The Influence of Deposition Temperature and Material Stress on Low-Loss Silicon Nitride Films for Integrated Quantum Optics

We report on an optimization procedure for depositing low-loss silicon nitride films at temperatures of 760 °C and 820 °C using low-pressure chemical vapor deposition. They were characterized in terms of quality and compositional proximity to stoichiometric silicon nitride. Films deposited at 760 °C showed a higher stoichiometry, with a silicon-to-nitrogen ratio of 0.744, when compared to the 820 °C film, which had a ratio of 0.77. We found the film deposited at the lower temperature had a smoother surface and exhibited lower optical losses. We investigated the impact of film stress on the refractive index of the film and found that removing the backside nitride from the wafer after deposition has a major effect on refractive index values. When using these films for integrated nonlinear and quantum applications, such as frequency conversion or soliton generation, knowledge of how the index changes with wafer and fabrication processing is critical for predicting the correct geometries, and the concomitant group velocities, needed to realize such quantum technologies.