Vacuum ultraviolet to visible emission from hydrogen plasma: Effect of excitation frequency

Abstract

The expanding use of low pressure (p⩽10 Torr), high frequency plasmas in various applications has stimulated research toward increased operating efficiency. In order to optimize a particular plasma process, the operator can vary several “external” (operator-set) parameters, among which the excitation frequency f has received relatively little attention in the literature over the years, probably due to the difficulties encountered in designing meaningful frequency-dependent experiments. These difficulties can be avoided by the use of surface-wave discharges (SWDs), which possess great flexibility: a very broad (continuous) range of excitation frequencies, and wide ranges of operating pressures and plasma densities, under noncritical, almost perfect impedance matching with the power source. In earlier work in these laboratories, we have examined the f dependence of plasma deposition and etching experiments; the present experiments have been designed to investigate the f dependence more “directly” by turning to the plasma through its optical emission. The vacuum ultraviolet to visible emission from SWD plasmas in pure hydrogen or 7%H2 in Ar mixture has been investigated over a broad range of excitation frequency (50⩽f⩽200 MHz) using a spectrophotometer with a known transfer function. The observed f dependence of emission intensity (atomic lines and molecular bands) as f is increased is interpreted in the case of the pure H2 discharge in terms of changes from a nonstationary to a stationary electron energy distribution function (EEDF) while, in the 7%H2/Ar mixture, it is related to changes in the form of the stationary EEDF.