Capillary-force driven self-assembly of silicon microstructures

Self-assembly, or the spontaneous organization of parts into larger structures via energy minimization, is an attractive solution to overcome packaging and integration challenges. Capillary forces constitute one self-assembly mechanism, and this dissertation explored the capillary-force based self-assembly of single crystal silicon parts with dimensions of less than 300 mum. In one example, a hydrophobic liquid polymer was attached to microfabricated parts in an aqueous environment, using a macroscopic solubility change to induce microscopic precipitation and selective polymer deposition. Selectivity was achieved by modification of silicon and gold surface energies using self-assembled monolayers. This polymer deposition technique was then used on microfabricated 20--100 mum sized silicon parts to assemble them into three-dimensional (3-D) structures. During the assembly process, a solutal Marangoni effect propelled fluid in toroidal patterns, which helped agitate and attract neighboring parts, and effectively extended the range of interaction between hydrophobic surfaces by four orders of magnitude. Using analytical and finite element analysis, the fluid motion was shown in be linearly proportional to the radius of the fluid interface, and for one 250 mum diameter droplet, the room temperature motion was equivalent to a thermocapillary effect from a 200°C/cm temperature gradient.; By using molten alloy contacts to provide capillary forces instead of a hydrophobic polymer, electrical connections could be made. Corrosion problems associated with scaling such contacts down in size were reduced by selecting a eutectic Sn-Bi alloy and glycerol. The self assembly of 1500 100 mum parts and 5000 40 mum parts was demonstrated, each in about 2.5 minutes. Thus, 40 mum square, 4 mum high contacts were shown to remain functional in glycerol at 180°C, which is five times smaller than any other examples of molten alloy based self-assembly using an alloy melting at temperatures above 47°C. Finally, a six-mask fabrication process was developed to fabricate free-standing parts which may participate in a full, 3-D self-assembly process. Fabrication details, results, and future challenges are discussed. Using parts with a subset of the features required for 3-D self-assembly, the electrical conductance of self-ssembled 20 mum diameter, 2.5 mum high alloy contacts was measured at 2/mO-cm2.