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Bio-inspired incorporation of phenylalanine enhances ionic selectivity in layer-by-layer deposited polyelectrolyte films

Soft Matter

Percival, Stephen P.; Russo, Sara R.; Priest, Chad; Hill, Ryan C.; Ohlhausen, J.A.; Small, Leo J.; Rempe, Susan R.; Spoerke, Erik D.

The addition of a common amino acid, phenylalanine, to a Layer-by-Layer (LbL) deposited polyelectrolyte (PE) film on a nanoporous membrane can increase its ionic selectivity over a PE film without the added amino acid. The addition of phenylalanine is inspired by detailed knowledge of the structure of the channelrhodopsins family of protein ion channels, where phenylalanine plays an instrumental role in facilitating sodium ion transport. The normally deposited and crosslinked PE films increase the cationic selectivity of a support membrane in a controllable manner where higher selectivity is achieved with thicker PE coatings, which in turn also increases the ionic resistance of the membrane. The increased ionic selectivity is desired while the increased resistance is not. We show that through incorporation of phenylalanine during the LbL deposition process, in solutions of NaCl with concentrations ranging from 0.1 to 100 mM, the ionic selectivity can be increased independently of the membrane resistance. Specifically, the addition is shown to increase the cationic transference of the PE films from 81.4% to 86.4%, an increase on par with PE films that are nearly triple the thickness while exhibiting much lower resistance compared to the thicker coatings, where the phenylalanine incorporated PE films display an area specific resistance of 1.81 Ω cm2in 100 mM NaCl while much thicker PE membranes show a higher resistance of 2.75 Ω cm2in the same 100 mM NaCl solution.

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Assessing atomically thin delta-doping of silicon using mid-infrared ellipsometry

Journal of Materials Research

Katzenmeyer, Aaron M.; Luk, Ting S.; Bussmann, Ezra B.; Young, Steve M.; Anderson, Evan M.; Marshall, Michael T.; Ohlhausen, J.A.; Kotula, Paul G.; Lu, Ping L.; Campbell, DeAnna M.; Lu, Tzu-Ming L.; Liu, Peter Q.; Ward, Daniel R.; Misra, Shashank M.

Hydrogen lithography has been used to template phosphine-based surface chemistry to fabricate atomic-scale devices, a process we abbreviate as atomic precision advanced manufacturing (APAM). Here, we use mid-infrared variable angle spectroscopic ellipsometry (IR-VASE) to characterize single-nanometer thickness phosphorus dopant layers (δ-layers) in silicon made using APAM compatible processes. A large Drude response is directly attributable to the δ-layer and can be used for nondestructive monitoring of the condition of the APAM layer when integrating additional processing steps. The carrier density and mobility extracted from our room temperature IR-VASE measurements are consistent with cryogenic magneto-transport measurements, showing that APAM δ-layers function at room temperature. Finally, the permittivity extracted from these measurements shows that the doping in the APAM δ-layers is so large that their low-frequency in-plane response is reminiscent of a silicide. However, there is no indication of a plasma resonance, likely due to reduced dimensionality and/or low scattering lifetime.

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Electrochemical aspects of copper atmospheric corrosion in the presence of sodium chloride

Electrochimica Acta

Schindelholz, Eric J.; Cong, H.; Jove Colon, Carlos F.; Li, S.; Ohlhausen, J.A.; Moffat, Harry K.

This study describes the evolving state of electrolyte and corrosion processes associated with sodium chloride on copper at the initial stage of corrosion and the critical implications of this behavior on controlling kinetics and damage distributions. Sodium chloride droplets were placed on copper in humid conditions and the resulting electrolyte properties, corrosion products and damage were characterized over time using time-lapse imaging, micro Raman spectroscopy, TOF-SIMS and optical profilometry. Within minutes of NaCl droplet placement, NaOH-rich films resultant from oxygen reduction advanced stepwise from the droplets, leaving behind concentric trenching attack patterns suggestive of moving anode-cathode pairs at the alkaline film front. Corrosion attack under these spreading alkaline films was up to 10x greater than under the original NaCl drops. Furthermore, solid Cu2Cl(OH)3 shells formed over the surface of the NaCl drops within hours of exposure. Thermodynamic modeling along with immersed electrochemical experiments in simulated droplet and films electrolytes were used to rationalize this behavior and build a description of the rapidly evolving corroding system.

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Effect of silicon and oxygen dopants on the stability of hydrogenated amorphous carbon under harsh environmental conditions

Carbon

Mangolini, Filippo; Krick, Brandon A.; Jacobs, Tevis D.B.; Khanal, Subarna R.; Streller, Frank; McClimon, J.B.; Hilbert, James; Prasad, Somuri V.; Scharf, Thomas W.; Ohlhausen, J.A.; Lukes, Jennifer R.; Sawyer, W.G.; Carpick, Robert W.

Harsh environments pose materials durability challenges across the automotive, aerospace, and manufacturing sectors, and beyond. While amorphous carbon materials have been used as coatings in many environmentally-demanding applications owing to their unique mechanical, electrical, and optical properties, their limited thermal stability and high reactivity in oxidizing environments have impeded their use in many technologies. Silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) films are promising for several applications because of their higher thermal stability and lower residual stress compared to hydrogenated amorphous carbon (a-C:H). However, an understanding of their superior thermo-oxidative stability compared to a-C:H is lacking, as it has been inhibited by the intrinsic challenge of characterizing an amorphous, multi-component material. Here, we show that introducing silicon and oxygen in a-C:H slightly enhances the thermal stability in vacuum, but tremendously increases the thermo-oxidative stability and the resistance to degradation upon exposure to the harsh conditions of low Earth orbit (LEO). The latter is demonstrated by having mounted samples of a-C:H:Si:O on the exterior of the International Space Station via the Materials International Space Station (MISSE) mission 7b. Exposing lightly-doped a-C:H:Si:O to elevated temperatures under aerobic conditions or to LEO causes carbon volatilization in the near-surface region, producing a silica surface layer that protects the underlying carbon from further removal. These findings provide a novel physically-based understanding of the superior stability of a-C:H:Si:O in harsh environments compared to a-C:H.

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Effect of Hydrogen and Oxygen Partial Pressure on the Tribochemistry of Silicon Oxide-Containing Hydrogenated Amorphous Carbon

Mangolini, Filippo M.; Mangolini, Filippo M.; Koshigan, Komlavi D.; Koshigan, Komlavi D.; Van Benthem, Mark V.; Van Benthem, Mark V.; Ohlhausen, J.A.; Ohlhausen, J.A.; McClimon, J B.; McClimon, J B.; Hilbert, James A.; Hilbert, James A.; Fontaine, Julien F.; Fontaine, Julien F.; Carpick, Robert W.; Carpick, Robert W.

Abstract not provided.

Effect of humidity on scintillation performance in Na and Tl activated CsI crystals

IEEE Transactions on Nuclear Science

Yang, Pin; Harmon, Charles D.; Doty, F.P.; Ohlhausen, J.A.

Time dependent photoluminescence and radioluminescence for sodium (Na) and thallium (Tl) activated cesium iodide (CsI) single crystals exposed to 50% and 75% relative humidity (RH) has been investigated. These results indicate that Tl activated crystals are more robust than the Na activated crystals against humidity induced scintillation degradation. The development of 'etching pits' and 'inactive' domains are the characteristics of deteriorated Na activated CsI crystals. These 'inactive' domains, bearing a resemblance to a polycrystalline appearance beneath the crystal surface, can be readily detected by a 250 nm light emitting diode. These features are commonly observed at the corners and deep scratched areas where moisture condensation is more likely to occur. Mechanisms contributing to the scintillation degradation in Na activated CsI crystals were investigated by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). ToF-SIMS depth profiles indicate that Na has been preferentially diffused out of CsI crystal, leaving the Na concentration in these 'inactive' domains below its scintillation threshold. © 1963-2012 IEEE.

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Synthesis and characterization of supported ferrites for thermochemical CO 2 splitting using concentrated solar energy

ACS National Meeting Book of Abstracts

Ambrosini, Andrea; Coker, Eric N.; Rodriguez, Marko A.; Ohlhausen, J.A.; Miller, James E.; Stechel-Speicher, Ellen B.

The Sunshine to Petrol effort at Sandia National Laboratories aims to convert CO 2 and water to liquid hydrocarbon fuel precursors using concentrated solar energy with redox-active metal oxide systems, such as ferrites: Fe 3O 4→3FeO+ 0.5O 2 (>1350°C) 3FeO + CO 2→Fe 3O 4 + CO (<1200°C). However, the ferrite materials are not repeatedly reactive on their own and require a support, such as yttria-stabilized zirconia (YSZ). The ferrite-support interaction is not well defined, as there has been little fundamental characterization of these oxides at the high temperatures and conditions present in these cycles. We have investigated the microstructure, structure-property relationships, and the role of the support on redox behavior of the ferrite composites. In-situ capabilities to elucidate chemical reactions under operating conditions have been developed. The synthesis, structural characterization (room and high- temperature x-ray diffraction, secondary ion mass spectroscopy, scanning electron microscopy), and thermogravimetric analysis of YSZ-supported ferrites will be discussed.

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The effect of chrome adhesion layer on quartz resonator aging

Wessendorf, Kurt O.; Ohlhausen, J.A.

This SAND report documents a late start LDRD designed to determine the possible aging effects of a quartz resonator gold adhesion layer. Sandia uses quartz resonators for applications. These applications require a very stable frequency source with excellent aging (low drift) characteristics. These parts are manufactured by one of our qualified vendors outside Sandia Laboratories, Statek Corp. Over the years we, Sandia and the vendor, have seen aging variations that have not been completely explained by the typical mechanisms known in the industry. One theory was that the resonator metallization may be contributing to the resonator aging. This LDRD would allow us to test and analyze a group of resonators with known differentiating metallization and via accelerated aging determine if a chrome adhesion layer used to accept the final gold plating may contribute to poor aging. We worked with our main vendor to design and manufacture a set of quartz resonators with a wide range of metallization thickness ratios between the chrome and gold that will allow us determine the cause of this aging and which plating thickness ratios provide the best aging performance while not degrading other key characteristics.

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Experiences with the High Energy Resolution Optics (HERO) update on a physical electronics 690 auger system

Wallace, William O.; Ohlhausen, J.A.; Brumbach, Michael T.

We will present our experiences with the new High Energy Resolution Optics (HERO) upgrade on a Physical Electronics Auger 690 system. This upgrade allows the single pass cylindrical analyzer in the Auger system to achieve higher energy resolution than in the standard mode. With this upgrade, it should be possible to separate chemical states for certain elements. Also, it should be possible to separate closely spaced peaks from selected elements that have been difficult or impossible to separate without the upgrade. Specifically, we will investigate practical use of this upgrade in the analysis of materials systems where overlapping peaks have historically been an issue, such as Kovar, which consists of the elements Ni, Fe and Co. Strategies for the successful use of the technique as well as its current limitations will be shown.

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The role of polymer formation during vapor phase lubrication of silicon

Dugger, Michael T.; Ohlhausen, J.A.; Dirk, Shawn M.

The lubrication of silicon surfaces with alcohol vapors has recently been demonstrated. With a sufficient concentration of pentanol vapor present, sliding of a silica ball on an oxidized silicon wafer can proceed with no measurable wear. The initial results of time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of wear surfaces revealed a reaction product having thickness on the order of a monolayer, and with an ion spectrum that included fragments having molecular weights of 200 or more that occurred only inside the wear tracks. The parent alcohol molecule pentanol, has molecular weight of 88amu, suggesting that reactions of adsorbed alcohols on the wearing surfaces allowed polymerization of the alcohols to form higher molecular weight species. In addition to pin-on-disk studies, lubrication of silicon surfaces with pentanol vapors has also been demonstrated using MicroElectroMechanical Systems (MEMS) devices. Recent investigations of the reaction mechanisms of the alcohol molecules with the oxidized silicon surfaces have shown that wearless sliding requires a concentration of the alcohol vapor that is dependent upon the contact stress during sliding, with higher stress requiring a greater concentration of alcohol. Different vapor precursors including those with acid functionality, olefins, and methyl termination also produce polymeric reaction products, and can lubricate the silica surfaces. Doping the operating environment with oxygen was found to quench the formation of the polymeric reaction product, and demonstrates that polymer formation is not necessary for wearless sliding.

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Characterization of the surface changes during the activation process of erbium/erbium oxide for hydrogen storage

Brumbach, Michael T.; Zavadil, Kevin R.; Snow, Clark S.; Ohlhausen, J.A.

Erbium is known to effectively load with hydrogen when held at high temperature in a hydrogen atmosphere. To make the storage of hydrogen kinetically feasible, a thermal activation step is required. Activation is a routine practice, but very little is known about the physical, chemical, and/or electronic processes that occur during Activation. This work presents in situ characterization of erbium Activation using variable energy photoelectron spectroscopy at various stages of the Activation process. Modification of the passive surface oxide plays a significant role in Activation. The chemical and electronic changes observed from core-level and valence band spectra will be discussed along with corroborating ion scattering spectroscopy measurements.

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Characterization of the surface changes during the activation of erbium/erbium oxide for hydrogen storage

Zavadil, Kevin R.; Snow, Clark S.; Ohlhausen, J.A.

Erbium is known to effectively load with hydrogen when held at high temperature in a hydrogen atmosphere. To make the storage of hydrogen kinetically feasible, a thermal activation step is required. Activation is a routine practice, but very little is known about the physical, chemical, and/or electronic processes that occur during Activation. This work presents in situ characterization of erbium Activation using variable energy photoelectron spectroscopy at various stages of the Activation process. Modification of the passive surface oxide plays a significant role in Activation. The chemical and electronic changes observed from core-level and valence band spectra will be discussed along with corroborating ion scattering spectroscopy measurements.

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Predicting fracture in micron-scale polycrystalline silicon MEMS structures

Boyce, Brad B.; Foulk, James W.; Field, Richard V.; Ohlhausen, J.A.

Designing reliable MEMS structures presents numerous challenges. Polycrystalline silicon fractures in a brittle manner with considerable variability in measured strength. Furthermore, it is not clear how to use a measured tensile strength distribution to predict the strength of a complex MEMS structure. To address such issues, two recently developed high throughput MEMS tensile test techniques have been used to measure strength distribution tails. The measured tensile strength distributions enable the definition of a threshold strength as well as an inferred maximum flaw size. The nature of strength-controlling flaws has been identified and sources of the observed variation in strength investigated. A double edge-notched specimen geometry was also tested to study the effect of a severe, micron-scale stress concentration on the measured strength distribution. Strength-based, Weibull-based, and fracture mechanics-based failure analyses were performed and compared with the experimental results.

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Spectroscopy and capacitance measurements of tunneling resonances in an Sb-implanted point contact

Bishop, Nathaniel B.; Stevens, Jeffrey S.; Childs, Kenton D.; Ohlhausen, J.A.; Lilly, Michael L.; Carroll, Malcolm; Young, Ralph W.; Bielejec, Edward S.; Ten Eyck, Gregory A.; Wendt, J.R.; Rahman, Rajib R.; Grubbs, Robert K.

We fabricated a split-gate defined point contact in a double gate enhancement mode Si-MOS device, and implanted Sb donor atoms using a self-aligned process. E-beam lithography in combination with a timed implant gives us excellent control over the placement of dopant atoms, and acts as a stepping stone to focused ion beam implantation of single donors. Our approach allows us considerable latitude in experimental design in-situ. We have identified two resonance conditions in the point contact conductance as a function of split gate voltage. Using tunneling spectroscopy, we probed their electronic structure as a function of temperature and magnetic field. We also determine the capacitive coupling between the resonant feature and several gates. Comparison between experimental values and extensive quasi-classical simulations constrain the location and energy of the resonant level. We discuss our results and how they may apply to resonant tunneling through a single donor.

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LDRD 140639 final report : investigation of transmutation claims

Reich, Jeffrey E.; Grant, Richard P.; Ohlhausen, J.A.

The Proton-21 Laboratory in the Ukraine has been publishing results on shock-induced transmutation of several elements, including Cobalt 60 into non-radioactive elements. This report documents exploratory characterization of a shock-compressed Aluminum-6061 sample, which is the only available surrogate for the high-purity copper samples in the Proton-21 experiments. The goal was to determine Sandia's ability to detect possible shock-wave-induced transmutation products and to unambiguously validate or invalidate the claims in collaboration with the Proton-21 Laboratory. We have developed a suitable characterization process and tested it on the surrogate sample. Using trace elemental analysis capabilities, we found elevated and localized concentrations of impurity elements like the Ukrainians report. All our results, however, are consistent with the ejection of impurities that were not in solution in our alloy or were deposited from the cathode during irradiation or possibly storage. Based on the detection capabilities demonstrated and additional techniques available, we are positioned to test samples from Proton-21 if funded to do so.

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Multivariate statistical analysis of three-spatial-dimension TOF-SIMS raw data sets

Analytical Chemistry

Smentkowski, V.S.; Ostrowski, S.G.; Braunstein, E.; Keenan, M.R.; Ohlhausen, J.A.; Kotula, Paul G.

Three-spatial-dimension (3D) time-of-flight-secondary ion mass spectrometry (TOF-SIMS) analysis can be performed if an X-Y image is saved at each depth of a depth profile. In this paper, we will show how images reconstructed from specified depths, depth profiles generated from specific X-Y coordinates, as well as three-spatial-dimensional rendering provide for a better understanding of the sample than traditional depth profiling where only a single spectrum is collected at each depth. We will also demonstrate, for the first time, that multivariate statistical analysis (MVSA) tools can be used to perform a rapid, unbiased analysis of the entire 3D data set. In the example shown here, retrospective analysis and MVSA revealed a more complete picture of the 3D chemical distribution of the sample than did the as-measured depth profiling alone. Color overlays of the MVSA components as well as animated movies allowing for visualization (in 3D) from various angles will be provided. © 2007 American Chemical Society.

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Spectrum Imaging Approaches for Bioforensics

Sandia journal manuscript; Not yet accepted for publication

Ohlhausen, J.A.; Kotula, Paul G.; Michael, Joseph R.

Spectrum imaging combined with multivariate statistics is an approach to microanalysis that makes the maximum use of the large amount of data potentially collected in forensics analysis. Here, this study examines the efficacy of using spectrum imaging-enabled microscopies to identify chemical signatures in simulated bioagent materials. This approach allowed for the ready discrimination between all samples in the test. In particular, the spectrum imaging approach allowed for the identification of particles with trace elements that would have been missed with a more traditional approach to forensic microanalysis. Finally, the importance of combining signals from multiple length scales and analytical sensitivities is discussed.

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Macro- to nanoscale wear prevention via molecular adsorption

Proposed for publication in Science.

Dugger, Michael T.; Ohlhausen, J.A.

As the size of mechanical systems shrinks from macro- to nanoscales, surface phenomena such as adhesion, friction, and wear become increasingly significant. This paper demonstrates the use of alcohol adsorption as a means of continuously replenishing the lubricating layer on the working device surfaces and elucidates the tribochemical reaction products formed in the sliding contact region. Friction and wear of native silicon oxide were studied over a wide range of length scales from macro- to nanoscales using a ball-on-flat tribometer (millimeter scale), sidewall microelectromechanical system (MEMS) tribometer (micrometer scale), and atomic force microscopy (nanometer scale). In all cases, the alcohol vapor adsorption successfully lubricated and prevented wear. Imaging time-of-flight secondary ion mass spectrometry analysis of the sliding contact region revealed that high molecular weight oligomeric species were formed via tribochemical reactions of the adsorbed linear alcohol molecules. These tribochemical products seemed to enhance the lubrication and wear prevention. In the case of sidewall MEMS tests, the lifetime of the MEMS device was radically increased via vapor-phase lubrication with alcohol.

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Acceleration of dormant storage effects to address the reliability of silicon surface micromachined Micro-Electro-Mechanical Systems (MEMS)

Walraven, J.A.; Cox, James C.; Skousen, Troy J.; Ohlhausen, J.A.; Jenkins, Mark W.; Jokiel, Bernhard J.; Parson, Ted B.; Tang, Michelle D.

Qualification of microsystems for weapon applications is critically dependent on our ability to build confidence in their performance, by predicting the evolution of their behavior over time in the stockpile. The objective of this work was to accelerate aging mechanisms operative in surface micromachined silicon microelectromechanical systems (MEMS) with contacting surfaces that are stored for many years prior to use, to determine the effects of aging on reliability, and relate those effects to changes in the behavior of interfaces. Hence the main focus was on 'dormant' storage effects on the reliability of devices having mechanical contacts, the first time they must move. A large number ({approx}1000) of modules containing prototype devices and diagnostic structures were packaged using the best available processes for simple electromechanical devices. The packaging processes evolved during the project to better protect surfaces from exposure to contaminants and water vapor. Packages were subjected to accelerated aging and stress tests to explore dormancy and operational environment effects on reliability and performance. Functional tests and quantitative measurements of adhesion and friction demonstrated that the main failure mechanism during dormant storage is change in adhesion and friction, precipitated by loss of the fluorinated monolayer applied after fabrication. The data indicate that damage to the monolayer can occur at water vapor concentrations as low as 500 ppm inside the package. The most common type of failure was attributed to surfaces that were in direct contact during aging. The application of quantitative methods for monolayer lubricant analysis showed that even though the coverage of vapor-deposited monolayers is generally very uniform, even on hidden surfaces, locations of intimate contact can be significantly depleted in initial concentration of lubricating molecules. These areas represent defects in the film prone to adsorption of water or contaminants that can cause movable structures to adhere. These analysis methods also indicated significant variability in the coverage of lubricating molecules from one coating process to another, even for identical processing conditions. The variability was due to residual molecules left in the deposition chamber after incomplete cleaning. The coating process was modified to result in improved uniformity and total coverage. Still, a direct correlation was found between the resulting static friction behavior of MEMS interfaces, and the absolute monolayer coverage. While experimental results indicated that many devices would fail to start after aging, the modeling approach used here predicted that all the devices should start. Adhesion modeling based upon values of adhesion energy from cantilever beams is therefore inadequate. Material deposition that bridged gaps was observed in some devices, and potentially inhibits start-up more than the adhesion model indicates. Advances were made in our ability to model MEMS devices, but additional combined experimental-modeling studies will be needed to advance the work to a point of providing predictive capability. The methodology developed here should prove useful in future assessments of device aging, however. Namely, it consisted of measuring interface properties, determining how they change with time, developing a model of device behavior incorporating interface behavior, and then using the age-aware interface behavior model to predict device function.

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Multivariate statistical analysis of concatenated time-of-flight secondary ion mass spectrometry spectral images. Complete description of the sample with one analysis

Analytical Chemistry

Smentkowski, V.S.; Keenan, Michael R.; Ohlhausen, J.A.; Kotula, Paul G.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) instruments are capable of saving an entire mass spectrum at each pixel of an image, allowing for retrospective analysis of masses that were not selected for analysis during data collection. These TOF-SIMS spectral images contain a wealth of information, but few tools are available to assist the analyst in visualizing the entire raw data set and as a result, most of the data are not analyzed. Automated, nonbiased, multivariate statistical analysis (MVSA) techniques are useful for converting the massive amount of data into a smaller number of chemical components (spectra and images) that are needed to fully describe the TOF-SIMS measurement. Many samples require two back-to-back TOF-SIMS measurements in order to fully characterize the sample, one measurement of the fraction of positively charged secondary ions (positive ion fraction) and one measurement of the fraction of negatively charged secondary ions (negative ion fraction). Each measurement then needs to be individually evaluated. In this paper, we report the first MVSA analysis of a concatenated TOF-SIMS date set comprising positive ion and negative ion spectral images collected on the same region of a sample. MVSA of concatenated data sets provides results that are intuitive and fully describe the sample. The analytical insight provided by MVSA of the concatenated data set was not obtained when either polarity data set was analyzed separately. © 2005 American Chemical Society.

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Combined surface analytical methods to characterize degradative processes in anti-stiction films in MEMS devices

Zavadil, Kevin R.; Ohlhausen, J.A.; Hankins, M.G.; Kent, Michael S.; Tallant, David T.

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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Multivariate statistical analysis of time-of-flight secondary ion mass spectrometry images - Looking beyond the obvious

Applied Surface Science

Smentkowski, Vincent S.; Ohlhausen, J.A.; Kotula, Paul G.; Keenan, Michael R.

Analytical instrumentation such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) provides a tremendous quantity of data since an entire mass spectrum is saved at each pixel in an ion image. The analyst often selects only a few species for detailed analysis; the majority of the data are not utilized. Researchers at Sandia National Laboratory (SNL) have developed a powerful multivariate statistical analysis (MVSA) toolkit named AXSIA (Automated eXpert Spectrum Image Analysis) that looks for trends in complete datasets (e.g., analyzes the entire mass spectrum at each pixel). A unique feature of the AXSIA toolkit is the generation of intuitive results (e.g., negative peaks are not allowed in the spectral response). The robust statistical process is able to unambiguously identify all of the spectral features uniquely associated with each distinct component throughout the dataset. General Electric and Sandia used AXSIA to analyze raw data files generated on an Ion Tof IV ToF-SIMS instrument. Here, we will show that the MVSA toolkit identified metallic contaminants within a defect in a polymer sample. These metallic contaminants were not identifiable using standard data analysis protocol. © 2004 Elsevier B.V. All rights reserved.

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Multivariate statistical analysis of time-of-flight secondary ion mass spectrometry images using AXSIA

Applied Surface Science

Ohlhausen, J.A.; Keenan, Michael R.; Kotula, Paul G.; Peebles, Diane E.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) by its parallel nature, generates complex and very large datasets quickly and easily. An example of such a large dataset is a spectral image where a complete spectrum is collected for each pixel. Unfortunately, the large size of the data matrix involved makes it difficult to extract the chemical information from the data using traditional techniques. Because time constraints prevent an analysis of every peak, prior knowledge is used to select the most probable and significant peaks for evaluation. However, this approach may lead to a misinterpretation of the system under analysis. Ideally, the complete spectral image would be used to provide a comprehensive, unbiased materials characterization based on full spectral signatures. Automated eXpert spectral image analysis (AXSIA) software developed at Sandia National Laboratories implements a multivariate curve resolution technique that was originally developed for energy dispersive X-ray spectroscopy (EDS) [Microsci. Microanal. 9 (2003) 1]. This paper will demonstrate the application of the method to TOF-SIMS. AXSIA distills complex and very large spectral image datasets into a limited number of physically realizable and easily interpretable chemical components, including both spectra and concentrations. The number of components derived during the analysis represents the minimum number of components needed to completely describe the chemical information in the original dataset. Since full spectral signatures are used to determine each component, an enhanced signal-to-noise is realized. The efficient statistical aggregation of chemical information enables small and unexpected features to be automatically found without user intervention. © 2004 Elsevier B.V. All rights reserved.

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Nanoscale morphology development in the passive oxide on aluminum and the relationship to pitting

Proceedings - Electrochemical Society

Zavadil, Kevin R.; Ohlhausen, J.A.; Kotula, Paul G.

Nanometer scale morphological changes in the passive oxide on aluminum have been tracked as a function of polarization in an aqueous, moderate chloride electrolyte. Nanoscale void formation has been detected and characterized in the passive oxide on both single crystal Al and nanocrystalline Al thin films. Void nucleation occurs at the metal/oxide interface and growth proceeds into the oxide. This void formation process correlates with the faradaic charge density produced due to Al oxidation indicating that the voids result from point defect saturation at the Al/oxide interface. The shape factors for the voids are inconsistent with two leading pit initiation models where stable pitting is argued to result from disruption of the remnant oxide over a void or void-like structures. Several experimental observations and measurements suggest this predominant structural feature is not sufficient alone in determining the stability of the passive oxide toward stable pitting. An experiment is proposed and conducted to clearly establish causality between voids and stable pitting, however, the results are inclusive.

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121 Results
121 Results