CMOS Image Sensor for the THESEUS X-ray Space Telescope

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Proposed by the European Space Agency, the Transient High-Energy Sky and Early Universe Surveyor (THESEUS) is a space telescope mission tasked with studying the early Universe by imaging in the X-ray, gamma-ray and infrared wavelengths. Onboard THESEUS will be a soft X-ray imager working in the 300 eV to 6 keV energy range, which is planned to use an array of specialised CMOS image sensors (CIS) as active devices. CIS are increasingly used in space applications due to their numerous advantages. Unlike Charge Coupled Devices (CCDs), CIS are largely immune to radiation-induced charge transfer inefficiency and can tolerate large doses of ionising radiation. In addition, CIS dissipate less power and integrate digitisation and other functions on-chip, significantly simplifying spacecraft electronics and reducing its mass. The requirements for the soft X-ray imager dictate that the sensor is built on a relatively thick, fully depleted silicon substrate, and has large pixel size (e.g. 40 micrometres square) combined with very low readout noise and image lag. This poses several challenges to the sensor design due to the conflicting interplay between the requirements and the limitations of the available CMOS technologies. In particular, the solutions for achieving negligible image lag make full depletion more difficult, and can make the sense node too large, which increases the readout noise