Characterization of capacitive micromachined ultrasonic transducers

This paper describes numerical and experimental characterization of capacitive micromachined ultrasonic transducers (CMUTs) for ultrasound transmission. Simulations based on a finite elements method to model the electromechanical behaviour of CMUTs and to determine the dimensions of elementary cells are presented. In particular, we analyse how the collapse voltage and the capacitance are affected by different parameters of a circular cell and by different bias voltages. The fill factor is defined as the ratio of the top electrode radius to the membrane radius and we study the influence of the fill factor in the performances of CMUTs. The fabrication process of a CMUT uses anodic bonding of a SOI wafer on a borosilicate glass substrate and we compare experimental results with numerical results in terms of eigenfrequencies, bandwidth, quality factor and capacitance for non-metallized and metallized membranes.     

Flexible piezoelectric micromachined ultrasonic transducer (pMUT) for application in brain stimulation

We propose a new flexible piezoelectric micromachined ultrasonic transducer (pMUT) array integrated on flexible polydimethylsiloxane (PDMS) that can be used in studying brain stimulation by ultrasound. To achieve the technical demands of a high sound pressure level and flexibility, a diaphragm-type piezoelectric ultrasound transducer array was manufactured with 55 μm-thick bulk lead zirconate titanate (PZT) that was thinned after bonding with a silicon wafer. The ultrasound transducer array was then strongly bonded onto a PDMS substrate using an oxygen-plasma treatment followed by precise dicing with a fixed pitch to achieve flexibility. The radius of curvature was smaller than 5 mm, which is sufficient for attachment to the surface of a mouse brain. After a thinning process for the PZT layer, we observed that the PZT layer still maintained a high ferroelectric property. The measured remnant polarization (P_r) and coercive field (E_c) were 28.26 μC/cm² and 79 kV/cm, respectively. The resonant frequencies of fabricated pMUT elements with different membrane sizes of 700, 800, 900, 1200 μm in diameter were measured to be 694.4, 565.4, 430.8, and 289.3 kHz, respectively. By measuring the ultrasound output pressure, a pMUT showed a sound intensity (I_sppa) of 44 mW/cm² at 80 V, which is high enough for low-intensity ultrasound brain stimulation.

     

Fabricating capacitive micromachined ultrasonic transducers with wafer-bonding technology

Introduces a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs) that uses a wafer bonding technique. The transducer membrane and cavity are defined on an SOI (silicon-on-insulator) wafer and on a prime wafer, respectively. Then, using silicon direct bonding in a vacuum environment, the two wafers are bonded together to form a transducer. This new technique, capable of fabricating large CMUTs, offers advantages over the traditionally micromachined CMUTs. First, forming a vacuum-sealed cavity is relatively easy since the wafer bonding is performed in a vacuum chamber. Second, this process enables better control over the gap height, making it possible to fabricate very small gaps (less than 0.1 /spl mu/m). Third, since the membrane is made of single crystal silicon, it is possible to predict and control the mechanical properties of the membrane to within 5%. Finally, the number of process steps involved in making a CMUT has been reduced from 22 to 15, shortening the device turn-around time. All of these advantages provide repeatable fabrication of CMUTs featuring predictable center frequency, bandwidth, and collapse voltage.     

Equivalent circuit for capacitive micromachined ultrasonic transducers to predict anti-resonances

Equivalent circuit models have been long used to evaluate the dynamics of the capacitive micromachined ultrasonic transducer (CMUT). An important parameter in the characterization of a CMUT is the anti-resonance frequency, which limits the immersion bandwidth. However, there is no equivalent circuit model that can accurately determine the anti-resonance frequency of a membrane. In this work, we present an improved lumped element parametric model for immersed CMUT. We demonstrate that the proposed equivalent circuit model accurately predicts anti-resonance and higher order mode frequencies, in addition to that of the fundamental mode. The proposed circuit model is in good agreement with device characteristics calculated using the finite element method and experimentally measured data.

     

Surface micromachined piezoelectric accelerometers (PiXLs)

The design, fabrication, and characterization of surface micromachined piezoelectric accelerometers are presented in this paper. The thin-film accelerometers employ zinc oxide (ZnO) as the active piezoelectric material, with different designs using either polysilicon or ZnO bimorph substrates. Sensitivity analyses are presented for two specific sensor designs. Guidelines for design optimization are derived by combining expressions for device sensitivity and resonant frequency. Two microfabrication techniques based on SiO₂ and Si sacrificial etching are outlined. Techniques for residual stress compensation in both fabrication processes are discussed. Accelerometers based on both processes have been fabricated and characterized. A sensitivity of 0.95 fC/g and resonant frequency of 3.3 kHz has been realized for a simple cantilever accelerometer fabricated using the sacrificial SiO₂ process. Sensors fabricated in the sacrificial Si process with discrete proof masses have exhibited sensitivities of 13.3 fC/g and 44.7 fC/g at resonant frequencies of 2.23 kHz and 1.02 kHz, respectively     

Piezoelectric micromachined ultrasonic transducer based on dome-shaped piezoelectric single layer

In this paper, we proposed a dome-shaped model for piezoelectric micromachined ultrasonic transducer structure. A finite element analysis was carried out to study the elecro-mechanical behaviour of the dome-shaped model as well as the conventional model. The result showed that a considerable improvement of electro-mechanical coupling performance was achieved with the dome-shaped model.     

Influences of perforation ratio in characteristics of capacitive micromachined ultrasonic transducers in air

Capacitive micromachined ultrasonic transducers (CMUTs) with a perforated membrane have been fabricated and characterized in air. Two types of CMUT device have been fabricated having perforation ratio (area ratio of holes = AR) of 10% and 20%, and analyzed about electrical and mechanical characteristics. The perforation ratio (AR) of membrane substantially influences on the electrostatic force and mechanical restoring force of the device since it leads to the area variation of electrode and membrane, it subsequently influences on the sensitivity and frequency response of the CMUT device. The electrostatic force and mechanical restoring force were improved by decreasing the AR and increasing the DC bias voltage. The open-circuit sensitivity of a CMUT having AR 10% membrane, 8.45 μV/Pa, is larger than that of AR 20%, 4.07 μV/Pa at DC 15 V. Furthermore, the resonance behaviors were observed in the range of 60-80 kHz, and the resonance frequency could be changed by varying the applied DC voltage and AR.     

Design and performance analysis of capacitive micromachined ultrasonic transducer (CMUT) array for underwater imaging

Microsystem Technologies - In this paper, a Receipt & Transmission CMUT array has been developed for underwater imaging purposes. To guide the array design more intuitively, directivity...     

压电式压力传感器热冲击效应的试验研究

论述了瞬变温度对压电式压力传感器具有热冲击作用,并能改变传感器的预紧力、自振频率及灵敏度,形成瞬变温度误差.传感器的特殊膜片结构可以大大减小瞬变温度误差.通过对某2种压电式传感器的模拟热冲击效应的试验研究,阐述了热冲击对传感器的实际影响,并给出了测量数据及分析结果.     

压电微固体模态陀螺的安装技术

压电微固体模态陀螺（ PMMG）利用压电PZT长方体的某高阶共振模态作为参考振动,是一种抗冲击、抗震动能力强的新型全固态陀螺。获得优化的参考振动振型对提高陀螺性能至关重要,但是大多数文献都没有涉及到封装对参考振动的影响。利用有限元方法（ FEM）分析了压电振子安装时支撑点位置与接触面积对参考振动的影响,分析结果表明：相对于接触面积的变化,支撑点位置的变化对陀螺参考模态的影响更大。该研究结果为压电微固体模态陀螺的安装方法提供了理论基础。     

Tonpilz型压电陶瓷超声传感器的设计

超声传感器是一种电声转换器件,其敏感元件压电陶瓷控制传感器的主要性能。设计了一种谐振频率为140 kHz的Tonpilz型压电陶瓷超声传感器,从压电方程入手,建立了不同的理论模型,对等效网络法和有限元法2种不同的设计方法进行了比较。相应的试验表明:有限元法的分析结果直观明了、建模快捷、分析准确,其误差可控制在5%以内。设计研制的Tonpilz型传感器工艺简单、造价低廉、性能稳定。     

Micromachined ultrasonic transducers based on lead zirconate titanate (PZT) films

The design, fabrication and measuring of piezoelectric micromachined ultrasonic transducers (pMUTs), including the deposition and patterning of PZT films, was investigated. The (100) preferential orientation of PZT film have been deposited on Pt/Ti/SiO₂/Si (100) substrates by modified sol–gel method. PZT film and Pt/Ti electrode were patterned by novel lift-off using ZnO as a sacrificial layer avoiding shortcomings of dry and wet etching methods. pMUT elements have been fabricated by an improved silicon micromachining process and their properties were also characterized. As measured results, the pMUT tends to operate in a standard plate-mode. The receive sensitivity and transmit sensitivity of pMUT element whose active area only has 0.25 mm² are ?218 dB (ref. 1 V/μPa) and 139 dB (ref. 1 μPa/V), respectively.     

静电激励硅微机械谐振压力传感器设计

设计了一种静电激励/电容检测的硅微机械谐振压力传感器,采用改进的侧向动平衡双端固支音叉谐振器,利用基于绝缘体上硅的加工工艺制作。为了抑制压力敏感膜片受压变形时谐振器的高度变化,在谐振器固定端设计了全新的桁架结构。针对传感器检测信号微弱和同频干扰严重的特点,在芯体和接口电路设计中采取添加屏蔽电极、降低交流驱动电压幅值、差动电容检测和高频载波调制解调方案等多项措施。同时基于该接口电路设计了开环测试系统,并在常压封装条件下对传感器进行了初步性能测试。实验结果表明:其基础谐振频率为33.886 kHz,振动品质因数为1222;测量范围为表压0~280 kPa,非线性为0.018%FS,迟滞为0.176%FS,重复性为0.213%FS;在-20~60℃的温度范围内,谐振器的平均温度漂移为-0.037%/℃。     

一种高灵敏度硅微机械陀螺的设计与制造

设计与制造了一种高灵敏度的硅微机械陀螺。陀螺用静电来驱动,用连接成惠斯顿电桥的压阻式力敏电阻应变计来检测。主梁、微梁 质量块结构实现了高灵敏度。比较硬的主梁提供了一定的机械强度,并且提供了高共振频率。微梁很细,检测时微梁沿轴向直拉直压。力敏电阻应变计就扩散在微梁上,质量块很小的挠动就能在微梁上产生很大的应力,输出很大的信号。5V条件下,陀螺检测部分的理论灵敏度达到27.45mV/gn。压阻式四端器件用来监测驱动振幅,可以反馈补偿压阻的温度系数。检测模态的Q值达260使陀螺能在大气下工作。陀螺利用普通的n型硅片制造,为了刻蚀高深宽比的结构,使用了深反应离子刻蚀(DRIE)工艺。     

Microbioreactor designed for integration with piezoelectric transducers for cellular diagnostics

This article reports a perfusion-based microbioreactor that can be integrated with a thin-film piezoelectric transducer array for rapid disease diagnosis, such as identifications of cancer cells and infection induced cell abnormality. By using the gap between suspended transducers as the high-aspect ratio barriers to establish high flow resistance into the culture chamber, we have verified that diffusion dominated the main transport mechanisms in the culture chamber. The fluid flow was successfully suppressed under 0.03, 0.3, and 3.0 μl/min volume flow rates, and a very low shear flow region was achieved on the transducer surfaces. This design offers minimal influences of mechanical forces on cellular detection and cells cultured on the surface of transducers. Finite element simulation showed that the shear stress on transducer surface could be maintained lower than milli-Pascal range. Detailed design, simulation results and experimental verifications of the microbioreactor are discussed.     

Design of piezocomposite materials and piezoelectric transducers using topology optimization— Part III

Summary Following the topic introduced in [1, 2] this paper discusses the design of piezoelectric transducers used in applications such as acoustic wave generation and resonators. These applications require goals in the transducer design such as high electromechanical energy conversion for a certain transducer vibration mode and narrowband or broadband response. The development of these devices has been based on the use of simple analytical models, test of prototypes, and analysis by the finite element method. However, in all cases the design is limited to a parametric optimization where only some dimensions of a chosen transducer configuration are optimized. By changing the topology of these devices or their components, we may obtain devices with better performance since the design space of solutions is enlarged. Based on this idea, we have proposed the use of topology optimization for designing these devices. This method consists of finding the distribution of the material and void phases in the design domain that optimizes a defined objective function. The optimized solution is obtained using Sequential Linear Programming (SLP). Considering acoustic wave generation and resonator applications, three kinds of objective functions were defined: maximize the energy conversion for a specific mode or a set of modes; design a transducer with specified frequencies; and design a transducer with narrowband or broadband response. Although only two-dimensional plane strain transducer topologies have been considered in order to illustrate the implementation of the method, it can be extended to three-dimensional topologies. Transducer designs were obtained that conform to the desired design requirements and have better performance characteristics than other common designs.     

Design of piezocomposite materials and piezoelectric transducers using topology optimization—Part I

Summary Currently developments of piezocomposite materials and piczoelectric actuators have been based on the use of simple analytical models, test of prototypes, and analysis using the finite element method (FEM), usually limiting the problem to a parametric optimization. By changing the topology of these devices or their components, we may obtain an improvement in their performance characteristics. Based on this idea, this paper discusses the application of topology optimization combined with the homogenization method and FEM for designing piezocomposite materials. The homogenization method allows us to calculate the effective properties of a composite material knowing its unit cell topology. New effective properties that improves the electromechanical efficiency of the piezocomposite material are obtained by designing the piezocomposite unit cell. This method consists of finding the distribution of the material and void phases in a periodic unit cell that optimizes the performance characteristics of the piezocomposite. The optimized solution is obtained using Sequential Linear Programming (SLP). A general homogenization method applied to piczoelectricity was implemented using the finite element method (FEM). This homogenization method has no limitations regarding volume fraction or shape of the composite constituents. The main assumptions are that the unit cell is periodic and that the scale of the composite part is much larger than the microstructure dimensions. Prototypes of the optimized piezocomposites were manufactured and experimental results confirmed the large improvement. Department of Mechanical Engineering and Applied Mechanics Department of Mechanical Engineering and Applied Mechanics     

Modeling the performance of a micromachined piezoelectric energy harvester

Piezoelectric energy microgenerators are devices that generate continuously electricity when they are subjected to varying mechanical strain due to e.g. ambient vibrations. This paper presents the mathematical analysis, modelling and validation of a miniaturized piezoelectric energy harvester based on ambient random vibrations. Aluminium nitride as piezoelectric material is arranged between two electrodes. The device design includes a silicon cantilever on which AlN film is deposited and which features a seismic mass at the end of the cantilever. Euler–Bernoulli energy approach and Hamilton’s principle are applied for device modeling and analysis of the operation of the device at various acceleration values. The model shows good agreement with the experimental findings, thus giving confidence into model. Both mechanical and electrical characteristics are considered and compared with the experimental data, and good agreement is obtained. The developed analytical model can be applied for the design of piezoelectric microgenerators with enhanced performance.     

Design of piezocomposite materials and piezoelectric transducers using topology optimization—Part II

Archives of Computational Methods in Engineering - In addition to the design of piezocomposite material, discussed by Silvaeet al. [1], another important issue is the design of the piezoelectric...