Argon metastable densities in radio frequency Ar, Ar/O2 and Ar/CF4 electrical discharges

The spatial distributions of excited states in radio frequency electrical gas discharges have been observed to be dynamic functions of gas mixture, pressure, and applied voltage. Recent measurements of two-dimensional profiles of excited states in the Gaseous Electronics Conference reference cell (GECRC) [McMillin and Zachariah, J. Appl. Phys. 77, 5538 (1995); 79, 77 (1996)] have shown that the spatial distribution of the $\mathrm{Ar}\text{(4s)}$ density varies considerably with operating conditions. The peak density of $\mathrm{Ar}\text{(4s)}$ systematically shifted in position, as well as changed in magnitude, with variations in pressure, applied voltage, and gas mixture. In this article, we present results from a two-dimensional computer simulation of Ar, ${\mathrm{Ar/O}}_{\mathrm{2}},$ and ${\mathrm{Ar/CF}}_{\mathrm{4}}$ discharges sustained in the GECRC with the intent of investigating the experimental trends. The simulations, performed with the Hybrid Plasma Equipment Model, agree well with experiments. They show that the shift in $\mathrm{Ar}\text{(4s)}$ densities is largely explained by the reduction in the electron mean free path, and local perturbations in the ambipolar electric field resulting from electrode structures. Additions of small amounts of ${\mathrm{O}}_{\mathrm{2}}$ and ${\mathrm{CF}}_{\mathrm{4}}$ decrease the $\mathrm{Ar}\text{(4s)}$ density due to quenching, and change its profile due to a transition to an electronegative plasma.