Soft X-ray performance of an optimised charge-coupled device for astronomy

Abstract

For the upcoming European Space agency (ESA)/Chinese Academy of Sciences (CAS) SMILE mission, launching in 2025, large-format soft X-ray optimised CCDs have been manufactured by Teledyne e2v, named the CCD370. The CCDs are approximately 8 cm × 8 cm and are comprised of 4510 × 4510 18 μm pitch pixels, with a store shield covering approximately 1/7th of the active imaging area to facilitate frame-transfer operation mode. To optimise quantum efficiency within the soft X-ray energy band, the device is 16 μm thick, back illuminated, and has an additional back surface passivation process. The focal plane of the soft X-ray imager (SXI) on the SMILE spacecraft will be comprised of 2 CCD370s, operating with 6×6 on-chip binning to mitigate against CTI-induced charge transfer losses and charge spreading throughout the 3-year mission lifetime.As part of the pre-flight testing and calibration, a CCD370 was characterised at the PTB beamline at the BESSY 2 synchrotron in Berlin, and key metrics such as quantum efficiency and energy resolution in the 0.2 – 1.8 keV energy band were assessed. The quantum efficiency measurements show expected performance within specification for the instrument, and also match a transmission-layer QE model. The energy resolution is 68 ± 2 eV FWHM @ 1.2 keV, which is an improvement over the current generation of X-ray telescopes such as XMM-Newton, Chandra, and the Swift X-ray Telescope. Although competing technologies such as DEPFETs and scientific CMOS image sensors now have similar performance to CCDs, the performance shown here can still easily satisfy requirements for novel X-ray instruments, and can still be useful in future astronomy missions given the rich heritage, high technology-readiness-level, and maturity of the technology.