Quantification of the Influence of Thin Film Microstructure on Adhesion for a Model Brittle-Ductile Interface

The influence of film microstructure on interface adhesion was studied on a model brittle-ductile interface consisting of thin Cu films on brittle glass substrates. Therefore, 110 nm thin Cu films were deposited on glass substrates using magnetron sputtering. While film thickness, residual stresses, and texture of the Cu films were maintained comparable in the sputtering processes, the film microstructure was varied during deposition and via isothermal annealing resulting in four different Cu films with bimodal grain size distributions. The interface adhesion of each Cu film was determined using stressed Mo overlayers, which triggered delamination of the Cu film from the glass substrates in the shape of straight spontaneous buckles. The model of Hutchinson and Suo for spontaneous buckles was applied to quantify the mixed mode adhesion energy for each film system ranging from 2.35 J/m² for the films with the highest amount of large grains to 4.90 J/m² for the films with the highest amount of nanosized grains. This surprising result could be clarified using an additional study of the buckles using focused ion beam cutting and quantification via confocal laser scanning microscopy to decouple and quantify the amount of elastic and plastic deformation stored in the buckled thin film. It could be shown that the films with smallest grains exhibit the possibility to absorb a higher amount of energy during delamination, which explains their improved adhesion.