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

Free standing polycrystalline thin films with a strong (111) texture were t ested 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 mu m (bilayer repeat length, lam bda, between 3 nm and 3 mu m). The films had a total thickness of about 3 m u m 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 gri d. The average values of the Young's moduli, determined from hundreds of me asurements, are 63 GPa for Ag, 102 GPa for Cu, 57 GPa for Al and 87.5 GPa f or Ag/Cu multilayers. In all cases, these values are about 20% lower than t hose calculated from the literature data and, for the Ag/Cu multilayers, ar e independent of lambda. No "supermodulus" effect was observed. The 20% red uction in modulus is most likely the result of incomplete cohesion ("microc racking") of the grain boundaries. The ductility of the Ag/Cu multilayers d ecreases when lambda is reduced. For lambda < 80 nm. the films are brittle at room temperature: they break without macroscopic plastic flow. For lambd a > 80 nm, the yield stress increases with decreasing lambda according to a Hall-Fetch-type relation. No softening with decreasing grain size was obse rved even at the lowest values of lambda. (C) 2000 Acta Metallurgica Inc. P ublished by Elsevier Science Ltd. All rights reserved.