Reliability of nanomaterials - Plasticity, fracture and fatigue of ultra thin metallic films


 Patric A. Gruber


Karlsruhe Institute of Technology

Institute for Applied Materials - Materials and Biomechanics

P.O. Box 3640

76021 Karlsruhe, Germany



Compliant substrates like polyimide enable the fabrication of flexible electronics for numerous applications like flexible displays, organic LEDs or flexible solar cells. However, the reliability of such devices is often limited by the extensibility of the inorganic components. So far, little experimental work has been carried out to investigate the mechanical properties of thin metallic films on compliant substrates at high strains and cyclic loading. Here, we present experimental results for the yield strength, fracture toughness and fatigue behavior of ultra thin Cu and Ta/Cu film systems on polyimide substrates with 20 to 1000 nm film thickness. The film systems have been tested by a synchrotron-based tensile testing technique (up to 7% total strain) as well as cycling loading (100 Hz, strain amplitude of 0.5 to 1%) and have been characterized by SEM and FIB microscopy. The synchrotron experiments yield the stress-strain curves of the Cu films (as prepared and fatigued) whereas the cyclic tests give the fatigue lifetime. On the other hand, in situ tensile tests in the SEM and stationary FIB investigations reveal the evolution of cracks and fatigue damage in the films. For the thinnest films the deformation behavior during the tensile tests and for cycling loading becomes more and more brittle and multiple cracking is observed. It is shown that the fracture toughness of the Cu films decreases with decreasing film thickness whereas the fatigue lifetime and the yield strength increase. The different size effects may be attributed to an increasing constraint and final lack of plastic deformation by full dislocations. This may be corroborated by the observation that the strain hardening is strongly reduced for the thinnest films. Finally, some results on the unexpectedly strong temperature dependence of different Au/SiNx film systems and the evolution and succession of specific deformation mechanisms in nanocrystalline AuCu films, which could be determined and separated by sophisticated peak shape analysis, will be shown in order to demonstrate the unique capabilities of the synchrotron-based testing technique.