【作者】 方东明；

【导师】 赵小林； 周勇；

【作者基本信息】 上海交通大学 ， 微电子学与固体电子学， 2008， 博士

【摘要】 随着无线通信系统向小型化、低电压、低功耗、多功能、高性能和更高频方向发展,现有的半导体工艺无法满足射频通信部件的高性能和小型化,导致现有的无线通信系统体积庞大、成本高、工作频率低。RF-MEMS采用MEMS技术研制各种无线通信器件及其集成模块,RF-MEMS制作的器件不仅在体积上小于传统的射频器件,易于集成,而且性能也优于传统的射频器件,MEMS技术为实现无线通信系统的小型化、低功耗、高频化提供了良好的解决方案。在RF MEMS元件中,电感和电容是重要的元器件,是滤波器、谐振器的重要组成部分,影响着谐振电路、阻抗匹配网络、放大器、压控振荡器的性能。因此,随着对高频电子器件的需求越来越大,提高和改善MEMS电感和MEMS电容的高频性能已成为国内外学者的研究重点。传统的微机械电感多采用平面螺旋结构,存在着电感值低、寄生损耗大、占用芯片面积大等缺点。同平面螺旋结构的电感相比,三维结构的螺线管微电感由于采用了立体结构,螺线管型微电感器所产生的磁通是平行于基片平面的,所以高频磁通在基片上产生的涡流损失比磁通垂直于基片的平面线圈结构微电感的要小,底导线与衬底的接触面积较小,寄生电容和寄生电阻引起的寄生损耗降低,提高了电感的Q值。为了提高螺线管微电感的性能,本文研究了微电感的集总参数电路模型以及其MEMS制作技术,设计并制作了射频螺线管微电感。另外,射频电路需要可调范围大、高Q值的可变电容器,以实现宽频调变和有关电路功能。与传统的变容二极管相比,用MEMS技术制作的微机械可调电容没有静态电流,信号损耗较小、Q值较高、调节范围更宽,在射频下具有很小的损耗,从而可以降低电路的损耗,提高电路的优值,可替代片外压控变容二极管来实现射频压控振荡器、调谐滤波器等的单片集成。本文采用Al2O3作为牺牲层,利用螺线管微电感相似的MEMS工艺制作出高性能的可变微机械电容。针对射频电路对微电感和微电容的要求,结合目前国内外研制微电感和微电容的现状,本论文采用MEMS技术,包括UV-LIGA、干法刻蚀、湿法刻蚀、抛光技术和电镀技术等,研制各种高性能的微电感和微电容。本文主要完成的工作和结论如下所述:(1)螺线管微电感的建模和仿真利用微电感两端口π网络模型和等效电路,从Greenhouse电感量计算公式和Q值理论计算公式出发,采用Matlab软件进行编程,对螺线管微电感进行建模设计,并与实验结果相比较,对微电感优化设计上起到理论指导作用。研究和分析了线圈结构参数线圈匝数、线圈导体长度、宽度和厚度等对微电感的性能影响,并进行了优化设计。用HFSS软件对微电感进行了仿真,从实验测量值与理论计算值、仿真值对比可以看出,计算结果、仿真结果与实验结果吻合得较好。因此,在今后微电感的应用时,可以根据实际应用要求,设计优化微电感器件的性能,然后进行实验验证和制备,从而达到应用的目的。(2)空芯螺线管微电感的制备和测试考虑和分析了空芯螺线管微电感的几何结构参数对微电感性能的影响,利用MEMS技术制作了不同几何结构和衬底的高性能射频螺线管微电感,所涉及的MEMS工艺主要包括溅射、涂胶、光刻、电镀、干法刻蚀、湿法刻蚀和抛光等工艺。在制备过程中,解决了以下两个关键制造技术:一是通过合理的掩模版设计和双面套刻对准标记、多层深度光刻技术,解决了多层掩膜工艺中的套刻问题和电镀用的光刻胶模具,获得了较好的高深宽比光刻胶模具;二是采用深层微电铸工艺解决了微电感线圈的绕线和连线,通过在电镀液中施加添加剂的手段,实现了高深宽比微小尺寸深度盲孔的电镀问题。这些微电感采用铜线圈以减小线圈寄生电阻。测试结果表明玻璃衬底上的微电感在较宽的工作频率范围内具有高Q值,在6GHz下,微电感Q峰值为38,对应的电感量为1.81nH;硅衬底上的微电感在较宽的工作频率范围内也具有高Q值,2.3GHz下微电感Q峰值为32.8,对应的电感量为1.79nH,HFSS仿真结果与实验结果大致相符,在2.3GHz下Q峰值为34.8,对应的电感量分别为1.68nH。(3)可变电容制备和测试和制作螺线管微电感工艺类似,利用MEMS技术制作了高性能的静电驱动射频可调微机械电容。可调微电容制作方法的最大优势是利用Al2O3作为牺牲层,可以很方便地控制微电容可动机板与固定极板的距离即空气隙的间距,而且Al2O3很容易被KOH溶液刻蚀掉。结构金属材料采用电镀镍,因为镍在常温下的应力比较低,这有利于微电容的结构可靠性。利用WYKO NT1100光学表面轮廓仪测量在不同外加直流电压下可变电容的表面轮廓和位移等信息。测试结果表明有效长度为500μm的方形板微电容吸合电压为13.5V,在1GHz下电容值为0.79pF,品质因数为51.6,电容可调比为1.31:1。(4)可变电容的形变分析和动力学分析在传统平行板可变电容的理论基础上,根据实际情况,引入了可动极板形变参数,对可动极板的形变进行了合理分析,对微电容的初始电容和吸合电压进行了某些修正,给出了具体情况下的偏移量。对于成功的MEMS器件来说,除了改进的实验方法,还要对器件的行为进行探讨。可变电容或开关中一个明显的现象就是吸合效应,为了达到平衡状态,静电力与弹性支撑力要保持相等。但是当外加电压加到两个极板后,静电力增加要比线性弹性支撑力要快,当外加电压超过一个定值时,平衡状态被打破,吸合现象就会发生。对于平行板可变电容这种质量-弹簧-阻尼系统模型,将动力学方程进行近似展开并配合边界条件,就得到可变电容的位移和速度等力学参量。当没有外加电压时,可变电容的质量-弹簧-阻尼系统处于稳定状态,当加上外加电压后,被悬臂梁支撑的可动极板向固定极板移动并达到其平衡位置,同时在稳定区域(线)出现一种不稳定的区域。当外加电压逐渐增大时,位移逐渐增大,达到的平衡位置离可动极板的距离也越来越大,不平衡区域逐渐偏离于平衡区域(线)甚至凸出尖化,说明外加电压增大促进速度快速增大。当外加电压达到吸合电压时,静电力超过弹簧的回复力,可动极板就被吸附到固定极板。更多还原

【Abstract】 With the development of the wireless communication systems towards minitype, low-voltage, low-power, multi-function, high-performance and high-frequency, traditional components and systems fabricated by current semiconductor processes can not meet the demands of high-performance and miniaturization of radio frequency communication components, leading to the current wireless communication systems have large volume, high cost and low-operation frequency. Radio frequency microelectromechanical systems (RF MEMS) study and fabricate the wireless communication components and systems and their integration using by MEMS technologies. The components and systems fabricated by RF MEMS have smaller volume, easier integration and higher performance than traditional RF devices. MEMS technologies provide good solutions for realizing miniature, low-power and high-frequency wireless communication systems. Among RF MEMS devices, inductor and capacitor are important components and vital parts of filters and resonators, affecting the performance of the resonant circuit, impedance matching network, amplifier and voltage-controlled oscillator (VCO). So with the greater requirement of electronic devices working at high frequency, domestic and overseas researchers gradually focus on developing and improving the high-frequency performance of MEMS inductors and capacitors. Traditional MEMS microinductors are planar spiral inductors, which have low inductance, large parasitic losses and occupy large on-chip area. Compared to planar spiral inductor, three-dimensional (3D) solenoid inductor has improved the quality (Q) factor because the main electromagnetic flux of the solenoid inductor is parallel to the substrate surface,thus eddy current loss is potentially reduced then that of the planar spiral inductor whose main electromagnetic flux is vertical to the substrate surface, and smaller overlap area of the conductors with the substrate results in lower parasitic losses which is caused by the parasitic capacitance and parasitic resistance. In order to improve the performance of the solenoid inductors and simplify the fabrication process, the lumped parameter circuit model was studied and RF solenoid inductors were designed and fabricated. Moreover, the RF circuit also needs variable capacitors with high tunable range and Q-factor so that wide band tuning and related circuit function can be realized. Compared to the traditional varactors, the micromechanical variable capacitor fabricated by MEMS technologies has no static current, low signal lasses, high Q-factor, wide tunable range and low losses at radio frequency, so that it can decrease circuit losses and improve the circuit merit as an alternative of off-chip voltage-controlled varactors to realize monolithic integration of RF VCOs and tuned filters. In this dissertation, Al2O3 was used as the sacrificial layer, the variable high-performance capacitors were fabricated by MEMS process which is similar to that of the fabrication of the solenoid inductors.In order to meet the demand of microinductors and microcapacitors in RF circuit combined with the current research status and the fabricated devices, in this work, high-performance microinductors and microcapacitors were fabricated by MEMS technologies including UV-LIGA, dry etching, wet etching, fine polishing and electroplating technique. In a word, the main work and the conclusions of this dissertation are as follows:(1) The modeling and simulation of solenoid inductorsThe solenoid inductors were modeled, designed and optimized according to the Greenhouse theory of calculating inductance and the theory of calculating Q-factor equations by using a two-portπ-network physical model and equivalent circuit of on-chip microinductor. The MATLAB software was programmed and used to get the performance of the modeled and designed microinductor compared with the experiment results. All these can be used for the guidance to design and optimize the solenoid inductors. The effect of structure parameters of the solenoid inductors such as coil turns, conductor length, conductor width and conductor thickness on the performance of microinductor was modeled, analyzed and optimized. Meanwhile, the performance of the solenoid inductors were simulated by the high frequency structure simulator (HFSS) software, and the calculated/simulated results were matched well with the experimental results. So the microinductors can be designed and optimized according to the demands of practical applications in the future, then the experiments can be validated and carried out to meet with the applications.(2) Fabrication and test of the air-core solenoid inductorsThe effect of the geometrical parameters on the air-core solenoid inductors were considered and analyzed. The MEMS technologies were used to fabricate different geometrical structure inductors on different substrates, including sputtering, coating, lithography, electroplating, dry etching technique, wet etching technique and fine polishing, etc. During the fabrication process, the following key issues have been studied: 1) by appropriate designs for mask and double-side mask alignment marks, the problem of double-side mask alignment photolithography was solved, and the photoresist mold with high aspect ratio was formed;2)The coil windings and link windings were realized by deep electroplating and high aspect ratio micro- scale blind holes electroplating were resolved through adding some additives in the electroplating solution. All the coils of the air-core inductors were copper in order to decrease the series resistance. The testing results indicate that the microindcutors have high Q-factor over wide range of operating frequency on glass substrate. The peak Q-factors at 6GHz is 38 and the corresponding inductance at peak-Q frequency is 1.81nH. The microindcutors fabricated on silicon substrate also have high Q-factor over wide range of operating frequency. The peak Q-factors at 2.3GHz is 32.8 and the corresponding inductance at peak-Q frequency is 1.79nH. The HFSS simulated results were approximate to the measured results. The simulated peak Q-factors at 2.3GHz is 34.8 and the corresponding inductance at peak-Q frequency is 1.68nH.(3) Fabrication and test of the variable capacitorsSimilar to the fabrication process of the solenoid inductors, the electrostatically driven RF variable micromechanical capacitors were fabricated by MEMS technologies. The most advantage of the fabrication is that using Al2O3 as the sacrificial layer so that it is easy to control the air gap between the movable plate and the fixed plate; moreover, Al2O3 can be easily etched by KOH solution. Electroplated nickel were used to form the structure because nickel has low stress at room temperature which will benefit to the reliability of the capacitors. The surface profile and displacement of the variable capacitor at different values of the applied voltage is measured by using WYKO NT1100 optical surface profiler. The measured results show that the pull-in voltage of the variable square capacitor with the effective length 500μm was 13.5V, the capacitance and Q-factor at 1GHz are 0.792pF and 51.6, and the tuning ratio of the capacitor is more than 1.31:1.(4) Deformation and dynamic analysis of the variable capacitorsBased on the traditional theory of the parallel plate variable capacitor and according to the practical situation, the deformed parameters were introduced. The deformation of the movable plate was properly analyzed, and the initial capacitance and the pull-in voltage of the capacitor were modified in some degree, giving their deviation for specific states. Successful MEMS devices rely not only on the fabrication technologies but also on the knowledge of device behaviors, based on which a favorable structure of the device can be forged. A well-known phenomenon of the electrostatic actuators or microbeams is pull-in. In order to achieve equilibrium, the electrostatic force must balance with spring force. However, as the applied voltage increases or the initial gap decreases, the electrostatic force increases much faster than the linear spring force. The stability of the equilibrium is broken and pull-in occurs when the applied voltage exceeds a threshold value. As for the mass–spring–damper system of the parallel plate variable capacitor, by approximately expanding the governing equation of motion for the dynamics of the system and combining with the boundary conditions, the dynamic parameters such as displacement and the velocity of the variable capacitor will be achieved. When a voltage is applied, the suspended plate moves to the fixed plate and finally arrives at the equilibrium position, meanwhile, an instability zone occurs around the stability zone (line). As voltage increases, the displacement of the equilibrium position increases and the instability zone deviates from the stability zone (line) even extrudes. When the applied voltage exceeds the pull-in voltage, the electric force may become higher than the spring restoring force and the movable plate will stick to the fixed plate.更多还原