Fracture mechanics of nanoscale thin films
Open Access
- Author:
- Kumar, Sandeep
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 24, 2012
- Committee Members:
- Md Amanul Haque, Dissertation Advisor/Co-Advisor
Md Amanul Haque, Committee Chair/Co-Chair
Asok Ray, Committee Member
Eric M Mockensturm, Committee Member
Sulin Zhang, Committee Member
Douglas Edward Wolfe, Committee Member - Keywords:
- Thin films
TEM
Fracture
mechanics
nanoscale - Abstract:
- Thin films are prevalently used in micro-electronics, data storage, sensors and actuators, energy conversion and micro-electro-mechanical systems, where they experience mechanical and thermal loading during device fabrication and operation. Fundamental understanding of the mechanics of deformation and fracture of thin films is therefore important from device design and reliability perspectives. This thesis pursues an experimental approach to understand deformation behavior of nanoscale thin films. A micro electro mechanical system (MEMS) based device has been developed to carry out fracture testing of metallic and multilayer thin films in-situ inside the transmission electron microscope (TEM). Nanofabrication techniques are utilized to fabricate a bent beam thermal actuator device integrated with notched fracture testing specimens. The experimental results suggest significant deviation from classical relationship between grain size and underlying deformation mechanism. A key contribution of this research is the size effect on stress concentration. Typically, geometrically necessary dislocations result in strain gradient at the notch tip. However, our in-situ selected area electron diffraction (SAED) studies clearly show the absence of any strain gradient at the notch tip. To explain this anomalous observation, we propose that when the grain size is below a critical size (for aluminum ~ 60 nm), dislocation based plasticity mechanisms are replaced by grain rotation, which homogenize the strain field. In the absence of strain (or stress) concentration, the material becomes flaw tolerant. This behavior is similar to behavior shown by naturally occurring biomaterials. We repeated the experiments on Platinum thin films with grain size favorable for dislocation based mechanics to support our hypothesis. Pure metals do not exhibit solid-solid phase transformation since they deform and fail far below the theoretically required stress levels that exceed hundreds of GPa. A key contribution of this research is to show experimentally that by controlling grain size and thickness, classical deformation modes can be suppressed to induce phase transformation in pure metal films at stresses few orders of magnitude lower than theoretical values. For the first time, we present in-situ transmission electron diffraction evidence of face-centered cubic (FCC) to hexagonal ω phase transformation in 99.99% pure nanocrystalline aluminum at room temperature and only 2.5 GPa of tensile stress. For 60 nm average grain size, the aluminum films did not show any appreciable diffusion-based processes such as grain growth, rotation and sliding. Facilitated by the absence of dislocation and diffusion based processes, the uniaxial nature of specimen loading results in phase transformation at stresses two orders of magnitude lower than that predicted for aluminum. We propose that in absence of dislocations and grain rotation, phase transformation could be the only way to relax the high strain energy at the notch tip without initiating failure, which can make the material flaw tolerant. We also developed an experimental setup for bulk materials to explore the deformation mechanics in thin films of complex multilayer materials, Ti/TiN in present case. We observed that microstructure plays a critical role in determining the performance of multilayer coatings. We also observed that TiN, even though a ceramic, might exhibit significant dislocation activity. Whereas a metal like Ti with small grain may not provide the sufficient ductility required for such materials. Such dislocation activity was observed at room temperature as compared to bulk behavior where dislocations are activated above 0.5Tm (melting point). Typically dislocations are generated at the notch tip and they move away from the notch tip resulting in increased radius of notch/crack tip and reduction in stress concentration. This behavior is called dislocation shielding of a crack. Our experiments suggest a second mechanism where dislocations generated away from the notch tip get attracted by notch tip could provide effective shielding. This behavior may give rise to flaw tolerance in TiN. The final topic of this dissertation explores whether deformation of nanoscale materials result in complex multi domain loading. We utilized electric field generated joule heating and electromigration induced stress to achieve thermo-electro-mechanical loading on the freestanding thin films. This complex loading helped us in mapping the deformation mechanisms in platinum thin films. At very small grain size (5-30 nm) grain growth is the predominant deformation mechanism. For grain size 30-60 nm, grain rotation is the predominant deformation mechanism. Above approximately 80 nm, conventional dislocation based deformation is predominant. This complex thermo-electro-mechanical loading resulted in complete amorphization in aluminum thin films. Such amorphization may be detrimental to performance of interconnects in microelectronic devices. This thesis also lays the foundation for future direction to carry out further research in thin film behavior. Future work includes understanding the coupling between different physical domains such as mechanical, thermal, electrical and optical. As a part of current work, we developed an experimental setup that can used to apply tensile loading on the specimen while carrying out four probe thermal and electrical measurements on the stressed thin film specimen. We also carried out preliminary experiments on aluminum thin films. These experiments suggest a strong length-scale induced coupling between mechanical and thermo-electrical domains.