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The round-trip sensitivity is a key metric for ultrasonic transducers. It is well known that the bimorph transducer design can effectively improve the sensitivity of a piezoelectric micromachined ultrasonic transducer (PMUT). However, inevitable process variations will lead to degraded sensitivity and frequency control issues in fabricated bimorph PMUTs. This letter presents a coupled cantilever and dual-cavity PMUT (CCDP) based on a bimorph structure. The design is insensitive to fabrication process variations, because the coupled cantilevers can deform to release residual stress while suppressing the sound pressure cancellation from misalignment. Moreover, dual cavities are adopted to precisely define the cantilever boundary and reduce the dimensional inconsistency at anchor among cantilevers. The typical measured transmitting and receiving sensitivities are 1.38 Pa/V/mm2 at a distance of 10 cm and 1.5 mV/Pa, respectively. The fabricated CCDP demonstrated the highest round-trip sensitivity per unit area among previously reported airborne PMUTs, and the sensitivity was more than 10 times greater than that of conventional ceramic ultrasonic transducers. Furthermore, the CCDP shows a resonant frequency uniformity of 4.9% across an 8-inch wafer. It is believed that even greater round-trip sensitivity could be achieved by combining the CCDP design with higher piezoelectric coefficient materials, such as PZT and AlScN.

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.

In this article, a 2.4-GHz micro-electromechanical systems (MEMS)-based oscillator is designed and demonstrated based on $g_{\text {m}}$ -boosting Colpitts structure. A dynamic self-body-biasing scheme is proposed to shorten startup time without introducing any extra units or additional parasitics to the resonant tank. Darlington cell is also employed to enhance negative transconductance and improve the oscillation stability, especially under a low supply voltage operating condition. The oscillator is integrated with a 2.4-GHz MEMS resonator with high $Q$ characteristic. The tail current source is designed at the boundary of the current- and voltage-limited regime for a superior phase noise performance. The measurement result shows that the phase noise at 10- and 100-kHz offsets is −112.92 and −138.23 dBc/Hz, respectively. A figure-of-merit of 226 dBc/Hz is achieved. The measured startup time is $\sim 5 \mu \text{s}$ . The core oscillator consumes a dc power of 0.98 mW from a 0.7-V supply voltage.

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]

It is a longstanding issue that high-order width-extensional (WE) piezoelectric microelectromechanical system (MEMS) resonators suffer low quality factors (Q-factors). In this study, it is observed that the mode distortion occurs in each unit when it couples with each other to constitute the high-order WE-mode resonator, leading to a decreased Q-factor. Based on this finding, we propose a new Q-factor boost strategy by improving mode matching degree between adjacent units from the view of dividing the high-order resonator into units. Both simulation and experimental results show a significant improvement in mode matching degree between adjacent units with varied unit length. Resonator’s Q-factor measured in air improved by 53% compared with the conventional resonator with constant unit length. It is believed that the proposed strategy could apply to other WE-mode MEMS resonators with different orders or dimensions; and Q-factor can be further improved by combining the proposed method with traditional energy reflection methods. [2023-0201]

It is a long-lasting issue that process variations, such as residual stress and poor dimension definition, deteriorate resonant frequency control of a piezoelectric micromachined ultrasonic transducer (PMUT). This letter presents a tapered cantilever cluster PMUT with tight frequency control. The PMUT could release residual stress via cantilever deformation and its front cavity could precisely define cantilever boundary. Theoretical analysis and test results show that the impact of residual stress on resonant frequency has been eliminated. Frequency precision and accuracy are evaluated across die, wafer and lot. Across wafer frequency uniformity and relative deviation of the proposed design are 0.8% and 1%, respectively, which are approximately an order of magnitude better than those of conventional design. The proposed PMUT demonstrates superior frequency control among reported PMUTs till date, meeting the 1% frequency control requirement for the first time. The measured transmitting and receiving sensitivities are 1.15 Pa/V/mm2 at a distance of 5 mm and 0.15 mV/Pa, respectively. The design is simple and cost effective to implement without stringent fabrication process requirements such as trimming or etch optimization. It is competitive in air-coupled applications where tight frequency control for frequency matching is a must, such as ultrasonic gas flowmeter and beam-forming array. [2023-0139]

Cointegration and coupling a perfect metamaterial absorber (PMA) together with a film bulk acoustic wave resonator (FBAR) in a monolithic fashion is introduced for the purpose of producing ultracompact uncooled infrared sensors of high sensitivity. An optimized ultrathin multilayer stack was implemented to realize the proposed device. It is experimentally demonstrated that the resonance frequency of the FBAR can be used efficiently as a sensor output as it downshifts linearly with the intensity of the incident infrared irradiation. The resulting sensor also achieves a high absorption of 88% for an infrared spectrum centered at a wavelength of 8.2 μm. The structure is compact and can be easily integrated on a CMOS-compatible chip since both the FBAR and PMA utilize and share the same stack of metal and dielectric layers.

Performance of electroanalytical lab-on-a-chip devices is often limited by the mass transfer of electroactive species toward the electrode surface, due to the difficulty in applying external convection. This article describes the powerful signal enhancement attained with a 2.54 GHz miniature acoustic resonator integrated with an electrochemical device in a miniaturized cell. Acoustic resonator and an on-chip gold thin-film three-electrode electrochemical cell were arranged facing each other inside a structured poly(methyl methacrylate) chamber. Cyclic voltammetric and chronoamperometric responses of 1 mM ferrocene-methanol were recorded under resonator’s actuation at powers ranging from 0 to 1 W. Finite element analysis was carried out to study the sono-electroanalytical process. Acoustic resonator’s actuation greatly enhances the mass transport of electroactive species toward the electrode surface. The diffusion limited cyclic voltammetric and chronoamperometric currents increase around 10 and 20 times, respectively, with an input power of 1 W compared to those recorded under stagnant conditions. The improvement in electroanalytical process is mainly associated with acoustic resonator’s vibration induced fluid streaming. The advantages of a miniaturized acoustic resonator, including the submillimeter small size, amenability for mass fabrication, cost effectiveness, low energy consumption, as well as outstanding enhancement of coupled electrochemical processes, will enable the production of highly sensitive compact electroanalytical devices.

In designing bioassay systems for low-abundance biomolecule detection, most research focuses on improving transduction mechanisms while ignoring the intrinsically fundamental limitations in solution: mass transfer and binding affinity. We demonstrate enhanced biomolecular surface binding using an acoustic nano-electromechanical system (NEMS) resonator, as an on-chip biomolecular concentrator which breaks both mass transfer and binding affinity limitations. As a result, a concentration factor of 105has been obtained for various biomolecules. The resultantly enhanced surface binding between probes on the absorption surface and analytes in solution enables us to lower the limit of detection for representative proteins. We also integrated the biomolecular concentrator into an optoelectronic bioassay platform to demonstrate delivery of proteins from buffer/serum to the absorption surface. Since the manufacture of the resonator is CMOS-compatible, we expect it to be readily applied to further analysis of biomolecular interactions in molecular diagnostics.

In this work, we presented a thin-film piezoelectric acoustic gas sensor with enhanced sensitivity by a surface modification strategy of oxygen plasma treated graphene oxide (GO) functionalization. By exposing to ammonia vapor (NH3) of various concentrations at controlled temperature and humidity, the characteristics of the GO-coated acoustic sensor were investigated, that is, sensitivity, linearity, response, and recovery time. Oxygen plasma treatment of the GO-coated sensor further enhanced the sensitivity compared with the freshly prepared GO-coated sensor. The mechanism of oxygen plasma treatment effect on the GO-coated sensor was discussed based on characterizations of X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscope (SEM), and precise weighing of the acoustic sensor. It was found that the oxygen plasma treatment introduces numerous defects to GO flakes, which are uniformly distributed across the GO surface, providing more gas molecule binding sites.