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We have been developing event driven X-ray Silicon-On-Insulator (SOI) pixel sensors, called "XRPIX", for the next generation of X-ray astronomy satellites. XRPIX is a monolithic active pixel sensor, fabricated using the SOI CMOS technology, and is equipped with the so-called "Event-Driven readout", which allows reading out only hit pixels by using the trigger circuit implemented in each pixel. The current version of XRPIX has lower spectral performance in the Event-Driven readout mode than in the Frame readout mode, which is due to the interference between the sensor layer and the circuit layer. The interference also lowers the gain. In order to suppress the interference, we developed a new device, "XRPIX6E" equipped with the Pinned Depleted Diode structure. A sufficiently highly-doped buried p-well is formed at the interface between the buried oxide layer and the sensor layer, and acts as a shield layer. XRPIX6E exhibits improved spectral performances both in the Event-Driven readout mode and in the Frame readout mode in comparison to previous devices. The energy resolutions in full width at half maximum at 6.4 keV are 236 ± 1 eV and 335 ± 4 eV in the Frame and Event-Driven readout modes, respectively. There are differences between the readout noise and the spectral performance in the two modes, which suggests that some mechanism still degrades the performance in the Event-Driven readout mode.

We have been developing event-driven SOI Pixel Detectors, named "XRPIX" (X-Ray soiPIXel) based on the silicon-on-insulator (SOI) pixel technology, for the future X-ray astronomical satellite with wide band coverage from 0.5 keV to 40 keV. XRPIX has event trigger output function at each pixel to acquire a good time resolution of a few µs and has Correlated Double Sampling function to reduce electric noises. The good time resolution enables the XRPIX to reduce Non X-ray Background in the high energy band above 10 keV drastically by using anti-coincidence technique with active shield counters surrounding XRPIX. In order to increase the soft X-ray sensitivity, it is necessary to make the dead layer on the X-ray incident surface as thin as possible. Since XRPIX1b, which is a device at the initial stage of development, is a front-illuminated (FI) type of XRPIX, low energy X-ray photons are absorbed in the 8 µm thick circuit layer, lowering the sensitivity in the soft X-ray band. Therefore, we developed a back-illuminated (BI) device XRPIX2b, and confirmed high detection efficiency down to 2.6 keV, below which the efficiency is affected by the readout noise. In order to further improve the detection efficiency in the soft X-ray band, we developed a backilluminated device XRPIX3b with lower readout noise. In this work, we irradiated 2-5 keV X-ray beam collimated to 4 µmφ to the sensor layer side of the XRPIX3b at 6 µm pitch. In this paper, we reported the uniformity of the relative detection efficiency, gain and energy resolution in the subpixel level for the first time. We also confirmed that the variation in the relative detection efficiency at the subpixel level reported by Matsumura et al.[1] has improved.

A: We have developed monolithic CMOS pixel sensor using fully-depleted (FD) siliconon-insulator (SOI) pixel process technology. The SOI substrates consist of high-resistivity silicon with p-n junctions and low-resistivity silicon layers for forming SOI-CMOS circuitry. Tungsten vias are used to make connections between p-n junctions in the silicon substrate and the first metal layers in the top-layer circuitry. Using this sensor construction, high sensor gain in small pixel areas can be achieved. In 2014, a high-resolution, integrated SOI pixel sensor, called INTPIX8, was developed with two types of substrates: a float-zone, p-type layer on a single SOI (SSOI) wafer and a Czochralski, p-type layer on a double SOI (DSOI) wafer. The X-ray spectra were obtained using Am-241 radiation source. The SSOI-based and DSOI-based sensors exhibited different levels of sensor gain and there were no large differences in the noise levels between them.

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