Fabrication and measurement of a SINIS-type Josephson array

Superconductor-insulator-normal metal-insulator-superconductor (SINIS)-type Nb/Al2O3/Al/Al2O3/Nb Josephson arrays with small hysteresis were fabricated to investigate the possibility of applications to a programmable Josephson voltage standard. When irradiated with microwaves in the frequency range between 6.5 GHz and 75 GHz, a vertical Josephson step up to 21 mV was observed in 140 junctions, in which the step size was large enough to ensure good stability as a reference voltage. With the 2,520 junctions irradiated with 73 GHz, an evident, but non-vertical, step up to 380 mV was observed. The experimental results suggest the possibility of making a few thousand SINIS junctions in a uniform way for the construction of a programmable voltage standard. In addition to the Josephson step, a, huge enhancement of the de critical current and the S-type negative differential resistance were also observed when our SINIS junctions were irradiated with low-power microwave.