Nb/Al STJ detectors with sub-nA subgap current

Superconducting tunnel junctions (STJ) are promising high-resolution radiation detectors because of their larger energy resolution with respect to conventional Si-based detectors. We have developed a fabrication process for Nb/Al-AlOx/Nb STJ devices which produces junctions with subgap currents below 50 pA at 300 mK, a factor of 10−6 less than the subgap current at 4.2 K. We report the details of the fabrication process and preliminary results on X-ray measurements at 300 mK with a 5 mC 55Fe source.