Uniformity and stability of Nb-aSi-Nb "SNAP" Josephson tunnel junctions

We have examined the suitability of Nb-aSi-Nb tunnel junctions, patterned by the selective niobium anodization process (SNAP), for large scale integrated circuit applications. Using sputtered SiO2as the SNAP anodization mask, the device area closely reproduces the mask area, with undercut < 50 nm, and no detectable perimeter effects. The critical current density increases with the radial distance from the center of the Si target, due to the quadratic decrease in the thickness of the sputtered Si barrier: at a distance of 1 cm, this increase is ∼ 10%. Arrays of 12 μm × 12 μm junctions have been fabricated with standard deviations of the critical current of ∼ 1%. These arrays have been thermally cycled between room temperature and 4.2 K over 600 times without change. Junctions can be annealed at 175°C for extended periods of time to increase the current density by up to ∼ 50% with no deterioration in junction quality.