Superconducting tunnel junction x-ray detectors with ultra-low subgap current

Superconducting tunnel junctions are promising as radiation detectors because of their larger energy resolution with respect to conventional Si-based detectors. We have developed a fabrication process for Nb-based STJ devices which allows to fabricate junctions with leakage currents well below 50 pA at 300 mK, a factor of 10-6 less than the leakage current at 4.2 K. The junctions are diamond-shaped with areas between 20 × 20 and 100 × 100 ?m2. We report the details of the fabrication process, together with an analysis of the structure of films by AFM, SEM and x-ray spectroscopy. We show the measured I - V curves at 300 mK, together with preliminary results on x-ray measurements at 300 mK with a 5 mC 55Fe source.