GAS-PHASE COMPOSITION AND FILM PROPERTIES OF HOT-FILAMENT DIAMOND SYNTHESIS FROM CH4-H-2-O-2 GAS-MIXTURES

The gas concentration profiles occurring in a hot filament (HF) reactor with well-defined Bow conditions were determined by laterally resolved mass spectroscopy analysis using a differentially pumped quadrupole mass spectrometer (QMS). The changing gas compositions (due to filament reactions, diffusion and flow processes) could be measured with the QMS relative to the filament position in the vertical direction. These data and calculations were complemented by the measurement of the film growth rate by laser interferometry and by film quality and morphology control using Raman and scanning electron microscopy techniques. Additional information about the radical concentrations, which cannot be measured with the QMS, could be obtained using the CHEMKIN software package. Without the addition of oxygen we obtained very good agreement between the experimental data and the theoretical calculations. For the H-2-CH4-O-2 system we developed a new mechanism comprising 33 reactions and 18 species. This mechanism calculates characteristic concentration profiles for the most important growth species CH4, CH3 and C2H2 with maximum or minimum values at the filament position. The CH3 mole fraction at the substrate position is found to be rather low, suggesting that CHx radicals for the growth process must be formed at the surface. Additionally, we found that oxygen reduces the concentration of acetylene much more than that of methane, which results in a decrease in growth rate and an increase in film quality. We conclude that in HFCVD CH3 is the most important growth species, while C2H2 is responsible for graphitic impurities.