Instabilities in low-pressure electronegative inductive discharges

Plasma instabilities have been studied in low-pressure inductive processing discharges with SF6 and Ar/SF6 mixtures, i.e. attaching gases. Oscillation s are seen in charged particle density, electron temperature and plasma pot ential 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 90 0 W. For pressures above 20 mTorr, increasing power is required to obtain t he instability with increasing pressure, with the frequency of the instabil ity increasing with pressure, mainly lying between 1 and 100 kHz. For Ar/SF 6 mixtures, we observe a similar low power transition, with an upper transi tion to a stable inductive mode. The instability windows become smaller as the argon partial pressure increases. For Ar/SF6 mixtures, we observe a sig nificant effect of the matching network. We improve a previously developed volume-averaged (global) model to describe the instability. We consider a c ylindrical discharge containing time varying electrons, positive ions, nega tive 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 deposi tion. 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 o scillations between higher and lower density states. The model qualitativel y agrees with experimental observations, and also shows a significant influ ence of the matching network.