Relative humidity effect on pull-off forces in MEMS flexible structures measured by AFM

Abstract

An atomic force microscope operating in contact mode is used in this paper for determining the pull-off forces as a function of relative humidity. The paper reports the measurements and the modeling of adhesion forces versus humidity in controlled ranges between 20 to 90%RH. In the low RH range (<20%) where the adsorbed water layer is insignificant, due to the absence of water meniscus, adhesion force increases slowly. However, in relative higher RH range (>20%), adhesion force increases very sharply once ‘liquid-like’ adsorbed water layer forms, because it increases the capillary force. After the relative humidity reaches about 70–80% the drop in the experimental values of the adhesive force is a result of desorption of water molecules and the corresponding decrease of water menisci contribution. To investigate the effect of relative humidity on adhesion for flexible structures such as aluminum microbridges with different lengths against silicon substrate we used an analytical method which encompasses the effect of capillarity as well as the solid-to-solid interaction. The capillary force is expressed as the sum between the Laplace force and the surface tension, while the solid-to-solid interaction is estimated using the DMT model. The analytical results are in good agreement with the experimental ones.