Thin film stress studies using microcantilevers and microcantilever sensors.

摘要

Adsorption-induced and absorption-induced stresses on coated microcantilevers can produce bending, which can be related to gas or vapor concentration for sensing applications. Experimental results demonstrate detection of ppb (parts-per-billion) or ppt (parts-per-trillion) levels of hydrogen or mercury, respectively, for specially coated cantilevers.; 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 on the number of adsorbate atoms adsorbed onto 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. Effects of environmental influences, such as relative humidity and temperature, were also investigated.; The investigation of mercury adsorption-induced stress on thin gold film undertaken here 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 dissertation. This model may serve as a guide for future studies in surface adsorption.