Performance of SOI Pixel Sensors Developed for X-ray Astronomy

We have been developing monolithic active pixel sensors for X-rays based on the silicon-on-insulator technology. Our device consists of a low-resistivity Si layer for readout CMOS electronics, a high-resistivity Si sensor layer, and a SiO2 layer between them. This configuration allows us both high-speed readout circuits and a thick (on the order of 100 m) depletion layer in a monolithic device. Each pixel circuit contains a trigger output function, with which we can achieve a time resolution of ≲ 10 s. One of our key development items is improvement of the energy resolution. We recently fabricated a device named XRPIX6E, to which we introduced a pinned depleted diode (PDD) structure. The structure reduces the capacitance coupling between the sensing area in the sensor layer and the pixel circuit, which degrades the spectral performance. With XRPIX6E, we achieve an energy resolution of ~ 150 eV in full width at half maximum for 6.4-keV X-rays. In addition to the good energy resolution, a large imaging area is required for practical use. We developed and tested XRPIX5b, which has an imaging area size of 21.9 mm 13.8 mm and is the largest device that we ever fabricated. We successfully obtain X-ray data from almost all the 608 384 pixels with high uniformity.