Materials Science & Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2792
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Item LOW TEMPERATURE PLASMA-METAL INTERACTIONS: PLASMA-CATALYSIS AND ELECTRON BEAM-INDUCED METAL ETCHING(2024) Li, Yudong; Oehrlein, Gottlieb G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Low-temperature plasma can generate different types of chemically reactive species at gas temperatures far below what is required to form such species from thermal excitation. Interactions between these reactive plasma-generated species and material surfaces have great potential for various applications, such as semiconductor etching or gas conversion. Synergistic effects, where the production rate with two inputs is greater than the sum of the consequences of each individually, have been demonstrated by combining the plasma with other energy inputs such as heat or kinetic energy from ions or electrons. Understanding the mechanisms by which these species interact with relevant surfaces is vital for the future development of plasma processing, chemistry and physics. In this work, we focus on the interaction of long-lived plasma species, particularly neutrals, with metal. A remote plasma-surface configuration was applied, where the plasma itself does not directly contact the surface. Two examples of plasma-metal interactions will be discussed, one taking place at atmospheric and the other at low pressure. The first case is plasma-assisted catalytic oxidation of methane (CH4) using a nickel (Ni) catalyst at atmospheric pressure, implemented by combining a remote plasma jet. The interrelation of real-time measurements of reaction products and surface adsorbates and plasma diagnostics allowed the identification of atomic oxygen as the key plasma-generated species that drives the synergistic plasma-catalytic reaction. The in-situ characterizations of the surface and gas phase reactions reveal the possible key reaction pathways for the plasma-catalysis reactions. We also observed the activation of the catalyst resulting from long-lasting catalyst surface modification induced by plasma species interaction. The second case is the damage-free etching of refractory metals, ruthenium (Ru) and tantalum (Ta), at low pressure. This was implemented by combining a remote plasma source (RPS) with an electron beam (EB) source. We investigated the effects of CF4 and Cl2 additions to Ar/O2 RPS effluents and we find that Ar/O2 with Cl2 addition induces the highest Ru etch rate (ER) and best removal selectivity over Ta. The surface chemistry characterization by spatially-resolved XPS reveals the possible mechanism of the electrons and neutrals induced materials etching. We also proposed a model that considers the fundamental aspects of the etching reaction and successfully predicts the major features of the electron and neutral induced etching reactions.Item PLASMA-BASED ATOMIC SCALE ETCHING APPROACHES USING EITHER ION OR ELECTRON BEAM ACTIVATION(2022) Lin, Kang-Yi; Oehrlein, Gottlieb; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma dry etching has been extensively employed in semiconductor manufacturing processes for anisotropic pattern transfer. With device miniaturization, the conventional approach utilizing continuous wave plasma etching does not meet the requirement for sub-nanometer processing nodes, including profile control and atomic-scale etching selectivity. Additionally, the direct plasma exposure of a substrate raises the concern of plasma damage and undesired material removal. We describe improvements of plasma-based etching techniques and identified novel ways for enabling material removal. We have systematically studied different precursor chemistries for atomic layer etching on etching selectivity of SiO2 to Si3N4 and SiO2 to Si and obtained an understanding of the surface chemistry evolution. Compared to the conventional approach that mixes fluorocarbon and hydrogen precursors, selected hydrofluorocarbon can deliver optimal plasma chemistry that produces a reduced F/C film in the deposition step and realizes atomic-scale etching selectivity. We also report a new approach for establishing etching selectivity of HfO2 over Si by integrating substrate-selective deposition into an atomic layer etching sequence. The optimal precursor chemistry can selectively deposit on the Si surface as a passivation layer and convert HfO2 to metal-organic compounds for desorption. Finally, we designed and built a system that consists of an electron flood gun and a remote plasma source to demonstrate the concept of a new etching approach by exploiting electron-neutral synergistic effects. This configuration achieves precisely controlled SiO2 or Si3N4 etching by co-introducing an electron beam and Ar/CF4/O2 remote plasma. This approach also addresses the issue of limited precursor chemistries in electron beam-induced etching.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 COLD ATMOSPHERIC PRESSURE PLASMA SURFACE INTERACTIONS WITH POLYMER AND CATALYST MATERIALS(2018) Knoll, Andrew Jay; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cold atmospheric pressure plasma (CAP) is an excellent source of reactive species because they are able to produce these species cheaply, in a variety of configurations, and in a way that can be distributed easily but there needs to be more understanding of how they specifically interact with surfaces. The goals of this dissertation are to understand what the critical reactive species reaching a surface are for particular applications. As a first step we find that a plasma in direct electrical contact with a polymer material shows high etching rate and non-uniform treatment whereas a remote regime treatment can lead to a relatively uniform treatment over the exposed to plasma area. The interaction of vacuum ultraviolet (VUV) light with polymer surfaces was found to be critical under conditions where local oxygen is displaced by noble gas flow. This VUV flux is also dependent on plasma source type, being highest for high voltage sources using noble gas flow. For a surface microdischarge (SMD) source we find high activation energy compared with atomic oxygen etching suggesting less reactive species reaching the surface are causing surface modification. However, for an atmospheric pressure plasma jet (APPJ) source we find that the activation energy changes over treatment distance, decreasing below the value expected for atomic oxygen as the jet gets closer to the surface. Additionally we find evidence of directional etching for the close distances which becomes less directional for further distance treatments suggesting we have a contribution from high energy species at closer distances despite there being no visible contact between the plasma plume and the polymer surface. Nickel catalyst materials interacting with plasma can be enhanced to show increased breakdown of methane and production of different product species such as CO compared to just the catalyst. This catalyst material also shows carbon deposition by CO and COO- groups by plasma treatment, though increased plasma power and temperature can then remove these groups as well.Item High Precision Plasma Etch for Pattern Transfer: Towards Fluorocarbon Based Atomic Layer Etching(2016) Metzler, Dominik; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A basic requirement of a plasma etching process is fidelity of the patterned organic materials. In photolithography, a He plasma pretreatment (PPT) based on high ultraviolet and vacuum ultraviolet (UV/VUV) exposure was shown to be successful for roughness reduction of 193nm photoresist (PR). Typical multilayer masks consist of many other organic masking materials in addition to 193nm PR. These materials vary significantly in UV/VUV sensitivity and show, therefore, a different response to the He PPT. A delamination of the nanometer-thin, ion-induced dense amorphous carbon (DAC) layer was observed. Extensive He PPT exposure produces volatile species through UV/VUV induced scissioning. These species are trapped underneath the DAC layer in a subsequent plasma etch (PE), causing a loss of adhesion. Next to stabilizing organic materials, the major goals of this work included to establish and evaluate a cyclic fluorocarbon (FC) based approach for atomic layer etching (ALE) of SiO2 and Si; to characterize the mechanisms involved; and to evaluate the impact of processing parameters. Periodic, short precursor injections allow precise deposition of thin FC films. These films limit the amount of available chemical etchant during subsequent low energy, plasma-based Ar+ ion bombardment, resulting in strongly time-dependent etch rates. In situ ellipsometry showcased the self-limited etching. X-ray photoelectron spectroscopy (XPS) confirms FC film deposition and mixing with the substrate. The cyclic ALE approach is also able to precisely etch Si substrates. A reduced time-dependent etching is seen for Si, likely based on a lower physical sputtering energy threshold. A fluorinated, oxidized surface layer is present during ALE of Si and greatly influences the etch behavior. A reaction of the precursor with the fluorinated substrate upon precursor injection was observed and characterized. The cyclic ALE approach is transferred to a manufacturing scale reactor at IBM Research. Ensuring the transferability to industrial device patterning is crucial for the application of ALE. In addition to device patterning, the cyclic ALE process is employed for oxide removal from Si and SiGe surfaces with the goal of minimal substrate damage and surface residues. The ALE process developed for SiO2 and Si etching did not remove native oxide at the level required. Optimizing the process enabled strong O removal from the surface. Subsequent 90% H2/Ar plasma allow for removal of C and F residues.Item Low and Atmospheric Pressure Plasma Interactions with Biomolecules and Polymers(2015) Bartis, Elliot Andrew James; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cold atmospheric plasma (CAP) sources have emerged as economical and environmentally friendly sources of reactive species with promising industrial and biomedical applications. Many different sources are studied in the literature for advanced applications including surface disinfection, wound healing, and cancer treatment, but the underlying mechanisms for these applications are not well-understood. The overall goals of this dissertation are to 1) identify how plasma treatments induce surface modifications and which plasma species are responsible for those modifications; 2) identify how changes in surface and plasma chemistry contribute to changes in biological activity of biomolecules; and 3) investigate how fluxes of reactive species produced by atmospheric pressure plasma devices can be controlled. As a first step, a well-studied low pressure plasma system was used to isolate the effects of ions, high energy photons, and radicals using Ar and H2 plasma. The finding that plasma-generated radicals can biodeactivate and modify films with negligible etching motivated further study at atmospheric pressure. Two very different CAP sources were used under mild, remote conditions to study the biological deactivation of two immune-stimulating biomolecules: lipopolysaccharide (LPS), found in bacteria such as Escherichia coli, and peptidoglycan, found in bacteria such as Staphylococcus aureus. The surface chemistry was measured to understand which plasma- generated species and surface modifications are important for biological deactivation. To simplify the complex molecular structure of the biomolecules and study specific moieties, model polymer films were studied including polystyrene, poly(methyl methacrylate), polyvinyl alcohol, and polypropylene. The interaction of the plasma plume with the environment was studied as a parameter to tune surface modifications. It was found that increasing ambient N2 concentrations in an N2/Ar ambient decreased surface modifications of LPS, similarly to how adding N2 to the O2/Ar feed gas decreased the plasma-generated O3 density and O atom optical emission. In this work, we first observed the formation of surface-bound NO3 after plasma treatment, which had not been reported in the literature. The plasma-ambient interaction was further studied using polystyrene as a model system. This detailed study demonstrated a competition between surface oxidation and nitridation, the latter of which occurs under very specific conditions. It was found that NO3 formed on all the materials studied in this dissertation after plasma treatment. This NO3 formed after treatment by both sources, but in different concentrations. The surface-bound NO3 correlated better with changes in biological activity than general oxidation, demonstrating its importance. Studying model polymers revealed that this surface moiety preferentially forms on – OH containing surfaces. Since the atmospheric pressure plasma jet (APPJ) operates with low N2/O2 admixtures to Ar and the surface microdischarge (SMD) operates with N2/O2 mixtures, the mechanisms that cause biological deactivation must be different, and are discussed.Item PLASMA-SURFACE INTERACTIONS DURING REACTIVE PLASMA PROCESSING OF HYDROCARBON FILMS(2013) Fox-Lyon, Nicholas; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Reactive plasma interactions with hydrocarbon-based surfaces play a critical role in future biological-plasma applications and for microelectronic device manufacture. As device dimensions get smaller and we require fine control of surfaces during plasma processing we will need to develop more understanding of fundamental plasma surface interactions. Through the use of plasma-deposited amorphous carbon films interacting inert/reactive plasmas (Ar/H2 plasmas) we explored etch rates and the formation of modified layers. Facing Ar and H2 plasmas mixtures, hydrocarbon surfaces can exhibit widely different properties, depending on plasma composition (ions, reactive species, fast neutrals) and initial film composition (graphitic, polymeric). Ar plasmas cause densification of hydrocarbon surface by selectively sputtering H atoms, while H2 plasmas cause incorporation/saturation of H atoms within the film surface. For hard amorphous carbon, we find that small amounts of H2 added to Ar plasma can completely negate ion-induced densification. Plasmas are also drastically changed by small impurities of H2 atoms. We investigated the plasma property effects of adding H2, D2, CH4, and surface derived hydrocarbon gases. We find that small amounts (as low as 1%) of H2/D2 in Ar cause a large decrease in electron density, increase in electron temperature, Ar metastable atoms, and radically different ion mass distributions. These effects are intensified at higher pressures, as neutral molecule-ion interactions in the plasma increase. These changes can be related to the surface modification caused by the plasma. Surface derived impurities into inert plasmas were also investigated. Hydrocarbon flow from the surface causes changes to plasma properties similar to the addition of CH4 gas. We applied the learning from these fundamental plasma-surface interaction studies to an applied problem of plasma-assisted shrink of asymmetric photoresist features. Using fluorocarbon-based plasmas, we successfully shrink asymmetric pattern features and find that lower concentrations of C4F8 in plasmas and shorter deposition thicknesses lead to more uniform shrink in L and W dimensions. To improve future plasma-assisted shrink processes, careful tuning of plasma composition and feature dimensions is critical.Item PLASMA INTERACTIONS WITH MASKING MATERIALS FOR NANOFABRICATION(2011) Weilnboeck, Florian; Oehrlein, Gottlieb; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma-based transfer of patterns into other materials is a key process for production of nano-scale devices used in micro-electronic technology. With the continuously decreasing feature-size of integrated circuits, manufacturing tolerances are becoming increasingly smaller and complex interactions of plasmas and patterned mask materials require an atomistic understanding to meet future processing tolerances. In this work, we investigated how plasma-material interactions in typical low-k pattern transfer processes depend on individual plasma components and properties of polymeric and metallic masks. First, we studied modifications of 193nm and 248nm photoresist (PR) by plasma ultraviolet/vacuum ultraviolet (UV) radiation, quantifying contributions of plasma radiation to the overall material modifications for direct interaction with plasma. Energetic ions (~125 eV) led to rapid (~3-5 s) formation of a graphitic ion-crust (~1.8 nm) and introduced together with simultaneous UV modifications of the material bulk significantly higher roughness for 193nm PR (~6 nm) than for 248nm PR (~1 nm). During ion-crust formation, 193nm PR softened by chain-scissioning and pendant group detachment in a depth of ~60 nm by UV radiation, while 248nm PR was radiation stable showing surface-close cross-linking (~4 nm). Pretreating 193nm PR with a radiationdominated He plasma and introducing UV modifications before ion-crust formation in the subsequent plasma etch reduces synergistic roughness formation as explained by wrinkling theory. Second, we studied interactions of fluorocarbon (FC) plasmas with Ti and TiN and compared these with organosilicate glass (OSG). Metal hardmasks are expected to provide improved etching selectivity (ES) and low-k damage compared to PR during pattern transfer. Erosion stages and dependencies of etch rates (ER) on FC layer thickness and energy deposition by ions were identified. ES were low (~4-8) in the diffusion-limited regime (thick FC layers) where OSG experienced strong reduction in ER, but high (up to 15) in the chemical sputtering regime (thin FC layers) at low ion energies where removal of Ti etch products was limited. TiN exhibited higher ER and lower ES than Ti due to increased surface reactivity after rapid removal of N. Overall, findings give directions for rational design of masking materials and plasma discharges for future nanofabrication pattern transfer processes.