Stress and Strength of free-standing 2-dimensional tetrahedral amorphous carbon bridge arrays
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Stress and Strength of free-standing 2-dimensional tetrahedral amorphous carbon bridge arrays Daniel H.C. Chua, T.H. Tsai and W.I. Milne Engineering Dept, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, England D. Sheeja and B.K. Tay School of Electrical and Electronics Engineering, Nanyang Technological University, Singapore ABSTRACT The fabrication of ultrathin (25nm) 2-dimensional free-standing arrays of tetrahedral amorphous Carbon (ta-C) microbridges is reported for the first time. The ta-C films were deposited by a Filtered Cathodic Vacuum Arc (FCVA) deposition system where the sp3 content in the film was measured to be in excess of 90% by high resolution XPS. Continuous arrays of free standing taC bridges whose length/width ratios ranged from 1:1 to 12:1 were successfully fabricated while maintaining the same thickness. Due to the naturally high compressive stress of ta-C films, the buckling of films was perpendicular to the length of the beam. The displacement of curvature obtained was in good agreement with FEM simulation results. Moreover, the curvature or arch of these ultrathin films, coupled with a high Young’s modulus (750GPa) and Hardness (60GPa), meant they could withstand a vertical force in excess of 8000µN without breaking. INTRODUCTION Micromechanical devices are currently fabricated primarily in silicon because of the available surface machining technology. A major problem with Si-based MEMS technology is that Si has poor mechanical properties, namely poor flexural strength and fracture toughness. Practical MEMS devices typically must be designed to circumvent these limitations. At the same time, many researchers are reporting on the miniaturization of MEMS microstructures from micron scales to nanometers scale [1,2]. One advantage of using nanomechanical structures over MEMS is the decrease in the size of the mechanical structures, which is usually accompanied by an increased resonant frequency in the case of mechanical resonators. [3,4] As such, a great variety of engineering applications can be realised with the decrease in dimensions but whether these microstructures can be engineered remains. Currently, majority of the smallest poly-Si microstructures are fabricated of the order of 200-400nm in thickness and probably several microns in length. A literature survey revealed that there is no design for tens of nanometer thick nanomechanical structures as the probability of such devices collapsing under their own weight is very high. In this study, we demonstrate a proof of concept that it is possible to engineer such structures by using tetrahedral amorphous Carbon (ta-C) rather than Si or poly-Si. Ta-C films have the unique advantages of high hardness, low coefficient of friction, thermal stability to 600oC, high electrical resistivity, high optical bandgap with transparency over a wide spectral range. They are also highly inert due to the high sp3 content (>90%) which makes them very useful in highly corrosive or hostile environments [5,6]. However, such films have
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