Activity of tungsten and rhenium filaments in CH₄/H₂ and C₂H₂/H₂ mixtures: Importance for diamond CVD

The resistance R, spectral emissivity ∊, and power consumption of W and Re filaments heated to 2500 °C in mixtures of CH₄ or C₂H₂ in H₂ have been measured in a series of experiments focusing on the state of the filament activity, i.e., its ability to dissociate the reactant gases. It has been found that these properties of the filaments, as well as the partial pressures of CH₄ and C₂H₂ in the reaction chamber, depend critically on both the filament temperature and the reactant ratio, e.g., C₂H₂/H₂. Specifically, both W and Re filaments show sharp jumps in power consumption at essentially the same temperature, signaling strong increases in filament activity and, hence, production of atomic hydrogen. These results are proposed to be due to the removal of nonreactive carbon from the surface of the filament via etching by atomic hydrogen and are consistent with the predictions of our thermodynamic model for the C–H system. Evidence for gas phase reactions is presented and the role of thermal diffusion is discussed. The emissivities of the W and Re filaments are observed to have significantly different temperature dependences which are attributed to differences in the phase diagrams for the W–C and Re–C systems. The implications of these results for hot-filament diamond CVD are discussed.