【作者】 贺训军；

【导师】 吴群；

【作者基本信息】 哈尔滨工业大学 ， 微电子学与固体电子学， 2008， 博士

【摘要】 作为一种新型的射频微机电系统(RF MEMS:Radio Frequency MicroElectroMechanical Systems)器件,分布式MEMS传输线(DMTL: Distributed MEMS Transmission Line)移相器因其具有损耗低、体积小、重量轻、以及与集成电路加工工艺相兼容等特点,已经越来越广泛地应用在相控阵雷达、微波通信、卫星通信以及微波测量等领域,其工作稳定性、可靠性以及射频性能在应用系统中起着关键作用。伴随着DMTL移相器的广泛应用,其射频性能问题一直是人们关注的焦点之一。特别是近年来,DMTL移相器朝多位数字式结构、更高频通信系统应用以及集成化方向发展,从而对DMTL移相器的机械性能、射频性能以及封装结构提出了新的要求。为此,本文对硅基DMTL移相器MEMS电容开关的下拉电压、响应时间和动态特性进行了理论分析和数值计算,并针对传统毫米波段DMTL移相器的射频性能、分析方法和封装结构在实际应用中存在的问题进行了深入系统地研究。鉴于DMTL移相器含有可动机械部件,其机械性能和射频特性等理论分析方法还不够完善,因此,本文首先针对静电驱动分布式MEMS传输线移相器的下拉电压和响应时间展开研究。采用理论模型和仿真验证相结合的方法从多个角度对DMTL移相器的MEMS电容开关进行机械性能分析,探讨和总结MEMS桥几何尺寸、材料性能等因素与下拉电压和响应时间之间的基本规律,目的是为RF MEMS器件的分析、设计和应用提供一定的理论依据。其次,本文针对DMTL移相器的MEMS桥动态特性展开研究,在考虑多种物理因素情况下建立MEMS桥的二维分布动态方程,并采用有限元多物理耦合场方法研究静态加载和动态加载下MEMS桥的动态特性,从而获得了在静态和动态加载情况下残余应力、伸长效应、加载电压、外界机械力的加载模式以及加载模式的振幅、周期和频率对MEMS桥的非线性动态特性影响的基本规律,为分布式MEMS传输线移相器MEMS桥设计和可靠性研究提供了理论基础。第三,本文针对传统毫米波段DMTL移相器射频性能改进展开研究,设计出一种硅基锯齿形DMTL移相器匹配电路结构。该锯齿形DMTL移相器结构特点是把加载MEMS金属桥处共面波导信号线设计成不连续,在传输线上引入串联电感抵消并联MEMS桥所产生的电容,从而有效实现传输线阻抗匹配和降低器件损耗。采用等效电路理论和软件仿真相结合的方法对器件的射频性能进行分析和仿真优化,结果表明:与传统DMTL移相器相比,所设计的硅基锯齿形DMTL移相器的射频性能和可靠性获得明显改进。第四,本文根据毫米波段DMTL移相器结构特点,设计出封装RF MEMS器件的芯片级微封装结构,该微封装结构采用相兼容的MEMS工艺直接在器件衬底上制备微封装结构,而不是在器件衬底键合上采用陶瓷材料、合金材料、金属、玻璃或硅等材料制成的封装帽。通过应用三维电磁仿真工具分析了器件衬底厚度、垂直互连线半径和微空腔高度对器件射频性能的影响,仿真结果表明:优化的微封装结构对器件射频性能影响非常小。最后,根据硅基锯齿形DMTL移相器MEMS电容开关的结构特点,设计了制备MEMS电容开关的表面微机械加工工艺流程以及相应的版图,且该工艺流程只需4块掩模板。测试结果表明:器件射频性能的模拟值和测试结果吻合很好,证明了等效电路理论与仿真工具相结合的方法分析RF MEMS器件的可行性;器件的微结构已完全成型,立体感强,与介质层无粘连,桥本身也非常挺直。更多还原

【Abstract】 As a new type of radio frequency microelectromechanical systems (RF MEMS) device, the distributed MEMS transmission line (DMTL) phase shifter has the advantages of lower loss, small volume, lighter weight and compatibility with IC fabrication process. These characteristics make DMTL phase shifter widely apply at phased-array radar, microwave communications, satellite communications and microwave measurement etc, and the stability, reliability and RF performances of DMTL phase shifter play key roles in those applied systems. However, The RF performance of DMTL phase shifter is always the focus of attention. Especially in recent years, the trends of development about DMTL phase shifter are multi-bit digital structure, high frequency system application and system integration, which put forward new requirements about mechanical characteristics, RF performance and packaging structure of DMTL phase shifter. Therefore, the theoretical analysis and numerical calculation of pull-down voltage, response time and dynamic characteristics about MEMS capacitive switch of DMTL phase shifter on silicon substrate are performed in this paper. Moreover, the existing problem of traditional millimeter-wave band DMTL phase shifter at practical application, including RF performance, analysis method and packaging structure, is deeply researched.Considering the fact that the DMTL phase shifter concludes the movable mechanical components, the theoretical analysis of mechanical characteristics and RF performances etc suffers from a series of limitations. Therefore, the pull-down voltage and response time of electrostatic driven DMTL phase shifter are investigated in this paper firstly. The mechanical characteristics of MEMS capacitive switch of DMTL phase shifter are analyzed using method combining theoretical model and simulation verification to discuss and conclude elementary relations geometric dimension and material performance of MEMS bridge with pull-down voltage and response time. Therefore, these results provide the theory basis for analysis, design and application of RF MEMS devicesSecondly, in order to investigate the dynamic characteristics of MEMS bridge about DMTL phase, the two-dimension distributed dynamic equation of MEMS bridge which takes into account multi-physical factors and effects is presented. The dynamic characteristics of MEMS bridge under static and dynamic loading conditions are analyzed by finite element multi-physical coupled field method to obtain elemental relations about influence of residual stress, stretching effects, applied voltage, different mechanical force loading modes and the amplitude, period and frequency of loading mode on nonlinear dynamic character of MEMS bridge. Therefore, these results provide the theory basis for design and reliability analysis of MEMS bridge about DMTL phase shifter.Thirdly, a circuit match structure of saw-shape DMTL phase shifter on silicon substrate is designed to improve the RF performances of traditional millimeter-wave band DMTL phase shifter. The structure characteristics of saw-shape DMTL phase shifter are that the center conductor of CPW, where the MEMS bridges are loaded, is discontinuous and narrow, so that the series inductance is introduced on the transmission line to offset shunt capacitance from the MEMS bridges, realize impedance match of transmission line and reduce the loss of device. The RF performances analysis and simulation optimization of saw-shape DMTL phase shifter on silicon substrate are performed using method combining equivalent circuit and software simulation. The results show that the RF performance and reliability of saw-shape DMTL phase shifter on silicon substrate obtain obvious improvements comparing with the traditional DMTL phase shifter.Fourth, a chip level micropackaging structure, which package RF MEMS devices, is designed based on structure character of millimeter-wave band DMTL phase shifter. The micropackaging structure is directly fabricated on substrate of devices by compatibility with MEMS fabrication processes and is different from bonding packaging cap on substrate of devices which are fabricated by the ceramic material, alloy, metal, glass or silicon etc. The influences of the substrate thickness, vertical feedthrough radius and microcavity height of micropackaging structure on RF performances of device are analyzed by 3-D EM simulation tool. The results show that optimized micropackaging structure slightly impact on RF performance of device.Finally, the surface micromechanical process flow fabricating MEMS capacitive switch and corresponding layout are designed according to the structure character of MEMS capacitive switch about saw-shape DMTL phase shifter on silicon substrate, and the process flow only requires 4 masks for MEMS capacitive switch. The measurement results show that the MEMS bridge fully suspend over the signal line of CPW, do not adhere to dielectric layer and has very strong three-dimension effect under up-state. Moreover the simulation results agree with test results very well, this demonstrates that the method combining equivalent cicuit theory and simulation tool, which analyze the RF performance of RF MEMS devices, is feasible.更多还原