bluuebunny/crossref_metadata_2025 huggingface dataset:
Steel coating by self-induced ion plating, a new high throughput metallization ion plating technique
Authors: Brande P. Vanden , Weymeersch A.
Published: July 2002

Ion plating techniques present major advantages for continuous steel coating in terms of throughput and product quality when compared, respectively, to sputtering and vacuum evaporation techniques. However, the ion plating systems available on the market today are cumbersome and have technological problems that prevent immediate implementation in high throughput air-to-air continuous steel coating plants. To address these problems, we have developed a new ion plating technique, referred to as “self-induced ion plating,” in order to produce continuous coating on flat products. This technique is based on the generation of a magnetron discharge in the sputtered and evaporated vapor produced by a tin cylindrical target. Very high deposition rates were achieved (⩽220 μm/min) with moderate values of the electrical mean power density (⩽45 W cm−2) applied to the tin target. The magnetron configuration reduced material side losses by reducing the metal escape zone area on the target side. Another feature of this technique is that the control of the heat transfer between the target and its backing plate allows control of the target surface temperature, and hence control of the sputtered and evaporated material fractions.
Effect of rapid thermal annealing temperature on the formation of CoSi studied by x-ray photoelectron spectroscopy and micro-Raman spectroscopy
Authors: Zhao Jin , Ballast Lynette K. , Hossain Tim Z. , Trostel Rebecca E. , Bridgman William C.
Published: July 2002

The effect of rapid thermal annealing (RTA) temperature on the formation of the cobalt silicide (CoSi) was investigated using x-ray photoelectron spectroscopy (XPS) and micro Raman spectroscopy. With 125 Å of Co deposited on single-crystalline Si wafers and capped by a Ti thin film, the wafers were rapid thermal annealed at 450, 460, 470, 480, and 490 °C. These wafers were then stripped with a sulfuric acid peroxide mixture treatment. XPS was used to determine the chemical composition of the CoSi thin films, and the Co Auger parameter was continuously monitored along with ion sputtering to provide a chemical-state depth profile. Micro-Raman spectroscopy was used as a nondestructive method to characterize the film thickness and uniformity of the CoSi thin films on Si wafers. The Raman shifts at 204 and 220 cm−1 due to CoSi and the Raman shift at 150 cm−1 due to Co2Si are reported. The product αd (α is the absorption coefficient, d is the film thickness), which indicates the CoSi film thickness, was calculated from the Si excitation wavelength of 520 cm1. The behavior of the calculated αd as a function of the RTA temperature confirms the XPS result.
Surface morphology and electronic structure of Ni/Ag(100)
Authors: Hite D. A. , Kizilkaya O. , Sprunger P. T. , Howard M. M. , Ventrice C. A. , Geisler H. , Zehner D. M.
Published: July 2002

The growth morphology and electronic structure of Ni on Ag(100) has been studied with scanning tunneling microscopy (STM) and synchrotron based angle resolved photoemission spectroscopy. At deposition temperatures at or below 300 K, STM reveals Ni cluster growth on the surface along with some subsurface growth. Upon annealing to 420 K, virtually all Ni segregates into the subsurface region forming embedded nanoclusters. The electronic structure of Ni d bands in the unannealed surface shows dispersion only perpendicular to the surface whereas the annealed surface has Ni d bands that exhibit a three-dimensional-like structure. This is a result of the increased Ni d–Ag sp hybridization bonding and increased coordination of the embedded Ni nanoclusters.
Kinetics of the ablative photodecomposition of organic polymers in the far ultraviolet (193 nm)
Authors: Srinivasan R.
Published: July 2002

The efficiency of ablative photodecomposition of poly(methyl methacrylate) caused by pulsed radiation at 193 nm from an excimer laser (argon–fluorine fill) has been studied as a function of fluence. A threshold for the onset of the process lies at 10 mJ/cm2 per pulse of 14 ns. The efficiency of the process increases rapidly with fluence up to 250 mJ/cm2 per pulse after which it levels off sharply. The etch depth of ∼3100 Å/pulse at 250 mJ/cm2 can be compared to a depth of 4800 Å/pulse at a fluence of >20 J/cm2/pulse. A model is suggested for the ablative photodecomposition of organic materials and the photochemistry of the process at different fluences is discussed. The variation in the efficiency with the composition of the polymer is briefly considered.
Self-aligned silicides or metals for very large scale integrated circuit applications
Authors: Murarka Shyam P.
Published: July 2002

The increases in the packing density and the resulting shrinkage of the silicon integrated circuit dimensions led to the investigation and successful application of the deposited refractory silicide layers as the gate and interconnection metallization. The continued shrinking of the device dimensions has now turned attention to further lowering of the resistance at the gate level and to finding a contact metallization for the shallow junctions. Although refractory metals are being considered for the former, self-aligned silicides of cobalt, titanium, platinum, and nickel offer the possibility of satisfying both the gate and interconnection and contact metallization requirements. This paper will review the present status of the refractory silicide and refractory metal technologies and compare them with the upcoming self-aligned silicide technology.
A vertical stepper for synchrotron x-ray lithography
Authors: Ishihara S. , Kanai M. , Une A. , Suzuki M.
Published: July 2002

A newly developed synchrotron x-ray stepper installed at a port of an electron storage ring of the NTT Synchrotron Facility1–3 is described. The stepper exposes wafers in a normal atmosphere to vertically scanned synchrotron x rays with continuous alignment control throughout the exposure. The key devices of the stepper are a vertical x–y stage and an optical-heterodyne alignment system. The x–y stage fully utilizes air lubricated ceramic components including air bearing lead screws and achieves 5 nm motion precision over a 200 mm stroke. The optical-heterodyne alignment system detects both lateral and gap displacements between the mask and wafer gratings with accuracies of 10 and 100 nm, respectively. After wafer step positioning using a laser interferometer, alignment between the mask and wafer is executed using the optical-heterodyne signal. In several exposure experiments, alignment capability is estimated to be better than ±0.14 μm as a 3σ value of machine repeatability. Also, several applications to device fabrication confirmed that the vertical stepper is feasible in quarter- to half-micron x-ray lithography.
Focused ion beam fabrication of submicron gold structures
Authors: Blauner Patricia G. , Ro Jae Sang , Butt Yousaf , Melngailis John
Published: July 2002

Because of their ability to both mill and deposit material with submicron resolution, focused ion beams are now used to repair photolithography masks and are of increasing technological interest in the repair of x-ray lithography masks and in integrated circuit restructuring. With the latter two applications in mind, we have fabricated milled and deposited Au features with linewidths of ≤0.1 μm using a 40 keV Ga focused ion beam. In addition, we present the results of a study parameterizing focused ion beam induced Au deposition under conditions of practical interest. Milling is accomplished by simple physical sputtering. Examples of milled microfeatures include a grating with a 210 nm period milled through a 5000 Å thick evaporated Au film. Deposition is accomplished by ion bombarding a SiO2 substrate on which a precursor gas, dimethyl gold hexafluoro acetylacetonate, is continuously being adsorbed. Examples of deposited Au features include a 3×3 μm patch 1-μm-thick with steep sidewalls. The deposition rate was measured at room temperature as a function of ion and precursor flux, and a simple model of the process is fitted to the data. Ion beam induced deposition efficiency is shown to depend critically on the time averaged beam current density and only weakly on the precursor flux. The maximum achievable growth rate is shown to be ∼10 Å/s. Deposited Au films contain 30–60 at. % carbon and have conductivities 200–600 times less than that of bulk Au. Those films formed using lower organometallic pressures or higher ion beam current densities are characterized by greater purity with more continuous microstructure.
Diamond membrane based x-ray masks
Authors: Löchel B. , Huber H.-L. , Klages C.-P. , Schäfer L. , Bluhm A.
Published: July 2002

Diamond layers are of special interest for x-ray mask fabrication because of their excellent optical and mechanical properties. Using a plasma activated chemical vapor deposition process, 2–5 μm thick diamond layers were deposited and characterized. Microhardness and Young’s modulus are found to be very close to the literature values of bulk diamond. A standard isotropic etching process was applied for membrane fabrication. The optical transparency and the thickness homogeneity were measured. Concerning surface roughness, scanning electron micrographs and atomic force measurements were evaluated. After irradiation of diamond masks with synchrotron light, a slight increase of optical transparency was found.
Optimization of partially coherent optical system for optical lithography
Authors: Inoue Soichi , Fujisawa Tadahito , Tamaushi Shuichi , Ogawa Yoji , Nakase Makoto
Published: July 2002

A new approach, based on the optimization algorithm of ‘‘simulated annealing,’’ is applied to maximize depth of focus for a partially coherent optical system. Optimization is carried out with practical constraints for optical lithography. A phase contrast lithography, consisting of an annular effective source and an annular phase filter on the pupil, is proposed as a definite result of the optimization procedure.
Proximity effect correction data processing system for electron beam lithography
Authors: Harafuji Kenji , Misaka Akio , Kawakita Kenji , Nomura Noboru , Hamaguchi Hiromitsu , Kawamoto Masahiro
Published: July 2002

A proximity effect correction system has been developed by utilizing an efficient dose modulation technique based on a double Gaussian proximity function. A shaped electron beam system is assumed to be used. Two improvements are made. First, an optimal exposure dose on each pattern is determined by a new fast iterative method. The optimal dose makes the development isocontour conform to the pattern specification fairly well. Second, a ‘‘simple cell unit algorithm’’ that one of identical cells is proximity-corrected, and the result is used to the other remaining cells is introduced. This offers to both decrease the processing time and save the memory/disk space. The present system is applied to the data processing of scaled-down version of an aluminum wiring layer pattern of 16 Mbit dynamic random access memory with its minimum dimension of 0.4 μm. The calculation is successfully completed within 1 h of CPU time on a 10 MIPS general-purpose computer. The dimensional accuracy of 10% is confirmed experimentally for the pattern including minimum features of 0.4 μm in the combination of trilayer resist process.
New selective molecular-beam epitaxial growth method for direct formation of GaAs quantum dots
Authors: Koguchi Nobuyuki , Ishige Keiko , Takahashi Satoshi
Published: July 2002

Numerous GaAs epitaxial microcrystals having an average base size of 700 Å×700 Å surrounded mainly by (111) and (110) facets were fabricated on a sulfer-terminated (S-terminated) GaAs (001) substrate by sequentially supplying Ga and As molecular beams. The S-terminated GaAs (001) surface was produced by exposing the surface to a sulfur vapor in the molecular-beam epitaxy system immediately after obtaining a Ga-stabilized surface. The growth of GaAs microcrystals on the S-terminated substrate is caused by a vapor–liquid–solid mechanism. The process consists of forming Ga droplets on the inert surface and reacting the droplets with As to produce GaAs microcrystals. This method termed droplet epitaxy is thought to be a promising growth method for fabricating the GaAs quantum dots.
Fundamental principles of phase shifting masks by Fourier optics: Theory and experimental verification
Authors: Ronse K. , Op de Beeck M. , Van den hove L. , Engelen J.
Published: July 2002

In this article, the imaging principles of projection steppers for optical lithography, using standard transmission masks, are reviewed, and compared with the image formation using various types of phase shifting masks (PSM), in order to get a better insight in the working principles of the various PSM techniques. The printability of periodic lines using a standard transmission mask is compared with the use of an alternating shifter PSM and the dependence of imaging on spatial coherence is discussed extensively. Furthermore the imaging of isolated lines is discussed, using a bright field standard transmission mask, a chromeless phase edge PSM, and a dark field rim PSM. The coherence dependence of an alternated shifter is verified experimentally. Experiments also confirm the superior performance of a chromeless phase edge PSM to print isolated lines.
Fabrication of silicon quantum wires by anisotropic wet chemical etching and thermal oxidation
Authors: Liu J. L. , Shi Y. , Wang F. , Zhang R. , Han P. , Mao B. H. , Zheng Y. D.
Published: July 2002

Ultrafine silicon quantum wires with high-quality Si/SiO2 heterointerfaces are successfully fabricated by utilizing anisotropic wet chemical etching and subsequent thermal oxidation. It is also found that the lateral dimensions of silicon quantum wires can be well controlled by selecting the temperature of the thermal oxidation process. The cross-sectional image from a scanning electron microscope shows silicon quantum wires of high quality with the linewidth down to 20 nm.
Probing complex low-dimensional solids with scanning probe microscopes: From charge density waves to high-temperature superconductivity
Authors: Liu Jie , Huang Jin-Lin , Lieber Charles M.
Published: July 2002

Scanning probe microscopy (SPM) studies of low-dimensional solids have yielded significant advances in the understanding of collective phenomena in these complex materials. In this article, SPM studies from the authors’ laboratory of charge density waves (CDWs) in metal dichalcogenide materials and of high-temperature copper–oxide superconductors (HTSCs) are reviewed. Temperature-dependent SPM studies of the structural properties of CDW phases in metal-doped tantalum disulfide (TaS2) and tantalum diselenide (TaSe2) have directly illuminated the problem of weak and strong CDW pinning. These studies have also led to the unexpected discovery of a hexatic CDW phase in niobium-doped TaS2. SPMs have also been used to probe the structural and electronic properties of HTSCs. SPM studies of metal-doped Bi2Sr2CaCu2O8 (BSCCO) superconductors have been used to elucidate the complex crystal chemistry of this system. Comparisons of these data with electron diffraction measurements also highlight the importance of local crystallography in these complex materials. In addition, SPM studies have been used to elucidate the interaction of magnetic flux lines with surface defects, thus enabling a quantitative assessment of pinning by surface roughness. Finally, etching has been studied by in situ SPM and shown to be a promising means for controlling the properties of HTSC surfaces.
Kelvin probe force microscopy for characterization of semiconductor devices and processes
Authors: Tanimoto Masafumi , Vatel Olivier
Published: July 2002

Kelvin probe force microscopy was applied to the characterization of Al0.3Ga0.7As/GaAs multilayer structures and Si-pn structures. The spatial resolution of Kelvin probe force microscopy measurement was investigated using cleaved Al0.3Ga0.7As/GaAs structures. A 40-nm-thick Al0.3Ga0.7As layer was resolved with potential difference of 15 mV. It was found that the measured potential is sensitive to the Al mole fraction for AlGaAs. Two-dimensional delineation of Si-pn structures was successfully carried out on the topmost and the cleaved surfaces. A decrease of a depletion-layer width with increasing the intensity of light illuminating the sample was observed. It was confirmed through investigating illumination effects on various kinds of structures that the measured Kelvin probe force microscopy potential reflects the surface band structure of the sample. A lateral impurity profile under the ion implantation mask was extracted from the two-dimensional potential image obtained on the cleaved pn junction by calibrating with the impurity concentration profile measured by the spreading resistance method.
Electron beam induced deposition from W(CO)6 at 2 to 20 keV and its applications
Authors: Hoyle P. C. , Cleaver J. R. A. , Ahmed H.
Published: July 2002

Electron beam induced deposition from W(CO)6 was studied for beam energies between 2 and 20 keV and a range of exposure doses, to investigate the dependence of deposit thickness and electrical conductivity on energy and the dependence of deposit conductance on cumulative exposure dose. Larger deposited thicknesses and higher conductivities were produced at the lower beam energies and were attributed to the higher secondary electron yield at lower energies. The deposit thickness was found to scale linearly with exposure dose. The initial dependence of conductance on exposure dose (and deposit thickness) was nonlinear and was attributed to the change from a discontinuous to a continuous film, and to increased backscattering. The subsequent dependence of conductance on exposure dose was linear for deposit thicknesses which were small compared with the electron range, implying that burial precludes the further decomposition of partially decomposed W(CO)6 molecules incorporated in the deposit. Transmission electron microscope examination showed that the structure of the deposits depended on the beam scanning conditions. Deposits were used to form a mask for CF4 plasma etching of Si, while deposits on a doped GaAs substrate were found to form a Schottky contact with an ideality factor of 1.40, enabling the repair of a metal–semiconductor field-effect transistor gate to be demonstrated.
Microtrenching resulting from specular reflection during chlorine etching of silicon
Authors: Hoekstra Robert J. , Kushner Mark J. , Sukharev Valeriy , Schoenborn Phillipe
Published: July 2002

In an effort to increase throughput, the microelectronics fabrication industry has transitioned to high plasma density etching reactors using large source (>800 W) and moderate substrate bias (>100 W) powers in which the ion to neutral radical flux is large compared to reactive-ion-etching systems. These conditions can lead to microtrenching where etch rates are largest at the base of the sidewalls. Microtrenching has been attributed to specular reflection of high energy particles, usually ions, at grazing angles on the sidewalls of the mask and trench. These reflections produce a “focusing” of flux to the corners of the trench which results in locally enhanced etching. In this letter, integrated plasma equipment and Monte Carlo feature profile models have been used to examine the processes and conditions which produce focused fluxes and microtrenching, including the degree of specular reflection and sidewall slope of the mask. Quantitative comparisons are made to experimental measurements of etch profiles.
Investigation of the chemistry and electronic properties of metal/gallium nitride interfaces
Authors: Wu C. I. , Kahn A.
Published: July 2002

We present a systematic investigation of the formation of Schottky barriers between n- and p-GaN(0001)−(1×1) grown by metalorganic chemical vapor deposition and a series of high and low work function metals (Mg, Al, Ti, Au, and Pt). Al, Ti, and Mg react at room temperature with nitrogen, whereas Au and Pt form abrupt, unreacted interfaces. We find that the Fermi level movement on both n- and p-GaN is consistent with variations in metal work functions, but limited by surface or interface states. Upon annealing, the incorporation of Mg increases the density of acceptors as seen on both n- and p-GaN. In spite of similar work functions and chemical reaction with nitrogen, Ti and Al show drastic differences in Schottky barrier formation due to differences in the nature of the products of reaction. AlN is a wide band gap semiconductor whereas TiN is a metallic compound.
Focused ion beam micromilling of GaN and related substrate materials (sapphire, SiC, and Si)
Authors: Steckl A. J. , Chyr I.
Published: July 2002

Micromilling of GaN films has been obtained using a Ga+ focused ion beam (FIB). The GaN micromilling has been investigated over a range of energies (15–70 keV), incident angles (0°–30°), and number of scans (10–50). At normal incidence, increasing the Ga+ energy up to 50 keV increases the milling rate, while higher energies produce the same (or a slightly decreased) milling rate. Increasing the angle of incidence increases the milling rate at all energies. The highest GaN milling rate of 0.6 μm3/nA s (corresponding to an average yield of 6.6 atoms/ion) has been obtained at 50 keV, 30° incidence, and 50 scans. The milling rate of current substrate materials (sapphire, Si and SiC) for GaN thin film growth is shown to be 2–5 times lower. The sputtering yield is found to vary inversely with the strength of the chemical bond in the materials investigated. Distributed Bragg reflection air/GaN gratings for short cavity lasers were fabricated to show the capability of FIB micromilling to produce optoelectronic devices based on GaN.
Characteristics of chemical vapor deposition diamond films for x-ray mask substrates
Authors: Brooks Cameron J. , Powers Lynn A. , Acosta R. E. , Moily Darius , Faili Firooz , Herb John A.
Published: July 2002

In this article we summarize the development and characterization of chemical vapor deposition diamond films used for x-ray mask substrates. Good control of the deposition parameters governing film stress and film thickness has been achieved, and optical transmission greater than 40% has been demonstrated. Novel polishing techniques were employed to reduce the diamond surface roughness to below 3.0 nm Ra. The crystal structure of the diamond material, determined by Raman spectroscopy, transmission electron microscopy and electron diffraction analysis, is polycrystalline, with grain sizes of approximately 10 nm. Accelerated radiation damage testing indicates a correlation between hydrogen content in the films and the degree of distortion. X-ray masks using diamond membranes have been successfully fabricated to provide early learning of image-size and image-placement performance.
XY stage driven by ultrasonic linear motors for the electron-beam x-ray mask writer EB-X3
Authors: Kunioka T. , Takeda Y. , Matsuda T. , Shimazu N. , Nakayama Y.
Published: July 2002

A new XY stage employing ultrasonic linear motors that is simple, compact, and completely nonmagnetic was developed for an e-beam x-ray mask writer employing step-and-repeat writing. The main performance results are: (1) position fluctuations during writing are less than ±4 nm; (2) the time for a 650 μm step movement is less than 270 ms; (3) the magnetic interference is less than ±1 nT at the e-beam deflection center; and (4) the vibration of the stage when moving at a constant speed is less than ±0.25 m/s2. The new stage is also very easy to maintain. It has been installed in the EB-X3 e-beam x-ray mask writer.
Free-Electron Lasers: Status and Applications
Authors: O'Shea Patrick G. , Freund Henry P.
Published: July 2002

A free-electron laser consists of an electron beam propagating through a periodic magnetic field. Today such lasers are used for research in materials science, chemical technology, biophysical science, medical applications, surface studies, and solid-state physics. Free-electron lasers with higher average power and shorter wavelengths are under development. Future applications range from industrial processing of materials to light sources for soft and hard x-rays.
Filopodial Calcium Transients Promote Substrate-Dependent Growth Cone Turning
Authors: Gomez Timothy M. , Robles Estuardo , Poo Mu-ming , Spitzer Nicholas C.
Published: July 2002

Filopodia that extend from neuronal growth cones sample the environment for extracellular guidance cues, but the signals they transmit to growth cones are unknown. Filopodia were observed generating localized transient elevations of intracellular calcium ([Ca
2+
]
i
) that propagate back to the growth cone and stimulate global Ca
2+
elevations. The frequency of filopodial Ca
2+
transients was substrate-dependent and may be due in part to influx of Ca
2+
through channels activated by integrin receptors. These transients slowed neurite outgrowth by reducing filopodial motility and promoted turning when stimulated differentially within filopodia on one side of the growth cone. These rapid signals appear to serve both as autonomous regulators of filopodial movement and as frequency-coded signals integrated within the growth cone and could be a common signaling process for many motile cells.
Structure of an Extracellular gp130 Cytokine Receptor Signaling Complex
Authors: Chow Dar-chone , He Xiao-lin , Snow Andrew L. , Rose-John Stefan , Garcia K. Christopher
Published: July 2002

The activation of gp130, a shared signal-transducing receptor for a family of cytokines, is initiated by recognition of ligand followed by oligomerization into a higher order signaling complex. Kaposi's sarcoma–associated herpesvirus encodes a functional homolog of human interleukin-6 (IL-6) that activates human gp130. In the 2.4 angstrom crystal structure of the extracellular signaling assembly between viral IL-6 and human gp130, two complexes are cross-linked into a tetramer through direct interactions between the immunoglobulin domain of gp130 and site III of viral IL-6, which is necessary for receptor activation. Unlike human IL-6 (which uses many hydrophilic residues), the viral cytokine largely uses hydrophobic amino acids to contact gp130, which enhances the complementarity of the viral IL-6–gp130 binding interfaces. The cross-reactivity of gp130 is apparently due to a chemical plasticity evident in the amphipathic gp130 cytokine-binding sites.
Hepatitis C Virus IRES RNA-Induced Changes in the Conformation of the 40
S
Ribosomal Subunit
Authors: Spahn Christian M. T. , Kieft Jeffrey S. , Grassucci Robert A. , Penczek Pawel A. , Zhou Kaihong , Doudna Jennifer A. , Frank Joachim
Published: July 2002

Initiation of protein synthesis in eukaryotes requires recruitment of the 40
S
ribosomal subunit to the messenger RNA (mRNA). In most cases, this depends on recognition of a modified nucleotide cap on the 5′ end of the mRNA. However, an alternate pathway uses a structured RNA element in the 5′ untranslated region of the messenger or viral RNA called an internal ribosomal entry site (IRES). Here, we present a cryo-electron microscopy map of the hepatitis C virus (HCV) IRES bound to the 40
S
ribosomal subunit at about 20 Å resolution. IRES binding induces a pronounced conformational change in the 40
S
subunit and closes the mRNA binding cleft, suggesting a mechanism for IRES-mediated positioning of mRNA in the ribosomal decoding center.
Despised Marvels of Deception and Adaptation
Authors: Bush Albert O.
Published: July 2002

Parasite Rex
Inside the Bizarre World of Nature's Most Dangerous Creatures. Carl Zimmer. Free Press, New York, 2000. 320 pp. $26, C$38.50. ISBN 0-684-85638-7.

Rather than just consider "worms, protozoa, and other terrifying creatures," Zimmer offers general readers an appreciation of the complexities of parasite ecologies, life histories, and evolution.
Binding of GGA2 to the Lysosomal Enzyme Sorting Motif of the Mannose 6-Phosphate Receptor
Authors: Zhu Yunxiang , Doray Balraj , Poussu Anssi , Lehto Veli-Pekka , Kornfeld Stuart
Published: July 2002

The GGAs are a multidomain protein family implicated in protein trafficking between the Golgi and endosomes. Here, the VHS domain of GGA2 was shown to bind to the acidic cluster–dileucine motif in the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor (CI-MPR). Receptors with mutations in this motif were defective in lysosomal enzyme sorting. The hinge domain of GGA2 bound clathrin, suggesting that GGA2 could be a link between cargo molecules and clathrin-coated vesicle assembly. Thus, GGA2 binding to the CI-MPR is important for lysosomal enzyme targeting.
Orbital Influence on Earth's Magnetic Field: 100,000-Year Periodicity in Inclination
Authors: Yamazaki Toshitsugu , Oda Hirokuni
Published: July 2002

A continuous record of the inclination and intensity of Earth's magnetic field, during the past 2.25 million years, was obtained from a marine sediment core of 42 meters in length. This record reveals the presence of 100,000-year periodicity in inclination and intensity, which suggests that the magnetic field is modulated by orbital eccentricity. The correlation between inclination and intensity shifted from antiphase to in-phase, corresponding to a magnetic polarity change from reversed to normal. To explain the observation, we propose a model in which the strength of the geocentric axial dipole field varies with 100,000-year periodicity, whereas persistent nondipole components do not.
X-ray Emission from Comets
Authors: Cravens T. E.
Published: July 2002

The discovery of x-ray emission from comet Hyakutake was surprising given that comets are known to be cold. Observations by x-ray satellites such as the Röntgen Satellite (ROSAT) indicate that x-rays are produced by almost all comets. Theoretical and observational work has demonstrated that charge-exchange collisions of highly charged solar wind ions with cometary neutral species can explain this emission. X-ray observations of comets and other solar system objects may be used to determine the structure and dynamics of the solar wind.
Polytype Distribution in Circumstellar Silicon Carbide
Authors: Daulton T. L. , Bernatowicz T. J. , Lewis R. S. , Messenger S. , Stadermann F. J. , Amari S.
Published: July 2002

The inferred crystallographic class of circumstellar silicon carbide based on astronomical infrared spectra is controversial. We have directly determined the polytype distribution of circumstellar SiC from transmission electron microscopy of presolar silicon carbide from the Murchison carbonaceous meteorite. Only two polytypes (of a possible several hundred) were observed: cubic 3C and hexagonal 2H silicon carbide and their intergrowths. We conclude that this structural simplicity is a direct consequence of the low pressures in circumstellar outflows and the corresponding low silicon carbide condensation temperatures.
Science in Japan: Competition on Campus
Authors: Mervis Jeffrey
Published: July 2002

Two opposing forces--competition and cooperation--are beginning to coexist at Japanese universities as they enter an era of increased funding, increased demand for graduate students, and increased pressure to contribute to the country's economic growth. The new environment promises to be a challenging one, with great rewards for those who succeed and great risks for those who fail to keep pace.

On the competitive side, there is the unprecedented bidding for talented students, the growing use of peer review to make awards, and the emerging recognition that lifetime tenure, tiny but equitable grants, and immobility are not compatible with cutting-edge science. Those below the top tier--dominated by the University of Tokyo and Kyoto University--liken the new atmosphere to the law of the jungle: "I have to get everything for my team by myself, like a small yakuza [organized crime] boss," says one researcher at Waseda University.

But cooperation is too deeply ingrained in Japanese culture to be driven out by the latest demands on science. From high school teachers sharing their ideas on improving the precollege science curriculum to universities putting out the welcome mat for foreign scientists, increased domestic and international collaboration are seen as vital to a successful transformation of academic science.

The stories in this special section are intended to be a snapshot of life on campus and the issues affecting academic research in Japan. It is part of our broader, ongoing effort to report on the country's vibrant and growing scientific enterprise. We welcome your comments and your ideas for future coverage.

Jeffrey Mervis

Choice of Ph.D. or job

This section was reported and written by Dennis Normile, head of the Japan news bureau of Science, and June Kinoshita, a science writer based in Belmont, Massachusetts.

For more information on career opportunities, see Science's Next Wave's forum on “s
Training Young Scientists in Asia for the 21st Century
.”
Tumor Cells Fight Back to Beat Immune System
Authors: Williams Nigel
Published: July 2002

One of the biggest puzzles of cancer is why the sufferer's immune system almost always fails to eliminate the tumor. New research published in this issue (p.
1363
) suggests that at least one kind of cancer cell can kill the immune cells that attack them by prompting them to commit suicide.
Disease Extinction and Community Size: Modeling the Persistence of Measles
Authors: Keeling M. J. , Grenfell B. T.
Published: July 2002

A basic issue in ecology is the relation between extinction and population size. One of the clearest manifestations of a population threshold for extinction is the critical community size below which infections like measles do not persist. The current generation of stochastic models overestimates the observed critical community size for measles, generating much less persistence of infection than is observed. The inclusion of a more biologically realistic model for the duration of infection produced a much closer fit to the actual critical community size and explains previously undescribed high-frequency oscillations in measles incidence.
Mass Spawning by Green Algae on Coral Reefs
Authors: Clifton Kenneth E.
Published: July 2002

Predawn episodes of mass spawning by green algae (up to nine species in five genera on a single morning) intermittently cloud Caribbean waters. Species- and sex-specific bouts of anisogamous gamete release occurred synchronously and predictably on a given morning, with closely related species spawning at different times. Algal sexual reproduction was seasonal, but, unlike the mass-spawning behavior of other sessile marine organisms, showed no lunar or tidal cycling. The discovery of mass-spawning behavior by these algae has important implications for future studies of the reproductive ecology and speciation of a vital, yet poorly understood, component of the coral reef community.
Paleomagnetic Evidence of a Low-Temperature Origin of Carbonate in the Martian Meteorite ALH84001
Authors: Kirschvink Joseph L. , Maine Altair T. , Vali Hojatollah
Published: July 2002

Indirect evidence for life on Mars has been reported from the study of meteorite ALH84001. The formation temperature of the carbonates is controversial; some estimates suggest 20° to 80°C, whereas others exceed 650°C. Paleomagnetism can be used to distinguish between these possibilities because heating can remagnetize ferrimagnetic minerals. Study of two adjacent pyroxene grains from the crushed zone of ALH84001 shows that each possesses a stable natural remanent magnetization (NRM), implying that Mars had a substantial magnetic field when the grains cooled. However, NRM directions from these particles differ, implying that the meteorite has not been heated significantly since the formation of the internal crushed zone about 4 billion years ago. The carbonate globules postdate this brecciation, and thus formed at low temperatures.
Liver Regeneration
Authors: Michalopoulos George K. , DeFrances Marie C.
Published: July 2002

Liver regeneration after the loss of hepatic tissue is a fundamental parameter of liver response to injury. Recognized as a phenomenon from mythological times, it is now defined as an orchestrated response induced by specific external stimuli and involving sequential changes in gene expression, growth factor production, and morphologic structure. Many growth factors and cytokines, most notably hepatocyte growth factor, epidermal growth factor, transforming growth factor-α, interleukin-6, tumor necrosis factor-α, insulin, and norepinephrine, appear to play important roles in this process. This review attempts to integrate the findings of the last three decades and looks toward clues as to the nature of the causes that trigger this fascinating organ and cellular response.
Insects on Plants: Macroevolutionary Chemical Trends in Host Use
Authors: Becerra Judith X.
Published: July 2002

Determining the macroevolutionary importance of plant chemistry on herbivore host shifts is critical to understanding the evolution of insect-plant interactions. Molecular phylogenies of the ancient and speciose
Blepharida
(Coleoptera)–
Bursera
(Burseraceae) system were reconstructed and terpenoid chemical profiles for the plant species obtained. Statistical analyses show that the historical patterns of host shifts strongly correspond to the patterns of host chemical similarity, indicating that plant chemistry has played a significant role in the evolution of host shifts by phytophagous insects.
Measurement Techniques in Gas-Phase Tropospheric Chemistry: A Selective View of the Past, Present, and Future
Authors: Roscoe Howard K. , Clemitshaw Kevin C.
Published: July 2002

Measurements of trace gases and photolysis rates in the troposphere are essential for understanding photochemical smog and global environmental change. Chemical measurement techniques have progressed enormously since the first regular observations of tropospheric ozone in the 19th century. In contrast, by the 1940s spectroscopic measurements were already of a quality that would have allowed the use of modern analysis techniques to reduce interference between gases, although such techniques were not applied at the time. Today, chemical and spectroscopic techniques complement each other on a wide range of platforms. The boundaries between spectroscopic techniques will retreat as more Fourier transform spectrometers are used at visible wavelengths and as wide-band lidars are extended, and combining chemical techniques will allow detection of more trace gases with better sensitivity. Other future developments will focus on smaller, lighter instruments to take advantage of new platforms such as unmanned aircraft and to improve the effectiveness of urban sampling.
Silicon and Oxygen Self-Diffusivities in Silicate Liquids Measured to 15 Gigapascals and 2800 Kelvin
Authors: Poe Brent T. , McMillan Paul F. , Rubie David C. , Chakraborty Sumit , Yarger Jeff , Diefenbacher Jason
Published: July 2002

Mass transport properties of silicate liquids exhibit complex behavior as a function of pressure, as the tetrahedral framework structure of the liquid shifts to a more compact arrangement of atoms. For highly polymerized aluminosilicate liquids, oxygen diffusivities pass through a maximum at pressures below 10 gigapascals, whereas up to 15 gigapascals diffusivities continue to increase for sodium tetrasilicate liquid. A diffusivity maximum indicates a change in the mechanism of formation of 5-coordinated silicon or aluminum in the liquid. In the case of aluminosilicate liquids, this mechanism is restricted to aluminum sites in the network, suggesting that not only degree of polymerization, but also the ratio of aluminum to aluminum plus silicon strongly influences the behavior of magmatic processes at depth.
North Siberian Lakes: A Methane Source Fueled by Pleistocene Carbon
Authors: Zimov S. A. , Voropaev Y. V. , Semiletov I. P. , Davidov S. P. , Prosiannikov S. F. , Chapin F. S. , Chapin M. C. , Trumbore S. , Tyler S.
Published: July 2002

The sizes of major sources and sinks of atmospheric methane (CH
4
), an important greenhouse gas, are poorly known. CH
4
from north Siberian lakes contributes ∼1.5 teragrams CH
4
year
−1
to observed winter increases in atmospheric CH
4
concentration at high northern latitudes. CH
4
emitted from these lakes in winter had a radiocarbon age of 27,200 years and was derived largely from Pleistocene-aged carbon.
X-ray Emissions from Comets Detected in the Röntgen X-ray Satellite All-Sky Survey
Authors: Dennerl Konrad , Englhauser Jakob , Trümper Joachim
Published: July 2002

After the unexpected discovery of x-rays emitted from comet C/1996 B2 (Hyakutake) with the Röntgen X-ray Satellite (ROSAT) in March 1996, x-ray emissions from comets C/1990 K1 (Levy), C/1990 N1 (Tsuchiya-Kiuchi), 45P (Honda–Mrkos–Pajdušáková), and C/1991 A2 (Arai), optically 300 to 30,000 times fainter than Hyakutake, were discovered in archival ROSAT data. These findings establish comets as a class of x-ray sources and allow their properties to be studied over a wide optical brightness range. The results indicate that charge exchange between highly charged heavy ions in the solar wind and cometary neutrals is the dominant process for the x-ray emission. Comets may thus be used as probes for monitoring the heavy-ion content of the solar wind.
A Tribosphenic Mammal from the Mesozoic of Australia
Authors: Rich Thomas H. , Vickers-Rich Patricia , Constantine Andrew , Flannery Timothy F. , Kool Lesley , van Klaveren Nicholas
Published: July 2002

A small, well-preserved dentary of a tribosphenic mammal with the most posterior premolar and all three molars in place has been found in Aptian (Early Cretaceous) rocks of southeastern Australia. In most respects, dental and mandibular anatomy of the specimen is similar to that of primitive placental mammals. With the possible exception of a single tooth reported as Eocene in age, terrestrial placentals are otherwise unknown in Australia until the Pliocene. This possible Australian placental is similar in age toProkennalestesfrom the late Aptian/early Albian Khoboor Beds of Mongolia, the oldest currently accepted member of the infraclass Placentalia.
Transitions Between Blocked and Zonal Flows in a Rotating Annulus with Topography
Authors: Weeks Eric R. , Tian Yudong , Urbach J. S. , Ide Kayo , Swinney Harry L. , Ghil Michael
Published: July 2002

The mid-latitude atmosphere is dominated by westerly, nearly zonal flow. Occasionally, this flow is deflected poleward by blocking anticyclones that persist for 10 days or longer. Experiments in a rotating annulus used radial pumping to generate a zonal jet under the action of the Coriolis force. In the presence of two symmetric ridges at the bottom of the annulus, the resulting flows were nearly zonal at high forcing or blocked at low forcing. Intermittent switching between blocked and zonal patterns occurs because of the jet's interaction with the topography. These results shed further light on previous atmospheric observations and numerical simulations.
Excitation Spectra of Circular, Few-Electron Quantum Dots
Authors: Kouwenhoven L. P. , Oosterkamp T. H. , Danoesastro M. W. S. , Eto M. , Austing D. G. , Honda T. , Tarucha S.
Published: July 2002

Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund's rule. These impurity-free quantum dots allow “atomic physics” experiments to be performed in magnetic field regimes not accessible for atoms.
Penny Ice Cap Cores, Baffin Island, Canada, and the Wisconsinan Foxe Dome Connection: Two States of Hudson Bay Ice Cover
Authors: Fisher David A. , Koerner Roy M. , Bourgeois Jocelyne C. , Zielinski Greg , Wake Cameron , Hammer Claus U. , Clausen H. B. , Gundestrup N. , Johnsen Sigfus , Goto-Azuma Kumiko , Hondoh Takeo , Blake Erik , Gerasimoff Mike
Published: July 2002

Ice cores from Penny Ice Cap, Baffin Island, Canada, provide continuous Holocene records of oxygen isotopic composition (δ18O, proxy for temperature) and atmospheric impurities. A time scale was established with the use of altered seasonal variations, some volcanic horizons, and the age for the end of the Wisconsin ice age determined from the GRIP and GISP2 ice cores. There is pre-Holocene ice near the bed. The change in δ18O since the last glacial maximum (LGM) is at least 12.5 per mil, compared with an expected value of 7 per mil, suggesting that LGM ice originated at the much higher elevations of the then existing Foxe Dome and Foxe Ridge of the Laurentide Ice Sheet. The LGM δ18O values suggest thick ice frozen to the bed of Hudson Bay.
Structural Basis of Plasticity in T Cell Receptor Recognition of a Self Peptide-MHC Antigen
Authors: Garcia K. Christopher , Degano Massimo , Pease Larry R. , Huang Mingdong , Peterson Per A. , Teyton Luc , Wilson Ian A.
Published: July 2002

The T cell receptor (TCR) inherently has dual specificity. T cells must recognize self-antigens in the thymus during maturation and then discriminate between foreign pathogens in the periphery. A molecular basis for this cross-reactivity is elucidated by the crystal structure of the alloreactive 2C TCR bound to self peptide–major histocompatibility complex (pMHC) antigen H-2Kb–dEV8 refined against anisotropic 3.0 angstrom resolution x-ray data. The interface between peptide and TCR exhibits extremely poor shape complementarity, and the TCR β chain complementarity-determining region 3 (CDR3) has minimal interaction with the dEV8 peptide. Large conformational changes in three of the TCR CDR loops are induced upon binding, providing a mechanism of structural plasticity to accommodate a variety of different peptide antigens. Extensive TCR interaction with the pMHC α helices suggests a generalized orientation that is mediated by the Vαdomain of the TCR and rationalizes how TCRs can effectively “scan” different peptides bound within a large, low-affinity MHC structural framework for those that provide the slight additional kinetic stabilization required for signaling.
Role of PML in Cell Growth and the Retinoic Acid Pathway
Authors: Gang Wang Zhu , Delva Laurent , Gaboli Mirella , Rivi Roberta , Giorgio Marco , Cordon-Cardo Carlos , Grosveld Frank , Paolo Pandolfi Pier
Published: July 2002

ThePMLgene is fused to the retinoic acid receptor α (RARα) gene in chromosomal translocations associated with acute promyelocytic leukemia (APL). Ablation of murine PML protein by homologous recombination revealed that PML regulates hemopoietic differentiation and controls cell growth and tumorigenesis. PML function was essential for the tumor-growth–suppressive activity of retinoic acid (RA) and for its ability to induce terminal myeloid differentiation of precursor cells. PML was needed for the RA-dependent transactivation of thep21WAF1/CIP1gene, which regulates cell cycle progression and cellular differentiation. These results indicate that PML is a critical component of the RA pathway and that disruption of its activity by the PML-RARα fusion protein may be important in APL pathogenesis.
Chemical Dynamics in Proteins: The Photoisomerization of Retinal in Bacteriorhodopsin
Authors: Gai Feng , Hasson K. C. , McDonald J. Cooper , Anfinrud Philip A.
Published: July 2002

Chemical dynamics in proteins are discussed, with bacteriorhodopsin serving as a model system. Ultrafast time-resolved methods used to probe the chemical dynamics of retinal photoisomerization in bacteriorhodopsin are discussed, along with future prospects for ultrafast time-resolved crystallography. The photoisomerization of retinal in bacteriorhodopsin is far more selective and efficient than in solution, the origins of which are discussed in the context of a three-state model for the photoisomerization reaction coordinate. The chemical dynamics are complex, with the excited-state relaxation exhibiting a multiexponential decay with well-defined rate constants. Possible origins for the two major components are also discussed.