|Publication number||WO2012138345 A1|
|Publication date||11 Oct 2012|
|Filing date||7 Apr 2011|
|Priority date||7 Apr 2011|
|Publication number||PCT/2011/31639, PCT/US/11/031639, PCT/US/11/31639, PCT/US/2011/031639, PCT/US/2011/31639, PCT/US11/031639, PCT/US11/31639, PCT/US11031639, PCT/US1131639, PCT/US2011/031639, PCT/US2011/31639, PCT/US2011031639, PCT/US201131639, WO 2012/138345 A1, WO 2012138345 A1, WO 2012138345A1, WO-A1-2012138345, WO2012/138345A1, WO2012138345 A1, WO2012138345A1|
|Inventors||Richard MCKEE, Richard MADONNA, Perry FATH, James Halvis|
|Applicant||Northrop Grumman Systems Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (5), Referenced by (4), Classifications (4), Legal Events (4)|
|External Links: Patentscope, Espacenet|
APPLICATION FOR PATENT
ADVANCED FOCAL PLANE ARRAY CONCEPT FOR OVERHEAD PERSISTENT IR APPLICATIONS
Background of the Invention.
1. Technical Field.
The invention relates to the field of focal plane arrays of detectors, and more particularly to large-area dual-color focal plane arrays of infrared detectors
incorporating foveal vision.
2. Background Art.
Focal plane arrays (FPA) are detectors which consist of a linear or two- dimensional matrix of individual typically rectangular sensor chip assembly (SCA) detectors and are used at the focus of imaging systems. Linear focal plane arrays consist of a single line of SCA detectors while area focal plane arrays consist of rows and columns of SCA detectors. In general, digital FPAs receive imaged radiation (usually in the visible or infrared spectral band) and transform that radiation into digital counts. These digital counts are proportional to the amount of radiation incident upon the pixels of the SCAs that constitute the FPA.
Focal plane arrays differ in terms of spectral range of detection and can typically detect X-ray, ultraviolet (UV), visible, near-infrared (NIR mid-infrared, far- infrared (FIR), and microwave radiations. In terms of applications, FPAs are typically used in astronomical imaging, aerial reconnaissance, aerial mapping, spectrographic analysis, star tracking, machine vision, X-ray diffraction, and measurement applications.
There are many surveillance applications where the FPA must cover a very large region, like full earth viewing coverage from geosynchronous orbit satellites. Presently, FPAs are not large enough to support high field of view optics with reasonable f numbers, needed to survey the full earth region. This is because individual SCA detectors typically have controlling circuitry located at the periphery of their active region. This prevents SCAs from being tiled or butted against each other with minimal pixel gaps to form large area rectangular FPA patterns. In FPAs with a field of view smaller than the full region to be surveilled, optical-mechanical step stare and scanning techniques can be used to view each point in the region. The time between successive returns of the FPA's field of view at a specific point in the region constitutes the revisit time. Typically, there is a requirement to see each point in the FPAs surveillance region within a certain amount of time, which corresponds to a specific revisit time requirement. It would be desirable to fabricate an SCA detector that is four- side buttable in order to accommodate large-area rectangular FPAs with minimal pixel gaps between their constituent modular SCAs. A four side buttable SCA would allow assembly of large-area modular FPAs with a field of view sufficiently large for the region to be surveilled, thereby eliminating the complexity and cost of the various scanning and revisit techniques.
Another desirable capability of large-area FPAs like those used in space applications, is the ability to simultaneously collect two infrared bands from the same earth location. Currently, full dual color simultaneity is done by having two detector layers vertically grown with differing spectral cutoffs. Typically, the broadband infrared radiation is incident on the first detector layer having a wideband spectral cutoff. This first layer then absorbs the higher energy infrared radiation, while passing the lower energy infrared radiation to the second detector layer. This second layer then absorbs the second, lower energy infrared radiation. The two detector layer vertical architecture requires vias between the layers to make electrical interconnects and these vias take away from detector area causing a decrease in sensitivity and modulation transfer function. Additional drawbacks of the vertical architecture include relatively high spectral cross talk, less than 100% fill factor, operability reduced by the product of the two detector layers, low spectral flexibility as band-gap engineering of two filter cut-offs is required, and significant complexity should collection of more than two spectral bands is engineered. The vertical architecture is used in tactical applications, which typically have more relaxed performance requirements than space-based infrared applications. For space based applications, it would be highly desirable to develop a large-area FPA capable of simultaneously collecting two infrared bands without the drawbacks and complexities of the vertical architecture.
Yet another desirable capability of large-area FPAs is foveal vision, i.e., the capability of higher resolution in certain regions of the FPA. In applications like whole earth staring coverage from geosynchronous orbit at high resolutions, the data rates can exceed 1 Gbit s. Existing communication channels have 10-100 times lower bandwidth than this. Foveal vision allows management of data rates; high resolution is only applied where it is needed and the rest of the scene is viewed at a coarser resolution. Thus, large-area FPA images can be transmitted using existing communication channels. This represents a major savings at the system level since communications infrastructure does not need to be upgraded to accommodate higher bandwidths. Foveal vision is typically not present in prior art smaller area FPAs, because they have a reasonable number of detectors to fit into existing communications channels.
A factor that complicates the design of large-area FPA optical systems is that the image surface must be planar so that the image can be recorded using currently available planar FPAs. The planar image surface constraint leads to off-axis aberrations that include astigmatism, field curvature and coma. These need to be corrected using additional optical elements, complicating the optical system design and resulting in higher cost. If the requirement that the image surface be planar can be relaxed, simpler, more compact and lower-cost optics can be used. As the field of view increases, it is increasingly harder to maintain a flat image plane. In large area FPA applications, single SCAs with different shim heights can be used to match the image plane curvature as best as possible. It is highly desirable for large-area FPAs like those used in space applications to accommodate curved image planes.
It should be noted that while a number of noteworthy advances and technological improvements have been achieved within the art of to large-area dual- color focal plane arrays of infrared detectors incorporating foveal vision, none completely fulfills the specific objectives achieved by this invention.
Disclosure of Invention.
In accordance with the present invention, a vertical-stack integrated sensor chip assembly having a multiplicity of pixels for detecting an infrared radiation comprises a micro-optics structure for processing infrared radiation, a photovoltaic detector layer situated below the micro-optics structure for converting the infrared radiation into a signal, an indium bump layer situated below the photovoltaic detector layer for electrical signal interconnection, a first readout integrated circuit input cell layer situated below the indium bump layer for signal processing, a metal bus layer situated below the first readout integrated circuit input cell layer for interfacing bias, clock, address, and read out lines, a second readout integrated circuit input cell layer situated below the metal bus layer and interfaced with the first readout integrated circuit input cell layer through wafer vias for digitizing and outputting signals to off-chip electronics, and a mechanical structure layer situated below the second readout integrated circuit input cell layer for mechanical support. The sensor chip assembly has spatial foveal vision capability such that high resolution is only applied where it is needed and the rest of the scene is viewed at a coarser resolution. The vertical stack integration enables four side butted modular assembly (tiling) of sensor chip assemblies with minimal gaps, which allows the formation of large-area focal plane arrays.
The micro-optics structure comprises a microlens at each pixel location for directing the infrared radiation to a first spectral filter, wherein a first spectral band of the infrared radiation passes through the first spectral filter into a first photovoltaic detector. A second photovoltaic detector receives at least a portion of the infrared radiation not passing through the first spectral filter. In one embodiment, a second spectral band of the infrared radiation not passing through the first spectral filter passes through a second spectral filter into the second photovoltaic detector. The first photovoltaic detector and the second photovoltaic detector collect spatially co- registered infrared radiation synchronously.
Multiple sensor chip assemblies are side butted with minimal gaps to form a modular focal plane array and are electronically synchronized for synchronous focal plane array image capture. The first focal plane array photovoltaic detector image and the second focal plane array photovoltaic detector image are spatially co-registered and are collected synchronously. The focal plane array is capable of spatial foveal vision that crosses sensor chip assembly boundaries. Furthermore, the focal plane array may have a segmented curved focal image plane for utilization with curved image optical systems. The focal plane array has a wide field of view focal image plane for large area coverage. In addition, provided are methods for forming the vertical-stack integrated sensor chip assembly and the modular focal plane array.
These and other objects, advantages and preferred features of this invention will be apparent from the following description taken with reference to the accompanying drawings, wherein is shown the preferred embodiments of the invention.
Brief Description of Drawings.
A more particular description of the invention briefly summarized above is available from the exemplary embodiments illustrated in the drawing and discussed in further detail below. Through this reference, it can be seen how the above cited features, as well as others that will become apparent, are obtained and can be understood in detail. The drawings nevertheless illustrate only typical, preferred embodiments of the invention and are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.
Figure 1 is a cross-sectional view of an SCA depicting an exemplary vertical stack of micro-optics, detector layer, silicon (Si) read out integrated circuit (ROIC) level 1, ROIC level 2, and mechanical mounting structure.
Figure 2 is an exemplary diagram of an 8 X 8 arrangement of sensor chip assemblies (SCA's) forming the focal plane array (FPA) of the present invention.
Figure 3 is another cross-sectional side view depicting one SCA.
Figure 4 is a cross-sectional diagram of micro-optic structure within each pixel for the present invention.
Mode(s) for Carrying Out the Invention.
So that the manner in which the above recited features, advantages and objects of the present invention are attained can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to the embodiment thereof that is illustrated in the appended drawings. In all the drawings, identical numbers represent the same elements.
The description below is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. The drawing figures are not necessarily to scale and certain features of the invention may be shown exaggerated in scale or in somewhat schematic form in the interest of clarity and conciseness. In the description, relative terms such as "front," "back," "up," "down," "top" and "bottom," as well as derivatives thereof, should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms concerning attachments, coupling and the like, such as "connected" and "attached," refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.
In describing various embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. It is to be understood that each specific element includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction may be made without departing from the spirit of the invention. Although the description of large-area focal plane arrays of infrared detectors incorporating foveal vision utilizes a micro-optics structure with dual color capability for illustration purposes, a person of ordinary skill in the art will readily recognize that the micro-optics structure is not limited to two colors.
With reference now to Fig. 1, shown is a cross-sectional view of an SCA 100 depicting an exemplary vertical stack of micro-optics 110, photovoltaic detector layer 120, silicon (Si) read out integrated circuit (ROIC) level 1 130, ROIC level 2 140, and mechanical mounting structure 150. The micro-optics 110 can be attached to the detector layer 120 using known wafer bonding techniques. The SCA 100 has a multiplicity of pixels for detecting incident infrared radiation. The micro-optics 110 structure receives the incident infrared radiation and has dual color simultaneous and co-registered processing capability, i.e., two infrared spectral bands can be detected synchronously at the same pixel location. The two infrared spectral bands and their photon flux are directed to two separate detectors in the photovoltaic detector layer 120. Typical infrared photovoltaic detector layer 120 material systems include Mercury Cadmium Telluride (MCT) and Cadmium Zinc Telluride (CZT), although other detector or materials may be used as desired. The photovoltaic detector layer 120 converts the infrared radiation into a signal. An indium bump 160 electrical connection is then used to connect photovoltaic detector layer 120 to a silicon LI (Level 1) ROIC input cell 130. This is the conventional, state of the art approach for moving signal charge (current) from the photovoltaic detector layer 120 to the LI ROIC input cell 130. A metal bus layer 170 situated below the LI ROIC input cell 130 interfaces bias, clock, address, and read out lines. This information is stored in capacitors located within the LI ROIC input cell 130. A second L2 ROIC input cell 140 digitizes and outputs signals to off-chip electronics. L2 ROIC input cell 140 is situated below the metal bus layer 170 and is interfaced with the LI ROIC input cell 130 through wafer vias. Thus, two spectral infrared signals from each pixel location can be accessed. A mechanical structure layer 150 situated below the L2 ROIC input cell 140 provides mechanical support. Each SCA 100 can be composed of at least 1024 x 1024 infrared pixels 410. Each pixel 410 of the SCA (figure 4) collects infrared radiation from two distinct infrared spectral bands and outputs digitally, proportionally to the amount of incident infrared radiation upon it.
Also incorporated within the SCA 100 is foveal vision capability. Foveal capability enables frame rate selectable regions of high and low resolution within the SCA 100 field of view. This enables precise resolution where required while not suffering the large bandwidth penalty of unneeded, high resolution across the entire SCA 100. The fovea control resides at the L2 ROIC input cell 140. Separate connections to small memory cell switches located within each pixel 410 allow flexible summations of nearest neighbor signal charge, which controls the overall SCA 100 resolution. Resolution is controlled by connecting or not connecting adjacent pixel 410 signals.
The vertical stack integration of the SCA 100 controlling electronic circuitry enables four side butted modular assembly of SCAs 100 with minimal gaps, i.e., a multiplicity of SCAs 100 can be tiled on all four sides to form a large-area FPA 200 with a precise rectangular pattern and minimal gaps between the constituent SCAs 100. The large area FPA 200 has a wide field of view focal image plane for large area coverage. This is an improvement over prior art SCAs that typically have controlling electronic circuitry located at the periphery of their active regions, thereby complicating modular assembly of large-area SCAs with minimal gaps. The SCAs 100 are electronically synchronized to each other for synchronous focal plane array image capture. Figure 2 shows an exemplary 8 8 arrangement of closely four side butted SCA's 100 forming FPA 200. The number of SCA's 100 utilized to form FPA 200 is dependent upon the application. To cover a very large region (like full earth viewing coverage from geosynchronous orbit satellites) with an FPA 200 field of view equal or larger than the region to be covered, an arrangement of a large number of closely four- side butted SCA's 100 is needed. In figure 2, each square 210 represents an SCA 100.
Incorporated within the FPA 200 is foveal vision capability, i.e., frame rate selectable regions of high and low resolution. Foveal capability regions can cross SCA 100 boundaries within the FPA 200 thus giving great flexibility to the user. This enables precise resolution where required while not suffering the large bandwidth penalty of unneeded, high resolution across the entire FPA 200. The size and shape of the high and low resolution regions can be controlled through outside input commands and can be updated at the frame rate of the FPA 200. The electronics controlling the FPA 200 would orchestrate which regions of the FPA 200, and consequently parts of different SCA's 100, would be regions of high resolution and which regions would be regions of low resolution. Ultimately, the size of the high resolution regions is limited by the overall sensor downlink data rate and bandwidth compression schemes employed. FPA 200 foveal capability reduces bandwidth requirements, thus simplifying system data recording, compression, and transmission for typical space- based and large field of view area applications. In space vehicle applications, the output data rate sent to the ground is hardware limited. High data-rate high-resolution regions need to be managed at the FPA level to ensure that the maximum amount of relevant information utilizes the existing space vehicle downlink bandwidth.
A modular FPA 200 consisting of four-side butted SCAs 100 with minimal pixel gaps could be employed for the image plane of space systems utilizing optics which form curved images. As the field of view increases, it is increasingly harder to maintain a flat image plane and the image plane becomes curved. By accommodating curved image planes, the modular FPA 200 simplifies optical system design and offers an advantage over prior art monolithic FPAs that are inherently flat and can't be used in curved image plane applications.
With reference now to figure 3, an exemplary cross-sectional side view is shown of a vertically integrated SCA 100 of the present invention. The focal plane chassis 300 is a structure that precisely locates the focal plane plate 310 and the backplane 320. Functionally, the focal plane chassis 300 provides the mechanical support of the focal plane plate 310 and the thermal path to cool the focal plane plate 310. The photovoltaic detector layer 120, readout and analog to digital converter are denoted by sensor assembly 330. The dotted lines 340 show a clearance hole for the connection cable 350 that comes from the sensor assembly 330 to the backplane 320. The connection ends of the cable 350 are shown by elements 380 and 390. A precision machined base 360 provides mechanical support for the sensor assembly 330. The dotted outline of the bolt 370 mechanically joining the sensor assembly base 360 to the focal plane plate 310 is also shown in figure 3.
With reference now to figure 4, shown is an exemplary cross-sectional diagram 400 of the SCA's 100 micro-optic structure 110 within each pixel 410 for the present invention. The micro-optics structure 110 is utilized to obtain two color, spatially co- registered, simultaneous infrared signal collection. A micro-lens 415 is used to direct incoming infrared spectral radiation 450 to the refractive interface 420 and then to the infrared spectral filter 425 located just above a first infrared photovoltaic detector 430 within each pixel 410. The micro-lens 415 can be comprised of low refractive index material. The infrared spectral filter 425 can be a low-pass, high-pass, or band-pass filter. The infrared spectral filter 425 passes a desired infrared spectral band into the first infrared photovoltaic detector 430, and redirects the remaining infrared radiation via reflection through mirror 435 to a second infrared photovoltaic detector 440. In one embodiment, a second infrared spectral filter is located just above the second infrared photovoltaic detector 440. Thus, the second infrared photovoltaic detector 440 receives at least a portion of the infrared radiation not passing through the infrared spectral filter 425. In this arrangement, a portion of the incoming infrared spectral radiation 450 incident upon pixel 410 is collected by the first infrared photovoltaic detector 430. The second infrared photovoltaic detector 440 receives all of the infrared radiation not passing through the first infrared spectral filter 425 in the case where there is no second infrared spectral filter located above it. In the embodiment wherein a second infrared spectral filter is located just above the second infrared photovoltaic detector 440, a spectral band of the infrared spectral radiation 450 not passing through the infrared spectral filter 425 passes through the second infrared spectral filter into the second infrared photovoltaic detector 440. The first infrared photovoltaic detector 430 and the second infrared photovoltaic detector 440 receive spatially co-registered infrared radiation as it originates from a single micro-lens 415 location. Also, the first infrared photovoltaic detector 430 and the second infrared photovoltaic detector 440 receive infrared radiation synchronously due to their proximity and the small light path separating them via reflection through mirror 435. Thus, SCA 100 composed of a plurality of infrared pixels 410 is capable of spatially co-registered and synchronous dual-color infrared radiation detection. Consequntly, FPA 200 composed of a plurality of electronically synchronized SCAs 100 is also capable of spatially co-registered and synchronous dual color infrared radiation detection.
In large field of view area applications, FPA 200 would be placed behind infrared optics, all embedded within a space qualified sensor as part of a space payload. This payload might occupy a geosynchronous orbit and view the earth in it's entirety using FPA 200 as the image plane. Through sensor line of sight knowledge relative to the earth and ground based mapping of each of the ~ 16 million pixels relative to the center line of sight of the sensor, it would be known where on the earth each of the FPA's 200 pixels are imaging. Consequently, when events in certain regions of the earth are of interest, these regions could be imaged in the high resolution foveal format, while regions of less interest would be imaged in low resolution. If needed, the foveal region could be changed at the frame rate of the FPA 200.
The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction may be made without departing from the spirit of the invention.
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|Cooperative Classification||H01L31/02327, H01L27/1465, H01L27/14649|
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