The intensity of the infrared radiation emitted by objects is mainly a function of their temperature. In infrared thermography, this feature is used for multiple purposes: as a health indicator in medical applications, as a sign of malfunction in mechanical and electrical maintenance or as an indicator of heat loss in buildings. This paper presents a review of infrared thermography especially focused on two applications: temperature measurement and non-destructive testing, two of the main fields where infrared thermography-based sensors are used. A general introduction to infrared thermography and the common procedures for temperature measurement and non-destructive testing are presented. Furthermore, developments in these fields and recent advances are reviewed.

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Infrared thermography for temperature measurement and non-destructive testing.

Active thermography has gained broad acceptance as a non- destructive evaluation method for numerous in-service and manufactur- ing applications in the aerospace industry. However, because of the dif- fusive nature of the process, it is subject to blurring and degradation of the signal as one attempts to image deeper subsurface features. Despite this constraint, active thermographic response is deterministic, to the extent that the postexcitation time evolution for a defect-free sample can be accurately predicted using a simple one-dimensional model. In the patented thermal signal reconstruction method, the time history of every pixel in the field of view is compared to such a model in the logarithmic domain, where deviations from ideal behavior are readily identifiable. The process separates temporal and spatial nonuniformity noise compo- nents in the image sequence and significantly reduces temporal noise. Time-derivative images derived from the reconstructed data allow detec- tion of subsurface defects at earlier times in the sequence than conven- tional contrast images, significantly reducing undesirable blurring effects and facilitating detection of low-thermal-contrast features that may not be detectable in the original data sequence. © 2003 Society of Photo-Optical In-

Reconstruction and enhancement of active thermographic image sequences

Active infrared thermography is a fast and accurate non-destructive evaluation technique that is of particular relevance to the aerospace industry for the inspection of aircraft and helicopters’ primary and secondary structures, aero-engine parts, spacecraft components and its subsystems. This review provides an exhaustive summary of most recent active thermographic methods used for aerospace applications according to their physical principle and thermal excitation sources. Besides traditional optically stimulated thermography, which uses external optical radiation such as flashes, heaters and laser systems, novel hybrid thermographic techniques are also investigated. These include ultrasonic stimulated thermography, which uses ultrasonic waves and the local damage resonance effect to enhance the reliability and sensitivity to micro-cracks, eddy current stimulated thermography, which uses cost-effective eddy current excitation to generate induction heating, and microwave thermography, which uses electromagnetic radiation at the microwave frequency bands to provide rapid detection of cracks and delamination. All these techniques are here analysed and numerous examples are provided for different damage scenarios and aerospace components in order to identify the strength and limitations of each thermographic technique. Moreover, alternative strategies to current external thermal excitation sources, here named as material-based thermography methods, are examined in this paper. These novel thermographic techniques rely on thermoresistive internal heating and offer a fast, low power, accurate and reliable assessment of damage in aerospace composites.

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Recent Advances in Active Infrared Thermography for Non-Destructive Testing of Aerospace Components.

In this article, the theoretical and experimental aspects of three active thermography approaches: pulsed thermography (PT), lock-in thermography (LT), and vibrothermography (VT), are discussed in relation to the nondestructive evaluation (NDE) of honeycomb sandwich structures. For this purpose, two standard specimens with simulated defects (delaminations, core unbonds, excessive adhesive, and crushed core) were tested, and results were processed, examined, and compared. As will be pointed out, the similarities and differences between these active approaches allow conclusions to be made about the most suitable approach for a particular application. In addition, results from NDE inspection by X-rays and c-scan ultrasounds are provided and discussed for reference.

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Comparative Study of Active Thermography Techniques for the Nondestructive Evaluation of Honeycomb Structures

The semiconductor industry continues to produce ever smaller devices that are ever more complex in shape and contain ever more types of materials. The ultimate sizes and functionality of these new devices will be affected by fundamental and engineering limits such as heat dissipation, carrier mobility and fault tolerance thresholds. At present, it is unclear which are the best measurement methods needed to evaluate the nanometre-scale features of such devices and how the fundamental limits will affect the required metrology. Here, we review state-of-the-art dimensional metrology methods for integrated circuits, considering the advantages, limitations and potential improvements of the various approaches. We describe how integrated circuit device design and industry requirements will affect lithography options and consequently metrology requirements. We also discuss potentially powerful emerging technologies and highlight measurement problems that at present have no obvious solution.

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Metrology for the next generation of semiconductor devices

Visualization and analysis of pulsed thermographic data for NDT has generally been based on simple image averaging, subtraction or slope operations. Quantitative contrast methods, based on comparison to a defect free reference point or region, have also been used to a lesser extent. Despite their widespread use, all of these methods are highly susceptible to noise, nonlinearity of the IR camera response, and the presence of surface features on the sample. More importantly, the ability of any of these methods to significantly improve the ability to retrieve deep or weak subsurface features beyond the original unmodified image is limited. In a previous paper, we introduced the concept of Thermographic Signal Reconstruction (TSR) as a means of enhancing defect to background contrast while reducing the amount of data that must be stored by an order of magnitude. The TSR method increases the depth range over which pulsed thermography can be applied, and also reduces the amount of blurring due to lateral diffusion that is typical of thermographic imaging. In this paper we compare TSR with conventional thermographic approaches and consider the mechanisms for the resulting performance improvements.

Enhancement and reconstruction of thermographic NDT data

Systems and methods for discrete polarization scatterometry are provided. One embodiment relates to an optical subsystem of a scatterometer. The optical subsystem includes one or more light sources configured to produce light having different polarizations. The optical subsystem also includes a polarizing beam splitter configured to separate the light into two different light beams having orthogonal and mutually exclusive polarizations. The optical subsystem further includes one or more second optical elements configured to control which one of the two different light beams illuminates the wafer during measurements. The optical subsystem also includes a detection subsystem configured to separately detect two different scattered light beams resulting from illumination of the wafer. The two different scattered light beams have orthogonal and mutually exclusive polarizations. All optical surfaces of the optical subsystem used for the measurements are stationary during the measurements.

Discrete polarization scatterometry

Flash thermography is widely used to inspect Thermal Barrier Coatings (TBC) during manufacturing and maintenance for defects such as delamination or contamination. However, attempts to use thermography to quantify TBC thickness have been less successful. In conventional thermographic NDT applications, the sample surface is opaque to an incident light pulse, and highly emissive in the infrared. The situation is more complex in TBC's, as the coatings are translucent to visible light and near-IR radiation (including the IR component of the flash). Furthermore, TBC's are translucent to the mid-IR wavelengths at which many IR cameras operate. Thus, in the absolute worst case, the flash pulse does not heat the coating, and the camera does not see the coating. Although the latter problem can be mitigated by judicious choice of camera wavelength, it must also be recognized that both the heating and cooling mechanisms in a flash-heated TBC are different from the usual thermography model, where transit time of a heat pulse from the sample surface to a layer interface is an indicator of coating thickness. The resulting time sequence is processed using the Thermographic Signal Reconstruction to generate thickness maps which are accurate to an accuracy of a few percent of the actual coating thickness.

Thermographic measurement of thermal barrier coating thickness

The subject invention relates to the design of a compact imaging spectrometer for use in thin film measurement and general spectroscopic applications. The spectrometer includes only two elements, a rotationally symmetric aspheric reflector and a plane grating. When employed in a pupil centric geometry the spectrometer has no coma or image distortion. Both spherical aberration and astigmatism can be independently corrected. The invention is broadly applicable to the field of optical metrology, particularly optical metrology tools for performing measurements of patterned thin films on semiconductor integrated circuits

Wang David Y

Papers

Reconstruction and enhancement of active thermographic image sequences

Enhancement and reconstruction of thermographic NDT data

Discrete polarization scatterometry

Thermographic measurement of thermal barrier coating thickness

Compact imaging spectrometer