Stable and unstable behavior of inductively coupled electronegative discharges

Abstract

Plasma instabilities have been observed in low-pressure inductive discharges, in the transition between low density capacitively driven and high density inductively driven discharges when attaching gases such as SF6 and Ar/SF6 mixtures are used. Oscillations of charged particles, plasma potential, and light, with frequencies from a few hertz to tens of kilohertz, are seen for gas pressures between 1 and 100 mTorr and discharge powers in the range of 75–1200 W. The region of instability increases as the plasma becomes more electronegative and the frequency of plasma oscillations increases as the power, pressure, and gas flow rate increase. Time-resolved measurements of electron temperature, and time averaged ion energy distribution at the wall have been made. A volume-averaged (global) model of the instability has been developed, for a discharge containing time varying densities of electrons, positive ions, and negative ions, and time invariant excited states and neutral densities. The particle and energy balance equations are integrated to produce the dynamical behavior. The model agrees well with experimental observations, and also shows a significant influence of the matching network. Values of plasma parameters and of chemical composition are measured to set the values used in the model, as required to obtain the good agreement with experiment. Capacitive coupling plays a crucial role in the instability process. A variable electrostatic (Faraday) shield has been used to control the capacitive coupling from the excitation coil to the plasma. The plasma instability disappears when the shielded area exceeds 65% of the total area of the coil. The model gives a slightly higher value of 85% for instability suppression with the same discharge conditions (Ar/SF6 1:1, 5 mTorr).