Infoscience

Thesis

RF MEMS Switches for Wide-band (5 - 40 GHz), Tuneable Filters and DMTL Phase Shifters

In wireless communications, radio frequency micro-electro-mechanical systems (RF MEMS) technology is attracting tremendous interest across the world as the needs of frequencies gets higher, data bandwidth gets larger and multiple broadband signals have to be handled in the same device. Compared to other technologies, components based on RF-MEMS provide superior RF performance and tuning, and can perform over a much broader range of operating frequencies. In particular, the RF MEMS switches enable the reconfigurability of RF front-end circuits and their excellent linearity, lower insertion losses and improved isolation recommend them for high frequency applications. Phase shifters and tuneable filters in X to Ka-band frequencies for beam steering/forming of antenna phase arrays and for bandpass or rejection can take full advantage of the RF MEMS switches. The present open challenges of RF MEMS devices are the reliability, the packaging and their 3D integration with RF ICs. In this context, the objectives of this thesis have been to develop novel RF MEMS switches, phase shifter and highly tuneable filters with improved characteristics in terms of high frequency performance, improved reliability, lifetime and repeatability. The airborne applications added supplementary constraints in terms of operation over a wide range of temperatures. These important challenges have been addressed in this work not only by introducing novel materials and technologies but also by innovating at the level of device architecture, compactness of the design and substrate engineering. RF switches and phase shifters have been conceived especially for applications in the aeronautical field and for Wireless Sensors Network (WSN) working at 17 GHz. Some of them have been designed even with 3D integration capability and co-fabricated with antenna elements for beam steering applications. On the other hand, RF MEMS filters were designed to feature highly tunable capabilities for radar applications. RF MEMS capacitive switches in shunt and series configuration used as core devices for both phase shifters and filter applications have been realized in two different fabrication processes: (1) Ni/Au-membrane AlN-dielectric process from Fraunhofer ISit and (2) Al/Al-based MEMS from CMI at EPFL. A key contribution of this work is the use of Aluminum Nitride (AlN) as dielectric in highly reliable RF MEMS. The charging and discharging mechanisms of AlN are reported to be particularly adapted for RF MEMS operating in air ambient conditions. The reliability studies have been performed for all types of devices and applications reported in the manuscript. Different DMTL phase shifter topologies have been optimized for minimizing the size and the mismatch at specific frequencies. Multi-bit DMTL phase shifters have been designed and fabricated with an interlaced-bit (IB-DMTL) configuration to enhance the matching in the intermediate states. In addition, 3-state phase shifters have been realized based on different spring constant bridges and a single control actuation command. Dipoles antennas and DMTL phase shifters have been integrated and fabricated in a single wafer process for beam steering applications. Another significant part of the work is dedicated to novel bandpass and band-stop RF tunable architectures operating above 8 GHz frequency. Bandpass filters based on distributed-lumped series and shunt MEMS switches have been investigated for independent control of the center frequency and bandwidth tuning. Band-stop filters based on compact lumped single MEM devices have been designed to extend the center frequency tuning range over state-of-the-art values reaching more than 50% tuning range by means of the meander arm inductors and their zipping capabilities.

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