Tomographic reconstruction of multiple‐layer specimens with the scanning tomographic acoustic microscope

Abstract

In this article we report the recent results of tomographic image reconstructions of multiple-layer specimens at 100 MHz with the Scanning Tomographic Acoustic Microscope (STAM) system. The experiment utilizes 12 uniformly spaced projections and the results show significant improvement over the holographic images.Acoustic microscopy has been a very important approach to high-resolution nondestructive evaluation of microscopic structures. The Scanning Laser Acoustic Microscope (SLAM) is one of the most widely used systems among the acoustic microscopes currently available. Based on the operating format, SLAM is a transmission-mode system with plane-wave illumination, The data acquisition is performed by a scanned laser beam that picks up the dynamic surface profile of the resultant acoustic waveform. The SLAM system has several fundamental advantages. It does not require focusing of the illumination beam. There is no mechanical movement of the specimen. The laser read-out approach provides a high-speed data acquisition capability. However, the SLAM remains limited to an orthographic imaging device because of the lack of subsurface imaging capability. This seriously limits the applications of the SLAM systems for specimens with certain thickness. Figure 1 illustrates the structure of the two-layer test specimen. Figure 2 is the orthographic image produced by a conventional SLAM system.With the addition of a quadrature receiver, the SLAM system is then capable of holographic subsurface imaging because of the detection of both the amplitude and phase of the dynamic wavefield [1]. For this experiment, the illumination acoustic wave is operated at 100MHz. The quadrature receiver is operated at 32.4 MHz after first down-converting the frequency from 100 MHz. The holographic image reconstruction is performed by the backward propagation algorithm [2]. Figure 3 shows the holographic images of the two layers. The holographic images demonstrate some improvement over the conventional SLAM image in terms of better separation of layers. However, leakage between layers still exists.The objective to upgrade the capability of the SLAM system to the tomographic level is to improve the depth resolution over the holographic reconstruction. Tomographic microscopy can be achieved by extending the spatial-frequen- cy content of the image information with a wider coverage of the observation angle. The experiment included in this article utilized 12 projections. These projections are evenly spaced with the spacing of 30 degrees to form a complete perspective coverage. The tomographic reconstruction is the superposition of the holographic images reconstructed from the 12 projections by backward propagation. The tomographic superposition process also requires an extensive sequence of preprocessing procedures, including data-acquisition-phase error removal, resampling for uniform data sample spacing, estimation of the initial phase of projections, and displacement estimation for accurate array alignment [3,4]. Figure 4 shows the tomograph...