Normal-Incidence PZT-LDV Instrumentation for Omnidirectional Single-Mode Lamb Wave Generation and Wavefield Characterization in Silicon Wafers

Ultrasonic Lamb waves are promising for the nondestructive evaluation of silicon wafers; however, their dispersive, multimode, and orientation-dependent propagation complicates multidirectional measurements in anisotropic media. This study presents a measurement system for wedge-free, multidirectional, A0-dominant Lamb-wave interrogation in a silicon wafer. The distinctive feature of the system is the integration of a fixed normal-incidence PZT source with non-contact scanning laser Doppler vibrometry, enabling wavefield acquisition along arbitrary in-plane directions without repeated wedge coupling or directional source reconfiguration. Frequency–wavenumber analysis demonstrates an A0-dominant wavefield, with the S0-associated power virtually indistinguishable from the baseline spectrum. A noise-adaptive Hilbert-envelope time-of-flight method and locally weighted scatterplot smoothing (LOWESS) reconstruct the orientation-dependent A0 group-velocity profile. The reconstruction shows a root-mean-square percentage deviation of 1.39% relative to the theoretical group velocities obtained from numerical simulations. This capability is practically important for wafer inspection as it reduces setup complexity, thereby improving measurement consistency.