Design and numerical simulation of a ring waveguide slot-coupled magnetic resonance cavity for large-area diamond film deposition

Microwave plasma chemical vapor deposition (MPCVD) is the preferred method for preparing large-area, high-quality diamond films. This paper presents a specifically configured and numerically optimized slot-coupled MPCVD system based on a magnetically coupled annular waveguide. The proposed system employs a magnetic coupling scheme using an annular waveguide slot antenna, in which microwave energy is coupled into the resonant cavity through uniformly distributed horizontal slots. This configuration significantly enhances both the size and intensity of the high-electric-field region above the deposition stage. The effects of key structural parameters—including slot depth, slot width, and deposition stage height—on the electric field and plasma distribution were systematically investigated through numerical simulations, and the cavity dimensions were accordingly optimized. Frequency perturbation analysis demonstrates that the optimized apparatus exhibits strong robustness against microwave frequency fluctuations. The microwave transmission section incorporates a waveguide magic-T tuner; by adjusting the short-circuit piston dimensions on the E-plane and H-plane branches, the reflected microwave power can be substantially reduced, with S11 reaching −32 dB. The system was operated stably for 280 h at a microwave frequency of 2.45 GHz, a microwave power of 3400 W, and a gas pressure of 105 Torr, enabling the growth of 3 in. diamond films. The deposited diamond films exhibited large grains and high crystalline quality, as confirmed by scanning electron microscopy, Raman, and x-ray diffraction characterizations.