Finite element modeling of stress variation in multilayer thin-film specimens for in situ transmission electron microsco
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J.H. An Materials Science and Engineering Program, University of Texas, Austin, Texas 78712
R. Huang Department of Aerospace Engineering and Engineering Mechanics, University of Texas, Austin, Texas 78712
P.J. Ferreiraa) Materials Science and Engineering Program, University of Texas, Austin, Texas 78712 (Received 28 January 2007; accepted 16 May 2007)
Multilayer thin-film materials with various thicknesses, compositions, and deposition methods for each layer typically exhibit residual stresses. In situ transmission electron microscopy (TEM) is a powerful technique that has been used to determine correlations between residual stresses and the microstructure. However, to produce electron transparent specimens for TEM, one or more layers of the film are sacrificed, thus altering the state of stresses. By conducting a stress analysis of multilayer thin-film TEM specimens, using a finite element method, we show that the film stresses can be considerably altered after TEM sample preparation. The stress state depends on the geometry and the interactions among multiple layers.
I. INTRODUCTION
Multilayer thin-film materials are used in many electronic and optoelectronic devices, wherein the thickness of each layer ranges from a few microns to several nanometers. With various chemical compositions and deposition methods used for each layer, multilayer thin films are normally subjected to residual stresses at room temperature.1 In addition, these multilayers undergo thermal cycling during processing, and are subjected to changes in stress due to differences in the coefficients of thermal expansion for the various materials. These stresses are often detrimental to the performance of the aforementioned devices, leading to excessive substrate curvature, void formation and/or electromechanical failure.2–4 Occasionally, the presence of stress and strain enhances performance of functional devices.5 In either case, to optimize the application of multilayer thin films, it is crucial to understand the nature of residual stresses. The use of in situ transmission electron microscopy (TEM) has been developed as one powerful technique in
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0341 J. Mater. Res., Vol. 22, No. 10, Oct 2007
establishing a direct link between residual stresses and induced microstructural evolution in multilayer thin films.6–9 The advent of electronic cameras and the continual development in electron optics and stage designs have greatly enhanced the capabilities of in situ TEM analyses. Currently, novel in situ experiments are able to observe and record micro-to-nanoscale behavior of materials in various heating, cooling, straining, or growth environments. These experiments are invaluable for characterizing and understanding thermomechanical properties along with the underlying dynamic processes. Despite these advantages, there are usually concerns with in situ TEM experiments because to produce electron transparent specimens in these multilayers, one or more
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