Quantum efficiency characterization of scientific CCDs

Characterization of CCDs is extremely important when developing scientific detectors. If CCD foundries are used to produce the devices, the foundries require feedback to maintain a quality process. In this case, the users require fairly automated testing to evaluate the large number of devices obtained from even a single lot run. We have developed a CCD characterization facility which is used to evaluate these foundry devices as well as commercial scientific images. Our test capabilities include automated QE measurements, X-ray CTE and gain calibration, optical illumination from 200 nm - 1200 nm, and dark current and read noise characterization. We can also make interferometric flatness measurements of the devices. A cryogenic probe station for wafer testing is being developed to extend some of these tests to the wafer level. We discuss in this paper our facilities and techniques to measure the quantum efficiency (QE) of scientific CCDs. QE (along with read noise) is perhaps the most important parameter for many classes of astronomical observations when working at very low light levels. It is also the most useful parameter for evaluating the quality of backside processing when developing back illuminated CCDs.