Foundation
GEC 2012 P.T.
AGENDA
UV/VUV photons during plasma material processing
Description of model
Pulsed Plasma
Photon/Ion flux ratios onto wafer surface in ICP
Duty cycle
Aspect ratio of the reactor
Gas pressure
Photon/Ion flux ratios onto wafer surface in CCP
Concluding Remarks
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
PHOTONS IN PLASMA PROCESSING
UV/VUV photon fluxes are ubiquitous in low pressure
plasma material processing.
The consequences of UV/VUV fluxes and the possible
synergies with other reactive species are just now
becoming apparent.
Controlling UV/VUV fluxes, or their relative values
compared to other reaction fluxes, may be important in
controlling these synergistic interactions.
In this talk, we discuss methods to control relative fluxes
of UV/VUV photons using pulsed plasmas.
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
SYNERGISTIC EFFECTS ON PR ROUGHNESS
The roughening of photo-resist (PR) is important to maintaining
critical dimensions.
PR roughening is
weakly dependent on
substrate temperature
with only ion
bombardment.
When adding VUV
fluxes of 1-10% of the
ion fluxes, roughening
was quite sensitive to
temperature.
D. Nest, D. B. Graves, S. Engelmann, R. L. Bruce, F. Weilnboech et al.
Appl. Phys. Lett. 92, 153113 (2008)
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
PHOTON PROMOTED ETCHING
Recent observations of VUV sustained etching in Cl plasmas below
accepted ion energy threshold.
Need to control ion and
photon fluxes – perhaps
independently.
University of Michigan
Institute for Plasma Science & Engr.
PULSED PLASMAS
Carrier frequency is modulated by a pulse, with controllable
duty cycle, pulse repetition frequency and fixed average power
over a period.
The fast rising edge can “over-shoot” the self sustaining E/N,
raising the “hot tail” in EEDF f().
Controllable rate coefficients compared to CW excitation
1/ 2
2
kCW f CW , t d
0
0
me
1
2
f
,
t
dt
0 Pulsed
m d k ave pulsed
e
Pmax
Power(t)
1/ 2
Duty Cycle
Pave
Pt dt
1
0
Pmin
= 1/
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Time
University of Michigan
Institute for Plasma Science & Engr.
HYBRID PLASMA EQUIPMENT MODEL
r
E r , ,
Br , z r ,
je r ,
k r , Te r
S r
Er , z r , N i r ,
ne r , Ti r
Anatural
Atrapped
Surface
Chemistry
Module
The Hybrid Plasma Equipment Model (HPEM) is a modular
simulator that combines fluid and kinetic approaches.
Radiation transport is addressed using a spectrally resolved
Monte Carlo simulation.
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
RADIATION TRANSPORT MODEL IN HPEM
Frequency resolved radiation transport in HPEM is modeled using a
Monte Carlo simulation that accounts for radiation trapping
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ATOMIC MODELS FOR Ar/Cl2
Threshold energy for
excitation and
ionization has a
notable difference.
With customized EEDF
f(), separate control
over ion and photon
fluxes is possible.
e + Ar(3s) e + Ar(1s2,3,4,5)
(11.55~11.83 eV)
e + Ar(3s) e + e + Ar+
(16.0 eV)
e + Cl(3s23p5) e + Cl(4s,4p,3d)
e + Cl(3s23p5) e + e + Cl+
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(8.90~10.90 eV)
(12.99 eV)
University of Michigan
Institute for Plasma Science & Engr.
ICP
ICP-f(), Te, PHOTON FLUX– TIME VARIATION
The tail of EEDF f() has been raised during pulse-on period.
f()
Te
VUV Photon
= cycle average
Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, duty cycle = 15%
MIN
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MAX
Animation Slide
University of Michigan
Institute for Plasma Science & Engr.
ICP-AVERAGED DENSITIES AND FLUXES
Excited state density and photon flux decay faster than ions. Even with
radiation trapping, excited states still have a shorter lifetime than ions.
Neutral and Ion Density
Total Ion and Photon Flux
(Smoothed data)
Ar/Cl2 = 80/20, 20 mTorr, 200 sccm, 10 MHz, 150 W, 15% DC, 50 kHz PRF
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
ICP-DUTY CYCLE
ICP-DUTY CYCLE – CUSTOMIZED f()
Animation Slide
= cycle average
f()
Te
With smaller duty cycle, a fairly hotter tail of EEDF can be generated.
Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, 15%, 35%, 55% DC
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
ICP-ION & PHOTON FLUX vs DUTY CYCLE
Longer lifetime of ions and lowered ambipolar diffusion during after glow
enables ion flux to stay at a relatively constant value.
With lower “over-shoot” E/N in larger duty cycle pulses, maximum photon
flux decreases with increasing duty cycle. The flux decays fast due to
shorter lifetime of excited states.
Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
ICP-PHOTON/ION FLUX vs DUTY CYCLE
Varying duty cycle provides control over coincidence of photon and
ion fluxes, and cycle averaged fluxes to substrate.
10 MHz, 150 W, Ar/Cl2 = 80/20, 20 mTorr, 200 sccm, 50 kHz PRF
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
ICP-ASPECT RATIO OF
REACTOR
ICP-ASPECT RATIO – TRANSPORT OF IONS vs PHOTONS
Although moderately trapped, the transport of
Ar resonance radiation is significantly less
diffusive than ions.
Animation Slide
Since sources are near the coils, aspect ratio
can be used to discriminate fluxes.
Ar/Cl2 = 80/20, 20 mTorr,
150 W, 50 kHz, duty cycle
= 15%
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MIN
MAX
University of Michigan
Institute for Plasma Science & Engr.
ICP-ION & PHOTON FLUX vs ASPECT RATIO
The ion fluxes are more sensitive to aspect ratio. The more
diffuse nature cause it decrease with greater height.
Photon fluxes traverse the reactor in a more “ballistic” way
and are less sensitive to height.
Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, 15% DC
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
ICP-PHOTON/ION FLUX vs ASPECT RATIO
Photon flux ratio can be controlled by varying aspect
ratio (height).
The ratio is more sensitive in pulsed plasmas.
Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, 15% DC
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
ICP-PRESSURE
ICP-PRESSURE – f() AND TRANSPORT OF IONS
= cycle average
Te
f()
Keeping power constant, higher pressure will decrease E/N value,
thus a “cooler” EEDF f().
Ar/Cl2 = 80/20, 20 mTorr, 150 Wave, 50 kHz, 15%, 35%, 55% DC
Animation Slide
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
ICP-ION & PHOTON FLUX vs PRESSURE
Lower rate of diffusion combined with more volumetric losses due
to ion-ion and e-ion recombination produces a decrease in flux.
A “cooler” EEDF f() leads to a decrease in photon flux.
Ar/Cl2 = 80/20, 150 Wave, 50 kHz, 15% DC
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
ICP-PHOTON/ION FLUX vs PRESSURE
Varying pressure provides another means of controlling the ratio
of photon to ion flux.
Ar/Cl2 = 80/20, 150 Wave, 50 kHz, 15% DC
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
CCP
CCP-f(), Te, PHOTON FLUX– TIME VARIATION
f()
1.6 cm
Te
VUV Photon
Ar/Cl2 = 80/20, 500 Wave, 100 kHz, 15% DC
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Stochastic heating
provide higher Te
near wafer (sheath).
University of Michigan
Institute for Plasma Science & Engr.
CCP-PHOTON/ION FLUX vs DUTY CYCLE
Ion Flux
Photon Flux
During after-glow, with thin sheath
and low plasma potential, photon
flux is higher than ion flux.
Ar/Cl2 = 80/20, 500 Wave, 100 kHz,
15% DC
Ratio
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
CCP-PHOTON/ION FLUX vs DUTY CYCLE
Similar to ICP, the flux ratio increases as duty cycle decreases.
However, the peak shifts with the falling edge of pulse.
Ar/Cl2 = 80/20, 500 Wave, 100kHz,
Varying DC
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
CCP-PHOTON/ION FLUX vs FREQUENCY
Flux ratio can also be controlled by RF frequency. However, the
difference is not large.
Ar/Cl2 = 80/20, 500 Wave, 100kHz,
Varying RF
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.
CONCLUDING REMARKS
A possible method to separately control photon/ion fluxes by
pulsed RF power in inductively coupled plasmas and
capacitively coupled plasmas has been investigated by
computational approach.
Photon/ion fluxes ratio to wafer surface in RIE reactors can be
controlled as they have different reaction rate to pulsed power
deposition.
In pulsed ICPs, photon/ion fluxes ratio can be controlled by
varying duty cycle, gas pressure and aspect ratio of the reactor.
In pulsed CCPs, photon/ion fluxes ratio can be controlled by
varying duty cycle and RF frequency.
GEC 2012 P.T.
University of Michigan
Institute for Plasma Science & Engr.