Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf
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Characterization of Ni-Ti SMA thin films: from annealing to in-situ sputteringMartins, R. M. S.; Schell, N.; Beckers, M.; Silva, R. J. C.; Braz Fernandes, F. M.
Shape Memory Alloy (SMA) NiTi thin films have attracted much interest as functional and smart materials due to their unique properties. However, there are still important issues unresolved like formation of film texture and its control as well as substrate effects. For microelectromechanical systems (MEMS) integration, there is a need for an electrically and thermally insulating or sacrificial layer like poly-Si. In the present study, NiTi thin films have been prepared by d.c. magnetron sputtering on Si(100) and poly-Si/Si(100) substrates to elucidate the substrate influence on the crystallization of these films. Following the structural development of the films during crystallization (at a constant temperature of 430 ºC) by GIXD, is was observed that the crystallization process is significantly enhanced for low target/substrate distances of 4 cm instead of 7 cm for Si(100) substrates. The presence of an intermediate layer of poly-Si furthermore drastically enhances the crystallization process.
In-situ XRD during the growth of Ni-Ti thin films was chosen in order to investigate their texture development using a deposition chamber installed at ROBL. Near-equiatomic films were obtained by co-sputtering from Ni-Ti and Ti targets. The texture evolution during deposition is clearly affected by the substrate type and the ion bombardment of the growing film. On naturally oxidized Si(100) substrates the NiTi B2 phase starts by stacking onto (h00) planes, and as the thickness increases evolves into a (110) fiber texture. This pronounced cross-over is only observed when a substrate bias voltage (-45 V) is applied for the deposition on thermally oxidized Si(100) substrates. The oxide layer plays an important role on the development of the (100) orientation of the B2 phase during deposition on heated substrates (~ 470ºC).
The effect of a TiN layer deposited on top of the SiO2/Si(100) substrate prior to the deposition of the NiTi films was also analysed. These experiments have shown that TiN acts not only as a diffusion barrier, but also induces different crystallographic orientations. These are promising results concerning the manipulation of the crystallographic orientations of NiTi thin films, since the texture has a strong influence on the extent of the strain recovery.
ESRF Science Days 2006, 07.-08.06.2006, Autrans, France