Process Development of Low- loss Thick Silicon Nitride Waveguide on 8- inch Wafer

Silicon nitride, a semiconductor material compatible with the CMOS process, offers numerous advantages such as a wide transparent window, a larger bandgap, non-existent two-photon absorption, and a smaller thermo-optic coefficient compared to silicon. These features have garnered significant attention in the field of silicon photonics. Among various silicon nitride deposition processes, those utilizing low-pressure chemical vapor deposition methods to create silicon nitride waveguides benefit from high film density stability, lower absorption loss, and reproducible processes. However, due to high process temperatures, the depsition of silicon nitride films exceeding 400 nm in thickness on 8-inch wafers results in induced tensile stress, leading to film cracking. In this study, a photonic Damascene process was used to fabricate low-loss silicon nitride photonic devices. Oxygen-buried layers are first deposited in two steps, which facilitates the formation of a dense oxide layer. We etched the pattern on the buried oxide with depth around 900 nm, following the depostion. The deposition process involves a two-step approach. In the first step, a 400 nm silicon nitride film is deposited, followed by Chemical Mechanical Polishing (CMP) to remove excess silicon nitride from the surface, thereby reducing stress accumulation. In the second step, an additional layer of silicon nitride is deposited, followed by a second CMP. After the completion of silicon nitride deposition, a high- temperature anneal is performed to break the Si-H and N-H bonds in the film, helping to reduce absorption losses in the waveguide core layer. Finally, a 2.6 mu m silicon dioxide layer is deposited as the top cladding layer using Plasma-Enhanced Chemical Vapor Deposition (PECVD). To relieve stress and prevent crack propagation, a 5 mu mx 5 mu m checkerboard structure was designed. The single mode condition of the 800 nm silicon nitride waveguide was analyzed by Lumerical software, and the waveguide width of 0.8 mu m was selected to satisfy the single mode condition of TE0 0 and TM0. 0 . Based on the simulated single-mode condition, the Finite-Difference Time-Domain (FDTD) simulation module was used to simulate the bending loss of the 0.8 mu m wide silicon nitride waveguide. The bending loss is 0.010 462 dB per 90 degrees degrees bend at a bending radius of 50 mu m and 0.006 302 dB per 90 degrees degrees bend at a bending radius of 80 mu m. We designed several sets of silicon nitride waveguide structures with different lengths and different numbers of bendings to test the propagation loss and bending loss of silicon nitride waveguides. The results show that the propagation loss is 0.087 dB/cm at 1 550 nm and 0.062 dB/cm at 1 580 nm, which are among the best in the field. The bending loss is 0.006 5 dB per 90 degrees degrees bend at a bending radius of 50 mu m and 0.006 dB per 90 degrees degrees bend at a bending radius of 80 mu m. Waveguides at different locations on the wafer were also tested and the waveguide porpagation loss remained within (0.096 +/- 0.009 2) dB/cm over the entire wafer. In this paper, an edge coupler was used to couple to the input light and a coupler with a coupling loss of 0.51 dB was designed by scanning the length and tip width of the edge coupler. In this study, low-loss thick silicon nitride waveguides were successfully fabricated on an 8-inch wafer. It was observed that annealing the core layer had the most significant effect on reducing the propagation loss of the waveguide, with the loss decreasing progressively with annealing temperature and time increased. The silicon nitride waveguides produced using the developed process exhibited excellent uniformity across the wafer with minimal variation, while maintaining extremely low propagation loss. The process can be integrated with other process platforms to expand its applications in nonlinear optics, narrow linewidth lasers, radar and other areas.