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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Novel Approaches to Control Surface Reactions in Plasma Etching of Electronic Materials(2019) Li, Chen; Oehrlein, Gottlieb S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Advanced semiconductor manufacturing requires precise plasma etching control for patterning complex semiconductor device structures. Pattern transfer into dielectric materials is one of the most frequently performed operation and traditionally done using continuous wave (CW) plasma etching processes based on fluorocarbon (FC) chemistries. Such etching methods are facing challenges when the critical dimension (CD) approach 10 nm. Issues include low materials etching selectivity, surface damage, roughness, and poor etching profile control. In this work, various aspects of low temperature plasma-based etching approaches are tailored for optimal plasma etching performance, including novel gaseous precursors for better control of gas phase and surface processes, tailoring the relative importance of radicals and ion bombardment at surface by sequential processes, and a new way to input energy to surfaces to stimulate etching reactions. We systematically studied the impact of molecular structure parameters of hydrofluorocarbon (HFC) precursors on plasma deposition of fluorocarbon (FC) and material etching performance. The HFC chemical composition and molecular structure such as ring structure, C=C, C≡C, C-O, C-H and degree of unsaturation have dramatic impacts on FC surface polymerization and material etching performance. Further, we report a new atomic layer etching (ALE) technique which temporally separates chemical reactant supply to a surface from ion bombardment induced etching. By this ALE method, the ion bombardment energy can be reduced to ensure low substrate damage and extremely high etching selectivity of two materials. Finally, we developed a hollow cathode electron beam etching system to reduce the energy and momentum input on the material surface by utilizing an electron-radical synergy effect. This present work has unveiled highly promising elements of a new roadmap of next generation semiconductor etching approaches and is expected to impact multiple areas of nanoscience and technology, including plasma etching of post-silicon materials. The use of specially selected gaseous precursor chemistry, temporal separation of radical exposure and energy-induced etching, and finally using electron bombardment for activation of surface etching, challenge our current understanding of semiconductor plasma processing and presents an important step forward in terms of the further industrial development of these approaches.Item SUBSCORE REPORTING FOR DOUBLE-CODED INNOVATIVE ITEMS EMBEDDED IN MULTIPLE CONTEXTS(2018) Li, Chen; Jiao, Hong; Measurement, Statistics and Evaluation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Reporting subscores is a prevalent practice in standardized tests to provide diagnostic information for learning and instruction. Previous research has developed various methods for reporting subscores (e.g. de la Torre & Patz, 2005; Wainer et al., 2001; Wang, Chen, & Cheng, 2004; Yao & Boughton, 2007; Yen, 1987). However, the existing methods are not suitable for reporting subscores for a test with innovative item types, such as double-coded items and paired stimuli. This study proposes a two-parameter doubly testlet model with internal restrictions on the item difficulties (2PL-DT-MIRID) to report subscores for a test with double-coded items embedded in paired-testlets. The proposed model is based on a doubly-testlet model proposed by Jiao and Lissitz (2014) and the MIRID (Butter, De Boeck, & Verhelst, 1998). The proposed model has four major advantages in reporting subscores— (a) it reports subscores for a test with double-coded items in complex scenario structures, (b) it reports subscores designed for content clustering, which is more common than subscores based on construct dimensionality in standardized tests, (c) it is computationally less challenging than the Multidimensional Item Response Theory (MIRT) models when estimating subscores, (d) it can be used to conduct Item Response Theory (IRT) based number-correct scoring (NCS, Yen, 1984a). A simulation study is conducted to evaluate the model parameter recovery, subscore estimation and subscore reliability. The simulation study manipulates three factors: (a) the magnitude of testlet effect variation, (b) the correlation between testlet effects for the dual testlets and (c) the percentage of double-coded items in the test. Further, the study compares the proposed model with other underspecified models in terms of model parameter estimation and model fit. The result of the simulation study has shown that the proposed 2PL-DT-MIRID yields more accurate model parameter and subscore estimates, in general, when the testlet effect variation is small, the dual testlets are weakly correlated and there are more double-coded items in a test. Across the study conditions, the proposed model outperforms other competing models in model parameter estimation. The reliability yielded from models ignoring dual testlets are spuriously inflated, the 2PL-DTMIRID produces higher overall score reliability and subscore reliability than models ignoring double-coded items, in most study conditions. In terms of model fit, none of the model fit indices investigated in this study (i.e. AIC, BIC and DIC) can achieve satisfactory rates of identifying the proposed true model as the best fitting model.