X-ray response evaluation in subpixel level for X-ray SOI pixel detectors

Abstract

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.

Performance of SOI Pixel Sensors Developed for X-ray Astronomy

“…Charge collection efficiency (CCE) should be high enough to achieve superior spectral performance. We irradiated XR-PIX1b and XRPIX3b with an X-ray pencil beam, and found that the CCE becomes lower when X-rays are absorbed near the pixel boundaries [11], [12]. Comparing the experimental data with results from device simulations that we carried out, we came to a conclusion that a part of signal charge is lost due to traps located close to the interface between the sensor and SiO 2 layers [11].…”

Development of the detector simulation framework for the Wideband Hybrid X-ray Imager onboard FORCE

“…sensors have been reported [16,6,7]. Also, the disruption and/or weakening of the electric field near the pixel edges, which enhances the charge cloud diffusion, has been discussed [5].…”

“…The detector response of the XRPIX sensors has been investigated in several works. Matsumura et al (2015) [5] and Negishi et al (2019) [6] studied the spatial distribution of the charge collection efficiency (CCE) and an electric field structure in the sensors in order to evaluate the XRPIX sensor performance. Hagino et al (2019) [7] modeled the charge cloud size and verified it by comparing it to measured number fractions of chargesharing events.…”

Low-Energy X-ray Performance of SOI Pixel Sensors for Astronomy, "XRPIX"

“…Here the "effective" thickness is the one measured with X-ray detection and is often different from the one defined in terms of semiconductor processes. So far, we have achieved a performance of detection of 2.1 keV Xrays and an effective thickness of 0.9-1 µm in the Frame readout mode [7,8]. However, the required threshold of 1 keV has not been reached yet.…”