The challenge of highly curved monolithic imaging detectors

In a recent optical design study of CODEX - a visible spectrograph planned for the European Extremely Large Telescope (E-ELT) - it was determined that a significant simplification of the optical design - accompanied by an improvement of the image quality - could be achieved through the application of large format (90mm square) concave spherically curved detectors with a low radius of curvature (500 to 250mm). Current assemblies of image sensors and optics rely on the optics to project a corrected image onto a flat detector. While scientific large-size CCDs (49mm square) have been produced unintentionally with a spherical radius of convex curvature of around 5m, in the past most efforts have concentrated onto flattening the light-sensitive detector silicon area as best as possible for both scientific state-of-the-art systems, as well as commercial low-cost consumer products. In some cases curved focal planes are mosaicked out of individual flat detectors, but a standard method to derive individual spherically curved large size detectors has not been demonstrated. This paper summarizes important developments in the area of curved detectors in the past and their different technical approaches mostly linked to specific thinning processes. ESO's specifications for an ongoing feasibility study are presented. First results of the latter are described with a link to theoretical and practical examinations of currently available technology to implement curved CCD and CMOS detectors for scientific applications.