Visible light propagation loss of LPCVD Si3N4 thin films and waveguides: impact of high-temperature annealing.

We present a systematic study of how thermal annealing (450-1000°C) affects optical propagation loss in low-pressure chemical vapor deposited (LPCVD) stoichiometric silicon nitride (Si3N4) thin films and waveguides at visible wavelengths. Slab-mode characterization using prism coupling shows that the loss remains within measurement uncertainty up to annealing temperatures of ∼900°C, above which it increases sharply at all measured wavelengths. After accounting for modeled scattering, the residual loss is consistent with an absorption-like increase of 11.1 ± 1.48 dB/cm at 447 nm following 975°C annealing. This behavior is corroborated by measurements on high-confinement waveguides. For the Si3N4 stack investigated here, annealing above 900°C substantially increases visible-wavelength propagation loss, highlighting a trade-off between infrared-optimized annealing and visible performance.