Fast simulation of ultrasound impulse responses and signals in layered structures by means of transient Green's functions.

In sound field simulations the short wavelength compared to the area of interest results in a large number of mesh cells. As a result, FEM-based simulation tools require high computational power and are still limited in the size and dimensionality of the simulated problems. Especially in high-frequency ultrasonic applications such as Scanning Acoustic Microscopy, where inspection frequencies exceed 100 MHz and broadband pulses are transmitted, fully numerical methods cannot be efficiently applied. In addition, when inspecting multi-material, multi-layer specimens such as modern 3D integrated semiconductor chips, mode conversion and surface waves must be taken into account. To overcome this we propose a simulation method that utilizes spatial convolution and transient Green's functions. This method is capable of calculating accurate impulse responses in layered structures from extended sources. The initial boundary problem is solved by a method that is part of the Generalized Ray Theory. Our proposed method does so by transforming it with a Laplace- and Hankel-transformation. Furthermore, the generalized transmission and reflection coefficients provide a comprehensive description of mode conversion at the layer interfaces. This facilitates the study of pressure and shear waves, as well as more complex wave phenomena, including head and interface waves. The efficiency of the simulation method enables parameter studies in high-frequency applications. We verify the proposed method by comparing its output to a half-space solution from previous literature and extend to a pseudo two-layered structure with almost identical properties. Finally, the method is demonstrated by calculating impulse responses in a solid-fluid structure.