Quantitative assessment of the effect of interlayers on the adhesion of copper thin-films

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 two thicknesses, nominally 430 nm and 1140 nm, and were deposited onto Si/SiO₂ wafers in three groups: no interlayer, a 7-10 nm titanium interlayer and a 7-12 nm chromium interlayer. Additionally, a second set of samples utilizing 600 nm tungsten overlayers were tested. The copper films had a residual tensile stress of 200 to 300 MPa whereas the tungsten films had a residual compressive stress of 100 to 300 MPa. Without the use of the tungsten overlayer, delamination rarely occurred. For those that did delaminate, interfacial adhesion energy was shown to increase with increasing thickness, inclusion of an interlayer, and inclusion of an overlayer. The average energies for fracture ranged from 4 to 50 J/m². There were also large variations in energies between tests at different loads, suggesting there were changes in micromechanisms affecting fracture as mode mixity (stress state) changed at the delamination boundary. The variation for those films with overlayers was higher, primarily due to successful delamination at all loads, thus testing a higher range of mode mixities. 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.