Browsing by Subject "Semiconductor"
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Item A bidirectional MEMS thermal actuator as the building block for a programmable metamaterial(2018-10-04) Zhao, Cheng, M.S. in Engineering; Cullinan, MichaelThis thesis presents a novel bidirectional MEMS thermal actuator that is intended to be implemented as the building block for a microarchitectured material. The successful proof of concept demonstrates the potential for a new level of miniaturization for the technology that would improve existing capabilities and enable new ones. The design is built upon the bent-beam type thermal actuators with an emphasis on large travel and force output. Sensing capabilities are accomplished through piezoresistive strain gauges that provide sufficient sensitivity and resolution. An analytical model was created to calculate the performance parameters of actuator designs and was used in conjunction with optimization software to arrive at four selected designs with minimal theoretical trade-offs. Successful fabrication of the devices was achieved with standard microfabrication techniques. Preliminary testing results have demonstrated the successful operation of bidirectional actuation and confirms the validity of the conceptItem Automatic semiconductor wafer map defect signature detection using a neural network classifier(2010-12) Radhamohan, Ranjan Subbaraya; Ghosh, Joydeep; El-Hamdi, MohamedThe application of popular image processing and classification algorithms, including agglomerative clustering and neural networks, is explored for the purpose of grouping semiconductor wafer defect map patterns. Challenges such as overlapping pattern separation, wafer rotation, and false data removal are examined and solutions proposed. After grouping, wafer processing history is used to automatically determine the most likely source of the issue. Results are provided that indicate these methods hold promise for wafer analysis applications.Item Control performance assessment of run-to-run control system used in high-mix semiconductor manufacturing(2012-08) Jiang, Xiaojing; Edgar, Thomas F.; Sanchez, Isaac C.; Masada, Glenn Y.; Stuber, John D.; Wang, Jin; Hwang, Gyeong S.Control performance assessment (CPA) is an important tool to realize high performance control systems in manufacturing plants. CPA of both continuous and batch processes have attracted much attention from researchers, but only a few results about semiconductor processes have been proposed previously. This work provides methods for performance assessment and diagnosis of the run-to-run control system used in high-mix semiconductor manufacturing processes. First, the output error source of the processes with a run-to-run EWMA controller is analyzed and a CPA method (namely CPA I) is proposed based on closed-loop parameter estimation. In CPA I, ARMAX regression is directly applied to the process output error, and the performance index is defined based on the variance of the regression results. The influence of plant model mismatch in the process gain and disturbance model parameter to the control performance in the cases with or without set point change is studied. CPA I method is applied to diagnose the plant model mismatch in the case with set point change. Second, an advanced CPA method (namely CPA II) is developed to assess the control performance degradation in the case without set point change. An estimated disturbance is generated by a filter, and ARMAX regression method is applied to the estimated disturbance to assess the control performance. The influence of plant model mismatch, improper controller tuning, metrology delay, and high-mix process parameters is studied and the results showed that CPA II method can quickly identify, diagnose and correct the control performance degradation. The CPA II method is applied to industrial data from a high-mix photolithography process in Texas Instruments and the influence of metrology delay and plant model mismatch is discussed. A control performance optimization (CPO) method based on analysis of estimated disturbance is proposed, and optimal EWMA controller tuning factor is suggested. Finally, the CPA II method is applied to non-threaded run-to-run controller which is developed based on state estimation and Kalman filter. Overall process control performance and state estimation behavior are assessed. The influence of plant model mismatch and improper selection of different controller variables is studied.Item Control-friendly scheduling algorithms for multi-tool, multi-product manufacturing systems(2011-12) Bregenzer, Brent Constant; Qin, Joe; Hasenbein, John J.; Edgar, Thomas F.; Hwang, Gyeong S.; Kutanoglu, Erhan; Bonnecaze, Roger T.The fabrication of semiconductor devices is a highly competitive and capital intensive industry. Due to the high costs of building wafer fabrication facilities (fabs), it is expected that products should be made efficiently with respect to both time and material, and that expensive unit operations (tools) should be utilized as much as possible. The process flow is characterized by frequent machine failures, drifting tool states, parallel processing, and reentrant flows. In addition, the competitive nature of the industry requires products to be made quickly and within tight tolerances. All of these factors conspire to make both the scheduling of product flow through the system and the control of product quality metrics extremely difficult. Up to now, much research has been done on the two problems separately, but until recently, interactions between the two systems, which can sometimes be detrimental to one another, have mostly been ignored. The research contained here seeks to tackle the scheduling problem by utilizing objectives based on control system parameters in order that the two systems might behave in a more beneficial manner. A non-threaded control system is used that models the multi-tool, multi-product process in a state space form, and estimates the states using a Kalman filter. Additionally, the process flow is modeled by a discrete event simulation. The two systems are then merged to give a representation of the overall system. Two control system matrices, the estimate error covariance matrix from the Kalman filter and a square form of the system observability matrix called the information matrix, are used to generate several control-based scheduling algorithms. These methods are then tested against more tradition approaches from the scheduling literature to determine their effectiveness on both the basis of how well they maintain the outputs near their targets and how well they minimize the cycle time of the products in the system. The two metrics are viewed simultaneously through use of Pareto plots and merits of the various scheduling methods are judged on the basis of Pareto optimality for several test cases.Item Controlling work in process during semiconductor assembly and test operations(2017-09-18) Zhang, Chuwen, active 21st century; Bard, Jonathan F.; Chacon, RodolfoIn the semiconductor industry, products go through a series of steps over a three- to four-month period that begins with the fabrication of chips and ends with assembly and test (AT) and shipment. This paper introduces a mid-term planning model for scheduling AT operations aimed at minimizing the difference between customer demand and product completions each day. A secondary objective is to maximize daily throughput. Typically, semiconductor companies have 1000s of products or devices in their catalog that can be organized into unique groups of up to 100 devices each. This simplifies the planning process because it is only necessary to consider the groups as a whole rather than the individual devices when constructing schedules. In all, we developed and tested three related models. Each provides daily run rates at each processing step or logpoint for each device group for up to one month at a time. The models are distinguished by how cycle time is treated. The first takes a steady-state approach and uses Little’s Law to formulate a WIP target constraint based on the average cycle time at each processing step. The second and third include integer and fractional cycle times in the variable definitions. To find solutions, raw production data are analyzed in a preprocessing step and then converted to input files in a standard format. FlopC++ from the COIN-OR open source software project is used to write and solve the model. Testing was done using three datasets from the Taiwan AT facility of a global semiconductor firm. By comparing model output with historical data for 6 device groups and 33 logpoints, we were able to realize decreases in shortages of up to 40% per month.Item Dynamic decision making under uncertainty for semiconductor manufacturing and healthcare(2019-05-09) Gupta, Shreya; Hasenbein, John J.; Clarke, Dave F; Kutanoglu, Erhan; Bard, Jonathan FThis dissertation proposes multiple methods to improve processes and make better decisions in manufacturing and healthcare. First, it investigates algorithms for controlling the automated material handling system (AMHS) in a wafer fab. In particular, this research examines algorithms that route vehicles for both the pickup and delivery of lots, with the goal of improving vehicle flow, cycle time, and avoiding congested segments in the AMHS. The proposed methods are simulated using both a stylized simulation model and a more detailed Automod model. These simulations demonstrate that algorithms designed specifically to anticipate congestion can significantly improve some fab metrics. Secondly, this research develops several algorithms for ranking tools in a manufacturing facility so that routes can be categorized and the best routes can be used for recipe probing. Ranking is performed using three different metrics: score-based metrics where higher implies better, target-based metrics where a balance has to be struck by the decision maker between accuracy and precision of a tool based on a target value, and count based metrics such as defect data where a lower number is better (e.g., zero defects is the best scenario). In this part of the dissertation, the ranking algorithms designed for count based metrics are the main contribution to the tool-ranking literature for the manufacturing industry. Finally, the dissertation addresses the problem of medical decision making under uncertainty during the treatment of epilepsy. Here the sequential decision making problem is modeled as an average cost Markov decision process (MDP) to maximize a patient's remaining quality of life. A crucial issue is the uncertainty in transition probabilities extracted from medical studies in epilepsy due to attrition of patients from studies, lack of data and lack of proper experimental design owing to the complexity in treatment procedure. This is addressed by formulating a robust MDP that suggests the best course of treatment for a patient.Item Embedded dielectric microstructures in molecular beam epitaxy : high-quality planar coalescence toward enhanced optoelectronic materials(2018-10-29) Ironside, Daniel Joseph; Bank, Seth Robert; Wasserman, Daniel; Li, Xiaoqin (Elaine); Yu, Edward T; Wang, ZhengSeamless integration of embedded dielectric microstructures in III-V crystal growth is a continued area of research due to its numerous high-impact applications. Historically, investigations into embedded dielectric microstructures within existing crystal growth techniques were focused on blocking dislocations at the III-V/dielectric interface in the production of low defect relaxed high mismatched heteroepitaxy. However, recent efforts have broadened the use of embedded dielectric microstructures for enhancement of optoelectronic device functionality and development of monolithic growth schemes toward integrated photonic circuits. The central challenge of embedding dielectric microstructures in III-V materials is achieving single-crystal high-quality planar coalescence within existing conventional III-V crystal growth techniques without defect. While prevalent in the field of III-V crystal growth, solid-source Molecular Beam Epitaxy (MBE) has a well-known "coalescence problem," historically lacking approaches that achieve planar coalescence over dielectric microstructures. Limited coalescence is in large part due to low diffusion of III-adatoms on dielectric surfaces, typically below 300nm, readily forming polycrystalline deposition on dielectric surfaces exceeding this diffusion length. Several solid-source MBE highly-selective growth and lateral epitaxial overgrowth (LEO) growth approaches have been reported; however, none demonstrating complete planar coalescence over dielectric microstructures. In this dissertation, to overcome the "coalescence problem," we demonstrate for the first time a general methodology for an all-MBE growth of high-quality planar coalescence over a variety of embedded dielectric microstructures. Underpinning the approach, we developed a two-stage all-MBE growth approach for GaAs and InAs on (001) substrates, producing highly selective LEO and planarization, returning the growth front to the (001) surface. Characterization of the growth approach demonstrates for the first time an all-MBE approach to planar coalescence. In application of the two-stage all-MBE growth approach towards photonics, we demonstrate enhancement of quantum emitters using buried silica gratings arrays and develop several methodologies for embedded high-contrast photonic materials through self-formed air voids and molded air channel processes. Lastly, in application to high-quality relaxed high mismatch heteroepitaxy, we demonstrate for the first time an all-MBE approach to III-V metamorphic heteroepitaxy, demonstrating threading dislocation reduction in InAs/GaAs metamorphics with high fill factor embedded silica gratings. Thus, from the material presented here, we provide several significant advances to the long-standing challenge of marrying high-quality semiconductor crystal growth with dielectric microstructures, unlocking several high-impact applications, including high-quality material pathways for enhanced quantum emitters and embedded metasurfaces as well as an all-MBE approach toward heterogeneous III-V integration on silicon.Item Exciton and valley properties in atomically thin semiconductors and heterostructures(2019-05) Tran, Kha Xuan; Li, Elaine; Shih, Chih-Kang; Dodabalapur, Ananth; Lai, Keji; Lu, NanshuTwo dimensional van der Waals (vdW) materials recently emerged as promising candidates for optoelectronic, photonic, and valleytronic applications. Monolayer transition metal dichalcogenides (TMD) are semiconductors with a band gap in the visible frequency range of the electromagnetic spectrum. Their unique properties include evolution from indirect band gap in bulk materials to direct band gap in monolayers, large exciton binding energy (few hundred meV), large absorption per monolayer (about 10%), strong spin-orbit coupling, and spin-valley locking. Moreover, two or more TMD monolayers can be stacked on top of one another to create vdW heterostructures with exciting new properties. Optical properties of semiconductors near the band gap are often dominated by the fundamental optical excitation: the exciton (Coulomb-bound electron-hole pair). Excitons in TMD monolayers (intralayer exciton) exhibit a large binding energy and a very short lifetime. The excitons in TMD monolayers are formed at the boundary of the Brillouin zone at the K and K' points. The time-reversal symmetry dictates that spins are oriented with opposite directions, leading to distinct optical selection rules for the excitons at these two valleys, a property known as the spin-valley locking. Valley polarization is often characterized by circularly polarized photoluminescence (PL). We show that the degree of valley polarization in a WSe2 monolayer depends on the degree of disorder evaluated by the Stokes shift between the PL and absorption spectra. Intrinsic valley dynamics associated with different optical resonances can only be evaluated using resonant nonlinear optical spectroscopy. We discovered exceptionally long-lived intra-valley trions in WSe2 monolayers using two-color, polarization resolved pump-probe spectroscopy. A different type of excitons (interlayer excitons) may rapidly form in TMD heterostructures with a type-II band alignment. Because of the spatial indirect nature, interlayer excitons have a much longer lifetime, which is tunable by the twist angle between the two layers. Especially, we discover that multiple interlayer excitons formed in a small twist angle heterobilayer exhibit alternating circular polarization - a feature uniquely pointing to Moiré potential as the origin. We assign these peaks to the ground state and excited state excitons localized in a Moiré potential and explain how the spatial variation of optical selection rule within the moiré superlattice can give rise to multiple peaks with alternative circular polarization. The twist angle dependence, recombination dynamics, and temperature dependence of these interlayer exciton resonances all agree with the localized exciton picture. Our results suggest the feasibility of engineering artificial excitonic crystal using vdW heterostructures for nanophotonics and quantum information applications.Item High-throughput, tip-based in-line nanometrology in semiconductor and nano-featured manufacturing(2018-08) Yao, Tsung-Fu; Cullinan, Michael; Djurdjanovic, Dragan; Sreenivasan, S.V.; Sarkar , NeilA high-throughput, tip-based, in-line nanometrology system that can be helpful for developing closed-loop process control in nanomanufacturing of semiconductor industries. One of the most significant barriers stands in the path to in-line inspection in nanomanufacturing is sample-preparation. After the beforehand inspection process detecting regions that have higher chance to be failed, known as “hotspots,” the operator may need to spend much time to position a high-resolution probe to there because small field-of-view (FOV) makes it hard to recognize its position from target. The other barrier to developing in-line inspection is the resolution limitation of conventional metrology technique, a.k.a. optical and e-beam inspections. Especially for technology nodes beyond 10-nm, the critical dimension is going close to resolving capability. As a result, the in-line inspection requires a higher resolution imaging technique with fast and precision method to position the probe. The methodology developed in this study overcomes those barriers by passive alignment methods and a state-of-the-art single-chip atomic force microscopy system. Instead of those prevalent active methods, the passive alignment uses kinematic method providing the wafer adequately constraint to limit degree of freedoms in place. Once the wafer sit into site, a preload applied then a sub-micron precision can be achieved. The passive mechanism almost instantaneously finishes alignment, so no time budget should be counted. Compared with AFM’s FOV, the sub-micron positioning precision guarantees the same location on wafer-by-wafer inspection. To enhance the metrology throughput, the proposed system uses multiple AFM chips distributed over the wafer footprint in order to image on multiple hotspots simultaneously. On the other hand, a flexure-based XY stage which is able to make nm-precision and mm-range is implemented to position AFM probe. The proposed system takes a serial of images neighboring to each other, image-stitching programmatically mosaic all images to generate a large area FOV measurement. This system applies several concepts to thoroughly enhance the throughput of advanced nanometrology and make it compatible with an in-line inspection methodology in the nanomanufacturing process. By the enhanced throughput of metrology, the nanomanufacturing will have a great potential to develop a feedback, process control and improve product’s quality and yield.Item III-V MOSFETs from planar to 3D(2013-08) Xue, Fei, active 2013; Lee, Jack Chung-YeungSi complementary metal-oxide-semiconductor (CMOS) technology has been prospered through continuously scaling of its feature size. As scaling is approaching its physical limitations, new materials and device structures are expected. High electron mobility III-V materials are attractive as alternative channel materials for future post-Si CMOS applications due to their outstanding transport property. High-k dielectrics/metal gate stack was applied to reduced gate leakage current and thus lower the power dissipation. Combining their benefits, great efforts have been devoted to explore III-V/high-k/metal metal-oxide-semiconductor field-effect-transistors (MOSFETs). The main challenges for III-V MOSFETs include interface issues of high-k/III-V, source and drain contact, silicon integration and reliability. A comprehensive study on III-V MOSFETs has been presented here focusing on three areas: 1) III-V/high-k/metal gate stack: material and electrical properties of various high-k dielectrics on III-V substrates have been systematically examined; 2) device architecture: device structures from planar surface channel MOSFETs and buried channel quantum well FETs (QWFETs) to 3D gate-wrapped-around FETs (GWAFETs) and tunneling FETs (TFETs) have been designed and analyzed; 3) fabrication process: process flow has been set up and optimized to build scaled planar and 3D devices with feature size down to 40nm. Potential of high performances have been demonstrated using novel III-V/high-k devices. Effective channel mobility was significantly improved by applying buried channel QWFET structure. Short channel effect control for sub-100nm devices was enhanced by shrinking gate dielectrics, reducing channel thickness and moving from 2D planar to 3D GWAFET structure. InGaAs TFETs have also been developed for ultra-low power application. This research work demonstrates that III-V/high-k/metal MOSFETs with superior device performances are promising candidates for future ultimately scaled logic devices.Item Improving process monitoring and modeling of batch-type plasma etching tools(2015-05) Lu, Bo, active 21st century; Edgar, Thomas F.; Stuber, John D; Djurdjanovic, Dragan; Ekerdt, John G; Bonnecaze, Roger T; Baldea, MichaelManufacturing equipments in semiconductor factories (fabs) provide abundant data and opportunities for data-driven process monitoring and modeling. In particular, virtual metrology (VM) is an active area of research. Traditional monitoring techniques using univariate statistical process control charts do not provide immediate feedback to quality excursions, hindering the implementation of fab-wide advanced process control initiatives. VM models or inferential sensors aim to bridge this gap by predicting of quality measurements instantaneously using tool fault detection and classification (FDC) sensor measurements. The existing research in the field of inferential sensor and VM has focused on comparing regressions algorithms to demonstrate their feasibility in various applications. However, two important areas, data pretreatment and post-deployment model maintenance, are usually neglected in these discussions. Since it is well known that the industrial data collected is of poor quality, and that the semiconductor processes undergo drifts and periodic disturbances, these two issues are the roadblocks in furthering the adoption of inferential sensors and VM models. In data pretreatment, batch data collected from FDC systems usually contain inconsistent trajectories of various durations. Most analysis techniques requires the data from all batches to be of same duration with similar trajectory patterns. These inconsistencies, if unresolved, will propagate into the developed model and cause challenges in interpreting the modeling results and degrade model performance. To address this issue, a Constrained selective Derivative Dynamic Time Warping (CsDTW) method was developed to perform automatic alignment of trajectories. CsDTW is designed to preserve the key features that characterizes each batch and can be solved efficiently in polynomial time. Variable selection after trajectory alignment is another topic that requires improvement. To this end, the proposed Moving Window Variable Importance in Projection (MW-VIP) method yields a more robust set of variables with demonstrably more long-term correlation with the predicted output. In model maintenance, model adaptation has been the standard solution for dealing with drifting processes. However, most case studies have already preprocessed the model update data offline. This is an implicit assumption that the adaptation data is free of faults and outliers, which is often not true for practical implementations. To this end, a moving window scheme using Total Projection to Latent Structure (T-PLS) decomposition screens incoming updates to separate the harmless process noise from the outliers that negatively affects the model. The integrated approach was demonstrated to be more robust. In addition, model adaptation is very inefficient when there are multiplicities in the process, multiplicities could occur due to process nonlinearity, switches in product grade, or different operating conditions. A growing structure multiple model system using local PLS and PCA models have been proposed to improve model performance around process conditions with multiplicity. The use of local PLS and PCA models allows the method to handle a much larger set of inputs and overcome several challenges in mixture model systems. In addition, fault detection sensitivities are also improved by using the multivariate monitoring statistics of these local PLS/PCA models. These proposed methods are tested on two plasma etch data sets provided by Texas Instruments. In addition, a proof of concept using virtual metrology in a controller performance assessment application was also tested.Item Li-ion and Na-ion battery anode materials and photoanodes for photochemistry(2015-08) Dang, Hoang Xuan; Mullins, C. B.; Heller, Adam; Hwang, Gyeong S.; Fan, Donglei; Korgel, Brian A.The current Li-ion technologies allow the popularity of Li-ion batteries as electrical energy storage for both mobile and stationary applications. The graphite-based anode is most commonly used in commercial Li-ion batteries. However, because lithium intercalation in graphite occurs very close to the redox potential of Li/Li+, accidental lithium plating is a known hazard capable of resulting in internal shorting, particularly when the battery is charged rapidly, requiring higher overpotentials to accomplish the Li-intercalation. Moreover, toward the next-generation battery, a growing interest is now on promising rechargeable Na-ion batteries. The main motivation for Na-ion alternative is that sodium is much more abundant and widely distributed on the earth’s crust than lithium. In the first part of this dissertation, we investigate safer, higher specific capacity anode materials for both Li-ion and Na-ion batteries. In a separated effort toward the efficient solar energy harvesting, the second part of the dissertation examines thin film photoanodes, active in the visible-light region, for photoelectrochemical water oxidation. This part also discusses in detail the synthesis, characterization, as well as the use of co-catalysts to improve the electrode’s photochemistry performance. The current Li-ion technologies allow the popularity of Li-ion batteries as electrical energy storage for both mobile and stationary applications. The graphite-based anode is most commonly used in commercial Li-ion batteries. However, because lithium intercalation in graphite occurs very close to the redox potential of Li/Li+, accidental lithium plating is a known hazard capable of resulting in internal shorting, particularly when the battery is charged rapidly, requiring higher overpotentials to accomplish the Li-intercalation. Moreover, toward the next-generation battery, a growing interest is now on promising rechargeable Na-ion batteries. The main motivation for Na-ion alternative is that sodium is much more abundant and widely distributed on the earth’s crust than lithium. In the first part of this dissertation, we investigate safer, higher specific capacity anode materials for both Li-ion and Na-ion batteries. In a separated effort toward the efficient solar energy harvesting, the second part of the dissertation examines thin film photoanodes, active in the visible-light region, for photoelectrochemical water oxidation. This part also discusses in detail the synthesis, characterization, as well as the use of co-catalysts to improve the electrode’s photochemistry performance.Item Light modulation of electric field driven semiconductor micromotors(2021-07-27) Liang, Zexi; Fan, Donglei; Manthiram, Arumugam; Yu, Edward; Chen, RayThe future micro/nanorobots require high degrees of freedom in motion control to perform complex tasks by individuals or by a swarm. It remains a great challenge to control the motions of an individual nanomachine amidst many, to switch the operation modes facilely, and it is even more difficult to actuate several components of a nanomachine coordinately for purposed actions. This high degree of versatility is essential for the future micro/nanorobots and requires investigation of innovative actuation mechanisms. In this dissertation, we report our recent finding about a new approach combining two types of stimulation to achieve such goal. The micromotors being studied are made of semiconductor silicon nanowires. Mechanical motion of the motors is driven by several types of AC electric field. Meanwhile, the electrical property of the nanowires can be locally and instantaneously modulated by visible light illumination in a reversible manner. We demonstrate that visible light is able to change the electric polarization of semiconductor nanowires under AC electric field, and reflected by the dramatic change of mechanical motions with very rich configurations. Under a rotating electric field, the rotation speed of semiconductor Si nanowires in electric fields can instantly increase, decrease, and even reverse the orientation by light illumination in the visible to infrared regime at various AC E-field frequencies. Under a linear AC electric field, instantaneous change of alignment direction and speed of semiconductor nanowires is observed under visible-light exposure. With theoretical analysis and simulation, the working principle can be attributed to the optically tuned imaginary-part (out-phase) and real-part (in-phase) electrical polarization of a semiconductor nanowire in aqueous suspension. Based on the understanding of this system, we further propose a new approach to control the semiconductor micromotor via light tunable dielectrophoresis. Localized control of collective behavior in a highly density silicon nanowire suspension is also investigated. Finally, we demonstrated how to utilize the mechanical motion at microscale for practical application of biosensing.Item Main group semiconducting materials : boron arsenide and an ester-functionalized salophen aluminum polymer(2013-05) Swingle, Sarah Faye; Cowley, Alan H.; Holliday, Bradley J.Boron arsenide is a compound main group semiconductor with a theoretical band gap in the range of 1.1 to 1.6 eV. Despite this ideal band gap, experimental studies of boron arsenide are very limited. In the present work, single source precursors with covalent bonds between boron and arsenic and labile ligands have been designed and synthesized. These precursors underwent thermal or chemical treatment to produce boron arsenide materials. Boron arsenide has also been prepared as a thin layer deposited on a boron substrate and a p-type photoelectrode was prepared from this material. The structure of the product was identified on the basis of X-ray diffraction and scanning electron microscopy, and the surface composition was determined by means of X-ray photoelectron spectroscopy. The electrode was found to be photoactive under both visible and UV-visible light irradiation and displayed a photocurrent of approximately 0.1 mA/cm² under UV-visible light irradiation at an applied potential of -0.25 V vs. Ag/AgCl. The valence band was estimated to be -5.1 eV. The indirect band gap, as determined from incident photo-to-electron conversion efficiency plots, is 1.46 eV. An ester-fuctionalized salophen aluminum complex that features a polymerizable bithiophene as the ester R group has been designed and synthesized. Metallopolymers of this type offer the additional advantages of processability and uniformity of the resulting films. The new salophen complex exhibited emission in the blue region at 491 nm with a quantum yield of 8.19%, which is significantly larger than that of the isolated ligand. Electropolymerization of this complex on a platinum button electrode resulted in the formation of an electrically conductive polymer that is also ionically conductive at low scan rates. In the polymeric form, the emission wavelength was found to be red-shifted to 505 nm.Item Mechanistic study of plasma damage to porous low-k : process development and dielectric recovery(2010-05) Shi, Hualiang; Ho, Paul S.; Niu, Qian; Shi, Li; Swift, Jack B.; Yao, ZhenLow-k dielectrics with porosity are being introduced to reduce the RC delay of Cu/low-k interconnect. However, during the O2 plasma ashing process, the porous low-k dielectrics tend to degrade due to methyl depletion, moisture uptake, and densification, increasing the dielectric constant and leakage current. This dissertation presents a study of the mechanisms of plasma damage and dielectric recovery. The kinetics of plasma interaction with low-k dielectrics was investigated both experimentally and theoretically. By using a gap structure, the roles of ion, photon, and radical in producing damage on low-k dielectrics were differentiated. Oxidative plasma induced damage was proportional to the oxygen radical density, enhanced by VUV photon, and increased with substrate temperature. Ion bombardment induced surface densification, blocking radical diffusion. Two analytical models were derived to quantify the plasma damage. Based on the radical diffusion, reaction, and recombination inside porous low-k dielectrics, a plasma altered layer model was derived to interpret the chemical effect in the low ion energy region. It predicted that oxidative plasma induced damage can be reduced by decreasing pore radius, substrate temperature, and oxygen radical density and increasing carbon concentration and surface recombination rate inside low-k dielectrics. The model validity was verified by experiments and Monte-Carlo simulations. This model was also extended to the patterned low-k structure. Based on the ion collision cascade process, a sputtering yield model was introduced to interpret the physical effect in the high ion energy region. The model validity was verified by checking the ion angular and energy dependences of sputtering yield using O2/He/Ar plasma, low-k dielectrics with different k values, and a Faraday cage. Low-k dielectrics and plasma process were optimized to reduce plasma damage, including increasing carbon concentration in low-k dielectrics, switching plasma generator from ICP to RIE, increasing hard mask thickness, replacing O2 by CO2 plasma, increasing CO addition in CO/O2 plasma, and increasing N2 addition in CO2/N2 plasma. By combining analytical techniques with the Kramers-Kronig dispersion relation and quantum chemistry calculation, the origin of dielectric loss was ascribed to the physisorbed water molecules. Post-ash CH4 plasma treatment, vapor silylation process, and UV radiation were developed to repair plasma damage.Item Modeling and optimization for energy efficient large scale cooling operation(2013-12) Kapoor, Kriti; Edgar, Thomas F.Optimal chiller loading (OCL) is described as a means to improve the energy efficiency of a chiller plant operation. It is formulated as a multi-period constrained mixed integer non-linear optimization problem to optimize the total cooling load distribution through accurate chiller models. OCL is solved as a set of quadratic programs using sequential programming algorithm (SQP) in MATLAB. Based on application of the methodology to chiller systems at UT Austin and a semiconductor manufacturing facility, OCL can result in an annual energy savings of about 8%. However, the savings may reduce considerably in case of additional physical constraints on overall plant operation. With the addition of thermal energy storage (TES) to the system, OCL can reduce the daily cooling costs in the case of time varying electricity prices by 13.45% on an average. The energy efficiency of a chiller plant as a function of its chiller arrangement is studied by using fitted chiller models. If all other variables are kept same, chillers operating in parallel consume up to 9.62% less power as compared to when they are operated in series. Otherwise, chillers may operate up to 12.26% more efficiently in series depending on their chilled water outlet temperature values. The answer to the optimal chiller arrangement can be straightforward in some cases or can be a complex optimization problem in others.Item Novel tools for ultrafast spectroscopy(2011-12) Jarvis, Thomas William; Li, Elaine; Fink, Manfred; Keto, John; Lim, Sang-Hyun; Shih, Chih-Kang; Sitz, GregExciton dynamics in semiconductor nanostructures are dominated by the effects of many-body physics. The application of coherent spectroscopic tools, such as two-dimensional Fourier transform spectroscopy (2dFTS), to the study of these systems can reveal signatures of these effects, and in combination with sophisticated theoretical modeling, can lead to more complete understanding of the behaviour of these systems. 2dFTS has previously been applied to the study of GaAs quantum well samples. In this thesis, we outline a precis of the technique before describing our own experiments using 2dFTS in a partially collinear geometry. This geometry has previously been used to study chemical systems, but we believe these experiments to be the first such performed on semiconductor samples. We extend this technique to a reflection mode 2dFTS experiment, which we believe to be the first such measurement. In order to extend the techniques of coherent spectroscopy to structured systems, we construct an experimental apparatus that permits us to control the beam geometry used to perform four-wave mixing reflection measurements. To isolate extremely weak signals from intense background fields, we extend a conventional lock-in detection scheme to one that treats the optical fields exciting the sample on an unequal footing. To the best of our knowledge, these measurements represent a novel spectroscopic tool that has not previously been described.Item Quantum coherent dynamics of excitons and valley pseudospins in atomically thin semiconductors(2018-10-09) Hao, Kai, Ph. D.; Li, Elaine; MacDonald, Allan; Downer, Michael; Shih, Chih-Kang; Tutuc, EmanuelMonolayer transition metal dichalcogenides (TMDCs) are new emerging van der Waals materials. Several TMDC materials go through with a transition from indirect to direct gap semiconductors when reduced to monolayer thickness limit with emission in the visible to near-infrared range, making them attractive materials for optoelectronic applications. Their near-gap optical properties are dominant by excitons (bound electron-hole pairs), charged excitons (known as trions) or higher order bound states (e.g., neutral and charged biexcitons). In this dissertation, we explored the quantum coherent dynamics of exciton, trions and their associated valley index using a powerful ultrafast spectroscopy tool known as the two-dimensional coherent spectroscopy (2DCS). We investigated the underlying mechanisms that determined the valley coherence associated with excitons and trions. In monolayer TMDCs, there are two inequivalent K and K’ points in momentum space, where the band extrema are located and the excitons are formed. The excitonic states in the two valleys are selectively coupled to light with opposite helicity. This valley contrasting optical selection rules allow one to address and manipulate the valley index readily, a unique property and advantage of TMDC materials for valleytronic applications. The valley coherence can be quantitatively evaluated in polarization resolved zero-quantum 2D spectra. We found that the exciton valley coherence is limited by the electron-hole exchange interaction in the system. In contrast, for the charged exciton (trion) states, where the inter-valley scattering is suppressed, it is the intra-valley pure dephasing limits the inter-valley coherence time. These results provide the insight of valley coherence dynamics in monolayer TMDCs and suggest possible approaches to improve the valley coherence time. Next, we investigate the coherence coupling between excitons and trions created in one valley. The trions are charged quasiparticles which contribute to the charge transport directly. Thus, the coupling between exciton and trion states can significantly influent the interpretation of transport measurements. We demonstrate that these two types of quasiparticles are coherently coupled to each other by the observation of the quantum beating of the cross-diagonal peaks in one-quantum 2D spectra. The coherence time between them can be extracted by monitoring the amplitude decay of the beating signal. We found that the coherent coupling dephasing rate between the exciton and trion equals to the sum of the exciton and trion dephasing rate, indicating uncorrelated dephasing process for excitons and trions. At longer time scale, the phonon-assisted energy transfer couples the two states incoherently. Finally, we studied the higher order correlated states in monolayer TMDCs. We used polarization resolved 2DCS to reveal bounded inter-valley neutral biexcitons and charged biexcitons as new peaks which spectrally shifted in 2D spectra. The binding energies of these biexcitons are ∼20 and ∼5 meV respectively. Unlike linear optical spectroscopy studies, the 2D spectra separate the different quantum pathways. Hence, these spectra provide unambiguous evidence of the biexciton states. The extracted binding energy of the biexciton states agrees with theoretical calculation and resolves controversies in the literature. Biexciton formation is important for applications such as lasers and generations of entangled photon pairs.Item Secondary functionalization of passivated Si(111) : pathways to stable photoelectrode systems(2021-05-06) Gurrentz, Joseph Martin; Rose, Michael J., Ph. D.; Roberts, Sean T; Milliron, Delia J; Ekerdt, John GSilicon surfaces functionalized with electrochemically-active metal complexes and clusters are an important family of functional interfaces. These systems have shown promise for solar energy conversion and storage, molecular electronics, and biological and chemical sensing applications. However, surface stability is a limiting factor that has persistently challenged the broad implementation and in-depth study of photoelectrochemistry at chemically-modified Si surfaces. The work presented herein demonstrates that secondary functionalization of passivated Si(111) surfaces is a versatile means to generate stable Si-based photoelectrodes comprised of surface-tethered metal-containing redox species. Organic mixed monolayers and metal oxide overlayers were used as platforms for the immobilization of a macro-chelated Ni(II)-bis-diphosphine complex and heteropolytungstate clusters on oxide-resistant Si(111). Rigorous physical and (photo)electrochemical analyses were used to probe analyte surface coverages, asses interfacial electron transfer kinetics, and indicate the extent of band bending in each sample. Thus, the high stabilities of these samples’ surfaces were revealed, which enabled deconvolution of kinetic and thermodynamic structure-function relationships that govern photoelectrochemical outcomes of these systems. These studies revealed the importance of band-edge modulation as a primary design consideration in the development of optimized chemically-modified photoelectrodes.Item Semiconductor manufacturing dashboard(2012-12) Collier, Scott Allen; Barber, K. Suzanne; Graser, ThomasThe semiconductor manufacturing process is a complex process that can consist of hundreds if not thousands of steps. During this process an enormous amount of data is generated and collected by several different systems. Analyzing this data can be complicated and time consuming. But, in order to optimize the manufacturing process, it is important to be able to process data quickly and provide data consumers an easy, meaningful way to view the data. Data consumers at a management level need to view data differently than someone who works in the semiconductor fabrication plant (FAB) operating the manufacturing equipment or a maintenance technician who fixes and maintains the equipment. So, it is important to provide these different data views to the users in a logical, organized way. This paper will discuss what a dashboard is, an overview of the semiconductor manufacturing process, and one implementation of a dashboard for the semiconductor industry, the Semiconductor Manufacturing Dashboard (SMD). An explanation of the systems involved in collecting and loading the data, the database structures, and the web servers used for development and production will also be discussed.