Photonic Damascene Process for Low-Loss, High-Confinement Silicon Nitride Waveguides

We report on fabrication of high-confinement and low loss silicon nitride (Si
₃
N
₄
) waveguides using the photonic Damascene process. This process scheme represents a novel fabrication approach enabling reliable, wafer-scale fabrication of highconfinement optical waveguides. A reflow step of the silica preform reduces sidewall scattering to values not attainable with conventional etching, and reduces losses and backscattering significantly, resulting in a waveguide attenuation of 5.5 dB/m. We discuss the critical aspects of the process in detail and demonstrate the fabrication of high stress Si
₃
N
₄
waveguides with unprecedentedly large dimensions (1.75 μm × 1.425 μm) providing high-confinement at midinfrared wavelengths. A device characterization strategy allowing for systematic extraction of statistically relevant loss values is discussed and reveals the effects of the sidewall smoothing.