Fundamental limits to growth rates in a methane-hydrogen microwave plasma

The fluxes of the major stable products, acetylene and ethylene, emanating from a 2.45 GHz methane-hydrogen plasma (0.2-5 vol.% methane; 800 W; 33 Torr) have been measured using quadrupole mass spectrometry. The diminution in the methane flux was also measured to allow the overall carbon balance to be determined. The ball plasma was sustained within a spherical ultrahigh vacuum chamber, out of contact with the substrate and chamber walls. Experiments were performed either in the presence of a heated (720°C) silicon substrate of 4 in diameter or with the substrate heater completely removed from the chamber. Within experimental error, the product yields and methane losses were identical for the two experimental arrangements. The carbon mass imbalance was compared with the growth rates of polycrystalline diamond using in-situ laser interferometry. The measured growth rates were of the order of 15% of the maximum permissible rates, for a 0.5 vol.% methane mixture, based on the carbon imbalance. Thermodynamic calculations, based on minimizing the Gibbs free energy, have been used to determine the relative amounts of the species involved in the decomposition of dilute mixtures 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 and pressure under which both phases coexist. Such calculations extending the phase diagrams to include oxygen enable the ternary diagrams developed by Bachmann to be rationalized. The conditions for the exclusive growth of diamond have been calculated, employing values for the surface enthalpy of diamond, taking into account the enhanced surface stability of hydrogenated diamond surfaces. © 1993.