Date of Award

Fall 2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Hai-Feng Ji

Abstract

Ultra-sensitive and selective moisture sensors are needed in various industries for processing control or environmental monitoring. As an outstanding sensor platform, microcantilevers have potential application in moisture detection due to their advantages, such as low-level moisture detection limits, high accuracy, quick response time, high reproducibility, good recovery rate and low in cost. Our research results will lead to the first of its kind for the commercialization of a microcantilever-based moisture sensor used for industrial and household applications. The novelty of the present work is the development of SiO2 and Si cantilevers, which were fabricated using developed processes and modified with Al2O3, for detecting moisture as low as ppm level.

To increase the deflection of the microcantilever under surface stress induced by specific reactions, a new SiO2 microcantilever, which consists of two SiO2 cantilever beams as the sensing and reference elements, two connecting wings and three guard arms, has been developed which features a much lower Young's modulus than conventional Si or SiNx microcantilevers. For comparing SiO2 cantilever with Si cantilevers, a model of the cantilever sensor is reported by using both analysis and simulation, resulting in good agreement with the experimental data. The results demonstrate that the SiO2 cantilever can achieve a much higher sensitivity than the Si cantilever due to its lower Young's modulus. In order to fabricate this device, a new fabrication process using isotropic combined with anisotropic dry etching to release the SiO2 microcantilever beam by Inductively Coupled Plasma (ICP) was developed and investigated. This new process not only obtains a high etch rate at 9.1 μm per minute, but also provides good etch profile controllability, and a flexibility of device design. Attributed to its high sensitivity, Al2O3 coated SiO2 microcantilevers demonstrated the capability of detecting moisture concentration levels down to 30 ppm using optical detection methods. It can be seen that the SiO2microcantilevers, with appropriate sensing material, can be utilized as ultra sensitive moisture sensors and are potentially able to detect the moisture concentration level as low as 1 to 10 ppm.

Although optical readout systems are most extensively used for measurement of cantilever deflections in labs, they have some disadvantages, such as its alignment system is expensive and involves great precision. Piezoresistive, capacitive, MOSFET-embedded and frequency readout methods, which are all fit for commercial application, have been investigated both in simulation and experiment. It is found that the Al2O3 modified microcantilever operating in frequency mode is able to meet the requirements of detecting low moisture levels. To make this device compatible with IC technology, the piezoelectric microcantilever is chosen as the platform for moisture sensing. A piezoelectric microcantilever vibrates at its resonant frequency upon applying an appropriate AC voltage and provides an electrical signal at the output via piezoelectric coupling, which can be fed back through the phase shift loop to determine the change in resonant frequency caused by any change in mass. In order to fabricate the piezoelectric microcantilever, the sputtering parameters for ZnO were reported and investigated. The piezoelectric microcantilevers, which consists of bottom electrode, ZnO piezoelectric layer, and two separate top electrodes as sensing and actuation elements, were designed and fabricated using a standard lithography process. Its resonant frequency shift is measured at 1.25 Hz/ppm, based on an optical detection method. Although both SiO 2 and Si piezoelectric cantilevers were fabricated successfully, the latter are more likely to be used in dynamic mode because of the higher fragility of SiO2. The developed cantilever sensor platform operating in dynamic mode, which can be integrated with on-chip electronic circuitry, is able to provide ultra-sensitive detection, not only for moisture sensing, but also for chemical and biological sensing with appropriate surface modification.

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