Dislocation Plasticity in Thin Metal Films
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Dislocation Plasticity in Thin Metal Films
O. Kraft, L.B. Freund, R. Phillips, and E. Arzt Abstract This article describes the current level of understanding of dislocation plasticity in thin films and small structures in which the film or structure dimension plays an important role. Experimental observations of the deformation behavior of thin films, including mechanical testing as well as electron microscopy studies, will be discussed in light of theoretical models and dislocation simulations. In particular, the potential of applying strain-gradient plasticity theory to thin-film deformation is discussed. Although the results of all studies presented follow a “smaller is stronger” trend, a clear functional dependence has not yet been established. Keywords: mechanical properties, metals, thin films.
Introduction The behavior of thin metal films has been the subject of intense study over the past decade or more, driven largely by the importance of small-scale metal features in the fabrication and performance of microelectronic and optoelectronic devices, multilayer configurations for magnetic recording, and microelectromechanical systems (MEMS). It is primarily the electrical conductivity and chemical bonding characteristics of metals that are exploited in these applications. The low resistivity of aluminum and copper, along with techniques that have been developed for the deposition of patterned structures of these metals (lithographic processing technology), have put these particular materials in the research spotlight. Although metals in the applications mentioned do not serve a load-bearing structural role, they are invariably subjected to high levels of mechanical stress as a result of the constraint on deformation imposed by other materials to which they are joined. Stress arises most commonly due to temperature change and is a consequence of the relatively high coefficients of thermal expansion of metals as compared with those of the semiconductors, glasses, and ceramics to which they are typically bonded. In general, metals under stress exhibit a propensity for inelastic deformation. At relatively low homologous temperatures, the physical mechanism giving rise to this inelastic deformation is predominantly dislocation nucleation and glide. Plastic
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deformation of a given material depends on many factors, including temperature, material microstructure, and material volume. It is the aim of this article to describe the current level of understanding of the mechanisms for plastic deformation in thin films and small structures, for which the connection between mechanical properties and material volume is a special characteristic. These issues are particularly important for assessing the reliability of metal structures integrated into smallscale systems during subsequent processing and long-term service.
film, and p is the equi-biaxial plastic strain, then rates of change are related by
˙/M ˙ p ( film sub)T˙ 0, (0) 0 ,
(1)
where is a coefficient of thermal expansion. M E/(1 )
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