Polycrystalline thin films, stress

Windischmann, H. (1987), An intrinsic stress scaling law for polycrystalline thin films prepared by ion beam sputtering , Journal of Applied Physics, 62, 1800-1807. 

Table 7.1. Representative values of the flow stress of polycrystalline thin films of metals and alloys as a function of film thickness (h ) and average grain size (g.s.). Data obtained from micro-tensile tests, microbeam deflection and x-ray diffraction measurements.

The results presented in Tables 7.1 and 7.2, and Figure 7.29 also reveal that the flow strength of a polycrystalline thin film is several times larger than that exhibited by a bulk material of the same composition and grain size. For example, a 1-yum thick continuous film of pure A1 on a relatively thick Si substrate, with a grain size comparable to film thickness, has a tensile flow stress of approximately 200 MPa at room temperature, whereas... 

Fig. 9.10. Schematic diagram of a polycrystalline thin film of thickness /t on a substrate. Grains are of width b and the system is periodic in the x—direction. Initially, the film supports a mismatch tensile stress in the direction the interface, which induces a configurational force tending to drive mass transport from the free surface into the grain boundaries. The mass fluxes along the free surface and jj, along the interface are estimated through the use of the variational principle based on the functional in (9.51).

Chason, E., Sheldon, B. W., Freund, L. B., Floro, J. A. and Hearne, S. J. (2002), Origin of compressive residual stress in polycrystalline thin films. Physical... 

Thompson, G. V. (1993), The yield stress of polycrystalline thin films. Journal of Materials Research 8, 237-238. 

The external manifestation of reorientation relaxation under an applied stress is the anelastic strain that accompanies a net change of orientational order. In contrast to the elastic strain, the anelastic strain develops in a time-dependent manner governed by the rate of the reorientation jump. Under a static stress, the relaxation may therefore be observed as a limited (and recoverable) creep process. Frequently, however, it is more desirable for reasons of sensitivity or convenience to observe the relaxation dynamically as a loss-peak, via internal friction measurements made as a function of temperature and/or vibration frequency. Figure 2 shows the oxygen Snoek peak in polycrystalline thin film niobium, tested in the same vibrating-reed apparatus" - used for our studies of... 

For polycrystalline specimens without texture, the line shift is proportional to the trace of the stress tensor. In the case of untextured thin films with free surface, the stress component perpendicular to the film plane vanishes.Thus, the line shift is given by... 

Vibrational Raman band intensities and frequencies are also dependent on temperature, applied pressure, and the intrinsic microstructure of the material. These second-order parameters may be extracted from measured spectra. Both X-ray diffraction lines and Raman bands from polycrystalline materials show increased broadening as the microcrystallite grain sizes decrease. In fact, for the hexagonal phase of BN, bandwidths vary linearly with the reciprocal grain size (13). Inherent stress in thin films is manifested in vibrational line shifts. Based on pressure-dependent measurements of vibrational frequencies in bulk solids, inherent stress and stress inhomogeneity can be determined in thin films. Since localized stress can influence the optical and electronic properties of a thin film, it appears to be an important parameter in film characterization studies. Vibrational features also exhibit temperature-dependent frequency shifts. Therefore, an independent measurement of temperature is sometimes necessary to deconvolute these effects. Reference to Figure 1 shows that the molecular temperature of a material may be determined from the Stokes/anti-Stokes... 

The results for characterizing stress in elastically anisotropic cubic single crystal films presented in Section 3.5 and in this section can also be adapted, under appropriate conditions, for textured polycrystalline films. An equi-biaxial mismatch strain in the (001) or (111) textured thin film of a cubic crystal results in an equi-biaxial mismatch stress. Because the response is transversely isotropic, the biaxial moduli derived for cubic crystals of these orientations can also be used to characterize polycrystalline thin... 

The various curvature measurement methods described in Section 2.3 provide convenient tools with which the film stress can be assessed by using the approaches outlined in Chapters 2 and 3 and Section 7.4. However, these approaches are often predicated on assumed microstructural conditions which may differ markedly from real behavior in some material systems. Microstructural evolution and competition between strain relaxation mechanisms can influence the evolution of substrate curvature during thermal excursions and the manner in which such effects are strongly affected by the geometry and material properties of the particular thin film-substrate system being considered. The following points are discussed in order in the remainder of the section, primarily in the context of polycrystalline metal Aims ... 

Fig. 7.29. Experimentally determined variation of the effect of film thickness on the tensile yield stress of polycrystalline A1 thin films on Si substrates at 60 °C. The data are shown for three different grain sizes. Adapted from Venkatraman and Bravman (1992).

As previously shown, polycrystalline aluminum (Al) thin films deposited and compressively stressed in an ultrahigh vacuum do not form nodules. Yet, when the same Al films are exposed to ambient and stressed again, nodules form [59]. 

FIG. 39 A schematic diagram of the cross section of a polycrystalline Al thin film under compression, (a) The arrows indicate the flow of atoms to relieve the stress on the basis of the Nabarro-Herring or Coble creep models in the case where there is no surface oxide, (b) If a surface oxide exists, stress relief is only possible if it can be broken locally and penetrated. (Courtesy of UCLA.)... 

The characteristics of monocrystalline diamond films are much more clearly defined. Still polycrystalline films are employed in most cases as the high price interferes with large scale application of the monocrystalline material. Even for thin layers there is no significant change to the essential characteristics of diamond. For this reason as well as to save further material, it is a common practice to employ coated substrates with a film the thickness of micrometers spread on their surface (Section 6.6.1). The endurance of such films against mechanical stress is essentially influenced by two factors Firstly, by delamination (peeUng off) of the film from the substrate, and secondly by normal, gradual wear. 

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