Effects of gas residence time on microwave plasma enhanced CVD of ultrananocrystalline diamond in mixtures of methane and argon without hydrogen or oxygen additives

For low-power and low-temperature microwave plasma CVD of UNCD in gas mixtures of methane and argon without hydrogen and oxygen additives, excessive carbon containing species often induce gas phase synthesis of non-diamond carbon phases, which fall on diamond growing surfaces to become part of the deposited diamond films. To prevent undesirable gas phase reactions, effects of gas residence time, or equivalently, the total gas flow rate at a fixed gas pressure and compositions, on the microwave plasma and its deposition of UNCD are studied. The gas residence time is increased by by-passing an increasing amount of a pre-set mixture of methane and argon at a fixed total flow rate through a mass flow controller to a vacuum pump while allowing the rest of the gas feed to flow through the reaction chamber. The gas composition, gas pressure, microwave power, and substrate temperature are kept constant. Optimization of the UNCD growth is, thus, achieved by increasing the gas residence time to deposit UNCD of high phase purity.