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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 article presents an 8.6-GHz oscillator utilizing the third-order antisymmetric overtone (A3) in a lithium niobate (LiNbO3) radio frequency microelectromechanical systems (RF-MEMS) resonator. The oscillator consists of an acoustic resonator in a closed loop with cascaded RF tuned amplifiers (TAs) built on Taiwan Semiconductor Manufacturing Company (TSMC) RF general purpose (GP) 65-nm complementary metal-oxide semiconductor (CMOS). The TAs bandpass response, set by on-chip inductors, satisfies Barkhausen’s oscillation conditions for A3 while suppressing the fundamental and higher order resonances. Two circuit variations are implemented. The first is an 8.6-GHz standalone oscillator with a source-follower buffer for direct 50-Ω-based measurements. The second is an oscillator-divider chain using an on-chip three-stage divide-by-two frequency divider for a ~1.1-GHz output. The standalone oscillator achieves a measured phase noise of −56, −113, and −135 dBc/Hz at 1 kHz, 100 kHz, and 1 MHz offsets from an 8.6-GHz output while consuming 10.2 mW of dc power. The oscillator also attains a figure-of-merit of 201.6 dB at 100-kHz offset, surpassing the state-of-the-art (SoA) oscillators-based electromagnetic (EM) and RF-MEMS. The oscillator-divider chain produces a phase noise of −69.4 and −147 dBc/Hz at 1 kHz and 1 MHz offsets from a 1075-MHz output while consuming 12 mW of dc power. Its phase noise performance also surpasses the SoA L-band phase-locked loops (PLLs). With further optimization, this work can enable low-power multistandard wireless transceivers featuring high speed, high sensitivity, and high selectivity in small-form factors. [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]

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]

In this work, we present gigahertz low-loss unidirectional acoustic focusing transducers in thin-film lithium niobate. The design follows the anisotropy of fundamental symmetric (S0) waves in X-cut lithium niobate. The implemented acoustic delay line testbed consisting of a pair of the proposed transducers shows a low insertion loss of 4.2 dB and a wide fractional bandwidth of 7.5% at 1 GHz. The extracted transducer loss is 1.46 dB, and the propagation loss of the S0 waves is 0.0126 dB/ $\mu \text{m}$. The design framework is readily extendable to other acoustic modes, given consideration on the optimal orientation for power flow and electromechanical transduction. [ABSTRACT FROM AUTHOR]

This article presents a comprehensive guide to codesign lithium niobate (LiNbO3) lateral overtone bulk acoustic resonators (LOBARs) and voltage-controlled oscillators (VCOs) using discrete components on a printed circuit board (PCB). The analysis focuses on understanding the oscillator-level tradeoffs between the number of locked tones, frequency stability, tuning range, power consumption, and phase noise. Moreover, this article focuses on understanding the relationship between the abovementioned specifications and the different LOBAR parameters, such as electromechanical coupling (${k}_{t}^{{2}}$), quality factor (${Q}$), transducer design, and the resonator size. As a result of this study, the first voltage-controlled MEMS oscillator (VCMO) based on LiNbO3 LOBAR is demonstrated. Our LOBAR excites over 30 resonant modes in the range of 100–800 MHz with a frequency spacing of 20 MHz. The VCMO consists of an LOBAR in a closed loop with two amplification stages and a varactor-embedded tunable LC tank. By adjusting the bias voltage applied to the varactor, the tank can be tuned to change the closed-loop gain and phase responses of the oscillator so that the Barkhausen conditions are satisfied for a particular resonant mode. The tank is designed to allow the proposed VCMO to lock to any of the ten overtones ranging from 300 to 500 MHz. These ten tones are characterized by average ${Q}\text{s}$ of 2100, ${k}_{t}^{{2}}$ of 1.5%, figure of merit ($\text {FOM} = {Q} {k}_{t}^{{2}}$) of 31.5 enabling low phase noise, and low-power oscillators crucial for Internet of Things (IoT). Due to the high ${Q}\text{s}$ of the LiNbO3 LOBAR, the measured VCMO shows a close-in phase noise of −100 dBc/Hz at 1-kHz offset from a 300-MHz carrier and a noise floor of −153 dBc/Hz while consuming 9 mW. With further optimization, this VCMO can lead to direct radio frequency (RF) synthesis for ultralow-power transceivers in multimode IoT nodes. [ABSTRACT FROM AUTHOR]

We present the first group of GHz low-loss acoustic radio frequency (RF) couplers using the fundamental symmetric (S0) mode in X-cut lithium niobate thin films. The demonstrated multistrip couplers (MSCs) significantly surpass the insertion loss (IL) and the operating frequency of the previous works in more compact structures, thanks to the large electromechanical coupling and low loss of S0 in lithium niobate. The design space of S0 MSCs is first explored. Devices with different coupling factors are fabricated using different numbers of strips. Based on the S0 testbed with an IL of 4.5 dB at 1 GHz, the hybrid coupler shows an IL of 7.5 dB, while the track changer shows an IL of 5.1 dB, over a 3-dB fractional bandwidth of 8%. Couplers at different frequencies (between 0.75 and 1.55 GHz) are also investigated. Upon further optimizations, the S0 MSC platform can potentially enable low-loss wideband signal processing functions toward an RF acoustic component kit. [ABSTRACT FROM AUTHOR]

In this work, we present a novel unidirectional transducer design for frequency scaling aluminum nitride (AlN)-based radio frequency (RF) microsystems. The proposed thickness-field-excited single-phase unidirectional transducers (TFE-SPUDT) adopt 5/16 wavelength electrodes and, thus, enable efficient piezoelectric transduction with better frequency scalability. The design space of the TFE-SPUDT is theoretically explored and validated using the acoustic delay line (ADL) testbeds. The ADL testbeds with a large feature size of ${3}~\mu \text{m}$ show a center frequency of 1 GHz, a minimum insertion loss (IL) of 4.9 dB, and a fractional bandwidth (FBW) of 5.3%, significantly surpassing the IL and frequency scalability of the previously reported AlN transducers. The design approach can potentially contribute to various AlN-based RF microsystems for signal processing, physical sensing, optomechanical interaction, and quantum acoustic applications, and are readily extendable to other piezoelectric platforms. [ABSTRACT FROM AUTHOR]

We present the first group of GHz broadband SH0 mode acoustic delay lines (ADLs). The implemented ADLs adopt unidirectional transducer designs in a suspended X-cut lithium niobate thin film. The design space of the SH0 mode ADLs at GHz is first theoretically investigated, showing that the large coupling and sufficient spectral clearance to adjacent modes collectively enable the broadband performance of SH0 delay lines. The fabricated devices show 3-dB fractional bandwidth ranging from 4% to 34.3% insertion loss between 3.4 and 11.3 dB. Multiple delay lines have been demonstrated with center frequencies from 0.7 to 1.2 GHz, showing great frequency scalability. The propagation characteristics of SH0 in lithium niobate thin film are experimentally extracted. The demonstrated ADLs can potentially facilitate broadband signal processing applications. [ABSTRACT FROM AUTHOR]

We present the first group of gigahertz S0 mode low loss and wideband acoustic delay lines (ADLs). The ADLs use a single-phase unidirectional transducers (SPUDT) design to launch and propagate the S0 mode in an X-cut lithium niobate thin film with large electromechanical coupling and low damping. In this work, the theoretical performance bounds of S0 mode ADLs are first investigated, significantly surpassing those in state-of-the-art. The design tradeoffs of S0 mode ADLs, when scaled to the gigahertz frequency range, are also discussed. The fabricated miniature ADLs show a fractional bandwidth (FBW) of 4% and a minimum insertion loss (IL) of 3.2 dB, outperforming the incumbent surface acoustic wave (SAW) counterparts, and covering a wide range of delays from 20 to 900 ns for digitally addressable delay synthesis. Multiple ADLs with center frequencies from 0.9 to 2 GHz have been demonstrated, underscoring their great frequency scalability. The propagation properties of S0 waves in lithium niobate at the gigahertz range are experimentally extracted. The demonstrated ADLs can potentially enable wide-range and high-resolution delay synthesis that is highly sought after for the self-interference cancellation in full-duplex radios. [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]