The effect of electron induced secondary electrons on the characteristics of low-pressure capacitively coupled radio frequency plasmas

Abstract

We investigate the effects of secondary electrons (SEs), induced by electrons impinging on the electrodes, on the characteristics of low-pressure single-frequency capacitively coupled plasmas (CCPs) by particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations. In a recent PIC/MCC simulation study, that incorporated a realistic description of the electron-surface interaction, such electron-induced SEs (δ-electrons) were found to have a remarkable impact on the ionization dynamics and the plasma parameters in argon at 0.5 Pa and 6.7 cm gap between SiO2 electrodes (Horváth et al 2017 Plasma Sources Sci. Technol. 26 124001). At such low pressure and at high voltage amplitudes, the ion-induced SEs (γ-electrons) emitted at one electrode can reach the opposite electrode with high energies, where, depending on the surface material and surface conditions, they can induce the emission of a high number of δ-electrons, which can cause significant ionization and a higher plasma density. Here, we study the influence of δ-electrons on the ionization dynamics and plasma parameters at various pressures and voltage amplitudes, assuming different SE yields for ions (γ-coefficient) in single-frequency 13.56 MHz argon discharges. The emission of SEs by electron impact is found to be an important plasma-surface process at low pressures, between 0.5 Pa and 3 Pa. Both the gas pressure and the value of the γ-coefficient are found to affect the role of δ-electrons in shaping the discharge characteristics at different voltage amplitudes. Their effect on the ionization dynamics is most striking at low pressures, high voltage amplitudes and high values of the γ-coefficient. However, in the whole parameter regime investigated here, the realistic description of the electron-surface interaction significantly alters the computed plasma parameters, compared to results obtained based on a simple model for the description of the electron-surface interaction, widely used in PIC/MCC simulations of low-pressure CCPs.