Operation of a CCD imager on a curved focal surface offers advantages to flat focal planes, especially for lightweight, relatively simple optical systems. The first advantage is that the modulation transfer function can approach diffraction-limited performance for a spherical focal surface employed in large field-of-view or large-format imagers. The second advantage is that a curved focal surface maintains more uniform illumination as a function of radius from the field center. Examples of applications of curved imagers, described here, include a small compact imager and the large curved array used in the Space Surveillance Telescope. The operational characteristics and mechanical limits of an imager deformed to a 15 mm radius are also described.

Dark current for back-illuminated (BI) charge-coupled-device (CCD) imagers at Lincoln Laboratory has historically been higher than for front-illuminated (FI) detectors. This is presumably due to high concentrations of unpassivated dangling bonds at or near the thinned back surface caused by wafer thinning, inadequate passivation and low quality native oxide growth. The high dark current has meant that the CCDs must be substantially cooled to be comparable to FI devices. The dark current comprises three components: frontside surface-state, bulk, and back surface. We have developed a backside passivation process that significantly reduces the dark current of BI CCDs. The BI imagers are passivated using molecular beam epitaxy (MBE) to grow a thin heavily boron-doped layer, followed by an annealing step in hydrogen. The frontside surface state component can be suppressed using surface inversion, where clock dithering reduces the frontside dark current below the bulk. This work uses surface inversion, clock dithering and comparison between FI and BI imagers as tools to determine the dark current from each of the components. MBE passivated devices, when used with clock dithering, have dark current reduced by a factor of one hundred relative to ion-implant/laser annealed devices, with measured values as low as 10-14 pA/cm2 at 20°C.

We report an array of Shack-Hartmann wavefront sensors using high-fill-factor Geiger-mode avalanche detector quad cells hybridized to all-digital CMOS counting circuits. The absence of readout noise facilitates fast wavefront sensing at low light levels. (C)2008 Optical Society of America

Massachusetts Institute of Technology, Lincoln Laboratory (MIT LL) has been developing both continuous and burst solid-state focal-plane-array technology for a variety of high-speed imaging applications. For continuous imaging, a 128 x 128-pixel charge coupled device (CCD) has been fabricated with multiple output ports for operating rates greater than 10,000 frames per second with readout noise of less than 10 e(-) rms. An electronic shutter has been integrated into the pixels of the back-illuminated (BI) CCD imagers that give snapshot exposure times of less than 10 ns.For burst imaging, a 5 cm x 5 cm, 512 x 512-element, multi-frame CCD imager that collects four sequential image frames at megahertz rates has been developed for the Los Alamos National Laboratory Dual Axis Radiographic Hydrodynamic Test (DARHT) facility. To operate at fast frame rates with high sensitivity, the imager uses the same electronic shutter technology as the continuously framing 128 x 128 CCD imager. The design concept and test results are described for the burst-frame-rate imager.Also discussed is an evolving solid-state imager technology that has interesting characteristics for creating large-format x-ray detectors with ultra-short exposure times (100 to 300 ps). The detector will consist of CMOS readouts for high speed sampling (tens of picoseconds transistor switching times) that are bump bonded to deep-depletion silicon photodiodes. A 64 x 64-pixel CMOS test chip has been designed, fabricated and characterized to investigate the feasibility of making large-format detectors with short, simultaneous exposure times.

The orthogonal-transfer array (OTA) is a new charge-coupled device (CCD) concept for wide-field imaging in ground-based astronomy based on the orthogonal-transfer CCD (OTCCD). This device combines an W array of small OTCCDs, each about 600x600 pixels with on-chip logic to provide independent control and readout of each CCD. The device provides spatially varying electronic tip-tilt correction for wavefront aberrations, as well as compensation for telescope shake. Tests of prototype devices have verified correct functioning of the control logic and demonstrated good CCD charge-transfer efficiency and high quantum efficiency. Independent biasing of the substrate down to -40 V has enabled fully depleted operation of 75-pm-thick devices with good charge PSF. Spurious charge or "glow" due to impact ionization from high fields at the drains of some of the NMOS logic FETs has been observed, and reprocessing of some devices from the first lot has resolved this issue. Read noise levels have been 10 - 20 e-, higher than our goal of 5 e-, but we have identified the likely sources of the problem. A second design is currently in fabrication and uses a 10-mu m pixel design resulting in a 22.6-Mpixel device measuring 50x50 mm. These devices will be deployed in the U. of Hawaii Pan-STARRS focal plane, which will comprise 60 OTAs with a total of nearly 1.4 Gpixels.

We describe results from recent enhancements to the performance of charge-coupled devices (CCDs) to both low- and high-energy soft X-rays. For improved low-energy (E<500 eV) sensitivity, we show that a low-temperature surface treatment on back-illuminated devices results in superior energy resolution compared to that of the devices flown on Chandra, which had a more process-intensive, high-temperature treatment. For improved high-energy response, we describe a design approach for MOS CCDs that allows high substrate biases for deep depletion (up to 160 /spl mu/m) and, thus, improved X-ray detection for E>5 keV.

A 512/spl times/512-element, multi-frame charge-coupled device (CCD) has been developed for collecting four sequential image frames at megahertz rates. To operate at fast frame rates with high sensitivity, the imager uses an electronic shutter technology developed for back-illuminated CCDs. The megahertz frame rates also required metal strapping of the polysilicon gate electrodes. Tested imagers have demonstrated multi-frame capture capability.

Lincoln Laboratory has developed 32 x 32-pixel ladar focal planes comprising silicon Geiger-mode avalanche photodiodes and high-speed all-digital CMOS timing circuitry in each pixel. In Geiger mode operation, the APD can detect as little as a single photon, producing a digital CMOS-compatible voltage pulse. This pulse is used to stop a highspeed counter in the pixel circuit, thus digitizing the time of arrival of the optical pulse. This "photon-to-digital conversion" simultaneously achieves single-photon sensitivity and 0.5-ns timing. We discuss the development of these focal planes and present imagery from ladar systems that use them.

Electronically shuttered solid-state imagers are being developed for high-speed imaging applications. A 5 cm×5 cm, 512×512-element, multiframe charge-coupled device (CCD) imager has been fabricated for the Los Alamos National Laboratory DARHT facility that collects four sequential image frames at megahertz rates. To operate at fast frame rates with high sensitivity, the imager uses an electronic shutter technology designed for back-illuminated CCDs. The design concept and test results are described for the burst-frame-rate imager. Also discussed is an evolving solid-state imager technology that has interesting characteristics for creating large-format x-ray detectors with short integration times (100 ps to 1 ns). Proposed device architectures use CMOS technology for high speed sampling (tens of picoseconds transistor switching times). Techniques for parallel clock distribution, that triggers the sampling of x-ray photoelectrons, will be described that exploit features of CMOS technology.

Silicon charge-coupled devices (CCDs) are extensively used for commercial and scientific imaging in the visible to near-infrared wavelength range of 450 to 850 nm. Ground-based astronomers require large-scale high-performance CCDs with high sensitivity at wavelengths from the 320 nm atmospheric cutoff to 900 nm. We report on wafer-scale low-temperature silicon molecular beam epitaxy to enable ultraviolet (UV) detection utilizing silicon-fabrication-facility-compatible surface preparation. Characterization of the UV response-enhanced backside-illuminated CCDs fabricated with this technique show near 100% internal quantum efficiency in the wavelength range of 200 to 900 nm.

We describe here the continuing research in large-area, back-illuminated CCD imagers at MIT Lincoln Laboratory in collaboration with the University of Hawaii Consortium. Among the developments are a two-layer antireflection coating of TiO2/Al2O3 combined with thick (>40 mum) substrates aimed at broader bandwidth and reduction of Fabry-Perot interference effects in the near infrared. Recent work on the orthogonal-transfer CCD (OTCCD) is described in which the previously noted problem of pockets has been solved with a four-layer polysilicon process. Two new larger OTCCDs (1024x1320 and 2kx4k) have been designed and are in fabrication. The larger OTCCD will use a recently designed four-side-buttable package.

We perform precise measurements of the x-ray transmission of the thin films comprising CCD gate structure, namely, phosphorus doped polysilicon, silicon dioxide, and silicon nitride. The x-ray transmission of these films shows large oscillations with small changes in energy in the vicinity of the following absorption edges: nitrogen K (400 eV), oxygen K (536 eV), silicon L and K (100 and 1840 eV, respectively). As a result, quantum efficiency of a CCD in the soft x-ray range deviates significantly from simple model predictions based on Henke et al. (1993) mass absorption coefficients. The measurements covered the range of energies from 60 to 3000 eV, using synchrotron beamlines at the Advanced Light Source (ALS; Berkeley), Physikalisch-Technische Bundesanstalt BESSY (Berlin), the Synchrotron Radiation Center (SRC; University of Wisconsin-Madison). Our model of the CCD response includes near edge x-ray absorption structure and predicts a very complicated shape of the energy dependence of the quantum efficiency around silicon and oxygen absorption edges. Experimental measurements of CCD quantum efficiency relative to a calibrated detector at the BESSY synchrotron confirmed our model predictions for both frontside and backside illuminated CCDs.

We describe recent work in the area of large, back-illuminated CCD imagers at M.I.T. Lincoln Laboratory as well as new technology applicable to astronomy. We completed in 1995 the development of a 2560 x 1960-pixel frame-transfer imager that filled a 100-mm wafer and several back-illuminated versions of this device were completed. More recently we have begun the development, in collaboration with the U. of Hawaii, of a three-side abuttable 2k x 4k CCD for a multi-chip focal plane. In the unused chord area of the wafer layout, we added test imagers as development vehicles for blooming control and for the demonstration of a CCD that is capable of charge transfer in all four directions. We expect the latter to find application as an electronic means of performing tip-tilt correction to compensate for atmospheric turbulence.

We have performed precise measurements of x-ray absorption constants for all the thin films comprising CCD gate structure, namely, phosphorous doped polysilicon, silicon dioxide, and silicon nitride. X-ray absorption of these films shows large oscillations around the corresponding absorption edges: nitrogen K, oxygen K, silicon L and K. As a result, quantum efficiency of a CCD in the soft x-ray range deviates significantly from the generally assumed simple model predictions. In order to cover the range of energies from 60 eV to 3000 eV transmission measurements were performed at several synchrotron beamlines at ALS, PTB BESSY, SRC. A model of the CCD response with near edge x-ray absorption structure taken into account predicts a very complicated shape of the energy dependence of the quantum efficiency around silicon and oxygen absorption edges. Experimental measurements of CCD quantum efficiency relative to a calibrated detector were performed at BESSY for both frontside illuminated and backside illuminated CCDs for energies around the oxygen absorption edge. Experimental results were found to be in a good agreement with our model.

We report monolithic arrays of silicon Geiger-mode avalanche photodiodes, and their integration with arrays of CMOS processing circuits. This is an enabling technology for a new class of focal plane arrays whose pixels can detect single photons as well as time their arrival with sub-ns precision. The CMOS circuits can be tailored to support either 3D laser radar applications, where the time of arrival of the first detected photon is of interest, or high-dynamic-range passive imaging, where the rate of arrival of photons is of interest. This combination of capabilities is not supported by traditional focal plane technologies such as CCDs and active-pixel sensors.

A charge modulation device (CMD) has been fabricated in a p-type epitaxial layer grown from the buried-channel silicon region of a charge-coupled device (CCD). Construction of the CMD directly above the CCD buried-channel and over the oxidized CCD transfer gates lowers the effective sense capacitance while providing isolation of the CMD source/drain regions. Responsivity values of 28 and 66 /spl mu/V/e for feedback and no feedback conditions, respectively, were measured dynamically on test devices. Input-referred noise values of approximately four electrons r.m.s. were calculated from noise spectral density measurements assuming a low-pass filter 3 dB cutoff frequency of 5 MHz and correlated double sampling.