Acceleration measurement has been an essential requirement in many commercial and industrial applications. One of the urgent needs is the low-power, reliable, and high-performance acceleration-sensing nodes for the Internet of Things (IoTs) in smart buildings and smart manufacturing. With the mature CMOS fabrication process in the industry, the low-cost reliable CMOS MEMS thermal accelerometers with the on-chip integrated circuits (ICs) will be promising for practical implementation for IoT.

This thesis mainly focuses on the systematic design and fabrication of the low-cost micromachined thermal convective accelerometers by using MEMS and CMOS-MEMS technology. Primarily, a unified design platform with the theoretical modeling and simulation was proposed to achieve a combined mu...[ Read more ]

Acceleration measurement has been an essential requirement in many commercial and industrial applications. One of the urgent needs is the low-power, reliable, and high-performance acceleration-sensing nodes for the Internet of Things (IoTs) in smart buildings and smart manufacturing. With the mature CMOS fabrication process in the industry, the low-cost reliable CMOS MEMS thermal accelerometers with the on-chip integrated circuits (ICs) will be promising for practical implementation for IoT.

This thesis mainly focuses on the systematic design and fabrication of the low-cost micromachined thermal convective accelerometers by using MEMS and CMOS-MEMS technology. Primarily, a unified design platform with the theoretical modeling and simulation was proposed to achieve a combined multiphysics study of the micro thermal convective accelerometers (MTCA). Thereby, a general one dimensional (1D) model of MTCA was proposed for the first time to predict the sensor output as a function of Rayleigh number, regard as the normalized input acceleration. This unified design platform could be significantly efficient for the design and optimization of the MTCA device system. Accordingly, two different designs of MTCAs were successfully fabricated with MEMS technology and commercial CMOS MEMS technology, respectively.

The first MTCA design was fabricated by using the MEMS techniques at HKUST, and the test results show good agreement with the prediction from the 1D model, which enables efficient design analysis and structure optimization. Besides, using this CMOS compatible fabrication process, a device with a sensitivity of 7,075μV/g and a normalized sensitivity/power ratio of 201.4μV/g/mW is achieved, which is twenty-fold larger than that of the state of the art. Besides, a temperature-compensated MTCA with the on-chip temperature sensor and the digital signal progressing algorithm in the MCU has been successfully developed with the excellent performance, i.e., the maximum normalized variation of 2% under changing T_a of 27℃~89℃ in comparison with the uncompensated counterpart (65%). Furthermore, a novel dual-axis micro thermal convective accelerometer with Coriolis-force compensation structures is demonstrated. The crossing effect error could be well controlled within 2% comparing with previously uncompensated results of 15%, demonstrating that this novel design is meant for improving the accuracy of MTCA and decoupling the signal between acceleration and gyroscope. Finally, a z-axis accelerometer with enhanced sensitivity (x-axis of 1,211μV/g and z-axis of 402μV/g) is proposed.

Another MTCA design was fabricated by using the techniques of commercial foundries, including AMS 0.35μm 2P4M CMOS MEMS technology and Global Foundry 0.18μm 1P6M CMOS MEMS technology. By leveraging the conformal parylene-C coating, a reliable liquid-based CMOS MEMS MTCA is fabricated, which demonstrated that a fluid with larger Rayleigh number Ra could significantly improve the sensitivity and would obtain an evident saturation phenomenon. The liquid-based MTCA with the working fluid of alcohol could achieve a two-order of magnitude better-normalized sensitivity of 3,344μV/g/mW and a one order of magnitude lower minimum detectable acceleration (MDA) of 61.9μg than the air-based MTCA. In addition, another CMOS based MTCA with reduced film thickness and low power consumption is designed and fabricated using a metal layer as the hard mask. Furthermore, a novel method of increasing the heater temperature is proposed to enhance the minimum detectable acceleration. Benefiting from these two methods, an MTCA is achieved with a high sensitivity of 1646μV/g and a low minimum detectable acceleration of 372μg comparing with the former AMS-based design of 757μg.

The performance achieved by these MEMS and CMOS micro thermal convective accelerometers, including the high sensitivity, low power consumption, large measurement range, and high reliability and high accuracy makes them possible to be implemented to the acceleration sensing nodes of Internet of Things (IoT) for the smart building health monitoring system.