Radiation Damage Effects in X-ray SOI Pixel Sensors with Pinned Depleted Diode Structure

We have been developing the X-ray SOI pixel sensor named "XRPIX" for future X-ray astronomical satellites. XRPIX is a monolithic active pixel sensor composed of a high-resistivity Si sensor, thin SiO
2
insulator, and CMOS pixel circuits by utilizing the silicon-on-insulator (SOI) technology. Since XRPIX is capable of event-driven readout, it can achieve a high timing resolution better than 10 μs, which enables an extremely low background observation by adopting the anticoincidence technique. However, it had a major issue with its radiation tolerance to the total ionizing dose (TID) effect because of its thick buried oxide layer due to the SOI structure. To improve radiation tolerance, we introduce a new device structure with pinned depleted diode (PDD). Thus, in this presentation, we report on the improvement of the radiation tolerance of the XRPIX with PDD structure based on proton-irradiation experiments. Even after the irradiation of 40 krad, the gain decreased only by about 1%, and the energy resolution at 5.9 keV in FWHM was still as good as 260 eV. In addition, we focus on the slight residual radiation effects seen even in the new XRPIX, and investigate its degradation mechanism utilizing device simulations. In particular, mechanisms of increase in readout noise and decrease in gain are investigated, taking into account the positive charge accumulation in the oxide layer. As a result, it is found that the increase in the sense-node capacitance plays an important role in the degradation of readout noise and gain.