Back-Illuminated, Fully-Depleted CCD Image Sensors for use in Optical and near-IR Astronomy

Charge-coupled devices (CCD’s) of novel design have been fabricated at Lawrence Berkeley National Laboratory (LBNL), and the rst large-format science-grade chips for astronomical imaging are now being characterized at Lick Observatory. They are made on 300-µm thick n-type high-resistivity (10,000 Ω-cm) silicon wafers, using a technology developed at LBNL to fabricate low-leakage silicon mi-crostrip 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 pho-tosensitive and ensuring that charge reaches the potential wells with minimal lateral diusion. The development of a thin, transparent back side contact compatible with fully depleted operation permits blue response comparable to that obtained with thinned CCD’s. Since the entire region is active, high quantum eciency is main-tained to nearly λ = 1000 nm, above which the silicon bandgap eectively truncates photoproduction. Early characterization results indicate a charge transfer eciency> 0.999995, readout noise 4 e’s at −132 C, full well capacity> 300, 000 e’s, and quantum eciency> 85 % at λ = 900 nm.