Microstructure to substrate self-assembly using capillary forces

Abstract

We have demonstrated the fluidic self-assembly of micromachined silicon parts onto silicon and quartz substrates in a preconfigured pattern with submicrometer positioning precision. Self-assembly is accomplished using photolithographically defined part and substrate binding sites that are complementary shapes of hydrophobic self-assembled monolayers. The patterned substrate is passed through a film of hydrophobic adhesive on water, causing the adhesive to selectively coat the binding sites. Next, the microscopic parts, fabricated from silicon-on-insulator wafers and ranging in size from 150/spl times/150/spl times/15 /spl mu/m/sup 3/ to 400/spl times/400/spl times/50 /spl mu/m/sup 3/, are directed toward the substrate surface under water using a pipette. Once the hydrophobic pattern on a part comes into contact with an adhesive-coated substrate binding site, shape matching occurs spontaneously due to interfacial free energy minimization. In water, capillary forces of the adhesive hold the parts in place with an alignment precision of less than 0.2 /spl mu/m. Permanent bonding of the parts onto quartz and silicon is accomplished by activating the adhesive with heat or ultraviolet light. The resulting rotational misalignment is within /spl sim/0.3/spl deg/. Using square sites, 98-part arrays have been assembled in less than 1 min with 100% yield. The general microassembly approach described here may be applied to parts ranging in size from the nano- to milliscale, and part and substrate materials including semiconductors, glass, plastics, and metals.