Acoustic holograms for beam focusing in immersed anisotropic silicon

Ultrasonic inspection of anisotropic materials presents challenges due to directionally dependent wave propagation and beam distortion. Specifically, conventional spherically focused probes, which aim to converge the beam to a small cross section within the solid and increase inspection resolution, can yield spatially and temporally varying focal profiles in anisotropic media. This variability can make interpreting signals from defects within the samples more difficult. To address this challenge, acoustic holograms were designed to enhance ultrasonic beam focusing in silicon wafers. Lens geometries were inversely calculated using ray tracing in pursuit of conical focusing in the solid. Analytical modeling using the angular spectrum approach predicted higher amplitude and more circular focal cross sections for the custom lenses compared to the spherical lenses. The custom lenses, along with conventional spherical lens designs, were fabricated using stereolithographic 3D printing and tested on [3 1 1] and [1 3 5] silicon wafers with etched flat bottom holes. Experimental validation showed the custom lenses produced higher contrast defect signatures with smaller cross-sectional areas from sub-wavelength defects, suggesting improved defect sensitivity and anisotropy-dependent scattering. The results showcase the potential of customized acoustic holograms to enhance ultrasonic inspection of anisotropic materials. The presented design and modeling methods provide the framework for further optimization of acoustic lenses tailored to anisotropic media.