Time and Temperature Creep Behaviour Measurement of Al and Al-Mg Alloy Thin Films Using Pressure Bulge Tests
Metal thin films are often used as capacitance switches in Micro Electro Mechanical Systems (MEMS). But long-term reliability is always the question to be solved. If thin films have better mechanical properties, it can not only reduce the creep behavior,
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Time and Temperature Creep Behaviour Measurement of Al and Al-Mg Alloy Thin Films Using Pressure Bulge Tests C.-H. Lu, S.-C. Wu, A.-W. Huang, and M.-T. Lin Abstract Metal thin films are often used as capacitance switches in Micro Electro Mechanical Systems (MEMS). But longterm reliability is always the question to be solved. If thin films have better mechanical properties, it can not only reduce the creep behavior, but also extend its lifetime. In this study, we use solid solution strengthening to improve the mechanism of the material, adding foreign atom Mg to Al thin films in order to increase the resistance to creep behavior, the more we add Mg, the more difficult for dislocation sliding, then the mechanism of the material become better. As the result, after adding Mg into Al, it can effectively reduce the creep behavior, so Al-Mg films are much better than pure Al films using in capacitance switches. Keywords Thin films • Mechanical properties • Creep of thin metal films
19.1
Introduction
Creep is a very important issue in the reliability of MEMS (microelectromechanical system) devices. MEMS devices are easily affected by creep, especially when MEMS devices are working in high stress and high temperature environments [1]. For example some RF MEMS capacitive switches may be endangered by creep or viscoelastic behavior if the material is not selected appropriately. Some low-melting materials are sensitive to creep and viscoelastic behavior; an example is Al alloy. But movable bridges of RF switches are often fabricated from Al alloy [2]. In bulk metal materials, it has little relaxation and small creep strain. On the contrary, the fine small grain material such as thin film has be strong influence on relaxation and creep. Many research report that several thin film material is time dependent deformation as the result of relaxation and creep. Yan et al. [3] reported that a 27 % reduction in effective elastic modulus occurs under constant strain conditions at room temperature over a 3 day period. Therefore, increasing temperature means increasing both the magnitude and rate of stress relaxation and this situation make RF switch easy to failure at high temperature. Al thin film has low resist to creep. The goal of this study is to add Mg atoms in pure Al thin films to improve the mechanism of Al thin films. These Mg atoms could hinder the movement of dislocation and decrease recrystallizes effect on the thin films. The Al-Mg alloy thin films are testing and hope to increase the resist of creep. If the mechanisms of Al-Mg alloy thin films being increase and the other mechanisms are not decreases, this alloy thin films can be replaced to pure Al thin films to increase the reliability and expand the useful time of MEMS.
19.2
Experimental
19.2.1 Sample Design and Fabrication The sample fabrication procedure is using standard clean room processing. The sample is fabricated on a standard 4 in. diameter double polished silicon wafer. The single test chip is designed as shown in Fig. 19.1 to fit in the bulge apparatus
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