Finite Element Method Analysis of Evaluation of Surface Micro Cracks Using Laser Ultrasound Generated by Phase Velocity Scanning Method

Abstract

Nondestructive evaluation of defects that reduce material strength is critical to maintain the reliability of micro electro mechanical system (MEMS) components. We developed a new laser ultrasonic method, the phase velocity scanning (PVS) method, for this purpose. In a feasibility study, 200-µm-long micro standard defects (slits) on a Si wafer were fabricated with various depths (6.8 to 28.7 µm) and widths (2 to 5 µm), and scattered waves from a 6.8-µm-deep slit were successfully detected using 60 MHz surface acoustic waves. To analyze the directivity of acoustic waves generated by the PVS method and scattered waves from the slits, we developed 2D and 3D finite element method (FEM) programs. Using these programs, we confirmed that scattered waves are generated from a slit of 200 µm length, 10 µm depth, and 5 µm width. A comparison with the simulation of a conventional laser ultrasonic method showed that the PVS method has better directivity to suppress disturbing echoes generated in small objects.