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Pyroelectric Thermal Diffusion Microscope

IP.com Disclosure Number: IPCOM000050436D
Original Publication Date: 1982-Oct-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 5 page(s) / 64K

Publishing Venue

IBM

Related People

Melcher, RL: AUTHOR [+2]

Abstract

The method detects localized thermal discontinuities in layered structures directly and measures the thermal wave rather than the associated thermoacoustic wave. Numerous publications (2-10) discuss the imaging of subsurface defects using photoacoustic, electron acoustic or thermoelastic techniques. Two types of imaging are presented: (a) Acoustic wave imaging uses an acoustic wave generated by thermoelastically using a laser or other energy source. The acoustic wave is scattered or otherwise disturbed by a defect, altering its properties. Analysis of the detected acoustic wave yields an acoustic image of the defect. (b) Thermal wave imaging uses an energy source (laser or electron beam, etc.) absorbed by the sample to produce a thermal wave. The thermal wave generates an acoustic wave thermoelastically which is detected.

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Pyroelectric Thermal Diffusion Microscope

The method detects localized thermal discontinuities in layered structures directly and measures the thermal wave rather than the associated thermoacoustic wave. Numerous publications (2-10) discuss the imaging of subsurface defects using photoacoustic, electron acoustic or thermoelastic techniques. Two types of imaging are presented: (a) Acoustic wave imaging uses an acoustic wave generated by thermoelastically using a laser or other energy source. The acoustic wave is scattered or otherwise disturbed by a defect, altering its properties. Analysis of the detected acoustic wave yields an acoustic image of the defect. (b) Thermal wave imaging uses an energy source (laser or electron beam, etc.) absorbed by the sample to produce a thermal wave. The thermal wave generates an acoustic wave thermoelastically which is detected. If the thermal wave encounters a defect, the thermal wave is altered so that the acoustic wave generated thermoelastically is altered. The detected acoustic wave then leads to an image of the thermal wave. In (1) the measurement of thermal diffusivity is described and demonstrated. The technique is based upon the direct measurement of the thermal wave using a pyroelectric detector when a sample (in intimate thermal contact with the pyroelectric) is heated by an energetic beam, such as a laser or E-beam. This system uses the thermal diffusivity technique described in (1) to develop a thermal wave image by detecting the thermal wave rather than by detecting the associated acoustic wave, as discussed in (4-10). The advantages of this system are: (1) It directly measures the relevant quantity (the thermal wave). (2) It is independent of the acoustic properties of the sample and detector. (3) It is readily useful under pulsed conditions under which the full frequency spectrum can be obtained in a single measurement rather than measuring each frequency independently.

Fig. 1 shows a pyroelectric detector 22 composed of a standard material, such as LiTaO(3), LiNbO(3) PZT ceramic (of several types) or any polar material. A focused and scanned beam 20, such as a laser or electron beam, is directed upon sample 21. Beam 20 scans to locate a flaw, such as defect 24. Pyroelectric detector 22 is secured to sample 21 by means of thermal adhesive layer 26 between the surfaces. Detector 22 is powered by means of a voltage V(t), and resistor R(L) 23 is a load resistor. Fig. 2 shows a response with the beam 20 aligned to strike position X(1) located to the right of defect 24. Fig. 3 shows a response with beam 20 aligned to strike position X(2) directly above defect 24. The choice of materials for detector 22 is determined by sensitivity, speed and flexibility. An accepted figure of merit for pyroelectic infrared sensors (which applies here) is the ratio of the pyroelectric coefficient to the dielectric constant (p/E). One of the better materials is LiTaO(3), although many materials have high...