Date of Award
Doctor of Philosophy
Ashraf F. Bastawros
Metallic interconnects and circuitry has been experiencing excessive deformation beyond their elastic limits in many applications, ranging from micro-electro-mechanical systems (MEMS) to flexible electronics. These applications are creating needs to understand the extent of strength and ductility of free standing 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 free-standing 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 2-50μm 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.
The results show that thickness effects on the strength (yield and flow stress) of freestanding films are primarily an outcome of the competing contributions from two distinct dislocation mechanisms i.e. intragranular Frank-Read (IG) type dislocations and grain boundary (GB) dislocations. At large grain sizes, typically ~O(10μm) or larger, in bulk materials and foil/plate type specimen, IG dislocations dominate the overall response. In this regime, reducing the film thickness leads to a reduction of the effective microstructural constraint. Consequently, prominent thickness dependent weakening is observed. On the other hand, for small grain sizes, ~O(1μm) or smaller, GB dislocations play an increasingly prominent role. In such cases, reducing grain boundary area per unit specimen volume, with reducing thickness, results in a reduction of the available GB dislocation source density. As a result, plasticity commences under source-limited conditions leading to thickness dependent, source starvation strengthening.
The role of film thickness on macroscopic ductility was more prominent for dg = 3.5μm and larger for the examined set of film microstructures. Generally, reducing ductility with reducing film thickness was observed. Surface scans showed that in specimens with many grains across the thickness, plastic deformation remains relatively uniform to higher level of macroscopic plastic deformation. On the other hand, for specimens with just one grain across the thickness, plastic deformation evolves into highly localized deformation bands at an early stage of deformation, leading to premature failure. It is speculated that local accommodation of deformation incompatibilities between neighboring grains are the primary driving mechanism for the observed trends. For films with the lowest dg = 1.8μm, the apparent ductility is inherently limited by the loss of strain hardening capability. These films show a limited ductility with maximum uniform strain of ~2%, irrespective of the thickness.
Shakti Singh Chauhan
Chauhan, Shakti Singh, "An experimental assessment of the size effects on the strength and ductility of freestanding Cu films under macroscopically homogenous deformation" (2008). Graduate Theses and Dissertations. 10412.