On the Influence of Substrate Temperature

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On the Influence of Substrate Temperature on the
Ion Energy Distribution Function
M.M. de Jong MsC. Final Talk 9-9-08
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Outline

• Introduction
• Thin film solar cells
• Plasma-Enhanced chemical vapor deposition
• Ion energies
• Why?
• How?
• Results
• Pressure variations
• Power variations
• Dilution variations
• Temperature variations
• Restore high T energy flux
• Conclusions

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Thin Film Solar Cells

• Positive, neutral and negative doped layers
• Thickness 1µm
• Deposite on any substrate
• Holy grail
• Roll to Roll on plastics
• Low T!

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Plasma Enhanced Chemical Vapour Deposition

• Plasma ingredients
• Gas
• Oscillating field
• Free electron
• Ionization
• e- A ? A 2e-
• Dissociation
• SiH4 e- ? SiH3 H
• Ambipolar diffusion leads to positive plasma
  bulk and plasma sheaths

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Introduction Why?

• Ions contribute to growth rate and create growth
  sites
• Ions transfer energy to the substrate (both
  kinetic and potential (10eV))
• Ions enhance surface diffusion, densifying the
  film

From Hamers, 1998, PhD thesis
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Retarding Field Ion Energy Analyzer (RFIEA)

• Retarding field ion energy analyzer

• V1 -30V to repel electrons
• V2 sweeps from 0V to 75V
• V3 -30V to repel secondary electrons

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RFIEA II
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Ion Energy Distributions Function (IEDF)

• Measures I as a function of V2
• Ion current
• Energy flux
• In a collisionless plasma
• heavy positive ions gain eVpl
• Light ions gain eVpl
• Elastic collisions will decrease ion energy
• Chemical reactions will broaden the IEDF

12.2W, 280C, FH225 sccm, 0.10 mbat
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RFIEA vs. MS

• Equally sensitive to ions with different masses
• No chromatic abberation, enabling calculation of
  average ion energies
• Less failure due to clogging (low T ? dust)
• Therefore better reproducibility

12.4W, 280C, FH245.5 sccm, FSi2H44.5 sccm,
0.10 mbar
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Results Pressure variations(in pure hydrogen)

• Increasing pressure will increase number of
  collisions and reduce ion energy and ion current

12.0W, 200C, FH220 sccm
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Results Dilution variations

• Increasing silane flow decreases ion energies and
  ion current
• Silane has a lower ionization energy
• Hydrogen atoms go faster, increasing ion current

12.3W, 280C, Ftotal50sccm, 0.13 mbar
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Results Power Variations(in pure hydrogen)

• Increasing dissipated power will increase Vpl
• Increasing power will increase ionization rate

25C, FH210 sccm, 0.10 mbar
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Results Temperature Variations

• Direct influence of substrate T on IEDF
• Therefore it is even harder to compensate for
  reduced surface mobility at low T
• Change in particle density?

12.3W, Ftotal50sccm, 0.10 mbar
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Results Temperature Variations II

• Constant p/T
• But still T dependent
• Overcompensation
• Sticking coefficients depend on T
• Check growth rate?

12.3W, Ftotal50sccm
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Results Restore High T Conditions

• Ion current is easily restored
• Ion energy is hard to restore

50C, Ftotal50sccm, 0.10 mbar
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Results Restore High T Conditions II

• Ion current can be restored
• Ion energy can be increased, not restored

12.4W, 50C, Ftotal50sccm
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Results Restore High Temp Conditions III

• Energy flux can be restored by both increasing
  power and decreasing pressure

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Results Restore High Temp Conditions IIII

• Although the energy flux can be reacquired at low
  T, the IEDF changes shape

FH245.5 sccm, FSi2H44.5 sccm
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Conclusions

• Decreasing substrate temperature will decrease
  energy flux to the substrate
• Therefore it is more difficult to compensate for
  decreased surface mobility at low T
• When going from 280C to 50C, this decrease can
  be compensated by reducing the pressure or
  increasing the power coupled into the plasma.
• Although the energy flux can be repaired, the
  IEDFs have a different shape.
• Increasing the power will mainly increase ion
  current.
• Decreasing the plasma pressure will both increase
  ion energies and ion current.