Acoustic Microscopy in Non-Destructive Testing
In the past few years, scanning acoustic microscopy has been recognized as a valuable method for observing microstructures which were hardly resolved using standard optical techniques. Indeed, it is the case of transparent biological samples which needs time consuming coloration techniques to be observed using optical microscopy. It is also the case for the observation of thick or opaque samples which gives only, when using optical techniques, informations about the surface structures. At the present time the spatial resolution of acoustical methods is comparable to that obtained with optical microscopes (say 0.5 µm). Moreover, in the pulse emission mode, it is possible to focus the acoustic beam under the sample surface and to detect structures like microdefects lying under the surface. A first practical example is the observation of microdefects near the surface of electronic microcircuits, which needs a very high spatial resolution (1µm) and a very small depth of focus under the surface sample (1 to 10 µm). The utilization of mercury as a coupling medium decreases the surface echo amplitude and allows the detection of the echo on the defect (1). Indeed the mechanical impedances of the mercury, the lens and the electronic microcircuit (say alumina) are very close to, and moreover the propagation losses in this liquid metal material are quite negligible in this high frequency domain of ultra sound.
KeywordsFocal Plane Velocity Ratio Geometrical Optic Ultra Sound Acoustic Velocity
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