Observation of the strengthening of Pt layers in Ni/Pt and Pd/Pt multilayers by in-situ substrate curvature measurement

The high strength of metal multilayers may be due to small layer thicknesses and grain sizes, elastic modulus differences between components of the multilayered structure and high dislocation densities, possibly due to a large lattice mismatch between the layers. This high strength is reflected in the large stresses often observed in the individual layers of metal multilayer structures. In this study, a detailed comparison of the stress behaviors of Pt in Ni/Pt and Pd/Pt multilayers is presented. Stress is measured by monitoring the curvature of the substrate continuously during film deposition. Pt is observed to be in compression in both systems with contributions from coherency and atomic peening due to bombardment by energetic particles during the sputtering process. Pt is able to withstand a higher stress level when grown on Ni than when grown on Pd, implying that the Pt layers are stronger in the Ni/Pt multilayer. Likely causes for the observed increase in strength are the higher shear stiffness of Ni compared to Pt and the large lattice mismatch between Ni and Pt. Cross-section TEM micrographs show structural differences between Ni/Pt and Pd/Pt multilayers. The stress evolution in the Pt layers is compared to model predictions based on the Matthews–Blakeslee critical stress theory.