Film Bulk Acoustic Resonator Filters Research Articles

Design and Fabrication of 3.5 GHz Band-Pass Film Bulk Acoustic Resonator Filter.

With the development of wireless communication, increasing signal processing presents higher requirements for radio frequency (RF) systems. Piezoelectric acoustic filters, as important elements of an RF front-end, have been widely used in 5G-generation systems. In this work, we propose a Sc0.2Al0.8N-based film bulk acoustic wave resonator (FBAR) for use in the design of radio frequency filters for the 5G mid-band spectrum with a passband from 3.4 to 3.6 GHz. With the excellent piezoelectric properties of Sc0.2Al0.8N, FBAR shows a large Keff2 of 13.1%, which can meet the requirement of passband width. Based on the resonant characteristics of Sc0.2Al0.8N FBAR devices, we demonstrate and fabricate different ladder-type FBAR filters with second, third and fourth orders. The test results show that the out-of-band rejection improves and the insertion loss decreases slightly as the filter order increases, although the frequency of the passband is lower than the predicted ones due to fabrication deviation. The passband from 3.27 to 3.47 GHz is achieved with a 200 MHz bandwidth and insertion loss lower than 2 dB. This work provides a potential approach using ScAlN-based FBAR technology to meet the band-pass filter requirements of 5G mid-band frequencies.

Temperature Cycle Reliability Analysis of an FBAR Filter-Bonded Ceramic Package.

On the background that the operating frequency of electronic devices tends to the radio frequency (RF) segment, a film bulk acoustic resonator (FBAR) filter is widely used in communication and military fields because of its advantages of high upper frequency, ample power capacity, small size, and low cost. However, the complex and harsh working environment puts higher requirements for packaging FBAR filters. Based on the Anand constitutive equation, the stress-strain response of the bonded ceramic package was studied by the finite element method for the FBAR filter-bonded ceramic package, and the thermal fatigue life of the device was predicted. We developed solder models with various spillage morphologies based on the random generation technique to examine the impact of spillage on device temperature reliability. The following are the primary conclusions: (1) Solder undergoes periodic deformation, stress, and strain changes throughout the cycle. (2) The corner of the contact surface between the chip and the solder layer has the largest stress at the end of the cycle, measuring 19.377 MPa. (3) The Engelmaier model predicts that the gadget will have a thermal fatigue life of 1928.67 h. (4) Expanding the layered solder area caused by any solder overflow mode may shorten the device's thermal fatigue life. The thermal fatigue life of a completely spilled solder is higher than that of a partially spilled solder.

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.

Design and Fabrication of Temperature-Compensated Film Bulk Acoustic Resonator Filter Based on the Stress Compensation Effect

To ensure that the performance of filters matches the continuous development in communication frequency bands, the influence of temperature on filter performance must be considered during the fabrication of filters. In this study, a cavity-type temperature-compensated film bulk acoustic resonator (TC-FBAR) device was prepared with an SiO2 temperature filter between the bottom electrode and the piezoelectric layer. A one-dimensional Mason model of the TC-FBAR was established. An advanced design system, a high-frequency structure simulator, and COMSOL software were used to optimize the design of the TC-FBAR. After the optimization, the out-of-band rejection was improved by 10 dB. To address the compensation effect of the tensile and compressive stresses, a multilayer film was implemented for low-stress control and a reduction in stress to |P| ≤ 150 MPa, thereby improving the orientation of the piezoelectric film. Moreover, the influence of the thickness of the SiO2 temperature-compensated layers on the temperature-compensated characteristics was studied. When the SiO2 thickness was 50 nm, the temperature coefficient of frequency (TCF) was ±1 ppm/°C. The center frequency and 3 dB bandwidth of TC-FBAR were 2.492 GHz and 15.02 MHz, respectively, and the center insertion loss was −3.1 dB. Moreover, the out-of-band rejection was greater than 40 dBc, and TCF was 0.8 ppm/°C.

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.

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.

Compact Intrinsically Switchable FBAR Filters Utilizing Ferroelectric BST.

Intrinsically switchable thin film bulk acoustic resonator (FBAR) filters based on ferroelectric barium strontium titanate (BST) are presented. A 1.5-stage -network intrinsically switchable FBAR filter unit cell with a 3-dB fractional bandwidth of 3% at 2 GHz is systematically designed, simulated, and fabricated. The minimum insertion loss (IL) for the filter unit cell is 2.25 dB, representing the lowest IL reported for BST bulk acoustic wave filters to date, which is mainly due to its high BST resonators. Two 1.5-stage filter unit cells are connected in series to form a 2.5-stage filter, providing more than 25 dB of out-of-band rejection and OFF-state isolation between the input and the output ports. The measured input third-order intercept point (IIP3) of the 2.5-stage filter is 47 dBm. Furthermore, the footprint of the fabricated filters is notably small, due to the high permittivity of BST.

An Investigation of Lateral Modes in FBAR Resonators.

Using first principles and the constitutive equations for a piezoelectric, we solve the 2D acoustic wave inside a single, infinite, piezoelectric membrane in order to study the dispersion of Thin Film Bulk Acoustic Resonator (FBAR) lateral modes, with and without infinitely-thin electrodes. The acoustic eigenfunction is a dual wave, composed of longitudinal and shear components, able to satisfy the 2D acoustic boundary conditions at the vacuum interfaces. For the single piezoelectric slab we obtain analytical expressions of the dispersion for frequencies near the longitudinal resonant frequency (Fs) of the resonator. These expressions are more useful for the understanding of dispersion in FBARs and more elegant than numerical methods like Finite Element Modeling (FEM) and various matrix methods. We additionally find that the interaction between the resonator's electrodes and the acoustic wave modifies the lateral mode dispersion when compared to the case with no electrodes. When correctly accounting for these interactions the dispersion zero is placed clearly at Fs, unlike what is calculated from a 2D model without electrodes where the dispersion zero is placed at Fp. This is important since all experimental evidence of measures FBAR resonators shows that the dispersion zero is at Fs. Furthermore, we introduce an electrical current flow model for the propagating acoustic wave inside the electroded piezoelectric and based on this model we can discuss an electrode-loss mechanism for FBAR lateral modes which depends on dispersion. From our model it results that lateral modes with real kx have higher electrode dissipation if they are closer to the resonant frequency. This is consistent with the typical behavior of measured FBAR filters where the maximum lateral mode damage on the insertion loss takes place for frequencies immediately below Fs.

Electrode influence on effective electromechanical coupling coefficient of thin film bulk acoustic resonators

In this paper, the finite element method was employed to study the electrode influence on the effective electromechanical coupling coefficient of thin film bulk acoustic resonators (FBARs). A three-dimensional FBAR model was built to characterise the resonator behaviour. The calculation results revealed that the electrode properties including electrode material, electrode configuration, electrode area and the thickness ratio of electrode/piezoelectric layers had a significant effect on the of the device. Since the electrode has a pronounced influence on the performance of FBAR, it is necessary to take electrode parameters into account during the subsequent design stage of FBAR filters or sensors based on FBAR.

Low-Loss Broadband Package Platform With Surface Passivation and TSV for Wafer-Level Packaging of RF-MEMS Devices

Packaging of radio frequency (RF) microelectromechanical system (MEMS) devices requires a good electrical performance, and thus requires the parasitic effects of packaging to be minimal. This paper presents the design, fabrication, and characterization of improved low-loss wafer-level packaging (WLP) platform with through-silicon-via (TSV) interposer for RF-MEMS packaging. The insertion loss because of parasitic effects is reduced using optimized grounding configuration and surface passivation. The coplanar waveguide (CPW) test vehicle, high-frequency antenna, RF tuner, and film bulk acoustic resonator (FBAR) filter are fabricated and characterized using the developed WLP platform with TSV. To determine the RF performance of the package, the CPW transmission lines are fabricated on high-resistivity Si substrates, and the grounding configuration is optimized. The fabrication of the RF-MEMS WLP involves the process of a TSV cap wafer and CPW transmission lines or RF MEMS on the substrate wafer. TSV cap process includes TSV etching, void-free TSV plating, and redistribution layer postprocess on thin TSV wafer. The electrical characterization of the fabricated devices is performed. The optimized model has a wide bandwidth of 26.5 GHz and a packaging loss of 0.1 dB at 10 GHz. This implies that the effect of packaging on the performance of RF device is expected to be minor. The developed WLP platform is used for the 94-GHz antenna, RF tuner, and FBAR filter packaging and the characterization of RF-MEMS devices is presented.

Simulation and Tuning of Film Bulk Acoustic Resonator Filters with Different Ladder Topologies

Different topologies and tuning of ladder filter made up of film bulk acoustic resonator (FBAR) using Modified Butterworth-Van Dyke (MBVD) model is studied. The influence to the filter by singly adding series or shunt FBAR is separately simulated. The three-stage ladder filter with different topologies is especially investigated. The five-stage ladder filter is studied by changing the inherent components of Lm and C0 in the FBAR.

Wideband Lithium Niobate FBAR Filters

Filters based on film bulk acoustic resonators (FBARs) are widely used for mobile phone applications, but they can also address wideband aerospace requirements. These devices need high electromechanical coupling coefficients to achieve large band pass filters. The piezoelectric material LiNbO3 complies with such specifications and is compatible with standard fabrication processes. In this work, simple metal—LiNbO3—metal structures have been developed to fabricate single FBAR elements directly connected to each other on a single chip. A fabrication process based on LiNbO3/silicon Au-Au bonding and LiNbO3 lapping/polishing has been developed and is proposed in this paper. Electrical measurements of these FBAR filters are proposed and commented exhibiting filters with 8% of fractional bandwidth and 3.3 dB of insertion losses. Electrical measurements show possibilities to obtain 14% of fractional bandwidth. These devices have been packaged, allowing for power handling, thermal, and ferroelectric tests, corresponding to spatial conditions.

An FBAR‐ and LTCC‐based RF front‐end module for Wi‐Fi and WiMAX dual‐mode systems

AbstractA compact quad‐band RF front‐end module (FEM) is proposed for Wi‐Fi and WiMAX dual‐mode systems using film bulk acoustic resonator (FBAR) and low temperature co‐fired ceramic (LTCC) technologies. The quad‐band RF FEM consists of three LTCC bandpass filters, two diplexers, one FBAR filter, two single‐pole‐double‐throw switches, and one single‐pole‐four‐throw switch. The diplexers with the bandpass filters for each mode are fully embedded in the LTCC substrate, whereas the FBAR filter and three RF switches are placed on top of the substrate. The measured performance of the embedded diplexers with the bandpass filters is compared to the simulated one, resulting in good agreement between both performances. The implemented quad‐band RF FEM provides relatively low insertion loss with high attenuation at other passband for each mode. The overall size of the module is 6.0 mm × 5.0 mm with a substrate thickness of 0.6 mm. © 2010 American Association of Anatomists Microwave Opt Technol Lett 52: 753–757, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25023

Easily adjustable FBAR filter circuit

A filter circuit employing film bulk acoustic resonators (FBAR) or other types of piezoelectric resonators is proposed. It consists of two FBARs and two amplifiers, and is equivalent to a lattice FBAR filter. The basic advantage of the new circuit is its easy tuning by using the amplifier output impedances and/or extra resistors.

Film bulk acoustic resonator filters with a coplanar waveguide

Multiple thin film bulk acoustic resonators (10, 11) configured in series (10) and parallel (11) within a coplanar waveguide line structure provides a compact ladder filter. The resonators (10, 11) are formed over an opening (28) in a substrate (20) and connected to associated circuitry by one or more transmission lines formed on the substrate (20). The arrangement of the resonators (10, 11) between the ground and signal lines of a coplanar line structure provides a means of minimising the area of the filter. Embedding a ladder filter within the coplanar transmission line structure eliminates the need for wire bonds, thus simplifying fabrication. Embodiments for 2×2, and hither order filters are described.

Closed-Form Expressions for the Design of Ladder-Type FBAR Filters

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Filters based on film bulk acoustic resonators (FBARs) promise to improve the performance of actual filters, however, to the authors' knowledge, their systematic implementation is not straightforward. This letter presents a collection of closed-form expressions for the systematic design of ladder-type filters using FBARs. The procedure is validated against the work of other authors, finding a very good agreement between results. </para>

Design and fabrication of integrated film bulk acoustic resonator and filter on silicon nitride membrane

AbstractIn this paper, fully integrated film bulk acoustic resonators (FBARs) and filter have been designed, fabricated, and characterized by using silicon bulk micromachining technology. The fabricated resonator is comprised of aluminum nitride (AlN) piezoelectric material sandwiched by two molybdenum (Mo) electrodes on top of silicon nitride thin membrane. The developed AlN piezoelectric film has 1.9° of FWHM (full width half maximum) of rocking curve and approximately 70 Mpa in tensile stress on top of the Mo electrode. The fabricated FBAR has a quality factor of 1530 in size of 200 × 200 μm2, insertion loss of −0.45 dB, and return loss of −28 dB, respectively. The fabricated FBAR ladder type filter is comprised of 7 resonators with 3 resonators in series and 4 resonators in parallel. It has insertion loss of −3.2 dB and ripple of 0.4 dB in pass band, absolute attenuation of 40 and 50 dB at 1.6–1.875 GHz and 2.110–2.170 GHz, respectively. The size of the fabricated FBAR filter is 1.2 mm × 1.4 mm before packaging and 3 mm × 3 mm after packaging, respectively. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2230–2233, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21905

Encapsulation of film bulk acoustic resonator filters using a wafer-level microcap array

A simple method of protecting film bulk acoustic resonator (FBAR) filters by wafer-level packaging is proposed. This method employs a reliable silicon microcap structure to encapsulate FBAR filters. Since only conventional processes and facilities were required to accomplish this work, it is a cost-effective way to package FBAR filters. The fabrication process consists of three steps: cavity etching, wafer bonding and wafer singulation. In this study, the design and fabrication processes are described in detail. Moreover, a filter designed for personal communication systems (PCS) transmitter band is measured to demonstrate the performance variation after the microcap process. The fabrication results and measurement results have ensured the effectiveness of this process.

Modeling and Simulation of Thin-Film Bulk Acoustic Resonators

The PZT-based thin film bulk acoustic resonator (FBAR) structure had been simulated and optimized. The results revealed that the resonant frequency of the FBAR can be controlled by adjusting the mono-layer thickness of the sandwich structure. Dependence of resonant frequency upon the thickness of the piezoelectric film was obtained. The relation of the buffer layer thickness and the quality factor (Q value) had been investigated. The structure of a ladder-type FBAR filter with the center frequency of 2 GHz and the 3-dB bandwidth of 100MHz had been designed. The CPW for the FBAR with characteristic impedance of 50 z was optimized.

High frequency thin film ferroelectric acoustic resonators and filters

There is a need for more compact filters that are able to meet the needs of the next generation of mobile phone operating in the 1-2GHz frequency band. These filters are required to have low insertion loss, high Q, low sensitivity to temperature and in some cases wider bandwidths than those currently available. In this paper we review some of our recent work on the application of PZT (sol-gel PbZr0.3Ti0.7O3) as the piezoelectric layer in FBARs (thin Film Bulk Acoustic Resonators) and FBAR filters. The high electromechanical coupling coefficients in PZT offer the possibility of realising wide band-width filters. It is shown possible to incorporate PZT into a FBAR structure previously designed for piezoelectric ZnO by the simple addition of a TiO2 layer that avoids the crazing that occurs when PZT is deposited directly on SiNx. PZT FBAR filters of extremely small area (<300μ m2) can be produced. Preliminary measurements suggest that the resonance frequencies in PZT FBARs are only weakly temperature dependent. The advantages and disadvantages of using PZT in FBAR structures is discussed.