CMOS Image Sensor for the THESEUS X-ray Space Telescope

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