A near-field scanning laser source technique and a microcantilever ultrasound receiver for detection of surface-breaking defects

Abstract

The scanning laser source (SLS) technique has been proposed recently as an effective way to investigate small surface-breaking defects. By monitoring the changes in amplitude and frequency of the ultrasound generated as a laser source scans over a defect, the SLS technique has provided enhanced signal-to-noise performance compared to the traditional pitch–catch or pulse–echo ultrasonic methods. In the previous work, the laser ultrasound source was in the near-field of a scatterer and an optical or piezoelectric detector was used to measure the ultrasound in the far-field. We now propose an extension of the SLS approach to map defects in microdevices by bringing both the generator and the receiver to the near-field scattering region of the defects. To facilitate near-field ultrasound measurement, silicon microcantilever probes are fabricated using microfabrication techniques and their acoustical characteristics are investigated. The fundamental frequency of the microcantilever is measured and compared with analytically calculated fundamental frequency. The performance of the fabricated microcantilevers as resonant ultrasound detectors is investigated. Bulk and surface acoustic waves are generated with specific narrowband frequencies and the surface ultrasonic displacements are detected using the microcantilever probe. Next, broadband ultrasound is generated by a laser source and the resulting surface acoustic displacements are monitored using the microcantilever probe in the near-field of the ultrasound source. Finally, both the laser-generated ultrasonic source and the microcantilever probe are used to monitor near-field scattering by a surface-breaking defect.