Gas Phase Synthesis of Diamond in Microwave Plasma

Abstract

This paper reports the formation of diamond particles and films on diamond and nondiamond substrates by means of microwave(2.45 GHz) glow discharge in a stream of the hydrogenmethane mixtures. The dependence of the feature of grown diamond on experimental conditions, such as methane concentration, pressure, substrate temperature, microwave power and gas flow rate, is discussed. Diamond was formed on silicon wafers, silica glasses and natural diamonds under the following conditions: methane concentration, 0.3∼2%; total pressure, 4∼26.6 kPa; microwave power, 150∼500 W. The temperature of the substrate ranged from 800°C to 1000°C, depending on the microwave power and the gas pressure. For identification, the deposits were studied by reflection electron diffraction and Raman spectroscopy. The diffraction pattern of the deposit on a silicon wafer consists of spotty rings. The interlayer spacings are in good agreement with the values reported for diamond(Table 1). A sharp Raman peak observed at 1333±1.5 cm^-1 agrees with the reporte d value of 1332.5 cm^-1 for diamond. The deposits were observed with scanning electron microscopy. The faces that predominantly appeared were {111} or {100} faces. Cubooctahedral, twinned and octahedral crystals, all truncated by the substrate, have been observed. When the methane concentration was constant, polyhedrons were formed at lower microwave powers and irregular shaped crystals were formed at higher powers(Fig.4). At h igher substrate temperatures than 980°C, the size of the deposits became small in comparison with the deposits at 930°C(Fig.4). It seems to be due to the vaporization reactions of the deposits with atomic hydrogen. A polycrystalline diamond film was formed on a mechanicall activated silicon wafer(Fig.5). The Raman spectrum of the film formed at the meth aney concentration of 0.3% and the microwave power of 500 W consists of only a sharp peak at 1333cm^-1 (Fig.6 b). The film is transparent in the infra-red region of 2.5-25μm (Fig.7), It seems that hydrogen atoms in microwave plasma react with carbons of graphitic structure t o vaporize the carbons, and bond to carbons of diamond structure to stabilize the st ructure. Th e decomposition of methane on the substrates may be the rate -controlling step for the diamond synthesis. The growth rate increased with an increase in the substrate temperature. A t higher temperatures than 950°C, apparent growth rate decreased because of the reaction of the deposits with atomic hydrogen.