Scanning acoustic microscopy

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Abstract
Generation and propagation of sound and ultrasound in materials are strongly affected by various quantities such as elastic and thermal, electronic and magnetic properties. Accordingly, local changes of one of these quantities can give information on the physical and chemical nature of that region which the acoustic signal is generated in or in which the propagation occurs. Based on this, different techniques of ‘scanning acoustic microscopy’ have been developed, important methods in this respect being conventional scanning acoustic microscopy (CSAM), scanning photo‐ or laser acoustic microscopy (SPAM) and scanning electron acoustic microscopy (SEAM). In CSAM ultrasound is focused onto the specimen surface, then its transmission or reflexion within the specimen is determined. Thus propagative properties and acoustic scattering are dominant in CSAM. In SPAM and SEAM a focused laser or electron beam, mostly intensity‐modulated with time, is used to generate an acoustic signal within the probed surface region. Recently these two techniques have been accompanied by the introduction of scanning ion acoustic microscopy (SIAM), which uses a focused ion beam for sound generation. In these three techniques the sound generation mechanism is the main source of information, as typical operation frequencies are low compared to CSAM and quite often very long acoustic wavelengths are resulting. However, for a detailed analysis of the acoustic image contrast, contributions of acoustic propagation to the signal have to be taken into account, too. Due to the different mechanisms involved in signal formation, the mentioned techniques allow determination of a large variety of material parameters, and they are not only competing methods, but add up to a complete set of techniques for material microcharacterization. The applicability of these methods, their advantage and disadvantages are discussed.