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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]

The characterization of Ar/H2/Ch4 microwave discharges suitable for nanocrystalline diamond (NCD) deposition by both plasma diagnostics and modeling is reported. Emission spectroscopy (ES) and broadband absorption spectroscopy (BAS) were carried out in the UV-visible spectral range in order to estimate the gas temperature and the C2 density within the plasma.

For the first time nanocrystalline diamond (NCD) films were deposited by the pulsed microwave plasma assisted chemical vapour deposition process starting from an Ar/H2/CH4 gas mixture. Comparisons with continuous mode deposition gave evidence for the improvement in film quality when the microwave power was modulated with a pulse repetition rate in the range 50–1000 Hz. A reduction in grain size and surface roughness, especially at low pulse repetition rate, accompanied by a decrease in soot particle formation was observed. A thermo-chemical plasma model, developed for pulsed Ar/H2/CH4 microwave discharges, provides evidence for the fact that the pulsed mode permits the enhancement of the mole fraction of the C2 dimer assumed to be the growth precursor of NCD. This may be responsible for a high secondary nucleation rate improving the nanostructure of the film in pulsed discharges. [ABSTRACT FROM AUTHOR]