Development of a new microwave plasma torch and its application to diamond synthesis

We have developed a new microwave plasma torch (MW plasma jet), which can generate a pure H2 plasma jet at atmospheric pressure from the end of a center electrode with an input power above 2 kW. The electric field has maximum strength at the end of the electrodes, so the plasma is initially ignited by the electric breakdown. The plasma jet seems to be powered and sustained by the electromagnetic field generated between the electrodes and/or in the chamber. Accordingly, the mechanism to sustain the plasma is considered to be different from that to initially ignite the plasma. In order to confirm the effectiveness of this plasma for CVD processes, the MW plasma jet has been applied to the synthesis of diamond from Ar-H2-CH4 system. Diamond particles or films were successfully synthesized at a higher rate (30 μm/h) on a larger area (25×25 mm2), compared with the usual synthesis methods. The growth rate was in proportion to substrate temperature and/or CH4/H2 ratio up to 1200 °C and/or 10 vol %, respectively. These conditions were a little different from those of other methods reported so far. The results derived confirmed that the MW plasma jet has unique characteristics and that the process developed here was effective for the synthesis of diamond.