โ Scribed by L Holland
No coin nor oath required. For personal study only.
The uses of cold cathode discharges in inert and active gases for improving the vacuum characteristics of systems and components are reviewed with the objective of analysing the processes which occur under the various complex conditions of treatment. Initially the effects and practical problems are considered of passing a glow discharge in an assembled metal system using grounded and non-grounded electrodes. Depending on the electrodes and discharge gas chosen it is possible to fix hydrocarbon gas as a polymer on the system walls or remove carbonaceous surface contaminants. Means of Iocalising a glow discharge to prevent its entry into pump orifices are discussed. Attention is then given to reported work on the glow discharge treatment of stainless steel envelopes as employed in particle accelerators. The effects are surveyed of ion bombarding tubular cathodes in inert and oxygen bearing gases with the cathode at normal or elevated temperature. Gases released by ion bombardment of thermally degassed stainless steel are hydrogen, carbon oxides, hydrocarbon and water. Mechanisms by which these are released or formed by reaction at the bombarded surface are discussed. Particular attention is given to removal of carbon in oxygen plasmas because of its importance in sorption processes. Stainless steel tubes treated whilst at elevated temperature as discharge cathodes have been reported as showing negligible or zero desorption when subsequently exposed to energetic ion impact. Also their low ion induced desorption yield is not impaired by air exposure if the treated tube is vacuum baked before ion re-exposure. It is suggested here, that these characteristics could arise during treatment from simultaneous sputtering and deposition in a hollow cathode discharge tube in the presence of atomic oxygen and ion impact which removes carbon by chemical sputtering and aids growth of a thin metal/metal oxide layer. Reactively sputtered oxides of Fe, Cr, etc., are known to possess glass-like structures over a range of Me : 02 ratios. They may have more passivating properties than natural oxide layers. If oxygen alone were released by ion impact it would be recaptured on surface collision. If this were correct then vacuum components could be treated in separate stages using chemical-sputter etching followed by reactive sputtering for depositing passivating materials.
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