Characterization, quantum efficiency, and radiation hardness 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 and Holland, Andrew D. (2023). Characterization, quantum efficiency, and radiation hardness of a CMOS image sensor optimized for soft x-ray astronomy. In: Proc. SPIE 12678, UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XXIII, 126780I.

DOI: https://doi.org/10.1117/12.2674828

Abstract

CIS221-X is the first in a new generation of monolithic CMOS image sensors optimised for soft X-ray applications. The pixels are built on 35 µm thick, high-resistivity epitaxial silicon and feature deep depletion extension (DDE) implants, facilitating over depletion by reverse substrate bias. When cooled to -40 °C, CIS221-X reports a readout noise of 3.3 e- RMS and 12.4 ± 0.06 e-/pixel/s of dark current. The 40µm pixels experience near-zero image lag. Following per-pixel gain correction, an energy resolution of 130 ± 0.4 eV FWHM has been measured at 5.9 keV. In the 0.3 – 1.8 keV energy range, the sensor achieves a quantum efficiency of above 80%. Radiation tests have shown that both the readout noise and dark current increase with total ionising dose and that the OBF can help to mitigate the increase in dark current. The measured electro-optical parameters and the preliminary ionising radiation results strongly support the use of the CIS221-X in soft X-ray applications.

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