Influences of ambient atmosphere on diamond synthesis using an oxygen-acetylene torch
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D. G. Goodwin Division of Engineering and Applied Sciences, California Institute of Technology, Pasadena, California 91125 (Received 6 April 1993; accepted 9 September 1993)
The influence of the ambient atmosphere on the synthesis of diamond by chemical vapor deposition using an oxygen-acetylene torch is investigated experimentally. Diamond synthesis in an air atmosphere is compared to that in an inert atmosphere. It is found that the quality of diamond deposited on substrates positioned in the burnt gases near the premixed flame of the torch (within 1 mm) is independent of the composition of the ambient gas. Farther downstream from the premixed flame in the feather region, however, diamond deposition is controlled by the combustion of incomplete products from the premixed flame with oxygen from the atmosphere. In this region diamond grows in an annulus on the substrate with an air atmosphere, but no film is grown with an inert atmosphere.
I. INTRODUCTION The first successful attempts to synthesize diamond on a commercial scale date back to the 1950s, when high-temperature, high-pressure techniques were used to produce diamond by recrystallization of metal-solvated carbon.1 This process accounted for over 90% (sixty tons) of industrial diamonds consumed in 1990.2 Recently, however, much progress has been made toward increasing the feasibility of vapor-phase diamond deposition at atmospheric and lower pressures. Hot filaments, turbulent and laminar flames, and plasmas have been used as activation energy sources, resulting in diamondfilm growth rates that range from low to high generally in this same order (typically 1-1000 yitm/h). Growth rates and diamond film quality have been found to be very sensitive to the chemistry of the activated species and to the substrate temperature.3"6 In the oxygen-acetylene-torch CVD process, a premixed flame burns a rich oxygen-acetylene gaseous mixture. In the most typical configuration the excess acetylene is burned in a primary diffusion flame, termed the feather, with oxygen which diffuses in from the ambient air. Surrounding the feather is a secondary diffusion flame, where the CO and H2 produced upstream in the premixed flame burn with O2 from the ambient atmosphere to form the complete combustion products CO 2 and H 2 O [cf. Fig. l(a)]. The substrate is positioned normal to the flow of hot gases between the premixed flame and the feather, as shown in [Fig. l(b)]. If the atmosphere surrounding the torch flow is oxidizer-free, then neither the feather nor the secondary diffusion flame 80
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J. Mater. Res., Vol. 9, No. 1, Jan 1994
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will exist and the incomplete products from the rich premixed flame will not be further burned. The absence of the feather is expected to result in a lower productgas temperature, because of the absence of heat release from the secondary flame and of transverse diffusion of heat from the product gases into the inert atmosphere. A reduction in the //-atom concentration downstream from the premixed
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