Femtosecond laser modification of LPCVD-SiN photonic devices: microstructural evolution and post-fabrication trimming

Femtosecond laser processing provides a localized and contactless approach for permanent post-fabrication trimming of integrated photonic devices. Here, we investigate dose-dependent microstructural evolution in low-pressure chemical vapor deposition silicon nitride (LPCVD-SiN) and apply the resulting refractive-index modification to resonance correction and performance tuning of microring resonators (MRRs). An 800 nm, 40 fs laser is focused through the SiO 2 upper cladding onto the SiN core, and the accumulated pulse dose is controlled by varying the scanning speed. Raman spectroscopy, transmission electron microscopy and energy-dispersive spectroscopy reveal Si–N bond dissociation, pronounced Si enrichment, and the coexistence of amorphous and nanocrystalline Si. These transformations increase the local effective refractive index and red-shift the MRR resonances. Calibration using individual MRRs yields a trimming resolution of less than 0.1 nm with an excess loss of approximately 0.15 dB. Localized laser re-annealing partially restores device performance and limits the resonance drift to below 20 pm over 30 days. The method corrects resonance mismatches in second-order and parallel-coupled MRRs and improves channel uniformity in wavelength-division multiplexing devices.