Back-illuminated, fully-depleted CCD image sensors for use in optical and near-IR astronomy

Charge-coupled devices (CCDs) of novel design have been fabricated at Lawrence Berkeley National Laboratory [LBNL]), and the first large-format science-grade chips for astronomical imaging are now being characterized at Lick Observatory. They are made on 300-mu m thick n-type high-resistivity( similar to 10000 Omega cm) silicon wafers, using a technology developed at LBNL to fabricate low-leakage silicon microstrip detectors for high-energy physics. A bias voltage applied via a transparent contact on the back side fully depletes the substrate, making the entire volume photosensitive and ensuring that charge reaches the potential wells with minimal lateral diffusion. The development of a thin, transparent back-side contact compatible with fully depleted operation permits blue response comparable to that obtained with thinned CCDs. Since the entire region is active, high quantum efficiency is maintained to nearly lambda = 1000 nm, above which the silicon band gap effectively truncates photoproduction. Early characterization results indicate a charge transfer efficiency > 0.999995, readout noise 4 e's at - 132 degrees C, full well capacity > 300 000 e's, and quantum efficiency > 85% at lambda = 900 nm. (C) 2000 Elsevier Science B.V. All rights reserved.