Abstract Body

Introduction
X-ray absorption spectroscopy (XAS) is a method of materials analysis which provides nanostructure information around specific element in a sample via observing high resolution XAS near the absorption edge. Soft X-rays (SX) are important for XAS, since measurements of light element dopants are only possible there. When measuring XAS of bulk materials with SX, the fluorescence yield is measured instead of the absorption due to the low X-ray transmittance. Because the fluorescence yield of SX is low and the characteristic X-rays of other elements overlap on the fluorescent X-ray spectrum, a spectrometer with high sensitivity and high resolution is required. Superconducting tunnel junction (STJ) detectors have attracted attention because they can simultaneously achieve higher resolution than semiconductor detectors, higher sensitivity than analyzing crystals, and high counting rate required to detect rare events due to dopants.

For these reasons, XAS using STJ detector array were developed in the past two decades [1-3]. We constructed an XAS equipped with a 100-pixel, 100-micron square STJ detector. The performance achieved was an area of ​​1 mm2, energy resolution of 10-20 eV for X-ray energy below 1000 eV, and count rate of 0.5 M cps with energy resolution of 20 eV at the C-K line [4-5]. The instrument was used to analyze compound semiconductor and structural materials [6-8]. The lowest dopant concentration was 80 ppm nitrogen in silicon carbide [8]. However, to analyze real materials for industries, it is necessary to be able to measure lower concentrations of dopants and to measure a large number of samples. Therefore, we developed an XAS apparatus equipped with a 200-pixel STJ detector to achieve the sensitivity twice. Since the new 200-pixel chip introduced problems with magnetic flux trapping, geomagnetic shielding methods are reconsidered. After solving these issues, we conducted performance tests at synchrotron radiation beamline BL-11A at the Photon factory of the High Energy Accelerator Research Organization. This paper reports the implementation of a 200-pixel STJ detector array and the performance of the XAS system using the STJ array.

Experiment
The STJ spectrometer used is as follows: The STJ chip was fabricated at Clean Room for Analog-digital superconductiVITY (CRAVITY) at the National Institute of Advanced Industrial Science and Technology. The area of ​​one pixel detector is 100 microns square, and the layer structure is Si substrate / 100 nm Nb / 70 nm Al / AlOx / 70 nm Al / 300 nm Nb, the passivation layer is SiO2, and the wiring is Nb. The characteristics of a detector with the same structure have already been reported [9]. Two hundred pixels are placed on a 17 mm square silicon die (Figure 1a). The layout is almost the same as in the literature [3-5]. The difference from the previous setup is that the material for the magnetic shield has been changed from permalloy C (PC) to A4K. The reason for changing the material was that the magnetic permeability of the PC magnetic shield decreases at low temperatures, causing fluxoid traps and resulting in variations in the current-voltage characteristics of each pixel. Tests at room temperature have shown that the A4K shield can reduce the effects of the earth's magnetic field to less than 50 nT, which is approximately one-thousandth of the earth's magnetic field.

The performance was evaluated by measuring the current-voltage characteristics, X-ray spectrum, and XAS at the synchrotron beamline, BL-11A. The current-voltage characteristics were measured by the two-terminal method. The X-ray measurements were carried out as follows. A set of charge amplifier and digital multichannel analyzer was used for each pixel. The energy resolution was evaluated using the full width at half maximum of the characteristic X-ray peak. The X-ray absorption spectrum was measured as the ratio of the characteristic X-ray intensity to the incident light intensity, with the energy of the incident light as a parameter.

Results and Discussion
The results of measuring the current-voltage characteristics of the 200-pixel STJ detector are as follows. Of the 200 pixels, 189 pixels showed a current value of 10 nA or less at a sub-gap voltage of 0.3 mV. For seven pixels, the sub-gap current was 20-30 nA. In the preliminary experiments, the distribution of　pixels showing high current values ​​was different. This fact indicates that random flux quantum traps are the cause of the excess current, and suggests that the magnetic field at the superconducting transition exceeds 200 nT, which is the flux quantum of 100 microns square. In contrast, according to the magnetic shielding characteristics measured at room temperature, the magnetic field around the STJ chip is less than 50 nT. In order to determine the cause of magnetic flux trapping and eliminate the adverse effects of the magnetic field, in-situ magnetic field measurements are required.

Figure 1c shows the X-ray spectra for some samples. The energy resolution of each pixel for the N-K line (392 eV) is 10.1 eV ± 0.9 eV in full width at half maximum. Figure 1d shows an example of measuring the N-K absorption edge of polyimide. By using a 200-pixel array, the time required to acquire data was cut in half.

Summary
XAS using STJ detector arrays is required for the analysis of light element dopants in advanced materials. In order to improve the detection sensitivity, we attempted to use a 200-pixel STJ detector. By improving the magnetic shielding and suppressing flux quantum traps, over 90% of the pixels are usable. Tests were performed at a synchrotron radiation beamline, and an energy resolution of 10.1 ± 0.9 eV for N-K lines was achieved, enabling XAS. The overall sensitivity is twice that of our previous 100-pixel system.

References

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[7] N. Isomura and Y. Kimoto, J. Synchrotron Rad. 28 (2021) 1114–1118, doi:10.1107/S1600577521004008

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[9] G. Fujii et al., J Low Temp Phys 184 (2016) 194–199, doi:10.1007/s10909-015-1433-4

Acknowledgment

The authors thanks to Tomoaki Ishizuka in AIST for his support in the experiments. The device was fabricated in the CRAVITY in National Institute of Advanced Industrial Science and Technology (AIST). A part of this work was supported by "Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), grant number JPMXP1222AT5041. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2021G666).

Figure 1. (a) Microphotograph of a 200-pixel, 100 micoron square STJ detector array. (b) Ssetup of experiment. (c) Fluorescence X-ray spectrum of polyimide, teflon, aluminium, and stainless steels taken by STJ detector array (d) Partial fluorescence yield X-ray absorption spectrum of polyimide at nitrogen K edge obtained using the 200-pixel STJ array.

Keywords: X-ray spectroscopy, superconducting tunnel junction, synchrotron radiation, materials analysis