UNT Theses and Dissertations - 8 Matching Results

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Selectivity Failure in the Chemical Vapor Deposition of Tungsten

Description: Tungsten metal is used as an electrical conductor in many modern microelectronic devices. One of the primary motivations for its use is that it can be deposited in thin films by chemical vapor deposition (CVD). CVD is a process whereby a thin film is deposited on a solid substrate by the reaction of a gas-phase molecular precursor. In the case of tungsten chemical vapor deposition (W-CVD) this precursor is commonly tungsten hexafluoride (WF6) which reacts with an appropriate reductant to yield metallic tungsten. A useful characteristic of the W-CVD chemical reactions is that while they proceed rapidly on silicon or metal substrates, they are inhibited on insulating substrates, such as silicon dioxide (Si02). This selectivity may be exploited in the manufacture of microelectronic devices, resulting in the formation of horizontal contacts and vertical vias by a self-aligning process. However, reaction parameters must be rigorously controlled, and even then tungsten nuclei may form on neighboring oxide surfaces after a short incubation time. Such nuclei can easily cause a short circuit or other defect and thereby render the device inoperable. If this loss of selectivity could be controlled in the practical applications of W-CVD, thereby allowing the incorporation of this technique into production, the cost of manufacturing microchips could be lowered. This research was designed to investigate the loss of selectivity for W-CVD in an attempt to understand the processes which lead to its occurrence. The effects of passivating the oxide surface with methanol against the formation of tungsten nuclei were studied. It was found that the methanol dissociates at oxide surface defect sites and blocks such sites from becoming tungsten nucleation sites. The effect of reactant partial pressure ratio on selectivity was also studied. It was found that as the reactant partial pressures are varied there are significant changes in the ...
Date: August 1994
Creator: Cheek, Roger W. (Roger Warren)
Partner: UNT Libraries

Spectroscopic Properties of Polycyclic Aromatic Compounds

Description: The fluorescence spectrum of many polycyclic aromatic compounds (PACs) depends upon solvent polarity. The emission spectrum of PAC monomers consists of several major vibronic bands labeled I, II, etc., in progressive order. Emission intensity enhancement of select bands is observed in polar solvents.
Date: May 1994
Creator: Tucker, Sheryl A. (Sheryl Ann)
Partner: UNT Libraries

A Study of the Processing Properties of Hard-Particle Reinforced Composite Solders

Description: The microstructural, mechanical and thermal properties of various composite solder formulations were investigated. Special interest was given in observing the processing properties, microstructural characteristics, fatigue behavior, tensile strength, and the effect of environmental ageing on the composite solder formulations. The solderability parameters wetting and speed of soldering, reflow temperature, and the thermal stability of the resulting composite solder were also examined.
Date: May 1994
Creator: Calderon, Jose Guadalupe
Partner: UNT Libraries

Substitution Chemistry of the Cobalt Complexes [Co₂(CO)₆(PhC≡CR) (R=Ph, H) and PhCCo₃(CO)₉] with the Diphosphine Ligands [Bis(diphenylphosphino)maleic Anhydride (BMA) and (Z)-Ph₂PCH=CHPPh₂]. Reversible Chelate-to-Bridge Diphosphine Ligand Exchange, Phosphorus-Carbon Bond Cleavage and Phosphorus-Carbon Bond Formation

Description: The tricobalt cluster PhCCo3(CO)9 (1) reacts with the bidentate phosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma) in the presence of added Me3NO to give the diphosphine-substituted cluster PhCCo3(CO)7(bma) (2). Cluster 2 is unstable in solution, readily losing CO to afford Co3(CO)6[(μ2-η2/η1-C(Ph)C=C(PPh2)C(O)OC(O)](μ2-PPh2) (3) as the sole observed product. VT-31P NMR measurements on cluster 2 indicate that the bma ligand functions as both a chelating and a bridging ligand. At -97 °C, 31P NMR analysis of 2 reveals a Keq of 5.7 in favor of the bridging isomer. The bridged bma cluster 2 is the only observed species above -50°C. The solid-state structure of 2 does not correspond to the major bridging isomer observed in solution but rather the minor chelating isomer. The conversion of 2 to 3 followed first-order kinetics, with the reaction rates being independent of the nature of the reaction solvent and strongly suppressed by added CO, supporting a dissociative loss of CO as the rate-determining step. The activation parameters for CO loss were determined to be ΔH≠ = 29.9 ± 2.2 kcal/mol and ΔS≠ = 21.6 ± 6 eu.
Date: December 1994
Creator: Yang, Kaiyuan
Partner: UNT Libraries

The Synthesis and Chemistry of Polyciclic Cage Compounds

Description: Chapter I describes the synthesis of a trishomocubyl helical tubuland diol and some aspects of its inclusion chemistry. Thus, all three isomers of 4,7-dimethylpentacyclo[6.3.0.0^2,6.0^3,10.0^5,9]undecane-4,7-diol have been prepared and their X-ray structures have been determined. The syn,syn-isomer crystallizes in a double-stranded hydrogen-bonded lattice, while anti,syn-isomer forms a hydrogen-bonded layer lattice. In contrast, the anti,anti-isomer is a new member of the helical tubuland diol host family; its crystal lattice consists of parallel canals with a trefoil-shaped cross-section of area 25.4 Å^2. Chapter II describes the synthesis of new molecular clefts. These molecular clefts have been synthesized via base-promoted reactions of 3,6-diaryl-l,2,4,5-tetrazines with tetracyclo[6.3.0.0^4,11.0^5,9]undecane-3,6-dione and with tricyclo[6.3.0.0^2,6]undecane-3,11-dione, respectively. Compounds of this type are of interest as a potential new class of host molecules for use in host-guest complexation studies. Chapter III reports the synthesis of stereospecifically deuterated spiro(oxetane-3,8'-pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecanes) and their acid-promoted ring opening and concomitant rearrangements. The deuterium-containing reaction products have been characterized via analysis of their NMR and mass spectra. The results strongly suggest that intramolecular 1,5-hydride shifts provide an important pathway through which the acid promoted rearrangements occur. Chapter IV reports the oxidation of heptacyclo-[6.6.0.0^2,6.0^3,13.0^4,11.0^5,9.0^10,14] tetradecane (HCTD) via application of Barton's "GoAgg" systems. The products have been characterized by NMR and mass spectral analysis. Under GoAgg^v conditions, oxidation of HCTD proceeds to afford heptacyclo [6.6.0.0^2,6.0^3,13.0^4,11.0^5,9.0^10,14]tetradecan-7-one in 1% yield.
Date: December 1994
Creator: Wang, Yanjun
Partner: UNT Libraries

Thermophysical and Mechanical Properties of Polymer Liquid Crystals and Their Blends

Description: Tensile properties, namely the elastic modulus, tensile strength, percent of elongation at yield and at the break were determined for the pure components and blends. The results are connected to the respective phase diagrams and demonstrate that blending makes property manipulation possible. Blends for which the mechanical properties are better than those of pure EPs can be obtained.
Date: May 1994
Creator: López, Betty Lucy
Partner: UNT Libraries