Large format infrared imaging sensors are required to achieve simultaneously high resolution and wide field of view image data. Infrared sensors are generally required to be cooled from room temperature to cryogenic temperatures in less than 10 min thousands of times during their lifetime. The challenge is to remove mechanical stress, which is due to different materials with different coefficients of expansion, over a very wide temperature range and at the same time, provide a high sensitivity and high resolution image data. These challenges are met by developing a hybrid where the indium antimonide detector elements (pixels) are unconnected islands that essentially float on a silicon substrate and form a near perfect match to the silicon read-out circuit. Since the pixels are unconnected and isolated from each other, the array is reticulated. This paper shows that the front side illuminated and reticulated element indium antimonide focal plane developed at L-3 Cincinnati Electronics are robust, approach background limited sensitivity limit, and provide the resolution expected of the reticulated pixel array.
The evolution of InSb Focal Plane Arrays (FPAs) at L-3 Communications Cincinnati Electronics (L-3 CE) has resulted in large format, high reliability, and high yields for 256x256, 640x512, 1Kx1K and even 2Kx2K formats using our patented front-side illuminated, reticulated pixel design. Baseline processes matured at 30um pitch and gradually were made producible at 25um pitch. Recent progress in process technology, specifically dry etch plasma processes and photolithography tools, has created a new set of processes/design capabilities which enable 15um pixel pitch FPAs, thus allowing us to develop a 15um pitch FPA with 4 times as many pixels, in the same foot print as the previous 30um pitch designs. We have developed a new 15um pitch, reticulated pixel design, implemented on a 512x512 format, which can then be sized into larger arrays, similar to the evolution that occurred on 30um pitch FPAs. As unit cell dimensions shrink by a factor of two, both the feature size and the alignment tolerances begin to limit optical fill factor. Addition of a novel micro-optic design, which optimizes signal collection to near 100% efficiency while maintaining near theoretical pixel MTF, will be presented.
KEYWORDS: Image enhancement, Image processing, Algorithm development, Video processing, Color and brightness control algorithms, Infrared imaging, Detection and tracking algorithms, Electronics, Imaging systems, Video
The MWIR imaging systems developed by L-3 Communications Cincinnati Electronics (L-3 CE) include several video processing algorithms designed to provide enhanced imagery that meets a variety of military and other application requirements. When IR imaging systems are confronted with varying IR conditions, video processing algorithms are designed and selected to optimize human interpretation of specific scene details. The Visual Difference Predictor model has been used and a derived Image Enhancement Score has been developed to provide an objective metric to evaluate the effects of processing algorithms on imagery. Comparing the Image Enhancement Score of the processed image gives an objective measure of the success of the video processing algorithm being evaluated. This paper will describe selected algorithms in the L-3 CE Video Processing Suite, evaluate them against several test scenes and present associated Image Enhancement Scores. These will include a novel local contrast enhancement, general sharpening, and display mapping algorithms. Finally, the direction of ongoing and future efforts in Video Processing Suite development will be discussed.
CMC Electronics Cincinnati (CMC) is now in production on 1Kx1K InSb focal plane arrays (FPAs), and continuing efforts on a third production run of 2Kx2K large format IR FPAs. These FPAs are based on our unique reticulated InSb architecture that has been shown to be inherently scalable across format size while maintaining performance properties. Performance in the 10mk to 15mk NETD range will be shown. The design and fabrication of these advanced FPAs has challenged the state of the art in fabrication processing, testing, and qualification of both InSb detectors and silicon ROICs. Program sponsored manufacturing improvement activities, as well as CMC internal R&D, continue to improve both the yields and the performance characteristics of these large arrays. The latest yield, operability, and performance data will be shown. Data will be drawn from a population of approximately 30 2Kx2K FPAs and 50 1Kx1K FPAs. A novel approach to rapid thermal cycling FPAs will we described and recent developments that enable the fabrication of reticulated, smaller pixel pitch devices and practical Ultra Large Format FPAs with additional capability and features will be discussed.
CMC Electronics Cincinnati (CMC) is now in production on 1Kx1K InSb focal plane arrays (FPAs), and continuing efforts on a third production run of 2Kx2K large format IR FPAs. These FPAs are based on our unique reticulated InSb architecture which has been shown to be inherently scalable across format size without losing performance properties. Current offerings range from 256x256 to 2Kx2K formats ranging in between 30um and 20um pixel pitch, with 15um pixel pitch FPAs in development. Performance in the 10mk to 15mk NETD range will be shown. The design and fabrication of these advanced FPAs has challenged the state of the art in fabrication processing of both InSb detectors and silicon ROICs. Improvements made to enable large format fabrication have improved the yields and lowered the cost of smaller format FPAs as well. Program sponsored manufacturing improvement activities, as well as CMC internal R&D, continue to improve both the yields and the performance characteristics of these large arrays. This has resulted in breakthroughs in FPA size, performance, reliability and yeilds. The latest yield, operability, and performance data will be shown. Data will be drawn from a population of approximately 30 2K FPAs and 50 1K FPAs. Recent developments in smaller pixel pitch and other R&D areas will be discussed.
Last year, CMC reported performance data on the first article large format Indium Antimonide (InSb) Focal Plane Arrays (FPAs) produced at CMC Electronics Cincinnati (CMCEC). CMCEC's FPA design contains novel, thermally matched elements, which allow scaling from 256 x 256 pixel FPAs up to and including 1Kx1K and 2Kx2K FPAs as shown in Figure 1. Since a common process and wafer size is used to fabricate 256 x 256 640 x 512, 1Kx1K and 2Kx2K FPAs, the main issue in providing 2Kx2K FPAs is one of yeild improvement, not invention. Approximately 30 of these large format 1Kx1K and 2Kx2K FPAs have been built and 18 have been integrated into deliverable systems over the last year.
Developing maximum image performance in an infrared imaging system without exceeding physical design parameters, such as size and weight, leads to system requirements for small and closely spaced detector pixels areas on the focal plane array. Small sensitive areas allow high resolution and close spacing that can result in high spatial sampling rates and the ability to discern objects at long distances. The imaging resolution performance of compact two-dimensional Indium Antimonide (InSb) arrays can be limited by the fact that photo generated carriers can diffuse and be collected by junctions removed from the point of generation. Carrier diffusion can limit resolution in compact sensor packages. This paper quantitatively discusses the effect of carrier diffusion on resolution and the advantages of a reticulated pixel design.
With the increased use of large format detector arrays in IR imaging systems, designers are seeing the limitations that array response nonuniformities place on overall system design, performance, and cost. In many designs where high sensitivities are required and temporal noise has been reduced with the use of high quantum efficiency detectors, response nonuniformity across the array begins to limit the overall system performance. In addition, changes in the pixel response or offset that occur after nonuniformity corrections show up as new nonuniformities that also limit system performance. Because the array response is not adequately stable over time, system designers must include updatable nonuniformity corrections (recorrections) in the hardware or algorithms in the software to remove nonuniformities. We have measured the stability of InSb 2D focal plane arrays including measurements of the residual spatial nonuniformities over time and with temperature cycling. 2D focal plane arrays with stability that allows a one time correction to be used over several days have been measured. To help understand the source of this stability, the sensitivity of the spatial nonuniformity to small changes in the array temperature, detector leakage current, and detector bias was measured. These results are presented here.
The effects of detector series resistance on the electrooptical performance of infrared detectors are calculated by means of a simple diode model with a parallel photocurrent source. Calculations are performed for a number of variables, and series resistance is shown to be an important parameter in the design of photovoltaic detectors because it can limit operation in high performance design applications.
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