Active Microcantilevers for High Material Contrast in Harmonic Atomic Force Microscopy

Atomic force microscope (AFM) probes are mechanical beams that can be used to simultaneously map topography and material properties. Upon contact of the tip with the sample surface at each cycle in the intermittent mode, higher harmonics are excited. The harmonics in the vicinity of higher eigenmodes are enhanced and present an amplified response, ultimately carrying information about the material properties. In this paper, active cantilevers with integrated actuation and sensing are used as a basis to create harmonic cantilevers for the signal-to-noise ratio improved measurement of time-varying forces. Focused ion beam milling is used to remove mass from specific areas in the cantilever such that the fundamental and higher eigenmodes are tuned toward each other. Two methods are tested, where the shape and location of mass removal is determined, first by simulation and second through an in situ approach. Higher harmonics of the harmonic cantilevers with piezoresistive deflection sensors indicate a significant response of up to 10% in respect to the first harmonic. The improved material contrast mapping abilities of the modified cantilevers are validated by characterization and AFM images.