Mechanical Properties of Thin Film Silicon Carbide
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Mechanical Properties of Thin Film Silicon Carbide Kamili M. Jackson, Richard L. Edwards1, Guy F. Dirras2, and William N. Sharpe, Jr. Department of Mechanical Engineering Johns Hopkins University, Baltimore, MD 21218 1 Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723 2 LPMTM-CNRS, Institut Galilee, University Paris XIII , Villetaneuse, France ABSTRACT Silicon carbide is a very attractive material for a variety of applications. Originally considered for use in high power and high temperature electronics because of its large bandgap, designers of MEMS are now considering use of silicon carbide because of its stability at high temperatures, resistance to corrosives, high stiffness, and radiation resistance. However, as with any new structural material, its mechanical properties must be measured for design information. This research measures the elastic modulus, strength, and Poisson’s ratio of two different silicon carbides using microtensile testing. One material is a 0.5-1µm thick film from Case Western Reserve University. Preliminary results give an average of 420 GPa for elastic modulus, a strength of 1.2 GPa, and a Poisson’s ratio of 0.19. The second material is from Massachusetts Institute of Technology with an average thickness of 30 microns. Preliminary results show an elastic modulus of 430 GPa, a strength of 0.49 GPa, and a Poisson’s ratio of 0.24. In addition to the most recent results, techniques used to obtain these results, microstructure investigations, and a comparison of the materials are detailed. INTRODUCTION The significant advancement of mechanical testing of thin films over the past ten years has enabled extensive mechanical testing of polysilicon, the dominant MEMS material. Test results include fracture strength, Young’s modulus, Poisson’s ratio, fatigue life, fracture toughness, temperature dependence, and size dependence. As MEMS technology grows, the properties of polysilicon are becoming a limiting factor in some designs. In order to adapt to new challenges, new materials with attractive electrical and mechanical properties like silicon carbide, are being considered. Characteristics like a larger bandgap, high temperature stability, corrosion resistance, and high stiffness make it a favorable material for high temperature sensors, micro power applications and some biological uses. A drawback to using thin film silicon carbide in MEMS is that knowledge of the mechanical properties is not readily available. However, design and fabrication with silicon carbide is increasing, which creates a need to characterize the mechanical properties of thin film silicon carbide. This research is focused on measuring the Young’s modulus, Poisson’s ratio and strength of thin film silicon carbide. An interesting feature of silicon carbide is the numerous polytypes that form with the same chemical composition. They can be cubic, hexagonal or rhombehedral. The silicon carbide used here is the only known cubic form, 3C.
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EXPERIMENTAL METHODS Specimen Preparation To provide these results, micros
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