Tema da tese: Low-pressure resonant MEMS accelerometers for automotive applications

Autor: Eurico Esteves Moreira

Programa Doutoral: Programa Doutoral em Sistemas Avançados de Engenharia para a Indústria

Orientador: João Carlos Azevedo Gaspar; Jorge Miguel Nunes dos Santos Cabral; Luís Alexandre Machado da Rocha (à memória de)

Data: 28/10/2020

Abstract: MEMS accelerometers are instrumental in the development of the automotive industry, where many applications improved the security and driving experience. However, better, cheaper, and smaller devices would enable even further use-cases. Encapsulation in vacuum has been studied to develop more affordable and smaller devices since it would allow the monolithic integration of the sensors composing an inertial measurement unit. Nevertheless, traditional accelerometers do not operate properly at low pressures.

This work focus on the development of a resonant accelerometer based on double-ended tuning fork (DETF) resonators. The accelerometer proposed is composed of two DETF on a differential configuration to minimise common-mode effects, and a force amplification mechanism was added to increase sensitivity. The designed devices were fabricated in a Bosch standard surface micromachining process and encapsulated in vacuum. A system was implemented to validate the accelerometers, operating the resonators in open-loop and closed-loop. In closed-loop, the driving frequency of the DETF is constantly updated, increasing the sensor linearity and full-scale. An extensive characterisation was performed and a good non-linearity (<0.63\% FS) and high sensitivity (170.7 Hz/g) on a small footprint (0.25 mm²) were highlighted as the main achievements. A bandwidth of 63 Hz and a measurement range of at least ±5 g were also reported.

A differential frequency-modulated magnetometer was designed, implemented and characterised to prove the feasibility of magnetic field sensing using DETF resonators. A novel Lorentz force transducer was proposed and optimised to increase the device’s sensitivity. The magnetometers were fabricated on an in-house two mask SOI-process, and the experimental validation of the sensor envisioned was performed.  Keywords: MEMS, resonant accelerometer, frequency-modulated magnetometer, vacuum encapsulation.