Spectroscopic diagnostics and modeling of Ar/H2/CH4 microwave discharges used for nanocrystalline diamond deposition.

In this paper Ar/H₂/CH₄ microwave discharges used for nanocrystalline diamond chemical vapor deposition in a bell-jar cavity reactor were characterized by both experimental and modeling investigations. Discharges containing 1% CH₄ and H₂ percentages ranging between 2% and 7% were analyzed as a function of the input microwave power under a pressure of 200 mbar. Emission spectroscopy and broadband absorption spectroscopy were carried out in the UV-visible spectral range in order to estimate the gas temperature and the C₂ density within the plasma. Infrared tunable diode laser absorption spectroscopy was achieved in order to measure the mole fractions of carbon-containing species such as CH₄, C₂H₂, and C₂H₆. A thermochemical model was developed and used in order to estimate the discharge composition, the gas temperature, and the average electron energy in the frame of a quasihomogeneous plasma assumption. Experiments and calculations yielded consistent results with respect to plasma temperature and composition. A relatively high gas temperature ranging between 3000 and 4000 K is found for the investigated discharge conditions. The C₂ density estimated from both experiments and modeling are quite high compared with what is generally reported in the literature for the same kind of plasma system. It ranges between 10¹³ and 10¹⁴ cm^-3 in the investigated power range. Infrared absorption measurements and model predictions indicate quite low densities of methane and acetylene, while the atomic carbon density calculated by the model ranges between 10¹³ and 10¹⁵ cm^-3. The methane and hydrogen introduced in the feed gas are subject to a strong dissociation, which results in a surprisingly high H-atom population with mole fraction ranging between 0.04 and 0.16. Result analysis shows that the power coupling efficiency would range between 70% and 90%, which may at least explain the relatively high values obtained, as compared with those reported in the literature for similar discharges, for gas temperature and C₂ population. The high H-atom densities obtained in this work would indicate that growing nanocrystalline diamond films would experience a very high etching. Simulation results also confirm that sp species would play a key role in the surface chemistry that governs the diamond growth. [ABSTRACT FROM AUTHOR]