  The design and fabrication of low-loss wide-bandwidth superconducting
vacuum-gap crossovers for high performance millimeter wave applications are
described. In order to reduce ohmic and parasitic losses at millimeter
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 micro-machining 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 DRIE is implemented in the same process step used to
complete the detector fabrication
