Diamond growth with locally supplied methane and acetylene
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Experiments have been conducted to examine the relative importance of different gas feed geometries and molecular species in the hot filament assisted vapor phase deposition of diamond. Remote and local (relative to the substrate) gas feeds of methane and acetylene were tested and it was found that although dramatic effects on uniformity and rate of deposition can be observed using a gas feed of methane local to the substrate surface, little or no variation in growth rate or deposition uniformity was produced by feeding acetylene locally. The growth rate observed on a defined area of the substrate using a local gas feed of methane was 0.50 mgms/h versus 0.17 mgms/h for a local feed of acetylene. These results, as well as the patterns of deposition observed, suggest that the major species contributing to growth in the hot filament assisted CVD of diamond is the methyl radical. This species is readily formed in the gas phase from methane and atomic hydrogen by hydrogen abstraction. It is, however, further suggested that numerous different hydrocarbon species may be of importance, with the nature of the species contributing most to the growth dependent on the method used.
I. INTRODUCTION There has been great interest in the precise nature of the hydrocarbon species that add to the growth surface of diamond during chemical vapor deposition. This issue is important in deciding the possible growth mechanisms and can also help identify critical parameters in the design and construction of large-scale reactors. Tsuda et al. argued that the most important species in diamond growth is CH3"1" or that a positively charged surface was important to diamond growth. 12 In one of the seminal papers on the subject Matsumoto et al? suggested the importance of neutral radical chemistry. They proposed from thermodynamic considerations that CH3, as well as C2H and C 2 H 2 , is potentially an active species. Frenklach et al. 4>5 proposed a reaction mechanism based on C 2 H 2 . Harris6'7 has proposed methyl radical mechanisms for growth on the (100) surface of diamond and has discussed some of the steric problems inherent in such a mechanism for growth on the (111) surface. Yarbrough8 has discussed a general CH3 mechanism for growth and suggested it as an additive species in growth along all three principal growth axes, (100), (110), and (111). Kline et al.9 modeled the chemical kinetics of CH4 dissociation in RF glow discharge plasmas. From the spatial profiles of the various species, their respective "sticking" coefficients, and experimental growth rates, they concluded that the concentrations of CH 3 and CH 3 + determine the deposition rate of amorphous carbon films. Chu et al.10 have shown that, in hot filament growth, a better rationalization of their data is obtained by assuming that CH 3 , not C 2 H 2 , is the dominant growth species in growth using hot filaments. However, they also found J. Mater. Res., Vol. 7, No. 2, Feb 1992 http://journals.cambridge.org
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