Sheath model for radio-frequency-biased, high-density plasmas valid for all omega/omega(i)

Abstract

A model is proposed for sheaths in high-density discharges, with radio-frequency (rf) bias applied at frequencies omega comparable to omega(i), the ion plasma frequency at the edge of the sheath. The model treats ion dynamics using fluid equations, including all time-dependent terms. Model predictions for current, impedance, and power were compared to measurements performed in high-density discharges in argon at 1.33 Pa (10 mTorr) at rf bias frequencies from 0.1 to 10 MHz (omega/omega(i) from 0.006 to 1.8) and rf bias voltages from 1 to 200 V. Model predictions were in good agreement with measurements, much better than that obtained by models that neglect time-dependent ion dynamics. In particular, differences of as much as 40-50 % between power measurements and the power predicted by previous models are now explained and eliminated. The model also explains why methods of extracting plasma parameters from electrical measurements using previous sheath models may fail, and it suggests more accurate methods of extracting these parameters.