Experiment and modeling of the deposition of ultrananocrystalline diamond films using hot filament chemical vapor deposition and Ar/CH₄/H₂ gas mixtures: A generalized mechanism for ultrananocrystalline diamond growth

Ar/CH₄/H₂ gas mixtures have been used to deposit nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) films using hot filament (HF) chemical vapor deposition. The Ar:H₂ concentration was maintained at Ar/(H₂+Ar)=80% while the CH₄ concentration was varied over the range CH₄/(H₂+CH₄)=0.3–6.0. For higher methane concentrations, the filament became coated in a graphitic layer which prevented film growth. For lower CH₄ additions, the film morphology depended upon the CH₄ concentration, with different gas mixing ratios producing microcrystalline diamond (MCD), NCD, or UNCD films. A two-dimensional computer model was used to calculate the gas phase composition for all these conditions at all positions within the reactor. Using the experimental and calculated data, we show that the observed film morphology can be rationalized using a model based on competition between H atoms, CH₃ radicals, and other C₁ species reacting with dangling bonds on the surface. The relative concentrations of each of these species close to the growing diamond surface determine the probability of a renucleation event occurring and hence the morphology of the subsequent film. This has been developed into a general mechanism for the deposition of MCD, NCD, and UNCD films from Ar/CH_4- /H₂ gas mixtures which is consistent with published experimental observations.