This paper presents a novel method for close-range, high-resolution ultrasonic Time-of-Flight (ToF) ranging using piezoelectric micromachined ultrasonic transducers (PMUTs) operating below the device resonance in air. The proposed method involves cross-correlation techniques to accurately detect the reflected echo signals despite the presence of ringdown signal interference. For the experiments, a high fill-factor array of silicon-on-nothing (SON) PMUTs was used to enhance the signal-to-noise ratio. A thorough investigation was conducted to determine the optimal driving frequency for below-resonance ToF ranging, to improve resolution and minimize detection errors. The results of the experiments showed that the system was able to accurately measure sub-µm vibrations of a metal plate placed 13 mm away from the PMUT array. The system exhibited the ability to detect target object vibrations with a peak-to-peak displacement under 6 µm and sub-µm floor noise. Moreover, the maximum detectable vibration frequency reached up to 1 kHz. This study highlights the potential of the proposed ToF ranging method in non-contact vibration monitoring applications across various fields, such as robotics and predictive maintenance.

This work demonstrates the imaging capabilities of a silicon-on-nothing (SON) ScAlN piezoelectric micromachined ultrasonic transducer (pMUT) array by a method of transmit beamforming followed by suitable signal postprocessing for 3-D image reconstruction. A 2.7-MHz pMUT array with 15% scandium-doped aluminum nitride (ScAlN) as the piezo thin film was implemented for this demonstration. A total of $16\,\,{3} {}\times {}{3}$ -mm dies consisting of ${34} {}\times {}{37}$ pMUT elements per die and operated as dual channels were arranged in an eight-column $\times $ two-row array configuration, resulting in a 32-channel experimental system with a $24\times6$ -mm coverage. In this work, experimental testing using single and multiple 6-mm-diameter reflectors at a different spacing from each other and a distance of 2.5 cm from the array was performed to validate the 3-D imaging capabilities of the developed 2-D SON ScAlN pMUT array with an attained lateral resolution of 5 mm.

This article presents an ultralow-power (ULP) microelectromechanical system (MEMS)-based phase-tracking frequency shift keying (FSK) transceiver (TRX) with an embedded envelop detector-based on–off keying (OOK) wake-up receiver (WuRX). The system supports a shared antenna interface of FSK-matched filter for TRX and a matching network for the FSK receiver (RX) and OOK WuRX. To consume less power, the transmitter employs an adaptive fast switching (AFS) technique for FSK/OOK modulator and uses a high-Q MEMS resonator to get frequency stability. In addition, for better interference filtering, the MEMS-based modulator is shared to provide LO in FSK RX as it owns inherent sidelobe suppression. To support higher blocker/interference rejection with relatively low power, MEMS-based FSK-matched filter plus matching network with current-reuse LNA is co-designed to provide additional passive gain. Fabricated in 65-nm CMOS process, the transmitter (TX) generates FSK signals of −11.2 dBm output power at ISM-915 MHz and the data rate is up to 6 Mb/s. At 1 MHz offset, the measured phase noise (PN) is −140.2 and −139.3 dBc/Hz at 926.3 and 932.4 MHz, respectively. By sharing the modulator, the RX exhibits −36.2 dB in-band signal-to-interference ratio (SIR), −77.5 dBm sensitivity at bit error rate (BER) of $10^{-3}$ working under 2 Mb/s data rate. The TX, FSK RX, and WuRX consume 642 $\mu \text{W}$ @6 Mb/s, 520 $\mu \text{W}$ @2 Mb/s, and 86 $\mu \text{W}$ @200 kb/s, leading to the energy efficiency of 107, 260, and 430 pJ/b, respectively. The proposed TX achieves over $1.5\times $ better energy efficiency than state-of-the-art results and the FSK RX attains the best FoM with the sensitivity of −77.5 dBm, which are well suited for energy-efficient pico-IoT applications.

We demonstrate a wireless sensing node (WSN) that is woken-up by a Microelectromechanical systems (MEMS) inertial switch with low-g threshold of 9.4g for Internet-of-Things (IoT) applications based on zero-power event-driven sensing. By virtue of the device design and fabrication of the switch contacts, the inertial switch in the WSN maintains a contact time long enough to wake up a Bluetooth Low Energy (BLE) module. Upon wake-up, the BLE module initiates data transmission to a remote display, when the WSN senses an external acceleration above the threshold. We also demonstrate remarkable contact reliability in the MEMS inertial switch as a significant step towards long lifetime zero-power sensors. The inertial switch output shows little degradation even after 100 cycles of testing under ambient conditions with an unpackaged MEMS device; a notable first among MEMS inertial switches.

Precision manipulation techniques in microfluidics often rely on ultrasonic actuators to generate displacement and pressure fields in a liquid. However, strategies to enhance and confine the acoustofluidic forces often work against miniaturization and reproducibility in fabrication. This study presents microfabricated piezoelectric thin film membranes made via silicon diffusion for guided flexural wave generation as promising acoustofluidic actuators with low frequency, voltage, and power requirements. The guided wave propagation can be dynamically controlled to tune and confine the induced acoustofluidic radiation force and streaming. This provides for highly localized dynamic particle manipulation functionalities such as multidirectional transport, patterning, and trapping. The device combines the advantages of microfabrication and advanced acoustofluidic capabilities into a miniature "drop-and-actuate" chip that is mechanically robust and features a high degree of reproducibility for large-scale production. The membrane acoustic waveguide actuators offer a promising pathway for acoustofluidic applications such as biosensing, organoid production, and in situ analyte transport.

We present a method to extract transverse elastic properties (Young's modulus, shear modulus and Poisson's ratio) and relative permittivity of 15% scandium (Sc) doped aluminum nitride (AIN) film from electrical measurements of resonators and parallel plate capacitors. The resonators comprise a vertical stack of $\boldsymbol{0.3\mu} \mathbf{m}$ thick $\mathbf{Sc}_{\boldsymbol{0.15}}\mathbf{Al}_{\boldsymbol{0.85}}\mathbf{N}$ being sandwiched between $\boldsymbol{0.2\mu} \mathbf{m}$ thick molybdenum (Mo) and $\boldsymbol{2\mu} \mathbf{m}$ thick degenerately doped pre-released silicon (Si) membrane on cavity as fabricated using our piezoelectric over silicon-on-nothing platform. Parallel plate capacitors followed the same vertical stack except that these were fabricated on unreleased but degenerately doped Si layers. Despite the large thickness ratio between pre-released degenerately doped Si membrane to $\mathbf{Sc}_{\boldsymbol{0.15}}\mathbf{Al}_{\boldsymbol{0.85}}\mathbf{N}$ film, we have successfully extracted 2 elements ( $\mathbf{S_{11}}$ and $\mathbf{S_{12}}$ ) from the compliance matrix using an iterative gradient descent method and relative permittivity of $\mathbf{Sc}_{\boldsymbol{0.15}}\mathbf{Al}_{\boldsymbol{0.85}}\mathbf{N}$ film.

In this paper, a MEMS torsional mode acceleration switch is proposed for the detection of moderately low levels of acceleration. The torsional mode structure provides for a large displacement to close a switch with a lower acceleration threshold. A nano-gap vertical TiN contact was introduced to implement a stable switch contact. We demonstrate activation of the acceleration switch by a 15g acceleration input with a contact time that is sufficient to trigger electric circuitry. The proposed acceleration switch can be used as a wake-up sensor for zero-power Internet-of-Things nodes.

The rapid development of the fifth-generation mobile networks (5G) and Internet of Things (IoT) is inseparable from a large number of miniature, low-cost, and low-power sensors and actuators. Piezoelectric microelectromechanical system (MEMS) devices, fabricated by micromachining technologies, provide a versatile platform for various high-performance sensors, actuators, energy harvesters, filters and oscillators (main building blocks in radio frequency front-ends for wireless communication). In this paper, we provide a comprehensive review of the working mechanism, structural design, and diversified applications of piezoelectric MEMS devices. Firstly, various piezoelectric MEMS sensors are introduced, including contact and non-contact types, aiming for the applications in physical, chemical and biological sensing. This is followed by a presentation of the advances in piezoelectric MEMS actuators for different application scenarios. Meanwhile, piezoelectric MEMS energy harvesters, with the ability to power other MEMS devices, are orderly enumerated. Furthermore, as a representative of piezoelectric resonators, Lamb wave resonators are exhibited with manifold performance improvements. Finally, the development trends of wearable and implantable piezoelectric MEMS devices are discussed.

INTRODUCTION The measured response of a passive resonator is fundamentally reciprocal: switching the drive and sense ports results in the same measured admittance. Previously, it has been shown that a non-reciprocal admittance (for producing isolator and circulator behavior) can be generated by applying spatiotemporal modulations on spatially extended systems composed of multiple resonators and waveguides [16]. In this work we show that, in fact, these spatial complexities are unnecessary, and that giant non-reciprocal admittances can be produced within a single microelectromechanical system (MEMS) device (Fig. 1a) by simply temporally modulating its degenerate degrees-offreedom (DoF), i.e. by considering the DoF axis as an extra dimension. We demonstrate this principle by repurposing a wafer-scale encapsulated MEMS gyroscope (Fig. 1b) which features two degenerate modes. By modulating these modes, we are able to induce a synthetic Hall effect [1, 2] resulting in a non-reciprocity ratio greater than 40 dB. This result is among the largest demonstrations of non-reciprocal MEMS including hybrid electronics-MEMS [7-10], acoustoelectric effect [11-13], parity-time symmetry [14], and piezoelectric nonlinear stiffening [15]. Moreover, we show that this giant non-reciprocal impedance is highly reconfigurable and can be easily implemented without the use of complicated electronic circuits or elaborate networks of coupled resonators.

This article presents a 915-MHz ultra-low-power (ULP) sub-sampling phase-tracking receiver (SSPT-RX). It is targeted for the power-constrained devices that need short range but medium high-speed data receiving, such as the multi-channel neural stimulator with arbitrary waveform generation. The zero-intermediate frequency (zero-IF) phase-tracking receiver (PT-RX) topology is adopted to simplify the sub-sampling RX architecture with direct demodulation of frequency-shift keying (FSK) signal while improving the image frequency issue. The frequency of local oscillator (LO) is reduced by ten times with the proposed architecture, which leads to greatly reduced power consumption. Fabricated in 65-nm CMOS process, the RX chip occupies an active area of 0.58 mm 2 . The RX consumes one of the lowest power consumptions of 164 $\mu \text{W}$ from 0.5-/1-V supplies. It achieves an energy efficiency of 32.8 pJ/bit at a data rate of 5 Mb/s, which is improved by $\sim 5.6\times $ compared to the state-of-the-art RXs. The measured sensitivity of 915-MHz FSK signal receiving is −69.5 dBm with an LO frequency of 91.5 MHz, which is 1/10 of the carrier frequency. The achieved RX sensitivity figure-of-merit (FoM) is 174.3 dB.

This letter presents a nanowatt-level detector-first wakeup receiver using microelectromechanical system (MEMS)-based matching network (MN). The aluminum nitride (AlN) piezoelectric MEMS resonator is utilized to implement the impedance MN for high passive gain to improve the receiver sensitivity. The high- $Q$ MEMS-based MN also greatly improves the receiver’s out-of-band interference rejection. Gate-biased envelope detector (ED) with small input capacitance is used for RF-to-baseband direct conversion. The wakeup receiver is implemented in a 180-nm CMOS process. It obtains −57-dBm sensitivity with 1-kbps ON-OFF keying (OOK) data rate at 789 MHz while consuming only 16-nW power consumption.

In this paper, a piezoelectric micromachined ultrasound transducer (PMUT) embedded microfluidic device is proposed for acoustic particle trapping in a microfluid channel. The proposed device traps microparticles in a pressure node along the vertical axis resulting from the reflection of acoustic waves generated by PMUT actuation to set up a standing wave. We have successfully demonstrated trapping of microbead spheroids/agglomerates according to their size. This study opens up a new avenue for particle manipulation based on PMUTs for integrated microfluidic systems.

This work presents a promising microfabrication technique employing the silicon-on-nothing (SON) process to form a $2\ \mu\mathrm{m}$ thick continuous monocrystalline silicon membrane over a vacuum cavity of $1\ \mu\mathrm{m}$ in depth. Utilizing the SON process, high fill-factor piezoelectric micromachined ultrasonic transducer (pMUT) arrays on an 8-inch silicon wafer with cavity widths ranging from $170\ \mu\mathrm{m}$ down to $38\ \mu\mathrm{m}$ have been demonstrated. Devices are designed with 15% scandium-doped aluminum nitride as the piezoelectric layer of the pMUT for both air-coupled and water-coupled applications. The air-coupled pMUTs show a peak displacement frequency from 0.8 to 1.6 MHz with a $Q$ -factor between 120 to 194. The water-coupled pMUT arrays show a transmit pressure measured by a needle hydrophone, in DI water at a distance of 20 mm, ranging between 0.4 to 6.9 kPa/V with peak frequency between 5 to 13.4 MHz and fractional bandwidth 56 to 36%, respectively. The piezoelectric-over-SON process proposed here has the potential to gain traction in low-cost and high-yield pMUT manufacturing.

Ultrasonic ranging with piezoelectric micromachined ultrasonic transducers (pMUTs) can be used in applications such as robotic systems and industrial machinery for precise distance measurements. This work presents a technique of operating pMUTs below the resonance frequency to obtain a wide bandwidth for high resolution time-of-flight (ToF) ranging. Digital signal processing was performed to remove the residual ringing at resonance and recover the wideband echo signal by bandpass filtering and cross-correlation in the frequency domain. The ToF was then determined after transforming back to the time domain. An array of pMUTs with resonance frequencies around 1.1 MHz was pulsed at 500 kHz, and a 4.3 µm standard deviation in ranging resolution was achieved with a pulse repetition frequency (PRF) of over 2 kHz. This enables ultra-precise non-contact distance or vibration measurements within a low-profile form factor.

This paper describes microelectromechanical systems (MEMS) contour mode resonators (CMRs) with 900MHz resonant frequencies suitable for radio frequency (RF) applications, using a ~2µm thick degenerately doped n-type silicon (Si) layer. The Silicon-on-Nothing (SON) process has been used to fabricate monocrystalline thin pre-released Si membranes over cavities with well-controlled thickness across the wafer. A thin layer of 15% scandium (Sc)-doped aluminum nitride (AIN) has been incorporated in the vertical stack of the CMRs for piezoelectric transduction. The CMRs were aligned to the <100> crystal orientation of a (100) Si substrate to obtain a reduced temperature coefficient of frequency (TCF) of - 7.4ppm/°C.

This paper presents a novel spring disk resonator gyroscope (SRG). The SRG consists of coupled concentric serpentine spring-like rings to reduce mode stiffness while maintaining a device footprint the same as that of existing standard DRGs: 600um in diameter. The $Q$ is increased by a factor of 2.5, and the ring down-time of 934ms is 4 times longer than that of a standard DRG.

Passive SAW sensors do not require circuitry or power and are ideal for harsh environments or rotating machinery. AlN-based thin film SAW resonators for temperature and pressure sensors have been developed in this work using AlGe wafer-scale encapsulation for device protection, pressure reference, high stability, and small size. Temperature and pressure co-testing of both sensors up to 160°C and 60 bar revealed ~1% full-scale accuracy.

We demonstrate NDIR CO2 gas sensing using CMOS compatible MEMS ScAlN-based pyroelectric detectors. The ScAlN-based pyroelectric detectors are fabricated using 8-inch wafer level technology with 12 % Sc-doped AlN deposited at a temperature of -200 degrees C. Together with a blackbody thermal emitter, a 10 cm long enclosed gas channel with only inlet and outlet holes connected to tubings, and testing using 2 different reference gases (N2 and synthetic air), measurements show voltage signal drop due to CO2 gas absorption at the 4.26 mu m wavelength at CO2 gas concentrations ranging from 5000 ppm down to 25 ppm. The signal change due to the CO2 gas response ranges from -2% at 100 ppm CO2 concentration to -40 % at 5000 ppm CO2 gas concentration for both CO2 gas measured in N2 and in synthetic air. CO2 gas response times are also measured for CO2 gas in N2 and in synthetic air at concentrations of 5000 ppm, 1000 ppm and 400 ppm. The gas response times measured around 2 s and lower. Introduction of humidity show some minor effect (<3%) to the CO2 gas response and seems most perturbed at 10 % relative humidity. To the best of our knowledge, this is the first demonstration using ScAlNbased pyroelectric detectors in NDIR CO2 gas sensing, towards practical sensor applications. The results obtained show promise in using CMOS-compatible MEMS ScAlN-based pyroelectric detectors for NDIR gas sensing, opening up possibilities for low cost, wafer-level, monolithic NDIR gas sensors with small footprint integrated with CMOS circuits.

This paper presents a theoretical derivation for the design parameters of wavefront computing (WFC) system in solids including the aperture/transducer sizes, focal length, and the ideal lens. To achieve Fourier transform at the expected focal plane based on the wavefront computing system with ideal lens, the design parameters are derived for various transducers arrays from 4 by 4 to 32 by 32. Based on the theory in geometrical optics, the equations of ideal thick plano-concave lens are derived in solids for a given focal length and material properties. To validate the derived design parameters and the ideal lens, time domain simulation is used to show the propagation of the elastic waves in fused silica block. It can be observed that the waves excited by four piezoelectric transducers will get focused at the expected focal plane after the ideal lens.

An 800-MHz OOK transmitter for short-range Internet-of-Things (IoT) applications is presented in this letter. The OOK transmitter utilizes a microelectromechanical system (MEMS)-based RF oscillator for carrier frequency generation, leading to a hardware- and power-efficient architecture. The transmitter is implemented in a 180-nm CMOS process and delivers −12.4-dBm output power. The power consumption of the MEMS oscillator is only 0.24-mW with 126.3-dBc/Hz phase noise at 100-kHz offset and a figure-of-merit (FoM) of 210.5 dB due to the optimized high- $Q$ aluminum nitride (AlN) piezoelectric MEMS resonator. The transmitter can provide a 120-Mb/s data rate of OOK modulation with an energy efficiency of 5 pJ/bit by directly switching ON/OFF the power amplifier (PA). The 175-ns startup time of the MEMS-based oscillator can support very heavy duty-cycle transmitter operation, leading to ultralow average power consumption of 50 nW.