Thin Film Bulk Acoustic Resonator Research Articles

Aluminium Scandium Nitride Thin Film BAW Resonators (FBAR) for 5G Applications

With the growing impact of 5G communication system, Radio-Frequency Micro-Electro-Mechanical-Systems (RF MEMS) technologies are becoming increasingly important in our daily lives. Development of filters in the 5G frequency range is being carried out tremendously. The rapid rise in smartphone usage, heightened data demands, congestion in the frequency spectrum, and advancements in technology have posed numerous challenges for the development of RF filters. The ultimate goal is to simplify the RF Front End Module used in smart phones, thereby achieving better selectivity. In this paper, a high frequency - high Q thin film bulk acoustic wave resonator of piezoelectric material Aluminium scandium nitride (AlScN)based FBAR is designed for 5G Application. Finite element analysis conducted using the COMSOL Multiphysics software assess the characteristics of the constructed FBAR. The outcomes indicate that the device resonates effectively at 5 GHz, exhibiting commendable return loss and a Q-factor of 1490. A high-resonance-frequency FBAR device offers advantages for 5G mobile technology.

Reliable formulas for accurately determining the resonance and antiresonance frequencies of thin film bulk acoustic resonators

Expanding upon Lakin’s theory by considering the phase shift of the acoustic waves in the metallic layers, we have developed an impedance formula for a piezoelectric layer covered by a finite thickness of metal layers, enabling the precise determination of its resonant and anti-resonant frequencies. Compared to experimental data and 3D finite element simulations, our formula can accurately and quickly predict resonant, anti-resonant frequencies, and bandwidth across a wide range of piezoelectric and metallic layer thickness combinations.

A cancer cell nanotherapy method based on GHz film bulk acoustic resonator and nanoscale CaO2.

Sonodynamic therapy (SDT) is an emerging cancer treatment method in recent years. However, the ultrasound signal utilized for SDT is usually located at a low-frequency spectrum (<2MHz), and in the field of SDT research, few studies have focused on the exploration and development of ultrasound frequency. Studies have shown that the GHz-level ultrasound can increase cell membrane permeability and have a negligible effect on cell vitality. Herein, we reported the study of a GHz thin film bulk acoustic resonator as an ultrasound source for synergistic treatment with nanoscale calcium peroxide (CaO2). It was discovered that this ultrasound source ultimately achieved an efficient therapeutic outcome on mouse breast cancer cell line 4T1. Such GHz-level ultrasound application in SDT is of high significance to broaden the cognition and application scope of SDT.

Novel mode-coupling vibrations of AlN thin film bulk acoustic resonator operating with thickness-extensional mode

Novel mode-coupling vibrations of AlN thin film bulk acoustic resonator operating with thickness-extensional mode

Alumina intercalation to enhance the performance of BAW magnetic sensors: Micromagnetic and finite element analysis

The bulk acoustic wave (BAW) magnetic sensor, which is based on a thin-film bulk acoustic wave resonator and magnetoelectric coupling, has the technological advantage of passive wireless magnetic detection and can be applied to medical diagnostics and resource exploration to achieve ultra-weak magnetic field detection. However, the enhancement of BAW magnetic sensors' performance is severely limited by the localized magnetization phenomenon and the significant eddy current loss suffered by single-layer magnetic films. In this paper, the effect and internal physical mechanism of inserting different alumina intercalation layers into a magnetic film on the performance of the magnetic film and the BAW sensor were investigated, and the optimal design of the BAW magnetic sensor was achieved. First, single-layer FeGaB films and composite FeGaB/(Al2O3 5 nm/FeGaB)n films (n = 1, 2, 4, 6, and 10) were fabricated to characterize the effect of intercalation on the static and dynamic magnetic properties of the films. When four layers of 5 nm alumina were inserted, the magnetic film was found to have the finest soft magnetic properties. Further micromagnetic simulation and microstructural characterization demonstrated that the alumina intercalation enhances the magnetic film's microstructure, resulting in a complex interlayer interaction. Second, the BAW magnetic sensor's finite element simulation model was developed using COMSOL Multiphysics. The relaxation response mechanism of the magnetic film to an external alternating magnetic field was investigated using micromagnetism, and the energy transfer and coupling between the magnetic film and the piezoelectric layer were further analyzed. The effect of the alumina intercalation on the magnetic sensor's performance, including sensitivity and linearity, was quantified. Eventually, the operating frequency of the BAW magnetic sensor was matched to 2.5 GHz by refining the design conditions of inserting four layers of 5 nm alumina into FeGaB thin films. Within the magnetic field testing range of 50000–60000 A/m, simulation results indicate that the sensitivity can reach 0.0028 Vm/A, and the linearity of the output voltage is better than 3.85%.

High-quality c-axis oriented Al(Sc)N thin films prepared by magnetron sputtering

In order to adapt to the upgrading of wireless communication system, thin film bulk acoustic resonator (FBAR) has become one of the research hotspots in the communication field. The piezoelectric film, such as aluminum nitride (AlN), is the core functional layer of this device. However, previous reports have only focused on specific characteristics of the piezoelectric film, such as preferred orientation, piezoelectric coefficient, residual stress and so on. In this work, AlN and 6% scandium doped AlN films with (002) preferred orientation were deposited on (100) silicon wafers by direct-current magnetron reactive sputtering. We studied the crystallinity, roughness, element distribution, stress and additional characteristics of the films, and compared the quality changes of AlN thin films doped with Sc. Our results showed that all the deposited films had a uniform and dense structure with high crystallinity. But, the doping of Sc significantly reduced the nonuniformity, roughness, and stress of AlN films. Our research is of great significance for the preparation of high-quality piezoelectric thin films to improve the performance of FBAR filters, and ultimately improve the yield of devices.

Correlated disorder by defects clusters in LiNbO3 single crystals after crystal ion-slicing

LiNbO3 (LNO) is a material suitable for high-frequency and wideband RF filters as well as photonic applications due to its outstanding piezoelectric and optical properties. Applications such as thin-film bulk acoustic wave resonators or highly confined electro-optic modulators require thin films, which are still challenging to be obtained with the necessary crystalline quality by deposition methods. Alternatively to deposition, crystal ion-slicing (CIS) or Smart CutTM enables obtaining thin films hundreds of nanometers thick from the bulk single-crystal utilizing high-energy and dose implantation of light atoms. The price to pay is the partial degradation of the initial crystalline perfection, which translates to a degradation of the piezoelectric performance of the material if the CIS-induced damage is not eliminated.This work aims to understand the physics of the recrystallization process of the CIS-damaged LNO crystal upon thermal annealing. The study of the coherent and diffuse scattering by high-resolution X-ray diffraction, complemented by micro-Raman backscattering spectroscopy and Transmission Electron Microscopy (TEM), allowed to study the recovery of the crystallinity from the CIS-induced lattice strain and stress and the progressive reduction of the defects clusters dimension.

A Micro-Acoustic Enhanced Low-Impedance Antenna System for IoT Wake-Up Receivers

In this work, we propose a passive radio frequency (RF) front-end tailored for wake-up receivers (WuRx) to be deployed in cellular IoT devices and wearables networks, featuring a low radiation resistance antenna and a high-Q matching network implemented with microacoustic resonators integrated to obtain a systematic higher node’s sensitivity at no cost in terms of power consumption. We show how these components can be co-designed to obtain high passive voltage gain, hardwarelevel blocker immunity, and increased resilience to integration parasitics, relaxing link budget for low-power IoT nodes. We report experimental validation of a PCB antenna with 2 dBi gain measured on an 11Ω input resistance at resonance, and an in-house fabricated Micro-Electro-Mechanical System (MEMS) thin-film aluminum nitride bulk acoustic resonator with a quality factor Q = 550 and a piezoelectric coupling coefficient k2 t =7%, hybridly integrated with a commercial off-the-shelf low-power WuRx circuit to benchmark the proposed RF front-end design at 850MHz. We demonstrate a passive voltage gain of 12 dB due to the MEMS resonator, and an additional 11 dB due to the proposed antenna design (for a total of an unprecedented 23 dB passive gain in this frequency range) leading to an over the air.61dBm minimum detectable input power and 23 dB blocker rejection.

Aluminum Nitride Based Film Bulk Acoustic Resonator With Anchor Column Structure

In order to reduce the lateral acoustic energy loss of thin film bulk acoustic resonators (FBARs), the trench structure for FBAR is generally designed to block the leakage of acoustic wave and help improving the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> -factor. One method the designers often used is etching the piezoelectric material around the active area of the FBAR to form the air-wall structure as a perfect reflection boundary condition. In this paper, the FBAR with trench structure is fabricated. The multilayer films in the FBAR with trench structure are easily warped due to excessive compressive stress. The numerical analysis and finite element simulation are used to verify the effects of different stresses on the devices. In order to avoid the degradation of FBAR performance caused by film warpage, the FBAR with anchor columns is proposed. The anchor columns are located on both sides of the FBAR, and they are designed to reduce film bending and the compressive stress of active area of the resonator. The measured results show that the Qs and Qp of the FBAR with anchor columns are increased by 55.8% and 21.1%, respectively, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{eff}{}^{2}$ </tex-math></inline-formula> improved from 5.6% to 6.7% compared with the FBAR without the anchor columns. [2022-0179]

Design and Fabrication of a Flexible Gravimetric Sensor Based on a Thin-Film Bulk Acoustic Wave Resonator.

Sensing systems are becoming less and less invasive. In this context, flexible materials offer new opportunities that are impossible to achieve with bulky and rigid chips. Standard silicon sensors cannot be adapted to curved shapes and are susceptible to big deformations, thus discouraging their use in wearable applications. Another step forward toward minimising the impacts of the sensors can be to avoid the use of cables and connectors by exploiting wireless transmissions at ultra-high frequencies (UHFs). Thin-film bulk acoustic wave resonators (FBARs) represent the most promising choice among all of the piezoelectric microelectromechanical system (MEMS) resonators for the climbing of radio frequencies. Accordingly, the fabrication of FBARs on flexible and wearable substrates represents a strategic step toward obtaining a new generation of highly sensitive wireless sensors. In this work, we propose the design and fabrication of a flexible gravimetric sensor based on an FBAR on a polymeric substrate. The resonator presents one of the highest electromechanical coupling factors in the category of flexible AlN-based FBARs, equal to 6%. Moreover, thanks to the polymeric support layer, the presence of membranes can be avoided, which leads to a faster and cheaper fabrication process and higher robustness of the structure. The mass sensitivity of the device was evaluated, obtaining a promising value of 23.31 ppm/pg. We strongly believe that these results can pave the way to a new class of wearable MEMS sensors that exploit ultra-high-frequency (UHF) transmissions.

An Approximation for Rapid Simulation of Thin-Film Bulk Acoustic Resonators (FBARs) with Sandwich-Layered Structure

An Approximation for Rapid Simulation of Thin-Film Bulk Acoustic Resonators (FBARs) with Sandwich-Layered Structure

Fabrication of a New Air-Gap FBAR on an Organic Sacrificial Layer through an Innovative Design Algorithm

Realizing thin-film acoustic wave resonators presents many design and fabrication challenges. Actual material specifications always differ from nominal material properties employed in simulations, as they depend on the deposition technique and parameters used and on equipment type and status. Moreover, each deposition process introduces a degree of uncertainty regarding the thicknesses of the layers. All these factors have a substantial impact on the resonance frequency, which often differs from the designed value. This work details the design and fabrication of an aluminum nitride (AlN)-based thin-Film Bulk Acoustic wave Resonator (FBAR) showing one of the highest products of Q-factor and electromechanical coupling of 6895. The design process is based on an innovative, fast, and scalable design and fabrication approach that considers fabrication tolerances. The algorithm returns very fast results on the order of seconds, and successfully estimates the resonance of a designed stack at 2.55 GHz with a very low error of 0.005 GHz (about 0.2%). The FBAR layer stack is suspended on a polymeric membrane and an innovative rapid dissolving sacrificial layer made of Lift-Off Resist (LOR). This new fabrication protocol obtains resonators with an electromechanical coupling factor of 4.7% and a maximum quality factor of 1467, respectively.

Aluminum Nitride-Based Adjustable Effective Electromechanical Coupling Coefficient Film Bulk Acoustic Resonator.

The arrival of the 5G era has promoted the need for filters of different bandwidths. Thin-film bulk acoustic resonators have become the mainstream product for applications due to their excellent performance. The Keff2 of the FBAR greatly influences the bandwidth of the filter. In this paper, we designed an AlN-based adjustable Keff2 FBAR by designing parallel capacitors around the active area of the resonator. The parallel capacitance is introduced through the support column structure, which is compatible with conventional FBAR processes. The effects of different support column widths on Keff2 were verified by finite element simulation and experimental fabrication. The measured results show that the designed FBAR with support columns can achieve a Keff2 value that is 25.9% adjustable.

Aluminum scandium nitride thin-film bulk acoustic resonators for 5G wideband applications

Bulk acoustic wave (BAW) filters have been extensively used in consumer products for mobile communication systems due to their high performance and standard complementary metal-oxide-semiconductor (CMOS) compatible integration process. However, it is challenging for a traditional aluminum nitride (AlN)-based BAW filter to meet several allocated 5G bands with more than a 5% fractional bandwidth via an acoustic-only approach. In this work, we propose an Al0.8Sc0.2N-based film bulk acoustic wave resonator (FBAR) for the design of radio frequency (RF) filters. By taking advantage of a high-quality Al0.8Sc0.2N thin film, the fabricated resonators demonstrate a large Keff2 of 14.5% and an excellent figure of merit (FOM) up to 62. The temperature coefficient of frequency (TCF) of the proposed resonator is measured to be −19.2 ppm/°C, indicating excellent temperature stability. The fabricated filter has a center frequency of 4.24 GHz, a −3 dB bandwidth of 215 MHz, a small insertion loss (IL) of 1.881 dB, and a rejection >32 dB. This work paves the way for the realization of wideband acoustic filters operating in the 5G band.

Demonstration of Thin Film Bulk Acoustic Resonator Based on AlN/AlScN Composite Film with a Feasible Keff2.

Film bulk acoustic resonators (FBARs) with a desired effective electromechanical coupling coefficient (Keff2) are essential for designing filter devices. Using AlN/AlScN composite film with the adjustable thickness ratio can be a feasible approach to obtain the required Keff2. In this work, we research the resonant characteristics of FBARs based on AlN/AlScN composite films with different thickness ratios by finite element method and fabricate FBAR devices in a micro-electromechanical systems process. Benefiting from the large piezoelectric constants, with a 1 μm-thick Al0.8Sc0.2N film, Keff2 can be twice compared with that of FBAR based on pure AlN films. For the composite films with different thickness ratios, Keff2 can be adjusted in a relatively wide range. In this case, a filter with the specific N77 sub-band is demonstrated using AlN/Al0.8Sc0.2N composite film, which verifies the enormous potential for AlN/AlScN composite film in design filters.

Investigation of film bulk acoustic resonators for sensing applications in liquid environment

The thin film bulk acoustic resonator (FBAR) has emerged as a promising choice for liquid sensors because of its high frequency and sensitivity. To investigate the potential of FBAR devices working as the liquid sensors, we study the operating law of FBAR in liquid environments and explore the different loading effects of liquid on the shear mode and longitudinal mode. By analyzing the device and liquid interactions, we modify the Mason model of FBAR in the liquid environment. Subsequently, the influence of the piezoelectric film with different tilt angles and liquids on the characteristics of FBAR is discussed. We also prepared Sc0.2Al0.8N film-based FBAR to confirm the influence of different liquid environments on the resonant performances. The results show that the frequency drift of FBAR in the shear mode is related to density and viscosity of liquid, and the frequency drift of FBAR in the longitudinal mode is related to bulk modulus and density of liquid. The resonant frequency of FBAR in the shear mode is more sensitive with glycerol solution than that of FBAR in the longitudinal mode. This work can provide a research basis for the application of FBAR liquid sensors.

A Compact Low-Loss High-Selectivity Excellent-Isolation FBAR Duplexer Integrated Chip for LTE Band-3 Industrial Applications

This brief presents a complete design flow for a compact low-loss high-selectivity excellent-isolation LTE Band3 (B3) duplexer (TX: 1710 MHz-1785 MHz, RX: 1805 MHz-1880 MHz). The proposed chip duplexer is based on the thin film bulk acoustic resonator (FBAR). In addition to the general design method, the effects of temperature drift, resonator performance distribution on wafer, the quality factor of inductance, device yield, and dissipated power density distribution on the design flow are described. These aspects are of great significance for the batch production of FBAR duplexers in the industry. Samples that pass the above industrial process are selected for testing. Measured results show reasonable consistency with the co-simulation results. 1.2/1.0-dB minimum IL in TX /RX band, 20-dB rejection in 3.5 GHz (second harmonic of TX), and more than 50 dB isolation in both TX and RX frequency are achieved with less massload simultaneously.

An in-tire-pressure monitoring SoC using FBAR resonator-based ZigBee transceiver and deep learning models

In automobiles, tires play an important role in ensuring safe driving. Hence tire pressure monitoring system (TPMS) has become the most important safety system in automobile vehicles. In this work, a new indirect TMPS is proposed that measures the pressure of all the tyres indirectly using the available sensors already mounted for typical vehicle dynamics control purposes. The available sensors in vehicles usually contain a power supply module, a microcontroller, and transceiver and resonator circuitry. The available wheel speed sensor is modified by reducing the wireless receiver system in communication protocols to minimize the power consumption. The transceiver structure is modified using a suitable local oscillator (LO) frequency plan based on a temperature-compensated thin film bulk acoustic wave resonator (FBAR). In addition, the proposed TPMS SoC uses a deep-stacked autoencoder to detect the adherence conditions of the tires and a pressure loss based on wheel speed. This module will initially pre-process the speed signals to remove the speed error caused by manufacturing error. After pre-processing, different features such as statistical, frequency-domain and the fitted frequency-domain characteristics are extracted from the wheel speed signal and fed to the learning model for the judgment of the tire state. The proposed system was designed in Verilog synthesis and layout were made in cadence tool. For ASIC implementation, we use TSMC 65 nm CMOS Library for LNA/hybrid mixer design. The performance measures of the pressure detection model in terms of accuracy and errors are evaluated. Also, the performance metrics of the transceiver architecture in terms of gain, NF, IIP3, S11 and FOM are evaluated. The experimental evaluation describes that the model procures a maximum gain of 58 dB, NF of 2.15 dB and IIP3 of 6.6 dBm respectively. The die area is 0.45 mm2with 0.8 supply voltage and 0.83 mW power. In addition to this, the relationship between peak amplitude and speed vehicle regarding time and frequency domain features are evaluated.

Design and modelling of thin film bulk acoustic resonator (FBAR) for RF application

Thin film bulk acoustic resonator (FBAR) is widely utilized as an RF filter (Q) in order to achieve minimal in-band losses and high Quality Factor in complex 5G wireless communication systems. This paper describes the basic notion of FBAR on top of a cavity device design for building an RF-filter. The top and bottom electrodes are made of aluminium (Al), the piezoelectric materials used are zinc oxide (ZnO) and barium titanate (BaTiO3), and the substrate is silicon (Si). The BaTiO3-based resonators can function better at high temperatures because BaTiO3 has the highest thermal stability of any piezoelectric material where as ZnO is mostly used to build sensors and actuators due to lower flaws and MEMS compatibility. Also, the resonator attains remarkable Quality factors of 1895 and 1780 for ZnO and BaTiO3 at 5.6 GHz, respectively. A lower rate of energy loss is indicated by a higher quality factor.

A theoretical study of thermal management of FBAR considering thickness- and temperature-dependent thermal conductivity of AlN

Reliable and long-term operation of thin film bulk acoustic resonators (FBARs) under high power relies on the optimization of thermal resistance. In this work, thermal design strategies for high power FBARs are explored theoretically. For accurate estimation of the thermal characteristics of FBARs, the thermal conductivity of the AlN epilayer with temperature and thickness dependence is included in the finite element simulation model, of which AlN thermal conductivity is calculated through normal-process, Umklapp, and boundary scattering. To further reduce thermal resistance and improve power capacity, the effects of aspect ratio, AlN thickness, the number of resonators, and pitch distance on thermal resistance are investigated. Compared with FBARs with a square electrode, the thermal resistance of the FBAR-on-diamond device is decreased by 43% at an aspect ratio of three. Meanwhile, the optimal AlN thickness is 2 µm, which maintains the balance between thermal resistance and electric performance. The power capacity is increased by 1.93 dB by substituting six resonators for four resonators. The improvement in power handling ability is attributed to the reduced thermal spreading resistance and lower power density. Our study can provide detailed thermal design strategies for high power FBARs toward high throughput data transmission.