Transition between different regimes of rf glow discharges

A self-consistent fluid model of radio-frequency glow discharges has been used to analyze the existence of two different discharge regimes and the transition between them. The existence of these regimes had been previously established by Levitskii [Sov. Phys. Tech. Phys. 2, 887 (1958)]. The self-sustaining and power-deposition mechanisms that characterize each of these regimes are drastically different. In the first regime, termed as the ‘‘wave-riding regime’’ corresponding to low discharge power, most of the power deposition is due to bulk plasma electrons heated by the sheath expansions. In the second regime termed as the ‘‘secondary electron regime’’ corresponding to higher discharge power, the discharge is sustained mainly by electrons emitted by the electrodes under ion bombardment and avalanching in the sheath regions. The numerical results are in good agreement with previous experimental measurements by Godyak and Kanneh [IEEE Trans. Plasma Sci. PS-14, 112 (1986)]. The results presented in this paper form the first self-consistent description of these different regimes and of the transition between them. The validity domain of the model is restricted to pressure higher than a fraction of Torr and frequency less than a few tens of MHz. The gas being considered is helium and the discharge power varies between 0 and 700 mW ${\mathrm{cm}}^{\mathrm{-}2}$. The model is based on solutions of electron and ion fluid equations describing charged particle transport coupled with Poisson’s equation for the electric field. A realistic description of the electron kinetics has been obtained by considering separately two electron groups representing, respectively, the tail and the bulk of the electron distribution function. The validity of the two-electron group fluid model has been checked with Monte Carlo simulations.