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.

A new model for film bulk acoustic wave resonators

Based on cavity resonance and sandwich composite plate （3D） theoretical model for frequency dispersion characterization theory, this paper presents a universal three-dimensional and displacement profile shapes of the film bulk acoustic resonator （FBARs）. This model provides results of FBAR excited thickness-extensional and flexure modes, and the result of frequency dispersion is proposed in which the thicknesses and impedance of the electrodes and the piezoelectric material are taken into consideration; its further simplification shows good agreement with the modified Butterworth-Van-Dyke （MBVD） model. The displacement profile reflects the vibration stress distribution of electrode shapes and the lateral resonance effect, which depends on the axis ratio of the electrode shapes a/b. The results are consistent with the 3D finite element method modeling and laser interferometry measurement in general.

Active control scheme for improving mass resolution of film bulk acoustic resonators

High mass resolution of sensors based on film bulk acoustic resonators （FBARs） is required for the detection of small molecules with the low concentration. An active control scheme is presented to improve the mass resolution of the FBAR sen- sors by adding a feedback voltage onto the driving voltage between two electrodes of the FBAR sensors, The feedback voltage is obtained by giving a constant gain and a constant phase shift to the current on the electrodes of the FBAR sensors. The acoustic energy produced by the feedback voltage partly compensates the acoustic energy loss due to the material damping and the acoustic scattering, and thus improves the quality factor and the mass resolution of the FBAR sensors. An explicit expression relating to the impedance and the frequency for an FBAR sensor with the active control is derived based on the continuum theory by neglecting the influence of the electrodes. Numerical simulations show that the impedance of the FBAR sensor strongly depends on the gain and the phase shift of the feedback voltage, and the mass resolution of the FBAR sensor can greatly be improved when the appropriate gain and the phase shift of the feedback voltage are used. The active control scheme also provides an effective solution to improve the resolution of the quartz crystal microbalance （QCM）.

Tunable Ba_(0.5) Sr_(0.5) TiO_3 film bulk acoustic resonators using SiO_2 /Mo Bragg reflectors

Tunable and switchable Ba 0.5 Sr 0.5 TiO 3 film bulk acoustic resonators（FBARs） based on SiO 2 /Mo Bragg reflectors are explored,which can withstand high temperature for the deposition of Ba x Sr 1 x TiO 3（BST） films at 800 C.The dc bias-dependent resonance may be attributed to the piezoelectricity of the BST film induced by an electrostrictive effect.The series resonant frequency is strongly dc bias-dependent and shifts downwards with dc bias increasing,while the parallel resonant frequency is only weakly dc bias-dependent and slightly shifts upwards at low dc bias（ 45 V） while downwards at higher dc bias.The calculated relative tunability of shifts at series resonance frequency is around 2.3% and the electromechanical coupling coefficient is up to approximately 8.09% at 60-V dc bias,which can be comparable to AlN FBARs.This suggests that a high-quality tunable BST FBAR device can be achieved through the use of molybdenum（Mo） as the high acoustic impedance layer in a Bragg reflector,which not only provides excellent acoustic isolation from the substrate,but also improves the crystallinity of BST films withstanding higher deposition temperature.

Quantitative measurement of local elasticity of SiO_x film by atomic force acoustic microscopy

In this paper the elastic properties of SiOx film are investigated quantitatively for local fixed point and qualitatively for overall area by atomic force acoustic microscopy （AFAM） in which the sample is vibrated at the ultrasonic frequency while the sample surface is touched and scanned with the tip contacting the sample respectively for fixed point and continuous measurements. The SiOx films on the silicon wafers are prepared by the plasma enhanced chemical vapour deposition （PECVD）, The local contact stiffness of the tip-SiOx film is calculated from the contact resonance spectrum measured with the atomic force acoustic microscopy. Using the reference approach, indentation modulus of SiOx film for fixed point is obtained. The images of cantilever amplitude are also visualized and analysed when the SiOx surface is excited at a fixed frequency. The results show that the acoustic amplitude images can reflect the elastic properties of the sample.

Development and Analysis of an RF Film Bulk Acoustic Resonator

A high-Q diaphragm-structure film bulk acoustic resonator （FBAR） with a flat support diaphragm, made of Si3 N4/SiOz/Si3 N4 composite films ,is proposed. The N/O/N composite diaphragm overcomes the wrinkling in the released support diaphragm caused by the residual stress of a single Si3N4 or SiO2 diaphragm. ZnO piezoelectric film deposited employing a DC reactive magnetron sputtering method is used as the piezoelectric material for the FBAR device. The XRD 0- 20 scan indicates that the ZnO film has the preferred c-axis orientation growth,implying good piezoelectric properties. The S parameter measurement shows that there＇ are three primary resonances in the frequency range from 0.4 to 2.6GHz. The series resonant frequency,parallel resonant frequency, Kett^2, and quality factors of the three resonances are calculated. The third one,with a frequency of about 2.4GHz,has the highest quality factor about 500. Thus,it is expected to be a candidate to form a 2.4GHz low-phase-noise oscillator.

Manipulations of micro/nanoparticles using gigahertz acoustic streaming tweezers

Contactless acoustic manipulation of micro/nanoscale particles has attracted considerable attention owing to its near independence of the physical and chemical properties of the targets,making it universally applicable to almost all biological systems.Thin-film bulk acoustic wave(BAW)resonators operating at gigahertz(GHz)frequencies have been demonstrated to generate localized high-speed microvortices through acoustic streaming effects.Benefitting from the strong drag forces of the high-speed vortices,BAW-enabled GHz acoustic streaming tweezers(AST)have been applied to the trapping and enrichment of particles ranging in size from micrometers to less than 100 nm.However,the behavior of particles in such 3D microvortex systems is still largely unknown.In this work,the particle behavior(trapping,enrichment,and separation)in GHz AST is studied by theoretical analyses,3D simulations,and microparticle tracking experiments.It is found that the particle motion in the vortices is determined mainly by the balance between the acoustic streaming drag force and the acoustic radiation force.This work can provide basic design principles for AST-based lab-on-a-chip systems for a variety of applications.

High Q,high frequency,high overtone bulk acoustic resonator with ZnO films

Bulk acoustic wave resonators with piezoelectric films have been widely explored for the small size and high quality factor （Q） at GHz. This paper describes a high overtone bulk acoustic resonator （HBAR） based on AI/ZnO/AI sandwich layers and c-axis sapphire substrate. ZnO film with high quality c-axis orientation has been obtained using DC magnetron sputtering. The fabricated HBAR presents high Q at the multiple resonances from a 0.5-4.0 GHz wide band with a total size （including the contact pads） of 0.6 mm×0.3 mm×0.4 mm, The device exhibits the best acoustic coupling at around 2.4 GHz, which agrees with the simulation results based on the one-dimensional Mason equivalent circuit model. The HBAR also demonstrates Q values of 30 000, 25 000, and 6500 at 1.49, 2.43, and 3.40 GHz, respectively. It is indicated that the HBAR has potential applications for the low phase noise high frequency oscillator or microwave signal source.

Preparation of AlN thin films for film bulk acoustic resonator application by radio frequency sputtering

Aluminum nitride （AlN） thin films with high c-axis orientation have been prepared on a glass substrate with an Al bottom electrode by radio frequency （RF） reactive magnetron sputtering. Based on the analysis of Berg＇s hysteresis model, the improved sputtering system is realized without a hysteresis effect. A new control method for rapidly depositing highly c-axis oriented AlN thin films is proposed. The N2 concentration could be controlled by observing the changes in cathode voltage, to realize the optimum processing condition where the target could be fixed stably in the transition region, and both stoichiometric film composition and a high deposition rate could be obtained. Under a 500 W RF power of a target with a 6 cm diameter, a substrate temperature of 450 ℃, a target-substrate distance of 60 mm and a N2 concentration of 25%, AlN thin film with preferential （002） orientation was deposited at 2.3 μm/h which is a much higher rate than previously achieved. Through X-ray diffraction （XRD） analysis, the full width at half maximum （FWHM） of AlN （002） was shown to be about 0.28°, which shows the good crystallinity and crystal orientation of AlN thin film. With other parameters held constant, any increase or decrease in N2 concentration results in an increase in the FWHM of AlN.

Low-Loss Dielectric Material Characterization and High-Q Resonator Design from Microwave to Millimetre Waves Frequencies

Dielectric resonators are key components in many microwave and millimetre wave circuits and applications, including high-Q filters and frequency-determining elements for precision frequency synthesis. Multilayered and bulk low-loss single crystal and polycrystalline dielectric structures have become very important for designing these devices. Proper design requires careful electromagnetic characterisation of low-loss material properties. This includes exact simulation with precision numerical software and precise measurements of resonant modes. For example, we have developed the Whispering Gallery mode technique, which has now become the standard for characterizing low-loss structures. This paper will review some of the common characterisation techniques used in the microwave to millimetre wave frequency regime.

框架结构对薄膜体声波谐振器性能提升的研究

本文研究了框架结构(Frame)对薄膜体声波谐振器(Film Bulk Acoustic Resonator,FBAR)提升性能的作用,从两种Frame结构对体声波能量的反射匹配基础理论出发,针对其功能特点,以实验的方式确定了低频和高频多组框架微结构的组合,对多组FBAR进行了版图绘制以及光刻流片,最终通过片上测试得到了所有分组的谐振器性能.对测试得到的性能进行了分组统计筛选,从测试结果来看,经过优选之后的Frame结构分组无论是对低频还是高频FBAR谐振器都具有提升性能的作用.对低频(1.7 GHz附近)FBAR来说,并联谐振品质因数可以提升1000以上.对高频(5.5 GHz附近)FBAR来说,并联谐振品质因数可以提升300以上.对横向寄生模式较强的高频FBAR,优选的Frame结构可以提升谐振器的横向寄生模式抑制,使串联谐振频率之下的阻抗相位波动减少5°以上.

背腔刻蚀型横向激励薄膜体声波谐振器制备技术研究

随着移动通信技术的快速发展,薄膜体声波滤波器逐渐向高频和大带宽方向发展。该文研究了POI(LiNbO_(3)/SiO_(2)/Si)基片上背腔刻蚀型横向激励薄膜体声波谐振器制作工艺,通过研究POI基IDT光刻、背腔硅刻蚀等工艺,确定了IDT层曝光量和背腔刻蚀等关键工艺参数。研制出的背腔刻蚀型横向激励薄膜体声波谐振器,其谐振频率为4565 MHz,反谐振频率为5035 MHz,机电耦合系数为20.86%。此制备工艺对研究高频、大带宽薄膜体声波滤波器具有重要的参考意义。

Effective electromechanical coupling coefficient of high-overtone bulk acoustic resonator

A high-overtone bulk acoustic resonator （HBAR） is composed of a substrate, a piezoelectric film and upper and lower electrodes, the influences of their structure parameter （thickness） and performance parameter （characteristic impedance） on effective electromechani- cal coupling coefficient K^2eff are investigated systematically. The relationship between K^2eff and these parameters is obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonant frequency, and K^2eff at the resonance frequency closest to the given frequency is analyzed. The results show that K^2eff declines rapidly and oscillatorily with the continuous increase of the substrate thickness when the piezoelectric film thickness is fixed, and decreases inversely proportion to the thickness when the substrate thick-ness is greater than a certain value. With the ratio of the characteristic impedance of the substrate to the piezoelectric layer increasing, the maximum of K^2eff obtained from the vari- ation curve of K^2eff with the continuous increase of the piezoelectric film thickness decreases rapidly before reaching the minimum value, and later increases slowly. Fused silica with low impedance is appropriate as the substrate of HBAR to get a larger K^2eff. Compared with Al electrode, Au electrode can obtain larger K^2eff when the appropriate electrode thickness is selected. The revealed laws above mentioned provide the theoretical basis for optimizing parameters of HBAR.

Mechanical quality factor of high-overtone bulk acoustic resonator

Mechanical quality factor Qm is a key characteristic parameter of High-overtone bulk acoustic resonator（HBAR）. The effects of structure parameter（thickness） and perfor？mance parameters（characteristic impedance and mechanical attenuation factor） of substrate,piezoelectric film and electrode constituting HBAR on Qm are carried out. The relationships between Qm and these parameters are obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonance frequency, and the an？alytical expressions oi Qm are given. The results show that Qm increases non-monotonically with the continuous increase of the substrate thickness for HBAR with certain piezoelectric film thickness, and it approaches to the substrate material mechanical quality factor as the substrate thickness is large. Qm decreases wavily with the continuous increase of the piezoelectric film thickness for HBAR with certain substrate thickness. Sapphire and YAG with low mechanical loss are appropriate as the substrate to get a larger Qm- The electrode loss must be considered since it can reduce Qm- Compared with Au electrode, A1 electrode with lower loss can obtain higher Qm when the appropriate electrode thickness is selected. In addition, Qm decreases with the increase of frequency. These results provide the theoretical basis for optimizing the parameters of HBAR and show that trade-oflFs between Qm and must be considered in the design because their changes are often inconsistent.

A Micro-Electro-Mechanical System-Based Bulk Acoustic Wave Piezoelectric Disk Resonator for Detecting z-axis Rotation Rate

This paper presents a bulk acoustic wave piezoelectric disk resonator based on a special pair of degenerative modes, to detect z-axis angular velocity. A single piezoelectric disk is operated in its appropriate modes in the k Hz frequency range to achieve this function. This design combines the bulk acoustic wave drive/sense mode with lead zirconate titanate resonator which improves device's performance and simplifies its structural complexity. The operation principle of piezoelectric disk resonator is given and validated by finite element method,and the scale factor of piezoelectric disk resonator is 0.977 μV/[(°) · s-1] without any amplification section.The results of impedance analysis for the prototype in the air, which is fabricated on lead zirconate titanate wafer by Micro-Electro-Mechanical System process, show that the resonant frequency of the piezoelectric disk resonator is about 190 k Hz. Moreover, the measured frequency split between drive and sense mode is about290 Hz without any tuning methods. At last, a closed-loop driving and detecting circuit system is designed and its modulation/demodulation method is studied, preliminary experiments show that this device is not sensitive to acceleration, but is sensitive to angular velocity, its performance parameters need follow-up experiments.

基于优化布拉格结构的固态装配型谐振器

体声波谐振器的有效耦合系数和品质因数决定了体声波滤波器的整体性能。有效耦合系数依赖于叠层结构和压电材料。而品质因数高度依赖于损耗机制,主要为电学损耗和声学损耗。对于固态装配型谐振器(SMR),声学损耗主要为透过衬底的能量泄漏。为提高SMR的品质因数,改进了布拉格堆叠结构,使纵波和剪切波同时被约束在压电堆叠结构中,以减少声能对衬底的泄漏。同时,为抑制谐振腔的杂散模式,优化了布拉格结构顶层薄膜厚度,器件色散特性由Ⅱ型变为Ⅰ型。通过仿真和实验表明:基于优化布拉格结构的SMR性能得到了明显改善。

边缘空气层薄膜体声波谐振器的设计与制备

压电薄膜存在固有的对压电效应的非线性迟滞响应,并且声波有向边缘传播的横波分量,二者都会产生声波的能量损耗。该文设计了一种具有边缘空气层结构的单晶氮化铝薄膜体声波谐振器,可以减小非线性迟滞的机电损耗并阻止横波能量泄露,提高谐振器的品质因数。使用COMSOL对设计的器件进行有限元模拟仿真,并成功制备出具有边缘空气层结构的谐振器进行测试验证。

新型波浪加权式高性能声表面波滤波器设计

射频滤波器是移动通信中信号传输频率选择、噪声滤除干扰的不可或缺的关键器件。压电薄膜多层衬底结构的声表面波滤波器因具有高矩形度、低损耗等优点在近年来被广泛应用,但其谐振器固有的横向模式会极大影响滤波器性能。为此提出了一种新型的波浪加权式叉指设计的声表面波谐振器,该谐振器通过使用正弦函数作为交叠长度加权包络曲线,能够在不恶化谐振器性能的前提下有效抑制谐振器中的横向模式。将波浪加权式叉指设计的谐振器按照梯形拓扑结构级联,设计并制备了一款尺寸为1.1 mm×0.9 mm×0.55 mm的高性能滤波器。该滤波器中心频率为2.593 GHz,相对带宽为8.8%,插入损耗为0.52 dB。实物测试结果与仿真基本一致。

微型FBAR器件性能优化设计

利用有限元仿真软件建立了薄膜体声波谐振器模型,研究了不同谐振面积(3600~10000μm^(2))条件下,电极形状(矩形、梯形、圆形和正五边形)及变迹角(30°、36°、40°和45°)对寄生谐振的影响,得到史密斯阻抗曲线和不圆度值,讨论了阶梯负载结构对横向声波泄露的抑制作用。仿真结果表明,电极形状为非正五边形,变迹角为40°时,对寄生谐振的抑制效果最好;在谐振面积为3600μm^(2)时,其不圆度为6.45%,与谐振面积为10000μm^(2)时矩形电极相当。设计的电极阶梯负载结构提升了并联谐振点处的品质因数,当电极横向尺寸为60μm时,二阶电极负载结构的品质因数为1378,比无电极负载结构的品质因数高10.07%。

双阶梯型薄膜体声波谐振器有限元仿真分析

为了抑制薄膜体声波谐振器(FBAR)的寄生谐振同时满足5G通信的高频需求,基于Comsol Multiphysics仿真软件建立薄膜体声波谐振器的二维和三维有限元模型,研究了压电材料、电极横向尺寸和电极形状对寄生谐振的影响,并讨论了电极框架结构对FBAR并联谐振频率(f_(p))处的品质因数(Q_(p))的影响。基于分析,提出并设计了一种双阶梯电极框架结构的FBAR,该结构的FBAR以AlN为压电材料,电极形状为五边形,中心频率为3.504 GHz,串联谐振频率为3.467 GHz,并联谐振频率为3.541 GHz,Q_(p)为1591。Q_(p)与未优化的FBAR相比提高了19.2%,实现了对寄生谐振的有效抑制。