Improving Actinometric Diagnostics for Application to CF₄-containing Plasmas

Actinometry is a widely used spectroscopic technique based on optical emission line ratios, commonly employed for the non-intrusive characterization of ground-state species densities in plasmas, for example, atomic hydrogen (H), oxygen (O), and fluorine (F). However, the method relies on several simplifying assumptions, such as the presumption of thermal (Maxwellian) electron energy distribution functions (EEDFs), which introduce significant limitations to its accuracy and applicability. In this work we present a computational framework for improving the reliability of actinometry-based diagnostics by integrating detailed electron kinetics into the computations. The proposed approach exploits the LisbOn KInetics Boltzmann solver, using a two-term approximation (LoKI-B) [1] and a Monte Carlo technique (LoKI-MC) [2]. Both solvers provide the EEDFs, which are used to compute the excitation rate coefficients, avoiding the assumption of thermal electrons and reflecting the discharge conditions more accurately. Implemented in low-pressure (1–100 Pa) radiofrequency (RF) discharges with noble gases (e.g., Ar and He), the framework uses the measured electron temperature and emission line ratios as a zero-approximation, to infer the reduced electric field (E/N) and radical densities. By comparing the outputs from LoKI-B and LoKI-MC and validating them against more robust diagnostics, we assess the solvers' applicability across a range of pressures and discharge regimes. This validated methodology sets the stage for future applications to the CF₄-containing plasmas, where it is expected to significantly enhance the accuracy of F atoms actinometry.

[1] A. Tejero-del-Caz et al Plasma Sources Sci. Technol. 28 (2019) 043001.

[2] T. C. Dias et al Computer Physics Communications 282 (2023) 108554.