Dual-Beam Differential Amplitude Contrast Scanning Acoustic Microscopy

Abstract

In the reflection acoustic microscope {1}, the large impedance mismatch at the coupler/object interface can lead to a large, but relatively constant, signal. This is because, despite the large reflectivity at this interface, in some specimens, the variations in object impedance and, therefore, in the reflectivity may be quite small. For example, it has been shown that the grain structure in certain solids results in only a few percent change in reflectivity {2}. In such situations, the large background signal can overshadow the signal which arises from the reflectivity variations, leading to a poor image contrast. These issues become more serious in gas medium {3} and cryogenic {4} systems to the extent that, in these forms of microscopy, a major part of the image contrast is purely due to topography. It is clear that, to attain the largest image contrast, only the changes in the signal have to be displayed. Also, it is clear that, ideally, one requires a system which yields no signal for a perfect reflector. In such a system — a differential amplitude microscope — the difference in the amplitudes from adjacent points of the sample constitute the imaging signal.