Significant enhancement of the strength-to-resistivity ratio by nanotwins in epitaxial Cu films

Abstract

Epitaxial nanotwinned Cu films, with an average twin spacing ranging from 7 to 16 nm, exhibit a high ratio of strength-to-electrical resistivity, ∼400 MPa(μΩ cm)−1. The hardness of these Cu films approaches 2.8 GPa, and their electrical resistivities are comparable to that of oxygen-free high-conductivity Cu. Compared to high-angle grain boundaries, coherent twin interfaces possess inherently high resistance to the transmission of single dislocations, and yet an order of magnitude lower electron scattering coefficient, determined to be 1.5–5×10−7 μΩ cm2 at room temperature. Analytical studies as well as experimental results show that, in polycrystalline Cu, grain refinement leads to a maximum of the strength-to-resistivity ratio, ∼250 MPa(μΩ cm)−1, when grain size is comparable to the mean-free path of electrons. However, in twinned Cu, such a ratio increases continuously with decreasing twin spacing down to a few nanometers. Hence nanoscale growth twins are more effective to achieve a higher strength-to-resistivity ratio than high-angle grain boundaries.