Fabrication and Preparation of Micro-Coaxial Wires: Electrodeposition of Conductive Shields
Authors: Barron Sara C , Sricharoenchaikit Prasit , Torres Daniela A , Romano Gregory , Morrison Richard H , Magyar Andrew P , Zhang Hongmei , Gray Caprice L 
Published: February 2020

We describe the fabrication of micro-coaxial cables with diameters of 25 to 300 micrometers. Draper Laboratory is developing a new microelectronics packaging platform that relies on such microcoax, in place of copper interconnects, for both power and signal distribution among the packaged electronic components. This platform could save design time and fabrication labor over conventional packaging techniques for custom electronics with small production volumes and offers a way to rapid prototype and/or edit circuits. In this work, we focus on the fabrication of microcoax for power distribution. The fabrication process of the metal-dielectric-metal cross-section of the microcoax begins with a commercial metal wire of 18 to 150 micrometer diameter, which is either purchased with a polymer insulation of 1 to 12 micrometer thickness, or is insulated with SiO2 or Al2O3 of 0.1 to 2 micrometer thickness by PECVD or ALD processes. Next, a thin Ti/Au bilayer is applied to the insulated wire by electron beam evaporation and acts both as an adhesive layer and as electrical contact to plate the outer metal shield to its full desired thickness. The metal shield is electroplated using a cyanide-free gold sulfite chemistry that produces a soft satin to bright electrodeposit, meeting Type IIIA of Mil-G-45204 C requirements (i.e., ≥ 99.9 % gold purity and Knoop hardness 90 maximum). Straight DC, temperature of 60 °C, current density of 2 mA/cm2and mild agitation were chosen as parameters for Au electroplating conditions. Gold electrodeposits exhibit very low roughness, good corrosion resistance, excellent solderability, high shine and with evaporated seed metal, good adhesion to the polymer dielectrics. The shield thickness is chosen to give a resistance equal to or less than the resistance of the metal core, e.g., 34 micrometers of plated Au for a 127 micrometer diameter Cu core.
The use of the microcoax for packaging requires lengths of microcoax of < 15 mm with the wire tips prepared for electrical connection, i.e., tips where the metal shield ends abruptly leaving the center metal wire exposed for bonding, and so we introduce at periodic intervals along the wire length regions where the outer layers are more easily stripped to expose the metal core. We have accomplished this either by ad hoc placement of a plating-resistant (nail polish or soft-baked photoresist) bead on the seed metal to pattern regions where no shield metal is electrodeposited; or by an evaporation mask during seed metal deposition to pattern regions where neither the metal seed nor the metal shield are deposited. The core metal is then exposed in the beaded or masked regions by wet etching the seed metal, if necessary, and UV laser ablation of the polymer dielectric.
In addition to the fabrication processes, we will also discuss the fixturing and handling protocols required for these fine diameter wires. Fixturing is designed to accomplish four criteria: to minimize handling among the various process steps (insulating the wire, e-beam evaporation, electroplating); to pattern the microcoax lengths and abrupt edges to the electrodeposited metal; to make electrical contact to the seed metal during electroplating; and for the necessary volume and yield. We will describe our scale-up from an initial proof-of-concept process that produced 0.1 meters of microcoax of arbitrary segment lengths to a simpler process that produces 2 meters of microcoax of well-defined segment length.