Angular Ion and Neutral Energy-Distribution in a Collisional Rf Sheath

A numerical study on the ion and neutral angular impact energy distribution at the rf-driven electrode of a reactive ion etcher is presented. The calculations for the ions are performed using a Monte Carlo method that includes charge exchange and elastic scattering. The contribution of both collision processes to the angular ion impact energy distribution is studied. For the case that charge exchange is the only collision process, the Monte Carlo results can be checked against those of a method based on a spatially uniform and time independent collision rate. In that case, both methods yield the same ion impact energy distribution. The position, velocity, and propagation angle of the energetic neutrals created in collisions of ions with the background gas are stored. These are used as input data for a separate code that follows the evolution of the angular neutral energy distribution, taking into account (multiple) neutral elastic scattering. From the ion and neutral distributions, the number of neutrals per ion, the average impact energy, and the energy-weighted average impact angle have been derived. It is shown that these parameters are well described by simple expressions. Finally, the sputter yield is calculated. The results show that the contribution of the angular distributions of both ions and neutrals to the yield can be neglected.