Acoustic Transducers and Lens Design for Acoustic Microscopy

The transducer-lens system is one major component in the performance of an acoustic microscope. The design criteria for the various types of applications of an acoustic microscope are different. For surface imaging applications, it is desired to have a small spot size and low sidelobe level. For materials characterization and subsurface imaging applications (sue~ as subsurface crack imaging), it is required to have high surface wave excitation efficiency. Several researchers addressed the problem of surface imaging.l-5 Also, some work has been done to investigate materials properties indirectly by using so-called V(z) curves.6-8 The resolution obtained in the latter case is always much worse than that of the corresponding lens. The direct measurement of surface wave velocity by using conventional transducer-lens systems also gives poor spatial resolution because it suffers from the interference of the specularly-reflected signal with the surface wave component caused by the low efficiency of surface wave excitation. 9 In order to increase the surface wave excitation efficiency, we first modified the design of the standard longitudinal transducer-lens system. Furthermore, we worked out a novel configuration, i.e, the shear transducerlens system. This gives very high surface wave excitation efficiency and anisotropic acoustic beam distribution. Therefore, it can be used for direct measurements of materials properties in different directions with much less defocus than in the case of conventional transducer-lens systems. This gives the potential of many new applications of materials characterization with excellent spatial resolution.