Radiation-Induced Degradation Mechanism of X-ray SOI Pixel Sensors with Pinned Depleted Diode Structure

The X-ray Silicon-On-Insulator (SOI) pixel sensor named XRPIX has been developed for the future X-ray astronomical satellite FORCE. XRPIX is capable of a wide-band X-ray imaging spectroscopy from below 1 keV to a few tens of keV with a good timing resolution of a few tens of $\mu$s. 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. Although new device structures introducing pinned depleted diodes dramatically improved radiation tolerance, it remained unknown how radiation effects degrade the sensor performance. Thus, this paper reports the results of a study of the degradation mechanism of XRPIX due to radiation using device simulations. In particular, mechanisms of increases in dark current and readout noise are investigated by simulation, taking into account the positive charge accumulation in the oxide layer and the increase in the surface recombination velocity at the interface between the sensor layer and the oxide layer. As a result, it is found that the depletion of the buried p-well at the interface increases the dark current, and that the increase in the sense-node capacitance increases the readout noise.