Fabrication Of Comb-drive MEMS Accelerometers

Micromachined silicon accelerometers are one of the earliest micro mechanical inertial sensors under investigation because of easy miniaturization, excellent mechanical properties of silicon, maturity of silicon micromachining technology and easy integration with IC. They are widely used in the field of air navigation, military weapons and so on. This paper aims to design and implement MEMS accelerometers based on the comb-drive capacitive detection and electrostatic feedback, targeted at a range of ± 15 g, and an equivalent acceleration of mechanical thermal noise less than 10?g / ?Hz.Based on investigating the development status of MEMS accelerometers, this thesis analyzes the models of traditional mechanical acceleration sensing and variable distance capacitive displacement sensing. We study the parameters which might affect the accelerometer performances, such as the intrinsic resonant frequency of the spring-proof-mass system, the rejection ratio along the acceleration-sensitive axis to non-sensitive axis, the mechanical thermal noise, the operation bandwidth and the acceleration response linearity. Hence, the model and the structural design of accelerometers for this thesis are established. The analytical results of the mechanical structures are consistent with the finite element simulation of Ansys modal analysis, confirming the reliability of the mechanical structure design. With respect to the device fabrication, this thesis discusses the process flow based on silicon on insulator(SOI) in details. The emphasis is put on the release of the device layer after deep reactive ion etching(DRIE). A variety of release methods are carried out and compared, including wet etching and dry etching. Finally, the proof mass and the moveable comb structures are successfully suspended by dry etching release.Finally, using the external detection circuitry, the acceleration sensing module is tested. The intrinsic frequency is measured to be 384.5Hz, and the quality factor to be 104.5. It could be derived that the equivalent acceleration of thermal mechanical noise is 0.7×10-7g/?Hz, much lower than the resolution requirement of 10-5g/?Hz. According to calibration, this acceleration sensor can respond to external acceleration with the sensitivity of 127.4m V/g when operating in the open loop mode. Considering the good consistency between the design structure and the real fabrication results, the module is expected to reach the detection range of 15 g in the closed loop operation. The work of this thesis has built a solid base for ultimately implementing a closed-loop accelerometer based on electrostatic feedback with a range of ±15g and a resolution of 10?g/?Hz.