TENSILE TESTING OF FREE-STANDING Cu, Ag AND Al THIN FILMS AND Ag/Cu MULTILAYERS

ÐFree standing polycrystalline thin ®lms with a strong h111i texture were tested in uniaxial tension. Studied were electron-beam deposited Ag, Cu and Al ®lms, and Ag/Cu multilayers consisting of alternating Ag and Cu layers of equal thickness, between 1.5 nm and 1.5 mm (bilayer repeat length, l, between 3 nm and 3 mm). The ®lms had a total thickness of about 3 mm. A thin polymeric two-dimensional di€raction grid was deposited on the ®lm surface by microlithographic techniques. Strains were measured in situ from the relative displacements of two laser spots di€racted 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 l. No ``supermodulus'' e€ect 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 l is reduced. For l<80 nm, the ®lms are brittle at room temperature: they break without macroscopic plastic ̄ow. For l > 80 nm, the yield stress increases with decreasing l according to a Hall±Petch-type relation. No softening with decreasing grain size was observed even at the lowest values of l. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.