bluuebunny/crossref_metadata_2025 huggingface dataset:
Experience with MCrAl and thermal barrier coatings produced via inert gas shrouded plasma deposition
Authors: Taylor T. A. , Overs M. P. , Gill B. J. , Tucker R. C.
Published: July 2002

Plasma spray deposition of MCrAlY coatings for corrosion resistance and rub tolerance on gas turbine blades, vanes and seal surfaces has received a great deal of attention in recent years. While most of the literature has described coatings produced with low pressure spray systems, substantial advantages can be realized using inert gas shrouded torches operating in the open air. This paper discusses some of these advantages and describes the coatings produced with efficient argon shrouded torches. In addition, specific examples of production MCrAlY and thermal barrier coatings are given.
An x-ray photoelectron spectroscopy study on ozone treated InP surfaces
Authors: Ingrey S. , Lau W. M. , McIntyre N. S. , Sodhi R.
Published: July 2002

The formation of sacrificial oxides on GaAs surfaces has become the normal procedure in the preparation of in situ cleaned substrates for molecular-beam epitaxial growth. In this paper we show that sacrificial oxides can be formed on InP by a UV/ozone treatment, yielding more controlled and reproducible properties than thermal and air formed oxides. Results from x-ray photoelectron spectroscopy studies on acid/DI water/air and UV/ozone treatments are compared. It is shown that thermal desorption of the former leaves residues containing carbon which can seriously degrade epitaxial growth, whereas UV/ozone formed sacrificial oxides could be removed at 494 °C leaving atomically clean, stoichiometric surfaces.
Surface reactions in the decomposition of dimethylzinc on Si(100)-2×1
Authors: Rueter M. A. , Vohs J. M.
Published: July 2002

The reaction of dimethylzinc on Si(100)-2×1 was studied using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Dimethylzinc dissociated on this surface at temperatures less than 400 K producing zinc metal and surface methyl groups. During TPD experiments the zinc metal desorbed completely by 600 K, leaving only methyl groups on the surface at higher temperatures. The predominate decomposition pathway for the surface methyl species was complete dehydrogenation producing molecular hydrogen and depositing carbon on the surface. HREELS results indicated that methyl group dehydrogenation occurred at temperatures greater than 700 K. For high initial coverages of dimethylzinc, in addition to complete dehydrogenation products, methyl radicals and acetylene were also detected. The hydrogen desorption temperature was found to increase as a function of the amount of carbon deposited on the surface.
The influence of preparation conditions on the optical properties of titanium nitride based solar control films
Authors: Georgson M. , Roos A. , Ribbing C.-G.
Published: July 2002

Single layers of TiO2 and TiN films, as well as triple layer structures of TiO2/TiN/TiO2, have been sputtered on substrates at temperatures ranging from room temperature to 450 °C. A new combination of materials was used to make the substrate heater. X-ray diffraction (XRD) was used to study the infiuence of substrate temperature upon the grain size. The optical constants n and k for TiO2 were evaluated. The oxide films grown at a higher deposition rate and on 300 °C substrates had a refractive index of 2.48 at 550 nm, compared to 2.29 for the film grown at a lower rate and at room temperature. A single layer of TiN exhibits improved optical selectivity with increased substrate temperature up to 300 °C. Further increase of the substrate temperature only caused a small effect on the heat mirror properties of the film. The triple layer is improved with increased substrate temperature. However, at temperatures of 300 °C and higher, a reduction of the heat mirror performance occurs and it is demonstrated by optical multilayer calculations that this is probably due to oxide growth on the outer TiN-TiO2 interface.
The curing kinetics of polyamic acid in the presence of water vapor
Authors: Buncick Milan C. , Denton Denice D.
Published: July 2002

Polyimides (PIs) are used extensively in integrated circuit fabrication due to their attractive physical properties. Variations in physical properties due to variations in processing and sensitivity to moisture can lead to reliability problems for microelectronic applications. We present the details of exposure of polyamic acid (PAA) to 85 °C/85%RH and to 85 °C/dryN2 as part of a long term study of PI degradation. The PAA under study is formed from the polycondensation of pyromellitic dianhydride (PMDA) and oxydianaline (ODA) in N-methyl pyrrolidone. The acid was spun onto two inch silicon wafers coated with chromium. The PAA cure was followed with electron spectroscopy for chemical analysis (ESCA) and Fourier transform infrared (FTIR) spectroscopy. Results show that the presence of water vapor during heating dramatically raises the initial rate of imidization. Measurements indicate the PAA is 6% cured after 72 h in the 85 °C/dry environment while the PAA is 68% cured after 72 h in the 85 °C/85%RH environment. Spectroscopic data confirm that both the surface and the bulk are affected by exposure to the humid environment.
Comparative scanning probe microscopy study of the surface morphology of Au films grown from the vapor onto glass, fused silica, and muscovite mica
Authors: DeRose J. A. , Lampner D. B. , Lindsay S. M. , Tao N. J.
Published: July 2002

In a previous paper [J. A. DeRose, T. Thundat, L. A. Nagahara, and S. M. Lindsay, Surf. Sci. 256, 102 (1991)], the conditions which can be used to grow Au films epitaxially on muscovite mica to obtain large area, flat faces are reported. Since then, it has been reported that Au films grown on an amorphous substrate [J. Hwang and M. A. Dubson, J. Appl. Phys. 72, 1852 (1992)] (nonepitaxy) have shown to be at least as smooth as those on mica. In this article, the results from scanning tunneling microscopy analysis of Au films grown on mica, glass cover slips, and fused silica are compared. By comparing the rms surface roughness (z data standard deviation, SD) values for each film type, we find that the mica films show the smoothest surfaces, i.e., smallest rms surface roughness values. We then attempt to explain these results by taking into account the accepted model of film growth from the vapor (physical vapor deposition) and the surface structure of bare mica, glass (optical microscope slide cover slip), and fused silica as shown with atomic force microscopy.
Depth profiling analysis of aluminum oxidation during film deposition in a conventional high vacuum system
Authors: Kim Jongmin , Weimer Jeffrey J. , Zukic Muamer , Torr Douglas G.
Published: July 2002

The oxidation of aluminum thin films deposited in a conventional high vacuum chamber has been investigated using x-ray photoelectron spectroscopy (XPS) and depth profiling. The state of the Al layer was preserved by coating it with a protective MgF2 layer in the deposition chamber. Oxygen concentrations in the film layers were determined as a function of sputter time (depth into the film). The results show that an oxidized layer is formed at the start of Al deposition and that a less extensively oxidized Al layer is deposited if the deposition rate is fast. The top surface of the Al layer oxidizes very quickly. This top oxidized layer may be thicker than has been previously reported by optical methods. Maximum oxygen concentrations measured by XPS at each Al interface are related to pressure to rate ratios determined during the Al layer deposition.
X-ray photoelectron spectroscopy and secondary ion mass spectrometry studies of some surface modified hydrocarbon films
Authors: Loh F. C. , Tan K. L. , Kang E. T. , Neoh K. G. , Pun M. Y.
Published: July 2002

Near-ultraviolet light induced graft copolymerization of water soluble monomers such as acrylamide (AAm), acrylic acid (AAc), and sodium salt of 4-styrenesulfonic acid (NaSS) onto pristine and ozone-pretreated polyethylene terephthalate (PET), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polystyrene (PS), polypropylene (PP), and polycarbonate (PC) had been carried out. An emeraldine (EM) base was then coated onto the AAc and NaSS polymer grafted substrates to give rise to self-protonated conductive surfaces. Using x-ray photoelectron spectroscopy and secondary ion mass spectrometry techniques, it was found that ozone pretreatment led to a substantial enhancement of the density of graft, and subsequently, the extent of protonation of the EM base on these polymer substrates. The models describing the graft structure or morphology are also presented.
Electronic structure of surface defects in K0.3MoO3
Authors: Breuer Klaus , Smith Kevin E. , Greenblatt Martha , McCarroll William
Published: July 2002

The nature of defects on the cleavage surface of the quasi-one-dimensional oxide conductor K0.3MoO3 has been studied by angle-resolved photoemission and Auger electron spectroscopy. Defects were created on the (201̄) surface by Ne+ ion bombardment and by photon-stimulated desorption (PSD). The crystal cleaves to expose K atoms on the surface. Initial sputtering of the cleaved surface leads to the disappearance of emission from the Mo 4d states at the Fermi level (EF) and the growth of a well-defined emission feature 2 eV below EF. Continued sputtering leads to the reappearance of emission at EF. We explain this behavior by the sequential removal of K and O, respectively, from the surface, and verify this model by using PSD to selectively create only O vacancies. This model is further verified by measuring work function changes during surface modifications and by the use of Auger spectroscopy.
Resonant neutralization of 7Li+ scattered from Cs/Al(100) as a probe of the local electrostatic potential
Authors: Weare C. B. , Yarmoff J. A.
Published: July 2002

Time-of-flight detection is used to measure the resonant neutralization probabilities for 1.2 and 2.0 keV 7Li+ ions singly scattered from Cs and Al atomic sites of the Cs/Al(100) surface. For the lowest Cs coverages, neutral fractions for scattering from Cs are ∼0.70 and ∼0.78, while the neutral fractions for scattering from Al are ∼0.43 and ∼0.55, respectively. The disparity in neutral fractions between scattering from Cs and Al demonstrates that the surface local electrostatic potential is significantly inhomogeneous. At higher Cs coverages [≳0.05 monolayer (ML)], however, the neutral fractions for scattering from Al and Cs sites become nearly equal, which indicates that the variations in the surface potential are greatly reduced. For Cs coverages above 0.075 ML, the formation of Li− negative ions via resonant tunneling is observed for scattering from Cs.
Failure analysis of high power GaAs-based lasers using electron beam induced current analysis and transmission electron microscopy
Authors: Mallard R. E. , Clayton R. , Mayer D. , Hobbs L.
Published: July 2002

We report on an extensive study of the defect structure associated with catastrophic failure of GaAs-based 980 nm pump lasers. Electron beam induced current (EBIC) analysis shows that catastrophic optical damage (COD) is characterized by the introduction of high densities of extended defects in the optical cavity of the laser, in the vicinity of the output facet. The heterostructure comprising the active region of the device is interdiffused in a spherical region surrounding the region of highest defect density. In some regions, melting of the laser cavity is observed. A “fast capture” laser degradation analysis demonstrates that the COD damage initiates at the laser facet, and propagates back along the cavity with continued device stressing. COD failure under pulsed operation results in a dramatically altered defect distribution consisting of periodic arrays of dislocation tangles along the laser cavity. Successive pulses following the initial failure event result in the formation of fresh defect “packets” which are separated from the damaged region generated due to the preceding pulse by a volume of relatively defect free material. The periodicity of these defective packets is related to the magnitude of the drive current pulse at the time of failure. Following the description of the defect distribution obtained using EBIC, we employed site-specific transmission electron microscope sectioning methods to form a detailed description of the structural modifications that the device undergoes at the onset of failure.
Hollow cathode magnetron
Authors: Wang Zhehui , Cohen Samuel A.
Published: July 2002

A new type of plasma sputtering device, named the hollow cathode magnetron (HCM), has been developed by surrounding a planar magnetron cathode with a hollow cathode structure. Operating characteristics of HCMs, current–voltage (I–V) curves for fixed discharge pressure and voltage–pressure (V–p) curves for fixed cathode current, are measured. Such characteristics are compared with their planar magnetron counterparts. New operation regimes, such as substantially lower pressures (0.3 mTorr), were discovered for HCMs. Cathode erosion profiles show marked improvement over planar magnetron in terms of material utilization. The use of HCMs for thin film deposition are discussed.
Mott–Schottky analysis of thin ZnO films
Authors: Windisch Charles F. , Exarhos Gregory J.
Published: July 2002

Thin ZnO films, both native and doped with secondary metal ions, have been prepared by sputter deposition and also by casting from solutions containing a range of precursor salts. The conductivity and infrared reflectivity of these films are subsequently enhanced chemically following treatment in H2 gas at 400 °C or by cathodic electrochemical treatment in a neutral (pH=7) phosphate buffer solution. While Hall-type measurements usually are used to evaluate the electrical properties of such films, the present study investigated whether a conventional Mott–Schottky analysis could be used to monitor the change in concentration of free carriers in these films before and after chemical and electrochemical reduction. The Mott–Schottky approach would be particularly appropriate for electrochemically modified films since the measurements could be made in the same electrolyte used for the post-deposition electrochemical processing. Results of studies on sputtered pure ZnO films in ferricyanide solution were promising. Mott–Schottky plots were linear and gave free carrier concentrations typical for undoped semiconductors. Film thicknesses estimated from the Mott–Schottky data were also reasonably close to thicknesses calculated from reflectance measurements. Studies on solution-deposited films were less successful. Mott–Schottky plots were nonlinear, apparently due to film porosity. A combination of dc polarization and atomic force microscopy measurements confirmed this conclusion. The results suggest that Mott–Schottky analysis would be suitable for characterizing solution-deposited ZnO films only after extensive modeling was performed to incorporate the effects of film porosity on the characteristics of the space-charge region of the semiconductor.
Application of chlorinated polymethylstyrene, CPMS, to electron beam lithography
Authors: Kamoshida Yoichi , Koshiba Mitsunobu , Yoshimoto Hiroshi , Harita Yoshiyuki , Harada Kunihiro
Published: July 2002

Swelling behavior and resist performance of a series of chlorinated polymethylstyrenes, CPMS, were investigated. Measurements of swelling ratio and resolution of the CPMS resist coating in a wide variety of organic solvents disclosed that resolution depends on the swelling ratio. In addition, it was found that Cellosolve acetate or methyl isobutyl ketone was the most suitable developer, and that the critical swelling ratio might be 3.2 (=1.473) for screening of a developer. Functional testings on CPMS showed the resist had considerably high sensitivity as well as high contrast and dry etch resistance. The selectivity ratio of the resist to aluminum, when the resist was used in a dry etching process, was 1.5 times as high as that of the conventional Novolak-type positive-working photoresist.
Analytical model of positive resist development applied to linewidth control in optical lithography
Authors: Watts M. P. C.
Published: July 2002

A new analytical model is described for positive resist development. Using the model, the linewidth that will be obtained at any development time is calculated directly from the incident image. The accuracy of the analytical model is verified by comparison to a numerical simulation of positive resist development (SAMPLE). The analytical model is used to obtain expressions for the rate of change of linewidth with respect to both process parameters and resist parameters. Some of the process parameters that are considered are developer time and temperature, prebake time and temperature, and exposure dose. Some of the resist parameters that are considered include sensitivity, contrast, absorption, and developer concentration.
Molecular-dynamics simulation of low-energy beam deposition of silicon
Authors: Dodson Brian W.
Published: July 2002

The mechanisms controlling low-energy (10–100 eV) beam deposition of silicon onto a relaxed (111) silicon substrate have been studied using a molecular dynamics technique. The atomic interaction was modeled using a many-body empirical potential so that the effects of the covalent Si–Si bonding could be accurately included. Silicon atoms (10 eV) with near-perpendicular incidence were studied to determine the energy-loss mechanism resulting in capture and the local lattice excitation near and subsequent diffusion of excess vibrational energy away from the impact point. Shallow angles of incidence (5°–30°) were studied for beam energies of 20–100 eV. For incidence angles less than an energy- and orientation-dependent critical value, a new phenomenon of ‘‘surface channeling’’ is observed, in which the trajectory of the incoming particle is steered parallel to, and roughly 2 Å above, the surface of the substrate by inelastic interaction with the surface atoms. The phenomena observed in low energy beam deposition offer considerable promise for precision control of the growth of nonequilibrium semiconductor structures.
Observation of the superstructure at the Al–GaAs(001) interface by synchrotron x-ray diffraction
Authors: Mizuki J. , Akimoto K. , Hirosawa I. , Hirose K. , Mizutani T. , Matsui J.
Published: July 2002

The superstructure at the Al–GaAs(001) interface has been observed for the first time by grazing incidence x-ray diffraction with the use of a synchrotron source. The Al overlayer of 150 Å in thickness was deposited on the (4×6)-reconstructed (001) GaAs surface. The Bragg peaks were observed at the fractional-order reciprocal lattice points of (n/4 l) in the [11̄0] direction and (0 m/6) in the [1̄1̄0] direction. The discussion is addressed to the correlation between interface reactions and the stability of the superstructure at the interface.
Formation of submicron silicon-on-insulator structures by lateral oxidation of substrate-silicon islands
Authors: Arney Susanne C. , MacDonald Noel C.
Published: July 2002

Fully isolated islands have been formed in silicon using selective, lateral thermal oxidation at the base of 250-nm-wide structures. The final structure consists of substrate silicon on thermal oxide on substrate silicon. The process begins with the definition of 250-nm-wide islands that are capped on the top and sidewall with a silicon dioxide/silicon nitride oxidation mask. The structure is then isotropically or anisotropically recess etched and thermally oxidized to produce isolated silicon islands. Our experiments show that the quality of the silicon-on-insulator structure depends on the oxidation mask, the island dimension, the profile of the recess etch, and the oxidation time and temperature. The degree of isolation can be tailored by controlling the lateral oxidation of the filament connecting the island to the underlying substrate. By this technique we have formed silicon filaments of 10 to 100 nm in width.
Polymethacrylonitrile as a resist in x-ray lithography
Authors: Schlegel L. , Schnabel W.
Published: July 2002

The resist performance of polymethacrylonitrile (PMCN) was tested with respect to applications in x-ray lithography by irradiating the polymer with x rays (λ: 0.2–1.5 nm) generated by the electron storage ring BESSY. Both radiation sensitivity and contrast were greatly improved by soaking the irradiated polymer films with a very weak solvent (e.g., diethylketone) prior to development. Fine structure features of 0.3 μm (lines and spaces) were obtained at exposure doses of about 200 mJ/cm2 using a mixture of cyclohexanone, o-xylene, and water (weight ratio 65:34:1) as developer. PMCN performed quite satisfactorily in anisotropically operating etch resistance tests using a CHF3/O2 plasma. The ratio of etch rates for PMCN and SiO2 was determined as RPMCN/RSiO2<0.8. The resist performance of PMCN was independent of the initial molecular mass and the initial molecular mass distribution of the polymer in the range M=1×105–2×106.
X-ray lithography: On the path to manufacturing
Authors: Warlaumont John
Published: July 2002

X-ray lithography has been an active area of research and development for the last 20 years, and recent spectacular results using complete, integrated x-ray lithography systems have demonstrated, at the research level, the viability of x-ray lithography, and have shown that x–ray lithography is on the path to manufacturing. This paper presents IBM’s recent successful development of a synchrotron-radiation-based x-ray lithography system. The individual components of the system are described, along with a brief description of available alternate components. Measurements of the lithographic performance of the integrated system are presented, and the use of this system for the fabrication of fully-scaled 0.5 μm CMOS circuits is described.
Influence of reactant transport on fluorine reactive ion etching of deep trenches in silicon
Authors: Arnold John C. , Gray David C. , Sawin Herbert H.
Published: July 2002

The reactive ion etching of silicon by fluorine in high-aspect ratio features was modeled to assess the relative importance of reactant transport on etching rate at the bottom of rectangular trenches. The flux of ions to the feature bottom was found by summing two components: ions arriving directly from the plasma and ions reflected from the sidewalls before reaching the bottom. The transport of neutral reactants within the feature was modeled with diffuse scattering and reaction rates following the kinetics reported by Gray et al. [J. Vac. Sci. Technol. B 11, 1243 (1993)] at all surfaces. The etching rate was found to depend most strongly upon the ion flux under typical process conditions, because of relatively low fluorine reaction probability and low reactant depletion within the feature. Reactant transport limitations are expected to be more important under conditions of low fluorine to ion flux ratio, high-substrate temperature, and high-ion energy.
Photoinduced surface reactions of reverse-biased n-type porous Si
Authors: Lee Eric J. , Bitner Theodore W. , Hall Anselm P. , Sailor Michael J.
Published: July 2002

Light induced surface reactions of reverse-biased n-type porous Si with H 2O, CH3OH or CF3COOH are studied by diffuse reflectance and transmission Fourier transform infrared spectroscopy. The goal of this work is to identify reagents which will react with the irradiated, electron deficient surface but not the non-irradiated, neutral surface of reverse-biased porous Si. The porous Si is the anode in an electrochemical cell with the desired reagent present as the electrolyte solution. H2O/0.5M NaCl is found to react with the reverse-biased Si in both the light and the dark to form the native oxide. CH 3OH/0.1M NaBF4 generates a small amount of methoxide-modified Si in both the light and the dark. CF3COOH/1M CF3COONa reacts with the surface when the porous Si is optically excited to create a surface-bound trifluoroacetic ester species. CF 3COOH/1M CF3COONa does not react with reverse biased Si in the dark. The Si surface can be photolithographically patterned with the trifluoroacetic ester species.
Effect of O2 on the electron emission characteristics of active molybdenum field emission cathode arrays
Authors: Chalamala Babu R. , Wallace Robert M. , Gnade Bruce E.
Published: July 2002

Electron emission from molybdenum field emission microcathode arrays is sensitive to the contamination of the emitters by the residual gases. We studied the effects of O2 exposures on the emission properties of several Spindt type molybdenum field emitter arrays. The arrays were exposed to 0–10 000 L of O2 and the resulting changes in electron emission characteristics were measured. Exposure of the field emitters to O2 resulted in emission degradation ranging from 6.2±1.3% for 10 L to 99.2±0.4% for a 10 000 L exposure. The emission is recoverable for low exposures, but exposures beyond 1000 L result in permanent loss in emission current. We found that the degradation effects are similar in both the dc and pulsed modes of operation of the device. Therefore, dc mode testing can be used as an effective acceleration method in establishing the device lifetimes under various vacuum conditions.
Challenges and progress in x-ray lithography
Authors: Silverman Jerome P.
Published: July 2002

X-ray lithography (XRL) is a very promising technique with the potential to be available for integrated circuit manufacturing as early as the 130 nm generation. As a result of many years of development, the technology is relatively mature. Synchrotron sources have demonstrated performance and reliability; preproduction aligners are available from multiple vendors; significant improvements are being made in mask fabrication; and high resolution imaging has been demonstrated at 100 nm and below. Established vendors already have experience with almost all of the tools that are needed, although improved performance is required for most in order to satisfy the error budgets at 130 nm and below. Significant development activities are continuing in both the United States and Japan, and numerous complex integrated circuits have been fabricated using XRL for one or more critical levels. Nonetheless, there are challenges still to be met. Among the most important are the development and commercial availability of an improved e-beam mask writer; the ability to fabricate defect-free masks satisfying the image placement and critical dimension control requirements with good yields; the stability of the masks in usage (including the issue of possible radiation damage); the ability to correct for magnification errors; and the ability to satisfy the industry’s desire for a technology extendible to 70 nm ground rules. These issues are primarily manufacturing issues, as opposed to issues related to demonstrating proof-of-concept or feasibility, although demonstrating extendibility is still needed before the industry can commit to using XRL at 70 nm ground rules. Because of the existing XRL facilities and experience, effective work to address these and other issues can be accomplished in a timely manner.
Gap-filling property of Cu film by chemical vapor deposition
Authors: Kobayashi Akiko , Sekiguchi Atsushi , Koide Tomoaki , Okada Osamu , Zhang Minjuan , Egami Akihiro , Sunayama Hideki
Published: July 2002

The gap-filling property of Cu deposited by chemical vapor deposition was investigated in 1/4-μm-wide trenches and 1/4-μm-diam holes. Attention was paid to the influences on the gap-filling property of both substrate temperature and partial pressure of the source gas. At a low deposition temperature of 180 °C, Cu could completely fill a hole with an aspect ratio of 7. A high deposition rate could be obtained at the high substrate temperature and high partial pressure of the source gas. However the coverage probably deteriorated because of the increase in sticking coefficient due to an increase in the reaction rate at high temperature. As the source gas pressure increased, the deposition rate increased and saturated in the high source pressure region. The good gap-filling property at high source pressures was attributed to the Langmuir–Hinshelwood reaction where almost the same deposition rate is realized from the top to the bottom of the trench. At a high temperature of 215 °C, voids were observed at both the center and along the wall of the trench. The voids along the wall were assumed to be formed at the initial stage of deposition. They disappeared with an increase in source gas flow rate. Therefore, the formation of voids could be due to an insufficient supply of source gas. By annealing the Cu film, its properties of increase in grain size and the reduction in the amount of C, O, and F contaminants were improved. After annealing and chemical mechanical polishing treatment, scanning electron microscopy observation showed Cu interconnecting lines with grains larger than the wire width.
Ultrafast Source-to-Surface Movement of Melt at Island Arcs from
226
Ra-
230
Th Systematics
Authors: Turner Simon , Evans Peter , Hawkesworth Chris
Published: July 2002

Island arc lavas have radium-226 excesses that extend to higher values than those observed in mid-ocean ridge or ocean island basalts. The initial ratio of radium-226 to thorium-230 is largest in the most primitive lavas, which also have the highest barium/thorium ratios, and decreases with increasing magmatic differentiation. Therefore, the radium-226 excesses appear to have been introduced into the base of the mantle melting column by fluids released from the subducting plate. Preservation of this signal requires transport to the surface arguably in only a few hundreds of years and directly constrains the average melt velocity to the order of 1000 meters per year. Thus, melt segregation and channel formation can occur rapidly in the mantle.
Role of Nonimmune IgG Bound to PfEMP1 in Placental Malaria
Authors: Flick Kirsten , Scholander Carin , Chen Qijun , Fernandez Victor , Pouvelle Bruno , Gysin Jurg , Wahlgren Mats
Published: July 2002

Infections with
Plasmodium falciparum
during pregnancy lead to the accumulation of parasitized red blood cells (infected erythrocytes, IEs) in the placenta. IEs of
P. falciparum
isolates that infect the human placenta were found to bind immunoglobulin G (IgG). A strain of
P. falciparum
cloned for IgG binding adhered massively to placental syncytiotrophoblasts in a pattern similar to that of natural infections. Adherence was inhibited by IgG-binding proteins, but not by glycosaminoglycans or enzymatic digestion of chondroitin sulfate A or hyaluronic acid. Normal, nonimmune IgG that is bound to a duffy binding–like domain β of the
P. falciparum
erythrocyte membrane protein 1 (PfEMP1) might at the IE surface act as a bridge to neonatal Fc receptors of the placenta.
Genetic Basis for Activity Differences Between Vancomycin and Glycolipid Derivatives of Vancomycin
Authors: Eggert Ulrike S. , Ruiz Natividad , Falcone Brian V. , Branstrom Arthur A. , Goldman Robert C. , Silhavy Thomas J. , Kahne Daniel
Published: July 2002

Small molecules that affect specific protein functions can be valuable tools for dissecting complex cellular processes. Peptidoglycan synthesis and degradation is a process in bacteria that involves multiple enzymes under strict temporal and spatial regulation. We used a set of small molecules that inhibit the transglycosylation step of peptidoglycan synthesis to discover genes that help to regulate this process. We identified a gene responsible for the susceptibility of
Escherichia coli
cells to killing by glycolipid derivatives of vancomycin, thus establishing a genetic basis for activity differences between these compounds and vancomycin.
Engineering Crystal Symmetry and Polar Order in Molecular Host Frameworks
Authors: Holman K. Travis , Pivovar Adam M. , Ward Michael D.
Published: July 2002

A crystal design strategy is described that produces a series of solid-state molecular host frameworks with prescribed lattice metrics and polar crystallographic symmetries. This represents a significant advance in crystal engineering, which is typically limited to manipulation of only gross structural features. The host frameworks, constructed by connecting flexible hydrogen-bonded sheets with banana-shaped pillars, sustain one-dimensional channels that are occupied by guest molecules during crystallization. The polar host frameworks enforce the alignment of these guests into polar arrays, with properly chosen guests affording inclusion compounds that exhibit second harmonic generation because of this alignment. This protocol exemplifies a principal goal of modern organic solid-state chemistry: the precise control of crystal symmetry and structure for the attainment of a specific bulk property.
A Fossil Lemur from the Oligocene of Pakistan
Authors: Marivaux Laurent , Welcomme Jean-Loup , Antoine Pierre-Olivier , Métais Grégoire , Baloch Ibrahim M. , Benammi Mouloud , Chaimanee Yaowalak , Ducrocq Stéphane , Jaeger Jean-Jacques
Published: July 2002

In the absence of a comprehensive fossil record, the origin and early evolution of Malagasy lemurs have been subject to much uncertainty. We report here the discovery of a strepsirrhine fossil with strong cheirogaleid lemur affinities,
Bugtilemur mathesoni
gen. et sp. nov., from early Oligocene deposits of the Bugti Hills (Balochistan, Pakistan).
Bugtilemur
represents the earliest record of Lemuriformes, which hence appear to have already diversified outside of Madagascar at least 30 million years ago. This fossil clearly enhances the critical role of the Indian subcontinent in the early diversification of lemurs and constrains paleobiogeographic models of strepsirrhine lemur evolution.
Superplumes from the Core-Mantle Boundary to the Lithosphere: Implications for Heat Flux
Authors: Romanowicz Barbara , Gung Yuancheng
Published: July 2002

Three-dimensional modeling of upper-mantle anelastic structure reveals that thermal upwellings associated with the two superplumes, imaged by seismic elastic tomography at the base of the mantle, persist through the upper-mantle transition zone and are deflected horizontally beneath the lithosphere. This explains the unique transverse shear wave isotropy in the central Pacific. We infer that the two superplumes may play a major and stable role in supplying heat and horizontal flow to the low-viscosity asthenospheric channel, lubricating plate motions and feeding hot spots. We suggest that more heat may be carried through the core-mantle boundary than is accounted for by hot spot fluxes alone.
An LRR Receptor Kinase Involved in Perception of a Peptide Plant Hormone, Phytosulfokine
Authors: Matsubayashi Yoshikatsu , Ogawa Mari , Morita Akiko , Sakagami Youji
Published: July 2002

The sulfated peptide phytosulfokine (PSK) is an intercellular signal that plays a key role in cellular dedifferentiation and proliferation in plants. Using ligand-based affinity chromatography, we purified a 120-kilodalton membrane protein, specifically interacting with PSK, from carrot microsomal fractions. The corresponding complementary DNA encodes a 1021–amino acid receptor kinase that contains extracellular leucine-rich repeats, a single transmembrane domain, and a cytoplasmic kinase domain. Overexpression of this receptor kinase in carrot cells caused enhanced callus growth in response to PSK and a substantial increase in the number of tritium-labeled PSK binding sites, suggesting that PSK and this receptor kinase act as a ligand-receptor pair.
RAG Mutations in Human B Cell-Negative SCID
Authors: Schwarz Klaus , Gauss George H. , Ludwig Leopold , Pannicke Ulrich , Li Zhong , Lindner Doris , Friedrich Wilhelm , Seger Reinhard A. , Hansen-Hagge Thomas E. , Desiderio Stephen , Lieber Michael R. , Bartram Claus R.
Published: July 2002

Patients with human severe combined immunodeficiency (SCID) can be divided into those with B lymphocytes (B
+
SCID) and those without (B
−
SCID). Although several genetic causes are known for B
+
SCID, the etiology of B
−
SCID has not been defined. Six of 14 B
−
SCID patients tested were found to carry a mutation of the recombinase activating gene 1 (
RAG-1
),
RAG-2
, or both. This mutation resulted in a functional inability to form antigen receptors through genetic recombination and links a defect in one of the site-specific recombination systems to a human disease.
Distinct Mechanisms for Synchronization and Temporal Patterning of Odor-Encoding Neural Assemblies
Authors: MacLeod Katrina , Laurent Gilles
Published: July 2002

Stimulus-evoked oscillatory synchronization of neural assemblies and temporal patterns of neuronal activity have been observed in many sensory systems, such as the visual and auditory cortices of mammals or the olfactory system of insects. In the locust olfactory system, single odor puffs cause the immediate formation of odor-specific neural assemblies, defined both by their transient synchronized firing and their progressive transformation over the course of a response. The application of an antagonist of ionotropic γ-aminobutyric acid (GABA) receptors to the first olfactory relay neuropil selectively blocked the fast inhibitory synapse between local and projection neurons. This manipulation abolished the synchronization of the odor-coding neural ensembles but did not affect each neuron's temporal response patterns to odors, even when these patterns contained periods of inhibition. Fast GABA-mediated inhibition, therefore, appears to underlie neuronal synchronization but not response tuning in this olfactory system. The selective desynchronization of stimulus-evoked oscillating neural assemblies in vivo is now possible, enabling direct functional tests of their significance for sensation and perception.
Quick-Change Pathogens Gain an Evolutionary Edge
Authors: Grady Denise
Published: July 2002

Bacteria have a prodigious ability to reinvent themselves, speedily adapting to new hosts, new conditions, and new antibiotic countermeasures. A group of researchers report in this issue (p. 1208) that they have found one explanation for that versatility in E. coli and another pathogen, Salmonella enterica. In strains from outbreaks of food poisoning, they detected an exceptionally high percentage of "mutators"--microbes with a genetic flaw that makes them a prolific source of new variants.
The Origin of Gravitational Lensing: A Postscript to Einstein's 1936
Science
Paper
Authors: Renn Jürgen , Sauer Tilman , Stachel John
Published: July 2002

Gravitational lensing, now taken as an important astrophysical consequence of the general theory of relativity, was found even before this theory was formulated but was discarded as a speculative idea without any chance of empirical confirmation. Reconstruction of some of Einstein's research notes dating back to 1912 reveals that he explored the possibility of gravitational lensing 3 years before completing his general theory of relativity. On the basis of preliminary insights into this theory, Einstein had already derived the basic features of the lensing effect. When he finally published the very same results 24 years later, it was only in response to prodding by an amateur scientist.
Microtubule Treadmilling in Vivo
Authors: Rodionov Vladimir I. , Borisy Gary G.
Published: July 2002

In vivo, cytoplasmic microtubules are nucleated and anchored by their minus ends at the centrosome and are believed to turn over by a mechanism termed dynamic instability: depolymerization and repolymerization at their plus ends. In cytoplasmic fragments of fish melanophores, microtubules were shown to detach from their nucleation site and depolymerize from their minus ends. Free microtubules moved toward the periphery by treadmilling—growth at one end and shortening from the opposite end. Frequent release from nucleation sites may be a general property of centrosomes and permit a minus-end mechanism of microtubule turnover and treadmilling.
Dynamics of Transport Processes in the Upper Troposphere
Authors: Mahlman J. D.
Published: July 2002

A number of key problems in atmospheric chemistry are shaped by the strength and character of the various mechanisms acting to move and mix air in the upper troposphere. These transport processes are examined from a mechanistic perspective, with primary emphasis on the tropopause and middle-troposphere regions in the extratropics. The roles of vertical and horizontal transport “barriers” are explored, including the processes by which such barriers are created and are overcome. These transport considerations lead to a hypothesis concerning the processes that shape the tropopause itself. Some perspectives are offered on the still immature subject of transport in the upper troposphere of the tropics.
Planetary Nebulae: Understanding the Physical and Chemical Evolution of Dying Stars
Authors: Weinberger Ronald , Kerber Florian
Published: July 2002

Planetary nebulae are one of the few classes of celestial objects that are active in every part of the electromagnetic spectrum. These fluorescing and often dusty expanding gaseous envelopes were recently found to be quite complex in their dynamics and morphology, but refined theoretical models can account for these discoveries. Great progress was also made in understanding the mechanisms that shape the nebulae and the spectra of their central stars. In addition, applications for planetary nebulae have been worked out; for example, they have been used as standard candles for long-range distances and as tracers of the enigmatic dark matter.
The Filamentous Phage pIV Multimer Visualized by Scanning Transmission Electron Microscopy
Authors: Linderoth Nora A. , Simon Martha N. , Russel Marjorie
Published: July 2002

A family of homomultimeric outer-membrane proteins termed secretins mediates the secretion of large macromolecules such as enzymes and filamentous bacteriophages across bacterial outer membranes to the extracellular milieu. The secretin encoded by filamentous phage f1 was purified. Mass determination of individual molecules by scanning transmission electron microscopy revealed two forms, a unit multimer composed of about 14 subunits and a multimer dimer. The secretin is roughly cylindrical and has an internal diameter of about 80 angstroms, which is large enough to accommodate filamentous phage (diameter of 65 angstroms).
The Mars Pathfinder Atmospheric Structure Investigation/Meteorology (ASI/MET) Experiment
Authors: Schofield J. T. , Barnes J. R. , Crisp D. , Haberle R. M. , Larsen S. , Magalhães J. A. , Murphy J. R. , Seiff A. , Wilson G.
Published: July 2002

The Mars Pathfinder atmospheric structure investigation/meteorology (ASI/MET) experiment measured the vertical density, pressure, and temperature structure of the martian atmosphere from the surface to 160 km, and monitored surface meteorology and climate for 83 sols (1 sol = 1 martian day = 24.7 hours). The atmospheric structure and the weather record are similar to those observed by the Viking 1 lander (VL-1) at the same latitude, altitude, and season 21 years ago, but there are differences related to diurnal effects and the surface properties of the landing site. These include a cold nighttime upper atmosphere; atmospheric temperatures that are 10 to 12 degrees kelvin warmer near the surface; light slope-controlled winds; and dust devils, identified by their pressure, wind, and temperature signatures. The results are consistent with the warm, moderately dusty atmosphere seen by VL-1.
Estimating the Mass of Asteroid 253 Mathilde from Tracking Data During the NEAR Flyby
Authors: Yeomans D. K. , Barriot J.-P. , Dunham D. W. , Farquhar R. W. , Giorgini J. D. , Helfrich C. E. , Konopliv A. S. , McAdams J. V. , Miller J. K. , Owen W. M. , Scheeres D. J. , Synnott S. P. , Williams B. G.
Published: July 2002

The terminal navigation of the Near Earth Asteroid Rendezvous (NEAR) spacecraft during its close flyby of asteroid 253 Mathilde involved coordinated efforts to determine the heliocentric orbits of the spacecraft and Mathilde and then to determine the relative trajectory of the spacecraft with respect to Mathilde. The gravitational perturbation of Mathilde on the passing spacecraft was apparent in the spacecraft tracking data. As a result of the accurate targeting achieved, these data could be used to determine Mathilde's mass as 1.033 (± 0.044) × 10
20
grams. Coupled with a volume estimate provided by the NEAR imaging team, this mass suggests a low bulk density for Mathilde of 1.3 grams per cubic centimeter.
True Polar Wander as a Mechanism for Second-Order Sea-Level Variations
Authors: Mound Jon E. , Mitrovica Jerry X.
Published: July 2002

Long-term wander of the rotation pole can be a significant contributor to second-order (time scales of ∼100 million years) sea-level variations. Numerical predictions based on realistic viscoelastic Earth models and paleomagnetically constrained polar motion yield global-scale, differential sea-level trends that can be as large as ∼200 meters. From the results presented here, it is argued that the well-documented, second-order, Cretaceous-Tertiary sea-level cycle should be reinterpreted as some combination of a eustatic and a regionally varying rotational signal.
Propagating Activity Patterns in Large-Scale Inhibitory Neuronal Networks
Authors: Rinzel J. , Terman D. , Wang X.-J. , Ermentrout B.
Published: July 2002

The propagation of activity is studied in a spatially structured network model of γ-aminobutyric acid–containing (GABAergic) neurons exhibiting postinhibitory rebound. In contrast to excitatory-coupled networks, recruitment spreads very slowly because cells fire only after the postsynaptic conductance decays, and with two possible propagation modes. If the connection strength decreases monotonically with distance (on-center), then propagation occurs in a discontinuous manner. If the self- and nearby connections are absent (off-center), propagation can proceed smoothly. Modest changes in the synaptic reversal potential can result in depolarization-mediated waves that are 25 times faster. Functional and developmental roles for these behaviors and implications for thalamic circuitry are suggested.
Shear-Wave Splitting and Implications for Mantle Flow Beneath the MELT Region of the East Pacific Rise
Authors: Wolfe Cecily J. , Solomon Sean C.
Published: July 2002

Shear-wave splitting across the fast-spreading East Pacific Rise has been measured from records of
SKS
and
SKKS
phases on the ocean-bottom seismometers of the Mantle Electromagnetic and Tomography (MELT) Experiment. The direction of fast shear-wave polarization is aligned parallel to the spreading direction. Delay times between fast and slow shear waves are asymmetric across the rise, and off-axis values on the Pacific Plate are twice those on the Nazca Plate. Splitting on the Pacific Plate may reflect anisotropy associated with spreading-induced flow above a depth of about 100 km, as well as a deeper contribution from warm asthenospheric return flow from the Pacific Superswell region.
A Molecular Phylogeny of Reptiles
Authors: Hedges S. Blair , Poling Laura L.
Published: July 2002

The classical phylogeny of living reptiles pairs crocodilians with birds, tuataras with squamates, and places turtles at the base of the tree. New evidence from two nuclear genes, and analyses of mitochondrial DNA and 22 additional nuclear genes, join crocodilians with turtles and place squamates at the base of the tree. Morphological and paleontological evidence for this molecular phylogeny is unclear. Molecular time estimates support a Triassic origin for the major groups of living reptiles.
Solar Cycle Variability, Ozone, and Climate
Authors: Shindell Drew , Rind David , Balachandran Nambeth , Lean Judith , Lonergan Patrick
Published: July 2002

Results from a global climate model including an interactive parameterization of stratospheric chemistry show how upper stratospheric ozone changes may amplify observed, 11-year solar cycle irradiance changes to affect climate. In the model, circulation changes initially induced in the stratosphere subsequently penetrate into the troposphere, demonstrating the importance of the dynamical coupling between the stratosphere and troposphere. The model reproduces many observed 11-year oscillations, including the relatively long record of geopotential height variations; hence, it implies that these oscillations are likely driven, at least in part, by solar variability.