Experimental benchmark of kinetic simulations of capacitively coupled plasmas in molecular gases

Abstract

We discuss the origin of uncertainties in the results of numerical simulations of low-temperature plasma sources, focusing on capacitively coupled plasmas. These sources can be operated in various gases/gas mixtures, over a wide domain of excitation frequency, voltage, and gas pressure. At low pressures, the non-equilibrium character of the charged particle transport prevails and particle-based simulations become the primary tools for their numerical description. The particle-in-cell method, complemented with Monte Carlo type description of collision processes, is a well-established approach for this purpose. Codes based on this technique have been developed by several authors/groups, and have been benchmarked with each other in some cases. Such benchmarking demonstrates the correctness of the codes, but the underlying physical model remains unvalidated. This is a key point, as this model should ideally account for all important plasma chemical reactions as well as for the plasma-surface interaction via including specific surface reaction coefficients (electron yields, sticking coefficients, etc). In order to test the models rigorously, comparison with experimental 'benchmark data' is necessary. Examples will be given regarding the studies of electron power absorption modes in O 2 , and CF 4 -Ar discharges, as well as on the effect of modifications of the parameters of certain elementary processes on the computed discharge characteristics in O 2 capacitively coupled plasmas.

Physics of E × B discharges relevant to plasma propulsion and similar technologies

“…At the end of the 90s, a standardized RF discharge experiment -socalled, GEC Reference Cell [320] was also specifically designed for validation and is still used as shown in the recent study performed in Ref. [321]. Recently, there has been different works on the benchmarking of codes.…”

Ion energy and angular distributions in low-pressure capacitive oxygen RF discharges driven by tailored voltage waveforms

“…Our model of the oxygen CCPs is largely based on the "xpdp1" set of elementary processes [73] and its recent revision [74], and is the same as described in our previous works [65,66,69]. Therefore, only a brief summary of the features of the model is given below and the reader is referred to [65] for details.…”

“…The code allows including the emission of secondary electrons from the electrodes; we use SEEC values of γ = 0 (disregard electron emission), γ = 0.06 (a value that is characteristic for metal surfaces), and γ = 0.4 (a value typical for dielectric surfaces). The gas temperature is fixed at T g = 350 K. For the surface quenching probability of O 2 (a 1 ∆ g ) singlet delta molecules we use the value of α = 6 × 10 −3 , which resulted in our previous studies in a good overall agreement between the experimental and simulation data for the ion fluxes and the ion flux-energy distribution functions at the electrodes [65,66,69]. In the cases when a DC self-bias voltage develops, its value is determined in an iterative manner to ensure equal losses of positive and negative charges at each electrode over one period of the fundamental driving frequency [31].…”

Challenges and opportunities in verification and validation of low temperature plasma simulations and experiments

“…Recent work includes sensitivity analysis of input model parameters on oxygen RF discharges by varying collision cross sections and surface quenching parameters [12]. In this study, secondary emission coefficients were also varied in a CF 4 -Ar discharge that demonstrated the sensitivity of the sheath to this parameter.…”