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.