Stress and preferred orientation in nitride-based PVD coatings

Nitride-based coatings are nowadays widely studied both from fundamental and technological point of views due to their unique physical and mechanical properties. Among the binary nitrides, TiN is the most stable thermodynamically and has been widely used due to the combination of its covalent and metal-like characteristics. Coatings produced by Physical Vapor Deposition (PVD) techniques generally exhibit a crystallographic texture, which in turn may strongly affect their properties, such as hardness, wear resistance, or diffusion barrier properties in microelectronic devices. Therefore great efforts have been made in recent years to understand the underlying mechanisms governing texture development in nitride thin films. In particular, the issue of stress build-up during PVD growth and its possible interplay with film preferred orientation is essential to address. We present a brief overview of stress and preferred orientation in nitride-based thin films, either in the form of single-, multi-layered or nanocomposite coatings. X-ray Diffraction (XRD) was used in the standard theta-2 theta configuration to study the texture development with film thickness, while the sin(2)psi method combined with linear elasticity theory was employed to determine the complete strain/stress state. XRD measurements were made in the framework of the crystallite group method, which is of prime importance in thin films exhibiting a mixed texture, as it enables to selectively measure the elastic strain in a given subset of grains. For PVD films grown with energetic particles, the appropriate modeling requires the use of a triaxial stress tensor, including a hydrostatic stress component to take into account the local distortions induced by growth-defects. This approach enables us to determine the 'stress-free and defect-free lattice parameter', a(0), solely linked to chemical effect. Illustrations will be given for fiber-textured TiN and ZrN films deposited on Si substrates, epitaxial TiN layers as well as epitaxial TiN sub-layers in TiN/Cu multilayers grown on (001) MgO single crystal substrates. Ternary TiN-based coatings, either in the form of solid solutions or nanocomposites will be also investigated. (c) 2007 Elsevier B.V. All rights reserved.