Formation of coherent structures and mechanical properties of AlN/TiN multilayers G. Allidi, F. Medjani, R. Sanjines, A. Karimi Institute of Complex Matter Physics, Faculty of Basic Science Swiss Federal Institute of Technology Lausanne (EPFL), Switzerland Abstract AlN/TiN multilayered thin films with layer thickness ranging from 1 nm to 50 nm were synthesized using rf magnetron sputtering at 400°C. Two series of samples were prepared at the substrate bias of V b = -25 V and -100 V to modify growth texture of individual layers and verify its influence on the formation of coherent structures. XRD and TEM observations showed that in large period films (t c ≥ 30 nm) each constituent grows under its own growth kinetic, leading to the formation of nano-crystalline film randomly oriented with no pronounced texture. Decreasing progressively the layer thickness favours the alignment of (0002) basal plane of w-AlN on (111) plane of TiN, and results in development of strong (111) texture, prerequisite for stabilisation of c-AlN and the formation of epitaxial coherent structures. The degree of crystallographic coherence was found to be higher in TiN(111) oriented films than for TiN(002) textured films. The increase of hardness coincides with the structure transition from a randomly oriented nanocrystalline films to a highly (111) textured multilayers, and the maximum hardness was obtained for epitaxially coherent nanolayers. 1. Introduction Decreasing the layer thickness in multilayered thin films is often accompanied by structural transitions in terms of length scales and their interactions, which can exert strong influence on the material parameters. The formation of epitaxial coherent structures in isostructural multilayers [1.2] and stabilisation of metastable phases in non-isostructural multilayers [3,4] have widely been used to optimize semiconductor materials [5] and magnetic thin films [6]. Such dimensionally induced structural transitions have also been investigated for mechanical applications and were found to give rise to a significant enhancement of hardness and a notable improvement of overall mechanical properties in many metallic and ceramic systems [7,8]. One of the nanoscale modulated structures which have been received a great deal of attention is the TiN/AlN multilayered system, highly interesting for fundamental point of view as well as for application purpose [9,10]. The TiN films show higher hardness and thermal stability, while AlN posses elevated thermal conductivity and bending strength [11]. AlN crystallises in würtzite-type structure that can be transformed into zincblend or rocksalt cubic under high pressure and epitaxial growth [12], and of that it allows the formation of coherent layers with TiN. Several studies on AlN/TiN system have reported the formation of superlattice structures when the thickness of AlN layers decreases below a critical value, often estimated to about t c ≈ 2 - 3 nm depending on deposition technique for example magnetron sputtering, pulsed laser deposition, cathodic arc, etc. [13.14]. Beyond his range, the AlN phase was found to exhibit usually hexagonal würtzite-type AlN. However, the mechanism by which such phase transformation is brought about is not yet completely clear. Some studies based on the x-ray diffraction evidence seem to assert that hcp to fcc transformation is a post-nucleation phenomenon [13] involving rearrangement, migration, and rotation of crystal lattice or stable clusters. In contrast some other Mater. Res. Soc. Symp. Proc. Vol. 890 © 2006 Materials Research Society 0890-Y02-05.1