Fluidic Self-Assembly of Microfabricated Parts to Substrates Using Capillary Forces

In this work, we present a fluidic microassembly technique which uses capillary forces to self-assemble microparts onto a substrate. We have demonstrated this technique using microfabricated silicon parts and silicon and quartz substrates. The binding sites for self-assembly are photolithographically-defined patterns of hydrophobic self-assembled monolayers. A substrate patterned with these sites is passed through a film of hydrophobic adhesive into water, causing the adhesive to coat the binding sites selectivelythrough interfacial energy minimization. The parts are then pipetted towards the submerged substrate surface. Once the hydrophobic pattern on a part makes contact with an adhesive-coated substrate site, capillary forces of the adhesive meniscus result in spontaneous shape matching. Using this technique, 100-part arrays were assembled in <1 min with >99.5% alignment yield and <0.2 um alignment precision.
The capillary forces were modelcd using the finite element program Surface Evolver. Good agreement was observed between experimental results and modeling predictions of the capillary thickness for both circular and square binding sites. According to the model, the capillary forces were orders of magnitude stronger than gravitational or fluidic drag forces. Also, for these site shapes, the total energy of the assembly was found to be approximately linearly proportional to the overlap area between the sites. This result provides a useful metric for estimating the shape of the energy wells corresponding to more complex binding sites.
In further process development, acrylate adhesives were formulated so permanent bonding of the assembled parts could be achieved using activation by heat or ultraviolet light. The role of binding site shape in alignment yield was also investigated using a geometric shape overlap model implemented with Matlab. Finally, this assembly technique was used to self-assemble silicon micromirrors onto surface-micromachined actuators for an adaptive optics application. Over a 464 um-diameter mirror, the mirror flatness was measured to be within 20 nm. This mirror quality is difficult to attain without the process decoupling afforded by microassembly.
The general microassembly approach described here may be applied to parts ranging in size from the nano- to the milli- scale, and a variety of part and substrate materials. Since materials and process incompatibilities often present a barrier to the cofabrication of different microdevices, this and other microassembly techniques are an enabling technology for the next generation of microsystems.
Susan J. Muller
Jeffery A. Reimer
Publication date: 
April 30, 2001
Publication type: 
Ph.D. Dissertation
Srinivasan, U. (2001). Fluidic Self-assembly of Microfabricated Parts to Substrates Using Capillary Forces. United States: University of California, Berkeley.

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