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MEMS inertial sensors design and fabrication based on an innovative process

Abstract : In the vast majority of commercial MEMS inertial sensors, both seismic mass and sensing elements are patterned in the same silicon layer. This sets stringent design trade-offs, in particular for a capacitive sensor: a large silicon thickness increases seismic mass and decreases the Brownian noise floor. A low silicon thickness on the other hand, allows smaller gaps between electrodes, higher capacitance variation and lower electrical noise floor. For this reason, several examples of multi-layer MEMS devices were presented in the past. Yet, increasing capacitance density while reducing mechanical noise floor has not been achieved so far. Breaking the single-layer trade-off could enable new emerging applications that require high-performance sensors within a consumer size.In this work, multi-layer, in-plane and out-of-plane accelerometer are presented. Thanks to the multi-layer process the devices can feature a thick layer for large inertial mass, as well as a thin layer for high capacitive density. These aspects, together with surface-variation detection, allow to obtain µg/√Hz resolution and large full-scale while keeping compact size.The sensors are designed through analytical modeling and finite elements method simulations in order to reach the highest dynamic range with the lowest noise at given footprint.Few critical aspects were encountered during the fabrication of the sensors, especially for out-of-plane accelerometers. The notching of the thick-layer etching coupled to the strong lag effect caused most of the z-axis sensors to fail. This forced a reduction of the process thickness and relative loss of performance for this type of sensors.The characterization of the sensors is performed both at wafer-level (static capacitance, resonance frequency) and at die level (scale factor, noise-floor, full-scale). The die-level measurements are carried out with a dedicated electronic circuit implemented with discrete components, developed during this work.In-plane accelerometers showed static capacitance and resonance frequency in line with theory. They achieved resolution smaller than 8 µg/rtHz and full scale in the order of 160g. These aspects together lead to a dynamic range of more than 145dB (BW=1Hz) for a device with a footprint of only 0.24 mm². This it more than 100 times larger than the DR of consumer device of similar size. These results are achieved while keeping a large bandwidth and working with an open-loop readout.Out-of-plane sensors showed resonance frequency higher than expected due to fabrication tolerances. The devices had both smaller mass and thicker springs explaining the observed mechanical behavior. Despite the loss of scale factor due to the larger resonance frequency, these sensors achieved resolution ranging from 50-80 µg/rtHz. Again, such performance was obtained while keeping large resonance frequency (>8 kHz), small footprint (down to 0.22 mm²) and a potential full-scale of more than 200g. In the future, design corrections and process improvement could lead to device with thicker inertial layer, aligning the performance of out-of-plane sensors to those of in-plane ones and leading to a high-performance 3-axis accelerometer.This type of sensor could address the demand of emerging applications for high-stability, low-noise and large DR accelerometers within consumer footprint.Finally, the proposed technology offers a fabrication platform for inertial MEMS sensors and actuators. New design possibilities and great potentialities have been demonstrated with the first fabricated accelerometers. In the future this new concept could be applied to several other types of MEMS, like gyroscope or micro-mirrors.
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Federico Maspero. MEMS inertial sensors design and fabrication based on an innovative process. Micro and nanotechnologies/Microelectronics. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAT126⟩. ⟨tel-02944092⟩

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