Acoustic microscopy enables one to image the interaction of acoustic waves with the elastic properties of a specimen with microscopic resolution. A lens with good focusing properties on axis can be used for both transmitting and receiving the signal, and an image is formed by scanning the lens mechanically over the specimen. In pressurised superfluid helium with nonlinear coupling to harmonics a resolution of 15 nm has been achieved, but for routine use 2 GHz is the highest practical frequency, which offers a resolution of about 0.7-mu-m. The information is contained in the way that the acoustic wave is reflected from the specimen. For subsurface imaging, especially in polymer based materials such as composites and electronic packaging, the enhanced depth resolution of a confocal imaging system can be exploited to give good contrast from the plane of interest even when the specimen contains many scatters. In higher stiffness specimens, including most metals, semiconductors and ceramics, a dominant role in the contrast can be played by Rayleigh waves in the surface. If the specimen has a surface layer, then the propagation of the Rayleigh waves is sensitive to the perturbing action of the layer. If the specimen is anisotropic, then there will be dependence on the orientation of the surface and the direction of propagation in it. If there are surface cracks or boundaries, then there will be strong contrast when they scatter the Rayleigh waves. Detailed theory is available to relate the elastic properties of the surface to the contrast, and these enable informed interpretation of the acoustic images to be made, and also provide a foundation for more quantitative acoustic microscopy.
