High Pressure PlasmaHigh Pressure Plasmawith a third electrodewith a third electrode

James RobertsJames Roberts

Physics TSP 2002Physics TSP 2002

Supervised by Dr Kerrie BallaSupervised by Dr Kerrie Balla

What is a plasma?What is a plasma?

Collection of Collection of electrons, charged electrons, charged ions and neutral ions and neutral atoms and atoms and moleculesmolecules

Overall plasma is Overall plasma is neutralneutral

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How are they made?How are they made?

Charged particles result from the Charged particles result from the interaction of gas with an applied interaction of gas with an applied electric fieldelectric field

Typically confined in a reaction Typically confined in a reaction chamber at pressures < 1 mbarchamber at pressures < 1 mbar

How are plasmas used?How are plasmas used?

Many usesMany uses SputteringSputtering

Used in making our electrodesUsed in making our electrodes Thin film depositionThin film deposition EtchingEtching

Advantages of higher Advantages of higher pressurepressure

Improved rates in thin film processesImproved rates in thin film processes More ions More ions faster process faster process

Less expensive apparatusLess expensive apparatus At atmospheric pressure no vacuum At atmospheric pressure no vacuum

equipment neededequipment needed 100 mbar could be sustained by an 100 mbar could be sustained by an

inexpensive pumpinexpensive pump

Vacuum systemVacuum system

Constant flow 440 sccm Constant flow 440 sccm balloon gas @ 100 mbarballoon gas @ 100 mbar

The chamberThe chamber

ConditionsConditions

0.1 mm gap microplanar reactor0.1 mm gap microplanar reactor Al on alumina substrateAl on alumina substrate Constant flow 440 sccm balloon gas Constant flow 440 sccm balloon gas

@ 100 mbar@ 100 mbar 320V peak to peak applied by power 320V peak to peak applied by power

supply at 1.7 kHzsupply at 1.7 kHz

The plasmaThe plasma

The third electrodeThe third electrode

Piece of aluminium 38x14x4 mmPiece of aluminium 38x14x4 mm Supported on two feet, 2 microscope Supported on two feet, 2 microscope

slides highslides high Negative potential applied from DC Negative potential applied from DC

supplysupply Voltage and current monitors allow VI Voltage and current monitors allow VI

characteristics to be measured on a CROcharacteristics to be measured on a CRO Langmuir probeLangmuir probe

CRO traceCRO traceElectron current

Ion current

0 0 V applied-24 V self bias

Increase applied –ve voltage to 3rd electrode

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Ion current

No electron current

As magnitude of voltage is increased CRO trace takes this form

VI curveVI curve

VI curveVI curve

Graph shows relatively large ion Graph shows relatively large ion currentcurrent Potentially useful for depositionPotentially useful for deposition

Similar to that of a Langmuir probe, Similar to that of a Langmuir probe, except no maximum ion currentexcept no maximum ion current As 3As 3rdrd electrode dimensions are electrode dimensions are

comparable to the plasma itselfcomparable to the plasma itself

ConclusionsConclusions

High pressure plasmas can be High pressure plasmas can be generated with a 1.7 kHz RF supply generated with a 1.7 kHz RF supply and microplanar reactorand microplanar reactor

Ion currents in excess of 30 mA are Ion currents in excess of 30 mA are achievedachieved

Investigation only at a preliminary Investigation only at a preliminary stage – there is much room for stage – there is much room for further studyfurther study