Various mechanisms for the growth and renucleation of ultrananocrystalline diamond (UNCD) films are discussed and evaluated in the light of experimental and theoretical evidences in recent publications. We propose that the most likely model for UNCD growth is that where most of the diamond is formed via a similar mechanism to that of microcrystalline diamond films, i.e., gas phase H atoms abstracting surface hydrogens, followed by a CHx, x=0–3, addition. Calculations of the gas composition close to the substrate surface in the microwave plasma reactor for both the microcrystalline diamond and the UNCD growth, at substrate temperatures of 1073 and 673K, suggest that CH3 and C atoms are the most likely precursors for the growth of UNCD. However, the deposition is interrupted by an event which prevents the smooth growth of a continuous layer, and instead creates a surface defect which changes the growth direction and acts as a renucleation site. The possible nature of this event is discussed in detail. Using estimates for reaction rates of various species (including H atoms, Ar* metastables, Ar+ and ArH+ ions) on the diamond surface, a number of mechanisms are discussed and discounted. We propose that the most likely causes for the renucleation required for the UNCD growth are (i) the attachment of C1 species (especially C atoms) followed by local surface restructuring, (ii) the reduction of the efficiency of the β-scission reaction resulting in an increase in the number of long-chained hydrocarbons on the surface, or (iii) a combination of these two processes.

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