Low-Current Moderate-Pressure RF Discharge with Secondary Electron Photoemission.

A model is proposed for a low-current RF discharge with secondary electron photoemission from the electrode surface caused by photons originating in the electrode sheath. The low-current state of RF discharges at moderate pressures is peculiar in that the electrons and ions produced during the preceding periods of the RF field promote the development of the discharge during subsequent periods. Since the ion space charge is induced during many periods of the RF field, even comparatively moderate fields in the electrode sheath are sufficient to ensure the conditions under which the current is self-sustaining, in which case the electron photoemission dominates over the remaining secondary processes at the electrode surface. In a low-current RF discharge, the ion–electron emission has essentially no impact on the formation of the electrode sheath because the half-period of the RF field is much shorter than the ion transit time through the sheath. The sheath results from the overlap of the secondary electron avalanches triggered by electron photoemission from the electrode surface. The sheath parameters are determined by the conditions under which the current in the sheath is self-sustaining due to the secondary electron photoemission from the electrode surface. The capacitive susceptance of the electrode sheath is substantially higher than its electrical conductance. Low-current RF discharges can only exist when the time required for the ions to drift through the sheath and reach the electrode is much longer than the half-period of the RF field.