Probing the plasma chemistry in a microwave reactor used for diamond chemical vapour deposition by cavity ring down spectroscopy

Absolute column densities of C-2(a) and CH radicals and H(n=2) atoms have been measured in a diamond growing microwave reactor operating with hydrocarbon/Ar/H-2 gas mixtures as functions of height (z) above the substrate surface and process conditions. The monitored species are each localized in the hot plasma region, where T-gas similar to 3000 K, and their respective column densities are each reproduced, quantitatively, by two-dimensional (r,z) modeling of the plasma chemistry. The H(n=2) distribution is seen to peak nearer the substrate, reflecting its sensitivity both to thermal chemistry (which drives the formation of ground state H atoms) and the distributions of electron density (n(e)) and temperature (T-e). All three column densities are found to be sensitively dependent on the C/H ratio in the process gas mixture but insensitive to the particular choice of hydrocarbon (CH4 and C2H2). The excellent agreement between measured and predicted column densities for all three probed species, under all process conditions investigated, encourages confidence in the predicted number densities of other of the more abundant radical species adjacent to the growing diamond surface which, in turn, reinforces the view that CH3 radicals are the dominant growth species in microwave activated hydrocarbon/Ar/H-2 gas mixtures used in the chemical vapor deposition of microcrystalline and single crystal diamond samples.