Back-illuminated CCD mosaics

One of the drawbacks of using charge-coupled devices (CCDs) for scientific imaging is their relatively small size compared to many optical systems in which they are used. Telescopes, large format cameras, and medical imaging often require detectors much larger than the few cm dimensions of modern CCDs. One solution to this problem is to closely butt several CCDs together in the focal plane of the optical system, creating a focal plane mosaic. We have developed techniques to produce back illuminated CCDs from commercial front illuminated devices for enhanced quantum efficiency and spectral coverage. In this paper we discuss our development of packages and packaging techniques to butt back illuminated CCDs together, creating mosaics of up to 64 million pixels. We have discovered several critical issues during our development of back illuminated edge-buttable CCDs which we discuss in this paper. These include the development of proper chip carriers and packages, the ability to uniformly heat the devices in the required oxidation process, the ability to uniformly match antireflection coatings for all devices in a mosaic, and the development of alternative bonding methods which allow easy bonding of edge-buttable CCDs, especially as they approach whole wafer size.