Studies on etching and polymer deposition in halocarbon plasmas
Plasma etching, the selective removal of materials by reaction with chemically active species formed in a glow-discharge, is widely used by the electronics industry because of the advantages over 'wet' processes. The full potential has yet to be realised because chemical processes occuring in the plasma and at the plasma/substrate interface are incompletely understood. In this work attention was focussed on the accumulation of polymers on surfaces during plasma etching in fluorocarbon gases. An apparatus was designed and constructed to explore the conditions which give rise to these deposits by: i) The detection of the excited species such as CF and CF2 (by optical emission spectroscopy); and ii) The rate of accumulation or removal of deposits (by means of a quartz crystal microbalance). The gases CF4, C2F6, C3F8 and CHF3 were used at pressures between 200-600mT, together with mixtures with H2 and a few runs with other gases to vary the partial pressures of etching and polymerizing species. Both substrate effect of, viz silicon and thermally oxidised silicon (SiO2), and electrode materials effects have been examined. Polymer production from C3F8 has been found to be more sensitive to electrode composition than that from CHF3, but the material formed is overall less thermally stable. On the other hand, polymers produced from C3F8 accumulate at similar rates on Si and SiO2, whereas those from CHF3 show a much greater liklihood of building up on Si than SiO2 . XPS and infra-red spectroscopy have been used to demonstrate that polymers arising from these two gases exhibit marked structural differences, which can be minimised by mixing H2 with C3F8. These effects can be correlated with the decomposition products expected in the plasma.