Combined Spatially Resolved Optical Emission Imaging and Modeling Studies of Microwave-Activated H₂/Ar and H₂/Kr Plasmas Operating at Powers and Pressures Relevant for Diamond Chemical Vapor Deposition

Microwave (MW) activated H2/Ar (and H2/Kr) plasmas operating under powers and pressures relevant to diamond chemical vapor deposition have been investigated experimentally and by 2-D modeling. The experiments return spatially and wavelength resolved optical emission spectra of electronically excited H2 molecules and H and Ar­(/Kr) atoms for a range of H2/noble gas mixing ratios. The self-consistent 2-D­(r, z) modeling of different H2/Ar gas mixtures includes calculations of the MW electromagnetic fields, the plasma chemistry and electron kinetics, heat and species transfer and gas–surface interactions. Comparison with the trends revealed by the spatially resolved optical emission measurements and their variations with changes in process conditions help guide identification and refinement of the dominant plasma (and plasma emission) generation mechanisms and the more important Ar–H, Ar–H2, and H–H2 coupling reactions. Noble gas addition is shown to encourage radial expansion of the plasma, and thus to improve the uniformity of the H atom concentration and the gas temperature just above the substrate. Noble gas addition in the current experiments is also found to enhance (unwanted) sputtering of the copper base plate of the reactor; the experimentally observed increase in gas phase Cu* emission is shown to correlate with the near substrate ArH+ (and KrH+) ion concentrations returned by the modeling, rather than with the relatively more abundant H3+ (and H3O+) ions

What [plasma used for growing] diamond can shine like flame?

The gas-phase chemistry underpinning the chemical vapour deposition of diamond from microwave-activated methane/hydrogen plasmas is surveyed.</p

Microwave plasma-activated chemical vapour deposition of nitrogen-doped diamond, II:CH₄/N₂/H₂ plasmas

[Image: see text] We report a combined experimental and modeling study of microwave-activated dilute CH(4)/N(2)/H(2) plasmas, as used for chemical vapor deposition (CVD) of diamond, under very similar conditions to previous studies of CH(4)/H(2), CH(4)/H(2)/Ar, and N(2)/H(2) gas mixtures. Using cavity ring-down spectroscopy, absolute column densities of CH(X, v = 0), CN(X, v = 0), and NH(X, v = 0) radicals in the hot plasma have been determined as functions of height, z, source gas mixing ratio, total gas pressure, p, and input power, P. Optical emission spectroscopy has been used to investigate, with respect to the same variables, the relative number densities of electronically excited species, namely, H atoms, CH, C(2), CN, and NH radicals and triplet N(2) molecules. The measurements have been reproduced and rationalized from first-principles by 2-D (r, z) coupled kinetic and transport modeling, and comparison between experiment and simulation has afforded a detailed understanding of C/N/H plasma-chemical reactivity and variations with process conditions and with location within the reactor. The experimentally validated simulations have been extended to much lower N(2) input fractions and higher microwave powers than were probed experimentally, providing predictions for the gas-phase chemistry adjacent to the diamond surface and its variation across a wide range of conditions employed in practical diamond-growing CVD processes. The strongly bound N(2) molecule is very resistant to dissociation at the input MW powers and pressures prevailing in typical diamond CVD reactors, but its chemical reactivity is boosted through energy pooling in its lowest-lying (metastable) triplet state and subsequent reactions with H atoms. For a CH(4) input mole fraction of 4%, with N(2) present at 1–6000 ppm, at pressure p = 150 Torr, and with applied microwave power P = 1.5 kW, the near-substrate gas-phase N atom concentration, [N](ns), scales linearly with the N(2) input mole fraction and exceeds the concentrations [NH](ns), [NH(2)](ns), and [CN](ns) of other reactive nitrogen-containing species by up to an order of magnitude. The ratio [N](ns)/[CH(3)](ns) scales proportionally with (but is 10(2)–10(3) times smaller than) the ratio of the N(2) to CH(4) input mole fractions for the given values of p and P, but [N](ns)/[CN](ns) decreases (and thus the potential importance of CN in contributing to N-doped diamond growth increases) as p and P increase. Possible insights regarding the well-documented effects of trace N(2) additions on the growth rates and morphologies of diamond films formed by CVD using MW-activated CH(4)/H(2) gas mixtures are briefly considered