Active cantilever with integrated actuation and sensing for use in off-resonance tapping AFM

At present, the imaging speed of atomic force microscopes is limited by the vertical positioning bandwidth, which includes the resonance frequency of the cantilever, and the bandwidth of the verticalnanopositoner. The cantilever resonance frequency can be increased by reducing its dimensions;however, the width is limited to approximately 10 micrometers due to the size of the laser spot that istypically used to measure deflection. Any further reduction in dimensions requires alternate methodsfor sensing deflection, such as integrated piezoelectric or piezoresistive sensors. The bandwidth of thevertical nanopositioner also limits the imaging speed in modes where the interaction force is controlled. Other modes that require vertical height modulation are also limited by the vertical nanopositioner, including force spectroscopy and off-resonance tapping modes. To address the low bandwidth of the vertical positioner, direct excitation of the cantilever canbe employed, while integrated sensing can overcome the cantilever width constraint. Integratedpiezoelectric actuation and piezoelectric sensing are both suitable for cantilever miniaturisation and arecompatible with commercially available fabrication processes. However, the drawback of integratedactuation and sensing is the cross-coupling between displacements induced by direct actuation and theforces that act on the cantilever tip. This thesis presents a novel microcantilever with integrated piezoelectric actuation and twopiezoelectric sensors optimised for use in off-resonance tapping mode. The dual sensing configurationallows the tip force and tip displacement to be measured simultaneously, while addressing the crosscouplingfrom the actuator. A mathematical model is introduced to describe the behaviour of the cantilever with integratedactuation for quasi-static modes. The electric feedthrough is modelled to address the effect on themeasurements. Finite element analysis is used to compare and validate the mathematical model. A MEMS cantilever device is designed and fabricated using a commercial foundry process,followed by post-processing for tip deposition. A calibration procedure is developed to determine theactuation and sensor sensitivities, and cross-coupling. Experimental results compare favourably withthe mathematical and finite element analysis. The fabricated device and calibration procedure are experimentally demonstrated by acquiringforce-distance curves from two samples, and successfully estimating their modulus of elasticity withoutthe use of a vertical positioner or external sensors. The results correlate closely with results from acommercial cantilever and standard atomic force microscope.