ADVANCED FREQUENCY OUTPUT RESONANT MEMS ACCELEROMETERS
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Shin, Seungyong
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Abstract
MEMS accelerometers are micro-scale devices that measure acceleration of an object with respect to its reference frame. Due to their small size and low cost, MEMS accelerometers have had great success in industrial, automotive and consumer including gesture recognition. However, MEMS accelerometers with extended performance are required by emerging applications like health informatics, robotics, shock measurement, missile guidance, and indoor navigation (or dead reckoning), which are not easily achievable with currently available conventional MEMS accelerometer technology. Therefore, breakthroughs in 3-axis MEMS accelerometer technology are highly desirable.
Conventional MEMS accelerometers utilizing capacitive acceleration sensing mechanism suffer from well-known trade-off between noise performance, the measurement bandwidth, and dynamic range. In order to solve those, one can leverage sub-micron capacitive transduction gap. However, due to the increased capacitive mismatch due to fabrication imperfections, improvement in noise performance and dynamic range are limited.
On the other hands, a mechanically frequency modulated (mFM) resonant accelerometer technology developed in recent years shows very high sensitivity with low noise. However, because of its large proof-mass requirement, the operational bandwidth is limited. Additionally, system nonlinearity is easily increased with very low input acceleration which limit linear dynamic range of the device.
This dissertation focuses on the design and implementation of multi-axis high dynamic range MEMS resonant accelerometers operating in vacuum that sense linear acceleration in wide frequency range (DC - 10 kHz) with considerable accuracy (< 10 μg/ Hz) by utilizing the electrostatic spring softening effect, or eFM transduction mechanism in advanced resonating transducers. A unique fabrication process enabling features required for the implementation of out-of-plane eFM accelerometer with differential output is utilized. The design challenges are identified, and the solutions are provided. Especially the physical principles behind temperature stability of the eFM resonant accelerometer is carefully examined and compensation techniques are demonstrated. The hermetically encapsulated out-of-plane eFM accelerometer demonstrates a velocity random walk (VRW) of smaller than 10 μg/ Hz with open-loop bandwidth greater than 10kHz.
Additionally, designs and implementations of two piezoelectric resonant accelerometers for two extreme applications are also presented. The first one is piezoelectric eFM accelerometer targeted to have extremely high linear dynamic range (> 180 dB). The implemented prototype accelerometer with area changing transduction mechanism in a 3D fabrication process demonstrates over 140dB of linear dynamic range. The other is piezo- electric mFM seismometer which enables extremely low noise performance. The implemented mFM seismometer proves the advantage of piezoelectric mFM accelerometer utilizing 3D structure and mechanical force amplifier in improving the sensitivity.
The experiments and characterizations performed on the presented resonant accelerometers establish new designs and structures to enable high performance MEMS accelerometer for emerging applications.
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Date
2021-05-01
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Dissertation