A micromachined zipping variable capacitor

Micro-electro-mechanical systems (MEMS) have become ubiquitous in recent years and are found in a wide range of consumer products. At present, MEMS technology for radio-frequency (RF) applications is maturing steadily, and significant improvements have been demonstrated over solid-state components. A wide range of RF MEMS varactors have been fabricated in the last fifteen years. Despite demonstrating tuning ranges and quality factors that far surpass solid-state varactors, certain challenges remain. Firstly, it is difficult to scale up capacitance values while preserving a small device footprint. Secondly, many highly-tunable MEMS varactors include complex designs or process flows. In this dissertation, a new micromachined zipping variable capacitor suitable for application at 0.1 to 5 GHz is reported. The varactor features a tapered cantilever that zips incrementally onto a dielectric surface when actuated electrostatically by a pulldown electrode. Shaping the cantilever using a width function allows stable actuation and continuous capacitance tuning. Compared to existing MEMS varactors, this device has a simple design that can be implemented using a straightforward process flow. In addition, the zipping varactor is particularly suited for incorporating a highpermittivity dielectric, allowing the capacitance values and tuning range to be scaled up. This is important for portable consumer electronics where a small device footprint is attractive. Three different modelling approaches have been developed for zipping varactor design. A repeatable fabrication process has also been developed for varactors with a silicon dioxide dielectric. In proof-of-concept devices, the highest continuous tuning range is 400% (24 to 121 fF) and the measured quality factors are 123 and 69 (0.1 and 0.7 pF capacitance, respectively) at 2 GHz. The varactors have a compact design and fit within an area of 500 by 100 μm

A micromachined zipping variable capacitor

Micro-electro-mechanical systems (MEMS) have become ubiquitous in recent years and are found in a wide range of consumer products. At present, MEMS technology for radio-frequency (RF) applications is maturing steadily, and significant improvements have been demonstrated over solid-state components.A wide range of RF MEMS varactors have been fabricated in the last fifteen years. Despite demonstrating tuning ranges and quality factors that far surpass solid-state varactors, certain challenges remain. Firstly, it is difficult to scale up capacitance values while preserving a small device footprint. Secondly, many highly-tunable MEMS varactors include complex designs or process flows.In this dissertation, a new micromachined zipping variable capacitor suitable for application at 0.1 to 5 GHz is reported. The varactor features a tapered cantilever that zips incrementally onto a dielectric surface when actuated electrostatically by a pulldown electrode. Shaping the cantilever using a width function allows stable actuation and continuous capacitance tuning. Compared to existing MEMS varactors, this device has a simple design that can be implemented using a straightforward process flow. In addition, the zipping varactor is particularly suited for incorporating a highpermittivity dielectric, allowing the capacitance values and tuning range to be scaled up. This is important for portable consumer electronics where a small device footprint is attractive.Three different modelling approaches have been developed for zipping varactor design. A repeatable fabrication process has also been developed for varactors with a silicon dioxide dielectric. In proof-of-concept devices, the highest continuous tuning range is 400% (24 to 121 fF) and the measured quality factors are 123 and 69 (0.1 and 0.7 pF capacitance, respectively) at 2 GHz. The varactors have a compact design and fit within an area of 500 by 100 µm

A micromachined zipping variable capacitor

Micro-electro-mechanical systems (MEMS) have become ubiquitous in recent years and are found in a wide range of consumer products. At present, MEMS technology for radio-frequency (RF) applications is maturing steadily, and significant improvements have been demonstrated over solid-state components. A wide range of RF MEMS varactors have been fabricated in the last fifteen years. Despite demonstrating tuning ranges and quality factors that far surpass solid-state varactors, certain challenges remain. Firstly, it is difficult to scale up capacitance values while preserving a small device footprint. Secondly, many highly-tunable MEMS varactors include complex designs or process flows. In this dissertation, a new micromachined zipping variable capacitor suitable for application at 0.1 to 5 GHz is reported. The varactor features a tapered cantilever that zips incrementally onto a dielectric surface when actuated electrostatically by a pulldown electrode. Shaping the cantilever using a width function allows stable actuation and continuous capacitance tuning. Compared to existing MEMS varactors, this device has a simple design that can be implemented using a straightforward process flow. In addition, the zipping varactor is particularly suited for incorporating a highpermittivity dielectric, allowing the capacitance values and tuning range to be scaled up. This is important for portable consumer electronics where a small device footprint is attractive. Three different modelling approaches have been developed for zipping varactor design. A repeatable fabrication process has also been developed for varactors with a silicon dioxide dielectric. In proof-of-concept devices, the highest continuous tuning range is 400% (24 to 121 fF) and the measured quality factors are 123 and 69 (0.1 and 0.7 pF capacitance, respectively) at 2 GHz. The varactors have a compact design and fit within an area of 500 by 100 μm.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

An investigation of structural dimension variation in electrostatically-coupled MEMS resonator pairs using mode-localization

If a pair of MEMS resonators are electrostatically coupled together, the vibration amplitude ratios at the resonant frequencies of the resulting coupled system are sensitive to stiffness perturbation. An imbalance between the two resonators causes the confinement of vibration energy when the system is resonating, an effect known as mode-localization. The degree of localization can be determined by extracting the amplitude ratio of the resonators through capacitive transduction. In this paper, we have fabricated MEMS devices, using a dicing-free silicon-on-insulator process, consisting of pairs of closely spaced microresonators. Each resonator consists of a clamped-clamped beam with a wider section in the middle, which is the location of the electrostatic coupling, instituted through the DC biasing of the resonators. Several devices have been fabricated, with the length of the anchor beams being varied, which influences the frequency of resonance. Stiffness imbalance between the resonators has been introduced through electrostatic spring softening, with the sensitivity of the amplitude ratio of the resonant mode shape being greater for the higher frequency, shorter anchor devices. The sensitivities of the devices in this study have been found to be 9 times greater than state-of-the-art two-degree-of-freedom mode-localized sensors

Mechanical characterisation of nanocrystalline graphite using micromechanical structures

Conductive nanocrystalline graphite has been deposited using plasma-enhanced chemical vapour deposition at 750 °C, directly onto silicon substrates without any catalyst and fabricated into micromechanical membrane and beam structures. Using the buckling profile of the membrane and beam structures, we measure a built-in strain of - 0.0142 and through wafer-bow measurement, a compressive stress of 436 MPa. From this we have calculated the Young's modulus of nanographite as 23.0 ± 2.7 GPa. This represents a scalable method for fabricating nanographite MEMS and NEMS devices via a microfabrication-compatible process and provides useful mechanical properties to enable design of future devices

Cracking the Code of Market Secrets: A Deep Dive into Financial Anomalies

This paper reviews the literature on market anomalies and puzzles, providing a comprehensive overview of these complex phenomena that challenge the traditional Efficient Market Hypothesis. The authors examine a wide range of anomalies, including long-term return irregularities, earnings management, information uncertainty, mutual fund performance, day-of-the-week returns, the January effect, weather-induced mood shifts, international asset pricing, weekend anomalies, cryptocurrency efficiency, social transmission bias, emotional finance, biased beliefs, investor optimism, sentiment, global market inefficiencies, the influence of unique events and seasonal factors, and disappearing anomalies in country and industry returns. The authors also discuss the evolving landscape of market anomalies research, including machine learning approaches, investor behavior challenges, and the disappearance of some anomalies over time. They conclude by setting the groundwork for a more holistic comprehension of market anomalies, suggesting future research directions such as exploring new data sources, developing comprehensive theoretical models, and examining the role of technology, market regulations, and environmental changes in market anomalies

Sensing performance of Nanocrystalline Graphite Based Humidity Sensors

Environmental sensors play a crucial role in a wide range of applications. Amongst them, humidity sensors that are stable and operational in harsh environments are incredibly important for process control and monitoring. Nanocrystalline graphite (NCG) is a type of carbon-based thin film material. Previous work has shown that NCG has excellent mechanical properties and is able to withstand high radiation doses. The granular structure of the NCG film makes it a good candidate for humidity sensing as the film consists of conductive graphitic grains with a high density of sp2 bonds and amorphous grain boundaries with high resistivity, adsorption of water molecule onto the film forms conductive pathways between grains through the Grotthuss mechanism which lowers the resistance of the film by a measurable amount. Here we report for the first time, a working humidity sensor with linear response, fabricated using NCG as the sensing material for harsh, real-world environments, which include exposure to weak acids via rainfall, UV radiation, mechanical wear, and high humidity environments. The calculated sensitivity of the best-fabricated sensor is S = 0.0334%, with a maximum resistance change of -4.4 kOhms, over the range of 15% RH to 85% RH. The response time of the sensor is 20ms with the current measurement setup. The baseline resistance value of the sensor at 15% RH is 210 kOhms. The sensor has the potential to be used as a humidity sensor for harsh environments due to the chemical, thermal and mechanical stability of the NCG film

Micromachined nanocrystalline graphite membranes for gas separation

Carbon nanoporous membranes show promising performance for the passive separation and sieving of different gases, for example for helium and hydrogen separation. In this paper, nanocrystalline graphite (or nanographite) has been evaluated as a membrane material for molecular sieving of helium and hydrogen from larger gas constituents. Nanographite of 350 nm thickness was prepared using plasma-enhanced chemical vapour deposition onto fused silica substrates, from which membranes were microfabricated using deep wet etching. Permeability of hydrogen and helium were 1.79 ×10-16 and 1.40×10-16 mol·m·m-2·s-1·Pa-1 at 150 °C respectively, and measured separation was 48 for He/Ne, >135 for H2/CO2 and >1000 for H2/O2. The gas separation properties of the nanographite membranes were tested in the temperature range of 25 to 150 °C, and the permeation measurements show nanographite to be highly selective of helium and hydrogen over all larger gas molecules, including neon

Design of an ultra-sensitive MEMS force sensor utilizing mode localization in weakly coupled resonators

In this paper, the design of a novel ultrasensitive MEMS resonant force sensor utilizing a mode localization effect is presented. This new type of resonant sensor is constituted of several weakly coupled resonators and by measuring the amplitude ratio of designated resonators, a significant improvement in sensitivity is observed, compared to conventional frequency shift measurements. Furthermore, compared with conventional cantilever force sensors, the sensor is shown to be less constrained by the trade-off between sensitivity and stability along the axis of sensitivity, thus higher sensitivity and better stability can be achieved at the same time