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Register a new userProton radiation hardness of X-ray SOI pixel sensors with pinned depleted diode structure
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X-ray SOI pixel sensors, XRPIX, are being developed for the next-generation X-ray astronomical satellite, FORCE. The XRPIX are fabricated with the SOI technology, which makes it possible to integrate a high-resistivity Si sensor and a low-resistivity Si CMOS circuit. The CMOS circuit in each pixel is equipped with a trigger function, allowing us to read out outputs only from the pixels with X-ray signals at the timing of X-ray detection. This function thus realizes high throughput and high time resolution, which enables to employ anti-coincidence technique for background rejection. A new series of XRPIX named XRPIX6E developed with a pinned depleted diode (PDD) structure improves spectral performance by suppressing the interference between the sensor and circuit layers. When semiconductor X-ray sensors are used in space, their spectral performance is generally degraded owing to the radiation damage caused by high-energy protons. Therefore, before using an XRPIX in space, it is necessary to evaluate the extent of degradation of its spectral performance by radiation damage. Thus, we performed a proton irradiation experiment for XRPIX6E for the first time at HIMAC in the NIRS. We irradiated XRPIX6E with high-energy protons with a total dose of up to 40 krad, equivalent to 400 years of irradiation in orbit. The 40-krad irradiation degraded the energy resolution of XRPIX6E by 25 $pm$ 3%, yielding an energy resolution of 260.1 $pm$ 5.6 eV at the full width half maximum for 5.9 keV X-rays. However, the value satisfies the requirement for FORCE, 300 eV at 6 keV, even after the irradiation. It was also found that the PDD XRPIX has enhanced radiation hardness compared to previous XRPIX devices. In addition, we investigated the degradation of the energy resolution; it was shown that the degradation would be due to increasing energy-independent components, e.g., readout noise.
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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 SiO$_2$ layer between them. This configuration allows us both high-speed readout circuits and a thick (on the order of $100~mu{rm m}$) depletion layer in a monolithic device. Each pixel circuit contains a trigger output function, with which we can achieve a time resolution of $lesssim 10~mu{rm s}$. 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 $sim 150$~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 $21.9~{rm mm} times 13.8~{rm mm}$ and is the largest device that we ever fabricated. We successfully obtain X-ray data from almost all the $608 times 384$ pixels with high uniformity.
We have been developing a monolithic active pixel sensor, ``XRPIX``, for the Japan led future X-ray astronomy mission ``FORCE`` observing the X-ray sky in the energy band of 1-80 keV with angular resolution of better than 15``. XRPIX is an upper part of a stack of two sensors of an imager system onboard FORCE, and covers the X-ray energy band lower than 20 keV. The XRPIX device consists of a fully depleted high-resistivity silicon sensor layer for X-ray detection, a low resistivity silicon layer for CMOS readout circuit, and a buried oxide layer in between, which is fabricated with 0.2 $mu$ m CMOS silicon-on-insulator (SOI) technology. Each pixel has a trigger circuit with which we can achieve a 10 $mu$ s time resolution, a few orders of magnitude higher than that with X-ray astronomy CCDs. We recently introduced a new type of a device structure, a pinned depleted diode (PDD), in the XRPIX device, and succeeded in improving the spectral performance, especially in a readout mode using the trigger function. In this paper, we apply a mesh experiment to the XRPIX devices for the first time in order to study the spectral response of the PDD device at the subpixel resolution. We confirmed that the PDD structure solves the significant degradation of the charge collection efficiency at the pixel boundaries and in the region under the pixel circuits, which is found in the single SOI structure, the conventional type of the device structure. On the other hand, the spectral line profiles are skewed with low energy tails and the line peaks slightly shift near the pixel boundaries, which contribute to a degradation of the energy resolution.
The experiment of the future electron-positron colliders has unprecedented requirements on the vertex resolution, such as around 3micron single point resolution for the inner most detector layer, with fast readout, and very low power-consumption density and material budget. Significant efforts have been put into the development of monolithic silicon pixel sensors, but there have been some challenges to combine all those stringent specifications in a small pixel area. This paper presents a compact prototype pixel sensor fabricated in LAPIS 200nm SOI process and focuses on the characterization of low capacitance of the sensing node with a pinned depleted diode structure adopting a novel method of forward bias voltage and AC coupling on the diode. Three PDD structures with 16 micron by 20 micron pixel size were designed and compared using radioactive sources and injected charge. The measured result shows that the designed PDD structure has very low leakage current and around 3.5fF of equivalent input capacitance.
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 $pm$ 1 eV and 335 $pm$ 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 $mu rm s$ 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 $rm mu$m 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 back-illuminated device XRPIX3b with lower readout noise. In this work, we irradiated 2--5 keV X-ray beam collimated to 4 $rm mu m phi$ to the sensor layer side of the XRPIX3b at 6 $rm mu m$ 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. has improved.
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