Functional organic adlayers for controlling the adhesion strength of electrolessly deposited copper on super-engineering plastics

Abstract

Effective metallization of super-engineering plastics such as liquid crystal polymers (LCP) and polyphenylene sulfides (PPS) via electroless deposition of copper thin films is an important step in the manufacturing of a wide range of devices. Copper deposition typically requires a pre-treatment step involving chemical and/or mechanical roughening to ensure high adhesion at the polymer/Cu interface; however such treatments are detrimental to patterns and topographic features of the polymer that possess µm resolution and/or high aspect ratio. Herein, we demonstrate that it is possible to regulate and improve adhesion of electrolessly deposited copper thin films at LCP and PPS materials via multilayer functionalization with aryldiazonium cations of a p-aminobenzoic acid precursor. We first demonstrate that aryldiazonium grafting conditions can be optimized to overcome the chemical inertness of LCP/PPS without resorting to harsh oxidative or mechanical treatments. We then show that the density of Ar-COOH groups can be regulated by modifying the number of functionalization cycles. X-ray photoelectron spectroscopy studies shows that this has a clear impact on the composition of the catalytic nanoparticle layer required for copper deposition. Adhesion strength studies demonstrate that such changes translate into improved adhesion strengths without any adverse effects on surface roughness. Control experiments with alternative chemical moieties suggest that specific chemical interactions between catalytic seeds and grafted carboxylate groups play an important role in the observed improved adhesion.