Ultra temperature-stable bulk-acoustic-wave resonators with SiO2 compensation layer

This paper describes temperature compensated bulk acoustic-wave resonators (BAR) with temperature coefficient of frequency (TCF) less than 1 ppm/degrees C at above 3 GHz. The temperature compensation is produced from the unique physical property of silicon dioxide's positive TCF, unlike most other materials that have negative TCF. Two types of resonators have been explored: film bulk acoustic resonator (FBAR) composed of Al/ZnO/Al/SiO2 on a surface micromachined cantilever that is released by XeF2 vapor etching and high-overtone acoustic resonator (HBAR) composed of an Al/ZnO/Al resonator on a bulk micromachined SiO2/Si/SiO2 supporting substrate.     

Detection of various self-assembled monolayers by AlN-based film bulk acoustic resonator

Various self-assembled monolayers such as carcinoembryonic antigen (CEA), beta actin, and bovine serum albumin (BSA) were detected using an AlN-based film bulk acoustic resonator (FBAR). AlN thin film was deposited by reactive RF magnetron sputtering, on a substrate of Mo (100 nm)/Ti (34 nm)/SiO₂ (480 nm)/Si (300 μm)/Si₃N₄ (300 nm). The film showed a strongly c-axis preferred orientation with a main (0 0 2) peak, as well as a good full width at half maximum (FWHM) of 2.50° in XRD and rocking curve results. The AlN-based FBAR was confirmed to have a resonant frequency of 2.477 GHz and a sensitivity of 3514 Hz cm²/ng. In beta actin, BSA, and CEA, the frequency properties showed variation values of 472.142, 932.573, and 685.421 kHz and mass sensitivities of 3530, 3506, and 3514 Hz-cm²/ng, respectively. The FBAR sensor was confirmed to be very useful for detecting target antigens through the binding of an antigen and an anti-body.     

Extracting the electromechanical coupling constant of piezoelectric thin film by the high-tone bulk acoustic resonator technique

In this paper, a new approach is proposed to rapidly and accurately measure the electromechanical coupling constant K(t)(2) of thin film piezoelectric material, which is critically important for real-time quality control of the piezoelectric film growth in mass production. An ideal lossy bulk acoustic resonator (LBAR) model is introduced and the theory behind the method is presented. A high-tone bulk acoustic resonator (HBAR) was fabricated on a silicon wafer. The impedance response of the resonator was measured, from which the K(t)(2) of the piezoelectric material was extracted. To illustrate the potential of the proposed technique to extract material properties, two HBAR devices employing AlN as the piezoelectric material were fabricated using an RF sputter system with known good and bad deposition conditions; the extracted K(t)(2) values of the piezoelectric material are compared.     

Recent developments in thin film electro-acoustic technology for biosensor applications

The article reviews recent developments of the thin film electro-acoustic (TEA) technology in view of the design and fabrication of micro-acoustic transducers for biosensor applications. The use of the TEA technology leads to transducer miniaturisation, compatibility with the IC technology, possibility for multiplexing, decrease in fabrication cost, reduction of consumables, mass fabrication, etc. Focus lies on the design, fabrication and evaluation of the transducer performance in liquid media as judged by electro-acoustic behaviour and ultimately by mass and viscosity resolution. The analysis draws the conclusion that the thickness excited quasi-shear thin film bulk acoustic resonator technology is far ahead in its development with regard to other alternative approaches in terms of both performance and level of maturity. Consequently, the main aspects of the quasi-shear thin film bulk acoustic resonator (FBAR) technology from film synthesis and fabrication through to performance evaluation and demonstration are reviewed in detail.     

Mercuric ion sensing by a film bulk acoustic resonator

This paper describes a film bulk acoustic resonator (FBAR) mass sensor for detecting Hg2+ ion in water with excellent sensitivity and selectivity. When a thin Au film was deposited on the surface of an FBAR, the resonant frequency shifted to a lower value when the film was exposed to Hg2+ in aqueous solution. The FBAR sensor detected as low as 10(-9) M Hg2+ (0.2 ppb Hg2+) in water. Other ions such as K+, Ca2+, Mg2+, Zn2+, and Ni2+ had little or no effect on the resonant frequency of the FBAR. Coating of the FBAR Au surface with a self-assembled monolayer (SAM) of 4-mercaptobenzoic acid decreased the Hg2+ response.     

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.     

2.4GHz射频薄膜体声波谐振器的研制

提出了基于ZnO压电薄膜多层结构的2.4GHz射频薄膜体声波谐振器,并进行了研究.采用修正后的Mason等效电路模型对器件的谐振特性进行了分析和模拟.给出了采用MEMS工艺制备器件的工艺流程,并利用射频网络分析仪对实验器件进行了测试.利用多点数值拟合的方法消除射频测试中引入的寄生分布参数,提取出器件的实际参数:器件的串联谐振频率fs和并联谐振频率fp分别为2.3714GHz和2.3772GHz,相应的有效机电耦合系数为0.598%;串联谐振频率处和并联谐振频率处的Q值分别为500.3和425.5,f·Q值乘积达到1.2×1012.该谐振器器件的有效直径为200μm,样品实际尺寸为1.2mm×1.2mm×0.3mm,可用来制备体积小、高性能和低相噪的射频振荡器.     

Extremely low phase noise UHF oscillators utilizing high-overtone, bulk-acoustic resonators

High-overtone, bulk acoustic resonators (HBAR) have been designed that exhibit 9-dB insertion loss and loaded Q values of 80000 at 640 MHz with out-of-phase resonances occurring every 2.5 MHz. These resonators have been used as ovenized frequency-control elements in very low phase noise oscillators. The oscillator sustaining stage circuitry incorporates low-1/f noise modular RF amplifiers, Schottky-diode ALC, and a miniature 2-pole helical filter for suppression of HBAR adjacent resonant responses. Measurement of oscillator output signal flicker-of-frequency noise confirms that state-of-the-art levels of short-term frequency stability have been obtained. Sustaining stage circuit contribution to resulting oscillator flicker-of-frequency noise is 7-10 dB below that due to the resonators themselves. At 16-dBm resonator drive, an oscillator output signal white phase noise floor level of -175 dBc/Hz is achieved.     

Analytical and Finite-Element Modeling of Localized-Mass Sensitivity of Thin-Film Bulk Acoustic-Wave Resonators (FBAR)

Analytical and finite-element models of a localized-mass sensor fabricated with thin-film bulk acoustic wave resonator (FBAR) are reported. While our group demonstrated FBAR-based localized-mass sensors, no previous modeling of these sensors is found in the literature. The finite-element model (FEM) defines the boundary conditions and performs parametric analysis of the sensor's mass loading, whereas the analytical approach takes advantage on a modified Mason's model to describe the transmission-line circuit of the sensor. The sensitivity of the resonance frequency to location and size of mass deposition has been studied. Both the experimental and modeled responsivities exhibit a nonlinear dependence on the location and size of the localized-mass.      

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.     

An Investigation on Efficient Acoustic Energy Reflection of Flexible Film Bulk Acoustic Resonators

This paper investigates the issues on acoustic energy reflection of flexible film bulk acoustic resonators(FBARs). The flexible FBAR was fabricated with an air cavity in the polymer substrate, which endowed the resonator with efficient acoustic reflection and high electrical performance. The acoustic wave propagation and reflection in FBAR were first analyzed by Mason model, and then flexible FBARs of 2.66 GHz series resonance in different configurations were fabricated. To validate efficient acoustic reflection of flexible resonators, FBARs were transferred onto different polymer substrates without air cavities. Experimental results indicate that efficient acoustic reflection can be efficiently predicted by Mason model. Flexible FBARs with air cavities exhibit a higher figure of merit(FOM). Our demonstration provides a feasible solution to flexible MEMS devices with highly efficient acoustic reflection(i.e. energy preserving) and free-moving cavities, achieving both high flexibility and high electrical performance.     

Positioning FBAR technology in the frequency and timing domain

This paper focuses on the technical differentiation of film bulk acoustic resonator (FBAR) technology from other mechanical resonator technologies for timing applications. The paper will touch on a recent modification of FBARs, the zero-drift resonator (ZDR), that is temperature compensated. One technology differentiator is the size of the chip-scale packaged resonator. Another is that the silicon lid is perfectly suitable for placement of integrated circuits and this is currently being done. Many factors (wide tuning range, high Q, high frequency, small size, integrated circuitry) are being used to differentiate potential products for the time and frequency markets.     

Characteristics of pure-shear mode BAW resonators consisting of (1120) textured ZnO films

Thickness pure-shear mode film bulk acoustic wave resonators (FBARs) made of (1120) textured ZnO films have been fabricated. We also have fabricated FBAR structure consisting of two layers of the (1120) textured ZnO film with opposite polarization directions. This FBAR structure operated in second overtone pure-shear mode and allowed shear-mode FBARs at higher frequency. The effective electromechanical coupling coefficients k2 of pure-shear mode FBAR and second overtone pure-shear mode FBAR in this study were found to be 3.3% and 0.8%, respectively. The temperature coefficient of frequency (TCF) of thickness extensional mode FBAR, pure-shear mode FBAR, and second overtone pure-shear mode FBAR were measured in the temperature range of 10-60 degrees C. TCF values of -63.1 ppm/degrees C, -34.7 ppm/degrees C, and -35.6 ppm/degrees C were found for the thickness extensional mode FBAR, the pure-shear mode FBAR, and the second overtone pure-shear mode FBAR, respectively. These results demonstrated that pure-shear mode ZnO FBARs have more stable temperature characteristics than the conventional thickness extensional mode ZnO FBARs.     

Improved crystallization characteristics of ZnO thin film grown onto a-C:H film used as a buffer layer

We employed a-C:H buffer layer to improve the crystalline property of ZnO thin film for the membrane film bulk acoustic resonator (FBAR). The a-C:H film as a buffer layer is prepared by applying dc bias of 200 V and also this sample showed a smoother surface roughness, higher hardness and Young's modulus when compared to the other samples. In addition, the FWHM value was improved from 7.5 to 4.3° on a-C:H film. The fabricated FBAR device showed the resistivity of 0.73 × 10⁸ Ω when compared with no buffer layer and the frequency characteristics of the FBAR were finally confirmed to be 1.15 GHz and 21.24 dB, respectively.     

Air gap type thin film bulk acoustic resonator fabrication using simplified process

In this study we propose a simplified process for making an air-gap-type film bulk acoustic resonator (FBAR) using the magnesium (Mg) sacrificial layer. The Mg sacrificial layer minimizes damage to other layers in the wet etching process because of its short etching time. Also the Mg sacrificial layer plays the role of etching aisle and air-gap simultaneous during the etching process. In addition, our proposed process can reduce the number of FBAR fabrication steps when compared with previous dry etching techniques. The FBAR's resonant frequency characteristics successfully show performance from 2.44 to 3.11 GHz.     

Thin film bulk acoustic wave filter

Thin film bulk acoustic wave (BAW) resonators (FBAR) are fabricated on a silicon nitride bridge using a ZnO piezolayer on a glass substrate and surface micromachining by standard thin film technology. These resonators exhibit a coupling constant k/sub t//sup 2/=7.8% at the first thickness extensional wave mode and are used as impedance elements in a ladder filter in the 1-GHz frequency band of mobile telecommunications. An electrical equivalent circuit is used to characterize the properties of the resonators and to show how the performance of the filter depends on the parameters of the resonators. 2.5% bandwidth, 2.8-dB insertion loss, and 35-dB selectivity are obtained in a filter with six resonators. The technology can be used to manufacture miniature microwave filters without any additional inductances.     

Design and Simulation of FBAR for Quality Factor Enhancement

Thin film bulk acoustic resonator (FBAR) higher quality factor (Q) provides steep skirt and low insertion losses in the pass band. In this paper, three different loss sources are identified and FBAR is design and simulated in order to reduce the losses. Firstly, the FBAR’s top electrode is simulated for the areas of 150 × 150, 300 × 300 and 400 × 400 μm² and the quality factor is improved as the area increased to 102, 432.6 and 743.7 respectively. The impedance of the FBAR is reduced as the area of the electrode is increased. Secondly, the anchor width is reduced from 60 to 45 μm and the quality factor is increased from 341 to 432.6 respectively. The losses through the anchor reduce as the anchor reduces. Electrode area and anchor area simulation are showing the notable effect and no other paper is reported for the comparison. Thirdly, the damping factor coefficient (β) is varied as 4.7e^?14, 3.84e^?14 and 2.5e^?14. The quality factor is increased as the damping factor reduces and reported as 283.2, 341.2 and 444.5 respectively. The damping reduction leads the FBAR structure to vibrate more freely at the resonance. The losses through damping are reduced and more energy has stored at the resonance so it increases the quality factor.     

Quantitative surface acoustic wave detection based on colloidal gold nanoparticles and their bioconjugates

The immobilization scheme of monodispersed gold nanoparticles (10-nm diameter) on piezoelectric substrate surfaces using organosilane molecules as cross-linkers has been developed for lithium niobate (LiNbO3) and silicon oxide (SiO2)/gold-covered lithium tantalate (LiTaO3) of Rayleigh and guided shear horizontal- (guided SH) surface acoustic wave (SAW) sensors. In this study, comparative measurements of gold nanoparticle adsorption kinetics using high-resolution field-emission scanning electron microscopy and SAW sensors allow the frequency responses of SAW sensors to be quantitatively correlated with surface densities of adsorbed nanoparticles. Using this approach, gold nanoparticles are used as the "nanosized mass standards" to scale the mass loading in a wide dynamical range. Rayleigh-SAW and guided SH-SAW sensors are employed here to monitor the surface mass changes on the device surfaces in gas and liquid phases, respectively. The mass sensitivity ( approximately 20 Hz.cm2/ng) of Rayleigh-SAW device (fundamental oscillation frequency of 113.3 MHz in air) is more than 2 orders of magnitude higher than that of conventional 9-MHz quartz crystal microbalance sensors. Furthermore, in situ (aqueous solutions), real-time measurements of adsorption kinetics for both citrate-stabilized gold nanoparticles and DNA-gold nanoparticle conjugates are also demonstrated by guided SH-SAW (fundamental oscillation frequency of 121.3 MHz). By comparing frequency shifts between the adsorption cases of gold nanoparticles and DNA-gold nanoparticle conjugates, the average number of bound oligonucleotides per gold nanoparticle can also be determined. The high mass sensitivity ( approximately 6 Hz.cm2/ng) of guided SH-SAW sensors and successful detection of DNA-gold nanoparticle conjugates paves the way for real-time biosensing in liquids using nanoparticle-enhanced SAW devices.     

Fabrication and performance analysis of film bulk acoustic wave resonators

In this paper, a radio frequency reactive sputtering deposition technique for piezoelectric aluminum nitride (AlN) thin film formation on a gold (Au) bottom electrode and its successful application in a film bulk acoustic resonator (FBAR) are investigated. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) measurements show that the AlN films were deposited onto an Au bottom electrode with highly c-axis-preferred orientation, well-textured columnar structure with a fairly uniform grain size of approximately 83 nm. The roughness is measured at a root-mean square (RMS) value of 5.4 nm and the average peak to valley of each grain column is 46.3 nm. The FBAR consists of an AlN piezoelectric thin film sandwiched between Au electrodes, all of which lie on a thin low-stress silicon nitride which serves as a support membrane on silicon. The performance of FBAR device exhibits a significant of the series quality factor (Q_s), the parallel quality factor (Q_p), the effective electromechanical coupling coefficient (k_eff²), and the bandwidths are 97, 120, 5.1%, and 24 MHz, respectively.     

Micromachined thin film plate acoustic resonators utilizing the lowest order symmetric lamb wave mode

Thin film integrated circuits compatible resonant structures using the lowest order symmetric Lamb wave propagating in thin aluminum nitride (AlN) film membranes have been studied. The 2-mum thick, highly c-oriented AlN piezoelectric films have been grown on silicon by pulsed, direct-current magnetron reactive sputter deposition. The films were deposited at room temperature and had typical full-width, half-maximum value of the rocking curve of about 2 degrees. Thin film plate acoustic resonators were designed and micromachined using low resolution photolithography and deep silicon etching. Plate waves, having a 12-mum wavelength, were excited by means of both interdigital (IDT) and longitudinal wave transducers using lateral field excitation (LW-LFE), and reflected by periodical aluminum-strip gratings deposited on top of the membrane. The existence of a frequency stopband and strong grating reflectivity have been theoretically predicted and experimentally observed. One-port resonator designs having varying cavity lengths and transducer topology were fabricated and characterized. A quality factor exceeding 3000 has been demonstrated at frequencies of about 885 MHz. The IDT based film plate acoustic resonators (FPAR) technology proved to be preferable when lower costs and higher Qs are pursued. The LW-LFE-based FPAR technology offers higher excitation efficiency at costs comparable to that of the thin film bulk acoustic wave resonator (FBAR) technology