A study on the interfacial composition of the electroless-copper-plated BPDA-PDA polyimide sheet

Abstract

This work studies the chemical composition at the interface generated through the electroless deposition of a copper layer onto a polyimide (PI) slice. Upilex-S^®-50 [poly(biphenyl dianhydride-p-phenylenediamine)] (BPDA-PDA) polyimide film was selected as the substrate, on which wet chemstry modifications were carried out before plating. The modifications comprise the alkali catalyzed imide ring-opening reaction and the subsequent acidification, through which polyamic acid segments are generated at the surface. A copper thin film was then developed on this surface using the electroless plating technique. Changes in composition of the surface/interface caused by the modification as well as by the plating were examined using XPS and XPS depth profiling techniques. It reveals that the copper deposited initially contains both Cu²⁺ and Cu⁺ species in addition to Cu⁰; among them Cu⁺ and Cu⁰ are much more abundant. Furthermore, XPS depth profiling analysis suggests that there is a layer consisting of copper species spreading in the PI matrix underneath the copper film. The etching on the PI slice is believed to assist with the filtering of the copper species. On the other hand, topographic changes in the course of copper plating were traced by the AFM. The results suggest that the copper film grew by following a “mountain–valley” template.