Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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Item MEMS SENSOR PLATFORMS FOR IN SITU CHARACTERIZATION OF LI-ION BATTERY ELECTRODES(2016) Jung, Hyun; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lithium-ion batteries provide high energy density while being compact and light-weight and are the most pervasive energy storage technology powering portable electronic devices such as smartphones, laptops, and tablet PCs. Considerable efforts have been made to develop new electrode materials with ever higher capacity, while being able to maintain long cycle life. A key challenge in those efforts has been characterizing and understanding these materials during battery operation. While it is generally accepted that the repeated strain/stress cycles play a role in long-term battery degradation, the detailed mechanisms creating these mechanical effects and the damage they create still remain unclear. Therefore, development of techniques which are capable of capturing in real time the microstructural changes and the associated stress during operation are crucial for unravelling lithium-ion battery degradation mechanisms and further improving lithium-ion battery performance. This dissertation presents the development of two microelectromechanical systems sensor platforms for in situ characterization of stress and microstructural changes in thin film lithium-ion battery electrodes, which can be leveraged as a characterization platform for advancing battery performance. First, a Fabry-Perot microelectromechanical systems sensor based in situ characterization platform is developed which allows simultaneous measurement of microstructural changes using Raman spectroscopy in parallel with qualitative stress changes via optical interferometry. Evolutions in the microstructure creating a Raman shift from 145 cm−1 to 154 cm−1 and stress in the various crystal phases in the LixV2O5 system are observed, including both reversible and irreversible phase transitions. Also, a unique way of controlling electrochemically-driven stress and stress gradient in lithium-ion battery electrodes is demonstrated using the Fabry-Perot microelectromechanical systems sensor integrated with an optical measurement setup. By stacking alternately stressed layers, the average stress in the stacked electrode is greatly reduced by 75% compared to an unmodified electrode. After 2,000 discharge-charge cycles, the stacked electrodes retain only 83% of their maximum capacity while unmodified electrodes retain 91%, illuminating the importance of the stress gradient within the electrode. Second, a buckled membrane microelectromechanical systems sensor is developed to enable in situ characterization of quantitative stress and microstructure evolutions in a V2O5 lithium-ion battery cathode by integrating atomic force microscopy and Raman spectroscopy. Using dual-mode measurements in the voltage range of the voltage range of 2.8V – 3.5V, both the induced stress (~ 40 MPa) and Raman intensity changes due to lithium cycling are observed. Upon lithium insertion, tensile stress in the V2O5 increases gradually until the α- to ε-phase and ε- to δ-phase transitions occur. The Raman intensity change at 148 cm−1 shows that the level of disorder increases during lithium insertion and progressively recovers the V2O5 lattice during lithium extraction. Results are in good agreement with the expected mechanical behavior and disorder change in V2O5, highlighting the potential of microelectromechanical systems as enabling tools for advanced scientific investigations. The work presented here will be eventually utilized for optimization of thin film battery electrode performance by achieving fundamental understanding of how stress and microstructural changes are correlated, which will also provide valuable insight into a battery performance degradation mechanism.Item Fabrication and Process Development for an Integrated Optical MEMS Microsystem in Indium Phosphide(2013) Siwak, Nathan Paul; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the design, fabrication, and evaluation of the first monolithically integrated MEMS resonant sensor system realized in the InP-InGaAs material family. The integration of a MEMS sensor along with the facilitating optical interrogation platform provides for increased manufacturing scalability, sensitivity, and reduced measurement noise and device cost. The MEMS device presented in this dissertation consists of an Indium Phosphide (InP) cantilever waveguide resonator whose displacement is measured optically via a vertically integrated laser diode and waveguide photodetector. All three major components of the sensor were integrated in a single 7.1 µm thick molecular beam epitaxy (MBE) epitaxial growth, lattice matched to an InP substrate. Full fabrication of the integrated MEMS device utilizes 7 projection lithography masks, 4 nested inductively coupled plasma (ICP) etches, and over 60 discrete processing steps. This dissertation focuses on the integration design and the development of specific III-V semiconductor fabrication processes in order to completely fabricate and realize these devices, including specialized ICP etching steps and a MEMS undercutting release etch. The fabricated devices were tested and characterized by investigating the separate component subsystems as well as the total combined system performance. Investigation of device failure and performance degradation is performed and related to non-idealities in the device fabrication and design. A discussion of future work to improve the performance of the system is presented. The work in this dissertation describing the successful fabrication process and analysis of such a complex system is a milestone for III-V based optical MEMS research and will serve as the groundwork for future research in the area of optical MEMS microsystems.Item A Platform Towards In Situ Stress/Strain Measurement in Lithium Ion Battery Electrodes(2012) Baron, Sergio Daniel; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis demonstrates the design, fabrication and testing of a platform for in situ stress/strain measurement in lithium ion battery electrodes. The platform - consisting of a Microelectromechanical System (MEMS) chip containing an electrochemical cavity and an optical sensing element, a custom electrochemical package and an experimental setup - was successfully developed. Silicon was used as an active electrode material, and a thin-film electrochemical stack was conceived and tested. Finally, multiple experiments showed correlation between the active material volume change inside the battery and a signal change in the optical sensing element. The experimental results, combined with the MEMS implementation of the sensing element provide a promising way to evaluate electrochemical reaction-induced stress monitoring in a simple and compact fashion, while experiments are carried out in situ.Item AN INTEGRATED ELECTROMAGNETIC MICRO-TURBO-GENERATOR SUPPORTED ON ENCAPSULATED MICROBALL BEARINGS(2011) Beyaz, Mustafa Ilker; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the development of an integrated electromagnetic micro-turbo-generator supported on encapsulated microball bearings for electromechanical power conversion in MEMS (Microelectromechanical Systems) scale. The device is composed of a silicon turbine rotor with magnetic materials that is supported by microballs over a stator with planar, multi-turn, three-phase copper coils. The micro-turbo-generator design exhibits a novel integration of three key technologies and components, namely encapsulated microball bearings, incorporated thick magnetic materials, and wafer-thick stator coils. Encapsulated microball bearings provide a robust supporting mechanism that enables a simple operation and actuation scheme with high mechanical stability. The integration of thick magnetic materials allows for a high magnetic flux density within the stator. The wafer-thick coil design optimizes the flux linkage and decreases the internal impedance of the stator for a higher output power. Geometrical design and device parameters are optimized based on theoretical analysis and finite element simulations. A microfabrication process flow was designed using 15 optical masks and 110 process steps to fabricate the micro-turbo-generators, which demonstrates the complexity in device manufacturing. Two 10 pole devices with 2 and 3 turns per pole were fabricated. Single phase resistances of 46Ω and 220Ω were measured for the two stators, respectively. The device was actuated using pressurized nitrogen flowing through a silicon plumbing layer. A test setup was built to simultaneously measure the gas flow rate, pressure, rotor speed, and output voltage and power. Friction torques in the range of 5.5-33µNm were measured over a speed range of 0-16krpm (kilo rotations per minute) within the microball bearings using spin-down testing methodology. A maximum per-phase sinusoidal open circuit voltage of 0.1V was measured at 23krpm, and a maximum per-phase AC power of 10µW was delivered on a matched load at 10krpm, which are in full-agreement with the estimations based on theoretical analysis and simulations. The micro-turbo-generator presented in this work is capable of converting gas flow into electricity, and can potentially be coupled to a same-scale combustion engine to convert high-density hydrocarbon energy into electrical power to realize a high-density power source for portable electronic systems.Item MODELING AND TESTING OF ETHERNET TRANSFORMERS(2011) Bowen, David; Mayergoyz, Isaak D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Twisted-pair Ethernet is now the standard home and office last-mile network technology. For decades, the IEEE standard that defines Ethernet has required electrical isolation between the twisted pair cable and the Ethernet device. So, for decades, every Ethernet interface has used magnetic core Ethernet transformers to isolate Ethernet devices and keep users safe in the event of a potentially dangerous fault on the network media. The current state-of-the-art Ethernet transformers are miniature (<5mm diameter) ferrite-core toroids wrapped with approximately 10 to 30 turns of wire. As small as current Ethernet transformers are, they still limit further Ethernet device miniaturization and require a separate bulky package or jack housing. New coupler designs must be explored which are capable of exceptional miniaturization or on-chip fabrication. This dissertation thoroughly explores the performance of the current commercial Ethernet transformers to both increase understanding of the device's behavior and outline performance parameters for replacement devices. Lumped element and distributed circuit models are derived; testing schemes are developed and used to extract model parameters from commercial Ethernet devices. Transfer relation measurements of the commercial Ethernet transformers are compared against the model's behavior and it is found that the tuned, distributed models produce the best transfer relation match to the measured data. Process descriptions and testing results on fabricated thin-film dielectric-core toroid transformers are presented. The best results were found for a 32-turn transformer loaded with 100Ω, the impedance of twisted pair cable. This transformer gave a flat response from about 10MHz to 40MHz with a height of approximately 0.45. For the fabricated transformer structures, theoretical methods to determine resistance, capacitance and inductance are presented. A special analytical and numerical analysis of the fabricated transformer inductance is presented. Planar cuts of magnetic slope fields around the dielectric-core toroid are shown that describe the effect of core height and winding density on flux uniformity without a magnetic core.Item Microturbopump Utilizing Microball Bearings(2008-08-05) Waits, Christopher Michael; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the development of a microfabricated turbopump capable of delivering fuel with the flow rates and pressures required for portable power generation. The device is composed of a spiral-groove viscous pump that is driven by a radial in-flow air turbine and supported using a novel encapsulated microball bearing. First, the encapsulated microball bearing and methods to investigate the wear and friction behaviors were developed. Two primary raceway designs, point-contact and planar-contact designs, were developed with the key design factor being wearing of the raceway. A modification to the planar-contact design was made for the final turbopump that reduced both wear and debris generation. Second, two air turbine platforms were developed using the encapsulated microball bearings to characterize both the bearing and the turbine drive mechanism. A tangential air turbine platform was first developed and characterized using the point-contact bearing mechanism. Rotational speeds >37,000 rpm were demonstrated and long-term operation (>24 hours) using this platform, but with large driving pressures (tens of psi) and large raceway wear (tens of microns). Furthermore, the circumferential asymmetry of the turbine design led to difficulty in measuring pressure distribution and sealing for pump applications. Results from the tangential air turbine platform led to an axisymmetric radial in-flow air turbine platform using a planar-contact bearing design. Rotational speeds greater than 85,000 rpm with turbine pressure differentials in the range of 1 psi were demonstrated using this platform. The wear of the raceway was observed to be on the order of single microns (a 10x improvement). The radial in-flow air turbine platform allowed an empirical model to be developed relating the friction torque to the rotational speed and load for the planar-contact bearing. This enabled calculation of the power balance for pumping and a method to characterize future bearing designs and materials. Lastly, a microfabricated turbopump was demonstrated based on a spiral-groove viscous pump and the radial in-flow turbine platform using the planar-contact bearing. Pumping operation was demonstrated with a differential pressure up to +0.3 psi and flow rates ranging from 35 mL/hour to 70 mL/hour, within the range relevant to portable power generation.Item Closed-Loop Control of a Micropositioner Using Integrated Photodiode Sensors(2008-08-11) Beyaz, Mustafa Ilker; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A closed-loop control system with photodiode position sensors has been implemented in a microball bearing supported linear electrostatic micromotor to improve accuracy and reliability. The fabrication sequence of the previously developed micromotor was modified to integrate a photodiode-based position sensing mechanism. Proportional control law is used in the control system and device step response is analyzed for several step sizes at various maximum applied voltages by varying the constant of proportionality. Two critical functions for micropositioning applications have been demonstrated; the device can establish a necessary frame of reference for coordinate-based positioning and autonomously respond to arbitrary disturbances. The closed-loop position control system presented in this work illustrates the feasibility and functionality of smart microsystems using integrated feedback sensors.Item Double-Exposure Gray-Scale Photolithography(2008-08-08) Mosher, Lance; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Three-dimensional photoresist structures may be realized by controlling the transmitted UV light intensity in a process termed gray-scale photolithography. Light modulation is accomplished by diffraction through sub-resolution pixels on a photomask. The number of photoresist levels is determined by the number of different pixel sizes on the mask, which is restricted by mask fabrication. This drawback prevents the use of gray-scale photolithography for applications that need a high vertical resolution. The double-exposure gray-scale photolithography technique was developed to improve the vertical resolution without increasing the number of pixel sizes. This is achieved by using two gray-scale exposures prior to development. The resulting overlay produces an exposure dose that is a combination of both exposures. Calibration is utilized to relate the pixel sizes and exposure times to the photoresist height. This calibration enables automated mask design for arbitrary 3D structures and investigation of other effects, such as misalignment between the exposures.Item Indium Phosphide MEMS Cantilever Waveguides with Integrated Readout for Chemical Sensing(2007-11-26) Siwak, Nathan Paul; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents the development towards an integrated, monolithic, micro-electro-mechanical system (MEMS) cantilever waveguide resonator chemical sensor using the III-V semiconductor indium phosphide (InP). Waveguide cantilevers with resonant frequencies as high as 5.78 MHz, a quality factor of 340, and a sensitivity of 4.4x10^16 Hz/g are shown for the first time in this system. The first demonstration of vapor detection using the sensor platform is performed utilizing an organic semiconductor Pentacene absorbing layer. Vapors are measured from mass shifts of 6.56x10^-14 and 7.28x10^-14 g exhibiting a mass detection threshold of 5.09x10^-15 g. The design, fabrication, and testing of an integrated waveguide PIN photodetector with an In0.53Ga0.47As absorbing layer is reported. Dark currents as low as 8.7 nA are measured for these devices. The first demonstration of a resonating cantilever waveguide measurement is also performed using the monolithically integrated waveguide photodiodes with uncertainty of less than ± 35 Hz. Finally, a future outlook is presented for this monolithic InP sensor system.Item On The Pointing And Jitter Characterization Of MEMS Two-Axis (Tip-Tilt) Mirrors(2007-10-08) Edwards, Clinton Lee; Davis, Christoper C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents three first-principles analytic, closed-form models that describe the pointing characteristics of MEMS Two-Axis (Tip-Tilt) Mirrors: (1) a 2D Torque Model, (2) a Micro-mirror Pointing Model (MPM), and (3) a Micro-mirror Jitter Model (MJM). The 2D Torque Model accounts for all of the fundamental electrodynamics inherent in the operation of a MEMS Two-Axis (Tip-Tilt) Mirror. The 2D Torque Model is utilized in the MPM model and the MJM model and is a function of both axis angles. These three models provide explicit relationships between MEMS mirror physical, electrical, and environmental design parameters, and mirror performance. The MPM model, consisting of coupled damped harmonic oscillators with the 2D Torque Model as an input, is used to analyze the dynamics of the mirror. This formulation is imposed by Euler equations and the mirror's rigid structure. A generalized torque function, "G", is presented that utilizes symmetry in the torsional expressions to facilitate software implementation. A methodology is explained for determining the dynamic constants for the mirror as well as an "effective length" which accounts for electric field fringing. Since MEMS fabrication leads to variations in physical properties, the MPM model can be calibrated for a particular mirror to compensate for this variation. Experimental measurements and the MPM model results are in close agreement for steady-state and transient mirror dynamics. The MJM model was created using the MPM model to address the effects of mirror facet jitter. The MJM model provides an explicit relationship between noise sources and the resulting mirror jitter. It can be used to simulate the effects of mirror jitter as a function of the originating noise sources which are: (1) control voltage fluctuation, (2) platform vibration, (3) Brownian motion noise. A methodology is developed to validate the MJM model. Measurements from the resulting experimental apparatus support the model. Additionally, the experimental apparatus permitted pressure dependent measurements to be made. Mirror jitter was recorded and analyzed for varying pressure and tip-tilt angles. Damping constants (for both axes) were measured. Brownian motion generated jitter was isolated and its variance observed to be pressure invariant as the model predicted.