Characterization, quantum efficiency, and radiation hardness of a CMOS image sensor optimized for soft x-ray astronomy

Abstract

CIS221-X is the first in a new generation of monolithic CMOS image sensors optimized 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.

A model for photon counting X-ray event reconstruction uncertainty

“…For Lab-based tests raw image frames are often available, permitting the overall effect of the reconstruction threshold to be investigated by re-running event reconstruction algorithms for different threshold values on the same dataset. During separate experimental campaigns high quality monochromatic X-ray data was collected with a CIS221-X [14] and multi-peak fluorescence X-ray spectra of comparable energy was collected from an EMCCD [3]. These datasets have permitted the observation of some of the behaviours predicted by the model.…”

“…The predicted FWHM includes expected reconstruction noise and Fano noise. This data did not include the pixel level gain correction of[14].…”