Passive devices for determining fracture strength of MEMS structural materials
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1052-DD02-02
Passive devices for determining fracture strength of MEMS structural materials H Kahn1, R Ballarini2, and A H Heuer1 1 Materials Science and Engineering, Case Western Reserve University, Cleveland, OH, 441067204 2 Civil Engineering, University of Minnesota, Minneapolis, MN, 55455 ABSTRACT In microelectromechanical systems (MEMS) device design and fabrication there exists a need for rapid determination of fracture strength. This report describes several passive devices that use the residual stresses contained within structural MEMS materials to determine fracture strength. Stress concentrations of varying degrees are generated at micromachined notch roots, and the critical stress required for failure indicates the fracture strength. A variety of devices have been fabricated from materials such as polysilicon, silicon nitride, and aluminum, with widely varying residual stresses, including devices with both tensile and compressive residual stresses.
INTRODUCTION To accurately design microelectromechanical systems (MEMS) devices and predict their reliabilities, the mechanical strengths of the structural materials must be known. Since MEMS are typically made from brittle materials such as polysilicon, it is essential that the strength measurements be made on materials that have gone through the same fabrication steps, namely deposition and etching, that would be seen in device processing. For brittle materials, the strength is governed by the inherent flaws that are generated during fabrication. Rapid determination of fracture strength could also then be used to verify that any changes in a fabrication process had not caused detrimental effects to the device integrity. It is highly desirable that the strength measurements be made using “passive” devices – devices that perform their function upon release with no further actuation. Then, visual inspection is all that is needed to obtain the results. Devices that fit this description (which have previously been reported) are clamped-clamped beams produced from materials that contain residual tensile stresses [1-3]. Upon release, the residual tension creates stresses within straight beams [1], notched beams [2], or beams with sharp pre-cracks [3]. If the stresses are high enough, the beams will fail catastrophically, and if not, no changes will be observed. Given accurate knowledge of the residual stress in the material, finite element analysis (FEA) can predict the stresses in the device. Then, upper and lower bounds can be established for the strength of the material. However, for a given device geometry, the usable range of residual stresses can be small. For some materials, the residual stress can be tailored by varying the deposition conditions, but in other cases, this ability is limited. In this report, we describe several passive device designs that can be used to determine the strength of materials with a wide range of residual stresses, including devices with both tensile and compressive residual stresses.
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