Low temperature synthesis of diamond thin films through graphite etching In a microwave hydrogen plasma

摘要

Diamond thin films are very promising in many applications including semiconductors, microelectronics, optics, biomedical and manufacturing engineering due to the exceptional properties of diamond. Chemical vapor deposition (CVD) in hydrogen mixed with hydrocarbon gas is commonly used to synthesize diamond films. However, in this method, a substrate temperature above 700 deg C is required for synthesis of high quality diamond films with reasonably high growth rate. Substrates which melt or undergo problematic phase transition at these temperatures cannot therefore be employed. Removing this limitation would expand applications of diamond films in such areas as microelectronics and optics, and thus the development of low temperature synthesis technology for high purity diamond films is of great current interest. Several methods have been tested for lowering CVD diamond growth temperature: adding oxygen 1, using a halogenated carbon-containing precursor 2,3, using gas mixtures of CO_2/CH_4 4 and CO/H-, 5, improving the seeding processes 6, and using both argon-rich plasma and improved seeding processes 7. However, to date, no direct evidence of the successful synthesis of a high-purity crystalline diamond film at temperatures below 300 deg C has been reported. In this paper, we report on transformation of graphite into diamond thin films at temperatures as low as 250 deg C in a microwave hydrogen plasma without adding hydrocarbon. The diamond synthesis at low temperatures is believed to be induced by activated transitional hydrocarbon radicals formed in situ through hydrogen etching of graphite. The methodology may open a pathway to coating pure diamond films on steels and on low softening- or melting-point materials including glass and polymers. This may significantly expand industrial applications of diamond films.