Interfacial Behaviour of Metal Films on Polymer Substrates

This study will have a direct impact on design and development of flexible devices. It will also provide an estimate of performance and reliability that currently is obtained through laboratory and field tests. Insight gained from the experiments proposed below can be used to build materials science-based predictive models of structure and behavior for design and development of high-performing nanostructured materials and devices. Currently little is known of the mechanical properties of the material systems that compose flexible electronic devices. The materials include ceramic and metal films deposited onto flexible polymer substrates. Metal thin films on rigid substrates, however, are known to have improved mechanical properties such as hardness and yield strength, as long as the films remain bonded to the substrate. Metal films on compliant substrates also show improved mechanical properties; these films can deform far beyond their freestanding counterparts. Once debonded, substrate constraint disappears leading to film fracture as a consequence of high internal stresses. Interfacial adhesion of metal films on rigid substrates can also be calculated using a variety of methods. The same methods cannot always be used to examine the mechanical and interfacial properties of metal films on polymer substrates due to complicated deformation mechanisms that are still not understood. In order to remedy this, an iterative program will be implemented to address the questions of interfacial adhesion, mechanical properties and deformation mechanisms of thin metal films deposited onto compliant polymer substrates. Experiments include, but are not limited to, tensile and cyclic loading of the material systems to determine stress-strain behavior and fatigue lifetime as well as examining the film`s failure mechanism. The adhesion of the metal-polymer interfaces will be measured using stressed overlayers and nanoindentation to induce buckling and delamination. Mechanics-based models and the size of the delaminated region will be used to calculate the adhesion energy of the interface. Finally, the metal- polymer deformation mechanism will be observed using in situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques that will be developed. The observation of the crack front initiation and propagation will provide vital information as to how these systems deform. The Department Materials Physics at the University of Leoben and its partner institute, the Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences are an ideal place for the project since its characterization facilities include tensile and cyclic loading stages that fit inside an SEM which allows for the observation of the failure mechanism at the required scale. Faculty member Dr. Gerhard Dehm has vast experience in mechanics at small scales which includes the structure and properties of interfaces, and utilizing in situ methods in the electron microscope. The Institute also has nanoindentation facilities to probe the mechanical properties of the film/substrate systems. The proposer, Megan J. Cordill, has in-depth experience in adhesion of thin films as well as knowledge of mechanical testing techniques. Cordill and Dehm`s extensive experience in small scale deformation, nanoindentation, interfaces, adhesion, and TEM make the team well-qualified to perform the proposed research. The project has been well thought out and organized by the PI (Cordill) and includes original experiments utilizing in situ SEM/TEM techniques, testing highly stressed films for use as stretchable interconnect lines, testing patterned films, and using unique materials as adhesion layers. The results of the research will be disseminated through presentations at international conferences by the research team. Prominent international journals will also be used to report the results. With improved understanding of how these materials systems fail, more thought can go into the design of the new technology to generate the best flexible electronic devices for the public sector.