Electron and ion kinetics in three-dimensional confined microwave-induced microplasmas at low gas pressures

The effects of the gas pressure (pg), microcavity height (t), Au vapor addition, and microwave frequency on the properties of three-dimensional confined microwave-induced microplasmas were discussed in light of simulation results of a glow microdischarge in a three-dimensional microcavity (diameter dh = 1000 μm) driven at constant voltage loading on the drive electrode (Vrf) of 180 V. The simulation was performed using the PIC/MCC method, whose results were experimentally verified. In all the cases we investigated in this study, the microplasmas were in the γ-mode. When pg increased, the maximum electron (ne) or ion density (nAr+) distributions turned narrow and close to the discharge gap due to the decrease in the mean free path of the secondary electron emission (SEE) electrons (λSEE-e). The peak ne and nAr+ were not a monotonic function of pg, resulting from the two conflicting effects of pg on ne and nAr+. The impact of ions on the electrode was enhanced when pg increased. This was determined after comparing the results of ion energy distribution function (IEDFs) at various pg. The effects of t on the peaks and distributions of ne and nAr+ were negligible in the range of t from 1.0 to 3.0 mm. The minimum t of 0.6 mm for a steady glow discharge was predicted for pg of 800 Pa and Vrf of 180 V. The Au vapor addition increased the peaks of ne and nAr+, due to the lower ionization voltage of Au atom. The acceleration of ions in the sheaths was intensified with the addition of Au vapor because of the increased potential difference in the sheath at the drive electrode.