Effect of gas pressure on plasma asymmetry and higher harmonics generation in sawtooth waveform driven capacitively coupled plasma discharge

TL;DR

This study uses PIC simulations to analyze how varying gas pressure affects plasma asymmetry, ionization, electron energy distribution, and higher harmonics in a symmetric capacitively coupled plasma driven by a sawtooth waveform.

Contribution

It provides detailed insights into the pressure-dependent behavior of plasma asymmetry, harmonics, and electron distributions in sawtooth-driven CCPs, highlighting the transition from asymmetric to symmetric discharges.

Findings

Plasma asymmetry decreases with increasing pressure, becoming symmetric at high pressures.

Higher harmonics are prominent at low pressure and diminish at higher pressures.

Electron energy distribution shifts from bi-Maxwellian to Maxwellian with increasing pressure.

Abstract

Using particle-in-cell (PIC) simulation technique, the effect of gas pressure (5-500 mTorr) on the plasma spatial asymmetry, ionization rate, metastable gas densities profile, electron energy distribution function and higher harmonics generation are studied in a symmetric capacitively coupled plasma discharge driven by a sawtooth-like waveform. At a constant current density of 50 A/m2, the simulation results predict a decrease in the plasma spatial asymmetry (highest at 5mTorr) with increasing gas pressure reaching a minimum value (at intermediate gas pressures) and then turning into a symmetric discharge at higher gas pressures. Conversely, the flux asymmetry shows an opposite trend. At a low gas pressure, the observed strong plasma spatial asymmetry is due to high frequency oscillation on the instantaneous sheath edge position near to one of the electrodes triggered by temporally asymmetry waveform, whereas the flux asymmetry is not present due to collisionless transport of charge particles. At higher pressures, multi-step ionization through metastable states dominates in the plasma bulk, causing a reduction in the plasma spatial asymmetry. Distinct higher harmonics (26th) are observed in the bulk electric field at low pressure and diminished at higher gas pressures. The electron energy distribution function changes its shape from bi-Maxwellian at 5 mTorr to nearly Maxwellian at intermediate pressures and then depletion of the high-energy electrons (below 25 eV) is observed at higher gas pressures. The inclusion of the secondary electron emission is found to be negligible on the observed simulation trend.