Nanocomposites and nanomaterials
The effect of interfaces number on solid state reactions in Ni-Ti thin films
A. Lozova, I. Kruhlov, S. Voloshko, A. Orlov
1 Department of Physical Materials Science and Heat Treatment, National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute" Peremohy Ave 37, Kyiv, Ukraine, 03056
E-mail: anja.lozova@gmail.com

NiTi shape memory thin films are highly attractive materials for microelectromechanical systems (MEMS). The actuality of thin-film NiTi is attributed to its large recovery strain and high work output per unit, being useful for the fabrication of microactuators, microsensors, micropumps etc. The common pathway for NiTi fabrication is the film sputtering from the NiTi alloy target. However, the alternative approach is the deposition of layered stack with Ni and Ti individual layers, which is helpful in terms of more precise tuning of the chemical composition of the intermetallic compound. Upon the post-deposition annealing, the intermetallic compound is typically formed through the stages of amorphization and next crystallization [1]. However, in this case the number of the layers involved in the initial stack could drastically influe the solid-state reactions kinetics due to the crucial role of interfaces on the diffusion in thin films. 
Present study is aimed to figure out the effect of interfaces number on the solid-state reactions taking place upon annealing in high vacuum (10-4 Pa) for 30 min of Ni(30 nm)/Ti(30 nm) and [Ni(15 nm)/Ti(15 nm)]2x thin films prepared onto Si(001) substrate using rf magnetron sputtering at room temperature. Structural study performed using XRD technique revealed the difference in the amorphization behavior at the first stages of annealing of two- and four-layered stacks. It was found that the onset temperature of both amorphization and crystallization is about 100 oC lower in the film consisted of four layers compared to the bilayer system. The following rise of the temperature resulted in the similar recrystallization behavior with the formation of the NiTi intermetallic compounds observed for both films. The findings of this study can be useful for better understanding of the solid-state reactions taking place during the fabrication of application relevant NiTi thin films governed by the interfacial reactions between metal layers.

1.	Lehnert T., Grimmer H., Boni P., Horisberger M., Gotthardt R. Characterization of shape-memory alloy thin films made up from sputter-deposited Ni/Ti multilayers // Acta mater., 2000.-48.-P.4065-4071.