Publications

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The pitting potential of pure aluminum thin films in 50 mM K2 S O4 was measured as a function of implanted Cl fluence. Samples were implanted with 35 keV Cl+ at room temperature using fluences from 2.25× 1016 to 3.25× 1016 ions cm-2 in increments of 0.25× 1016. An empirical relationship between pitting potential and fluence was found which suggests a critical Cl concentration in the oxide is necessary for pit initiation. No correlation between pitting potential and the measured Cl concentration or distribution in the metal was found. © 2006 The Electrochemical Society. All rights reserved.

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We have developed a new nanotagging technology for detecting and imaging the self-organization of proteins and other components of membranes at nanometer resolution for the purpose of investigating cell signaling and other membrane-mediated biological processes. We used protein-, lipid-, or drug-bound porphyrin photocatalysts to grow in-situ nanometer-sized metal particles, which reveal the location of the porphyrin-labeled molecules by electron microscopy. We initially used photocatalytic nanotagging to image assembled multi-component proteins and to monitor the distribution of lipids and porphyrin labels in liposomes. For example, by exchanging the heme molecules in hemoproteins with a photocatalytic tin porphyrin, a nanoparticle was grown at each heme site of the protein. The result obtained from electron microscopy for a tagged multi-subunit protein such as hemoglobin is a symmetric constellation of a specific number of nanoparticle tags, four in the case of the hemoglobin tetramer. Methods for covalently linking photocatalytic porphyrin labels to lipids and proteins were also developed to detect and image the self-organization of lipids, protein-protein supercomplexes, and membrane-protein complexes. Procedures for making photocatalytic porphyrin-drug, porphyrin-lipid, and porphyrin-protein hybrids for non-porphyrin-binding proteins and membrane components were pursued and the first porphyrin-labeled lipids was investigated in liposomal membrane models. Our photocatalytic nanotagging technique may ultimately allow membrane self-organization and cell signaling processes to be imaged in living cells. Fluorescence and plasmonic spectra of the tagged proteins might also provide additional information about protein association and membrane organization. In addition, a porphyrin-aspirin or other NSAID hybrid may be used to grow metal nanotags for the pharmacologically important COX enzymes in membranes so that the distribution of the protein can be imaged at the nanometer scale.

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The performance and reliability of microelectromechanical (MEMS) devices can be highly dependent on the control of the surface energetics in these structures. Examples of this sensitivity include the use of surface modifying chemistries to control stiction, to minimize friction and wear, and to preserve favorable electrical characteristics in surface micromachined structures. Silane modification of surfaces is one classic approach to controlling stiction in Si-based devices. The time-dependent efficacy of this modifying treatment has traditionally been evaluated by studying the impact of accelerated aging on device performance and conducting subsequent failure analysis. Our interest has been in identifying aging related chemical signatures that represent the early stages of processes like silane displacement or chemical modification that eventually lead to device performance changes. We employ a series of classic surface characterization techniques along with multivariate statistical methods to study subtle changes in the silanized silicon surface and relate these to degradation mechanisms. Examples include the use of spatially resolved time-of-flight secondary ion mass spectrometric, photoelectron spectroscopic, photoluminescence imaging, and scanning probe microscopic techniques to explore the penetration of water through a silane monolayer, the incorporation of contaminant species into a silane monolayer, and local displacement of silane molecules from the Si surface. We have applied this analytical methodology at the Si coupon level up to MEMS devices. This approach can be generalized to other chemical systems to address issues of new materials integration into micro- and nano-scale systems.

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Thermionic energy conversion in a miniature format shows potential as a viable, high efficiency, micro to macro-scale power source. A microminiature thermionic converter (MTC) with inter-electrode spacings on the order of microns has been prototyped and evaluated at Sandia. The remaining enabling technology is the development of low work function materials and processes that can be integrated into these converters to increase power production at modest temperatures (800 - 1300 K). The electrode materials are not well understood and the electrode thermionic properties are highly sensitive to manufacturing processes. Advanced theoretical, modeling, and fabrication capabilities are required to achieve optimum performance for MTC diodes. This report describes the modeling and fabrication efforts performed to develop micro dispenser cathodes for use in the MTC.

A key factor in our ability to produce and predict the stability of metal-based macro- to nano-scale structures and devices is a fundamental understanding of the localized nature of corrosion. Corrosion processes where physical dimensions become critical in the degradation process include localized corrosion initiation in passivated metals, microgalvanic interactions in metal alloys, and localized corrosion in structurally complex materials like nanocrystalline metal films under atmospheric and inundated conditions. This project focuses on two areas of corrosion science where a fundamental understanding of processes occurring at critical dimensions is not currently available. Sandia will study the critical length scales necessary for passive film breakdown in the inundated aluminum (Al) system and the chemical processes and transport in ultra-thin water films relevant to the atmospheric corrosion of nanocrystalline tungsten (W) films. Techniques are required that provide spatial information without significantly perturbing or masking the underlying relationships. Al passive film breakdown is governed by the relationship between area of the film sampled and its defect structure. We will combine low current measurements with microelectrodes to study the size scale required to observe a single initiation event and record electrochemical breakdown events. The resulting quantitative measure of stability will be correlated with metal grain size, secondary phase size and distribution to understand which metal properties control stability at the macro- and nano-scale. Mechanisms of atmospheric corrosion on W are dependent on the physical dimensions and continuity of adsorbed water layers as well as the chemical reactions that take place in this layer. We will combine electrochemical and scanning probe microscopic techniques to monitor the chemistry and resulting material transport in these thin surface layers. A description of the length scales responsible for driving the corrosion of the nanocrystalline metal films will be developed. The techniques developed and information derived from this work will be used to understand and predict degradation processes in microelectronic and microsystem devices critical to Sandia's mission.

Electrostatic actuators exhibit fast response times and are easily integrated into microsystems because they can be fabricated with standard IC micromachining processes and materials. Although electrostatic actuators have been used extensively in 'dry' MEMS, they have received less attention in microfluidic systems probably because of challenges such as electrolysis, anodization, and electrode polarization. Here we demonstrate that ac drive signals can be used to prevent electrode polarization, and thus enable electrostatic actuation in many liquids, at potentials low enough to avoid electrochemistry. We measure the frequency response of an interdigitated silicon comb-drive actuator in liquids spanning a decade of dielectric permittivities and four decades of conductivity, and present a simple theory that predicts the characteristic actuation frequency. The analysis demonstrates the importance of the native oxide on silicon actuator response, and suggests that the actuation frequency can be shifted by controlling the thickness of the oxide. For native silicon devices, actuation is predicted at frequencies less than 10 MHz, in electrolytes of ionic strength up to 100 mmol/L, and thus electrostatic actuation may be feasible in many bioMEMS and other microfluidic applications.

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Density functional theory is used to predict workfunctions, {psi}. For relaxed clean W(1 0 0), the local density approximation (LDA) agrees with experiment better than the newer generalized gradient approximation, probably due to the surface electron self-energy. The large Ba metallic radius indicates it covers W(1 0 0) at about 0.5 monolayer (ML). However, Ba{sup 2+}, O{sup 2-}, and metallic W all have similar radii. Thus 1 ML of BaO (one BaO unit for each two W atoms) produces minimum strain, indicating commensurate interfaces. BaO (1 ML) and Ba (1/2 ML) have the same {psi} to within 0.02 V, so at these coverages reduction or oxidation is not important. Due to greater chemical activity of ScO vs. highly ionic BaO, when mixing the latter with this suboxide of scandia, the overlayer always has BaO as the top layer and ScO as the second layer. The BaO/ScO bilayer has a rocksalt structure, suggesting high stability. In the series BaO/ScO/, BaO/YO/, and BaO/LaO/W(1 0 0), the latter has a remarkably low {psi} of 1.3 V (LDA), but 2 ML of rocksalt BaO also has {psi} at 1.3 V. We suggest BaO (1 ML) does not exist and that it is worthwhile to attempt the direct synthesis and study of BaO (2 ML) and BaO/LaO.

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A physics-based understanding of material aging mechanisms helps to increase reliability when predicting the lifetime of mechanical and electrical components. This report examines in detail the mechanisms of atmospheric copper sulfidation and evaluates new methods of parallel experimentation for high-throughput corrosion analysis. Often our knowledge of aging mechanisms is limited because coupled chemical reactions and physical processes are involved that depend on complex interactions with the environment and component functionality. Atmospheric corrosion is one of the most complex aging phenomena and it has profound consequences for the nation's economy and safety. Therefore, copper sulfidation was used as a test-case to examine the utility of parallel experimentation. Through the use of parallel and conventional experimentation, we measured: (1) the sulfidation rate as a function of humidity, light, temperature and O{sub 2} concentration; (2) the primary moving species in solid state transport; (3) the diffusivity of Cu vacancies through Cu{sub 2}S; (4) the sulfidation activation energies as a function of relative humidity (RH); (5) the sulfidation induction times at low humidities; and (6) the effect of light on the sulfidation rate. Also, the importance of various sulfidation mechanisms was determined as a function of RH and sulfide thickness. Different models for sulfidation-reactor geometries and the sulfidation reaction process are presented.

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Thermionic energy conversion in a microminiature format shows potential as a viable, high efficiency, on-chip power source. Microminiature thermionic converters (MTC) with inter-electrode spacings on the order of microns are currently being prototyped and evaluated at Sandia. The remaining enabling technology is the development of low work function materials and processes than can be integrated into these converters. In this report, the authors demonstrate a method of incorporating thin film emitters into converters using rf sputtering. They find that the resultant films possess a minimum work function of 1.2 eV. Practical energy conversion is hindered by surface work function non-uniformity. They postulate the source of this heterogeneity to be a result of limited bulk and surface transport of barium. Several methods are proposed for maximizing transport, including increased film porosity and the use of metal terminating layers. They demonstrate a novel method for incorporating film porosity based on metal interlayer coalescence.

Engineered Cu-rich islands were fabricated on an Al thin film to investigate pit initiation mechanisms at noble particles. X-ray photoelectron spectroscopy confirms that the thin film Cu-rich islands interdiffuse with the underlying Al substrate to form Al{sub 2}Cu islands. The defect arrays exhibit open circuit potential fluctuations whose magnitude and frequency increase as defect spacing decreases for constant island size and cathode/anode ratio. Post-exposure examination by energy dispersive spectroscopy (EDS) shows that the Al beneath the Cu-rich island dissolves with a crevice geometry. Engineered Al islands fabricated under identical conditions do not induce crevice corrosion in the vicinity of the Al defects. These results suggest that the Al dissolution is driven by the galvanic coupling between the noble island and matrix, and/or by a local change in chemistry, rather than by the presence of a defective oxide in the vicinity of the island.

We have successfully developed a method for fabricating scandate-based thermionic emitters in thin film form. The primary goal of our effort is to develop thin film emitters that exhibit low work fimction, high intrinsic electron emissivity, minimum thermal activation properties and that can be readily incorporated into a microgap converter. Our approach has been to incorporate BaSrO into a SqOq matrix using rf sputtering to produce thin films. Diode testing has shown the resulting films to be electron emissive at temperatures as low as 900 K with current densities of 0.1 mA.cm-2 at 1100 K and saturation voltages. We calculate an approximate maximum work function of 1.8 eV and an apparent emission constant (Richardson's constant, A*) of 36 mA.cm-2.K-2. Film compositional and structural analysis shows that a significant surface and subsurface alkaline earth hydroxide phase can form and probably explains the limited utilization and stability of Ba and its surface complexes. The flexibility inherent in sputter deposition suggests alternate strategies for eliminating undesirable phases and optimizing thin film emitter properties.

The generation of surface defects on electron cyclotron resonance (ECR) plasma derived aluminum oxide films has been studied. We find that Cl active O vacancies can be generated using electron and ion irradiation yielding surface concentrations of 3 xl 013 to 1X1014 sites"cm-2. These values correspond to surface defect concentrations of 3 to 10% when compared to ordered, crystalline u-alumina. The vacancies appear to be responsible for increased surface O concentrations when immersed in water. Anodic polarization of irradiated films yields a decrease in the stable pitting potential which correlates with electron dose.