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

Abstract

Silicon nitride (Si₃N₄) 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 Si₃N₄ processes have been extensively developed, many nonlinear and quantum optics applications require the use of thick Si₃N₄ films with engineered dispersion, high mode confinement, and low optical loss. However, high tensile stress in thick Si₃N₄ 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 Si₃N₄ PICs using amorphous silicon (a-Si) hardmask etching. This approach enables smooth etching of thick Si₃N₄ waveguides while ensuring the long-term storage of crack-free Si₃N₄ wafers. We achieve intrinsic quality factors (Q_i) as high as 25.6 × 10⁶, 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 Si₃N₄ PICs for a wide spectrum of applications.