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Low-driven design and simulation of RF MEMS switches

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In recent years, RF Micro Systems (RF MEMS) devices for its small size, low power consumption and the widespread concern, especially in MEMS switches and phase shifters built antenna, is the realization of phased array radar million units of the key technologies Army Things are important. Also in the communications field with ultra-low loss, high isolation and low cost has been applied on the phone. However, prevalence of RF MEMS switches with high voltage, switching time length of the question, inferior FET FET switches and PIN diode switches. Compared to the results achieved abroad, the domestic research is still in its infancy. MEMS switches below the defect for some improvements. 1 RF MEMS switch of the general considerations When the MEMS switch beam or membrane by electrostatic attraction to a certain extent offset downward to reach the threshold voltage, the beam or membrane quickly shift to the next plate, the voltage depends on material parameters, the switch size and structure. Beam or membrane material Young's modulus need better and yield strength, Young's modulus greater the higher the resonant frequency to ensure stable and high-speed switches work life; size is designed to consider the electrostatic driving force of the size effect; structural natural vibration Fixed switching frequency of the maximum operating speed. Solely from the structural point of view, ways to reduce the drive voltage: lower plate distance; increase drive space; reduce the beam or membrane elasticity. The structure of common series and parallel suspension Arm beam switch, to reverse the arm switch and capacitive switch, the first three for the contact resistance, the metal outside the line of contact with the signal, such as insertion loss and large there is a lot of problems, and capacitive touch switch also hit in the insulating medium Wear problems. Research has shown that the higher the applied voltage the shorter the life of the switch, the drive voltage reduces the switching speed will inevitably lead to slow, how to meet the driving voltage and switching speed requirements is the current difficulty. 2 RIF MEMS switches Simulation and Optimization For capacitive switches, membrane voltage with the increasing length of the bridge fell, the greater the residual stress of the bridge membrane voltage is also greater. Young tensor usually 78 GPa, Poisson's ratio O. 44 Au as a bridge film material, in order to obtain good Requirements of the isolation switch has a large permittivity, dielectric constant of 7.5 here S3N4 selected as the dielectric layer, the bridge membrane unit is Solid98, plus 5 V voltage, medium is air, the bottom plate plus OV voltage. Then use the ANSYS Modeling, meshing, load and solve the electrostatic coupling and modal analysis. 5 V switching voltage deformation of about O. About 2 μm, still fail to low drive requirements. First five modes are extracted switch shown in Figure 1. Switch from low to high visible resonance frequency increases, respectively, 79.9 kHz, 130.3 kHz, 258.8 kHz, 360.7 kHz, 505.6 kHz, the first order modes from the other mode, that is not easy to be outside Interference, only the control of the switching frequency is less than one order mode resonant frequency to ensure its stability. As the actual switching time is still not ideal, so dig holes in the membrane module to reduce the compression damping, thereby increasing the switching speed . Although the off-state capacitance decreased, but the hole can reduce the weight of the beam, a higher mechanical resonant frequency. The final model of 100 holes were dug, and made both ends of the curved handle to reduce the drive voltage, the simulated Shape into a 5 V voltage than 1μm, stability, switching time 5μs following capacitive switch. Taking into account the capacitive switch dielectric breakdown problems that still exist here to improve its structure, will reverse the arm lever and punch combination of membrane capacitance, voltage and increase in reducing the switching speed at the same time, without affecting the capacitance Ratio, to some extent inhibited the electrical breakdown. Its working principle is: push lever elevation when the applied voltage electrodes, dielectric film and the contact membrane resulting in increased distance between the coupling capacitance is small, the signal through a transmission line; pull electrode power Pressure when the lever down, the coupling capacitance becomes larger, the microwave signal is reflected. Material selection and S3N4 Au is still the main parts of A1 can be used instead of Au. The design of the structure and size of the transcendental equation and the capacitor under switch-off Equation estimates, the bottom plate is 25 × 25 (units with μMKSV, the length unit is μm, the same below), with a dielectric layer on which the hole is 3.4 × 3.4, leverage is 100x30, the structure layer is 20 × 20, plate Thickness of 1. ANSYS to do in the electrostatic coupling and modal analysis using ANSOFT HFSS 3D electromagnetic field simulation of the switch, and further obtain the insertion loss and isolation, determine the CPW and the contact membrane structure to improve the switch RF performance. Ignore the switch curve modeling, the definition of material properties and air radiation boundary, simulated using wave port port were untied states seeking off-state insertion loss and isolation. Dielectric layer thinner, Switch in the vicinity of 10 GHz with good isolation and insertion loss in 1 dB or less. 3 RF MEMS switches Preparation Reasonable choice of growth medium membrane technology has great influence on the switching performance, RF MEMS switch of this article need to grow a layer of silicon nitride film substrate, usually selected LP-CVD process, while the dielectric film is selected for the PECVD process Yi, the performance requirements of the metal film is relatively low, with the sputtering method can be. Taking into account the requirements of the leakage current and loss of substrate as small as possible, select high resistivity silicon substrate with silicon dioxide to do, which is to ensure that the insulation requirements. Gold signal lines And the bottom plate through the plastic strip is formed by electron beam evaporation of aluminum on the plate. However, considering the feasibility of, part of the program process Shixian there for domestic processing difficulty, only the expense of micro-system performance to achieve Processing conditions. Four Conclusion This article from the structure on innovation, obtained by computer-aided design simulation of a theoretical solution, to some extent designed to meet, but in the process, not yet mature. Lower voltage and higher switching Frequency remains a critical problem, the other how to ensure the reliability of the actual product, availability is also a focus of future research.
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