Characterization of the strain rate dependent behavior of nanocrystalline gold films
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The strain rate dependence of freestanding, nanocrystalline gold films was evaluated by a microtensile technique with applied strain rates on the order of 10−4 to 10−6 s−1. Film thickness ranged from 0.25 to 1.00 m with corresponding grain sizes of 40 to 100 nm. The plastic properties were found to be particularly sensitive to strain rate, film thickness, and grain size, while the elastic property remained relatively unchanged. The thinner films exhibited significant strain rate sensitivity, while the thicker film exhibited only marginal changes. Hall–Petch boundary hardening was observed and dominated plastic flow at larger strain rates, while diffusion-controlled deformation mechanisms appeared to be activated with increasing influence as strain rate decreased. Analysis of dislocation-based and grain-boundary diffusion-related creep suggested that the films were likely experiencing power-law creep as the dominant deformation mechanism in this grain size regime at lower strain rates.
I. INTRODUCTION
Thin gold films have been and continue to be widely employed in semiconductor devices as conductive layers, interconnects, electrodes, etc. In these applications, their electrical properties define their primary function; however, equally important are their mechanical properties, which play a significant role in their reliability and life cycle. Typically, these films are many orders of magnitude smaller than the devices/components they are a part of, and loading conditions can vary depending on the length scale. In order for them to be of practical engineering use, their elastic and plastic response must be robust enough to endure loading conditions that may include high strain rates, cyclic loading, macroscopically triggered gradient effects, thermal, and many other conditions. The mechanical response of these films, in turn, can depend on many factors such as film thickness, grain morphology and texture, deposition technique and parameters, micromachining procedures, substrate influences, etc. One of the more important loading conditions that has been receiving recent attention is low applied strain rates that have been shown to significantly alter the mechanical response of thin-film gold1–3 and other thinfilm metals.4,5 The films studied in this work were nanocrystalline in scale, meaning they have the possibility to exhibit cona)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0032 J. Mater. Res., Vol. 23, No. 1, Jan 2008
siderable creep behavior at room temperature under low applied strain rates. An early observation of creep in nanocrystalline gold at room temperature was observed by Sakai et al.6 when bulk scale specimens were loaded with holding times of 103 to 105 s. They surmised that the preferable plastic deformation of nanocrystalline gold with grain size of 15 to 60 nm was dominated partially but inevitably by the applied strain rate. Although the gold studied in Sakai et al.’s work was in bulk form, their assessment of Coble creep at room temperature in
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