Mechanical Engineering
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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 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 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.Item MICROFABRICATION AND MODELLING OF DIELECTRIC ELASTOMER ACTUATORS(2012) Balakrisnan, Bavani; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dielectric elastomer actuators (DEAs) are a class of polymeric actuators that have gained prominence over the last decade. A DEA is comprised of a polymer sandwiched between two compliant electrodes. When voltage is applied between the two electrodes, electrostatic attraction between the electrodes compresses the elastomer in that direction and stretches it in the other two directions. DEAs produce dimensional changes (strains) up to 300% upon application of an electric field. DEAs have tremendous potential for applications requiring large displacements and have been demonstrated for many macro-scale (centimeter and larger) applications such as robots, loudspeakers, and motors. There are potentially many useful applications for micro-scale DEAs (less than millimeter-sized devices with micron-sized actuators) in the fields of micro-robotics, micro-optics, and micro-fluidics. However, miniaturization of DEAs is challenging because many of the materials and DEA fabrication methods used on the macro-scale cannot be adapted for micro-scale fabrication of DEAs. This thesis explores the feasibility of developing fabrication strategies for micro-scale DEAs by adapting micro-electromechanical systems (MEMS) technology. In addition, fabrication protocols for micro-scale DEAs have been developed. The other aspect of this thesis is the design of bending DEAs. Benders are useful because for a given actuation strain, greater deflection can be observed by controlling the stiffnesses and thicknesses of different layers. A general guideline for designing bending DEA configurations such as unimorph, bimorph, and multilayer stacks was developed using a multilayer analytical model. The design optimization is based on the effect of thickness and stiffness of different layers on curvature, blocked force, and work. Complaint electrodes and their design are important for DEAs to enable the elastomer to stretch unrestricted. Thus, design criteria for the fabrication of crenellated electrodes and crenellated elastomers with electrodes were investigated. This guideline enabled design of structures with appropriate axial or bending stiffnesses based on the amplitude, angle, length, and thickness. Simple analytical equations for axial and bending stiffness for crenellated electrodes with different shapes were derived. In addition, numerical simulations of crenellated elastomer with stiff electrode were performedItem MODELING THE PHYSICS OF FAILURE FOR ELECTRONIC PACKAGING COMPONENTS SUBJECTED TO THERMAL AND MECHANICAL LOADING(2011) Sharon, Gilad; Barker, Donald D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents three separate studies that examined electronic components using numerical modeling approaches. The use of modeling techniques provided a deeper understanding of the physical phenomena that contribute to the formation of cracks inside ceramic capacitors, damage inside plated through holes, and to dynamic fracture of MEMS structures. The modeling yielded numerical substantiations for previously proposed theoretical explanations. Multi-Layer Ceramic Capacitors (MLCCs) mounted with stiffer lead-free solder have shown greater tolerance than tin-lead solder for single cycle board bending loads with low strain rates. In contrast, flexible terminations have greater tolerance than stiffer standard terminations under the same conditions. It has been proposed that residual stresses in the capacitor account for this disparity. These stresses have been attributed to the higher solidification temperature of lead free solders coupled with the CTE mismatch between the board and the capacitor ceramic. This research indicated that the higher solidification temperatures affected the residual stresses. Inaccuracies in predicting barrel failures of plated through holes are suspected to arise from neglecting the effects of the reflow process on the copper material. This research used thermo mechanical analysis (TMA) results to model the damage in the copper above the glass transition temperature (Tg) during reflow. Damage estimates from the hysteresis plots were used to improve failure predictions. Modeling was performed to examine the theory that brittle fracture in MEMS structures is not affected by strain rates. Numerical modeling was conducted to predict the probability of dynamic failure caused by shock loads. The models used a quasi-static global gravitational load to predict the probability of brittle fracture. The research presented in this dissertation explored drivers for failure mechanisms in flex cracking of capacitors, barrel failures in plated through holes, and dynamic fracture of MEMS. The studies used numerical modeling to provide new insights into underlying physical phenomena. In each case, theoretical explanations were examined where difficult geometries and complex material properties made it difficult or impossible to obtain direct measurements.Item Microfabrication and Analysis of Manifold Microchannel Coolers for Power Electronics(2011) Boteler, Lauren Marie; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research presents the analysis and realization of a single phase high performance manifold microchannel cooler for improving the thermal and hydrodynamic performance of multi-chip power electronic modules. This heat exchanger, microfabricated directly into the substrate, enables higher power density electronic products by more efficiently removing the high levels of heat generated. The improved thermal performance and efficiency of the heat exchanger is demonstrated using both numerical and experimental techniques. The improved heat removal is due to the reduction in the number of packaging layers between the device and the heat exchanger and by improvement in convective heat transfer. In addition, the efficiency of the device is enhanced by minimizing fluid pressure drop through the use of large manifold channels to transport fluid to the cooling area and smaller crossover microchannels in the active cooling area. This combination of channels also improves the uniformity of the temperature distribution across the device. The manifold microchannel coolers were fabricated and tested both with and without electrical isolation between the chip and the coolant. Experimentally, the coolers without electrical isolation demonstrated thermal resistivity values as low as 0.06 K/(W/cm2), which is up to a 50X improvement over a standard power package with significant size and weight reduction. The coolers with an incorporated aluminum nitride electrical isolation layer experimentally demonstrated up to a 15X improvement. In addition to experimental results, the interaction between the manifold channels and multiple microchannels was numerically modeled and compared to simpler, one-dimensional approximations based on the Hagen-Poiseuille equation. The comparison shows that the one-dimensional model, while under-predicting total pressure drops, can provide insight into the effect of varying dimensions on system performance. The numerical models were used to identify the impact of varying dimensions across the entire length of the cooler, and a sensitivity analysis was performed with respect to system pressure drop, thermal resistance and uniformity. Additionally, large microchannel velocity gradients, some larger than 10X, were observed along the length of the device which impacts the chip non-uniformity. The simulations showed that when comparing the manifolded design to a comparable straight microchannel cooler, there is a 38X reduction in system pressure drop for similar thermal performance.