Inverse effect of thickness on the ductility in nanocrystalline Cu films

The ductility of nanocrystalline Cu films with different thickness ranging from 40 nm to 600 nm was tested by uniaxial tension. The ductility of the films was determined by the changes of the statistical crack density and the electrical resistance with tensile strain. It was found that the ductility decreases with increasing the film thickness, exhibiting an inverse size effect compared to the previous results. The 40-nm-thick Cu films sustain strains up to 20% without cracks, while the 600-nm-thick Cu films fracture at only 5% with channel cracks. The effective interfacial bond strength between the films and the substrate is suggested to be responsible for the ductility of Cu films.

► The ductility of Cu films decreases with increasing film thickness.
► The effect of thickness on ductility is contrary to the previous results.
► The effective interfacial bond strength is responsible for this inverse result.
► This finding may play an important role in the working stability of MEMS.