A Nanoindentation Study of Thermally-Grown-Oxide Films on Silicon

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A Nanoindentation Study of Thermally-Grown-Oxide Films on Silicon Fatih Helvaci and Junghyun Cho Dept. of Mechanical Engineering Thomas J. Watson School of Engineering State University of New York Binghamton, NY 13902, USA ABSTRACT We explore the effect of the substrate on mechanical behavior of thin films using a depthsensing indentation. For this purpose, nanoindentation has been performed on thermally grown silicon oxide films on the silicon substrate. One primary goal of this study is to extract ‘filmonly’ mechanical properties from the nanoindentation data by subtracting the substrate effect. It is also shown that elastic modulus of the film is more influenced by the substrate than hardness due to a larger elastic extension beyond a plastically deformed region, as well as a larger elastic mismatch between the SiO2 film and the Si substrate. Further, an inverse analysis of indentation data is proposed to estimate a thickness of thin films. Consequently, this study provides a fundamental understanding in mechanical phenomena of thin films occurring at nanoscales. INTRODUCTION Thin films have been a key technology in many areas including electronics, magnetic recording media, optical devices, MEMS, and chemical and mechanical protection of engineering components [1]. In such developments, a good understanding of mechanical properties of thin films is essential [2,3]. It is, however, not trivial to evaluate mechanical properties of a thin structural unit of the sub-micron scale by conventional testing methods. Given that, the depth-sensing indentation (nanoindentation) has received great attention as it provides necessary resolution besides its easy adaptation to localized structures [4]. In particular, silicon oxide thin films are of great importance in microelectronics and optics industry [5-7]. For example, the films can be used as inter-level dielectrics and gate dielectrics in metal-oxide-semiconductor field effect transistors (MOSFETs) and thin films transistors (TFTs), protective layers in metallic mirrors, anti-reflection coatings, and low index layers in multilayer coatings for the mid-IR range [8-13]. Although nanoindentation is an excellent tool, a good interpretation of the indentation data is important to have reliable and repeatable results for mechanical behavior of thin films. In addition, influence of substrates often makes it difficult to establish ‘film-only’ mechanical properties. Previously it was reported that 10-25% of the film thickness would be a proper range for indentation to avoid the substrate effect [14]. It will be therefore challenging to do nanoindentation on ultrathin films since the indentation will be performed beyond this range. Numerous models have been developed in order to obtain true thin film properties from nanoindentation data [14]. Accurate determination of the reduced modulus and projected contact area prior to application of the nanoindentation models would be critical. One purpose of this study is then to characterize the SiO2 thin films grown on the Si subs