Browsing by Subject "Raman spectroscopy"
Now showing 1 - 18 of 18
Results Per Page
Sort Options
Item Development of a multiple-pass Raman spectrometer for flame diagnostics(2013-05) KC, Utsav; Varghese, Philip L.A multiple-pass cell is developed and applied to enhance the Raman signal from methane-air flames for temperature measurements. Stable operation of the cell was demonstrated and studied in two alignment modes. In the ring mode, the beams are focused into a ring of ~ 3 mm diameter at the center of the cell, and spectra were recorded at low dispersion (0.26 nm/pixel). Temperature is calculated from the ratio of the intensity of Stokes to anti-Stokes signal from nitrogen. Temperature is also inferred from the shapes of the Stokes and anti-Stokes peaks in the spectrum. The uncertainty in the value of flame temperature in these measurements was ±50 K. The signal gain from 100 passes is a factor of 83. Signal to noise ratio (SNR) improved by a factor of 9.3 in room temperature air with an even higher factor in flames. The improvement in SNR depends on the acquisition time and is best for short acquisition times. In the two point mode, multi passing is achieved simultaneously with high spatial resolution as the laser is focused at two small regions separated by ~ 2 mm at the center of the cell. The probe regions are 300 [mu]m × 200 [mu]m. The vast improvement in the spatial resolution is achieved at the cost of a reduced number of passes and signal gain. The two point mode is operated with 25 passes at each point with a signal gain factor of ~20; the SNR gain depends on the data acquisition time. Spectra were recorded at high dispersion (~0.03 nm/pixel). Temperature is inferred from curve fitting to the high resolution Stokes spectrum of nitrogen in methane-air flames. The curve fit is based on very detailed simulation of Raman spectrum of nitrogen. The final model includes the angular dependence of Raman scattering, electrical and mechanical anharmonicity in the polarizability matrix elements, and the presence of a rare isotope of nitrogen in air. The uncertainty in the value of temperature in the least noisy data is ±9 K. The sources of uncertainty in temperature and their contribution to the total uncertainty are also identified.Item Electric field manipulation of polymer nanocomposites: processing and investigation of their physical characteristics(2009-05-15) Banda, SumanthResearch in nanoparticle-reinforced composites is predicated by the promise for exceptional properties. However, to date the performance of nanocomposites has not reached its potential due to processing challenges such as inadequate dispersion and patterning of nanoparticles, and poor bonding and weak interfaces. The main objective of this dissertation is to improve the physical properties of polymer nanocomposites at low nanoparticle loading. The first step towards improving the physical properties is to achieve a good homogenous dispersion of carbon nanofibers (CNFs) and single wall carbon nanotubes (SWNTs) in the polymer matrix; the second step is to manipulate the well-dispersed CNFs and SWNTs in polymers by using an AC electric field. Different techniques are explored to achieve homogenous dispersion of CNFs and SWNTs in three polymer matrices (epoxy, polyimide and acrylate) without detrimentally affecting the nanoparticle morphology. The three main factors that influence CNF and SWNT dispersion are: use of solvent, sonication time, and type of mixing. Once a dispersion procedure is optimized for each polymer system, the study moves to the next step. Low concentrations of well dispersed CNFs and SWNTs are successfully manipulated by means of an AC electric field in acrylate and epoxy polymer solutions. To monitor the change in microstructure, alignment is observed under an optical microscope, which identifies a two-step process: rotation of CNFs and SWNTs in the direction of electric field and chaining of CNFs and SWNTs. In the final step, the aligned microstructure is preserved by curing the polymer medium, either thermally (epoxy) or chemically (acrylate). The conductivity and dielectric constant in the parallel and perpendicular direction increased with increase in alignment frequency. The values in the parallel direction are greater than the values in the perpendicular direction and anisotropy in conductivity increased with increase in AC electric field frequency. There is an 11 orders magnitude increase in electrical conductivity of 0.1 wt% CNF-epoxy nanocomposite that is aligned at 100 V/mm and 1 kHz frequency for 90 minutes. Electric field magnitude, frequency and time are tuned to improve and achieve desired physical properties at very low nanoparticle loadings.Item Enhancement of Raman signals : coherent Raman scattering and surface enhanced Raman spectroscopy(2012-05) Chou, He-Chun; Vanden Bout, David A.; Lim, Sang-Hyun; Peter, Rossky J.; Willets, Katherine A.; Li, Xiaoqin E.Raman spectroscopy is a promising technique because it contains abundant vibrational chemical information. However, Raman spectroscopy is restricted by its small scattering cross section, and many techniques have been developed to amplify Raman scattering intensity. In this dissertation, I study two of these techniques, coherent Raman scattering and surface enhanced Raman scattering and discuss their properties. In the first part of my dissertation, I investigate two coherent Raman processes, coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS). In CARS project, I mainly focus on the molecular resonance effect on detection sensitivity, and I find the detection sensitivity can be pushed into 10 [micromolar] with the assistance of molecular resonance. Also, I am able to retrieve background-free Raman spectra from nonresonant signals. For SRS, we develop a new SRS system by applying spectral focusing mechanism technique. We examine the feasibility and sensitivity of our SRS system. The SRS spectra of standards obtained from our system is consistent with literature, and the sensitivity of our system can achieve 10 times above shot-noise limit. In second part of this dissertation, I study surface enhanced Raman scattering (SERS) and related plasmonic effects. I synthesize different shapes of nanoparticles, including nanorod, nanodimer structure with gap and pyramids by template method, and study how electric field enhancement effects correlate to SERS by two photon luminescence (TPL). Also, I build an optical system to study optical image, spectra and particle morphology together. I find that SERS intensity distribution is inhomogeneous and closely related to nanoparticle shape and polarization direction. However, TPL and SERS are not completely correlated, and I believe different relaxation pathways of TPL and SERS and coupling of LSPR and local fields at different frequencies cause unclear correlation between them.Item First principles calculations of Raman spectra for nanostructures and improved high order forces(2015-12) Bobbitt, Nathaniel Scott; Chelikowsky, James R.; Demkov, Alexander A; Ekerdt, John G; Hwang, Gyeong S; Korgel, Brian AAdvances in computing technology coupled with theoretical developments on the electronic structure problem have laid the foundation for the rapidly growing field of computational materials science. Modern supercomputers are able to perform ab initio calculations of realistic systems containing thousands of atoms. This is an important step forward in the maturation of the field because computational insight can be used to make predictions about or predict experimental data. This dissertation aims to address contemporary theory and practice of solving the electronic structure problem for a variety of nanoscale systems, most of which are of interest for energy application such as photovoltaics or Li-ion batteries. Our calculations are performed within density functional theory using real-space pseudopotentials. In the first part, we examine nanocrystals. We calculate size-dependent properties for ZnO nanocrystals with Al and Ga dopants. Next, we calculate Raman spectra for Si nanocrystals with Li impurities and Si-Ge core-shell structures, which gives us insight into the structure of these nanocrystals. In the second portion, we examine in depth the calculation of interatomic forces within density functional theory and propose a new integration scheme which we demonstrate calculates more accurate bond lengths and vibrational frequencies and improves the stability of molecular dynamics simulations.Item Irradiation Stability of Carbon Nanotubes and Related Materials(2012-09-28) Aitkaliyeva, Assel 1985-Application of carbon nanotubes (CNTs) in various fields demands a thorough investigation of their stability under irradiation. Open structure, ability to reorganize and heal defects, and large surface-to-volume ratio of carbon nanotubes affect materials' radiation response so that it differs from their bulk counterparts. Despite the work conducted to this date, radiation damage and mechanisms governing the evolution of CNTs under irradiation are still deficient in fundamental understanding. This dissertation is aimed to comprehend and characterize radiation response and crystalline-to-amorphous transition in ion and electron irradiated carbon nanotubes using various techniques, including but not limited to, transmission electron microscopy (TEM) and Raman spectroscopy. It shows that ion irradiation can be used to engineer properties of nanotubes in a controllable manner and significantly improve thermal diffusivity and conductivity of the material. This work also establishes the role of nuclear and electronic stopping powers in thermal diffusivity enhancement: thermal properties of irradiated CNTs are governed by nuclear stopping power of bombarding species. The change of thermal properties with irradiation is driven by two competing mechanisms: inter-tube displacement-mediated phonon transport and defect-induced phonon scattering. In addition to experiments, molecular dynamic simulations are used to confirm validity of the obtained results. Radiation damage in CNTs at various temperatures as a function of ion energy, flux and fluence is examined. Mechanisms governing crystalline-to-amorphous transition under electron and ion irradiations are explored, applicability of previously suggested models discussed, and new models introduced. The results show enhanced defect annealing at elevated irradiation temperatures, which delays the formation of amorphous regions. Investigation of nanotube stability after various processing techniques and irradiation indicated that radiation response of CNTs in a composite is similar to that of individual nanotubes.Item Modification and Reactivity Studies of the Blue Copper Protein Stellacyanin(Texas Tech University, 1980-08) Harsh, Claudia ElizabethNot Available.Item Molecular and Biochemical Characterization of Hydrocarbon Production in the Green Microalga Botryococcus braunii(2012-10-19) Weiss, Taylor LeighBotryococcus braunii (Chlorophyta, Botryococcaceae) is a colony-forming green microalga that produces large amounts of liquid hydrocarbons, which can be converted into transportation fuels. While B. braunii has been well studied for the chemistry of the hydrocarbon production, very little is known about the molecular biology of B. braunii. As such, this study developed both apparatus and techniques to culture B. braunii for use in the genetic and biochemical characterization. During genetic studies, the genome size was determined of a representative strain of each of the three races of B. braunii, A, B, and L, that are distinguished based on the type of hydrocarbon each produces. Flow cytometry analysis indicates that the A race, Yamanaka strain, of B. braunii has a genome size of 166.0 +/- 0.4 Mb, which is similar to the B race, Berkeley strain, with a genome size of 166 +/- 2.2 Mb, while the L race, Songkla Nakarin strain, has a substantially larger genome size at 211.3 +/- 1.7 Mb. Phylogenetic analysis with the nuclear small subunit (18S) rRNA and actin genes were used to classify multiple strains of A, B, and L races. These analyses suggest that the evolutionary relationship between B. braunii races is correlated with the type of liquid hydrocarbon they produce. Biochemical studies of B. braunii primarily focused on the B race, because it uniquely produces large amounts of botryococcenes that can be used as a fuel for internal combustion engines. C30 botryococcene is metabolized by methylation to generate intermediates of C31, C32, C33, and C34. Raman spectroscopy was used to characterize the structure of botryococcenes. The spectral region from 1600?1700 cm^-1 showed v(C=C) stretching bands specific for botryococcenes. Distinct botryococcene Raman bands at 1640 and 1647 cm^-1 were assigned to the stretching of the C=C bond in the botryococcene branch and the exomethylene C=C bonds produced by the methylations, respectively. A Raman band at 1670 cm^-1 was assigned to the backbone C=C bond stretching. Finally, confocal Raman microspectroscopy was used to map the presence and location of methylated botryococcenes within a living colony of B. braunii cells.Item Novel uses of organized media in chemical analysis(Texas Tech University, 1987-08) Spino, Larry AngeloMicelles and cyclodextrins have been utilized for years in a variety of chemical analysis techniques. This work considers some modern applications of micelles and cyclodextrins in chemical analysis. Micellar and cyclodextrin mobile phases (pseudophases) were used in liquid chromatography (LC). Cyclodextrin mobile phases were used in microcolumn LC for separating optical isomers of racemic nicotine and eight other racemic analogues of nicotine. This was the first reported facile and direct separation of these racemates. Synchronous luminescence was used as the detection method for liquid chromatography to identify polynuclear aromatic components that co-elute in the course of a LC separation. As many as five compounds could be identified from a single chromatographic peak. Several scans can be made easily for every eluting peak, however, a single scan is sufficient to produce a complete synchronous luminescence spectra of a complex mixture. The micellar matrix served as the mobile phase and produced an enhancement effect on the luminescence signals. Equations were derived which allow one to determine alphacyclodextrin: substrate complex stoichiometries as well as primary and secondary binding constants by using LC retention values. An equation was also derived which describes the binding of a monoprotic species in which either its ionized or unionized form could bind to one or two cyclodextrin molecules. Becaused multiple binding constants are difficult to evaluate graphically, a non-linear least squares computer program was utilized. The approach works equally well forthe determination of binding cqnstants in micellar media. Resonance enhancement of Raman signals requires excitation on an absorption band of a molecule. This frequently produces background fluorescence from which it is difficult or impossible to extract a vibrational spectrum. Carrying out the resonance Raman analysis in certain dilute aqueous micellar solutions allows one to circumvent the luminescence problem in many cases. Several difÃerent micellar effects can be used simultaneously to enhance Raman signals relative to the background. Both the laser excitation line and the micellar system must be properly chosen so as to produce the best signal to noise ratio. The first examples of micelle mediated resonance Raman analysis of fluorescent compounds using UV and visible laser excitation was shown. Obtaining resonance Raman spectra from thin-layerchromatography plates was also demonstrated.Item Radiolysis of Amino Acids: A Study Using Raman Spectroscopy, Ultraviolet-Visible Spectrophotometry and Electrospray Ionization Mass Spectrometry(2014-12-17) Lou, JijieAmino acids are basic components of proteins and play a critical role in the development and treatment of chronic diseases. Raman spectroscopy, as a non-destructive tool with little sample preparation, has been widely used in vitro and vivo studies. In the current study, twenty amino acids in solid state and aqueous/0.5 M hydrogen chloride (HCl) solution were previously irradiated to 10 kGy, 25 kGy and 50 kGy by electron beam and analyzed using Raman spectroscopy, ultraviolet-visible (UV-Vis) spectrophotometry and electrospray ionization mass spectrometry (ESI-MS). Raman spectroscopy revealed spectral signatures of radiolysis of amino acids which related to the specific molecular vibration could be used for non-invasive analysis of radiation damage in vivo. Correlation between the absorbed dose and absorbance of amino acids was modeled by exponential saturation to explain the nature of radical production during irradiation. The results from ESI-MS revealed the modification of molecules after radiation. The relative stability to radiolysis of twenty amino acids was estimated from the results of those three techniques and compared with previous work. The ability of three techniques to identify and analyze the effects of radiolysis of amino acids in solution was evaluated. This first attempt to combine these three techniques to explore the radiation effect on amino acids, is promising for the further application in vivo studies.Item Raman studies of heavily carbon doped GaAs(Texas Tech University, 1999-08) Seon, MoonsukOptical measurements, mainly Raman spectroscopy, are used to study GaAs heavily doped with carbon. Hole concentration in these samples ranges from 2.3 x 10 to 1x10 cm^20^-3. Three main Raman features are investigated: the longitudinal-optic (LO) phonon mode, the substitutional carbon-at-arsenic local vibrational mode (CAS LVM), and the coupled plasmon-LO phonon mode (LOPC). CAS LVM intensity is directly proportional to carrier concentration. This implies that CAS LVM intensity is a good carrier density indicator, even though its practical use is limited by its weakness. Only one phonon-like coupled mode is observed due to the large plasmon damping and high effective carrier masses. The coupled mode is seen to systematically red shift as p increases even though the peak width of the mode stays constant. This behavior is described by a model which includes the effects of high hole concentrations on the dielectric function and an additional shift in the LO phonon we attribute to carbon size effect. Interestingly, the local mode intensity shows good correlation with that of LOPC mode as a function of p. Based on these results, the intensity of LOPC to that of the LO phonon is determined to be a good indicator of the carrier concentration. ILOPC/ILO decreases upon annealing, implying p reduction. Simultaneously, two peaks around 1375 and 1600 cm"' appear in all the annealed samples. The two peaks are assigned to carbon precipitates. From the observation, it is believed that carbon precipitates into a nanocrystalline graphite phase upon annealing. The crystal size of the carbon precipitates was estimated from the peak intensity ratio. Emission due to a conduction band to acceptor level transition (e.A) was observed from photoluminescence (PL) spectra. The band redshifts as carrier concentration increases due to band gap shrinkage. PL intensity of the (e.A) transition in annealed samples is drastically decreased. Carrier concentration reduction and the formation of nonradiative recombination centers are suggested as the cause of the behavior, and carbon precipitates observed in Raman spectra are suggested as the possible nonradiative recombination centers.Item Raman study of liquid methyl iodide and methyl fluoride(Texas Tech University, 1986-08) Zyung, TaehyoungThe DqukJs, CH3I and CH3F, were studied using a Raman spectrometer. The polarized and depolarized components of the V2 and V3 bands of CH3I were measured in the whole range of the temperature of the liquid. The recorded spectra were Fourier transformed to provide vibrational autocorrelation functions and memory functions. The vibrational autocorrelation functions and the experimental memory functions were analyzed by the methods of a memory function procedure and an autoregressive analysis, respectively. The Raman spectra of neat liquid CH3F were obtained as a function of temperature and the spectra in the dissolved state were also obtained. The spectra were explained qualitatively by using some theoretical models arid densities of states were calculated classically with a simple theoretical model in order to be compared with the observed spectra.Item The search for metastables and molecular ions in discharges(2002-05) McCluskey, Craig William, 1950-; Keto, John W.Improvements to and use of an existing Raman Induced Kerr Effect (RIKE) spectrometer [Bhatia et al., J. Opt. Soc. Am. B, 14(2):263–270, February, 1997.] are described. Primary improvements were the use of wedged windows on the sample chamber, a new method of monitoring birefringence, and the addition of a photomultiplier tube (PMT) and double monochromator for monitoring Coherent Anti-Stokes Raman Spectroscopy (CARS) signals. This spectrometer is controlled through a Computer Automation and Control (CAMAC) crate. The construction and operation of a Linear Discharge Cell (LDC), a High Voltage Constant Current Sink for consistent operation of the LDC, and a Transverse Electric Atmospheric (TEA) discharge chamber are described in detail, as is synchronization of the pulsed discharge in the TEA with the pulsed output of the YAG laser using Hewlett-Packard Versatile Link fiber optic components. The atmospheric gases oxygen, carbon dioxide, and nitrogen were investigated with CARS in both discharge and non-discharge conditions. The influence of nuclear spin on the spectra and line strengths observed for all three gases is discussed. The origins of oxygen’s triplet ground state are discussed as well as simultaneous transitions in the visible of two colliding, excited oxygen molecules whose individual energies are in the infrared. The oxygen metastable singlet delta was observed, though with insufficient signal-to-noise ratio to extract molecular constant information. Also discussed for carbon dioxide are the profusion of state naming conventions, Fermi splitting, the calculation of the temperature of the discharge, quantum interference in the change of relative intensity of the two peaks in the ν1/2ν2 Fermi dyad from non-discharge to discharge conditions, and upper level hotband lines that appear when the discharge is turned on. Quantum interference in carbon dioxide was consistently observed in the LDC but not in the TEA discharge, most likely because the amount of power dissipated in the TEA was on the order of 1% of that dissipated in the LDC and the gas temperature was much lower. The molecular radical N3 was sought without success, though spectrometer characteristics set an upper bound on its concentration in the discharge.Item Strain and modulation doping in epitaxial Si/Ge core-shell nanowire heterostructures(2015-12) Dillen, David Carl; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; Dodabalapur, Ananth; Yu, Edward T; Korgel, Brian AFor over five decades, silicon based electronics relied on scaling of individual field-effect transistors (FETs) for improvements in integrated circuit performance. Recently, however, further enhancement of packing density and switching speed was limited by the increase in power consumption of short channel devices. New materials and device geometries were introduced to help expand CPU performance while also decreasing power dissipation. Semiconducting nanowires have also been recognized for potential applications as channel material in highly scaled FETs. These structures present opportunities for strain and energy band engineering through the use of radial, or core-shell, heterostructures. To fully exploit the benefits of radial heterostructures, however, requires knowledge of elastic strain distributions and energy band alignments, necessitating the development of new characterization methods. This is especially true in Si/Ge material systems, where a large lattice mismatch over 4% is possible. In this thesis, we grow Si/Ge core-shell nanowires and demonstrate multiple techniques to characterize the nanoscale heterostructure, including strain measurements and extraction of valence band offsets. We grow Ge-SixGe1-x core-shell nanowires and measure the elastic strain using Raman spectroscopy. The Ge core’s Raman spectrum is consistent with a compressive strain in this region due to lattice mismatch with the SixGe1-x shell. The strain distribution and expected Raman peak positions are calculated using continuum elasticity models and lattice dynamic theory, finding excellent agreement to experimental data. We also demonstrate radial modulation doping in Ge-SixGe1-x core-shell nanowire heterostructures by doping a portion of the SixGe1-x shell with boron during growth. The modulation doped nanowire FETs show an enhanced low temperature hole mobility and also a decoupling of transport between core and shell. Through comparison to finite-element calculations, we extract the valence band offset at the core-shell interface. Lastly, we grow coherently strained Si-SixGe1-x core-shell nanowires and characterize the structure using Raman spectroscopy. We first optimize the Si nanowire growth process to favor the diamond crystal structure and to minimize sidewall coverage by Au catalyst, followed by epitaxial growth of the SixGe1-x shell using the Si nanowire as substrate. Raman measurements on core-shell samples indicate a tensile strain in the Si core and a compressive strain in the SixGe1-x shell, both consistent with calculations of the strain and the strain-induced shift of the Raman peaks in this structure.Item Surface enhanced Raman spectroscopy of olivine type battery cathode LiFePO4(2010-08) Delone, Nicholas Ryan; Stevenson, Keith J.; Vanden Bout, David A.This thesis explores the use of Raman Spectroscopy to study the battery cathode material LiFePO4. Surface Enhanced Raman Spectroscopy (SERS) was incorporated into the study due to fluorescence that traditionally plagues Raman. By imaging LiFePO4 nanoparticles, an understanding can be gained of the complex chemistry taking place when the material is lithiated and delithiated at the nanoscale level and the phase changes of the material that occur during this process. The use of bimetallic (Au/Ag) SERS substrates allowed for more stable substrates with longer shelf life compared single metal Ag substrates. Further tuning of these substrates can be applied to the ever evolving science of energy storage material technology as a way to track phase changes in the material.Item Synthesis and characterization of carbon nanotubes using scanning probe based nano-lithographic techniques(2009-05-15) Gargate, Rohit VasantA novel process which does not require the traditional Chemical Vapor Deposition (CVD) synthesis techniques and which works at temperatures lower than the conventional techniques was developed for synthesis of carbon nanotubes (CNT). The substrates used for this study involved MEMS (Micro Electrical Mechanical Systems) elements and passive elements. These were coated with Fullerene using Physical Vapor Deposition or through a solution in an organic solvent. Catalyst precursors were deposited on these Fullerene coated substrates using ?wet processes?. These substrates were then heated using either the integrated microheaters or external heaters in an inert atmosphere to obtain CNT. Thus, in this process we tried to obviate the Chemical Vapor Deposition (CVD) process for synthesis of CNT (SWCNT and MWCNT). The synthesized CNT will be characterized using Scanning Electron Microscopy and Raman spectroscopy techniques. Also, conductivity measurements were carried out for the synthesized tubes using Dry (contact based) and Wet (electro-chemical) methods. This work also proves the concept for the feasibility for a portable hand held instrument for synthesis of CNT with tunable ?on demand? chirality.Item Thermal-mechanical stress measurement and analysis in three dimensional interconnect structures with Raman spectroscopy(2013-12) Zhao, Qiu; Ho, P. S.; Huang, RuiThree-dimensional (3-D) integration as an effective method to overcome the wiring limit imposed on device density and performance with continued scaling. The application of TSV (Through Silicon Via) is essential for 3D silicon integration and 3D IC integration. TSV are embedded into the silicon substrate to form vertical, electrical connections between stacked IC chips. However, due to the large CTE mismatch between Silicon and Copper, thermal stresses are induced by various thermal histories from the process, and they have caused serious concerns regarding the thermal-mechanical reliability. In this thesis, we mainly used Micro- Raman spectroscopy characterized the thermal-mechanical stress distribution at both near surface and cross-section of Si around TSV. First we use the conventional Raman, to measure the linear combination of in-plane stress components. The local distribution of near-surface stress in Si has been measured, in comparison with stress analysis of the TSV structure based on a semi-analytic approach and finite element analysis. The effects of residual stress in surface oxide layer and the interaction of stress field of neighboring TSVs are evaluated experimentally. Second, the limitation of conventional Raman measurement is discussed, and two different kinds of innovative Raman measurements have been developed and employed to study the normal stress components separately by taking advantages of different laser polarization configuration. The top view Raman measurements utilize so called "High NA effect" to obtain additional information, and can resolve all 3 normal stress components near the top surface of silicon. From the cross-section Raman measurements, the in-plane stress distribution in Silicon on the cross-section can be determined with reasonable assumption based on FEA simulation. There shows good agreements between FEA simulation and experimental data in general.Item Thermo-mechanical stress analysis and interfacial reliabiity for through-silicon vias in three-dimensional interconnect structures(2011-12) Ryu, Suk-Kyu; Huang, Rui, doctor of civil and environmental engineering; Liechti, Kenneth; Landis, Chad; Ho, Paul S.; Im, Jang-HiContinual scaling of devices and on-chip wiring has brought significant challenges for materials and processes beyond the 32-nm technology node in microelectronics. Recently, three-dimensional (3-D) integration with through-silicon vias (TSVs) has emerged as an effective solution to meet the future interconnect requirements. Among others, thermo-mechanical reliability is a key concern for the development of TSV structures used in die stacking as 3-D interconnects. In this dissertation, thermal stresses and interfacial reliability of TSV structures are analyzed by combining analytical and numerical models with experimental measurements. First, three-dimensional near-surface stress distribution is analyzed for a simplified TSV structure consisting of a single via embedded in a silicon (Si) wafer. A semi-analytic solution is developed and compared with finite element analysis (FEA). For further study, the effects of anisotropic elasticity in Si and metal plasticity in the via on the stress distribution and deformation are investigated. Next, by micro-Raman spectroscopy and bending beam technique, experimental measurements of the thermal stresses in TSV structures are conducted. The micro-Raman measurements characterize the local distribution of the near-surface stresses in Si around TSVs. On the other hand, the bending beam technique measures the average stress and viii deformation in the TSV structures. To understand the elastic and plastic behavior of TSVs, the microstructural evolution of the Cu vias is analyzed using focused ion beam (FIB) and electron backscattering diffraction (EBSD) techniques. To study the impacts of the thermal stresses on interfacial reliability of TSV structures, an analytical solution is developed for the steady-state energy release rate as the upper bound of the driving force for interfacial delamination. The effect of crack length and wafer thickness on the energy release rate is studied by FEA. Furthermore, to model interfacial crack nucleation, an analytical approach is developed by combining a shear lag model with a cohesive interface model. Finally, the effects of structural designs and the variation of the constituent materials on TSV reliability are investigated. The steady state solutions for the energy release rate are developed for various TSV designs and via materials (Al, Cu, Ni, and W) to evaluate the interfacial reliability. The parameters for TSV design optimization are discussed from the perspectives of interfacial reliability.Item Ultrafast Cooperative Phenomena in Coherently Prepared Media: From Superfluorescence to Coherent Raman Scattering and Applications(2012-07-16) Gombojav, AriunboldTechnological progress in commercializing ultrafast lasers and detectors has allowed realization of cooperative processes on an ultrashort time scale, which demand a re-evaluation of the conventional cooperative phenomena with a new insight. Ultrafast cooperative phenomena in coherently prepared media and various applications of super?uorescence and coherent Raman scattering are studied in this dissertation. In particular, a simple theoretical testimony on analogy between a cooperative emission and coherent Raman scattering is presented by o?ering an opportunity to perform parallel research on these two processes from a uni?ed point of view. On one hand, the super?uorescent pulse with a time duration of a few tens of picoseconds (ps) from alkali metal vapor is observed for the ?rst time, even though cooperative phenomena in atomic vapor have been extensively studied for more than ?ve decades. A dense rubidium vapor pumped by ultrashort (100 femtosecond, fs) pulses allows a realization of the ultrafast super?uorescence while a time-resolved study of super?uorescence is accomplished by using a streak camera with 2 ps time resolution. Experimental research on quantum nature of cooperative emissions has been ?frozen? over the years (three decades) possibly because of the technical di?culties. Quantum ?uctuations of super?uorescence development are explored experimentally by taking advantage of the ultra fast streak camera. Presumable applications of the super?uorescent pulse in e.g., a remote sensing, and an ultraviolet upconversion of the input infrared laser pulse are presented. The quantum interference due to di?erent excitation pathways is revealed by the temporal coherent control technique while observing interferometric signals from alkali metal vapors. On the other hand, a new spectroscopic technique based on ultrafast coherent Raman scattering is developed. The key advantage of the presented technique is to suppress the non-resonant background noise which usually obscures possible applications of the other conventional coherent Raman techniques in practice. A reduction of the background noise is achieved by shaping and delaying the third pulse which probes the coherence of the medium (i.e., an enhancement of speci?c vibrations of the target molecules in unison) ?rstly prepared by two broadband pulses. We demonstrate a robustness and superiority of signal-to-noise ratio of the developed technique by identifying as few as 10000 bacterial spores at a single laser shot level. Finally, several comparative studies between cooperative and uncooperative processes are presented. A picosecond cooperative phenomenon in a three-photon resonant medium induced by a single as well as two-color ultrashort pulses is investigated. A time-resolved study shows that a picosecond cooperative e?ect is crucial in the well-established ?elds of resonant-enhanced multiphoton ionizations and harmonic generations. We also present a quantitative analysis for spontaneous versus broadband coherent Raman scattering on pyridine molecules. The spontaneous Raman signal is enhanced by 5 orders as a result of cooperative phenomena.