Effects of Film Thickness on the Yielding Behavior of Polycrystalline Gold Films
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Effects of Film Thickness on the Yielding Behavior of Polycrystalline Gold Films
H.D. Espinosa and B.C. Prorok Department of Mechanical Engineering, Northwestern University Evanston, IL 60208-3111, USA
ABSTRACT A Membrane Deflection Experiment was used to test the mechanical response of freestanding thin film gold specimens. We present stress-strain curves obtained on films 0.3, 0.5 and 1.0 µm thick. Elastic modulus was consistently measured in the range of 53-55 GPa. Several size effects on the mecha nical properties were observed including yield stress variations with membrane width and film thickness. It was observed that thickness plays a key role in deformation behavior with a major transition in the material inelastic response occurring between a thickness of 0.5 and 1.0 µm. The size effects here reported are the first of their kind in the sense that the measurements were performed under a macroscopically homogeneous axial deformation, i.e., in the absence of macroscopic deformation gradients. INTRODUCTION Thin films with thickness of a fraction of a micron are commonly employed in microelectromechanical systems (MEMS) and microelectronic devices. These films frequently serve essential device functions. In these applications, the films are subjected to various thermomechanical conditions that may result in fracture, plasticity, friction and wear, creep, fatigue, etc. It is well known that most knowledge of bulk material behavior fails to describe material response in the submicron size regime. Hence, several groups have experimental and theoretical programs to study such features [1-4]. Frequently, investigations involving microsystems tends to be device dependent. For instance, substrate material and etching techniques play a major role on film grain structure as well as the presence of initial defects. Whatever the application, successful development of a thin film material requires an in-depth understanding of its mechanical properties through carefully designed experiments and their analysis. The quality and mechanical response of these films depends on many factors. Of relevance is the existence of film thickness and width effects that arise because of geometrical constraints to motion of defects. This paper uses the Membrane Deflection Experiment developed by Espinosa and co-workers to examine size effects in polycrystalline thin film gold specimens [5-9].
EXPERIMENTAL PROCEDURE Specially designed thin film gold specimens were microfabricated on (100) Si wafers. Specimen shape was defined on the topside by photolithography and lift-off. On the bottom-side windows were etched through the wafer, underneath the specimens, with the purpose of creating suspend membranes. The geometry of the suspended thin- film membranes can be described best L8.3.1
as a double dog-bone tensile specimen. A detailed description of their fabrication and shape is given in Espinosa and Prorok [8]. Figure 1 shows an optical image of the Au membranes. Membrane in-plane dimensions were varied but geometrical ratios were preserv
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