In-situ study of Ni-Ti film growth by synchrotron radiation scattering

Ni-Ti thin films are promising high performance materials in the field of micro-electro-mechanical system (MEMS) applications. Their preferential orientation is a crucial factor in determining the shape memory behavior since it has a strong influence on the extent of the strain recovery. The relationship between structure and deposition parameters is of extreme importance for future device applications. Our approach is in-situ x-ray diffraction during deposition carried out in a process chamber installed at a synchrotron radiation beamline. Near-equiatomic films (800nm) were co-sputtered from Ni-Ti and Ti targets on heated substrates (~470C) without applying a substrate bias voltage. The texture evolution during deposition is clearly affected by the substrate type. On naturally oxidized Si(100) substrates the Ni-Ti B2 phase starts by stacking onto (h00) planes, and, as the thickness increases, evolves to a (110) fiber texture. An initial significant change of the lattice parameter, as calculated from d(200) , is observed and its tendency for stabilization is coincident with the deposition time where the preferential stacking of B2 phase on (110) planes is starting.
For the deposition on thermally oxidized Si(100) there is a strong preferential stacking on (h00) planes of B2 leading to a (100) fiber texture. The measured lattice constants do not exhibit such a strong initial variation as for the sample deposited on naturally oxidized Si(100) substrate, but a continuous slight decrease of this value is perceptible.
Ni-Ti films were also deposited on top of a TiN buffer layer. There is a preferential growth of <110> oriented grains of the Ni-Ti B2 phase [ grains are defined as grains with a plane from the {hkl} family parallel to the film surface] from the beginning of the deposition, with a constant growth rate during the whole deposition for a Ni-Ti film deposited on TiN with a topmost layer formed mainly by <111> oriented grains. The Ni-Ti films deposited on top of a TiN layer where a dominating orientation could not be identified (primarily <001> and <111> oriented grains nucleate and grow) exhibit a different behavior. In this case <110> oriented grains of the Ni-Ti B2 phase dominate at small thicknesses while <211> oriented grains take over at larger thicknesses. The decrease of the lattice parameter suggests that the films experience compressive stress which is significantly relaxed with increasing film thickness.