Studies On RF MEMS Phase Shifters And Terahertz Waveguide Filters

The appearance of RF MEMS technology provides a new way to design andfabricate microwave devices. With high performances including integrability, low loss,high linearity and miniature, RF MEMS devices have broad prospects in applying bothfor civil use and for military use. In the dissertation, taking account of both movable andfixed devices, RF MEMS Phase Shifters and Terahertz Waveguide Filters are studied insome aspects of performance optimization and process realization. The main researchesand innovative achievements are as follows.(1) Extracting parasitic parameters by analyzing the discontinued of distributedMEMS phase shifter with MAM capacitor, equivalent-circuit models are presented. Thefitting degree of simulation results shows that, compared with existingequivalent-circuit model, the model’s accuracy was improved greatly, which couldreflect the performances accurately.A modeling idea of transmission line decomposition is advanced, that is, RFMEMSdevice is divided into some typical structures, which can be considered as differenttransmission lines and be modeled respectively to get equivalent circuit model of thedevice by cascading. The verification results indicate that the equivalent-circuit modelobtained by the idea could reflect the device performance over a wide frequency band.This method turns the complex three-dimensional electromagnetic field analysis into thetransmission line problem，which is easier and more applicable. With the similarity ofbasic structures of RF MEMS devices, the obtained circuit models of typical structureshave versatility.(2) The design method to optimize the physic length of MEMS distributed phaseshifters is presented. The researches are carried out on how to achieve designs with lowdependence on processing. Based on those, an X-band5-bit distributed phase shifterwas designed, processed and tested. After solving problems including the residual stressof the structural layer, the reliability of MEMS beam anchor and process accuracy, andbreaking through critical processing steps like the sacrificial layer, the structure layer and the dielectric layer, the formed surface micromachining process could be used to fabr icate RF MEMS devices that contain suspended beam structure. Compared with MEMSdistributed phase shifters reported in the literature, the fabricated phase shifter hascharacteristics of easier process, smaller size and lower actuated voltage.(3) A novel meander distributed phase shifter is advanced. By loading MEMSbridges on meander CPW lines making full use of common ground plane, the length ofdevices are greatly reduced to get a compact structure, thus ensuring processconsistency. Besides, aperiodic distributed phase shifters are advanced, in which unitcells with different phase shifts was cascaded, design flexibility was added and thenumber of MEMS bridges was reduced. Making use of the proposed structures, periodicand aperiodic5-bit distributed phase shifters are designed and simulated to achievesatisfactory performances and in the area of5.36mm*4.72mm and4.8mm*3.75mm.(4) Aimed to the particularity of multi-bit MEMS distributed phase shifters,sources of phase error are investigated comprehensively. The key factor influencing thephase error of phase shifters is proposed and verified, which is mismatch betweenadjacent bits in the phase states switching process. An optimal arrangement of bits withdifferent phase shifts for multi-bit MEMS distributed phase shifters is put forward toimprove phase error.A novel distributed phase shifter with more biased pads is proposed, by controllingthe phase shifter from the unit cell level, multiple reflection of the signal resulted frommismatch between bits is avoided. The contradiction of miniaturization and return lossis also alleviated to some extent. The design results show that phase error of distributedphase shifters using the structure was improved. The return loss and insertion loss arealso improved.Combining the structure of more biased pads with aperiodic structure, thenumber of MEMS bridges in distributed phase shifter is decreased sharply and5-bitphase shifter with only16MEMS bridges is obtained. The structure of distributed phaseshifter in which single bias control many MEMS bridges is given. So bias pads can bereduced three or four times and the interconnection parasitic effects and performancedeterioration in package are alleviated.(5) Another type of RF MEMS phase shifters---switched-line phase shifters areresearched. A compact5-bit phase shifter is designed which adopts SP2T and SP4T. Thecircuit model of MEMS switches and the effects of the bonding wire connectedswitches and peripheral circuits are analyzed. Good measurements results are got after processing and testing.(6) The method of fabricating Terahertz iris band-pass filter by adopting MEMStechnology is presented. Based on the existing process conditions, through analyzing theinfluence of process factors on electromagnetic performace of filters, the process anddesigning parameters are mutually compromised to obtain optimized design. Sharpdeterioration of device performance due to process factors is avoided. Meanwhile, somecritical processing steps like MEMS ICP etching, electroplating gold and bonding andso on were researched, thus solving the problems like iris sidewall angle, metalthickness and roughness. The micromachining process of the filters is proposed.Ultimately the integratable THz filters of small insertion loss and high reliability isobtained by fabrication and test，whose power attenuation are below1dB at the centerfrequency140GHz and170GHz.