Planarized vacuum-gap capacitors for improved quantum optomechanics

Since their creation over a decade ago, superconducting vacuum-gap capacitors have enabled several breakthroughs in the field of quantum optomechanics. The mechanically-compliant capacitor provides strong optomechanical coupling, g0, between a superconducting microwave circuit and a mechanical resonator. This coupling is the direct result of fabrication methods that can achieve small capacitor plate separations (~30 nm) at low temperature. However, the traditional fabrication technique [1] creates kinks in the suspended film that result in inconsistent plate separations. Recent work [2] has shown that planarization can provide reproducible vacuum-gap separation and improve its mechanical quality (Q) factor, however g0 is still limited by the achievable gap size. Here we report on the progress toward an alternative planarized fabrication process that combines the benefits of the traditional methods with the advantages of planarization. Incorporating a planarized silicon nitride sacrificial layer into an etched sapphire substrate should allow for concurrent optimization of both Q and g0, enabling quantum coherent interactions between the cavity and mechanics at single photon levels.