Thickness dependent critical strain in submicron Cu films adherent to polymer substrate

For the polymer-supported metal thin films that are finding increasing applications, the critical strain to nucleate microcracks (ε_C) should be more meaningful than generally measured rupture strain. In this letter, the ε_C values of polymer-supported Cu films are simply but precisely determined by measurements of both electrical resistance and statistical microcrack density changes on the film surface. Significant thickness dependence of ε_C, i.e., the thinner the film the lower the ε_C, has been revealed for the Cu films with a thickness range from 700 down to 60 nm, which is suggested to result from the constraint effect of refining grain size on the dislocation movability.