COMPARISON OF MICROWAVE AND RF PLASMAS - FUNDAMENTALS AND APPLICATIONS

It is now generally accepted that the frequency omega/2pi at which a high frequency (HF) discharge is excited has considerable influence on the properties of the plasma. For example, the electron density n obtained for a given HF power density P(A) in the plasma is usually higher at microwave (MW) than at radio frequencies. This, in turn, strongly affects the outcome of a given plasma process, for example etching or deposition. The analysis which has been developed to account for this calls on the dependence of the electron energy distribution function (EEDF) upon omega. This paper is divided into three main parts. In a first section, following a brief introduction, we review recent work on the kinetic modelling of low pressure HF discharges, which is based on the determination of the EEDF from the Boltzmann equation for electrons under the action of the HF electric field. This allows us to calculate parameters which can be compared with experimental data, for example the mean power theta absorbed by an electron from the HF field. The second section reviews a series of experiments in these laboratories which have been specifically designed to investigate the influence of omega on plasma processing of materials, namely plasma etching of a polymer (polyimide), and deposition of thin plasma polymer films. Finally, in the third section we briefly review our recent research on ion bombardment effects in dual frequency (MW-r.f.) plasmas, where the beneficial effects of MW excitation and r.f.-induced bias voltage act to enhance synergistically plasma processes.