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Stress-Strain Response of Free-Standing Nano-Crystalline Gold Thin-Films Sauri Gudlavalleti∗ , Sharvan Kumar† , and Lallit Anand∗ ∗ Massachusetts Institute of Technology, Cambridge, MA 02139, USA † Brown University, Providence, RI 02912, USA ABSTRACT Free-standing gold thin-film tension specimens of thicknesses in the range 500-700 nm were fabricated by e-beam deposition methods. The films have grains which have thicknesses essentially equal to the film thickness, and an in-plane grain-size distribution which is bi-modal, with a collection of small grains with an average size of 130 nm, and another collection of larger grains with an average size of 380 nm. The films possess a strong texture with the {111} crystallographic planes aligned with the plane of the film. The thin-film specimens have been tested in simple tension using a novel testing machine which enables measurement of the elastic-plastic stress-strain response of the films with high resolution. The experiments show that the gold thin-films have an initial yield strength of around 100 MPa, which is followed by rapid strain hardening to a stress level of 360 MPa at a strain of only ≈1.2%, at which point the specimens fail abruptly. Transmission electron microscopy (TEM) investigations of the deformed films shows that the rapid strain hardening is attributable to extensive dislocation activity in the larger grains. The reason behind the low ductility in our gold specimens is still unclear, but it may be attributable to the observed lack of dislocation activity in the smaller grains, whose boundaries then serve as easier sites for alternative inelastic mechanisms such as grain boundary sliding and decohesion leading to low macroscopic strains to failure. INTRODUCTION Thin-films of face-centered-cubic (fcc) metals such as Cu, Al, Ag, and Au find wide use in microelectronics applications as conductive elements. It is now well recognized that polycrystalline metallic thin-films exhibit a substantially higher strength and a much lower ductility, as compared to the values of these properties obtained from conventional experiments on bulk specimens. Yield and ultimate strengths have been observed to increase with decreasing film thickness and grain size. For example, (i) Povirk et al. [1] report an ultimate strength of 350 MPa for 0.8 µm thick gold tension specimens;(ii) Weihs et. al [2] report yield strengths of 260 MPa for gold micro-cantilever beams (0.87 µm thick, 27 µm long and 20 µm wide), tested in bending using a nano-indenter; (iii) Huang [3], Hoffman [4] and Kang et al. [5], report ultimate strengths in the range 200 - 400 MPa, for free-standing polycrystalline aluminum thin-films in tension. The purpose of this brief paper is to present the results from our own experiments on freestanding gold thin-films (500 - 700 nm thick) on our recently developed micro-mechanical testing machine. Gold is chosen as a model fcc metal for this program for its chemical inertness; since gold does not oxidize easily, the free-standing films are uncapped by surface layers.

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