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A. J. GRIFFIN, JR.,* J. D. EMBURY,** M. F. HUNDLEY,* T. R. JERVIS,* H. H. KUNG,* W. K. SCARBOROUGH,* K. C. WALTER,* J. WOOD"* AND M. NASTASI* *Los Alamos National Laboratory, Los Alamos, NM 8754:5 **McMaster University, Ontario, Canada

ABSTRACT The effect of compositional wavelength on the residual stress, electrical resistivities and mechanical properties of Cu/Nb thin-film multilayers sputtered onto single-crystal Si substrates was evaluated. Electrical resistivities were measured down to 4 'K using a standard four-point probe measurement system. A differential specimen-curvature technique was used to detennine residual stress, and a mechanical-properties microprobe was employed to obtain hardness and elastic modulus. Characterization techniques included profilometry, Ion-Beam Analysis (IBA) and Transmission Electron Microscopy (TEM). The hardness of the Cu-Nb multilayers increased with decreasing compositional wavelength so that the layered structures had hardness values in excess of either of the constituents and the hardness predicted by the rule of mixtures. A peak in the net residual compressive stress of the multilayers was observed at a compositional wavelength of 100 nm. No resistivity plateau was observed within the composition wavelength range studied.

INTRODUCTION Copper-niobium microcomposites have been studied over the last three decades because of the exceptional combinations of high strength and excellent thermal and electrical conductivity that can be obtained and optimized for a variety of applications." 3 Much of the work centered initially on the production and evaluation of in-situ composites that were formed by successive wire drawing or rolling of Cu-Nb(10-30 volume percent) alloys.45 The microstructures of these in-situ composites were thus composed of Nb filaments or platelets within a Cu matrix - a result of the insolubility of Nb and the presence of Nb dendrites within the as-cast Cu-Nb alloy starting materials. However, a true Cu-Nb microlaminate composite consisting of alternating, uniformly parallel Cu and Nb films of known thickness can be easily produced by either evaporation or sputtering. The properties of Cu-Nb thin-film multilayer structures lend themselves well to being evaluated as a function of compositional wavelength, volume fraction and film microstructure instead of the average dendrite, filament or platelet spacing. Compositionally modulated thin-film structures can exhibit strengths that are in excess of that predicted by the rule of mixtures.7' 8 The strength can be tailored to suit a variety of applications by varying the thickness of the individual component films. Cu-Nb thin-film multilayer structures can therefore be used as high-strength thermal and electrical conductors as well as high-temperature, wear-resistant conductive coatings. Although previous investigations have been performed on Cu-Nb multilayers, these have involved phenomena of superconductivity as well as the anomalies exhibited in both the phonon dispersion curves and the folded Brillouin zones. 9