Size effect on stretchability of Cu films on wrinkled polydimethylsiloxane substrates

Stretchable electronics demand that the metal thin film electrodes have the capacity to experience large level of strains without obvious changes in electrical performance. This work investigated the effect of film thickness and wrinkle dimension on the stretchability of Cu films on polydimethylsiloxane substrates. It is revealed that the wrinkle dimension highly depends on the ultraviolet/ozone treatment time. The fracture strain increases until reaching a maximum and then decreases with increasing film thickness for smaller wrinkles. By contrast, the fracture strain monotonically increases as the film thickness increases for the larger wrinkles within the thickness range studied. Generally, the larger wrinkle dimension leads to a higher fracture strain for a given film thickness. The evolution of fracture strain can be rationalized in terms of the competition between the intrinsic stretchability of Cu films related to the grain size and the extrinsic strain concentration related to film thickness and wrinkle dimension.