Research Of The Design And The Reliability Of The Reconfigurable MEMS Microwave Power Coupler

Microwave power coupler is widely used in military and civilian microwave communication systems. The coupling of the traditional microwave power coupler is fixed, thus this paper proposed a novel reconfigurable MEMS microwave power coupler and this coupler was simulated, designed and fabricated. However, due to the use of movable micro-mechanical structure in MEMS devices, the reliability problems of MEMS devices which are caused by some microscopic effects is the key for their application, so the research about the reliability problems of MEMS beam structure has been studied. The main works and the innovations are described as follows:(1) A reconfigurable MEMS microwave power coupler is designed based on GaAs MMIC technology. The size of the coupler is optimized by HFSS 12.0 software. The center frequency of the coupler is 35 GHz, and the coupling of the coupler is 7.989 dB under’down’state and 10.62 dB under’up’ state at the center frequency. What’s more, from 30 to 40 GHz, the S11 is less than-15dB and the isolation of the coupler is more than 15 dB under the two states. The innovation is that the vice line of this coupler has a doubly-supported beam with two cantilevers on its free edge to control the coupling and the reconfigurable MEMS microwave power coupler is much smaller than other reconfigurable couplers because it has the same size with the traditional coupled line directional coupler with comparable performance.(2) A resonant method is presented to identify the adhesion based on the fact that adhesion will affect the vibration spectrum of a MEMS beam. According to the experimental results, the response frequency spectrums of a series of beams can be roughly divided into three groups. It can be demonstrated by observing the top surface profiles of the corresponding test beams that only the beams have the response frequency spectrums consistent with the theoretical prediction have no adhesion, while the MEMS beams under other cases adhere to the substrate partly or completely. In addition, the adhesion of the perforated beams with different drying temperatures has been studied by taking advantage of the resonant method. The experimental results show that high temperature can depress the adhesion of the beam. This is identical with the theory. The innovation is that the proposed resonant method is non-destructive and has good applicability for wide beams, while it is difficult for the SEM method and the interference method to identify the adhesion of some wide beams.A mathematical model has been developed to analyze the vibrational behavior of the step-up supported beams. This model accounts for the step-up boundary condition by replacing it with a simply-supported boundary condition in conjunction with equivalent torsional stiffness. The analytical formulas of the resonant frequencies and mode shape are derived. In addition, the effect of the Young’s modulus and support beam thickness on resonant frequencies is discussed by this model. The discussion shows that the resonant frequencies will decrease with Young’s modulus or the support beam thickness decreasing. What’s more, the model prediction is compared with the finite element analysis using ANSYS 12.0 software. When the length of the beam is larger than 200μm, the difference between the model prediction and the finite element analysis is less than 5%. The innovation is that our vibration model can give more effective prediction for the case that the length of the horizontal beam is much larger than the length of the support beam compared with the clamped model.(3) An improved test structure with four rotation structures has been designed to measure the residual stress of MEMS beams. The improved test structure is simulated by ANSYS software, and fabricated based on GaAs MMIC process. The experimental results show that the residual stress of the MEMS beams based on GaAs MMIC process is so small that the effect of the residual stress on the structures can be ignored and the residual stress has little impact on the reliability of the MEMS beam based on GaAs MMIC process. The innovation is that the improved rotating technique measures the rotating angles of the indicator beams instead of the deflection to calculate the residual stress, so it does not need vernier and is easy to be performed compared with the typical rotating technique.The design theory and implementation of this reconfigurable MEMS microwave power coupler fill the blank of GaAs MMIC-based microwave MEMS devices and systems in china and have a number of potential applications. Meanwhile, the related papers about the experimental method and model have been accepted or under review by the international Journals in MEMS field.