Electron and negative ions dynamics in a pulsed 100 MHz capacitive discharge produced in O2 and Ar/O2/C4F8 gas mixture

This paper presents an experimental study of electron and negative ion dynamics in a pulsed 100 MHz capacitive discharge. The plasma is produced in O2 and Ar/O2/C4F8 gas mixture at 2 pascal gas pressure. The laser photo-detachment combined with microwave resonance probe is used to diagnose absolute densities of multiple negative ion species, F- and O-. In pure oxygen discharge, both quasi-steady state electron and negative ion density during the active glow increases with an increase in rf power level. In the afterglow plasma, the electron density decay rate is ~10 15 s, whereas, the O- decays more slowly with a decay time constant in the range of 30-35 s. At a low rf power, a peak in the O- density is observed in the early afterglow plasma. In Ar/O2/C4F8 gas mixture, the quasi-steady state electron density in the active glow plasma increases with rf power, whereas, the O- density decreases and F- density increases. In comparison to pure O2 discharge, in Ar/O2/C4F8 gas mixture the electron density decay is faster; In the afterglow the F- density is 10-times higher than the O- density. These results suggests a change in the dominant negative ion production mechanism within discharge pulse from dissociative electron attachment of CxFy in the active on-phase to electron attachment to fluorine atom in the afterglow plasma respectively.