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In this paper, recent developments in the design and fabrication of micromachined fused quartz hemi-toroidal shells are presented. The fabrication is based on micro glassblowing process, demonstrated to enable the realization of high-Q MEMS resonators and gyroscopes. The design optimization of the shell geometry is performed using parametric finite element analysis. The effect of geometric parameters on the scaling of the resonant frequencies and energy dissipation are discussed. Three variations of the micro-glassblowing process are studied in the paper, concluding that shell resonators with a broad operational frequency range without losing the symmetry and Q-factor are feasible. Finite element models are presented to simulate the presented glassblowing processes, which are used to predict the final geometry of shell resonators accurately. Operational frequency as low as 5 kHz and Q-factor as high as 1.7 million is demonstrated on the fabricated shell resonators. The proposed process modifications demonstrate a low-cost and scalable fabrication of 3D shells for resonators and gyroscopes, which can be used in inertial navigation and timing applications. [2019-0179].

This paper reports on the design, fabrication, and characterization of $S_{0}$ mode X-cut Lithium Niobate Laterally Vibrating Resonators (LVRs) operating between 100 MHz and 1 GHz. In the first Section of this work, analytical and numerical models are implemented to investigate the impact of topology and geometrical features on the Electromechanical Coupling ($k_{t}^{2}$) and Quality Factor ($Q_{s}$) of this class of devices. The models are used to define the design of experiments aimed at characterizing the resonator performance. The resonators are fabricated and tested in atmospheric, vacuum, and cryogenic ($T$ = 10 K) conditions to assess their performance and identify the main sources of discrepancies in $k_{t}^{2}$ and $Q_{s}$ with respect to the analytical and numerical models. Under vacuum quality factors exceeding a few thousands are recorded across the investigated spectrum, with measured $k_{t}^{2}$ as high as 28.9%. At 100 MHz, measured $Q_{s}$ and $k_{t}^{2}$ of 8,279 and 28.5%, respectively, combine into a record-breaking Figure of Merit ($FOM$) of 2,688, while cryogenic testing highlighted $Q_{s}$ as high as 26,511. In the last section, by analyzing the differences in $k_{t}^{2}$ and $Q_{s}$ between experiments and models, best design approaches to maximize the $FOM$ and minimize spurious modes responses are reported, with a focus on the 100 to 550 MHz range. [2022-0081] [ABSTRACT FROM AUTHOR]

In this work, we present a Silicon on Insulator (SOI) mechanically coupled MEMS device composed of three rotating ring resonators for enhancing its sensitivity. It shows the exploration of Thermal Piezoresistive (TPR) transduction in mechanically coupled ring resonators for mass sensing applications. Mass sensing experiments are performed by dropping the silver nano drops on the surface of the resonator. It includes the first-time investigation of a three degree of freedom (3-DOF) differential ring-shaped TPR (DR-TPR) MEMS coupled-resonator and involves the selection of the best configuration of the differential drive and sense scheme that enhances the signal-to-noise ratio (SNR) using feedthrough cancellation. Amplitude change and amplitude ratio offer an enhancement in normalized sensitivity by 92 and 131 times respectively along with the common-mode rejection in comparison to their frequency shift counterparts. This coupled-resonator can show a quality factor of more than 1300 in ambient conditions, facilitating the resolution and DC tunability improvement. This work paves the way to realize various sensors for ambient environment applications. [2022-0045] [ABSTRACT FROM AUTHOR]

This study reports a closed-loop system (CLS) for resonant micro-electromechanical systems (MEMS) sensors subject to blue-sideband excitation (BSE) for the first time. Compared with the conventional excitation scheme, which uses an AC signal with a frequency that is very close to or equal to that of the desired MEMS resonator mode, the BSE scheme applies an AC signal with a frequency equal to or near the sum of two resonant frequencies to excite the resonator. This requires a CLS to provide a sum-frequency excitation signal to serve as the resonator’s feedback driving signal for practical applications. The proposed CLS mainly consists of a digital lock-in amplifier (DLA), a phase-locked loop (PLL) and a BSE control module (BSE-CM), and is implemented using a field-programmable gate array (FPGA). Experiments showed that the system can track the combined frequency of two flexural modes of the resonator in real-time. In addition, a potential application of the CLS for resonant MEMS sensors subject to BSE is preliminarily demonstrated, showing that the sensitivity of a vacuum-packaged resonant MEMS accelerometer can be enhanced by 166%. This approach could also be applied to reduce the effects of the feedback phase, or to simultaneously monitor multiple parameters. This study represents critical progress for practical sensing using the BSE scheme. [ABSTRACT FROM AUTHOR]

In this paper, a semi-analytical model is derived to calculate the anchor loss of TPoS resonators operating in higher-order length extensional modes. This model provides valuable insights on the effect of order, device dimensions, position and number of anchors on the quality factor (Q) of these resonators. Results from this model are in close agreement with the FEM simulations. The optimum position, dimension, and the number of anchors to deliver the highest Q are also discussed. Predictions of the model are validated through characterization of TPoS resonators fabricated to operate at a resonant frequency of around 1 GHz with different number of anchor pairs and position. A $23^{rd}$ order resonator with a W/L ratio of 1.5 with three pairs of anchors placed at positions suggested by the model gave a loaded Q of 4431, which is 65% more than the conventional case where anchors are uniformly placed towards the edge, and also an improvement in the insertion loss by 5.5 dB compared to the latter. For resonators where the anchor loss is predominant, our results show that the performance can be improved by placing the anchors at the optimum position, closer to the middle of the resonator than the conventional scheme where anchors are uniformly placed towards the edge. [ABSTRACT FROM AUTHOR]

This work reports on the measured performance of an Aluminum Scandium Nitride (AlScN) Two-Dimensional Resonant Rods resonator (2DRR), fabricated by using a Sc-doping concentration of 24%, characterized by a low off-resonance impedance ($\sim $ 25 $\Omega $) and exhibiting a record electromechanical coupling coefficient (${k}_{t}^{\textit {2}}$) of 23.9% for AlScN resonators. In order to achieve such performance, we identified and relied on optimized deposition and etching processes for highly-doped AlScN films, aiming at achieving high crystalline quality, low density of abnormally oriented grains in the 2DRR’s active region and sharp lateral sidewalls. Also, the 2DRR’s unit-cell has been acoustically engineered to maximize the piezo-generated mechanical energy within each rod and to ensure a low transduction of spurious modes around resonance. Due to its unprecedented ${k}_{t}^{\textit {2}}$ , the reported 2DRR opens exciting scenarios towards the development of next generation monolithic integrated radio-frequency (RF) filtering components. In fact, we show that $5^{th}$ -order 2DRR-based ladder filters with fractional bandwidths (BW) of $\sim $ 11%, insertion-loss (I.L) values of $\sim $ 2.5 dB and with $>$ 30 dB out-of-band rejections can now be envisioned, paving an unprecedented path towards the development of ultra-wide band (UWB) filters for next-generation Super-High-Frequency (SHF) radio front-ends. [2022-0024] [ABSTRACT FROM AUTHOR]

Aluminum nitride (AlN) has gained wide interest owing to its high values of elastic modulus, band gap, dielectric strength, resistivity, thermal conductivity and acoustic velocities, especially because it retains most of its properties and versatility in the thin-film form. This review focuses on applications where the CMOS integration of AlN MEMS has been effectively demonstrated. First, the fundamental concepts of piezoelectricity on polycrystalline $c$ -axis oriented thin-films are introduced and AlN is compared to other common piezoelectric materials, namely LiNbO3, LiTaO3, quartz, lead zirconate titanate (PZT), ZnO and GaN by thoroughly discussing the material properties, processing and technological implications. After presenting the possible MEMS-CMOS integration strategies, recent demonstrations of AlN-based devices are reviewed, namely energy harvesters, film bulk acoustic resonators (FBAR), contour mode resonators (CMR), gas sensors, imagers, microphones, transducers for chip-scale communication and calorimetric sensors. Finally, other recent applications/integration opportunities are outlined for AlN-based micro-mirrors, flexible sensors and transducers for liquid media and harsh environments. [2022-0006] [ABSTRACT FROM AUTHOR]

Doped AlN thin films, especially high Sc-ratio AlScN film, have been reported to significantly improve the piezoelectric properties and draw attention for high performance resonators, transducers and integrated ferroelectric applications. However, many demonstrated devices were limited by poor film stress control, abnormal oriented grains and lack of a good etching profile. Compared to costly single alloy target, co-sputtered AlScN films can benefit customized doping concentrations and provide a unique solution for high Sc-ratio AlScN film quality and device studies. In this work, the optimized co-sputtering deposition and ICP etching processes of 500 nm AlScN thin film were developed and released AlScN Lamb-wave resonators were demonstrated. The influence of stress control by changing N2 process gas on the crystalline orientation, abnormal orientation grains, film roughness and piezoelectric property of AlScN thin films were discussed in detail. The AlScN film with a high Sc content requires a lower deposition pressure to obtain good crystalline quality. Al0.85Sc0.15N thin films with FWHM of 1.75°, an average stress of −14.5 MPa and a stress range of 156 MPa were obtained. 130 nm/min etching rate and 77° sidewall profile were achieved by optimized ICP etching of Al0.78Sc0.22N film. Lamb-wave resonators were fabricated based on both Al0.78Sc0.22N and Al0.85Sc0.15N thin films, achieving a quality factor of over 1000 at resonant frequency of approximately 300 MHz. The electromechanical coupling coefficients were improved by 152% and 80% compared to pure AlN devices.[2021-0210] [ABSTRACT FROM AUTHOR]

In this paper, the design, modeling and characterization of width-extensional (WE) mode piezoelectric microelectromechanical systems (MEMS) resonators are presented. The resonators consist of piezoelectric stacked layers for transductions, highly doped single crystal silicon (HDS) device layer as vibrating structures, and oxide layers on top and bottom surfaces for passive temperature compensation. Dependencies of quality factors ($Q\text{s}$) of the WE mode resonators on structure designs, such as device length, thickness as well as crystal orientation of the HDS device layer are investigated. Effects of crystal orientation of the HDS device layer and oxide thickness on the temperature stability of resonant frequency of the WE mode resonators are also analyzed. Both aluminum nitride (AlN) and scandium-doped aluminum nitride (ScxAl1−xN, $x=9.5\%$) are adopted as the piezoelectric transduction layer in this work, in order to investigate the performance improvements by Sc doping. The designed WE mode MEMS resonators are fabricated based on a piezoelectric on cavity-silicon-on-insulator (CSOI) platform. A fabricated ScAlN-based MEMS resonator exhibits a $Q$ of 11086 and motional impedance as low as 28 $\Omega $ at its series resonant frequency of 25.044 MHz. [2021-0222] [ABSTRACT FROM AUTHOR]

This paper examines 1-port thickness-shear mode quartz resonators for use in RF wake-up receiver (WUR) front-ends within the very high frequency (VHF) range up to 200 MHz. Although RF voltage gain has been demonstrated in these devices only at frequencies ≤ 76 MHz to date, models and measurements in this work indicate that they provide high voltage gain and interference rejection over a broad frequency range. To assess performance, this work examines devices with varying frequency (50-200 MHz), geometry, and shunt capacitance, focusing on designs with moderate gain or higher (≥ 10 V/V) at load impedances around the input impedance of a near-zero-power envelope detector (around $R_{\mathrm {L}} =10\,\,\text{M}\Omega \vert \vert C_{\mathrm {L}} =120$ fF). An active probe presents these load impedances while precisely measuring voltage gain and quality factor ($Q_{\mathrm {m}}$). Overall, loaded gain, $Q_{\mathrm {m}}$ , and figure of merit (FoM) measurements validate model predictions. Furthermore, devices around 200 MHz demonstrate higher $Q_{\mathrm {m}}$ than previously predicted. Voltage gain 140 V/V is demonstrated at $f_{\mathrm {r}} \approx ~184.5$ MHz, the highest gain directly measured in a piezoelectric microresonator in this frequency range. Subject to post-fabrication variation, these results are repeatable within a commercial quartz process, enabling widespread utility of this technology. [2021-0133] [ABSTRACT FROM AUTHOR]

This work presents an in-depth study of simulation and measurement results of laterally excited shear bulk acoustic resonators (XBAR) in Lithium Niobate, at 5 GHz with high electromechanical coupling factor ($k_{t}^{2}$) as high as 25%, and with impedances at resonance close to $2~\Omega $. Loaded Quality factors of up to 340 and 150 are obtained at resonance and anti-resonance, respectively. Experimental00 dispersion behaviors of main mode and spurious are presented. Several geometric parameters affecting resonator performance are studied in order to improve figure of merits (FoM) of the device for 5G filter applications. The modified fabrication process presented shows a high yield of over 90% of devices which can be scalable for mass production of high frequency, sub-6 GHz, wide band filters. [2021-0202] [ABSTRACT FROM AUTHOR]

In this paper, a triple mass resonator (TMR) with a high capability to independently tune both resonant frequency and quality factor (Q-factor) is reported. An additive oscillating structure is designed into a conventional dual-mass resonator to control the modal shape. The theoretical studies revealed that the amplitude ratio between the outer masses, which mainly determine the Q-factor through the anchor loss, is sensitive to both suspension and inner springs because of mode coupling, while the resonant frequency is only sensitive to the suspension spring. Through this mechanism, the quality factor related to the anchor loss, $Q_{\mathrm {Anchor}}$ , can be adjusted by electrostatic tuning of the inner spring, while the resonant frequency keeps almost constant. On the other hand, the eigenfrequency could be significantly tuned by the electrostatic tuning of the suspension spring. The experimental results showed the Q-factor could be tuned as large as 19% by a DC bias of 15 V at the inner electrode, while the resonant frequency change was only as small as 162 ppm. In contrast, when the DC bias was applied to soften the suspension stiffness, the resonant frequency could be tuned as high as 7023 ppm with Q-factor change of 16%. [2021-0167] [ABSTRACT FROM AUTHOR]

The development of 5G has put forward a higher demand for filters, and the purpose of this work is fabricating surface acoustic wave (SAW) filters with excellent temperature stability and high power handling using LiTaO3/SiC bonded wafers. SAW resonators with different wavelengths are fabricated on LiTaO3/SiC, and LiTaO3/Si, bulk LiTaO3 as well. We evaluate these resonators, concluding that the bilayer substrates can enhance the performance. Subsequently, SAW filters are designed and fabricated. For the filters based on LiTaO3/SiC, the center frequency is around 2.62 GHz and the minimum insertion loss is 1.90 dB. The whole passband is flat and larger than 200 MHz, and the return loss is larger than 10 dB. SiC is more effective than Si in the enhancements of temperature stability and power handling. SiC reduces the temperature coefficient of the frequency at the left side of the passband to about half of that on LiTaO3/Si and one-sixth of that on bulk LiTaO3, and enlarges the peak power handling to 35.7 dBm. The time-to-failure of the filters on LiTaO3/SiC is 3.8 times as long as LiTaO3/Si and 11.3 times the level of bulk LiTaO3, when an input power of 30 dBm is applied ceaselessly. This work demonstrates the potential of SAW filters based on LiTaO3/SiC for RF filters in 5G. [2021-0233] [ABSTRACT FROM AUTHOR]

Clamping loss in micromechanical resonators can strongly depend on the boundary conditions far away from the actual vibrating structure because the acoustic wavelength greatly exceeds the device dimensions. We demonstrate a scheme for post-fabrication tuning of the clamping loss in flexural-mode and bulk-mode resonators by modifying the boundary conditions of the chip with the frame. The measured quality factor increases by more than an order-of-magnitude for the microcantilevers and more than a factor of three for the bulk-mode resonators when frame contact is minimized via suspension of the chip by wirebonds. We propose a two-degree-of-freedom fluctuation-dissipation model to describe the thermomechanical noise and forced response in the presence of this tunable anchor damping. By studying the thermomechanical displacement spectrum with tunable clamping loss, we show that variable clamping loss tunes the mechanical quality factor, modifying both the resonator transfer function and thermomechanical noise force. We delineate the dependence of the tunable clamping loss mechanism on microcantilever beam length and ambient temperature from 300 K down to 40 K, and observe potential temperature dependence to clamping loss with reducing temperature. [2021-0141] [ABSTRACT FROM AUTHOR]

This letter reports temperature compensated single crystal silicon (SCS) bulk-mode capacitive microelectromechanical system (MEMS) resonators with high temperature stability of less than ±20 ppm over industrial temperature range. Degenerate doping is adopted to change the temperature coefficient of frequency ($TCF$) and adjusting of crystal orientation is implemented to tune the turnover points for the MEMS resonators, thanks to the different temperature coefficient of elastic constants ($TCE$) of the resonators along different crystal orientation. The adjustment effects are verified by using finite element method (FEM) simulations as well as measurement results. Two bulk-modes, namely length-extensional (LE) and square-extensional (SE) mode resonators along various crystal orientations in a degenerate-doped (100) SCS wafer are designed and fabricated. The measurement frequency shifts are approximately ±20 ppm for the LE mode resonator and ±16 ppm for the SE mode resonator as the resonators are placed along 22.5° from the <110> direction over industrial temperature range of −40°C to 85 °C, respectively. It highlights an effective way to reduce the $TCF$ via degenerate doping and precisely adjust the turnover point by crystal orientation tuning for MEMS resonators. [2022-0046] [ABSTRACT FROM AUTHOR]

Suppression of spurious vibrations and accurate control of mechanical damping is critical in MEMS devices to ensure optimal performance, sensitivity and reliability. Conventional control schemes such as feedback-based mechanical vibration absorbers or viscous dampers are either complex to implement at the micro scale or result in a higher sensor noise floor due to thermomechanical processes. In this paper, we present a tunable electrical autoparametric vibration control scheme that is applied to a capacitive MEMS resonator. In the proposed method, energy is transferred from the primary mechanical resonating element to a secondary electrical resonator via parametric mode coupling, thereby enabling dramatic reduction in the amplitude of mechanical vibration without introducing significant linear damping. As the threshold for parametric coupling is determined by electrical resistance, the system can be tuned easily to adjust the magnitude, rate and bandwidth of vibration suppression in the mechanical structure. A 1D lumped parameter model is developed to numerically analyse the behavior of the autoparametric vibration control system followed by experiments performed using a micromachined resonator array to validate the proposed model. The proposed circuit-based scheme provides an easily integratable and tunable solution to control damping in MEMS sensors and actuators. [2021-0095] [ABSTRACT FROM AUTHOR]

This work reports a comparative study of acoustic loss between different Lamb waves at GHz in thin-film lithium niobate (LiNbO3). The propagation loss (PL) of higher-order Lamb waves in thin-film LiNbO3 is studied for the first time using acoustic delay line (ADL) testbeds. The acoustic wave attenuation and quality factors ($Q$) of different Lamb waves are extracted, showing higher $Q\text{s}$ for higher-order Lamb modes than fundamental modes in air and vacuum at room temperature. The extracted $Q\text{s}$ are higher than those reported in thin-film LiNbO3 resonators, implying the electrode and anchor-induced loss as the dominant loss factors. The ADL-based loss analysis framework is readily extendable to acoustic damping study in other microwave acoustic platforms. [2021-0165] [ABSTRACT FROM AUTHOR]

Magnetometers are ubiquitous in applications covering many aspects of modern life, such as navigation, smart devices, biomedical systems, geological surveying and aerospace. This work reports a resonant Lorentz-force magnetometer featuring structural topology, in which cavity slots are incorporated in the device structure to improve the thermoelastic dissipation. Such a device is capable of sensing a small Lorentz-force and can be used as a sensitive magnetic field sensor. The most prominent property of this subject is implementing sensing tasks under a large bias magnetic field. The proposed magnetometer achieves a quality factor of 43000 in a vacuum of 4 mPa, a Lorentz-force sensitivity of 0.2 Hz/nN, and a magnetic field change sensitivity of 3375 Hz/T. The topology of the double-ended tuning fork (DETF) using cavity slots in the tine beams resulted in a 5.9-fold enhancement of the Q-factor compared to a common DETF resonator with the same geometry. Experimental characterization of the proposed magnetometer confirms its feasibility and functionality. Experiments about an application scenario with a large pre-existing bias magnetic field were carried out. The prototype device demonstrated a magnetic field change sensitivity of 2585 Hz/T and a noise-limited resolution of 210 nT/ $\surd $ Hz, under a bias magnetic field of 0.13 T. [2021-0162] [ABSTRACT FROM AUTHOR]

In this paper, we propose a radially pleated disk resonator for gyroscopic application. The structure is made up of concentrically nested radially pleated rings that are interconnected by straight spokes, which has the potential to provide higher thermo-elastic quality factor and larger vibration amplitude. Decaying time constant of 31 s is experimentally obtained at resonant frequency of 3 kHz. A frequency-tuning model is proposed and verified, based on which, the frequency-tuning characters of the radially pleated disk resonator are thoroughly investigated. The Coriolis coupling factor $\kappa $ of the radially pleated disk resonator is precisely measured by constructing a concise quadrature frequency-modulation operation, which gives the values of $\kappa $ = 0.7687 and agrees well with the calculated value of 0.7768. This work provides a new strategy for designing high performance MEMS gyroscope structure, and gives a good template for gyroscopic resonator characterization. [2021-0096] [ABSTRACT FROM AUTHOR]

This work reports an experimental study of acoustic loss in thin-film lithium niobate (LiNbO3) using acoustic delay lines (ADLs). Unlike prior resonator-based quality factor (Q) studies, this approach directly extracts the damping in thin-film LiNbO3, avoiding the influence of other intricate loss mechanisms, e.g., anchor loss and electrode-induced loss. Acoustic attenuation of fundamental symmetric (S0) and shear horizontal (SH0) waves are studied in suspended LiNbO3 thin films of different thicknesses. The attenuation is significantly higher in thinner LiNbO3 films, suggesting the LiNbO3 crystal degradation during the microfabrication as the primary loss origin. Nevertheless, the extracted equivalent Q in thin-film LiNbO3 is still higher than reported values, suggesting that anchor design and electrode quality remain the bottlenecks for higher Q. The proposed loss extraction framework is readily extendable to other acoustic thin-film structures. [2021-0107] [ABSTRACT FROM AUTHOR]

We report on the first experimental characterization and analysis of the thermal response and temperature coefficient of resonance frequency (TC ƒ) of gallium nitride/aluminum nitride (GaN/AlN) heterostructure micro string resonators, in a wide temperature range from −10 °C up to 325 °C. Thanks to its excellent electrical and mechanical properties and chemical inertness, GaN has recently stimulated growing interests in GaN microelectromechanical systems (MEMS) for emerging high-power, high-temperature, and harsh-environment applications. GaN films on Si wafers often require AlN buffer layers, thus the residual tensile stress profile in the GaN epilayers and GaN/AlN hetero-layers can play a key role in affecting the MEMS specifications and performance. Here we design and fabricate GaN/AlN heterostructure micro string resonators with length L = 100, 200 and 300 μm to probe the stress and thermal effects on resonance behavior. All out-of-plane flexural modes show clear string behavior, and the multimode resonance frequencies downshift almost linearly with increasing temperature up to 325 °C. The linear temperature dependence and TC ƒ values of GaN/AlN heterostructure resonators can be directly employed for thermal sensing. Comparison among different devices indicates that higher tensile stress levels contribute to smaller TC ƒ values, suggesting strain engineering may be exploited for intentionally regulating the TC ƒ. [ABSTRACT FROM AUTHOR]

Silicon MEMS resonators are increasingly being adopted for applications in timing and frequency control, as well as precision sensing. It is well established that a key limitation to performance is associated with sensitivity to environmental variables such as temperature and pressure. As a result, technical approaches to address these factors such as vacuum sealing and ovenization of the resonators in a temperature controlled system have been introduced. However, residual sensitivity to such effects can still serve as a significant source of frequency fluctuations and drift in precision devices. This is experimentally demonstrated in this paper for a precision oven-controlled and vacuum-sealed silicon resonators. The frequency fluctuations of oscillators constructed using two separate nearly-identical co-located resonators on the same chip are analysed and differential frequency fluctuations are examined as a means of reducing the impact of common-mode effects such as temperature and pressure. For this configuration, our results show that the mismatch of temperature and pressure coefficients between the resonators ultimately limits the frequency stability. [2020-0395] [ABSTRACT FROM AUTHOR]

An increasing demand for surface acoustic wave filters with high performances contributes to the necessity of reducing sizes and costs. The present work proposes a multilayered structure of high-velocity layer/SiO2/interdigital transducers (IDTs)/piezoelectric-single-crystal substrate to further validate the strengths of packageless devices. Resonators based on the SiNx/SiO2/IDTs(Cu)/15°Y-X LiNbO3 structure are simulated and analyzed. One-port resonators are designed, fabricated and tested on the basis of the simulation, and results validates the feasibility of no encapsulation. Finally, by using this multilayered structure, a packageless filter with a center frequency of 2.14 GHz and an ultra-small size of 0.695 × 0.490 × 0.122 mm3 is successfully designed and realized, maintaining well-balanced performances. This work has demonstrated the strengths of packageless devices which can be obtained by utilizing the proposed multilayered structure, and may promote the development of device miniaturization. [2021–0035] [ABSTRACT FROM AUTHOR]

In this work, the effects of physical dimensions such as length, width and thickness as well as the mode of vibration and the number of anchors on the performance of longitudinal thin film piezoelectric on silicon (TPoS) MEMS resonators have been studied. TPoS resonators, designed for a resonant frequency of around 1 GHz, were fabricated with a 4 mask CMOS compatible process. A 225 μm wide resonator excited in its 23rd order had an unloaded quality factor of 9453 (in vacuum), which is the highest value reported so far for similar resonators, motional resistance of 107 Ω and linear thermal coefficient of frequency of −28.4 ppm. We have also studied and modeled the different loss mechanisms in these devices. The model matches well with measured results for resonators of different geometries, modes of vibrations and number of anchors. [2020-0397] [ABSTRACT FROM AUTHOR]

Recently we presented a parametric resonator which is constructed from a double-sided comb-drive transducer with an electrostatically floating rotor. That device had a natural frequency of ~2.3 kHz. In the present study we present a parametric resonator of the same type, but with a natural frequency of ~30 kHz, and a more compact design. The higher frequency is relevant for several applications, and the increased stiffness may contribute to enhancing fabrication yield. However, due to the more compact design, the electrostatic modulation of stiffness is less effective. Because of the drastic reduction of the ratio between modulated stiffness and average stiffness, the new resonator cannot be driven in high-order instability windows, without reverting to excessively high driving voltages. Since it could only be driven in the first instability window, the differential sensing signal is at the same frequency as the driving signal. This makes it difficult to distinguish between motional and feed-through currents. We demonstrate that the 3rd harmonic of the differential current is unaffected by feed-through and is therefore preferable for sensing the device response. We show that this higher harmonic component of current, is a unique characteristic of the resonator, and it is not due to a nonlinear mechanical effect, such as Duffing stiffening. [2021-0032] [ABSTRACT FROM AUTHOR]

This paper reports a frequency trimming method for a resonator used in a quad-mass gyroscopes (QMG) by focused ion beam (FIB) etching. The effects of the suspension stiffness, coupling stiffness and mass perturbations on the anti-phase mode resonant frequency were studied theoretically and numerically, proving the effectiveness and merit of the suspension stiffness trimming. The asymmetry caused by the suspension stiffness mismatch was further considered and its effects on frequency and Q-factor were investigated. Then the relationship between the trimming geometry of the suspension springs and the resonant frequency was studied by finite element analysis (FEA). The proposed trimming was demonstrated by using a QMG device fabricated by a standard SOI process. The resonant frequency mismatch was initially about 2810 ppm. After the proposed FIB trimming on the suspension springs, the frequency mismatch decreased to be as small as 3 ppm and the uncertainties do not exceed 75 ppm in the variation of ambient temperature between ±2 K. A slight change of Q-factor was observed in the trimmed axis from 100,000 to 90,000. [ABSTRACT FROM AUTHOR]

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 (kt2) 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] [ABSTRACT FROM AUTHOR]

A design of electric split-ring metamaterial (eSRM) with tunable terahertz (THz) free spectra range (FSR) characteristic is presented. The proposed eSRM is composed of double semicircle-shaped ring resonators with two connected metallic bars. By changing the radius of the semicircle-shaped ring resonators, the tuning ranges of FSR are 0.187 THz and 0.024 THz in transverse electric (TE) and transverse magnetic (TM) modes, respectively, which indicates eSRM could operate stably in TE and TM modes. Furthermore, eSRM exhibits switching and polarization-dependent characteristics by exploiting comb driver to control the distances between ring resonators along x- and y-axis directions. The electromagnetic responses could be tuned and switched between dual- and quad-resonance in both TE and TM modes. eSRM shows polarization angle-sensitive characteristic and the corresponding FSR could be tuned from 919 GHz to 336 GHz by changing the polarization angle of incident THz wave. In view of the above optical characteristics of eSRM, it is used for the sensing application exposed on different surrounding refraction index. The sensitivity, quality factor (Q-factor), and figure of merit (FOM) are calculated. The sensitivity is 0.457 THz/RIU and the average Q-factor and FOM are 40.65 and 35.47, respectively. These results indicate the proposed eSRM could be used in widespread applications, such as dual-/quad-resonance switches, polarization switches, and high-efficient environmental sensors. [2021-0006] [ABSTRACT FROM AUTHOR]

We present a study on the quantification of energy dissipation of Micro-Electro-Mechanical System (MEMS) resonators. Toroidal Ring Gyroscope (TRG) was used as a platform to conduct the study. The main energy dissipation mechanisms in TRG include viscous air damping, Thermo-Elastic Damping (TED), anchor loss, and surface loss. During our experimental study, these energy dissipation mechanisms were minimized and controlled by venting the encapsulation and actively pumping down to high vacuum, cooling the temperature down to around 123 K, adjusting modal balance by electrostatic tuning, and pre-baking the device at high temperature (425 °C), respectively. At room temperature, the quality factor related to viscous air damping was measured to be 625,000, TED to be 170,000, and anchor loss to be 1,350,000. Finite Element Analysis (FEA) was conducted to support these findings. Relation between the anchor loss and electrostatic tuning was also explored. The effects of moisture-related surface loss have also been demonstrated by monitoring characteristics over a 2-year period of time. High temperature bake-out was proven to be effective in removing the moisture and reducing the surface loss. This paper combines topics that are scattered in literature on identification of energy dissipation mechanisms in kilohertz-range silicon MEMS resonators and presents the topic as a single methodology illustrating how the contribution of each energy dissipation mechanism can be quantified independently. To the best of our knowledge, this study is the first to experimentally quantify all major energy dissipation mechanisms in a kilohertz-range silicon MEMS resonator. [2020-0294] [ABSTRACT FROM AUTHOR]

Thermal-actuation and piezoresistive-detection effects have been employed to pump the effective quality factor of MEMS resonators, targeting better mass sensing performance in air. In this paper, frequency resolution (bias instability) of a thermal-piezoresistive resonator operating in air at room temperature is experimentally investigated. It is found that the dynamic range decreases when increasing the bias direct current whereas the effective quality factor rises. The measurement results indicate a maximum effective quality factor of 169k with a dynamic range of 47.8 dB for a bias current of 6.25 mA, and a minimum effective quality factor of 11.3k with a dynamic range of 70.1 dB for a bias current is 5.8 mA. Our work also shows that the frequency and amplitude bias instabilities are significantly lower due to the dynamic range decrease for a high bias current. [2021-0250] [ABSTRACT FROM AUTHOR]

This paper presents a technique for suppressing specific resonance modes of micromechanical resonators by mechanical means. In particular, attaching multiple miniaturized beam structures with properly designed dimensions acting as anti-resonating tuned mass dampers (TMD’s) to a micromechanical clamped-clamped beam (CC-beam) resonator based on a CMOS-MEMS process platform successfully attenuate the dynamic frequency response of the fundamental mode while retaining the $2^{\mathrm {nd}}$ harmonic mode of the CC-beam. The measured results show that the frequency transmission response of the TMD-embedded CC beam drops as much as $\sim 12$ dB compared to that of a reference resonator. An analytical model is used to study the effects of the parameter variations of the TMD structures. This technique provides an alternative approach to mechanically suppressing vibration modes for the micromechanical resonators that cannot employ the conventional quarter-wavelength support technique. [2020-0289] [ABSTRACT FROM AUTHOR]

This paper reports a rate integrating gyroscope (RIG) using independently controlled clockwise (CW) and counter clockwise (CCW) modes on a single MEMS resonator. The rotation angle is read out by the phase difference between these modes. A CW/CCW mode separator was used to independently contol these modes superposed on the resonator. The frequency and Q-factor mismatches were compensated by the phases and amplitudes of driving signals. A control system including the mode separator, feedback controllers, signal generators, mismatch compensators were implemented in a field programmable gate array (FPGA). Using the proposed mismatch compensation technique, the equivallent aniso-damping term became 1/100. As a result, the scale factor became constant and the non-linearity became less than 0.2% even when the slow angular rate region around 5°/s. In addition, the temperature coefficient of scale factor as small as −1.84 ± 0.62 ppm/K was achieved without any temperature correction. [2020-0011] [ABSTRACT FROM AUTHOR]

This work reports a monolithic RF front-end module integrating bulk acoustic wave (BAW) filters, Lamb acoustic wave filters, and electronic RF silicon-on-insulator (RFSOI) switches to deliver single-chip multiband RF front-end module (RF-FEM) manufactured on commercial 200mm RF silicon-on-insulator (RFSOI) foundry technology. BAW and Lamb filters built in the same chip and within the same process enable multiband operation. Vertical System-on-Chip (SoC) integration of MEMS and RFSOI components contributes to footprint reduction up to 50%, compared to system-in-package (SiP) modules, and reduces the integration and design complexity of the modules. At its current state of development, this technology is suitable for diversity receive modules (DRX) for 4G/LTE and 5G bands. Extensive characterization results and case studies demonstrate the robustness of the integrated platform. Further productization of this technology will enable the next generation of hundred-filter 5G sub-6GHz RF-FEMs. [2020-0304] [ABSTRACT FROM AUTHOR]

Numerical modelling of MicroElectroMechanical Systems (MEMS) resonators is still attracting increasing interest from the sensors community especially when the nonlinear regime is activated. Here, the dynamic response of two different types of double-ended tuning fork MEMS resonators is studied both in the linear and nonlinear regimes. A one Degree Of Freedom (1 dof) model able to predict the frequency response of the device is proposed. Geometric and electrostatic nonlinearities are simulated through Finite Elements and Integral Equations, respectively. The total dissipation of the resonator is computed by taking into account both the thermoelastic and the nonlinear fluid contributions. Experimental measurements performed on resonators fabricated in polysilicon and single crystal silicon validate the proposed model showing a very good agreement with theoretical predictions. [2020-0240] [ABSTRACT FROM AUTHOR]

This paper investigates the impact of resonant frequency ($f_{\mathrm {s}}$) mismatch on the quality factor ($Q_{\mathrm {as}}$) of large arrays of X-cut Lithium Niobate (LN) Laterally Vibrating Resonators (LVRs) operating around 50 MHz and 400 MHz. The statistical distributions of key device parameters, including resonant frequency, quality factor ($Q_{\mathrm {s}}$), electromechanical coupling ($k_{\mathrm {t}}^{2}$), and static capacitance ($C_{0}$), are collected from replicas of identical resonators. A Monte Carlo approach is later implemented to simulate the impact of statistical variability on the quality factor of arrays of resonators. By including the effect of interconnects series resistance on the arrays, an excellent agreement between experimental and simulated $Q_{\mathrm {as}}$ is achieved, demonstrating that frequency mismatch is the major mechanism of quality factor degradation in arrays of parallel resonators. The experimental validation of the model confirms that it can be used as a predictive tool to establish the frequency tolerance requirements in order to attain high $Q_{\mathrm {as}}$ in arrays. [2019-0210] [ABSTRACT FROM AUTHOR]

Hexagonal 4H-silicon carbide (4H-SiC) is a transversely isotropic substrate garnering interest for precision MEMS devices such as resonant gyroscopes. This paper investigates the elastic anisotropy of 4H-SiC by utilizing capacitive bulk acoustic wave (BAW) resonators with ultra-high mechanical quality factors ($Q$) enabled by phononic crystals. We directly measure the value of $C_{66}$ using Lamé mode resonators for the first time and numerically fit the values of $C_{11}$ and $C_{12}$ using BAW elliptical modes in center-supported solid disk resonators. We compare (0 0 0 1) 4H-SiC to (1 1 1) Si, another in-plane isotropic material and validate (0 0 0 1) 4H-SiC’s superior robustness to fabrication and design variations. Measurement of in-plane BAW elliptical modes in multiple disk resonators with as-born frequency splits as low as 3 ppm reveal (0 0 0 1) 4H-SiC’s transverse isotropy across process corners. Lamé mode resonators display a temperature coefficient of frequency (TCF) three times lower compared to its Si counterpart. Finally, this paper provides a modified set of elastic constants for 4H-SiC with a view towards monocrystalline SiC MEMS devices. [2020-0254] [ABSTRACT FROM AUTHOR]

This article reports Lithium Niobate (LN) based shear horizontal (SH0) resonators utilizing suspended and solidly mounted structures for radio frequency (RF) applications. The solidly mounted SH0 structure (also termed guided SH0 structure) is advantageous in obtaining reduced temperature coefficient of frequency (TCF), reduced high frequency overtone spurious responses, and improved power handling. The demonstrated solidly mounted guided SH0 resonators exhibit a mechanical Q as high as 1316 around 950 MHz, and electromechanical coupling factor (${k} _{\mathrm {t}}^{\mathrm {2}}$) of around 21.8 % - resulting in a figure of merit ($\mathrm {FoM}=k_{\mathrm {t}}^{\mathrm {2}}\cdot \mathrm {Q}$) of 288. [ABSTRACT FROM AUTHOR]

This paper presents a 1-port thickness-shear (TS) mode quartz resonator with very high voltage gain, suitable for use in “near-zero-power” radios. These devices offer voltage gains >100x (referenced to $50~\Omega $) at ~50 MHz and >300x at ~75 MHz with ~60 fF load, which is the highest voltage gain directly measured to date in a piezoelectric micromechanical resonator. Six resonators with resonance ${f}_{{\text {r}}}\approx 50$ MHz and identical electrode dimensions are characterized by S11 measurements and by directly measuring the voltage gain using a high-impedance active probe. Quality factor (${Q}$) up to 140,000 at ~50 MHz and electromechanical coupling (${k}_{{\text {eff}}}^{2}$) up to 0.445% are demonstrated. In several devices, voltage gain around or above 100 V/V is observed for load ${C}~\approx ~60$ fF, and the gain changes as expected with decreasing load impedance. Additionally, a similar 1-port quartz resonator design at ${f}_{{\text {s}}}~\approx ~75$ MHz is demonstrated to have voltage gain over 300 V/V (referenced to $50\Omega $) and ${Q}$ around 335,000. These quartz resonators are fabricated in a mature, commercially available process, promoting widespread integration into radio-frequency (RF) wake-up receivers (WURs). [2020-0196] [ABSTRACT FROM AUTHOR]

We study thermal-piezoresistive cooling in silicon micromechanical resonators at large currents and high temperatures. Crossing a thermal transition region corresponds to a steep reduction in resonance frequency, an abrupt plateauing in the effective quality factor, and a large increase in thermomechanical fluctuations. Comparing measurements with simulations suggests that the second-order temperature coefficients of elasticity of doped silicon are not sufficient to capture the drop in resonance frequency at large currents. Overall, our results show that there are clear thermal limits to cooling a resonant mode using current-controlled thermal-piezoresistive feedback in silicon. [2020-0205] [ABSTRACT FROM AUTHOR]

We report on a non-destructive, on-chip technique for determining the elastic modulus ($E_{\mathrm {Y}}$) of silicon carbide (SiC) thin diaphragms by measuring their nonlinear resonances. Departing from the conventional static load-deflection techniques (e.g., beam bending, membrane bulging and nanoindentation), the nonlinear resonance approach enables characterizing mechanical properties without risk to the microdevices, bypassing complicated contact-mode sample preparation, and bulky, expensive apparatus. We derive the mode-dependent Duffing resonances of the diaphragms in the ‘membrane’ regime, and correlates $E_{\mathrm {Y}}$ with the Duffing ‘backbone’ curve. To verify our model, we fabricate SiC square diaphragms (1mm $\times 1$ mm $\times 2~\mu \text{m}$) that exhibit multimode resonances up to 500kHz and quality ($Q$) factors up to 16,000. Taking the device’s (2,2) mode as an example, we obtain $E_{\mathrm {Y}} = 436\,\,\pm \,\,27$ GPa via its Duffing nonlinear response. The technique can be readily and widely extended to other thin films and MEMS/NEMS resonators. [2020-0209] [ABSTRACT FROM AUTHOR]

An overview of microelectromechanical systems (MEMS) resonators for frequency reference and timing applications is presented. The progress made in the past few decades in design, modeling, fabrication and packaging of MEMS resonators is summarized. In particular, the state-of-the-art technologies for improving the overall performance of MEMS resonators, such as quality factor ($Q$), motional impedance, temperature sensitivity, and initial frequency uniformity, are reviewed in detail. The challenges and opportunities during the commercialization of MEMS resonators are also stated, and future development trends driven either by technology or market are outlined. This paper intends to provide an outlook for possible research directions of MEMS resonators in frequency reference and timing applications. With outstanding reliability, unique multi-frequency functionality on a single chip, and high accuracy, MEMS resonators show great potential for replacing the quartz crystal resonators which have been dominating the timing market since 1920s. [2020-0106] [ABSTRACT FROM AUTHOR]

In this work, we study experimentally, analytically, and numerically the dynamics of electrothermally actuated micro-electro-mechanical V-shaped micro-beam resonators. Upon actuating the micro-beams electrothermally, we examine the various scenarios of natural frequencies crossing and avoided-crossing (veering) among the various vibration modes. The effect of various parameters is experimentally investigated on the veering and crossing including the micro-beam width and the initial rise at the mid-point. An analytical study is presented based on a reduced order model of a nonlinear Euler–Bernoulli shallow arch beam model. The dynamic behavior is also confirmed through a multi-physics finite-element model (FEM). The static and dynamic behaviors of the V-shaped micro-beams are compared against those of Arc-shaped micro-beams. [2020-0232] [ABSTRACT FROM AUTHOR]

Phononic crystals/metamaterials are attracting increasing interest because of their large variety of applications at both the macro and micro scales. In this work, a new metamaterial plate (metaplate) composed of innovative phononic crystal unit-cells is presented, numerically studied, fabricated at the microscale and experimentally tested. Numerical simulations and experimental tests demonstrate a complete 3D phononic bandgap that guarantees a complete vibration isolation in a certain range of frequency. Moreover, its compatibility with Micro Electro Mechanical Systems (MEMS) fabrication processes suggests applications for vibration isolation of MEMS resonant devices. The measured transmission diagram shows a -30 dB attenuation level, which is in good agreement with numerical predictions. The proposed design opens up new perspectives for the development of vibration isolation applications for MEMS resonators. [2020-0069] [ABSTRACT FROM AUTHOR]

We report the implementation of a passive temperature compensation technique in thermally actuated, silicon-based, resonant cantilever gas sensors vibrating in their fundamental in-plane resonant mode. The temperature compensation technique utilizes oxide-filled trenches along the edges of the cantilever structure and adds a single additional mask to the overall fabrication process. The trench width for effective temperature compensation was optimized using finite element simulation. The fabricated resonators exhibit a temperature coefficient of frequency (TCF) as low as 1.7 ppm/°C, which represents a 15x improvement compared to the same resonators without oxide trenches. Quality factors of devices with oxide compensation are similar to those measured in non-compensated counterparts. The temperature compensation technique addresses a key limitation of silicon-based, mass-sensitive chemical microsensors, namely their temperature instability due to inherent temperature-dependent material properties. Compared to our previous work, the improved frequency and baseline stability yields an almost order-of-magnitude improvement in the extrapolated limit of detection, approaching 100 ppb for toluene. [2020-0154] [ABSTRACT FROM AUTHOR]

We present analytic and experimental evidence highlighting the importance of acoustic impedance matching for efficient power transfer in RF-MEMS composite resonators such as high-overtone bulk acoustic mode resonators (HBARs) and thin-film piezoelectric on substrate (TPoS) resonators. We show that materials used for the piezoelectric film and the bottom metal electrode in a composite resonator can be chosen or tailored for specific low-loss substrates, resulting in efficient acoustic power transmission across the interfaces of the acoustic source (piezoelectric transducer), intermediate layers including the bottom electrode, and into the acoustic cavity (substrate). We find that a composite resonator with good interfacial acoustic matching exhibits characteristic free spectral range (FSR) variations that are not well modeled in the literature, clearly differentiating it from resonators with poor acoustic matching. We verify this model by comparing the FSR spectra of the first experimentally demonstrated epitaxially grown Sc0.18Al0.82N/AlN/TaN/SiC HBARs (with a mismatched TaN bottom electrode) with epitaxial GaN/AlN/NbN/SiC HBARs where all constituent layers are acoustically matched to the substrate. Historically, the choice and quality of materials used for composite resonators has been limited by process constraints, but advances in epitaxial growth and heterogeneous integration techniques allow us to integrate multiple high quality, acoustically matched layers to form multi-functional composite resonators. [2020-0247] [ABSTRACT FROM AUTHOR]

We present a Lamé mode resonator whose limiting damping mechanism depends on its anchor geometry. The device is anchor-limited when the anchors are stiffer and is Akhiezer-limited with more compliant anchors. This result is determined by observing the temperature dependence of the quality factor (Q) for devices with different lateral dimensions and different anchor designs. The total measured Q increases by an order of magnitude with the more compliant anchors and reaches a room temperature fxQ product of $\boldsymbol {2.2\times 10^{13}}$. We studied the relationship between the device design and the anchor design and the measured Q(T) results to identify the contributions from different dissipation mechanisms. This investigation provides insight into how anchor design affects anchor damping. [2020-0195] [ABSTRACT FROM AUTHOR]

Functionalized gravimetric sensors are a promising path to small, versatile, real-time vapor sensors for volatile organic compounds. Many of these compounds can be dangerous to human health, but their nonreactive nature makes them notoriously difficult to sense. Unlike bulk acoustic resonators, chemiresistive devices have been investigated extensively and many researchers have used innovative synthesis strategies to functionalize these devices. In this work, we demonstrate how modifying a particular sensitizer for use with a bulk acoustic resonator significantly improves the sensitivity of the device (5 ppm vs. 1.11 ppm). Additionally, readout circuitry is described to avoid some problems that typically plague gravimetric sensors while simplifying the overall system. These strategies create a playbook for simple, fast, and sensitive systems for sensing volatile organic compounds, while also demonstrating the lowest limit of detection for cyclohexanone outside of gas chromatography/mass spectrometery in the literature. [2020-0053] [ABSTRACT FROM AUTHOR]

In this paper, the near-carrier enhancement of phase-noise (PN) at turnover temperature (${T} _{to}$) in a quasi-thickness-Lamé (QTL) mode thin-film piezoelectric-on-silicon (TPoS) oscillator is reported for the first time. QTL-TPoS resonators fabricated on degenerately-doped n-type silicon offer a ${T} _{to}$ greater than 80°C and are suitable for implementation of highly-stable ovenized oscillators. In this work, a ~123MHz QTL-TPoS resonator is heated up to ${T} _{to}$ (~90°C) by injecting current through the silicon body of the resonator. It is experimentally observed that at turnover temperature, the phase-noise slope close to the carrier frequency ($\Delta $ f < 100Hz) decreases substantially in contrast to the expected trends. A ~10dB improvement in phase noise at 10Hz offset and a ~25dB improvement at 1Hz offset is recorded when the oscillator is operating at ${T} _{to}$ compared to room temperature. [2020-0206] [ABSTRACT FROM AUTHOR]