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Published in:
2015 | OriginalPaper | Chapter
14. Anelasticity in Al-Alloy Thin Films: A Micro-mechanical Analysis
Authors : J. P. M. Hoefnagels, L. I. J. C. Bergers, M. G. D. Geers
Published in: Fracture, Fatigue, Failure, and Damage Evolution, Volume 5
Publisher: Springer International Publishing
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Abstract
Micro-electromechanical systems enable many novel high-tech applications. Aluminum alloy thin films would be electrically favorable, but mechanical reliability forms fundamental challenges. Notably, miniaturization reveals detrimental time-dependent anelasticity in free-standing Al-alloy thin films. Yet, systematic experimental studies are lacking, perhaps due to challenges in microscale testing.
To this end, a microbeam bending methodology (with <4 με strain resolution) and nano-tensile tester (with 70 nN and <6 με resolution) have been developed for reproducible long-duration characterization of anelasticity of on-wafer 5 μm-thick Al-(1 wt%)Cu test structures under real-time in situ microscopy. Time-dependent anelasticity was indeed observed, both in bending and in tension, and a multi-mode visco-elastic model was found to described and predict the non-linear anelastic behavior between 1 and 105 s.
To gain insight in the underlying micro-mechanisms, time-dependent anelasticity was characterized under systematic variation of the grain boundary density and copper precipitate state/distribution (carefully analyzed using HRTEM/EBSD/ WAXD/EDX/SEM), yielding a wealth of information. Surprisingly, both microstructural features were excluded as the (primary) cause of the time-dependent anelasticity. Based on dynamic strain aging effects observed in nano-indentation, an underlying micromechanism responsible for time-dependent anelasticity was hypothesized.