FUNDAMENTAL LIMITS TO GROWTH-RATES IN A METHANE-HYDROGEN MICROWAVE PLASMA

The fluxes of the major stable products, acetylene and ethylene, emana ting from a 2.45 GHz methane-hydrogen plasma (0.2-5 vol.% methane; 800 W; 33 Torr) have been measured using quadrupole mass spectrometry. Th e diminution in the methane flux was also measured to allow the overal l carbon balance to be determined. The ball plasma was sustained withi n a spherical ultrahigh vacuum chamber, out of contact with the substr ate and chamber walls. Experiments were performed either in the presen ce of a heated (720 degrees C) silicon substrate of 4 in diameter or w ith the substrate heater completely removed from the chamber. Within e xperimental error, the product yields and methane losses were identica l for the two experimental arrangements. The carbon mass imbalance was compared with the growth rates of polycrystalline diamond using in-si tu laser interferometry. The measured growth rates were of the order o f 15% of the maximum permissible rates, for a 0.5 vol.% methane mixtur e, based on the carbon imbalance. Thermodynamic calculations, based on minimizing the Gibbs free energy, have been used to determine the rel ative amounts of the species involved in the decomposition of dilute m ixtures of methane in hydrogen. Both diamond and graphite phases have been included in the heterogeneous reactions. Isotherms based on quasi -equilibrium theory are used to define the conditions of temperature a nd pressure under which both phases coexist. Such calculations extendi ng the phase diagrams to include oxygen enable the ternary diagrams de veloped by Bachmann to be rationalized. The conditions for the exclusi ve growth of diamond have been calculated, employing values for the su rface enthalpy of diamond, taking into account the enhanced surface st ability of hydrogenated diamond surfaces.