S. Holland

Quantum efficiency of a back-illuminated CCD imager: an optical approach

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 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. 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 CCDs, while the measured QE of prototype LBNL totally depleted thick CCDs is well described with no adjustable parameters. Application to the new LBNL CCDs indicates that these device swill have QE > 70 percent at (lambda) equals 1000 nm and negligible fringing in optical system faster than approximately f4.0.

A 200/spl times/200 CCD image sensor fabricated on high-resistivity silicon

A charge coupled device (CCD) image sensor fabricated on high-resistivity silicon is described. The resistivity, about 10,000 /spl Omega/-cm, allows for operation of the CCD with the entire 300 /spl mu/m substrate depleted. This results in better red to near infrared response when compared to conventional and thinned CCDs. In addition the CCD has good blue response when back illuminated. Since the substrate is fully depleted, thinning, with its inherent difficulties, is not necessary in order to enhance blue response.

Proton radiation damage in high-resistivity n-type silicon CCDs

A new type of p-channel CCD constructed on high-resistivity n-type silicon was exposed to 12 MeV protons at doses up to 1 X 1011 protons/cm2. The charge transfer efficiency was measured as a function of radiation dose and temperature. We previously reported that these CCDs are significantly more tolerant to radiation damage than conventional n-channel devices. In the work reported here, we used pocket pumping techniques and charge transfer efficiency measurements to determine the identity and concentrations of radiation induced traps present in the damaged devices.

Point-spread function in depleted and partially depleted CCDs

The point spread function obtainable in an astronomical instrument using CCD readout is limited by a number of factors, among them the lateral diffusion of charge before it is collected in the potential wells. They study this problem both theoretically and experimentally, with emphasis on the thick CCDs on high-resistivity n-type substrates being developed at Lawrence Berkeley National Laboratory.

Characterization of a fully depleted CCD on high-resistivity silicon

Most scientific CCD imagers are fabricated on 30-50 (Omega) - cm epitaxial silicon. When illuminated form the front side of the device they generally have low quantum efficiency in the blue region of the visible spectrum because of strong absorption in the polycrystalline silicon gates as well as poor quantum efficiency in the far red and near infrared region of the spectrum because of the shallow depletion depth of the low-resistivity silicon. To enhance the blue response of scientific CCDs they are often thinned and illuminated from the back side. While blue response is greatly enhanced by this process, it is expensive and it introduces additional problems for the red end of the spectrum. A typical thinned CCD is 15 to 25 micrometers thick, and at wavelengths beyond about 800 nm the absorption depth becomes comparable to the thickness of the device, leading to interference fringes from reflected light. Because these interference fringes are of high order, the spatial pattern of the fringes is extremely sensitive to small changes in the optical illumination of the detector. Calibration and removal of the effects of the fringes is one of the primary limitations on the performance of astronomical images taken at wavelengths of 800 nm or more. In this paper we present results from the characterization of a CCD which promises to address many of the problems of typical thinned CCDs. The CCD reported on here was fabricated at Lawrence Berkeley National Laboratory (LBNL) on a 10-12 K

SNAP focal plane

OMega-cm n-type silicon substrate.THe CCD is a 200 by 200 15-micrometers square pixel array, and due to the very high resistivity of the starting material, the entire 300 micrometers substrate is depleted. Full depletion works because of the gettering technology developed at LBNL which keeps leakage current down. Both front-side illuminated and backside illuminated devices have been tested. We have measured quantum efficiency, read-noise, full-well, charge-transfer efficiency, and leakage current. We have also observed the effects of clocking waveform shapes on spurious charge generation. While these new CCDs promise to be a major advance in CD technology, they too have limitations such as charge spreading and cosmic-ray effects. These limitations have been characterized and are presented. Examples of astronomical observations obtained with the backside CCD on the 1-meter reflector at Lick Observatory are presented.

High-performance CCD on high-resistivity silicon

The proposed SuperNova/Acceleration Probe (SNAP) mission will have a two-meter class telescope delivering diffraction-limited images to an instrumented 0.7 square-degree field sensitive in the visible and near-infrared wavelength regime. We describe the requirements for the instrument suite and the evolution of the focal plane design to the present concept in which all the instrumentation -- visible and near-infrared imagers, spectrograph, and star guiders -- share one common focal plane.The proposed SuperNova/Acceleration Probe (SNAP) mission will have a two-meter class telescope delivering diffraction-limited images to an instrumented 0.7 square-degree field sensitive in the visible and near-infrared wavelength regime. We describe the requirements for the instrument suite and the evolution of the focal plane design to the present concept in which all the instrumentation -- visible and near-infrared imagers, spectrograph, and star guiders -- share one common focal plane.

Wide-Field Surveys from the SNAP Mission

In this paper we present new results from the characterization of a fully depleted CCD on high resistivity silicon. The CCD was fabricated at Lawrence Berkeley National Laboratory on a 10-12 K(Omega) -cm n-type silicon substrate. The CCD is a 200 by 200 15-micrometers square pixel array. The high resistivity of the starting material makes it possible to deplete the entire 300 micrometers thick substrate. This results in improved red and near IR response compared to a standard CCD. Because the substrate is fully depleted, thinning of the CCD is not required for backside illumination, and the result presented here were obtained with a backside illuminated device. In this paper we present measured quantum efficiency as a function of temperature, and we describe a novel clocking scheme to measure serial charge transfer efficiency. We demonstrate an industrial application in which the CCD is more than an order of magnitude more sensitive than a commercial camera using a standard CCD.

An integral field spectrograph for SNAP supernova studies

The Supernova / Acceleration Probe (SNAP) is a proposed space-borne observatory that will survey the sky with a wide-field optical/near-infrared (NIR) imager. The images produced by SNAP will have an unprecedented combination of depth, solid-angle, angular resolution, and temporal sampling. For 16 months each, two 7.5 square-degree fields will be observed every four days to a magnitude depth of AB=27.7 in each of the SNAP filters, spanning 3500-17000Å. Co-adding images over all epochs will give AB=30.3 per filter. In addition, a 300 square-degree field will be surveyed to AB=28 per filter, with no repeated temporal sampling. Although the survey strategy is tailored for supernova and weak gravitational lensing observations, the resulting data will support a broad range of auxiliary science programs.

SNAP NIR detectors

A well-adapted spectrograph concept has been developed for the SNAP (SuperNova/Acceleration Probe) experiment. The goal is to ensure proper identification of Type Ia supernovae and to standardize the magnitude of each candidate by determining explosion parameters. An instrument based on an integral field method with the powerful concept of imager slicing has been designed and is presented in this paper. The spectrograph concept is optimized to have very high efficiency and low spectral resolution (R {approx} 100), constant through the wavelength range (0.35-1.7{micro}m), adapted to the scientific goals of the mission.