Practical Shear Wave Lens Design for Improved Resolution with Acoustic Microscope

Abstract

This paper presents a practical new lens design for acoustic microscopy. The new lens provides a factor-of-2 higher resolution than currently available commercial lenses for acoustic microscopy, and a reduction in the influence of surface roughness on the image formation. Analysis, computer simulations, and demonstration examples provide convincing evidence that new lens design works efficiently. Whereas most current lens designs emphasize the use of longitudinal waves, the designs presented here focus on the use of transverse or shear waves. In the present study, two kinds of lens designs have been developed: One is a "center-sealed" acoustic lens used at the center frequency of 400 MHz and 1 GHz for use with acoustic tone bursts, and the other is a "high-NA acoustic lens" used in the center frequency of 30 MHz for use with short pulses. The center-sealed acoustic lens has its center area aperture sealed to prevent longitudinal waves from traveling into the sample so that the acoustic image is substantially composed of shear wave components. The "high-NA" acoustic lens has an aperture with a large aperture angle for exciting shear waves in the object. In this study, the mechanisms of image formation with both of these lenses are described and their features are evaluated.