Drift-Diffusion models for RF-CCPs at intermediate pressure: estimating transport coefficients

RF-CCPs at gas pressures above 1Torr are often simulated by fluid models using the drift-diffusion approximation, since Particle-in-Cell (PIC) models are too computationally intensive. Fluid models require the electron transport coefficients (mobility, diffusion, energy mobility and energy diffusion), as well as rate constants (for ionisation, energy loss, etc.). These are calculated from the electron energy distribution function (EEDF), which is commonly estimated using either the local energy approximation or by a simple Maxwellian distribution. We compare these approaches to a full PIC model in a RF-CCP in 1Torr Ar. Here the local-energy EEDF is calculated as a function of the electron mean energy, using a Boltzmann solver (Loki [2, 3]). This assumption is well justified for homogeneous DC glow discharges, but its applicability to time-varying fields (e.g. RF-CCP) is more questionable. Due to numerical reasons, previous researchers employed the local-energy mobility and calculate the other coefficients from the mean electron energy using the Einstein relations (which are only strictly valid for a Maxwellian distribution) assuming a constant momentum-transfer collision frequency.