Annealing-induced defects and optical degradation in sputter-deposited silicon nitride: Implications for photonic applications

Silicon nitride (SiNx) is a critical material for advanced photonic applications. This study investigates the effect of annealing on sputter-deposited silicon nitride. The results demonstrate that blisters and holes appear in annealed SiNx, which compromise its integrity and lead to increased roughness. These defects are closely related to the annealing process. Specifically, the blister diameter expands from 5.17 μm at 600 °C to more than 25 μm at 800 °C. Additionally, the defect ratio increases significantly from 1.25 % to 14.82 % for full annealing. Rapid thermal annealing induces a 12 % defect ratio and a surface roughness of 13.1 nm. X-ray photoelectron spectroscopy results suggest that defects originate from unsaturated silicon sites, particularly in silicon-rich SiNx. Raman spectroscopy reveals a blueshift in the silicon peak after annealing, indicating biaxial tensile stress applied to the coatings, which is supported by the diffraction peaks observed in grazing incidence X-ray diffraction patterns. This mechanical stress may contribute to blistering and bursting. Moreover, annealing significantly alters the refractive index and dispersion properties of SiNx, with transmission losses reaching up to 20 %, presenting a considerable challenge for photonic applications. Notably, the performance of silicon nitride-based Bragg reflectors deteriorates markedly after annealing. This study elucidates the balance between annealing conditions and their effects on sputter-deposited silicon nitride, ensuring its continued viability in the rapidly expanding field of photonics.