# Performance of SOI Pixel Sensors Developed for X-ray Astronomy

## TL;DR

This paper presents the development of monolithic SOI pixel sensors for X-ray astronomy, achieving high energy resolution and large imaging areas through innovative structures like PDD, suitable for advanced space-based X-ray observations.

## Contribution

Introduction of a PDD structure in SOI pixel sensors to improve energy resolution and demonstration of a large-area sensor with high pixel uniformity.

## Key findings

- Energy resolution of ~150 eV at 6.4 keV achieved with XRPIX6E.
- Successful fabrication of a large-area sensor (XRPIX5b) with high pixel uniformity.
- High-speed readout with time resolution ≤10 μs.

## Abstract

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.