Stability of high-quality Nb/AlOx/Nb Josephson junctions

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Abstract
We fabricated Nb/AlOx/Nb Josephson junctions which showed excellent I‐V characteristics (Vm = 80 mV at the critical‐current density of 1500 A/cm2). We attribute these characteristics to the clear junction interface, obtained by depositing Nb and Al layers with a minimum of thermal and physical damage. We fabricated 104 junctions connected in series on a single chip with an area of 5×5 mm2. Each junction is 10×10 μm2. These junctions have excellent uniform critical currents, have not failed, and have not changed after room‐temperature storage for 4 years. These results indicate that Nb/AlOx/Nb junctions are excellent for use in Josephson LSI circuits. However, it may be further necessary to improve the uniformity of Al and Al‐oxide layers if they are used in an actual large‐scale integrated circuit, where more than 105 junctions are contained. The influence of the flux trap on current uniformity and deterioration of the I‐V characteristics by annealing may be critical for large‐scale integration.