Theses and Dissertations from UMD
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Item Fabrication and Characterization of Nanoscale Shape Memory Alloy MEMS Actuators(2020) Knick, Cory R.; Bruck, Hugh; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The miniaturization of engineering devices has created interest in new actuation methods capable of large displacements and high frequency responses. Shape memory alloy (SMA) thin films have exhibited one of the highest power densities of any material used in these actuation schemes with thermally recovery strains of up to 10%. With the use of a biasing force, such as from a passive layer in a “bimorph” structure, homogenous SMA films can experience reversible shape memory effect provided they are thick enough that the crystal structure is capable of transforming. However, thick films exhibit lower actuation displacements and speeds because of the larger inertial resistance. Therefore, there is a need to find a way to process thinner SMA films with grain structures that are capable of transformation in order to realize larger actuation displacements at higher speeds. In this work, a near-equiatomic NiTi magnetron co-sputtering process was developed to create nanoscale thick SMA films as thin as 120 nm. By using a metallic seed layer, it was possible to induce the crystallization of epitaxial, columnar grains exhibiting the shape memory effects in nanoscale films ranging from 120 – 400 nm. It was also possible to crystalize these SMA films at lower processing temperatures (as low as 325 °C) compared to directly sputtering thicker films onto Si wafers. The transformation behavior associated with the SME in these films were characterized using x-ray diffraction (XRD), differential scanning calorimetry (DSC), and stress-temperature measurements at wafer level. After quantifying the shape memory effects at wafer-level, the SMA films were used to fabricate various microscale MEMS actuators. The SMA films were mated in several “bimorph” configurations to induce out of plane curvature in the low-temperature Martensite phase. The curvature radius vs. temperature was characterized on MEMS cantilever structures to elucidate a relationship between residual stress, recovery stress, radius of curvature, and degree of unfolding. SMA MEMS actuators were fabricated and tested using joule heating to demonstrate rapid electrical actuation of NiTi MEMS devices at some of the lowest powers (5-15 mW) and operating frequencies (1-3 kHz) ever reported for SMA or thermal actuators. By developing a process to create nanoscale thickness NiTi SMA film, we enabled the fabrication of MEMS devices with full, reversible, actuation as low as 0.5 V. This indicated the potential of these devices to be used for high frequency, low power, and large displacement applications in power constrained environments (i.e. on chip).Item HIGH-FORCE ELECTROSTATIC INCHWORM MOTORS FOR MILLIROBOTICS APPLICATIONS(2019) Penskiy, Ivan; Bergbreiter, Sarah; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to scaling laws and ease of fabrication, electrostatic actuation offers a promising opportunity for actuation in small-scale robotics. This dissertation presents several novel actuator and motor designs as well as new techniques by which to characterize electrostatic gap closing actuators. A new motor architecture that uses in-plane electrostatic gap-closing actuators along with a flexible driving arm mechanism to improve motor force density is introduced, optimized, manufactured, and tested. This motor operates similarly to other inchworm-based microactuators by accumulating small displacements from the actuators into much larger displacements in the motor. Using an analytical model of the inchworm motor based on the static force equilibrium condition, optimizations of a full motor design were performed to maximize motor force density. In addition, force losses from supporting flexures were included to calculate the theoretical motor efficiency for different motor designs. This force density optimization analysis of the gap-closing actuators and supporting motor structures provided the basis for designing and manufacturing inchworm motors with flexible driving arms and gap-closing actuators. The motor required only a single-mask fabrication and demonstrated robust performance, a maximum speed of 4.8mm/s , and a maximum force on the shuttle of 1.88mN at 110V which corresponds to area force density of 1.38mN/mm2. In addition, instead of estimating motor force based on drawn or measured dimensions which often overestimates force, the demonstrated maximum motor force was measured using calibrated springs. The efficiency of the manufactured motor was measured at 8.75% using capacitance measurements and useful work output. To further increase force output from these motors, several new designs were proposed, analyzed, and tested. Thick film actuators that take advantage of a through-wafer etch offered a promising opportunity to increase force given the linear increase in force with actuator thickness. However, fabrication challenges made this particular approach inoperable with current manufacturing capabilities. New actuator designs with compliant and zipping electrodes did demonstrate significant increases in force, but not the order of magnitude increase promised by modeling and analysis. In order to study and understand this discrepancy, several new techniques were developed to electrically and electromechanically characterize the force output of these new actuator designs. The first technique identifies parameters in an equivalent circuit model of the actuator, including actuator capacitance. By monitoring change in capacitance along the travel range of the motor, electrostatic force in equilibrium can be estimated. Charge transferred to and from the actuator can also provide an estimate of actuator efficiency. The second technique uses a constant rate spike to more thoroughly explore the rapid dynamics of actuator pull-in and zipping. New characterization methods allowed for collecting large amounts of data describing performance of motors with zipping and compliant electrodes. The data was used to back up the main hypothesis of force output discrepancy between theory and practice. Also, it was used to highlight extreme sensitivity of proposed motors toward manufacturing process and its tolerances.Item Force Sensing by Electrical Contact Resistance in SOI-DRIE MEMS(2018) Rauscher, Scott Gibson; Bruck, Hugh; DeVoe, Don; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)MEMS force sensors employ microfabricated elements to convert applied external forces to electrical signals, typically by piezoelectric, piezoresistive, or capacitive transduction. While existing force sensors based on these sensing principles have commercial success, system dynamics inherent to displacement and strain-based sensing can limit force and frequency ranges. This work explores an alternative force-sensing principle in silicon-based MEMS devices that exploits changes in electrical contact resistance (ECR) during loading between two silicon surfaces, with the aim to determine if ECR can be used to sense force in SOI-DRIE microsystems containing only Silicon and bond pads. While several analytic models were combined to create an ECR-force model for predicting ECR-force sensitivity in systems containing differing contact geometry, topology, and electrical properties, experimental testing is the focal point of this work. The feasibility of using ECR to sense force in bare DRIE silicon contacts is initially evaluated using force applied by simple thermal actuation, which indicated that ECR behavior during applied cyclic loading was erratic and occasionally nonmonotonic with increasing load, while absolute contact resistance varied significantly chip-to-chip (200 Ω – 15 kΩ) and increased asymptotically as contact was removed. Results from further investigation using manual spring elongation show a consistent pre-load of at least 5 mN is critical to obtaining repeatable ECR-force curves, “break-in” cycling is required prior to consistent ECR-force behavior, and sidewall fracture occurs in 100 µm line contacts with radii less than 50 µm. Results from testing of packaged chips through inertial acceleration of embedded proof masses show that minimizing contact area during line contact loading reduces relative standard deviation (RSD) and increases sidewall fracture. When normalized to initial contact resistance, chips subjected to inertial loading exhibited linearized sensitivities of 2.0 %/mN and 2.1% hysteresis, with 1.6% RSD. The use of DRIE, as opposed to additive poly-Silicon-based fabrication, allows a tailorable force range through proof mass sizing and aspect ratio changes, adjustable pre-load through simple design, and integration of an ECR force sensor into existing systems. The successful use of a proof mass to apply force by acceleration indicates ECR between SOI-DRIE interfaces is a viable method to measure acceleration in the future. As with piezo-sensors, calibration of ECR force sensors is expected to improve chip-to-chip repeatability. Compared to commercially available force sensors, the realized ECR force sensor has several advantages (smaller size, lower force range, and simpler fabrication) that may be further leveraged in future development.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 MEMS Conveyance: Piezoelectric Actuator Arrays for Reconfigurable RF Circuits(2015) Tellers, Mary; Bergbreiter, Sarah E; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An array of piezoelectric cantilevers was designed, fabricated, and characterized for use as a micromanipulation surface in a reconfigurable RF circuit micro-factory. The project, known as RFactory, is an effort by the U.S. Army Research Laboratory to create environmentally adaptable, rapidly upgradeable RF systems. The RFactory actuator surface uses unimorph lead zirconate titanate cantilevers with metal posts at the tip that exaggerate the horizontal deflection produced by out-of-plane bending. The motion of a circuit component on the surface has been modeled and observed experimentally. By varying the waveform, voltage amplitude, and frequency of the drive signal, as well as the actuator length and width, the speed and precision of the motion can be controlled. From these characterization efforts, operating conditions that create speeds above 1 mm/s and low positional error (<200 microns after 5 mm translation) have been identified. Finally, full system RF reconfigurability has been demonstrated.Item Design of three degrees-of-freedom motion stage for micro manipulation(2014) Kim, Yong-Sik; Gupta, Satyandra K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A miniaturized translational motion stage has potentials to provide not only performances equivalent to conventional motion stages, but also additional features from its small form factor and low cost. These properties can be utilized in applications requiring a small space such as a vacuum chamber in a scanning electron microscopy (SEM), where hidden surface can decrease by manipulating objects to measure. However, existing miniaturized motion stages still have several cm3 level volumes and provide simple operations. In this dissertation, Micro-electro-mechanical systems (MEMS)-based motion stages are utilized to replace a miniaturized motion stage for micro-scale manipulation and possible applications. However, most MEMS fabrication methods remain in monolithic fabrication methods and a lot of MEMS based multiple degrees-of-freedom (DOFs) motion stage also remain for in-plane motions. In this dissertation, a nested structure based on a serial kinematic mechanism is implemented in order to overcome these constraints and implement out-of-plane motion, where one independent stage is embedded into the other individual stage with additional features for structurally and electrically isolations among the engaged stages. MEMS actuators and displacement amplifiers are also investigated for reasonable performance. 3-axis motions are divided into two in-plane motions and one out-of-plane motion; an in-plane 1 DOF motion stage (called an X-stage) and one out-of-plane 1 DOF motion stage (called a Z-stage) are designed and characterized experimentally. Based on the two stages, the XY-stage is designed by merging one X-stage into the motion platform of the other X-stage with a different orientation (called an XY-stage). With this nested approach, the fabricated XY-stage demonstrated in-plane motions larger than 50 µm with ignorable coupled motion errors. Based on this nested approach, the 3-axis motion stage is also implemented by utilizing the nested structure twice; integrating the Z-stage with the motion platform of the XY-stage (called an XYZ-stage). The XYZ-stage demonstrated out-of-plane motions about 23 µm as well as the in-plane motions. Two presented motion stages have been utilized in the manipulation of micro-scale object by the cooperation of the two XY-stages inside a SEM chamber. The large motion platform of the X-stage is also utilized in a parallel plate type rheometer to measure the material properties of viscoelastic materials.Item PERFORMANCE ASSESSMENT OF MEMS GYROSCOPE AND SHOCK DURABILITY EVALUATION OF SAC305-X SOLDERS FOR HIGH TEMPERATURE APPLICATIONS(2014) Patel, Chandradip Pravinbhai; McCluskey, F.Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent advances in MEMS technology have resulted in relatively low cost MEMS gyroscopes. Their unique features compared to macro-scale devices, such as lighter weight, smaller size, and less power consumption, have made them popular in many applications with environmental conditions ranging from mild to harsh. This dissertation aims to address a gap in the literature on MEMS gyroscopes by investigating the effects of elevated temperatures on the performance of MEMS gyroscopes. MEMS gyroscopes are characterized at room and elevated temperatures for both stationary and rotary conditions. During the test, MEMS gyroscopes are subjected to five thermal cycles at each of four temperature ranges (viz. 25degC to 85degC, 25degC to 125degC, 25degC to 150degC and 25degC to 175degC). A simulation model is developed in MATLAB Simulink to simulate the temperature effect on the MEMS gyroscope. Simulation results show good agreement with experimental results and confirm that Young's modulus and damping coefficient are the dominant factors responsible for temperature-dependent bias at elevated temperatures. Solder interconnects are one of the weakest elements in MEMS devices. Thus, the reliability of solder interconnects is separately studied in this dissertation. Though, SAC305 (96.5%Sn3.0%Ag0.5%Cu) is the industry preferred solder in combined thermal cycling and shock/drop environments, it exhibits better thermal cycling reliability than drop/shock reliability. One of the ways to improve drop/shock reliability of SnAgCu solder is by microalloy addition of various dopants such as Mn, Ce, Ti, Y, Ge, Bi, Zn, In, Ni, Co etc. Thus, the second part of this dissertation aims to evaluate the shock durability of SAC305 and SAC305-X (where X refers to two different concentrations of Mn and Ce dopants). High temperature isothermal aging tests are conducted on selected solders using QFN44, QFN32 and R2512 package types at 185degC and 200degC up to 1000 hours. Isothermal aging test results showed that interfacial IMC growth reduction can be achieved by microalloy addition of selected dopants in SAC305 on both copper and nickel leaded package types. Shock durability of selected solders is examined on as-reflowed and thermally aged test boards. Mechanical shock is performed using a custom shock machine that utilizes a shock pulse of 500G with a 1.3 millisecond duration. The shock test results showed that the mechanical shock reliability of SAC305 was significantly improved on both as-reflowed and thermally aged test boards by microalloy addition of one of the selected dopant in SAC305.Item Optimization of PZT (52/48) through Improved Platinum Metallization, Use of a PbTiO3 Seed Layer, and Fine Tuning of Annealing Conditions for Applications in Multilayer Actuator MEMS Technology(2014) Sanchez, Luz Miriam; Takeuchi, Ichiro; Polcawich, Ronald G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Using a systematic approach, the processing of PZT (52/48) was optimized to achieve both a high degree of {001} texture and high piezoelectric properties. Initial experiments examined the influence of Ti/Pt and TiO2/Pt thins films used as the base-electrode for chemical solution deposition PZT thin film growth. The second objective was to achieve highly {001}-textured PZT using a seed layer of PbTiO3 (PTO). A comparative study was performed between Ti/Pt and TiO2/Pt bottom electrodes. The results indicate that the use of a highly oriented TiO2 led to highly {111}-textured Pt, which in turn improved both the PTO and PZT orientations. A third objective was to determine the effects of lead excess in the starting PTO and PZT solution on the films orientations and piezoelectric properties. During the annealing of PZT (52/48), lead (Pb) is volatilized from the films leading to a non stoichiometric state which ultimately reduces the electrical properties. To remedy this issue, a percentage of Pb-excess is added to the PZT solution prior to deposition to compensate for the Pb that is lost during the thermal treatment. This study thoroughly examines the effects of the Pb-excess in the PTO seed layer with percentages between 0% and 30% and PZT (52/48) with Pb-excesses between 0% and 10%. The final objective, leveraged the texture optimization on single 500nm thick PZT thin films, to deposit high quality PZT films in multiple Pt/PZT/Pt layers for use in multilayer actuators (MLA). Efforts have been focused on developing actuators using a four 250 nm layer stack of PZT using 10% lead excess in solution. By performing x-ray diffraction (XRD) measurements between each layer, the texture within the films could be monitored during the growth process. To electrically measure the quality of the PZT multilayer stack, a series of six-sided capacitors were fabricated. In addition to capacitors, cantilever actuators were fabricated so as to measure the piezoelectric induced deformation. These measurements on MLA PZT films demonstrate high piezoelectric coefficients that are suitable for tactile radio and mm-scale robotic devices.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 EXTREME VERTICAL DISPLACEMENT, HIGH FORCE, SILICON MICROSTAGE ZIPPER ACTUATORS(2013) Felder, Jason; DeVoe, Don L; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Large vertical deflection, high force microactuators are desired in MEMS for a variety of applications. This thesis details a novel large-displacement electrostatic "zipper" microactuator capable of achieving hundreds of microns of out-of-plane deflection and delivering high forces, fabricated entirely from SOI (silicon-on-insulator). This technology is novel in its use of SiO2 as both a high quality dielectric and the stressed layer of the bimorph. Geometries are explored analytically, numerically and experimentally to provide the greatest electromechanical output while constraining the device footprint to 1mm2. Device performance was benchmarked against previously established out-of-plane microactuators. We report the first instance of zipper-inspired electrostatic "microstage" actuators whose flat center stage and vertical actuation mode is ideal for carrying and moving a load. Fabricated microstages are capable of achieving out-of-plane deflections up to 1.2 mm, force outputs up to 1 mN, pull-in voltage as low as 20 V, and switching times of 1 ms.