ION ENERGY AND ANGULAR DISTRIBUTIONS INTO SMALL FEATURES IN PLASMA ETCHING REACTORS:

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ION ENERGY AND ANGULAR DISTRIBUTIONS INTO SMALL
FEATURES IN PLASMA ETCHING REACTORS THE WAFER-
FOCUS RING GAP 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 AV
S 54th International Symposium October 2007
Work supported by Semiconductor Research Corp.,
Applied Materials and NSF
AVS2007_Natalie_01
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AGENDA

• Wafer edge effects
• Description of the model
• Ion energy and angular distribution on different
  surfaces in wafer-focus ring gap for focus ring
• Capacitance
• Height
• Conductivity
• Concluding remarks

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PENETRATION OF PLASMA INTO WAFER-FOCUS RING GAP

• Gap (lt 1 mm) between wafer and focus ring in
  plasma tools for mechanical clearance.
• Beveled wafers allow for under wafer
  plasma-surface processes.
• Penetration of plasma into gap can deposit of
  contaminating films.

• Orientation of electric field and ion
  trajectories, energy and angular distributions
  depend on details of the geometry and materials.

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INVESTIGATION OF IEADs INTO WAFER-FOCUS RING GAP

• The ion energy and angular distributions (IEADs)
  into the wafer-focus ring gap are important
• Angular distribution determines erosion (e.g.,
  maximum sputtering at 60o.
• Time between replacement of consumable parts
  depends on erosion.
• Spacing, materials (e.g., dielectric constant,
  conductivity) determine electric field in gap and
  so IEADS.
• In this presentation, results from a
  computational investigation of IEADs onto
  surfaces in wafer-focus ring gap will be
  discussed.
• Model nonPDPSIM using unstructured meshes.
• Goal How does one control the IEADs?

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nonPDPSIM BASIC EQUATIONS

• Poisson equation Electric potential
• Transport of charged species j
• Surface charge balance
• Full momentum for ion fluxes
• Neutral transport Navier-Stokes equations.
• Improvements to include Monte Carlo simulation of
  Ion Energy and Angular distributions (IEADs).

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MESHING TO RESOLVE FOCUS RING GAP

• 2-dimensional model using an unstructured mesh to
  resolve wafer-focus ring gaps of lt 1 mm.
• Numbering indicates materials and locations on
  which IEADs are obtained.
• Ar, 10 MHz, 100 mTorr, 300 V, 300 sccm

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POTENTIAL, ELECTRIC FIELD, IONS
Potential

• Off-axis maximum in Ar is due to electric
  field enhancement near focus ring and is
  uncorrelated to gap.
• Ar, 10 MHz, 100 mTorr, 300 V
• Gap 1 mm

E/N
Ar
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POTENTIAL AND CHARGES (RF CYCLE)
? 1.0 mm Gap
? Surface Charges
? Cycle averaged potential
-1.1 x 1011 cm-3
Powered Electrode
Powered Electrode

• Highly conductive wafer with small capacitance
  charges and discharges rapidly.
• Focus ring acquires larger negative surface
  charges.
• Large potential drop in focus ring.

Animation Slide
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ION FLUX VECTORS (RF CYCLE)
? 1.0 mm Gap
Powered Electrode

• Directions of electric fields near surfaces
  evolve slowly during rf cycle due to slowly
  changing surface charge.
• Direction of ion fluxes changes during rf cycle
  from nearly vertical to perpendicular to surface
  with transients in electric field.

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Animation Slide
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ION ENERGY AND ANGULAR DISTRIBUTIONS

• Broad IEAD on top bevel due to ions arriving
  during positive and negative parts of rf cycle.
• Grazing angles for ions striking vertical
  surfaces.

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ION FLUXES AT DIFFERENT PHASE OF RF CYCLE
? Cathodic rf cycle
? 1.0 mm Gap

• Cathodic cycle
• High energy ions at grazing incident on side
  wall.
• Near vertical to bevel.
• Anodic rf cycle
• Low energy ions near vertical on side wall.
• High energy angles a large angle to bevel.

5
9
? Anodic rf cycle
5
9
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CAPACITANCE OF FOCUS RING ION DENSITY AND CHARGES

• Wafer charges quickly (almost anti-phase with
  focus ring).
• More surface charges collected on focus ring with
  larger capacitance.
• Ions penetrate into gap throughout rf cycle with
  larger capacitance.

? 1.0 mm Gap
-7.8 x 1010 cm-3
-1.2 x 1011 cm-3
Powered electrode
Powered electrode
Ar
Ar
Powered electrode
Powered electrode
Animation Slide
? 0.5 mm Gap
? ?/?o 4
? ?/?o 20
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CAPACITANCE OF FOCUS RING IEAD
? ?/?o 4

• Penetration of potential into focus ring with low
  capacitance produces lateral E-field.
• IEAD on substrate is asymmetric.

? ?/?o 20
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FOCUS RING HEIGHT ION DENSITY AND FLUX
? 1.0 mm Gap

• Ions do not fully penetrate into the gap with
  high focus ring.
• Ion focusing on edges.
• Substantial penetration of ion flux under bevel
  with low focus ring.

Powered Electrode
Powered Electrode
Animation Slide
Powered Electrode
Powered Electrode
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FOCUS RING HEIGHT IEAD
? 1.0 mm Gap
? 0.25 mm Gap

• Open edge produces skewed IEADs

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DESIGN TO CONTROL IEADs

• Configuration of wafer-focus ring gap can be used
  to control IEADS.
• Example Extension of biased substrate under
  dielectric focus ring of differing conductivity.

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EXTENDED ELECTRODE CHARGE, E-FIELD AND ION FLUX

• Same conductivity wafer and FR.
• More uniform and symmetric sheath and plasma
  parameters.
• 0.1 Ohm-1 cm-1

Powered Electrode
Powered Electrode
Animation Slide
Powered Electrode
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EXTENDED ELECTRODE IEAD

• Wafer 0.1 Ohm-1 cm-1
• Ring 10-8 Ohm-1 cm-1

• On all surfaces more narrow and symmetric IEAD
  with uniform electrical boundary condition.

• Wafer and Ring 0.1 Ohm-1 cm-1

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BROADENING OF IEAD ON TOP BEVEL EFFECT OF FR

• FR Conductivity

? ?/?o 4
? ?/?o 20
? High FR
? Low FR

• Always broad and asymmetric IEAD on tilted
  surface.

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CONCLUDING REMARKS

• Ion energy and angular distributions were
  investigated on surfaces inside wafer-focus ring
  gap.
• Different regions of the IEADs are generated
  during different parts of the rf cycle. Even
  vertical surfaces receive some normal angle ion
  flux.
• Narrow IEAD are obtained with
• High focus ring
• High focus ring capacitance
• High focus ring conductivity.
• Uniform electrical boundary conditions leads to
  more symmetric sheath over the gap and narrows
  IEADs.
• On tilted surfaces broad and asymmetric IEADs are
  obtained for most conditions.

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