<title>Quantum efficiency of a back-illuminated CCD imager: an optical approach</title>

Abstract

We have developed an optical approach for modeling the quantum efficiency (QE) of back-illuminated CCD optical imagers for astronomy. Beyond its simplicity, it has the advantage of providing a complete fringing description for a real (wide-aperture) system. Standard thin-film calculations are extended by (a) considering the CCD itself as a thin film, and (b) treating the refractive index as complex. The QE is approximated as the fraction of the light neither transmitted nor reflected, which basically says that all absorbed photons produce e-h pairs and each photoproduced e or h is collected (recombination is negligible). Near-surface effects relevant to blue response must still be treated by standard semiconductor modeling methods. A simple analytic expression describes the QE of a CCD without antireflective (AR) coatings. With AR coatings the system is more easily described by transfer matrix methods. A two-layer AR coating is tuned to give a reasonable description of standard thinned CCD's, while the measured QE of prototype LBNL totally-depleted thick CCD's is well described with no adjustable parameters. Application to the new LBNL CCD's indicates that these devices will have QE > 70% at A = 1000 nm and negligible fringing in optical systems faster than f 4.0.

Fully depleted, back-illuminated charge-coupled devices fabricated on high-resistivity silicon

“…The ITO functions as part of an antireflection (AR) coating and improves the conductivity of the back side, where an equipotential is desired as described below. A second AR coating of ≈ 1000Å of SiO 2 is added to form a 2-layer AR coating optimized for near-IR detection [21]. Figure 6 shows measured QE of an ≈ 280 µm thick, 1980 × 800, 15 µm pixel back-illuminated CCD with the two-layer AR coating.…”

“…The QE exceeds 90% at nearinfrared wavelengths, and is still ≈ 60% at a wavelength of 1 µm. For detailed discussion of QE modeling results for these CCD's the reader is referred to reference [21].…”

“…where N 0 is the peak photon flux, k = 2πf where f is the spatial frequency, and R is the wavelength-dependent reflectivity, which can be determined from optical calculations [21]. The total hole flux J p (x) consisting of the hole diffusion current entering the depletion region plus the number of holes per unit time and area that are optically generated in the depletion region is given as…”

“…In addition, a thick CCD reduces fringing [21]. In thinned CCD's fringing arises due to multiple reflections at long wavelengths when the absorption length of the incident light is greater than the CCD thickness.…”

CCD Development Progress at Lawrence Berkeley National Laboratory

“…Since first described by Holland et al (1996), the virtues of such a CCD have been well established. The typical thickness of 200-300 µm results in good near-infrared quantum efficiency (QE) and greatly reduces fringing (Groom et al, 1999). The point spread function (PSF) is well defined and is determined by the transit time of the photo-generated holes in an electric field that extends throughout the thickness of the device.…”