This letter introduces a micro-electromechanical system (MEMS) resonant pressure gauge based on aluminum nitride piezoelectric resonators that can operate in an atmospheric environment. The pressure is measured in terms of the change in the resonant frequency of a double-ended tuning fork (DETF) resonator, where the aluminum nitride thin film serves as both a piezoelectric layer and a passive layer. The resonator exhibits a resonant frequency of 719 kHz and a Q value of 1764 in air. To our knowledge, this Q value surpasses those of previously reported in-air MEMS DETF resonators that operate in the 100 kHz frequency range. The electronic oscillator circuit for the resonant sensor exhibits a phase noise level of -120 dBc/Hz. The measured sensitivity is 16.8 Hz/kPa within the pressure range of 0-90 kPa, which is consistent with the simulation results obtained from finite element modeling. The resolution of the sensor is 14.9 Pa based on Allan variance measurements. This work thus demonstrates a new class of MEMS sensors for use in harsh environments.

This paper reports a new MEMS temperature sensor based on piezoelectric membrane resonator for temperature measurement and compensation applications. For the first time, it theoretically analyzed and experimentally validated that the main factor of temperature coefficient of frequency (TCF) is the difference in coefficient of thermal expansion (CTE) between the resonating structure and silicon substrate. The engineered temperature sensor achieved a TCF, i.e. sensor sensitivity, as high as -660 ppm/degrees C Because of the high sensitivity and low noise oscillating circuit, the sensor reached an extremely high temperature resolution of 10 mu K which is the best among state-of-the-art MEMS resonant temperature sensors; and the sensor also reached a superior resolution FoM of 0.06 pJK(2). In addition, the MEMS resonator operates at atmosphere pressure without vacuum packaging, and the cost-effective solution should have better reliability and long-time drift.

Flexural mode MEMS resonators are ideal candidates for resonant microsensors. However, their high motional resistance in air restricts the performance of corresponding oscillators and consequently the sensor performance. In this work, we report a piezoelectric MEMS oscillator based on a flexural mode membrane resonator array for in-air resonant sensors. Array design and piezoelectric transduction of the membrane resonators facilitate a low motional resistance and a high power handling capability. At the resonator level, the electrode pattern is optimized to further reduce the motional resistance, and the nonlinearity of the resonator is analyzed to fully exploit its high power handling for oscillator design. At the oscillator level, transimpedance and Pierce circuits are designed, analyzed and characterized. Theoretical calculations well fit measured results, both for the white and 1/f2 phase noise of the transimpedance oscillator and for the Allan deviation below an integration time of 0.1 s of the Pierce oscillator. The Pierce oscillator achieves a phase noise of −119 dBc/Hz at a 1 kHz offset and a −151 dBc/Hz noise floor. The frequency resolution of the Pierce oscillator reaches 0.024 Hz. To the best of our knowledge, the measured phase noise and frequency resolution are the best among reported low-frequency piezoelectric MEMS oscillators for in-air resonant sensors. The proposed solution could be applied for a variety of sensing scenarios, such as mass, pressure, acceleration and strain sensing. A theoretical resolution as low as 15 $\text{p}\varepsilon $ is expected if it is utilized as a strain sensor. [2023-0128]

Stretchable electronic skin (e-skin) paves the way for applications that exceed the scope of intrinsic rigid devices and hard-to-stretch sensors. The broad application range of flexible e-skins benefits from device architectures that can simultaneously provide mechanical flexibility and superior sensitivity. Classic fractal design provides a simple architecture to achieve the desired flexibility through structural design for improved wear comfort, but at the expense of sensor sensitivity. In this study, the proposed method addresses the trade-off between stretchability and sensitivity in fractal design. A high-sensitivity e-skin is obtained by eliminating the effect of negative charge on the output by applying the concept of electromechanical coupling. This concept for designing e-skins with high sensitivity is validated through the delicate patterning of hard-to-stretch functional materials. Further, human speech signals are acquired through the integration of e-skin with signal processing circuits, and speech pattern recognition is realized using machine learning. The stretchable e-skin with an enhanced gauge factor illustrates the wider application of this concept for improving the sensitivity of stretchable electronic functional materials.

Resonant sensors based on flexural mode MEMS resonators are promising for mass, strain, acceleration and pressure sensing applications. However, low quality factor and high motional resistance of these resonators in air limit oscillator and hence sensor performance. In this work, we report a piezoelectric membrane resonator array with two-port configuration, leveraging its high power handling capability and low motional resistance. An oscillator was designed based on it and achieved a phase noise of -115 dBc/Hz and -145 dBc/Hz at offset frequencies of 1 kHz and 100 kHz, respectively. The lowest Allan deviation of the oscillator was measured to be 0.21 Hz. The phase noise and Allan deviation are the best among low frequency piezoelectric MEMS oscillators for resonant sensors working in air.