SOI RF-MEMS Based Variable Attenuator for Millimeter-Wave Applications

The most-attractive feature of microelectromechanical systems (MEMS) technology is that it enables the integration of a whole system on a single chip, leading to positive effects on the performance, reliability and cost. MEMS has made it possible to design IC-compatible radio frequency (RF) devices for wireless and satellite communication systems. Recently, with the advent of 5G, there is a huge market pull towards millimeter-wave devices. Variable attenuators are widely employed for adjusting signal levels in high frequency equipment. RF circuits such as automatic gain control amplifiers, broadband vector modulators, full duplex wireless systems, and radar systems are some of the primary applications of variable attenuators. This thesis describes the development of a millimeter-wave RF MEMS-based variable attenuator implemented by monolithically integrating Coplanar Waveguide (CPW) based hybrid couplers with lateral MEMS varactors on a Silicon–on–Insulator (SOI) substrate. The MEMS varactor features a Chevron type electrothermal actuator that controls the lateral movement of a thick plate, allowing precise change in the capacitive loading on a CPW line leading to a change in isolation between input and output. Electrothermal actuators have been employed in the design instead of electrostatic ones because they can generate relatively larger in-line deflection and force within a small footprint. They also provide the advantage of easy integration with other electrical micro-systems on the same chip, since their fabrication process is compatible with general IC fabrication processes. The development of an efficient and reliable actuator has played an important role in the performance of the proposed design of MEMS variable attenuator. A Thermoreflectance (TR) imaging system is used to acquire the surface temperature profiles of the electrothermal actuator employed in the design, so as to study the temperature distribution, displacement and failure analysis of the Chevron actuator. The 60 GHz variable attenuator was developed using a custom fabrication process on an SOI substrate with a device footprint of 3.8 mm x 3.1 mm. The fabrication process has a high yield due to the high-aspect-ratio single-crystal-silicon structures, which are free from warping, pre-deformation and sticking during the wet etching process. The SOI wafer used has a high resistivity (HR) silicon (Si) handle layer that provides an excellent substrate material for RF communication devices at microwave and millimeter wave frequencies. This low-cost fabrication process provides the flexibility to extend this module and implement more complex RF signal conditioning functions. It is thus an appealing candidate for realizing a wide range of reconfigurable RF devices. The measured RF performance of the 60 GHz variable attenuator shows that the device exhibits attenuation levels (|S21|) ranging from 10 dB to 25 dB over a bandwidth of 4 GHz and a return loss of better than 20 dB. The thesis also presents the design and implementation of a MEMS-based impedance tuner on a Silicon-On-Insulator (SOI) substrate. The tuner is comprised of four varactors monolithically integrated with CPW lines. Chevron actuators control the lateral motion of capacitive thick plates used as contactless lateral MEMS varactors, achieving a capacitance range of 0.19 pF to 0.8 pF. The improvement of the Smith chart coverage is achieved by proper choice of the electrical lengths of the CPW lines and precise control of the lateral motion of the capacitive plates. The measured results demonstrate good impedance matching coverage, with an insertion loss of 2.9 dB. The devices presented in this thesis provide repeatable and reliable operation due to their robust, thick-silicon structures. Therefore, they exhibit relatively low residual stress and are free from stiction and micro-welding problems

Enabling Solutions for Integration and Interconnectivity in Millimeter-wave and Terahertz Systems

Recently, Terahertz (THz) systems have witnessed increasing attention due to the continuous need for high data rate transmission which is mainly driven by next-generation telecommunication and imaging systems. In that regard, the THz range emerged as a potential domain suitable for realizing such systems by providing a wide bandwidth capable of achieving and meeting the market requirements. However, the realization of such systems faces many challenges, one of which is interconnectivity and high level of integration. Conventional packaging techniques would not be suitable from performance perspective above 100 GHz and new approaches need to be developed. This thesis proposes and demonstrates several approaches to implement interconnects that operate above 100 GHz. One of the most attractive techniques discussed in this work is to implement on-chip coupling structures and insert the monolithic microwave integrated circuit (MMIC) directly into a waveguide (WG). Such approach provides high level of integration and eliminates the need of galvanic contacts; however, it suffers from a major drawback which isthe propagation of parasitic modes in the circuit cavity if the MMIC is large enough to allow such modes to propagate. To mitigate this problem, this work suggests and investigates the use of electromagnetic bandgap (EBG) structures that suppresses those modes such as bed of nails and mushroom-type EBG structures. The proposed techniques are used to implement several on-chip packaging solutions that have an insertion loss as low as 0.6 dB at D-band (110-170 GHz). Moreover, the solutions are demonstrated in several active systems using various commercial MMIC technologies. The thesis also investigates the possibility of utilizing the commercially available packaging technologies such as Embedded Wafer Level Ball Grid Array (eWLB) packaging. Such technology has been widely used for integrated circuits operating below 100 GHz but was not attempted in the THz range before. This work attempts to push the limits of the technology and proposes novel solutions based on coupling structures implemented in the technology’s redistribution layers. The proposed solutions achieve reasonable performance at D-band that are suitable for low-cost mass production while allowing heterogeneous integration with other technologies as well. This work addresses integration challenges facing systems operating in the THz range and proposes high-performance interconnectivity solutions demonstrated in a wide range of commercial technologies and hence enables such systems to reach their full potential and meet the increasing market demands

Additively Manufactured RF Components, Packaging, Modules, and Flexible Modular Phased Arrays Enabling Widespread Massively Scalable mmWave/5G Applications

The 5G era is here and with it comes many challenges, particularily facing the high frequency mmWave adoption. This is because of the cost to implement such dense networks is much greater due to the high propagation losses of signals that range from 26 GHz to 40 GHz. Therefore there needs to be a way to utilize a method of fabrication that can change with the various environments that 5G will be deployed in, be it dense urban areas or suburban sprawl. In this research, the focus is on making these RF components utilized for 5G at low cost and modular with a focus on additive manufacturing. Since additive manufacturing is a rapid prototyping technique, the technology can be quickly adjusted and altered to meet certain specifications with negligible overhead. Several areas of research will be explored. Firstly, various RF passive components such as additively manufactured antennas and couplers with a combination hybrid inkjet and 3D printing will be discussed. Passive components are critical for evaluating the process of additive manufacturing for high frequency operation. Secondly, various structures will be evaluated specifically for packaging mmWave ICs, including interconnects, smart packaging and encapsulants for use in single or multichip modules. Thirdly, various antenna fabrication techniques will be explored which enables fully integrated ICs with antennas, called System on Antenna (SoA) which utilizes both inkjet and 3D printing to combine antennas and ICs into modules. These modules, can then be built into arrays in a modular fashion, allowing for large or smaller arrays to be assembled on the fly. Finally, a method of calibrating the arrays is introduced, utilizing inkjet printed sensors. This allows the sensor to actively detect bends and deformations in the array and restore optimal antenna array performance. Built for flexible phased arrays, the sensor is designed for implementation for ubiquitous use, meaning that its can be placed on any surface, which enables widespread use of 5G technologies.Ph.D

Development of micromachined millimeter-wave modules for next-generation wireless transceiver front-ends

This thesis discusses the design, fabrication, integration and characterization of millimeter wave passive components using polymer-core-conductor surface micromachining technologies. Several antennas, including a W-band broadband micromachined monopole antenna on a lossy glass substrate, and a Ka-band elevated patch antenna, and a V-band micromachined horn antenna, are presented. All antennas have advantages such as a broad operation band and high efficiency. A low-loss broadband coupler and a high-Q cavity for millimeter-wave applications, using surface micromachining technologies is reported using the same technology. Several low-loss all-pole band-pass filters and transmission-zero filters are developed, respectively. Superior simulation and measurement results show that polymer-core-conductor surface micromachining is a powerful technology for the integration of high-performance cavity, coupler and filters. Integration of high performance millimeter-wave transceiver front-end is also presented for the first time. By elevating a cavity-filter-based duplexer and a horn antenna on top of the substrate and using air as the filler, the dielectric loss can be eliminated. A full-duplex transceiver front-end integrated with amplifiers are designed, fabricated, and comprehensively characterized to demonstrate advantages brought by this surface micromachining technology. It is a low loss and substrate-independent solution for millimeter-wave transceiver integration.Ph.D.Committee Chair: John Papapolymerou; Committee Chair: Manos Tentzeris; Committee Member: Gordon Stuber; Committee Member: John Cressler; Committee Member: John Z. Zhang; Committee Member: Joy Laska

Wireless Applications of Radio Frequency Micro-Electro-Mechanical Systems

With mass proliferation of wireless communication technologies, there is a continuous demand on fast data transmission rate and efficient use of frequency spectrum. As a result, reconfigurable systems are of significant importance and research is being conducted in numerous universities. The purpose of this research is to develop novel RF MEMS based reconfigurable wireless systems. By utilizing the RF MEMS switches as a basic building block, this thesis focus on developing a unique design technique for the design and development of RF MEMS delay line phase shifter, frequency reconfigurable antennas and pattern reconfigurable antennas. This thesis work is divided into four parts: 1. Investigation and development of nano-electro-mechanical systems (NEMS) based 3-bit phase shifter. Analyzing the slow wave structure to further reduce the size of delay line phase shifter. 2. Development of frequency reconfigurable antennas to compete with broadband and multi-band antennas. Two novel MEMS-loaded frequency reconfigurable antennas were designed with spectrum switchable between WPAN band (57 to 66 GHz) and the whole E-band (71 to 86 GHz). 3. Investigation of microstrip-to-coplanar striplines (CPS) transition balun used for antennas to explain the inherent phase delay of this type of structure. Based on the discovery, a pattern reconfigurable quasi-Yagi antenna was designed. The antenna exhibits excellent RF performance, compact size and switchable end-fire radiation pattern with the goal to replacing existing phased array antennas. It has the full functionality of a multi-antenna phased array plus phase shifting network while its size is same as a fixed single Yagi antenna. 4. Development of full seven masks all metal fabrication process of the RF MEMS integrated reconfigurable antennas. The fabrication processes are optimized based on Australian National Fabrication Facility (ANFF) New South Wales node’s equipment

High efficiency planar microwave antennas assembled using millimetre thick micromachine polymer structures

Communication systems at microwave and millimetre wave regimes require compact broadband high gain antenna devices for a variety of applications, ranging from simple telemetry antennas to sophisticated radar systems. High performance can usually be achieved by fabricating the antenna device onto a substrate with low dielectric constant or recently through micromachining techniques. This thesis presents the design, fabrication, assembly and characterisation of microstrip and CPW fed micromachined aperture coupled single and stacked patch antenna devices. It was found that the micromachining approach can be employed to achieve a low dielectric constant region under the patch which results in suppression of surface waves and hence increasing radiation efficiency and bandwidth. A micromachining method that employs photolithography and metal deposition techniques was developed to produce high efficiency antenna devices. The method is compatible with integration of CMOS chips and filters onto a common substrate. Micromachined polymer rims (SU8 photoresist) was used to create millimetre thick air gaps between the patch and the substrate. The effect of the substrate materials and the dimensions of the SU8 polymer rims on the performance of the antenna devices were studied by numerical simulation using Ansoft HFSS electromagnetic field simulation package. The antenna structures were fabricated in layers and assembled by bonding the micromachined polymer spacers together. Low cost materials like SU8, polyimide and liquid crystal polymer films were used for fabrication and assembly of the antenna devices. A perfect patch antenna device is introduced by replacing the substrate of a conventional patch antenna device with air in order to compare with the micromachined antenna devices. The best antenna parameters for a perfect patch antenna device with air as a substrate medium are ~20% for bandwidth and 9.75 dBi for antenna gain with a radiation efficiency of 99.8%. In comparison, the best antenna gain for the simple micromachined patch antenna device was determined to be ~8.6 dBi. The bandwidth was ~20 % for a microstrip fed device with a single patch; it was ~40 % for stacked patch devices. The best bandwidth and gain of 6.58 GHz (50.5%) and 11.2 dBi were obtained for a micromachined sub-array antenna device. The simulation results show that the efficiency of the antenna devices is above 95 %. Finally, a novel high gain planar antenna using a frequency selective surface (FSS) was studied for operation at ~60 GHz frequency. The simulation results show that the novel antenna device has a substantial directivity of around 25 dBi that is required for the emerging WLAN communications at the 60 GHz frequency band

Millimeter Wave Traveling Wave Tubes for the 21st Century

Traveling wave tubes are rapidly evolving to provide unprecedented power level in comparison to solid state devices in the millimeter waves region of the spectrum (80 – 300 GHz) thus enabling a wide range of applications. Wireless communications, imaging, security, plasma diagnostics, healthcare and many others will gain substantial features if high power in the millimeter wave region would be available from compact sources. The development of fabrication technologies is proving crucial for introducing new topologies and structures for millimeter wave vacuum electronic devices, compatible with the dimensions dictated by the short wavelength that poses substantial challenges due to tight tolerances and surface quality. This review paper will provide an overview of the principles, evolution and state of the art of one of the most widely utilized vacuum electronic devices, the traveling wave tube (TWT). The wide band, high gain features of TWTs make those devices the most promising solutions for high power at millimeter waves and THz frequencies

Millimeter-Wave Gan Mmic Integration with Additive Manufacturing

This thesis addresses the analysis, design, integration and test of microwave and millimeter-wave monolithic microwave integrated circuits (MMIC or MMICs). Recent and ongoing progress in semiconductor device fabrication and MMIC processing technology has pushed the upper limit in MMIC frequencies from millimeter-wave (30 - 300 GHz) to terahertz (300 - 3000 GHz). MMIC components operating at these frequencies will be used to improve the sensitivity and performance of radiometers, receivers for communication systems, passive remote sensing systems, transceivers for radar instruments and radio astronomy systems. However, a serious hurdle in the utilization of these MMIC components, and a main topic presented in this thesis, is the development and reliable fabrication of practical packaging techniques. The focus of this thesis is the investigation of first, the design and analysis of microwave and millimeter-wave GaN MMICs and second, the integration of those MMICs into usable waveguide components. The analysis, design and testing of various X-band (8-12 GHz) thru H-band (170-260 GHz) GaN MMIC power amplifier (PA or PAs), including a V-band (40-75 GHz) voltage controlled oscillator, is the majority of this work. Several PA designs utilizing high-efficiency techniques are analyzed, designed and tested. These examples include a 2nd harmonic injection amplifier, a Class-E amplifier fabricated with a GaN-on-SiC 300 GHz fT process, and an example of the applicability of supply-modulation with a Doherty power amplifier, all operating at 10 GHz. Two H-band GaN MMIC PAs are designed, one with integrated CPW-to-waveguide transitions for integration. The analysis of PA stability is especially important for wideband, high-fT&nbsp;devices and a new way of analyzing stability is explored and experimentally validated. Last, the challenges of integrating MMICs operating at millimeter-wave frequencies are discussed and assemblies using additive and traditional manufacturing are demonstrated.</p