Creep behavior of gold thin films investigated by bulge testing at room and elevated temperature
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EARLY CAREER SCHOLARS IN MATERIALS SCIENCE 2019 Creep behavior of gold thin films investigated by bulge testing at room and elevated temperature Benoit Merlea) Materials Science & Engineering, Institute I, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen D-91058, Germany (Received 1 June 2018; accepted 20 July 2018)
The creep behavior of 200-nm thick gold films was investigated by plane-strain bulge testing between 23 and 100 °C. The polycrystalline gold films were produced by thermal evaporation and their columnar microstructure was stabilized by a preliminary heat treatment at 120 °C. The creep tests were performed at constant stress values between 80 and 300 MPa over 12 h, using a custom-built bulge tester. The stress exponent calculated from the creep data decreased from 4.3 to 2.8 between 23 and 100 °C, suggesting a possible transition in deformation mechanisms. The stress exponent and activation energy measured around room temperature point toward the climb of dislocations at grain boundaries being the rate-limiting deformation mechanism. Above 75 °C, scanning electron microscope inspections of tested membranes suggest an increased contribution of diffusion and of grain-boundary mediated deformation, as evidenced by the formation of grooves along grain boundaries. This is presumably the reason for the decrease of the apparent stress exponent and sudden increase of the apparent activation energy. Benoit Merle is leading the research group on nanomechanics and thin film mechanics at the Materials Science & Engineering Department of the University of Erlangen-Nürnberg (FAU), Germany. He obtained his Master of Engineering degree from the Ecole Centrale de Lyon in France in 2005. After spending a few years as an engineer with Siemens AG, he joined the University of Erlangen-Nürnberg to pursue a Ph.D. on the mechanical behavior of thin films, which he completed in 2013. He received a Young Researcher Award from the German Materials Society in 2013 as well as a Ph.D. award by the Staedtler Foundation. His current research focusses on the development of small scale methods enabling the local measurement of mechanical properties, which he uses for investigating mechanical size effects. This research involves thin film specific methods, such as bulge testing or in situ tensile testing, and nanoindentation-based methods, such as micropillar compression and microbeam bending. Benoit Merle
I. INTRODUCTION
Knowledge about the creep properties of metallic freestanding thin films is still scarce.1–7 This is especially true for the lower submicrometric range, due to the fragility8 of the corresponding specimens, which makes their production and testing extremely challenging. This lack of data is detrimental to the design of reliable microchips and MEMS based on freestanding thin films. Mechanical creep is especially a critical issue for the radiofrequency microswitches providing essential signal processing functionalities to smartphones and other telecommunication systems.9,10
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