Electro-mechanical fatigue in metal films on polymer substrates

One of the general trends in science and technology during last few decades is a strong tendency to miniaturize electronic components and devices. This tendency pushes the material science community towards the development of new testing techniques to reveal the properties of micro- and nano-sized structures. A new emerging technology allowing for fabrication of electronic devices on flexible plastic substrates requires integration of thin film materials with very different mechanical properties: from hard and brittle barrier layers and transparent conductors such as indium tin oxide, over ductile metal contacts and conductive interconnects to elastic polymer substrates. Although mechanical properties of single materials are typically well investigated, fundamental understanding of fatigue and fracture of composite structures is required. The aim of the proposed project is the systematic and comprehensive investigation of the fatigue of composite metal-polymer thin film structures. Considerable efforts will be directed to detailed examination of how and where the fatigue-induced damage in a metal film on polymer substrate initiates and develops with the cycle number. An advanced characterization technique which includes alternating stages of cyclic straining and microstructure characterization will be applied. The fatigue life of two different metals (Au, Cu) on two polymer substrates (polyethylene terephthalate, polyimide) will be determined and typical failure regimes characterized. All fatigue tests will be performed with in-situ resistance monitoring and the damage induced by cyclic straining will be characterized both microscopically (formation of surface extrusions, intrusions, and cracks) and electrically (growth of electrical resistance). A 2D finite-element model will be used to simulate the current flow in a metal sheet containing cracks in order to reveal the influence of crack parameters (i.e. crack density) on the overall resistance of the film. The results of the simulations will be compared to experimentally measured crack density and resistance to understand the relation between structural and electrical changes. Finally, fatigue of narrow metal lines which are oriented perpendicularly to straining direction will be investigated. Due to the large difference in Youngs moduli, patterned metals on polymers may exhibit a higher fatigue life then continuous metal films. A model describing the transfer of stress from a polymer to patterned metal components will be made by subjecting specially designed metal lines to cyclic loading and examining the damage induced within the lines. Successful accomplishment of the project is expected to have a great impact on understanding of fundamental fatigue properties of metal films and metallization lines on polymer substrates.

The project Electro-mechanical fatigue in metal films of polymer substrates was intended to understand the processes of structural changes and damage development in polymer- supported thin metal films subjected to cyclic mechanical loading. The main scientific results can be described by three milestones which were reached. - The controversial and not well understood effect of grain boundary migration and grain coarsening at room temperature was investigated systematically. The analysis of experimental data allowed formulation of a new model which explains grain coarsening by the existence of the driving force which is based on local difference in elastic strains between individual grains. - An explicit relationship which connects the growth of electrical resistance of thin films due to cracking with the parameters of the crack pattern (i.e. crack density and lengths) was discovered using a combination of finite element modeling and direct experimental measurements. With the help of this relationship it was shown that for correct interpretation of the correlation between mechanical and electrical degradation, it is insufficient to determine crack density evolution. A new parameter, called cracking factor, was shown to give an adequate description as it can be put in direct and unambiguous relation with the growth of electric resistance. - Systematic study of the mechanisms of fatigue damage initiation was performed during the project. Altogether 22 thin film systems with different parameters (such as material, thickness, texture, grain size) were fabricated, mechanically tested and characterized. Four different mechanisms of fatigue damage initiation are formulated. The grain size, texture and the fraction of coherent twin boundaries are found to be the most important parameters defining the fatigue damage initiation sites. On the basis of scientific results obtained during the project duration six papers were published in international peer-reviewed journals. One further paper was submitted and is currently under review, three more papers are in preparation. The results were presented in 11 scientific conferences. Apart from scientific results, the project led to several technical advancements. Digital image correlation technique was successfully applied for non-contact strain measurements and strain mapping. An in-situ heating system to perform high-temperature mechanical testing with in-situ resistance measurements was designed.