Strain, thin-film systems

Huse has pointed out that strain is to be expected in most thin-film systems, since even in the incommensurate case the intrinsic surface stress will strain the film (18). As a result, we conclude that incomplete wetting is expected for all crystalline films, except in the case where there is an epitaxial relationship between film and substrate and that the film is maintained at its bulk equilibrium lattice spacing. 

Stainless steel substrates are used because of their suitable mechanical properties and stability against corrosion. Thin-film technology on steel substrates achieves high accuracy and long-term stability. A schematic cross section of a thin-film system for the creation of strain gauges on stainless steel is shown in Figure 5.4.1. The thin-film system contains four functional layers the isolation layer, which also forms the interface to the steel substrate the sensing layer the contact layer and finally, the passivation layer. 

For even a relatively small number of interacting dislocations which have independent degrees of freedom in a thin film system, the behavior becomes very complex. In order to deal with this complexity, Nicola et al. (2003) reformulated the problem of dislocation formation and interaction in a strained single crystal thin film to allow for determination of its features by numerical methods. A single crystal thin film on an elastic substrate was considered to deform under plane strain conditions. Edge dislocations... 

For the case of a strained thin film of mean thickness / on a relatively thick unstrained substrate, the depth of the valleys in this equilibrium configuration determine whether the material separates into distinct islands or it reaches equilibrium before doing so. The valley depth in this two degree of freedom idealization is a distance ai - - a2 below the mean film thickness. Therefore, the material will separate into islands if h < ai + a2 and it will not do so otherwise. For the case of A = Acr, this depth is approximately 0.135A = O.lSAcr- This observation implies that a strained film will divide up into islands only if the mean thickness is less than about 18% of the system specific critical wavelength. The particular model is too idealized... 

Various experimental methods have been developed for investigating the magnetoelastic properties of thin films and nanoscale magnetic systems. In the following subsections, we discuss the most important ones (i) the magnetoelastic cantilever, (ii) strain induced anisotropy, (iii) magnetostriction in spin valves, (iv) strain modulated ferromagnetic resonance, (v) secondary-electron spin-polarisation, and (vi) strain-induced anisotropy due to the spontaneous strains. 

Stagnation flows represent a very important class of flow configurations wherein the steady-state Navier-Stokes equations, together with thermal-energy and species-continuity equations, reduce to systems of ordinary-differential-equation boundary-value problems. Some of these flows have great practical value in applications, such as chemical-vapor-deposition reactors for electronic thin-film growth. They are also widely used in combustion research to study the effects of fluid-mechanical strain on flame behavior. 

One of the most frequently observed phenomena in epitaxial growth is the formation of strain relief patterns. These are caused by the mismatch of the unit cell size of the substrate and the deposited film. In many cases the strain or stress, which is imposed on the thin film by fhe subsfrafe lattice is relieved by reconstruction of the film. This reconsfrucfion can but must not necessarily lead to a nanopatterned film. An inferesfing example is the growth of Ag on Pt(l 11) (see Fig. 10) [41]. It has been shown for this particular system that the first Ag layer grows pseudomorphically exhibiting an isotropic compressive strain of 4.3% whereas in higher layers this strain is relieved by the formation of a dislocation network [42-47]. In order to improve the long-... 

Fig. 13.34 Engineering stress-strain curves of free-standing thin films of Nylon-6, crystallized between PS layers. The film of thickness 0.15 rm has the largest flow stress since the best slip system of (001) [010] is perpendicular to the axis of the film (from Muratoglu et al. (1995b) ...

Hay, R. S., Coherency strain energy and thermal strain energy of thin films in any crystal system, Scripta MetalL, 26, 535-40, 1992. 

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