Adhesion criterion for center-anchored circular plates in microstructures

Abstract

During wet etching of sacrificial layer in surface micromachining fabrication, a microstructure may be pinned on the substrate due to the capillary force of liquid. This paper investigates the adhesion criterion between a center-anchored circular plate and its underlying substrate caused by strong capillary forces. Using catastrophe theory, a non-dimensional elastocapillary number NEC, in terms of structure sizes and material properties, is derived. Theoretically, when NEC>1 the center-anchored circular plate will restore to its desired position from substrate, while NEC<1, it will be attached to substrate. The proposed devices have been successfully fabricated through surface micromachining to examine the proposed NEC and the results show the feasibility of using NEC to determine the adhesion criterion. From experimental data, the ratio of inner and outer radii of plates ri/ro, ranging from 0.4 to 0.7, can be used as the first approach of the adhesion criteria. By observing the experimental results, a novel adhesion criterion NAD, in terms of material properties, plate thickness, gap, ri/ro, and residual stress, is therefore defined and used to estimate the adhesion effect. The experimental results showed that only 8.87% error occurs if NAD is employed.