Stable elasticity of epitaxial Cu thin films on Si

In epitaxial films, large elastic strain originating from the lattice misfit with the substrate remains, and it could affect the elastic stiffness. However, we found that epitaxially grown Cu thin films on (001) Si show exceptionally stable stiffness: the average out-of-plane elastic constant throughout the film thickness was independent of the film thickness and of the annealing procedure up to 150 degrees C. We calculated the strain dependence of the elastic constants using the higher-order elastic constants and confirmed that the misfit strain with the substrate hardly affects the stiffness in the particular case of Cu(001) thin films.