Physical chemistry of multi-filament diamond CVD processes

A multi-filament diamond CVD process with an extended array of hot wires differs from the single- filament version in several respects. Most importantly, the transition from a two-dimensional to a quasi-one-dimensional geometry changes the diffusive mass transport of carbon-bearing species towards the film growth front. As lateral diffusion is virtually excluded, acetylene may become the prevalent hydrocarbon species, although a methane/ hydrogen feed gas is used. Under suitable conditions a strong decrease of the linear film growth rate from the edge towards the center region of the substrate is therefore observed. The chemical growth environment changes considerably, if the mass transport is no longer diffusion-controlled, as in conventional HF-CVD, but if the gas is fed into the reaction zone as a jet striking the substrate vertically. At flow velocities in the range of 1000 to 10000 cm/s, the diffusion of acetylene into the reaction zone as well as its formation within the jet can be suppressed sufficiently to make methane the dominant stable species at the substrate. The diamond growth rate is at maximum at the stagnation point of the gas jet.