The three improvements to the baseline CCD have been included in the design of a breadboard model for Euclid. Charge injection schemes will be studied in detail using the baseline operating conditions for the Euclid CCDs. It is envisaged that each image will be acquired with an integration time of at least 400 seconds at a temperature of - 120°C. The low temperature is specified to minimise dark current to such a level that the long exposure time enables faint sources to be detected.
Although there can be some advantage in optimising the read-out rates and temperature, the charge transfer efficiency is fundamentally limited by the different trap species generally present in silicon. However, these detectors will see a diffuse optical background that may serve to keep the traps filled. The combination of the diffuse optical background and charge injection described earlier may prove to limit the effects of radiation damage and extend the performance and potential duration of the mission.
The incorporation of the charge injection structure will sacrifice 4 rows of the image area, representing less than 0.1% of each device. If the improvement is considered to be less important than the loss of image area, the structure can be replaced by imaging pixels for the flight devices.
The devices will be back illuminated for the highest quantum efficiency and thinned sufficiently to be fully depleted, thereby eliminating the field-free regions that could lead to charge diffusion and loss of resolution. A low voltage process will be used to reduce power dissipation and as an added benefit, will decrease the threshold voltage shift experienced as a result of ionising radiation.
The lightweight silicon carbide package has been designed for 4-side buttability with the image area comprising more than 80% percent of the total footprint on the focal plane.
A temperature sensor has been incorporated in the side walls of the package to allow calibration of the science data without impacting on the focal plane assembly and integration.
The Euclid VIS instrument will benefit from an optimised sensor design that is built on extensive CCD heritage. The low noise amplifier, charge injection structure and serial register design change will lead to an improved signal to noise ratio and point spread function. This will improve the accuracy with which the instrument will measure gravitationally distorted galaxies.
M. Cropper et al., “VIS: the visible imager for Euclid”, Proc. SPIE 7731-55, , 2010.