bluuebunny/crossref_metadata_2025 huggingface dataset:
Process optimization of the advanced negative electron beam resist SAL605
Authors: Fedynyshyn Theodore H. , Cronin Michael F. , Poli Louis C. , Kondek Christine
Published: July 2002

Electron beam lithography requires a high resolution resist system capable of maintaining tight linewidth tolerances. These tolerances require a resist process which yields the greatest process latitude. SAL601-ER7 is an example of an electron beam resist which has demonstrated the ability to provide submicron resolution coupled with high resist sensitivity. This paper will focus on developing an optimized process for a new advanced negative electron beam resist (ANR), SAL605 which also utilizes a chemically amplified crosslinking system. The SAL605 electron beam resist is a next generation version of SAL601-ER7, and as such has many of the same processing requirements. The effect of developer normality and development time on sensitivity and linewidth control with MF-312 based developers will be presented. The effect of post-exposure bake (PEB) temperature and time with respect to sensitivity and linewidth control will also be presented. We will show that there are processing windows for submicron resolution which give high sensitivity as well as a large exposure latitude.
A statistically based model of electron-beam exposed, chemically amplified negative resist
Authors: Tam N. N. , Liu H. Y. , Spanos C. , Neureuther A. R.
Published: July 2002

A methodology based on statistically designed factorial experiments has been developed to model the dissolution rate of an electron-beam exposed chemically amplified resist, Shipley SAL-601-ER7, for different post-exposure bake (PEB) and developer concentrations. A statistical experiment of dissolution rate measurements were performed in the Perkin-Elmer development rate monitor using a central composite design with PEB temperature, PEB time, and developer concentration as the factors. These measurements were combined with Monte Carlo simulation of electron energy deposition to generate dissolution rate data as a function of absorbed energy. These data were then fitted to a semiempirical rate equation using nonlinear regression. Statistical analysis of the effects of the three processing factors on the parameters of the rate functions yielded empirical models relating the parameters to the factors. As a result, a rate equation can be determined for any processing conditions, and thus a complete model is obtained for the simulation and optimization of the resist.
Low-temperature growth of midwavelength infrared liquid phase epitaxy HgCdTe on sapphire
Authors: Johnston S. , Blazejewski E. R. , Bajaj J. , Chen J. S. , Bubulac L. , Williams G.
Published: July 2002

Consistently low dislocation density midwavelength infrared (x=0.31) liquid phase epitaxy HgCdTe epitaxial layers with excellent morphology were grown on 2 in. sapphire substrates (PACE-1) using a new low-temperature (420 °C) Te melt process. Surface etch pit densities (EPDs) between 5×105 and 9×105 cm−2 were revealed using a previously reported chemical etch [J. S. Chen, US Patent No. 4897152] on a 20-layer sample set. Cross-sectional EPD profiles reveal a more rapid decrease of defects from the CdTe buffer layer interface as compared to conventionally grown (500 °C) material. X-ray rocking curve widths from 43 to 66 arcsec were routinely observed. 77 K electron mobilities as high as 51 000 cm2/V-s were measured. Secondary-ion mass spectrometry profiles show a minimum of impurity gettering at the HgCdTe/CdTe buffer layer interface.
The application of the helicon source to plasma processing
Authors: Perry A. J. , Vender D. , Boswell R. W.
Published: July 2002

The results of a study of the mode transitions in the helicon source when used in the geometry required for plasma processing are presented. We find that the basic characteristics of high density (>5×1011 cm−3 in the processing chamber at 500 W) and low plasma potential (∼15 V) are observed in this configuration. The mode transitions can be interpreted in terms of the dispersion relation for the helicon wave. A study of the initial plasma breakdown has also been made and the results have aided in the understanding of the operation of the helicon source during pulsed plasma etching.
Novel indices characterizing resolution power of photoresist for half-micron feature size photolithography
Authors: Ito Tetsuo , Okano Sadao , Takahashi Shigeru , Sugimoto Aritoshi , Kadota Kazuya
Published: July 2002

The photoactive and development parameters of commercially available g-line photoresists which are TOKs OFPR800, OFPR5000, TSMR8900, and TSMR-V3, were measured. From the simulated and experimental linewidth linearity data, it was found that the photoactive parameters have very little effect on the resolution power (linearity limit) of photoresists, while the development parameters (dissolution rate characteristics) greatly affect the resolution power. New indices, which can properly characterize the resolution power of photoresists, were extracted from the dissolution rate characteristics curves. One is Cd, which is the contrast of the dissolution rate and the other is Rd, which is the range of the dissolution rate. These indices are closely related to the resist resolution power. A larger Cd or Rd gives a higher resolution power to the photoresist. The necessary value of Cd⋅Rd for the 0.5-μm feature size photolithography process was derived from experimental data under the present highest NA(=0.55) g-line stepper for mass production and Cd⋅Rd must be larger than 39. Both Cd and Rd are useful parameters for estimation of the photoresist resolution power.
Molecular-beam epitaxy growth and uniformity test of modulation-doped Al0.3Ga0.7As/GaAs heterostructure on 4-in. diameter GaAs(100)
Authors: Yang K. , So K. C. , Taylor A. P. , Schowalter L. J.
Published: July 2002

A modulation-doped Al0.3 Ga0.7 As/GaAs film with high mobility (11×104 cm2/V s at 77 K) has been grown on a 4-in. diam GaAs (100) substrate in a MBE system from Vacuum Generators. The variations of electrical parameters and thickness over the wafer are about ±2%, indicating good uniformities. The flux transient variations which influence the uniformity of stoichiometry in the growth direction are also investigated.
Dislocation density variations in HgCdTe films grown by dipping liquid phase epitaxy: Effects on metal–insulator–semiconductor properties
Authors: Chandra D. , Tregilgas J. H. , Goodwin M. W.
Published: July 2002

The dislocation density variations in HgCdTe films grown by liquid phase epitaxy (LPE) from tellurium-rich melts were measured and were found to depend on film growth temperature, substrate dislocation density, epitaxial film thickness, and postgrowth annealing in Hg vapor. Thin films, less than about 30–40 μm in thickness, exhibit dislocation densities which generally follow the dislocation densities of the substrates. In thicker epitaxial films, however, dislocation multiplication can accompany Te precipitation and subsequent low temperature annealing in Hg vapor, thereby, raising the density of dislocations by as much as a factor of 3–10. Use of a high temperature preanneal in Hg vapor can be employed to eliminate dislocation multiplication during low temperature annealing. When this preanneal is used in conjunction with very thick film growth with thicknesses greater than about 80 μm, dislocation densities in the LPE films can fall to values below those in the substrate. The dependence of metal–insulator–semiconductor storage times on dislocation densities in these films has also been measured.
A new high-speed simulation method for electron-beam critical dimension metrology profile modeling
Authors: Wang Xinlei , Joy David C.
Published: July 2002

Critical dimension metrology relies on the analysis of electron-beam signal profiles from structures of arbitrary size and shape. Conventional modeling techniques for simulating such profiles use Monte Carlo techniques which, while very accurate, require considerable computing time since any change in any parameter necessitates a complete new calculation. This paper describes an alternative approach in which a Monte Carlo simulation is used to derive the coefficients of a diffusion matrix. From this matrix the backscattering and secondary electron profiles from a surface of given geometry can be found by simple summation, and the results of a change in the geometry can be determined immediately without the need for further computation. The predictions of this method are in excellent agreement with those of the conventional approach.
Fabrication of compact 100 nm-scale silicon metal–oxide–semiconductor field effect transistors
Authors: Wind S. J. , Reeves C. M. , Bucchignano J. J. , Lii Y. T. , Newman T. H. , Klaus D. P. , Keller J. , Volant R. P. , Tebin B. , Hohn F. J.
Published: July 2002

The fabrication of exploratory silicon metal–oxide–semiconductor field effect transistors in which all device levels meet 100 nm groundrules is reported. The device design incorporates several novel features which allow for an ultracompact structure. These features include shallow trench isolation, over-the-gate contacts, fully overlapped source, drain and gate contacts and shallow source and drain extensions. This article offers a detailed description of the fabrication process, with an emphasis on high resolution, high accuracy electron beam nanolithography and new reactive ion etching processes. Complete process integration is described which culminates in the successful fabrication of functional compact devices.
Effect of Mo oxide layer on electromigration resistance of Al–Si–Cu/Mo bilayered interconnections
Authors: Fukada Shin-ichi , Suwa Motoo
Published: July 2002

The influence of the molybdenum oxide (MoOx) layer between the Al–Si–Cu layer and the Mo underlayer on the electromigration resistance of the Al–Si–Cu/Mo interconnection has been investigated. A homogeneous MoOx layer improves the electromigration resistance of the Al alloy/Mo interconnection. The simulation of current flow in the interconnection shows that the oxide layer relieves the current crowding at the voids in the Al alloy layer and suppresses void growth.
Effect of reactive ion beam etching damage on exciton absorption recovery time of multiple quantum well wires
Authors: Endoh Akira , Arimoto Hiroshi , Kitada Hideki , Tackeuchi Atsushi , Muto Shunichi
Published: July 2002

We studied the effect of process damage on exciton absorption recovery time of multiple quantum well narrow wires. The wires were fabricated to the order of 100 nm by focused ion beam lithography and subsequent reactive ion beam etching. The recovery time decreases with increased damage. The recovery times of the damaged samples slightly increase after thermal annealing. The effect of etching damage on the recovery time was discussed by the numerical calculation assuming nonradiative recombination by defects.
Properties of electromagnetic fields in x-ray lithographic masks: Guided modes and beam propagation calculus
Authors: Kupka R. K. , Chen Y. , Rousseaux F. , Haghiri-Gosnet A. M. , Launois H.
Published: July 2002

The modal fields within x-ray lithographic masks are analyzed using standard optical waveguide theory and a ‘‘beam propagation method (BPM).’’ From the properties of the guided modes, it is shown that for small feature sizes the field no longer resembles the geometrical shape of the absorber structure, but strongly the shape of the modes. TE and TM modes behave almost identically and, thus, scalar diffraction calculations are generally valid in the domain of soft x rays. Mode cut-off of the fundamental mode never occurs. Using such guiding properties it is possible to generate spatially concentrated (16 nm) x-ray fields with much larger absorber openings (40 nm). A BPM with fast Fourier transforms is employed to efficiently calculate the accurate wave front propagation inside the mask, to determine the physical boundary condition downstream of the absorber structure and to calculate the diffracted image in the wafer plane. The BPM in free space is equivalent with the Rayleigh–Sommerfeld diffraction formula. The method turns out to be very useful in field image calculations of x-ray lithographic systems using phase and absorption mask patterns of arbitrary shapes and surface roughnesses.
Effect of Mg(OH)2 on YBa2Cu3O7 thin film on MgO substrate studied by atomic force microscope
Authors: Kim B. I. , Hong J. W. , Jeong G. T. , Moon S. H. , Lee D. H. , Shim T. U. , Khim Z. G.
Published: July 2002

We observed the surface degradation of MgO single-crystal substrates stored in humid air after cleavage by atomic force microscope. Annealing at 1000 °C in dry oxygen removed local defects though residual subgrain structure remains. The morphology of YBa2Cu3O7−δ (YBCO) films on the annealed and degraded substrates reflects these changes of the substrates. We confirmed that the annealing process of degraded substrate improves the film quality by measuring transition temperature Tc and critical current Jc of each film.
Effect of MeV electron irradiation on gold atom implantation into silicon carbide and silicon nitride
Authors: Mori H. , Sakata T. , Yasuda H. , Maeda M.
Published: July 2002

Electron-irradiation-induced gold atom implantation into α-SiC and β-Si3N4 has been studied by ultrahigh voltage electron microscopy (UHVEM). Bilayer films of Au(target atom)/α-SiC (substrate) and of Au (target atom)/β-Si3N4 (substrate) were irradiated with 2-MeV electrons in a UHVEM with the electron beam incident on the gold layer. In the Au/α-SiC system, irradiation with 2-MeV electrons first induces amorphization of the α-SiC substrate, and then with continued irradiation gold atoms which have been knocked-off from the gold layer by the collision with 2-MeV electrons are implanted into the amorphous SiC substrate. The distribution of gold in amorphous SiC is not uniform and there exists a concentration fluctuation of gold with the average wavelength of a few to several nm. In the Au/β-Si3N4 system, it seems difficult to implant gold atoms into the β-Si3N4 substrate by MeV electron irradiation. The ease with which gold implantation into ceramic substrates takes place has been discussed in terms of the chemical constraint effect.
Dependence of current-voltage characteristics on Al mole fraction in GaAs/AlxGa1−xAs asymmetric double barrier structures
Authors: Blue L. J. , Daniels-Race T. , Kendall R. E. , Schmid C. R. , Teitsworth S. W.
Published: July 2002

The effect of barrier Al mole fraction, 0.2⩽x⩽0.8, on tunneling currents has been studied for a set of asymmetric GaAs/AlxGa1−xAs double barrier structures. The barrier widths of each sample were scaled so that barrier transmission coefficients for different samples should be approximately equivalent at the first resonant tunneling peak. Structures were grown by molecular beam epitaxy; by adjusting Ga and Al cell temperatures, the full range of Al mole fractions could be achieved in AlxGa1−xAs barrier layers while maintaining a nearly constant growth rate of about 1 μm/h. Current-voltage measurements are in agreement with theoretical estimates and indicate good sample quality.
Gas source molecular beam epitaxy growth of high quality InGaAsP for 0.98 μm Al-free InGaAs/InGaAsP/InGaP laser diodes
Authors: Liu Jin-Shung , Chen Jian-Guang , Lin Hao-Hsiung , Tu Yuan-Kuang
Published: July 2002

The properties of gas source molecular beam epitaxy grown InGaAsP bulk layers and InGaAs/InGaAsP strain-compensated multiple quantum well (SCMQW) structures on GaAs were studied by double crystal x-ray diffraction and photoluminescence measurements. It was found that high quality of SCMQW can be obtained below a critical growth temperature. Above the temperature, an immiscible growth for the InGaAsP alloy degraded both the optical and structural qualities of the MQW structures. Based on these findings, a high performance 0.98 μm Al-free InGaAs/InGaAsP/InGaP two-step graded index separate confinement hetrostructure quantum well laser was prepared at the optimal growth temperature.
Mathematical modeling of focused ion beam microfabrication
Authors: Nassar R. , Vasile M. , Zhang W.
Published: July 2002

A mathematical model for sputtering a shape or cavity with an arbitrary cross-sectional profile has been developed for focused ion beam milling. The ion beam is assumed to have a Gaussian intensity distribution and a submicron width. The model solves for ion beam dwell times on a pixel grid which yields the desired feature depth as a function of the pixel (x,y) coordinate. The solution is unique and accounts for the ion beam flux contribution at any point from all other pixels in the address matrix. A semiempirical sputter yield treatment allows for a very wide range of ion beam/solid combinations and for yield variations with ion energy and angle of incidence. Solutions have been obtained for parabolic surfaces of revolution, a parabolic trench (with a plane of symmetry) and a hemispherical pit. Either a square or a circular pixel matrix was used for the parabolic shapes. Correspondence between the predictions of the model and experimental 20 keV Ga+ sputtering of a parabolic cross-section trench in Si(100) was within the limits of the accuracy of the experimental control.
Microstructure of novel superhard nanocrystalline-amorphous composites as analyzed by high resolution transmission electron microscopy
Authors: Christiansen S. , Albrecht M. , Strunk H. P. , Veprek Stan
Published: July 2002

The recently developed, novel superhard nanocrystalline composites nc-TiN/a-Si3N4 have been investigated by means of high resolution transmission electron microscopy. The microstructure consisting of nanocrystalline TiN imbedded within a⩽1 nm thin amorphous Si3N4, the relative amount of both phases, and the TiN-crystallite size which were previously determined by means of x-ray diffraction and energy dispersive analysis of x rays have been directly confirmed.
Multiple layers of silicon-on-insulator for nanostructure devices
Authors: Neudeck Gerold W. , Pae Sangwoo , Denton John P. , Su Tai-chi
Published: July 2002

A new method for silicon-on-insulator (SOI) is presented that has very few stacking fault defects and produces multiple layers of single crystal silicon surrounded by thermal SiO2. The technique requires selective epitaxial growth, epitaxial lateral overgrowth, and chemical mechanical planarization to form SOI islands stacked in multiple layers. Islands of silicon as small as 150×150×40 nm thick were fabricated. Larger SOI islands in two SOI layers, with grown vertical interconnections between layers, were 5×500×0.1 μm. Only one stacking fault was observed in 85 000 μm2 of the first layer and none in the second layer. P-channel metal–oxide–semiconductor field effect transistors with gate lengths of less than ∼100 nm were fabricated in the thin SOI islands. They had normal current–voltage plot characteristics with less than 0.2 pA/μm of leakage current, illustrating the quality of the material in both SOI layers and at the silicon to thermal-oxide interfaces. The devices had measured subthreshold slopes of 76 mV/decade and good saturated current drives.
Active Normal Faulting in the Upper Rhine Graben and Paleoseismic Identification of the 1356 Basel Earthquake
Authors: Meghraoui Mustapha , Delouis Bertrand , Ferry Matthieu , Giardini Domenico , Huggenberger Peter , Spottke Ina , Granet Michel
Published: July 2002

We have identified an active normal fault in the epicentral area of the Basel (Switzerland) earthquake of 18 October 1356, the largest historical seismic event in central Europe. The event of 1356 and two prehistoric events have been characterized on the fault with geomorphological analysis, geophysical prospecting, and trenching. Carbon-14 dating indicates that the youngest event occurred in the interval 610 to 1475 A.D. and may correspond to the 1356 Basel earthquake. The occurrence of the three earthquakes induced a total of 1.8 meters of vertical displacement in the past 8500 years for a mean uplift rate of 0.21 millimeters per year. These successive ruptures on the normal fault indicate the potential for strong ground movements in the Basel region and should be taken into account to refine the seismic hazard estimates along the Rhine graben.
Physiological Regulation of the Immunological Synapse by Agrin
Authors: Khan Adil A. , Bose Christian , Yam Lung S. , Soloski Mark J. , Rupp Fabio
Published: July 2002

T cell activation is dependent on both a primary signal delivered through the T cell receptor and a secondary costimulatory signal mediated by coreceptors. Although controversial, costimulation is thought to act through the specific redistribution and clustering of membrane and intracellular kinase-rich lipid raft microdomains at the contact site between T cells and antigen-presenting cells. This site has been termed the immunological synapse. Endogenous mediators of raft clustering in lymphocytes have not been identified, although they are essential for T cell activation. We now demonstrate that agrin, an aggregating protein crucial for formation of the neuromuscular junction, is also expressed in lymphocytes and is important in reorganization of membrane lipid microdomains and setting the threshold for T cell signaling. Our data show that agrin induces the aggregation of signaling proteins and the creation of signaling domains in both immune and nervous systems through a common lipid raft pathway.
Living with Genome Instability: Plant Responses to Telomere Dysfunction
Authors: Riha Karel , McKnight Thomas D. , Griffing Lawrence R. , Shippen Dorothy E.
Published: July 2002

Loss of telomere function in metazoans results in catastrophic damage to the genome, cell cycle arrest, and apoptosis. Here we show that the mustard weed
Arabidopsis thaliana
can survive up to 10 generations without telomerase. The last five generations of telomerase-deficient plants endured increasing levels of cytogenetic damage, which was correlated with developmental anomalies in both vegetative and reproductive organs. Mutants ultimately arrested at a terminal vegetative state harboring shoot meristems that were grossly enlarged, disorganized, and in some cases, dedifferentiated into a callusoid mass. Unexpectedly, late-generation mutants had an extended life-span and remained metabolically active. The differences in plant and animal responses to dysfunctional telomeres may reflect the more plastic nature of plant development and genome organization.
A Mutant
Drosophila
Insulin Receptor Homolog That Extends Life-Span and Impairs Neuroendocrine Function
Authors: Tatar M. , Kopelman A. , Epstein D. , Tu M.-P. , Yin C.-M. , Garofalo R. S.
Published: July 2002

The
Drosophila melanogaster
gene
insulin-like receptor
(
InR
) is homologous to mammalian insulin receptors as well as to
Caenorhabditis elegans daf-2
, a signal transducer regulating worm dauer formation and adult longevity. We describe a heteroallelic, hypomorphic genotype of mutant
InR
, which yields dwarf females with up to an 85% extension of adult longevity and dwarf males with reduced late age-specific mortality. Treatment of the long-lived
InR
dwarfs with a juvenile hormone analog restores life expectancy toward that of wild-type controls. We conclude that juvenile hormone deficiency, which results from
InR
signal pathway mutation, is sufficient to extend life-span, and that in flies, insulin-like ligands nonautonomously mediate aging through retardation of growth or activation of specific endocrine tissue.
Extension of Life-Span by Loss of CHICO, a
Drosophila
Insulin Receptor Substrate Protein
Authors: Clancy David J. , Gems David , Harshman Lawrence G. , Oldham Sean , Stocker Hugo , Hafen Ernst , Leevers Sally J. , Partridge Linda
Published: July 2002

The
Drosophila melanogaster
gene
chico
encodes an insulin receptor substrate that functions in an insulin/insulin-like growth factor (IGF) signaling pathway. In the nematode
Caenorhabditis elegans
, insulin/IGF signaling regulates adult longevity. We found that mutation of
chico
extends fruit fly median life-span by up to 48% in homozygotes and 36% in heterozygotes. Extension of life-span was not a result of impaired oogenesis in
chico
females, nor was it consistently correlated with increased stress resistance. The dwarf phenotype of
chico
homozygotes was also unnecessary for extension of life-span. The role of insulin/IGF signaling in regulating animal aging is therefore evolutionarily conserved.
The Evolution of Climate Over the Last Millennium
Authors: Jones P. D. , Osborn T. J. , Briffa K. R.
Published: July 2002

Knowledge of past climate variability is crucial for understanding and modeling current and future climate trends. This article reviews present knowledge of changes in temperatures and two major circulation features—El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO)—over much of the last 1000 years, mainly on the basis of high-resolution paleoclimate records. Average temperatures during the last three decades were likely the warmest of the last millennium, about 0.2°C warmer than during warm periods in the 11th and 12th centuries. The 20th century experienced the strongest warming trend of the millennium (about 0.6°C per century). Some recent changes in ENSO may have been unique since 1800, whereas the recent trend to more positive NAO values may have occurred several times since 1500. Uncertainties will only be reduced through more extensive spatial sampling of diverse proxy climatic records.
Structural Basis of Transcription: RNA Polymerase II at 2.8 Ångstrom Resolution
Authors: Cramer Patrick , Bushnell David A. , Kornberg Roger D.
Published: July 2002

Structures of a 10-subunit yeast RNA polymerase II have been derived from two crystal forms at 2.8 and 3.1 angstrom resolution. Comparison of the structures reveals a division of the polymerase into four mobile modules, including a clamp, shown previously to swing over the active center. In the 2.8 angstrom structure, the clamp is in an open state, allowing entry of straight promoter DNA for the initiation of transcription. Three loops extending from the clamp may play roles in RNA unwinding and DNA rewinding during transcription. A 2.8 angstrom difference Fourier map reveals two metal ions at the active site, one persistently bound and the other possibly exchangeable during RNA synthesis. The results also provide evidence for RNA exit in the vicinity of the carboxyl-terminal repeat domain, coupling synthesis to RNA processing by enzymes bound to this domain.
Recent Origin of
Plasmodium falciparum
from a Single Progenitor
Authors: Volkman Sarah K. , Barry Alyssa E. , Lyons Emily J. , Nielsen Kaare M. , Thomas Susan M. , Choi Mehee , Thakore Seema S. , Day Karen P. , Wirth Dyann F. , Hartl Daniel L.
Published: July 2002

Genetic variability of
Plasmodium falciparum
underlies its transmission success and thwarts efforts to control disease caused by this parasite. Genetic variation in antigenic, drug resistance, and pathogenesis determinants is abundant, consistent with an ancient origin of
P. falciparum
, whereas DNA variation at silent (synonymous) sites in coding sequences appears virtually absent, consistent with a recent origin of the parasite. To resolve this paradox, we analyzed introns and demonstrated that these are deficient in single-nucleotide polymorphisms, as are synonymous sites in coding regions. These data establish the recent origin of
P. falciparum
and further provide an explanation for the abundant diversity observed in antigen and other selected genes.
New Ages for the Last Australian Megafauna: Continent-Wide Extinction About 46,000 Years Ago
Authors: Roberts Richard G. , Flannery Timothy F. , Ayliffe Linda K. , Yoshida Hiroyuki , Olley Jon M. , Prideaux Gavin J. , Laslett Geoff M. , Baynes Alexander , Smith M. A. , Jones Rhys , Smith Barton L.
Published: July 2002

All Australian land mammals, reptiles, and birds weighing more than 100 kilograms, and six of the seven genera with a body mass of 45 to 100 kilograms, perished in the late Quaternary. The timing and causes of these extinctions remain uncertain. We report burial ages for megafauna from 28 sites and infer extinction across the continent around 46,400 years ago (95% confidence interval, 51,200 to 39,800 years ago). Our results rule out extreme aridity at the Last Glacial Maximum as the cause of extinction, but not other climatic impacts; a “blitzkrieg” model of human-induced extinction; or an extended period of anthropogenic ecosystem disruption.
MgB
2
Superconducting Thin Films with a Transition Temperature of 39 Kelvin
Authors: Kang W. N. , Kim Hyeong-Jin , Choi Eun-Mi , Jung C. U. , Lee Sung-Ik
Published: July 2002

We fabricated high-quality
c
axis–oriented epitaxial MgB
2
thin films using a pulsed laser deposition technique. The thin films grown on (1 1̄ 0 2) Al
2
O
3
substrates have a transition temperature of 39 kelvin. The critical current density in zero field is ∼6 × 10
6
amperes per cubic centimeter at 5 kelvin and ∼3 × 10
5
amperes per cubic centimeter at 35 kelvin, which suggests that this compound has potential for electronic device applications, such as microwave devices and superconducting quantum interference devices. For the films deposited on Al
2
O
3
, x-ray diffraction patterns indicate a highly
c
axis–oriented crystal structure perpendicular to the substrate surface.
Induction of Apoptosis by a Secreted Lipocalin That is Transcriptionally Regulated by IL-3 Deprivation
Authors: Devireddy Laxminarayana R. , Teodoro Jose G. , Richard Fabien A. , Green Michael R.
Published: July 2002

Many hematopoietic cells undergo apoptosis when deprived of specific cytokines, and this process requires de novo RNA/protein synthesis. Using DNA microarrays to analyze interleukin-3 (IL-3)–dependent murine FL5.12 pro–B cells, we found that the gene undergoing maximal transcriptional induction after cytokine withdrawal is
24p3
, which encodes a secreted lipocalin. Conditioned medium from IL-3–deprived FL5.12 cells contained 24p3 and induced apoptosis in naı̈ve FL5.12 cells even when IL-3 was present. 24p3 also induced apoptosis in a wide variety of leukocytes but not other cell types. Apoptotic sensitivity correlated with the presence of a putative 24p3 cell surface receptor. We conclude that IL-3 deprivation activates
24p3
transcription, leading to synthesis and secretion of 24p3, which induces apoptosis through an autocrine pathway.
Ultrafast Manipulation of Electron Spin Coherence
Authors: Gupta J. A. , Knobel R. , Samarth N. , Awschalom D. D.
Published: July 2002

A technique is developed with the potential for coherent all-optical control over electron spins in semiconductors on femtosecond time scales. The experiments show that optical “tipping” pulses can enact substantial rotations of electron spins through a mechanism dependent on the optical Stark effect. These rotations were measured as changes in the amplitude of spin precession after optical excitation in a transverse magnetic field and approach π/2 radians. A prototype sequence of two tipping pulses indicates that the rotation is reversible, a result that establishes the coherent nature of the tipping process.
A Gene Expression Map for
Caenorhabditis elegans
Authors: Kim Stuart K. , Lund Jim , Kiraly Moni , Duke Kyle , Jiang Min , Stuart Joshua M. , Eizinger Andreas , Wylie Brian N. , Davidson George S.
Published: July 2002

We have assembled data from
Caenorhabditis elegans
DNA microarray experiments involving many growth conditions, developmental stages, and varieties of mutants. Co-regulated genes were grouped together and visualized in a three-dimensional expression map that displays correlations of gene expression profiles as distances in two dimensions and gene density in the third dimension. The gene expression map can be used as a gene discovery tool to identify genes that are co-regulated with known sets of genes (such as heat shock, growth control genes, germ line genes, and so forth) or to uncover previously unknown genetic functions (such as genomic instability in males and sperm caused by specific transposons).
Interpretation of High Projections for Global-Mean Warming
Authors: Wigley T. M. L. , Raper S. C. B.
Published: July 2002

The Intergovernmental Panel on Climate Change (IPCC) has recently released its Third Assessment Report (TAR), in which new projections are given for global-mean warming in the absence of policies to limit climate change. The full warming range over 1990 to 2100, 1.4° to 5.8°C, is substantially higher than the range given previously in the IPCC Second Assessment Report. Here we interpret the new warming range in probabilistic terms, accounting for uncertainties in emissions, the climate sensitivity, the carbon cycle, ocean mixing, and aerosol forcing. We show that the probabilities of warming values at both the high and low ends of the TAR range are very low. In the absence of climate-mitigation policies, the 90% probability interval for 1990 to 2100 warming is 1.7° to 4.9°C.
An Essential Role for BAFF in the Normal Development of B Cells Through a BCMA-Independent Pathway
Authors: Schiemann Barbara , Gommerman Jennifer L. , Vora Kalpit , Cachero Teresa G. , Shulga-Morskaya Svetlana , Dobles Max , Frew Erica , Scott Martin L.
Published: July 2002

The B cell activating factor BAFF (BlyS/TALL-1/zTNF4) is a tumor necrosis factor (TNF)–related ligand that promotes B cell survival and binds to three receptors (BCMA, TACI, and the recently described BAFF-R). Here we report an absolute requirement for BAFF in normal B cell development. Examination of secondary lymphoid organs from BAFF-deficient mice revealed an almost complete loss of follicular and marginal zone B lymphocytes. In contrast, mice lacking BCMA had normal-appearing B lymphocyte compartments. BAFF therefore plays a crucial role in B cell development and can function through receptors other than BCMA.
Atlantic Water Flow Pathways Revealed by Lead Contamination in Arctic Basin Sediments
Authors: Gobeil Charles , Macdonald Robie W. , Smith John N. , Beaudin Luc
Published: July 2002

Contaminant lead in sediments underlying boundary currents in the Arctic Ocean provides an image of current organization and stability during the past 50 years. The sediment distributions of lead, stable lead isotope ratios, and lead-210 in the major Arctic Ocean basins reveal close coupling of the Eurasian Basin with the North Atlantic during the 20th century. They indicate that the Atlantic water boundary current in the Eurasian Basin has been a prominent pathway, that contaminant lead from the Laptev Sea supplies surface water in the transpolar drift, and that the Canadian and Eurasian basins have been historically decoupled.
Mapping of Contralateral Space in Retinotopic Coordinates by a Parietal Cortical Area in Humans
Authors: Sereno M. I. , Pitzalis S. , Martinez A.
Published: July 2002

The internal organization of a higher level visual area in the human parietal cortex was mapped. Functional magnetic resonance images were acquired while the polar angle of a peripheral target for a delayed saccade was gradually changed. A region in the superior parietal cortex showed robust retinotopic mapping of the remembered target angle. The map reversed when the direction of rotation of the remembered targets was reversed and persisted unchanged when study participants detected rare target reappearances while maintaining fixation, or when the eccentricity of successive remembered targets was unpredictable. This region may correspond to the lateral intraparietal area in macaque monkeys.
Solar Forcing of Regional Climate Change During the Maunder Minimum
Authors: Shindell Drew T. , Schmidt Gavin A. , Mann Michael E. , Rind David , Waple Anne
Published: July 2002

We examine the climate response to solar irradiance changes between the late 17th-century Maunder Minimum and the late 18th century. Global average temperature changes are small (about 0.3° to 0.4°C) in both a climate model and empirical reconstructions. However, regional temperature changes are quite large. In the model, these occur primarily through a forced shift toward the low index state of the Arctic Oscillation/North Atlantic Oscillation as solar irradiance decreases. This leads to colder temperatures over the Northern Hemisphere continents, especially in winter (1° to 2°C), in agreement with historical records and proxy data for surface temperatures.
Induction of Dendritic Cell Differentiation by IFN-α in Systemic Lupus Erythematosus
Authors: Blanco Patrick , Palucka A. Karolina , Gill Michelle , Pascual Virginia , Banchereau Jacques
Published: July 2002

Dendritic cells (DCs) are important in regulating both immunity and tolerance. Hence, we hypothesized that systemic lupus erythematosus (SLE), an autoimmune disease characterized by autoreactive B and T cells, may be caused by alterations in the functions of DCs. Consistent with this, monocytes from SLE patients' blood were found to function as antigen-presenting cells, in vitro. Furthermore, serum from SLE patients induced normal monocytes to differentiate into DCs. These DCs could capture antigens from dying cells and present them to CD4-positive T cells. The capacity of SLE patients' serum to induce DC differentiation correlated with disease activity and depended on the actions of interferon-α (IFN-α). Thus, unabated induction of DCs by IFN-α may drive the autoimmune response in SLE.
Blazing Pathways Through Genetic Mountains
Authors: Gifford David K.
Published: July 2002

It is now widely accepted that high-throughput data sources will shed essential understanding on the inner workings of cellular and organism function. One key challenge is to distill the results of such experiments into an interpretable computational form that will be the basis of a predictive model. A predictive model represents the gold standard in understanding a biological system and will permit us to investigate the underlying cause of diseases and help us to develop therapeutics. Here I explore how discoveries can be based on high-throughput data sources and discuss how independent discoveries can be assembled into a comprehensive picture of cellular function.
Germ Cell Survival Through Carbohydrate-Mediated Interaction with Sertoli Cells
Authors: Akama Tomoya O. , Nakagawa Hiroaki , Sugihara Kazuhiro , Narisawa Sonoko , Ohyama Chikara , Nishimura Shin-Ichiro , O'Brien Deborah A. , Moremen Kelley W. , Millán José Luis , Fukuda Michiko N.
Published: July 2002

Spermatogenesis is a precisely regulated process in which the germ cells closely interact with Sertoli cells. The molecular basis of this cell-cell adhesion is unknown. Here, we demonstrate that targeted disruption of
Man2a2
, a gene encoding α-mannosidase IIx (MX), an enzyme that forms intermediate asparagine-linked carbohydrates (
N
-glycans), results in
Man2a2
null males that are largely infertile. The
Man2a2
null spermatogenic cells fail to adhere to Sertoli cells and are prematurely released from the testis to epididymis. We identified an
N
-glycan structure that plays a key role in germ cell–Sertoli cell adhesion and showed that a specific carbohydrate was required for spermatogenesis.
Activation of Orphan Receptors by the Hormone Relaxin
Authors: Hsu Sheau Yu , Nakabayashi Koji , Nishi Shinya , Kumagai Jin , Kudo Masataka , Sherwood O. David , Hsueh Aaron J. W.
Published: July 2002

Relaxin is a hormone important for the growth and remodeling of reproductive and other tissues during pregnancy. Although binding sites for relaxin are widely distributed, the nature of its receptor has been elusive. Here, we demonstrate that two orphan heterotrimeric guanine nucleotide binding protein (G protein)–coupled receptors, LGR7 and LGR8, are capable of mediating the action of relaxin through an adenosine 3′,5′-monophosphate (cAMP)–dependent pathway distinct from that of the structurally related insulin and insulin-like growth factor family ligand. Treatment of antepartum mice with the soluble ligand-binding region of LGR7 caused parturition delay. The wide and divergent distribution of the two relaxin receptors implicates their roles in reproductive, brain, renal, cardiovascular, and other functions.
Trithorax and dCBP Acting in a Complex to Maintain Expression of a Homeotic Gene
Authors: Petruk Svetlana , Sedkov Yurii , Smith Sheryl , Tillib Sergei , Kraevski Vladislav , Nakamura Tatsuya , Canaani Eli , Croce Carlo M. , Mazo Alexander
Published: July 2002

Trithorax (Trx) is a member of the trithorax group (trxG) of epigenetic regulators, which is required to maintain active states of Hox gene expression during development. We have purified from
Drosophila
embryos a trithorax acetylation complex (TAC1) that contains Trx, dCBP, and Sbf1. Like CBP, TAC1 acetylates core histones in nucleosomes, suggesting that this activity may be important for epigenetic maintenance of gene activity. dCBP and Sbf1 associate with specific sites on salivary gland polytene chromosomes, colocalizing with many Trx binding sites. One of these is the site of the Hox gene
Ultrabithorax
(
Ubx
). Mutations in either
trx
or the gene encoding dCBP reduce expression of the endogenous
Ubx
gene as well as of transgenes driven by the
bxd
regulatory region of
Ubx
. Thus Trx, dCBP, and Sbf1 are closely linked, physically and functionally, in the maintenance of Hox gene expression.
Beta-Diversity in Tropical Forest Trees
Authors: Condit Richard , Pitman Nigel , Leigh Egbert G. , Chave Jérôme , Terborgh John , Foster Robin B. , Núñez Percy , Aguilar Salomón , Valencia Renato , Villa Gorky , Muller-Landau Helene C. , Losos Elizabeth , Hubbell Stephen P.
Published: July 2002

The high alpha-diversity of tropical forests has been amply documented, but beta-diversity—how species composition changes with distance—has seldom been studied. We present quantitative estimates of beta-diversity for tropical trees by comparing species composition of plots in lowland terra firme forest in Panama, Ecuador, and Peru. We compare observations with predictions derived from a neutral model in which habitat is uniform and only dispersal and speciation influence species turnover. We find that beta-diversity is higher in Panama than in western Amazonia and that patterns in both areas are inconsistent with the neutral model. In Panama, habitat variation appears to increase species turnover relative to Amazonia, where unexpectedly low turnover over great distances suggests that population densities of some species are bounded by as yet unidentified processes. At intermediate scales in both regions, observations can be matched by theory, suggesting that dispersal limitation, with speciation, influences species turnover.
Direct Synthesis of Long Single-Walled Carbon Nanotube Strands
Authors: Zhu H. W. , Xu C. L. , Wu D. H. , Wei B. Q. , Vajtai R. , Ajayan P. M.
Published: July 2002

In the processes that are used to produce single-walled nanotubes (electric arc, laser ablation, and chemical vapor deposition), the typical lengths of tangled nanotube bundles reach several tens of micrometers. We report that long nanotube strands, up to several centimeters in length, consisting of aligned single-walled nanotubes can be synthesized by the catalytic pyrolysis of
n
-hexane with an enhanced vertical floating technique. The long strands of nanotubes assemble continuously from arrays of nanotubes, which are intrinsically long.
Protein Nanoarrays Generated By Dip-Pen Nanolithography
Authors: Lee Ki-Bum , Park So-Jung , Mirkin Chad A. , Smith Jennifer C. , Mrksich Milan
Published: July 2002

Dip-pen nanolithography was used to construct arrays of proteins with 100- to 350-nanometer features. These nanoarrays exhibit almost no detectable nonspecific binding of proteins to their passivated portions even in complex mixtures of proteins, and therefore provide the opportunity to study a variety of surface-mediated biological recognition processes. For example, reactions involving the protein features and antigens in complex solutions can be screened easily by atomic force microscopy. As further proof-of-concept, these arrays were used to study cellular adhesion at the submicrometer scale.
Unique and Redundant Connexin Contributions to Lens Development
Authors: White Thomas W.
Published: July 2002

Connexin genes encode intercellular channels that help to coordinate development. In mice, the targeted deletion of different connexins produces disparate effects on ocular growth and differentiation in the lens, and the need for multiple channel subunits is poorly understood. Knockout of
Cx46
causes a loss of homeostasis and cataracts. Deletion of
Cx50
results in reduced ocular growth and cataracts. Targeted replacement of
Cx50
with
Cx46
by genetic knock-in corrected defects in cellular differentiation and prevented cataracts, but did not restore normal growth. These data show that intrinsic properties of
Cx50
were required for cellular growth, whereas nonspecific restoration of communication by
Cx46
maintained differentiation.
Functional Materials Based on Self-Assembly of Polymeric Supramolecules
Authors: Ikkala Olli , ten Brinke Gerrit
Published: July 2002

Self-assembly of polymeric supramolecules is a powerful tool for producing functional materials that combine several properties and may respond to external conditions. We illustrate the concept using a comb-shaped architecture. Examples include the hexagonal self-organization of conjugated conducting polymers and the polarized luminance in solid-state films of rodlike polymers obtained by removing the hydrogen-bonded side chains from the aligned thermotropic smectic phase. Hierarchically structured materials obtained by applying different self-organization and recognition principles and directed assembly form a basis for tunable nanoporous materials, smart membranes, preparation of nano-objects, and anisotropic properties, such as proton conductivity.
Asparagine Hydroxylation of the HIF Transactivation Domain: A Hypoxic Switch
Authors: Lando David , Peet Daniel J. , Whelan Dean A. , Gorman Jeffrey J. , Whitelaw Murray L.
Published: July 2002

The hypoxia-inducible factors (HIFs) 1α and 2α are key mammalian transcription factors that exhibit dramatic increases in both protein stability and intrinsic transcriptional potency during low-oxygen stress. This increased stability is due to the absence of proline hydroxylation, which in normoxia promotes binding of HIF to the von Hippel–Lindau (VHL tumor suppressor) ubiquitin ligase. We now show that hypoxic induction of the COOH-terminal transactivation domain (CAD) of HIF occurs through abrogation of hydroxylation of a conserved asparagine in the CAD. Inhibitors of Fe(II)- and 2-oxoglutarate–dependent dioxygenases prevented hydroxylation of the Asn, thus allowing the CAD to interact with the p300 transcription coactivator. Replacement of the conserved Asn by Ala resulted in constitutive p300 interaction and strong transcriptional activity. Full induction of HIF-1α and -2α, therefore, relies on the abrogation of both Pro and Asn hydroxylation, which during normoxia occur at the degradation and COOH-terminal transactivation domains, respectively.
Neural Mechanisms of Saccadic Suppression
Authors: Thiele A. , Henning P. , Kubischik M. , Hoffmann K.-P.
Published: July 2002

In normal vision our gaze leaps from detail to detail, resulting in rapid image motion across the retina. Yet we are unaware of such motion, a phenomenon known as saccadic suppression. We recorded neural activity in the middle temporal and middle superior temporal cortical areas during saccades and identical image motion under passive viewing conditions. Some neurons were selectively silenced during saccadic image motion, but responded well to identical external image motion. In addition, a subpopulation of neurons reversed their preferred direction of motion during saccades. Consequently, oppositely directed motion signals annul one another, and motion percepts are suppressed.
Solution of a 20-Variable 3-SAT Problem on a DNA Computer
Authors: Braich Ravinderjit S. , Chelyapov Nickolas , Johnson Cliff , Rothemund Paul W. K. , Adleman Leonard
Published: July 2002

A 20-variable instance of the NP-complete three-satisfiability (3-SAT) problem was solved on a simple DNA computer. The unique answer was found after an exhaustive search of more than 1 million (2
20
) possibilities. This computational problem may be the largest yet solved by nonelectronic means. Problems of this size appear to be beyond the normal range of unaided human computation.