Comparison of measurements and particle-in-cell simulations of ion energy distribution functions in a capacitively coupled radio-frequency discharge

The ion dynamics in the high-voltage sheath of a capacitively coupled radio-frequency plasma has been investigated using mass-resolved ion energy analysis in combination with a two-dimensional particle-in-cell (PIC) code. A symmetric confined discharge is designed allowing highly accurate comparisons of measured ion energy distribution functions in high-voltage sheaths with simulation results. Under the conditions investigated, the sheaths are not only collisional, but also chemically complex. This situation is common in applications but rare in laboratory experiments. Excellent agreement has been found for a hydrogen discharge benchmarking the code. Hydrogen is of particular interest since its light mass gives detailed insight into sheath dynamics, and an extensive database of collisional cross sections is available. The H3+ ion was found to be the dominant ion in the sheaths and the plasma bulk under most conditions investigated. H3+ exhibits the typical saddle-shaped ion energy distribution function indicative of ions created in the plasma bulk and traversing the entire sheath potential. H+ and H2+ are predominantly formed through collisions in the high-voltage sheath. H2+ ion energy distribution functions show structures resulting from symmetric charge exchange collisions with the background gas. Minor discrepancies between the experimental results and PIC simulations indicate slightly lower plasma densities in the simulation, resulting in larger sheath width.