Stresses, in thin films

A. Segmuller and M. Murakami. X-Ray Diffraction Analysis of Strains and Stresses in Thin Films. In Analytical Techniques for Thin Films. (K.N. Tu and R. Rosenberg, eds.) Academic, San Diego, 1988, p.l43. 

Stress in crystalline solids produces small shifts, typically a few wavenumbers, in the Raman lines that sometimes are accompanied by a small amount of line broadening. Measurement of a series of Raman spectra in high-pressure equipment under static or uniaxial pressure allows the line shifts to be calibrated in terms of stress level. This information can be used to characterize built-in stress in thin films, along grain boundaries, and in thermally stressed materials. Microfocus spectra can be obtained from crack tips in ceramic material and by a careful spatial mapping along and across the crack estimates can be obtained of the stress fields around the crack. ... 

There are also models assuming the electrostrictive input of incorporated anions into the breakdown initiation,285,299 ionic drift models,300 and many others reviewed elsewhere.283,293 However, the majority of specialists agree that further work is necessary in order to properly understand the physics of the electric breakdown in growing oxide films and that caused by electric stress in thin-film structures. 

The easiest way to measure stress in thin films after deposition is to analyze the change in the radius of curvature of the wafers before and after film deposition on one side, as first proposed by Stoney [7]. However, this technique usually requires the use of test wafers. After complete processing of the wafers, the stress can be obtained by measuring the deflection of membranes or indicator structures [8], To measure compressive stress, the buckling technique on double-side supported bridges [9] and harp-like structures [10] can be applied. 

The stress behaviour of films is very important in all applications of thin films with respect to durability and hence use. This fact was recognized very early on. The first investigations regarding mechanical stresses in thin films were carried out in 1877 by Mills [132] on chemically deposited films. Thirty years later, more quantitative studies were undertaken by Stoney [133]. During further investigation, it was soon found that almost all films, whatever the production method, have mechanical stresses. Tensile and compressive stresses should be distinguished tensile stresses arise if the film tends to contract parallel to its surface. Analogously, compressive stress is present if the film expands parallel to the surface. 

X-ray and electron diffraction methods are applied in order to measure atomic distances in the crystal lattice and their changes. Hence, the diffraction methods are also basically suitable for measuring the strain/stress behaviour in thin films. However, since the film thickness and the crystallite size in thin films are small, some line broadening already arises from this. In order to determine what contribution the mechanical stresses have on the diffuse lines, careful analysis of the line profiles must be undertaken [148, 151]. This method is less suitable for routine determination of stresses in thin films. In some cases, it is possible though rarely applied to determine the stresses in the films through their influence on other, known film properties, at least approximately. Such properties are, for example, the position of an absorption edge [152], the Hall effect [153], electron spin resonance spectra [155] and in the case of superconducting films, variations in the critical transition temperature [156]. However, these effects can, unfortunately, also arise for other reasons, and thus these techniques can usually only be used as supplemental experiments. 

As already mentioned, the stresses in thin films may present problems in many industrial applications of the films. Early observations of thin films in optical applications showed that particularly when the film thickness was large, cracks arose, which had cloudy marks and sometimes the films even became detached from the substrate. Exact measurements of evaporated single films and film systems indicated a partial stress compensation, especially in dielectric multilayers, since low refractive films often have tensile stresses, whilst high refractive films have compressive stresses [157,158]. 

J.D. Wilcock, Stress in thin films, Thesis, Imperial College, University of London,... 

The details of thin-film formation by PVD or CVD on the atomic and molecular scale are unknown in most cases, but such knowledge would be very helpful to design new processes and to tailor film properties. Information is lacking due to the high reactivities, short lifetimes, and low concentrations of the relevant transient gas-phase species. Furthermore, many of the thin-film properties in statu nascendi are unknown due to the experimental difficulties of thin-film characterization during deposition, particularly with non-crystalline films and if established methods such as RHEED and FEED cannot be applied. Among the film properties, mechanical stress in thin films can lead to unwanted (uncontrolled) instabilities and peel-off phenomena. Therefore, in situ diagnostic methods have been developed to... 

Figure 3.3. Scheme of optical set-up to measure substrate bending for the in situ determination of mechanical stress in thin films. ... 

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... 

In addition, based on their ion impact approach, Mirkarimi et al. [17] established a quantitative model of defect generation during ion-assisted film growth. They found that the maximum height of the defect production distribution exhibits the same (mion ion) dependence as cBN formation. Moreover, as the authors emphasize, this is the very same dependence as that found by Windischmann [51] for the formation of intrinsic compressive stress in thin films deposited under ion bombardment. 

The thermal and dynamic mechanical behaviors of triblock copolymers with a styrene/isoprene/styrene architecture were investigated in order to understand their adhesive properties. Both copolymer free films and films bonding together two titanium alloy plates were found to have thermal and mechanical response that was strongly dependent on joint preparation. Microphase separation in the melts of these triblock materials was felt to contribute to the observed phenomena namely, the presence of residual stresses in thin films which had been cooled while under high pressure. 

It is important to determine stresses in thin films in relation to mechanical stability as a result of the deposition process and the temperatures involved. The stress developed in a film is made up of three components. The first component is intrinsic, which is the result of factors such as deposition, structure, and mode of growth the second is the result of the mismatch in thermal expansion between film and substrate and the third is related to externally applied stresses (Lepienski et al., 2004). Once the values of intrinsic stress ( thermal stress ( (thermal)), and externally applied stress ((T(extemal)) are determined, the stress developed within a film can be calculated using the following equation ... 

Lepienski, C.M., Pharr, G.M., Park, Y.J., Watkins, T.R., Misra, A., Zhang, X., 2004. Factors limiting the measurement of residual stresses in thin films by nanoindentation. Thin SoUd Films 447, 215-257. 

F. Spaepen, Interfaces and stresses in thin films, Acta Mater, 48 (2000), 31-42. 

As noted in previous sections, the development of life prediction models for the reliability of patterned features such as periodic lines on substrates inevitably requires knowledge of intrinsic stress and mismatch stress generated during film growth, patterning, passivation and service. In this section, three prominent experimental methods for determining stress in thin films with patterned lines are considered the substrate curvature method, the x-ray diffraction method, and the micro-Raman spectroscopic method. The advantages and limitations of each of these techniques are also briefly addressed. 

Laugier, M. (1981), Intrinsic stress in thin films of a vacuum evaporated LiF an.d ZnS using an improved cantilevered plate technique, Vacuum 31, 155-157. 

Another way to asses the residual stress in thin films is to utilize their critical thickness,... 

R.P. Netterfield, P.J. Martin, T.J. Kinder, Real-time monitoring of optical properties and stress in thin films, in Proceedings of the 36th Annual Technical Conference, Society of Vacuum Coaters, 1993, p. 41. 

Figure 11.22 An example of the effeet of sputtering system pressure on residual stress in thin films. Redrawn with permission from D. W. Hoffman and John A. Thornton, Journal of Vaeuum Seienee Teehnology, 17, 380 (1980). Copyright 1980, AVS The Science Technology Society.

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