Site-Selective Solder Deposition on Multi-Segment Nanowires for Directed Assembly and Nanoscale Joining

Electronic device packaging involves complex wiring assemblies for transport of electrical and thermal energy, enabling increasingly sophisticated circuit designs in an industry favoring aggressive device miniaturization. Nanoscale miniaturization of the wire morphology has become increasingly sophisticated in recent years with regards to synthesis; however, the ability to arrange and manufacture these small-scale materials to fully exploit their novel properties has been quickly outpaced. The purpose of this research is to establish a synthesis technique for a core/shell nanowire structure with tunable compositions via electrodeposition which can pass through this bottleneck via directed assembly techniques. A gold-nickel-gold nanowire was utilized as the core, synthesized via electrodeposition into an anodic aluminum oxide (AAO) template; the nickel segment was chosen as a magnetic vehicle for directed assembly, while the bordering gold tips were coated with a solder shell. Gold-selective deposition of this tin solder shell was achieved by establishing a chemisorbed nickel-protective monolayer via immersion in azelaic acid solution. Next, the protected nanowires were dispersed in an aqueous reaction vessel with excess sodium borohydride reducing agent. Controlled addition of tin sulfate and sodium dodecyl sulfate solution to the vessel resulted in the formation of tin seeds which deposited along the gold tips of the nanowires, growing into a “dumbbell” shape nanowire with tin deposited on both gold tips. Composition and confirmation of the solder deposition was performed with SEM and EDS analysis. Crystal structure of the core/shell nanowires was performed with x-ray diffraction to identify intermetallic formation between the tin and gold. The core/shell nanowires were arranged magnetically across an interdigitated electrode and melted by an infrared heating under flux atmosphere, establishing ohmic contact tip-to-tip between nanowires and in wire-to-pad morphologies. This work establishes an effective and scalable method for site-specific nanowire functionalization while simultaneously addressing miniaturization-associated assembly issues in its desired applications.