Strains and stresses in an epitaxial Ni(111)/Mo(110) multilayer grown by direct ion beam sputtering

doi:10.1016/S0169-4332(01)00736-X

Applied Surface Science

Volume 188, Issues 1–2, 13 March 2002, Pages 90-96

https://doi.org/10.1016/S0169-4332(01)00736-X Get rights and content

Abstract

An epitaxial Ni(111)/Mo(110) multilayer was grown on a $(1 1 2 ̄ 0)$ oriented monocrystalline sapphire substrate in a high vacuum sputtering deposition system. The strain and stress state of the layers has been measured with X-ray diffraction in symmetric and asymmetric geometries. Non-equal biaxial coherency stresses due to the Nishiyama–Wassermann epitaxial relation between both lattices are clearly evidenced. The values of the stress-free lattice parameters of molybdenum and nickel sublayers, deduced from the global stress analysis, supports the hypothesis of an interfacial alloying effect between layers with diffusion of nickel in the Mo layers and, to a lesser degree, of molybdenum in Ni layers. On the other hand, the Ni layers appear nearly fully relaxed. A detailed analysis of the stresses in the film shows a strong contribution of a compressive stress field due to possible post-growth atomic rearrangements occurring inside the multilayered film.

Introduction

Mechanical behavior of superlattices has received a great deal of attention in the past few years mainly due to the technological interest of such structures in the fields of magnetic recording and semiconductor devices. Large anomalies have been reported in the elastic constants, generally related to highly strained states of the constituents in metallic multilayers [1]. The stress–strain relation in such multilayered systems is very important for a clear understanding of the origin of these anomalies. As it appears that the future of this field lies in part in the control of the amount of stress necessary to reach particular properties, the influence of the deposition method on the final state is a critical issue. Sputtering deposition under a low working pressure leads generally to extreme stress state in the films allowing studies of far-from-equilibrium systems. X-ray diffraction is a very efficient tool for the determination of strains and stresses [2]. The epitaxial relation between layers facilitates the measurements and allows the proper characterization of the stress state in the case of non-equal biaxial stress encountered in f.c.c./b.c.c. stacking [3]. The stress and strain states in each constituent can be totally determined and the stress-free lattice parameter deduced for each material.

Here, we present the complete investigation of the strain and stress state of an epitaxial Mo/Ni multilayer elaborated by direct ion beam sputtering on a $(1 1 2 ̄ 0)$ oriented sapphire substrate.

Section snippets

Multilayer growth

A Ni(111)/Mo(110) multilayer with a 7.7 nm wavelength has been grown at room temperature on a $(1 1 2 ̄ 0)$ oriented sapphire substrate in a high vacuum computer-controlled NORDIKO-3000 sputtering system equipped with an RF-plasma source and two cryogenic pumps. A molybdenum buffer layer of ∼5 nm was deposited at $T=650 ° C$ prior to the first Ni layer. Pressure in the deposition chamber or deposition rates may dramatically change the final state of the deposited material. In the present case, an initial

Superlattice structure

Spectra in the symmetric geometry were taken on a Bruker D5005 θ–θ diffractometer. This geometry allows to probe the interplanar distances in the growth direction. The first and second order spectra were then fitted using the program SUPREX [4] until a consistent solution for both orders was found. The main parameters used during the fit process were the interplanar distances of both layers and the numbers of atomic planes in each elemental sublayer. No interdiffusion profile was introduced at

Discussion and conclusion

The strain–stress analysis gives evidence for significant changes in the stress-free lattice parameters. For Mo, this parameter is lower than the bulk value while it is higher in Ni. This effect can be explained by an alloying effect with introduction of nickel in molybdenum reducing the Mo stress-free parameter and of molybdenum in nickel leading to an expansion of the Ni parameter. This alloying effect takes place during the growth at the interfaces. The relative change is larger for

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