SUPERCONDUCTING NB ALOX/NB TUNNEL-JUNCTIONS ON MICROMACHINED SILICON SUBSTRATES/

Combining superconducting circuit and microfabrication technology, sup erconducting Nb/AlO(X)/Nb tunnel junctions (STJs) have been fabricated on Si-membrane windows of 1-2 mum thickness acting as substrates. The membrane windows with areas of (10 mum)2 to (1000 mum)2 remained unda maged after the different fabrication steps. Furthermore, the STJs pos itioned on the membranes and on the bulk silicon substrate show simila r current-voltage characteristics. The investigation of a large area S TJ entirely covering a (120 mum)2 membrane window allowed to compare t he detector performance in the membrane area and on bulk substrate for the same STJ under identical operation conditions. Low Temperature Sc anning Electron Microscopy was applied to image the time evolution of the two-dimensional signal distribution of the device for the detectio n of 5 keV electrons. At T = 2.1 K the number of quasiparticles genera ted and counted in the membrane area is increased by a factor of 10 as compared to identical energy deposition into the detector regions cov ering the bulk substrate. However, the strong improvement of the count ing efficiency in the membrane geometry under study is combined with a spatially inhomogeneous signal distribution. Advantages and disadvant ages of the membrane configuration are discussed with respect to appli cations in x-ray spectroscopy.