SPECTROSCOPIC DETERMINATION OF CARBON DIMER DENSITIES IN AR-H-2-CH4 AND AR-H-2-C-60 PLASMAS

In contrast to conventional methods of diamond chemical vapour deposit ion (CVD), nanocrystalline diamond CVD takes place with only a small f raction of feed gas hydrogen. Minimal amounts of CH3, believed critica l in hydrogen-rich CVD, are expected to be produced in hydrogen-defici ent systems and alternative mechanisms for diamond growth must be cons idered. The carbon dimer, C-2, is believed to be an important species in these growth environments. We have experimentally determined the de nsity of gas phase C-2 in Ar-H-2-CH4 and Ar-H-2-C-60 microwave plasmas used to deposit nanocrystalline diamond. The C-2 density is monitored using high-sensitivity absorption spectroscopy of the d(3)Pi <-- a (3 )Pi (0, 0) band as chamber pressure, microwave power, substrate temper ature and feed gas mixtures are varied for these two chemical systems. The absolute density of C-2 is most sensitive to the total chamber pr essure and fraction of carbon in all molecular species in the feed gas in Ar-H-2-C-60 discharges and to the total chamber pressure and subst rate temperature in Ar-H-2-CH4 plasmas. We discuss possible C-2 produc tion channels in both chemical systems. The efficiency of C-2 producti on from fullerene precursors is over an order of magnitude greater tha n that from hydrocarbon precursors.