Instabilities in low pressure inductive discharges with attaching gases

Plasma instabilities have been observed in low pressure inductive processing discharges with attaching gases. For SF₆, instability windows in pressure and driving power have been explored for gas pressures between 2.5 and 100 mTorr and absorbed powers between 150 and 900 W. For most pressures increasing power is required to obtain the instability with increasing pressure, with the frequency of the instability increasing with pressure, mainly lying between 10 and 100 kHz. A volume-averaged (global) model of the instability has been developed, considering idealized inductive and capacitive energy deposition. As pressure or power are varied to cross a threshold, the instability is born at a Hopf bifurcation, with relaxation oscillations between inductive and capacitive modes causing modulations of charged particle densities, electron temperature, and plasma potential. The theoretical predictions are in qualitative agreement with experimental observations. SF₆/argon mixtures have also been investigated, obtaining the instability bounds and frequencies. The experimental results and comparison with theory will be presented.