Development of a low actuation voltage electrostatic RF MEMS switch

The research focused on the design, fabrication and measurement of a low actuation voltage micro electro mechanical high frequency switch. The fabricated micro switch offers outstanding radio frequency parameters for a very large frequency band, with actuation voltage and switching time less than 20 volts and 3 micro seconds, respectively

Towards an on-chip power supply: Integration of micro energy harvesting and storage techniques for wireless sensor networks

The lifetime of a power supply in a sensor node of a wireless sensor network is the decisive factor in the longevity of the system. Traditional Li-ion batteries cannot fulfill the demands of sensor networks that require a long operational duration. Thus, we require a solution that produces its own electricity from its surrounding and stores it for future utility. Moreover, as the sensor node architecture is developed on complimentary metal-oxide-semiconductor technology (CMOS), the manufacture of the power supply must be compatible with it. In this thesis, we shall describe the components of an on-chip lifetime power supply that can harvest the vibrational mechanical energy through piezoelectric microcantilevers and store it in a reduced graphene oxide (rGO) based microsupercapacitor, and that is fabricated through CMOS compatible techniques. Our piezoelectric microcantilevers confirm the feasibility of fabricating micro electro- mechanical-systems (MEMS) size two-degree-of-freedom systems which can solve the major issue of small bandwidth of piezoelectric micro-energy harvesters. These devices use a cut-out trapezoidal cantilever beam to enhance the stress on the cantilever’s free end while reducing the gap remarkably between its first two eigenfrequencies in 400 - 500 Hz and 1 - 2 kHz range. The energy from the M-shaped harvesters will be stored in rGO based microsupercapacitors. These microsupercapacitors are manufactured through a fully CMOS compatible, reproducible, and reliable micromachining processes. Furthermore, we have also demonstrated an improvement in their electrochemical performance and yield of fabrication through surface roughening from iron nanoparticles. We have also examined the possibility of integrating these devices into a power management unit to fully realize a lifetime power supply for wireless sensor networks

Microelectromechanical Systems and Devices

The advances of microelectromechanical systems (MEMS) and devices have been instrumental in the demonstration of new devices and applications, and even in the creation of new fields of research and development: bioMEMS, actuators, microfluidic devices, RF and optical MEMS. Experience indicates a need for MEMS book covering these materials as well as the most important process steps in bulk micro-machining and modeling. We are very pleased to present this book that contains 18 chapters, written by the experts in the field of MEMS. These chapters are groups into four broad sections of BioMEMS Devices, MEMS characterization and micromachining, RF and Optical MEMS, and MEMS based Actuators. The book starts with the emerging field of bioMEMS, including MEMS coil for retinal prostheses, DNA extraction by micro/bio-fluidics devices and acoustic biosensors. MEMS characterization, micromachining, macromodels, RF and Optical MEMS switches are discussed in next sections. The book concludes with the emphasis on MEMS based actuators

Fabrication and characterization of suspended pyrolytic carbon microstructures in various pyrolysis temperatures

The aim of this Master’s thesis is to fabricate and study the issues related to the fabrication of suspended C-MEMS microstructures, as well as to investigate the properties of unpatterned pyrolytic carbon films in relation to the pyrolysis temperatures. In recent years, suspended pyrolytic carbon microstructures have started to emerge as part of the next generation C-MEMS devices. Although the use of such structures can greatly improve the quality and expands the application of C-MEMS devices, suspended pyrolytic carbon microstructures are far more susceptible to fabrication issues than substrate-bound structures. So, in order to further advance the C-MEMS process we must first understand the underlying fabrication issues that these structures face. Suspended SU-8 microstructures with varying shapes and sizes were prepared with the use of sacrificial layers and pyrolyzed in an inert atmosphere, in order to obtain suspended pyrolytic carbon microstructures. The structures were then analyzed in terms of their structural stability (optical microscope, SEM) and contraction (profilometer). The pyrolytic carbon films were prepared by pyrolyzing unpatterned SU-8 films at four different pyrolysis temperatures between 800 and 1100 °C. The films were characterized in terms of their electrical resistivity (4-point probe), crystallinity (Raman spectroscopy) and surface roughness (AFM). During the fabrication process various issues were observed. This allowed us to determine a correlation between the shape and size of the microstructures with the specific fabrication issue, a potential reasoning as to why these issues would occur and how they can be avoided in the future. Based on the obtained results, a new analysis of the pyrolysis process was performed from a structural standpoint of SU-8 microstructures. Novel microstructures were also presented in the form of pyrolytic carbon cups, which show great promise as structures used for the trapping of micro and nanoparticles. Analysis of the pyrolytic carbon films show an increase in the electrical conductivity, surface roughness and crystallinity of the material with higher pyrolysis temperatures. The electrical resistivity drops from 1.29·10-4 to 2.92·10-5 Ωm as the pyrolysis temperature is increased from 800 to 1100 °C. At the same time, the surface roughness of the pyrolytic carbon films increases from 0.33 to 1.27 nm. The Raman spectra indicate a very high level of structural disorder and small crystallinity of the material. The crystallite size was calculated to increase from 6.45 to 9.15 nm with higher pyrolysis temperatures. Furthermore, detailed analysis of the Raman spectra also indicates a buildup of intrinsic stress at temperatures up to 1000 °C. Upon increasing the pyrolysis temperature further, the stress is gradually reduced from the material as the structure begins to anneal

Flexible Thermoelectric Generators and 2-D Graphene pH Sensors for Wireless Sensing in Hot Spring Ecosystem

abstract: Energy harvesting from ambient is important to configuring Wireless Sensor Networks (WSN) for environmental data collecting. In this work, highly flexible thermoelectric generators (TEGs) have been studied and fabricated to supply power to the wireless sensor notes used for data collecting in hot spring environment. The fabricated flexible TEGs can be easily deployed on the uneven surface of heated rocks at the rim of hot springs. By employing the temperature gradient between the hot rock surface and the air, these TEGs can generate power to extend the battery lifetime of the sensor notes and therefore reduce multiple batteries changes where the environment is usually harsh in hot springs. Also, they show great promise for self-powered wireless sensor notes. Traditional thermoelectric material bismuth telluride (Bi2Te3) and advanced MEMS (Microelectromechanical systems) thin film techniques were used for the fabrication. Test results show that when a flexible TEG array with an area of 3.4cm2 was placed on the hot plate surface of 80°C in the air under room temperature, it had an open circuit voltage output of 17.6mV and a short circuit current output of 0.53mA. The generated power was approximately 7mW/m2. On the other hand, high pressure, temperatures that can reach boiling, and the pH of different hot springs ranging from 9 make hot spring ecosystem a unique environment that is difficult to study. WSN allows many scientific studies in harsh environments that are not feasible with traditional instrumentation. However, wireless pH sensing for long time in situ data collection is still challenging for two reasons. First, the existing commercial-off-the-shelf pH meters are frequent calibration dependent; second, biofouling causes significant measurement error and drift. In this work, 2-dimentional graphene pH sensors were studied and calibration free graphene pH sensor prototypes were fabricated. Test result shows the resistance of the fabricated device changes linearly with the pH values (in the range of 3-11) in the surrounding liquid environment. Field tests show graphene layer greatly prevented the microbial fouling. Therefore, graphene pH sensors are promising candidates that can be effectively used for wireless pH sensing in exploration of hot spring ecosystems.Dissertation/ThesisDoctoral Dissertation Exploration Systems Design 201

High Frequency Thermally Actuated Single Crystalline Silicon Micromechanical Resonators with Piezoresistive Readout

Over the past decades there has been a great deal of research on developing high frequency micromechanical resonators. As the two most common and conventional MEMS resonators, piezoelectric and electrostatic resonators have been at the center of attention despite having some drawbacks. Piezoelectric resonators provide low impedances that make them compatible with other low impedance electronic components, however they have low quality factors and complicated fabrication processes. In case of electrostatic resonators, they have higher quality factors but the need for smaller transductions gaps complicates their fabrication process and causes squeezed film damping in Air. In addition, the operation of both these resonators deteriorates at higher frequencies. In this presented research, thermally actuated resonators with piezoresistive readout have been developed. It has been shown that not only do such resonators require a simple fabrication process, but also their performance improves at higher frequencies by scaling down all the dimensions of the structure. In addition, due to the internal thermo-electro-mechanical interactions, these active resonators can turn some of the consumed electronic power back into the mechanical structure and compensate for the mechanical losses. Therefore, such resonators can provide self-Q-enhancement and self-sustained-oscillation without the need for any electronic circuitry. In this research these facts have been shown both experimentally and theoretically. In addition, in order to further simplify the fabrication process of such structures, a new controlled batch fabrication method for fabricating silicon nanowires has been developed. This unique fabrication process has been utilized to fabricate high frequency, low power thermal-piezoresistive resonators. Finally, a new thermal-piezoresistive resonant structure has been developed that can operate inside liquid. This resonant structure can be utilized as an ultra sensitive biomedical mass sensor

Silicon Membranes for Extracorporeal Membrane Oxygenation (ECMO)

In cases of severe lung or heart failure, extracorporeal membrane oxygenation (ECMO) is a life-saving therapy in which a patient’s blood is passed into a circuit outside of their body to provide respiratory support. The circuit’s main component is the membrane oxygenator that drives oxygen into the blood from a sweep gas source and removes excess carbon dioxide from the blood. At present, clinical use of ECMO is limited by its high risk profile, owing to two intertwined risks: thrombosis from the large circuit, and bleeding from the anticoagulation needed to prevent thrombosis. Improvements to the gas exchange efficiency and hemocompatibility of the oxygenator could enable the development of a longer-term supportive ECMO therapy, intended as a bridge-to-transplant or destination therapy for chronic lung failure. Here we describe a novel blood oxygenator concept based on parallel plate silicon membranes developed for high precision geometry, mechanical rigidity, and high efficiency membrane transport. Using these membranes, we create blood oxygenator prototypes consisting of arrays of silicon membranes, and endeavor to improve the efficiency and hemocompatibility of this concept.First, multiple types of silicon membranes were evaluated systematically for mechanical rigidity and oxygen exchange efficiency, indicators of suitability for a future oxygenator. The combination of a silicon micropore membrane (SµM) and a 5 µm-thick polydimethylsiloxane (PDMS) layer maximized both qualities, withstanding over 260 cmHg of applied pressure and producing 0.03 mL/min of O2 flux. These membranes were then assembled into prototype flow cells, and tested for in vitro and in vivo oxygenation, successfully yielding an oxygen permeability of 1.92 ± 1.04 ml O2 STP/min/m2/cmHg. From this benchmark, we then attempted to optimize the surface hemocompatibility of the Si-PDMS composite through application of multiple polyethylene glycol (PEG)-based coatings. Although successful application of PEG to the surfaces was demonstrated, none of the coatings appeared to reduce protein adhesion to the SµM -PDMS membranes. Finally, we inserted turbulence-inducing spacer meshes into the channels of the SµM-PDMS prototypes to disrupt the transport boundary layer adjacent to the membranes, with the goal of substantially improving oxygenation. Though a threefold increase in oxygen flux was observed in vitro with the spacer meshes, the disruptive turbulence resulted in thrombosis and channel occlusion within the channels despite heavy anticoagulation of the blood. In summary, the work in this dissertation demonstrates the successful construction and testing of SµM-PDMS oxygenator prototypes, laying the foundation for future work to optimize this concept and create a large-scale blood oxygenator that can expand the clinical use of this life-saving therapy

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

Modeling and characterization of non-ideal effects in high-performance RF MEMS tuners

The emerging standards for the next-generation wireless communication system demand for multi-band RF front-ends. Reconfigurable RF devices based on MEMS technology have emerged with the potential to significantly reduce the system complexity and cost. Robust operation of RF MEMS tuners under the non-ideal effects due to fabrication uncertainties and environmental variations is critical in achieving reliable RF MEMS reconfigurable devices. Therefore, it is essential to model and characterize these non-ideal effects, and further to alleviate these non-ideal effects by design optimization.^ In this dissertation, the effects of non-perfect anchor support, residual stress, and temperature sensibility of MEMS tuners have been studied. The anchor supports of MEMS beams, which are widely used as tunable components, are often far from the ideally assumed built-in or step-up conditions. An equation-based nonlinear model for inclined supports in non-flat fixed-fixed beams has been developed and validated by experimental results. Residual stress developed during the fabrication presents the major challenges in developing reliable MEMS tuners. An efficient extraction method for in-plane residual stress has been proposed using a single beam test structure. This method has been demonstrated by wafer-scale measurements of electrostatically actuated beams. The statistic and spatial distribution of extracted residual stresses on a quarter wafer is presented, and the accuracy of this method is evaluated by uncertainty analysis. With the awareness the residual stress effects, the design optimization has been conducted for designing stress-tolerant micro-corrugated diaphragm tuners used in tunable cavity resonators/filters. Furthermore, the temperature sensitivity issue results from the mismatch of material properties between the structure material and substrate has been discussed and a thermally-stable RF MEMS tuner based on a nonuniform micro corrugated diaphragm has been proposed and experimentally validated over a wide temperature variation