The role of electron induced secondary electron emission from SiO₂surfaces in capacitively coupled radio frequency plasmas operated at low pressures

Abstract

The effects of electron induced secondary electron (SE) emission from SiO 2 electrodes in singlefrequency capacitively coupled plasmas (CCPs) are studied by particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations in argon gas at 0.5 Pa for different voltage amplitudes. Unlike conventional simulations, we use a realistic model for the description of electron-surface interactions, which takes into account the elastic reflection and the inelastic backscattering of electrons, as well as the emission of electron induced SEs (δ-electrons). The emission coefficients corresponding to these elementary processes are determined as a function of the electron energy and angle of incidence, taking the properties of the surface into account. Compared to the results obtained by using a simplified model for the electron-surface interaction, widely used in PIC/MCC simulations of CCPs, which includes only elastic electron reflection at a constant probability of 0.2, strongly different electron power absorption and ionization dynamics are observed. We find that ion induced SEs (γ-electrons) emitted at one electrode and accelerated to high energies by the local sheath electric field propagate through the plasma almost collisionlessly and impinge on the opposing sheath within a few nanoseconds. Depending on the instantaneous local sheath voltage these energetic electrons are either reflected by the sheath electric field or they hit the electrode surface, where each γ-electron can generate multiple δ-electrons upon impact. These electron induced SEs are accelerated back into the plasma by the momentary sheath electric field and can again generate δ-electrons at the opposite electrode after propagating through the plasma bulk. Overall, a complex dynamics of γand δ-electrons is observed including multiple reflections between the boundary sheaths. At high voltages, the electron induced SE emission is found to strongly affect the plasma density and the ionization dynamics and, thus, it represents an important plasma-surface interaction that should be included in PIC/MCC simulations of CCPs under such conditions.

Heavy-particle induced secondary electrons in capacitive radio frequency discharges driven by tailored voltage waveforms

“…Other types of electron-surface processes, e.g., the emission of SEs due to electron impact, are neglected. We note that recent studies have found considerable effects of the electron induced SEs on the plasma parameters at specific discharge conditions 57,58 , i.e., at very low pressures (p ( 3 Pa) and high voltage amplitudes, for dielectric surfaces. However, in the present study, metal electrodes are assumed (compared to dielectric surfaces, the maximum of the electron induced SE yield is much lower for metal surfaces), and the discharge conditions covered here are significantly different (higher pressures, lower f tot ) compared to those in Refs.…”

Material dependent modeling of secondary electron emission coefficients and its effects on PIC/MCC simulation results of capacitive RF plasmas

“…However, powerful models already exist for δ and ρ [21][22][23]. The implementation of δ in PIC/MCC simulations and its effects on the calculated discharge characteristics have already been studied for SiO 2 boundary surfaces [24,25]. Other electron ejection processes from surfaces by fast neutrals, metastable atoms and VUV photons are also suggested to be important [13,26].…”

Modelling vacuum flashover mitigation with complex surface microstructure: mechanism and application

“…In many past studies concerning the electron dielectric interaction, only true SEE is taken into consideration [19,20], while this neglects the fact that not all incident electrons can penetrate the dielectric surface. The recent study by Horváth et al indicated that electron reflection/backscattering can be remarkable when incident electrons have relatively low energy [29]. A similar algorithm was also employed in previous Hall Thruster simulation code EDIPIC compiled by Sydorenko et al [39].…”

“…Pierron et al [27] have proved that the SEY of the grooved surface greatly reduces. Swanson et al obtained similar results but with a different surface structure [28–30]. Meanwhile theoretical results, such as the saturated charge density of SSEE stage and the relation between the charge density and the flashover voltage, were also studied by Pillai and Hackam [31].…”