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 ssongs_at_umich.ed
u b)Department of Electrical Engineering and
Computer Science University of Michigan, Ann
Arbor, MI 48109, USA mjkush_at_umich.edu http//uige
lz.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(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(?)

• Controlling the generation of reactive species
  for technological devices benefits from
  customizing the electron energy (velocity)
  distribution function.

• Need SiH3 radicals

• LCD

• Solar Cell

Ref Tatsuya Ohira, Phys. Rev. B 52 (1995)
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
Te, S, k
Fluid Kinetics Module Fluid equations (continuity,
momentum, energy) Poissons equation
Electron Monte Carlo Simulation
E, Ni, ne, Ti

• Fluid Kinetics Module
• Heavy particle and electron continuity, momentum,
  energy
• Poissons 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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• 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.

Pmax
Power(t)
Duty Cycle
Pmin
Time
? 1/PRF

• 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(?).
• LF 10 MHz, 300 W
• HF 40 MHz, pulsed 500 W
• PRF 100 kHz, Duty-cycle 25

ANIMATION SLIDE-GIF

• e

VHF 226 V VLF 106 V
f(e)

• 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

• Duty cycle 25

• Cycle Average

• Duty cycle 50

VHF 128 V VLF 67 V
VHF 226 V VLF 106 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

• 10 mTorr

• Cycle Average

• 40 mTorr

VHF 226 V VLF 106 V
VHF 274 V VLF 146 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.
• 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.

• PRF415 kHz

• PRF100 kHz

• 40 mTorr, CF4/O280/20, 200 sccm
• LF 10 MHz, 300 W
• HF 40 MHz, 500 W (CW or pulse)

ANIMATION SLIDE-GIF
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• CW

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.
• Only at the start of the pulse-on cycle, is there
  a positive electron source due to the overshoot
  of E/N.

• PRF415 kHz

• PRF100 kHz

• 40 mTorr, CF4/O280/20, 200 sccm
• LF 300 W, HF 500 W

ANIMATION SLIDE-GIF
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• CW

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.
• The electron source by beam electrons compensates
  the electron losses and sustains the plasma.

• PRF415 kHz

• PRF100 kHz

• 40 mTorr, CF4/O280/20, 200 sccm
• LF 10 MHz, 300 W
• HF 40 MHz, 500 W (CW or pulse)

ANIMATION SLIDE-GIF
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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.
• 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.

VHF 226 V VLF 106 V
VHF 203 V VLF 168 V
ANIMATION SLIDE-GIF

• 40 mTorr, 200 sccm
• LF 10 MHz, 300 W
• HF 40 MHz, 500 W (25 dc)

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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

• Flux Ratios
• Poly (CF3CF2CFC) / 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.
• Extension of tail of f(e) beyond CW excitation
  during pulsing produces different excitation and
  ionization rates, and different mix of fluxes to
  wafer.

VHF 203 V VLF 168 V
ANIMATION SLIDE-GIF

• 40 mTorr, CF4/O280/20, 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.
• Polymer layer thickness may be reduced by pulsed
  excitation because poly to ion flux ratio
  decreases.

CW
100
CW
100
415
415 kHz
100
CW
415
F O Poly

• 40 mTorr, CF4/O280/20, 200 sccm, Duty-cycle
  25
• LF 10 MHz, 300 W
• HF 40 MHz, 500 W

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f(e) CF4/O2, DUTY CYCLE

• Control of average f(?) over with changes in duty
  cycle is limited if keep power constant.

ANIMATION SLIDE-GIF

• Duty cycle 25

• Cycle Average

• Duty cycle 50

VHF 191 V VLF 168 V
VHF 203 V VLF 168 V

• 40 mTorr, CF4/O280/20, 200 sccm
• LF 10 MHz, Pulsed HF 40 MHz, PRF 100 kHz

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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.

CW
50
25
50
CW
25
50
25
CW
F O Poly

• LF 10 MHz, Pulsed HF 40 MHz, PRF 100 kHz
• 40 mTorr, CF4/O280/20, 200 sccm

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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

• 10 mTorr

• Cycle Average

• 40 mTorr

VHF 191 V VLF 168 V
VHF 233 V VLF 188 V

• CF4/O280/20, 200 sccm, PRF 100 kHz
• LF 10 MHz, Pulsed HF 40 MHz

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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.

40
P Pulsed excitation CW CW excitation
CW
40 mTorr
P
CW
10
P
10
P
CW
CW
P
40
10
P
CW
CW
P
F O Poly

• CF4/O280/20, 200 sccm
• LF 10 MHz, 300 W
• HF 40 MHz, 500 W

• 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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