Elasticity study of very thin Cu films

The elastic properties and the surface morphology of Cu/Ta films and Ta/Cu/Ta films sputter-deposited on Si reed substrates were studied for the Cu film thickness, tCu, between 7 and 1000 nm. A monotonic increase in the internal friction, Qf−1, in the constituent Cu film above 200 K was commonly observed. For Cu/Ta films, Young's modulus of a Cu film, Ef, showed good agreement with the theoretical value for tCu > ∼80 nm and a decrease with decreasing tCu below ∼80 nm. A decrease in Qf−1 after 400 K annealing, ΔQf,400K−1 and the surface roughness showed a local minimum and a local maximum near 80 nm, respectively, indicating that the properties of the Cu film were modified by subsequent deposition of a Cu layer (the self capping effect). For Ta/Cu/Ta films, Ta capping brought about an increase in the grain size parallel to the film surface, a decrease in the internal stress and a decrease in Ef but no changes in the grain size normal to the film surface, Qf−1, ΔQf,400K−1 and the tCu-dependence of the surface roughness. These results are discussed in the light of the anelastic responses of grain boundaries and interface in nano-scale Cu films which may be important for down-sizing of electric devices.