Experimental study on tensile bifurcation of nanoscale Cu film bonded to polyethylene terephthalate substrate

• The elastic modulus of the Cu films is 120 GPa and does not change with thickness.
• The optimal thickness of the Cu films is about 500 nm.
• The critical strain tends to ascend then to descend as film thickness increases.
• The interfacial strength changes in accordance with the critical strain.
• Microcrack spacing is proportional to film thickness after the microcrack saturates.

Cu films are widely used in flexible electronic products. Tensile mechanical properties of the film determine product performance. In this paper, tensile experiments of sputtered Cu films on a polyethylene terephthalate (PET) substrate were carried out under an optical microscope. In the experiments, three changes took place under tension: uniform deformation, microcrack initiation and propagation, and microcrack saturation. The elastic modulus of the Cu film is 120 GPa and is independent of film thickness since the film is formed to be continuous in the nanoscale range. Film thickness is an important parameter to decide the tensile properties. The critical fracture strain, the interfacial bonding strength, and the crack spacing after saturation are related to film thickness. The critical strain and the interfacial bonding strength of the nanoscale Cu film tend to ascend then to descend as film thickness increases. The microcrack spacing is in direct proportion to film thickness after the microcrack saturates. The optimum thickness of the sputtered Cu films on the PET substrate is about 500 nm.