INTERACTION OF HYDROGEN, METHANE, ETHYLENE, AND CYCLOPENTANE WITH HOT TUNGSTEN - IMPLICATIONS FOR THE GROWTH OF DIAMOND FILMS

Modulated-beam mass spectrometry and x-ray photoelectron spectroscopy (XPS) have been used to investigate the interaction of CH4, C2H4, C5H10, and H-2 with carburized and uncarburized tungsten. It is shown that significant evaporation of C1, C2, and C3 occurs for carburized tungsten at temperatures above 1900-degrees-C. The temperature dependence of the carbon evaporation rate was found to be similar to the temperature dependence of the diamond film deposition rate observed in chemical vapor deposition (CVD) reactors, similar to the temperature dependence for the carbon deposition rate observed in the present experiments, and similar to the expected evaporation rate of carbon from graphite and tungsten carbide. The desorption of hydrocarbon species (other than the incident gas) was not clearly observed under any conditions for methane or ethylene. In contrast, it is quite likely that cyclopentane decomposes at the surface to produce new species which are subsequently desorbed into the gas phase. The reaction of ethylene with tungsten is believed to result in complete decomposition with the hydrogen being desorbed as atoms or molecules while the carbon remains on the surface where there is competition between carburization and evaporation. The reaction probability of ethylene with tungsten was found to be close to unity while the reaction probability of methane was small. The removal of carbon from carburized tungsten via an etching reaction involving hydrogen was not observed. The production of hydrogen atoms from H-2 Was found to be largest on clean tungsten, less on carburized tungsten, and not observable on graphite. Evaporation of tungsten from carburized tungsten was seen at temperatures below 2500-degrees-C but not below 2200-degrees-C. XPS measurements indicated that slightly carburized tungsten contained some graphite in the surface region while heavily carburized tungsten contained much more graphite. The surface concentration of carbon was found to depend in a complicated manner on the balance between carbide and graphite growth and carbon evaporation. The reaction probability of the incident gas is also a determining factor. In addition, computer simulations were used to calculate the concentrations of various species in the gas phase under conditions which are typical of those used in diamond hot-filament CVD reactors. Calculated gas-phase species distributions near the substrate for carbon-atom/H-2 mixtures are found to be similar for most species to those calculated for CH4/H-2 mixtures. It appears that the fast H-2 and H chemistry determines the equilibrium mixture and that it is nearly independent of the type of carbon containing species introduced near the filament. Literature results obtained in typical diamond hot-filament CVD reactors are compared and interpreted on the basis of the present data.