A convergent beam of ultrasonic waves at very high frequencies can be used in an acoustic microscope to achieve resolutions of the order of 1 μm at depths of up to 1 mm. One application of the acoustic microscope are welding tests (e.g. the Figure which shows an Si/Mo structure welded at 475°C). magnified image
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Fonseca et al. (1993) studied this question.
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