Oxygen atom density and kinetics in intermediate-pressure radiofrequency capacitively-coupled plasmas in pure O₂

We have studied radiofrequency (13.56 MHz) capacitively-coupled plasmas in pure O₂ at intermediate pressures (67–800 Pa) using single-mode laser cavity ring-down spectroscopy (CRDS) of oxygen atoms at 630 nm. The absolute atom densities and translational temperatures (from Doppler broadening) in continuous plasma were determined over a range of pressures and RF power (50–900 W). At higher pressures (267–800 Pa), the O-atom mole-fraction increases with RF power and decreases with pressure, reaching a maximum of 15%. However, at lower pressures (67–133 Pa) it passes through a distinct maximum with power before decreasing significantly. The atom recombination processes were probed by time-resolved measurements in the afterglow of pulse-modulated plasmas. At 67 and 133 Pa the atom loss is dominated by surface recombination, and we see clear evidence that this rate is increased by energetic ion bombardment. However, this effect only partially explains the observed decrease in dissociation at high RF power. At higher pressures gas phase recombination mechanisms dominate oxygen atom loss, although gas convection driven by gas cooling in the afterglow complicates quantitative analysis of the time-resolved data. Time-resolved CRDS measurements of the continuum absorption adjacent to the O atom resonance allow the O⁻ negative ion density to be determined, as well as indicating the creation of ozone in the afterglow. At 133 Pa, the trends with RF power of the O₂ dissociation, O^- density and gas temperature suggest a transition to a plasma mode at high power which has fewer electrons at energies capable of dissociating O₂.