Aspect Ratio Dependent Twisting and Mask Effects During Plasma Etching of SiO2 in Fluorocarbon Gas Mixture*

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Aspect Ratio Dependent Twisting and Mask Effects
During Plasma Etching of SiO2 in Fluorocarbon Gas
Mixture Mingmei Wang1 and Mark J. Kushner2
1Iowa State University, Ames, IA 50011
USA mmwang_at_iastate.edu 2University of Michigan,
Ann Arbor, MI 48109 USA mjkush_at_umich.edu http//u
igelz.eecs.umich.edu 55th AVS, October 2008,
Boston, MA
Work supported by the SRC, Micron Inc. and Tokyo
Electron Ltd.
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AGENDA

• Issues in high aspect ratio contact (HARC)
  etching.
• Approaches and Methodologies
• Electric field buildup due to charge deposition.
• Feature twisting trench to trench variation when
  etching at critical dimension (CD).
• High energy electron (HEE) effects on feature
  twisting in SiO2 etching over Si.
• Varied mesh resolution due to computing
  limitation.
• Photo resist sputtering and redeposition.
• Twisting and bowing during etch in features
  patterned with photo resist (PR) and hard mask
  (HM).
• Concluding Remarks

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CHALLENGES IN HARC ETCHING
Ref Oxford Instruments
Mask Erosion
Bowing
Twisting
Ref ULVAC Technologies
Ref JJAP, 46, p7873 (2007)

• Etched features for advanced micro-electronic
  devices have aspect ratios (AR) approaching 100.
• Twisting, bowing and consequences of mask erosion
  challenge maintaining CD.
• In this poster, results from a computational
  investigation of these processes are presented.

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HYBRID PLASMA EQUIPMENT MODEL (HPEM)

• Electromagnetics Module Antenna generated
  electric and magnetic fields.
• Electron Energy Transport Module Beam and bulk
  generated sources and transport coefficients.
• Fluid Kinetics Module Electron and Heavy
  Particle Transport.
• Plasma Chemistry Monte Carlo Module
• Ion, Higher Energy Electron (HEE) and Neutral
  Energy and Angular Distributions.
• Fluxes for feature profile model.

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MONTE CARLO FEATURE PROFILE MODEL

• Monte Carlo techniques address plasma surface
  interactions and evolution of surface profiles.
• Electric potential is solved using Successive
  Over Relaxation (SOR) method.

Ions, HEE, radicals and neutrals
Mask
SiO2
Polymer
Si
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SURFACE REACTION MECHANISM

• Etching of SiO2 is dominantly through a formation
  of a fluorocarbon complex.
• SiO2(s) CxFy(g) ? SiO2(s) CxFy(g)
• SiO2(s) CxFy(g) ? SiO2CxFy(s)
• SiO2CxFy (s) CxFy(g) ? SiFy(g) CO2 (g)
  CxFy(g)
• Further deposition by CxFy(g) produces thicker
  polymer layers.
• Sputtering of photo resist and redeposition.
• PR(s) CxFy(g) ? PR(g) CxFy(g)
• PR(g) SiO2CxFy(s) ? SiO2CxFy(s) PR(s)

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FLUOROCARBON ETCHING OF SIO2

• DC augmented single frequency capacitively
  coupled plasma (CCP) reactor.
• DC Top electrode RF Substrate

• Plasma tends to be edge peaked due to electric
  field enhancement.
• Plasma densities in excess of 1011 cm-3.
• Ar/C4F8/O2 80/15/5, 300 sccm, 40 mTorr, RF 1 kW
  at 10 MHz, DC 200 W/-250 V.

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10 MHz LOWER, DC UPPER PLASMA POTENTIAL

• LF electrode passes rf current. DC electrode
  passes combination of rf and dc current with
  small modulation of sheath potential.
• Ar, 40 mTorr, LF 10 MHz, 300 W, 440V/dc-250V
• DC 200 W, -470 V

ANIMATION SLIDE-GIF
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HIGH ENERGY ELECTRON (HEE) FLUXES

• HEE fluxes increase with increasing RF bias power
  due to increase in plasma density.
• 40 mTorr, RF 10 MHz, DC 200 W/-250 V, Ar/C4F8/O2
  80/15/5, 300 sccm

• HEE flux increases with increasing DC voltage.
• HEE is naturally generated by RF oscillation
  (when VDC0 V).
• 40 mTorr, RF 4 kW/1.5 kV at 10 MHz, Ar/C4F8/O2
  80/15/5, 300 sccm

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ION ENERGY ANGULAR DISTRIBUTIONS (IEADs)

• IEADs for sum of all ions.
• Peak in ion energy increases with increasing rf
  bias power while IEAD narrows.
• Higher energy ions increase maximum positive
  charging of feature.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF 10
  MHz, DC 200 W/-250 V.

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HEE ENERGY ANGULAR DISTRIBUTIONS

• HEE energy increases with increasing rf bias
  power.
• Narrower angular distribution (-20 20) than for
  ions.
• Peak at maximum energy with long tails.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF 10
  MHz, DC 200 W/-250 V.

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HEE EFFECTS ON TWISTING FINE MESH

• Atomic scale mesh size (3 Å).
• Ions hitting the surface deposit charge.
  Electrons may scatter. Statistical composition
  of fluxes into small features produces occasional
  twisting.
• Twisting occurs randomly without considering HEE
  (3/20).
• HEE neutralizes charge effectively deep into the
  trench.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF 1 kW
  at 10 MHz, DC 200 W.

Aspect Ratio 125
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HEE EFFECTS on TWISTING COARSE MESH

• Coarse mesh (5 nm) with photo resist erosion on
  the top.
• Bowing occurs at later stage of etching due to
  reflection from sloped profile of eroded PR.
• HEE fluxes improve feature profiles.
• Trench to trench differences due to small opening
  (75nm) to the plasma and statistican nature of
  fluxes.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF 5
  kW at 10 MHz.

Aspect Ratio 120
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HEE ENERGY ANGULAR DISTRIBUTIONS

• HEE energy increases with increasing DC voltage.
• Narrower angular distribution is obtained at high
  voltage with longer tails.
• At low energy region (lt500 eV), low DC voltage
  causes broader angular distribution and lower
  particle density.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF
  1.5 kV at 10 MHz.

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TWISTING ELIMINATION DC VOLTAGE

• Two group of profiles are selected from 21 cases
  with different random seed number generators.
• HEE neutralizes positive charge deep into the
  trench.
• Higher HEE energy and flux produce better
  profiles and higher etch rates
• VDC0 V, twisting probability7/21.
• VDC500 V, twisting probability5/21.
• VDC750 V, twisting probability3/21.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm,
  RF 1.5 kV at 10 MHz.

Aspect Ratio 120
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PHOTO RESIST SPUTTERING and PROFILE BOWING

• Time sequence of feature etching.
• Photo resist is eroded during process broadening
  view-angle to plasma.
• Bowing occurs at later stage of etching as
  view-angle and slope of PR increases.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF 5 kW
  at 10 MHz.

Aspect Ratio 130
ANIMATION SLIDE-GIF
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PHOTO RESIST SPUTTERING and PROFILE BOWING

• Time sequence of feature etching.
• Photo resist is eroded during process broadening
  view-angle to plasma.
• Bowing occurs at later stage of etching as
  view-angle and slope of PR increases.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF 5 kW
  at 10 MHz.

Aspect Ratio 130
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MASK MATERIAL EFFECTS

• Hard mask is not etched or sputtered easily.
• PR has an etching selectivity of 10 over SiO2.
• Bowing occurs at the middle height of trench with
  the hard mask.
• Bowing occurs right under the PR layer.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF 5 kW
  at 10 MHz.

(AR30)
(AR30)
(AR40)
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BOWING MECHANISM

• With hard mask, as etch depth increases, ions
  with a small incident angle hit the side wall.
• Statistical deposition of charge produces
  deflection of narrow angle ions.
• With photo resist etching, ions hitting PR
  surface reflect to the side wall of trench.
• 40 mTorr, Ar/C4F8/O2 80/15/5, 300 sccm, RF 5 kW
  at 10 MHz.

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PROPOSED METHODS OF BOWING ELIMINATION

• Many methods have been proposed to address
  bowing.

• Deposit a protective layer onto PR.
• Sputtering protective layer away at later stage
  of etching.

• Multiple layers of mask materials (upper PR,
  lower hard mask).

• Increase HEE flux and energy to further
  neutralize positive charge on trench bottom and
  side walls.
• Control ion energy as the etch proceeds to
  utilize selectivity difference between PR and
  SiO2 etching.

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CONCLUDING REMARKS

• HEE effects on eliminating twisting in HARC
  etching have been computationally investigated in
  fluorocarbon plasmas.
• Statistical nature of ion fluxes into small
  features produce lateral electric fields which
  deflect ions.
• HEE neutralizes positive charge deep into the
  trench to eliminate ion trajectory change and
  accelerate etching.
• Photo resist sputtering leads to bowing at top of
  feature profile.
• Bowing occurs at middle of feature in HARC
  (AR40) etching.

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