Microscopic imaging of residual stress using a scanning phase-measuring acoustic microscope

A high-resolution scanning phase-measuring acoustic microscope (SPAM) has been developed and used to image the near-surface residual stress field around features etched in sputtered alumina via the acoustoelastic effect. This microscope operates at 670 MHz and has a resolution of 5-10 μm, depending upon the amount of defocus. Relative velocity changes of sample surface waves as small as 50 ppm are resolved. Images of the stress field at the tip of a 400-μm-wide slot etched in alumina are presented and compared with a finite element simulation. The SPAM uses an unconventional acoustic lens with an anisotropic illumination pattern which can measure anisotropic effects and map residual stress fields with several μm resolution and a stress sensitivity of 1/3 MPa in an alumina film.