Thickness and grain size dependence of the strength of copper thin films as investigated with bulge tests and nanoindentations

Abstract

Thin films have often been reported to exhibit increased strength compared to their bulk counterparts. In this study, copper films with a thickness ranging from 400 to 2400 nm were investigated via bulge testing and nanoindentation, focusing on the influence of thickness and microstructure on the mechanical properties. Bulge tests on freestanding membranes show that the yield strength follows Hall–Petch behaviour, as bulk materials do. This also holds true for films on a substrate, as long as the microstructure contains several grains in the thickness direction. The determined Hall–Petch coefficient of 0.16 MPa m^−1/2 is also similar to bulk materials. Texture was found to have very limited influence on mechanical behaviour. Nanoindentation data, evaluated by the Han–Nix method, showed that the hardness of thin films is higher than that of bulk materials with the same grain size.

Keywords:

• size effect
• thin films
• copper
• bulge test
• nanoindentation

Acknowledgements

The authors thank Verena Maier and Tobias Schilling for their contributions to the nanoindentation measurements. The authors also gratefully acknowledge the funding of the German Research Council (DFG), which, within the framework of its ‘Excellence Initiative’, supports the cluster of excellence ‘Engineering of Advanced Materials’ at the University of Erlangen-Nürnberg.