Film Bulk Acoustic Resonator Devices Research Articles

High-Q film bulk acoustic resonator with high quality AlN film based on transfer method

Thin film bulk acoustic resonator (FBAR) with high quality factor (Q) is preferred for many communication applications. AlN film deposited in conventional FBAR fabrication process is poor, resulting in unsatisfactory device performance. This study demonstrated the comparison of two Si-substrate FBARs, adopting the same AlN deposition process by physical vapor deposition (PVD) method, however using different device fabrication method. Instead of depositing AlN film on Mo/SiO2/Si substrate in existing fabrication process, we deposited AlN directly on Si substrate by PVD to get better crystal quality piezoelectric layer of the FBAR. The full width at half maximum (FWHM) of rocking curve and surface roughness of AlN directly deposited on Si substrate are 1.4° and 1.96 nm, while those of AlN deposited on Mo/SiO2/Si substrate are 8.5° and 5.54 nm, respectively, indicating AlN deposited directly on Si substrate has a better crystal quality than that on Mo/SiO2/Si substrate. The dielectric loss of FBAR using AlN developed on Si substrate decreases from 0.4 Ω to 0.11 Ω leading to the increase of Q m from 470 to 830. By preserving the high quality AlN deposited on Si substate with film transfer method, the FBAR device enjoys an increasing of Q-values as high as 76%.

2D scalar wave modelling of apodized RF BAW resonators for transverse mode analysis

This paper proposes the use of a two-dimensional scalar wave model for fast analysis of apodized AlN-based radio frequency bulk acoustic wave devices. First, the working principle and model setup is introduced. Only one particular Lamb mode (S1−) is considered, and its behavior is modeled by a simple scalar wave equation looking from the top surface. Multiple layers are added to the side boundaries so that mode conversion to other modes is treated as leakage to surroundings. The wavenumber of field distribution for each resonance mode is calculated, and then its excitation coefficient, quality factor, and resonance frequency are derived. The device input admittance is calculated by applying derived parameters to the equivalent circuit. A series of oval-shaped AlN-based film bulk acoustic resonator devices are simulated and compared with experimental results. The theoretical results agreed well with the experimental ones, and accuracy and effectiveness of the present analysis are verified.

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.

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

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

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.

A 2-GHz FBAR-Based Transformer Coupled Oscillator Design With Phase Noise Reduction

The oscillator architecture is proposed to reduce the close-in phase noise as well as power consumption. The proposed Colpitts oscillator removes the current source which is the main source of the 1/ $f$ noise conversion. The removal of the current source is compensated by using the transformer and cross-coupled capacitors. The 2-GHz film bulk acoustic resonator (FBAR) is used for the high Q frequency selective elements in the oscillator design. According to measurement results, this FBAR-based oscillator shows $\geq$ 10 dB reduction in close-in phase noise. Specifically, 12-dB lower phase noise is achieved at 100 Hz offset frequency. Measured power consumption is 350 uW, which is almost half of the conventional Colpitts oscillator using same FBAR device.

Process optimization and device variation of Mg-doped ZnO FBARs

Thin film bulk acoustic resonator (FBAR) plays a very important role in radio frequency (RF) filters used in cell phone and other wireless systems. Although FBAR is commercialized, the design/process interactions on the frequency response variation in FBAR device are still lacking. Design and fabrication are two crucial aspects affecting FBAR device performance. In this report, various solidly mounted resonators (SMR) were designed, fabricated and analyzed to study wafer-level site-to-site RF variation on design and fabrication process. As a key process step for SMR FBAR, the optimization process of Mg-doped ZnO piezoelectric thin film deposition was studied by varying thin film sputtering conditions using various sputtering targets and by post annealing treatment after the deposition. The quality of this crucial layer was verified by XRD on its (0 0 2) crystallization and wafer-level FBAR RF characterization. FBAR devices with high quality were fabricated with an excellent resonant behavior near 2 GHz and a maximum return loss of −15 to 25 dB. Quality factor Q ranges from 400 to 800, with a coupling coefficient keff2 of 1.5–3%. Wafer-level and wafer-to-wafer variation of central frequency are within 1.8–2.1 GHz. Computer simulation verified that this frequency variation correlates to the piezoelectric film variation of 1.6–1.9 μm. Process control on this piezoelectric thin film is essential to maintaining the resonator frequency-controlled value when building duplexer RF circuits. The dependency of RF performance on FBAR size, density and orientation is not obvious, compared to that of the wafer-level FBAR device variation on fabrication process. Regarding to the Mg-doping effect in MgxZn1-xO piezoelectric film, the amount of Mg in MgxZn1-xO film during the sputtering process must be properly controlled within 30% to keep the piezoelectric quality. The average acoustic speed of the Mg-doped ZnO film is 6870 m/s with the estimated range of 5760–7980 m/s, which is better than that of pure ZnO film (6330 m/s).

Temperature calibrated on-chip dual-mode film bulk acoustic resonator pressure sensor with a sealed back-trench cavity

We propose an on-chip film bulk acoustic resonator (FBAR) device based pressure sensor with a simple cavity structure. By sealing the back trench cavity of the FBAR device, the FBAR device become sensitive to the outside environmental pressure. The pressure sensor possesses two resonant peaks with high quality factors and coupling coefficients, which exhibit different temperature coefficients of frequency, and could be used as on-chip temperature compensation. The results show that mode 1 resonant peak of the FBAR pressure sensor has a pressure sensitivity of 1.642 ppm kPa−1, while that of mode 2 resonant peak is 0.1764 ppm kPa−1. Both of the two resonant peaks have high pressure response linearity above 0.999 in the pressure ranges of 17 kPa to 101 kPa and 101 kPa to 400 kPa. A method for temperature calibration is provided. The pressure sensor is simple in package and small in dimension, and the output data is frequency, which can easily be transmitted wirelessly, demonstrating great potential for applications such as the tire pressure monitor system.

Biosensor for human IgE detection using shear-mode FBAR devices.

Film bulk acoustic resonators (FBARs) have been evaluated for use as biosensors because of their high sensitivity and small size. This study fabricated a novel human IgE biosensor using shear-mode FBAR devices with c-axis 23°-tilted AlN thin films. Off-axis radio frequency (RF) magnetron sputtering method was used for deposition of c-axis 23°-tilted AlN thin films. The deposition parameters were adopted as working pressure of 5 mTorr, substrate temperature of 300°C, sputtering power of 250 W, and 50 mm distance between off-axis and on-axis. The characteristics of the AlN thin films were investigated by X-ray diffraction and scanning electron microscopy. The frequency response was measured with an HP8720 network analyzer with a CASCADE probe station. The X-ray diffraction revealed (002) preferred wurtzite structure, and the cross-sectional image showed columnar structure with 23°-tilted AlN thin films. In the biosensor, an Au/Cr layer in the FBAR backside cavity was used as the detection layer and the Au surface was modified using self-assembly monolayers (SAMs) method. Then, the antigen and antibody were coated on biosensor through their high specificity property. Finally, the shear-mode FBAR device with kt2 of 3.18% was obtained, and the average sensitivity for human IgE detection of about 1.425 × 105 cm2/g was achieved.

Micro Through-hole Array in top Electrode of Film Bulk Acoustic Resonator for Sensitivity Improving as Humidity Sensor

This paper reports a Film Bulk Acoustic Resonator (FBAR) with micro through-hole array in the top electrode. When the FBAR device is used as a sensor based on adsorption and its piezoelectric actuation layer under the top electrode acts as sensitive layer, the through-hole array provide paths for gas to reach the sensitive layer directly and improves sensitivity of the sensor. In addition, thermal modules, a temperature sensor and a heater, are added to monitor and control working temperature of the resonator, and high temperature is available to enhance thermal desorption. We applied this FBAR system to a humidity sensor. The results show that the micro through-hole array design made sensitivity rise to 18.6ppm/1%, namely 21.2kHz/1%, which was 18 times higher than samples without. Meanwhile, the thermal modules kept working temperature stable and made it vary with the power supplied.

Electromagnetic Compatibility Analysis Method for FBAR Devices

With the increasing of resonant frequency and integration of film bulk acoustic resonator (FBAR), the electromagnetic interference (EMI) of FBAR devices appears to be extremely important. The electric fields of the common electrical model and finite element model are assumed to be quasi-static and cannot simulate the electromagnetic (EM) property. A 3-D EM model of FBAR is achieved by means of high frequency EM simulation software HFSS. A real-time simulation of EM distributed effects, EM coupling and piezoelectric effects is achieved by employing an effective permittivity in the EM model. The effects of the high frequency EM distributed effects and the EM coupling on resonant characteristics are analyzed. By optimizing the distance between FBAR and components, introducing substrate materials with different permittivity, the EM coupling is effectively reduced.

ZnO based SAW and FBAR devices for bio-sensing applications

ZnO thin film based surface and bulk acoustic wave devices are reviewed in this paper. The films were initially produced using a standard RF sputtering technique. However in order to produce lower stress, smoother films at low temperatures, a novel High Target Utilisation Sputtering (HiTUS) system has also been utilised. The ZnO acoustic devices have been used to move, mix, pump, eject and atomise liquids depending upon the amplitude of the signal and the condition of the surface. Such Surface Acoustic Wave (SAW) devices have also been shown to act as bio sensors but more sensitive detection is obtainable by the use of Film Bulk Acoustic Resonators (FBARs), the design and operation of which are described at the end of this review. Whilst the interaction of the acoustic wave with a fluid on its surface allows its rheological properties to be measured, such as viscosity, it is speculated that the combination of SAW and FBAR technologies may also provide new opportunities for rheometry on the microscale where fluids generally follow a non-Newtonian behaviour.

A Multiple Resonant Mode Film Bulk Acoustic Resonator Based on Silicon-on-Insulator Structures

We report a multiple resonant mode film bulk acoustic resonator (FBAR) with different AlN film thicknesses of 605 nm, 640 nm and 680 nm. With the tilted c-axis orientation of the AlN piezoelectric film providing polarization vertical to the c-axis, acoustic wave resonant peaks have been observed for both the thickness shear modes (TSM 0th, TSM 1st, TSM 2nd) and the thickness extension modes (TEM 0th, TEM 1st). The corresponding parallel resonant frequencies are around 1.60 GHz, 2.41 GHz, 3.45 GHz, 2.75 GHz and 4.10 GHz, respectively. The latter two TEM modes also have good quality factors, and high equivalent electromechanical coupling coefficients Keff2 of 647, 3.13% and 113, 6.23%, respectively. By etching the 1.8 μm silicon sacrificial layer, the air gap FBAR devices have been fabricated in an easier and cleaner way resulting in a low insertion loss of −2.2 dB. The overall device structure of the top electrode/AlN film/bottom electrode on SiO2/silicon-on-insulator (SOI) substrate potentially enables CMOS compatibility. These multiple resonant mode FBAR devices will promote the integration of multi-band filters on a single chip. Improvements of the fabrication process, the influence of different AlN film thicknesses and theoretical analyses of the coexistence of multiple resonant modes are presented.

Effect of design parameters on thin film bulk acoustic resonator performance

Film bulk acoustic resonators (FBARs) have received considerable attention for micro-electromechanical systems (MEMS) applications such as RF filters, resonators, and recently, bio-sensors. FBARs consisting of a piezoelectric thin film and an acoustic reflector for acoustic isolation from the surrounding medium are advantageous due to the intrinsic high sensitivity to mechanical, chemical, or electrical perturbation and compatibility with standard CMOS processing. In this work, we investigated the resonance characteristics of acoustic Bragg reflector-type FBARs by the model-based parametric analysis. The effect of various thicknesses and materials of electrodes and structural variation of acoustic Bragg reflectors on the key performance characteristics was studied to identify optimal design parameters for high performance FBAR devices. The key performance characteristics of FBARs include impedance characteristics, reflection coefficients, electro-mechanical coupling coefficients, and Q factors. These results not only offer new insights into design guidelines for high performance FBAR devices but also help better understanding of the fundamental physical phenomenon at resonances of the device.

Direct comparison of the gravimetric responsivities of ZnO-based FBARs and SMRs

Film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs) have the potential to significantly improve upon the sensitivity and minimum detection limit of traditional gravimetric sensors based on quartz crystal microbalances (QCMs) and surface acoustic wave resonators (SAWs). To date, neither FBAR nor SMR devices have been demonstrated to be superior to the other; hence the choice between them depends primarily on the users’ ability to design/fabricate membranes and/or Bragg reflectors. In this work, it is shown that identically designed FBAR and SMR devices resonating at the same frequency exhibit different responsivities to mass loadings, Rm, and that the SMRs are less responsive than the FBARs. For the specific device design and resonant frequency (~2GHz) of the resonators presented here, the FBARs’ mass responsivity is ~20% greater than that of the SMRs’, and although this value is not universal for all possible device designs, it clearly shows that FBAR devices should be favoured over SMRs in gravimetric sensing applications where the FBARs’ fragility is not an issue. Numerical calculations based on Mason's model offer an insight into the physical mechanisms behind the greater FBARs responsivity, and it was shown that the Bragg reflector has an effect on the acoustic load at one of the facets of the piezoelectric films which is in turn responsible for the SMRs’ lower responsivity to mass loadings.

웨이퍼 레벨 패키지를 적용한 저가격 고성능 FBAR 듀플렉서 모듈

본 논문에서는 US-PCS(US-personal communications services)를 위해 사용이 가능한 저가격 고성능 FBAR (film bulk acoustic resonator) 듀플렉서(duplexer) 모듈(module)을 제시하였다. FBAR 소자는 일반적인 실리콘(Si) 기반의 공정보다 가격경쟁력이 우수한 유리(glass) 웨이퍼 기반의 패키지를 개발하여 적용하였다. FBAR 듀플렉서 모듈의 전송단(Tx)과 수신단(Rx)에서 얻어진 최대 삽입손실 특성은 각각 1.9 dB와 2.4 dB이다. 전송단 및 수신단 FBAR 소자와 본딩(bonding)된 유리 기반의 웨이퍼 및 PCB 기판과 몰딩(molding) 물질을 모두 포함하는 FBAR 듀플렉서 모듈의 전체 두께는 1.2 mm이다. This paper presents a cost-effective and high-performance film bulk acoustic resonator (FBAR) duplexer module for US-PCS handset applications. The FBAR device uses a glass wafer level packaging process, which is a more cost-effective alternative to the typical silicon capping process. The maximum insertion losses of the FBAR duplexer at the Tx and Rx bands are of 1.9 and 2.4 dB, respectively. The total thickness of the duplexer module is 1.2 mm, including the glass-wafer bonded Tx/Rx FBAR devices, PCB board, and transfer molding material.

Application of ICP Deep Trenches Etching in the Fabrication of FBAR Devices

It is presented a fabrication processing of a two step method in deep silicon etching for MEMS applications using an the UK company Surface Technology Systems plc (STS), inductively-coupled plasma (ICP) etch technique STS ICP deep dry etching system. A brief introduction of schematic process of etching deep trenches on silicon substrate is first given, then with two step method for etching deep trenches. The film bulk acoustic resonator (FBAR) devices have been fabricated using STS ICP deep dry etching system with maximum etch rate of 4.6μ m/min, depth more than 450μm and sidewall roughness no more than 0.14μm. At the end of the second step process, the etch selective ratio of silicon to silicon oxygen is enhanced to ensure the device yield. At the same time, the negative effects such as microloads effects, footing effects, lag effects and micrograss effects are suppressed effectively.

ZnO-Based FBAR resonators with carbon nanotube electrodes

Film bulk acoustic resonator (FBAR) devices with carbon nanotube (CNT) electrodes directly grown on a ZnO film by thermal chemical vapor deposition have been fabricated. CNT electrodes possess a very low density and high acoustic impedance, which reduces the intrinsic mass loading effect resulting from the electrodes¿ weight and better confines the longitudinal acoustic standing waves inside the resonator, in turn providing a resonator with a higher quality factor. The influence of the CNTs on the frequency response of the FBAR devices was studied by comparing two identical sets of devices; one set comprised FBARs fabricated with chromium/ gold bilayer electrodes, and the second set comprised FBARs fabricated with CNT electrodes. It was found that the CNTs had a significant effect on attenuating traveling waves at the surface of the FBARs' membranes because of their high elastic stiffness. Three-dimensional finite element analysis of the devices fabricated was carried out, and the numerical simulations were consistent with the experimental results obtained.

Investigation of Liquid Sensor Using a Dual-Mode Thin Film Bulk Acoustic Resonator (FBAR) Combined with Au/Cr Layers

This paper reports a dual-mode film bulk acoustic resonator (FBAR) device combined with Au/Cr layer to construct a liquid sensor. In order to obtain a liquid sensor with high sensitivity, the dual-mode FBAR with piezoelectric layer of ZnO thin film has been investigated. The dual-mode characteristics of FBAR appeared by off-axis deposition of ZnO thin film with the RF magnetron sputtering. The top and bottom electrodes consisted of a titanium (Ti) and a platinum (Pt) layers were deposited by dual-gun DC sputtering system. Owing to the chemical stability, the Pt layer is not only a perfect bottom electrode but also an etching stop layer. The gold layer (Au) is adopted for the formation of sensory area. In order to improve the adhesion between the Au layer and the Pt layer, a chromium (Cr) layer was deposited onto the backside cavity of the FBAR device before the Au layer was deposited by a DC sputter. The Au/Cr layer is used as the binding layer with analytes of DI water because of its hydrophile. The frequency response is measured using an HP8720 network analyzer and a CASCADE probe station. The resonant frequencies of longitude and shear modes in air appear at 2262.5 MHz and 1012.5MHz, whereas the resonant frequencies of longitude and shear modes in DI water are 2062.5 MHz and 955MHz, respectively.