WAFER EDGE EFFECTS CONSIDERING ION INERTIA IN CAPACITIVELY COUPLED DISCHARGES*

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WAFER EDGE EFFECTS CONSIDERING ION INERTIA IN
CAPACITIVELY COUPLED DISCHARGES Natalia Yu.
Babaeva and Mark J. Kushner Iowa State
University Department of Electrical and Computer
Engineering Ames, IA 50011, USA
natalie5_at_iastate.edu mjk_at_iastate.edu
http//uigelz.ece.iastate.edu June 2006
Work supported by Semiconductor Research Corp.
and NSF
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AGENDA

• Wafer Edge effects and their origin.
• Description of the model
• Improvement of nonPDPSIM to include ion momentum
  equation
• Effect of wafer-focus ring gaps on Ar and Ar/Cl2
  CCPs
• Plasma penetration
• Ion focusing
• Concluding remarks

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WAFER EDGE EFFECTS

• It is desirable to use wafer area to the edge of
  the wafer to maximize utilization.
• Perturbation of fluxes may occur by method of
  terminating wafer and matching to tool material
• Wafer is beveled at edge with small gap (lt 1 mm)
  between wafer and focus ring.
• Penetration of plasma into gap is bad due to
  formation of particles and deposition of
  contaminating films.

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ION MOMENTUM EQUATION IN nonPDPSIM

• Goal is to computationally investigate edge
  effects and penetration of plasma into
  wafer-focus ring gap.
• Large dynamic range (gt 100) requires unstructured
  mesh.
• Large Knudson number in gap requires accounting
  for inertia.
• nonPDPSIM, a 2-dimensional plasma hydrodynamics
  model, was improved by adding ion momentum
  equations on unstructured mesh.
• The coupling between the dynamics of charged and
  neutral transport is through the species resolved
  collision terms in momenta equations.

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nonPDPSIM CHARGE PARTICLE TRANSPORT

• Poisson equation for the electric potential
• Transport equations for conservation of the
  charged species j
• Surface charge balance
• Full momentum for ion fluxes of species j
• Equations are simultaneously solved using a
  Newtons iterations.

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2-D GEOMETRY AND CONDITIONS

• Conditions
• Ar, 90 mTorr, 300 sccm, 500 V
• Ar/Cl2 70/30, 90 mTorr, 300 sccm, 500 V
• Biased substrate, grounded housing
• Showerhead to wafer distance 4 cm
• Transport of energetic secondary electrons from
  biased substrate is addressed with a Monte Carlo
  simulation.

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MESHING TO RESOLVE WAFER-FOCUS RING GAP

• Unstructured mesh with multiple refinement zones
  was used to resolve wafer-focus ring gap.
• Gaps of lt 1 mm were investigated.

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ELECTRON DENSITY NEAR THE GAPS
? 0.9 mm Gap
? 0.3 mm Gap
106 108 cm-3
106 108 cm-3

• Electron penetration into the gaps is nominal due
  to surface charging and sheath formation.
• Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V

Electrons (106 3 x109 cm-3)
Animation slide
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EDGE REGION NEGATIVE CHARGE
? 0.9 mm Gap
? 0.3 mm Gap

• Negative charging of wafer surface (and focus
  ring) extends beyond edge of bevel in large gap.
• Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V

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EDGE REGION IONS
? 0.9 mm Gap
? 0.3 mm Gap
106 3x108 cm-3
108 3 x108 cm-3

• Ions are modulated by 10 MHz e-field variation.
• Ions penetrate into the large gap reaching the
  biased substrate.
• Ions do not penetrate into the small gap but do
  respond to sentinal surface charge.
• Ar, 90 mTorr, 10 MHz,300 sccm, 500 V

Animation slide
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EDGE REGION ELECTRON TEMPERATURE
? 0.9 mm Gap
? 0.3 mm Gap

• Te is higher near the small gap due to
  overlapping os sheaths and higher local electric
  fields.
• Electron temperature (and electron density) is
  negligibly small inside the gaps.
• Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V

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Ar/Cl2 DISCHARGE
e
Cl2
Cl-
Ar

• Maximum electron density shifts towards the focus
  ring.
• Negative ion density comparable to electron
  density, though are trapped in the plasma bulk
  and do not reach the wafer
• Ar/Cl2 85/15, 90 mTorr, 300 sccm, 500 V

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EDGE REGION Ar AND Cl2 FLUXES
? 0.9 mm Gap
? 0.9 mm Gap

• Cl2 flux is larger and less collisional than Ar
  due to lower rate of charge exchange.
• There is some focusing of flux to the corner of
  the bevel that could lead to excessive heating
  and sputtering.
• Some ion trajectories terminate on the lower
  bevel.
• Ar/Cl2 85/15, 90 mTorr, 300 sccm, 500 V

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EDGE REGION Ar AND Cl2 FLUXES
? 0.3 mm Gap
? 0.3 mm Gap

• Less focusing of ion fluxes to corner of bevel
  occurs with the smaller gap due to lack of
  charging of wafer into wafer-focus ring cavity.
• Ar/Cl2 85/15, 90 mTorr, 300 sccm, 500 V

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EDGE REGION Ar FLUX STREAMTRACES
? 0.3 mm Gap
? 0.3 mm Gap

• Streamlines penetrate into large gap throughout
  rf cycle.
• In small gap, momentary penetration occurs at
  peak of cathode cycle. Slightly conductive wafer
  is able to dissipate that charge.
• Ar/Cl2 85/15, 90 mTorr, 300 sccm, 500 V

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Animation slide
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EDGE REGION Cl2 FLUX STREAMTRACES
? 0.9 mm Gap
? 0.3 mm Gap

• Focusing of ion flux streamlines to edge of wafer
  is more severe for Cl2 than Ar due to lower
  collisionality.
• Periodic flux into gap is also larger.
• Ar/Cl2 85/15, 90 mTorr, 300 sccm, 500 V

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Animation slide
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CONCLUDING REMARKS

• Penetration of plasma into narrow wafer-focus
  ring gap of a capacitively coupled discharge was
  computationally investigated.
• Gap sizes gt 0.5 mm allow significant penetration
  of the plasma.
• Charging and ion fluxes may penetrate to bottom
  side of bevel.
• Focusing of ion flux to the corner of the bevel
  depends on the ion species and collisionality
  chemically enhanced sputtering is problematic.

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