Film Bulk Acoustic Wave Resonator Devices 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.

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.

Influence of Sputtering Pressure for Radio Frequency Magnetron Sputtering (103) Oriented AlN Films on (100) Silicon Substrate

The thin film of (103)-oriented aluminum nitride (AlN) is an attractive piezoelectric material for the applications in film bulk acoustic wave resonator (FBAR) devices. Due to the bulk acoustic wave (BAW) properties of (103) oriented AlN films, it can excite a quasi-shear mode (velocity = 5,957 m/s, K2 = 3.8%) that can be used for FBAR liquid sensor and even loss less than the FBAR device with (002) oriented AlN films. In this research, the (103) oriented AlN films were successfully deposited onto (100) silicon substrate by radio frequency (RF) magnetron sputtering. Different sputtering pressures (1m torr, 3m torr, 5m torr, and 7m torr) were discussed in this experiment process. Comparisons were made on their crystalline structures with X-ray diffraction (XRD) and the surface morphologies was investigated by the atomic force microscopy (AFM). The result exhibited the optimal sputtering pressure is 5m torr. The optimal (103) oriented AlN films have the strongest XRD intensity, the smallest full width at half maximum (FWHM) value (0.6°), the largest grain size (15.78nm) and the smooth surface roughness (Ra=3.259nm).

Development of High-Quality FBAR Devices for Wireless Applications Employing Two-Step Annealing Treatments

In this letter, a new two-step annealing technique is presented that can more effectively improve the resonance characteristics of the film bulk acoustic wave resonator (FBAR) devices in terms of return loss, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor, and effective electromechanical coupling coefficient ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). In the case of the SMR-type FBAR devices, the use of this approach has considerably improved the resonance performance (~8000 of Q-factor value, ~ 2% of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) at the operating frequency of ~1.8 GHz, as compared to the conventional annealing techniques.

Frequency domain analysis of lamb wave scattering and application to film bulk acoustic wave resonators

This paper demonstrates a novel frequency domain analysis (FDA) to evaluate the scattering behavior of a waveguide mode at arbitrary scattering geometries by a time harmonic simulation based on the finite element method (FEM). To this end, we add an injection-damping mechanism (IDM) to avoid interference at the acoustic input port. The IDM can be easily constructed by a numerical operation. Our approach offers improved time consumption and calculation power necessary over the established method in the time domain. After checking the validity of the proposed method, we discuss the importance of considering wave scattering phenomena in film bulk acoustic wave resonator (FBAR) devices by applying the proposed method to two simplified models of an FBAR device.

Electrode optimization for bulk acoustic wave resonators based on ZnO

In this study we have employed a one-dimensional transfer matrix method to obtain the input electrical impedance (Zin) for a four layer (metal/piezoelectric/metal/substrate) thin film bulk acoustic wave resonator (FBAR). The input electrical impedance was calculated taking into account the electromechanical properties of the ZnO thin films, the metal used for the contacts and the silicon oxide (SiO2) supporting layer in order to calculate the electromechanical effective coupling coefficient keff and the quality factor of the device (QD). We use a figure of merit (FOM) defined as the product of keff2×QD to optimize both parameters simultaneously for their use in microwave band-pass filters. In this analysis, several metals were employed as electrodes in the FBAR device, and we have found that for gold we obtain a higher value for the FOM than for aluminum, copper or silver. In this case, the optimal metal thickness is around 0.15 μm. In addition, the calculated values show that for copper and silver electrodes the FOM is smaller but close to that obtained for gold. Then, copper or electrodes can substitute aluminum electrodes for achieving low cost filters with good electrical performance.

Effects of thermal annealing condition on N-incorporated ZnO films in FBAR devices

A new approach to fabricate the film bulk acoustic wave resonator (FBAR) devices is presented that employs high-quality piezoelectric ZnO films, particularly sputter-deposited in a mixture of N2O and Ar gases as the reactive and sputtering gases, respectively. Various thermal annealing treatments were performed on the deposited ZnO films and their effects on the resonance characteristics of the FBAR devices were investigated. With a process optimisation, the FBAR devices could be fabricated to have excellent resonance characteristics in terms of return loss.

Fabrication of piezoelectric AlN thin film for FBARs

This paper focuses on the fabrication of film bulk acoustic-wave resonator (FBAR) comprising an aluminum nitride (AlN) piezoelectric thin film sandwiched between two metal electrodes and located on a silicon substrate with a low-stress silicon nitride (Si3N4) support membrane for high frequency wireless applications, and analyzes the optimization of the thin AlN film deposition parameters on Mo electrodes using the reactive RF magnetron sputter system. Several critical parameters of the sputtering process such as RF power and Ar/N2 flow rate ratio were studied to clarify their effects on different electrodes characteristics of the AlN films. The experiment indicated that the process for Mo electrode was easier compared with that of the Pt/Ti or Au/Cr bi-layer electrode as it entailed only one photo resist and metal deposition step. Besides, Pt/Ti or Au/Cr electrodes reduced the resonance frequency due to their high mass density and low bulk acoustic velocity. Compared with the case of the Al bottom electrode, there is no evident amorphous layer between the Mo bottom electrode and the deposited AlN film. The characteristics of the FBAR devices depend not only upon the thickness and quality of the AlN film, but also upon the thickness of the top electrode and the materials used. The results indicate that decreasing the thickness of either the AlN film or the top electrode increases the resonance frequency. This suggests the potential of tuning the performance of the FBAR device by carefully controlling AlN film thickness. Besides, increasing either the thickness of the AlN film or higher RF power has improved a stronger c-axis orientation and tended to promote a narrower rocking curve full-width at half-maximum (FWHM), but increased both the grain size and the surface roughness. An FBAR device fabricated under optimal AlN deposition parameters has demonstrated the effective electromechanical coupling coefficient (k eff 2 ) and the quality factor (Q f x ) are about 1.5% and 332, respectively.

High-quality 2.5 GHz ZnO-based FBAR devices for broadband WiMAX applications

The improvements of resonance characteristics of ZnO-based film bulk acoustic-wave resonator (FBAR) devices operating at 2.5 GHz are presented for the first time. Because of thermal annealing processes, the resonance characteristics could be significantly improved. At about 2.5 GHz, excellent resonance characteristics are observed in terms of good return loss and high quality factor.

Design and Fabrication of ZnO-Based FBAR Microwave Devices for Mobile WiMAX Applications

In this letter, we present the design and fabrication of a novel ZnO-based film bulk acoustic wave resonator (FBAR) microwave devices. The novel FBAR devices employ a new-type of Bragg reflector with very thin chromium (Cr) layer formed between SiO2 and W films. The Cr layer seems to enhance the adhesion between SiO2 and W layers. The novel FBAR devices show good return losses (S11) and high Q-factors at the frequency range of 2.7-3.0 GHz. This approach will be very helpful for mobile worldwide interoperability for microwave access applications.

High-Performance Film Bulk Acoustic Wave Pressure and Temperature Sensors

A film bulk acoustic wave resonator (FBAR)-based sensor for the simultaneous measurement of temperature and pressure with high sensitivity is fabricated and characterized. Temperature or pressure sensing is determined by the change in the series resonant frequency of the FBAR device when exposed to a measurement environment. For temperature sensing, measurement results show a sensitivity of 25.02 ppm/°C, a nonlinearity less than ±0.005% over the measurement range of 10 to 80 °C, and a hysteresis within ±0.005% in one temperature cycle. In pressure sensing, measured results show a sensitivity of 336.2 ppm/bar, a nonlinearity less than ±0.004% over the measurement pressure range of 0 to 2.07 bar, and a hysteresis within ±0.007% in one pressure cycle.

Approach to improve ZnO-based FBAR devices

Presented is a new technique to improve the resonance characteristics of film bulk acoustic-wave resonator (FBAR) devices. The FBAR devices were fabricated on multilayer Bragg reflectors into which ultra-thin chromium (Cr) adhesion layers were inserted, followed by several kinds of thermal annealing processes. This technique resulted in excellent device improvement in terms of return loss and Q-factors.

A comprehensive investigation of thermal treatment effects on resonance characteristics in FBAR devices

This paper presents some methods to improve the resonance characteristics of film bulk acoustic-wave resonator (FBAR) devices. The FBAR devices were fabricated on Bragg reflectors. Thermal treatments were done using sintering and/or annealing processes. The measurement shows a considerable improvement of return loss (S11) and quality factor (Qs/p). These thermal treatments seem very promising for enhancing the FBAR's resonance performance. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 47: 459–462, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21199

Performance Characterization of Thin AlN Films deposited on Mo Electrode for Thin-Film Bulk Acoustic-Wave Resonators

In this paper, we discuss the implementation and testing of a film bulk acoustic-wave resonator (FBAR) comprising an AlN piezoelectric thin film sandwiched between two metal electrodes and located on a silicon substrate with a low-stress silicon nitride (Si3N4) support membrane. In the present study, we choose molybdenum as the bottom electrode rather than the conventionally used Pt/Ti, Au/Cr or Al because it has a low acoustic attenuation and a good electrical conductivity, and provides a good adhesion between the thin AlN film and the low-stress Si3N4 membrane. We investigate the influence of AlN film thickness and top-electrode thickness on the resonance frequency of the FBAR device. The results indicate that decreasing the thickness of either the AlN film or the top electrode increases the resonance frequency. This suggests the potential of tuning the performance of the FBAR device by the carefully controlling AlN film thickness. Furthermore, it is also determined that the resonance frequency of the device can be increased by specifying a higher RF power for the magnetron sputtering process used to deposit the AlN film. This study employs an X-ray diffraction (XRD) technique to investigate the influence of AlN film thickness and RF power on the c-axis orientation of the thin film. It is shown that increasing either the thickness of the AlN film or the power of the RF sputtering process improves c-axis orientation. However, scanning electron microscopy (SEM) and atomic force microscopy (AFM) results reveal that a thicker AlN film exhibits a rougher surface and a larger grain size, both of which decrease the coupling factor of the FBAR device.

Finite element modelling of nanostructured piezoelectric resonators (NAPIERs)

A new modification to the traditional piezoelectric thin film bulk acoustic wave resonator (FBAR) and solidly mounted acoustic wave resonator (SMR) is proven to significantly improve their performances. The proposed design involves the surface micro/nano structuring of planar piezoelectric thin films to realize an array of a large number of rod-like structures. In contrast to the plate-like thickness extensional resonance in traditional FBAR and SMR devices, the rod-like structures can be excited in their length extensional resonance, yielding a higher electromechanical coupling factor and effectively eliminating the spurious resonances from lateral modes of vibration. The designs have been investigated by two and three-dimensional finite element analyses and one-dimensional transmissionline modelling. The results show that significant increases in the electromechanical coupling factor of ca. 40% can be achieved by using the rod-like length extensional resonances as compared with the plate-like thickness extensional resonances in traditional devices. Simulations show that rod width-to-thickness aspect ratios of less than 0.5 could result in an electromechanical coupling factor (k2eff) of over 10% for a zinc oxide device, compared with approximately 7% for a conventional design.

Improved Resonance Characteristics by Thermal Annealing of W/SiO2 Multi-Layers in Film Bulk Acoustic Wave Resonator Devices

In this paper, we, for the first time, present the effects of the thermal annealing of the W/SiO2 multi-layer Bragg reflectors on the resonance characteristics of the ZnO-based film bulk acoustic wave resonator (FBAR) devices. In order to improve the resonance characteristics of the FBAR devices, we employed a thermal annealing process after the Bragg reflectors were formed on a silicon substrate using a radio frequency (RF) magnetron sputtering technique. As a result, the resonance characteristics of the FBAR devices were observed to strongly depend on the annealing conditions applied to the Bragg reflectors. The FBAR devices with the Bragg reflectors annealed at 400°C/30 min showed excellent resonance characteristics as compared to those with the non-annealed (as-deposited) Bragg reflectors. The newly proposed simple thermal annealing process will be very useful to more effectively improve the resonance characteristics of the future FBAR devices with multi-layer Bragg reflectors.