Film Bulk Acoustic Resonator Research Articles

A Switchless Quad Band Filter Bank Based on Ferroelectric BST FBARs

An intrinsically switchable ferroelectric barium strontium titanate (Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(x)</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(1-x)</sub> TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , BST)-based film bulk acoustic resonator (FBAR) quad channel filter bank is presented for the first time. The design incorporates a matching network free of tunable elements and provides a high isolation >30 dB across all four filter paths when the resonators are switched off. The measured insertion loss for the four 2.5 stage BST FBAR filters range from 6 to 7 dB with an out-of-band rejection >30 dB, which is an improvement over previously reported BST ladder-type structure. The filter bank, realized through T-networks, occupies an area of 840 μm × 340 μm and operates across four different bands solely through the BST-based acoustic resonators.

Dual functionality metamaterial enables ultra-compact, highly sensitive uncooled infrared sensor

Abstract Cointegration and coupling a perfect metamaterial absorber (PMA) together with a film bulk acoustic wave resonator (FBAR) in a monolithic fashion is introduced for the purpose of producing ultracompact uncooled infrared sensors of high sensitivity. An optimized ultrathin multilayer stack was implemented to realize the proposed device. It is experimentally demonstrated that the resonance frequency of the FBAR can be used efficiently as a sensor output as it downshifts linearly with the intensity of the incident infrared irradiation. The resulting sensor also achieves a high absorption of 88% for an infrared spectrum centered at a wavelength of 8.2 μm. The structure is compact and can be easily integrated on a CMOS-compatible chip since both the FBAR and PMA utilize and share the same stack of metal and dielectric layers.

The research of dual-mode film bulk acoustic resonator for enhancing temperature sensitivity

A dual-mode film bulk acoustic resonator (DM-FBAR) temperature sensor modulated by different phosphorous-doped silica insertion layers was reported in this paper. The relative drift of the second and third resonance peaks of FBAR could improve the temperature sensitivity through the dual-mode beat frequency calculation method. The temperature sensitivity can be regulated by different the PH3 doping flow in the SiO2 insertion layer. Among the fabricated devices, FBAR with a SiO2 insertion layer doped 4 sccm PH3 has the highest temperature sensitivity of 64.8 kHz °C−1. It is discovered that the greater the Young’s modulus of the insert film changes with temperature, the higher the temperature sensitivity of DM-FBAR device. Therefore, an important technical means to improve the performance of DM-FBAR devices is using the material whose Young’s modulus is more sensitive to temperature in the future.

Analytical Study of the Film Bulk Acoustic Resonators Based on Single Crystal LiNbO3 with Different Crystal Orientations

Film bulk acoustic resonator (FBAR) based on single crystal LiNbO3 are promising for next-generation high Q, wideband filters in telecommunications. The crystal orientation of LiNbO3 has significant impacts on the FBAR performance, and the bulk acoustic wave (BAW) velocity was calculated by solving the Christoffel’s equation in each crystal orientation. The electromechanical coupling coefficient of every cut was obtained. Moreover, the impedance information and the film displacement of various crystal orientations were analyzed by the finite element method (FEM). Analytical results demonstrate that the effective coupling coefficient and the number of BAW modes vary with the crystal orientation.

Research and Design of High Sensitivity FBAR Micro-mass Sensors

Micro-mass sensors have important application in chemical and biological sensing. Based on the theory of thin film bulk acoustic resonator (FBAR), this paper represents four designs of different micro-mass sensor models. COMSOL Multiphysics™ software is used to simulate the FBAR sensor sensitivity, which with low-impedance acoustic layer and additional electrode frame. In the finate element simulation models, five different values of mass-loading were applied. From the experimental results, the characteristic parameters and the sensitivity of the FBAR sensors were obtained. It is known to us that the FBAR sensor’s frequency quality factor can be improved by using the electrode frame, and its sensitivity can be enhanced by using the low-impedance acoustic layer. To sum up, the above analysis shown that the best sensitivity is 1.351 × 10-12ng/μm2/Hz and the frequency quality factor is 1415, which is the FBAR sensor with a single-step electrode frame and a low-impedance acoustic layer.

Modeling, fabrication, and structural characterization of thin film ZnO based film bulk acoustic resonator

This work reports the finite element modeling, fabrication and structural characterization a film bulk acoustic resonator (FBAR) devices using shadow mask technique. For the performance analysis of the FBAR device, simulations were performed on COMSOL Multiphysics platform. RF sputtering technique is used to deposit c-axis oriented zinc oxide (ZnO) thin film on Aluminum electrodes. The Aluminum electrodes were deposited using e-beam evaporation technique. The X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) techniques were used to examine the surface morphologies of the deposited piezoelectric film (ZnO). The quality of the deposited piezoelectric material film was measured with the help of XRD spectra and AFM is used to measure surface roughness of the deposited ZnO film and its measured value is 6.94 nm. The resonant characteristics of fabricated FBAR device are evaluated with the help of COMSOL Multiphysics simulations and the obtained values of effective electromechanical coupling constant (k2eff), quality factors (Qs and Qp) and Figure of merit are 3.0025%, 1811.3, 1711.4 and 54 (approximately), respectively.

High Q Lateral-Field-Excited Bulk Resonator Based on Single-Crystal LiTaO₃ for 5G Wireless Communication

The paper presents the design and fabrication of lateral-field-excited (LFE) resonators based on 42° Y-cut single-crystal LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (LT) on silicon dioxide (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). A simple coplanar top electrode is defined to excite the bulk acoustic wave modes in the suspended LT/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> structure, and the fabrication process that only involves two lithography steps is more simplified compared to that of commercial film bulk acoustic wave resonators. For a model structure consisting of LT(670 nm)/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (1500 nm) thin film, two types of acoustic modes are both piezoelectrically active in the LT film: the first one is the thickness-shear mode with a resonance frequency of 2.46 GHz, an electromechanical coupling ( <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> ) of 1.4%, a high quality factor ( <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> ) of 1690, and the second one corresponds to longitudinal mode with a resonance frequency of 4.39 GHz, <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> of 1.2%, a high <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> of 1590, which is among the highest reported for piezoelectric MEMS resonators operating at this frequency range. The excellent performance would enable application scenarios including high-resolution sensors, low-phase-noise oscillators, and low-loss, high selectivity filters in the sub-6 GHz range for the fifth-generation (5G) wireless communication.

Dual-mode thin film bulk acoustic wave resonator and filter

Bulk acoustic wave (BAW) filters have been extensively used in consumer products for mobile communication systems due to their high performance and a standard CMOS compatible integration process. However, a traditional BAW filter generally presents only one frequency transmission band; hence, it cannot meet the demands of multi-mode wireless communication systems. In this work, we propose a radio frequency dual-band filter based on a dual-mode film bulk acoustic resonator (FBAR). The dual-mode FBAR consists of molybdenum electrodes and an aluminum nitride film with a c axis tilt angle, which leads to the coexistence of the longitudinal mode and the shear mode in the resonator. The influence of the tilt angle on the resonant frequency and electromechanical coupling coefficient of the dual-mode FBAR is investigated. Subsequently, the behaviors of the dual-mode FBAR have been emulated by a modified Butterworth–Van Dyke model with two motional branches, which is further used to design and optimize the dual-band filter. The dual-mode FBAR-based filters connected in a ladder configuration present two dual passbands of 43 MHz and 105 MHz, respectively. The developed filters with dual passbands can give an inspiration to the design of BAW devices in modern wireless communications.

A Film Bulk Acoustic Resonator Based on Ferroelectric Aluminum Scandium Nitride Films

This work reports on the first demonstration of the frequency tuning and intrinsic polarization switching of film bulk acoustic resonators (FBARs), based on sputtered AlScN piezoelectric thin films with Sc/(Al + Sc) ratio of approx. 30%. A box-like ferroelectric hysteresis behavior of 900 nm-thick Al0.7Sc0.3N sputtered films is obtained, showing a coercive electric field at ~3 MV/cm. The fundamental thickness-mode resonance of the bulk acoustic wave (BAW) resonator is measured at 3.17 GHz frequency with an excellent electromechanical coupling coefficient ( $k_{t}^{2}$ ) of 18.1%. The FBAR frequency response is studied, in both (i) the linear tuning regime, upon application of DC electric fields below the coercive field; as well as (ii) the polarization switching regime, upon application of electric fields above the coercive field. A large linear tuning range of 215 ppm $\times \,\,\mu \text{m}$ /V is obtained in case (i), resulting from the high scandium content. The series resonance frequency of the FBARs is switched ON and OFF in (ii) upon application of 350 V unipolar waveform across the Al0.7Sc0.3N thickness. This is the first demonstration of the intrinsically switchable AlN-based FBARs with a large tuning range; and record high $k_{t}^{2}$ reported for AlN-based FBARs to date. Furthermore, this work paves the way for realization of tunable and switchable wideband acoustic filters operating at super high frequency ranges (SHF). [2020-0203]

Effect of c-axis tilted orientation ZnO thin film on shear-mode bulk acoustic resonator in liquid environment

ZnO has been widely used as piezoelectric materials in bulk acoustic resonators, especially in shear-mode film bulk acoustic resonators (FBAR). In this paper, we studied the effect of c-axis tilted ZnO thin film on shear mode FBAR in liquid environment. Mechanical impedances of FBARs are compared between shear-mode and longitudinal-mode wave due to their different orientations of ZnO thin film. It is found ZnO thin film with 43° c-axis tilted angels induces quasi-shear-mode acoustic wave only. The Q values of FBAR with 43° c-axis tilted ZnO are 256.94 in air at 0.8148 GHz, and 34.34 in water at 0.814 GHz, respectively. The shifts of the resonant frequencies are proportional to the square root of the product of viscosity and density of liquid medium, which is consistent with Kanazawa-Gordon theory and verified by actual experiment.

Active area optimisation of film bulk acoustic resonator for improving performance parameters

In this Letter, an active area optimisation technique to improve the performance parameters of the film bulk acoustic resonator (FBAR) is proposed. The active area of the back trench membrane-based FBAR is optimised to remove the spurious modes, higher harmonic modes, and to confine the acoustic signal at its central part during the resonance. The effect of thickness variation of SiO2 layer on the performance parameters was studied using finite-element analysis (FEA) simulation. The SiO2 film, on silicon substrate, was used as the support layer and zinc oxide was used as the piezoelectric film for the resonator. The authors have successfully demonstrated the FBAR through FEA for an optimised active area of 320 × 320 µm2, series resonance frequency (f s) 1.249 GHz, and parallel resonance frequency (f p) 1.273 GHz with an effective electromechanical coupling coefficient ( k eff 2 ) of 4.65%.

Ultrathin single‐crystalline LiNbO 3 film bulk acoustic resonator for 5G communication

This Letter reports a high-performance film bulk acoustic resonator (FBAR) that can be applied in 5G wireless communication. The FBAR has a back-etched free-standing structure using an ultra-thin single-crystalline lithium niobate (LiNbO3) as the piezoelectric film. The thin LiNbO3 was obtained by the smart cut method and FBARs were fabricated by a standard MEMS process. Owing to the superior bulk-like properties of the single-crystal thin film, the fabricated FBARs have a resonant frequency of 5.0 GHz, a quality factor above 1800 and an electromechanical coupling coefficient of 2.66%, demonstrating a promising potential of FBAR filters in 5G and future 6G applications.

Mo/Ti multilayer Bragg reflector for LiNbO3 film bulk acoustic wave resonators

A 43° rotated Y-cut LiNbO3 (LN) single-crystal thin film coated with a bottom electrode and a Mo/Ti Bragg reflector is successfully transferred onto the 4-in. LN wafer substrate by means of crystal-ion-slicing (CIS) technique and benzocyclobutene bonding method. The multilayer film is crack-free, and the Mo/Ti reflector shows good acoustic reflection efficiency. A thin film bulk acoustic wave resonator based on the Mo/Ti Bragg reflector and the LN thin film (LN-BAW) is prepared, and the properties are further analyzed. The results indicate that the LN-BAW provides a large Kt2 of ∼16.63% and the Q-factor and figure-of-merit of the LN-BAW reach 183 and 30.4, respectively.

A Hybrid Film-Bulk-Acoustic-Resonator/Coupled-Line/Transmission-Line High Selectivity Wideband Bandpass FBAR Filter

A hybrid wideband filter composed of two sections with sharp frequency selectivity is proposed in this article. The first section consists of a coupled line (CL), two transmission lines (TL), and a film bulk acoustic resonator (FBAR) with lower frequency. This section mainly controls the lower roll-off. The other section is composed of two CLs and two FBARs with higher frequencies. This section mainly controls the roll-off of the upper band. The insertion loss and rejection outside the passband are well controlled because of the CLs. The analysis of the proposed filter is realized by combining microwave network theory with a modified Mason model for FBAR. Through analytical analysis, conditions for generating transmission zeros and poles are obtained. The methods of enhanced out-of-band rejection and bandwidth control are proposed through FBAR networks and tuning of FBARs, respectively. To verify the proposed theory, a wideband filter with a central frequency of 2.14 GHz is designed, simulated, and assembled. The FBARs, TLs, and CLs are assembled through a 1-mil wire bond technique. The measured results show good consistency with the simulation results; 24.9% of 3-dB relative bandwidth, 1.87-dB in-band insert loss, and an excellent 0.91 shape factor are obtained.

Analogue gravity on a superconducting chip.

We describe how analogues of a Hawking evaporating black hole as well as the Unruh effect for an oscillatory, accelerating photodetector in vacuum may be realized using superconducting, microwave circuits that are fashioned out of Josephson tunnel junction and film bulk acoustic resonator elements. This article is part of a discussion meeting issue 'The next generation of analogue gravity experiments'.

The thin film bulk acoustic wave resonator based on single-crystalline 43○Y-cut lithium niobate thin films

43°Y-cut lithium niobate (LiNbO3, LN) single-crystalline thin films, which are integrated with two types of Bragg reflectors of W/SiO2 and Mo/SiO2, respectively, are fabricated by means of the crystal-ion-slicing technique. Crack-free LN thin films are obtained when the Mo/SiO2 reflector is adopted, while crumples take place when the LN thin film is combined with W/SiO2. Moreover, solidly-mounted-type thin film bulk acoustic wave resonators (LN-SMRs) based on the single-crystalline LN film and Bragg reflector layers of Mo/SiO2 are prepared and characterized. The results indicate that the LN thin film combined with the Mo/SiO2 reflector has good thin film quality, and keff2 of the LN-SMR reaches 20%.

Design Considerations for Frequency Shifts in a Laterally Finite FBAR Sensor in Contact With the Newtonian Liquid.

Mode-coupled vibrations in a thickness-shear (TSh) mode and laterally finite film bulk acoustic resonator (FBAR) with one face in contact with Newtonian (linearly viscous and compressional) liquid are investigated. With boundary conditions and interface continuity conditions, exact dispersion curves in FBAR sensors contacting with two kinds of liquids are obtained, and they are compared with the dispersion curves in a bare sensor without liquid contact. Frequency spectra, describing mode couplings between the main TSh modal branch and undesirable modal branches, are calculated by employing weak boundary conditions at lateral free edges constructed based on the variational principle. Mode shapes of mechanical displacements in both the sensor and liquid layer are presented, and mode transformations are observed due to the liquid contact and lateral edge effect. The effect of liquid thickness on frequency spectra is also studied. Numerical results reveal that the generation of shear wave in the liquid layer results in the shifts of spectrum curves along the frequency axis and hence it is the main factor of frequency shifts of FBAR sensors. The compressional wave causes the shifts of spectrum curves along the lateral aspect ratio axis. Then for a given FBAR sensor, the liquid thickness changes could also cause frequency shifts. Therefore, desirable vibration modes should be chosen based on the frequency spectra to avoid strong mode couplings and to eliminate frequency shifts induced by the liquid thickness changes in real applications.

Highly Tunable Film Bulk Acoustic Wave Resonator Based on Pt/ZnO/Fe65Co35 Thin Films.

We have developed a highly tunable film bulk acoustic wave resonator (TFBAR) using magnetostrictive (MS) Fe65Co35 thin films in acoustic layer stack. The resonator acoustic layer stack consists of Pt/ZnO/Fe65Co35 layers to tune the devices. Due to ∆E effect, TFBAR resonance frequency was up-shifted ~106.9 MHz (4.91%) in the presence of 2-kOe magnetic field. From experimental measurement, ∆E enhancement was estimated to be ~35 GPa. Further, it is observed that return loss ( S11 ), phase response, and quality factor were improved in the presence of magnetic field. This improvement is due to the field-induced stiffness in the magnetic layer. Equivalent-modified Butterworth-Van Dyke (mBVD) circuit model was developed and fit with the experimental data, and circuit parameters were extracted. The proposed resonator is compact, low loss, power efficient, and highly tunable. This method also facilitates a new method of tuning FBAR devices using MS thin films.

Two-Dimensional Plate Theory for the Analysis of Coupling Vibrations in Shear Mode FBARs

The shear mode film bulk acoustic wave resonators (FBARs) have been developed rapidly in recent years due to the increased interest in biosensors. In this paper, the mode coupling effects which are unavoidable in real devices are theoretically analyzed and discussed. Mindlin's approximation method of expanding the displacement fields with power series along the plate thickness direction is employed and a system of two-dimensional plate equations is obtained. The accuracy of the plate theory is validated through the comparison of dispersion curves with the results calculated by three-dimensional exact theory. Free coupling vibrations of the cross-sectional plane of finite-sized shear mode FBAR plates are further studied with the derived two-dimensional theory. Results in this paper show that the presented plate theory is accurate and efficient for the structural analyses of shear mode FBARs. Besides, the selection of aspect ratios can be used to avoid strong mode couplings, which is helpful for device designs.

Bulk acoustic wave resonator based wireless and passive pressure sensor

In this work, a wireless and passive pressure sensor based on film bulk acoustic wave resonator (FBAR) has been proposed and investigated experimentally. Finite element analysis (FEA) was performed to determine the vibration mechanism and modes of the film in the device. The FBAR device was fabricated using traditional MEMS process and the pressure sensor was obtained by sealing the back-etched cavity of the device on a glass substrate. The network analyzer was used to characterize the wireless and passive performance of the sensor. The results showed that the pressure variation of the surroundings can be detected by measuring the frequency shift of the sensor wirelessly. The sensor has a fast response time of about 1 s, and a pressure sensitivity of 1.5 ppm/kPa with an excellent linearity of 0.997 in the pressure range from 17 kPa to 200 kPa at the temperature of 25 °C. Moreover, the sensor also possesses an excellent linearity frequency response with temperature change, demonstrated its potential for wireless passive sensing applications.