X-ray Radiation Damage Effects on Double-SOI Pixel Detectors for the Future Astronomical Satellite "FORCE"

We have been developing the monolithic active pixel detector "XRPIX" onboard the future X-ray astronomical satellite "FORCE". XRPIX is composed of CMOS 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 the celestial objects as well as cosmic rays. From previous studies, positive charges trapped in the SiO2 insulator are known to cause the degradation of the detector performance. To improve the radiation hardness, we developed XRPIX equipped with 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 to cosmic rays of the D-SOI detectors has been evaluated, the radiation effect due to the X-ray irradiation has not been evaluated. Then, we 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 $\pm$ 2.8% and the dark current increases by 89 $\pm$ 13%. We also investigate the physical mechanism of the increase in the dark current due to X-ray irradiation using TCAD 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.Comment: 15 pages, 12 figures, accepted for publication in Journal of Astronomical Telescopes, Instruments, and System

Proton radiation damage experiment for X-ray SOI pixel detectors

In low earth orbit, there are many cosmic rays composed primarily of high energy protons. These cosmicrays cause surface and bulk radiation effects, resulting in degradation of detector performance. Quantitativeevaluation of radiation hardness is essential in development of X-ray detectors for astronomical satellites. Weperformed proton irradiation experiments on newly developed X-ray detectors called XRPIX based on silicon-on-insulator technology at HIMAC in National Institute of Radiological Sciences. We irradiated 6 MeV protonswith a total dose of 0.5 krad, equivalent to 6 years irradiation in orbit. As a result, the gain increases by 0.2%and the energy resolution degrades by 0.5%. Finally we irradiated protons up to 20 krad and found that detectorperformance degraded significantly at 5 krad. With 5 krad irradiation corresponding to 60 years in orbit, thegain increases by 0.7% and the energy resolution worsens by 10%. By decomposing into noise components, wefound that the increase of the circuit noise is dominant in the degradation of the energy resolution