A Model-based Synthetic Aperture Focusing Technique for Acoustic Microscopy

This contribution is concerned with a technique to precisely measure the physical dimensions of flaws in solid specimen by pulse-echo mode acoustic microscopy. We have implemented a synthetic aperture focusing technique (SAFT), which is based on a semi-numerical simulation approach (SIRFEM). In acoustic microscopy, one normally applies single-element transducers with center frequencies above 10 MHz. Due to the very short wavelengths, numerical simulations demand a very fine discretized measurement setup. This would lead to extensive calculation costs. Therefore, we model the sound propagation in the fluid coupling medium between transducer surface and specimen analytically by the spatial impulse response (SIR) method. Only the mechanical wave propagation in the solid specimen is then simulated by a higher-order finite element method (FEM). This coupled SIRFEM method can be used to calculate the RF signal output (i.e., echo data) of ultrasonic transducers. The presented model-based SAFT algorithm is based on an optimized filter-kernel, which is generated by an adapted SIRFEM simulation.