Infoscience

Thesis

Microwave periodic structures based on MicroElectroMechanical Systems (MEMS) and micromachining techniques

As a result of the ever growing number of functionalities and standards to be supported by communication systems, as well as the constant development of radar and imaging technologies, a key research area in the field of microwaves and millimeter waves is the achievement of reconfigurability capabilities. In recent years, the progress of MicroElectroMechanical Systems (MEMS) fabrication techniques has allowed radically challenging the performances of reconfigurable devices based on established technologies such as controllable ferrite material, semiconductor pin diodes or FET transistors. Consequently, there is presently significant effort to apply MEMS technology to the microwave field; in the case of high-cost applications (e.g. radars, satellites), the main reason is the state-of-the-art performances that MEMS technology can offer; namely, low losses, high linearity, large bandwidth. In the case of mass market (e.g.: mobile phone, GPS receiver), it is rather pushed by the increasing demand for the integration of numerous microwave functionalities into a monolithic, small, low-power consuming and low-cost device. In this context, the objective of this thesis is to contribute to the development of new periodic or cascadable microwave devices reconfigurable by means of MEMS. Indeed, numerous microwave devices take advantage of the particular propagation properties of a wave in periodic structures to achieve given functionalities (e.g. phase shifters, frequency selective surfaces, periodic antennas, antenna arrays and reflectarrays, metamaterials). For this purpose, analysis and design methods were developed based on the theory of waves propagating in periodic structures to help in dealing with different kinds of periodic or cascadable MEMS structures in an integrated approach. The method comprises the following main steps: the setup of efficient full-wave simulations of MEMS blocks, the derivation of physical and accurate circuit models, and the development of hybrid full-wave-circuit model design methods based on periodic structure modeling. It is noticeable that several theoretical developments presented are not restricted to micromachined and MEMS devices, but could be of use for many other microwave designs. Three main classes of devices have been studied and designed to illustrate the versatility of the approach, as well as the various potentialities of MEMS in microwave applications. The first structure addressed is an existing microwave MEMS structure, the distributed MEMS transmission line (DMTL), for which design methods based on the periodic structure modeling were developed. Analog and digital devices were fabricated, showing excellent agreement with the circuit modeled results. We also introduce and analyze a new topology for the reduction of the mismatch in multi-bit DMTLs. The results presented next consist mainly in theoretical developments on the metamaterial composite right/left handed transmission line (CRLH-TL) structure, carried out to overcome the limitations of existing models, which were shown to be inappropriate in the case of MEMS CRLH-TL implementations. Fixed micromachined devices were successfully designed based on the new theory, which also allowed the demonstration of the possibility to design especially low/high impedance CRLH-TLs. Next, MEMS implementations of variable CRLH-TLs are presented. Analog and digital devices were designed, and excellent agreements between full-wave simulations and circuit models are obtained in both cases. For fabrication reasons, only the analog device could be measured to exhibit the expected performances. This constitutes –to the author's knowledge– the first implementation of a MEMS-reconfigurable metamaterial structure. The last device studied is a MEMS-reconfigurable reflectarray cell. A comprehensive assessment of the numerous requirements for such a cell with regard to the functioning of a reconfigurable reflectarray is first presented, as well as detailed discussions on the rigorous simulation and measurement of the device. A monolithic MEMS reflectarray cell was then designed based on these considerations, and exhibits excellent performances in comparison with other reconfigurable reflectarray cells based on MEMS and other technologies.

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