Size-dependent tensile failure of epitaxial TiN/Cu/TiN sandwich pillar structures: A combined experimentation – Atomistic simulation study

Abstract

A combined experimentation - molecular dynamics simulation study was conducted to understand tensile failure of TiN/Cu/TiN interfacial regions. Tensile loading was conducted on micro-pillar specimens fabricated from TiN/Cu/TiN thin film sandwich structures. The Cu layer and the TiN layer underneath were grown epitaxially on MgO(001) substrates, with Cu[110]//TiN[001] in the growth direction and Cu<111>//TiN<100> and Cu<112>//TiN<100> within the growth plane. The Cu layer contains numerous nanotwins with the {111} twin plane parallel to the growth direction, with 2–10 nm wide twin bands rotated in-plane by 90° in different yet symmetry-equivalent epitaxial domains. Tensile loading in-situ a scanning electron microscope measured tensile fracture stress ∼1.5 GPa and revealed a surprising failure mode transition. At a larger Cu layer thickness, ductile tensile fracture occurred within the Cu layer. At smaller Cu layer thicknesses, apparently brittle fracture occurred close to or at the Cu/TiN interface. The accompanying molecular dynamics simulations illustrate a significant dependence of the failure mode on the aspect ratio of Cu pillars under tensile loading. With pillars of small height-to-diameter ratios, tensile loading leads to a significant hydrostatic tension within, as well as significant plasticity throughout the Cu pillar, in particular near the top and bottom Cu/TiN interfaces. The high degree of dislocation activities close to or at the interface, combined with dislocation pile-up, serves to create nanovoids. The high hydrostatic tension furnishes a driving force for growth of such nanovoids, leading to rapid tensile fracture. The simulation results offer an analogy to experimental observations and mechanistic understanding of tensile failure mechanisms for ceramic/metal/ceramic interfacial regions.