High sensitivity resonance frequency measurements of individual microcantilevers using fiber optical interferometry

We describe a setup for the resonance frequency measurement of individual microcantilevers. The setup displays both high spatial selectivity and sensitivity to specimen vibrations by utilizing a tapered fiber tip. The high sensitivity to specimen vibrations is achieved by the combination of optical Fabry-Perot interferometry and narrow band rf detection. Wave fronts reflected on the specimen and on the fiber tip end face interfere, thus no reference plane on the specimen is needed, as demonstrated with the example of freestanding silicon nitride microcantilevers. The resulting system is integrated in a DB-235 dual beam focused ion beam (FIB) system, thereby allowing the measurement of microcantilever responses during observation in scanning electron microscopy mode. The FIB was used to modify the optical fiber tip. At this point of our rf system development, the microcantilevers used to characterize the detector were not modified in situ.