Nanometer-scale multilayer coatings combining a soft metallic phase and a hard nitride phase: study of the interface structure and morphology

The present study reports on the microstructure and interface morphology of multilayered films consisting of inherently soft metal layers (Cu or Ag) separated by hard titanium nitride (TiN) layers. Dual ion beam sputtering technique was used to synthesise multilayers with modulation periods (Λ ranging between 2.5 and 50 nm). Both high-angle X-ray diffraction (XRD), X-ray reflectivity (XRR) combined with grazing incidence small angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM) techniques were used to globally characterise the multilayers structure as well as the nature of the interfaces. Symmetric XRD profiles revealed that all the constituents in TiN/Cu and TiN/Ag multilayers exhibited a [002]-out-of-plane texture, while pure Ag and Cu films had a (111) preferred orientation. For TiN/Cu, a cube-on-cube epitaxial growth was found for all Λ values, with semi-coherent interfaces. High resolution TEM observations showed the presence of interfacial misfit dislocations and micro-twins, which were formed to relax the large misfit strain (15.9%) between the two fcc lattices. They also revealed the presence of facetted islands in the Cu layers. For TiN/Ag, in spite of the much smaller lattice mismatch (3.6%), no strong preferred orientations were observed for Λ>20 nm. In both multilayer systems, XRR spectra exhibited a large number of superlattice reflections, while the high-angle spectra did not reveal such features, indicating that although a fairly constant bilayer thickness over the multilayer stack was achieved, the structural coherency length was rather small. The interfacial roughness was found to be much larger in TiN/Ag than TiN/Cu.