Particle-in-Cell Simulations of Multi-Peak Ion Impact Energy Distributions in RF Capacitively-Coupled Discharges using the hPIC2 code

Capacitively-Coupled Plasma (CCP) discharges exhibit complex behavior in their ion impact energy distribution function, exhibiting multiple peaks at different discharge regimes. Such multi-peak behavior of CCPs has been observed both experimentally [Wild; Koidl, Appl. Phys. Lett. 54, 1989] and numerically [Sharma et. al., J. Phys. D: Appl. Phys., 2022]. In this work we systematically analyze the occurrence of multiple ion peaks through detailed Particle-in-Cell simulations using the hPIC2 code [Meredith, Comput. Phys. Commun. 283, 2023]. We consider the case of an argon CCP operating between 5 and 200 mTorrs of neutral pressure. The energy peaks are a result of radio-frequency sheath modulation and ion-neutral interactions, dominated by inelastic charge exchange collisions. Ion-neutral collisions create low energy ions within the sheath, which cannot respond to the full sheath potential. It is observed that the number of low-energy peaks roughly scales with the ratio between the ion transit time across the sheath and the RF period.