Scanning electron acoustic microscopy and its applications

Abstract

Scanning electron acoustic microscopy (SEAM) is a relatively new technique for imaging and characterization of thermal and elastic property variations on the scale of a few micrometres. A megahertz-chopped, focused electron beam in a scanning electron microscope (SEM) generates sound waves in the sample, and the signal from a transducer attached to the specimen is used to form a scanned image in parallel with the normal SEM image. Although the acoustic wavelengths are typically several millimetres, lateral and depth resolution may be only a few micrometres. This is because image contrast is mainly derived from the micrometre-sized acoustic generation volume just under the beam. In this generation volume, both elastic variations and thermal scattering of the critically damped ‘thermal waves’, with wavelength of a few micrometres (due to the periodic beam heating), may lead to contrast. There is also evidence for non-thermoelastic contrast generation mechanisms, and these show finer resolution, SEAM shows both advantages and drawbacks compared to conventional scanning acoustic microscopy (SAM). Although understanding of the detailed contrast mechanisms in seam is at present only at a qualitative level, it is clear that they are generally different from those in SAM. The technique is now beginning to attract commercial attention, and applications include imaging of cracks and voids, grains and grain boundaries and second phases in polycrystalline materials, regions of plastic deformation, and magnetic domain structures. Images can be obtained from ics and semiconductor materials which show doped and implanted regions, near-surface manufacturing defects, and even individual dislocations. The acoustic waves excite vibrational patterns in the specimen, and these can be used as low-resolution probes of, for example, bonding integrity.