Fabrication of Superconducting Vacuum-Gap Crossovers for High Performance Microwave Applications

The fabrication of low-loss wide-bandwidth superconducting vacuum-gap crossovers for high performance millimeter wave applications is described. In order to reduce ohmic and parasitic losses atmillimeter wavelengths a vacuum gap is preferred relative to dielectric spacer. Here, vacuum-gap crossovers were realized by using a sacrificial polymer layer followed by niobium sputter deposition optimized for coating coverage over an underlying niobium signal layer. Both coplanar waveguide and microstrip crossover topologies have been explored in detail. The resulting fabrication process is compatible with a bulk micromachining process for realizing waveguide coupled detectors, which includes sacrificial wax bonding, and wafer backside deep reactive ion etching for creation of leg isolated silicon membrane structures. Release of the vacuum-gap structures along with the wax bonded wafer after deep reactive ion etching is implemented in the same process step used to complete the detector fabrication.