Electron series resonant discharges: Comparison between simulation and experiment

Abstract

Plasma discharges driven at the electron series resonance (ESR) frequency have many desirable properties. The input resistance is small and the drive voltage and current are in-phase. Also, the drive voltage is small (∼Te) and the average plasma potential is low (∼10Te). Particle-in-cell simulations with Monte Carlo collisions (PIC-MCC) show that a strongly kinetic phase space bunching process provides electrons of sufficient energy for ionization, allowing discharge operation at low neutral pressures and low electron temperatures. Simulations also show that, at these low pressures, the ion flux to the wall has a narrow angular spread about the normal and the ion bombarding energy distribution has a sharp peak at the plasma potential. In this paper PIC-MCC simulations recreating the ESR discharge experiment by Godyak are performed. The driven V-I characteristics of a plasma diode (magnitude and phase angle) are measured in simulation and found to compare well with experimental results. Steady state characteristics and the response of an ESR discharge to slow changes in RF drive amplitude and frequency are shown as well. The steady state properties of an ESR sustained discharge are demonstrated in simulations at different pressures. The applied RF voltage in these discharges is comparable to the electron temperature and well below the ion sheath potential. This is in stark contrast to capacitive discharges where the ion sheath potential is comparable to ∼0.8 VRF. Scaling laws at fixed pressure (n ∞ ωRF3, s ∞ ωRF-1) are shown to hold when RF is varied smoothly ('chirping'), demonstrating continuous density control. Hysteresis is observed in the V-I characteristics. The behaviour about hysteresis suggests resonant discharges should be initiated from a capacitive state rather than from an inductive state.