Combined experimental and modeling studies of microwave activated CH₄/H₂/Ar plasmas for microcrystalline, nanocrystalline, and ultrananocrystalline diamond deposition

A comprehensive study of microwave (MW) activated CH4/H-2/Ar plasmas used for diamond chemical vapor deposition is reported, focusing particularly on the effects of gross variations in the H-2/Ar ratio in the input gas mixture (from H-2/Ar mole fraction ratios of > 10:1, through to similar to 1:99). Absolute column densities of C-2(a) and CH(X) radicals and of H(n=2) atoms have been determined by cavity ringdown spectroscopy, as functions of height (z) above a substrate and of process conditions (CH4, H-2, and Ar input mole fractions, total pressure, p, and input microwave power, P). Optical emission spectroscopy has also been used to explore the relative densities of electronically excited H atoms, and CH, C-2, and C-3 radicals, as functions of these same process conditions. These experimental data are complemented by extensive 2D (r, z) modeling of the plasma chemistry, which provides a quantitative rationale for all of the experimental observations. Progressive replacement of H-2 by Ar (at constant p and P) leads to an expanded plasma volume. Under H-2-rich conditions, > 90% of the input MW power is absorbed through rovibrational excitation of H-2. Reducing the H-2 content (as in an Ar-rich plasma) leads to a reduction in the absorbed power density; the plasma necessarily expands in order to accommodate a given input power. The average power density in an Ar-rich plasma is much lower than that in an H-2-rich plasma operating at the same p and P. Progressive replacement of H-2 by Ar is shown also to result in an increased electron temperature, an increased [H]/[H-2] number density ratio, but little change in the maximum gas temperature in the plasma core (which is consistently similar to 3000 K). Given the increased [ H]/[ H-2] ratio, the fast H-shifting (CyHx + H <-> CyHx-1 + H-2; y=1-3) reactions ensure that the core of Ar-rich plasma contains much higher relative abundances of "product" species like C atoms, and C-2, and C-3 radicals. The effects of Ar dilution on the absorbed power dissipation pathways and the various species concentrations just above the growing diamond film are also investigated and discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3562185]