Fabrication of Nb/Al-Al/sub 2/O/sub 3//Nb junctions with extremely low leakage currents

Nb/Al-Al/sub 2/O/sub 3//Nb trilayer films were deposited using DC magnetron sputtering guns in a UHV (ultrahigh vacuum) system which is capable of 5*10/sup -10/ Torr. SIS (superconductor-insulator-superconductor) junctions as small as 3.2*3.2 mu m/sup 2/ were isolated from the trilayer by standard photolithography. The junctions typically have V/sub m/=70-90 mV at 4.2 K, while at 2.0 K, V/sub m/ is as large as 1 V. This corresponds to a subgap current of 0.15% of the quasiparticle current rise. The subgap leakage current is compared to the predictions of the BCS (Bardeen-Cooper-Schrieffer) theory. The specific capacitance is preliminarily measured to be 45+or-5 fF/ mu m/sup 2/.<
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