Dynamics of argon metastables in Ar-CH₄ radio frequency capacitively-coupled plasma: real-time monitoring with neural network-augmented broadband optical emission spectroscopy

In moderate-pressure radio frequency (RF) capacitively coupled plasmas generated in argon-methane mixtures, the density of argon metastable atoms (Ar 1 s₅) exhibits a non-monotonic dependence on methane (CH₄) concentration. Laser-induced fluorescence (LIF) was used to measure and compare local Ar 1 s₅ densities in Ar and Ar-CH₄ plasmas at 2.6 Torr and RF powers of 17-117 W. The addition of 1% CH₄ increases the metastable density, and 2% CH₄ triggers a strong depletion by an order of magnitude, compared to 1% CH₄ case. This non-monotonic behavior demonstrates the sensitivity of metastable populations to small gas admixtures, which is critical for processes where metastables drive precursor dissociation. For real-time monitoring of metastable population, broadband optical emission spectroscopy (OES) is augmented with a feedforward neural network (NN) to predict Ar s1₅ densities from spectral features. When trained on LIF data, the NN replicates the absolute densities and the dynamic trends of Ar 1 s₅ density variation. The NN-augmented broadband OES approach can be used as a simple and cost-effective tool for tracking Ar metastables in Ar-rich plasmas, facilitating industrial-scale optimization.