Kinetic effects on the radio-frequency sheath dynamics

Abstract

Summary form only given. The collisionless rf sheath dynamics is studied using a hybrid simulation with kinetic ions and drift-diffusion electrons. The ion velocity distribution function is obtained by solving the ion Vlasov equation using the cubic interpolated propagation (CIP) scheme. The results are compared with the solutions for the ion fluid equations in different rf frequency regimes. We consider two kinds of ion source term: uniform source and non-uniform source which is proportional to the electron density, for different source temperatures. We find that the dynamics variables in the sheath such as the ion density, ion velocity, electric field and potential are insensitive to the source temperature. In the different frequency regime, the kinetic ions respond to the rf field in a similar way as in the fluid theory. For the uniform source, if the generation rate of the cool ions is significant, a low-energy tail due to the creation of the cool ions in the sheath is formed in the ion velocity distribution function. In this case, the ion pressure term in the sheath is no longer negligible and the ion velocity is then affected. In the low rf frequency regime where the rf frequency is much lower than the ion plasma frequency, the cold ions born in the sheath can result in the formation of a low-energy peak in the ion velocity distribution function and multi-peaks structure in the time-averaged ion velocity distribution function which represents the experimentally measurable ion energy distribution (IED). In contrast, if the ion source term is proportional to the electron density, much fewer cold ions are created in the sheath since the electron density decays rapidly. Consequently, the low-energy tail disappears and the predictions of the kinetic and fluid theories are in good agreement.