Thin-film Bulk Acoustic Wave Resonator Research Articles

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%.

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.

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.

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.

Investigation on high-frequency and mode-coupling vibrations in thickness-extensional piezoelectric thin-film resonators with initial stress

Mode-coupling vibrations in thickness-extensional piezoelectric thin-film bulk acoustic wave resonators (FBARs) with initial stress operating in the ultra-high frequency (UHF) range are investigated by employing the Frequency Spectrum Quantitative Prediction (FSQP) method. The uniaxial uniform initial stresses along the in-plane and thickness directions are separately considered. The dispersion relation of the bulk wave in the FBAR with initial stress is obtained and then the solutions of physical fields are constructed by superimposing the eigen-modes in the dispersion curve. The variational formulation considering the initial stress is derived through the perturbation method. Then frequency spectra that predict the coupling strength between the thickness-extensional mode and other eigen-modes are obtained by substituting the solutions into the variational formulation. Mode shapes of mechanical displacements are presented to illustrate the influence of initial stress on mode-coupling behaviors in FBARs. The results show that the initial stress could cause the shifts of frequency spectrum curves, reflecting that the in-plane initial stress leads to the shifts of spectrum curves along the lateral aspect ratio axis, while the out-of-plane initial stress leads to the shifts along the frequency axis (i.e., frequency shifts) and lateral aspect ratio axis simultaneously. Desirable structural parameters should be chosen based on frequency spectra to avoid the increases of mode-coupling strength induced by initial stress. The out-of-plane initial stress could be used to tune the resonance frequency of the FBARs without changing the mode-coupling behaviors. The coupling strength factor is first defined for the convenient and fast judgment of mode-coupling intensity of all data points in the frequency spectra. The obtained results provide a crucial reference to the manipulation of resonance frequency and the mode-coupling strength in FBAR devices.

Manipulations of micro/nanoparticles using gigahertz acoustic streaming tweezers

Contactless acoustic manipulation of micro/nanoscale particles has attracted considerable attention owing to its near independence of the physical and chemical properties of the targets, making it universally applicable to almost all biological systems. Thin-film bulk acoustic wave (BAW) resonators operating at gigahertz (GHz) frequencies have been demonstrated to generate localized high-speed microvortices through acoustic streaming effects. Benefitting from the strong drag forces of the high-speed vortices, BAW-enabled GHz acoustic streaming tweezers (AST) have been applied to the trapping and enrichment of particles ranging in size from micrometers to less than 100 nm. However, the behavior of particles in such 3D microvortex systems is still largely unknown. In this work, the particle behavior (trapping, enrichment, and separation) in GHz AST is studied by theoretical analyses, 3D simulations, and microparticle tracking experiments. It is found that the particle motion in the vortices is determined mainly by the balance between the acoustic streaming drag force and the acoustic radiation force. This work can provide basic design principles for AST-based lab-on-a-chip systems for a variety of applications.

Phenomenological Circuit Modeling of Ferroelectric-Driven Bulk Acoustic Wave Resonators

A nonlinear circuit model has been developed to model electrostriction in thin-film ferroelectric bulk acoustic wave (BAW) resonators. This new model is based on nonlinear coupling relations among strain, stress, electric field, and polarization, derived from the Landau–Devonshire theory. The coupling relations and the mechanical vibrations are then represented as lumped circuit elements using an improved approximation procedure along with an enhanced model for dielectric loss. The response from the simulation is compared with the measurement results obtained for a barium strontium titanate (BST) thin-film bulk acoustic resonator (FBAR) at different dc bias levels. The improved approximation procedure enables one to achieve accurate simulation results with reduced circuit model complexity.

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.

Suppression of Spurious Lateral Modes and Undesired Coupling Modes in Frame-Like FBARs by 2-D Theory.

Thin-film bulk acoustic wave resonators (FBARs) are of significant interest for wireless applications due to their good performance. To achieve excellent characteristics, the frame-like FBARs were previously invented by researchers in this field to suppress the resonances of spurious lateral modes (or spurious modes). Another aspect that has an impact on the working performance of FBAR is the undesired coupling modes, which were rarely studied due to the lack of proper theoretical tools. In this article, we thoroughly investigate the coupling modes and spurious modes in frame-like FBARs by 2-D theory, which was already derived in our previous work. The widely used state-vector approach is applied here and both free and forced vibration problems are analyzed. Frequency spectra and mode shapes obtained from free vibration analysis show the coupling vibration characteristics in this structure. Moreover, the admittance resonance obtained from the forced vibration analysis shows the efficiency of the designed structure on the suppression of both undesired coupling and spurious modes. Results in this article provide a thorough understanding of the vibration in frame-like FBAR, and the method presented can work as an effective tool in engineering design process.

Two-Dimensional Coupling Vibration Analysis of Laterally Acoustically Coupled Two-Port Thin-Film Bulk Acoustic Resonators

In this paper, we present an approach to studying the mode coupling vibrations in two-port thin-film bulk acoustic wave resonator (FBAR) devices with two pairs of electrodes deposited on the zinc oxide film. The two-dimensional plate theory established in our previous work is employed, which takes into account the coupling of the operating thickness-extensional mode with the extensional, flexural, fundamental and second-order thickness-shear modes. The propagation of straight-crested waves in the plate is studied, and the state-vector approach is successfully used to simplify the derivation process. For a structurally symmetric device, the modes are separated into quasi symmetric and antisymmetric ones. Frequency spectra and corresponding mode shapes are obtained under the stress-free boundary conditions, respectively, and then coupling effects and energy trapping phenomenon are discussed in detail. Some results for structures with asymmetric electrode distributions are also shown. It is found that the choice of aspect ratio has a great effect on mode couplings of FBAR devices. This study will be useful for the design of FBAR filters and sensors.

Effects of mode couplings on the vibration characteristics of partially electroded thin-film bulk acoustic wave resonators

Mode coupling vibrations in thin-film bulk acoustic wave resonators (FBARs) were systematically analyzed in the series of our work. Previously, the two-dimensional theory of multi-layered FBAR plate has been established and used for the analysis of both free and forced vibrations of a fully electroded plate. In this paper, coupling vibrations in a partially electroded FBAR were taken into account, where the top electrode only covers the central part of the film. The two-dimensional equations were written into matrix form based on the state-vector approach. Transfer matrix functions of the global structure were obtained when continuous and traction-free boundary conditions were satisfied. Aspect ratios of both electroded and unelectroded regions were varied to research the influence of them on the coupling effects. The admittance versus frequency response was also obtained through forced vibration analysis to investigate the coupling effects. Results in this paper show the existence of energy-trapping behavior and the importance of structure size on mode coupling effects, which is important to the design of real FBAR devices.

Design of 1.8-mW PLL-Free 2.4-GHz Receiver Utilizing Temperature-Compensated FBAR Resonator

This paper presents a 1.8-mW 2.4-GHz channelized receiver for ISM-band applications. Unlike traditional ISM-band radios which typically require a phase-locked loop (PLL) for channelization, we propose a modified sliding-IF receiver architecture with a suitable local oscillator (LO) frequency plan utilizing a temperature-compensated thin film bulk acousticwave resonator (FBAR). This strategy completely eliminates the need for a PLL by directly dividing down the fixed FBAR oscillator frequency. An inductor-less current-reuse balun LNA is proposed allowing a low-power wideband matching as well as noise cancelling. The frequency conversion is achieved by a hybrid mixer, which stacks a switching mixer on a switched-gm mixer for current reuse. It also features good voltage headroom and common-mode noise rejection. The FBAR-based Colpitts oscillator achieves the phase noise of -144 dBc/Hz at 3.5-MHz offset. The measured RX gain, noise figure, and in-band IIP3 are 57.8 dB, 15.7 dB, and -18.5 dBm, respectively, without external crystal and on-chip inductors, which allows us to reduce the size and weight of the receiver system. It dissipates 0.86 mW (RX) and 0.92 mW (LO) from a single 1-V supply.

Design and Optimization of SHF Composite FBAR Resonators.

We propose an apodization technique-based composite thin-film bulk acoustic wave resonator (c-FBAR) design to enable the displacement and strain energy confinement at the central section of the resonator while in operation at the resonance mode. Sinc-shaped AlN on Silicon on Insulator apodized c-FBARs is designed to attain close to 90% energy localization. In this paper, a single crystal silicon as the mechanical layer and an AlN piezoelectric material as the transducer layer of the resonator implemented by InvenSense Inc.'s AlN MEMS-CMOS platform renders an asymmetric feature to the traditional FBAR resonator. The nature of composite thin-film piezoelectric on substrate FBAR resonators in the super high-frequency (SHF) range is studied in detail. Furthermore, a complete deembedding procedure to extract the resonator parameters from the CMOS + MEMS measured data is also explained meticulously. A complete equivalent circuit modeling for c-FBAR operating in the SHF is provided. Measurement data statistics show that sinc c-FBAR features superior electromechanical coupling coefficient ( ) than that of pentagon c-FBAR. As a result, we successfully demonstrate a sinc c-FBAR resonator operating at 3.264 GHz with an electromechanical coupling coefficient of 2.12%, a loaded quality factor ( ) of 790 and an unloaded of 2507.

Effects of aspect ratio on the mode couplings of thin-film bulk acoustic wave resonators

We studied mode couplings in thin film bulk acoustic wave resonators of a piezoelectric film on a dielectric layer operating with the fundamental thickness-extensional mode. A system of plate equations derived in our previous paper was used which includes the couplings to the unwanted in-plane extension, flexure, fundamental and second-order thickness shear modes. It was shown that the couplings depend strongly on the plate length/thickness ratio. For a relatively clean operating mode with weak couplings to unwanted modes, a series of discrete values of the plate length/thickness ratio should be avoided and these values were determined in the present paper. The results can be of great significance to the design and optimization of film bulk acoustic wave resonators.

High-Quality-Factor and Low-Temperature-Dependence SMR FBAR Based on BST Using MOD Method.

This paper presents the design, fabrication, and experimental evaluation of switchable and tunable thin-film bulk acoustic wave (BAW) resonators and filters with metal-organic solution deposited barium strontium titanate (BST) for radio-frequency (RF) applications. The switchability and tunability of these devices come from utilizing the electrostrictive effect of ferroelectric materials such as BST with the application of an external dc bias voltage. The BAW resonators and filters in this paper were fabricated on a high-resistivity silicon substrate as a solidly mounted resonator structure with eight layers of silicon dioxide and tantalum pentoxide as a Bragg reflector. A 170-nm BST thin-film thickness with Br/Sr (70/30) composition was deposited using the metal-organic deposition solution chemical technique. The measurements of the RF transmission characteristics showed no resonance (OFF state) with zero bias voltage across the terminals of the resonator. At nonzero applied bias, a switching response (ON state) was observed at near 5.44 GHz. With the increasing magnitude of the applied bias across the film, the resonance increased in intensity by showing a quality factor of 746 and shifted to a lower frequency of 5.375 GHz with a tunability of 1.3%. The temperature dependence of the fabricated device was measured from room temperature up to 80 °C and showed a temperature coefficient of frequency of -12.595 ppm/°C.

One watt gallium arsenide class‐E power amplifier with a thin‐film bulk acoustic resonator filter embedded in the output network

Integration of a class-E power amplifier (PA) and a thin-film bulk acoustic wave resonator (FBAR) filter is shown to provide high power added efficiency in addition to superior out-of-band spectrum suppression. A discrete gallium arsenide pseudomorphic high-electron-mobility transistor is implemented to operate as a class-E amplifier from 2496 to 2690 MHz. The ACPF7041 compact bandpass FBAR filter is incorporated to replace the resonant LC tank in a traditional class-E PA. To reduce drain voltage stress, the supply choke is replaced by a finite inductance. The fabricated PA provides up to 1 W of output power with a peak power added efficiency (PAE) of 58%. The improved out-of-band spectrum filtering is compared to a traditional class-E with discrete LC resonant filtering. Such PAs can be combined with linearisation techniques to reduce out-of-band emissions.

Composite ferroelectric FBARs that are switchable between the first and second harmonics: experimental demonstration.

Digital switching between the first and second harmonics, in a composite thin-film bulk acoustic wave resonator (FBAR), is demonstrated experimentally. The FBAR consists of two 180-nm-thick paraelectric-phase Ba0.25Sr0.75TiO3 films separated by a 50-nm-thick SrRuO3 conducting layer. The resonant frequency of this composite resonator (with Pt bottom and Al top electrodes) is switched from 3.6 GHz to 7.6 GHz, where the polarity of the 5 V dc bias is reversed on one of the ferroelectric films. The frequency switching ratio (f2/f1 ≥ 2) depends on the thickness of the electrodes. Some adjustment of f2/f1 is possible by changing the applied dc bias.

Sensitivity analysis of multi-layered C-axis inclined zigzag zinc oxide thin-film resonators as viscosity sensors

This paper presents a theoretical analysis of a new zigzag C-axis inclined multi-layer ZnO thin-film bulk acoustic wave resonator (FBAR) as a viscosity sensor to monitor the lubrication performance of engine oil and other liquids. Free vibration and forced vibration for the FBAR loaded with liquids are analyzed. Equations necessary to calculate the sensitivity are derived. The numerical analysis shows that as the number of layers increases, the absolute sensitivity increases as well. The influences on the sensitivity of C-axis inclined angle, Q-factor, and thickness are also investigated. The results provide a foundation for further design of multi-layer FBAR viscosity sensors.

Analysis of Thickness Vibrations of C-Axis Inclined Zig-Zag Multi-layered Zinc Oxide Thin Film Resonators

The free vibrations of a multilayer C-axis inclined zig-zag ZnO thin-film bulk acoustic wave resonator (FBAR) connected to external impedance have been analyzed; the frequency equation and mode shape for this resonator were derived based on the linear piezoelectric theory. The impedance characteristics of the FBAR are derived and compared with previous experimental results. The analytical result shows that as the layer number increases, the thickness shear mode remains stable while the longitudinal mode has shifted to a lower value. Sensitivity analysis has been performed to explain the discrepancy between our analytical result and previous experiment result. The analytical results provide an insight for the multi-layered zig-zag FBAR design and applications.

Parametric study of laterally acoustically coupled bulk acoustic wave filters

Acoustically coupled thin-film bulk acoustic wave resonator filters, in which the coupling takes place mechanically in the lateral direction between closely-spaced narrow resonators, are a promising approach to passband filtering at gigahertz frequencies. In this paper, filters with interdigital electrode structures are studied. Electrode number, electrode width, and coupling gap width are varied. The resonators are solidly mounted, having an acoustic mirror isolating the resonator from a Si substrate and providing the means to engineer the acoustic dispersion properties of the resonators. The center frequency of the filters is around 2 GHz. Electrical frequency responses of the filters are measured and the strength of the lateral acoustic coupling is calculated from the measurements. The effects of device parameters on the acoustic coupling and the obtainable filter bandwidth are analyzed in detail. A bandpass filter with 4.9% bandwidth, minimum insertion loss of 2 dB and sharp transition from passband to suppression band, is presented.