Oxygen atom kinetics in pulsed Radiofrequency Capacitively-coupled plasmas in pure O₂ at intermediate pressures

We present an experimental study of a highly-symmetric CCP (Ø 50cm gap 2,.5cm), excited at 13.56 MHz (100-900W), operating in 0.5-6 Torr pure O₂. We used monomode laser cavity ringdown spectroscopy (CRDS) at 630 nm in cw and pulsed plasmas to measure oxygen atom densities and kinetics, as well as the gas temperature, the O^- negative ion density and ozone generation in the afterglow. Electron densities were estimated using a microwave hairpin probe. Above 2 Torr the oxygen atom density increases with pressure and RF power, reaching a mole-fraction of 15%. However, at lower pressures the oxygen atom density passes through a maximum with power. The electron density (and therefore O₂ dissociation rate) increases with power, and at low-pressure shows a sharp increase, indicating a transition into a high density, low (bulk) electron energy mode. The O atom trend indicates that the oxygen atom loss rate (principally due to recombination at the electrode surfaces) increases with power at lower pressures. Time-resolved O atoms measurements in the afterglow confirm that the O atom loss rate increases with the RF power. This corresponds to conditions of increasing energy and flux of ion surface bombardment, suggesting that surface recombination is activated by energetic ion bombardment. Nevertheless, the decays are surprisingly slow, corresponding to surface reaction coefficients of the order several x10^-4. These small values are rather surprising on bare metallic (aluminium) surfaces, and very comparable to the values observed on borosilicate glass, showing the importance of surface oxidation. At higher O₂ pressures the oxygen atom decays become faster (and non-exponential), indicating the onset of gas-phase reactions. However, kinetic models with existing reaction sets are unable to explain our observations, implying that these models may need to be significantly revised