Intermolecular interactions govern molecular self-recognition and self-assembly, giving rise to 3-dimensional solids. Thus, independent intermolecular interactions should result in a predictable crystal packing. Our working hypothesis is the crystal packing of symmetrically substituted tetra-n-alkoxy-bis-indane-piperazinediones of H-shaped topography will be governed by three factors: First, assembly of molecules into "one-dimensional" tapes through establishment of reciprocal amide hydrogen bonds. Second, assembly of the tapes into "two-dimensional" grooved sheets through establishment of edge-to-face arene interactions. Third, assembly of the sheets into "three-dimensional" solids by "tongue-in-groove" interdigitation of the n-alkyl "tails" in extended conformations independent of chain length. Liquid crystal properties are anticipated for molecules with hydrocarbon chains of sufficient length. Rupture of one or more of the three structure-determining interactions may be independent of the others and observable. "Dissection" of the melting process by correlation of crystallographic data with thermochemical data would permit formulation of a mechanism for melting. Compounds of this homologous series with methoxy, ethoxy, butoxy, hexyloxy, octyloxy and nonyloxy substituents have been synthesized, crystallized, and studied by X-ray crystallography, differential scanning calorimetry (DSC) and optical microscopy. The solid-state structures for the first three in the series were determined and each engaged in intermolecular amide-to-amide hydrogen bonding interactions that established "ladder-like" parallel tapes. Intertape organization was governed by the development of arene and/or van der Waals contact interactions. The melting behaviors of the series were remarkable in that these were not only high melting solids but most also demonstrate transitions by DSC at lower temperatures and magnitude consistent with liquid crystalline behavior. Also remarkable and consistent with liquid crystalline behavior are the optical properties of the longer alkoxy substituted compounds of the series, demonstrating birefringence of cross-polarized fight. In the case of the tetra-n-hexyloxy compound a transition was observed by optical microscopy that correlates with DSC data. Design, syntheses and characterizations of this new family of compounds, which are highly ordered at the molecular level, supporting our hypothesis of liquid crystalline behavior, are discussed. Using these data a mechanism of melting is postulated and discussed.

Thin ceramic substrates are widely used in engineering applications in modern industry. For example, they are used as molecular filters in fuel cells and solid oxide electrolyzers for oxygen generation. Development of high-reliability substrate materials inevitably requires the accurate characterization of their mechanical properties. The loading conditions in service on the ceramic substrates, such as the solid oxide electrolytes with a thickness of much less than 2 mm, often involve multiaxial bending instead of simple tension or bending. In this dissertation, the ASTM standard piston-on-3-ball experimental technique at ambient temperature is employed to investigate the quasi-static biaxial flexural strength of pure 8YSZ and Al₂O₃ or 3YSZ doped 8YSZ ceramic substrates. Furthermore, this piston-on-3-ball experimental technique is developed into a dynamic piston-on-3-ball technique at ambient temperature and a quasi-static piston-on-3-ball technique at elevated temperatures. Stress distribution functions in the tensile surface of a specimen under piston-on-3-ball loading condition are formulated and used to develop statistical models, which are proven to be in the form of a Weibull distribution function, to describe the biaxial flexural strength behavior of ceramic substrates under piston-on-3-ball loading condition. Analytical modeling was conducted on the dynamic piston-on-3-ball loading configuration. This analytical model can be used to guide the experimental design and judge the validity of experimental results. A new material constitutive model is developed to give a good description of the dynamic strength behavior of ceramic materials under constant stress-rate loading. Quasi-static experiments under piston-on-3-ball loading are conducted at both ambient temperature and elevated temperatures, while dynamic experiments are conducted at ambient temperature. Experimental results, as well as observations from SEM microstructure images and values of fracture toughness measured using a newly developed Vickers micro-indentation toughness technique, lead to a conclusion that no obvious overall improvement to the SYSZ ceramic substrates in the biaxial flexural strength can be observed by adding Al₂O₃ additive with amount up to 3 mol% or 3YSZ additive with amount up to 30 wt%.

The optical properties of planar semiconductor microstructures and three-dimensional nanostructures containing narrow linewidth In₀.₀₄Ga₀.₉₆As quantum wells are studied in this dissertation. The interaction of quantum-well excitons with light in environments different from free space gives a pronounced effect on the optical response. N periodically arranged quantum wells are coupled to each other by light leading to N exciton-polariton eigenmodes. Each eigenmode is characterized by a distinct energy and radiative lifetime depending on the periodicity of the quantum wells. For a period of about half the excitonic transition wavelength, linear measurements of reflection, transmission, and absorption show significant features of the light-coupled eigenmodes. At Bragg periodicity, the oscillator strengths of all quantum well excitons are concentrated into one superradiant mode resulting in an N times increased radiative decay rate. The slope of the reflectivity linewidths versus N gives the radiative linewidth of the quantum well exciton. For off-Bragg periodicity, however, other eigenmodes become optically active and show their features in reflection and absorption spectra. Oxide-aperture three-dimensional nanocavities containing a single quantum well are investigated. The discrete transverse modes due to the lateral confinement of the optical field are observed in empty cavities with various aperture sizes. The linewidth measurements of the cavity modes show quality-factor values around 2000 for aperture diameters down to 2 μm. This is high enough to give a strong light-coupling effect in the nonperturbative regime, named normal mode coupling or vacuum Rabi splitting. The anti-crossing behavior of exciton and cavity modes for a 2 μm diameter aperture cavity is measured in transmission by temperature tuning of the exciton resonance through the lowest transverse cavity mode. A minimum splitting value of 3.9 meV and a splitting-to-linewidth ratio of 4.9 are obtained. Then, nonlinear pump-probe measurements on nanocavities with several aperture sizes are performed. The transition from the nonperturbative regime to the weak coupling regime is observed as the pump power increases. From the measured saturation powers for various aperture diameters, a photon density of 90 photons/μm² is found necessary to saturate the normal-mode peaks. The effect of quantum fluctuations of the light field in the nonperturbative regime of planar semiconductor microcavities containing quantum wells is studied. A pronounced third transmission peak lying spectrally between the two normal modes is observed in resonant single-beam-transmission and pump-probe measurements. Measurements on three-dimensional nanocavities confirm the important role of guided modes for this intriguing effect.

Surface micromachining using deposited polysilicon films is a technology that is widely used for the fabrication of micro-electromechanical structures. One of the biggest yield and reliability problems in the fabrication of such structures is "stiction" or adhesion to the substrate. This may occur during the drying step that is required after wet processing and/or during use of a device. Deposition of self-assembled monolayer coatings is one of the most successful approaches to chemical modification of silicon surfaces to reduce stiction. This approach involves making the surfaces of pre-oxidized polysilicon highly hydrophobic. As a result, microstructures come out of the final water rinse extremely dry without being broken or adhered to the substrate. Available technology requires that these coatings are applied from organic media . However, increasing pressure on semiconductor companies to reduce the generation of organic wastes has sparked interest in the feasibility of applying these coatings from aqueous media. The objective of this research was to develop the chemistry and techniques for the application of hydrophobic coatings on polysilicon from aqueous media. The results obtained from three commercially available water dispersible silanes and cationic alkoxysilanes are discussed. Key experimental variables that were investigated are concentration of reactive silane, type of oxidation pretreatment of polysilicon, pH and temperature of the silane dispersion and curing temperature of the coating. The stability of the dispersions was characterized by viscosity measurements. The formation and quality of the films were studied using atomic force microscopy (AFM), ellipsometry, dynamic contact angle measurements and electrochemical impedance spectroscopy (EIS). The coatings showed contact angles greater than 100°. It was found using AFM that the structure of these films is a continuous film with some particulates attributed to bulk polymerization of the precursor molecule in water. EIS results indicated that the coatings had low porosity as well as high charge transfer resistance across the silicon/HF interface. Ellipsometric analysis showed that thickness of these coatings is roughly a (statistical) monolayer. The stability improvement of the dispersions by the addition of quaternary ammonium cationic surfactants is also discussed.

Effects of pore, secondary dendrite arm spacings (SDAS ), hot isostatic pressing (Hipping), and strontium-modification on fatigue behavior were studied in an aluminum casting alloy (A356.2). Microstructures were revealed by X-ray radiography, light microscopy and scanning electron microscopy. Small-cracks were monitored by taking replicas of the surfaces with which the cracks intersected. As the SDAS increases from 15 to 55 μm, fatigue life decreases by a factor of 3 in low-cycle fatigue, and 100 in high-cycle fatigue. When SDAS is less than 30 mum, the pore size is below a critical size of ∼80 μm and large eutectic constituents initiate cracks; and the initiation life is as high as 70% of the fatigue life. As the SDAS increases beyond 30 μm, pores are the main crack-initiation sites; the initiation life is as low as 5% of the fatigue life. Near-surface oxides initiate the fatigue crack regardless of SDAS. When crack initiated at pore and oxides, fatigue life is well correlated with the size of the initiation site and the effect of SDAS is overshadowed by the effect of pore. Non-hipped A356.2 without Sr shows better fatigue life and the deleterious effect of pores overshadowed the beneficial effect that Sr-modification might have had. Hipping significantly increased the initiation life and small-crack propagation life of A356.2 with Sr as a result of the elimination of the porosity. However, hipping did not significantly improve the fatigue life of A356.2 without Sr. After hipping, Sr-modification is beneficial in improving the crack initiation life, and increasing both small-crack and long-crack propagation lives. Fracture mechanics models (Newman-Raju, and Trantina-Barishpolsky models) yielded similar results on the crack-propagation rate against the effective stress-intensity factor range. In the micro-mechanics model, the theory of continuously distributed dislocations was applied to represent crack and crack-tip plastic zone, and the propagation rate was related to the length of the crack-tip plastic zone. When the grain size is used as the characteristic length of the microstructures, the model predicts the oscillations of the propagation rates and the predicted rates agreed reasonably well with those from experiments.

This dissertation describes research conducted to apply the Fractional Step Method to finite-element simulations of directional solidification. The Fractional Step Method (FSM) is also referred to as a projection method and as a splitting method, and has been applied commonly to high Reynolds number flow simulations. However, it is less common for low Reynolds number flows, such as occur in an alloy undergoing directional solidification (DS). The FSM offers increased speed and reduced memory requirements by allowing non-coupled solution of the pressure and velocity components. The FSM provides significant benefits for predicting flows in a DS alloy, since other methods presently employed are not computationally efficient. Previously, the most suitable finite-elements based methods for predicting flow in a DS alloy has been the penalty method for two-dimensional simulations and Galerkin least-squares (GLS) for three-dimensional simulations. The penalty method and GLS have the disadvantage that they require the coupled solution of the velocity components. The FSM allows decoupled iterative solution of the finite element equations, thereby greatly increasing the efficiency of the method, both in terms of memory and CPU requirements. Numerical simulations are now commonly used to predict macrosegregation in directionally solidified (DS) castings, which are used in jet and spacecraft engines. In particular, the finite-element simulations can predict the existence of "channels" within the processing mushy zone and subsequently "freckles" within the fully processed solid, which are known to result from macrosegregation. This macrosegregation is a direct result of thermosolutal convection of the melt during the solidification process. Freckles cause strong material non-uniformities in the castings that are therefore scrapped. The phenomenon of thermosolutal convection in an alloy undergoing DS is explained, along with applications for DS alloys. Next, the momentum and continuity equations for a binary alloy undergoing DS, and the application of the FSM to these equations are presented, along with characteristics of the FSM that make its application to DS challenging. Finally, results of applying the FSM to simulations of DS in a binary alloy are given for two-dimensional and three-dimensional geometries, including performance improvements over methods previously applied.

This dissertation describes the design and manufacturing techniques of antireflection coatings for the optical components of the MAESTRO spectrograph. This work was motivated by the need for a high-performance, robust and inexpensive anti-reflection (AR) coating for deep-space, faint object astronomical observations. I have created a new line of AR-filters, optimal for several different glass substrates, with a small number of layers. The possible ease of mass-manufacture of the CoyoteRTM coating is also illustrated here. I also describe a possible new technique for overcoating aluminum and silver reflective films on glass substrates, as well as future directions for research.

Chemical mechanical polishing (CMP) of dielectric and metal films has become a key process in manufacturing devices with ultra large scale integration (ULSI). In a CMP process, planarization is achieved by polishing a wafer with uneven topography using colloidal slurry consisting of sub-micron sized abrasive particles, oxidant and various additives. Hydrogen peroxide and hydroxylamine are commonly used oxidants in copper CMP process. To achieve planarization, the low lying areas have to be protected while the higher areas are polished away. This requires low static dissolution rate of copper in low areas. Another major issue in copper CMP is galvanic corrosion during barrier polishing step where both copper and the barrier metal are exposed to the slurry. The main goal of the research reported in this dissertation is to understand the dissolution and corrosion issues during the removal of copper in hydroxylamine based chemistries. Electrochemical and physical methods such as profilometry were used to obtain copper removal rates. Among the variety of organic compound tested, benzotriazole and salicylhydroxamic acid were identified as potential corrosion inhibitors for copper. The passive film formed on the copper surface by the addition of benzotriazole and salicylhydroxamic acid was characterized by X-ray photoelectron spectroscopy and atomic force microscopy. The passivation and repassivation kinetics were investigated in detail and a passivation mechanism for copper in hydroxylamine in the presence of benzotriazole and salicylhydroxamic acid chemistries is proposed. Copper removal experiments were performed on a specially designed electrochemical abrasion cell (EC-AC) in the presence and absence of inhibitors. The effect of anodic potentials on the dissolution of copper in various chemistries was studied to identify suitable conditions for electro-chemical mechanical planarization process. The extent of galvanic corrosion between copper and tantalum was estimated using electrochemical polarization measurements. A novel setup was designed to make direct measurement of the galvanic current between copper and tantalum and was successfully used to measure galvanic current in various chemical systems. CMP and post CMP cleaning operations account for almost twenty five percent of the total water usage at semiconductor fabrication plants. The waste water has to be treated to remove copper and unused oxidants and organic additives before it can be recycled or disposed. Fundamental studies on the treatment of copper CMP waste water using boron doped diamond electrodes was performed. The feasibility of copper deposition and organic oxidation was established and a design for a novel reactor is proposed.

Rutherford backscattering spectrometry (RBS) is a very versatile and popular technique in materials characterization. ⁴He⁺ ion beams in the keV or MeV range have been widely used to obtain quantitative information regarding the composition and depth distribution of elemental constituents and impurities in thin films. However, in many cases, RBS is ineffective for light element analysis due to overlapping signals caused by heavy elements in the film or backing material. This project proposes using the (³He,p) and (α,p) reactions to develop nuclear reaction techniques for light element analysis in cases where regular RBS cannot accurately determine elemental content. The (³He,p) nuclear reaction for boron, nitrogen, carbon and oxygen in thin films was investigated using incident beams between 2 and 4 MeV. Absolute cross sections were measured at reaction angles of 90° and 135°. These reactions were observed to have regions of constant cross section suitable for elemental content determination. The B(³He,p)C and ¹⁴N(³He,p)¹⁶O reactions were applied to thin films containing boron and nitrogen, and were proven to be an accurate means of determining elemental areal density in thin films in cases where regular RBS was ineffective due to signal interference from heavier elements in the film or backing substrate. Advantages and limitations of the application of the (³He,p) reaction to B, N, C and O will be discussed for each of these elements. The ¹⁹F(α,p)²²Ne nuclear reaction was investigated over the energy range 2200-2500 keV. Cross sections for the ¹⁹F(α,p₀) reaction were measured at a reaction angle of 135°. A strong, isolated resonance near 2315 keV was observed which is suitable for fluorine depth profiling. A computer program was also used to generate simulated yield curves. Resonance parameters were empirically fit to the yield curve obtained using a target with known areal density (atoms/cm²). The program, with these parameters, was applied to accurately simulate yield curves obtained from other targets. The advantages, limitations and applications of this reaction will be discussed.

Particle formation during low pressure SF6/argon etching of silicon in a single wafer parallel plate reactor is studied. Particles are extracted from the exhaust and collected on the wafer. Particle composition and morphology depend on plasma power, etch time, gas composition, and pressure. Primary particles are tens of nanometers in diameter and spherical and chain aggregates as large as 5 mum are observed. Critical powers and etch times are required for the formation of these aggregates. The presence of major gas phase species is determined using mass spectroscopy and optical emission spectroscopy. A three stage mechanism for describing the particle formation (nucleation, heterogeneous growth, and coagulation) is presented. Particle precursor and heterogeneous sources are determined from plasma-dependent, homogeneous, gas-phase reactions and etch product distributions predicted from electrical and etch rate measurements. Dissociation of SF6 into lower molecular weight SFx species and unsaturated SiFx species are primarily responsible for nuclei formation and subsequent, rapid heterogeneous growth by attachment of positive ions.