Uniformity and Reproducibility of Submicron 20kA/cm² Nb/AlOx/Nb Josephson Junction Process

Submicron Nb/AlO_x/Nb junctions having high critical current density of more than 20 kA/cm² are required for realizing single-flux-quantum (SFQ) circuits operating at a high speed of more than 100 GHz. We have developed a 20 kA/cm² process having submicron circular junctions. 1000-serially-connected circular shape junctions having six different diameters of 0.5 μm, 0.564 μm, 0.7 μm, 0.9 μm, 1.1 μm, and 1.3 μm have been designed. The 0.564 μm junction is designed to be 50 μA with critical current density of 20 kA/cm², which is a minimum current value of a target SFQ circuit design. We totally fabricated 12 wafers with the 20 kA/cm² process. The 1σ spread of critical currents (I_c) depending on junction sizes and the run-to-run reproducibility of critical current densities are reported as a summary of these fabrications. We obtained I_c scattering of 1σ≤ 2% even in the minimum circular junction of 0.25 μm² in several fabrication runs.