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 20times20 and 100times100 mum². 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.