Effect of the admixture of N 2 to low pressure, low temperature H 2 -CH 4 -CO 2 microwave plasmas used for large area deposition of nanocrystalline diamond films

In a distributed antenna array reactor, microwave H 2 -CH 4 -CO 2 plasmas with admixture of N 2 used for the low-temperature deposition of nanocrystalline diamond (NCD) films are studied by in situ infrared laser absorption spectroscopy (LAS) and optical emission spectroscopy techniques. The experiments are carried out in order to analyze the dependence of temperatures and species densities as a function of the admixture of nitrogen. The evolution of the concentrations of the methyl radical (CH 3 ), and of five stable molecules (NH 3 , HCN, CH 4 , C 2 H 2 , and CO), are monitored in the plasma processes by LAS using tunable lead salt diode lasers and external-cavity quantum cascade lasers (EC-QCL) as radiation sources. OES is performed simultaneously to obtain complementary information about (i) the degree of dissociation of H 2 precursor gas, (ii) the gas temperature and therefore (iii) the density of atomic hydrogen, a key species in the chemistry of NCD deposition plasmas. The species temperatures are not significantly affected by the nitrogen addition. The concentrations of the various species are in the range between 10 11 to 10 15 molecules cm −3 . HCN and CO are the major products in the plasma besides atomic hydrogen. The analysis of the nitrogen and carbon mass balances of the measured species shows that in addition to NH 3 and HCN other nitrogen containing species are produced in the plasma which were not probed. It is shown that the formation of HCN consumes C atoms that can be provided from hydrocarbon species and from the deposition of carbon-containing films on the reactor walls, which results in a decrease of the measured densities of hydrocarbon species.