Comparison of microwave interferometry and impedance analysis measurements of electron concentrations in low-frequency sulphur hexafluoride glow discharges

Abstract

Electron concentrations in low-frequency (100 kHz) sulphur hexafluoride discharges have been measured by microwave interferometry. To obtain high signal-to-noise ratios, the plasma was switched on and off with a 150 Hz pulse. Reproducible average electron concentration measurements were obtained for pressures ranging from 0.15 to 0.45 Torr and powers ranging from 13 to 92 W. The lowest time-averaged electron concentration measured is 4.3*107 cm-3 (corresponding to a line average density of 6.0*108 cm-2). Electron concentration variation within the 100 kHz cycle is obtained after correcting for response times of the interferometer. These time-averaged and time-dependent measurements are compared with measurements from an impedance analysis technique. Although the two types of measurements agree within a factor of two for all measurements, the impedance technique gives a stronger dependence of electron concentration on power and a weaker dependence on pressure. The large time-dependent variation of the electron concentration from zero to high concentrations is attributed to high attachment and ionization rates in sulphur hexafluoride discharges under these conditions.