Fabrication of superconducting tunnel junctions for soft X-ray spectroscopy

Abstract

Superconducting tunnel junction (STJ) array detectors with a new design, which has a minimum junction edge coverage of an SiO 2 insulation, passivation layer and an asymmetric tunnel junction layer structure, have been fabricated for a soft X-ray region between 100 eV and 1 keV. The sensitive area was patterned by removing the SiO 2 deposition layer by a lift-off technique that ensured no contamination layer on the top Nb electrode surface. The width of the passivation rim was as narrow as 0.5 µm at the junction edge. The clean Nb surface and the narrow SiO 2 rim resulted in almost no artifact photon events in a low-energy region. The asymmetric layer design is effective in solving a problem of double peak response to monochromatic X-rays, which is commonly observed in STJ detectors. The performance of a 100 pixel array detector was investigated by the fluorescent X-ray analysis of oxides and nitrides: an energy resolution of about 30 eV for the total absorption of the Kα lines of oxygen and nitrogen. We plan to realize an energy resolution of better than 20 eV and a counting rate of over 1 Mcps for fluorescence-yield X-ray absorption spectroscopy for light trace elements.

Superconducting Molecule Detectors Overcoming Fundamental Limits of Conventional Mass Spectrometry

“…Elements, simple molecules, and biomolecules were ionized and then separated according to the m/z values. The fabrication of the 100 pixel STJ detectors can be found in [8]. Each STJ simultaneously measures the arrival time and the deposited kinetic energy of individual ions, using field programmable gate array (FPGA)-based digital signal processing with 5 ns time resolution and 12 bit pulse height resolution.…”

X-ray Detection Performance of 100-pixel Superconducting Tunnel Junction Array Detector in the Soft X-ray Region

“…However, the problem of spectral artifacts remains unsolved. Previously, we reported an improved STJ array detector, in which the STJ pixels had a minimum junction edge coverage of SiO 2 insulation and an asymmetric tunnel junction layer structure, which eliminated both the double-peak response and the low-energy artifacts [4]. This detector had 100 Nb/Al STJs, each with a size of 100 × 100 µm 2 .…”

X-ray Spectrometry

“…In this case, the microcalorimeter can be very powerful due to its low detection limits for target elements. Ukibe et al 10 at the Photon Factory at KEK, Japan, fabricated a superconducting tunnel junction (STJ) detector for the soft X-ray region. They patterned the STJ detector by removing the SiO 2 deposition layer using a lift-off technique to avoid contamination of the top Nb electrode, resulting in almost no artifact photon events.…”