Experimental study of expanding hollow cathode discharges
Date
2014
Authors
Dixon, Sam
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This thesis details work performed in the Distributed Array Shower Head (DASH) and Pocket Rocket systems over the past three years, and is targeted at use in surface processing applications. The DASH source produces plasma in an array of hollow cathodes. Pocket Rocket is a geometrically similar analogue to a single hollow cathode in the DASH source, used for its superior optical access. The work aims to investigate the physics that underpins the operation of such a plasma source. To characterise the expansion plumes from the hollow cathodes, two dimensional maps of ion densities in argon are measured using a Langmuir probe. A peak density of 1E18 per cubic metre is measured at the interface between hollow cathode and expansion chamber. Downstream of the plasma plume, densities are as low as 1E14 per cubic metre. Plasma density in the plume has a 1/z squared dependence, as plasma freely diffuses into the expansion chamber. Further downstream, there is an abrupt change in behaviour as the plasma density becomes nearly constant with increasing distance from the hollow cathode. A compensated Langmuir probe is used in conjunction with an emissive probe to make spatially resolved measurements of the plasma potential, floating potential, and electron energy probability function (EEPF). Inside the hollow cathode a Maxwellian distribution is observed with an electron temperature of 2 eV. The EEPFs measured in the plume show bi-Maxwellian behaviour, before becoming a single Maxwellian with temperatures as high as 11 eV far from the source. To investigate its etching capabilities, the DASH source is also operated using SF6 to produce atomic fluorine and to chemically etch unbiased silicon. Etch rates of up to 3.2 micrometres per minute are measured using a profilometer. Scanning Electron Microscopy of some processed wafers demonstrates the isotropic profile expected of the chemical etching process. It is shown that scaling of the DASH source to process a 300 mm wafer with uniformity of plus-minus 5% uniformity of etch rate is possible. To investigate the possible use of DASH in deposition processes, optical experiments on hydrogen plasma with a pressure on the order of 1 Torr are performed in Pocket Rocket. Two operational modes are observed, named the diffuse and the bright modes after their respective appearances. Fulcher alpha spectroscopy yields gas temperatures of approximately 350 K for the diffuse mode and 500 K in the bright mode. Temperature is observed to be independent of pressure but strongly dependent on power. Pocket Rocket is also investigated using an intensified CCD (ICCD) camera for spatially resolved, sub-rf cycle imaging of Balmer alpha emission. The diffuse mode is found to be operating as a typical gamma capacitive discharge, while the bright mode displays evidence of some degree of inductive coupling. The bright mode has high enough gas temperatures to precipitate the formation of a supersonic shock as gas expands from the hollow cathode. The shock is stationary, but is only apparent once per rf cycle as a burst of energetic electrons passes through it.
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Distributed Array Shower Head (DASH), Pocket Rocket systems, surface processing, plasma, array, hollow cathodes, physics
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Thesis (PhD)
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