Lamb Wave Resonators Research Articles

Piezoelectric MEMS—evolution from sensing technology to diversified applications in the 5G/Internet of Things (IoT) era

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.

High Figure-of-Merit Lamb Wave Resonators Based on Al0.7Sc0.3N Thin Film

This work reports the Lamb wave resonator based on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> N thin films using magnetron sputtering with a single alloy target. The resonator fabrication process based on high Sc doping concentration is discussed. Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> N thin films with a 1.2° (0002) rocking curve were obtained with improved crystalline quality and reduced abnormal orientation grains (AOGs). The etching process has been optimized to achieve an etch rate of 127 nm/min and a profile angle of 72°. The dispersion properties of Lamb waves and the influence of different electrode metals on the coupling coefficient in Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> N thin films were simulated. Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> N Lamb wave resonators operating at approximately 300 MHz and 600 MHz were fabricated. A high electromechanical coupling coefficient ( k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of 7.74% is reported, with the loaded quality factor of 1119, respectively. A high Figure-of-Merit (FOM) of 86.6 has been achieved for AlScN film based Lamb wave resonators below 1 GHz. The application potential of high scandium concentration (>25%) in resonators and filters is demonstrated.

Near-Zero Drift and High Electromechanical Coupling Acoustic Resonators at &gt; 3.5 GHz

In this article, near-zero drift and high electromechanical coupling acoustic resonators have been designed and demonstrated. The acoustic resonator is based on Lamb acoustic waves in a bimorph composed of lithium niobate on silicon dioxide. Our approach breaks through a performance boundary in conventional Lamb-wave resonators by introducing the bimorph while operating at higher order resonant modes. This enables the resonator to achieve frequency scalability, a low-temperature coefficient of frequency, and high electromechanical coupling altogether. The electromechanical coupling and temperature coefficient of the resonator were analytically optimized for the A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> mode through adjusting the thicknesses of different materials in the bimorph. Resonators with different dimensions and stack thickness were fabricated and measured, resulting in a temperature coefficient of frequency ranging from -17.6 to -1.1 ppm/°C, high electromechanical coupling ranging from 13.4% to 18%, and quality factors up to 800 at 3.5 GHz. The achieved specifications are adequate for fifth-generation (5G) sub-6-GHz frequency bands n77 and n78.

A MEMS Fabrication Process with Thermal-Oxide Releasing Barriers and Polysilicon Sacrificial Layers for AlN Lamb-Wave Resonators to Achieve fs·Qm > 3.42 × 1012.

This paper presents a micro-electro-mechanical systems (MEMS) processing technology for Aluminum Nitride (AlN) Lamb-wave resonators (LWRs). Two LWRs with different frequencies of 402.1 MHz and 2.097 GHz by varying the top interdigitated (IDT) periods were designed and fabricated. To avoid the shortcomings of the uncontrollable etching of inactive areas during the releasing process and to improve the fabrication yield, a thermal oxide layer was employed below the platted polysilicon sacrificial layer, which could define the miniaturized release cavities well. In addition, the bottom Mo electrode that was manufactured had a gentle inclination angle, which could contribute to the growth of the high-quality AlN piezoelectric layer above the Mo layer and effectively prevent the device from breaking. The measured results show that the IDT-floating resonators with 12 μm and 2 μm electrode periods exhibit a motional quality factor (Qm) as high as 4382 and 1633. The series resonant frequency (fs)·Qm values can reach as high as 1.76 × 1012 and 3.42 × 1012, respectively. Furthermore, Al is more suitable as the top IDT material of the AlN LWRs than Au, and can contribute to achieving an excellent electrical performances due to the smaller density, smaller thermo-elastic damping (TED), and larger acoustic impedance difference between Al and AlN.

Non-degenerate parametric mixing and Q-enhancement in ALN Lamb wave resonator

In this Letter, we explore a non-degenerate phase independent parametric quality factor (Q)-enhancement technique for aluminum nitride (AlN) Lamb wave resonators. Unlike other active Q-enhancement techniques which require precise phase control of the electronic feedback loop, this technique is implemented by parametrically pumping AlN material stiffness to realize a negative resistance seen at the signal path. The negative resistance is dependent on the nonlinear material modulation and multi-resonance coupling in the device. A nonlinear circuit model is developed to simulate the parametric coupling of each resonance and extract the nonlinearity of AlN from experimental data. With proper pump frequency and pump power, the device quality factor is boosted in both simulation and experiment. The demonstrated Q-enhancement method is simple to implement and can be applied to other types of resonators that have nonlinear behavior and support multi-resonance operation.

Al0.68Sc0.32N Lamb wave resonators with electromechanical coupling coefficients near 10.28%

Aluminum scandium nitride (Al1−xScxN/AlScN) (x = 0.32) Lamb wave resonators (LWR) have been fabricated and tested to demonstrate electromechanical coupling coefficients (kt2) in excess of 10%. The resonators exhibited an average kt2 and unloaded quality factor (Qu) of 10.28% and 711, respectively, when calculated from the measured data. Applying the Butterworth Van-Dyke (BVD) model to the measured data enabled the extraction of the resonator's lumped element parameters to calculate the motional quality factor (Qm), which neglects the contributions of the electrical traces. For the best measured resonator response, results from the BVD model showed a Qm of 1184 and a resulting figure-of-merit (FOM = K2 · Qm) of 100. Comparing the response of similar AlScN and AlN resonators shows that the AlScN LWR has a significantly lower motional resistance (Rm), suggesting that AlScN has a strong potential for use in piezoelectric microelectromechanical oscillators.

Optimization of Inactive Regions of Lithium Niobate Shear Mode Resonator for Quality Factor Enhancement

This work presents the development of a novel process flow for the fabrication of Lithium niobate Lamb wave resonator that allows full control of the shape and size of the released area (undercut) necessary for stabilizing quality factor. We then investigate the influence of the inactive regions of the resonator (i.e. undercut, anchor, bus and gap) on Q; and determine the optimum dimensions for those regions in order to limit the flow of energy escaping from the resonator's body resulting in overall improvement in Q. We report devices with electromechanical coupling ( k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of 41% and quality factor ( Q) of 1900 at around 290 MHz resulting in the highest-ever achieved Figure-of-Merit (FoM) of 780 for SH0 mode resonators in X-cut LN. [2020-0363]

Characterization of AlN and AlScN film ICP etching for micro/nano fabrication

We investigate the inductively coupled plasma (ICP) etching characteristics of (0002) Aluminum Nitride (AlN) and Aluminum Scandium Nitride (Al0.94Sc0.06N) piezoelectric thin films as well as the implementation on piezoelectric lamb wave resonators. A profile of 84° is obtained on AlN thin film, with an etching rate of approximately 230 nm/min, and a selectivity of 0.77:1 relative to photoresist mask. A nearly 80° profile and over 30:1 selectivity are accomplished using Ni film as the etching mask. Al0.94Sc0.06N film has achieved a profile of 77° by optimizing the RF power. Finally, AlN contour mode resonators (CMRs) are fabricated and characterized, and a CMR operating at about 400 MHz with a quality factor exceeding 1600 has been demonstrated.

Anchor Loss Reduction of Lamb Wave Resonator by Pillar-Based Phononic Crystal.

Energy leakage via anchors in substrate plates impairs the quality factor (Q) in microelectromechanical system (MEMS) resonators. Most phononic crystals (PnCs) require complicated fabrication conditions and have difficulty generating a narrow bandgap at high frequency. This paper demonstrates a pillar-based PnC slab with broad bandgaps in the ultra high frequency (UHF) range. Due to Bragg interference and local resonances, the proposed PnC structure creates notably wide bandgaps and shows great advantages in the high frequency, large electromechanical coupling coefficient () thin film aluminum nitride (AlN) lamb wave resonator (LWR). The dispersion relations and the transmission loss of the PnC structure are presented. To optimize the bandgap, the influence of the material mechanical properties, lattice type, pillar height and pillar radius are explored. These parameters are also available to adjust the center frequency of the bandgap to meet the desirable operating frequency. Resonators with uniform beam anchors and PnC slab anchors are characterized. The results illustrate that the Q of the resonator improves from 1551 to 2384, and the mechanical energy leakage via the anchors is significantly decreased using the proposed PnC slab anchors. Moreover, employment of the PNC slab anchors has little influence on resonant frequency and induces no spurious modes. Pillar-based PnCs are promising in suppressing the anchor loss and further improving the Q of the resonators.

Towards Acoustic Radiation Free Lamb Wave Resonators for High-Resolution Gravimetric Biosensing

A novel Lamb wave resonator with fully suppressed acoustic radiation in water is proposed for high-resolution mass-sensitive detection of biomolecules. The elimination of acoustic radiation is achieved by slowing down the Lamb wave to a velocity lower than the sound speed in water. This enables high-quality-factor resonance in water and reduces the sensing frequency noise. High aspect ratio electrodes (HAREs) are used to slow down the Lamb wave. The elastic resonance and large surface area of the HAREs can also enhance the mass sensitivity of the device. The improved mass sensitivity together with the low frequency noise substantially improves the overall sensing resolution. Although reducing the plate thickness can also slow down the Lamb wave, it makes the device very fragile and not practical to use. In contrast, slowing down the Lamb wave by increasing electrode height allows the use of thick plates which is robust. In this article, the behavior and performance of the proposed high aspect ratio electrode Lamb wave resonator (HARE-LWR) are theoretically analyzed using finite element method simulations. Optimum design parameters were found through the simulations. Reported results show that a significant figure of merit improvement was achieved by the proposed HARE-LWR design.

AlN MEMS filters with extremely high bandwidth widening capability

This paper presents radio frequency (RF) microelectromechanical system (MEMS) filters with extremely high bandwidth widening capability. The proposed filtering topologies include hybrid configurations consisting of piezoelectric MEMS resonators and surface-mounted lumped elements. The MEMS resonators set the center frequency and provide electromechanical coupling to construct the filters, while the lumped-element-based matching networks help widen the bandwidth (BW) and enhance the out-of-band rejection. Aluminum nitride (AlN) S0 Lamb wave resonators are then applied to the proposed filtering topologies. AlN S0 first- and second-order wideband filters are studied and have shown prominent performance. Finally, the AlN S0 first-order wideband filter is experimentally implemented and characterized. The demonstrated first-order filter shows a large fractional bandwidth (FBW) of 5.6% (achieved with a resonator coupling of 0.94%) and a low insertion loss (IL) of 1.84 dB. The extracted bandwidth widening factor (BWF) is 6, which is approximately 12 times higher than those of the current ladder or lattice filtering topologies. This impressive bandwidth widening capability holds great potential for satisfying the stringent BW requirements of bands n77, n78, and n79 of 5G new radio (NR) and will overcome an outstanding technology hurdle in placing 5G NR into the marketplace.

Epitaxial Aluminum Scandium Nitride Super High Frequency Acoustic Resonators

This paper demonstrates super high frequency (SHF) Lamb and surface acoustic wave resonators based on single-crystal orientation Aluminum Scandium Nitride (AlScN) thin films grown on silicon substrates by molecular beam epitaxy (MBE). We report on the experimental frequency response and electromechanical properties of 400 nm-thick crystalline AlScN acoustic resonators with up to 12% Sc/(Sc+Al) ratio. The film thickness is optimized for operation at the SHF range, targeting emerging wireless communication standards, such as 4G LTE/5G. We report on high-performance acoustic devices that take advantage of the crystallinity, and high piezoelectric properties of 400 nm-thick epitaxial AlScN films. Our work presents enhanced effective electromechanical coupling coefficients (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) up to 5.3% and unloaded quality factors (Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) of ~192 at 3-10 GHz. However, fabrication challenges due to the high-stress levels of sub-micron AlScN epi-layers grown on Si substrates remain challenging and will be discussed in this paper. [2019-0231].

Smartphone-enabled Dynamic Chemiluminescence Biomarker Quantitation Using Acoustic Tweezers Approach.

Quantitation of protein biomarker featured with portability, rapidity, high sensitivity is critical for the point-of-care testing (POCT) application. Herein, a novel smartphone-enabled microfluidic chemiluminescence platform for the quantitation of prostate specific antigen (PSA) was proposed based on acoustic tweezers approach. The primary antibodies labeled polystyrene microparticles (Ab1-PSs), target samples, the horseradish peroxidase labeled secondary antibodies (Ab2-HRP) were injected into the microfluidics simultaneously. Under the actuation of Lamb Wave Resonator (LWR), they were dynamically trapped and concentrated in the acoustic streaming; meanwhile, the biomolecular binding was enhanced. After the injection of chemiluminescent substrate, the concentrated immuno-particles catalyzed hydrogen peroxide (H2O2) reaction so that the emitted blue light was directly captured by smartphone. Besides, the flow rate and the applied power of LWR were optimized for the signal amplification. The chemiluminescence immunoassay exhibited a dynamic linear range from 0.5 ng/mL to 10 ng/mL with a limit of detection of 0.1 ng/mL in PBS buffer. The portable immunosensor will be utilized for the quantitation of PSA in serum samples to demonstrate the clinical significance.Clinical Relevance-The smartphone-enabled detection platform realizes the quantitation of biomarker within 10 min, which reveals a valuable potential tool for the early diagnosis of various diseases, even in resource-limited regions.

Spectrum-clean S1 AlN Lamb wave resonator with damped edge reflectors

A concept for the development of spectrum-clean S1 Lamb wave resonator (LWR) utilizing an aluminum nitride (AlN) plate with damped edge reflectors is demonstrated. Numerical analysis is performed on the dispersive characteristics of the Lamb modes propagating in AlN plates, and the design parameters to enable zero group velocity (vg) for the S1 mode are identified. The simulation results based on finite element analysis verify that the use of the damped edge reflectors, instead of the conventional edge reflectors, can efficiently de-couple most of the Lamb waves such as the S0 mode by mode conversion upon edges, but not the S1 mode characterizing zero vg. Specifically, the measured frequency response of a 1.38-GHz AlN S1 LWR with damped edge reflectors yields a clean spectrum up to 6 GHz and excellent resonance passband performance.

Frequency-scalable fabrication process flow for lithium niobate based Lamb wave resonators

In this paper, we demonstrate the fabrication and characterization of lithium niobate (LN) based Lamb wave mode (S0 and SH0) resonators that address the stringent requirements of RF filtering for modern communication systems. The devices consist of a 400 nm-thick X-cut LN membrane anchored from two ends with interdigitated transducer (IDT) on the top. We present a frequency-scalable process flow using e-beam lithography for the fabrication of ultra-high frequency ( to Hz) devices. Our fabricated devices yield the highest reported electromechanical coupling () of 31% and 40% for S0 and SH0 modes, respectively, in X-cut LN. We also investigate the influence of IDT material and coverage on and quality factor () by fabricating identical devices with aluminum and platinum electrodes with electrode coverages ranging from 20% to 70%.

High frequency guided mode resonances in mass-loaded, thin film gallium nitride surface acoustic wave devices

We demonstrate high-frequency (&amp;gt;3 GHz), high quality factor radio frequency (RF) resonators in unreleased thin film gallium nitride (GaN) on sapphire and silicon carbide substrates by exploiting acoustic guided mode (Lamb wave) resonances. The associated energy trapping, due to mass loading from gold electrodes, allows us to efficiently excite these resonances from a 50 Ω input. The higher phase velocity, combined with lower electrode damping, enables high quality factors with moderate electrode pitch and provides a viable route towards high-frequency piezoelectric devices. The GaN platform, with its ability to guide and localize high-frequency sound on the surface of a chip with access to high-performance active devices, will serve as a key building block for monolithically integrated RF front-ends.

Narrowband Impedance Transformer With Extremely High Transformation Ratio of 200

This letter presents a hybrid narrowband radio frequency (RF) impedance transformer using an aluminum nitride (AlN) microelectromechanical system (MEMS) resonator and lumped elements. The hybrid transformer consists of an AlN Lamb wave resonator, a parallel inductor, and a set of three lumped-element networks on a printed circuit board (PCB). The AlN resonator and the parallel inductor form the narrowband response while the lumped element networks fulfill the high impedance transformation. The transformer has been theoretically predicted and experimentally shown to achieve a narrow fractional bandwidth (FBW) of 2.2% and an extremely high impedance transformation ratio (ITR) of 200.

Ultralow Acoustic Loss Micromachined Butterfly Lamb Wave Resonators on AlN Plates

This study reports the design of a novel Butterfly Lamb wave resonator (LWR) employing the S0 mode in the AlN plate, and for the first time, its ultrahigh parallel-resonance quality factor ( Qp ) of 4,021 is demonstrated, indicating an ultralow acoustic loss. Although the series resonance quality factor ( Qs ) is widely used for various loss comparisons, it is inconclusive since Qs is always dominated by the routing resistance ( Rs ), which is normally huge without the thick metal rewiring. Instead, Qp is a precise representation of the acoustic loss level for its independence of Rs and as it is closer to Qmax of the Bode Q -curve in the IDT-excited devices. A butterfly-shaped resonance cavity, theoretically predicted to reduce the anchor loss and suppress the transverse spurious mode, has been applied to the AlN LWR and experimentally shown to boost the Qp by 2.3 times. In addition, a directly measured Bode- Q curve for the LWR is reported for the first time, showing superior Q profile for the Butterfly-LWR than the conventional-LWR and good agreement with the 3 dB- Qp 's.

Numerical analysis of anchor loss and thermoelastic damping in piezoelectric AlN-on-Si Lamb wave resonators

Even though many thin-film piezoelectric aluminum nitride (AlN) on silicon (Si) Lamb wave resonators have been proposed in the literature, little focus has been set on the modeling of the quality factors Q. Their Qs are associated with numerous dissipation sources, which are often difficult to separate from each other in experiments. Besides, the values of Q measured in experiments can largely deviate from sample-to-sample of the same design. In order to gain better insight into these issues, we have applied numerical models to estimate anchor losses and thermoelastic damping to a large set of AlN-on-Si resonators specifically designed to have significantly different Qs. The data set includes biconvex resonators of different curvatures (designed to reduce anchor losses), regular flat-edge resonators, and different electrode patterns. For the broad range of devices tested, we show that the computed values of Q agree well with the experimental data. In particular, the experimentally measured values of Q in regular flat-edge Lamb wave resonators are due to comparable contributions of the two types of losses.

Flexible lamb wave resonators with high figure of merit

This paper presents a 429 MHz flexible Lamb wave resonator based on lithium niobate thin films with a high figure of merit (FoM) of 205, which is about 10 times higher than the FoM of the flexible resonator presented in our previous work. The measured corresponding quality factor (Q) and electromechanical coupling coefficient (Kt2) are 1268 and 16.2%, respectively. The resonant frequency, Q, and Kt2 of the flexible resonator show maximum changes of only 0.11%, 0.37%, and 0.31%, respectively, under a repeated mechanical bending condition up to 10 000 times at a bending radius of 3 mm. We also found that FlexMEMS technology we proposed not only endows Lamb wave resonators with mechanical flexibility but also improves FoM by ∼180% compared to their counterpart, conventional Lamb wave resonators on rigid silicon substrates. The flexible resonators with much improved FoM will find applications as low-power radio frequency key components in emerging applications, especially Internet of Things.