Piezoelectric Microelectromechanical Systems Research Articles

Miniaturized Multi-Cantilever MEMS Resonators with Low Motional Impedance

Microelectromechanical system (MEMS) cantilever resonators suffer from high motional impedance (Rm). This paper investigates the use of mechanically coupled multi-cantilever piezoelectric MEMS resonators in the resolution of this issue. A double-sided actuating design, which utilizes a resonator with a 2.5 μm thick AlN film as the passive layer, is employed to reduce Rm. The results of experimental and finite element analysis (FEA) show agreement regarding single- to sextuple-cantilever resonators. Compared with a standalone cantilever resonator, the multi-cantilever resonator significantly reduces Rm; meanwhile, the high quality factor (Q) and effective electromechanical coupling coefficient (Kteff2) are maintained. The 30 μm wide quadruple-cantilever resonator achieves a resonance frequency (fs) of 55.8 kHz, a Q value of 10,300, and a series impedance (Rs) as low as 28.6 kΩ at a pressure of 0.02 Pa; meanwhile, the smaller size of this resonator compared to the existing multi-cantilever resonators is preserved. This represents a significant advancement in MEMS resonators for miniaturized ultra-low-power oscillator applications.

Unsealed Piezoelectric MEMS Speaker with Rigid-Flexible Composite Membrane

Abstract In this paper, we propose an unsealed piezoelectric microelectromechanical systems (MEMS) speaker with rigid-flexible composite membrane, which can eliminate the membrane separation and the vibration displacement limitation at high driving voltage compared to that with the sealed rigid-flexible coupling membrane demonstrated in our previous work. Measurements performed on encapsulated prototypes mounted to an artificial ear simulator have revealed that in the human audible range of 20 Hz-20 kHz, higher than 68.5 dB SPL are obtained at 2 V, and greater than 89.6 dB SPLs are achieved at 10 V. Moreover, the SPL distribution and effective SPLs at each moment when playing the same song exhibit similar characteristics to those of a commercial electromagnetic one. This piezoelectric MEMS speaker achieves high SPLs meeting the basic hearing needs of the human, and will have excellent prospects for future wearable audio electronics.

Metalorganic Chemical Vapor Deposition of AlN on High Degree Roughness Vertical Surfaces for MEMS Fabrication

AbstractAluminum nitride (AlN) grown on vertical surfaces can be utilized for the fabrication of advanced piezoelectric microelectromechanical systems (MEMS). The in‐plane motion of the parts of a MEMS element is possible when AlN is deposited on the vertical surfaces of the moving structure. For the best device performance, AlN must have high crystal quality, uniform coverage of the vertical sidewalls, and c‐axis crystalline orientation perpendicular to the plane of a vertical surface. The impact of the surface roughness (Rq) of the vertical sidewalls formed in Si using wet and dry etching methods on the crystal quality, crystallographic orientation, and uniformity of the metalorganic chemical vapor deposited (MOCVD) AlN thin films is studied in this paper. In both cases, AlN films demonstrated full sidewall coverage and grew crystalline in the c‐axis direction. AlN films grown on vertical Si surfaces achieved using anisotropic wet etching are highly crystalline and oriented in [0001] direction, while the films grown on vertical surfaces achieved using dry etching displayed a lower level of alignment with the Si sidewalls.

Multi-Material Radial Phononic Crystals to Improve the Quality Factor of Piezoelectric MEMS Resonators.

In this paper, a multi-material radial phononic crystal (M-RPC) structure is proposed to reduce the anchor-point loss of piezoelectric micro-electro-mechanical system (MEMS) resonators and improve their quality factor. Compared with single-material phononic crystal structures, an M-RPC structure can reduce the strength damage at the anchor point of a resonator due to the etching of the substrate. The dispersion curve and frequency transmission response of the M-RPC structure were calculated by applying the finite element method, and it was shown that the M-RPC structure was more likely to produce a band-gap range with strong attenuation compared with a single-material radial phononic crystal (S-RPC) structure. Then, the effects of different metal-silicon combinations on the band gap of the M-RPC structures were studied, and we found that the largest band-gap range was produced by a Pt and Si combination, and the range was 84.1-118.3 MHz. Finally, the M-RPC structure was applied to a piezoelectric MEMS resonator. The results showed that the anchor quality factor of the M-RPC resonator was increased by 33.5 times compared with a conventional resonator, and the insertion loss was reduced by 53.6%. In addition, the loaded and unloaded quality factors of the M-RPC resonator were improved by 75.7% and 235.0%, respectively, and at the same time, there was no effect on the electromechanical coupling coefficient.

AlN-based piezoelectric MEMS deformable mirror.

An AlN-based piezoelectric micro-electromechanical system (MEMS) continuous membrane deformable mirror (DM) prototype is presented for the first time. Its effective aperture diameter is 5 mm and it is equipped with 25 independently controlled actuators. Owing to the advantages associated with the AlN piezoelectric thin-film technology, attractive characteristics including CMOS compatible fabrication, bidirectional linear and negligible hysteresis actuation, and excellent linear superposition control capability have been successfully demonstrated. Moreover, good optical aberration correction performance is also validated via the surface contour fitting experiment to the Zernike polynomials up to the first 14 orders despite the non-optimized device structure design, representing great application perspective.

A Micro-Oven Controlled Dual-Mode Piezoelectric MEMS Resonator With ± 190 ppb Stability Over −40 to 105 °C Temperature Range

This letter presents a dual-mode piezoelectric microelectromechanical system (MEMS) resonator with an in-chip micro-oven for resonator temperature control. The piezoelectric MEMS resonator simultaneously operates in width-extensional (WE) mode with a high temperature turnover point, and torsion-bending (TB) mode with a large temperature coefficient of frequency (TCF) based on a phase-locked loop based system. Exploiting the difference in frequency-temperature characteristics of these two modes, accurate temperature sensing and precise close-loop micro-oven heating control are achieved by phase locking the resonator at its turnover point temperature of WE mode. Measured real-time frequency stability of the reported micro-oven controlled MEMS resonator is ±190 ppb over an extended industrial temperature range of -40 °C to 105 °C, indicating potential applications in high-end timing field, such as macro base stations and instrumentations.

Extending atomic layer deposition for use in next-generation piezoMEMS: Review and perspective

The objective of this work is to describe the current state of the rapidly evolving field of 3D piezoelectric microelectromechanical systems (piezoMEMS), and where it needs to go to fully leverage the potential performance benefits offered by atomic layer deposition (ALD). We define 3D piezoMEMS as the application of piezoelectric ALD films to 3D, high aspect-ratio, mechanically pliable structures. Since there are so few existing reports of 3D piezoMEMS, a literature review of ALD films applied to conventional microelectromechanical system (MEMS) devices is given. ALD processes for piezoelectric thin films are reviewed in the context of relevant applications such as transducers and actuators. Examples include aluminum nitride, hafnium zirconate, doped-hafnia, lead zirconate-titanate, lead hafnate, and lead hafnate-titanate. New concepts for ALD-enabled 3D piezoMEMS actuators are presented with supporting theoretical calculations that show that chip-scale mechanical work densities could be improved by ≫10× compared to conventional planar piezoMEMS. 3D fabrication methods are also discussed, while the future needs of atomic layer processing are highlighted.

Investigation on anchor and base design for aluminum nitride flexural mode resonators

This paper investigates the impact of base and anchor on the quality factor (Q) of piezoelectric microelectromechanical system (MEMS) tuning fork resonators vibrating in in-plane flexural mode, and proposes a new strategy to improve Q significantly. Finite element method simulation and measured results reveal that base and anchor geometries have a significant impact on the thermoelastic damping (TED) which dominates the overall resonator quality factor. For the first time, we accurately extract Q value related to TED of piezoelectric tuning fork resonators by cryogenic temperature experiment, which is consistent with FEM simulation results. According to the analysis, a wider anchor or a longer base should promote overall Q by restraining the thermal conduction induced by TED. On the other hand, increase of width or length of anchor will force the out-of-plane flexural modes closer to the target mode, reducing the overall Q by multimode effect. With a wide anchor and a pillar structure under the base to suppress multimode effect, the resonator Q could be improved by 65% to more than 11 000 and impedance at series resonant frequency (f s) could be reduced by 78% to 6.5 kΩ. To our knowledge, the proposed resonator exhibits the highest f× Q and lowest motional impedance among the reported in-plane mode piezoelectric MEMS tuning fork resonators.

A High-Sensitivity MEMS Accelerometer Using a Sc0.8Al0.2N-Based Four Beam Structure.

In this paper, a high-sensitivity microelectromechanical system (MEMS) piezoelectric accelerometer based on a Scandium-doped Aluminum Nitride (ScAlN) thin film is proposed. The primary structure of this accelerometer is a silicon proof mass fixed by four piezoelectric cantilever beams. In order to enhance the sensitivity of the accelerometer, the Sc0.2Al0.8N piezoelectric film is used in the device. The transverse piezoelectric coefficient d31 of the Sc0.2Al0.8N piezoelectric film is measured by the cantilever beam method and found to be -4.7661 pC/N, which is approximately two to three times greater than that of a pure AlN film. To further enhance the sensitivity of the accelerometer, the top electrodes are divided into inner and outer electrodes; then, the four piezoelectric cantilever beams can achieve a series connection by these inner and outer electrodes. Subsequently, theoretical and finite element models are established to analyze the effectiveness of the above structure. After fabricating the device, the measurement results demonstrate that the resonant frequency of the device is 7.24 kHz and the operating frequency is 56 Hz to 2360 Hz. At a frequency of 480 Hz, the sensitivity, minimum detectable acceleration, and resolution of the device are 2.448 mV/g, 1 mg, and 1 mg, respectively. The linearity of the accelerometer is good for accelerations less than 2 g. The proposed piezoelectric MEMS accelerometer has demonstrated high sensitivity and linearity, making it suitable for accurately detecting low-frequency vibrations.

Fabrication, Characterization, and Signal Processing Optimization of Flexible and Wearable Piezoelectric Tactile Sensors

Piezoelectric microelectromechanical systems (MEMS) meet the growing demand for sensors with small sizes and low power consumption. For tactile applications and pressure measurements, they are applied in various fields from robotics to healthcare. In this context, flexible devices are very important for their high responsivity and ability to conform to the analyzed surface. This work reports on miniaturized flexible and compliant piezoelectric devices to increase the number of integrable sensors for detecting and discriminating localized pressures and contacts per unit area with minimal crosstalk. For this purpose, a series of Aluminium Nitride-based (AlN) piezoelectric sensors with different diameters (from 5 μm to 500 μm) was realized and the generated electrical signal by sensor deformation was amplified by a differential voltage amplifier circuit. By the analysis of the shape of the piezoelectric signal as a function of the applied pressure, oscillations due to piezoelectric deformations, superimposed on the initial peak signal corresponding to the touch event, have been observed. The integral of the electrical signal was calculated for the most accurate representation to describe the sensor’s response. The best responsivity was obtained in samples with diameters of 200 μm and 500 μm. Furthermore, it was also found that the minimum distance between the edges of closed sensors, observing crosstalk below -20 dB, was around 500 μm.

Self-aligned single-electrode actuation of tangential and wineglass modes using PMN-PT

Considering the evolution of rotation sensing and timing applications realized in micro-electro-mechanical systems (MEMS), flexural mode resonant shapes are outperformed by bulk acoustic wave (BAW) counterparts by achieving higher frequencies with both electrostatic and piezoelectric transduction. Within the 1–30 MHz range, which hosts BAW gyroscopes and timing references, piezoelectric and electrostatic MEMS have similar transduction efficiency. Although, when designed intelligently, electrostatic transduction allows self-alignment between electrodes and the resonator for various BAW modes, misalignment is inevitable regarding piezoelectric transduction of BAW modes that require electrode patterning. In this paper transverse piezoelectric actuation of [011] oriented single crystal lead magnesium niobate–lead titanate (PMN–PT) thin film disks are shown to excite the tangential mode and family of elliptical compound resonant modes, utilizing a self-aligned and unpatterned electrode that spans the entire disk surface. The resonant mode coupling is achieved by employing a unique property of [011] PMN–PT, where the in-plane piezoelectric coefficients have opposite signs. Fabricating 1-port disk transducers, RF reflection measurements are performed that demonstrate the compound mode family shapes in the 1–30 MHz range. Independent verification of mode transduction is achieved using in-plane displacement measurements with Polytec’s laser Doppler vibrometer (LDV). While the tangential mode achieves a 40o/s dithering rate at 335 kHz resonant frequency, the n = 2 wine-glass mode achieves 11.46 nm tip displacement at 8.42 MHz resonant frequency on a radius of 60 μm disk resonator in air. A single electrode resonator that can excite both tangential and wine-glass modes with such metrics lays the foundation for a BAW MEMS gyroscope with a built-in primary calibration stage.

Physicochemical Properties of Organic Molecular Ferroelectric Diisopropylammonium Chloride Thin Films.

We fabricated ferroelectric films of the organic molecular diisopropylammonium chloride (DIPAC) using the dip-coating technique and characterized their properties using various methods. Fourier-transform infrared, scanning electron microscopy, and X-ray diffraction analysis revealed the structural features of the films. We also performed ab-initio calculations to investigate the electronic and polar properties of the DIPAC crystal, which were found to be consistent with the experimental results. In particular, the optical band gap of the DIPAC crystal was estimated to be around 4.5 eV from the band structure total density-of-states obtained by HSE06 hybrid functional methods, in good agreement with the value derived from the Tauc plot analysis (4.05 ± 0.16 eV). The films displayed an island-like morphology on the surface and showed increasing electrical conductivity with temperature, with a calculated thermal activation energy of 2.24 ± 0.03 eV. Our findings suggest that DIPAC films could be a promising alternative to lead-based perovskites for various applications such as piezoelectric devices, optoelectronics, sensors, data storage, and microelectromechanical systems.

A tip-coupled, two-cantilever, non-resonant microsystem for direct measurement of liquid viscosity

We report a non-resonant piezoelectric microelectromechanical cantilever system for the measurement of liquid viscosity. The system consists of two PiezoMEMS cantilevers in-line, with their free ends facing each other. The system is immersed in the fluid under test for viscosity measurement. One of the cantilevers is actuated using the embedded piezoelectric thin film to oscillate at a pre-selected non-resonant frequency. The second cantilever, the passive one, starts to oscillate due to the fluid-mediated energy transfer. The relative response of the passive cantilever is used as the metric for the fluid’s kinematic viscosity. The fabricated cantilevers are tested as viscosity sensors by carrying out experiments in fluids with different viscosities. The viscometer can measure viscosity at a single frequency of choice, and hence some important considerations for frequency selection are discussed. A discussion on the energy coupling between the active and the passive cantilevers is presented. The novel PiezoMEMS viscometer architecture proposed in this work will overcome several challenges faced by state-of-the-art resonance MEMS viscometers, by enabling faster and direct measurement, straightforward calibration, and the possibility of shear rate-dependent viscosity measurement.

A Piezoelectric MEMS Speaker with a Combined Function of a Silent Alarm.

To explore the versatility of speakers, a piezoelectric micro-electro-mechanical system (MEMS) speaker combining the function of a silent alarm is proposed, which mainly comprises a lead zirconate titanate (PZT) actuation layer and a rigid-flexible coupling supporting layer. Measurements performed on encapsulated prototypes mounted to an artificial ear simulator have revealed that, compared to a speaker with a rigid supporting layer, the sound pressure level (SPL) of the proposed piezoelectric MEMS speaker with a rigid-flexible coupling supporting layer is significantly higher and is especially higher by 4.1-20.1 dB in the frequency range from 20 Hz to 4.2 kHz, indicating that the rigid-flexible coupling supporting layer can improve the SPL significantly in low frequency. Moreover, the spectral distribution characteristic of its playback audio is similar to that of the commercial electromagnetic type. The device can also function as a silent alarm based on oral airflows in dangerous situations, as it performs well at recognizing words according to their unique voltage-signal characteristics, and can avoid the effects of external sound noise, body movement, long distance, and occlusion. This strategy provides inspiration for functional diversification of piezoelectric MEMS speakers.

Aluminum Nitride Out-of-Plane Piezoelectric MEMS Actuators

Integrating microelectromechanical systems (MEMS) actuators with low-loss suspended silicon nitride waveguides enables the precise alignment of these waveguides to other photonic integrated circuits (PICs). This requires both in-plane and out-of-plane actuators to ensure high-precision optical alignment. However, most current out-of-plane electrostatic actuators are bulky, while electrothermal actuators consume high power. Thus, piezoelectric actuators, thanks to their moderate actuation voltages and low power consumption, could be used as alternatives. Furthermore, piezoelectric actuators can provide displacements in two opposite directions. This study presents a novel aluminum nitride-based out-of-plane piezoelectric MEMS actuator equipped with a capacitive sensing mechanism to track its displacement. This actuator could be integrated within PICs to align different chips. Prototypes of the device were tested over the range of ±60 V, where they provided upward and downward displacements, and achieved a total average out-of-plane displacement of 1.30 ± 0.04 μm. Capacitance measurement showed a linear relation with the displacement, where at -60 V, the average change in capacitance was found to be -13.10 ± 0.89 fF, whereas at 60 V the change was 11.09 ± 0.73 fF. This study also investigates the effect of the residual stress caused by the top metal electrode, on the linearity of the displacement-voltage relation. The simulation predicts that the prototype could be modified to accommodate waveguide routing above it without affecting its performance, and it could also incorporate in-plane lateral actuators.

On the importance of the SrTiO3 template and the electronic contact layer for the integration of phase-pure low hysteretic Pb(Mg0.33Nb0.67)O3-PbTiO3 layers with Si

The rapid advent of the piezoelectric microelectromechanical systems (PiezoMEMS) field has created a tremendous demand for low hysteretic piezoelectric thin films on Si. In this work, we present the integration of epitaxial Pb(Mg0.33Nb0.67)O3-PbTiO3 (PMN-PT) thin films with Si to enable device fabrication using state of the art methods. With optimized buffer layers and electronic contacts, high-quality low hysteretic PMN-PT thin films are integrated with Si, which is a significant stride towards employing PMN-PT thin film for PiezoMEMS devices. It is found that the processing of the necessary SrTiO3 buffer layer is crucial to achieve the growth of phase-pure perovskite PMN-PT layers on Si. Furthermore, we propose the engineering of the electronic contact for the PMN-PT-on-Si capacitors to obtain low hysteretic polarization and displacement responses.

A piezoelectric mechanically coupled Lamé mode resonator with ultra-high Q

A piezoelectrically actuated high quality factor (Q) mechanically coupled Lamé mode resonator based on a thin-film piezoelectric-on-silicon (TPoS) platform is demonstrated. The reported coupled resonator consists of two identical square plates of single-crystal silicon (SCS) coupled with a SCS beam having thin-film piezoelectric stacked layers on top. The coupled resonator is piezoelectrically driven into resonance, and the two square plates are excited into vibrations in a coupled Lamé mode through mechanical coupling. An impressive passive Q-enhancement is obtained by such a scheme based on mechanical coupling of two SCS square plates with a piezoelectrically transduced beam. A fabricated mechanically coupled Lamé mode resonator exhibits an ultra-high Q of 309 894 while still maintaining a moderate impedance (Rm) of 6652 Ω at its resonant frequency of 18.2838 MHz, corresponding to the record highest f-Q product of 5.67 × 1012 for TPoS resonators. The reported high Q microelectromechanical system (MEMS) resonator overcomes the inherent low-Q nature of piezoelectric MEMS resonators, indicating an application for providing a high-performance timing solution in a MEMS timing field.

MEMS piezoelectric resonant microphone array for lung sound classification

This paper reports a highly sensitive piezoelectric microelectromechanical systems (MEMS) resonant microphone array (RMA) for detection and classification of wheezing in lung sounds. The RMA is composed of eight width-stepped cantilever resonant microphones with Mel-distributed resonance frequencies from 230 to 630 Hz, the main frequency range of wheezing. At the resonance frequencies, the unamplified sensitivities of the microphones in the RMA are between 86 and 265 mV Pa−1, while the signal-to-noise ratios (SNRs) for 1 Pa sound pressure are between 86.6 and 98.0 dBA. Over 200–650 Hz, the unamplified sensitivities are between 35 and 265 mV Pa−1, while the SNRs are between 79 and 98 dBA. Wheezing feature in lung sounds recorded by the RMA is more distinguishable than that recorded by a reference microphone with traditional flat sensitivity, and thus, the automatic classification accuracy of wheezing is higher with the lung sounds recorded by the RMA than with those by the reference microphone, when tested with deep learning algorithms on computer or with simple machine learning algorithms on low-power wireless chip set for wearable applications.

Acoustic Performance of Stress Gradient-Induced Deflection of Triangular Unimorphic/Bimorphic Cantilevers for MEMS Applications

This paper reports two piezoelectric materials of lead zirconium titanate (PZT) and aluminum nitride (AlN) used to simulate microelectromechanical system (MEMS) speakers, which inevitably suffered deflections as induced via the stress gradient during the fabrication processes. The main issue is the vibrated deflection from the diaphragm that influences the sound pressure level (SPL) of MEMS speakers. To comprehend the correlation between the geometry of the diaphragm and vibration deflection in cantilevers with the same condition of activated voltage and frequency, we compared four types of geometries of cantilevers including square, hexagon, octagon, and decagon in triangular membranes with unimorphic and bimorphic composition by utilizing finite element method (FEM) for physical and structural analyses. The size of different geometric speakers did not exceed 10.39 mm2; the simulation results reveal that under the same condition of activated voltage, the associated acoustic performance, such as SPL for AlN, is in good comparison with the simulation results of the published literature. These FEM simulation results of different types of cantilever geometries provide a methodology design toward practical applications of piezoelectric MEMS speakers in the acoustic performance of stress gradient-induced deflection in triangular bimorphic membranes.

Design and testing of ultraminiature microelectromechanical systems middle ear accelerometers

Moderate to severe hearing loss is a debilitating condition that affects over 5% of the world’s population. Hearing aids and cochlear implants positively impact the lives of those who suffer from sensorineural hearing loss. However, both display numerous limitations that affect their adoption and use rates, notably including those associated with the external elements of these devices (e.g., microphone and signal processor). These external components impact device safety, appearance, acoustic performance, and ease-of-use. A totally implantable auditory prosthesis would help to address these issues by eliminating external components. A major barrier to progress toward this goal is the lack of a completely implantable acoustic sensor capable of matching or exceeding the performance of commercial external microphones. Our previous studies have indicated that piezoelectric microelectromechanical systems (MEMS) accelerometers have the potential to function as implantable sensors within the middle ear meeting a 20-phon noise floor over a 100–8 kHz range. In this paper, we describe a process to design and fabricate dual and tri-resonance devices that are comprised of piezoelectric cantilever bimorph beams tip-loaded by a proof mass, can meet or exceed the 20-phon noise floor, and can produce a measurable voltage output when deflected by sound-induced ossicular vibrations.