Strength and interface-constrained plasticity in thin metal films
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Strength and interface-constrained plasticity in thin metal films Y-L. Shen Department of Mechanical Engineering, University of New Mexico, Albuquerque, New Mexico 87131 (Received 28 March 2003; accepted 8 July 2003)
This study seeks to provide a mechanistic rationale for the substrate confinement effect on the strength and plasticity of thin metal films. Atomistic simulations of tensile loading of the freestanding and substrate-bonded films were carried out. Particular attention was devoted to correlating the overall mechanical response and the defect mechanisms on the atomic scale. The existence of an interface with the underlying substrate was observed to constrain significantly the dislocation motion in the film. The extent of film strengthening due to the substrate was dictated by the capability of atoms to slide along the interface.
Thin-film materials, when attached to mechanically stiff substrates, derive their strength in part from the interfacial constraint caused by the underlying layer. This is manifested by the high plastic flow stress that can be carried by polycrystalline metal films on substrates compared to the case of free-standing films.1,2 A schematic is shown in Fig. 1, where the plastic flow stress of the film is plotted against the grain size. The schematic, based on experimental data,1,2 illustrates that a substrate-attached film can support a much greater stress than its freestanding counterpart with the same average grain size. Prevailing theories for substrate strengthening and interface-constrained plasticity in metal films have invoked the concept of misfit dislocations left behind at the interface caused by dislocation gliding inside a crystal.1,3–5 Such model is predicated on the balance of the work done by the film stress and the stored energy of the interfacial misfit dislocation. However, there is a general lack of experimental evidence of misfit dislocations in recent electron microscopy examinations of aluminum and copper films.6,7 This is not surprising because in most cases the metal film is in direct contact with an amorphous layer (such as oxidized silicon and silicon nitride barrier layer) of the substrate, so a true “misfit” dislocation at the interface is not likely to exist. Clearly, a different physical picture concerning strength and interface-constrained plasticity is needed. In this study, we use atomistic simulations to provide a mechanistic rationale for the substrate confinement effect in thin metal films. Attention is devoted to correlating incipient plasticity, defect mechanisms, and the overall stress-bearing capability. Because the focus is J. Mater. Res., Vol. 18, No. 10, Oct 2003
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solely on the substrate effect, the model system is a two-dimensional crystal without any grain boundary. The initial configuration of atomic arrangement having a close-packed structure is shown in Fig. 2. To assist in triggering the onset of crystal plasticity and avoid brittle fracture, an artificial defect, in the form of
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