Microstructure–property relationship in highly ductile Au–Cu thin films for flexible electronics

The new and fast emerging field of flexible electronic devices requires highly ductile materials. Deposition of thin metal films on flexible substrates is a suitable method to create highly ductile interconnects. In this study, thin films consisting of a graded composition of Au–Cu were co-deposited by direct-current magnetron sputtering on polyimide (Kapton®) substrate for in situ SEM tensile testing, while silicon wafer supported thin film spreads were characterized by nanoindentation, XRD and EDX. Substrate quality turned out to be extremely important for strain delocalization to allow for uniform deformation characterized by high ductility. No cracking was observed up to the maximal strain of 30% for films consisting of pure gold and alloys with a low copper content up to 10 at.%, while cracking was more prevalent in films with higher copper contents and with applied heat treatment. In the most ductile thin films shear bands are the precursors of ductile cracks.

Research highlights▶ Nanocrystalline AuCu alloy thin films were co-sputter deposited on polyimide. ▶ In situ SEM tensile tests were performed. ▶ The most ductile films did not crack up to 30% applied tensile strain. ▶ Deformation localizes in periodic and oriented shear bands. ▶ Shear bands are the precursors for cracks.