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

Abstract

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-μm thick n-type high-resistivity (∼10000Ωcm) 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 λ=1000nm, 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°C, full well capacity >300000e's, and quantum efficiency >85% at λ=900nm.