Influence of IR Sensor Technology on the Military and Civil Defense

Influence of IR Sensor Technology on the Military and Civil Defense

By

Latika Becker

U.S. Army Space and Missile Defense Command

ABSTRACT Advances in basic infrared science and developments in pertinent technology applications have led to mature designs being incorporated in civil as well as military area defense systems. Military systems include both tactical and strategic, and civil area defense includes homeland security. Technical challenges arise in applying infrared sensor technology to detect and track targets for space and missile defense. Infrared sensors are valuable due to their passive capability, lower mass and power consumption, and their usefulness in all phases of missile defense engagements. Nanotechnology holds significant promise in the near future by offering unique material and physical properties to infrared components. This technology is rapidly developing. This presentation will review the current IR sensor technology, its applications, and future developments that will have an influence in military and civil defense applications.

1. Introduction Infrared (IR) sensors, including focal plane detector arrays, are used in military systems as a major component for acquisition, tracking, and kill assessment, and for seekers in interceptors. Early infrared systems used single detectors and small arrays. The current day challenge of sensing at longer ranges with better resolution demands larger array sizes. With smaller array sizes, surveillance systems required scanning the scene often. With usage of large format arrays, staring avoids the need for scanning. Sensors are being developed to operate through the atmosphere or in space. Sensor designs are progressing as fast as the need for their use in the critical infrared spectrum is. Infrared sensors are valuable because they operate in the passive mode, unlike radar which is active. Additionally, IR sensors possess the advantages of low mass and low power consumption. Mid and long wave IR sensors have proven to be useful for applications requiring target discrimination. It appears that the next century will witness significant advances in infrared sensor technology as the needs for military and civil defense become more critical. As we attack the global warming and climate changes that are occurring today in many subcontinents of the world, multi-band and hyper-spectral IR technology will play a vital role in years to come in providing evidence of changes. The state-of –the-art performance of these devices makes it possible to sense several types of gaseous plumes and emissions enabling chemical warfare agent detection/discrimination. Over the next decade, a succession of infrared sensors with evolving improvements will provide us information on the background phenomenology—sensing of auroras and space backgrounds, operation of sensors in space, ground and sea counterparts etc. Due to enhanced computation capability and technology advancements in computer simulation tools, the physics of IR sensor technology can now be investigated more thoroughly than before. IR spectrometers or spectral radiometers can be utilized to understand the spectral characteristics of the material to be examined for machine-vision applications. The type of IR camera used in machine vision will depend upon its suitability for the spectral region of application. Depending on the specific application, the detector material system used in the camera will vary. A new generation of high performance IR spectrometers with high reliability and small package size is being developed by Lawrence Livermore National Laboratory (LLNL) at Livermore, CA, as a field instrument for chemical sensing [1]. It uses dispersive or immersion gratings (of Si and Ge). Radiance Technologies, Inc. [2] is developing and demonstrating the “WeaponWatch” system using IR sensors.

Invited Paper

Quantum Sensing and Nanophotonic Devices III, edited by Manijeh Razeghi, Gail J. Brown,Proc. of SPIE Vol. 6127, 61270S, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.640529

Proc. of SPIE Vol. 6127 61270S-1

Spectroscopy and spectro-polarimetry applications that need IR detector technology, assist many investigations in planetary, galactic and extraterrestrial sciences [3,4]. These applications employ large format (1024x1024) detector array technology using HgCdTe or extrinsic Si as the detector material. The HgCdTe CMOS camera is used in the near IR (NIR) spectral region, whereas blocked impurity band (BIB) detector arrays fabricated in arsenic doped Si and hybridized with indium bump bonds onto CMOS multiplexers are used in the mid IR (MIR) spectral region. As focal plane arrays, the megapixel MIR FPA CMOS-based image sensors cost significantly less than Si CCDs and can produce sharper images. Advances in materials research have contributed significantly to improvement in the performance of IR devices. The demand for low voltage, low power, and high performance poses challenges for engineering of subnanometer gate length CMOS devices which are an integral part of hybridized focal plane arrays comprising detectors operating in the photovoltaic mode. Alternative device architecture options and implementation of new material systems in the IR detector technology along with compatible CMOS technology are all perceived to be factors critical to improving IR detector performance. IR sensor technology together with processor technology enables realization of many novel applications. Advances in IR sensors for use in missile seeker have necessitated improvement of CMOS technology used in hybrid focal plane arrays. CMOS technology improvements have made it possible to put more functions on the IRFPA chip, including intelligent signal processing. The fabrication process and the design rules have evolved considerably; revolutionizing the technology of CMOS based sensors. The CMOS readout electronics integrated circuit design can be used in applications requiring hybrid focal plane arrays in visible to very long wavelengths. Thus there can be a great deal of simplification for the system designer. Table 1, showing the National Technology Roadmap for Semiconductors (NTRS) [5], demonstrates the progression of denser lithography. Very fine geometry will allow packing more transistors into a single pixel thereby resulting in improved resolution and sensitivity. There will be several new challenges to be solved. Rockwell Scientific’s HyViSITM and ProCam-HDTM are successful examples of low power, high performance CMOS implementation exhibiting superior noise performance for high frame rates and large formats [6]. Figure 1 shows the next generation of camera systems and their sizes being developed at Rockwell Scientific Company.

Table 1: National Technology Roadmap for Semiconductors

Tech. ( µm ) 0.35 0.25 0.18 0.13 0.10 0.07

Year # transistors Clock (MHz) Area (mm2) Wiring levels

1995 12M 300 250 4-5

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2004 150M 800 430 6

2007 350M 1000 520 6-7

2010 800M 1100 620 7-8

V2M Camera (July 2005) V12M Camera (Feb2006) V59M Camera (Nov 2006) (1920x1080) (3375x3648) (7680x7680) In Development

Figure 1: High Definition Camera roadmap using high performance CMOS (Courtesy: Rockwell Scientific)


With the impetus of modernizing many devices, miniaturization is required for various applications- particularly for biochemical/biological discrimination and detection. Miniaturization of devices in the field of avionics has proliferated since the time of heightened securities and concern about terrorism. Platforms for IR sensor technologies range from the hand held ‘Raven’ to the next generation micro air vehicles (MAVS) which will be on the order of the size of birds (or even insects?). Uncooled IR sensor technology will be the driving force for these micro air vehicles and the next generation of unmanned air vehicles (UAVs) that provide surveillance and aerial reconnaissance capability. IR sensors on board the UAV (small or big) make UAV aware of the traffic, presence of structures and obstacles in the area so that intelligent decisions can be made accordingly (e.g. keeping personnel out of harm’s way). Thermal IR sensor cameras (that use no active light source to illuminate the target or the scene) will replace the current visible detectors in closed circuit television, thus enhancing the capabilities of security systems. These IR sensor adapted systems will enable observation from longer distances and through haze or fog without the need for visible light. IR sensor technology will also provide security at airports and harbor ports with the adaptability to biometrics of a person. Thermal IR cameras allow the users to have day and night continuous operation (24/7) through external environment (e.g. smoke, firefighting inside building, security and surveillance systems). This technology can be extended to include hyperspectral imaging sensors which are proving to be useful for passive standoff detection of chemical species. The implementation of cost and weight reduction features in IR sensor systems for the military is also resulting in benefits of these sensor systems for commercial usage. Optics plays an important part in the IR sensor system and the cost and weight of the optics determine the overall cost of the system. As the pixel sizes are reduced to improve the resolution of the detector array, the optics size can be decreased and reduction of the cost can result. Thus the total sensor package is significantly smaller and less expensive. And the trades of sensor performance versus cost will be essential to adapt the military technology for its application to civil defense systems. Some of the military technologies are slowly becoming commercial technologies also. Products that continue technology progression in performance and cost are in the micro-scale dimension regime and are driving nanotechnology development, which is essential to continue the miniaturization process and efficiency improvement.

2. IR Sensor Technologies

Although many commercially available IR cameras use uncooled focal plane arrays (FPAs) based on microbolometer technology, there are several other IR sensor technologies available to meet specific imaging applications. Many applications for military and domestic defense systems demand high speed and improved sensitivity in specific spectral ranges. This section presents some of the pertinent IR sensor developments. Table 2 presents a summary of the various sensor technologies along with advantages and issues associated with each one of them. Sensor technologies that evolved during the 20th century are now being upgraded for 21st century space and missile defense. All IR systems require a basic set of subsystems to perform the useful function of the seeker in missile defense and space satellites. IR imaging systems are valuable to US military forces in their respective missions—strategic or tactical. The overall goal of the IR sensor technology - for applications such as night vision, remote surveillance, mine detection, scout platforms, law enforcement, search and rescue, intelligent traffic monitoring, intrusion detection, space, ground and sea based defense - is to provide cost effective miniaturized systems with improved performance. Several government laboratories and industries are developing uncooled and cooled IR sensor technologies and are demonstrating their manufacturing capabilities via improved fabrication processes, materials, tooling and testing to support high volume production with economies of scale. Uncooled FPAs are usually manufactured on the combined military- commercial production line. And the trend has been to demonstrate the uncooled technology for dual use. The term “uncooled” is used for FPAs that operate at or near room temperature to distinguish from “cooled” FPAs that operate at cryogenic temperatures. Commercial products such as Raytheon’s MicrosightTM and MicrocamTM, and BAE system’s MicroIRTM handheld cameras are the examples of system integration of this technology. Uncooled IR sensors are devoid of some problems associated with cryogenically cooled IR sensors. Although the sensitivity performance of uncooled sensors is not quite the same as that of the cooled IR sensors, applications in specific spectral regions of space and ground based defense systems have resulted in their


Table 2: Summary of Detector Types Used in Sensor Systems

Detector Type Advantages Issues

Thermal Detectors: Semiconductor Bolometer VOx , a-Si Pyroelectric (BST) Extrinsic (III-V): Si:As Si:Sb Photon Detectors: II-VI: HgCdTe/CdZnTe HgCdTe/Si III-V: InGaAs/InP InGaAs/GaAs Type II SLS: InAs/InGaSb QWIP AlGaAs/GaAs QDIP InAs/InGaAs/InP InGaAs/GaAs

Room Temp. Operation Dual Purpose Use -(commercial/military) Very long wavelength Mature technology Cooled or uncooled Well developed theory Bandgap tunability Multicolor capability Monolithic, Hybrid Dual Purpose Use Advanced technology Good material Monolithic, Hybrid Dual Purpose Use Reduced Auger recombination rate Reduced Tunneling current Bandgap tunability Multicolor Capability Mature material Multicolor capability GaAs Substrate (low cost, high volume availability) Commercial applications Normal incidence Low thermal generation Potential variety of applications Small size, compact

Low D* at high Freq. High thermal constant Smaller pixel size Very low temp. Operation Low QE High thermal generation Non-uniformity over large area Very long wavelength Expensive Growth & process improving Long Wavelength Large lattice mismatch Design and growth complexity Interface sensitivity Passivation Low QE Complexity due to 450 incidence (grating involved) High thermal generation Array uniformity Complex design and growth issues

continued progress and their use in homeland security applications. To make use of these technologies in compact systems, costs associated with the optics components will need to be reduced. There are currently several efforts in developing low cost optics/lenses so that the cost of the total sensor package comprising the detector, readout/imaging electronics, and associated optics will be reduced substantially. Some of the factors that affect the design of the optics, depending on specific application, are Field of View (FOV), Resolution, Imaging sensor format and depth of field. As military requirements evolve, and technical capabilities improve, infrared sensor technology using integrated architectures holds promise in several fields.


3. Uncooled Infrared Detectors There are several types of uncooled Infrared detectors varying in the method of operation. There have been some excellent reviews written by Kruse [7] and Wood [8]. Lifetime and cost issues make these detectors very attractive. One of the attractive features of these detectors is the ability for monolithic integration and compatibility with CMOS technology. Figure 2 illustrates the IR uncooled thermal detector [9] whose key characteristic is the thermal conductance between the pixel and the substrate. In the thermal detector, absorption of the incident radiation changes the temperature of the material, which in turn changes the physical property of the material. The resultant change is then used to generate an electrical output. Thermal detectors only respond to the intensity of the absorbed radiant power and therefore the detector sensitivity has no dependence on wavelength. Thus responsivity curves for thermal detectors are flat, as a function of wavelengths. Currently Raytheon, DRS (previously Boeing), and BAE systems are the producers of compact microbolometer cameras.

In the uncooled microbolometer, the detector element is suspended on two legs from the CMOS readout integrated circuit (ROIC). These provide support and contact to the ROIC. The choice of detector material depends upon its Temperature Coefficient of Resistance (TCR) at room temperature. The resistance change is sensed in the ROIC. Thermal isolation of the detector is necessary and is usually provided by the support legs having low thermal conductivity. In order to obtain high signal to noise ratio, low noise electrical contact is necessary. This creates a dilemma of the requirement of a support structure possessing both low thermal conductivity and high electrical conductivity characteristics. Si3 N4 is used as the support structure due to its excellent properties. Vanadium Oxide (VOx) bridge material with a high TCR value is used in the MicroIRTM uncooled Microbolometer FPA technology developed by BAE systems [10]. VOx material characteristics such as resistivity changes as a function of temperature and optical properties depend on the fabrication compatibility and the stoichiometry of the material. With higher resistivity of the material, heating becomes a problem. The microbolometer structure is designed to minimize thermal conductivity coupling to the silicon readout electronics. Figure 3 shows a micro-bolometer unit cell structure, along with the photograph of a 640x480 FPA and some characteristics of the FPA. All FPAs have on-chip analog to digital conversion (14 bit digital output). BAE Systems has significantly improved their uncooled thermal imaging technology over three generations of development. Over a decade ago, Honeywell in Minneapolis, MN, developed novel micromachining techniques for silicon. Honeywell (BAE systems’ parent company) has licensed the microbolometer FPA technology to several companies. Technology improvements in multiplexing, thermal isolation, and implementation of smaller pixel sizes could enhance the performance for some of the commercial applications. BAE systems’ team comprising Norton et al. [11] have recently reported the advances in the uncooled imaging technology demonstrating excellent sensitivity in the MWIR (NEDT ~ 180mK, 3-5µm) and LWIR (NEDT ~ 15mK, 8-12µm) wavebands with pixel pitch of 28µm and use of the planar pixel structure. In these detectors, a pulse of current senses the resistance change of the VOx bridge material caused by its temperature change. Recent advances in micromachining fabrication process have led to the development and manufacturing of the microbolometers monolithically on a silicon ROIC using surface micromachining techniques. Raytheon’s team (comprising Radford and Murphy et al), has implemented several improvements into the microbolometer fabrication process and has reported [12,13] VOx 320x240 pixel arrays of 50µm square pixels. The average noise equivalent temperature difference, NEDT, in these arrays was demonstrated to be < 20mk (f#1 optics, LWIR). Since NEDT is inversely proportional to the pixel area, for pixel sizes of 25µm or lower, some improvements in readout electronics will be needed and maximizing both optical area and thermal isolation will be required to maintain high sensitivity.

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In order to maximize the optical absorption area, DRS’s team comprising Han et al [14] added a top layer, termed the “umbrella” layer, on a microbridge structure. Incorporation of this feature relaxes photolithographic requirements in the thermal isolation structures and greatly improves absorption efficiency over the 8-12 µm range (estimated 75%). The sensitivity of microbolometers is limited either by Johnson noise, 1/f, or readout noise, but the ultimate limit of sensitivity will be set by the noise limit due to thermodynamic fluctuations often referred to as “phonon noise” [15-17]. There is a significant challenge in fabrication and processing technology in order to lower the pixel sizes below 40-50µm. Raytheon, BAE systems and DRS Infrared Technologies have reported performances for 25-28 µm pixel sizes enabling the development of 640x480 FPA array [18-21]. Figure 4a shows a photograph of the multi-level 25 µm microbolometer pixels and Figure 4b shows a microbridge structure with an umbrella layer for enhanced absorption. Table 3 summarizes the design and performance parameters for the advanced 25µm pixels for two array configurations. Several application- specific ROIC designs have been demonstrated by these companies to improve the performance of these microbolometers. Figure 5 shows a dual field of view (WFOV and NFOV) imaging camera and excellent image quality obtained with 25µm 640x480 arrays. The electronics package incorporates a low-cost single stage thermoelectric cooler for temperature stabilization.

28 µm

FPA Performance Characteristics: Detector Fill Factor >70% Detector Spectral Response 7-14 µm F/# Compatibility 0.8 to 4.0 IR absorption 80% NEDT (f/0.8, 60Hz) < 50mK Array Format 640x480 Pixels Detector Time constant 4-15ms typical Pixel Pitch 28 µm

Figure 3: Micro-bolometer Unit Cell Structure Sketch (BAE Systems)

Figure 4a: SEM Photograph of the multi-level 25µm Figure 4b: SEM Photograph of a microbridge structure Microbolometer pixels (Raytheon Vision Systems) with Umbrella layer (DRS Infrared Technologies)


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There are several efforts worldwide, continuing work in the uncooled infrared detector technology. Other examples of TCR material are amorphous silicon (α-Si), semiconducting YBCO, thin film ferroelectrics, etc. The Laboratoire d’Electronique de Technologie at d’Instrumentaion (LETI) in France has developed microbolometer technology based on amorphous silicon (α-Si) and this technology is now covered by a license agreement with Sofradir [22]. Mottin et al [23] have presented their results on surface micromachined microbolometer 320x240 FPAs with 35µm pitch, as NEDT ~35mK (f#1 optics) using Si CMOS integrated circuit (IC) compatible material α-Si.. In this technology, the bias is continuous rather than pulsed. (Pulsed bias reduces joule heating). The ‘amorphous silicon’ approach has the benefit of having a low thermal constant that improves image quality. Yon et al., [24] have recently reported their achievement of a 25µm 320x240 IRFPA with NETD of 63mK and 7 ms time constant. Degradation of α-silicon over time when exposed to light can be minimized by using multiple junction structures and controlled layer thickness (Staebler-Wronski effect). Camera packages using these IRFPA sensors include lenses such as Ge, ZnSe, GeZnSe or chalcogenide glass [25]. There is ongoing effort at improving the designs of thermal detectors using microelectromechanical (MEMs) technology. Next generation of detectors may use new materials and new concepts/architectures. New materials with high responsivity in the infrared will achieve high absorption in thin layers. One such example is that of the uncooled microcantilever thermal detector that is being developed at Oak Ridge National Laboratory by J.L. Corbeil et al [26]. Several of the important issues such as thermal isolation and non-uniformity of these arrays are being researched; design/geometry considerations are aimed at avoiding the use of thermoelectric coolers to stabilize the detector temperature and eliminating the ambient temperature fluctuations. In addition to MEMs technology, as microelectronics and nanoelectronics technologies play a larger role, low cost and high quality thermal imagers in two dimensional arrays may have a better future. Although uncooled IR detectors exhibit reduced sensitivity compared to cooled IR detectors, they will have many foreseeable applications in the civil, domestic arena. Applications could range from transportation

Table 3: Summary of performance characteristics for Raytheon microbolometer FPAs (320x240 and 640x480)

Figure 5: Dual field-of-view imaging camera and image quality obtained with this 25µm 640x480 array


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(night vision for cars, boats and aircraft, security and firefighting) to medicine. Proliferation of uncooled microbolometer technology is forthcoming with anticipation that the quantities could approach several million a year around 2010. Technology doesn’t stand still and users’ need continues to grow. Murphy et al. [27] have recently reported success in the development of a 640x512 array with 20µm pixel sizes to achieve the ultimate performance near the temperature-fluctuation-limited NETD (<20mk, f#1, 30Hz). In recent years, there is significant attention focused on the development of MEMs tunable optical filters based on Fabry-Perot Interferometers (FPI). These can be integrated with the imaging focal plane array (cooled or uncooled as an adaptive device) as hyperspectral sensors. Research is ongoing in the areas of bolometer and pyroelectric detector technology [28]. Currently, one of the applications of pyroelectric technology (vidicon) is being widely used by firefighters. In pyroelectric detectors, when the detector absorbs radiation, its temperature changes, which in turn, results in a change in its surface charge. The change in polarization basically results from the change in temperature and this temperature coefficient is called the pyroelectric coefficient. .A suitable material will have a large pyroelectric coefficient, a high resistivity, a small dielectric constant and a low thermal capacity. The thermal conductance G must be made as small as possible by design. Two-dimensional pyroelectric arrays hybridized to silicon ROICs has been researched by Hanson et al [29]. 320x240 hybrid arrays with 48.5µm pixel sizes have been demonstrated with NEDT of 40mK [30]. The sensor is of barium strontium titanate ferroelectric material. Research is in progress to drive the temperature of operation of HgCdTe into uncooled regime, using the principle of the so called ‘High Operating Temperature (HOT) detector’. This concept was first proposed by Ashley and Elliott [31]. In such detectors, thermal generation and recombination in HgCdTe at near room temperature will be determined by Auger mechanisms and from Shockly-Reed (S-R) centers [9]. Figure 6 shows the associated dark current for the state of the art HOT detectors for both LWIR and MWIR HgCdTe. With an arsenic doping concentration of 5x1014 cm -3 it is anticipated that the background-limited photodetector (BLIP) operation can be achieved at close to room temperature [9]. Kinch of DRS has shown a comparison of NEDT of uncooled thermal and photon detectors operating at 290K, for the LWIR and MWIR bands in Figure 7. The HOT detector is assumed to be Auger-limited (p-type) with doping concentration of 5x1014 cm -3 as mentioned above. Further optimization of the device structures, growth parameters and device processing are expected to result in improved performance. Piotrowski [32] has pointed out that the thermoelectrically cooled and optically immersed MWIR photodetectors closely approach the BLIP performance limit. For these devices to operate at room temperatures and exhibit higher detectivity values, 1/f noise will need to be reduced [33]. Such detectors can potentially become competitive with thermal detectors operating at or near room temperature.

Other material systems used for intrinsic photodetectors include binary alloys (e.g. InSb), ternary narrow-gap HgCdTe, PbSnTe and other tunable bandgap semiconductors and superlattices (Type II, Type III). The performance of these detector material systems operating at near room temperature has been progressing at a slower pace.

Figure 6: Dark Current Vs. temperature for MWIR and LWIR HgCdTe with Na = 5x1014 /cm3


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4. Cooled Infrared Detectors

In photon detectors, the absorption of infrared radiation depends on the semiconductor material used with the number of photons per second per watt being directly proportional to wavelength. Since they utilize direct transition of electrons, they have high response speeds compared with thermal detectors. Thus they find various applications where high response speed detector system is needed. Photon detectors have been fabricated in various material systems such as doped Si, II-VI compounds, III-V compounds, and quantum well and superlattice structures. These detectors commonly operate at cryogenic temperatures in order to reduce the thermally generated dark current. To date, the II-VI compound HgCdTe material system has demonstrated extreme flexibility of operation in a wide range of the IR spectrum both in the photoconductive and photovoltaic modes of operation. Also, in this material system, single color and multicolor devices can be fabricated with ease. Device operation as avalanche photodiodes (APDs), HOT detectors, and Heterojunction photovoltaic devices has already been demonstrated. For all these reasons, it still remains as the most thoroughly researched detector material. HgCdTe FPAs have been mostly used for military systems; however, recent advances in fabrication have led to the commercial availability of HgCdTe –based IR imaging systems for limited use. This technology is also finding use in many scientific applications requiring operation in the wavelength ranging of SWIR to VLWIR. Figure 8 illustrates two hybridization schemes for HgCdTe IRFPA. In the indium bump hybridization technique for backside illuminated FPAs, photons pass through the transparent detector substrate, whereas in the frontside illuminated loophole (vias) technique photons pass through the transparent Si multiplexer (Si ROIC). Direct bandgap HgCdTe IR imaging sensor systems offer many advantages over other sensors in applications that require imaging in the LWIR spectral band. These applications include spectroscopy, signature analysis and phenomenology. For third generation of IR sensor systems, where extending the imaging range is critical, research in improving the sensor performance in the LWIR spectral region is continuing at several industries and government laboratories. Because of the high responsivity and detectivity achievable in the HgCdTe material system, most current

Figure 7: Theoretical NEDT comparison of Uncooled thermal and HgCdTe uncooled photon detectors for LWIR and MWIR


state of the art detectors are those fabricated in this material. The current status of HgCdTe technology is summarized in a recent paper [35]. The f# optics requirement of an IR system is determined by the sensitivity of the detector, and for a thermal detector (microbolomer or pyroelectric) an extremely fast lens design is needed (f#1 optics), due to its low sensitivity. Because of their high sensitivity, HgCdTe detectors do not need so fast a lense.

5. New Class of Detector Materials For utilizing IR detectors in a compact system, the most important parameter for the selection of a particular semiconductor material is its wavelength-dependent optical absorption coefficient. This coefficient indicates the depth of radiation absorption needed to achieve the highest sensitivity. Direct bandgap materials (such as compound semiconductors) exhibit higher absorption coefficient than indirect bandgap materials such as Silicon. With higher absorption coefficient in direct bandgap materials, higher quantum efficiency is achievable with thinner absorption regions, enhancing the ability to realize high-speed photodiodes. However innovative solutions are being explored to achieve high speed and high sensitivity even in indirect bandgap materials. An example of this is doped silicon. Because of the use of extrinsic silicon detectors (λ >20µm) in several of the space (exoatmospheric) astronomy applications, continuous performance improvement is being sought in this technology for many applications. The spectral range of these indirect bandgap photodetectors is determined by the type of dopant and base material in which it

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is introduced. Large format VLWIR FPAs based on doped silicon (Si:As) Blocked Impurity Band (BIB) detectors hybridized with indium bump bonding to a matching Si CMOS ROIC have been developed at DRS (formerly Rockwell International) [36]. This technology has been demonstrated in various flight experiments (e.g. SPIRIT II, Midcourse Space Experiment (MSX/SPIRIT III), infrared space observatory (ISO), Wide-Field infrared Experiment (WIRE), and SIRTF and the first integrated flight test of the Ground Based Interceptor). Antimony -doped silicon (Si:Sb) BIB arrays having spectral response greater than 40µm have also been demonstrated [36]. Figure 9 illustrates the structure of a BIB detector. Because of the presence of the blocking layer, the behavior of the BIB detectors is closer to that of a reverse biased photodiode, with the exception that photoexcitation of electrons occurs from the donor impurity band to the conduction band (not the intrinsic bandgap). Thermal excitation of electrons across the narrow bandgap leads to dark current and thus limits the detector operation to temperatures lower (< 13K) than the normal cryogenic operation temperature of 77 K achievable with HgCdTe.

Another alternative to direct bandgap detectors is the intersubband transitions in quantum-confined heterostructures using III-V technology. The advantage of this quantum well infrared photodetector (QWIP) approach is the availability of a mature III-V fabrication technology and multispectral capability. These heterostructures have a narrow absorption spectrum that can be tuned (3 to 20µm) by varying the quantum well width and the barrier layer compositions. Several research efforts are continuing in this technology for single and dual color detection using the GaAs/AlGaAs material system. In the III-V compound semiconductors, there is a large range of material combinations available allowing the tailoring of band-gaps through stacking of thin layers. Fabrication of two-layer InGaAs/GaAs/AlGaAs quantum well stacks has been demonstrated recently [37]. Low cost, large format GaAs substrate-based QWIP technology is also being developed by Gunapala et al for NASA applications [38]. The draw back of the QWIP technology is that the selection rules prevent normal incidence detection resulting in shorter carrier lifetimes. The detectivity and quantum efficiency of these devices are low at room temperature and therefore need operation at cryogenic temperatures to minimize dark current effects. A new detector structure for normal incidence light coupling – corrugated QWIP (C-QWIP) - has been extensively researched at the Army Research Laboratory (ARL) [39]. Due to some limitations of applicability of HgCdTe, especially in the longer wavelengths, intensive efforts are ongoing on the use of type II (III-V materials) and III (II-VI material) superlattices to replace HgCdTe. Razeghi and her group at Northwestern University have demonstrated 256x256 FPAs with a pitch of 30µm and cutoff wavelength of 8µm at 77K for type II strained-layer superlattice (InAs/GaSb) hybridized to Si CMOS ROICs. Peak NEDT obtained was 18mK [40]. Although this technology results in reduced Auger generation rate, epitaxial growth technique for Sb-based devices is

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not very mature and surface passivation is a very big show stopper. Device processing improvements are being researched to minimize the surface passivation problems. Newest in the competition for infrared photodetector is the normal incidence quantum-dot photodetectors (QDIPs) in semiconductors such as InGaAs/InP, InGaAs/GaAs or SiGe/Si. These devices are expected to perform well at elevated temperatures due to their 3-dimensional carrier confinement characteristics [41]. In these low dimensional structures (LDS), the movement of charge carriers is constrained by potential barriers. QDIPs are inherently sensitive to normal incidence IR radiation. Dark current also is much lower than that of QWIPs. This technology will be useful in remote sensing and chemical and biological detection application. Some researchers use long wavelength infrared (LWIR) QDIPs for spectroscopic applications. Several universities are engaged in improving the QDIP technology for its use in single color and multispectral imaging camera systems, combining electronically tunable QDIPs with signal processing strategies [42]. In a recent paper Bhattacharya et al [43] presented several heterostructure designs for obtaining improved responsitvity in In(Ga)As/Ga(Al)As mid and far infrared QDIPs. Figures 10a,b,c illustrate the schematic of band structure, devices heterostructure and spectral responses for Mid, Long and far infrared wavelengths. It is noted that AlGaAs barriers when appropriately inserted in these heterostructures, can perform wonders to the design and performance improvement of the QDIPs. A variety of issues remains to be addressed in order to make these nano-devices cost effective. Application of these devices spurring from night vision to environmental monitoring is generating a lot of interest around the world.

6. Summary and Conclusions There is greatly improved performance available in the IR sensor technology. The choice of detection material and device architectures has expanded the use in many defense and civil applications. The technology advances are exhibiting potential to field infrared sensors and associated systems in a layered defense. Several sensor types will be available for surveillance/space situation awareness suites and homeland security systems. As some specific sensor technology delivers unexpectedly improved results, new applications emerge and the process for improvements continues. Often no one solution/technology is right for all applications. Also, the next generation of researchers may find new ways to improve the old technology. Each technology will find and maintain its own market niche in its intended application.

Figure 10 a): Schematic of 70-layer InGaAs/GaAs MWIR QDIP conduction band profile, device heterostructure, and Spectral response


O.2um n GaAs contact —80K p

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IR sensor technology has influenced our everyday life and many opportunities will revolve around finding new ways and innovative uses of IR sensor technology not only for military but also for civil applications. Homeland security is the emerging market for dual purpose driven IR sensors. The market potential of portable IR sensor camera systems will be driven by the desire for reduced system cost, size and mass concurrently improving the performance. 3-D stacking of signal processing application-specific ICs on to IRFPAs will reduce the overall system weight, volume and power [44]. And still more progress lies ahead as the detector technologies move forward. Wide variety of applications and numerous end uses for uncooled IR camera is foreseen. The uncooled IR FPAs will require improvements in order to reduce thermal time constant and build large format arrays. Signal and image processing in the camera will provide a significant performance improvement. Image fusion technology will be required for security systems. Hyperspectral and polarimetric imaging will complement uncooled IR sensors [45]. Passive and active sensors can be combined to benefit future systems. Antenna-coupled IR sensor technology may find applications requiring a fast uncooled response and polarization tuning capability [46].

7. Acknowledgements The author would like to thank Bill Radford, Dan Murphy, Steve Anderson, and Scott Johnson of Raytheon Vision Systems, Peter Norton of BAE Systems in Lexington, Massachusetts, Jeffery L. Johnson of Rockwell Scientific Company, Jim Robinson of DRS Infrared Technologies, H. Vydyanath of Avyd Devices, Ashok Sood of Magnolia Optical, M. Razeghi of Northwestern University, and Gene Ezell of Teledyne Solutions for their assistance in providing information for this review and for many valuable technical discussions.

Figure 10 b): Schematic of 30-layer InGaAs/GaAs LWIR QDIP heterostructure and Spectral response as a function of bias at 80K, 150K, and 200K

Figure 10 c): Schematic of the conduction band profile of InGaAs/GaAs tunneling QDIP (T-QDIP) using resonant AlGaAs double barriers, device heterostructure and Spectral response as a function of wavelength at different temperatures.


References 1) Lawrence Livermore laboratory

2) www.radiancetech.com

3) W. Cao et al., “ 1024x1024 HgCdTe CMOS camera for infrared imaging magnetograph of Big Bear Solar Observatory”, SPIE, vol. 5881 (2005)

4) A.K. Mainzer. et al, “A new large-well 1024x1024 Si:As detector for the mid-infrared”, SPIE, vol. 5881 (2005)

5) Semiconductor Industry Association, National Technology Roadmap for Semiconductors, 1994

6) Tom Chuh, “FPA Technology Advancements at Rockwell Scientific,” Proc. of SPIE vol. 5783, 907-922 (2005)

7) P.W.Kruse, “Principles of Uncooled Infrared Focal Plane Arrays,” Uncooled Infrared Imaging Arrays and Systems, in Semiconductors and Semimetals, P.W.Kruse and D.D. Skatrud, Editors, Vol 47 chap.2, Academic Press, (1997)

8) R.A.Wood, “Monolithic Silicon microbolometer arrays,” Uncooled Infrared Imaging Arrays and Systems, in Semiconductors and Semimetals, P.W.Kruse and D.D. Skatrud, Editors, Vol 47 pp. 43-121, Academic Press, (1997)

9) M.A.Kinch, “Fundamental Physics of Infrared Detector Materials,” Journal of Elec.Mat., Vol 29, No.6, (2000).

10) S.A.Jamison, “Use of IR Microbolometer Systems for Border Security,” SPIE conference on Homeland Security, Washington DC, Dec.4, 2003

11) Peter W. Norton and Margaret Kohin, “Technology and Application Advancements of Uncooled Imagers,” Proc. SPIE, Vol 5783 (2004), Brian Backer, “High Resolution Uncooled Imaging –PMC300TM “ Proc. SPIE Vol. 5778, 2005 12) W.Radford et al., “Microbolometer Uncooled Infrared Camera with 20 mK NETD,” Proc. SPIE national meeting in San Diego, CA (1998) 13) W. Radford et al., “Sensitivity Improvements in Uncooled microbolometer FPAs,” Proc. SPIE 3698, 119-130 (1999) 14) C.J.Han et al., “320x240 and 640x480 UFPA for TWS and DVE Applications,” Proc. SPIE 5783, 559-565 (2005) 15) Paul Kruse, “Can the 300K radiating background noise limit be attained by uncooled thermal imagers,” Proc. of SPIE 5405, (2004) 16) Margaret Kohin and Neal Butler, “Performance Limits of Uncooled VOx Microbolometer Focal Plane Arrays,” Proc.of SPIE Vol 5406-48 (2004) 17) C.M. Hanson, “Barriers to background-limited performance for uncooled IR Sensors,” Proc. SPIE 5406, 49 (2004) 18)D.Murphy et al., “Resolution and Sensitivity Improvements for VOx Microbolometer FPAs,” 2003 SPIE Aerosense Conference in Orlando, Florida. 19) D.Murphy et al., Proc. SPIE 4369 20) M.N.Gurnee et al., “Developments in uncooled IR technology at BAe systems,” Proc. SPIE 4369 (2001) 287-296. 21) C.J.Han et al., “320x240 and 640x480 UFPA for TWS and DVE Applications,” Proc. SPIE 5783, 559-565 (2005) 22) Opto &Laser Europe, May 2001 23) E. Mottin et al., “Enhanced amorphous silicon technology for 320x240 microbolometer arrays with a pitch of 35µm,” Proc.of SPIE 4369, 250-256 (2001) 24) J.J.Yon et al., “First demonstration of 25µm pitch uncooled amorphous silicon microbolometer IRFPA at LETI-LIR,” Proc. of SPIE Vol 5783 (2005)


25) www.cedip-infrared.com 26) J.L.Corbeil, N.V.Lavrikand S.Rajic, P.G. Datskos, “Self-leveling uncooled microcantilever thermal detector,” Appl.Phys.Lett.,Vol 81, No. 7, 1306-1308, (2002) 27) D. Murphy et al., “Extended Applications for High performance VOx Microbolometer FPAs,” Proc. SPIE 5783,49 (2005) 28) R.E.Flannery, J.E.Miller, “Status of uncooled infrared imagers,” Proc. of SPIE 1689 pp 379-395 (1992) 29) C.M.Hanson, “Hybrid pyroelectric-ferroelectric bolometer arrays,” Uncooled Infrared Imaging Arrays and Systems, in Semiconductors and Semimetals, P.W.Kruse and D.D. Skatrud, Editors, Vol 47 pp. 123-174, Academic Press, (1997) 30) C.M.Hanson, “Uncooled thermal imaging at Texas Instruments,” Proc. of SPIE 2020, 330-339 (2002). 31) T.Ashley and C.T.Elliott, Electronics Letters, 21, 451 (1985) 32) J. Piotrowski, Photonics Spectra, May 2004, J.Piotrowski and A. Rogalski, “New generation of infrared photodetectors,” Sensors and Actuators, A67, 146-152 (1998) 33) C.T.Elliott et al., J.Electr.Mater. 26, 643, (1997) 34) R.Rawe et al., “Novel high fill-factor, small pitch, reticulated InSb IR FPA design,” Proc. SPIE 5783, 899-906 (2005) 35) L.Becker, “Current and Future trends in infrared focal plane array technology,” proc. SPIE 5881, 06, (2005) 36) M.G.Stapelbroek et al., “Silicon for Visible-to-VLWIR Photon Detection,” Proc. SPIE 5074, 166-172 (2003) 37) Image shows prototype of Dual-color IR camera at SPIE 2005 Defense and Security symposium (Vision Systems Design, June 1, 2005) 38) S.D.Gunapala et al., “MWIR and LWIR megapixel QWIP focal plane arrays,” Proc. of SPIE Vol 5563-25 (2004), S.D.Gunapala et al., “1024x1024 pixel MWIR and LWIR QWIP Focal Plane Arrays and 320x256 MWIR:LWIR Pixel Colocated Simultaneous Dualband QWIP Focal plane Arrays,” Proc.of SPIE vol 5783 (2005) 39) K.K.Choi et al., “Performance of corrugated quantum well infrared photodetectors,” Proc. SPIE 3379, 441-452, (1998). 40) M.Razeghi, “High performance Type II InAs/GaSb superlattices for mid,long and very long wavelength infrared focal plane arrays,” Proc. SPIE 5783 (2005) 41) P.Bhattacharya, et al., Applied Physics Letters, Vol 86, 191106, May 2005 42) Unal Sakoglu et al., “Spectrally adaptive infrared photodetectors with bias-tunable quantum dots,” J.Opt. Soc. Am. B, Vol 21, No.1, January 2004 43) P.Bhattacharya, Extended abstracts 2005 US workshop of the Physics and Chemistry of II-VI materials 44) www.irvine-sensors.com 45) R.W.McMillan et al., “IR polarimetry and far IR imaging,” Proc. of SPIE vol. 5563, 111-117 (2004) 46) Pashang Esfandiari et al., “Tunable Antenna-Coupled metal-oxide-metal (MOM) uncooled IR detector,” Proc. SPIE 5783, 470-482 (2005)


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Influence of IR Sensor Technology on the Military and Civil Defense