Investigation of Plasma Characteristics in VHF Capacitive Discharges Operated at Extremely Low Pressures and High Voltages (kV Range)*

In recent years, capacitively coupled discharges (CCP) operated at very high frequencies (VHF) have garnered significant attention. Present study reports on the plasma properties in collisionless CCP discharges, where the electron and ion mean free paths are significantly larger than the system length, operated at very low pressures and applied potentials in the kiloVolt range, triggered by VHF (60 MHz). Understanding the plasma parameters that can sustain plasma at such extreme low pressures and characterizing it in detail is valuable for industries. It has been observed that very high-density plasma can be produced in the collisionless limit, which may hold significant interest for various industrial applications. Interestingly, the presence of higher harmonics at the center of the discharge has been observed, which significantly alters the bulk plasma properties. This finding has been verified by analyzing the shape of the electron energy distribution function (EEDF) in the bulk plasma. The electron heating mechanism and ion energy distribution function (IEDF) in the discharge is also investigated in this study.