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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. [ABSTRACT FROM AUTHOR]

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 mm2. 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. [ABSTRACT FROM AUTHOR]

In this paper, we present a single-chip $65 \times 42$ element ultrasonic pulse-echo fingerprint sensor with transmit (TX) beamforming based on piezoelectric micromachined ultrasonic transducers directly bonded to a CMOS readout application-specific integrated circuit (ASIC). The readout ASIC was realized in a standard 180-nm CMOS process with a 24-V high-voltage transistor option. Pulse-echo measurements are performed column-by-column in sequence using either one column or five columns to TX the ultrasonic pulse at 20 MHz. TX beamforming is used to focus the ultrasonic beam at the imaging plane where the finger is located, increasing the ultrasonic pressure and narrowing the 3-dB beamwidth to 50~\mu \text{m} , a factor of 6.4 narrower than nonbeamformed measurements. The surface of the sensor is coated with a poly-dimethylsiloxane (PDMS) layer to provide good acoustic impedance matching to skin. Scanning laser Doppler vibrometry of the PDMS surface was used to map the ultrasonic pressure field at the imaging surface, demonstrating the expected increase in pressure, and reduction in beamwidth. Imaging experiments were conducted using both PDMS phantoms and real fingerprints. The average image contrast is increased by a factor of 1.5 when beamforming is used. [ABSTRACT FROM AUTHOR]

A fully integrated 3-D ultrasonic fingerprint sensor-on-a-chip is presented. The device consists of a 110\times 56 piezoelectric micromachined ultrasonic transducer (PMUT) array bonded at the wafer level to custom readout electronics fabricated in a 180-nm CMOS process with a HV (24 V) transistor option. With the 24 V driving signal strength, the sensor consumes 280 \mu \text{J} to image a 4.73 mm \times3.24$ mm section of a fingerprint at a rate of 380 fps. A wakeup mode that detects the presence of a finger at 4 fps and dissipates 10 \mu \text{W} allows the proposed sensor to double as a power switch. The sensor is capable of imaging both the surface epidermal and subsurface dermal fingerprints and is insensitive to contaminations, including perspiration or oil. The 3-D imaging capability combined with the sensor’s sensitivity to the acoustic properties of the tissue translates into excellent robustness against spoofing attacks. [ABSTRACT FROM PUBLISHER]

We present an experimental study of conservative nonlinearities in bulk acoustic mode (bulk-mode) silicon MEMS resonators with degenerate doping. Three types of bulk acoustic mode resonators oriented in $\langle 110\rangle $ and \langle 100\rangle $ silicon crystalline directions are used to analyze both linear and nonlinear elastic behavior of silicon with p- ( N \cong 4.19e+18$ to 1.63e+20 ) and n-type ( N \cong 1.58e+18$ to 5.91e+19$ cm ) doping. For accurate characterization of the amplitude-dependent nonlinear stiffness constant, we employ two methods: amplitude-frequency response and ringdown measurement. Experimental results show that the nonlinear behavior of these resonators is dominated by material-dependent mechanical nonlinearities and strongly depends on the doping type and crystal orientation. These results are useful for understanding the material-induced nonlinear properties of doped silicon and design of MEMS resonators with desired dynamic behavior, and may provide a new avenue for tailoring resonator response characteristics. [2015-0301] [ABSTRACT FROM AUTHOR]

This paper presents a thin-film wafer-level encapsulation process based on an epitaxial deposition seal that incorporates both narrow and wide lateral transduction gaps (0.7–50 \mu \textm ), both in-plane and out-of-plane electrodes, and does not require release etch-holes in the device layer. Resonant structures fabricated in this process demonstrate high-quality factors ( $f\times Q$ products of up to $2.27e+13$ Hz) and exceptional stability (±18 ppb over one month) with no obvious aging trends. Studies on cavity pressure indicate that vacuum levels better than 0.1 Pa can be achieved after final encapsulation, thus reducing gas damping for high surface-to-volume devices. The vast diversity of functioning devices built in this process demonstrates the potential for combinations of high-performance MEMS devices in a single process and/or single chip. [2015-0278] [ABSTRACT FROM PUBLISHER]

A systematic experimental and theoretical evaluation of stiction between intermittently contacting silicon surfaces in an ultra-clean encapsulation process is presented, evaluating magnitude of stiction forces, the reversible nature of sidewall contact, and repeatability of results. The uniquely stable environment and the lack of native oxide are leveraged to enable reliable collision and contact models, which confirm the nature of the asperity contact. In addition, we demonstrate a series of dynamic mechanical anti-stiction solutions and the mechanisms by which they mitigate stiction. These devices are shown to reduce susceptibility to stiction-related failure by 50%. [2015-0291] [ABSTRACT FROM PUBLISHER]

Bias voltages are typically required for the operation of many capacitive microelectromechanical systems (MEMS) devices. A stored charge can be used in place of a bias voltage supply, granting benefits in terms of power consumption and circuit area. However, charges stored on a device typically tend to leak out over time to the environment, and in this paper, the charge storage capability of resonators fabricated in the epi-seal encapsulation platform is evaluated. Charges were introduced on the resonators using a mechanical switch located within the epi-seal cavity, and no leakage was observed over a year at room temperature. At higher temperatures, the leakage follows an Arrhenius relationship with an activation energy of 1.87 eV, with the leakage pathway believed to be through the buried silicon dioxide support. A 3-dB drop in the resonant peak requires a leakage time of approximately two days at 350 °C, indicating extreme charge trapping capabilities at lower temperatures are possible within the epi-seal platform. [2015-0091] [ABSTRACT FROM PUBLISHER]

In this paper, we present the fabrication and test results of an oxide-coated polysilicon disk resonator gyroscope in a wafer-scale encapsulation process. We demonstrate the effects of an oxide coating on the device structure and the key performance parameters of resonant-based sensors, such as the temperature coefficient of frequency and quality factor ( $Q$ ). Results from the as-fabricated device show that a thin oxide coating reduces the surface roughness of a fully encapsulated device by $10\times $ , compared with a polysilicon device without the oxide coating. This increases the uniformity across the wafer, providing higher process yield. The open-loop rate measurement mode reveals an angle random walk of 0.18°/ $\surd $ hr and a bias instability of 1.43°/hr. [2015-0124] [ABSTRACT FROM AUTHOR]

We report on characterization of Si-to-Si contact microswitches fabricated in an ultraclean encapsulation process. This three-terminal microswitch relies on a curved beam (source) that actuates toward the contact terminal (drain) by charging the control terminal (gate). The operation range of this switch from −60 °C to 300 °C is investigated, which approximately yields a resistance drift of $- 200~\Omega $ /K. Our experiments include tests of the high-temperature lifetime during continuous ON–OFF cycles. By reducing Joule heating at the contact, preliminary results demonstrate at least 10^{\mathrm {\mathbf {6}}} cycles longer lifetime at 400 °C. Subsequently, the failure mode is investigated and reported. The study of ultraclean Si-to-Si contact-based microswitches provides a crucial guideline to the field of mechanical and electrical failure mechanisms for harsh environment applications. [2014–0373] [ABSTRACT FROM AUTHOR]

We present multifunctional integrated sensors (MFISES) that combine temperature, humidity, pressure, air speed, chemical gas, magnetic, and acceleration sensing on a single 2-mm $\times 2$ -mm die. We fabricate the MFISES in a wafer scale encapsulation process to hermetically seal the sensor functions with moving parts at low vacuum, and then surface micromachine the environmental sensors on top of the sealed layer. The encapsulation process provides very stable conditions for the pressure, magnetic, and acceleration sensors, and enables the deployment of the MFISES in dynamic environmental conditions without special postprocess packaging. We quantify the performance requirements for sensor applications in weather stations, indoor climate control, road activity monitoring, chemical sensing, and parking monitoring. We compare the performance of the MFISES to the application requirements to demon- strate the utility of an integrated sensor in a wide range of applications. [2014-0117] [ABSTRACT FROM PUBLISHER]

Resonators fabricated in heavily doped silicon have been noted to have a reduced frequency-temperature dependence compared with lightly doped silicon. The resonant frequency of silicon microelectromechanical systems (MEMS) resonators is largely governed by the material’s elastic properties, which are known to depend on doping. In this paper, a suite of different types and orientations of resonators were used to extract the first- and second-order temperature dependences of the elastic constants of p-doped silicon up to 1.7e20 \mathrmcm^\mathrm \mathbf -3 , and n-doped up to 6.6e19 \mathrmcm^\mathrm \mathbf -3 . It is shown that these temperature-dependent elastic constants may be used in finite element analysis to predict the frequency-temperature dependence of similarly doped silicon resonators. [2013-0331] [ABSTRACT FROM AUTHOR]

In this paper, we present four design methods to overcome (100) silicon crystalline anisotropy and achieve mode-matching in wineglass-mode disk resonator gyroscope (DRG). These methods were validated through experimental characterization of more than 145 different devices that arose from simulations. With the proposed methods, the frequency split of the 250-kHz DRG wineglass modes in (100) silicon was reduced from >10 kHz to as low as 96 Hz (<0.04% of 250-kHz resonant frequency) without any electrostatic tuning. Perfect mode-matching is then achieved using electrostatic tuning. Mode-matching was maintained within ±10 Hz over a temperature range from −20 °C to 80 °C. The temperature dependence of quality factor is also discussed in this paper. These results allow for the development of high-performance miniature DRGs tuned for degenerate wineglass mode operation from high-quality crystalline silicon material. [2013-0303] [ABSTRACT FROM AUTHOR]

The fatigue lifetime of single crystal silicon (SCS) was characterized in an environment free of oxygen, humidity, and organics. Long-term ( > 10^10 Hz) fatigue experiments performed with smooth-walled SCS devices showed no signs of fatigue damage up to 7.5 GPa. In contrast, experiments using SCS devices with a silicon dioxide (SiO2) coating and rough sidewalls due to scalloping from deep reactive ion etching exhibited fatigue drift at 2.7 GPa and suffered from short-term (< 10^10 Hz) fatigue failure at stress levels >3 GPa. In these SCS-SiO2 experiments, the initiation of fracture occurs in the SiO2 layer. It is concluded that fatigue in this case is likely attributed to a subcritical cracking mechanism; not reaction-layer nor dislocation related. A cross-comparison with other works from literature is developed to show that packaging a pristine device in an inert environment is necessary in order to operate devices at high-stress levels. [2013-0267] [ABSTRACT FROM AUTHOR]

A vacuum-encapsulated silicon switch with a curved electrode is characterized for operation in harsh environments. An ultraclean vacuum encapsulation process (episeal) seals the switch after release, providing a pristine operating environment for switching operations. In these devices, the curved beam of the actuator enhances the overdrive voltage tolerance to be more than 100 V. The ON/OFF cycle tests were carried out up to 105 cycles at room temperature, and at least 104 cycles under an elevated temperature of 300°C. Throughout the 300°C tests, an average contact resistance of ~ 28 kΩ is measured, demonstrating the stability of the contact. Finally, high speed pulse I-V monitoring unit was used to observe 13-μs switching speed. [ABSTRACT FROM AUTHOR]

Micromechanical resonators are promising replacements for quartz crystals for timing and frequency references owing to potential for compactness, integrability with CMOS fabrication processes, low cost, and low power consumption. To be used in high performance reference application, resonators should obtain a high quality factor. The limit of the quality factor achieved by a resonator is set by the material properties, geometry and operating condition. Some recent resonators properly designed for exploiting bulk-acoustic resonance have been demonstrated to operate close to the quantum mechanical limit for the quality factor and frequency product (Q-f ). Here, we describe the physics that gives rise to the quantum limit to the Q-f product, explain design strategies for minimizing other dissipation sources, and present new results from several different resonators that approach the limit. [ABSTRACT FROM AUTHOR]

The frequency stability of single-crystal silicon microelectromechanical systems resonators encapsulated with epitaxial polysilicon (epi-seal) is investigated. As silicon resonators have significant temperature dependence, the inherent frequency stability of the resonators is masked by temperature-induced noise. Using two resonators side-by-side and assuming identical temperatures and fluctuations, temperature effects are eliminated, resulting in the two resonators tracking each other within \pm10~ppb, or 3\,\times 10^-4^\circC, over a month. Power and thermal cycling the resonators produced no observable effects on the resonant frequency. This result indicates that silicon resonators make excellent on-chip thermometers, or high stability timing references if temperature is compensated well. [ABSTRACT FROM PUBLISHER]

There is an error in one set of values listed in Table III, page 735. The cell in the 4th row from the bottom (Ring <110>) and rightmost column (P6.6) should read “-3.05 / -58.10 / 19.46”, not “-58.10 / -30.99 / 19.46”. [ABSTRACT FROM AUTHOR]

The increasing population of the elderly and motion-impaired people brings a huge challenge to our social system. However, the walking stick as their essential tool has rarely been investigated into its potential capabilities beyond basic physical support, such as activity monitoring, tracing, and accident alert. Here, we report a walking stick powered by ultra-low-frequency human motion and equipped with deep-learning-enabled advanced sensing features to provide a healthcare-monitoring platform for motion-impaired users. A linear-to-rotary structure is designed to achieve highly efficient energy harvesting from the linear motion of a walking stick with ultralow frequency. Besides, two kinds of self-powered triboelectric sensors are proposed and integrated to extract the motion features of the walking stick. Augmented sensing functionalities with high accuracies have been enabled by deep-learning-based data analysis, including identity recognition, disability evaluation, and motion status distinguishing. Furthermore, a self-sustainable Internet of Things (IoT) system with global positioning system tracing and environmental temperature and humidity amenity sensing functions is obtained. Combined with the aforementioned functionalities, this walking stick is demonstrated in various usage scenarios as a caregiver for real-time well-being status and activity monitoring. The caregiving walking stick shows the potential of being an intelligent aid for motion-impaired users to help them live life with adequate autonomy and safety.

This paper presents a 591×438-DPI ultrasonic fingerprint sensor. The sensor is based on a piezoelectric micromachined ultrasonic transducer (PMUT) array that is bonded at wafer-level to complementary metal oxide semiconductor (CMOS) signal processing electronics to produce a pulse-echo ultrasonic imager on a chip. To meet the 500-DPI standard for consumer fingerprint sensors, the PMUT pitch was reduced by approximately a factor of two relative to an earlier design. We conducted a systematic design study of the individual PMUT and array to achieve this scaling while maintaining a high fill-factor. The resulting 110×56-PMUT array, composed of 30×43-μm 2 rectangular PMUTs, achieved a 51.7% fill-factor, three times greater than that of the previous design. Together with the custom CMOS ASIC, the sensor achieves 2 mV kPa -1 sensitivity, 15 kPa pressure output, 75 μm lateral resolution, and 150 μm axial resolution in a 4.6 mm×3.2 mm image. To the best of our knowledge, we have demonstrated the first MEMS ultrasonic fingerprint sensor capable of imaging epidermis and sub-surface layer fingerprints., Competing Interests: The authors declare no conflict of interest.