New paradigms in microcantilever-based sensing

Matthew A Spletzer, Purdue University

Abstract

This thesis focuses on theory and experiments on the structural dynamics of microcantilever structures used for chemical, biological, and radiation detection as well as in Atomic Force Microscopy (AFM). ^ Microcantilever-based sensors have emerged over the past decade as a versatile tool for the detection of chemical and biological substances. Improving the sensitivity of microcantilever-based devices to target particles, while simultaneously decreasing their sensitivity to contamination or changing environmental conditions, is a constant challenge. In order to achieve increases in sensitivity, a new type of sensor consisting of an array of mechanically coupled microcantilevers is discussed. The attachment of a target particle to one cantilever in such an array leads not only to changes in the resonance frequencies of the device, but changes in eigenmodes as well. Both theoretical and experimental results show that the sensitivity of the eigenmodes to an added mass particle is two to three orders of magnitude greater than the resonance frequency shifts of a single cantilever. A new method for detecting non-uniform analyte binding or radiation exposure with a single microcantilever is also discussed. By monitoring the shifts in resonance frequencies of several modes, the location and size of areas undergoing changes in viscoelastic properties can be determined. A theoretical model is developed to explain how resonance frequencies and Q-factors shift with changes in viscoelastic properties, and experimental results are presented for a microcantilever non-uniformly coated with an adhesive that cures on exposure to ultraviolet radiation. ^ Microcantilevers as force sensors also form the backbone of a common nanoscale metrology tool: the Atomic Force Microscope (AFM). As use of the AFM increases, the need for reduced order models that accurately predict the motion of the cantilever under a variety of experimental conditions has increased as well. While the Proper Orthogonal Decomposition (POD) has been used to generate reduced order models from experimental data of macroscale structures, its use in the field of micro-electromechanical systems (MEMS) has been limited to numerical, and not experimental, data. AFM represent a special class of resonant MEMS where the nonlinear forces are spatially confined to within a few nanometers of the sharp tip of an AFM microcantilever. The results of applying POD to the experimentally measured vibrations of an AFM microcantilever are presented, and the basis of modes obtained using POD is found to be more efficient at capturing the dynamics of the microcantilever, potentially leading to a substantial decrease in the complexity of a reduced order model of the AFM microcantilever without any sacrifice in accuracy. The experimental procedure and analysis technique discussed could be applied to a wide variety of devices, including radio frequency (RF) microswitches, resonant mass sensors, micro-gyroscopes, and inertial sensors.^

Degree

Ph.D.

Advisors

Arvind Raman, Purdue University.

Subject Area

Engineering, Mechanical

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