Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 149-153). Available also in a digital version from Dissertation Abstracts.

The Ultra-low-k material is required to reduce the RC time delay in the integrated circuits. However, the integration of the porous low-k material into the on-chip interconnects was impeded by the plasma induced damage during etching and photoresist stripping processes. This dissertation aims to study the mechanism of plasma damage to porous ultra-low-k dielectrics with the objective to minimize the damage and to develop methods and processes to restore the low-k dielectric after the plasma damage. First, the plasma etching induced surface roughening was studied on blanket porous SiCOH films in the fluorocarbon based plasma. Substantial surface roughening was found in the low polymerization region, where the surface roughening process was initiated by the unevenly distribution of surface fluorocarbon polymers in the pore structure and enhanced by ion induced surface densification. With oxygen addition, the surface densification layer increased the radial diffusion rate difference between the top and the bottom of the pits, resulting in further increase of the surface roughness. The best process optimization was found at a "threshold point" where the surface polymerization level is just high enough to suppress the roughness initiation. The second part of this dissertation investigates the mechanism of the oxygen plasma damaging process. The roles of plasma constituents (i.e. ions, radicals and photons with different wavelengths) were differentiated by an on-wafer filter system. Oxygen radical was identified as the most critical and its damage effect was enhanced by photons with wavelength smaller than 185nm. The oxygen radical kinetics in the porous structure of low-k, including diffusion, reaction and recombination, was described analytically with a plasma altered layer model and then simulated with a Monte Carlo computational method, which give guidelines to minimize the damage. The analytical model of oxygen radical kinetic process is also used to investigate the oxygen plasma damage to patterned low-k structure, which is confirmed by experiments. Finally, the dielectric recovery was studied using silylation and UV broadband thermal treatment, both individually and in combination. After both vapor and supercritical CO₂ silylation, surface carbon and hydrophobicity were partially recovered. However, the recovery effect was limited to the surface. In comparison, UV treatment can effectively remove water from the bulk of the damaged film and consolidate the silanol bonds with the help of thermal activation. The combination of UV and silylation treatments is more effectively for dielectric recovery than UV or silylation alone. The "UV first" treatment provided a better recovery in sequential processes. Under the same conditions, simultaneous treatments by silylation and UV irradiation achieved better bulk and surface recovery than the sequential process. / text

This thesis describes the analysis, fabrication and testing of a novel magnetohydrodynamic plasma actuator for aerodynamic flow control, specifically, retreating blade stall. A magnetohydrodynamic plasma actuator is comprised of two parallel rail electrodes embedded chord-wise on the upper surface of an airfoil. A pulse forming network generates a low-voltage, high-current repetitive pulsed arc. Self-induced electromagnetic fields force the pulsed arc along the length of the rail electrodes at high velocities, transferring momentum to the surrounding air, creating a high-velocity pulsed air wall jet. A systematic experimental investigation of the effect of plasma actuators on the surrounding air is conducted in stagnant air conditions to gain an understanding of the physical characteristics. These characteristics include voltage and current measurements, pulsed arc velocity measurements, and high speed video imaging. The results show typical pulsed arc velocities of about 100 m/s can be induced with discharge energies of about 300 J per pulse. Additional experimental studies are conducted to quantify the performance of the pulsed arc for potential use in subsonic flow control applications. To gain an estimate of the momentum transferred from the pulsed arc to the surrounding air the plasma actuator is placed in a subsonic open-circuit wind tunnel at a Reynolds number of 4.5 x 105. The induced velocity of the pulsed wall jet is measured using a Laser Doppler Anemometer. The measurements show that the pulsed arc creates a high-velocity pulsed wall jet that extends 40 mm above the airfoils surface and has an induced velocity of 15 m/s greater than the unaltered air flow over the airfoil, with peak velocities of 32 m/s. The magnetohydrodynamic plasma actuator proved to induce velocities an order of magnitude greater than the velocities attained by current state-of-the-art plasma actuators. Moreover, the RailPAc is found to posses the potential for alleviation of retreating blade stall. Future work will include experiments to gain a detailed understanding of the improvements to the static stall angle, the optimal actuator geometry, excitation duty cycle, magnetic field augmentation, and behavior of the plasma armature at high Mach/Reynolds numbers. Particle Image Velocimetry (PIV) will be utilized to improve the induced flow velocity measurements acquired with the LDA. / text

This document presents a study of the numerical simulation of non-equilibrium plasma discharges in air mixtures in the atmospheric pressure regime. Such plasma is formed by applying a very high electric field over a very short time duration (nano-microsecond) which preferentially heats the electrons to very high temperatures (10 electron Volts or more) while preventing thermalization of the gas. Preferentially heating the electrons to very high temperatures allows the discharge to efficiently and rapidly ionize and dissociate the gas mixture without losing too much energy to thermalization or vibrational excitation. Consequently, two useful characteristics of these discharges are low gas temperatures and rapid electron chemistry. This study focuses on two applications of interest: ignition of fuel-air mixtures and plasma enhanced medicine. For ignition, there are two situations that arise where it is difficult for traditional spark ignition systems to operate. The first is at the supersonic flow regime where the residence time of the flow in the engine is low. The second is high pressure ignition of lean fuel-air mixtures. For plasma medicine and surface treatment, non-equilibrium plasma is an effective means of delivering reactive radical species to the surface while limiting damage due to thermal heating. The problems of interest are characterized by the formation of weakly ionized plasma in the presence of flow fields such as supersonic boundary layers or low speed jets. To simulate the coupled plasma-fluid flow physics of these discharges, two numerical tools are utilized. The first is a two-temperature, multiple species, self-consistent plasma solver with finite rate chemistry which is used to simulate the plasma as it forms in a neutral background gas. The second tool is a multiple-species compressible flow solver which calculates the flow field properties of the background gas mixture. / text

An intense laser or charged particle pulse propagating through matter excites light-speed refractive index structures in its wake via Kerr effect, ionization, or displacement of electrons from background ions. Examples include plasma wakes used to accelerate charged particles and self-guided filaments used for atmospheric analysis and micromachining. Such applications constrain the shape, size and evolution of the index structure, yet often these are known in detail only through intensive computer simulations based on estimated initial conditions. Here we develop and demonstrate three methods for visualizing evolving light-speed structures directly in the laboratory in a single shot : (1) frequency-domain streak camera, (2) frequency-domain tomography, and (3) multi-object-plane phase-contrast imaging. All three methods are based on analyzing phase perturbations that an evolving object imprints on one or more probe laser pulses that cross its path obliquely. The methods are tailored to different propagation lengths, material densities, and dimensionality of imaging. Using these techniques, evolving laser-driven filaments in glass and air and plasma wakes in helium gas driven by laser pulses up to petawatt peak power are visualized in one shot, revealing underlying nonlinear laser-plasma interaction physics that is compared in detail to computer simulations. / text

With the scaling of devices, integration of porous ultra low-κ dielectric materials into Cu interconnect becomes necessary. Low-k dielectric materials usually consist of a certain number of methyl groups and pores incorporated into a SiO₂ backbone structure to reduce the dielectric constant. They are frequently exposed to various plasmas, since plasma is widely used in VLSI semiconductor fabrication such as etching, ashing and deposition. This dissertation is aimed at exploring the interaction between plasma and low-κ dielectric surfaces. First, plasma assisted the atomic layer deposition (ALD) of Ta-based Cu barriers. Atomic layer deposition of Ta barriers is a self-limited surface reaction, determined by the function groups on the low-κ dielectric surface. But it was found TaCl₅ precursor could not nucleate on the organosilicate low-κ surface that was terminated with methyl groups. Radical NH[subscript x] beam, generated by a microwave plasma source, could activate the surface through exchanging with the methyl groups on the low-κ surface and providing active Si-NH[subscript x] nucleation sites for TaCl₅ precursors. Results from Monte Carlo simulation of the atomic layer deposition demonstrated that substrate chemistry was critical in controlling the film morphology. Second, the properties of low-κ dielectric materials tended to degrade under plasma exposure. In this dissertation, plasma damage of low-κ dielectric surface was investigated from a mechanistic point of view. Both carbon depletion and surface densification were observed on the top surface of damaged low-κ materials while the bulk remained largely uninfluenced. Plasma damage was found to be a complicated phenomenon involving both chemical and physical effects, depending on chemical reactivity and the energy and mass of the plasma species. With a downstream plasma source capable of separating ions from the plasma beam and an in-situ x-ray photoelectron spectroscopy (XPS) monitoring of the damage process, it was clear that ions played a more important role in the plasma damage process. Increase of dielectric constant after plasma damage was mainly attributed to moisture uptake and was confirmed with quantum chemistry calculation. Annealing was found to be effective in mitigating moisture uptake and thus restoring κ value. Finally, oxygen plasma damage to blanket and patterned low-κ dielectrics was studied in detail. Energetic ions in oxygen plasma contributed much to the loss of film hydrophobicity and dielectric constant through the formation of C=O and Si-OH. Based on results from residual gas analyses (RGA), three possible reaction paths leading to carbon depletion were proposed. This was followed by analytical solution of the evolution of carbon concentration during O₂ plasma damage. O₂ plasma damage to patterned CDO film was studied by TEM/EELS. And the damage behavior was simulated with Monte Carlo method. It was found that the charging potential distribution induced by plasma was important in determining the carbon loss in patterned low-k films. The charging potential distribution was mainly related to the geometry of low-k trench structures. To recover the dielectric constant, several recovery techniques were tried and briefly discussed. / text

<p> We report the observation of a net inward, up-gradient turbulent particle flux which occurs when a collisional drift waves generate a sufficiently strong radially sheared azimuthal zonal flow in a cylindrical magnetized plasma. At low magnetic fields (B&le;1.0 kG), particle transport is outward at all radii. As the magnetic field is further increased to 1200G, an up-gradient inward particle flux develops between the peak of the velocity shear and the maximum density gradient. The mean density gradient is also observed to steepen in response to this inward flux. Time-domain and bispectral Fourier domain analysis shows that at the peak of the velocity shear, where the particle flux is outward, the turbulent Reynolds stress acts to reinforce the shear flow. In contrast, in the region of the inward particle flux, the zonal flow drives the fluctuations, and a transient increase in the shearing rate is occurs prior to an increase in the magnitude of the inward flux. The results suggest a hypothesis in which the shear flow is responsible for the up-gradient particle flux and the corresponding steepening in the mean density gradient. However, a linear instability analyses using experimentally measured density and E&times;B flow profiles in a linear, modified Hasegawa-Wakatani theory model with the coupled potential and density fluctuations failed to reproduce the essential elements of our experimental observations, suggesting some other mechanism is responsible for the inward flux. We summarize recent new experimental results which point towards the possible role of finite ion temperature gradient effects, possibly combined with parallel flow shear, in driving up-gradient particle flux.</p>

Kinetic microinstabilities in the solar wind arise when its non-thermal properties become too extreme. This thesis project focused specifically on the four instabilities associated with ion temperature anisotropy: the cyclotron, mirror, and parallel and oblique firehose instabilities. Numerous studies have provided evidence that proton temperature anisotropy in the solar wind is limited by the actions of these instabilities. For this project, a fully revised analysis of data from the Wind spacecraft's Faraday cups and calculations from linear Vlasov theory were used to extend these findings in two respects. First, theoretical thresholds were derived for the \(\alpha\)-particle temperature anisotropy instabilities, which were then found to be consistent with a statistical analysis of Wind \(\alpha\)-particle data. This suggests that \(\alpha\)-particles, which constitute only about 5% of ions in the solar wind, are nevertheless able to drive temperature anisotropy instabilities. Second, a statistical analysis of Wind proton data found that proton temperature was significantly enhanced in plasma unstable due to proton temperature anisotropy. This implies that extreme proton temperature anisotropies in solar wind at 1 AU arise from ongoing anisotropic heating (versus cooling from, e.g., CGL double adiabatic expansion). Together, these results provide further insight into the complex evolution of the solar wind's non-fluid properties. / Astronomy

A tokamak plasma near equilibrium can be perturbed with modulated power sources, such as modulated electron cyclotron heating, or repeated cold pulse application. Temperature response to cyclical changes in profiles parameters that are induced by modulated power deposition can be used to test theoretical transport models as well as improve experimental phenomenology used to optimize tokamak performance. The goal of this document to discuss some methods of analyzing electron temperature data in the context of energy transport. Specific experiments are considered in order to demonstrate the methods discussed, as well as to examine the electron energy transport properties of these shots. Electron cyclotron emission provides a convenient way to probe electron temperature for plasmas in thermal equilibrium. We can show that in tokamak devices,barring harmonic overlap, we can associate a particular frequency with a particular location in a tokamak, by carefully selecting the detection frequency and line of sight of the responsible antenna. ECE radiometers typically measure temperature at tens of locations at a time with a spatial resolution on the order of a few centimeters. Tracking the evolution of electron energy flux depends on careful analysis of the resulting data. The most straightforward way to analyze temperature perturbations is to simply consider various harmonics of the driving source and consider the corresponding harmonics in the temperature. We can analyze the phase and amplitude of the response to find the effective phase velocity of the perturbation which can in turn be related to parameters in the selected heat flux model. The most common example is to determine , the diffusion coefficient that appears in the linearized energy transport equation. The advantages and limitation of this method will be discussed in detail in Section 3. A more involved approach involves using the perturbed temperature data to compute modulated heat flux at any given point in the perturbation cycle, rather than using the temperature data directly. As before the heat flux can then be related to measured profile parameters and theoretical predictions. The advantages and limitations of this approach will be discussed in more detail. Both of the mentioned analysis methods are used to probe electron energy transport in a quiescent H mode (QH mode) shot conducted at DIIID. The nature of the internal transport barrier that is present in the shot is considered in light of the results. / text

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