Atomic carbon vapor as a diamond growth precursor in thermal plasmas

A detailed surface chemistry mechanism is proposed for chemical vapor deposition of diamond films, which extends the growth-by-methyl mechanism proposed by Harris to treat any CH_m radical, m=0-3, as a growth monomer. Numerical computations were performed in which the mechanism was coupled to a model for the boundary layer above the substrate, for conditions typical of diamond deposition in an atmospheric-pressure thermal plasma. The predicted linear growth rate increases strongly as the boundary layer thickness δ is decreased, and the results indicate a strong dependence of the diamond growth chemistry on δ. For relatively thick boundary layers (modest velocities of the reactant jet) growth is dominated by CH₃. For very thin boundary layers (high velocities) the model predicts that growth is dominated by C. For the transition region where C and CH₃ each contribute about 40% to growth, CH₂ also contributes about 17%. The carbon conversion efficiency is also predicted to peak in the transition region, and drops sharply for very thin boundary layers.