A microindentation method for estimating interfacial shear strength and its use in studying the influence of titanium transition layers on the interface strength of epitaxial copper films on sapphire

An approximate elastic model is derived that converts measurements of indentation depth as a function of the applied load in a microindentation experiment, to an estimate of the interfacial yield strength of a metal film on a ceramic substrate. The analysis is done in two stages. In the first stage the indenter lies entirely within the overgrowth. In the second, the indenter pierces through the interface and into the substrate. The load is predicted to increase as the second power of the displacement in Stage I. In Stage II the net load supported in the film is predicted to increase linearly with the displacement. The coefficients for the parabolic and linear functions are related to the effective interfacial shear strength. Experiments conducted on epitaxial films of (111) copper on (0001) sapphire, show excellent fits with the Stage I and Stage II models, yielding self consistent values for the interfacial shear strength. The technique was used to measure the influence of titanium transitional layers, varying in thickness from 0.7 nm (∼ 3 monolayers) to 110 nm. Just 0.7 nm thick titanium produced a 40% increase in the interfacial strength of the copper/sapphire interface, with thicker titanium layers producing only a marginal further improvement in the interfacial strength. Possible reasons why the elastic analysis provides a good description of the data, even through the deformation underneath the indenter is plastic, are discussed.