GEOMETRY-EFFECTS ON THE PERFORMANCE OF SUPERCONDUCTING TUNNEL-JUNCTIONS

Superconducting tunnel junctions are predicted to have high energy res olving power at X-ray energies (similar to 4 eV FWHM at 6 keV). Possib le mechanisms for the degradation of the energy resolution in an exper imental configuration are spatial non-uniformities in the response of the STJs, induced by the loss of quasiparticles into the connecting le ads or at the edges of the detectors. In order to investigate the effe cts of the device geometry on its performance as an X-ray detector, ST Js of different sizes (from 10 X 10 to 200 X 200 mu m(2)) and with dif ferent widths of the connecting leads (1 and 3 mu m) have been fabrica ted on the same chip. The STJs are niobium based devices with 110 nm t hick epitaxial base electrodes and 180 nm thick polycrystalline top el ectrodes. The response of devices to 6 keV X-rays is discussed.