A computationally assisted technique to measure material-specific surface coefficients in capacitively coupled plasmas based on characteristics of the ion flux-energy distribution function

Abstract

We present a new method for the determination of surface coefficients, more specifically the effective ion-induced secondary electron yield, γ eff, and the effective elastic electron reflection coefficient, r eff, by means of a synergistic combination of energy-selective mass spectrometry measurements and numerical particle-in-cell/Monte Carlo collisions simulations of the ion flux-energy distribution function (IEDF) in a symmetric capacitively coupled plasma (CCP). In particular, we analyze the bimodal peak structure of the IEDF, which is caused by ions crossing the sheath without collisions. The position and width of this structure on the energy scale are defined by the time-averaged sheath potential and the ion transit time through the sheath, respectively. We find that both characteristics are differently influenced by γ eff and r eff. The ion-induced secondary electrons are accelerated in the large sheath potential and mainly influence the plasma density, sheath width and, consequently, the ion transit time and in this way the bimodal peak separation. Electron reflection from the electrodes acts mainly at times of sheath collapse, where low energy electrons can reach the surfaces. Their contribution to the plasma density increase is small, however, their longer residence time in the vicinity of the electrodes modifies the space charge density and the potential gradient. Additionally, the charge balance at the electrode requires an incident electron flux that is correlated to the flux of emitted ion induced secondary electrons and reflected electrons, which is realized by a change of the electron repelling sheath voltage. As a consequence, the electron reflection coefficient mainly influences the sheath potential and, hence, the position of the bimodal peak structure. These effects allow the simultaneous in situ determination of both surface parameters. The parameter values determined for stainless steel and Al2O3 surfaces are in good agreement with literature data. Our method opens a straightforward way of obtaining γ eff and r eff under realistic plasma conditions.