bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication of silicon and metal nanowires and dots using mechanical atomic force lithography
Authors: Hu S. , Hamidi A. , Altmeyer S. , Köster T. , Spangenberg B. , Kurz H.
Published: July 2002

A novel bilayer resist system consisting of a 3 nm thick titanium (Ti) layer on top of a 65 nm thick poly(methylmethacrylate) (PMMA) layer was developed for mechanical nanolithography with the atomic force microscope. The ultrathin Ti layer allowed 20 nm resolution patterning with conventional silicon cantilevers, provided a proper force-depth calibration was performed before lithography. Techniques of pattern transfer were applied to fabricate chromium nanostructures and silicon nanowires from the patterned Ti/PMMA resist.
Reflow of copper in an oxygen ambient
Authors: Lee Seung-Yun , Kim Dong-Won , Rha Sa-Kyun , Park Chong-Ook , Park Hyung-Ho
Published: July 2002

In order to investigate the reflow characteristics of copper, copper was deposited on hole and trench patterns by metal organic chemical vapor deposition and it was annealed in nitrogen and oxygen ambients with the annealing temperatures ranging from 350 to 550 °C. Upon annealing in an oxygen ambient at higher than 450 °C, copper was reflowed into the trench patterns whose line- width and aspect ratio were 0.2 μm and 4:1, respectively. Copper oxide was found with a thickness of less than a fifth of the total film thickness. The resistivity of the copper film increased when reflow occurred. It is thought that the reflow of copper in an oxygen ambient takes place because of enhanced surface diffusion.
Elimination of gate oxide damage during electron cyclotron resonance plasma etching of the tungsten polycide gate structure (WSi/poly-Si)
Authors: Chen Rui , Koretsky Milo D.
Published: July 2002

Oxide damage during the over etch step of a polycide gate structure is studied. Seven process parameters of the electron cyclotron resonance (ECR) plasma etching system are systematically investigated. These parameters include: rf power, microwave power, main coil and subcoil currents, Cl2 and O2 gas flow rate, and pressure. The results show that damage depends strongly on only three parameters: rf power, subcoil current, and O2 flow rate. As a result, a highly anisotropic, damage and poly-Si residue free etch of the polycide gate structure, with a high etch rate, has been achieved. These etch characteristics are critical for manufacturing the next generation of IC products.
Nonstatistical degradation and development characteristics of poly(methylmethacrylate) based resists during electron beam exposure
Authors: Uhl A. , Bendig J. , Leistner J. , Jagdhold U. , Bauch L. , Böttcher M.
Published: July 2002

We present an investigation to study the degradation of poly(methylmethacrylate) (PMMA) based resists during electron beam exposure and their development characteristics. After exposure to the electron beam we detect for ARP 610 resist (PMMA 74%, PMAA 26%) and for a homopolymeric PMMA resist, a similar bimodal respectively multimodal molecular weight distribution curve characterized by a shift of the maximum from 105 to 103 g/mol and an increase of low molecular weight fractions (≈103 g/mol) with increasing exposure dose. The model of Greeneich can only be applied to lower deposited energy densities (ARP 610<4 eV/nm3, homopolymeric PMMA <8 eV/nm3). The difference found between experimental data and modeling values at higher deposited energy densities for both resists, results from a formation of stable low molecular weight fractions. Comparing the measured dissolution rate with the calculated one, the determined difference proves the nonapplicability of the empirical formula for the dissolution rate given by Greeneich. The low molecular weight fractions influence the dissolution rate significantly more than given by their portion in the molecular weight distribution.
Comparative study of AZPN114 and SAL601 chemically amplified resists for electron beam nanolithography
Authors: Cui Zheng , Gerardino Annamaria , Gentili Massimo , DiFabrizio Enzo , Prewett Phil D.
Published: July 2002

With the minimum feature dimension of integrated circuits (ICs) shrinking to below 130 nm, the IC industry has to choose one among a number of nonoptical lithography tools. Electron beam direct write or mix-and-match lithography is an attractive choice because of the maturity of the technology and the potential of using the tool for several generations of ICs. The low throughput associated with e-beam direct writing of wafers can be significantly reduced by using chemically amplified resists. Two commercially available negative tone chemically amplified resists, AZPN114 and SAL601, were investigated and compared for their performance in e-beam direct writing of wafers at 150, 100, and sub-50 nm resolutions. A number of factors influencing the throughput issue were evaluated. The latitudes of postexposure bake and postexposure delay were compared. It is found that both resists are of high resolution and high contrast, with the highest resolution, 30 nm, achieved for AZPN114. AZPN114 has higher sensitivity and is more stable during postexposure delay than SAL601 but less stable during postexposure bake. A 6 in. industrial wafer was exposed with AZPN114 at gate level at 150 nm resolution using e-beam lithography and it showed feature uniformity across the entire wafer.
Large area high density quantized magnetic disks fabricated using nanoimprint lithography
Authors: Wu Wei , Cui Bo , Sun Xiao-yun , Zhang Wei , Zhuang Lei , Kong Linshu , Chou Stephen Y.
Published: July 2002

A new low-cost, high throughput method was developed for fabricating large area quantized magnetic disks (QMDs) using nanoimprint lithography (NIL), electroplating, and chemical mechanical polishing. Perpendicular QMDs with a density of 18 Gbit/in.2 and good uniformity over an area of 4 cm×4 cm (total 45 Gbit) have been achieved, as well as longitudinal QMDs of 30 Gbit/in.2 The NIL molds for the perpendicular QMDs were fabricated using double NIL with a grating mold. The magnetic properties of both types of QMDs were studied by magnetic force microscopy.
Problems of the nanoimprinting technique for nanometer scale pattern definition
Authors: Scheer H.-C. , Schulz H. , Hoffmann T. , Sotomayor Torres C. M.
Published: July 2002

We have tested nanoimprint lithography, a new and promising technique for nanometer-scale pattern definition. Preliminary experiments reveal that, besides severe sticking and adhesion problems, the problem of material transport is one inherent to this technique. There are clear indications that most of the effects found may be understood in terms of material transport. We performed experiments within a well defined pressure and temperature window which ranged from 60 to 100 bar and from 50 to 90 °C above the glass transition temperature of the poly(methylmethacrylate)-like polymer used. As a result, the quality of imprint is evaluated with respect to full area pattern transfer, based on a qualitative scanning electron microscope investigation of the fully imprinted area of 2 cm × 2 cm patterned with features of different size and shape. Optimum conditions for imprint quality are found around 100 bar and 90 °C above Tg for the specific polymer used. Although material transport will limit nanoimprint performance in general, it is found that periodic patterns and isolated or small area negative stamp relief patterns are most suitable for high quality nanoimprinting.
Environmental stability of 193 nm single layer chemically amplified resists
Authors: Timko A. G. , Houlihan F. M. , Cirelli R. A. , Nalamasu O. , Yoshino Hiroshi , Itani Toshiro , Tanabe Hiroyoshi , Kasama Kunihiko
Published: July 2002

The principle of chemical amplification was introduced to develop fast and high resolution resist materials, primarily for deep-ultraviolet (UV) lithography. Excellent sensitivity in CA resists emanates from the utilization of a photogenerated species, typically an acid, to cause several catalytic crosslinking or deblocking events during a postexposure bake (PEB) reaction. Deactivation of the photoacid by airborne basic contaminants or other pathways typically changed the feature width and/or the profile as a function of the delay between exposure and the PEB. Decreasing the photoacid strength, use of low activation energy protecting groups, a decrease in the catalytic turnover rates or a combination of these has helped alleviate this. Today, deep-UV resists that show several hours of postexposure stability are available. We have examined the postexposure delay stability of several 193 nm resists that varied in matrix polymer, photoacid generator, and protecting group chemistry as a function of different concentrations of ammonia and N-methyl pyrrolidone (NMP) at three different relative humidity conditions. In this article we will discuss the experimental setup and describe the environmental contamination stability of these different resists as well as describe in detail the design methodology employed in formulating the resist that showed no significant variation in its linewidth of 0.16 μm line/space pairs with 13 ppb of ammonia or 14 ppb of NMP.
Remote microwave plasma source for cleaning chemical vapor deposition chambers: Technology for reducing global warming gas emissions
Authors: Raoux S. , Tanaka T. , Bhan M. , Ponnekanti H. , Seamons M. , Deacon T. , Xia L.-Q. , Pham F. , Silvetti D. , Cheung D. , Fairbairn K. , Jonhson A. , Pearce R. , Langan J.
Published: July 2002

The semiconductor industry uses a large amount of perfluoro compounds (PFCs), and their impact on global warming has become a major environmental concern. In the semiconductor industry, PFC are used to periodically remove deposits from the chamber walls of chemical vapor deposition (CVD) reactors after film deposition. These chamber clean processes account for typically 50%–70% of the PFC usage in a semiconductor wafer fabrication site, the rest being mainly used for wafer-etching processes. With a conventional parallel plate radio frequency (rf) plasma reactor, the PFC gas utilization is incomplete and a large fraction of unreacted gas can be emitted in the atmosphere. This paper describes a microwave plasma source that provides as high as 99.9% utilization removal efficiency (URE) of the reactant gas (NF3) during chamber clean. This technology brings the million metric tons carbon equivalent (MMTCE) of a chamber clean to negligible levels and also enhances the chamber clean efficiency and the system throughput. Here we review the requirements for the manufacturability of a remote plasma clean process. Gaseous Fourier transform infrared and quadrupole mass spectroscopy techniques have been used to characterize the clean process, the by-products of the reaction, and the efficiency in reducing the MMTCE of CVD chamber cleans.
Field emission properties of the polycrystalline diamond-coated silicon emitters
Authors: Yang Su-Hua , Yokoyama Meiso
Published: July 2002

This work examines the electron emission characteristics of a polycrystalline diamond-coated silicon field emitter fabricated by hot filament chemical vapor deposition. The electron emission behavior of a polycrystalline diamond-coated Si field emission array (D/Si-FEA) was compared with that of an uncoated Si field emission array (Si-FEA) and flat-diamond film coated on Si substrate (flat-D/Si). From Raman spectrum and x-ray diffraction pattern analyses, it was revealed that the overcoated diamond film involved structural defects, graphite inclusion, grain boundaries, and a negative electron affinity surface. The direct current current–voltage (I–V) characteristics exhibited linear ohmic emission behavior at low anode voltages and Fowler–Nordheim emission behavior at high anode voltages. The threshold voltages for the D/Si-FEA, Si-FEA, and flat-D/Si were about 250, 666, and 800 V, respectively. The maximum emission current for the D/Si-FEA, 0.23 mA, was obtained for a 500 V anode bias. The low onset voltage of the D/Si-FEA was attributed to the cone-shaped emitter and the excellent properties of the overcoated diamond film.
Minimization of chemical-mechanical planarization (CMP) defects and post-CMP cleaning
Authors: Zhang Liming , Raghavan Srini , Weling Milind
Published: July 2002

Chemical-mechanical planarization (CMP) is inherently a dirty process. Defect minimization in CMP is of great interest to the overall success of the manufacturing of sub-0.20 μm devices. In this article, some general aspects of contamination in CMP and the effect of CMP defects on the manufacturing of devices will be reviewed. Defects that are created during a CMP process can be classified into four categories: particulate, metallic, organic, and others. Additives to CMP slurries that can modulate interfacial chemistry and electrochemistry of a solid film/solution system will be reviewed. CMP defects and their minimization are also associated to CMP process itself. General approaches that can be used for removing contaminants after CMP will be analyzed. Because defects are mostly formed as a result of the interaction between the slurry and the film being polished during CMP, the reduction of defects can be achieved through a fundamental understanding of the surface chemistry and electrochemistry aspects of the CMP system.
InP-based photonic micro-sensor for near field optical investigations
Authors: Bélier B. , Castagne M. , Falgayrettes P. , Bonnafé J. , Santoso A. , Leclercq J. L.
Published: July 2002

This article describes the fabrication process for an InP-based atomic force microscopy (AFM)-type cantilever that is equipped with a micro-photonic sensor. This cantilever, intended to be associated with a diamond supertip, will make it possible to obtain synchronous AFM and near field optical (NFO) images in a scanning operation. The mechanical, electrical and optical performance of the photonic sensor is detailed. This photonic sensor was successfully used as a photodetector. It is possible to integrate the system into a hybrid data processing circuit. Such a cantilever could be used simultaneously in the NFO mode and in AFM operations.
Structural properties of AlGaN/GaN heterostructures on Si(111) substrates suitable for high-electron mobility transistors
Authors: Kaiser S. , Jakob M. , Zweck J. , Gebhardt W. , Ambacher O. , Dimitrov R. , Schremer A. T. , Smart J. A. , Shealy J. R.
Published: July 2002

Transmission electron microscopy (TEM) investigations of metal organic vapor phase deposition grown AlxGa1−xN/GaN heterostructures on Si(111) containing an AlN high-temperature buffer layer have been carried out. The structural properties at the interface and in the epilayer as well as the electronic properties suitable for a high electron mobility transistor (HEMT) were analyzed and compared with systems grown on Al2O3(0001). High resolution TEM (HRTEM) at the AlN/Si(111) interface reveals a 1.5–2.7 nm thick amorphous SiNx layer due to the high growth temperature of TAlN=1040 °C. Therefore, a grain-like GaN/AlN region extending 40–60 nm appears and it is subsequently overgrown with (0001) orientated GaN material because of geometrical selection. The residual strain at the AlN/Si(111) interface is estimated to be εr=0.3±0.6% by Fourier filtering of HRTEM images and a moiré fringe analysis. This indicates almost complete relaxation of the large mismatch f(AlN/Si)=+23.4% which seems to be supported by the SiNx layer. Weak beam imaging and plan view TEM show typical threading dislocations in the epilayer with a density of 3×109 cm−2 extending along 〈0001〉 which sometimes form grain boundaries. An AlxGa1−xN/GaN interface roughness of 3 monolayers is estimated and a small AlxGa1−xN surface roughness of 1.5 nm is obtained by HRTEM and atomic force microscopy investigations which correspond to two-dimensional growth. C–V and Hall measurements reveal two-dimensional electron gas at the Al32Ga68N/GaN interface that has a sheet carrier concentration of 4×1012 cm−2. The electron mobility of 820 cm2/Vs measured at room temperature is applicable for a HEMT grown on Si(111).
Characterization and nanometer-scale modifications of Bi2Te3 surface via atomic force microscopy
Authors: Czajka R. , Horák J. , Lošt’ák P. , Karamazov S. , Vaniš J. , Walachová J.
Published: July 2002

The structure of the basal plane of a Bi2Te3 crystal surface was studied by means of the atomic force microscopy (AFM) technique. The measured heights of steps, ranging from 0.2 to 1.4 nm, were in agreement with other structural data of Bi2Te3. The images of freshly cleaved surface, obtained with atomic resolution, revealed a plane of tellurium atoms. The recorded interatomic distances of 0.42±0.02 nm were also in accordance with the structural data. We also showed that there is a possibility of surface nanostructures formation using the AFM probe, when working above force threshold value.
Comparison between tantalum carbosulfide and TaCl5-graphite intercalation compound by scanning tunneling microscopy
Authors: Walter J. , Shioyama H. , Hara S.
Published: July 2002

The layered compounds of TaCl5-graphite intercalation compound, second-stage stacking sequence: C–TaCl5–C–C and tantalum carbosulfide were studied by scanning tunneling microscopy. Although both materials consist of layers the information on their content gathered by scanning tunneling microscopy is quite different for both. The second-stage TaCl5-graphite intercalation compound provides detailed information concerning the superstructure and positions of adjacent chlorine atoms at the upper graphene layer. Tantalum carbosulfide is a layered compound with a stacking sequence of S–Ta–C–Ta–S. Direct information from the first (sulfur) layer only could be obtained. Measured amplitudes are distinguishable for the sulfur layer, this may indicate the occurrence of charge density waves.
Mapping the field-emission tunneling barrier of organic adsorbates on tungsten
Authors: Condon G. R. , Panitz J. A.
Published: July 2002

The field-emission tunneling barrier has been mapped for the doublet and quadruplet emission patterns associated with organic adsorbates on tungsten. The tunneling barrier was mapped by photometric probe-hole field-emission electron microscopy (PhotoFEEM). The adsorbates were deposited by evaporating a film of the molecule copper-phthalocyanine (CuPc) but it may be polymeric groups of CuPc or decomposition products. The tunneling barrier displays unexpected structures that are not seen in the corresponding field-emission images. Doublet patterns observed in the FEEM image can display singlet structures in the corresponding PhotoFEEM barrier maps. Similarly, quadruplet patterns in the FEEM image can display doublet structures in the PhotoFEEM barrier maps. This behavior is similar to supply limited tunneling previously observed in the field-emission barrier of a clean tungsten emitter. An analysis of deviations from purely linear Fowler–Nordheim behavior indicates that the observed structures arise from independent emitters that are spatially superimposed on the surface.
Outgassing Behavior and Composition of Comet C/1999 S4 (LINEAR) During Its Disruption
Authors: Bockelée-Morvan Dominique , Biver Nicolas , Moreno Raphaël , Colom Pierre , Crovisier Jacques , Gérard Éric , Henry Florence , Lis Dariusz C. , Matthews Henry , Weaver H. A. , Womack Maria , Festou Michel C.
Published: July 2002

The gas activity of comet C/1999 S4 (LINEAR) was monitored at radio wavelengths during its disruption. A runaway fragmentation of the nucleus may have begun around 18 July 2000 and proceeded until 23 July. The mass in small icy debris (≤30-centimeter radius) was comparable to the mass in the large fragments seen in optical images. The mass budget after breakup suggests a small nucleus (∼100- to 300-meter radius) that had been losing debris for weeks. The HNC, H
2
CO, H
2
S, and CS abundances relative to H
2
O measured during breakup are consistent with those obtained in other comets. However, a deficiency in CH
3
OH and CO is observed.
Evolution of Cell Recognition by Viruses
Authors: Baranowski Eric , Ruiz-Jarabo Carmen M. , Domingo Esteban
Published: July 2002

Evolution of receptor specificity by viruses has several implications for viral pathogenesis, host range, virus-mediated gene targeting, and viral adaptation after organ transplantation and xenotransplantation, as well as for the emergence of viral diseases. Recent evidence suggests that minimal changes in viral genomes may trigger a shift in receptor usage for virus entry, even into the same cell type. A capacity to exploit alternative entry pathways may reflect the ancient evolutionary origins of viruses and a possible role as agents of horizontal gene transfers among cells.
On Atmospheric Loss of Oxygen Ions from Earth Through Magnetospheric Processes
Authors: Seki K. , Elphic R. C. , Hirahara M. , Terasawa T. , Mukai T.
Published: July 2002

In Earth's environment, the observed polar outflow rate for O
+
ions, the main source of oxygen above gravitational escape energy, corresponds to the loss of ∼18% of the present-day atmospheric oxygen over 3 billion years. However, part of this apparent loss can actually be returned to the atmosphere. Examining loss rates of four escape routes with high-altitude spacecraft observations, we show that the total oxygen loss rate inferred from current knowledge is about one order of magnitude smaller than the polar O
+
outflow rate. This disagreement suggests that there may be a substantial return flux from the magnetosphere to the low-latitude ionosphere. Then the net oxygen loss over 3 billion years drops to ∼2% of the current atmospheric oxygen content.
Controlling Chemical Turbulence by Global Delayed Feedback: Pattern Formation in Catalytic CO Oxidation on Pt(110)
Authors: Kim Minseok , Bertram Matthias , Pollmann Michael , Oertzen Alexander von , Mikhailov Alexander S. , Rotermund Harm Hinrich , Ertl Gerhard
Published: July 2002

Control of spatiotemporal chaos is one of the central problems of nonlinear dynamics. We report on suppression of chemical turbulence by global delayed feedback using, as an example, catalytic carbon monoxide oxidation on a platinum (110) single-crystal surface and carbon monoxide partial pressure as the controlled feedback variable. When feedback intensity was increased, spiral-wave turbulence was transformed into new intermittent chaotic regimes with cascades of reproducing and annihilating local structures on the background of uniform oscillations. The global feedback further led to the development of cluster patterns and standing waves and to the stabilization of uniform oscillations. These findings are reproduced by theoretical simulations.
Genetic Evidence for Two Species of Elephant in Africa
Authors: Roca Alfred L. , Georgiadis Nicholas , Pecon-Slattery Jill , O'Brien Stephen J.
Published: July 2002

Elephants from the tropical forests of Africa are morphologically distinct from savannah or bush elephants. Dart-biopsy samples from 195 free-ranging African elephants in 21 populations were examined for DNA sequence variation in four nuclear genes (1732 base pairs). Phylogenetic distinctions between African forest elephant and savannah elephant populations corresponded to 58% of the difference in the same genes between elephant genera
Loxodonta
(African) and
Elephas
(Asian). Large genetic distance, multiple genetically fixed nucleotide site differences, morphological and habitat distinctions, and extremely limited hybridization of gene flow between forest and savannah elephants support the recognition and conservation management of two African species:
Loxodonta africana
and
Loxodonta cyclotis
.
Hemichannel-Mediated Inhibition in the Outer Retina
Authors: Kamermans Maarten , Fahrenfort Iris , Schultz Konrad , Janssen-Bienhold Ulrike , Sjoerdsma Trijntje , Weiler Reto
Published: July 2002

An essential feature of the first synapse in the retina is a negative feedback pathway from horizontal cells to cones. Here we show that at this synapse, connexin26 forms hemichannels on horizontal cell dendrites near the glutamate release site of the cones. Blocking these hemichannels hyperpolarizes horizontal cells, modulates the Ca
2+
channels of the cones, and abolishes all feedback-mediated responses. We propose a feedback mechanism in which the activity of the Ca
2+
channels and the subsequent glutamate release of the cones are modulated by a current through these hemichannels. Because the current through the hemichannels depends on the polarization of the horizontal cells, their activity modulates the output of the cones.
Representation of Perceived Object Shape by the Human Lateral Occipital Complex
Authors: Kourtzi Zoe , Kanwisher Nancy
Published: July 2002

The human lateral occipital complex (LOC) has been implicated in object recognition, but it is unknown whether this region represents low-level image features or perceived object shape. We used an event-related functional magnetic resonance imaging adaptation paradigm in which the response to pairs of successively presented stimuli is lower when they are identical than when they are different. Adaptation across a change between the two stimuli in a pair provides evidence for a common neural representation invariant to that change. We found adaptation in the LOC when perceived shape was identical but contours differed, but not when contours were identical but perceived shape differed. These data indicate that the LOC represents not simple image features, but rather higher level shape information.
High-Temperature Ferromagnetism in CaB
2
C
2
Authors: Akimitsu J. , Takenawa K. , Suzuki K. , Harima H. , Kuramoto Y.
Published: July 2002

We report a high Curie–temperature ferromagnet, CaB
2
C
2
. Although the compound has neither transition metal nor rare earth ions, the ferromagnetic transition temperature
T
c
is about 770 Kelvin. Despite this high
T
c
, the magnitude of the ordered moment at room temperatures is on the order of 10
−4
Bohr magneton per formula unit. These properties are rather similar to those of doped divalent hexaborides, such as Ca

1−
x

La

x

B
6
. The calculated electronic states also show similarity near the Fermi level between CaB
2
C
2
and divalent hexaborides. However, there is an important difference: CaB
2
C
2
crystallizes in a tetragonal structure, and there are no equivalent pockets in the energy bands for electrons and holes—in contrast with CaB
6
. Thus, the disputed threefold degeneracy, specific to the cubic structure, in the energy bands of divalent hexaborides turns out not to be essential for high-temperature ferromagnetism. It is the peculiar molecular orbitals near the Fermi level that appear to be crucial to the high-
T
c
ferromagnetism.
Triallelic Inheritance in Bardet-Biedl Syndrome, a Mendelian Recessive Disorder
Authors: Katsanis Nicholas , Ansley Stephen J. , Badano Jose L. , Eichers Erica R. , Lewis Richard Alan , Hoskins Bethan E. , Scambler Peter J. , Davidson William S. , Beales Philip L. , Lupski James R.
Published: July 2002

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous disorder characterized by multiple clinical features that include pigmentary retinal dystrophy, polydactyly, obesity, developmental delay, and renal defects. BBS is considered an autosomal recessive disorder, and recent positional cloning efforts have identified two
BBS
genes (
BBS2
and
BBS6
). We screened our cohort of 163 BBS families for mutations in both
BBS2
and
BBS6
and report the presence of three mutant alleles in affected individuals in four pedigrees. In addition, we detected unaffected individuals in two pedigrees who carry two
BBS2
mutations but not a
BBS6
mutation. We therefore propose that BBS may not be a single-gene recessive disease but a complex trait requiring three mutant alleles to manifest the phenotype. This triallelic model of disease transmission may be important in the study of both Mendelian and multifactorial disorders.
Identification of Critical Staphylococcal Genes Using Conditional Phenotypes Generated by Antisense RNA
Authors: Ji Yinduo , Zhang Barbara , Van Stephanie F. , Horn  , Warren Patrick , Woodnutt Gary , Burnham Martin K. R. , Rosenberg Martin
Published: July 2002

Comprehensive genomic analysis of the important human pathogen
Staphylococcus aureus
was achieved by a strategy involving antisense technology in a regulatable gene expression system. In addition to known essential genes, many genes of unknown or poorly defined biological function were identified. This methodology allowed gene function to be characterized in a comprehensive, defined set of conditionally growth-defective/lethal isogenic strains. Quantitative titration of the conditional growth effect was performed either in bacterial culture or in an animal model of infection. This genomic strategy offers an approach to the identification of staphylococcal gene products that could serve as targets for antibiotic discovery.
Mediation of Hippocampal Mossy Fiber Long-Term Potentiation by Presynaptic
I
h
Channels
Authors: Mellor Jack , Nicoll Roger A. , Schmitz Dietmar
Published: July 2002

Hippocampal mossy fiber long-term potentiation (LTP) is expressed presynaptically, but the exact mechanisms remain unknown. Here, we demonstrate the involvement of the hyperpolarization-activated cation channel (
I
h
) in the expression of mossy fiber LTP. Established LTP was blocked and reversed by
I
h
channel antagonists. Whole-cell recording from granule cells revealed that repetitive stimulation causes a calcium- and
I
h
-dependent long-lasting depolarization mediated by protein kinase A. Depolarization at the terminals would be expected to enhance transmitter release, whereas somatic depolarization would enhance the responsiveness of granule cells to afferent input. Thus,
I
h
channels play an important role in the long-lasting control of transmitter release and neuronal excitability.
Correlation Between Histone Lysine Methylation and Developmental Changes at the Chicken β-Globin Locus
Authors: Litt Michael D. , Simpson Melanie , Gaszner Miklos , Allis C. David , Felsenfeld Gary
Published: July 2002

Methylation of histones at specific residues plays an important role in transcriptional regulation. Chromatin immunoprecipitation of dimethylated lysine 9 on histone H3 across 53 kilobases of the chicken β-globin locus during erythropoiesis shows an almost complete anticorrelation between regions of elevated lysine 9 methylation and acetylation. Lysine 9 is methylated most over constitutive condensed chromatin and developmentally inactive globin genes. In contrast, lysine 4 methylation of histone H3 correlates with H3 acetylation. These results lead us to propose a mechanism by which the insulator in the β-globin locus can protect the globin genes from being silenced by adjacent condensed chromatin.
Pathway Databases: A Case Study in Computational Symbolic Theories
Authors: Karp Peter D.
Published: July 2002

A pathway database (DB) is a DB that describes biochemical pathways, reactions, and enzymes. The EcoCyc pathway DB (see
http://ecocyc.org
) describes the metabolic, transport, and genetic-regulatory networks of
Escherichia coli
. EcoCyc is an example of a computational symbolic theory, which is a DB that structures a scientific theory within a formal ontology so that it is available for computational analysis. It is argued that by encoding scientific theories in symbolic form, we open new realms of analysis and understanding for theories that would otherwise be too large and complex for scientists to reason with effectively.
Matching Spiracle Opening to Metabolic Need During Flight in
Drosophila
Authors: Lehmann Fritz-Olaf
Published: July 2002

The respiratory exchange system of insects must maximize the flux of respiratory gases through the spiracles of the tracheal system while minimizing water loss. This trade-off between gas exchange and water loss becomes crucial when locomotor activity is increased during flight and metabolic needs are greatest. Insects that keep their spiracles mostly closed during flight reduce water loss but limit the flux of oxygen and carbon dioxide into and out of the tracheal system and thus attenuate locomotor performance. Insects that keep their spiracles completely open allow maximum gas exchange but face desiccation stress more quickly. Experiments in which water vapor was used as a tracer gas to track changes in the conductance of the respiratory system indicated that flying fruit flies minimize potential water loss by matching the area of the open spiracles to their gas exchange required for metabolic needs. This behavior maintained approximately constant pressure for carbon dioxide (1.35 kilopascals) and oxygen (19.9 kilopascals) within the tracheal system while reducing respirometric water loss by up to 23% compared with a strategy in which the spiracles are held wide open during flight. The adaptive spiracle-closing behavior in fruit flies has general implications for the ecology of flying insects because it shows how these animals may cope with environmental challenges during high locomotor performance.
Advances in Magnetic Microscopy
Authors: Freeman M. R. , Choi B. C.
Published: July 2002

A remarkable number of methods for direct, real-space imaging in magnetic microscopy have been demonstrated over the past decade and a half, and the pace of development shows no sign of slowing. Our understanding of magnetism increases as each striking new image of surface and thin-film magnetization is obtained. The continued development of high-performance magnetic information technologies also requires detailed study of the magnetostatics and dynamics of microscopic magnetic structures. Both fundamental curiosity and practical interest now drive us toward innovations in magnetic microscopy for nanometer-length scale and femtosecond temporal resolutions, which are beyond the limits of traditional imaging techniques. This survey is intended to provide an overview of the motivations, accomplishments, and future prospects for this discipline.
Two Essential DNA Polymerases at the Bacterial Replication Fork
Authors: Dervyn Etienne , Suski Catherine , Daniel Richard , Bruand Claude , Chapuis Jérome , Errington Jeff , Jannière Laurent , Ehrlich S. Dusko
Published: July 2002

DNA replication in bacteria is carried out by a multiprotein complex, which is thought to contain only one essential DNA polymerase, specified by the
dnaE
gene in
Escherichia coli
and the
polC
gene in
Bacillus subtilis
.
Bacillus subtilis
genome analysis has revealed another DNA polymerase gene,

dnaE
BS

, which is homologous to
dnaE
. We show that, in
B. subtilis
,

dnaE
BS

is essential for cell viability and for the elongation step of DNA replication, as is
polC,
and we conclude that there are two different essential DNA polymerases at the replication fork of
B. subtilis
, as was previously observed in eukaryotes.

dnaE
BS

appears to be involved in the synthesis of the lagging DNA strand and to be associated with the replication factory, which suggests that two different polymerases carry out synthesis of the two DNA strands in
B. subtilis
and in many other bacteria that contain both
polC
and
dnaE
genes.
Continuous Wave Operation of a Mid-Infrared Semiconductor Laser at Room Temperature
Authors: Beck Mattias , Hofstetter Daniel , Aellen Thierry , Faist Jérôme , Oesterle Ursula , Ilegems Marc , Gini Emilio , Melchior Hans
Published: July 2002

Continuous wave operation of quantum cascade lasers is reported up to a temperature of 312 kelvin. The devices were fabricated as buried heterostructure lasers with high-reflection coatings on both laser facets, resulting in continuous wave operation with optical output power ranging from 17 milliwatts at 292 kelvin to 3 milliwatts at 312 kelvin, at an emission wavelength of 9.1 micrometers. The results demonstrate the potential of quantum cascade lasers as continuous wave mid-infrared light sources for high-resolution spectroscopy, chemical sensing applications, and free-space optical communication systems.
Immunoglobulin-Domain Proteins Required for Maintenance of Ventral Nerve Cord Organization
Authors: Aurelio Oscar , Hall David H. , Hobert Oliver
Published: July 2002

During development, neurons extend axons along defined routes to specific target cells. We show that additional mechanisms ensure that axons maintain their correct positioning in defined axonal tracts. After termination of axonal outgrowth and target recognition, axons in the ventral nerve cord (VNC) of
Caenorhabditis elegans
require the presence of a specific VNC neuron, PVT, to maintain their correct positioning in the left and right fascicles of the VNC. PVT may exert its stabilizing function by the temporally tightly controlled secretion of 2-immunoglobulin (Ig)–domain proteins encoded by the
zig
genes. Dedicated axon maintenance mechanisms may be widely used to ensure the preservation of functional neuronal circuitries.
Formation of Recent Martian Debris Flows by Melting of Near-Surface Ground Ice at High Obliquity
Authors: Costard F. , Forget F. , Mangold N. , Peulvast J. P.
Published: July 2002

The observation of small gullies associated with recent surface runoff on Mars has renewed the question of liquid water stability at the surface of Mars. The gullies could be formed by groundwater seepage from underground aquifers; however, observations of gullies originating from isolated peaks and dune crests question this scenario. We show that these landforms may result from the melting of water ice in the top few meters of the martian subsurface at high obliquity. Our conclusions are based on the analogy between the martian gullies and terrestrial debris flows observed in Greenland and numerical simulations that show that above-freezing temperatures can occur at high obliquities in the near surface of Mars, and that such temperatures are only predicted at latitudes and for slope orientations corresponding to where the gullies have been observed on Mars.
Long-Range Interactions Within a Nonnative Protein
Authors: Klein-Seetharaman Judith , Oikawa Maki , Grimshaw Shaun B. , Wirmer Julia , Duchardt Elke , Ueda Tadashi , Imoto Taiji , Smith Lorna J. , Dobson Christopher M. , Schwalbe Harald
Published: July 2002

Protein folding and unfolding are coupled to a range of biological phenomena, from the regulation of cellular activity to the onset of neurodegenerative diseases. Defining the nature of the conformations sampled in nonnative proteins is crucial for understanding the origins of such phenomena. We have used a combination of nuclear magnetic resonance (NMR) spectroscopy and site-directed mutagenesis to study unfolded states of the protein lysozyme. Extensive clusters of hydrophobic structure exist within the wild-type protein even under strongly denaturing conditions. These clusters involve distinct regions of the sequence but are all disrupted by a single point mutation that replaced residue Trp
62
with Gly located at the interface of the two major structural domains in the native state. Thus, nativelike structure in the denatured protein is stabilized by the involvement of Trp
62
in nonnative and long-range interactions.
Visualizing Chromatin Dynamics in Interphase Nuclei
Authors: Gasser Susan M.
Published: July 2002

Real-time fluorescence microscopy has emerged as a powerful tool for examining chromatin dynamics. The initial lesson is that much of the genome, particularly in yeast, is highly dynamic. Its movement within the interphase nucleus is correlated with metabolic activity. Nonetheless, the nucleus is an organelle with conserved rules of organization. Determining the distribution and regulation of mobile domains in interphase chromosomes, and characterizing sites of anchorage, will undoubtedly shed new light on the function of nuclear order.
Regulatory Role of
SGT1
in Early
R
Gene-Mediated Plant Defenses
Authors: Austin Mark J. , Muskett Paul , Kahn Katherine , Feys Bart J. , Jones Jonathan D. G. , Parker Jane E.
Published: July 2002

Animal SGT1 is a component of Skp1-Cullin–F-box protein (SCF) ubiquitin ligases that target regulatory proteins for degradation. Mutations in one (
SGT1b
) of two highly homologous
Arabidopsis SGT1
genes disable early plant defenses conferred by multiple resistance (
R
) genes. Loss of
SGT1b
function in resistance is not compensated for by
SGT1a
.
R
genes differ in their requirements for
SGT1b
and a second resistance signaling gene,
RAR1
, that was previously implicated as an SGT1 interactor. Moreover,
SGT1b
and
RAR1
contribute additively to
RPP5
-mediated pathogen recognition. These data imply both operationally distinct and cooperative functions of SGT1 and RAR1 in plant disease resistance.
X-ray Imaging of Shock Waves Generated by High-Pressure Fuel Sprays
Authors: MacPhee Andrew G. , Tate Mark W. , Powell Christopher F. , Yue Yong , Renzi Matthew J. , Ercan Alper , Narayanan Suresh , Fontes Ernest , Walther Jochen , Schaller Johannes , Gruner Sol M. , Wang Jin
Published: July 2002

Synchrotron x-radiography and a fast x-ray detector were used to record the time evolution of the transient fuel sprays from a high-pressure injector. A succession of 5.1-microsecond radiographs captured the propagation of the spray-induced shock waves in a gaseous medium and revealed the complex nature of the spray hydrodynamics. The monochromatic x-radiographs also allow quantitative analysis of the shock waves that has been difficult if not impossible with optical imaging. Under injection conditions similar to those found in operating engines, the fuel jets can exceed supersonic speeds and result in gaseous shock waves.
Asteroid 1950 DA's Encounter with Earth in 2880: Physical Limits of Collision Probability Prediction
Authors: Giorgini J. D. , Ostro S. J. , Benner L. A. M. , Chodas P. W. , Chesley S. R. , Hudson R. S. , Nolan M. C. , Klemola A. R. , Standish E. M. , Jurgens R. F. , Rose R. , Chamberlin A. B. , Yeomans D. K. , Margot J.-L.
Published: July 2002

Integration of the orbit of asteroid (29075) 1950 DA, which is based on radar and optical measurements spanning 51 years, reveals a 20-minute interval in March 2880 when there could be a nonnegligible probability of the 1-kilometer object colliding with Earth. Trajectory knowledge remains accurate until then because of extensive astrometric data, an inclined orbit geometry that reduces in-plane perturbations, and an orbit uncertainty space modulated by gravitational resonance. The approach distance uncertainty in 2880 is determined primarily by uncertainty in the accelerations arising from thermal re-radiation of solar energy absorbed by the asteroid. Those accelerations depend on the spin axis, composition, and surface properties of the asteroid, so that refining the collision probability may require direct inspection by a spacecraft.
A LAT Mutation That Inhibits T Cell Development Yet Induces Lymphoproliferation
Authors: Sommers Connie L. , Park Cheung-Seog , Lee Jan , Feng Chiguang , Fuller Claudette L. , Grinberg Alexander , Hildebrand Jay A. , Lacaná Emanuela , Menon Rashmi K. , Shores Elizabeth W. , Samelson Lawrence E. , Love Paul E.
Published: July 2002

Mice homozygous for a single tyrosine mutation in LAT (linker for activation of T cells) exhibited an early block in T cell maturation but later developed a polyclonal lymphoproliferative disorder and signs of autoimmune disease. T cell antigen receptor (TCR)–induced activation of phospholipase C–γ1 (PLC-γ1) and of nuclear factor of activated T cells, calcium influx, interleukin-2 production, and cell death were reduced or abrogated in T cells from LAT mutant mice. In contrast, TCR-induced Erk activation was intact. These results identify a critical role for integrated PLC-γ1 and Ras-Erk signaling through LAT in T cell development and homeostasis.
A Role for Skin γδ T Cells in Wound Repair
Authors: Jameson Julie , Ugarte Karen , Chen Nicole , Yachi Pia , Fuchs Elaine , Boismenu Richard , Havran Wendy L.
Published: July 2002

γδ T cell receptor–bearing dendritic epidermal T cells (DETCs) found in murine skin recognize antigen expressed by damaged or stressed keratinocytes. Activated DETCs produce keratinocyte growth factors (KGFs) and chemokines, raising the possibility that DETCs play a role in tissue repair. We performed wound healing studies and found defects in keratinocyte proliferation and tissue reepithelialization in the absence of wild-type DETCs. In vitro skin organ culture studies demonstrated that adding DETCs or recombinant KGF restored normal wound healing in γδ DETC-deficient skin. We propose that DETCs recognize antigen expressed by injured keratinocytes and produce factors that directly affect wound repair.
The Shaping of Continental Slopes by Internal Tides
Authors: Cacchione D. A. , Pratson L. F. , Ogston A. S.
Published: July 2002

The angles of energy propagation of semidiurnal internal tides may determine the average gradient of continental slopes in ocean basins (∼2 to 4 degrees). Intensification of near-bottom water velocities and bottom shear stresses caused by reflection of semi-diurnal internal tides affects sedimentation patterns and bottom gradients, as indicated by recent studies of continental slopes off northern California and New Jersey. Estimates of bottom shear velocities caused by semi-diurnal internal tides are high enough to inhibit deposition of fine-grained sediment onto the slopes.
Superconductivity-Induced Transfer of In-Plane Spectral Weight in Bi
2
Sr
2
CaCu
2
O
8+δ
Authors: Molegraaf H. J. A. , Presura C. , van der Marel D. , Kes P. H. , Li M.
Published: July 2002

Optical data are reported on a spectral weight transfer over a broad frequency range of Bi
2
Sr
2
CaCu
2
O
8+δ
, when this material became superconducting. Using spectroscopic ellipsometry, we observed the removal of a small amount of spectral weight in a broad frequency band from 10
4
cm
−1
to at least 2 × 10
4
cm
−1
, due to the onset of superconductivity. We observed a blue shift of the
ab
-plane plasma frequency when the material became superconducting, indicating that the spectral weight was transferred to the infrared range. Our observations are in agreement with models in which superconductivity is accompanied by an increased charge carrier spectral weight. The measured spectral weight transfer is large enough to account for the condensation energy in these compounds.
Induction and Suppression of RNA Silencing by an Animal Virus
Authors: Li Hongwei , Li Wan Xiang , Ding Shou Wei
Published: July 2002

RNA silencing is a sequence-specific RNA degradation mechanism that is operational in plants and animals. Here, we show that flock house virus (FHV) is both an initiator and a target of RNA silencing in
Drosophila
host cells and that FHV infection requires suppression of RNA silencing by an FHV-encoded protein, B2. These findings establish RNA silencing as an adaptive antiviral defense in animal cells. B2 also inhibits RNA silencing in transgenic plants, providing evidence for a conserved RNA silencing pathway in the plant and animal kingdoms.
Dominant-Negative Mutants of a Toxin Subunit: An Approach to Therapy of Anthrax
Authors: Sellman Bret R. , Mourez Michael , John Collier R.
Published: July 2002

The protective antigen moiety of anthrax toxin translocates the toxin's enzymic moieties to the cytosol of mammalian cells by a mechanism that depends on its ability to heptamerize and insert into membranes. We identified dominant-negative mutants of protective antigen that co-assemble with the wild-type protein and block its ability to translocate the enzymic moieties across membranes. These mutants strongly inhibited toxin action in cell culture and in an animal intoxication model, suggesting that they could be useful in therapy of anthrax.
Resistance to
Leishmania major
Induced by Tolerance to a Single Antigen
Authors: Julia Valérie , Rassoulzadegan Minoo , Glaichenhaus Nicolas
Published: July 2002

In mice, susceptibility to
Leishmania major
is associated with the early expansion of T helper 2 cells (T
H
2) cells, but nothing is known of the specificity of these cells. A previously identified antigen,
Leishmania
homolog of receptors for activated C kinase (LACK), was found to be the focus of this initial response. Mice made tolerant to LACK by the transgenic expression of the antigen in the thymus exhibited both a diminished T
H
2 response and a healing phenotype. Thus, T cells that are activated early and are reactive to a single antigen play a pivotal role in directing the immune response to the entire parasite.