Electro-optical characterization of a CMOS image sensor optimized for soft x-ray astronomy

Townsend-Rose, Charles; Buggey, Thomas; Ivory, James; Stefanov, Konstantin D.; Jones, Lawrence; Hetherington, Oliver; Holland, Andrew D. and Prod’homme, Thibaut (2023). Electro-optical characterization of a CMOS image sensor optimized for soft x-ray astronomy. Journal of Astronomical Telescopes, Instruments, and Systems, 9(4), article no. 046001.

DOI: https://doi.org/10.1117/1.JATIS.9.4.046001

Abstract

CIS221-X is a prototype complementary metal-oxide-semiconductor (CMOS) image sensor, optimized for soft x-ray astronomy and developed for the proposed ESA Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission. The sensor features 40 μm pitch square pixels built on a 35 μm thick, high-resistivity epitaxial silicon that is fully depleted by reverse substrate bias. Backside illumination processing has been used to achieve high x-ray quantum efficiency, and an optical light-blocking filter has been applied to mitigate the influence of stray light. A comprehensive electro-optical characterization of CIS221-X has been completed. The median readout noise is 3.3 e⁻ RMS with 90% of pixels reporting a value <3.6 e⁻ RMS. At −40 ° C, the dark current is 12.4 ± 0.06 e⁻ / pixel / s. The pixel photo-response is linear to within 1% for 0.3 to 5 keV photons (82 to 1370 e⁻) with <0.1 % image lag. Following per-pixel gain correction, an energy resolution of 130.2 ± 0.4 eV has been measured at 5898 eV. In the 0.3 to 1.8 keV energy range, CIS221-X achieves >80 % quantum efficiency. With the exception of dark current, these results either meet or outperform the requirements for the THESEUS mission, strongly supporting the consideration of CMOS technology for soft x-ray astronomy.