Review of relaxation oscillations in plasma processing discharges

Relaxation oscillations due to plasma instabilities at frequenciesranging from a few Hz to tens of kHz have been observed in varioustypes of plasma processing discharges. Relaxation oscillations havebeen observed in electropositive capacitive discharges between apowered anode and a metallic chamber whose periphery is groundedthrough a slot with dielectric spacers. The oscillations oftime-varying optical emission from the main discharge chamber show,for example, a high-frequency (sim 40~kHz) relaxationoscillation at 13.33Pa, with an absorbed power being nearly theperipheral breakdown power, and a low-frequency ( sim 3 Hz)oscillation, with an even higher absorbed power. The high-frequencyoscillation is found to ignite plasma in the slot, but usually not inthe peripheral chamber. The kilohertz oscillations are modelled usingan electromagnetic model of the slot impedance, coupled to a circuitanalysis of the system including the matching network. The modelresults are in general agreement with the experimental observations,and indicate a variety of behaviours dependent on the matchingconditions. In low-pressure inductive discharges, oscillations appearin the transition between low-density capacitively driven andhigh-density inductively driven discharges when attaching gases suchas SF₆ and Ar/SF₆ mixtures are used. Oscillations ofcharged particles, plasma potential, and light, at frequenciesranging from a few Hz to tens of kHz, are seen for gas pressuresbetween 0.133 Pa and 13.33 Pa and discharge powers in a range of75--1200 W. The region of instability increases as the plasma becomesmore electronegative, and the frequency of plasma oscillationincreases as the power, pressure, and gas flow rate increase. Avolume-averaged (global) model of the kilohertz instability has beendeveloped; the results obtained from the model agree well with theexperimental observations.