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Register a new userLow-Energy X-ray Performance of SOI Pixel Sensors for Astronomy, XRPIX
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We have been developing a new type of X-ray pixel sensors, XRPIX, allowing us to perform imaging spectroscopy in the wide energy band of 1-20 keV for the future Japanese X-ray satellite FORCE. The XRPIX devices are fabricated with complementary metal-oxide-semiconductor silicon-on-insulator technology, and have the Event-Driven readout mode, in which only a hit event is read out by using hit information from a trigger output function equipped with each pixel. This paper reports on the low-energy X-ray performance of the XRPIX6E device with a Pinned Depleted Diode (PDD) structure. The PDD structure especially reduces the readout noise, and hence is expected to largely improve the quantum efficiencies for low-energy X-rays. While F-K X-rays at 0.68 keV and Al-K X-rays at 1.5 keV are successfully detected in the Frame readout mode, in which all pixels are read out serially without using the trigger output function, the device is able to detect Al-K X-rays, but not F-K X-rays in the Event-Driven readout mode. Non-uniformity is observed in the counts maps of Al-K X-rays in the Event-Driven readout mode, which is due to region-to-region variation of the pedestal voltages at the input to the comparator circuit. The lowest available threshold energy is 1.1 keV for a small region in the device where the non-uniformity is minimized. The noise of the charge sensitive amplifier at the sense node and the noise related to the trigger output function are ~$18~e^-$ (rms) and ~$13~e^-$ (rms), respectively.
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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 the X-ray silicon-on-insulator (SOI) pixel sensor called XRPIX for future astrophysical satellites. XRPIX is a monolithic active pixel sensor consisting of a high-resistivity Si sensor, thin SiO$_2$ insulator, and CMOS pixel circuits that utilize SOI technology. Since XRPIX is capable of event-driven readouts, it can achieve high timing resolution greater than $sim 10{rm ~mu s}$, which enables low background observation by adopting the anti-coincidence technique. One of the major issues in the development of XRPIX is the electrical interference between the sensor layer and circuit layer, which causes nonuniform detection efficiency at the pixel boundaries. In order to reduce the interference, we introduce a Double-SOI (D-SOI) structure, in which a thin Si layer (middle Si) is added to the insulator layer of the SOI structure. In this structure, the middle Si layer works as an electrical shield to decouple the sensor layer and circuit layer. We measured the detector response of the XRPIX with D-SOI structure at KEK. We irradiated the X-ray beam collimated with $4{rm ~mu mphi}$ pinhole, and scanned the device with $6{rm ~mu m}$ pitch, which is 1/6 of the pixel size. In this paper, we present the improvement in the uniformity of the detection efficiency in D-SOI sensors, and discuss the detailed X-ray response and its physical origins.
The X-ray SOI pixel sensor onboard the FORCE satellite will be placed in the low earth orbit and will consequently suffer from the radiation effects mainly caused by geomagnetically trapped cosmic-ray protons. Based on previous studies on the effects of radiation on SOI pixel sensors, the positive charges trapped in the oxide layer significantly affect the performance of the sensor. To improve the radiation hardness of the SOI pixel sensors, we introduced a double-SOI (D-SOI) structure containing an additional middle Si layer in the oxide layer. The negative potential applied on the middle Si layer compensates for the radiation effects, due to the trapped positive charges. Although the radiation hardness of the D-SOI pixel sensors for applications in high-energy accelerators has been evaluated, radiation effects for astronomical application in the D-SOI sensors has not been evaluated thus far. To evaluate the radiation effects of the D-SOI sensor, we perform an irradiation experiment using a 6-MeV proton beam with a total dose of ~ 5 krad, corresponding to a few tens of years of in-orbit operation. This experiment indicates an improvement in the radiation hardness of the X- ray D-SOI devices. On using an irradiation of 5 krad on the D-SOI device, the energy resolution in the full-width half maximum for the 5.9-keV X-ray increases by 7 $pm$ 2%, and the chip output gain decreases by 0.35 $pm$ 0.09%. The physical mechanism of the gain degradation is also investigated; it is found that the gain degradation is caused by an increase in the parasitic capacitance due to the enlarged buried n-well.
We report on a measurement of the size of charge clouds produced by X-ray photons in X-ray SOI (Silicon-On-Insulator) pixel sensor named XRPIX. We carry out a beam scanning experiment of XRPIX using a monochromatic X-ray beam at 5.0 keV collimated to $sim 10$ $mu$m with a 4-$mu$m$phi$ pinhole, and obtain the spatial distribution of single-pixel events at a sub-pixel scale. The standard deviation of charge clouds of 5.0 keV X-ray is estimated to be $sigma_{rm cloud} = 4.30 pm 0.07$ $mu$m. Compared to the detector response simulation, the estimated charge cloud size is well explained by a combination of photoelectron range, thermal diffusion, and Coulomb repulsion. Moreover, by analyzing the fraction of multi-pixel events in various energies, we find that the energy dependence of the charge cloud size is also consistent with the simulation.
We have been developing monolithic active pixel sensors, X-ray Astronomy SOI pixel sensors, XRPIXs, based on a Silicon-On-Insulator (SOI) CMOS technology as soft X-ray sensors for a future Japanese mission, FORCE (Focusing On Relativistic universe and Cosmic Evolution). The mission is characterized by broadband (1-80 keV) X-ray imaging spectroscopy with high angular resolution ($<15$~arcsec), with which we can achieve about ten times higher sensitivity in comparison to the previous missions above 10~keV. Immediate readout of only those pixels hit by an X-ray is available by an event trigger output function implemented in each pixel with the time resolution higher than $10~{rm mu sec}$ (Event-Driven readout mode). It allows us to do fast timing observation and also reduces non-X-ray background dominating at a high X-ray energy band above 5--10~keV by adopting an anti-coincidence technique. In this paper, we introduce our latest results from the developments of the XRPIXs. (1) We successfully developed a 3-side buttable back-side illumination device with an imaging area size of 21.9~mm$times$13.8~mm and an pixel size of $36~{rm mu m} times 36~{rm mu m}$. The X-ray throughput with the device reaches higher than 0.57~kHz in the Event-Driven readout mode. (2) We developed a device using the double SOI structure and found that the structure improves the spectral performance in the Event-Driven readout mode by suppressing the capacitive coupling interference between the sensor and circuit layers. (3) We also developed a new device equipped with the Pinned Depleted Diode structure and confirmed that the structure reduces the dark current generated at the interface region between the sensor and the SiO$_2$ insulator layers. The device shows an energy resolution of 216~eV in FWHM at 6.4~keV in the Event-Driven readout mode.
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