Influence of phase stability on the in situ growth stresses in Cu/Nb multilayered films

Abstract

As the length scale of individual layers were reduced in a Cu/Nb multilayer, a face centered cubic (fcc)-to-body centered cubic (bcc) and bcc-to-fcc transformations were noted for Cu and Nb respectively. In addition, at equal fractions of Cu/Nb and at a very thin thickness, the layers vitrified. These phase transformations have been modeled using a thermodynamic phase diagram where strain, interfacial, and volumetric energy considerations were used to determine the phase as a function of the bilayer thickness and volume fraction in the multilayer. Using HRTEM, the evolution of the interface from semicoherent-to-coherent was observed as a function of phase and bilayer spacing and is discussed in terms of the interfacial energy contributions towards phase stability. The vitrification has been rationalized by the positive enthalpy of mixing with clustering quantified by atom probe tomography between these two species. All of these phase transformations have been related to real-time, intrinsic growth stress measurements. Upon phase transforming, a reduction in the compressive stress within all of the multilayers were noted. This relaxation was captured in a molecular dynamics simulation of the multilayers and is contributed to the change in interfacial stress and stress distributions within the Cu layer. This demonstrates that phase transformations provide a means to tailor film stress in nanolaminate materials.