The effect of ambipolar electric fields on the electron heating in capacitive RF plasmas

TL;DR

This paper explores how ambipolar electric fields outside sheaths influence electron heating in RF plasmas, revealing their significant role alongside classical sheath expansion heating through simulations and an analytical model.

Contribution

It introduces a detailed analytical model for electric fields outside sheaths and highlights the importance of ambipolar fields in electron heating mechanisms in RF plasmas.

Findings

Ambipolar electric fields significantly contribute to electron heating outside sheaths.

Electron ionization is mainly caused by electrons backscattering during sheath expansion.

Time modulation of ambipolar fields enhances overall electron heating.

Abstract

We investigate the electron heating dynamics in electropositive argon and helium capacitively coupled RF discharges driven at 13.56 MHz by Particle in Cell simulations and by an analytical model. The model allows to calculate the electric field outside the electrode sheaths, space and time resolved within the RF period. Electrons are found to be heated by strong ambipolar electric fields outside the sheath during the phase of sheath expansion in addition to classical sheath expansion heating. By tracing individual electrons we also show that ionization is primarily caused by electrons that collide with the expanding sheath edge multiple times during one phase of sheath expansion due to backscattering towards the sheath by collisions. A synergistic combination of these different heating events during one phase of sheath expansion is required to accelerate an electron to energies above the threshold for ionization. The ambipolar electric field outside the sheath is found to be time modulated due to a time modulation of the electron mean energy caused by the presence of sheath expansion heating only during one half of the RF period at a given electrode. This time modulation results in more electron heating than cooling inside the region of high electric field outside the sheath on time average. If an electric field reversal is present during sheath collapse, this time modulation and, thus, the asymmetry between the phases of sheath expansion and collapse will be enhanced. We propose that the ambipolar electron heating should be included in models describing electron heating in capacitive RF plasmas.