Instabilities in low-pressure electronegative inductive discharges

Abstract

Plasma instabilities have been studied in low-pressure inductive processing discharges with SF₆ and Ar/SF₆ mixtures, i.e. attaching gases. Oscillations are seen in charged particle density, electron temperature and plasma potential using electrostatic probe and optical emission measurements. For SF₆, instability onset in pressure and driving power has been explored for gas pressures between 2.5 and 100 mTorr and absorbed powers between 150 and 900 W. For pressures above 20 mTorr, increasing power is required to obtain the instability with increasing pressure, with the frequency of the instability increasing with pressure, mainly lying between 1 and 100 kHz. For Ar/SF₆ mixtures, we observe a similar low power transition, with an upper transition to a stable inductive mode. The instability windows become smaller as the argon partial pressure increases. For Ar/SF₆ mixtures, we observe a significant effect of the matching network. We improve a previously developed volume-averaged (global) model to describe the instability. We consider a cylindrical discharge containing time varying electrons, positive ions, negative ions, and time invariant excited states. The driving power is applied to the discharge through a conventional L-type capacitive matching network, and we use realistic models for the inductive and capacitive energy deposition. The particle and energy balance equations are integrated, considering quasi-neutrality in the plasma volume and charge balance at the walls, to produce the dynamical behaviour. As pressure or power is varied to cross a threshold, the instability is born at a Hopf bifurcation, with relaxation oscillations between higher and lower density states. The model qualitatively agrees with experimental observations, and also shows a significant influence of the matching network.