Theory of Continuous Wave Scanning Acoustic Microscope

Abstract

The very first reflection Scanning Acoustic Microscope (SAM) was built on the principles of continuous waves [1]. Though it produced good images it was abandoned because of sever degradation in images due to the standing waves [2]. The next and present generations of SAMs use the most popular pulse-echo technique for imaging as well as for quantitative measurements. It is capable of producing good images and the contrast has been explored in detail. Ray and wave theories have been worked out to explain the contrast [3,4,5,6]. Very good agreement between the theory and experiment have been obtained for the V(z) curves[5,7]. Robust software techniques have been developed to analyze the V(z) curves which can determine the Surface wave velocity and attenuation to an accuracy better than one part in thousand[8]. Although the pulse echo SAM is a powerful tool in materials characterization, a valuable information of phase is lost in this technique. Several alternatives have been proposed [9,10] to get obtain both amplitude and phase information. One such microscope has been Continuous Wave Reflection Scanning Acoustic Microscope (SAMCRUW)[11]. It has been used to obtain both amplitude and phase images. It can also provide quantitative information through unique CWV(z) as well as conventional V(z). It is quite versatile because it can be used as a CW SAM or even like a conventional pulse echo SAM with quite ease. Moreover it has been built basically with off the shelf commercial equipment. It has been used for characterization of different kinds of materials [12,13,14].