Relationship between film stress and dislocation microstructure evolution in thin films
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1052-DD07-06
Relationship between film stress and dislocation microstructure evolution in thin films Ray S. Fertig, and Shefford P. Baker Materials Science and Engineering, Cornell University, Bard Hall 214, Ithaca, NY, 14853 ABSTRACT Metals with one or more dimensions in the submicron regime are widely used in MEMS devices. Device stresses often exceed the strength of the corresponding bulk material by an order of magnitude and can lead to a variety of mechanical failures. At moderate temperatures, high stresses occur partly because complex dislocation behavior, such as junction formation, annihilation, and nucleation, prevents dislocation motion. In this report, we present results from analytical models, cellular automata simulations, and large-scale dislocation dynamics simulations of thin films to examine the relationship between dislocation interactions and material strength. Our results reveal a complex relationship between dislocation interactions and stress inhomogeneity that arises from the stress fields of the dislocations. We show that the stress inhomogeneity increases both the likelihood of interactions and acts to increase the strain hardening rate. INTRODUCTION Thin metal films are used in applications ranging from chemical barriers to MEMS devices. However, the stresses supported by thin films often exceed bulk yield stresses by an order of magnitude [1, 2], which increases the likelihood of device failure and makes metal films not viable for some MEMS applications [3]. To increase reliability and expand the usefulness of metal films in MEMS applications, residual stresses must be reduced. For this to be realized, the origin of high film stresses must be better understood. At moderate temperatures, stress relaxation occurs via dislocation motion; in thin films, relaxation occurs via motion of threading dislocations (threads), which extend through the thickness of the film. As a thread moves it creates a misfit dislocation (misfit) or surface step, which reduces the film stress. Thus, high film stress depends on preventing thread motion. Three mechanisms that prevent thread motion have been identified. First, the onset of thread motion is controlled by a dimensional constraint, whereby a thread may not move until some channeling stress τch (or strain εch), which is inversely proportional to the film thickness, is exceeded [4, 5]. Second, the thread may be impeded by interaction with grain or twin boundaries. Finally, a thread may be stopped by interaction with other dislocations [6]. For example, a misfit can stop a moving thread up to a film stress of about 1.3τch [6, 7]. Most models of film stress only consider interactions of threads with misfits [7]. At higher stresses, the thread breaks free and continues moving through the film. Three other important features of film relaxation have been observed but their effect on film strength has not been studied. First, in metal films on amorphous substrates misfits are observed to disappear over time [8], indicating dislocation core spreading into the int
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