Doctoral Dissertations

Zhiyu Hu

Date of Award

5-2000

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Robert J. Warmack

Committee Members

Marianne Breinig, William R. Hamel

Abstract

Microcantilever-based sensors have attracted much attention due to their potential as a platform for the development of myriad physical, chemical, and biological sensorsStudies have shown that thin bi-material microcantilevers undergo bending (deflection) due to differential stresses caused by exposure to such environments. Because of their very small spring constants (less than 0.01 N/m), microcantilevers are sensitive to stress differentials between the substrate and coating layer. Taking advantage of the ultra-high stress sensitivity of micrometer-sized microcantilevers, vapor adsorption on solid surfaces and adsorption-induced stress in thin metal films were investigated. Experimental results show that experimental systems can detect ppb (part-per-billion) or ppt (part-per-trillion) levels of hydrogen or mercury, respectively.

Thin, coated cantilevers undergo bending if they are exposed to various biological, chemical and physical environments. This bending is due to a differential surface stress caused by stress changes that are induced during the adsorption process. Depending on the nature of analyte-substrate interactions (adsorption or absorption) the adsorption-induced stress in the thin film can be expressed as either a surface stress (N/m) or a bulk stress (N/m) Such stress effects can be used to create extremely sensitive sensors and can be much larger than mass-induced frequency shifts of the fundamental resonance.

During the experiments, thin metal films were coated on one side of silicon or silicon-nitride based microcantilevers. Both optical and electrical cantilever deflection detection methods were employed. The two systems were selected to study two different interaction mechanisms:

1. Palladium-coated microcantilevers were employed to investigate the bulk-like absorption of hydrogen that diffuses into palladium and causes volume expansion.

2. Gold-coated microcantilevers were employed to study surface-like adsorption when mercury atoms adsorb onto the gold surface, thus changing the film stress.

Adsorption-induced stress on bi-material microcantilevers can produce bending, which can be related to gas or vapor concentration. The sensitivity of microcantilevers (Si or SiN based) is in the pico-newton range. Bi-material cantilevers can be used to measure thin film stress or as a chemical sensor platform using selected coatings.

For bulk-like absorption, vapors do not stop at the gas/solid interface on the film but actually penetrate the entire thickness of the film. An example is hydrogen adsorption in palladium that induces a volume expansion of the palladium film. The sensitivity of coated cantilevers is adjustable in a certain range by controlling the coating layer thickness:

For surface-like adsorption, both stress and resistance changes depend only upon the number of adsorbate atoms adsorbed on the adsorbent surface. Film thickness has little effect on the sensitivity of coated cantilevers. An example is mercury adsorption onto a gold surface that causes a stress decrease.

Novel simultaneous cantilever bending and electrical resistance measurements indicate that adsorption onto or absorption into thin metal films can induce stress changes and resistance changes at different rates. These differences may or may not vary with gas or vapor concentration depending on the interaction mechanisms; and they imply more complex chemical reactions during the adsorption process than were known before this study. Additional investigations will be required to ascertain such details.

The investigation of mercury adsorption-induced stress on thin gold film described is believed to be the first complete work in this area. A surface adsorption model is proposed and shows excellent agreement with experimental data as well as that reported outside this dissertationThe model may serve as a guide for future studies in surface adsorption.

Bi-material microcantilevers have shown ultra-high stress sensitivity that may be utilized to study thin film stress or employed as a sensor platform. Investigations of the two-adsorption mechanisms help to provide a clear understanding of gas (or vapor) adsorption onto solid surfaces as well as the associated stress. Both theoretical models and experimental results could be used to design and improve the performance of microcantilever-based sensors. Effects of environmental influences, such as relative humidity and temperature, were also investigated.

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