Gas-phase characterization in diamond hot-filament CVD by infrared tunable diode laser absorption spectroscopy

In hot-filament CVD the gas-phase composition is a vital parameter for diamond coating results. Concentrations of carbon-containing species have significant influence on growth rate, quality and morphology of deposited diamond. To learn more about the correlations between process parameters and gas species concentrations we applied the highly sensitive infrared tunable diode laser absorption spectroscopy (IR-TDLAS) technique. With a sophisticated compact IR-TDLAS unit, relative and absolute concentrations of CH4, C2H2 and CO were simultaneously measured. Also, the absolute concentration of the methyl radical was determined in dependence on process parameters. Concentrations of CO2, C2H6 and HCN were investigated but found to be lower than the detection limit. The influence of the typical diamond CVD-process parameters on various species concentrations is discussed. The applicability of IR-TDLAS for hot-filament process monitoring is evaluated. In context with diamond growth results, information for CVD process refinement was deducted from the IR-TDLAS measurements.