Thermal Imaging

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What is Infrared Radiation?

The Latin prefix "infra" means "below" or "beneath." Thus "infrared" refers to the region beyond or beneath the red end of the visible color spectrum. The infrared region is located between the visible and microwave regions of the electromagnetic spectrum. Because heated objects radiate energy in the infrared, it is often referred to as the heat region of the spectrum. All objects radiate some energy in the infrared, even objects at room temperature and frozen objects such as ice.

The higher the temperature of an object, the higher the spectral radiant energy, or emittance, at all wavelengths and the shorter the predominant or peak wavelength of the emissions. Peak emissions from objects at room temperature occur at 10 µm. The sun has an equivalent temperature of 5900 K and a peak wavelength of 0.53 µm (green light). It emits copious amounts of energy from the ultraviolet to beyond the far IR region.

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Much of the IR emission spectrum is unusable for detection systems because the radiation is absorbed by water or carbon dioxide in the atmosphere. There are several wavelength bands, however, with good transmission.

  • The long wavelength IR (LWIR) band spans roughly 8-14 µm, with nearly 100% transmission on the 9-12 µm band. The LWIR band offers excellent visibility of most terrestrial objects.
  • The medium wavelength IR (MWIR or MIR) band (3.3-5.0 µm) also offers nearly 100% transmission, with the added benefit of lower, ambient, background noise.
  • Visible and short wavelength IR (SWIR or near IR, NIR) light (0.35-2.5 µm) corresponds to a band of high atmospheric transmission and peak solar illumination, yielding detectors with the best clarity and resolution of the three bands. Without moonlight or artificial illumination, however, SWIR imagers provide poor or no imagery of objects at 300K.

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Infrared Detectors

An infrared detector is simply a transducer of radiant energy, converting radiant energy in the infrared into a measurable form. Infrared detectors can be used for a variety of applications in the military, scientific, industrial, medical, security and automotive arenas. Since infrared radiation does not rely on visible light, it offers the possibility of seeing in the dark or through obscured conditions, by detecting the infrared energy emitted by objects. The detected energy is translated into imagery showing the energy differences between objects, thus allowing an otherwise obscured scene to be seen. For example, the left image below is what you may see in ordinary light on a dark night. The image at right is the same scene but as seen with an Infrared camera. Hot objects such as people stand out from the typically cooler backgrounds regardless of the available visible light.

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Under infrared light, the world reveals features not apparent under regular visible light. People and animals are easily seen in total darkness, weaknesses are revealed in structures, components close to failure glow brighter, visibility is improved in adverse condition such as smoke or fog.


Infrared Detector Types

There are two fundamental methods of IR detection, energy and photon detection. Energy detectors respond to temperature changes generated from incident IR radiation through changes in material properties. Photon detectors generate free electrical carriers through the interaction of photons and bound electrons. Energy detectors are low cost and typically used in single detector applications; common applications include fire detection systems and automatic light switches. However, the simplicity of fabricating large 2D focal plane arrays in semiconductors has lead to the use of photon detectors in almost all advanced IR detection systems. Recent advances in micromachining and materials science have lead to the exciting field of uncooled detectors which promise lower system and operation costs.


Energy Detectors

The absorption of IR energy heats the detection element in energy or thermal detectors, leading to changes in physical properties which can be detected by external instrumentation and which can be correlated to the scene under observation. Energy detectors contain two elements, an absorber and a thermal transducer. The following are examples of energy detectors.

Thermocouples / Thermopiles

Thermocouples are formed by joining two dissimilar metals which create a voltage at their junction. This voltage is proportional to the temperature of the junction. When a scene is optically focused onto a thermocouple, its temperature increases or decreases as the incident IR flux increases or decreases. The change in IR flux emitted by the scene can be detected by monitoring the voltage generated by the thermocouple. For sensitive detection, the thermocouple must be thermally insulated from its surroundings. For fast response, the thermocouple must be able to quickly release built up heat. This tradeoff between sensitivity of detection and the ability to respond to quickly changing scenes is inherent to all energy detectors.

A thermopile is a series of thermocouples connected together to provide increased responsivity.

2 Pyroelectric Detectors

Pyroelectric detectors consist of a polarized material which, when subjected to changes in temperature, changes polarization. These detectors operate in a chopped system; the fluctuation in the exposure to the scene generates a corresponding fluctuation in polarization and thus an alternating current that can be monitored with an external amplifier.

Ferroelectric Detectors

Similar to pyroelectric detectors, ferroelectric detectors are based on a polarized material which, when subjected to changes in temperature, changes polarization.

Thermistors / Bolometers / Microbolometers

In thermistors, the resistance of the elements varies with temperature. One example of a thermistor is a bolometer. Bolometers function in one of two ways: monitoring voltage with constant current or monitoring current with constant voltage.

Advances in the micromachining of silicon have lead to the exciting field of microbolometers. A microbolometer consists of an array of bolometers fabricated directly onto a silicon readout circuit. This technology has demonstrated excellent imagery in the IR. Although the performance of microbolometers currently falls short of that of photon detectors, development is underway to close the performance gap. Microbolometers can operate near room temperature and therefore do not need vacuum evacuated, cryogenically cooled dewars. This advantage brings with it the possibility of producing low cost night vision systems for both military and commercial markets.

Microcantilevers

Microcantilevers are based on the bimetal effect to measure IR radiation. This effect utilizes the difference in thermal expansion coefficients of two different bimetals to cause a displacement of a microcantilever. In combination with a reference plate, this cantilever forms a capacitance. When infrared light is absorbed by the microcantilever, the microcantilever deflects and thus alters the capacitance of the structure. This change in capacitance is a measure for the incident infrared radiation.


Photon Detectors

Light interacts directly with the semiconductors in photon detectors to generate electrical carriers. Because these detectors do not function by changing temperature, they respond faster than energy detectors. However, these detectors will also pick up the IR radiation generated by their own mountings and accompanying optics and thus must be cooled to cryogenic temperatures to minimize background noise. The following are examples of photon detectors.

Intrinsic Detectors

Photovoltaic Intrinsic Detectors
Photovoltaic (PV) detectors generate photocurrents which can be monitored with a trans-impedance amplifier. These photocurrents are created when incident light with energy greater than or equal to the energy gap, or diode junction, of the semiconductor strikes the detector causing excited, minority, electrical carriers to be swept across the photodiode's electrical junction.

PV devices operate in the diode's reverse bias region; this minimizes the current flow through the device which in turn minimizes power dissipation. In addition, PV detectors are low noise because the reverse bias diode junction is depleted of minority carriers. The highest performance PV detectors are fabricated from Si, Ge, GaAs, InSb, InGaAs, and from HgCdTe (MCT).

Photoconductive Intrinsic Detectors
Photoconductive (PC) detectors function similarly to PV detectors. Incident light with energy greater than or equal to the energy gap of the semiconductor generates majority electrical carriers. This results in a change in the resistance, and hence conductivity, of the detector. Examples of PC detector materials are Lead sulfide (PbS), Lead selenide (PbSe) and MCT.

Extrinsic Detectors

Extrinsic detectors are based on Si (SiX) or Ge (GeX) doped with impurities such as Boron, Arsenic and Gallium. They are similar to intrinsic detectors. However, in extrinsic detectors carriers are excited from the impurity levels and not over the bandgap of the basic material. Both photovoltaic and photoconductive types exist.

Photo-emissive Detectors

Photo-emissive detectors are based on the emission of carriers from a metal into a semiconductor material through the absorption of light. A typical example is Platinum Silicide (PtSi) on Si.

Quantum Well Infrared Photodetector

The Quantum Well Infrared Photodetector (QWIP) is an infrared detector that consists of multiple alternating thin gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) layers. Carriers are generated by absorption of IR light inside quantum wells.


Detector Types and Materials Overview

The table below summarizes the main detector types and materials.

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Many of these IR materials are based on compound semiconductors made of III-V elements such as indium, gallium, arsenic, antimony, or on the II-VI elements mercury, cadmium and telluride, or on the IV-VI elements lead, sulfur and selenide. They can be combined into binary compounds such as GaAs, InSb, PbS and PbSe or into ternaries such as InGaAs or HgCdTe.


Infrared Detector Formats and Architectures

Infrared detectors are available as single element detectors in circular, rectangular, cruciform, and other geometries for reticle systems, as linear arrays, and as 2D focal plane arrays (FPAs).

Single element detectors are normally frontside illuminated and wire bonded devices. Linear and 2D arrays may be fabricated with a variety of device and signal output architectures.

First generation linear arrays were usually frontside illuminated, with the detector signal output connected by wire bonding to each element in the array. The signal from each element was then brought out of the vacuum package and connected to an individual room temperature preamplifier prior to interfacing with the imaging system display. Gain adjustments were usually made in the preamplifier circuitry. This approach limited first generation linear arrays to less than two hundred elements.

Second generation arrays, both linear and 2D, are frequently backside illuminated through a transparent substrate. Several alternative focal plane architectures are illustrated in the graph below.

The diagram below illustrates a detector array which is electrically connected directly to an array of preamplifiers and/or switches called a readout. The electrical connection is made with indium "bumps" which provide a soft metal interconnect for each pixel. This arrangement, commonly referred to as a "direct hybrid", facilitates the interconnection of large numbers of pixels to individual preamplifiers coupled with row and column multiplexers.

Indirect hybrid configurations (b) may be used with large linear arrays to interface the detector with a substrate having a similar thermal coefficient of expansion. These hybrids may also be used for serial hybridization, allowing the detector to be tested prior to committing the readout, and/or to accommodate readout unit cells having dimensions larger than the detector unit cell, increasing the charge storage capacity and thereby extending the dynamic range. Readouts and detectors are electrically interconnected by a patterned metal bus on a fanout substrate.

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Monolithic detector arrays (c) have integrated detector and readout functions. Generally, in these arrays, the command and control signal processing electronics are adjacent to the detector array, rather than underneath. In this case, the signal processing circuits may be connected to the detector by wire bonds. In the monolithic configuration it is not necessary for the signal processing circuits to be on the same substrate as the detector/readout (as shown in the figure) or at the same temperature as the detector. Monolithic PtSi detector arrays can be made with signal processing incorporated on the periphery of the detector/readout chip through the use of silicon-based detector technology.

Z technology, as illustrated in figure (d), provides extended signal processing real estate for each pixel in the readout chip by extending the structure in the orthogonal direction. In the approach illustrated, stacked, thinned readout chips are glued together, and the detector array is connected to the edge of this signal processing stack with indium.

Finally, a "Loophole" approach, as illustrated in figure (e), relies on thinning the detector material after adhesively bonding it to the silicon readout. Detector elements are connected to the underlying readout with vias, which are etched through the detector material to contact pads on the readout and metallized.


History and Trends of Infrared Detectors

Infrared detectors are in general used to detect, image, and measure patterns of the thermal heat radiation which all objects emit. Early devices consisted of single detector elements that relied on a change in the temperature of the detector. Early thermal detectors were thermocouples and bolometers which are still used today. Thermal detectors are generally sensitive to all infrared wavelengths and operate at room temperature. Under these conditions, they have relatively low sensitivity and slow response.

Photon detectors were developed to improve sensitivity and response time. These detectors have been extensively developed since the 1940's. Lead sulfide (PbS) was the first practical IR detector. It is sensitive to infrared wavelengths up to ~3 µm.

Beginning in the late 1940's and continuing into the 1950's, a wide variety of new materials were developed for IR sensing. Lead selenide (PbSe), lead telluride (PbTe), and indium antimonide (InSb) extended the spectral range beyond that of PbS, providing sensitivity in the 3-5 µm medium wavelength (MWIR) atmospheric window.

The end of the 1950's saw the first introduction of semiconductor alloys, in the chemical table group III-V, IV-VI, and II-VI material systems. These alloys allowed the bandgap of the semiconductor, and hence its spectral response, to be custom tailored for specific applications. MCT (HgCdTe), a group II-VI material, has today become the most widely used of the tunable bandgap materials.

As photolithography became available in the early 1960's it was applied to make IR sensor arrays. Linear array technology was first demonstrated in PbS, PbSe, and InSb detectors. Photovoltaic (PV) detector development began with the availability of single crystal InSb material.

In the late 1960's and early 1970's, "first generation" linear arrays of intrinsic MCT photoconductive detectors were developed. These allowed LWIR forward looking imaging radiometer (FLIR) systems to operate at 80K with a single stage cryoengine, making them much more compact, lighter, and significantly lower in power consumption.

The 1970's witnessed a mushrooming of IR applications combined with the start of high volume production of first generation sensor systems using linear arrays.

At the same time, other significant detector technology developments were taking place. Silicon technology spawned novel platinum silicide (PtSi) detector devices which have become standard commercial products for a variety of MWIR high resolution applications.

The invention of charge coupled devices (CCDs) in the late 1960's made it possible to envision "second generation" detector arrays coupled with on-focal-plane electronic analog signal readouts which could multiplex the signal from a very large array of detectors. Early assessment of this concept showed that photovoltaic detectors such as InSb, PtSi, and MCT detectors or high impedance photoconductors such as PbSe, PbS, and extrinsic silicon detectors were promising candidates because they had impedances suitable for interfacing with the FET input of readout multiplexers. PC MCT was not suitable due to its low impedance. Therefore, in the late 1970's through the 1980's, MCT technology efforts focused almost exclusively on PV device development because of the need for low power and high impedance for interfacing to readout input circuits in large arrays. This effort has been paying off in the 1990's with the birth of second generation IR detectors which provide large 2D arrays in both linear formats. These detectors use TDI for scanning systems; in staring systems, they come in square and rectangular formats.

Monolithic extrinsic silicon detectors were demonstrated first in the mid 1970's. The monolithic extrinsic silicon approach was subsequently set aside because the process of integrated circuit fabrication degraded the detector quality. Monolithic PtSi detectors, however, in which the detector can be formed after the readout is processed, are now widely available.

Second generation devices have now been demonstrated with many detector materials and device types, including PbS, PbSe, InSb, extrinsic Si, PtSi, and PV MCT.

It has taken nearly two decades since the invention of the CCD to mature the integration of IR detectors coupled with electronic readouts on the focal plane. This progress brought with it the transition from first generation to second generation device production. The size and complexity of infrared image detectors corresponds to the evolution of silicon integrated circuit size and complexity; this can be seen through comparison to dynamic random access memory chip trends (see graph below). Note that DRAMs require just one transistor per unit cell, whereas infrared sensor readouts require three or more, one of which must be a low noise analog device.

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