The role of secondary electron emission in capacitive rf plasmas at low pressure

The correct choice of the ion induced secondary electron emission coefficient, $\gamma$, is of high importance to obtain realistic results by PIC/MCC-simulations of capacitive rf plasmas. In most studies, this coefficient is set to $\gamma=0.1$ without taking into account the energy of the incident particles, the electrode material, and the surface conditions. Recently, studies showed that using a more realistic, energy dependent $\gamma$-coefficient strongly influences the outcome of computational investigations at high pressure. In CCPs used for sputtering a much lower pressure of approx. 1Pa is used. In this regime, the plasma-surface interaction can lead to a change of the surface conditions, e.g. by target poisoning. This can result in process drifts. This effect is usually linked to the change of $\gamma$ but it is not understood how $\gamma$ affects the plasma at such low pressures, where the multiplication of secondary electrons within the sheath is negligible. This work investigates the effect of different $\gamma$-coefficients on the discharge by PIC/MCC-simulations at low pressures in argon. It is found that the confinement of $\gamma$-electrons by multiple reflections at the sheaths strongly influences the ionization rate.