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 Direct Laser Writing-Enabled Microstructures with Tailored Reflectivity for Optical Coherence Tomography Phantoms(2023) Fitzgerald, Declan Morgan; Sochol, Ryan D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As the continuous push to improve medical imaging techniques produces increasingly complex systems, so too must the phantoms critical to the accurate evaluation of these systems evolve. The inclusion of precise geometries is a well documented gap in optical coherence tomography (OCT) phantoms, a gap felt more severely as the technology improves. This thesis seeks to investigate the feasibility of utilizing new manufacturing techniques in the production of OCT phantoms with complex geometries while developing a phantom to determine the sensitivity of OCT systems. The new manufacturing methods include the replication of microstructures printed via direct laser writing into PMMA photoresist, the tailored smoothing of surface roughness inherent to direct laser writing, and the selective retention of surface roughness in certain regions. Each of these methods were implemented in the manufacture of an OCT sensitivity phantom and were found to be effective in each of their respective goals.The efficacy of the sensitivity phantom in evaluating the minimum reflectance still detectable by an OCT system shows promise. Effective reflectivity ranging from 0 to ~1 was accomplished within a single angled element and should provide a basis for determining the minimum reflectivity that results in a signal-to-noise ratio of 1. Further improvements must be made to the phantom footprint and manufacturing before the phantom’s reliability is certain.Item Plasma-Surface Interactions of Model Polymers for Advanced Photoresist Systems(2008-08-28) Engelmann, Sebastian Ulrich; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma processing of advanced photoresist (PR) materials is a critical step in nano-manufacturing. We have studied the interactions of PRs and polymers in fluorocarbon/Ar discharges. The effects of process time, PR material, bias and source power, pressure and gas chemistry (C4F8/Ar, CF4/Ar and CF4/H2/Ar) were studied by ellipsometry, atomic force microscopy and x-ray photoelectron spectroscopy. Additionally, patterned structures of 193nm PR were examined using scanning electron microscopy. Polymer destruction in the top surface, oxygen and hydrogen loss along with fluorination were observed for all materials initially, which was followed by steady state etch conditions. A strong dependence of plasma-induced surface chemical and morphological changes on polymer structure was observed. In particular, the adamantane group of 193 nm PR showed poor stability. Two linked mechanisms for the roughening behavior of the films during processing were identified: A physical pattern transfer mechanism enhances initial roughness by non-uniform removal. Additional to that, roughness formation occurred linear to the energy density deposited during processing. For adamantyl polymers, a higher roughening constant was found. Additionally, fluorocarbon (FC) deposition on the damaged PR affected roughening in two opposing ways: Ion-induced mixing with the damaged PR increased roughening, whereas for simple FC precursor deposition a reduction of roughness was seen. Fluorination of the PR surfaces using plasma increased etching yields, which were found to improve the roughness of 193nm PR after etch. The fluorination of the PR prevented the formation of characteristic small scale roughness features at the cost of large scale roughness introduction. Use of low energy density processes suppressed the roughness growth by ion-induced transfer. Examining 3-dimensional trenches and contact holes patterned in PR showed that the sidewall roughness changed with process parameters similar to that seen for blanket films. The close correlation suggested that our model of polymer surface roughening also applies to resist sidewall evolution during etch. All process conditions can be combined in the energy density roughening model. Even for various feedgas chemistries adamantyl containing polymers show enhanced roughening rates, suggesting that the instability of the adamantyl structure used in 193nm PR polymers is the performance limiting factor for processing PR materials.