Microwave discharges of H2 admixed with CH4 in a moderate-pressure quartz bell jar reactor used for diamond deposition are studied numerically. Special attention was devoted to high-power densities which provide the most effective way for producing high-quality diamond films. First, a one-dimensional radial model describing the coupled phenomena of chemistry, energy transfer, as well as species and energy transport along the reactor’s radial coordinate was developed. Species densities predicted with the model were compared with measurements with infrared tunable diode laser spectroscopy, resulting in validation of the model. Second, a one-dimensional axial model was used to describe the plasma flow along the reactor axis in a region between the reactor end wall and the substrate surface. This model was particularly useful for studying the plasma behavior in the vicinity of the substrate surface, where thermal and composition gradients are large. Both the radial and axial transport models are based on the same discharge model in which the plasma is described as a thermochemically nonequilibrium flow with different energy distributions for heavy species and electrons. The chemistry was described with a model containing 28 species and 131 reactions. The electron temperature, the gas temperature, and the species concentration were determined by solving a coupled set of equations. A wide range of experimental conditions used for diamond deposition was simulated, from low microwave power density (9Wcm−3, i.e., 600W, 2500Pa, and Tg∼2200K) to high-power density (30Wcm−3, i.e., 2kW, 12000Pa, and Tg∼3200K). The main chemical paths were identified, and the major species, transport effects, and reaction pathways that govern diamond deposition plasmas are discussed.

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A compact and transportable infrared multicomponent acquisition (IRMA) system based on infrared absorption spectroscopy has been developed for plasma diagnostics and control. The IRMA system contains four independent tunable diode lasers which can be temporally multiplexed and directed into plasma reactors or into a multipass cell for exhaust gas detection. Rapid scan software with real-time line shape analysis provides simultaneous measurements of the absolute concentrations of several molecular species.

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and nine stable molecules, CH 4 , CH 3 OH, C 2 H 2 , C 2 H 4 , C 2 H 6 , NH 3 , HCN, CH 2 O and C 2 N 2 , in H 2 -Ar-N 2 microwave plasmas containing up to 7% of methane or methanol, under both flowing and static conditions. The degree of dissociation of the hydrocarbon precursor molecules varied between 20% and 97%. The methyl radical concentration was found to be in the range 10 12 -10 13 molecules cm −3 . By analysing the temporal development of the molecular concentrations under static conditions it was found that HCN and NH 3 are the final products of plasma chemical conversion. The fragmentation rates of methane and methanol (R F (CH 4 ) = (2-7) × 10 15 molecules J −1 , R F (CH 3 OH) = (6-9) × 10 15 molecules J −1 ) and the respective conversion rates to methane, hydrogen cyanide and ammonia (R Cmax (CH 4 ) = 1.2 × 10 15 molecules J −1 , R Cmax (HCN) = 1.3 × 10 15 molecules J −1 , R Cmax (NH 3 ) = 1 × 10 14 molecules J −1 ) have been determined for different hydrogen to nitrogen concentration ratios. An extensive model of the chemical reactions involved in the H 2 -N 2 -Ar-CH 4 plasma has been developed. Model calculations were performed by including 22 species, 145 chemical reactions and appropriate electron impact dissociation rate coefficients. The results of the model calculations showed satisfactory agreement between calculated and measured concentrations. The most likely main chemical pathways involved in these plasmas are discussed and an appropriate reaction scheme is proposed.

Infrared diode laser spectroscopy has been used as a diagnostic probe to measure the concentrations of the methyl radical and stable products in an ac methane/hydrogen/oxygen (CH4−H2−O2) plasma. Among the products detected were all of the stable C-2 hydrocarbons and oxygen-containing species including methanol, formaldehyde, formic acid, carbon monoxide, and carbon dioxide. A simple one-dimensional chemical modeling program has been written to calculate and compare the model concentrations of all the detected species with their observed concentrations. Good agreement between these values has been obtained which enables some insights to be gained into the gas-phase mechanism in mixed methane plasmas.