Global model Simulations of low-pressure oxygen discharges

We use a global model (volume averaged) to study plasma discharges in molecular oxygen gas in the 1-100 mTorr pressure range. This model determines densities of positive ions O-2(+) and O+, negative ion O-, electrons, ground state O-2 and O atoms, and metastables O-2(a(1)Delta g) and O(D-1), and electron temperature as function of gas pressure and input power, for a cylindrical discharge. We apply the model to O-2 discharges and the results are compared to a particle-in-cell simulation (PIC), experimental data and a volume-averaged global model developed at the University of California at Berkeley. We find that the total positive ion density increases with pressure at low pressures (up to approximately 30 mTorr), and decreases at higher pressures. The electronegativity decreases with increased power and increased pressure as predicted by the global models presented in the literature. The predictions for electron temperature are also in agreement with these models. However, there is a discrepancy betweeen these global models and PIC simulations and experimental data, for 20 and 40 mTorr cases, concerning electronegativity calculations. PIC simulations yield much higher electronegativities. There are strong indications that this is due to the assumption of Maxwellian electron energy distribution functions in the global model, while in the PIC simulations this is clearly not the case.