High‐Stress Si 3 N 4 Reflective Membranes Monolithically Integrated With Cavity Bragg Mirrors

High‐stress silicon nitride () membranes represent the state‐of‐the‐art for cavity optomechanics, combining ultralow dissipation, optical transparency, and full compatibility with wafer‐scale nanofabrication. Yet their integration into high‐finesse optical cavities has remained difficult, typically requiring bonding or alignment‐sensitive assembly that limits scalability and long‐term stability. Here, we introduce a monolithic, wafer‐level integration strategy that directly suspends high‐stress photonic‐crystal membranes above thermally compatible SiN/ distributed Bragg reflectors (DBRs) capable of withstanding the high temperatures required for stoichiometric growth. An amorphous‐silicon sacrificial layer and stiction‐free plasma undercut yield vertically coupled cavities with sub‐micron spacing—forming self‐aligned resonators within seconds of release. Owing to the intrinsic tensile stress, the suspended membranes exhibit atomic‐scale sagging, ensuring near‐ideal cavity parallelism and long‐term stability. Optical reflectivity measurements reveal cavity finesse above with wavelength‐scale gaps between mirrors. Mechanical ringdown measurements show , indicating that DBR integration preserves the low‐dissipation character of high‐stress . This supports straightforward extension to many high‐Q nanomechanical architectures. The resulting –DBR platform unites optical and mechanical coherence with high fabrication yield and design flexibility, enabling scalable optomechanical devices for precision sensing and quantum photonics.