Deposition of diamond films in arc plasma jet at atmospheric pressure

Study of diamond growth was made in atmospheric pressure dc are jet with high ratio of carbon to hydrogen flow rates. A plasma torch was designed for operation with mixture of argon, hydrogen and methane. Supply of reaction gases, hydrogen and methane, upstream of the cathode together with argon assures good mixing of gases and complete dissociation in a high temperature are core. The disadvantage of this arrangement consists in the fact that arc power is substantially affected by flow rates of reacting gases. The methane to hydrogen concentration has pronounced effect on the quality and growth rate of deposited diamond films. Films deposited on the cooled Mo substrate were analyzed using scanning electron microscopy and Raman spectroscopy. Polycrystalline films were created with typical crystal size of several pm, the ratio of content of diamond structures to non-diamond forms of carbon was strongly dependent on ratio of flow rates of hydrogen and methane and on orientation of surface with respect to plasma flow. Effect of parameters of plasma jet as well as effect of position and orientation of substrate surface in the plasma flow on composition and growth rate of films was investigated. Carbon was deposited on all surfaces of molybdenum tube. Diamond growth was observed only on the surfaces orthogonal to the axis of plasma jet while amorphous carbon was deposited on the side and back surfaces. Generally the growth rate increases with increased methane to hydrogen ratio but this is accompanied by increasing content of non-diamond forms of carbon in the film. The threshold value of the volumetric ratio hydrogen/methane for onset of diamond growth in our experiments was about 3.7:1 which corresponds to ratio of atomic concentrations C:H approximately 1:11. At this ratio the growth rate of the film was about 130 mum/hour. The film at these conditions was composed of amorphous graphite with zones of diamond crystals.