Power dynamics in a low pressure capacitively coupled plasma discharge

A one-dimensional coupled particle-in-cell and fluid model is used to understand power dynamics at low gas pressures in a capacitively coupled Ar discharge. For the range of gas pressure (5-500 mTorr) and excitation frequency (30-120 MHz) examined, the electrons absorb power at the sheath edge during sheath expansion. Energetic electron beams are generated at the edge of the expanding sheath, which are responsible for plasma production and sustenance. These energetic electrons are able to reach the opposite sheath at low gas pressures and return some of their energy during deceleration in the sheath. As a result, peak electron density decreases significantly below 10 mTorr. Above 50 mTorr, peak electron density is relatively insensitive to pressure as beam electrons deposit most of their energy in the plasma bulk. Secondary electron emission is found critical for plasma sustenance at 30 MHz, while sheath electron heating is the dominant electron heating mechanism at higher frequencies.