This paper reviews the research and development of micromachined accelerometers with a noise floor lower than 1 µg/√Hz. Firstly, the basic working principle of micromachined accelerometers is introduced. Then, different methods of reducing the noise floor of micromachined accelerometers are analyzed. Different types of micromachined accelerometers with a noise floor below 1 µg/√Hz are discussed. Such sensors can mainly be categorized into: (i) micromachined accelerometers with a low spring constant; (ii) with a large proof mass; (iii) with a high quality factor; (iv) with a low noise interface circuit; (v) with sensing schemes leading to a high scale factor. Finally, the characteristics of various micromachined accelerometers and their trends are discussed and investigated.

With the introduction of the working principle of coupled resonators, the coupled bulk acoustic wave (BAW) Micro-Electro-Mechanical System (MEMS) resonators have been attracting much attention. In this paper, coupled BAW MEMS resonators are discussed, including the coupling theory, the actuation and sensing theory, the transduction mechanism, and the applications. BAW MEMS resonators normally exhibit two types of vibration modes: lateral (in-plane) modes and flexural (out-of-plane) modes. Compared to flexural modes, lateral modes exhibit a higher stiffness with a higher operating frequency, resulting in a lower internal loss. Also, the lateral mode has a higher Q factor, as the fluid damping imposes less influence on the in-plane motion. The coupled BAW MEMS resonators in these two vibration modes are investigated in this work and their applications for sensing, timing, and frequency reference are also presented.

The MEMS microphone is a representative device among the MEMS family, which has attracted substantial research interest, and those tailored for human voice have earned distinct success in commercialization. Although sustained development persists, challenges such as residual stress, environmental noise, and structural innovation are posed. To collect and summarize the recent advances in this subject, this paper presents a concise review concerning the transduction mechanism, diverse mechanical structure topologies, and effective methods of noise reduction for high-performance MEMS microphones with a dynamic range akin to the audible spectrum, aiming to provide a comprehensive and adequate analysis of this scope.

This study reports a closed-loop system (CLS) for resonant micro-electromechanical systems (MEMS) sensors subject to blue-sideband excitation (BSE) for the first time. Compared with the conventional excitation scheme, which uses an AC signal with a frequency that is very close to or equal to that of the desired MEMS resonator mode, the BSE scheme applies an AC signal with a frequency equal to or near the sum of two resonant frequencies to excite the resonator. This requires a CLS to provide a sum-frequency excitation signal to serve as the resonator’s feedback driving signal for practical applications. The proposed CLS mainly consists of a digital lock-in amplifier (DLA), a phase-locked loop (PLL) and a BSE control module (BSE-CM), and is implemented using a field-programmable gate array (FPGA). Experiments showed that the system can track the combined frequency of two flexural modes of the resonator in real-time. In addition, a potential application of the CLS for resonant MEMS sensors subject to BSE is preliminarily demonstrated, showing that the sensitivity of a vacuum-packaged resonant MEMS accelerometer can be enhanced by 166%. This approach could also be applied to reduce the effects of the feedback phase, or to simultaneously monitor multiple parameters. This study represents critical progress for practical sensing using the BSE scheme.