abstract: A highly uniform and repeatable method for synthesizing the single-layer transition metal dichalcogenides (TMDs) molybdenum disulfide, MoS2, and tungsten disulfide, WS2, was developed. This method employed chemical vapor deposition (CVD) of precursors in a custom built cold-wall reaction chamber designed to allow independent control over the growth parameters. Iterations of this reaction chamber were employed to overcome limitations to the growth method. First, molybdenum trioxide, MoO3, and S were co-evaporated from alumina coated W baskets to grow MoS2 on SiO2/Si substrates. Using this method, films were found to have repeatable coverage, but unrepeatable morphology. Second, the reaction chamber was modified to include a pair of custom bubbler delivery systems to transport diethyl sulfide (DES) and molybdenum hexacarbonyl (MHC) to the substrate as a S and Mo precursors. Third, tungsten hexacarbonyl (WHC) replaced MHC as a transition metal precursor for the synthesis of WS2 on Al2O3, substrates. This method proved repeatable in both coverage and morphology allowing the investigation of the effect of varying the flow of Ar, varying the substrate temperature and varying the flux of DES to the sample. Increasing each of these parameters was found to decrease the nucleation density on the sample and, with the exception of the Ar flow, induce multi-layer feature growth. This combination of precursors was also used to investigate the reported improvement in feature morphology when NaCl is placed upstream of the substrate. This was found to have no effect on experiments in the configurations used. A final effort was made to adequately increase the feature size by switching from DES to hydrogen sulfide, H2S, as a source of S. Using H2S and WHC to grow WS2 films on Al2O3, it was found that increasing the substrate temperature and increasing the H2S flow both decrease nucleation density. Increasing the H2S flow induced bi-layer growth. Ripening of synthesized WS2 crystals was demonstrated to occur when the sample was annealed, post-growth, in an Ar, H2, and H2S flow. Finally, it was verified that the final H2S and WHC growth method yielded repeatability and uniformity matching, or improving upon, the other methods and precursors investigated. / Dissertation/Thesis / Doctoral Dissertation Physics 2019

This diploma thesis deals with layers of tungsten oxide, which are prepared from a nanoparticle suspension and a binder. The coating composition consisted in the first case of ground tungsten oxide nanoparticles alone, then of acetylated peroxotungstic acid alone and finally of a mixture of ground tungsten oxide mixed with acetylated peroxotungstic acid. The compositions thus formed were applied to conductive FTO glass and thus act as photoanodes. A stainless steel needle is used as a counter electrode for volt-ampere measurements. The work focuses on the study of physicochemical properties of prepared photoanodes and their optimalization in order to obtain the highest possible values of generated photocurrents. The electrical properties of all cells were determined using voltammetric characteristics.

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In this paper, a study of tungsten metallization in advanced BEOL interconnects is presented. A mature 10 nm process is used for comparison between the tungsten and conventional copper metallization. Wafers were processed together till M1 dual-damascene etch then separated for different metallization. Tungsten metal line of 24 nm width is showing a 1.6X wire resistance comparing to the copper metal line. Comparable opens/shorts yield were obtained on a 0.8 M comb serpentine, Kelvin-via and 4K via chains. Similar physical profile were also achieved. This study has demonstrated the feasibility of replacing the copper by tungsten at BEOL using the conventional tungsten metallization tools and processes. This could be a cost- effective solution for the low-power products.

Plasma centrifugal atomization is a method widely used in the production of spherical powders of metals and alloys with relatively low melting points. A novel plasma centrifugal atomization process suitable for high melting point materials (i.e. 3500 ᵒC and above) was developed by Metasphere Technology AB, currently Höganäs Sweden AB. In this process, feedstock material in the form of crushed powder with particle sizes in the range 400-1000 µm is fed into a rotating crucible and subsequently melted by the glow discharge of a plasmatron. Due to high rotational speeds, a melt film forms at the edge of the crucible and breaks into fine droplets that are ejected into the reactor chamber and solidified in a whirl of cold inert gases. Capability of the plasmatron to reach very high temperatures, combined with extremely rapid cooling of the ejected droplets, allow for the fabrication of fine powders of refractory alloys exhibiting metastable phases that cannot be obtained otherwise.  Oil drilling, ore processing and metal shaping applications, among other, require tool materials capable of withstanding harsh working conditions under heavy loads. Owing to their physical, chemical and mechanical properties, tungsten-carbon alloys are among the most suited materials for such applications. Melting followed by rapid solidification of tungsten-carbon mixtures with 3.9 wt.% C results in a biphasic structure composed of WC lamellae inserted in a W2C matrix, known as cast tungsten carbide (CTC). Due to the metastable nature of both phases present, CTC exhibits exceptional mechanical properties. CTC is mainly used as reinforcing dispersed phase in metal matrix composite hardfacing overlays, which are deposited by plasma transferred arc (PTA) welding or laser cladding onto steel tools. High-entropy alloys (HEAs) are defined as multi-component solid solutions with equimolar or near-equimolar concentration of all principal elements. Owing to their outstanding mechanical, corrosion, erosion, oxidation and radiation resistance properties compared to conventional alloys, HEAs are among the most suited materials for aerospace and nuclear applications. Several processing routes have allowed for laboratory-scale production of HEAs. Nevertheless, size and shape of bulk components that can be thus produced are largely limited. In a quest for up-scaling the processing of high-end bulk HEA components, plasma centrifugal atomization of pre-alloyed refractory HEA spherical powders suitable for additive manufacturing was envisaged. In this work, capabilities of the novel plasma centrifugal atomization for processing of refractory alloys into fine spherical powders have been evaluated based on two different material systems, namely CTC and a refractory HEA containing Ti, V, Zr, Nb, Mo, Hf, Ta, W. Challenges of local mechanical characterization of micron-sized powders have been addressed and a robust method for testing of individual particles has been developed. Mechanical properties such as hardness and fracture toughness of plasma atomized CTC powders have been extensively investigated and related to the corresponding thermal stories. Experimental results suggest significant straining of the crystal lattice in the case of as-atomized CTC, possibly due to extremely high cooling rates experienced by the solidifying particles. This has been ruled out the main reason for the outstanding mechanical properties of plasma atomized CTC compared to both spheroidized CTC and conventional cast & crushed CTC. Effective stress relieve was possible upon heat treatment. Plasma atomization of the refractory HEA yielded similar results, where an extremely fine microstructure with no noticeable chemical segregation was obtained. Indentation hardness of this novel microstructure was found to be approximately 25% higher than that of similar alloys reported in literature. HEA powder thus produced was then consolidated into bulk HEAs with very simple geometries, proving that this powder can be further processed into components of more or less complexity for pre-defined applications.

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Plasma spray processing is a technique that is used extensively in thermal barrier coatings on gas and steam turbine components, biomedical implants and automotive components. Many processing parameters are involved to achieve a coating with certain functionality. The coating could be required to function as thermal barrier, wear resistant, corrosion resistant or a high temperature oxidation resistant coating. Various parameters, such as, nozzle and electrode design, powder feeding system, spray distances, substrate temperature and roughness, plasma gas flow rates and others can greatly alter the coating quality and resulting performance. Feedstock (powder or solution precursor) composition and morphology are some of the important variables, which can affect the high end coating applications. The amount of heat a plasma plume has to offer to the particles being processed as a coating depends primarily on the dissociation of the atoms of gaseous mixtures being used to create the plasma and the residence time required for the particle to stay in the flame. The parameters that are conducive for nanostructured retention could be found out if the residence time of the particles in the flame and the available heat in the plume for various gas combinations could be predicted. If the feedstock is a liquid precursor instead of a powder feedstock, the heat that has to be offered by the plasma could be increased by suitable gas combination to achieve a good quality coating. Very little information is available with regard to the selection of process parameters and processing of nano materials feedstock to develop nanostructured coatings using plasma spray. In this study, it has been demonstrated that nano ceramics or ceramic composites either in the form of coatings or bulk free form near net components could be processed using DC plasma spray. For powder feedstock, analytical heat transfer calculations could predict the particle states for a given set of parameters by way of heat input from the plasma to the particles. The parameter selection is rendered easier by means of such calculations. Alumina nano ceramic particles are processed as a coating. During Spray drying, a process of consolidation of nano alumina particles to spherical agglomerates, parameter optimization for complete removal of moisture has been achieved. The parameters are tested for alumina nanoparticles with a plasma torch for the veracity of calculations. The amount of heat transfer from the surface of the agglomerates to the core has been quantified as a function of velocity of particles. Since preparation of nanostructured feedstock for plasma spray is expensive and cumbersome, alternative solution precursor route for direct pyrolysis of precursor to coating has been studied in case of nanocrystalline rare earth oxides. Thus, it has also been shown by this research that nanostructured coatings could be either from a powder feedstock or a solution precursor feedstock. MoSi2-Si3N4, Ni-Al2O3, W-HfC nano ceramic composite systems have been processed as a bulk free form nanocomposite with 60-70% retained nanostructures. The importance of selection of substrates, roughness and the substrate temperature for development of free form bulk components has been highlighted. The improvement in mechanical and high temperature properties associated with having such nanostructured coatings or bulk nanocomposites are revealed. These nanostructured coatings are known for their low thermal conductivity, high wear resistance and can be potentially used as steam and gas turbines coatings for improved thermal efficiency. In summary, bulk nanocomposite through plasma spray processing is a viable alternative to conventional processes such as sintering, HIP for high fracture toughness and hardness applications.

Extensive attention has been paid to consolidate nanoparticles into nanocrystalline components that possess better properties than their coarse-grained counterparts. Nanocrystalline monolithic tungsten (W) has been envisaged to possess better properties than coarse-grained tungsten and to improve the performance of many military components. Commercially available nano-W powders were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and Brunauer, Emmett, and Teller (BET) measurement. While the bulk of nano-W powders consisted of bcc-W as confirmed by XRD and TEM, much of their surface consisted of WO3 with traces of WO2 and WC. Despite the irregular morphology and agglomerates greater than 1 m in size, the diameter of individual nano-W powders ranged from 30 to 100 nm with a surface area of 10.4 m2/g. To obtain green bodies of higher densities and more homogeneous microstructures after consolidation, W nanopowders were de-agglomerated in water and slip cast in plaster molds. De-agglomeration in water was conducted by repeated ultrasonication, washing, centrifuge and pH adjustment. The change in particle size and morphology was examined via SEM. After the initial surface oxide was removed by repeated washing, the reactivity of W nanoparticles to water was somewhat inhibited. Increasing the number of cycles for ultrasonication and washing increased the pH, the degree of de-agglomeration and the stability of W suspension. The zeta potential was more negative with increasing pH and most negative at pH values close to 5. Viscosity also decreased with increasing pH and reached a minimum at a pH 5. To obtain the highest solid loading with the lowest viscosity, the pH value of W suspension was adjusted to 5 using aqueous tetramethylammonium hydroxide solutions. The relative density of the slip cast increased with longer ultrasonic time, increasing slurry pH up to 5, and consequent increase in solids loading. Smaller particles were separated from larger ones by ultrasonication, washing with water and centrifugation. At a 27.8 vol.% solids loading, the size-separated fine W slurry was slip cast into pellets with relative green densities up to 41.3 % and approximate particle sizes of 100 nm. W powders were also ultrasonicated in aqueous poly (ethyleneimine) (PEI) solutions with various concentrations. SEM examinations of particle sizes showed that 1 wt.% PEI led to the optimum dispersion and ultrasonication for longer time with a low power resulted in better dispersion. 0.5 g of W powders were ultrasonicated in 10 ml aqueous poly (allylamine hydrochloride) (PAH) solutions with molar concentrations ranging from 0.01 to 0.05 M. W suspensions with 0.03 M and 0.04 M PAH after two washing cycles showed improved dispersion. Cold isostatic pressing can further increase the green density following slip casting. Sintered slip casts made from de-agglomerated nanoparticle W showed a lower density, more uniform microstructure, smaller grains and smaller pores than the sintered dry pressed pellets.