HR-TEM observation of the interfacial diffusion zone in magnetron sputtered Ni-Ti thin films deposited on different Si substrates

NiTi Shape Memory Alloy (SMA) thin films have been recognized as promising materials in the field of microelectromechanical systems (MEMS). Previous studies on the NiTi/Si(100) interface have shown that during high temperature deposition there exist interfacial diffusion and chemical interactions at the interface where Ni and Ti silicides form [1]. Since the NiTi film required for MEMS applications is usually a few micrometers thick, a relatively thin reaction layer could have significant adverse effects on shape memory properties. An intermediate SiO2 layer was found to serve as an effective diffusion barrier but at the expense of film adhesion [2].

For the present study, NiTi thin films were prepared by magnetron co-sputtering from NiTi and Ti targets in a specially designed chamber mounted on the 6-circle goniometer of the ROssendorf BeamLine (ROBL-CRG) at ESRF, Grenoble (France) [3-5]. The depositions were made at a temperature of  470°C on Si(100), Si(111) and poly-Si substrates and a detailed High-Resolution TEM (Philips TEM CM300) analysis of the interfacial structure has been performed. When NiTi is deposited on Si(100) substrate, a considerable diffusion of Ni into the substrate takes place, resulting in the growth of semi-octaeder NiSi2 silicide (Fig. 1). In the case of NiTi deposited on Si(111), there appears an uniform thickness plate, due to the alignment between substrate orientation and the [111]-growth front (Fig. 2). For NiTi deposited on poly-Si, the diffusion is inhomogeneous. Preferential diffusion is found along the columnar grains of poly-Si, which are favorably aligned for Ni diffusion (Fig. 3). These results show that for the NiTi/Si system, the morphology of the diffusion interface is strongly dependent on the type of substrates.