Tuning the Mechanical Properties of SiO 2 Thin Film for MEMS Application
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Tuning the Mechanical Properties of SiO2 Thin Film for MEMS Application Wang-Shen Su, Weileun Fang and Ming-Shih Tsai 1 Institute of Microelectromechanical Systems, National Tsing-Hua Uninersity No. 101, Sec. 2, Kuang-Fu Rd., Hsinchu 300, Taiwan 1 National Nano Device Laboratory, 1001-1 Ta-Hsueh Road, Hsinchu 300, Taiwan ABSTRACT This study reported a novel method for tuning thin film mechanical properties by means of plasma surface modification. In order to demonstrate the feasibility of this approach, various plasma treatments, including O2, H2, NH3 atmospheres, were implemented to tune the Young’s modulus and residual stress of SiO2 film. Without plasma treatment, the static tip deflection of 200µm long SiO2 cantilever was 9.01µm. After treatment with H2, O2, and NH3 plasma, the tip deformation of the treated cantilevers became 10.22µm, 8.28µm, and -6.84µm respectively. The Young’s modulus of the SiO2 cantilever without plasma treatment was 76.3GPa. After treated with H2, O2, NH3 plasma, the Young’s modului of those treated cantilevers became 70.8 GPa, 74.7 GPa, and 71.4 GPa, respectively. Hence, after H2 and NH3 plasma treatment, the equivalent elastic modulus of SiO2 cantilever could be reduced about 7%. INTRODUCTION Thin films on semiconductor substrates are of special interest to academics and industries involved in microelectronic devices and micro-electro-mechanical systems (MEMS). The mechanical properties of thin film, such as Young’s modulus and residual stress, are very critical for the performance of MEMS devices. However, thin films have, in general, different mechanical properties from their bulk counterparts. In this regard, the mechanical properties of thin films have attracted lots of attentions [1]. The thermally grown SiO2 film is particularly useful for dielectric isolation and chemical passivation. It surfers from the higher residual stress ranging from 100 to 400 Mpa [2], and could result in the degradations of functioning and reliability of devices. It is important to reduce the residual stress of SiO2 thin films, especially for MEMS and other optical devices applications [3]. Plasma treatment to produce a various functional surface for surface cleaning and modifications are widely implemented in the advanced semiconductor manufacturing [4]. Thus, it is important to exploit the effects of various plasma treatments and the changes of surface bonding and structure to tune the nano-mechanical properties of SiO2 thin films. In this study, various plasma treatments, including O2, H2 and NH3 plasma, on the SiO2 surface were investigated. The chemical bonding and mechanical properties of the treated SiO2 micro-cantilevers, such as static tip deflection, Young’s modulus, hardness, surface bonding and depth profile analysis, were characterized. EXPERIMENT SiO2 cantilever beam preparation The process flow with various plasma treatments was schematically shown in Figure 1. The 1 µm thick SiO2 film was thermally grown at 1050 on the 4” bare silicon wafer. Various plasma treatments were employed
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