Fabrication of NbN/MgO/NbN tunnel junctions

Abstract

NbN/MgO/NbN superconductor tunnel junction devices of area 5 μmx5 μ and 10 μmx10 μm have been fabricated using a bactch-process self-aligning technique. NbN deposition was performed by a new technique involving control and optimization of the target voltage-nitrogen flow hysteresis curve in reactive planar magnetron sputtering. The choice of NbN deposition conditions reflected a compromise between optimum critical temperature, film smoothness, and intrinsic film stress. After patterning of NbN/MgO/NbN trilayers to define top and bottom NbN electrodes, evaporated SiO was deposited as the passivation layer. A liftoff technique was used to define the vias for NbN interconnects through the SiO. Each quartz substrate contained 65 structures, including single junctions and multiple junctions in series. Characterization of these devices has shown a gap voltage of 4.1 mV, and critical current densities near 1000 A cm -2.