Process-structure-property relationships in subtractive fabrication of silicon nitride microresonators for nonlinear photonics

Abstract

Silicon nitride (SiN) has emerged as a popular platform for nonlinear photonics due to its large bandgap, relatively large nonlinear index of refraction, and high degree of CMOS-compatibility. Nonlinear optical processes in microresonators require both large quality factors to promote build-up of large intracavity powers and sufficient control over resonator dispersion to implement phase-matching, which is generally achieved through careful design and fabrication of the resonator cross-sectional dimensions. Additionally, generating dissipative Kerr soliton optical frequency combs requires operation in the anomalous dispersion regime, which calls for such thick layers of SiN as to make achieving accuracy during etching a challenge. In this work, we investigate the relationship between fabrication process details, including lithography and etching, and optical properties, such as optical loss and resonator dispersion, for two different subtractive fabrication approaches: one utilizing a polymer-based electron beam resist as the protective mask during etching and the other employing a thin metallic etch-mask. Using both approaches, we identify the advantages and disadvantages of polymer and metal etch-masks, determine the origins of optical loss through variations in electron beam lithography parameters, and connect resonator sidewall roughness to optical loss. Finally, we compare frequency comb generation in resonators fabricated using either approach.