Fabrication of Ultra-Low-Loss, Dispersion-Engineered Silicon Nitride Photonic Integrated Circuits via Silicon Hardmask Etching

Silicon nitride (Si 3 N 4 ) photonic integrated circuits (PICs) have emerged as a versatile platform for a wide range of applications, such as nonlinear optics, narrow-line-width lasers, and quantum photonics. While thin-film Si 3 N 4 processes have been extensively developed, many nonlinear and quantum optics applications require the use of thick Si 3 N 4 films with engineered dispersion, high mode confinement, and low optical loss. However, high tensile stress in thick Si 3 N 4 films often leads to cracking, making the fabrication challenging to meet these requirements. In this work, we present a robust and reliable fabrication method for ultralow-loss, dispersion-engineered Si 3 N 4 PICs using amorphous silicon (a-Si) hardmask etching. This approach enables smooth etching of thick Si 3 N 4 waveguides while ensuring the long-term storage of crack-free Si 3 N 4 wafers. We achieve intrinsic quality factors ( Q i ) as high as 25.6 × 10 6, corresponding to a propagation loss of 1.6 dB/m. The introduction of a-Si hardmask etching along with novel crack-isolation trench designs and fabrication strategies offers notable advantages including high etching selectivity, long-term wafer storage, high yield, and full compatibility with existing well-developed silicon-based semiconductor processes. We demonstrate frequency comb generation in the fabricated microring resonators, showcasing the platform’s potential for applications in optical communication, nonlinear optics, metrology, and spectroscopy. This stable and efficient fabrication method offers high performance with significantly reduced fabrication complexity, representing a remarkable advancement toward mass production of Si 3 N 4 PICs for a wide spectrum of applications.