Effect of Different Additive Gases on Diamond Synthesis by Microwave Plasma CVD

Diamond was synthesized by microwave plasma CVD from CH4-H2 based mixed gases containing varying proportions of either O2, CO2 or CO as an additive. H2 flow rate and CH4 concentration were fixed at 1.67cm3/s and 3.0% respectively and microwave power was 220W. Addition of minute amounts of all of the additives resulted in increasing deposition rate. O2 and CO2 were effective in improving the morphology, while CO had no appreciable effect. CO2 was found most effective in both increasing the deposition rate and improving the morphology. An increased concentration of O2 or CO2 resulted in more idiomorphic diamond crystals with less secondary growth on the surface. The effectiveness of CO2 addition in improving both deposition rate and morphology of diamond is thought to be due to the increase in both carbon and oxygen concentration in the reaction gas system. Amorphous carbon would be eliminated by active oxygen in the plasma, and methyl radical, an intermediate product on the way to diamond, would be formed. Raman spectroscopy is especially effective in evaluating amorphous carbon.