Quantitative measurement of the effect of annealing on the adhesion of thin copper films using superlayers

Nanoindentation at loads from 100 to 750 mN was used to measure the interfacial adhesion energy of sputtered copper thin-films, for which the release of elastic strain energy drives delamination. The as-sputtered films were of four thicknesses, nominally 225 nm, 400 nm, 600 nm and 1000 nm, and were deposited onto Si/SiO₂ wafers; one wafer of each two-wafer run was annealed at 600 °C for 2 hrs in a nitrogen atmosphere. Subsequently, a nominally 700 nm thick sputtered superlayer of tungsten was deposited over all wafers. The tungsten overlayer was used to promote delamination at the copper-SiO₂ interface. The energies for fracture ranged from 1 to 80 J/m², increasing with increased indentation depth, and a trend of higher adhesion energy for annealed films. The large values of adhesion energy with respect to the thermodynamic work of adhesion are attributed primarily to plasticity and/or void nucleation within the copper film, which appears to be strongly influenced by the constraint of an overlayer.