SiO2 ETCH PROPERTY CONTROL USING PULSE POWER IN CAPACITIVELY COUPLED PLASMAS*

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SiO2 ETCH PROPERTY CONTROL USING PULSE POWER IN
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.edu b)Department of Electrical
Engineering and Computer Science University of
Michigan, Ann Arbor, MI 48109, USA
mjkush_at_umich.edu http//uigelz.eecs.umich.edu Nov
. 2011 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/CF4/O2 pulsed plasma properties
• Etch rate with variable blocking capacitor
• Etch property with different PRF
• Etch rate, profile, and selectivity
• 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.

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ETCH RATE vs. FLUX RATIOS

• Large fluorine to ion flux ratio enhances etching
  yield of Si.
• Large fluorocarbon to ion flux ratio reduces
  etching yield of Si.

Etching Yield (Si/Ar)
Etching Yield (Si/Ar)
Flux Ratio (F/Ar)
Flux Ratio (CF2/Ar)
Ref D. C. Gray, J. Butterbaugh, and H. H. Sawin,
J. Vac. Sci. Technol. A 9, 779 (1991)
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ETCH PROFILE vs. FLUX RATIOS

• Large chlorine radical to ion flux ratio produces
  an undercut in etch profile.
• Etch profile result in ECR Cl2 plasma after 200
  over etch with different flux ratios

• Flux Ratio (Cl / Ion) 0.3

• Flux Ratio (Cl / Ion) 0.8

Ref K. Ono, M. Tuda, H. Ootera, and T. Oomori,
Pure and Appl. Chem. Vol 66 No 6, 1327 (1994)
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HYBRID PLASMA EQUIPMENT MODEL (HPEM)
Te, Sb, Seb, k
Fluid Kinetics Module Fluid equations (continuity,
momentum, energy) Poissons equation
Electron Monte Carlo Simulation
E, Ni, ne

• 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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MONTE CARLO FEATURE PROFILE MODEL (MCFPM)

• The MCFPM resolves the surface topology on a 2D
  Cartesian mesh.
• Each cell has a material identity. Gas phase
  species are represented by Monte Carlo
  pseuodoparticles.
• Pseuodoparticles are launched with energies and
  angles sampled from the distributions obtained
  from the HPEM
• Cells identities changed, removed, added for
  reactions, etching deposition.

HPEM
PCMCM
Energy and angular distributions for ions and
neutrals

• Poissons equation solved for charging

MCFPM
Etch rates and profile
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REACTOR GEOMETRY 2 FREQUENCY CCP

• 2D, cylindrically symmetric
• Ar/CF4/O2 75/20/5, 40 mTorr, 200 sccm
• Base conditions
• Lower electrode LF 10 MHz, 500 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 ON 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 50, 100, 200, 415, 625
  kHz
• Average Power 500 W

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VARIABLE BLOCKING CAPACITOR

• Due to the different area of two electrodes, a
  dc bias is produced on the blocking capacitor
  connected to the substrate electrode.
• The temporal behavior of dc bias is dependent
  on the magnitude of the capacitance due to RC
  delay time.

• We investigated variable blocking capacitor of 10
  nF, 1 mF, and 100 F
• 100 F of blocking capacitor results in NO dc
  bias on the substrate.

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Typical Plasma Properties
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PULSED CCP Electron Density Temperature

• Electron Density (x 1011 cm-3)

• Electron Temperature (eV)

• Pulsing with a moderate PRF duty cycle produces
  nominal intra-cycles changes in e but does
  modulate Te.

• 40 mTorr, Ar/CF4/O275/20/5
• PRF 100 kHz, Duty-cycle 25
• HF 40 MHz, pulsed 500 W
• LF 10 MHz, 250 V

ANIMATION SLIDE-GIF
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PULSED CCP ELECTRON SOURCES

• by Bulk Electrons (x 1014 cm-3 s-1)

• by Secondary Electrons

• The electrons have two groups bulk low energy
  electrons and beam-like secondary electrons.
• The bulk electron source is negative due to
  electron attachment and dissociative
  recombination.
• The electron source by beam electrons compensates
  the electron losses and sustains the plasma.

ANIMATION SLIDE-GIF

• 40 mTorr, Ar/CF4/O275/20/5
• LF 250 V, HF 500 W

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PULSED CCP E-SOURCES and f(e)

• Rate coefficient of e-sources is modulated
  between electron source (electron impact
  ionization) and loss (attachment and
  recombination) during pulsed cycle.

ANIMATION SLIDE-GIF

• 40 mTorr, Ar/CF4/O275/20/5
• PRF 100 kHz, Duty-cycle 25
• LF 10 MHz, 250 V
• HF 40 MHz, pulsed 500 W

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Etch PropertiesVariable Blocking Capacitor
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PULSED CCP PLASMA POTENTIAL dc BIAS

• A small blocking capacitor allows the dc bias
  to follow the change during the pulse period.
• Maximum ion energy gain Plasma Potential dc
  Bias

• 1 mF

• 10 nF

• PRF 100 kHz, Duty-cycle 25
• LF 10 MHz, 250 V
• HF 40 MHz, pulsed 500 W

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ETCH PROFILE IN SiO2 IEAD 1 mF

• With constant voltage, bias amplitude is constant
  but blocking capacitor determines dc bias.

• Cycle Average IEAD

• Etch Profile (600 sec)

Energy (eV)
Height (mm)
Angle (degree)
Width (mm)
ANIMATION SLIDE-GIF
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• Pulsed HF 40 MHz 500 W
• LF 10 MHz 250 V, Blocking Cap. 1 mF

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ETCH PROFILE IN SiO2 IEAD 10 nF

• With smaller blocking capacitor, dc bias begins
  to follow the rf power and so produces a
  different IEAD.

• Cycle Average IEAD

• Etch Profile (600 sec)

Energy (eV)
Height (mm)
Angle (degree)
Width (mm)
ANIMATION SLIDE-GIF
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• Pulsed HF 40 MHz 500 W
• LF 10 MHz 250 V, Blocking Cap. 1 nF

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ETCH PROFILE IN SiO2 IEAD NO dc BIAS

• In absence of dc bias and for constant voltage,
  pulse power and is effect on f(?) in large part
  determine etch properties.

• Cycle Average IEAD

• Etch Profile (600 sec)

Energy (eV)
Height (mm)
Angle (degree)
Width (mm)
ANIMATION SLIDE-GIF
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• Pulsed HF 40 MHz 500 W
• LF 10 MHz 250 V, Blocking Cap. 100 F

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POWER NORMALIZED ER Blocking Capacitor

• Power normalized etch rate is dependent not only
  on the pulse repetition frequency (PRF), but also
  the value of the blocking capacitor on the
  substrate at lower PRF.

• F to Poly Flux ratio

C
B
A
CW 250 100 50 kHz
B
C
A

• Pulsed HF 40 MHz 500 W
• LF 10 MHz 250 V

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E-SOURCES and FLUX RATIO PRF

• Electron source rate coefficient is modulated
  with f(e) by pulse power.
• Modulation is enhanced with smaller PRF.

• F to Poly Flux ratio

• Pulsed HF 40 MHz 500 W
• LF 10 MHz 250 V
• Blocking Cap. 1 mF

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ETCH RATE POWER NORMALIZED

• Power normalized etch rate is large at 250 kHz
  with ion distribution extending to higher
  energies.

• Cycle Average IEAD

• Normalized Etch Rate

Energy (eV)
CW 250 100 50 kHz
Angle (degree)

• Pulsed HF 40 MHz 500 W
• LF 10 MHz 250 V
• Without DC Bias on LF electrode

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ETCH PROFILE CRITICAL DIMENSION

• CD is compared at the middle and bottom of
  feature.
• CW excitation produces bowing and an undercut
  profile.
• Pulse plasma helps to prevent the bowing and
  under-cutting.
• Smaller PRF has a tapered profile.

• EPD Over Etch 50

A
(1/A)
1
(2/A)
CW 250 100 50 kHz
2

• Pulsed HF 40 MHz 500 W
• LF 10 MHz 250 V
• Blocking Cap. 1 mF

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ETCH SELECTIVITY Between SiO2 and Si

• Silicon damage depth is compared in 2-D etch
  profile.
• Pulsed operation helps to prevent the silicon
  damage.
• Lower damage appears to be correlated with
  smaller F flux ratio at 250 kHz.

• EPD Over Etch 50

CW 250 100 50 kHz

• Pulsed HF 40 MHz 500 W
• LF 10 MHz 250 V
• Blocking Cap. 1 mF

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CONCLUDING REMARKS

• Extension of tail of f(e) beyond that obtained
  with CW excitation produces a different mix of
  fluxes to substrate.
• Etch rate can be controlled by pulsed operation
  with different pulse repetition frequencies.
• Blocking capacitor is another variable to control
  ion energy distributions and etch rates. Smaller
  capacitance allows dc bias to follow the plasma
  potential in pulse period more rapidly.
• Etch rate is enhanced by pulsed power operation
  in CCP.
• Etch profile is improved with pulsed operation
  preventing undercut.
• Etch selectivity of SiO2 to Si is also improved
  with PRF of 250 kHz with a smaller fluorine flux
  ratio.

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