Bulk Acoustic Resonator Research Articles

Three-Dimensional Finite Element Analysis and Characterization of Quasi-Surface Acoustic Wave Resonators.

In this work, three-dimensional finite element analysis (3D FEA) of quasi-surface acoustic wave (QSAW) resonators with high accuracy is reported. The QSAW resonators consist of simple molybdenum (Mo) interdigitated transducers (IDT) on solidly mounted stacked layers of AlN/Mo/Si. Different to the SAW resonators operating in the piezoelectric substrates, the reported resonators are operating in the QSAW mode, since the IDT-excited Rayleigh waves not only propagate in the thin piezoelectric layer of AlN, but also penetrate the Si substrate. Compared with the commonly used two-dimensional (2D) FEA approach, the 3D FEA method reported in this work shows high accuracy, in terms of the resonant frequency, temperature coefficient of frequency (), effective coupling coefficient () and frequency response. The fabricated QSAW resonator has demonstrated a of 0.291%, series resonant frequency of 422.50 MHz, and of −23.418 ppm/°C in the temperature range between 30 °C and 150 °C, for the design of wavelength at 10.4 m. The measurement results agree well with the simulations. Moreover, the QSAW resonators are more mechanically robust than lamb wave devices and can be integrated with silicon-based film bulk acoustic resonator (FBAR) devices to offer multi-frequency function in a single chip.

Super-high sensitivity FBAR temperature sensor based on size effect of Ti insertion layer

The temperature sensitivity is one of the critical parameters for the thin film bulk acoustic resonator (FBAR) based temperature sensors. In this work, FBARs with Au/Fe0.8Ga0.2/Ti/AlN/Mo structure are developed. The size effect of the Ti insertion layer on the temperature sensitivity of the devices is systematically investigated. The devices were fabricated by MEMS process and characterized by a network analyzer under variable temperatures. It is found that the temperature sensitivity of the devices is strongly related to the thickness of the Ti insertion layer. A super-high temperature sensitivity up to 546 kHz °C−1 was obtained with 20 nm Ti inserted thin film; that feature can even reach 825 kHz °C−1 for some devices, showing great potential for ultra-sensitive temperature monitoring. Mason model is used to analyze the extraordinary characteristics of the device and finite element method (FEM) is used to analyze the strain distribution in the device. The supreme performance of the temperature sensor can be explained by the size effect of the temperature coefficient of Young’s modulus (TCE) of Ti film, which means that the TCE was enhanced when the thickness of the Ti film is around 20 nm. This work provides a new approach for the design of high sensitivity temperature sensor based on FBAR.

AlN FBAR Particle Sensor With a Thermophoretic Sampling Mechanism

Pollution by particulate matter (PM) poses a serious and growing risk to human health. Film Bulk Acoustic Resonators (FBARs) have been proposed as a low-cost way to monitor particle concentration. This paper presents particle detection by a 1.1 GHz aluminium nitride based FBAR designed and fabricated by SilTerra Malaysia. The FBARs are used in a differential configuration with one sensing and one reference device in a system comprising a microchannel with an electric microfan and a thermophoretic precipitator microhotplate array for improved sampling. A custom-built test chamber was designed to characterise the device at different levels of particulate matter concentration in air. The particle feed rate into the test chamber was varied between 9.5, 19, 43, 66 and 94 μg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s. The FBAR frequency was found to decrease with increasing particulate matter concentration in the test chamber air. The experiments were conducted with and without the microchannel and it was found that the sampling channel increased sensitivity of FBAR resonant frequency to particle concentration per cubic meter of air from 5 Hz/ μgm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 20 Hz/ μgm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The detection limit in this test was estimated at ca. 50 μg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , which is around current European limits for PM10 pollution. In addition, the use of the sampling microfan to aid cleaning of FBARs after particle measurement was also investigated and found to be feasible especially for lower particle concentration.

Coherent Pulse Echo in Hybrid Magnonics with Multimode Phonons

The hybridization of magnons and phonons is playing a critical role in the emerging field of hybrid magnonics because it combines the high tunability of magnetism with the long lifetime of mechanics for comprehensive coherent information processing. Recently there has been increasing interest in thin-film bulk acoustic waves because of their long lifetimes at high frequencies. However, the unique multimode nature of such phonon modes has not been exploited as an important resource for coherent information processing. In this Letter we study the simultaneous hybridization of multiple high-overtone bulk acoustic resonances with a magnon and a microwave mode. The demonstrated multimode hybridization allows us to observe coherent pulse echoes, opening opportunities for both fundamental studies and practical applications of hybrid magnonics.

The BAW and SAW coupling analysis of patterned piezoelectric films as inter-digital transducers on diamond substrates using delta-function model

Patterned piezoelectric films as inter-digital transducer (PP-IDT) structures on diamond substrates have been attracting extensive attention due to the increased working frequency, electromechanical coupling, and quality factor of piezoelectric devices. In this study, by conceiving every single patterned piezoelectric IDT bar as the longitudinal bulk acoustic wave (BAW) vibrating source, we study how the BAW excites and couples with the surface acoustic wave (SAW) on the surface of diamond substrates and resonates simultaneously. The classic delta-function model of the SAW and electric impedance theory of the film bulk acoustic resonator were innovatively combined to develop this conception. Resonating frequency and corresponding piezoelectric IDT thickness were calculated using MATLAB software based on the developed delta-function model. The resonance results coincided well with finite element method simulations, validating this conception and the developed delta-function model for PP-IDT structures.

Analytical Modeling of AIN-Based Film Bulk Acoustic Wave Resonator for Hydrogen sulfide Gas detection Based on PiezoMUMPs

Aluminium nitride (AIN) thin film bulk acoustic resonator (FBAR) sensor for hydrogen sulfide gas detection has been designed and mathematically modelled using CoventorWare and MATLAB software, respectively. The designed FBAR sensor is based on the PiezoMUMPs fabrication technology. The detection principle of the FBAR gas sensor is based on the resonant frequency changes detection due to the mass change on the top electrode of the sensor induced by the absorbed gas molecules by the nanomaterial deposited on the surface of the top electrode device. Reduced graphene oxide hybrid with copper oxide was considered as the sensitive nanomaterials and their mass loaded was evaluated in the theoretically calculation. The resonant frequency of the shear mode of the FBAR sensor has been calculated theoretically and found to be 9.4524 GHz. The effects of the gas molecules on the resonant frequency have been investigated using a mathematical equation and it shown that the increasing of the gas mass on the sensor surface will reduce the sensor resonant frequency. Furthermore, the sensitivity of the sensor was calculated to be 0.22615 Hz/fg.

The Recent Progress of MEMS/NEMS Resonators.

MEMS/NEMS resonators are widely studied in biological detection, physical sensing, and quantum coupling. This paper reviews the latest research progress of MEMS/NEMS resonators with different structures. The resonance performance, new test method, and manufacturing process of single or double-clamped resonators, and their applications in mass sensing, micromechanical thermal analysis, quantum detection, and oscillators are introduced in detail. The material properties, resonance mode, and application in different fields such as gyroscope of the hemispherical structure, microdisk structure, drum resonator are reviewed. Furthermore, the working principles and sensing methods of the surface acoustic wave and bulk acoustic wave resonators and their new applications such as humidity sensing and fast spin control are discussed. The structure and resonance performance of tuning forks are summarized. This article aims to classify resonators according to different structures and summarize the working principles, resonance performance, and applications.

Ferroelectric-on-Si Super-High-Frequency Fin Bulk Acoustic Resonators With Hf0.5Zr0.5O2 Nanolaminated Transducers

This letter reports, for the first time, on the ferroelectric-on-semiconductor fin bulk acoustic resonators (FoS-FinBARs) with lithographically scaled frequencies up to 18 GHz and 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> . Q as high as 17. The FoS-FinBARs benefit from the large electromechanical coupling of the ferroelectric hafnium zirconium oxide (Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) nanolaminates created by atomic layering. The ferroelectric transducer is conformally deposited on silicon fins with sub-1- μm widths enabling excitation of super-high-frequency bulk acoustic wave resonance modes with increasing 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> at higher frequencies. FoS-FinBARs, created by a CMOS-compatible process, are presented with frequencies over 2-18 GHz, Qs exceeding 500, and 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> s as high as 4.7%.

Reducing high carrier concentration in rocksalt-AlxSc1-xN with Mg acceptor doping

Aluminum scandium nitride (AlxSc1-xN) is an emerging III-nitride semiconductor that has attracted significant interest in recent years in surface and bulk acoustic resonators for its high piezoelectric coefficient and applications in high-power electronic devices. AlxSc1-xN stabilizes in the rock salt phase for x &amp;lt; 0.52 when deposited directly on (001) MgO substrates and has been utilized as a semiconductor in single-crystalline TiN/AlxSc1-xN metal/semiconductor superlattices for thermionic energy conversion, optical hyperbolic metamaterials, and the fundamental studies on heat and current transport in materials. However, due to the presence of oxygen impurities and native defects, such as nitrogen vacancies, sputter-deposited rock salt-AlxSc1-xN exhibits a high carrier concentration in the (2–4) × 1020 cm−3 range that leads to its Ohmic tunneling contact with metals and prevents observation of thermionic emission. In this Letter, we demonstrate that magnesium (Mg) acts as an efficient hole-dopant in r-AlxSc1-xN, increases its resistivity, and reduces its carrier concentration as a function of Mg concentration to as low as 1.4 × 1018 cm−3. A combination of spectroscopy, microscopy, and first-principles modeling demonstrate (a) epitaxial 001 oriented AlxSc1-xN:Mg growth for the first 35–75 nm and subsequent pyramidal growth with multiple in-plane orientations, (b) MgxNy to form a uniform and homogeneous solid solution with r-AlxSc1-xN without any precipitation, phase separation, or secondary phase formation, and (c) Mg-defect states are located deep inside the valence and conduction bands that leave behind a pristine r-AlxSc1-xN bandgap and band edges. The demonstration of Mg-hole doping in r-AlxSc1-xN marks significant progress in r-AlxSc1-xN thin film and superlattice-based devices.

A New Class of High-Overtone Bulk Acoustic Resonators Using Lithium Niobate on Conductive Silicon Carbide

This letter presents a new class of High-overtone Bulk Acoustic Resonators (HBARs) with only top electrodes but excellent performance using lithium niobate (LN) thin films on conductive silicon carbide (SiC). The 4-inch single-crystalline lithium niobate thin film on conductive SiC (LNCSiC) substrate was prepared by ion-cutting process. Since the conductive SiC serves as both the resonant cavity and the floating electrode, the HBARs based on LNCSiC with only top electrodes were designed and fabricated. The devices show a wide frequency response span exceeding 2.0 GHz, a frequency spacing of 10 MHz, an excellent f $\cdot $ Q $_{\textit {LAKIN}}$ product of ${9.6}\times {10} ^{{13}}$ Hz ( f $\cdot $ Bode-Q of ${1.1}\times {10} ^{{14}}$ Hz), and a low temperature coefficient of frequency (TCF) of −18.9 ppm/°C. The frequency response shift of the proposed HBAR can be achieved by just adjusting the in-plane orientation of the top electrodes. Additionally, a multi-band filter was numerically synthesized based on the proposed HBARs, showing a spectral spacing of 10 MHz and a passband span of more than 2.0 GHz. Upon further optimizations, the proposed HBARs with a wide frequency response, a high- f $\cdot $ Q , and a low-TCF will be very promising for radio-frequency applications.

A solidly mounted, high frequency, thickness shear mode bulk acoustic wave resonator using X-LiTaO3 thin plate and SiO2/Ta multilayer acoustic films

This paper describes a bulk acoustic wave (BAW) high-frequency resonator with a solidly mounted structure using a single crystal LiTaO3 (LT) thin plate. A Bragg reflector solidly supports the LT thin plate, which is fragile if self-suspended. The BAW resonator uses a thickness shear mode in 1.5 μm thick X-LT. The Bragg reflector is made of four pairs of SiO2 and Ta films, i.e. eight layers in total. The fabricated device works at a resonance frequency of 1.14 GHz and an antiresonance frequency of 1.23 GHz, showing a bandwidth of 7.9% and a low-temperature coefficient of frequency of −16 ppm °C−1 and −26 ppm °C−1 at the maximum and minimum admittances, respectively. The measured impedance ratio is 61 dB, although the structure and fabrication process are not optimized yet, suggesting a high potential for this device.

Ferroelectric Aluminum Scandium Nitride Thin Film Bulk Acoustic Resonators with Polarization‐Dependent Operating States

Given the recent discovery of ferroelectricity in highly doped AlN films and their widespread use in bulk acoustic wave (BAW) filters, the ferroelectric and acoustic performance of AlScN‐based devices at two distinct polarization states, i.e., nitrogen (N)‐polar and metal‐polar states, is investigated. Unreleased metal−ferroelectric−metal (MFM) capacitors, as well as suspended MFM structures in the form of thin‐film bulk acoustic resonators (FBARs), are fabricated based on Mo/AlScN/Mo‐sputtered films with ≈30% Sc concentration. The hysteresis polarization−electric field (P−E) loops and internal resistance of the MFM capacitors are characterized. The FBAR devices are characterized at the two polarization states, where the polarization inversion is accompanied by the modulation of thin‐film stress and resistance. Furthermore, the as‐fabricated N‐polar FBAR shows a fundamental thickness‐mode resonance frequency of 3.17 GHz, with a quality factor (QBode) of 572 and a high effective electromechanical coupling coefficient (kt2) of 11.4%. The FBAR frequency response changes when the polarization is switched from N‐polar to metal‐polar. Herein, the first investigations of electric and acoustic properties of N‐polar and metal‐polar AlScN‐based devices are demonstrated, paving the way for the realization of high‐kt2‐reconfigurable acoustic filters for next‐generation communication systems operating at the super‐high‐frequency (SHF) range.

Increase of electromechanical coupling coefficient k t 2 in (0001)-oriented AlN films by chromium doping

Piezoelectric AlN films possess high bulk acoustic wave velocity, low acoustic attenuation, and good temperature characteristics. However, AlN film bulk acoustic wave resonators (FBARs) have a relatively small electromechanical coupling coefficient k eff 2. It was recently reported that Cr doping in AlN films increased the piezoelectric constant d 33. The k t 2 of AlN FBARs may thus be enhanced by Cr doping. In this study, we investigated the effect of Cr doping in (0001)-oriented AlN films on k t 2 from the frequency characteristics of high overtone bulk acoustic wave resonators. k t 2 of the Cr-doped AlN films was increased for Cr contents of less than 3%. The maximum k t 2 observed for the Cr0.012Al0.088N film was 5.9%, which was approximately 1.4 times higher than that of pure AlN film (k t 2 = 4.4%).

Application of free side edges to thickness shear bulk acoustic resonator on lithium niobate for suppression of transverse resonances

This paper discusses the applicability of free side edges to a thickness shear bulk acoustic resonators (TSBARs) on a rotated Y-cut lithium niobate plate for suppression of lateral energy leakage and the transverse mode resonances. It is shown both theoretically and experimentally that free edges are effective to suppress lateral leakage and transverse mode resonances in free-standing TSBARs. Although this technique is applicable only to the X side boundaries, it is shown that the Y′ side perpendicular to the X side can be designed for the piston mode operation independently by the simple two-dimensional analysis ignoring wave propagation toward the X direction. Then the discussion is extended to the solidly-mounted TSBAR. It is shown that this technique works well also for this case, and removal of the first few Bragg layers is enough for the transverse mode suppression.

Thermal Conductivity of Aluminum Scandium Nitride for 5G Mobile Applications and Beyond.

Radio frequency (RF) microelectromechanical systems (MEMS) based on Al1-xScxN are replacing AlN-based devices because of their higher achievable bandwidths, suitable for the fifth-generation (5G) mobile network. However, overheating of Al1-xScxN film bulk acoustic resonators (FBARs) used in RF MEMS filters limits power handling and thus the phone's ability to operate in an increasingly congested RF environment while maintaining its maximum data transmission rate. In this work, the ramifications of tailoring of the piezoelectric response and microstructure of Al1-xScxN films on the thermal transport have been studied. The thermal conductivity of Al1-xScxN films (3-8 W m-1 K-1) grown by reactive sputter deposition was found to be orders of magnitude lower than that for c-axis-textured AlN films due to alloying effects. The film thickness dependence of the thermal conductivity suggests that higher frequency FBAR structures may suffer from limited power handling due to exacerbated overheating concerns. The reduction of the abnormally oriented grain (AOG) density was found to have a modest effect on the measured thermal conductivity. However, the use of low AOG density films resulted in lower insertion loss and thus less power dissipated within the resonator, which will lead to an overall enhancement of the device thermal performance.

3D Simulation-Based Acoustic Wave Resonator Analysis and Validation Using Novel Finite Element Method Software.

This work illustrates the analysis of Film Bulk Acoustic Resonators (FBAR) using 3D Finite Element (FEM) simulations with the software OnScale in order to predict and improve resonator performance and quality before manufacturing. This kind of analysis minimizes manufacturing cycles by reducing design time with 3D simulations running on High-Performance Computing (HPC) cloud services. It also enables the identification of manufacturing effects on device performance. The simulation results are compared and validated with a manufactured FBAR device, previously reported, to further highlight the usefulness and advantages of the 3D simulations-based design process. In the 3D simulation results, some analysis challenges, like boundary condition definitions, mesh tuning, loss source tracing, and device quality estimations, were studied. Hence, it is possible to highlight that modern FEM solvers, like OnScale enable unprecedented FBAR analysis and design optimization.

Correlation of Electrical Properties and Acoustic Loss in Single Crystalline Lithium Niobate-Tantalate Solid Solutions at Elevated Temperatures

Electrical conductivity and acoustic loss Q−1 of single crystalline Li(Nb,Ta)O3 solid solutions (LNT) are studied as a function of temperature by means of impedance spectroscopy and resonant piezoelectric spectroscopy, respectively. For this purpose, bulk acoustic wave resonators with two different Nb/Ta ratios are investigated. The obtained results are compared to those previously reported for congruent LiNbO3. The temperature dependent electrical conductivity of LNT and LiNbO3 show similar behavior in air at high temperatures from 400 to 700 °C. Therefore, it is concluded that the dominant transport mechanism in LNT is the same as in LN, which is the Li transport via Li vacancies. Further, it is shown that losses in LNT strongly increase above about 500 °C, which is interpreted to originate from conductivity-related relaxation mechanism. Finally, it is shown that LNT bulk acoustic resonators exhibit significantly lower loss, comparing to that of LiNbO3.

Finite element modeling and simulation of ultrasensitive film bulk acoustic resonator enabled by micropillar structure

Portable and ultra-sensitive film bulk acoustic resonator (FBAR) is a promising device to satisfy the requirement of detecting gas and biological molecule. In this work, a novel sensing device was developed to achieve ultrahigh sensitivity, by coupling polymer micropillars with a FBAR substrate to form a two-degrees-of-freedom resonance system (FBAR-micropillars). We systematically investigated the effects of micropillar structure on the characteristics of FBAR-micropillars device by finite element method (FEM). It was found that the resonant frequency shift increased with increasing the height of micropillars (h) within a certain range, and the FBAR-micropillars device displayed nonlinear frequency response, which was opposite to the linear response of conventional FBAR devices. In addition, a positive resonant frequency shift was captured near the “coupled resonant point” of the FBAR-micropillars device. The geometric parameters of micropillars, including micropillar diameter and micropillar spacing could also cause a change of Q-factor and mass sensitivity. The optimized design of the proposed device achieved a threefold improvement in sensitivity relative to conventional FBAR without pillars, suggesting a feasible method to improve the mass sensitivity of acoustic resonators.

Label-Free Protein Detection by Micro-Acoustic Biosensor Coupled with Electrical Field Sorting. Theoretical Study in Urine Models

Diagnostic devices for point-of-care (POC) urine analysis (urinalysis) based on microfluidic technology have been actively developing for several decades as an alternative to laboratory based biochemical assays. Urine proteins (albumin, immunoglobulins, uromodulin, haemoglobin etc.) are important biomarkers of various pathological conditions and should be selectively detected by urinalysis sensors. The challenge is a determination of different oligomeric forms of the same protein, e.g., uromodulin, which have similar bio-chemical affinity but different physical properties. For the selective detection of different types of proteins, we propose to use a shear bulk acoustic resonator sensor with an additional electrode on the upper part of the bioliquid-filled channel for protein electric field manipulation. It causes modulation of the protein concentration over time in the near-surface region of the acoustic sensor, that allows to distinguish proteins based on their differences in diffusion coefficients (or sizes) and zeta-potentials. Moreover, in order to improve the sensitivity to density, we propose to use structured sensor interface. A numerical study of this approach for the detection of proteins was carried out using the example of albumin, immunoglobulin, and oligomeric forms of uromodulin in model urine solutions. In this contribution we prove the proposed concept with numerical studies for the detection of albumin, immunoglobulin, and oligomeric forms of uromodulin in urine models.

All-Oxide Varactor Electromechanical Properties Extracted by Highly Accurate Modeling Over a Broad Frequency and Electric Bias Range.

Since the dielectric permittivity of ferroelectric materials depends on the electric field, they allow designing switchable and continuously tunable devices for adaptive microwave front ends. Part of the ongoing research is the field of all-oxide devices, where epitaxial oxide conductors are used instead of polycrystalline metal electrodes, leading to epitaxial ferroelectric layers and resulting in high device performance. In particular, they allow engineering the acoustic properties separated from the electric ones due to the structural similarity between the dielectric and conducting oxide films. Two major results are reported in this work. First, a highly accurate model for the microwave impedance of ferroelectric varactors is derived that tracks the superposition of induced piezoelectricity and field extrusion into the substrate caused by thin electrodes. In difference to previous works, this model covers both a wide frequency and biasing range up to 12 GHz and 100 V/ [Formula: see text]. Second, the high model accuracy enables the determination of all relevant electric and mechanic properties based on a mere microwave characterization. This approach will be especially valuable when independent measurements of mechanical properties of the thin-film materials are impeded by a high integration of the devices. Though derived for all-oxide varactors, the presented model can as well be adapted for thin-film bulk acoustic wave resonators (FBARs) and varactors with conventional metal electrodes when eventual dead layers at the interface are modeled correctly.