Solution-based, additive fabrication of flush metal conductors in plastic substrates by printing and plating in two-level capillary channels

A strategy to control the structure of electroless copper deposition in confined features was developed for applications in printed electronic devices and interconnects. This work builds on and refines a previous process of additively manufacturing metal conductors using a combined imprint, print, and plate strategy. A two-level imprinted channel geometry, consisting of inner, parallel capillary channels running along the base surface of a main channel, was designed to control electroless copper deposition. The inner capillary channels contain and precisely position the silver ink seed layer, and the main channel confines the electroless copper as it deposits from the seed layer, resulting in flat copper surfaces that are flush with the surface of the substrate or at a controlled depth. This process produces metal conductors with aspect ratios (height/width) up to ≈2 and low edge roughnesses as defined by the imprinting process. The resistance of the final conductors varies linearly with the length and inversely with the width of the main channel, suggesting high uniformity throughout. Conductive lines with a width of 40 μm and depth of 10 µm had low resistance per unit length (≈1Ω cm-1) and retained their performance with repeated mechanical flexing. The methods described here to confine the growth of electroless plated copper are adaptable to form conductors for numerous applications in flexible electronics.