This paper describes a robust liquid crystal alignment layer that can be applied to the interior surfaces of a preformed cavity. In this paper, we describe a method of infusing a dye into a microcavity to produce an effective photodefinable alignment layer. Additionally, we demonstrate that after the application of a diffused RM layer, the alignment of the liquid crystal can be rendered insensitive to subsequent light exposure. In this work we make clear the effect of the RM is not stabilizing the azodye layer, but becomes the stable alignment layer. This is demonstrated by using the process described above with the additional step of realigning of the azodye layer to be perpendicular to the surface through photo-bleaching; and showing the alignment of the LC is unaffected by this process. This versatile alignment layer method, offers significant promise for new photonics applications.
A germanium charge-coupled device (CCD) offers the advantages of a silicon CCD for X-ray detection – excellent uniformity, low read noise, high energy resolution, and noiseless on-chip charge summation – while covering an even broader spectral range. Notably, a germanium CCD offers the potential for broadband X-ray sensitivity with similar or even superior energy resolution than silicon, albeit requiring lower operating temperatures (≤ 150K) to achieve sufficiently low dark noise due to the lower band gap of this material. The recent demonstration of high-quality gate dielectrics on germanium with low surface-state density and low gate leakage is foundational for realization of high-quality imaging devices on this material. Building on this advancement, MIT Lincoln Laboratory has been developing germanium CCDs for several years, with design, fabrication, and characterization of kpixel-class front-illuminated devices discussed recently. In this article, we describe plans to scale these small arrays to megapixel-class imaging devices with performance suitable for scientific applications. Specifically, we discuss our efforts to increase charge-transfer efficiency, reduce dark current, improve fabrication yield, and fabricate backside-illuminated devices with excellent sensitivity.
Silicon charge-coupled devices (CCDs) are commonly utilized for scientific imaging in wavebands spanning the near infrared to soft X-ray. These devices offer numerous advantages including large format, excellent uniformity, low read noise, noiseless on-chip charge summation, and high energy resolution in the soft X-ray band. By building CCDs on bulk germanium, we can realize all of these advantages while covering an even broader spectral range, notably including the short-wave infrared (SWIR) and hard X-ray bands. Since germanium is available in wafer diameters up to 200 mm and can be processed in the same tools used to build silicon CCDs, large-format (>10 MPixel, >10 cm2 ) germanium imaging devices with narrow pixel pitch can be fabricated. Furthermore, devices fabricated on germanium have recently demonstrated the combination of low surface state density and high carrier lifetime required to achieve low dark current in a CCD. At MIT Lincoln Laboratory, we have been developing germanium imaging devices with the goal of fabricating large-format CCDs with SWIR or broadband X-ray sensitivity, and we recently realized our first front-illuminated CCDs built on bulk germanium. In this article, we describe design and fabrication of these arrays, analysis of read noise and dark current on these devices, and efforts to scale to larger device formats.
An uncooled thermal imager is being developed based on a liquid crystal (LC) transducer. Without any electrical connections, the LC transducer pixels change the long-wavelength infrared (LWIR) scene directly into a visible image as opposed to an electric signal in microbolometers. The objectives are to develop an imager technology scalable to large formats (tens of megapixels) while maintaining or improving the noise equivalent temperature difference (NETD) compared to microbolometers. The present work is demonstrating that the LCs have the required performance (sensitivity, dynamic range, speed, etc.) to enable a more flexible uncooled imager. Utilizing 200-mm wafers, a process has been developed and arrays have been fabricated using aligned LCs confined in 20×20-μm cavities elevated on thermal legs. Detectors have been successfully fabricated on both silicon and fused silica wafers using less than 10 photolithographic mask steps. A breadboard camera system has been assembled to test the imagers. Various sensor configurations are described along with advantages and disadvantages of component arrangements.
We report on two recently developed charge-coupled devices (CCDs) for adaptive optics wavefront sensing, both designed to provide exceptional sensitivity (low noise and high quantum efficiency) in high-frame-rate low-latency readout applications. The first imager, the CCID75, is a back-illuminated 16-port 160×160-pixel CCD that has been demonstrated to operate at frame rates above 1,300 fps with noise of < 3 e-. We will describe the architecture of this CCD that enables this level of performance, present and discuss characterization data, and review additional design features that enable unique operating modes for adaptive optics wavefront sensing. We will also present an architectural overview and initial characterization data of a recently designed variation on the CCID75 architecture, the CCID82, which incorporates an electronic shutter to support adaptive optics using Rayleigh beacons.
Gigahertz (GHz) imaging technology will be needed at high-luminosity X-ray and charged particle sources. It is
plausible to combine fast scintillators with the latest picosecond detectors and GHz electronics for multi-frame hard Xray
imaging and achieve an inter-frame time of less than 10 ns. The time responses and light yield of LYSO, LaBr3, BaF2 and ZnO are measured using an MCP-PMT detector. Zinc Oxide (ZnO) is an attractive material for fast hard X-ray
imaging based on GEANT4 simulations and previous studies, but the measured light yield from the samples is much
lower than expected.
We have demonstrated a wafer-scale back-illumination process for silicon Geiger-mode avalanche photodiode arrays
using Molecular Beam Epitaxy (MBE) for backside passivation. Critical to this fabrication process is support of the thin
(< 10 μm) detector during the MBE growth by oxide-bonding to a full-thickness silicon wafer. This back-illumination
process makes it possible to build low-dark-count-rate single-photon detectors with high quantum efficiency extending
to deep ultraviolet wavelengths. This paper reviews our process for fabricating MBE back-illuminated silicon Geigermode
avalanche photodiode arrays and presents characterization of initial test devices.
For adaptive optics systems, there is a growing demand for wavefront sensors that operate at higher frame rates and with
more pixels while maintaining low readout noise. Lincoln Laboratory has been investigating Geiger-mode avalanche
photodiode arrays integrated with CMOS readout circuits as a potential solution. This type of sensor counts photons
digitally within the pixel, enabling data to be read out at high rates without the penalty of readout noise. After a brief
overview of adaptive optics sensor development at Lincoln Laboratory, we will present the status of silicon Geigermode-
APD technology along with future plans to improve performance.
We present a unique hybridization process that permits high-performance back-illuminated silicon Geiger-mode
avalanche photodiodes (GM-APDs) to be bonded to custom CMOS readout integrated circuits (ROICs) - a hybridization
approach that enables independent optimization of the GM-APD arrays and the ROICs. The process includes oxide
bonding of silicon GM-APD arrays to a transparent support substrate followed by indium bump bonding of this layer to
a signal-processing ROIC. This hybrid detector approach can be used to fabricate imagers with high-fill-factor pixels and
enhanced quantum efficiency in the near infrared as well as large-pixel-count, small-pixel-pitch arrays with pixel-level
signal processing. In addition, the oxide bonding is compatible with high-temperature processing steps that can be used
to lower dark current and improve optical response in the ultraviolet.
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.
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 × 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 × 5 cm, 512 × 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 × 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 × 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 tip-tilt correction system at the Infrared Optical Telescope Array (IOTA) has been upgraded with a new star tracker camera.
The camera features a backside-illuminated CCD chip offering doubled overall quantum efficiency and a four times higher system gain compared to the previous system. Tests carried out to characterize the new system showed a higher system gain with a lower read-out noise electron level. Shorter read-out cycle times now allow to compensate tip-tilt fluctuations so that their error imposed on visibility measurements becomes comparable to, and even smaller than, that of higher-order aberrations.
A 512x512-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. Device-level simulations were done to estimate the CCD collection well spaces for sub-microsecond photoelectron collection times. Also required for the high frame rates were process
enhancements that included metal strapping of the polysilicon gate electrodes and a second metal layer. Tests on finished back-illuminated CCD imagers have demonstrated sequential multi-frame capture capability with integration intervals in the hundreds of nanoseconds range.
We report on the design of a system used to measure the multispectral intrapixel response of imaging sensor arrays. An Airy disk spot size of approximately 4 micrometers has been achieved for wavelength bands that extend from the visible blue to near IR. The automated system does rapid intrapixel row and/or column spatial mapping of individual pixels as well as rastered 2D spatial scans over multi-pixel girds. Commercially available equipment including a photometric eyepiece, a reflective objects, programmable pushers, and light-emitting diodes are utilized in the system. Scanned results using the system are presented for both front- and back-illuminating charge-coupled device imagers. The intrapixel response of a front-illuminated device shows good correlation with the physical cross section of the devices tested.
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.
A new technology is introduced for developing potentially low cost, high throughput DNA sequence analysis. This approach utilizes novel bioelectronic genosensor devices to rapidly detect hybridization events across a DNA probe array. Detection of DNA probe/target hybridization has been achieved by two electronic methods. The first method utilizes a permittivity chip which interrogates the miniature test fixtures with a low voltage alternating electric field. The second method, which is the emphasis of this paper, utilizes a charge- coupled device (CCD) to detect the hybridization of appropriately tagged (radioisotope, fluorescent, or chemiluminescent labels) target DNA to an array of DNA probes immobilized above the pixels. Such direct electronic-biologic coupling is shown to provide a tenfold sensitivity improvement over conventional lens-based detection systems.