Micro-scale metal contacts for capillary force-driven self-assembly

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 from a molten alloy can be used to both bond micro-scale components and to make electrical connections between them in a self-assembly process. Here, we present a systematic study of a number of metal alloys and self-assembly media with the aim of reducing the metal contact size between micro-scale components. We consider six different alloys or pure metals with melting points below 160 °C, and nine different fluids with boiling points above 160 °C. Tin-based alloys were generally found to be highly susceptible to corrosion at elevated temperatures above the alloy melting point, with Sn being the primary component to corrode and react with the underlying base metal. Using a eutectic Sn–Bi alloy and glycerol at 180–200 °C, we demonstrated the self-assembly of 1500 100-µm parts and 5000 40-µm parts, each in about 2.5 min. Thus, 40-µm square, 4-µm high contacts remained functional for self-assembly. The electrical conductance of self-assembled 20-µm diameter, 2.5-µm high alloy contacts based on this Sn–Bi–glycerol system was 1.9 mΩ −1 cm −2 .