4. Quantitative acoustic microscopy

Abstract

Abstract This chapter deals with the elastic characterization of solid media by acoustic microscopy. Three types of microscopy are considered. The first type is conventional acoustic microscopy, which uses monochromatic illumination for measuring elastic properties. The use of conventional acoustic microscopes for evaluating the elastic properties of layered anisotropic solids exploits the fact that this technique is able to measure the velocity of surface acoustic waves (SAWs). In conventional acoustic microscopy, the velocity of the surface acoustic wave is determined from the so-called V(z) curve, the variation of the voltage signal that is measured when the lens of the microscope is translated toward the sample. For isotropic layered materials, the SAW velocity is dependent on the frequency; i.e., there is dispersion. For anisotropic solids, the SAW velocity also exhibits a variation with the direction of propagation, called angular dispersion. The elastic properties of anisotropic solids can be extracted from measured dispersion curves. Acoustic microscopes usually operate within the frequency range of 100 MHz to 2 GHz. At such frequencies the elastic properties of thin layers from several hundred nanometers up to a few tens of microns can be determined. The second type is time-resolved microscopy, which uses pulses for material characterization. This type of microscopy provides an opportunity to measure velocities of bulk waves inside solid layers. A new (third type) type of microscopy, where two lenses are used for quantitative measurements, will be reviewed briefly. The chapter is organized into six sections. A short history of acoustic microscopy is presented in the Introduction (Section 4.1). The basic principles of quantitative acoustic microscopy and the accuracy of measuring SAW velocities by acoustic microscopy are described in Section 4.2. Sections 4.3 and 4.4 concern the acoustic microscopy measurement of dispersion curves of different types of materials and the extraction of the elastic properties from experimental acoustic data. The application of acoustic microscopy to elastic characterization of nonplane bodies is reviewed in Section 4.5. Finally, acoustic microscopy is compared to other techniques in Section 4.6.