Structure of Velocity Distribution of Sheath-Accelerated Secondary Electrons in Asymmetric RF-DC Discharge

Low-pressure capacitively-coupled discharges with additional DC bias applied to a separate electrode find important industrial applications. Prime examples are plasma-assisted etching and deposition technologies. An interesting and important property of such discharges, observed in experiments, is an enhanced and non-monotonic high-energy tail of the electron velocity distribution function (EVDF) near the surface of the RF (a.k.a. powered) electrode. Such structures, at energies of several hundred eV, are possibly caused by secondary electrons emitted from the electrodes and interacting with two high-voltage sheaths; a stationary sheath at the DC electrode and an oscillating, self-biased sheath at the powered electrode. We have performed particle simulations where the features in the EVDF of electrons impacting the RF electrode are fully resolved at all energies. An analytic model has been developed to predict existence of peaked and step-like structures in the EVDF. The latter electrons can be grouped by the number of bounces between the sheaths during their lifetime in the discharge. Each of the groups may give rise to an individual peak in the distribution. Initial particle-in-cell simulations of these effects will be reported.