Determination of gas temperature and C2 absolute density in Ar/H2/CH4 microwave discharges used for nanocrystalline diamond deposition from the C2 Mulliken system

The spectroscopic characterization of Ar/H-2/CH4 discharges suitable for the synthesis of nanocrystalline diamond using the microwave plasma assisted chemical vapour deposition process is reported. The experiments are realized in a moderate-pressure bell jar reactor, where discharges are ignited using a microwave cavity coupling system. The concentration of CH4 is maintained at 1% and the coupled set of hydrogen concentration/microwave power (MWP) ranges from 2%/500 W to 7%/800 W at a pressure of 200 mbar. Emission spectroscopy and broadband absorption spectroscopy studies are carried out on the C-2 (D(1)Sigma(u)(+)-(XEg+)-E-1) Mulliken system and the C-2(d(3)Pi(g)-a(3)Pi(u)) Swan system in order to determine the gas temperature and the C-2 absolute density within the plasma. For this purpose, and since the Swan system is quite well-known, much importance is devoted to the achievement of a detailed simulation of the Mulliken system, which allows the determination of both the rotational temperature and the density of the X(1)Sigma(g)(+) ground state, as well as the rotational temperature of the D(1)Sigma(u)(+) state, from experimental data. All the experimental values are compared to those predicted by a thermochemical model developed to describe Ar/H-2/CH4 microwave discharges under quasi-homogeneous plasma assumption. This comparison shows a reasonable agreement between the values measured from the C-2 Mulliken system, those measured from the C-2 Swan system and that calculated from plasma modelling, especially at low hydrogen concentration/MWP. These consistent results show that the use of the Mulliken system leads to fairly good estimates of the gas temperature and of the C-2 absolute density. The relatively high gas temperatures found for the conditions investigated, typically between 3000 K and 4000 K, are attributed to the low thermal conductivity of argon that may limit thermal losses to the substrate surface and reactor wall. The measured C-2 absolute densities range from 1013 to 10(14) cm(-3) depending on the experimental conditions. These high values may result from an enhanced thermal conversion of hydrocarbon species due to the high gas temperature.