Large Area Plasma Processing System (LAPPS)

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Large Area Plasma Processing System (LAPPS)

• R. F. Fernsler, W. M. Manheimer, R. A. Meger, D.
  P. Murphy, D. Leonhardt, R. E. Pechacek,
• S. G. Walton and M. Lampe
• Naval Research Laboratory
• Presented at ICOPS, June 1999
• Work supported by the Office of Naval Research

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Outline

• General description of LAPPS
• Experiments
• See Poster 6P01 for details.
• Physics
• Comparison with other sources
• Summary

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LAPPS Overview

• A magnetically confined, sheet electron beam is
  used to ionize a gas.
• Purpose Create a cold, weakly ionized, planar
  plasma for material processing.
• Key distinguishing features
• Beam ionization replaces plasma heating.
• Large processing area ( 1 m2).
• Limited by the size of the vacuum chamber and
  power.

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Experimental Program

• Status
• ne 5x1012 cm-3 over 60 x 60 x 2 cm.
• Wide range of gases, 20-300 mtorr.
• Etch rate in oxygen gt 6 mm/min.
• Grounded stage (no bias).
• Immediate Plans
• Extend area to 1 m2.
• Etch with rf bias.
• Investigate other beam sources.
• Present source is a hollow-cathode discharge.
• Diagnostics Langmuir probe, m-wave and optical,
  electron energy analyzer, mass spectrometer, SEM.

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Processing Objectives

• Large area.
• But thin to minimize electrical power.
• Independent control of ion and free-radical
  fluxes.
• Wide operating range in power, gas and bias.
• High uniformity.
• Efficient.
• Low Te.
• To maximize ion anisotropy.

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Design Constraints

• Beam current and energy.
• Gas pressure and composition.
• Magnetic field.
• Particle fluxes.

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Beam Energy

• Energy loss rate
• N gas density, Z atomic number, eo 100 eV.
• Beam range
• Increase R by raising eb or reducing N.
• R gt 1 m in 100 mtorr ? eb 3 keV.
• N 20 mtorr to avoid two-stream instability.

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Beam Current

• Gas ionization rate
• jb beam current density, wi _at_ 30 eV per e-i
  pair.
• Loss rate, worst case
• Where b 5x10-8 cm3/s (for eAB ? AB).
• ne 1012 cm-3 requires jb _at_ 10 mA/cm2.
• nb/ne lt 10-4.
• Dissociation rate
• Metastable rate Sm ltlt Si.

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Beam Confinement

• Elastic collisions cause the beam to spread.
• Apply Bz to keep the gyroradius rc lt dxb.
• dxb beam thickness.
• rc lt 1 cm at 3 keV ? Bz 200 G.

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Effect of Bz on the Plasma

• Outward plasma flow produces a diamagnetic
  current circulating around the plasma
• Bz ?
• Only the electrons are strongly magnetized.
• Outward flow is impeded if
• m mass, n collision frequency of ions
  electrons.
• n ? N ? Flow is impeded when Bz/N 2 kG/torr.

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Electron Temperature Te

• Initial mean energy of secondary electrons
• Ii ionization energy.
• Effective heating rate
• Collisional cooling Te Te(Q).
• Predict Te lt 1 eV in molecular gases.
• Consistent with Langmuir probe.
• Predict Te gt 1 eV in noble gases.
• Can raise Te with external heating.

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Particle Fluxes

• Fluxes
• Bz and recombination limit the ion flux to
• Increase Bz or Dx to decrease Fi.
• Add an atomic gas (b ? 0) to increase Fi.
• Free-radical flux
• Independent of Bz, Dx, or atomic additives.

beam/plasma
Dx
Fi ,Fr
substrate
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Uniformity

• jb _at_ constant but eb falls with z.
• Si and ne thus rise (smoothly) with z.
• One solution is to increase eb(0).
• Decreases efficiency, unless the energy is
  recovered prior to the beam dump.
• Other solutions
• Vary Bz(z) or Dx(z) to make Fi more uniform.
• Vary N(z) to make Fi and Fr more uniform.
• Move the stage.

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Conventional Sources

• Heat the plasma to produce ionization.
• Te gt 2 eV.
• Inefficient wi gtgt I1 (in processing gases).
• Energy goes mainly to low-lying excitation.
• Restricted operating range.
• In gas type, pressure and power.
• Limited control.
• Species with low Ii are ionized preferentially.
• Heating zone is determined by E-to-ne coupling.

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LAPPS Issues

• Requires
• Beam source.
• Isolation of the source from the processing
  region.
• Bz.
• Beam-plasma instabilities.
• Suppressed by collisions and beam velocity
  spread.
• RF bias
• Small ground electrode(s).
• Effect of Bz on the sheath.

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LAPPS Summary

• Plasma density ne ? 5x1012 cm-3 (above 20 mtorr).
• Area 1 m2.
• Limited by chamber size and power.
• Works in all gases over a wide pressure range.
• Ionization rate depends mainly on concentration.
• Independent control of ion and radical fluxes.
• Smooth (no hot spots).
• But some variation along Bz.
• Efficient.
• Thin.
• wi _at_ 30 eV, depending on tradeoff for uniformity
  along Bz.
• Te lt 1 eV.