Infrared technology is critical for military and security applications, as well as increasingly being used in many commercial products such as medical diagnostics, drivers’ enhanced vision, machine vision and a multitude of other applications, including consumer products. They are used for thermal efficiency analysis, environmental monitoring, industrial facility inspections, remote temperature sensing, short-ranged wireless communication, spectroscopy, and weather forecasting.
Infrared Cameras consist of several key elements such as Lens, Detector (Focal Plane Array), electronics for image processing and display for image presentation or video signal out. The key enablers of such infrared products are the detector materials and designs used to fabricate focal plane arrays (FPAs).
The word “infrared” refers to a broad portion of the electromagnetic spectrum that spans a wavelength range from 1.0 um to beyond 30 um everything between visible light and micro‐ wave radiation. Much of the infrared spectrum is not useful for ground- or sea-based imaging because the radiation is blocked by the atmosphere. The remaining portions of the spectrum are often called “atmospheric transmission windows,” and define the infrared bands that are usable on Earth.
The IR portion of spectrum is loosely segmented into frequency bands corresponding to “atmospheric transmission windows,” that are usable on earth, namely near infrared (NIR, 0.8-1.1um), short wave infrared (SWIR, 0.9-2.5um), mid wave infrared (MWIR, 3-5um), long wave infrared (LWIR, 8-14um), very long wave infrared (VLWIR, 12- 25um) and far infrared (FIR, > 25um).
The MWIR- LWIR wavebands are important for the imaging of objects that emit thermal radiation, while the NIR-SWIR bands are good for imaging scenes that reflect light, similar to visible light. Therefore NIR and SWIR imaging systems typically require some type of artificial illumination like sun‐ light, moonlight, starlight, and an atmospheric phenomenon called “nightglow.” While imaging system that operates in the MWIR and LWIR ranges can be completely passive, requiring no external illumination.
The infrared (IR) wavelengths are an important focus of military and defense research: it enables high-resolution vision and identification, target acquisition, surveillance, homing and tracking in near and total darkness. They are also useful to security personnel in Border Security to detect track and identify individuals and to detect contraband and hidden objects.
The trend in Military Infrared Imaging Systems is towards longer operating ranges, with larger arrays for increased resolution and standoff. DARPA has funded several new approaches for detector development using carbon nanotube, graphene, nanoparticles and other nanomaterials. These researches demonstrate high potential for future detector technologies that could be very beneficial for both, military and commercial sectors.
IR technology is following Moore’s law to a certain extent. The result is, and will continue to be, imaging systems that are smaller, less expensive, consume less power, are easier to use and offer. Resolution has reduced above the range of QCIF (Quarter Common Intermediate Format), typically 200 × 150, and sensitivity (noise-equivalent temperature difference) is now more in the order of 0.1 K when it used to be more than 0.2 K.
Smartphones with built in Thermal Imaging Camera
2014 saw a game-changing realization for the IR imaging market; thermal imaging sensors and systems that were extremely expensive tool used only by law enforcement are now built right into smartphones and now accessible to anyone.
Flir Systems Inc., introduced the first thermal IR camera to be plugged into an iPhone 5S. By the end of the year, Seek Thermal followed up with its own thermal camera for iOS and Android smartphones. Seek’s thermal camera is used for applications related to home improvement, personal safety, hunting and boating, and first response equipment.
“Thermal imaging will soon become ubiquitous and part of our everyday lives, like the adoption of PCs, cellphones and GPS before it.” That’s the prediction from Tim Fitzgibbons, consumer electronics pioneer and co-founder of Seek Thermal Inc., a startup IR specialist company that has just launched the latest thermal camera for smartphones.
IR detectors and FPA
IR detectors can be categorized as being either a quantum or thermal device. In a quantum or photon detector, electromagnetic radiation absorbed in a semiconductor material generates electron hole pairs (EHP), which are sensed by an electronic readout circuit (ROIC). In a thermal detector, on the other hand, the incident IR photons are absorbed by a thermally isolated detector element, resulting in an increase in the temperature of the element. The temperature is sensed by monitoring an electrical parameter such as resistivity or capacitance.
A Focal Plane Array (FPA) is a sensor with a two-dimensional array of detector pixel matrix, mated with a silicon readout integrated circuit (ROIC). i.e. for infra-red light, positioned in the focal plane of an optical system. The array of detectors converts photons into electrical signals measured by the silicon CMOS ROIC and read out the image from the array to a receiver.
The quantum or photon detectors show a selective wavelength dependence of response per unit incident radiation power. They exhibit both a good signal−to−noise performance and a very fast response. But to achieve this, the photon IR detectors require cryogenic cooling and are also called cooled detectors. This is necessary to prevent the thermal generation of charge carriers.
In a thermal detector, on the other hand, the incident IR photons are absorbed by a thermally isolated detector element, resulting in an increase in the temperature of the element. The temperature is sensed by monitoring an electrical parameter such as resistivity or capacitance. Thermal detectors require a temperature change to produce a signal and do not generally need cooling, hence also called un cooled detectors.
Cooled thermal imaging cameras are normally costlier than the un-cooled ones and are primarily used in defense applications for their sharp thermal contrast and effectiveness in long distance imaging for man-sized targets. “Extremely fast optics are required for uncooled detector technologies, which leads to unmanageable optics size and weight for longer-range applications,” said Mark Goodnough, chief scientist at Lockheed Martin Corporation’s Santa Barbara Focal plane facility.
Cooled FPAs, Cameras and Systems
Hg1-xCdxTe (MCT) is the most widely used infrared (IR) detector material in military applications, since it has high quantum efficiency due to its direct band gap and ability to tune the band gap of MCT enables IR detectors to operate in the wavelength bands ranging from SWIR to VLWIR (0.7-30 microns).
Heavier cooled systems are used in tanks and helicopters for targeting and in base outpost surveillance and high-altitude reconnaissance from aircraft. “Because of the higher operating temperatures of MCT, we can reduce the size, weight and power of systems in helicopters and aircraft,” says Scholten.
For SWIR imaging, InGaAs is one of the widely used detector materials due to its low dark current. SiGe is another example of material that can be used for the fabrication of SWIR detectors. The primary motivation for SiGe SWIR FPA development is the CMOS-like fabrication allowing for very low cost technology
In the SWIR and MWIR (1 to 5 microns wavelength), state-of-the-art devices include monolithic arrays of PtSi, and hybrid arrays of InSb and HgCdTe. Platinum silicide arrays are now commercially available with 640 by 480 elements, arranged on 24-micron centers; devices with up to 1040 by 1040 elements on 17-micron centers have also be reported. Indium antimonide arrays with over 1 million elements, in a 1024 by 1024 format on 27-micron centers, and HgCdTe arrays, with 1024 by 1024 pixels, are now undergoing laboratory testing.
In the LWIR (8 to 12 microns) and beyond, detector types include long-wave mercury cadmium telluride and extrinsic silicon. Mercury cadmium telluride devices are available in 256 by 256 format on approximately 40-micron centers, with up to 640 by 480 devices under research. Extrinsic silicon devices that respond in the wavelength range 2 to 28 microns will soon be available in a 320 by 240 format, on 50-micron centers.
Researchers are also exploring a wide range of group III and V detector materials for mid- and long-wave solutions have come up with nBn (n-type absorber, Barrier, n-type contact) and strained layer superlattice detectors, which provide higher operating temperatures with multifunction capabilities that yield smaller pixels with larger formats.
3rd-generation MW/LWIR sensor engine for advanced tactical systems developed at Raytheon Vision Systems
Raytheon Vision Systems (RVS) has developed and demonstrated the first-ever 1280 x 720 pixel dual-band MW/LWIR focal plane arrays (FPA) to support 3rd-Generation tactical IR systems under the U.S. Army’s Dual-Band FPA Manufacturing (DBFM) program. The MW/LWIR detector arrays are fabricated from MBE-grown HgCdTe triple-layer heterojunction (TLHJ) wafers.
Compared to single-band sensors, the capability to detect scene radiance in both the MWIR and LWIR spectral bands offers important advantages over a wide range of weather conditions, and in the presence of battlefield obscurants, and/or active infrared countermeasures. Coupling these dual-band detector arrays with wide-area-search sensor modes and dual-band detection algorithms presently under development should provide future combat systems with on-the-move aided target detection and cuing against cluttered IR backgrounds. Two additional key characteristics of 3rd-Generation FPAs are large array format and small pixel size.
While the LWIR is expected to play a dominant role in target detection mode, the need to identify potential targets at extended ranges emphasizes the importance of MWIR capability. Compared to LWIR, the shorter wavelengths of MWIR radiation equate to reduced optics diffraction, which in turn means that higher optical F/# numbers can be used effectively. MWIR imaging with high magnification optics at F/6 and higher has proven effective for identifying target at very long ranges. Effective use of dual-band MW/LWIR FPAs therefore demands that they be capable of operating in multiple dual- and single-band modes and are coupled with multi-focal-length optics.
Thus, a key attribute of the third-generation sensor vision is the incorporation of an in-dewar mechanism to permit the cold-aperture to be varied in size between two (or more) pre-determined settings while maintaining near-100% shielding efficiency, as the external system optics F/# is varied to optimize performance in each mod
The RVS dual-band FPA architecture provides highly simultaneous temporal detection in the MWIR and LWIR bands using time-division multiplexed integration (TDMI) incorporated into the readout integrated circuit (ROIC). The TDMI ROIC incorporates a high degree of integration and output flexibility, and supports both dual-band and single-band full-frame operating modes, as well as high-speed LWIR “window” operation at 480 Hz frame rate.
Raytheon Vision Systems
Raytheon Vision Systems in Goleta, Calif., is working on the next generation of E/O-IR detectors with the development of advanced discriminating focal plane array (FPA) technologies in the 3-to-5-micron mid-wavelength infrared (MWIR) and the 8-to-12-micron long-wavelength infrared (LWIR) spectral regions by using a variety of semiconductor materials and FPA architectures.
“These include advanced III-V1 semiconductor strained-layer-superlattice (SLS) FPAs, II-VI semiconductor mercury cadmium telluride (HgCdTe) dual-band FPAs and uncooled bolometer FPAs using micro-electromechanical systems (MEMS). Each of these FPA technologies address different mission requirements and are being developed to not only increase sensor capability and performance, but also to provide improved manufacturability and reduce costs,” Raytheon officials reported in the January 2014 issue of the company’s Technology Today magazine.
“MWIR FPAs with higher operating temperatures are advantageous for reducing the SWaP of cooled systems. For this reason, high operating temperature FPAs are being developed based on molecular beam epitaxy-grown indium arsenide/gallium antimonide (InAs/GaSb) SLS bandgap-engineered barrier device structures. The goal is to develop materials that have the producibility advantage of traditional III-V indium antimonide technology, but the operating temperature advantage of HgCdTe… Together, these factors offer the promise of improved performance, higher yield, and reduced cost for FPAs fabricated on SLS material.”
Uncooled Focal Plane Arrays (FPAs), Cameras and Systems
For now, next-generation systems for defense are moving to 17-μm pitch. The Reconnaissance, Surveillance and Target Acquisition (RSTA) group at DRS Technologies (Dallas, Texas, U.S.A.) has developed a VOx uncooled focalplane array (UFPA) consisting of 17-μm pixel-pitch detectors measuring 1,024 × 768. The imager, called U8000, was developed for the Army for use in next-generation military systems such as thermal weapon sights, digitally fused enhanced night-vision goggles, driver’s vision enhancers and unmanned aerial systems.
The 17-μm-pixel-pitch UFPA provides sensor systems with size, weight and power (SWaP) savings as well as cost advantages over existing devices.The microbolometer sensor used in the U8000 is a key enabling technology. Sensitive to the LWIR range between 7 to 14 μm,
Sensors Unlimited of Princeton, New Jersey began to produce small, uncooled cameras using a new semiconductor material, indium gallium arsenide (InGaAs). This remarkable material has peak sensitivity in the SWIR band, from 900 to 1700 nm.
DARPA program, LCTI-M addresses the development of an advanced low cost room temperature IR cameras based upon cell phone CMOS camera technology, where the imaging sensor, optics and electronics are fabricated at the wafer level.
The cost of thermal cameras is one of the key factors limiting the availability of high performance IR imagers at consumer level. Further, current form-factors are unacceptable for new applications in smaller handheld devices (such as PDAs) and glasses similar to GoogleGlass.
Availability of very low cost and small form-factor IR cameras will enable a variety ofapplications such as fire-fighting, security, medical and gaming industry.
Under the DARPA-funded DUDE (Dual-Mode Detector Ensemble) program, DRS and Goodrich/Sensors Unlimited are co developing an integrated two-color image system by combining a VOx microbolometer (for 8 to 14 μm) and InGaAs (0.7 to 1.6 μm) detectors into a single focal plane array.
Other two-color work at DRS includes the distributed aperture infrared countermeasure system. “The small system uses a two-color sensor to detect and track a missile launch while directing a laser to defeat it,” says Mike Scholten, vice president of sensors at DRS’s RSTA group.
HRL will research wafer scale infrared detectors for DARPA
The Defense Advanced Research Projects Agency (DARPA) has awarded HRL Laboratories, LLC, funding to research novel ways to synthesize semiconductors for sensing in the infrared spectrum, and methodologies to cost effectively integrate the infrared materials with silicon read-out integrated circuits (ROIC). Unlike high resolution, visible light cameras that can be manufactured quickly and inexpensively, making miniature infrared cameras and sensors is both time-consuming and costly at present.
According to Senior Scientist Dr. Rajesh Rajavel, who manages HRL’s infrared detector team, wafer-level direct integration of the infrared detector material with ROIC should result in a drastic reduction in the cost of infrared imaging focal plane arrays. “Time-consuming, conventional die-level serial processes will be replaced with streamlined wafer-level processes,” he said.
The DARPA Wafer Scale Infrared Detectors (WIRED) program seeks to drastically reduce the cost of infrared focal plane arrays. “Our research and implementation of innovative methodologies will draw on HRL’s core competencies in infrared sensing, semiconductor electronics, and heterogeneous integration,” said Rajavel.
DARPA’s WIRED program to develop affordable wafer-scale infrared sensors
U.S. Defense Advanced Research Projects Agency (DARPA) Wafer Scale Infrared Detectors (WIRED) program is asking industry to develop infrared sensors and cameras for low-cost, large-format, and high-performance imaging in the short-wave infrared (SWIR), medium-wave infrared (MWIR), and long-wave infrared (LWIR) spectral bands. These infrared detectors must be able to be fabricated directly on silicon-based readout integrated circuit (ROIC) substrates at the wafer scale.
FPAs that respond in the SWIR and MWIR spectral bands are currently manufactured using complex and time consuming processes that typically involve several manual processing steps, including single-die processes. Because this process produces a single FPA at a time and multiple steps contribute to yield loss, the cost of individual cameras is prohibitive for many applications.
Cooling MWIR, and LWIR detectors with reasonable power consumption, moreover, typically requires a cryogenic cooler, which adds size, weight, and power consumption, and cost (SWaP-C).
SWIR generally doesn’t require cryogenic coolers, but focal plane array costs are still high because of complex processing. SWIR imagers are not widely available with formats greater than 2 megapixels because the size of the die as well as the resulting optics become prohibitively large and expensive.
The WIRED program focuses on three technical areas: SWIR, MWIR, and LWIR detectors. SWIR detector research zeroes-in on appropriate materials and wafer-scale processing techniques to produce focal plane arrays with 3-micron pixel pitch without the need for cryogenic cooling or hybrid bump bonding
MWIR detector research concerns materials and wafer-scale processing techniques for 10-micron focal plane arrays without cryogenic cooling or hybrid bump bonding. LWIR work, meanwhile, focuses on 12-micron focal plane arrays without cryogenic cooling or hybrid bump bonding.
Growth in Infrared Imaging market
Maxtech International, Inc. is releasing the 2016 edition of its market research report on commercial and dual-use infrared imaging equipment markets (Vol. IRW-C). According to report, World infrared imaging markets have entered a new period of unprecedented growth. In 2015, the number of uncooled detectors shipped increased by more than 40% for the second year in a row, although market revenues grew at only 2% to $3.2 billion. This is almost entirely due to severe price reductions of uncooled infrared detectors which are being produced with higher yields, low cost wafer-level-packaging and highly integrated electronics.
The traditional major application markets include Predictive Maintenance, Building Inspection, R&D, Process Control, Nondestructive Testing (NDT), Fire Fighting, Automotive Night Vision, Aircraft Enhanced Vision Systems (EVS), Security/surveillance, Law Enforcement/Personal Vision Systems and dual-use uncooled sights (Thermal Weapon Sights, Driver’s Vision Enhancers, etc.)
Some of the traditional application markets fared significantly better than others in 2015. But they are all expected to benefit over the next five years from lower cost uncooled FPAs from detector suppliers that have establishing partnerships with semiconductor fabs, implementing Wafer Level Packaging (WLP), Wafer Level Optics and integrating electronics functions on ASICs (Application Specific Integrated Circuits).
In addition, market shares of suppliers of uncooled focal plane arrays have undergone dramatic changes. Spot Infrared Thermometry sensors (pyrometers) continue to have broad acceptance in special high temperature industrial process control applications and at the very low-end in consumer markets as prices decrease. Also as prices drop, a convergence between the imaging and spot markets is already starting to occur. Nonetheless, the high-end pyrometer market continues to be strong in steel, glass and aluminum manufacturing and has been relatively untouched by imaging infrared.
At the same time, new cooled detector technologies are being developed for high-end thermal imaging. Developments are accelerating in superlattice detectors and high operating temperature (“HOT”) FPAs. Cameras based on new superlattice-based nBn and XBn detectors and T2SL (Type Two Superlattice) detectors are now available in the commercial market.
A report on “The Global Military E/O-IR Market 2014-2024” released by Strategic Defence Intelligence predicts that market to reach $7.3 billion by the end of 2014, rising to $10 billion by 2024. Maxtech International, Inc. in its 2015 edition of its market research report on military infrared detectors and systems markets (Vol. IRW-M) has detailed following military infrared systems:
Ground-based Systems, including Night Sights, Fire Control Systems, Driver’s Vision Enhancers, Thermal Weapon Sights, Fused IR/I2 Systems, Unattended Ground Sensors, Soldier Systems, Hostile Fire Indicators (HFI);
Airborne Systems, including Targeting/Navigation Pods, Targeting/Piloting Systems, Reconnaissance Systems, Infrared Countermeasures, Infrared Search and Track (IRST) Systems and Persistent Surveillance Systems (aerostats, fixed wing, rotary wing and UAVs);
Naval Systems including optronic masts, thermal night sights, and shipboard IRST;
Tactical Missile Seekers, including air-to-air, surface-air, air-surface, ground-ground; and smart munitions;
Strategic Surveillance and Missile Defense including infrared satellite early warning systems and ballistic missile defense systems.
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