UMD Theses and Dissertations
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Item Microfabricated Bulk Piezoelectric Transformers(2017) Barham, Oliver M.; DeVoe, Don L; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Piezoelectric voltage transformers (PTs) can be used to transform an input voltage into a different, required output voltage needed in electronic and electro- mechanical systems, among other varied uses. On the macro scale, they have been commercialized in electronics powering consumer laptop liquid crystal displays, and compete with an older, more prevalent technology, inductive electromagnetic volt- age transformers (EMTs). The present work investigates PTs on smaller size scales that are currently in the academic research sphere, with an eye towards applications including micro-robotics and other small-scale electronic and electromechanical sys- tems. PTs and EMTs are compared on the basis of power and energy density, with PTs trending towards higher values of power and energy density, comparatively, indicating their suitability for small-scale systems. Among PT topologies, bulk disc-type PTs, operating in their fundamental radial extension mode, and free-free beam PTs, operating in their fundamental length extensional mode, are good can- didates for microfabrication and are considered here. Analytical modeling based on the Extended Hamilton Method is used to predict device performance and integrate mechanical tethering as a boundary condition. This model differs from previous PT models in that the electric enthalpy is used to derive constituent equations of motion with Hamilton’s Method, and therefore this approach is also more generally applica- ble to other piezoelectric systems outside of the present work. Prototype devices are microfabricated using a two mask process consisting of traditional photolithography combined with micropowder blasting, and are tested with various output electri- cal loads. 4mm diameter tethered disc PTs on the order of .002cm^3 , two orders smaller than the bulk PT literature, had the following performance: a prototype with electrode area ratio (input area / output area) = 1 had peak gain of 2.3 (± 0.1), efficiency of 33 (± 0.1)% and output power density of 51.3 (± 4.0)W cm^-3 (for output power of 80 (± 6)mW) at 1MΩ load, for an input voltage range of 3V-6V (± one standard deviation). The gain results are similar to those of several much larger bulk devices in the literature, but the efficiencies of the present devices are lower. Rectangular topology, free-free beam devices were also microfabricated across 3 or- ders of scale by volume, with the smallest device on the order of .00002cm^3 . These devices exhibited higher quality factors and efficiencies, in some cases, compared to circular devices, but lower peak gain (by roughly 1/2 ). Limitations of the microfab- rication process are determined, and future work is proposed. Overall, the devices fabricated in the present work show promise for integration into small-scale engi- neered systems, but improvements can be made in efficiency, and potentially voltage gain, depending on the applicationItem INTEGRATION OF CMOS TECHNOLOGY INTO LAB-ON-CHIP SYSTEMS APPLIED TO THE DEVELOPMENT OF A BIOELECTRONIC NOSE(2015) Datta-Chaudhuri, Timir Baran; Abshire, Pamela A; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work addresses the development of a lab-on-a-chip (LOC) system for olfactory sensing. The method of sensing employed is cell-based, utilizing living cells to sense stimuli that are otherwise not easily sensed using conventional transduction techniques. Cells have evolved over millions of years to be exquisitely sensitive to their environment, with certain types of cells producing electrical signals in response to stimuli. The core device that is introduced here is comprised of living olfactory sensory neurons (OSNs) on top of a complementary metal-oxide-semiconductor (CMOS) integrated circuit (IC). This hybrid bioelectronic approach to sensing leverages the sensitivity of OSNs with the electronic signal processing capability of modern ICs. Intimately combining electronics with biology presents a number of unique challenges to integration that arise from the disparate requirements of the two separate domains. Fundamentally the obstacles arise from the facts that electronic devices are designed to work in dry environments while biology requires not only a wet environment, but also one that is precisely controlled and non-toxic. Design and modeling of such heterogeneously integrated systems is complicated by the lack of tools that can address the multiple domains and techniques required for integration, namely IC design, fluidics, packaging, and microfabrication, and cell culture. There also arises the issue of how to handle the vast amount of data that can be generated by such systems, and specifically how to efficiently identify signals of interest and communicate them off-chip. The primary contributions of this work are the development of a new packaging scheme for integration of CMOS ICs into fluidic LOC systems, a methodology for cross-coupled multi-domain iterative modeling of heterogeneously integrated systems, demonstration of a proof-of-concept bioelectronic olfactory sensor, and a novel event-based technique to minimize the bandwidth required to communicate the information contained in bio-potential signals produced by dense arrays of electrically active cells.Item ULTRA-SMALL SCALE MECHANICAL PROPERTIES MEASUREMENT(2011) Gaither, Michael Scott; Lloyd, Isabel K; Cook, Robert F; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In order for the microelectromechanical systems (MEMS) industry to continue to grow and advance, it is critical that methods are developed to determine the mechanical reliability of MEMS devices. This is particularly so for advanced devices with contacting, moving components, for which component strength is a key factor in determining reliability. The etching processes used to produce MEMS devices leave residual surface features that typically limit device strength and, consequently, device lifetime and reliability. In order to optimize MEMS device reliability, it is therefore necessary to understand and characterize the effects these etching processes have on MEMS-scale device strengths. At the micro and nano scales, however, conventional strength testing methods cannot be used, and a standardized test method for MEMS-scale strength measurement has yet to be established. The micro-scale theta specimen, shaped like the Greek-letter theta, acts as a tensile test specimen when loaded in compression by generating a uniform tensile stress in the central web of the specimen. Utilizing the theta specimen for strength measurements allows for simple micro-scale strength testing and assessment of etching effects, while removing the difficulties associated with gripping and loading specimens as well as minimizing potential misalignment effects. Micro-scale silicon theta samples were fabricated using techniques relevant to MEMS processing. Processing-structure relationships were determined with microscopy techniques measuring sample dimensional variations, etch quality, and surface roughness. Structure-properties relationships were determined using three techniques. Samples were tested by instrumented indentation testing (IIT) and finite element analysis determined sample strength. Sample set strength data were examined via Weibull statistics. Fractographic analysis determined initial fracture locations and fracture propagation behavior. Key scientific findings included: (1) directly relating the processing-induced etching quality of fabricated samples to sample strength, and (2) critical flaw size calculations from sample strength measurements that were consistent with sample surface roughness. Technical contributions included development of the micro-scale theta specimen fabrication methodology, super-resolution dimensional measurements, and extension of IIT to strength measurements. The micro-scale theta specimen and corresponding testing methodology have enabled successful determination of processing-structure-mechanical properties relationships for three processing approaches. This is vital to the determination of properties-performance relationships in MEMS devices.Item Manifold microchannel cooling of photovoltaic cells for high efficiency solar energy conversion systems(2008) Kermani, Elnaz; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Several works have been published on the concentration of solar radiation by mirrors and lenses onto smaller sized solar panels, which reduce cost and increase conversion efficiency at higher concentration ratio. One of the challenges in this technology is active and uniform cooling of high heat flux solar arrays, because conversion efficiency is dependent on device temperatures and drops with increase in temperature. This research is targeted at cooling small, high concentrated solar cells. Benefits of manifold microchannel are attractive for cooling of electronic devices but have not been studied for cooling of high concentrated solar cells which is the target of this thesis, where the microchannel can be microfabricated and etched on the backside of the silicon solar cell to form a sealed heat sink with the manifold fabricated in the silicon substrate. This design minimizes the pressure drop, and also maximizes the heat transfer on the device.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.