Sub-pixel Response of Double-SOI Pixel Sensors for X-ray Astronomy

TL;DR

This paper presents the development and testing of a Double-SOI structure in XRPIX X-ray sensors, significantly improving detection uniformity and response for astrophysical applications.

Contribution

Introduction of a Double-SOI structure in XRPIX sensors to reduce electrical interference and enhance detection uniformity in X-ray astronomy.

Findings

Improved detection efficiency uniformity with D-SOI structure.

Successful measurement of X-ray response at KEK.

Enhanced understanding of physical response mechanisms.

Abstract

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 $4{\rm ~\mu m\phi}$ 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.