Indentation-induced debonding of ductile films

Thin film adhesion can be measured by means of the nanoindentation technique. In the case of a ductile film (Cu, Al, Au, etc.) well adhered to a brittle substrate, plastic deformation in the film acts as an energy dissipation mechanism, preventing film debonding. Depositing a brittle layer of W (about 1 micron thick) on top of the film of interest increases the driving force for delamination, thus solving the problem. Indentation produces circular delaminations (blisters), sometimes two orders of magnitude bigger than the indenter contact radius. Thin film adhesion was shown to scale with the film thickness, approaching the true work of adhesion of 0.8 J/m² for Cu films less than 100 nm thick. Conceptually it is important to know along what interface the fracture occurs during the blister formation. Auger electron spectroscopy (AES) has been used to determine where fracture occurs for different film systems. Cu films on SiO₂ failed along the Cu/SiO₂ interface. Fracture of Cu films with a 10 nm adhesion-promoting Ti underlayer occurred along the Ti/Cu interface. Significantly, Ti increased the thin Cu film adhesion by a factor of ten. Blisters were removed from the substrate, and the fracture surface was analyzed. In the case of thin Cu films, crack arrest (fiducial) marks were found upon blister removal, and represent the shape of the crack tip. AFM has been used to determine the geometry of the marks. The main component of the arrest marks is carbon, which comes either from the diamond tip or from the hydrocarbons adsorbed on the newly formed surfaces in the indentation process.