Thickness and grain size dependent mechanical properties of Cu films studied by nanoindentation tests

The hardness and the elastic modulus of Cu films with thickness (t) and grain size (d) have been investigated by nanoindentation tests. The d and the indentation depth increase linearly with the increase in t. The hardness rises with the decrease in t, whereas the elastic modulus is independent of t and it is about 20% less than conventional coarse-grained Cu. The enhanced hardness is attributed to the smaller d and the indentation depth. The analysis of load-displacement curves indicates that the scope of the critical shear stress for different thick Cu films ranges from 3.2 to 4.1 GPa, which is similar to the theoretical shear stress of single crystalline Cu. The present results are explained by the dislocation mediated mechanism even if d reaches about 16.4 nm for the Cu film with t = 180 nm.