TY - BOOK

T1 - Locally resolved deformation and fracture processes near interfaces

AU - Alfreider, Markus

N1 - no embargo

PY - 2021

Y1 - 2021

N2 - With the building blocks of modern technological devices ever decreasing in size, e.g. microelectronic components, the investigation of lifetime and durability becomes more and more challenging due to considerably higher stresses given day-to-day mechanical or thermal loads. Especially interfaces between two different materials are oftentimes the source for device failure, due to e.g. loss of conductive properties or spallation of protective layers. Therefore, a general approach to determine interfacial fracture properties of such very confined systems would be highly desired. Various methodologies have already been developed for macroscopic testing, e.g. brazil nut specimen or four-point bending specimen. However down-scaling such techniques to the micron or even sub-micron regime proves challenging. Therefore, specific techniques based on nowadays widely spread nanoindentation devices have been developed. However, these techniques as well as the associated evaluation schemes are commonly only applicable to specific material combinations, with the prevalent restriction that only linear elastic deformation takes place. Given that many industrially relevant material combinations contain at least one constituent which is prone to plastic deformation, e.g. Cu, Al or various polymers, a new methodological approach, incorporating plasticity is highly demanded. This work presents a study on the interface fracture behaviour of a Si-SiOx-WTi-Cu model material system, as commonly found in the microelectronics industry, utilizing novel micro mechanical approaches to take plasticity of the Cu phase into account. The methodology is based on the notched microcantilever bending specimen geometry in conjunction with a transducer equipment capable of continuous stiffness measurement for the steady determination of crack length, mounted inside a scanning electron microscope (SEM) for in situ observation during the test. This thesis covers the investigation of the mechanical testing setup inside the vacuum chamber of an SEM, the development of a mathematical framework for evaluation based on elastic-plastic fracture mechanical considerations, and finally successfully conducts experiments on the model material system. Experiments with the focus on determining the fracture properties of the SiOx/WTi interface as well as the WTi/Cu interface revealed a distinct difference from purely brittle interface cleavage to extensive crack tip plasticity without any crack extension. This confirmed for the first time the applicability of the technique for the determination of fracture mechanical parameters in spatially confined heterogeneous structures. Further investigations on interfaces with intentional exposition to atmosphere showed a transition from crack tip plasticity to interface cleavage, which proves a weakening of interface cohesion from oxygen accumulation. In conjunction with analytical dislocation plasticity models a shift in local mode mixity was conjectured, leading to a final study focusing on the different deformation behaviour upon pure normal- or shear loading on the interface utilizing transmission scanning electron microscopy. While normal loading led to pure ductile failure in the Cu phase, shear loading reveled the nucleation and extension of an interface crack. In summary, this thesis presents a detailed picture of the interface deformation and fracture behaviour of a model material system, with the caveat that the local microstructure, especially in the Cu phase remains oftentimes unknown. However, the obtained results showed that the developed methodologies were able to resolve previously inaccessible information with regards to interface failure and can act as a basis for further studies and improvements on similar materials systems.

AB - With the building blocks of modern technological devices ever decreasing in size, e.g. microelectronic components, the investigation of lifetime and durability becomes more and more challenging due to considerably higher stresses given day-to-day mechanical or thermal loads. Especially interfaces between two different materials are oftentimes the source for device failure, due to e.g. loss of conductive properties or spallation of protective layers. Therefore, a general approach to determine interfacial fracture properties of such very confined systems would be highly desired. Various methodologies have already been developed for macroscopic testing, e.g. brazil nut specimen or four-point bending specimen. However down-scaling such techniques to the micron or even sub-micron regime proves challenging. Therefore, specific techniques based on nowadays widely spread nanoindentation devices have been developed. However, these techniques as well as the associated evaluation schemes are commonly only applicable to specific material combinations, with the prevalent restriction that only linear elastic deformation takes place. Given that many industrially relevant material combinations contain at least one constituent which is prone to plastic deformation, e.g. Cu, Al or various polymers, a new methodological approach, incorporating plasticity is highly demanded. This work presents a study on the interface fracture behaviour of a Si-SiOx-WTi-Cu model material system, as commonly found in the microelectronics industry, utilizing novel micro mechanical approaches to take plasticity of the Cu phase into account. The methodology is based on the notched microcantilever bending specimen geometry in conjunction with a transducer equipment capable of continuous stiffness measurement for the steady determination of crack length, mounted inside a scanning electron microscope (SEM) for in situ observation during the test. This thesis covers the investigation of the mechanical testing setup inside the vacuum chamber of an SEM, the development of a mathematical framework for evaluation based on elastic-plastic fracture mechanical considerations, and finally successfully conducts experiments on the model material system. Experiments with the focus on determining the fracture properties of the SiOx/WTi interface as well as the WTi/Cu interface revealed a distinct difference from purely brittle interface cleavage to extensive crack tip plasticity without any crack extension. This confirmed for the first time the applicability of the technique for the determination of fracture mechanical parameters in spatially confined heterogeneous structures. Further investigations on interfaces with intentional exposition to atmosphere showed a transition from crack tip plasticity to interface cleavage, which proves a weakening of interface cohesion from oxygen accumulation. In conjunction with analytical dislocation plasticity models a shift in local mode mixity was conjectured, leading to a final study focusing on the different deformation behaviour upon pure normal- or shear loading on the interface utilizing transmission scanning electron microscopy. While normal loading led to pure ductile failure in the Cu phase, shear loading reveled the nucleation and extension of an interface crack. In summary, this thesis presents a detailed picture of the interface deformation and fracture behaviour of a model material system, with the caveat that the local microstructure, especially in the Cu phase remains oftentimes unknown. However, the obtained results showed that the developed methodologies were able to resolve previously inaccessible information with regards to interface failure and can act as a basis for further studies and improvements on similar materials systems.

KW - interface fracture

KW - micromechanics

KW - thin films

KW - elastic-plastic fracture mechanics

KW - dislocation plasticity

KW - Grenzflächenbruch

KW - Mikromechanik

KW - Dünnschichten

KW - elastisch-plastische Bruchmechanik

KW - Versetzungsplastizität

M3 - Doctoral Thesis

ER -