RF-MEMS Switched Varactors for Medium Power Applications

In RF (Radio Frequency) domain, one of the limitations of using MEMS (Micro Electromechanical Systems) switching devices for medium power applications is RF power. Failure phenomena appear even for 500 mW. A design of MEMS switched capacitors with an enhanced topology is presented in this paper to prevent it. This kind of device and its promising performances will serve to fabricate a MEMS based phase shifter able to work under several watts.Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/handle/2042/16838

RF-MEMS for high-performance and widely reconfigurable passive components – A review with focus on future telecommunications, Internet of Things (IoT) and 5G applications

Abstract Since its first discussions in literature during late '90s, RF-MEMS technology (i.e. Radio Frequency MicroElectroMechanical-Systems) has been showing uncommon potential in the realisation of high-performance and widely reconfigurable RF passives for radio and telecommunication systems. Nevertheless, against the most confident forecasts sparkling around the successful exploitation of RF-MEMS technology in mass-market applications, with the mobile phone segment first in line, already commencing from the earliest years of the 2000s, the first design wins for MEMS-based RF passives have started to be announced just in late 2014. Beyond the disappointment of all the most flattering market forecasts and, on the other hand, the effective employment of RF-MEMS in niche applications (like in very specific space and defence scenarios), there were crucial aspects, not fully considered since the beginning, that impaired the success of such a technology in large-market and consumer applications. Quite unexpectedly, the context has changed rather significantly in recent years. The smartphones market segment started to generate a factual need for highly reconfigurable and high-performance RF passive networks, and this circumstance is increasing the momentum of RF-MEMS technology that was expected to take place more than one decade ago. On a broader landscape, the Internet of Things (IoT) and the even wider paradigm of the Internet of Everything (IoE) seem to be potential fields of exploitation for high-performance and highly reconfigurable passive components in RF-MEMS technology. This work frames the current state of RF-MEMS market exploitation, analysing the main reasons impairing in past years the proper employment of Microsystem technology based RF passive components. Moreover, highlights on further expansion of RF-MEMS solutions in mobile and telecommunication systems will be briefly provided and discussed

Tuneable RF MEMS components using SU-8

With the rapid progress in the wireless communication field, radio frequency microelectro- mechanical systems (RF MEMS) are seen as one of the promising technologies to replace the existing high power communication systems. MEMS based tuneable devices such as varactors and phase shifters offer many advantages over their conventional diode-based counterparts including low loss, low power consumption and high linearity. MEMS varactors in particular can be integrated into many reconfigurable modules such as switching and reconfigurable matching networks. Moreover, distributed MEMS transmission line (DMTL) phase shifters with their linear phase characteristic can be applied to wideband phased array antennas for microwave medical imaging which requires beam steering and high gain antenna systems. This thesis focuses on the design and development of two RF MEMS devices which are a high tuning ratio digital MEMS varactor and a low frequency DMTL phase shifter using SU-8 polymer. The design and simulation of a 4-bit and a 5-bit digital MEMS varactors have been carried out in the first phase of this study. One of the limitations of the digital MEMS varactors fabricated on silicon substrates is the high fringing field capacitance that reduces the overall capacitance ratios of the devices. To reduce the effect of the fringing fields, two methods have been proposed to elevate the varactors from the silicon substrate. In the first method, a 26.35 μm deep trench is etched in the silicon substrate under the 4-bit digital MEMS varactor which is able to achieve a high capacitance ratio of 35.7. In the 5-bit digital MEMS varactor design, SU-8 material is used to form a 20 μm thick separation layer between the varactor and the silicon substrate instead of the deep trench method applied in the 4-bit MEMS varactor. The simulated capacitance ratio of the 5-bit digital MEMS varactor is 34.8. Additionally, the SU-8 also serves as a sacrificial layer to release the MEMS bridges on the devices hence reducing the fabrication process compared to the conventional MEMS release process that uses oxide as the sacrificial material. To verify the performance of using the thick SU-8 dielectric layer in reducing the fringing field capacitance in the varactor design, single-bridge varactors with different lengths and widths have been fabricated and analysed. A novel truss bridge structure has been proposed in order to reduce the pull-in voltage of the varactors. It is found that by using the truss structure, the measured pull-in voltage of the bridge can be reduced by 12.5% compared to the conventional solid fixed-fixed bridge structure. However, due to the high residual stress from the fabrication process which causes the bridge to warp over its width, the achievable average down-state capacitance of the fabricated single-bridge varactor is limited to 211 fF compared to the simulated value of 1.28 pF. Nevertheless, the capacitance ratio of the device fabricated on the SU-8 layer increases by 56.75% over a similar device fabricated without the polymer which proves that the fringing field capacitance has been reduced. Furthermore, fabrication of the single-bridge MEMS varactors on low-resistivity silicon has been carried out with the use of SU-8 as the passivation layer without affecting the performances of the varactors. This finding can lead to the realisation of low-cost MEMS varactors in the future. The second part of this thesis investigates the development of distributed MEMS transmission line (DMTL) phase shifters for operation in the frequency range of 2 GHz to 4 GHz (S-band). The proposed phase shifters are a 2-bit and 3-bit digital DMTL phase shifters. One of the potential applications of the proposed phase shifters is for phased array antenna systems for microwave head imaging that requires wideband performance. The 2-bit and 3-bit DMTL phase shifters have been designed and simulated with 41 MEMS bridges and 105 MEMS bridges respectively. The simulated phase shifts of the 2-bit phase shifter design are 00, 900, 1800 and 2700 whereas for the 3-bit phase shifter, 8 phase shifts have been achieved namely 00, 450, 900, 1350, 1800, 2250, 2700 and 3150. To validate the performance of the proposed low frequency DMTL phase shifter, the 2-bit phase shifter design has been fabricated and analysed. The measured impedance matching of the phase shifter shows good performance with reflection coefficients of less than -10 dB across the operating frequency range for all the states of the phase shifter. The measured differential phase shifts of the device are 00, 17.890, 34.510 and 52.390. The lower measured differential phase shifts compared to the simulated values can be attributed to the warping of the bridges over their width which causes a formation of an air gap between the bridge and dielectric layer hence reducing the down-state capacitance of the varactors in the phase shifter. Nevertheless, this is the first DMTL phase shifter to achieve a maximum differential phase shift of 52.390 at 2.45 GHz. Based on the measured differential phase shifts, the phase shifter can provide a maximum steering angle of ±5.730 for a 4-element phased array antenna at 2.45 GHz. The maximum measured transmission loss of the phase shifter is -10.51 dB at 2.45 GHz. The high loss of the phase shifter is due to the skin depth effect since the co-planar waveguide (CPW) transmission line of the phase shifter is fabricated using 300 nm thick aluminium. Therefore, further investigation has been carried out to provide better estimation of the transmission loss of the phase shifter by fabricating a CPW transmission line with the same configuration to that of the transmission line in the fabricated phase shifter by using 2 μm thick aluminium. The measured loss of the transmission line is -2.39 dB which shows significant improvement over the loss obtained from the phase shifter. Moreover, several CPW transmission lines with different centre conductor’s widths have been fabricated and analysed to further reduce the losses of the transmission lines. An attenuation loss of only 0.122 dB/cm has been achieved using a 500 μm-width centre conductor in the fabricated CPW transmission line which can lead to the realisation of a low-loss DMTL phase shifter for low microwave frequency range. The characterisation and optimisation of the varactors and phase shifters using SU-8 provide the initial step towards the development of tuneable RF MEMS devices for wide range of applications including wireless communications and radar systems. Moreover, the proposed DMTL phase shifters for operation at the lower end of microwave spectrum particularly in the frequency range of 2 GHz to 4 GHz are vital for the realisation of wideband phased array antennas for microwave medical imaging applications

Fluidic, Solid-State, and Hybrid Reconfiguration Techniques in a Frequency and Polarization Reconfigurable Antenna

This work presents the development of a hybrid reconfiguration technique used to achieve both frequency and polarization diversity in a 2.4 – 2.5 GHz microstrip antenna. This hybrid solution for the first time combines current state-of-the-art fluidic and solid-state reconfiguration mechanisms in a collaborative effort. Two orthogonally-crossed and co-located narrow microstrip patches with gap discontinuities separating a central probe-fed section from the radiating slots provides the base antenna structure. The fluidic mechanisms use high strength dielectric fluids or liquid metal loaded across the gap discontinuities and the solid-state mechanisms uses readily available RF PIN and varactor diodes integrated across the gaps to enable reconfiguration. Accurate and robust circuit modeling concepts are presented to provide insight on antenna performance and loss mechanisms from each reconfiguration technique. A polarization-only reconfigurable version of this antenna utilizing dielectric fluids, RF PIN didoes, and liquid metal in separate design iterations were examined to introduce design and circuit modeling concepts and provide a first comparison between the reconfiguration techniques. While all iterations achieved good linear polarization switching, dielectric fluids and the RF PIN didoes are found to have large negative impacts on radiation performance due to ohmic losses (radiation efficiencies between 8 – 35%). In the liquid metal iteration, ohmic losses are significantly reduced to boost radiation efficiencies near that of a tradition patch antenna (near 80%). The hybrid reconfiguration solution utilizes liquid metal and solid-state varactors for polarization and frequency diversity, respectively. Non-hybrid design iterations using only dielectric fluids and solid-state RF PIN diodes with varactors provide a comparison between all reconfiguration techniques and demonstrate the advantages of the hybrid solution. It was found that broadly variable dielectric strength fluids used as a sole reconfiguration mechanism can achieve a wide frequency tuning range of 700 MHz, maintain linear polarization switching, and have radiation efficiencies near 60%. However, the fluids must have loss tangents less than 0.02 and are currently not readily available. The RF PIN and varactor diode combination provides a realizable solution, however, suffers from excessive DC control power requirements, a limited tuning range of 100 MHz, and low radiation efficiency around 16%. The hybrid solution combines the best aspects of all subsequent design iterations to achieve a realizable frequency and polarization reconfigurable antenna with a tuning range of 263 MHz and 41.7% radiation efficiency average across reconfiguration states

3D BEAMSTEERING LOW COMPLEXITY RECONFIGURABLE MULTILEVEL ANTENNA

The main idea of the thesis is to develop a new reconfigurable antenna that makes beamsteering in 3D, with the minimum number of possible switches (maximum 9) and as simple as possible for use in a car vehicle. The design will explore an active dipole located in the center of the antenna (which is fed by a tapered balun), and 4 parasitic dipoles around, placed so that the steering can be done in 9 3D directions according to which parasites we activate by means of switches. The basic idea is to study the physical principle of double reflection, the first reflection due toBeamforming, in its many variants, is a key spatial processing technique to improve user throughput, system capacity, system coverage as well as reducing interference. Simple architectures enabling beamforming either in predefined or arbitrary directions are very desirable for the Fifth Generation of Mobile Communications (5G) to boost power efficiency. Furthermore, it is expected that the number of 5G mobile subscribers grows from 5 million in 2019 to nearly 600 million by 2023, increasing traffic, connections density, and latency which will increase the demand of capacity to the network. Therefore, a broadband intelligent antenna must be at the basis to provide reliable data service, capable to adapt the antenna's capabilities to environment changes. The scope of this thesis focuses on a novel multilevel reconfigurable antenna incorporating beamsteering capabilities by using the lowest number of switches possible

RF-MEMS Technology for High-Performance Passives (Second Edition) - 5G applications and prospects for 6G

The focus of this book develops around hardware, and in particular on low-complexity components for Radio Frequency (RF) applications. To this end, microsystem (MEMS) technology for RF passive components, known as RF-MEMS, is employed, discussing its potentialities in the application frame of 5G. The approach adopted is practical, and a significant part of the content can be directly used by scientists involved in the field, to put their hand on actual design, optimization and development of innovative RF passive components in MEMS technology for 5G and beyond applications. This update (which includes a review of the main approaches to the modelling and simulations of MEMS and RF-MEMS devices) is timely and will find a wider readership as it crosses into the translational aspects of applied research in the subject. Key features • With over 50 pages of new content, the book will be 1/3 larger than the 1st edition. • New chapter on simulation and modelling techniques. • Practical approach to the design and development of RF-MEMS design concepts for 5G and upcoming 6G. • Includes case studies. • Video figures. • Includes a review of the business landscape