Ion energy distributions in oxygen and argon in a pulsed mode RF discharge

The positive and negative ion energy distributions were measured at the grounded electrode of a square wave modulated RF discharge in O2 and Ar at between 1.5 and 5 Pa gas pressure. The resulting ion energy distributions were strongly affected by the modulation frequency according to the existence of different discharge periods during the modulation cycle. These discharge periods were directly represented in the energy distribution of the positive ions which is determined by the plasma sheath voltage in front of the grounded electrode. Negative ions generated in the plasma sheath at the RF electrode were accelerated towards the bulk plasma and detected with a high energy at the opposite grounded electrode. Additionally, during the afterglow with decaying sheath potential, low energy negative ions reached the electrodes. The intensity ratio between the two groups of negative ions allowed conclusions on the negative ion generation process.