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

Abstract

We 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.

Current-voltage characteristics of 100-pixel 200-μm-square superconducting tunnel junction X-ray detector array

“…One group exhibits a low subgap current. The I-V curves of the low-current group are similar to that of a previously reported 100-m STJ that showed excellent X-ray detection properties [8]. In the other group, the curves show an excess current at a voltage greater than 0.1 mV.…”

“…The yield of the 200-m STJ, which is defined by the portion of pixels having a subgap current less than 10 nA, is approximately 90%. The typical subgap current values in the present study are two orders of magnitude lower than those in a previous study [8].…”

“…In a single-element study of a 20-500-µm square, the detector showed a subgap current due to trapping of fluxoids in a 500-µm STJ, although detailed magnetic shielding was used during cooling [7]. In mass spectrometer studies using 100-element 200-µm-square STJs, 85% of pixels showed a typical subgap current less than 1 A [8]. Although the performance was sufficient for mass spectrometry, it was insufficient for X-ray spectroscopy, which requires high energy resolution and precise readout noise.…”

Fabrication of superconducting tunnel junctions for soft X-ray spectroscopy

“…It is already known that the SiO 2 residual layer on the top Nb surface causes low‐energy artifact events 5. Therefore, in our fabrication processes, the sensitive area was patterned by removing the SiO 2 isolation layer with a lift‐off technique 6. The sensitive area is defined by a photoresist pattern and then the SiO 2 layer is deposited.…”

Soft X-ray Detection Performance of Superconducting Tunnel Junction Arrays with Asymmetric Tunnel Junction Layer Structure

“…8,9) However, single conventional STJs are unsatisfactory for the fluorescence XAFS measurement of light trace elements because of three disadvantages. 5,6,[10][11][12][13][14][15][16][17][18] First, their sensitive areas should be limited to $0:01 mm 2 to maintain the intrinsic good energy resolution due to their capacitances and leakage currents. Second, the counting rate is limited to $10 kcps by tunneling times longer than 0.1 s. Third, spectral artifacts are generated in soft X-ray spectra obtained by conventional STJs, and interfere with the detection of low-concentration light elements.…”