Tensile testing of free-standing Cu, Ag and Al thin films and Ag/Cu multilayers

Abstract

Free standing polycrystalline thin films with a strong 〈111〉 texture were tested in uniaxial tension. Studied were electron-beam deposited Ag, Cu and Al films, and Ag/Cu multilayers consisting of alternating Ag and Cu layers of equal thickness, between 1.5 nm and 1.5 μm (bilayer repeat length, λ, between 3 nm and 3 μm). The films had a total thickness of about 3 μm. A thin polymeric two-dimensional diffraction grid was deposited on the film surface by microlithographic techniques. Strains were measured in situ from the relative displacements of two laser spots diffracted from the grid. The average values of the Young’s moduli, determined from hundreds of measurements, are 63 GPa for Ag, 102 GPa for Cu, 57 GPa for Al and 87.5 GPa for Ag/Cu multilayers. In all cases, these values are about 20% lower than those calculated from the literature data and, for the Ag/Cu multilayers, are independent of λ. No “supermodulus” effect was observed. The 20% reduction in modulus is most likely the result of incomplete cohesion (“microcracking”) of the grain boundaries. The ductility of the Ag/Cu multilayers decreases when λ is reduced. For λ<80 nm, the films are brittle at room temperature: they break without macroscopic plastic flow. For λ>80 nm, the yield stress increases with decreasing λ according to a Hall–Petch-type relation. No softening with decreasing grain size was observed even at the lowest values of λ.