Methyl Concentration Measurements During Microwave Plasma-Assisted Diamond Deposition

Absolute line-of-sight averaged measurements of methyl radical concentrationsin a microwave plasma-assisted diamond deposition reactor arepresented. The measurements are based on the use of broadband ultravioletabsorption spectroscopy to characterize the distinguishing absorptionfeature of methyl at 216 nm associated with the X(²A₂)(²A₁) electronictransition. The dependence of the line-of-sight methyl concentration andmole fractions with the percentage of methane in the feed-gas, plasma powerdensity, and position of substrate relative to the optical probe volume isstudied. The measurements suggest that the near-substrate methyl molefraction is only weakly sensitive to changes in substrate temperature andare largely influenced by the gas-phase temperature. A comparison is madebetween the measured mole fractions and recent predictions based on aone-dimensional model of this process. The measured mole fractions areconsistently greater than those predicted by about a factor of ten. Thisdiscrepancy is explained in part by the line-of-sight limitations in theexperimental facility.