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The heartbeat is one of the main human physiological activities and is closely related to psychological and physiological states. Since limited solutions exist for convenient heart rate measurements at the fingertip, such as photoplethysmography, this article provides an alternative solution enabled by a piezoelectric micromachined ultrasonic transducer (PMUT). With fingertip vascular expansion and retraction, the change in ultrasound time-of-flight (TOF) between the PMUT and phalanx is used to extract heart rate data. A PMUT with a resonant frequency of 4.3 MHz is designed and fabricated as a 25 by 25 array of elements, and Parylene and polydimethylsiloxane (PDMS) are used for packaging and better coupling with fingertips, respectively. The emitting ultrasound has a spatial-peak temporal average power density of 36 nW/mm2, far below the Food and Drug Administration (FDA) safety limit. The proposed solution paves the way for ultrasound-based heart rate monitoring at the fingertip, and it features convenient to use, easy integration, and biosafety. [ABSTRACT FROM AUTHOR]

This letter presents a piezoelectric tuning fork resonator vibrating in in-plane flexural mode with a high quality factor (Q) and low motional impedance. A novel double-sided actuating design is implemented based on unimorph structure with a 2.5- $\mu \text{m}$ -thick AlN film as passive layer. The design significantly improves the Q and motional impedance compared with the conventional single-sided actuating design thanks to an increase of static capacitance without change of material interfaces. The proposed double-sided actuating device shows an excellent Q measured as 11272 with an extracted motional impedance of 7.2 $\text{k}\Omega $ in vacuum. The measured Q-pressure relationship shows that a vacuum level <100 Pa would ensure a sufficiently high Q value, which can be easily achieved through wafer-to-wafer bonding with getters in the future. Furthermore, the impact of AlN passive layer thickness on the resonator performance is analyzed. The proposed device would find applications in sensing fields as miniaturized and high-performance resonator-based solutions, such as gyroscopes, accelerometers and pressure sensors. [ABSTRACT FROM AUTHOR]

Sensitivity is a key parameter for microspeakers, especially in low-power applications such as consumer electronics, medical devices and Internet of Things. However, the sensitivities of most reported piezoelectric MEMS speakers are inferior to those of commercially available electrodynamic speakers. Design of high-sensitivity piezoelectric MEMS speakers requires an analytical expression of sensitivity, which is not yet available. In this paper, the analytical expression is derived, and an ultrahigh-sensitivity piezoelectric MEMS speaker is designed, fabricated and characterized. The sensitivities of the speaker in an ear simulator are 107 dB/mW, 104 dB/mW and 105 dB/mW at 250 Hz, 1 kHz and 10 kHz, respectively, which are the highest among piezoelectric MEMS speakers. The analytical calculation results and measurement data are perfectly matched. Furthermore, the speaker is based on thin-film AlN instead of commonly used thin-film PZT. This work demonstrates that the ultrahigh-sensitivity piezoelectric AlN MEMS speaker consumes 10 times less power than a typical 6 mm electrodynamic speaker; and the developed analytical expressions provide valuable design insights into piezoelectric MEMS speakers for high sensitivity and low power. [2022-0063] [ABSTRACT FROM AUTHOR]

Cardiac auscultation is an orthodox method to prevent and diagnose cardiovascular diseases. Traditional stethoscopes cannot offer a highly reliable diagnosis, and electronic stethoscopes suffer from low signal-to-noise ratio (SNR) and high cost. This paper reports an electronic stethoscope based on piezoelectric bimorph MEMS transducers for cardiac sound monitoring. The stethoscope prototype, measuring 37 $\times 28\times12$ mm3, contains a preamplifier, holders and polydimethylsiloxane (PDMS) packaging. The transducer is designed using a lumped element model, fabricated by MEMS process, packaged, and assembled for final use. A triangular cantilever is chosen as a basic element for the transducer, with analytical analysis showing high sensitivity per unit area as a figure of merit (FoM). Performance parameters such as sensitivity and noise are experimentally characterized and consistent with simulation results. Owing to the outstanding receiving performance of the aluminum nitride (AlN) material and bimorph triangular cantilever structure of the transducer, compared with the commercial electronic stethoscopes, the proposed stethoscope demonstrates real-time phonocardiogram (PCG) with higher SNR. With its small size, high performance and low cost, the stethoscope is developed for portable and wearable cardiac sound monitoring. [2022-0007] [ABSTRACT FROM AUTHOR]

The development of Internet of Things (IoT) requires demanding accurate and low-power indoor localization. In this article, a high-precision 3-D ultrasonic indoor localization system with ultralow power is proposed. A new piezoelectric micromachined ultrasonic transducer (PMUT) chip with a slotted membrane is designed and fabricated as a receiver, breaking the dilemma of low resonant frequency and wide field of view required for indoor localization. The system works based on the time difference of arrival (TDoA), and an improved quantum genetic algorithm (QGA) is used to estimate the location. The results show that the system achieves centimeter-level positioning precision, which is among the best solutions nowadays. Due to the high performance and small size endowed by the PMUT, the receiver footprint reaches as small as 0.25 cm2 and power consumption could reach as low as 0.1 mW, which are far better than that of current indoor localization systems. [ABSTRACT FROM AUTHOR]

This article proposes a novel structured aluminum nitrate (AlN) thin film based piezoelectric micromachined ultrasonic transducer (PMUT) with superior ultrasonic performances that can directly be applied for pulse-echo imaging. The bottom electrode of the PMUT is thickened and serves both as the electrical layer and supporting layer. Large numbers of elements are parallelly connected to form square arrays. This method can not only improve the acoustic output but also reduce the total impedance. The ultrasonic transmitting, receiving behaviors of our developed 6.37 MHz (in air) PMUT array containing vibration elements of horizontal x vertical $= 20 \times 20$ are investigated. It can generate superior acoustic output with a normalized value of 1.71 kPa $\cdot \text{V}^{\mathbf {-1}}\,\,\cdot $ mm−2 at 3.6 mm away in deionized water media. Thanks to the lager acoustic output and reduced electrical impedance, ultrasonic pulse-echo imaging with our developed PMUT array is realized only with the assistance of a switch to separate the ultrasound transmission and reception process. We have mechanically scanned a mark in steel plate and generate a B-mode image with great accuracy using the recorded pulse-echo signals. These demonstrate the superior acoustic behavior of our developed PMUT array and open an avenue to the development of portable ultrasonic imaging devices. [ABSTRACT FROM AUTHOR]

This letter reports a multi-spectral infrared chemical sensor based on a virtual array of bulk acoustic wave resonators with different frequencies. The plasmonic metasurfaces with different patterns on the top surface of resonators provide high IR absorption without degradation of resonating performance. The metasurfaces enhance the frequency responses of the resonant sensor to the specific infrared by about ten times. In a demonstration, four kinds of polymers and their mixtures were detected by the sensor to show its capability for distinguishing unknown compounds. The proposed chemical sensor shows great potential for fast, accurate and portable infrared spectrum microsystems for chemical composition analysis. [ABSTRACT FROM AUTHOR]

We report on Lamb wave resonators (LWRs) on a flexible substrate made of polyethylene terephthalate (PET) by transferring method. LWRs with three different topologies were fabricated on the same silicon wafer and then were transferred simultaneously to a flexible substrate with air cavities using a polydimethylsiloxane stamp. The resonators were aligned accurately with the air cavities on the PET, and presented equivalent performance to those on the silicon wafer. The flexible resonator implementing thickness field excitation exhibited a quality factor of 1294 and a coupling coefficient of 2.2%. The flexible two-port resonator presented a low insertion loss of 3.8 dB. The resonator demonstrated excellent mechanical stability under a series of deformations. These results pave the way for flexible electronic filters and oscillators in future wearable electronic systems. [ABSTRACT FROM AUTHOR]