Improvement of anisotropy sensitivity in the scanning acoustic microscope

Abstract

Thr response of the conventional scanning acoustic microscope (SAM) to anisotropic materials is theoretically investigated. For this purpose, the reflection coefficient of plane acoustic naves incident on a liquid-solid interface is numerically calculated for a general anisotropic solid oriented in an) arhitrar) direction. In general, the reflection coefficient depends on polar and azimuthal angles of incidence. For the case of a circularly symmetric acoustic microscope lens, a mean reflectance function can he defined that depends only on the polar angle. With this mean reflectance function it is ver) easy to predict the anisotropic material response of the acoustic microscope. Moreover, one can explore the effects of changing lens parameters such as the acoustic fields at the back side of the lens. It is found that under certain conditions, the amplitude response of the acoustic microscope can depend heavily on the orientation of the solid material under investigation. The amplitude of the acoustic microscope signal is influenced by the orientation of the material because there is a cancellation of acoustic rays reflected from the object surface at different azimuthal angles. This cancellation i5 revealed as a minimum in the mean reflectance function. It is shown by numerical simulation that, the sensitivity to orientation can he increased by use of a ring-shaped insonification at the hack of the acoustic lens. W'ith such lenses it may he possible to determine the orientation of crystallites in a material with a high lateral resolution.