金属リフトオフ法による金属エッチングマスクを用いた低損失窒化ケイ素ケルマイクロ共振器の作製

Stoichiometric silicon nitride has emerged as a widely used integrated photonic material owing to its high index of refraction, nonlinear optical properties, and broad transparency window spanning visible to mid-IR frequencies. However, silicon nitride is generally more resistant to reactive ion etching than are typical etch masks made of polymer-based resist. This necessitates resist layers that are significantly thicker than the silicon nitride and results in mask patterns which are tall and narrow. These high-aspect-ratio patterns inhibit the plasma transport of reactive ion etching, which leads to difficulties in accurately reproducing dimensions and creating well-defined, vertical waveguide sidewalls. In this work, we overcome these challenges by developing a metallic etch mask deposited via metal lift-off that provides a 30 : 1 nitride-to-metal etch rate ratio, representing a near 45-fold reduction in the required mask thickness. We demonstrate the validity of this technique by etching microring resonators with near-vertical waveguide sidewalls and intrinsic quality factors of over 1 million. Leveraging the low optical loss of our resonators, we generate optical frequency combs with more than an octave of bandwidth and dual dispersive waves. These results establish metal lift-off as a viable and easy-to-implement technique capable of producing low optical loss waveguides.