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Uncooled Infrared Detectors Achieve New Performance Levels and Cost Targets
CONTENTS:  Background  |  Uncooled Detectors Mature Into the Mainstream  |  Technology Advancements of Amorphous Silicon Microbolometers  |  Applications  |  Summary  |  References

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Figure 1
Figure 1: Thermal image obtained with 1024x768 a-Si microbolometer detector shows both high sensitivity and resolution. Courtesy ULIS.

Background

Uncooled infrared detectors have realized many significant technology advances, improved reliability, better manufacturability and lower costs. This has fueled the availability of a wide variety of infrared cameras based on those detectors. Low cost portable and fixed infrared cameras as well as high performance systems have been introduced for a variety of thermal imaging applications. As a result of the new price points and better overall performance, traditional markets for infrared cameras have exploded while new markets have been created that benefit from the steady improvements in performance.

This article reviews how uncooled detectors have matured into the mainstream markets for infrared imaging. Significant performance improvements are described such as sensitivity, resolution, thermal time constant and uniformity as well as the benefits to system complexity and cost. Primary applications for thermal imaging cameras are also reviewed.

Manufacturers of infrared cameras find a wide choice of uncooled infrared detectors available for use in different model designs intended for a variety of commercial, industrial and military products.


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Uncooled Detectors Mature Into the Mainstream

Infrared camera manufacturers have available two possible classes of infrared detectors to be used to manufacture infrared cameras, cooled and uncooled. Cooled detectors (named because of their reliance on a cryogenic cooling mechanism) are considered extraordinarily sensitive to infrared radiation. The cryocooler substantially reduces thermal noise (infrared radiation from sources other than the objects being observed) down to very low levels. Mercury Cadmium Telluride (HgCdTe or MCT) and Indium Antimonide (InSb) are the most common materials used in cooled detectors. Because the cryocooler unit is a mechanical device with moving parts, cooled detectors usually require maintenance after a period of time, often for every 8,000 to 10,000 hours of operation.

Amorphous silicon microbolometers developed using MEMS technology have all the advantages of silicon processing including cost and yield, plus are highly sensitive to infrared radiation.

Uncooled infrared detectors have become an excellent alternative to the cooled detectors and are much more commonly used in many commercial, industrial and military IR camera products. Since they do not require the use of a cryogenic cooling unit, infrared cameras that use uncooled detectors enjoy substantial advantages in maintainability as well as a significant reduction in the size, complexity and cost.

The primary type of uncooled detector today is the microbolometer, a device based on microelectromechanical (MEMS) technology. When infrared radiation in the wavelength range between 7-13μm strikes the microbolometer’s detector material and is absorbed, it heats up and the resulting change in its electrical resistance is the basic sensing technique. These changes are processed by separate core electronics to create a thermal image. These detectors are quite sensitive and are able to sense heat radiated from objects depending on their temperature.

The two most common microbolometer detector materials are amorphous silicon (a-Si) and vanadium oxide (VOx), referring to the material on the outermost thin film layer. While the two materials function in a similar manner, there are many differences between the two.

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Figure 2: Uncooled Microbolometer Detector having small, low-cost ceramic package.

Microbolometers having a-Si as the thermometer material have been developed using sophisticated surface micromachining techniques to produce very thin membranes that are very sensitive. As shown in Figure 2, these detectors can be economically packaged in an extremely thin ceramic package specifically designed for the detector. The array is integrated and subsequently sealed under vacuum without the need for any pinch-off tube. The design of the package allows for a 15-year storage lifetime at room temperature of the detector package.

These detectors have become very widely integrated into many commercial and military products. Because of the use of a-Si, they have benefited from widely available silicon manufacturing processes. In addition, they have also benefited from a great deal of research performed in improving the performance of similar silicon-based devices, such as solar cells and flat panel displays, with particular attention on pixel operability and uniformity. While other detector materials such as VOx have shown slightly better noise figures, the technological benefits of amorphous silicon pixels and high yield silicon manufacturing processes have rapidly popularized this technology. Thirty years of manufacturing in the silicon industry have resulted in a mature process that enjoys high manufacturing yields with low defect rates.

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Figure 3: Drawing of amorphous silicon microbolometer pixel

Technology Advancements of Amorphous Silicon Microbolometers

Since their initial development, amorphous silicon (a-Si) microbolometers have substantially matured. Technological breakthroughs have occurred at a constant pace, resulting in improvements in performance (such as resolution and sensitivity) as well as in detector packaging. Pixel uniformity and operability have also continued to improve as well due to production changes.

Reduced Pixel Geometries

Because amorphous silicon is inherently stiff, microbolometers manufactured with this material can be made with very small bridge structures. As a result, a-Si microbolometers exhibit higher effective thermal insulation compared with VOx based devices due to thinner membranes. This reduction in bridge size allows for the development of small pixels and therefore more pixels per area, smaller physical sizes and lower costs. As shown in Figure 3, the amorphous silicon microbolometer pixel is quite simple in structure, so reducing pixel size does not require a complex microbridge structure since the structure scales appropriately. As a result, the number of fabrication operations in the manufacturing process is optimized.

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Figure 4: Pixel pitch for amorphous silicon microbolometers have continued to decrease due to technological advancements.

As shown in the chart in Figure 4, over the past ten years, a-Si IR detector arrays have seen a steady reduction in pixel pitch from 45μm back in 2000 to 17μm pixel pitch in 2008. The next generation arrays are projected to have a 12μm pixel pitch geometry, further reducing the size of detectors and optics as well as the size of infrared cameras.

In conjunction with pixel pitch reductions, arrays offering higher resolution have been introduced. Many microbolometer array sizes are available as shown in Figure 5, from a low resolution 160x120 array to a large format 1024x768 array. In light of the reduction in pixel size and the increase in resolution, the active detection area for these arrays remains nearly constant for certain comparative array sizes. As shown in Figure 6, the width of a detector having 320x240 pixels with 45µm pixels (14.4mm) is quite similar to the width of a detector having 640x480 pixels with 25µm pixels (16.0mm) and also to the width of a detector having 1024x768 pixels with 17µm pixels (17.4mm). With similar detection areas, production and packaging costs are maintained and the size and cost of infrared camera housings and optics can also be stabilized.

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Figure 5: A wide range of array sizes are now available, from 160x120 pixel arrays, for use in economical infrared cameras, to 1024x768 pixel arrays for very high resolution products.
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Figure 6: As pixel pitch decreases and pixel counts increase, detector active area stays fairly stable.

Thermal Time Constant and Sensitivity

For portable 30Hz infrared cameras, it is highly desirable to have pixel response times shorter than 10 ms. This is easily achieved with a-Si pixels.

Thermal time constant is one of the most important parameters in the design of an infrared imaging detector exceeding the importance of other parameters. Again, because of the intrinsic properties of silicon, a-Si pixels can be produced with very thin microstructures because of their rigid behavior. With these thin microstructures, pixels exhibit a very low thermal conductance between the pixel and substrate and low suspended thermal pixel mass. This results in pixels having a very short time constant and very fast response time which is an enormous benefit for any infrared detector. It is commonly accepted that pixel response times should not exceed one-third of the reciprocal of the frame time (i.e. 10ms for an array operating at 30Hz). This is easily achieved with a-Si pixels.

As pixel pitch decreases, other parameters need to be appropriately modified in order to keep pixel sensitivity from decreasing since the active detection area becomes smaller. Because sensitivity has a great impact on image quality, certain design changes are important to assure that detector sensitivity remains at the desired level. For example, when the pixel pitch of a-Si microbolometers was reduced from 45μm to 25μm, improvements were made in the photolithographic resolution without adversely impacting fill factor. While 1.5µm minimum design rules were used for 45µm pitch detectors, 25µm detectors required 0.8 µm minimum design rules, and 0.5µm for 17µm pitch detectors. However, improvements in the design of the support legs as well as a reduction in both the width and thickness of the patterned legs resulted in higher thermal insulation. The result was a microbolometer array having the same sensitivity but with all the advantages of smaller pixels.

In order to further reduce the pixel pitch, modifications were required to the design of pixels in order to maintain the pixel time constant and detector sensitivity. It was necessary to reduce the thickness of the microbridge films in order to reduce its thermal mass. In order to maintain good noise immunity, the microbridge was thinned down at the same rate of decrease as the detector active area. At the same time (but more challenging), the thermal conductance also needed to be reduced at a rate corresponding to the square of the pixel area. Because of the simplicity of the silicon substrate, the thickness of the a-Si microbridge could be easily reduced with very few restrictions.

As an example of the importance of pixel time constant, compare the results of different a-Si focal planes as shown in the table below, one standard in design, and another designed for high sensitivity, narrow intra-scene dynamic range applications such as would be required in astronomical missions.

 
Temporal NETD
Pixel time constant
Intra-scene dynamic range
Standard design
30mK
7ms
200K
MERTIS design
11mK
13.6ms
19K

Image Uniformity and Operability

As with other imaging arrays, each microbolometer pixel has a slightly different response given by the pixel gain and pixel offset. During the manufacturing process, each array has a resulting distribution of gain and offset values from each pixel. Because of the well-established silicon manufacturing methods and the simplicity of its structure, amorphous silicon microbolometers exhibit excellent image uniformity as shown by the very similar behavior of all of the pixels. Excellent array uniformity means that the array gain can be increased prior to digitization in order to improve overall system signal-to-noise performance.

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Figure 7: Amorphous silicon detectors are well known for their excellent array uniformity, resulting in improved image quality, reduced system complexity and lower overall costs.

The operability of an IR detector array is defined by the number of defective pixels in the array, or the percentage of defective pixels. Highly uniform arrays have a smaller number of bad pixels since pixels are defined as defective if their characteristics are significantly different than the average. To achieve the best image, array pixel response should be as uniform as possible across the surface of the device prior to any non-uniformity correction. Amorphous silicon detectors traditionally offer high operability rates partly because of the simplicity of silicon technology and the benefits of established processes. Operability of better than 99.5% is standard in amorphous silicon detectors and rates as high as 99.9% are not uncommon.

Amorphous silicon detectors traditionally offer high pixel operability partly because of the simplicity of silicon technology and the benefits of well-established manufacturing processes. Operability of better than 99.5% is typical and rates as high as 99.9% are not uncommon.

Because of their intrinsic uniformity, operating a-Si detectors has other important advantages over other types of microbolometer detectors. For example, these detectors can be operated quite easily without a thermoelectric cooler (“TEC-less”). Since the a-Si pixels can only have one activation energy that defines the bolometer resistance as a function of temperature, knowing the pixel resistance at one temperature is sufficient for predicting the resistance at other temperatures. Microbolometer arrays manufactured with a composite of materials (e.g. VOx) have numerous factors that impact the response of the pixels in the array and are not so easily characterized. Consequently, they exhibit higher non-uniformity characteristics and are more difficult to operate in TEC-less mode since the wide variation in response cannot be easily predicted. In addition, if not appropriately compensated, some of the pixels in the array may appear saturated, having a response outside the detector digitization range. Because of the high uniformity of a-Si detector arrays, the raw signal delivered by the detector is sufficiently uniform to be processed without any additional electronics, which reduces system complexity, calibration, yield, power consumption, size and finally cost of the overall system.

Applications

Amorphous silicon detectors are ideally suited for many IR applications due to their high performance, high reliability and lower cost. Applications for microbolometers exist in both commercial and military sectors. Some of the top applications are as follows:

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Figure 8: Infrared cameras are ideal for night-time surveillance applications.

Surveillance

Because of the high contrast between humans and vehicles, uncooled IR detectors are an ideal choice for surveillance application. Surveillance applications encompass many different areas including, night vision, security, and basic surveillance. Because of the technological advances including 17µm pixel pitch and large format detectors, a-Si microbolometers are finding their way into many more military and commercial night vision applications. For example, one military application that benefits from these advances is unmanned aerial vehicles (UAVs) which require lightweight and reliable IR detectors that can stand physical punishment including high vibrations and high G force loads. These requirements make uncooled a-SI microbolometers ideally suited for this application.

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Figure 9: Because infrared cameras can see through smoke, they are a valuable tool for firefighters.

Firefighting

Another application for IR detectors is firefighting. Firefighters are often thrust into situations where visible light cannot be used due to smoky conditions. In low visibility conditions, finding trapped victims or downed fire fighters can be near impossible. Thermal imaging cameras have allowed fire fighters to see through the smoke and locate trapped victims and firefighters. Additionally, Thermal imaging allows for the detection of hot spots after a fire is extinguished, as well as to find the source of active fires.

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Figure 10: Identifying unexpected heat loss in electrical connections has been found to be key in preventing costly failures to occur.

Industrial Inspection

Certain infrared cameras can be calibrated so that they provide for the non-contact measurement of object temperature. These find plentiful applications in industrial inspection. One of the most common uses of these radiometric infrared cameras is in industrial environments having high power usage. Inspectors use IR cameras for thermal analysis of electrical and mechanical components for problem prevention and detection. In these systems, excess heat can be a sign of a potential problem. Infrared cameras are used to scan for many different issues including loose or dirty connections, overloaded circuits, plugged cooling lines in transformers, and much more. Additionally, the technology is non-invasive and non-destructive and allows for the testing of many points per day.

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Figure 11: With rising energy costs, identifying heat loss in homes and buildings has become a valued application for infrared cameras.

Energy Conservation

According to the US Department of Energy “heating and cooling account for 50 to 70% of the energy used in the average American home. Inadequate insulation and air leakage are leading causes of energy waste in most homes” [See Reference 1]. Energy conservation is a booming industry fueled by higher energy costs, global warming, and dwindling natural resources. In an effort to make homes and businesses more energy efficient, energy audits are becoming more commonplace. IR detectors and thermal imaging technologies provide an easy to use tool for conducting energy audits. Energy audits primarily focus on evaluating the effectiveness of insulation and seals around exterior doors and windows in addition to looking for gaps or cracks that are leaking energy. In addition, thermal imaging cameras can save time and money in detecting leaks in pipes as well. Both in wall and buried pipes can be quickly scanned without the need to destroy a wall or dig up a pipe. Common applications include: hot water leaks in buried pipe, oil and gas pipeline leaks, and leak detection in municipal water mains.

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Figure 12: Fever screening cameras are now commonplace at airports and other transit locations to monitor travelers having elevated body temperature.

Medical Monitoring

Infrared cameras have become quite popular at airports and other transit locations for non-contact “fever screening” where public health officials can swiftly scan and measure the skin temperature of people as they pass. Individuals showing an elevated temperature can be evaluated in more detail to help prevent the spread of disease.

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Figure 13: Infrared cameras are now available as an option on many new cars for improved night vision.

Automotive Night Vision

Over the last decade IR detectors have become more commonplace in civilian applications. In addition to being used for surveillance, IR detectors and cameras are now becoming standard in automobiles as automotive night vision systems. Automotive night vision systems are used to assist drivers in dark conditions, as well as in bad weather. Automotive night vision systems increase a driver’s seeing distance and perception in these conditions beyond the reach of the vehicle’s headlights.

   

Summary

Significant technological improvements have occurred in uncooled microbolometers over the last few years fueling the growth of infrared camera designs that use them. With improvements in sensitivity and resolution and because of a wide variety of available pixel sizes, infrared cameras meeting a variety of application-specific requirements have been introduced. In particular, amorphous silicon microbolometers are shown to have specific benefits in performance (including thermal time constant and uniformity) and cost due to steady technical advances, silicon-based manufacturing advantages as well as yield improvements. Their simplified design, small size and mature silicon-based manufacturing technology have been leveraged to make everything from handheld thermal imaging cameras to night vision systems. Thermal imaging products have found their way into a variety of different applications, including: surveillance, firefighting, industrial inspection, energy conservation, medical monitoring and automotive night vision.

References

  1. US Dept. of Energy Insulation Fact Sheet: www.ornl.gov/sci/roofs+walls/insulation/ins_01.html.
  2. Jean-Jacques Yon, Eric Mottin, Jean-Luc Tissot, “Latest amorphous silicon microbolometer developments at LETI-LIR, Proc. SPIE Vol. 6940, May 2008.
  3. C. Trouilleau, B. Fièque, S. Noblet, “High-performance uncooled amorphous silicon TEC less XGA IRFPA with 17μm pixel-pitch”, Proc. SPIE Vol. 7298, May 2009.
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