Vibrational Temperature Estimation of Nitrogen Molecules in Radio-Frequency (RF) Produced Plasma

A capacitive coupled radio frequency (RF) plasma system has been developed for producing tungsten coated graphite tiles using plasma assisted chemical vapor deposition (PACVD) technique. To characterize the deposition chamber for optimal plasma parameters, small amount of air is released into the hydrogen plasma purposefully to measure its gas temperature using spectral bands of nitrogen molecule. Optical emission spectra in the wavelength range 350 to 900 nm have been recorded with a miniature spectrometer. Molecular spectral bands of N-2 (B-3 Pi(g) -A(3) Sigma(+)(u)) have been observed and identified as three bands from the nitrogen 1PS (Delta v = 2, 3 & 4). These bands are simulated using MATLAB code developed in-house by considering Boltzmann distribution of particles in the vibrational states. The experimental spectra have been modelled with the simulated spectrum through the best-fit technique by iterating the latter one with different temperature values. Boltzmann plot method is also utilized to evaluate plasma gas temperature using identified vibrational bands. The estimated temperature using spectral modelling method matches fairly well with Boltzmann plot method. The estimated vibrational temperatures are in the range of similar to 7000 - 8000 K, an order higher than the room temperature similar to 300 K. (C) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research