Optimization of the refractive index of silicon nitride films in magnetron sputtering and annealing control for low-roughness

Low-loss silicon nitride (SiNx) films deposited at low temperatures hold substantial potential for photonic integrated circuits, especially for hybrid integration with thermally sensitive substrates such as Complementary Metal Oxide Semiconductor (CMOS)-compatible silicon-on-insulator (SOI) and lithium niobate on insulator (LNOI). In this work, silicon nitride thin films were synthesized at room temperature via radiofrequency (RF) reactive sputtering. Systematic optimization of deposition parameters, including RF power, nitrogen flow rate, and working pressure, was performed to enhance the optical properties of the films. The variation in nitrogen flow rate was confirmed to result in changes in the film's chemical composition, as evidenced by X-ray photoelectron spectroscopy, which in turn led to variations in the film's refractive index. We further examined the impact of annealing time at the CMOS back-end-of-line (BEOL) compatible temperature (400 °C) on the surface roughness of the silicon nitride film. The results indicate that an optimized annealing time can substantially reduce film roughness, effectively mitigating surface scattering losses in waveguides and facilitating the development of ultralow-loss silicon nitride waveguides.