A new method for measuring surface acoustic wave speeds by acoustic microscopes and its application in characterizing laterally inhomogeneous materials

The surface acoustic wave (SAW) speed of a material is conventionally measured by an acoustic microscope from the V(z) curve (voltage V versus the defocus distance z). V(z) curves are generated by moving the lens of an acoustic microscope vertically and thus varying its distance from the specimen. Alternate constructive and destructive interferences between two rays received by the receiver produce oscillating patterns in the V(z) curves. The SAW speed is measured from the distance between two successive dips (or peaks) of the V(z) curve. Recently it has been shown that the SAW speed can also be measured from the V(f) curve. V(f) curves are generated by changing the signal frequency while the lens-specimen distance remains constant. In this paper an alternative method of measuring the SAW speed is proposed. Accuracy of different methods used to measure the SAW speed is investigated. In the new method the SAW speed is measured from the first dips of the V(z) or V(f) curves instead of the distance between two successive dips. Hence, the new methods do not need several clearly defined dips and peaks of the V(z) or V(f) curves, unlike the traditional methods.