Ion and electron dynamics in the sheath of radio-frequency glow discharges

Abstract

The distribution of ion energies and the temporal modulation of ion and electron currents were measured at the cathode of an asymmetric, capacitively coupled rf discharge system operated at 13.56 MHz. The ion energy distributions (IEDs) were found to exhibit pronounced features, namely a characteristic series of peaks and double peaks, their position and intensity depending strongly on process parameters such as self-bias voltage and pressure. Those structured IEDs have been observed for a variety of gases including argon, oxygen, hydrogen, and benzene. The IED features are explained by the rf modulation of the sheath potential in combination with the creation of thermal ions by charge-exchange processes in the sheath. A model for the ion transport through collisional rf sheaths is presented which satisfactorily explains the observed IEDs. It will be shown that the observed features are an inherent property of capacitively coupled rf discharges. By analyzing these structures detailed information about the spatial field distribution in the sheath and the transport of ions through rf plasma sheaths is obtained. The time-resolved measurements revealed that the ion current behind the cathode is strongly modulated. This temporal modulation has been described by the same theory confirming the applicability of the present model. Finally the electron current was found to be confined to short periodic pulses of 2–3 ns width.