Interdependence between stress, preferred orientation, and surface morphology of nanocrystalline TiN thin films deposited by dual ion beam sputtering

Abstract

To clarify the underlying mechanisms that cause the preferred orientation in TiN films, we investigated the evolution with the thickness of the texture, surface morphology, and residual stress in TiN thin films deposited by dual ion beam sputtering. The films, with thickness h ranging from 50to300nm, were grown on oxidized Si substrates using a primary Ar ion beam accelerated under 1.2kV and different voltages Va of the (Ar+N2) assistance beam: 25, 50, and 150V. The influence of temperature was also investigated by varying the substrate temperature Ts (25–300°C) during growth or by performing a postdeposition annealing. X-ray diffraction (XRD) as well as transmission electron microscopy were used to study the microstructure and changes of texture with thickness h, while x-ray reflectivity and atomic force microscopy measurements were performed to determine the surface roughness. Residual stresses were measured by XRD and analyzed using a triaxial stress model. The crystallite group method was used for a strain determination of crystallites having different fiber axis directions, i.e., when a mixed texture exists. The surface roughness is found to increase with Va and Ts due to the resputtering effect of the film surface. XRD reveals that for a small thickness (h∼50nm) the TiN films exhibit a strong (002) texture independent of Va. For a larger thickness (100<h<300nm), the development of a (111) preferred orientation is observed together with a grain size increase, except at Ts=300°C, where the predominant texture remains (002). A minor (220) texture is also found, but its contribution strongly decreases with Va and Ts. The residual stresses are highly compressive, ranging from −8to−5GPa, depending on the deposition conditions. When a mixed texture exists, the analysis reveals that (111)-oriented grains sustain stresses that are about 20% more compressive than those sustained by (002)-oriented grains. The present results suggest that the change in the preferred orientation from (002) to (111) is not correlated with a strain energy minimization or with a systematic increase in surface morphology. Rather, kinetically driven mechanisms occurring during growth and linked to anisotropies in surface diffusivities, adatom mobilities, and collisional cascades effects are likely to control the texture development in TiN thin films produced with energetic ionic species. This interpretation is supported by in situ temperature XRD measurements.