Fabrication and characterization of SNS Josephson junctions with an aluminum barrier

We report the current status of our SNS technology aimed at the development of programmable voltage standard devices. Using the simple Nb/Al/Nb trilayer process, with an Al barrier thickness of the order of 100 nm, we have fabricated SNS Josephson junctions whose electrical properties can be changed by varying the morphology of the Al film. The major role in determining the electrical behavior of the junctions is played by the roughness of the thick Al barrier. AFM analysis shows that the Al roughness is strongly reduced by increasing its deposition rate. The critical current density varies by two orders of magnitude, from 10/sup 3/ A/cm/sup 2/ up to 10/sup 5/ A/cm/sup 2/, with correspondingly normal resistances from 1 /spl Omega/ down to few m/spl Omega/. The magnetic field dependence of the critical current is also affected by the barrier structure, while all the junctions show regular Shapiro-like rf-induced steps at 70 GHz.