Microbridge Tests on Bilayer Thin Films
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Microbridge Tests on Bilayer Thin Films XU-SHENG WANG and TONG-YI ZHANG In the present work, we report a microbridge testing method for a bilayer microbridge beam initially buckled by a residual compressive resultant force and a residual moment. A theoretical formula is derived in closed form with the consideration of substrate deformation. Measuring the profile of the initial buckling, one can evaluate the Young’s modulus and residual stress of each layer of the bilayer beam. Alternatively, the Young’s modulus and residual stress can be evaluated also from the loaddeflection curve of the microbridge. Experimentally, Si3N4(200 nm)/SiO2(400 nm) microbridge samples were fabricated on a silicon wafer by the micromachining technique. The Young’s modulus and residual stress were determined from the microbridge test to be 35 6 2 GPa and 380 6 20 MPa for the silicon oxide layer and 250 6 7 GPa and 470 6 15 MPa for the silicon nitride layer.
I. INTRODUCTION
A microbridge testing method was developed to characterize mechanical properties of asymmetrical trilayer (or multilayer) thin films, in which there was a residual moment that induced initial deflection.[1] In the previous work,[1] the residual resultant force in the asymmetrical trilayer films was intentionally designed to be tensile such that the initial deflection was induced only by the residual moment. The initial deflection caused by a residual moment is not due to buckling. Buckling is induced by a compressive resultant force in a multilayer film when the magnitude of the compressive resultant force exceeds a critical value. Film buckling has been studied without any applied loads.[2,3,4] Usually, the classic Euler beam theory is adopted in the analysis and fitting a buckling profile with the theoretical result yields residual stress in the buckled film, such as in polysilicon[2,4] and silicon dioxide[3] films. Cao et al.[5] conducted the microbridge test[6] on buckled silicon oxide single-layer films. The initial deflection was taken into account in the data analysis and the unloading curve was used to determine the Young’s modulus and the residual stress of the silicon dioxide film.[5] Huang and Zhang[7] systematically studied microbridge tests on a buckled gold single-layer film. In single-layer films, there is no residual moment if residual stress in the film is homogeneously distributed, which is usually assumed in the study of film stresses. In multilayer thin films, however, there may coexist a residual moment and a compressive resultant force even if residual stress in each layer of the film is homogeneously distributed. A residual moment and a compressive resultant force may jointly cause microbeams and other structures made of multilayer thin films to buckle and bend. To date, microbridge tests on such multilayer thin films with a residual moment and a compressive resultant force XU-SHENG WANG, Postdoctoral Student, and TONG-YI ZHANG, Professor, are with the Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Kowloon, Hong Kong. Contact
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