Microbridge Testing of Silicon Oxide/Silicon Nitride Bilayer Films
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MICROBRIDGE TESTING OF SILICON OXIDE/SILICON NITRIDE BILAYER FILMS C.-F. Qianc, Y.-J. Su, M.-H. Zhaob , T.-Y. Zhanga Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ABSTRACT The present work further develops the microbridge testing method to characterize mechanical properties of bilayer thin films. A closed-form formula for deflection versus load under small deflection is derived with consideration of the substrate deformation and residual stress in each layer. The analysis shows that the solution for bending a bilayer beam is equivalent to that for bending a single-layer beam with an equivalent bending stiffness, an equivalent residual force and a residual moment. One can estimate the Young’s modulus and residual stress in a layer if the corresponding values in the other layer are known. The analytic results are confirmed by finite element calculations. The microbridge tests are conducted on low-temperature-silicon oxide (LTO)/silicon nitride bilayer films as well as on silicon nitride single-layer films. All microbridge specimens are prepared by the microfabricating technique. The tests on the single-layer films provide the material properties of the silicon nitride films. Then, applying the proposed method for bilayer films under small deflection yields the Young’s modulus of 37 GPa and the residual stress of -148 MPa for LTO films. INTRODUCTION Characterizing, understanding and controlling mechanical properties of thin films have become one of the most active areas of research, as applications of thin films in microelectronic devices and microelectromechanical systems (MEMS) increase dramatically. One of the major difficulties, encountered in studying the mechanical behavior of thin films, is that they are not amenable to testing by conventional means because of their size and configuration. Therefore, new methods are being developed [1-7], such as uniaxial tensile testing of free-standing films, beam bending, bulge testing and the microbridge testing method. The microbridge testing method [7] is able to simultaneously characterize the Young’s modulus, residual stress and the bending strength. The microbridge method can be applied to multilayer films. The present paper reports the results for small deformation and silicon oxide/silicon nitride bilayer films deposited on silicon substrates are used as a model system to demonstrate the proposed method. For large deformation, readers may refer to reference [8]. ANALYSIS Figure 1 schematically shows the microbridge tests, where the microbridge is made of LTO/silicon nitride bilayer films and a silicon substrate with a support angle of 54.74o, which is caused by wet etching during the specimen fabrication. The thickness, Young’s modulus and residual stress in the two layers are denoted by h, E and σ, respectively, with the subscripts “1” and “2” representing the lower and the upper layers, respectively. a
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