Metallic interconnects and circuitry has been experiencing excessive deformation beyond their elastic limits in many applications, ranging from micro-electromechanical systems (MEMS) to flexible electronics. These broad applications are creating needs to understand the extent of strength and ductility of freestanding metallic films at scales approaching the micron and sub micron range.
This work aims to elucidate the effects of microstructural constraint as well as geometric dimensional constraint on the strength and ductility of freestanding Cu films under uniaxial tension. Two types of films are tested (i) high purity rolled films of 12.5-100µm thickness and average grain sizes of 11-47µm and (ii) electroplated films of 2-50 µm thickness and average grain sizes of 1.8-5µm. Several experimental tools including residual electrical resistivity measurements, surface strain measurements and surface roughness measurements are employed to highlight the underlying deformation mechanisms leading to the observed size effects.
With respect to the strength of the specimens, we find that the nature and magnitude of thickness effects is very sensitive to the average grain size. In all cases, coupled thickness and grain size effects were observed. This study shows that this observed coupling, unique to the case of freestanding specimen, arises because the observed size effects are an outcome of the size dependence of two fundamental microstructural parameters i.e. volume fraction of surface grains and grain boundary area per unit specimen volume.
For films having thickness and grain sizes greater than 5µm, thickness dependent weakening is observed for a constant grain size. Reducing thickness results in an increase in the volume fraction of grains exposed to the free surface as well as a reduction in the grain boundary area per unit specimen volume. The former effect leads to a reduction in the effective microstructural constraint on the intragranular dislocation activity in individual grains. This free surface related effect is the origin of a weakening contribution to the overall specimen strength with reducing thickness. For specimens with grain sizes ∼ O (10-50µm), this effect was found to be dominating i.e. reducing thickness resulted in reducing strength. A phenomenological model employing the flow strength of surface and bulk grains is proposed to model the observed trends.
For films having thickness and grain sizes smaller than 5µm, size dependent strengthening is observed for a constant grain size. At this scale, grain boundary dislocations dominate. As a consequence, thickness effects arise because grain boundary dislocation source density per unit specimen volume reduces with reducing specimen thickness. This statistical reduction in dislocation source density leads to increasing specimen strength via source starvation strengthening. Our results show that such increasing specimen strength with reducing thickness, which has only been observed previously for nanocrystalline thin films, first appears at average grain size of ∼5µm or xx smaller. The measurements showed a characteristic length scale of about 5µm, which defines the size dependent strengthening or weakening of the film.
With respect to the thickness effects on ductility, it was found that both thickness and average grain size affect ductility. While prominent thickness effects persist at larger grain sizes, for specimens with grain size approaching 1µm, the loss of strain hardening ability at such fine microstructures dominates and a limiting ductility of ∼2% is seen irrespective of the thickness. The observed thickness effects on ductility were investigated via surface roughness measurements that allow the characterization of initiation and evolution of deformation heterogeneities. It was found that thickness has a strong influence on the characteristic heterogeneity of deformation. At small specimen thicknesses, the deformation was found to be highly localized i.e. widely spaced regions showing substantial thickness reduction, hence increasing the vulnerability to the onset of plastic instabilities. At larger thicknesses, however, the increasing microstructural constraint delocalizes the strain and thereby precludes the early onset of instability, leading to enhanced ductility.
|Advisor:||Bastawros, Ashraf F.|
|Commitee:||Hong, Wei, Rudolphi, Thomas J., Shrotriya, Pranav, Ustundag, Ersan|
|School:||Iowa State University|
|School Location:||United States -- Iowa|
|Source:||DAI-B 70/01, Dissertation Abstracts International|
|Subjects:||Mechanical engineering, Materials science|
|Keywords:||Copper films, Deformation mechanisms, Dislocations, Ductility, Plastic deformation, Size effects, Strength|
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