Martin H. Ettenberg

High-resolution SWIR arrays for imaging at night

We describe innovations in short wave infrared (SWIR) InGaAs focal plane arrays and cameras which now allow imaging under starlight only conditions at video rates. These lattice matched In.53Ga.47As imagers detect 0.9 μm to 1.7 μm SWIR band light, which is generally reflected from the imaged target. At night, the sources of light are the night glow, stars, the moon, or light pollution from nearby towns and cities. Detectivities, D*, greater than 6 x 1013 cm-√Hz/W and no image lag are necessary to image under starlight only conditions at RS-170 video rates. The InGaAs arrays are now commercially available in formats as large as 640 x 512 on a 25 μm pitch, and custom arrays are being manufactured on a 15 μm pitch with pixel counts as large as 1280 x 1024. The cameras are capable of adapting to the different light conditions that may occur in a scene over a 24-hour period, without the need for new corrections; this illumination variation can be over 5 orders of magnitude. The InGaAs material is stable, making new field corrections unnecessary for the life of the camera and eliminating the need for mechanical parts. The cameras have a dual output design to produce corrected analog output at video rates without the assistance of a computer, and corrected digital output through a 14 bit Camera Link interface.

Room temperature 640x512 pixel near-infrared InGaAs focal plane array

We report on the performance of a 640 X 512 pixel, indium gallium arsenide (In53Ga47As) focal plane array (FPA). The device has 25 micrometer pixels and represents the largest and finest pitched imager demonstrated in this material system. The device is sensitive to the 0.9 micrometer-to-1.7 micrometer short wave infrared band and features a room temperature detectivity, D*, greater than 5 X 1012 cm- (root)Hz/W with greater than 98% of the pixels operable. The performance of the In53Ga47As photodiode array is such that at room temperature the focal plane array is read noise- limited. The presentation will include a description of the FPA fabrication and assembly as well as characterization of dark current versus temperature, spectral response, and resolution. The implications of these results to applications such as passive night vision imaging, active illumination, covert surveillance, target designation using eye safe lasers, and target acquisition and tracking will be discussed.

Indium gallium arsenide imaging with smaller cameras, higher-resolution arrays, and greater material sensitivity

Indium Gallium Arsenide (InGaAs) photodiode arrays have numerous commercial, industrial, and military applications. During the past 10 years, great strides have been made in the development of these devices starting with simple 256-element linear photodiode arrays and progressing to the large 640 x 512 element area arrays now readily available. Linear arrays are offered with 512 elements on a 25 micron pitch with no defective pixels, and are used in spectroscopic monitors for wavelength division multiplexing (WDM) systems as well as in machine vision applications. A 320 x 240 solid-state array operates at room temperature, which allows development of a camera which is smaller than 25 cm3 in volume, weighs less than 100 g and uses less than 750 mW of power. Two dimensional focal plane arrays and cameras have been manufactured with detectivity, D*, greater than 1014 cm-(root)Hz/W at room temperature and have demonstrated the ability to image at night. Cameras are also critical tools for the assembly and performance monitoring of optical switches and add-drop multiplexers in the telecommunications industry. These same cameras are used for the inspection of silicon wafers and fine art, laser beam profiling, and metals manufacturing. By varying the Indium content, InGaAs photodiode arrays can be tailored to cover the entire short-wave infrared spectrum from 1.0 micron to 2.5 microns. InGaAs focal plane arrays and cameras sensitive to 2.0 micron wavelength light are now available in 320 x 240 formats.

InGaAs focal plane arrays and cameras for man-portable near-infrared imaging

During this presentation, the status of the technology will be described and prototype applications will be demonstrated and discussed. Included in the discussion will be: (1) the ability to distinguish camouflage from the surrounding environment, (2) the ability to see through fog that is opaque to visible imagers, (3) the ability to image eye-safe lasers for range-finding and target-acquisition, and (4) the use in conjunction with NIR flood lights for both covert surveillance and search and rescue operations. The high room-temperature D* makes indium gallium arsenide focal plane arrays excellent candidates for inclusion in small, light-weight, low-power, and low-cost NIR imaging modules. This type of development will enable additional applications such as the use in gun sights and micro-unmanned aerial vehicle surveillance. The presentation will conclude with the discussion of ongoing development activities.

Range-gated imaging with an indium-gallium-arsenide-based focal plane array

Range-gated imaging using indium gallium arsenide based focal plane arrays enables both depth and intensity imaging with eye-safe lasers while remaining covert to night vision goggles. We report on a focal plane array consisting of an indium gallium arsenide photodiode array hybrid-integrated with a CMOS readout circuit, resulting in an all solid state device. A 5 V supply avoids the complication of high voltage supplies and improves reliability, while also allowing the device to be small and lightweight. The spectral sensitivity of InGaAs extends from 0.9 microns to 1.7 microns, allowing the use of commercially available pulsed lasers with 1.5 micron wavelength, several millijoule pulse energies, and nanosecond scale pulse durations. SUI is developing a 320 x 256 pixel imager with the ability to conduct range gated imaging with sub-100 ns gates, while also allowing a 16 ms integration time for imaging in a staring mode. The pixels are fabricated on a 25 micron pitch for a compact device, and all pixels are gated simultaneously for “snapshot” exposure. High in-pixel gain with nearly noiseless amplification and low dark current enable high sensitivity imaging from ultra-short gates to video rate imaging.

Miniaturized 320x256 indium gallium arsenide SWIR camera for robotic and unmanned aerial vehicle applications

We describe a new InGaAs SWIR microcamera developed for robotic and UAV applications. The camera has a volume less than 27 cm3, weighs less than 100 g, and consumes less than 1.4 W. The camera operates with the focal plane array at room temperature and is sensitive to the 0.9 μm to 1.7 μm SWIR band with a detectivity, D*, greater than (formula available in paper). The InGaAs focal plane array has 320x256 pixels on a 25 μm pitch. It features snapshot-mode integration with a minimum exposure time of 500 ns making it ideally suited for all-solid-state range-gated imaging. The full-frame readout rate is greater than 400 frames per second. The built-in windowing feature is highly flexible with as many as 8 arbitrarily shaped regions-of-interest can be located anywhere (including overlapping) on the imager. Eight 64 x 64 regions of interest (ROIs), for example, can be read out faster than 1000 frames per second with a single 64 x 64 ROI read out faster than 5000 frames per second enabling high speed target acquisition and tracking applications.

Correlation of shunt resistance with InGaAs layer photoluminescence intensity for 2200 nm cutoff InGaAs photodiodes

This paper discusses techniques developed for predicting electrical properties of photodiodes fabricated from chloride vapor phase epitaxy-grown 2200 nm cutoff In/sub 0.72/Ga/sub 0.28/As/InAs/sub y/P/sub 1-y/ heterostructures with y compositionally graded from 0.0 - 0.4. Scanning electron microscopy (SEM) was used to examine the epitaxial layers in cross-section to determine their thickness uniformity over the wafer. Cross-sectional transmission electron microscopy (XTEM) was used to show that although strain in the structure was well accommodated within the InAs/sub y/P/sub 1-y/ graded layers, the cap, active and buffer layers were not completely lattice-matched to each other. In/sub 0.72/Ga/sub 0.28/As photoluminescence (PL) intensity data showed a strong dependence on the lattice-mismatch between the cap and active layers. Photodiode shunt resistance normalized to the active region area, R/sub 0/A, was found to increase dramatically with increasing PL intensity. We propose that PL intensity from the In/sub 0.72/Ga/sub 0.28/As layer on pre-processed wafers is a faithful measure of ultimate device performance.

High resolution 1.3 megapixel extended wavelength InGaAs

Extended wavelength InGaAs detectors grown on InP substrates have been generally used only in single element detectors and low resolution linear arrays. The extended wavelength InGaAs is no longer lattice matched to the InP substrate so it requires buffer layers to be used in the epitaxial growth process to accommodate the strain of the mismatched material. These detectors are generally front side illuminated with wire bonded pads. This work describes the results of extended wavelength InGaAs detector arrays that are backside illuminated which presents many more challenges, including imaging through the buffer layers as well as hybridization to Si Readout Integrated Circuits (ROICs). The buffer layers absorb shorter wavelength light making NIR response challenging. The arrays produced in this work are at high resolution, 1.3 megapixels on small pitch of 12 µm. The imagers have response from 700 nm to <2000 nm while imaging via backside illumination. New processing methodologies were developed to extend the short wavelength response to allow for NIR response by removing the substrate and most of the buffer layers from the structure after hybridization. This has produced material with quantum efficiencies <50% across most of its detection range during TEC cooled operation.

Monolithic planar InGaAs detector arrays for uncooled high-sensitivity SWIR imaging

There are few choices when identifying detector materials for use in the SWIR wavelength band. We have exploited the direct-bandgap InGaAs material system to achieve superior room temperature (293°K) dark current. We have demonstrated sensitivity from 400nm through 2.6um with this material system and thus provide the opportunity to sense not only the visible, but also the J-band (1.25um), H-band (1.65um) and K-band (2.2um) windows. This paper discusses the advantages of our hybridized CMOS-InGaAs material system versus other potential SWIR material systems. The monolithic planar InGaAs detector array enables 100% fill factor and thus, high external quantum efficiency. We have achieved room-temperature pixel dark current of 2.8fA and shot noise of 110 electrons per pixel per second. Low dark current at +300K allows uncooled packaging options, affording the system designer dramatic reductions in size, weight (cameras <28grams), and power (<2.5W). Commercially available InGaAs pin arrays have shown diode lifetime mean time between failures (MTBF) of 1011hours for planar InGaAs detectors1, far exceeding telecom-grade reliability requirements. The use of a hybrid CMOS-InGaAs system allows best of breed materials to be used and permits efficient, cost-effective, volume integration. Moreover, we will discuss how the InGaAsP material system is compatible with CMOS monolithic integration. Taken together, these advantages, we believe, make InGaAs the obvious choice for all future SWIR systems.

A customizable commercial miniaturized 320×256 indium gallium arsenide shortwave infrared camera

The design and performance of a commercial short-wave-infrared (SWIR) InGaAs microcamera engine is presented. The 0.9-to-1.7 micron SWIR imaging system consists of a room-temperature-TEC-stabilized, 320x256 (25 μm pitch) InGaAs focal plane array (FPA) and a high-performance, highly customizable image-processing set of electronics. The detectivity, D*, of the system is greater than 1013 cm-&sqrt;Hz/W at 1.55 μm, and this sensitivity may be adjusted in real-time over 100 dB. It features snapshot-mode integration with a minimum exposure time of 130 μs. The digital video processor provides real time pixel-to-pixel, 2-point dark-current subtraction and non-uniformity compensation along with defective-pixel substitution. Other features include automatic gain control (AGC), gamma correction, 7 preset configurations, adjustable exposure time, external triggering, and windowing. The windowing feature is highly flexible; the region of interest (ROI) may be placed anywhere on the imager and can be varied at will. Windowing allows for high-speed readout enabling such applications as target acquisition and tracking; for example, a 32x32 ROI window may be read out at over 3500 frames per second (fps). Output video is provided as EIA170-compatible analog, or as 12-bit CameraLink-compatible digital. All the above features are accomplished in a small volume < 28 cm3, weight < 70 g, and with low power consumption < 1.3 W at room temperature using this new microcamera engine. Video processing is based on a field-programmable gate array (FPGA) platform with a soft-embedded processor that allows for ease of integration/addition of customer-specific algorithms, processes, or design requirements. The camera was developed with the high-performance, space-restricted, power-conscious application in mind, such as robotic or UAV deployment.