On the possibility of making a geometrically symmetric RF-CCP discharge electrically asymmetric

Abstract

A fundamental problem in technological plasmas has been how to control the ion energy and the ion flux (plasma density) independently of one another. A simple, but previously overlooked asymmetry effect is reported that should allow a high degree of control of the ion energy. The idea is that when a temporally symmetric, multi-frequency voltage waveform containing one or more even harmonics is applied to a discharge, even a geometrically symmetric one, the two sheaths are necessarily asymmetric. To balance the charged particle fluxes, a dc self-bias develops. Optimally, this is achieved with a dual frequency discharge that uses the phase locked fundamental and its second harmonic. The resulting dc self-bias and hence the ion energy are a nearly linear function of the phase angle between the two applied RF voltages. This works even for geometrically symmetric discharges, and the roles of the two electrodes can be reversed using the phase. This means that the technique can be used to increase or decrease the ion energy striking a substrate while leaving the applied RF voltage and frequency and thereby the discharge parameters effectively unchanged.