Process Development of Low-Loss LPCVD Silicon Nitride Waveguides on 8-Inch Wafer

Silicon nitride is a material compatible with CMOS processes and offers several advantages, such as a wide transparent window, a large forbidden band gap, negligible two-photon absorption, excellent nonlinear properties, and a smaller thermo-optic coefficient than silicon. Therefore, it has received significant attention in the field of silicon photonics in recent years. The preparation of silicon nitride waveguides using low-pressure chemical vapor deposition methods results in lower loss and better process repeatability. However, due to the higher temperature of the process, when the thickness of the silicon nitride film exceeds 300 nm on an 8-inch wafer, it is prone to cracking due to the high stress generated by the film. Limited by this high stress, silicon nitride waveguide devices are typically developed on wafers with a thickness of 4 inches or less. In this work, we successfully fabricated a 400 nm-thick silicon nitride waveguide on an 8-inch wafer using a Damascene method similar to the CMOS process for copper interconnects and demonstrated propagation losses of only 0.157 dB/cm at 1550 nm and 0.06 dB/cm at 1580 nm.