Fracture toughness of freestanding copper films with a thickness of 39 nm

In situ field emission scanning electron microscope (FESEM) fracture toughness experiments on 39-nm-thick freestanding copper (Cu) films were conducted. We developed methods for fabricating and testing the fracture toughness of ultrathin freestanding metallic film specimens. Although 39-nm-thick Cu films showed lower critical crack tip opening displacement (CTOD) than 100-2600-nm-thick films, both films had similar critical CTODs when normalized by their film thickness. This independence of normalized CTOD on film thickness indicates that geometric similarity of local fracture morphologies exists in films with a thickness of the order of 10 nm. Crack initiation in ultrathin films was accompanied by ductile fracture.