It is now generally accepted that the frequency ω/2π at which a high frequency (hf) discharge is sustained has considerable influence on the properties of the plasma. For example, the electron density obtained for a given hf power density deposited into the plasma is usually higher at microwave than at radio frequencies (rfs). This paper reviews a series of experiments at the Université de Montréal designed to investigate the influence of ω on the power balance between the hf field and the plasma, and the plasma processing of materials. For the two particular etching and deposition processes which are described here, the ‘‘optimum’’ frequency (at which the process is most efficient) appears to be in the range between 50 and 100 MHz. This suggests that converting a plasma process from 13.56 to 2450 MHz does not necessarily lead to the greatest possible process enhancement, and that optimization may require the plasma reactor to be constructed in such a way as to allow the excitation frequency to be changed. To provide insight into these results, the most recent models dealing with the influence of ω on the electron energy distribution are reviewed and extended to calculate parameters that can be compared with our experimental data.

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Abstract

A plasma sustained by electromagnetic surface waves (SW’s) has been used to study the deposition rate R of hydrocarbon and fluorocarbon plasma polymer films as a function of the applied field frequency f in the range 12–400 MHz. The SW technique allows one to vary only f while keeping constant all other parameters known to influence R, for example, power density P in the plasma. A plot of R/P at a total pressure of 200 mTorr (27 Pa) displays two plateaus, that at f<30 MHz being about five times lower than that at f>100 MHz. This is attributed to the fact that electron energy distribution functions differ fundamentally at radio and microwave frequencies, for the gas pressure range considered.