Fatigue Behaviors Of Thin Gold Films In Flexible Electronics

Due to the demands for higher performance,portability,light weight,flexibility and the capability of being deformed into any shape,flexible electronics have been developed for many applications in recent years,such as thin-film solar cells,wearable electronic systems,flexible displays and biomedical sensors.In a practical application,flexible electronics usually suffer from tension,compression and cyclic deformation which would cause fatigue damages and consequent fatigue failure.Serving as electrodes or electrical interconnects in flexible electronics,the mechanical reliability of thin metal films is a precondition for electronics to function well.With a dramatic decrease in the appearance size and the higher and higher level of integration,the characteristic size of materials in electronics decrease to the microscale even to the nanoscale where fatigue damage behaviors are quite different from those in bulk materials.A thorough understanding of the mechanical behaviors and the fatigue mechanisms in the thin films with micron and less scale is one of the key scientific problems to be solved in the research area of material fatigue.Besides,when the grain sizes decrease to nanoscale,the microstructure of nanocrystalline(NC)metals is usually unstable and prone to undergo grain growth,which may influence the fatigue properties.The mechanisms of the grain growth behaviors under cyclic loading are unclear and remain to be studied further.In this thesis,we deposited Au films on polyimide substrates using magnetron sputtering.Fatigue tests were performed on thin Au films to investigate the fatigue properties using homemade dynamic bending fatigue test set-up.Some microstructure characterization methods were also performed on the fatigued Au films to investigate the fatigue damage behaviors and grain growth behaviors under cyclic loading.In order to evaluate bending fatigue reliability of thin metal films,dynamic bending fatigue test set-up is designed and established.This evaluation method can determine the critical failure strain amplitudes of the thin metal films at the applied cyclic cycles based on the direct observation of fatigue damage sites,and thus it can obtain the applied strain range-fatigue life curves.The investigation of fatigue damage behaviors in the 930 nm-thick Au films under dynamic bending cyclic loading shows that the dominant fatigue damage behaviors of thin Au films transformed from extrusions to intergranular cracks with decreasing applied strain ranges and increasing cyclic cycles.The extrusion heights of fatigued Au films increased with applied strain ranges and the number of cycles,while the densities of intergranular cracks increased with decreasing applied strain ranges and increasing the number of cycles.The different fatigue damage behaviors may be related to the process of edge dislocation annihilation and vacancy formation during cyclic deformation.Depositing 10 nm-thick Ti interlayers between the PI substrates and 1?m-thick annealed Au films is effective to suppress strain localization and increase the rupture strain and the fatigue properties of thin Au films.The investigation of grain growth behaviors in the 930 nm-thick Au films under dynamic bending cyclic loading shows that two different types of grain growth behaviors occurred during fatigue test.In the uncracked area far away from the fatigue cracks,the average grain sizes were in the sub-micro scale,and they increase with the applied cumulative cyclic strain.Besides,no significant grain orientation transition happened in this area.Theoretical analysis shows that the difference in elastic strain energy densities within adjacent grains caused by Hall-Petch effect is the driving force for grain growth under the applied loading.However,the abnormally grown grains around fatigue crack tips exhibited<001>orientation(along loading direction)and grown to micron scale.The high stress in front of fatigue crack tips is the driving force for grain growth in this area.Elastic anisotropy of gold and lattice rotation caused by quantitative irreversible dislocation slips accounted for the texture transition in the abnormally grown grains around fatigue crack tips in the fatigued Au films.Thermodynamic and kinetical consideration demonstrates the feasibility of abnormal grain growth behaviors in two regions.