Time-resolved measurements of electron and ion concentrations in low-frequency sulfur hexafluoride discharges

Abstract

Electron and ion concentrations in low-frequency SF6 discharges were measured with an impedance technique in which a small high-frequency power perturbation is added to the low-frequency powering signal. The impedances for the high frequency signals are measured and interpreted with an electrical analog of the plasma that gives bulk ion concentration and electron concentration with time within the low frequency cycle. Ion concentrations, which are about 5×1011 cm−3 at 0.3 W/cm2, 2 cm electrode spacing and 350 mTorr, increase approximately with (power)0.8 but change little with pressure from 250 to 450 mTorr. Electron concentrations change from essentially 0 to about 109 cm−3 during the low frequency cycle and increase approximately with (power).1.5 An overall negative ion balance in which the creation of negative ions by electron attachment equals loss by ion–ion recombination is used to compare the ion and electron concentrations. The calculated ion concentrations as determined from measured electron concentrations and literature-reported rate constants are within 25% of the measured concentrations without using any adjustable fitting parameters.