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

J. Schulze, Z. Donkó, A. Derzsi, I. Korolov, E. Schuengel

Research output: Contribution to journalArticle

41 Citations (Scopus)

Abstract

We investigate the electron heating dynamics in electropositive argon and helium capacitively coupled RF discharges driven at 13.56MHz by particle-in-cell simulations and by an analytical model. The model allows one 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 toward 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.

Original languageEnglish
Article number015019
JournalPlasma Sources Science and Technology
Volume24
Issue number1
DOIs
Publication statusPublished - Feb 1 2015

Keywords

  • ambipolar electric fields
  • capacitive radio frequency plasmas
  • electron heating
  • particle in cell simulations
  • plasma generation
  • stochastic heating

ASJC Scopus subject areas

  • Condensed Matter Physics

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