PIC/MCC simulation of capacitively coupled discharges in helium: boundary effects

Abstract

Particle-in-cell/Monte Carlo collision simulations considering one space and three velocity coordinates are applied to analyse the impact of surface properties on plasma parameters in low-pressure capacitively coupled radio-frequency discharges operating in α and γ mode, respectively. In particular, it is studied how electron reflection and secondary electron emission (SEE) contribute to the characteristics of sinusoidally excited, geometrically symmetric discharges under different operating conditions in the pressure range from 4 to 80 Pa and how such discharges respond to asymmetric SEE at the electrodes. It is found that electron reflection and SEE similarly control the important plasma parameters such as plasma density and mean electron energy at lower pressures (α mode). Furthermore, different materials of the electrodes involving asymmetric SEE do not induce the formation of asymmetric discharge features under these conditions. When increasing the pressure, a transition from α to γ mode is observed which causes a decrease of the relevance of electron reflection and an initiation of the secondary electron asymmetry effect, i.e., asymmetric SEE, results in asymmetric plasma distributions. This provides an additional option to control, e.g., the velocity distribution of ions at one of the electrodes.