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Stress Partitio I G I Fcc Thi Metal Films With Mixed Texture
Thin FCC metal films are used in a wide range of micro- and nano-fabricated devices. These films may support stresses up to an order of magnitude greater than would be predicted based on bulk scaling laws. These stresses drive failure mechanisms and are one of the main limitations on the reliability of film-containing devices. Thus, there has been a great deal of interest in thin film mechanical behavior. In experimental studies, a homogeneous equal-biaxial stress state in the plane of the film is typically assumed and data are analyzed accordingly. However, annealed FCC metal films tend to have columnar grained microstructures with grain size on the order of the film thickness and strong fiber texture with (111) and/or (100) planes parallel to the plane of the film. Due to anisotropy, these orientations often have very different elastic stiffnesses both in and out of the film plane. Thus, for a uniform applied strain (e.g. thermal strain), very different three-dimensional stress states should be expected in each orientation. In this thesis experimental and analytical methods for accurately determining the stress states in films with mixed (111)/(100) texture are presented. Synchrotron x-ray diffraction was used to characterize the strains in each texture component individually during thermal cycling of Cu films on Si substrates. A new analysis method is presented that allows the 3-D stress states in the different texture components to be determined with reasonable accuracy. Stress states are found to be dramatically different from the simple biaxial stresses typically assumed. Further analysis allowed the plastic strain and the geometrically necessary dislocation density in each texture component to be determined throughout the temperature cycle. Again, results are quite different from those found following the biaxial assumption. Finite element simulations of stress states as a function of grain aspect ratio and texture volume fractions confirmed that the experimental results were reasonable, and detailed studies were conducted to explore the effects of inhomogeneous stresses on x-ray peak widths and on texture formation. This work suggests that many models of thin film mechanical behavior should be revised.
dissertation or thesis