We are developing monolithic active pixel sensors, x-ray SOIPIXs based on a Silicon-On-Insulator CMOS technology. Its event trigger output function offers a high time resolution better than ~10 usec. (1) In 2022-23, we and evaluated large sensors, XRPIX-X, with a pixel array size of 14mm x 22mm. We report its design and the results of the performance evaluation. (2) We are developing "Digital X-ray SOIPIXs" for satellite use, featuring on-chip ADCs, DACs, and BGRs for noise robustness. An on-chip clock pattern generator is also included to simplify the readout digital circuits. (3) XRPIXs are increasingly being utilized in various scientific applications beyond x-ray astronomy, and a brief introduction will be provided.
We evaluate the single event tolerance of the x-ray silicon-on-insulator (SOI) pixel sensor named XRPIX, developed for the future x-ray astronomical satellite FORCE. In this work, we measure the cross-section of single event upset (SEU) of the shift register on XRPIX by irradiating heavy ion beams with linear energy transfer (LET) ranging from 0.022 to 68 MeV / ( mg/cm2 ) . From the SEU cross-section curve, the saturation cross-section and threshold LET are successfully obtained to be 3.4−0.9+2.9×10−10 cm2/bit and 7.3−3.5+1.9 MeV/(mg/cm2), respectively. Using these values, the SEU rate in orbit is estimated to be ≲ 0.1 event / year primarily due to the secondary particles induced by cosmic-ray protons. This SEU rate of the shift register on XRPIX is negligible in the FORCE orbit.
Xtend is a soft x-ray imaging telescope developed for the x-ray imaging and spectroscopy mission (XRISM). XRISM is scheduled to be launched in the Japanese fiscal year 2022. Xtend consists of the soft x-ray imager (SXI), an x-ray CCD camera, and the x-ray mirror assembly (XMA), a thin-foil-nested conically approximated Wolter-I optics. The SXI uses the P-channel, back-illuminated type CCD with an imaging area size of 31mm on a side. The four CCD chips are arranged in a 2×2 grid and can be cooled down to −120 °C with a single-stage Stirling cooler. The XMA nests thin aluminum foils coated with gold in a confocal way with an outer diameter of 45 cm. A pre-collimator is installed in front of the x-ray mirror for the reduction of the stray light. Combining the SXI and XMA with a focal length of 5.6m, a field of view of 38′ × 38′ over the energy range from 0.4 to 13 keV is realized. We have completed the fabrication of the flight model of both SXI and XMA. The performance verification has been successfully conducted in a series of sub-system level tests. We also carried out on-ground calibration measurements and the data analysis is ongoing.
We have been developing the SOI pixel detector “INTPIX” for space use and general purpose applications such as the residual stress measurement of a rail and high energy physics experiments. INTPIX is a monolithic pixel detector composed of a high-resistivity Si sensor, a SiO2 insulator, and CMOS pixel circuits utilizing Silicon-On- Insulator (SOI) technology. We have considered the possibility of using INTPIX to observe X-ray polarization in space. When the semiconductor detector is used in space, it is subject to radiation damage resulting from high-energy protons. Therefore, it is necessary to investigate whether INTPIX has high radiation tolerance for use in space. The INTPIX8 was irradiated with 6 MeV protons up to a total dose of 2 krad at HIMAC, National Institute of Quantum Science in Japan, and evaluated the degradation of the performance, such as energy resolution and non-uniformity of gain and readout noise between pixels. After 500 rad irradiation, which is the typical lifetime of an X-ray astronomy satellite, the degradation of energy resolution at 14.4 keV is less than 10%, and the non-uniformity of readout noise and gain between pixels is constant within 0.1%.
We have been developing the monolithic active pixel detector XRPIX onboard the future x-ray astronomical satellite FORCE. XRPIX is composed of complementary metal-oxide-semiconductor pixel circuits, SiO2 insulator, and Si sensor by utilizing the silicon-on-insulator (SOI) technology. When the semiconductor detector is operated in orbit, it suffers from radiation damage due to x-rays emitted from celestial objects as well as cosmic rays. From previous studies, positive charges trapped in the SiO2 insulator are known to cause degradation of the detector performance. To improve the radiation hardness, we developed XRPIX equipped with a double-SOI (D-SOI) structure, introducing an additional silicon layer in the SiO2 insulator. This structure is aimed at compensating for the effect of the trapped positive charges. Although the radiation hardness of the D-SOI detectors to cosmic rays has been evaluated, the radiation effect due to x-ray irradiation has not been evaluated. Thus, we then conduct an x-ray irradiation experiment using an x-ray generator with a total dose of 10 krad at the SiO2 insulator, equivalent to 7 years in orbit. As a result of this experiment, the energy resolution in full-width half maximum for the 5.9 keV x-ray degrades by 17.8 % ± 2.8 % and the dark current increases by 89 % ± 13 % . We also investigate the physical mechanism of the increase in the dark current due to x-ray irradiation using technology computer-aided design simulation. It is found that the increase in the dark current can be explained by the increase in the interface state density at the Si / SiO2 interface.
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