Extending microwave plasma assisted CVD SCD growth to pressures of 400 Torr
The fundamental operational behavior of a microwave cavity plasma reactor was studied for pressures between 120 and 400 Torr, i.e. the plasma dimensions, the absorbed power density and the operational field map were measured and the reactor growth performance was evaluated by growing single crystalline diamond (SCD). For pressures that exceeded 300 Torr the reactors' behavior differed from previous observations at lower pressures. As pressures were increased up to 400 Torr, the discharge became visually intense and absorbed power densities increased at a higher rate versus pressure up to 670 W/cm3 at 400 Torr. The higher discharge absorbed power densities were necessary for reactor operation at these higher pressures. The intense plasma shrunk and pulled away from the substrate holder as pressure increased. Efficient reactor operation required repositioning of the discharge with respect to the substrate holder by retuning of the reactor geometry. Successful growth of SCD was demonstrated over the entire pressure range. A designed pocket holder suppressed the formation of a polycrystalline rim for moderate methane concentrations and the SCD top surface expanded up to 40%. Freestanding SCD plates showed high crystalline quality without birefringence and had nitrogen incorporation levels below 160 ppb. The SCD growth rate tripled with increasing pressure and was as high as 28 mm/h for 380 Torr and increased to 51 mm/h after retuning the reactor geometry. Increasing the methane concentration from 5 to 9% increased the growth rate by 40%, but the crystalline quality deteriorated, i.e. the grown SCD material contained birefringence and visible defects. Additionally, the polycrystalline rim reappeared for methane concentrations of 7% and above.