Synthesis of diamond by thermal plasma processing under low pressure

To realize the large area deposition of diamond films at a high rate, synthesis of diamond with the use of plasma jets under medium pressure, which are in the transition region from thermal plasmas to cold plasmas, is studied. Under low pressure, the morphology of deposited particles strongly depends on the ratio of methane gas flow rate to hydrogen gas flow rate, CH4/H-2, and on the distance from the feed ring exit to the substrate, L. The euhedral diamond particles are synthesized when CH4/H-2 is lower and L is shorter. To clarify the relationship between the morphology of the prepared particles and the chemical species in the plasma jet, emission spectra from the plasma jet have been studied. The spectra of the Balmer series of hydrogen, hydrocarbons and dicarbon have attracted special interest. Among these spectra, the spectral intensity ratio of CH to H-alpha at 1 mm above the substrate has a strong correlation with the quality of synthesized diamond and its deposition area.