Electron and negative ion dynamics in a pulsed 100 MHz capacitive discharge produced in an 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 an O-2 and Ar/O-2/C4F8 gas mixture at 2 Pa gas pressure. Laser photo-detachment combined with a microwave resonance probe is used to diagnose the absolute densities of multiple negative ion species, F- and O-. In pure oxygen discharge, both the quasi-steady state electron and negative ion densities increase during the active glow with an increase in RF power level. In the afterglow plasma, the electron density decay rate is similar to 10-15 mu s, whereas the O- decays more slowly with a decay time constant in the range of 30-35 mu s. At a low RF power, a peak in the O- density is observed in the early afterglow plasma. In the Ar/O-2/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 O-2 discharge, in the Ar/O-2/C4F8 gas mixture the electron density decay is faster. In the afterglow the F- density is ten times higher than the O- density. These results suggest a change in the dominant negative ion production mechanism within the discharge pulse from dissociative electron attachment of CxFy in the active on-phase to electron attachment to fluorine atoms in the afterglow plasma.