Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item THE APPLICATIONS OF MULTIPHOTON ABSORPTION POLYMERIZATION(2013) Qin, Sijia; Fourkas, John T.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Among various nano/micro-fabrication techniques, multiphoton absorption polymerization (MAP) stands out for its high resolution and its capability of creating arbitrary three-dimensional micro-structures. This thesis will focus on the applications of, and improvements to, MAP. MAP was used to fabricate polymer double-ring resonators (DRRs) because MAP's high resolution allows for precise control of the coupling gap sizes. Pedestal acrylic DRRs with 33 nm free spectral range and -15 dB isolation were fabricated, and their properties showed qualitative agreement with simulation results. Single-mode, acrylic microring resonator devices were fabricated on a low-index substrate using MAP and were coupled to side-polished fibers as in-line devices. High-finesse spectral notches with low insertion loss were observed at the fiber output. Surface mapping of the polished face of the fiber was accomplished by moving a microring resonator device across and along the fiber core. The optimal coupling region on the polished face of the fiber could be identified through the change of modulation depth. Spectral modulation induced by varying the pressure on a microring resonator device coupled to a side-polished fiber was also investigated. Efficient multiphoton radical generation chemistry has been developed for use in aqueous media and has been applied to the fabrication, manipulation, and assembly of 3-D polymeric and biomolecular structures through a combination of MAP and optical tweezers. The combination of MAP and optical tweezers allows for the realization of structures such as tape-like and rope-like microthreads that can be used in complex microfabrication techniques such as microbraiding and microweaving. These capabilities enhance the toolbox of methods available for the creation of functional microstructures in aqueous media. MAP-fabricated and UV-cured acrylic patterns were treated with reactive ion etching (RIE) to create high-roughness "nanograsses." The nanograss patterns have shown the potential to be used as superhydrophobic materials. The density and dimension of the nanograss depends on the total exposure dose. Different etch angles gave different etch structures.Item Electrostatic MEMS Actuators using Gray-scale Technology(2006-08-30) Morgan, Brian Carl; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The majority of fabrication techniques used in micro-electro-mechanical systems (MEMS) are planar technologies, which severely limits the structures available during device design. In contrast, the emerging gray-scale technology is an attractive option for batch fabricating 3-D structures in silicon using a single lithography and etching step. While gray-scale technology is extremely versatile, limited research has been done regarding the integration of this technology with other MEMS processes and devices. This work begins with the development of a fundamental empirical model for predicting and designing complex 3-D photoresist structures using a pixilated gray-scale technique. A characterization of the subsequent transfer of such 3-D structures into silicon using deep reactive ion etching (DRIE) is also provided. Two advanced gray-scale techniques are then introduced: First, a double exposure technique was developed to exponentially increase the number of available gray-levels; improving the vertical resolution in photoresist. Second, a design method dubbed compensated aspect ratio dependent etching (CARDE) was created to anticipate feature dependent etch rates observed during gray-scale pattern transfer using deep reactive ion etching (DRIE). The developed gray-scale techniques were used to integrate variable-height components into the actuation mechanism of electrostatic MEMS devices for the first time. In static comb-drives, devices with 3-D comb-fingers were able to demonstrate >34% improvement in displacement resolution by tailoring their force-engagement characteristics. Lower driving voltages were achieved by reducing suspension heights to decrease spring constants (from 7.7N/m to 2.3N/m) without effecting comb-drive force. Variable-height comb-fingers also enabled the development of compact, voltage-controlled electrostatic springs for tuning MEMS resonators. Devices in the low-kHz range demonstrated resonant frequency tuning >17.1% and electrostatic spring constants up to 1.19 N/m (@70V). This experience of integrating 3-D structures within electrostatic actuators culminated in the development of a novel 2-axis optical fiber alignment system using 3-D actuators. Coupled in-plane motion of electrostatic actuators with integrated 3-D wedges was used to deflect an optical fiber both horizontally and vertically. Devices demonstrated switching speeds <1ms, actuation ranges >35μm (in both directions), and alignment resolution <1.25μm. Auto-alignment to fixed indium-phosphide waveguides with <1.6μm resolution in <10 seconds was achieved by optimizing search algorithms.