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, γeff, and the effective elastic electron reflection coefficient, reff, 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 γeff and reff. 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 emitted ion induced secondary electron flux, 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 γeff and reff under realistic plasma conditions.

Metastable argon atom kinetics in a low-pressure capacitively coupled radio frequency discharge

“…[22]. We note, that the basic PIC/MCC code has been benchmarked with other independent codes [78] and validated with experiments [72,73].…”

“…These simulation settings respect the stability criteria of the PIC/MCC scheme [70,71] for the given discharge parameters. At the electrode surfaces, an ion-induced effective secondary electron yield of γ = 0.07 and an effective electron reflection coefficient of r = 0.7 are adopted [72,73]. The (time-averaged) Electron Energy Probability Function, EEPF, f(ε), normalized as ´f(ε) √ εdε = 1 is computed in the PIC/MCC code for the central 10%-wide domain of the discharge.…”

Review: Machine learning for advancing low-temperature plasma modeling and simulation

“…More recent efforts include the combination of both (i.e., PIC/MCC simulations, phase resolved OES) and was framed as γ-CAST. 208,209 Moreover, a review of MC methods to simulate the electron scattering in solids was put together by Chang et al, 210 who studied the low-energy electron deposition in W surfaces. Electron force field simulations have been demonstrated to predict electron emissions due to the Auger process.…”

“…Although experimental as well as theoretical approaches have been pursued in the past, the availability of such coefficients is scarce. More recent efforts include the combination of both (i.e., PIC/MCC simulations, phase resolved OES) and was framed as γ-CAST 208 , 209 . Moreover, a review of MC methods to simulate the electron scattering in solids was put together by Chang et al., 210 who studied the low-energy electron deposition in W surfaces.…”

Fine structure resolved excitation cross sections of singly ionized Ga for the modeling and diagnostics of Ga plasmas

“…This data plays a pivotal * Author to whom any correspondence should be addressed. role in accurately determining number densities and plasma species fluxes, particularly in scenarios involving the impact of plasma on microorganisms and food [3] and in the computation of surface coefficients for materials within capacitively coupled plasma (CCP) systems [4]. Furthermore, collision cross section data is invaluable for investigating the dielectric properties of ternary mixtures that include CO 2 , N 2 , and O 2 [5].…”