Oxygen atom density and kinetics in intermediate-pressure radiofrequency capacitively-coupled plasmas in pure O2
We have studied radiofrequency capacitively coupled plasmas in pure O2 using single mode laser cavity ringdown spectroscopy of oxygen atoms at 630 nm. The absolute atom densities and translational temperatures were determined over a range of pressures and RF power . At pressures of 267 Pa and above, the O atom mole fraction increases with RF power and decreases with pressure, reaching a maximum of 15 percent. However, at 133 and 67 Pa it passes through a distinct maximum with power before decreasing significantly. The atom recombination processes are probed by time resolved measurements in the afterglow of pulse modulated plasmas. At 133 and 67 Pa the atom loss is dominated by surface recombination, and we see clear evidence that this rate is increased by energetic ion bombardment, in agreement with a study from Bill Graham group. This effect partially explains the observed decrease in dissociation at high RF power. The time-resolved results also allow the O negative ion density to be determined and indicate the creation of ozone in the afterglow. At 133 Pa, the trends with RF power of the O2 dissociation, O negative ion density and gas temperature suggest a transition at high power to a plasma mode with fewer high energy electrons. At higher pressures gas phase recombination mechanisms become dominant, however gas convection driven by gas cooling in the afterglow makes it complex to analyse the time-resolved data.