First results from a novel curving process for large area scientific imagers

Observations in seeing limited imaging conditions with an extremely large telescope - such as the European Extremely Large Telescope (E-ELT) - will require large detectors and very fast cameras (around F/1.0). The correction of field curvature is a complex task, requiring numerous optical elements operating with high incidence angles. Large format (60 to 90 mm square) concave detectors with a curvature radius between 500 and 250 mm would considerably simplify the optical design, while improving image quality and cutting cost of optical components. Potential applications are not limited to astronomy exclusively. The associated advantages of curved image sensors inside (mosaicked) focal planes have been described in our paper "The challenge of highly curved monolithic imaging detectors", presented at SPIE 2010 [1]. This paper compares in a first step important developments in the area of curving CCD and CMOS detectors using different technical approaches linked to specific thinning processes with a novel approach followed after ESO's initial feasibility study: First results of the latter are described with a report on the chosen curving technology aimed at producing 500 to 250 mm radius of curvature silicon detectors of approximately 60 mm square format (typical astronomical 4k × 4k CCDs). The curvature technique has been developed for front-illuminated devices with the goal of extending the process to back-illuminated sensors in the near future. We will discuss the fabrication process of curving the devices as well as the difficulties encountered during development. Characterization results from a curved detector, including metrology, and electrical performance before and after curvature are presented.