Theses and Dissertations from UMD
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Item Plasma-Surface Interaction at Atmospheric Pressure: from Mechanisms with Model Polymers to Applications for Sterilization(2018) Luan, Pingshan; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cold temperature atmospheric pressure plasma (APP) produces many types of chemically reactive species and is capable of modifying materials at atmospheric pressure. Studying plasma-surface interaction (PSI) at such pressure has been challenging due to the small mean-free-path (< 100 nm) which prohibits the conventional method of using independently controlled beams of ions/neutrals to isolate the role of each species. In this dissertation, we developed an alternative approach of studying PSI at atmospheric pressure using well-controlled source-ambient-sample systems and comprehensive surface/gas phase characterization techniques. In this new approach, we emphasize the controlled generation of reactive species from the plasma source, the regulated transportation of reactive species to the target surfaces, as well as the simplified material structure subjected to plasma treatment. To isolate and identify the role of certain reactive species on materials, a plasma source is selected with its operating conditions carefully tuned for the delivery of such species to target surface. Plasma-induced effects on model polymers and biomolecules were characterized and then quantitatively correlated to the gas phase species. Due to the multi-phase nature of PSI, many characterization techniques, including that of plasma/gas phases such as optical emission spectroscopy (OES), Fourier transform infrared spectroscopy (FTIR) and UV absorption, and that of material surfaces such as X-ray photoelectron spectroscopy (XPS), attenuated total reflection (ATR) FTIR and Ellipsometry were adopted. Using this approach, we were able to evaluate the effect of both short- and long-lived reactive neutrals on many types of surface moieties. For example, we find that atomic O and OH radicals are able to cause fast material removal but moderate oxidation on the etched surface. We also find that O3 can participate in the chemical modification of aromatic rings, i.e. cleavage and their conversion into ether, ester carbonyls and surface organic nitrate groups, both on surface and in the polymer bulk. We also find evidence for (1) the competition between etching and surface modification processes when a high density of short-lived reactive species is involved, and (2) three polymer transformation stages when large fluxes of long-lived reactive species are interacting with styrene-based polymers. Lastly, we extended our work to explore the potential application of APP reactors for disinfecting raw foods and evaluated bacterial inactivation mechanisms.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.