Exploring the Plasma Chemistry in Microwave Chemical Vapor Deposition of Diamond from C/H/O Gas Mixtures

Microwave (MW)-activated CH4/CO2/H-2 gas mixtures operating under conditions relevant to diamond chemical vapor deposition (i.e., X-C/Sigma = X-elem(C)/(X-elem(C) + X-elem(O)) approximate to 0.5, H-2 mole fraction = 0.3, pressure, p = 150 Torr, and input power, P = 1 kW) have been explored in detail by a combination of spatially resolved absorption measurements (of CH, C-2(a), and OH radicals and H(n = 2) atoms) within the hot plasma region and companion 2-dimensional modeling of the plasma. CO and H-2 are identified as the dominant species in the plasma core. The lower thermal conductivity of such a mixture (cf. the H-2-rich plasmas used in most diamond chemical vapor deposition) accounts for the finding that CH4/CO2/H-2 plasmas can yield similar maximal gas temperatures and diamond growth rates at lower input powers than traditional CH4/H-2 plasmas. The plasma chemistry and composition is seen to switch upon changing from oxygen-rich (X-C/Sigma < 0.5) to carbon-rich (X-C/Sigma > 0.5) source gas mixtures and, by comparing CH4/CO2/H-2 (X-C/Sigma = 0.5) and CO/H-2 plasmas, to be sensitive to the choice of source gas (by virtue of the different prevailing gas activation mechanisms), in contrast to C/H process gas mixtures. CH3 radicals are identified as the most abundant C1Hx [x = 0-3] species near the growing diamond surface within the process window for successful diamond growth (X-C/Sigma approximate to 0.5-0.54) identified by Bachmann et al. (Diamond Relat. Mater. 1991, 1, 1). This, and the findings of similar maximal gas temperatures (T-gas similar to 2800-3000 K) and H atom mole fractions (X(H)similar to 5-10%) to those found in MW-activated C/H plasmas, points to the prevalence of similar CH3 radical based diamond growth mechanisms in both C/H and C/H/O plasmas.