Impedance characteristics of an rf parallel plate discharge and the validity of a simple circuit model

Abstract

Measurements of the electrical characteristics of a parallel plate rf discharge at pressures from 0.1 to 10 Torr in argon and at frequencies from 7.1 to 20 MHz are reported. The discharge impedance reaches a minimum for all frequencies at a pressure of about 1 Torr and the impedance decreases with increasing frequencies. The phase variation over the pressure range is as high as 40°. With added attaching gas, the impedance increases and the minimum occurs at lower pressures. A simple equivalent circuit with a capacitance representing the electrode sheaths can model the measured impedance behavior at pressures up to 1 Torr when the capacitance is made proportional to pressure, analogous to the sheath characteristics. At higher pressures, a good fit can be achieved by making the resistance, which represents the power losses in the discharge volume, a function of pressure. Derived sheath dimensions correlate with observed variations versus pressure and frequency. The fact that at the lower pressures the resistive part of the impedance is independent of pressure and that the resistance no longer properly represents the voltage drop across the discharge volume indicates that collisional losses in the volume no longer dominate the power deposition. The ‘‘wave-riding’’ mechanism is suggested as the dominant process in this pressure range. Its frequency characteristic correlates well with the frequency characteristic of the measured impedance.