Diffraction-corrected subsurface imaging with a scanning laser acoustic microscope

A back projection (acoustic holography) algorithm has been developed and implemented on a computer-controlled scanning laser acoustic microscope. The algorithm permits the reconstruction of clear well-defined images of acoustic features which are far below (by tens of acoustic wavelengths) the laser-scanned surface of the substrate. In essence, the algorithm corrects for the diffraction of the incident acoustic beam caused by the feature being insonified. Experimental results obtained using an electron microscope locator grid as the test feature and a compressional wave source of 100 MHz are presented.