Barry E. Burke

An abuttable CCD imager for visible and X-ray focal plane arrays

A frame-transfer silicon charge-coupled-device (CCD) imager has been developed that can be closely abutted to other imagers on three sides of the imaging array. It is intended for use in multichip arrays. The device has 420*420 pixels in the imaging and frame-store regions and is constructed using a three-phase triple-polysilicon process. Particular emphasis has been placed on achieving low-noise charge detection for low-light-level imaging in the visible and maximum energy resolution for X-ray spectroscopic applications. Noise levels of 6 electrons at 1-MHz and less than 3 electrons at 100-kHz data rates have been achieved. Imagers have been fabricated on 1000- Omega cm material to maximize quantum efficiency and minimize split events in the soft X-ray regime. >

CCD soft X-ray imaging spectrometer for the ASCA satellite

We describe the development of a charge-coupled device (CCD) array for use as a soft X-ray (0.4-12 keV) imaging spectrometer for the ASCA (formerly Astro-D) satellite. The CCDs are 420/spl times/420-pixel frame-transfer devices designed to be closely abutted to other chips on three sides of the imaging array. The imagers are made on 6500-/spl Omega//spl middot/cm p-type float-zone silicon for depletion depths of about 50 /spl mu/m under typical CCD bias conditions. The read noise of the CCD is typically 3-4 e/sup /spl minus// rms at data rates of 50 KHz resulting in an energy resolution E//spl delta/E/spl ap/50 at 5.9 keV. The complete focal-plane sensor consists of a 2/spl times/2 array of these devices mounted on a common substrate. Radiation damage from energetic protons is mitigated by the use of a narrow potential trough along the center of the CCD channel to confine the small X-ray event charge to a reduced volume and thereby minimize trapping effects. Charged-particle events from the non-X-ray space background are minimized by using a junction on the back of the chip to deplete most of the neutral bulk and draw background charge away from the CCD. Wafer-level device screening at low temperatures and the focal-plane packaging methods are also described. >

Soft-X-ray CCD imagers for AXAF

We describe the key features and performance data of a 1024/spl times/1026-pixel frame-transfer imager for use as a soft-X-ray detector on the NASA X-ray observatory Advanced X-ray Astrophysics Facility (AXAF). The four-port device features a floating-diffusion output circuit with a responsivity of 20 /spl mu/V/e/sup -/ and noise of about 2 e/sup -/ at a 100-kHz data rate. Techniques for achieving the low sense-node capacitance of 5 fF are described. The CCD is fabricated on high-resistivity p-type silicon for deep depletion and includes narrow potential troughs for transfer inefficiencies of around 10/sup -7/. To achieve good sensitivity at energies below 1 keV, we have developed a back-illumination process that features low recombination losses at the back surface and has produced quantum efficiencies of about 0.7 at 277 eV (carbon K/spl alpha/).

Dynamic suppression of interface-state dark current in buried-channel CCDs

It is shown that the time dependence of the carrier generation rate at a depleted surface can be exploited to completely suppress interface-state dark current in buried-channel charge-coupled devices (CCDs). When a surface is switched from an inverted to a depleted state, the generation current recovers with a time constant which is strongly temperature dependent and varies from a few milliseconds at room temperature to nearly 3 h at -80 degrees C. This property can be applied to three- and four-phase CCDs by exchanging charge packets between adjacent phases within a cell at a rate that ensures that each phase remains out of inversion for time that is short in comparison to the recovery time. Measurements of this effect have been made on a CCD imager over the temperature range from -40 degrees C to +22 degrees C, and the results agree well with theory. >

Progress in x-ray CCD sensor performance for the Astro-E2 x-ray imaging spectrometer

We have developed X-ray CCD sensors for the Astro-E2 X-ray Imaging Spectrometer. Here we describe the performance benefits obtained from two innovations implemented in the CCD detectors developed for this instrument. First, we discuss the improved radiation tolerance afforded by a novel charge-injection structure. Second, we demonstrate for the first time the potential of a previously-developed chemisorption charging backside treatment process to produce back-illuminated X-ray sensors with excellent soft X-ray spectral resolution as well as improved quantum efficiency. We describe the changes in X-ray event detection algorithms required to obtain this improved performance, and briefly compare the performance of XIS sensors to that of back-illuminated detectors currently operating on-orbit.

Fano-Noise-Limited CCDs

Recent developments of scientific CCDs have produced sensors that achieve ultra low read noise performance (less than 2 electrons rms) and near perfect charge transfer efficiency (0.9999996) without the addition of a fat-zero. This progress has now made it possible to achieve Fano-noise-limited performance in the soft x-ray where the detectors energy resolution is primarily limited by the statistical variation in the charge generated by the interacting x-ray photon. In this paper, Fano-noise-limited test data is presented for two different CCD types and a CCD derived estimate of the Fano factor is determined. By evaluating ultra low-modulation images (less than 1 electron peak-to-peak) it is shown that the CCDs global CTE is now superior to its read noise floor. To capitalize on this capability CCD manufacturers are now focusing their attention on reducing the noise floor below the 1 electron level thereby matching the sensors CTE performance. This improvement, if accomplished, will push Fano-noise-limited performance for the CCD into the extreme ultra-violet.

CCD Charge Injection Structure at Very Small Signal Levels

A frame transfer charge-coupled device (CCD) designed for X-ray detection on board the SUZAKU spacecraft includes an input serial register and a charge injection structure which allows a very uniform injection of extremely small charge packets into the imaging section of the device. A variation of the fill-and-spill method was implemented to inject charge into the CCD. Very small charge packets (down to just a few electrons) can be reproducibly injected with noise as low as five-electron rms. The operation of the structure is described, and the results of the measurements are compared with the simulations. We have measured electron ldquoevaporationrdquo over potential barrier as a function of time, results being in excellent agreement with our model. By fitting a model to the data, it is possible to determine the internal capacitance of the input node. Charge injection noise as a function of signal charge was measured, and the results are also in agreement with theory. The designed structure can be used as a tool for studying and mitigating radiation damage effects in CCDs.

Progress on color night vision: visible/IR fusion, perception and search, and low-light CCD imaging

We report progress on our development of a color night vision capability, using biological models of opponent-color processing to fuse low-light visible and thermal IR imagery, and render it in realtime in natural colors. Preliminary results of human perceptual testing are described for a visual search task, the detection of embedded small low-contrast targets in natural night scenes. The advantages of color fusion over two alterative grayscale fusion products is demonstrated in the form of consistent, rapid detection across a variety of low- contrast (+/- 15% or less) visible and IR conditions. We also describe advances in our development of a low-light CCD camera, capable of imaging in the visible through near- infrared in starlight at 30 frames/sec with wide intrascene dynamic range, and the locally adaptive dynamic range compression of this imagery. Example CCD imagery is shown under controlled illumination conditions, from full moon down to overcast starlight. By combining the low-light CCD visible imager with a microbolometer array LWIR imager, a portable image processor, and a color LCD on a chip, we can realize a compact design for a color fusion night vision scope.

Rubber focal plane for sky surveys

We describe progress in removing image motion over large fields of view. A camera using a new type of CCD has been commissioned and we report first results which are very promising for wide field imaging. We are embarking on a project to build a new type of astronomical CCD which should provide image motion compensation over arbitrarily large fields of view, very fast readout, autoguiding capability, good red sensitivity, and should be significantly less expensive than the present generation of CCDs.

CCD soft x-ray imagers for ASCA and AXAF

We describe the development at Lincoln Laboratory of large-area CCD imager arrays for soft x-ray astronomy. One such array consists of four, closely abutted, 420 X 420-pixel CCDs for the ASCA (formerly Astro-D) satellite that was launched on February 20, 1993. The CCDs were fabricated on p-type 6500-(Omega) -cm material in order to attain the deep depletion depths needed for the higher-energy (> 4 keV) photons. The use of high- resistivity material and the effects of space-radiation are among the principal technical issues which will be discussed. We are also developing the next-generation CCD sensors for the Advanced X-ray Astrophysics Facility which is currently scheduled for launch in 1998. This mission will use two multichip focal planes comprising ten chips, each of a larger format (approximately 1000 X 1000 pixels). In addition to a new CCD, this program will require other technology developments such as an innovative packaging method for the nonplanar focal planes.