Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item REACTION NETWORK ANALYSIS FOR THIN FILM DEPOSITION PROCESSES(2016) Ramakrishnasubramanian, Krishnaprasath; Adomaitis, Raymond; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the growth of thin films produced by Atomic Layer Deposition (ALD) and Chemical Vapor Deposition (CVD) has been one of the most important challenge for surface chemists over the last two to three decades. There has been a lack of complete understanding of the surface chemistry behind these systems due to the dearth of experimental reaction kinetics data available. The data that do exist are generally derived through quantum computations. Thus, it becomes ever so important to develop a deposition model which not only predicts the bulk film chemistry but also explains its self-limiting nature and growth surface stability without the use of reaction rate data. The reaction network analysis tools developed in this thesis are based on a reaction factorization approach that aims to decouple the reaction rates by accounting for the chemical species surface balance dynamic equations. This process eliminates the redundant dynamic modes and identifies conserved modes as reaction invariants. The analysis of these invariants is carried out using a Species-Reaction (S-R) graph approach which also serves to simplify the representation of the complex reaction network. The S-R graph is self explanatory and consistent for all systems. The invariants can be easily extracted from the S-R graph by following a set of straightforward rules and this is demonstrated for the CVD of gallium nitride and the ALD of gallium arsenide. We propose that understanding invariants through these S-R graphs not only provides us with the physical significance of conserved modes but also give us a better insight into the deposition mechanism.Item Combinatorial Experiments Using a Spatially Programmable Chemical Vapor Deposition System(2007-05-02) Sreenvivasan, Ramaswamy; Adomaitis, Raymond; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A CVD reactor concept featuring a segmented design allows individual regions of a wafer to be exposed to different precursor concentrations simultaneously during a run resulting in different thickness profiles on the wafer and a thickness gradient at the boundaries between segment regions. Different recipes were cycled through each of the segments in a sequence of deposition experiments to develop a model relating precursor concentration to film thickness in each segment region. As a demonstration of spatial programmability, the system was re-programmed using this model to produce uniform thickness amongst the segments; inter-segment uniformity approaching 0.48 % (thickness standard deviation) was demonstrated. In a subsequent study, segmented CVD reactor designs enabling spatial control of across-wafer gas phase composition were evaluated for depositing graded films suitable for combinatorial studies. Specifically two reactor designs were evaluated with experiments and response surface model (RSM) based analysis to quantify the reactor performance in terms of film thickness uniformity, sensitivity to adjustable reactor operating conditions, range of thickness over which uniformity could be achieved and each reactor's ability to control the thickness gradient across the wafer surface. Design features distinguishing the two reactor systems and their influence on gradient control versus deposition rate performance are summarized. Response Surface (RS) models relating wafer state properties to process recipes are shown to be effective tools to quantify, qualify and compare different reactor designs.Item DEVELOPMENT OF AN OBJECT-ORIENTED FRAMEWORK FOR MODULAR CHEMICAL PROCESS SIMULATION WITH SEMICONDUCTOR MANUFACTURING APPLICATIONS(2006-04-27) Chen, Jing; Adomaitis, Raymond A.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chemical Vapor Deposition (CVD) processes constitute an important unit operation for micro electronic device fabrication in the semiconductor industry. Simulators of the deposition process are powerful tools for understanding the transport and reaction conditions inside the deposition chamber and can be used to optimize and control the deposition process. This thesis discusses the development of a set of object-oriented modular simulation tools for solving lumped and spatially distributed models generated from chemical process design and simulation problems. The application of object-oriented design and modular approach greatly reduces the software development cycle time associated with designing process systems and improves the overall efficiency of the simulation process. The framework facilitates an evolutionary approach to simulator development, starting with a simple process description and building model complexity and testing modeling hypothesis in a step-by-step manner. Modularized components can be easily assembled to form a modeling system for a desired process. The framework also brings a fresh approach to many traditional scientific computing procedures to make a greater range of computational tools available for solving engineering problems. Two examples of tungsten chemical vapor deposition simulation are presented to illustrate the capability of the tools developed to facilitate an evolutionary simulation approach. The first example demonstrates how the framework is applied for solving systems assembled from separate modules by simulating a tungsten CVD deposition process occurring in a single wafer LPCVD system both at steady-state and dynamically over a true processing cycle. The second example considers the development of a multi-segment simulator describing the gas concentration profiles in the newly designed Programmable CVD reactor system. The simulation model is validated by deposition experiments conducted in the three-segment prototype. To facilitate the CVD system design, experimental data archiving, and distributed simulation, a three-tier Java and XML-based integrated information technology system has also been developed.Item Development of a Spatially Controllable Chemical Vapor Deposition System(2005-01-28) Choo, Jae-Ouk; Adomaitis, Raymond A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Most conventional chemical vapor deposition (CVD) systems do not have the spatial actuation and sensing capabilities necessary to control deposition uniformity, or to intentionally induce nonuniform deposition patterns for single-wafer combinatorial CVD experiments. In an effort to address these limitations, a novel CVD reactor system has been developed that can explicitly control the spatial profile of gas-phase chemical composition across the wafer surface. In this thesis, the simulation-based design of a prototype reactor system and the results of preliminary experiments performed to evaluate the performance of the prototype in depositing tungsten films are presented. Initial experimental results demonstrate that it is possible to produce spatially patterned wafers using a CVD process by controlling gas phase reactant composition. Based on the evaluation of the first prototype, a second prototype system was designed and constructed, enabling for greater control and programmability. The capability of this prototype for performing combinatorial CVD experiments is discussed. Finally, improvement of intra-segment uniformity and film thickness together with micro structure or composition is discussed.