Comparison of Soft and Hard Mask Etch Templates for Subtractive Processing of High-Q SiN Microresonators

Optical frequency combs created in chip-scale photonic resonators (microcombs) have a wide range of applications in optical metrology, ranging, and frequency synthesis. Silicon nitride (SiN) microring resonators, popular for microcomb platforms, can be printed directly on silicon wafer substrates, which allows for geometric dispersion control and planar integration with other CMOS-compatible processes. However, SiN is resistant to most etching recipes, and it can be difficult to subtractively pattern SiN devices, where waveguides are lithographically printed onto a mask and transferred to a thick SiN layer via etching. It is typically necessary to increase either mask thickness or etch selectivity for such processes. Here, we present a comparison of two subtractive processes for the fabrication of silicon nitride waveguide resonators, one using a polymer-based photoresist “soft” mask and the other using a metallic “hard” mask. This work will focus on advantages and drawbacks of using both techniques and compare the optical performance of the fabricated waveguides. This work illustrates two reliable methods for the fabrication of high-Q on-chip resonators and lays the foundation for future fabrication of nonlinear photonic devices.