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.
We have been 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) We report the device structure optimized to achieve both low noise, a thick depletion layer, and to non-punch-through. (2) We succeeded in achieving an energy resolution of ~300 eV (FWHM) at 6 keV with a depletion layer of 300 um at room temperature by operating frequent reset to reduce the shot noise. (3) We present the development of "Digital X-ray SOIPIXs" having on-chip ADCs, DACs and readout sequencers.
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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