Richard J. Stover

The DEIMOS spectrograph for the Keck II Telescope: integration and testing

The DEIMOS spectrograph is a multi-object spectrograph being built for Keck II. DEIMOS was delivered in February 2002, became operational in May, and is now about three-quarters of the way through its commissioning period. This paper describes the major problems encountered in completing the spectrograph, with particular emphasis on optical quality and image motion. The strategies developed to deal with these problems are described. Overall, commissioning is going well, and it appears that DEIMOS will meet all of its major performance goals.

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

Charge-coupled devices (CCDs) have been fabricated on high-resistivity, n-type silicon. The resistivity, on the order of 10 000 /spl Omega//spl middot/cm, allows for depletion depths of several hundred micrometers. Fully depleted, back-illuminated operation is achieved by the application of a bias voltage to an ohmic contact on the wafer back side consisting of a thin in situ doped polycrystalline silicon layer capped by indium tin oxide and silicon dioxide. This thin contact allows for a good short-wavelength response, while the relatively large depleted thickness results in a good near-infrared response.

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

High-performance CCD on high-resistivity silicon

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.

Characterization and optimization of MIT Lincoln Laboratories CCID20 CCDs

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.

Packaging design for Lawrence Berkeley National Laboratory high-resistivity CCDs

CCID20 CCDs are designed and produced by personnel at MIT/Lincoln Laboratory. The CCDs are thinned, back illuminated, 4096 X 2048 15-micrometer square pixel, three- side buttable devices. Some CCDs have been made on high- resistivity bulk silicon and others have been made on standard resistivity epitaxial silicon. Recently many devices from the first round of production of these CCDs have been tested at the UCO/Lick Observatory Detector Development Laboratory. In this paper we present the results of the measurements of horizontal and vertical charge transfer efficiency, low- temperature dark current, localized charge traps, full well, responsive quantum efficiency, and fringing. We present performance measurements of the on-chip amplifier including measurements of read-out noise, gain and linearity with different bias voltages. Cross-talk between the two on-chip amplifiers is discussed. High resistivity CCDs made by MIT/Lincoln show higher QE and less QE variation at long wavelengths than regular thin CCDs. However, they are subject to additional lateral charge diffusion and cosmic-ray effects. We will give a comparison between the two kinds of CCID20 CCDs. CCID20 CCDs are not MPP devices. It is much more difficult to get high full well, low spurious charge, low dark current and low residual image, simultaneously. We present optimized parallel clocks and a special erasing procedure to help solve these problems. Devices from this first round of CCID20 CCDs exhibit a rectangular pattern of QE variations caused by backside surface treatment problems.

Large-area edge-buttable CCDs for mosaic focal plane applications

The Lawrence Berkeley National Laboratory has been developing fully-depleted high resistivity CCDs. These CCDs exhibit very high red quantum efficiency, no red fringing, and very low lateral charge diffusion, making them good candidates for astronomical applications that require better red response or better point spread function than can typically be achieved with standard thinned CCDs. For the LBNL 2Kx4K CCD we have developed a four-side mosaic package fabricated from aluminum nitride. Our objectives have been to achieve a flatness of less than 10 micrometers peak-to-valley and a consistent final package thickness variation of 10 micrometers or less in a light-weight package. We have achieved the flatness objective, and we are working toward the thickness variation objective.

Design and Fabrication of Large CCDs for the Keck Observatory Deimos Spectrograph

This paper will describe in some detail tow new large area CCD image sensors designed specifically to be used either as a single imager or assembled in mosaics of CCDs. The devices have 2048 X 4096, 15 micrometers pixels; the difference being the orientation of the serial register. Performance data are presented on both front- and back-illuminated parts. In addition, a new production camera test system will also be described which is being used to screen test the Advanced Camera CCDs for the wide field and high resolution channels.

A USB 2.0 computer interface for the UCO/Lick CCD cameras

The Keck II Deep Imaging Multi-Object Spectrograph (DEIMOS) is a general purpose, faint object, multi-slit, double-beam spectrograph which offers wide spectral coverage, high spectral resolution, high throughput, and long slit length on the sky. This powerful instrument will be the principal optical spectrograph on the Keck II telescope. DEIMOS is optimized for faint-object spectroscopy of individual point sources, low-surface-brightness extended objects, or widely distributed samples of faint objects on the sky. To obtain high resolution (∼1 A) and wide spectral coverage (up to 5000 A) the spectrograph uses wide angle cameras and large CCD detectors with many pixels.