High Pressure Plasma with a third electrode James Roberts Physics TSP 2002 Supervised by Dr Kerrie Balla

What is a plasma? Collection of electrons, charged ions and neutral atoms and molecules Collection of electrons, charged ions and neutral atoms and molecules Overall plasma is neutral Overall plasma is neutral

How are they made? Charged particles result from the interaction of gas with an applied electric field Charged particles result from the interaction of gas with an applied electric field Typically confined in a reaction chamber at pressures < 1 mbar Typically confined in a reaction chamber at pressures < 1 mbar

How are plasmas used? Many uses Many uses Sputtering Sputtering Used in making our electrodes Used in making our electrodes Thin film deposition Thin film deposition Etching Etching

Advantages of higher pressure Improved rates in thin film processes Improved rates in thin film processes More ions  faster process More ions  faster process Less expensive apparatus Less expensive apparatus At atmospheric pressure no vacuum equipment needed At atmospheric pressure no vacuum equipment needed 100 mbar could be sustained by an inexpensive pump 100 mbar could be sustained by an inexpensive pump

Vacuum system Constant flow 440 sccm balloon 100 mbar Constant flow 440 sccm balloon 100 mbar

The chamber

Conditions 0.1 mm gap microplanar reactor 0.1 mm gap microplanar reactor Al on alumina substrate Al on alumina substrate Constant flow 440 sccm balloon 100 mbar Constant flow 440 sccm balloon 100 mbar 320V peak to peak applied by power supply at 1.7 kHz 320V peak to peak applied by power supply at 1.7 kHz

The plasma

The third electrode Piece of aluminium 38x14x4 mm Piece of aluminium 38x14x4 mm Supported on two feet, 2 microscope slides high Supported on two feet, 2 microscope slides high Negative potential applied from DC supply Negative potential applied from DC supply Voltage and current monitors allow VI characteristics to be measured on a CRO Voltage and current monitors allow VI characteristics to be measured on a CRO Langmuir probe Langmuir probe

CRO trace Electron current Ion current 0 0 V applied -24 V self bias Increase applied –ve voltage to 3 rd electrode 0 Ion current No electron current As magnitude of voltage is increased CRO trace takes this form

VI curve

Graph shows relatively large ion current Graph shows relatively large ion current  Potentially useful for deposition Similar to that of a Langmuir probe, except no maximum ion current Similar to that of a Langmuir probe, except no maximum ion current  As 3 rd electrode dimensions are comparable to the plasma itself

Conclusions High pressure plasmas can be generated with a 1.7 kHz RF supply and microplanar reactor 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 Ion currents in excess of 30 mA are achieved Investigation only at a preliminary stage – there is much room for further study Investigation only at a preliminary stage – there is much room for further study