Detection of Defects in Micro-Machine Elements by Using Acoustic Waves Generated by Phase Velocity Scanning of Laser Interference Fringes

High aspect ratio microstructuring is a key process for fabricating microelectromechanical systems (MEMSs). Many microfabrication methods have been developed. Among them, inductively coupled plasma (ICP) source etching is especially attractive because it can fabricate high aspect ratio microstructures, at high speeds with a dry etching process. However, in such microstructures, it is difficult to nondestructively evaluate the quality of high aspect ratio etched bottoms. Therefore, we developed a technique for detecting defects in high aspect ratio microstructures on Si wafers machined by ICP source etching. Using acoustic waves generated by phase velocity scanning of laser interference fringes, we nondestructively detected 13-µm-high defects located on the bottom of narrow and deep grooves from the other side of the Si wafer, which could not be detected by means of optical techniques or a scanning electron microscope.