The integration of copper (Cu) and dielectric materials has been outlined in the International Technology Roadmap for Semiconductors (ITRS) as a critical goal for future microelectronic devices. A necessity toward achieving this goal is the development of diffusion barriers that resolve the Cu and dielectric incompatibility. The focus of this research examines the potential use of tungsten nitride as a diffusion barrier by characterizing the interfacial properties with Cu and evaluating its process capability for industrial use. Tungsten nitride (β-W2N) development has been carried out using a plasma enhanced chemical vapor deposition (PECVD) technique that utilizes tungsten hexafluoride (WF6), nitrogen (N2), hydrogen (H2), and argon (Ar). Two design of experiments (DOE) were performed to optimize the process with respect to film stoichiometry, resistivity and uniformity across a 200 mm diameter Si wafer. Auger depth profiling showed a 2:1 W:N ratio. X-ray diffraction (XRD) showed a broad peak centered on the β-W2N phase. Film resistivity was 270 mohm-cm and film uniformity < 3 %. The step coverage (film thickness variance) across a structured etched dielectric (SiO2, 0.35 mm, 3:1 aspect ratio) was > 44 %. Secondary ion mass spectroscopy (SIMS) measurements showed good barrier performance for W2N between Cu and SiO2 with no intermixing of the Cu and silicon when annealed to 390o C for 3 hours. Cu nucleation behavior and thermal stability on clean and nitrided tungsten foil (WxN = δ-WN and β-W2N phases) have been characterized by Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) under controlled ultra high vacuum (UHV) conditions. At room temperature, the Auger intensity ratio vs. time plots demonstrates layer by layer Cu growth for the clean tungsten (W) surface and three-dimensional nucleation for the nitride overlayer. Auger intensity ratio vs. temperature measurements for the Cu/W system indicates a stable interface up to 1000 K. For the Cu /WxN/W system, initial Cu diffusion into the nitride overlayer is observed at 550 K.

Tungsten (W) based materials can be used in fusion reactors due to several advantages. Different fabrication routes can be applied to develop tungsten materials with intended microstructure and properties for specific application including nanostructured grades. Therein, innovative chemical routes are unique in their approach owing numerous benefits. This thesis summarizes the development of W-based composites dispersed-strengthened by rare earth (RE) oxides and their evaluation for potential application as plasma facing armour material to be used in fusion reactor. Final material development was carried out in two steps; a) fabrication of nanostructured metallic tungsten powder dispersed with RE-oxides and b) powder sintering into bulk oxide-dispersed strengthened (ODS) composite by spark plasma process. With the help of advanced characterization tools applied at intermediate and final stages of the material development, powder fabrication and sintering conditions were optimized. The aim was to achieve a final material with a homogenous fine microstructure and improved properties, which can withstand under extreme conditions of high temperature plasma. Two groups of starting materials, synthesized via novel chemical methods, having different compositions were investigated. In the first group, APT-based powders doped with La or Y elements in similar ways, had identical particles’ morphology (up to 70 μm). The powders were processed into nanostructured composite powders under different reducing conditions and were characterized to investigate the effects on powder morphology and composition. The properties of sintered tungsten materials were improved with dispersion of La2O3 and Y2O3 in the respective order. The oxide dispersion was less homogeneous due to the fact that La or Y was not doped into APT particles. The second group, Ydoped tungstic acid-based powders synthesized through entirely different chemistry, contained nanocrystalline particles and highly uniform morphology. Hydrogen reduction of doped-tungstic acid compounds is complex, affecting the morphology and composition of the final powder. Hence, processing conditions are presented here which enable the separation of Y2O3 phase from Y-doped tungstic acid. Nevertheless, the oxide dispersion reduces the sinterability of tungsten powders, the fabricated nanostructured W-Y2O3 powders were sinterable into ultrafine ODS composites at temperatures as low as 1100 °C with highly homogeneous nano-oxide dispersion at W grain boundaries as well as inside the grain. The SPS parameters were investigated to achieve higher density with optimum finer microstructure and higher hardness. The elastic and fracture properties of the developed ODS-W have been investigated by micro-mechanical testing to estimate the materials’ mechanical response with respect to varying density and grain size. In contrast from some literature results, coarse grained ODS-W material demonstrated better properties. The developed ODS material with 1.2 Y2O3 dispersion were finally subjected to high heat flux tests in the electron beam facility “JUDITH-1”. The samples were loaded under ELM-like thermal-shocks at varying base temperatures up to an absorbed power density of 1.13 GW/m2, for armour material evaluation. Post mortem characterizations and comparison with other reference W grades, suggest lowering the oxide contents below 0.3 wt. % Y2O3. As an overview of the study conducted, it can be concluded that innovative chemical routes can be potential replacement to produce tungsten based materials of various composition and microstructure, for fusion reactor applications. The methods being cheap and reproducible, are also easy to handle for large production at industrial scale. / <p>QC 20120827</p>

Thesis (Ph.D.(Chemistry))--University of Pretoria, 2004. / Includes summary. Also available online.

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Student Number : 0420699F - MSc Dissertation - School of Physics - Faculty of Science / This dissertation presents for the first time the conditions for the synthesis of VO2 by ultrasonic nebula-spray pyrolysis (UNSP) from a precursor solution of NH4VO3+VCl3 optimized as follows: a carrier gas of argon at a flow rate of 11 liters per minute, a furnace temperature of 400 to 700oC. This work also incorporates thermodynamic variables of Tpr-P-V into the equations that relate the mean diameter of droplets, D, to frequency of the exciting ultrasound waves, f, the density of the precursor solution, #26; and the surface tension, #27;, previously worked on independently by Lang and Jokanovic. The incorporation results in the diameters of the droplets (and consequently the collected grains) being smaller as p and Tpr are increased in a non- linear form. The variable V, however, increases the diameter of the droplets as it is allowed to increase. This study shows the departures many authors find of the theory from experiment but it also shows that the departure does not lie in the equations but rather on post- synthesis and annealing effects. From X-ray diffraction, scanning electron microscopy (SEM) and Raman spectroscopy, this study shows that as furnace temperature is increased the morphology of the sample surfaces for both VO2 and WO3 transforms from amorphous to crystalline, from spherical grains to plate-like structures, with grain mean diameter increasing non-linearly in some cases and decreasing non-linearly in other cases confirming previous findings, the latter enjoying the majority vote. In Raman spectra of the as- obtained WO3, asymmetric broadening of the Raman peaks was observed in some samples and a phonon confinement model was employed in the size distribution prediction. These findings prompted the re- workout of the phonon confinement model. In this dissertation an equation has been derived based on the Faucet-Campbell equation of the PC model. The new equation relates the ratio of neighboring peaks in a material’s Raman spectrum to the mean diameter of the grains. The present modification allows the PCM model to predict the grain size beyond the current limiting range of 0 to 100 nm. Analysis of the experimental data using this equation unveils two different equations- one for particles of size below 100 nm and the other equation for particles with larger that 100 nm. Also this analysis has enabled the present study to evaluate the phonon dispersion relations for WO3.

Master of Science / Department of Geology / Saugata Datta / Tungsten (W) has become an element of greater concern in recent years. Investigations by the Centers for Disease Control implicated W as a possible link to several cases of childhood acute lymphatic leukemia (ALL) clusters in the western United States. In Fallon NV, 17 cases of ALL were reported from 1997-2001. Previously, it was difficult to attain knowledge about the geochemical behavior of W due to low concentrations and difficulties in detection in natural environments. Modern analytical techniques allow for a greater range of sensitivity, allowing for in depth W analysis. Elucidating information on the factors contributing to the fate and transport of W in low temperature environments will provide insight into how W moves through the environment and provide information to help mitigate W contamination in the future. Three sites were chosen for comparison of W concentration and how that may be linked to local geochemical factors. Fallon NV, Sierra Vista AZ, and Cheyenne Bottoms Refuge KS were chosen based on published literature and personal communications. The objectives for this study were to characterize W concentrations in these three climatologically distinct areas followed by using methods to speciate and semi-quantitatively characterize W phase association within the surficial sediments and using synchrotron X-Ray methods to define W valencies and elemental associations within the sediments. Tungsten occurs in varying concentrations in the study areas, from 17.8 mg/kg to ~25,907 mg/kg. Fallon has the highest average W concentration of the three sites as well as the highest amount of W associated with phases other than the organic matter or residual phase. Speciation of soluble W revealed no polytungstates, however tungstates are present in the samples as well as undefined W species, suggesting there are other forms of W that are readily soluble in water, hence bioavailable. Tungsten has a very heterogeneous distribution in sediments, creating dispersed but highly concentrated clumps of W hotspots. Spot analyses under X-Ray mapping reveal W may co-localize with other metals such as Ti, Co, and Zn.

Although the terrestrial flux of solar energy is enough to support human endeavors, storage of solar energy remains a significant challenge to large-scale implementation of solar energy production. One route to energy storage involves the capture and conversion of sunlight to chemical species such as molecular hydrogen and oxygen via water splitting devices. The oxygen evolution half-reaction particularly suffers from large kinetic overpotentials. Additionally, a photoactive material that exhibits stability in oxidizing conditions present during oxygen evolution represents a unique challenge for devices. These concerns can be potentially addressed with a metal oxide photoanode coupled with efficient water oxidation electrocatalysts. Despite decades of research, structure-composition to property relationships are still needed for the design of metal oxide oxygen evolution materials. This dissertation investigates transition metal oxide materials for the oxygen evolution portion of water splitting devices. Chapter I introduces key challenges for solar driven water splitting. Chapter II elucidates the growth mechanism of tungsten oxide (WOX) nanowires (NWs), a proposed photoanode material for water splitting. Key findings include (1) a planar defect-driven pseudo-one-dimensional growth mechanism and (2) morphological control through the supersaturation of vapor precursors. Result 1 is significant as it illustrates that common vapor-phase syntheses of WOX NWs depend on the formation of planar defects through NWs, which necessitates reconsideration of WOX as a photoanode. Chapter III presents work towards (1) single crystal WOX synthesis and characterization and (2) WOX NW device fabrication. Chapter IV makes use of the key result that WOX NWs are defect rich and therefore conductive in order to utilize them as a catalyst scaffold for oxygen evolution in acidic media. Work towards utilizing NW scaffolds include key results such as stability under anodic potentials and strongly acidic conditions used for oxygen evolution. Chapter V includes work characterizing nickel oxide/oxyhydroxide oxygen evolution catalysts at near-neutral pH. Key findings include (1) previous reports of anodic conditioning resulting in greater catalytic activity are actually due to incidental incorporation of iron impurities from solution and (2) through intentional iron incorporation via electrochemical co-deposition, catalytic activity is increased ~50-fold over Fe-free catalysts. This dissertation contains previously published coauthored material.

Lam Lung Yeung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1989. / Bibliography: leaves 73-76.

β-W is an A15 structured phase commonly found in tungsten thin films together with the bcc structured W, and it has been found that β-W has the strongest spin Hall effect among all metal thin films. Therefore, it is promising for application in spintronics as the source of spin-polarized current that can be easily manipulated by electric field. However, the deposition conditions and the formation mechanism of β-W in thin films are not fully understood. The existing deposition conditions for β-W make use of low deposition rate, high inert gas pressure, substrate bias, or oxygen impurity to stabilize the β-W over α-W, and these parameters are unfavorable for producing β-W films with high quality at reasonable yield. In order to optimize the deposition process and gain insight into the formation mechanism of β-W, a novel technique using nitrogen impurity in the pressure range of 10-5 to 10-6 torr in the deposition chamber is introduced. This techniques allows the deposition of pure β-W thin films with only incorporation of 0.4 at% nitrogen and 3.2 at% oxygen, and β-W films as thick as 1μm have been obtained. The dependence of the volume fraction of β-W on the deposition parameters, including nitrogen pressure, substrate temperature, and deposition rate, has been investigated. The relationship can be modeled by the Langmuir-Freundlich isotherm, which indicates that the formation of β-W requires the adsorption of strongly interacting nitrogen molecules on the substrate. The dependence of β-W formation on the choice of underlayer materials has also been investigated. The β-W phase can only be obtained on the underlayer materials containing non-metallic elements. The dependence is explained by the existence of strong covalent bonds in β-W compared with that in α-W. The nickel and permalloy underlayers are the only exception to the above rule, and β-W has been successfully deposited on permalloy underlayer using very low deposition rate for spin-diffusion length measurement of β-W. The permalloy thin films usually take the (111) texture, since its (111) planes have the lowest surface energy. However, permalloy thin films deposited on β-W underlayer can achieve (002) texture using amorphous glass substrates. Therefore, the permalloy/β-W bilayer system can work as a seed layer for the formation of (002) textured films with fcc or bcc structure. The mechanism of the (002) texture formation cannot be explained by the existing models. The β-W to α-W phase transition was characterized by differential scanning calorimetry. The enthalpy of transformation is measured to be 8.3±0.4 kJ/mol, consistent with the value calculated using density functional theory. The activation energy for the β-W to α-W phase transformation kinetics is 2.2 eV, which is extremely low compared with that of lattice and grain boundary diffusion in tungsten. The low activation energy might be attributed to a diffusionless shuffle transformation process.