A scanning laser source and a microcantilever ultrasound receiver for detection of surface flaws in microdevices

In recent work at Northwestern University, we have shown that near-field scattering of ultrasound generated by a Scanning Laser Source (SLS) can be used to effectively identify surface flaws in macroscale structures. In past work, the laser ultrasound source was in the near-field of a scatterer and a piezoelectric detector was used to measure the ultrasound in the far field. It was observed that distinct variations are observed in the far-field signals as the SLS scans past surface-breaking flaws. These changes were attributed to the near-field scatterer redirecting parts of the ultrasonic beam (which might otherwise have gone into the bulk of the object) towards the far-field detector. 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. For the purpose of near-field ultrasound measurement, the receiving transducer has to be made very small as well. To facilitate this, silicon microcantilever probes are fabricated and their acoustical characteristics are first investigated. Silicon cantilevers with tip and chip body are fabricated using isotropic reactive ion etching and anisotropic KOH etching. To characterize the free cantilever vibration, the chip body with the microcantilever is excited by an ultrasonic transducer and a Michelson interferometer is used to monitor the cantilever motion. The fundamental frequency of the microcantilever is measured and compared with analytically calculated fundamental frequency assuming the cross sections of the cantilevers are rectangular. Next, the performance of the fabricated microcantilevers as ultrasound detectors is investigated. The microcantilever is used essentially as a profilometer by contacting it to the specimen surface. Surface and bulk 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 source. Finally, both the laser-generated ultrasonic source and the microcantilever probe are used to monitor near-field scattering by a surface-breaking defect.