Radio Frequency Characteristics Of Ka Band Distributed MEMS Transmission Line Phase Shifter On Silicon Substrate

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.