Two-dimensional modeling of diamond growth by microwave plasma activated chemical vapor deposition: Effects of pressure, absorbed power and the beneficial role of nitrogen on diamond growth

Abstract

A two-dimensional (2D(r, z)) self-consistent model developed and validated in previous studies of diamond deposition processes in microwave (MW) plasma activated chemical vapor deposition reactors is applied in a systematic study of ways in which the gas pressure (over the range p = 75–350 Torr) and absorbed power (P = 1–3 kW) affect the plasma parameters, species distributions and diamond deposition processes from a gas mixture (1%CH4/H2 with 60 ppm added N2) at substrate temperatures (Ts = 1073 and 1323 K) and diameters (ds = 32–100 mm). A more limited set of process conditions for a 0.006%N2/4%CH4/H2 gas mixture and Ts = 1038–1153 K are investigated also. The study traces variations in the global distributions of electron concentration, electron and gas temperature, absorbed power density, key species concentrations and the hydrocarbon interconversion reactions, with particular focus on the radial profiles of CH3 radical and H atom concentrations just above the substrate and on predicted diamond growth rates, G(r). The results and the trends revealed when varying p and P should help guide future optimizations of deposition regimes. The absorbed power density is shown to exhibit quite steep gradients in both the radial (r) and axial (z) directions. This, and the finding of significant power absorption beyond the glowing plasma region, limits the utility of the oft-quoted ‘averaged power density’ as a parameter. The modeling also highlights the essential 2D character of the hydrocarbon interconversion and diffusion transfer processes, which challenges the applicability of any 1D modeling of such MW plasmas. Trace additions of N2 to a MW plasma activated CH4/H2 gas mixture have negligible effect on the plasma parameters or chemistry yet are known to boost the diamond growth rate. A semi-empirical expression for G(r) is developed further to explicitly include the effects of added N2 and a new mechanistic picture presented to account for the observed N-induced enhancements in G. This picture invokes stable moieties such as that formed by CH2 insertion into a CN dimer bond on the 2 × 1 reconstructed (100) diamond surface as ‘anchor’ sites that enable shorter CH2 surface migration lengths and more step-edges for irreversible incorporation of such migrating groups on the growing diamond surface.