Fabrication of array detectors with 100 superconducting tunnel junctions and Ta x‐ray absorbers

Abstract

AbstractWe have developed superconducting tunnel junction (STJ) x‐ray array detectors with a large detection area, high detection efficiency, and high counting rate. The STJ array detectors consist of 100 Nb/Al‐AlOx/Al/Nb STJs with a size of 200 × 200 µm. Each STJ has a 2 µm thick Ta absorber connected to the top Nb layer through a thin insulating layer. The absorbers act as an energy converter from high x‐ray energies to phonon energies, which are compatible with superconducting energy gaps. The array detectors realize a total detection area of 4 mm2 and a detection efficiency of ∼70% for x‐rays of energy up to 6 keV. In addition, it is expected that the spatial uniformity of the x‐ray signal outputs of the Nb‐based STJ by using the Ta absorber may be improved by a fast diffusion of quasiparticles in the Ta absorbers. In order to minimize cross talk, we have adapted a coplanar transmission waveguide (CPW) structure having an 8 µm wide signal Nb line, 7 µm wide ground gaps, 12 µm wide ground Nb lines, and a 50 Ω impedance. The fabrication processes and the detector response to x‐rays are discussed. Copyright © 2007 John Wiley & Sons, Ltd.