Enhancing back illuminated performance of astronomical CCDs

Back illuminated CCDs have been the detectors of choice for most astronomical imagers and spectrographs during the past decade. In recent years, we have developed processes to improve the performance of these detectors. Recent work has resulted in improved absolute quantum efficiency (QE), QE stability with temperature, QE stability with environmental contamination, and enhanced near-IR response. We demonstrate that QE near 100% can be achieved which is stable against hydrogen, dewar outgassing, and water contamination. We show that QE decrease with temperature can be eliminated for blue/visible optimized CCDs using a backside passivation layer, and significantly reduced for UV optimized CCDs which require very thin backside films. We also show that a 20% QE increase at 900 nm can be obtained by coating the frontside of back illuminated CCDs with a reflective metal film, without increasing interference fringing.