Liquid polymer deposition on free-standing microfabricated parts for self-assembly

In this paper, we present a method for the self-assembly of micrometer-scale silicon parts, using a cross-linkable liquid polymer as the intermediate for binding into three-dimensional (3-D) structures. A method to deposit the liquid polymer on free-standing micrometer-scale parts was developed and thoroughly characterized. The polymer deposition was achieved by the following: a) tailoring surface energies of target areas on micrometer-scale parts by formation of self-assembled molecular monolayers; b) suspending the parts in a polymer-solvent solution; and c) gradually forcing the polymer to leave the solution and deposit on areas with lower surface energy by introducing a third liquid miscible with the polymer solvent but not with the polymer itself. The polymer-deposition method presents a unique capability for targeted placement of liquid polymer on micrometer-scale parts after their release from substrates. The polymer-deposition process was modeled considering a precipitation step as a breakup of a polymer thin film into droplets, and considering forces acting on the droplets. The polymer-deposition process on microfabricated and surface energy-tailored templates was studied using normal and high-speed video microscopy. The effect of parameters such as surface energy of template regions, polymer concentration, and size of template regions on the polymer-deposition process was quantified. The observed experimental trends of deposited polymer thickness as a function of polymer concentration were in good agreement with modeling results. We microfabricated 20-100-mu m-sized silicon parts, functionalized their surfaces with self-assembled monolayers, employed the polymer-deposition technique to place volumes of a cross-linkable polymer on predefined part areas, and showed their self-assembly into 3-D structures.