We describe recent advances in backside passivation of large-format charge-coupled devices (CCDs) fabricated on 200- mm diameter wafers. These CCDs utilize direct oxide bonding and molecular-beam epitaxial (MBE) growth to enable high quantum efficiency in the ultraviolet (UV) and soft X-ray bands. In particular, the development of low-temperature MBE growth techniques and oxide bonding processes, which can withstand MBE processing, are described. Several highperformance large-format CCD designs were successfully back-illuminated using the presented process and excellent quantum efficiency (QE) and dark current are measured on these devices. Reflection-limited QE is measured from 200 nm to 800 nm, and dark current of less than 1e- /pixel/sec is measured at 40°C for a 9.5 μm pixel.

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.

We report our progress toward optimizing backside-illuminated silicon PIN CMOS devices developed by Teledyne Imaging Sensors (TIS) for far-UV planetary science applications. This project was motivated by initial measurements at Southwest Research Institute (SwRI) of the far-UV responsivity of backside-illuminated silicon PIN photodiode test structures described in Bai et al., SPIE, 2008, which revealed a promising QE in the 100-200 nm range as reported in Davis et al., SPIE, 2012. Our effort to advance the capabilities of thinned silicon wafers capitalizes on recent innovations in molecular beam epitaxy (MBE) doping processes. Key achievements to date include: 1) Representative silicon test wafers were fabricated by TIS, and set up for MBE processing at MIT Lincoln Laboratory (LL); 2) Preliminary far-UV detector QE simulation runs were completed to aid MBE layer design; 3) Detector fabrication was completed through the pre-MBE step; and 4) Initial testing of the MBE doping process was performed on monitoring wafers, with detailed quality assessments. Early results suggest that potential challenges in optimizing the UV-sensitivity of silicon PIN type CMOS devices, compared with similar UV enhancement methods established for CCDs, have been mitigated through our newly developed methods. We will discuss the potential advantages of our approach and briefly describe future development steps.

This paper summarizes progress of a project to develop and advance the maturity of photon-counting detectors for NASA exoplanet missions. The project, funded by NASA ROSES TDEM program, uses a 256×256 pixel silicon Geigermode avalanche photodiode (GM-APD) array, bump-bonded to a silicon readout circuit. Each pixel independently registers the arrival of a photon and can be reset and ready for another photon within 100 ns. The pixel has built-in circuitry for counting photo-generated events. The readout circuit is multiplexed to read out the photon arrival events. The signal chain is inherently digital, allowing for noiseless transmission over long distances. The detector always operates in photon counting mode and is thus not susceptible to excess noise factor that afflicts other technologies. The architecture should be able to operate with shot-noise-limited performance up to extremely high flux levels, >106 photons/second/pixel, and deliver maximum signal-to-noise ratios on the order of thousands for higher fluxes. Its performance is expected to be maintained at a high level throughout mission lifetime in the presence of the expected radiation dose.

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.

The CCD detectors in the X-ray Imaging Spectrometers (XIS) aboard Suzaku have been equipped with a precision charge injection capability. The purposes of this capability are to measure and reduce the detector degradation caused by charged particle radiation encountered on-orbit. Here we report the first results from routine operation of the XIS charge injection function. After 12 months' exposure of the XIS to the on-orbit charged particle environment, charge injection already provided measurable improvements in detector performance: the observed width of the 5.9 keV line from the onboard calibration source was reduced from 205 eV to less than 145 eV. The rate of degradation is also significantly smaller with charge injection, so its benefit will increase as the mission progresses. Measured at 5.9 keV, the radiation-induced rate of gain degradation is reduced by a factor of 4.3 ± 0.1 in the front-illuminated sensors when injecting charge greater than 6 keV equivalent per pixel. The corresponding rate of degradation in spectral resolution is reduced by a factor 6.5 ± 0.3. Injection of a smaller quantity of injected charge in the back-illuminated XIS sensor produces commensurately smaller improvement factors. Excellent uniformity of the injected charge pattern is essential to the effectiveness of charge injection in the XIS.

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.

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.

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.

There have been many recent developments in the attributes and capabilities of silicon-based CCD detectors for use in space and ground-based astronomy. The imagers used as X-ray detectors require very low noise and excellent quantum efficiency over the energy range of 200-10 000 eV. This is achieved using a combination of front and back-illuminated imagers fabricated on a 5000 Omega-cm resistivity material. A requirement for ground-based imagers is very good sensitivity between 350 and 1000 nm, as well as low noise and a high degree of spatial uniformity. We will describe the fabrication and performance of these imagers. Special features integrated into the CCD pixel architecture have increased the capability of the imagers. A fast electronic shutter has been developed for a wavefront sensor in an adaptive optics system. An orthogonal transfer CCD has been designed to compensate for the image motion relative to the CCD focal plane. Also, an antiblooming drain process has been developed so bright sources do not extend spatially into adjacent pixels in back- and front-illuminated imagers. Aspects of the design, fabrication, and performance of imagers with these features will be described. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

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 a back-illuminated 640/spl times/480 CCD imager which operates at 30-Hz frame rates with 5 e/sup -/ noise and which is capable of high resolution down to near starlight illumination levels. A new process for fabricating a compact blooming control is also described.

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.

High-sensitivity, low-noise, and high-frame-rate charge-coupled-devices (CCD) and camera system electronics have been developed for imaging applications in the visible and near infrared spectra. Back-illuminated CCD imagers are used in the camera system for sensitive signal detection. A fabrication technology, using a refractory process, has been developed for CCD back-illumination that has quantum efficiencies as high as 90% from 500 to 700 nm and 50% from 200 to 400 nm. The CCD imagers use multiple output ports, with single-port pixel rates up to 5 MHz, for low-noise operation at high frame rates. An electronic shutter has been integrated into the pixel structure that has switching times of approximately 50 ns and an extinction ratio value of 104 (signal detected shutter opened to shutter closed) at wavelengths below 540 nm. A flexible electronic board set, providing various operating modes, has been designed to run the CCD at high frame rates while maintaining the detection sensitivity of the imager.

We describe the key features and performance data of a 1024×1026-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 μV/e- and noise of about 2 e- 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 10-7. 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 Kα)