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This paper presents a comprehensive guide to co-design piezoelectric RF-micro-electromechanical system (MEMS) resonators and CMOS for enabling voltage-controlled MEMS oscillators (VCMOs) that harness the best benefits out of both platforms. The analysis, focusing on understanding different tradeoffs among the tuning range, power consumption, gain, and phase noise, is generic to any kind of piezoelectric resonators and specific for Colpitts VCMOs. As a result of this paper, the first VCMO based on the heterogeneous integration of a high-Q lithium niobate (LiNbO3) micromechanical resonator and CMOS has been demonstrated. A LiNbO3 resonator array with a series resonance at 171.1 MHz, a Q of 410, and an electromechanically coupling factor of 12.7% is adopted, while the TSMC 65-nm RF LP CMOS technology is used to implement the feedback and tuning circuitry with an active area of $220\times 70\,\,\mu \text{m}^{2}$. The frequency tuning of the VCMO is achieved by programming a binary weighted digital capacitor bank and a varactor that are both connected in series to the resonator. The best measured phase noise performance of the VCMO is −72 and −153 dBc/Hz at 1 kHz and 10-MHz offsets from 178.23- and 175.83-MHz carriers, respectively. The VCMO consumes a dc current of $60~\mu \text{A}$ from a 1.2-V supply while realizing a tuning range of 2.4 MHz (~1.4% fractional tuning range). Such VCMOs can be applied to enable ultralow power, low phase noise, and wideband RF signal synthesis for emerging applications in Internet of Things. [ABSTRACT FROM AUTHOR]

This paper reports on the demonstration of wideband spurious-free radio frequency micromechanical systems filters based on one-port shear horizontal (SH0) mode lithium niobate (LiNbO3) laterally vibrating resonators (LVRs). The fabricated filter has been demonstrated with an unprecedented spectral range of spurious-free response (290% of the center frequency), a low insertion loss of 2.1 dB, an excellent 30-dB shape factor of 1.48, and a 3-dB fractional bandwidth of 8.9%. The spurious free response is achieved by optimizing the building blocks of the filters, namely the LVRs, and employing a spurious mitigation design based on arraying single resonators with only two electrodes. The low loss and excellent shape factor are enabled by the demonstrated high electromechanical coupling ( \mathit kt^{2}=16.1 %), high quality factor ( Q =1093 ), and high figure of merit (FoM = 154) of the fabricated resonator arrays. [2016-0250] [ABSTRACT FROM PUBLISHER]

This work studies the influence of substrate loss on the performance of acoustic resonators and on-chip inductors and investigates the effective substrate resistivity of seven commonly used substrates in silicon-based devices. The substrates include X-cut lithium niobate (LiNbO3) film with two different thicknesses (400 nm and 1.6 $\mu \text{m}$) on high-resistivity Si (HR-Si) and amorphous Si wafers, SiO2 film with two different thicknesses on HR-Si, and bare HR-Si. The effective resistivities of these substrates are extracted using coplanar waveguides (CPWs) over a frequency range from 1 to 40 GHz. Using the effective resistivity approach, the efficiency of two substrate loss reduction techniques—Si wafer removal and amorphous Si—in reducing substrate loss is quantified. Comparison of the extracted substrate resistivities of the suspended and un-suspended dielectric-on-Si structures and comparison of LiNbO3 on HR-Si and amorphous Si are carried out. Substrate loss reduction techniques are more advantageous for a thinner dielectric film and at a lower frequency range due to the higher filling factor of the electric field in the Si wafer. Finally, by comparison of the effective substrate resistivity of SiO2 film on an HR-Si with bare HR-Si, thick plasma-enhanced chemical vapor deposition (PECVD) SiO2 film is found to be a good insulation layer to reduce substrate loss. [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]

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]

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 A3 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. [ABSTRACT FROM AUTHOR]

This article presents the design approach and the first demonstration of a wideband hybrid monolithic acoustic filter in the K-band, which exceeds the limitation of electromechanical coupling on the fractional bandwidth (FBW) of acoustic filters. The hybrid filter utilizes the codesign of electromagnetic (EM) and acoustic to attain wide bandwidth while keeping the advantages of small sizes and high Q in the acoustic domain. The performance trade space and design flow of the hybrid filter are also presented in this article, which allows this technology to be applied for filters with different center frequencies and FBWs. The hybrid filter is simulated by hybridizing the EM and acoustic finite element analysis, which are carried out separately and combined at a system level. The fabricated filter built with resonators having an electromechanical coupling of 0.7% based on the seventh-order antisymmetric Lamb wave mode (A7) has a 3-dB FBW of 2.4% at 19 GHz and a compact footprint of 1.4 mm2. [ABSTRACT FROM AUTHOR]

This work presents a class of micro-electro-mechanical system (MEMS)-driven radio frequency filters in the X-band. The X-band center frequencies are achieved by resorting to the third-order antisymmetric Lamb wave mode (A3) in a 650-nm-thick Z-cut lithium niobate thin film. A novel bandwidth (BW) widening technique based on using the self-inductance of the top interdigital transducers and bus lines is proposed to overcome the limitations set by the electromechanical coupling (${k} _{t}^{2}$) and satisfy the demands in miniaturization and wide BW. Four different designs of the filters are designed and fabricated to show the trade-off among BW, insertions loss (IL), out-of-band rejections, and footprint. Due to the spurious-free and high- ${Q}$ performance of the A3 lithium niobate resonators, the fabricated A3 lithium niobate filters have demonstrated small in-band ripples and sharp roll-offs. One of these fabricated has demonstrated a 3-dB BW of 190 MHz, an IL of 1.5 dB, and a compact footprint of 0.56 mm2. Another design is fabricated to demonstrate a 3-dB BW of 170 MHz, an IL of 2.5 dB, an out-of-band rejection of 28 dB, and a compact footprint of 1 mm2. [ABSTRACT FROM AUTHOR]

This paper reports on the analysis and removal of transverse spurious modes in shear horizontal (SH0) mode lithium niobate (LiNbO3) laterally vibrating resonators (LVRs). A length-controlled electrode configuration that employs an optimized overlapping length between the adjacent electrodes has been developed to subdue the spurious modes. The technique modifies the stress and electric field distribution in the resonator body, consequently enabling near zero electromechanical coupling ( kt^{2} ) for the higher order transverse spurious modes. The length-controlled electrode configuration also reduces the loss at the interface between the metal electrodes and the piezoelectric layer, demonstrating a more than 2\times enhancement in quality factors {Q} . The fabricated LiNbO3 LVR with the length-controlled electrode configuration has shown an unprecedented spectral range (100–200 MHz) of spurious-free response, a high \mathit {k}_{t}^{2}$ value of 20.1%, a {Q} value of 1150, and, thereby, a remarkably high figure of merit of 230. [ABSTRACT FROM AUTHOR]

This work presents an improved design that exploits dispersion matching to suppress the spurious modes in the lithium niobate first-order antisymmetric (A1) Lamb wave mode resonators. The dispersion matching in this work is achieved by micro-machining the lithium niobate thin film to balance the electrical and mechanical loadings of electrodes. In this article, the dispersion matchings of the A1 mode in lithium niobate based on different metals are analytically modeled and validated with finite-element analysis. The fabricated devices exhibit spurious-free responses with a quality factor of 692 and an electromechanical coupling coefficient of 28%. The demonstrated method herein could overcome a significant hurdle that is currently impeding the commercialization of A1 devices. [ABSTRACT FROM AUTHOR]

In this article, we explore how acoustic wave filters starting with shunt resonator require particular reflection phase conditions to ensure that the synthesized filter is feasible. The position of transmission zeros (TZs) along with the phase of the objective filter function might lead to nonfeasible solutions where the first and last resonators require elements with positive reactance slope in the static branch, or equivalently, a nonphysical negative static capacitor. Since the reflection phase of a duplexer-oriented filter is fixed to reduce loading effects, the feasibility problem is solved by bringing the resonance frequency of the first resonator beyond the central frequency of the counter band. However, this entails surpassing the limits of the electromechanical coupling coefficient. We demonstrate how two reactive elements at the input overcome this resonance position limitation and provide a simple rule to decide the right topology. The position of the first TZ of duplexers will play an important role. Moreover, we provide rules on the reflection phase values that ensure that all resonators have a capacitive static branch. [ABSTRACT FROM AUTHOR]

This work presents a new class of microelectromechanical system (MEMS) resonator toward 60 GHz for the fifth-generation (5G) wireless communications. The wide range of the operating frequencies is achieved by resorting to different orders of the antisymmetric Lamb wave modes in a 400-nm-thick Z-cut lithium niobate thin film. The resonance of 55 GHz demonstrated in this work marks the highest operating frequency for piezoelectric electromechanical devices. The fabricated device shows an extracted mechanical $Q$ of 340 and an $f\times Q$ product of $1.87\times 10^{13}$ in a footprint of $2 \times 10^{-3}$ mm2. The performance has shown the strong potential of LiNbO3 antisymmetric mode devices for front-end applications in 5G high-band. [ABSTRACT FROM AUTHOR]

In this work, we present a new paradigm for enabling gigahertz higher-order Lamb wave acoustic devices using complementarily oriented piezoelectric (COP) thin films. Acoustic characteristics are first theoretically explored with COP lithium niobate (LiNbO3) thin films, showing their excellent frequency scalability, low loss, and high electromechanical coupling ($k^{2}$). Acoustic resonators and delay lines are then designed and implemented, targeting efficient excitation of higher-order Lamb waves with record-breaking low loss. The fabricated resonator shows a $2^{\mathbf {nd}}$ -order symmetric (S2) resonance at 3.05 GHz with a high quality factor ($Q$) of 657, and a large $k^{2}$ of 21.5% and a $6^{\mathbf {th}}$ -order symmetric (S6) resonance at 9.05 GHz with a high $Q$ of 636 and a $k^{2}$ of 3.71%, both among the highest demonstrated for higher-order Lamb wave devices. The delay lines show an average insertion loss (IL) of 7.5 dB and the lowest reported propagation loss of 0.014 dB/ $\mu \text{m}$ at 4.4 GHz for S2. Notable acoustic passbands up to 15.1 GHz are identified. Upon further optimizations, the proposed COP platform can lead to gigahertz low-loss wideband acoustic components. [2020-0127] [ABSTRACT FROM AUTHOR]

In this work, we present first-order antisymmetric (A1) mode resonators in 128° Y-cut lithium niobate (LiNbO3) thin films with electromechanical coupling coefficients ($k^{2}$) as large as 46.4%, exceeding the state-of-the-art. The achievable $k^{2}$ of A1 in LiNbO3 substrates of different orientations is first explored, showing X-axis direction in 128° Y-cut LiNbO3 among the optimal combinations. Subsequently, A1 resonators with spurious mode mitigation are designed and fabricated. In addition to the large $k^{2}$ , the implemented devices show a maximum quality factor ($Q$) of 598 at 3.2 GHz. Upon further optimization, the reported platform can potentially deliver a wideband acoustic-only filtering solution in 5G New Radio. [2020–0003] [ABSTRACT FROM AUTHOR]

This work presents an extensive study on the temperature coefficient of frequency (TCF) for thin-film lithium niobate (LiNbO3) resonators. To capture the temperature-frequency behavior for each vibration mode, a one-dimensional (1D) multi-section TCF model is proposed and verified by both the finite-element method (FEM) and experiments. By partitioning the metallized and non-metallized sections of the interdigitated transducers (IDT) as distinct TCF blocks and properly considering the spatial energy distribution, the TCF of each mode can be accurately predicted. Two higher lateral-order resonators based on the fundamental shear-horizontal mode (SH0) are designed and fabricated on a LiNbO3-on-SiO2 wafer to provide odd- and even-order harmonics. TCFs of the first 6 overtones (from 84 to 515 MHz) are calculated using the material properties given in the literature, exhibiting a maximum TCF difference of 3 ppm/K between the theoretical prediction and measurement results. Although only SH0 resonators are selected for experimental verification, the proposed multi-section TCF model can be applied to other plate wave MEMS resonators. [2019-0112] [ABSTRACT FROM AUTHOR]

Recent advancements in the field of piezoelectric micro-resonators have produced devices, such as lithium niobate laterally vibrating resonators, with very high electromechanical coupling factors ($k_{t}^{2}$) and respectable quality factors (${Q}$). As a result, the records of the figure of merit (FoM) for radio-frequency MEMS resonators have been broken several times in the past five years. As exciting as these high FoMs are, they impose unique caveats in accurately extracting the electromechanical coupling often due to the presence of spurious modes. It is a less noted issue for micro-resonators with moderate ${k} _{t}^{2}$ , as spurious modes either are absent or do not significantly affect the common extraction technique based on identifying resonances and anti-resonances. This paper will first theoretically analyze how disregarding spurious modes can potentially lead to inaccurate extraction of ${k} _{t}^{2}$ of the intended mode and then offer a framework that accounts for spurious modes and accurately extracts electromechanical coupling using a multi-resonance recursive fitting. [2018-0209] [ABSTRACT FROM AUTHOR]

This paper presents the first demonstration of nanowatt-level CMOS wakeup receiver (WuRx) front ends (FEs) that utilize microelectromechanical system (MEMS)-based matching networks (MNs). The first FE uses a fully MEMS-based MN (MMN) that operates at 88.8 MHz. It consists of a large array of lithium niobate resonators that are fabricated on the same die. Measurements of the integrated WuRx FE with the MMN indicate that its loaded voltage gain achieves a bandwidth of 0.78 MHz with a quality factor of 114. The second FE uses a hybrid MN (HMN) that operates at 457 MHz. It consists of an aluminum nitride laterally vibrating resonator as well as eight discrete inductors and capacitors surrounding the MEMS device. Measurements of the integrated WuRx FE with the HMN indicate that it achieves a sensitivity of −54 dBm with 7 nW of average dc power consumption without a digital correlator. [ABSTRACT FROM AUTHOR]

This paper demonstrates a passive low-insertion-loss (IL) RF filter with periodic passbands and capable of sparsifying the spectrum from 238 to 526 MHz for sparse Fourier transform (SFT)-based spectrum sensing. The demonstrated periodic filter employs LiNbO3 lateral overtone bulk acoustic resonators (LOBARs) with high-quality factors (Qs), large electromechanical coupling ($k_{t}^{2}$), and multiple equally spaced resonances in a ladder topology. To demonstrate the periodic filter, the LOBARs is first modeled to predict $k_{t}^{2}$ of various tones accurately. The fabricated LOBARs show $k_{t}^{2}$ larger than 1.5% and figure of merits ($Q\cdot k_{t}^{2}$) more than 30 for over 10 tones simultaneously, which agree with our modeled response, and are both among the highest demonstrated in overmoded resonators. The multi-band filter centered at 370 MHz has then been obtained with a passband span of 291 MHz, a spectral spacing of 22 MHz, an IL of 2 dB, FBWs around 0.6%, and a sparsification ratio between 7 and 15. An out-of-band rejection around 25 dB has also been achieved for more than 14 bands. The great performance demonstrated by the RF filter with 14 useable periodic passbands will serve to enable future SFT-based spectrum sensing. [2018-0155] [ABSTRACT FROM AUTHOR]

This paper presents a new class of micro-electro-mechanical system (MEMS) C-band filters for 5G front-ends. The filter is comprised of resonators based on the first-order asymmetric Lamb wave (A1) mode in thin-film lithium niobate. Two filters have been demonstrated at 4.5 GHz with sharp roll-off, flat in-band group delay, and spurious-free response over a wide frequency range. The first design shows a 3-dB fractional bandwidth (FBW) of 10%, an insertion loss (IL) of 1.7 dB, an out-of-band (OoB) rejection of −13 dB, and a compact footprint of 0.36 mm2, while the second design shows a 3-dB FBW of 8.5%, an IL of 2.7 dB, an OoB rejection of −25 dB, and a footprint of 0.9 mm2. The demonstrations herein mark the largest FBW achieved for acoustic-only filters at 5G frequencies. [2019-0083] [ABSTRACT FROM AUTHOR]

This paper reports the first demonstration of phononic crystals (PnCs) in suspended lithium niobate thin films, which exhibit band gaps for tailoring the performance of laterally vibrating devices. Transmission and reflection properties of lithium niobate PnCs for both shear-horizontal (SH0) and length-extensional (S0) modes have been investigated and subsequently explored in two applications. In the first case, PnC-embedded delay lines were designed for filtering with stopbands, while in the second case, PnC-bounded resonators were implemented for spurious mode suppression. Equivalent circuit models incorporating acoustic scattering parameters of the designed PnCs and Mason’s model of the transducers have been built for each application. Benchmarked to reference devices without PnCs, the measured PnCs embedded in delay lines show 20-dB attenuation in the stopbands and less than 2-dB loss in the passbands for the SH0 mode, and 30-dB attenuation in the stopbands and less than 10-dB loss in the passbands for the S0 mode. The fabricated piezoelectric PnC-bounded resonator has shown a quality factor of 434 at 142.7 MHz with undesired spurious modes significantly suppressed. These demonstrations show that lithium niobate PnCs for laterally vibrating devices can potentially lead to wideband and low-loss acoustic functions for radio frequency signal processing. [ABSTRACT FROM AUTHOR]