Transcript High Pressure Plasma with a third electrode

High Pressure Plasma
with a third electrode
James Roberts
Physics TSP 2002
Supervised by Dr Kerrie Balla
What is a plasma?
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Collection of
electrons, charged
ions and neutral
atoms and molecules
Overall plasma is
neutral
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How are they made?
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Charged particles result from the
interaction of gas with an applied electric
field
Typically confined in a reaction chamber at
pressures < 1 mbar
How are plasmas used?
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Many uses
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Sputtering
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Used in making our electrodes
Thin film deposition
Etching
Advantages of higher pressure
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Improved rates in thin film processes
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More ions  faster process
Less expensive apparatus
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At atmospheric pressure no vacuum
equipment needed
100 mbar could be sustained by an
inexpensive pump
Vacuum system
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Constant flow 440 sccm
balloon gas @ 100 mbar
The chamber
Conditions
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0.1 mm gap microplanar reactor
Al on alumina substrate
Constant flow 440 sccm balloon gas @
100 mbar
320V peak to peak applied by power
supply at 1.7 kHz
The plasma
The third electrode
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Piece of aluminium 38x14x4 mm
Supported on two feet, 2 microscope
slides high
Negative potential applied from DC supply
Voltage and current monitors allow VI
characteristics to be measured on a CRO
Langmuir probe
CRO trace
Electron current
0 V applied
-24 V self bias
0
Ion current
Increase applied
–ve voltage to
3rd electrode
No electron current
0
Ion current
As magnitude of voltage
is increased CRO trace
takes this form
VI curve
Ion current and Voltage to third electrode
Ion current to 3rd electrode (mA)
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Voltage on 3rd e le ctrode (V)
VI curve
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Graph shows relatively large ion current
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Potentially useful for deposition
Similar to that of a Langmuir probe,
except no maximum ion current
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As 3rd electrode dimensions are comparable to
the plasma itself
Conclusions
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High pressure plasmas can be generated
with a 1.7 kHz RF supply and microplanar
reactor
Ion currents in excess of 30 mA are
achieved
Investigation only at a preliminary stage –
there is much room for further study