Synchrotron x-ray diffraction studies during co-sputtering deposition of Ni-Ti films: texture development, self-shadowing and surface diffusion effects

A better understanding of the underlying growth mechanisms of shape memory alloy films and their microstructural development requires sophisticated in-situ techniques. In-situ studies of Ni-Ti film growth were performed using a sputter deposition chamber inserted into a diffractometer installed at a synchrotron-radiation facility. Crystalline films were deposited onto TiN buffer layers grown onto oxidized Si(100) substrates. X-ray diffraction investigations carried out during Ni-Ti films processing allowed us to assess the evolution of their structure. The role of self-shadowing on the inhibition of the development of grains of the Ni-Ti austenitic (B2) phase with the more densely packed crystallographic planes [(110) planes] parallel to the substrate surface was observed in-situ. The application of a bias voltage (V_b) of -45 V to the substrate during film growth enhanced the mobility of the adatoms on the substrate surface, which opposes shadowing, resulting in the preferential stacking of (110) planes of the B2 phase parallel to the substrate since the beginning of the deposition. The in-situ analysis of the variation of the lattice parameter values (a₀) for the Ni-Ti film deposited without V_b shows a continuous decrease of this parameter, which suggests that the film experiences compressive stress that was significantly relaxed with increasing film thickness. The application of a V_b of -45 V led to a reasonable stability of a₀ along the processing steps and to an overall decrease of this value. Ex-situ analyses show a relationship between the nanostructure/structure of the Ni-Ti films and their phase transformation behaviour.