The influence of electron reflection/sticking coefficients at the electrodes on plasma parameters in particle-in-cell simulations of capacitive radio-frequency plasmas

Abstract

Capacitive radio-frequency discharges are frequently used to process different materials. In these systems, plasma-surface interactions are known to affect the discharge by particle emission and reflection. In simulations, the corresponding surface coefficients are input parameters, which are often unknown and are, therefore, roughly estimated or ignored. Electron reflection at boundary surfaces is typically either neglected completely or the reflection coefficient, , where S is the sticking coefficient, is assumed to be small, independently of the surface material and its conditions, although it is known to cover a wide range depending on the material. Here, we systematically investigate the effect of changing ρ in particle-in-cell simulations on plasma parameters such as the plasma density, electric field, and ionization rates, in geometrically symmetric single- and dual-frequency discharges operated in argon at a fundamental frequency of 13.56 MHz and at pressures of 5 Pa–20 Pa. We find that the plasma density strongly depends on the reflection coefficient. High coefficients cause electric field reversals during sheath collapse at the electrodes and an enhanced generation of energetic electron beams during sheath expansion, which lead to additional ionization and higher plasma densities. Different reflection coefficients at both electrodes are found to induce a discharge asymmetry that leads to the generation of a DC self-bias and different mean ion energies at the electrodes. In dual-frequency discharges, the electrical generation of the DC self-bias as a function of the phase between two consecutive driving harmonics via the electrical asymmetry effect can be significantly enhanced by choosing electrode materials with different reflection coefficients. In this way the electrical control range of the mean ion energy via phase control is shifted to different energies.