Superconducting Nb/AlO_X/Nb tunnel junctions on micromachined silicon substrates

Combining superconducting circuit and microfabrication technology, superconducting Nb/AlO_X/Nb tunnel junctions (STJs) have been fabricated on Si-membrane windows of 1-2 μm thickness acting as substrates. The membrane windows with areas of (10 μm)² to (1000 μm)² remained undamaged after the different fabrication steps. Furthermore, the STJs positioned on the membranes and on the bulk silicon substrate show similar current-voltage characteristics. The investigation of a large area STJ entirely covering a (120 μm)² membrane window allowed to compare the detector performance in the membrane area and on bulk substrate for the same STJ under identical operation conditions. Low Temperature Scanning Electron Microscopy was applied to image the time evolution of the two-dimensional signal distribution of the device for the detection of 5 keV electrons. At T=2.1 K the number of quasiparticles generated and counted in the membrane area is increased by a factor of 10 as compared to identical energy deposition into the detector regions covering the bulk substrate. However, the strong improvement of the counting efficiency in the membrane geometry under study is combined with a spatially inhomogeneous signal distribution. Advantages and disadvantages of the membrane configuration are discussed with respect to applications in x-ray spectroscopy.