Improved microwave plasma cavity reactor for diamond synthesis at high-pressure and high power density

Microwave plasma assisted synthesis of diamond is experimentally investigated using high purity, 2–5% CH4/H2 input gas chemistries and operating at high pressures of 180–240 Torr. A microwave cavity plasma reactor (MCPR) was specifically modified to be experimentally adjustable and to enable operation with high input microwave plasma absorbed power densities within the high-pressure regime. The modified reactor produced intense microwave discharges with variable absorbed power densities of 150–475 W/cm3 and allowed the control of the discharge position, size, and shape thereby enabling process optimization. Uniform polycrystalline diamond films were synthesized on 2.54 cm diameter silicon substrates at substrate temperatures of 950–1150 °C. Thick, freestanding diamond films were synthesized and optical measurements indicated that high, optical-quality diamond films were produced. The deposition rates varied between 3 and 21 μm/h and increased as the operating pressure and the methane concentrations increased and were two to three times higher than deposition rates achieved with the MCPR operating with equivalent input methane concentrations and at lower pressures (≤ 140 Torr) and power densities.

Research Highlights
► Microwave cavity plasma reactor performance is studied in the 180–240 Torr pressure regime.
► Absorbed microwave power density varied between 150 and 475 watts/cubic centimeter.
► Uniform one inch diameter polycrystalline diamond plates of optical quality were synthesized.
► Growth rates and uniformity were optimized by varying the substrate position and reactor tuning.
► Deposition rates were 2–3 times higher than with a similar reactor operating at lower pressures.