Compliant MEMS Motion Characterization by Nanoindentation
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Compliant MEMS Motion Characterization by Nanoindentation Joseph Goerges Choueifati1, Craig Lusk1, Xialou Pang1,2, and Alex A. Volinsky1 1 Mechanical Engineering, University of South Florida, Tampa, FL, 33620 2 Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing, 100083, China, People's Republic of ABSTRACT Large out-of-plane displacements can be achieved when compliant mechanisms are utilized in MEMS. While mathematical and macroscopic modeling is helpful in building original designs, the actual MEMS device motion needs to be characterized in terms of the forces and displacements. A nanoindentation apparatus equipped with Berkovich diamond tip was used in an attempt to actuate and characterize the motion of the Bistable Spherical Compliant Micromechanism with a nonlinear (approximately cubic) mechanical response. Based on the obtained lateral force-displacement data it was concluded that the Berkovich diamond tip was too sharp, thus cutting through the polysilicon material of the MEMS device. INTRODUCTION The most common technique used in building MEMS is surface micromachining [1, 2] due its simplicity and low cost. A challenge in using surface micromachining is that it produces essentially two-dimensional products. The ratio of the length and width with respect to the thickness of the elements created is high, thus most MEMS have a planar working space, where the motion of their links traces a single plane [3]. In some applications such as active Braille [4] and micro-optical systems [5], it may be useful for MEMS to achieve accurate three-dimensional motion. This paper provides the results obtained from testing a bistable compliant MEMS device with out-of-plane motion using a nanoindenter. A description of the bistable spherical micromechanism is also presented. Mechanisms that rely on elastic deformation of their flexural members to carry out mechanical tasks of transforming and transferring energy, force and motion are called compliant mechanisms [6]. Furthermore, compliant mechanisms combine energy storage and motion, thus eliminating the need for separate components of joints and springs [6]. Many products currently on the market such as nail clippers, shampoo cap hinges and mechanical pens utilize compliant segments in their designs. In addition, studies have shown that one of the main reasons behind MEMS failure is joints wear [7]; thus replacing rigid multi-pieces joints with compliant single member joints will likely increase the device’s lifespan [8-10]. A bistable mechanism is a mechanism that has two stable equilibrium points within its range of motion. A mechanism is considered to be in stable equilibrium if it returns to its equilibrium position after being subjected to small forces or disturbances. A mechanism is in an unstable equilibrium when a small force causes the mechanism to change positions, usually to a position of stable equilibrium. According to Lagrange-Dirichlet theorem, an object is in a stable equilibrium when its potenti
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