Metal lift-off for fabrication of silicon nitride photonic microring resonators

Chip-scale photonic microring resonators based on Kerr nonlinearity are powerful tools for the generation of broadband optical frequency combs with applications including precision spectroscopy, low-noise frequency synthesis, and optical clocks. To support comb generation in these devices, waveguides hundreds of nanometers tall are often required. This poses a challenge for subtractive fabrication in materials such as silicon nitride (SiN), where the desired pattern is chemically etched from a solid layer of bulk material utilizing a protective mask template. While organic polymer-based masks are a common choice, they offer low resistivity to the etch process and are prone to inconsistencies in waveguide sidewall angle and depth. Alternatively, we demonstrate a novel method for subtractive processing of thick SiN waveguides with metallic chromium masks through the application of a metal lift-off technique. By leveraging the high etch resistivity of the metallic mask, we fabricate SiN microring resonators that exhibit near-vertical waveguide sidewall angles and uniform etch depth. This work serves to highlight both the benefits and drawbacks to this technique as a robust approach to achieving high-quality optical microring resonators.