An Investigation of Film Thickness Effect on Mechanical Properties of Au Films Using Nanoindentation Techniques
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An Investigation of Film Thickness Effect on Mechanical Properties of Au Films Using Nanoindentation Techniques Yifang Cao, Zong Zong, and Wole Soboyejo Princeton Institute of Materials Science and Engineering(PRISM) and Department of Mechanical and Aerospace Engineering Princeton University Princeton, NJ 08544
ABSTRACT This paper presents the results of nanoindentation experimental studies of Au thin films with different thicknesses. The effects of film thickness and microstructure on the hardnesses of electron-beam deposited Au films were studied in terms of Hall-Petch relationship. The effects of different thicknesses on indentation size effects (ISE) are explained within the framework of mechanism-based strain gradient (MSG) theory using the concept of microstructural length scale. I. INTRODUCTION In recent years, significant efforts have been made to develop micro-electronics and micro-electromechanical systems (MEMS) structures that include metallic contacts at the micro- and nano-scales [1– 3]. In most cases, gold has been used due to its exceptional combination of oxidation resistance and electrical conductivity [4]. Within this context, the physics of contact-induced deformation of Au films needs to be fully investigated. In addition, the mechanical properties of the small structures may be significantly different from those of bulk materials [5]. Therefore, there is a need to obtain measurements of the mechanical properties at the appropriate scales for Au films. Several prior experimental efforts have been made to measure the nano/micro-scale mechanical properties of polycrystalline Au using nanoindentation [6,7], uniaxial tensile testing [8], membrane deflection experiment [9], and bimaterial microcantilever testing [10]. However, a complete understanding of mechanical properties of Au films in terms of film thickness effect, microstructure effect, substrate effect is yet to emerge. This paper presents the results of an experimental study of film thickness effect on the mechanical properties of Au films using nanoindentation techniques. Following a brief description of sample preparation and surface microstructure, film mechanical properties (hardnesses) are characterized using nanoindentation techniques. The thickness effect on yield strength of Au films is discussed in terms of Hall-Petch relationship. The effects of different thicknesses on hardness indentation size effects (ISE) are analyzed within the framework of the mechanism-based strain gradient (MSG) theory. Electron-beam deposited thin films Au with thicknesses of 100 nm, 500 nm, 1000 nm and 2000 nm on Si substrates are examined in this study. II. A NALYSIS T HEORIES Doerner and Nix [11], and later on Oliver and Pharr [12,13] developed a most comprehensive method for determining the hardness and modulus from depth sensing indentation (DSI) load-displacement data. In the theory, the Meyer’s definition of hardness, H was adopted. This is given by: H=
Pmax A
(1)
where Pmax is the maximum load, A is the projected contact area, which ca
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