Development of a three-axis MEMS accelerometer

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Title: Development of a three-axis MEMS accelerometer
Author: Leveto, Jacob
Abstract: While originally developed to deploy air bags for the automotive industry, Microelectromechanical Systems (MEMS) based accelerometers have found their way into everything from video game controllers to cells phones. As prices drop and capabilities improve, it is expected that the use of accelerometers will further expand in the coming years. Accelerometers currently have the second highest MEMS sales volume, trailing only pressure sensors [1]. In this work several single and three-axis accelerometers are designed, fabricated, and tested under a variety of conditions. The designed accelerometers are all based off of the piezoresistive effect, where the value of a resistor changes with applied mechanical stress [2]. When accelerated, the inertia of a suspended proof mass causes stress on piezoresistors placed on support arms. The corresponding changes in these resistor values are then converted to an output voltage using a Wheatstone bridge. To sense acceleration independently in all three axes, structures with three distinct modes of vibration and three sets of Wheatstone bridges are used. Devices were fabricated at the Semiconductor and Microsystems Fabrication Laboratory (SMFL), located at RIT. A modified version of the RIT bulk MEMS process was used, consisting of 65 steps, 7 photolithography masks, bulk silicon diaphragm etch, and top hole release etch [3]. Unfortunately the finished chips show poor aluminum step coverage into contact vias and over polysilicon lines. This results in open circuits throughout the chip, prohibiting proper operation. Process corrections have been identified, and with proper fabrication the designs are still expected to yield working devices. Since the finished accelerometers were not functional, several commercial accelerometers have been tested to characterize sensitivity, linearity, cross-axis sensitivity, frequency response, and device lifetime.
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Date: 2010-06

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