bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociative surface reactions at Schottky and heterojunction interfaces with AlAs and GaAs
Authors: Bauer R. S. , Bachrach R. Z. , Hansson G. V. , Chiaradia P.
Published: July 2002

We have conducted synchrotron radiation excited photoemission measurements of the initial stages of Au, Ge, and Ga deposition on AlAs(110) and (100) surfaces grown in situ by molecular beam epitaxy (MBE). Au evaporated onto room temperature AlAs produces a larger dissociation of the substrate surface then reported for Au on GaAs. However, the net Au–AlAs interdiffusion is smaller than for GaAs as seen by the more rapid approach of the Au(5d) and (4f) spin-orbit splittings to the bulk metallic values. Ge depositied onto AlAs(110) produces a dissociative outdiffusion of As at growth temperatures where abrupt, epitaxial Ge–GaAs(110) interfaces are produced. Since the bulk heat of formation of AlAs is over 50% greater than that of GaAs, the bulk compound heat of reaction incorrectly predicts the behavior of both Schottky and heterojunction interfaces. We conclude that it is the surface adatom bonding to the individual substrate constituents which determines the ultimate interface composition and extent. When the AlAs surface dissociates during interface formation, the Al does not substantially diffuse through the growing Au or Ge overlayers, in marked contrast to Ga from GaAs. Therefore, if metal adlayers are evaporated onto surfaces to influence the subsequent formation of Schottky interfaces, their mobility at the nominal interface is controlled by the bonding of the individual atomic species.
Hot hollow cathode and its applications in vacuum coating: A concise review
Authors: Kuo Y. S. , Bunshah R. F. , Okrent D.
Published: July 2002

A concise description of the hollow cathode (HC) effect is given to clarify the confusing terminology and their diverse applications. The cold and hot types of HC’s are then discussed. It is followed by a summary of the behavior of the hot HC as an arc generating device, with emphasis on its use for vacuum coating purposes. Two major coating devices, one developed by Ulvac Corp. (Japan) and the other at the Rocky Flats Plant (RFP) of Rockwell International (USA), are then reviewed with their respective applications. Review of the latter includes recent results of plume effect upon both the unusual deposition rate distribution and the extremely high substrate (ion) current. The coating morphology and microstructure can be best explained using the Krikorian concept and, when optimized, can help to render very high bonding strength (as high as 90 ksi for 304 stainless steel substrates), with the coatings used as the bonding interlayer. Finally, comments are made concerning the directions of future technology development based on the system features reviewed.
X-ray photoelectron spectroscopy investigation of ion beam sputtered indium tin oxide films as a function of oxygen pressure during deposition
Authors: Nelson A. J. , Aharoni H.
Published: July 2002

X-ray photoelectron spectroscopy analysis was performed on ion beam sputter deposited films of indium tin oxide as a function of O2 partial pressure during deposition. The oxygen partial pressure was varied over the range of 2.5×10−6–4.0×10−5 Torr. Changes in composition as well as in the deconvoluted In 3d5/2, Sn 3d5/2, and O 1s core level spectra were observed and correlated with the variation of the oxygen partial pressure during deposition. Results show that the films become increasingly stoichiometric as PO2 is increased and that the excess oxygen introduced during deposition is bound predominantly to the Sn and has little or no effect on the In–O bonding.
Reactive ion etching related Si surface residues and subsurface damage: Their relationship to fundamental etching mechanisms
Authors: Oehrlein Gottlieb S. , Lee Young H.
Published: July 2002

The role of silicon near-surface modifications caused by reactive ion etching in obtaining oxide-to-silicon etch selectivity has been studied by combining Si etch rate and plasma optical emission spectroscopy data with results from the analysis of etched Si surfaces. The nature of the near-surface modifications was established from the study of Si crystals subjected to overetching during CF4/x% H2 selective reactive ion etching of SiO2 on Si. The experimental techniques used included x-ray photoemission spectroscopy, He ion channeling, nuclear reaction techniques, and ellipsometry. Impurity penetration has been verified by nondestructive depth profiling using nuclear reaction and angular dependent core level photoemission techniques. Fluorocarbon film deposition, a highly disordered near-surface region (∼30 Å), and etching gas related impurity penetration into the Si lattice are among the Si substrate modifications observed. By nuclear reaction analysis, hydrogen at concentrations of the order 0.1 at. % has been detected at a depth of several 100 Å from the Si surface. By substituting deuterium for hydrogen in reactive ion etching and conducting secondary ion mass spectroscopy, deuterium at concentrations greater than 1017 atoms/cm3 has been detected more than 2000 Å from the Si surface after 1 min plasma exposure.

Direct observation of silicon–carbon bonds and silicon–hydrogen bonding in the modified Si layer was possible by photoemission and Raman scattering. The function of the Si near-surface modifications in the reactive ion etching process, specifically their role in achieving oxide-to-Si etch selectivity, has been elucidated. The etch rate of silicon is controlled by the thickness of the fluorocarbon layer on the silicon surface. It is consistent with a diffusion limited supply of F across the C,F film to the Si/C,F film interface and/or outdiffusion of etch product. The Si subsurface modifications, e.g., lattice disorder and impurities, do not contribute to the decrease of the Si etch rate; reactive ion etching (RIE) process modifications which minimize damage to the Si substrate are therefore discussed. Our results also demonstrate directly the recombinant/passivant model of etch anisotropy for the CF4/H2 RIE-Si etching system. Anisotropic etching of Si in a CF4/H2 plasma occurs via ion bombardment mediated C,F-layer thickness control. Energetic ion bombardment minimizes the thickness of the C,F layer and Si etching can proceed. Areas exposed to little ion bombardment (sidewalls) are covered by thicker C,F layers and no Si etching occurs.
Structural transitions in ballistic aggregation simulation of thin-film growth
Authors: Brett M. J.
Published: July 2002

Computer simulations of thin-film growth were performed by random ballistic aggregation of hard two-dimensional disks. A simple model incorporating sputtering, surface relaxation, and redeposition of sputtered atoms was used to simulate the effects of ion bombardment during growth. Without ion bombardment the deposits had a voided columnar structure typical of a zone 1 growth. With increasing bombardment flux, a densification of the deposit and an elimination of continuous voids were observed. These results reproduce the general features of a bombardment induced microstructural transition from zone 1 to zone T, and also demonstrate that redeposition of sputtered atoms is essential to this transition.
Arsenic on GaAs: Fermi-level pinning and thermal desorption studies
Authors: Chiang T. T. , Spicer W. E.
Published: July 2002

Arsenic overlayers are grown on freshly cleaved, clean GaAs(110) surface at room temperature and studied with ultraviolet and x-ray photoelectron spectroscopy. The coverage of As on the GaAs surface appears to saturate quickly, with saturation occurring at <10 L As exposure. The saturation coverage is estimated to be a monolayer of chemisorbed As and it pins the surface Fermi level at 0.55±0.05 eV above the valence-band maximum (VBM) for both n- and p-type GaAs. Upon annealing at 300 °C, the chemisorbed As overlayer is desorbed from the surface and the surface Fermi level shifts back almost to the bulk position [0.2 eV below the conduction-band minimum (CBM) for n-GaAs and 0.1 eV above VBM for p-GaAs; CBM and VBM are the bulk Fermi-level positions]. Since all the various models of Schottky barrier formation predict similar pinning positions, it is difficult to distinguish between them based solely on this present result. However, this result does emphasize the fact that excess As on the surface can be responsible for surface Fermi-level pinning. Furthermore, this result also shows that the ‘‘As capping’’ technique used for surface passivation of GaAs is extremely nonintrusive.
An investigation of electrowetting-based microactuation
Authors: Colgate Ed , Matsumoto Hirofumi
Published: July 2002

Electrical control of the interfacial tension between a liquid and solid (‘‘electrowetting’’) has been studied as a means of actuation in the microdomain (characteristic dimensions 1 μm–1 mm). Electrowetting provides a means of direct fluid pumping with no moving mechanical parts, which may prove useful in a number of application areas, most notably the liquid cooling of high-density microelectronic devices. A detailed model of a test device for the study of electrowetting is presented. The model describes liquid flow in a small channel, as governed as the Navier–Stokes equations, Young’s equation, and Lippmann’s equation (for the effect of an applied potential on interfacial tension). Simulation results are presented. Preliminary results indicate that electrowetting is a viable approach to microactuation: in a 10 μm radius channel, it may be used to generate pressures on the order of 0.01 MPa, comparable to pressures generated by existing micropump designs, which are orders of magnitude larger in size.
Hardness and elastic modulus of TiN based on continuous indentation technique and new correlation
Authors: Stone D. S. , Yoder K. B. , Sproul W. D.
Published: July 2002

Young’s modulus and hardness of TiN films are reported. The films, deposited on 440C stainless steel, range between 0.25 and 1.0 μm in thickness. They fall to within 3% of stoichiometry and have a 〈111〉 texture. Within experimental resolution, the properties of films with different thickness are indistinguishable: calculated values of Young’s modulus and hardness are 550±50 GPa and 31±4 GPa, respectively. Properties are obtained from a continuous indentation technique. A new correlation is used for identifying whether all films have the same properties, independent of thickness, and for measuring hardness of thin films. The same correlation is utilized for measuring machine compliance and obtaining the profile of the indenter tip. An elasticity analysis aids in obtaining elastic modulus from compliance data.
Electromagnetic fields in a radio-frequency induction plasma
Authors: Hopwood J. , Guarnieri C. R. , Whitehair S. J. , Cuomo J. J.
Published: July 2002

The electromagnetic fields which drive a radio-frequency induction plasma are both modeled and measured. The plasma source consists of a planar, square coil separated from a low pressure plasma chamber by a 2.54-cm-thick quartz window. A small loop antenna, which is sealed in a pyrex tube, is immersed in the discharge to determine the magnitude and direction of the rf magnetic field. The measured B field is primarily radial and axial. Typical rf field strengths vary from 2 to 7 G for rf powers of 0.1–1 kW. The radial B field decays exponentially in the axial direction. The skin depth of the electromagnetic field is 1.6–3.6 cm which is consistent with Langmuir probe measured ion densities (typically 3×1011 cm−3) in argon. Invoking Maxwell’s equations to deduce the rf electric field from the measured B field, we find the E field to be primarily azimuthal. Peak field strengths increase from 100 V/m at 100 W to 200 V/m at 600 W where they saturate for higher powers. Finally, we present a 3D finite element solution for the fields produced by this plasma source which employs a cold, collisionless plasma model to relate the relative plasma permittivity εr to the electron plasma frequency, ωpe, using εr=1−(ωpe/ω)2. The measured fields support this numerical solution.
Model for the outgassing of water from metal surfaces
Authors: Li Minxu , Dylla H. F.
Published: July 2002

Numerous measurements of outgassing from metal surfaces show that the outgassing obeys a power law of the form Q=Q10t−α, where α is typically near unity. For unbaked systems, outgassing is dominated by water. This work demonstrates that α is a function of the water vapor exposure during venting of the system, and the physical properties of the passivation oxide layer on the surface. An analytic expression for the outgassing rate is derived based on the assumption that the rate of water diffusing through the passivation oxide layer to the surface governs the rate of its release into the vacuum. The source distribution function for the desorbing water is assumed to be a combination of a Gaussian distribution centered at the interior surface driven by atmospheric exposure, and a uniform concentration throughout the bulk. We have measured the outgassing rate from a clean stainless-steel (type 304) chamber as a function of water exposure to the chamber surface from <1 to 600 monolayers. The measured outgassing rate data show that α tends to 0.5 for low H2O exposures and tends to 1.5 for high H2O exposures as predicted by the model.
Correlation of outgassing of stainless steel and aluminum with various surface treatments
Authors: Dylla H. F. , Manos D. M. , LaMarche P. H.
Published: July 2002

A number of important applications of ultrahigh vacuum technology involve the use of vacuum systems which cannot be baked in situ, e.g., materials processing equipment and various designs of accelerators and fusion devices. A choice of vessel material and surface treatment which minimizes outgassing in unbaked systems has obvious advantages. We report on a systematic comparison of outgassing measurements from type 304 stainless steels and type 6061/63 aluminum alloys after various standard and advanced surface treatments. Correlations were made with high-resolution secondary ion mass spectroscopy and x-ray photoelectron spectroscopy profiles of the passivation oxide layer and profilometry measurements of the surface roughness. The observed outgassing rates Q obeyed a power law Q=Q0t−n where n varied from 0.94 to 1.27. Over the standard observation period (50 h), the outgassing was dominated (≳85%) by H2O. The absolute outgassing rates varied by only a factor of 2.8 for the stainless steels and by a factor of 1.4 for the aluminum alloys, even though the surface roughness differed by more than two orders of magnitude. The high-value-added surface treatments (i.e., electropolishing) did not have significantly lower outgassing in comparison to simple detergent washes.
Modeling an inductively coupled plasma source
Authors: Paranjpe A. P.
Published: July 2002

Inductively coupled plasma (ICP) sources are becoming increasingly popular for etching submicrometer features, soft-cleans for physical vapor deposition cluster tools, and plasma-enhanced chemical-vapor deposition. Here, we report on a discharge model for argon for the ICP which combines an electromagnetic field distribution model with an ambipolar diffusion model. The model provides a self-consistent solution for the spatial distribution of the charged species in the bulk plasma, which permits the spatial distribution of the ion flux to the wafer surface to be calculated. The variations of the peak electron concentration, the ion flux, and its uniformity at the wafer have been simulated as a function of the rf power, antenna geometry, and chamber geometry. The electron number density increases linearly with rf power in accordance with experimental observations. The ion flux uniformity is primarily dependent on the diameter of the antenna and is only slightly dependent on the number of turns. The uniformity generally is relatively invariant with rf power above a threshold power level. Contouring the antenna or the dielectric separating the antenna from the plasma is useful for improving ion flux uniformity. The minimum thickness of the dielectric cap is constrained by the maximum allowable sheath potential. The induced electric field at the wafer can cause substantial wafer heating at high antenna power levels. Increasing the wafer to antenna spacing mitigates this problem without sacrificing the ion flux and its uniformity.
Nonevaporable getter films for ultrahigh vacuum applications
Authors: Benvenuti C. , Chiggiato P. , Cicoira F. , L’Aminot Y.
Published: July 2002

The vacuum behavior of stainless steel vacuum chambers, ex situ sputter coated with a thin film (∼1 μm) of getter material, has been studied to determine if after air exposure the getter film could be activated by a bakeout so as to transform the coated vacuum chamber into a pump. The materials studied so far are Ti, Zr, Hf, and some of their binary alloys. They all display an activation temperature lower than 400 °C, i.e., within the reach of the baking temperature of stainless steel vacuum chambers. The lowest activation temperature of 200–250 °C, measured for an equiatomic alloy of Ti and Zr, allows extension of this method to chambers made of copper and aluminum alloys. The experimental results, described here in detail, indicate that the values of the activation temperature obtained using electron stimulated desorption, pumping speed, and Auger spectroscopy measurements are self-consistent.
Comparison of two-dimensional and three-dimensional models for profile simulation of poly-Si etching of finite length trenches
Authors: Hoekstra Robert J. , Kushner Mark J.
Published: July 2002

The development of two-dimensional (2D) profile simulators for fabrication of microelectronics features has significantly progressed during the past few years and now enables modeling of etch profile evolution for many different plasma processing conditions. Increasingly complex devices which have three-dimensional (3D) (that is, asymmetric) structures are now being designed. These structures require improved dimensionality in profile simulators to capture their most important features. Under many conditions, such as circular via etching, two-dimensional profile simulators can be used to address 3D structures. A legitimate issue is to what degree these 2D approaches can indeed be applied to truly 3D structures. In this article, we present results from a 3D profile simulator for the purpose of comparing profiles for innately 3D features to results obtained from a 2D profile simulation. It has been found that profiles obtained from the 3D simulators exhibit greater sidewall sloping in three-plane corners than predicted by the 2D simulator. The implication to process design is that a greater degree of overetching will be required to clear these corners than predicted by the 2D simulators. Asymmetries as well as the angular spread of the ion flux distribution are examined to determine their role in 3D profile evolution.
Control of the structure and properties of aluminum oxide coatings deposited by pulsed magnetron sputtering
Authors: Kelly P. J. , Arnell R. D.
Published: July 2002

Alumina coatings offer the potential to protect metallic components operating in hostile corrosive, or oxidative environments. Until recently, though, the high rate deposition of fully dense, defect-free oxide coatings has proved extremely difficult. Oxide coatings can be produced by the reactive direct current (dc) sputtering of a metallic target in an oxygen atmosphere, or by radio frequency sputtering of an oxide target. The latter process results in very low deposition rates and is difficult to scale up for commercial applications. Reactive dc sputtering of highly insulating materials, such as alumina, is also problematic. The build up of an insulating layer on the surface of the target causes arcing. Arc events affect the stability of the deposition process and can adversely effect the structure and properties of the growing film. However, a new technique, pulsed magnetron sputtering, has been developed which significantly increases the viability of producing this type of material. It has been shown that pulsing the magnetron discharge in the 10–200 kHz range prevents the formation of arcs at the target. The process is stable and high deposition rates can be achieved. This technique has, therefore, been used to deposit coatings with compositions ranging from pure aluminum to stoichiometric Al2O3. The coatings were characterized by scanning and transmission electron microscopy, x-ray diffraction, four-point probe, and nano-indentation techniques. The observed variations in structure and properties are considered in terms of the composition of the coatings and expressed schematically in the form of a novel structure zone model.
X-ray photoelectron spectroscopy study of bombardment-induced compositional changes in ZrO2, SiO2, and ZrSiO4
Authors: Iacona Fabio , Kelly Roger , Marletta Giovanni
Published: July 2002

We have studied the chemical and compositional modifications induced by 8 keV Ar° irradiation at the surface layers of ZrO2, SiO2, and ZrSiO4 (zircon). The surfaces were examined by in situ x-ray photoelectron spectroscopy (XPS). We have found that while pure SiO2 evolved to SiO1.95 and pure ZrO2 evolved to ZrO1.85, involving the appearance of reduced Si and Zr species in the XPS spectra. On the other hand, ZrSiO4 underwent a major loss of SiO2, to yield a final Si/Zr ratio of 0.63, and a strong degree of ZrIV reduction (to yield ZrO1.70) with respect to pure ZrO2. The experimental results are discussed in terms of several possible mechanisms, including the kinetic loss of O due to high diffusivity in the perturbed collision cascade volume, the occurrence of “bombardment-induced segregation” (BIS) phenomena, the occurrence of redox reactions, as well as the “lattice relaxation with chemical guidance,” related to the thermodynamic stability of irradiation-induced compounds. In particular, the compositional modifications for simple oxides seem to be well explained mainly in terms of the model of lattice relaxation, i.e., in terms of the “thermodynamically allowed” formation of the intermediate valence species ZrIII and SrIII, clearly demonstrated by XPS. For the effects seen in the mixed oxide ZrSiO4, the preferential depletion of SiO2 has been tentatively explained in terms of BIS phenomena, while the enhanced ZrIV reduction (with respect to pure ZrO2) is explained in terms of competing O–Zr and O–Si recombination reactions.
A study of the mechanism of metal deposition by the laser-induced forward transfer process
Authors: Adrian F. J. , Bohandy J. , Kim B. F. , Jette A. N. , Thompson P.
Published: July 2002

The mechanism of the laser-induced forward transfer (LIFT) technique for transferring metal features from a film to a substrate is examined by using the one-dimensional thermal diffusion equation with a moving solid-melt boundary to model the heating, melting, and vaporization of the metal film by the laser. For typical LIFT conditions the calculations show that the back of the film (i.e., the part exposed to the laser) will reach the boiling point before the film melts through, which supports the qualitative picture that the LIFT process involves vapor-driven propulsion of metal from the film onto the target.
Molybdenum etching with chlorine atoms and molecular chlorine plasmas
Authors: Fischl D. S. , Hess D. W.
Published: July 2002

Thin films of molybdenum were etched both within and downstream from a Cl2 plasma at 200-mTorr pressure and temperatures below 180 °C. When samples were positioned downstream from the discharge, etching proceeded solely by chemical reaction of the film with chlorine atoms. Without a discharge, molecular chlorine did not etch molybdenum. Downstream and in-discharge etch rates were <20 nm/min and etching by atoms was not observed below 100 °C. The chemical reaction between chlorine atoms and molybdenum was proportional to the gas phase Cl atom mole fraction.
Microscopic uniformity in plasma etching
Authors: Gottscho Richard A. , Jurgensen C. W. , Vitkavage D. J.
Published: July 2002

As we enter the era of ultra-large-scale integrated circuit manufacture, plasma etching grows more important for fabricating structures with unprecedented dimensions. For feature sizes below 1 μm and aspect ratios (depth/width) much larger than one, etching rates have been observed to depend on aspect ratio and pattern density. Such dependencies tend to increase the cost of manufacturing because even small changes in device design rules, cell design, or wafer layout can result in time-consuming, new plasma process development. In addition, microscopically nonuniform etching affects the trade-off between chips lost from failure to clear and chips lost by damage from overetching. Although aspect ratio and pattern dependent etching have been observed for a large variety of material systems and processing conditions, the fundamental causes underlying these effects are poorly understood. Partly, this results from use of confusing and conflicting nomenclature and a lack of careful, quantitative comparisons between experiment and theory. In this article we review recent literature on microscopic uniformity in plasma etching and carefully define terminology to distinguish between aspect ratio dependent etching (ARDE) and the pattern dependent effect known as microloading. For ARDE, we use dimensional analysis to narrow the range of proposed mechanisms to four which involve ion transport, neutral transport, and surface charging. For microloading, we show that it is formally equivalent to the usual loading effect, where the reactant concentration is depeleted as a result of an excessive substrate load.
Influence of cathode material on emission characteristics of field emitters for microelectronics devices
Authors: Ishikawa Junzo , Tsuji Hiroshi , Gotoh Yasuhito , Sasaki Takahiro , Kaneko Takashi , Nagao Masayoshi , Inoue Kazunori
Published: July 2002

In order to find out the cathode material suitable to vacuum microelectronics devices, dependence of cathode material of field emitters was investigated with respect to the emission characteristics. Since the field emitters for vacuum microelectronics devices are fabricated by thin film processes, the characteristics of the electron emission from deposited materials should be examined. In the present study, a dozen materials were deposited onto the tungsten needle fabricated by well-controlled electrochemical etching. Measurement of the emission was performed at the pressure of 10−9 Torr range. The current–voltage characteristics and the stability measurements revealed that the gold emitters indicated excellent properties: stable and high current at low extraction voltage. The effective surface work function and the effective emission area were evaluated from the Fowler–Nordheim theory, assuming that the emission area rapidly decreases with reducing the apex radius. From this analysis, it is clarified that the gold emitter had the lowest effective work function among the examined emitters. The results can be interpreted as that in order to obtain stable emission characteristics, materials with inert surface should be selected.
Comparative study of the elastic properties of silicate glass films grown by plasma enhanced chemical vapor deposition
Authors: Carlotti G. , Doucet L. , Dupeux M.
Published: July 2002

The Brillouin light scattering technique has been used to study the elastic properties of a number of silicon dioxide films deposited by plasma-enhanced chemical vapor deposition on Si substrates. In addition to stoichiometric undoped glass films produced from either silane or tetraethylorthosilicate, we have also studied nonstoichiometric Si-rich films and P-doped films. The phase velocity of both the surface Rayleigh mode and the longitudinal bulk wave in the film material has been measured and the two independent elastic constants c11 and c44 have been evaluated. The derived values of the Young modulus and the Poisson ratio show appreciable deviations from the values we measured on thermally grown oxide. Moreover, the evolution of the stress during thermal cycles has been analyzed using the substrate curvature method. This permitted us to estimate the thermal expansion coefficient of the films and to distinguish between the intrinsic and thermal components of the stress.
Interface characterization of Si3N4/Si/GaAs heterostructures after high temperature annealing
Authors: Park Dae-Gyu , Wang Zhonghui , Morkoç Hadis , Alterovitz Samuel A. , Smith David J. , Tsen S.-C. Y.
Published: July 2002

We present data on interface characteristics of Si3N4/Si/GaAs metal–insulator–semiconductor (MIS) structures and correlate electrical properties with spectroscopic ellipsometry, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) observations. The interface of Si3N4/Si/GaAs heterostructures was electrically characterized by a combination of capacitance–voltage and conductance methods. The nature of an insulator/GaAs interface and the microstructure of Si3N4/Si/GaAs interfaces after high temperature annealing were investigated by variable angle spectroscopic ellipsometry and high resolution TEM, respectively. The evolution of chemical species in Si3N4/Si/GaAs heterostructures was examined using in situ angle-resolved XPS. The interface trap density (Dit) of the Si3N4/Si MIS capacitor was in the 2×1010 eV−1 cm−2 range near the Si midgap after rapid thermal annealing at 550 °C in N2. However, this density increased to high 1010 eV−1 cm−2 with annealing at 800 °C. The interface characteristics of Si3N4/Si/GaAs structures with Dit in the 7×1010 eV−1 cm−2 range also degraded after annealing at 750 °C in N2 with Dit increasing to 5×1011 eV−1 cm−2 near the GaAs midgap. The spectroscopic ellipsometry results together with high resolution TEM observations appear to suggest that the degradation is due in part to the interface changing from crystalline to amorphous through chemical reaction. XPS measurements revealed that the as-deposited Si interlayer is nitridated during the initial stages of silicon nitride deposition, thus the thinned Si cannot prevent the outdiffusion of Ga and As species. We circumvented thermally induced interface degradation of Si3N4/Si/GaAs structures by employing a novel ex situ/in situ growth approach.
Quantum Mechanical Actuation of Microelectromechanical Systems by the Casimir Force
Authors: Chan H. B. , Aksyuk V. A. , Kleiman R. N. , Bishop D. J. , Capasso Federico
Published: July 2002

The Casimir force is the attraction between uncharged metallic surfaces as a result of quantum mechanical vacuum fluctuations of the electromagnetic field. We demonstrate the Casimir effect in microelectromechanical systems using a micromachined torsional device. Attraction between a polysilicon plate and a spherical metallic surface results in a torque that rotates the plate about two thin torsional rods. The dependence of the rotation angle on the separation between the surfaces is in agreement with calculations of the Casimir force. Our results show that quantum electrodynamical effects play a significant role in such microelectromechanical systems when the separation between components is in the nanometer range.
Differentiation of Embryonic Stem Cell Lines Generated from Adult Somatic Cells by Nuclear Transfer
Authors: Wakayama Teruhiko , Tabar Viviane , Rodriguez Ivan , Perry Anthony C. F. , Studer Lorenz , Mombaerts Peter
Published: July 2002

Embryonic stem (ES) cells are fully pluripotent in that they can differentiate into all cell types, including gametes. We have derived 35 ES cell lines via nuclear transfer (ntES cell lines) from adult mouse somatic cells of inbred, hybrid, and mutant strains. ntES cells contributed to an extensive variety of cell types, including dopaminergic and serotonergic neurons in vitro and germ cells in vivo. Cloning by transfer of ntES cell nuclei could result in normal development of fertile adults. These studies demonstrate the full pluripotency of ntES cells.
Exponential Gain and Saturation of a Self-Amplified Spontaneous Emission Free-Electron Laser
Authors: Milton S. V. , Gluskin E. , Arnold N. D. , Benson C. , Berg W. , Biedron S. G. , Borland M. , Chae Y.-C. , Dejus R. J. , Den Hartog P. K. , Deriy B. , Erdmann M. , Eidelman Y. I. , Hahne M. W. , Huang Z. , Kim K.-J. , Lewellen J. W. , Li Y. , Lumpkin A. H. , Makarov O. , Moog E. R. , Nassiri A. , Sajaev V. , Soliday R. , Tieman B. J. , Trakhtenberg E. M. , Travish G. , Vasserman I. B. , Vinokurov N. A. , Wang X. J. , Wiemerslage G. , Yang B. X.
Published: July 2002

Self-amplified spontaneous emission in a free-electron laser has been proposed for the generation of very high brightness coherent x-rays. This process involves passing a high-energy, high-charge, short-pulse, low-energy-spread, and low-emittance electron beam through the periodic magnetic field of a long series of high-quality undulator magnets. The radiation produced grows exponentially in intensity until it reaches a saturation point. We report on the demonstration of self-amplified spontaneous emission gain, exponential growth, and saturation at visible (530 nanometers) and ultraviolet (385 nanometers) wavelengths. Good agreement between theory and simulation indicates that scaling to much shorter wavelengths may be possible. These results confirm the physics behind the self-amplified spontaneous emission process and forward the development of an operational x-ray free-electron laser.
The Morphological Evolution of Galaxies
Authors: Abraham Roberto G. , van den Bergh Sidney
Published: July 2002

Many galaxies have taken on their familiar appearance relatively recently. In the distant Universe, galaxy morphology deviates significantly (and systematically) from that of nearby galaxies at redshifts (z) as low as 0.3. This corresponds to a time ∼3.5 × 109years in the past, which is only ∼25% of the present age of the Universe. Beyondz= 0.5 (5 × 109years in the past), spiral arms are less well developed and more chaotic, and barred spiral galaxies may become rarer. Atz= 1, around 30% of the galaxy population is sufficiently peculiar that classification on Hubble's traditional “tuning fork” system is meaningless. On the other hand, some characteristics of galaxies have not changed much over time. The space density of luminous disk galaxies has not changed significantly sincez= 1, indicating that although the general appearance of these galaxies has continuously changed over time, their overall numbers have been conserved.
Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth
Authors: Catling David C. , Zahnle Kevin J. , McKay Christopher P.
Published: July 2002

The low O
2
content of the Archean atmosphere implies that methane should have been present at levels ∼10
2
to 10
3
parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH
4
is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH
4
implies that hydrogen escapes to space (↑space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O
2
and H, and methanogenesis transfers the H into CH
4
. Hydrogen escape after CH
4
photolysis, therefore, causes a net gain of oxygen [CO
2
+ 2H
2
O → CH
4
+ 2O
2
→ CO
2
+ O
2
+ 4H(↑space)]. Expected irreversible oxidation (∼10
12
to 10
13
moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized.
Pollen Tube Attraction by the Synergid Cell
Authors: Higashiyama Tetsuya , Yabe Shizu , Sasaki Narie , Nishimura Yoshiki , Miyagishima Shin-ya , Kuroiwa Haruko , Kuroiwa Tsuneyoshi
Published: July 2002

In flowering plants, guidance of the pollen tube to the embryo sac (the haploid female gametophyte) is critical for successful fertilization. The target embryo sac may attract the pollen tube as the final step of guidance in the pistil. We show by laser cell ablation that two synergid cells adjacent to the egg cell attract the pollen tube. A single synergid cell was sufficient to generate an attraction signal, and two cells enhanced it. After fertilization, the embryo sac no longer attracts the pollen tube, despite the persistence of one synergid cell. This cessation of attraction might be involved in blocking polyspermy.
Constraint to Adaptive Evolution in Response to Global Warming
Authors: Etterson Julie R. , Shaw Ruth G.
Published: July 2002

We characterized the genetic architecture of three populations of a native North American prairie plant in field conditions that simulate the warmer and more arid climates predicted by global climate models. Despite genetic variance for traits under selection, among-trait genetic correlations that are antagonistic to the direction of selection limit adaptive evolution within these populations. Predicted rates of evolutionary response are much slower than the predicted rate of climate change.
Fully State-Resolved Differential Cross Sections for the Inelastic Scattering of the Open-Shell NO Molecule by Ar
Authors: Kohguchi Hiroshi , Suzuki Toshinori , Alexander Millard H.
Published: July 2002

State-resolved differential cross sections (DCSs) for the inelastic scattering of NO(j" = 0.5, Ω" = 1/2) + Ar → NO(j′, Ω′ = 1/2, 3/2) + Ar were obtained at a collision energy of 516 cm−1, both experimentally and theoretically. A crossed molecular beam ion-imaging apparatus was used to measure DCSs for 20 final (j′, Ω′) states, including spin-orbit conserving (ΔΩ = 0) and changing (ΔΩ = 1) transitions. Quantum close-coupling scattering calculations on ab initio coupled-cluster CCSD(T) and CEPA (correlated electron pair approximation) potential energy surfaces were also performed. Although small discrepancies were found for the ΔΩ = 1 transitions, we find marked agreement between theory and experiment for the collision dynamics of this system, which is the paradigm for the collisional relaxation of a molecular radical.
Atomic-Resolution in Situ Transmission Electron Microscopy of a Promoter of a Heterogeneous Catalyst
Authors: Hansen Thomas W. , Wagner Jakob B. , Hansen Poul L. , Dahl Søren , Topsøe Haldor , Jacobsen Claus J. H.
Published: July 2002

Insight into the location, state, and function of a promoter in heterogeneous catalysis was obtained through atomic-resolution in situ transmission electron microscopy. In the most active ruthenium catalyst for ammonia synthesis known so far, the barium promoter is shown to be located in two different phases in the catalyst. The increased activity is suggested to be related to a two-dimensional barium-oxygen overlayer on the ruthenium crystals. The possibility for conducting such studies for other reactions could add substantially to our current understanding of heterogeneous catalysis. Heterogeneous catalysis plays an increasingly important role in environmental protection processes, in fuel upgrading, and in providing the majority of the chemical building blocks required by contemporary society. Most heterogeneous catalysts of industrial importance are multicomponent materials that are designed by trial-and-error experimentation. Application of even the most sophisticated physical-chemical characterization techniques is usually not sufficient to obtain a complete understanding of the structure of the active site, the reaction mechanism and kinetics, the structural dynamics, and the specific roles of all catalyst components.
Dynamics of the 2001 UK Foot and Mouth Epidemic: Stochastic Dispersal in a Heterogeneous Landscape
Authors: Keeling Matt J. , Woolhouse Mark E. J. , Shaw Darren J. , Matthews Louise , Chase-Topping Margo , Haydon Dan T. , Cornell Stephen J. , Kappey Jens , Wilesmith John , Grenfell Bryan T.
Published: July 2002

Foot-and-mouth is one of the world's most economically important livestock diseases. We developed an individual farm–based stochastic model of the current UK epidemic. The fine grain of the epidemiological data reveals the infection dynamics at an unusually high spatiotemporal resolution. We show that the spatial distribution, size, and species composition of farms all influence the observed pattern and regional variability of outbreaks. The other key dynamical component is long-tailed stochastic dispersal of infection, combining frequent local movements with occasional long jumps. We assess the history and possible duration of the epidemic, the performance of control strategies, and general implications for disease dynamics in space and time.
Genome Sequence of the Plant Pathogen and Biotechnology Agent
Agrobacterium tumefaciens
C58
Authors: Goodner Brad , Hinkle Gregory , Gattung Stacie , Miller Nancy , Blanchard Mary , Qurollo Barbara , Goldman Barry S. , Cao Yongwei , Askenazi Manor , Halling Conrad , Mullin Lori , Houmiel Kathryn , Gordon Jeffrey , Vaudin Mark , Iartchouk Oleg , Epp Andrew , Liu Fang , Wollam Clifford , Allinger Mike , Doughty Dahlia , Scott Charlaine , Lappas Courtney , Markelz Brian , Flanagan Casey , Crowell Chris , Gurson Jordan , Lomo Caroline , Sear Carolyn , Strub Graham , Cielo Chris , Slater Steven
Published: July 2002

Agrobacterium tumefaciens
is a plant pathogen capable of transferring a defined segment of DNA to a host plant, generating a gall tumor. Replacing the transferred tumor-inducing genes with exogenous DNA allows the introduction of any desired gene into the plant. Thus,
A. tumefaciens
has been critical for the development of modern plant genetics and agricultural biotechnology. Here we describe the genome of
A. tumefaciens
strain C58, which has an unusual structure consisting of one circular and one linear chromosome. We discuss genome architecture and evolution and additional genes potentially involved in virulence and metabolic parasitism of host plants.
Abrupt Decrease in Tropical Pacific Sea Surface Salinity at End of Little Ice Age
Authors: Hendy Erica J. , Gagan Michael K. , Alibert Chantal A. , McCulloch Malcolm T. , Lough Janice M. , Isdale Peter J.
Published: July 2002

A 420-year history of strontium/calcium, uranium/calcium, and oxygen isotope ratios in eight coral cores from the Great Barrier Reef, Australia, indicates that sea surface temperature and salinity were higher in the 18th century than in the 20th century. An abrupt freshening after 1870 occurred simultaneously throughout the southwestern Pacific, coinciding with cooling tropical temperatures. Higher salinities between 1565 and 1870 are best explained by a combination of advection and wind-induced evaporation resulting from a strong latitudinal temperature gradient and intensified circulation. The global Little Ice Age glacial expansion may have been driven, in part, by greater poleward transport of water vapor from the tropical Pacific.
T Cell Receptor Signaling Precedes Immunological Synapse Formation
Authors: Lee Kyeong-Hee , Holdorf Amy D. , Dustin Michael L. , Chan Andrew C. , Allen Paul M. , Shaw Andrey S.
Published: July 2002

The area of contact between a T cell and an antigen-presenting cell (APC) is known as the immunological synapse. Although its exact function is unknown, one model suggests that it allows for T cell receptor (TCR) clustering and for sustained signaling in T cells for many hours. Here we demonstrate that TCR-mediated tyrosine kinase signaling in naı̈ve T cells occurred primarily at the periphery of the synapse and was largely abated before mature immunological synapses had formed. These data suggest that many hours of TCR signaling are not required for T cell activation. These observations challenge current ideas about the role of immunological synapses in T cell activation.
Natural Iron Isotope Variations in Human Blood
Authors: Walczyk Thomas , von Blanckenburg Friedhelm
Published: July 2002

Isotopic analysis of human blood and liver and muscle tissue indicates that each individual bears a long-term iron (Fe) isotope signature in the blood. Blood and tissue differ slightly in isotopic composition and are depleted by up to 2.6 per mil in
56
Fe relative to
54
Fe when compared to dietary Fe. The
56
Fe/
54
Fe isotope ratio in the blood of males is, on average, lower by 0.3 per mil than that of females. These results suggest that Fe isotope effects in the blood reflect differences in intestinal Fe absorption between individuals and genotypes.
Recruitment of a 19
S
Proteasome Subcomplex to an Activated Promoter
Authors: Gonzalez Fernando , Delahodde Agnes , Kodadek Thomas , Johnston Stephen Albert
Published: July 2002

The 19
S
proteasome regulatory particle plays a critical role in cellular proteolysis. However, recent reports have demonstrated that 19
S
proteins play a nonproteolytic role in nucleotide excision repair and transcription elongation. We show by chromatin immunoprecipitation assays that proteins comprising the 19
S
complex are recruited to the
GAL1-10
promoter by the Gal4 transactivator upon induction with galactose. This recruited complex does not contain proteins from the 20
S
proteolytic particle and includes a subset of the 19
S
proteins. This subset is also specifically retained from an extract by the Gal4 activation domain. These data indicate that in vivo, the base of the 19
S
complex functions independently of the larger complex and plays a direct, nonproteolytic role in RNA polymerase II transcription.
A Pathway in Primate Brain for Internal Monitoring of Movements
Authors: Sommer Marc A. , Wurtz Robert H.
Published: July 2002

It is essential to keep track of the movements we make, and one way to do that is to monitor correlates, or corollary discharges, of neuronal movement commands. We hypothesized that a previously identified pathway from brainstem to frontal cortex might carry corollary discharge signals. We found that neuronal activity in this pathway encodes upcoming eye movements and that inactivating the pathway impairs sequential eye movements consistent with loss of corollary discharge without affecting single eye movements. These results identify a pathway in the brain of the primate
Macaca mulatta
that conveys corollary discharge signals.
Structural Basis of Transcription Initiation: An RNA Polymerase Holoenzyme-DNA Complex
Authors: Murakami Katsuhiko S. , Masuda Shoko , Campbell Elizabeth A. , Muzzin Oriana , Darst Seth A.
Published: July 2002

The crystal structure ofThermus aquaticusRNA polymerase holoenzyme (α2ββ′ωσA) complexed with a fork-junction promoter DNA fragment has been determined by fitting high-resolution x-ray structures of individual components into a 6.5-angstrom resolution map. The DNA lies across one face of the holoenzyme, completely outside the RNA polymerase active site channel. All sequence-specific contacts with core promoter elements are mediated by the σ subunit. A universally conserved tryptophan is ideally positioned to stack on the exposed face of the base pair at the upstream edge of the transcription bubble. Universally conserved basic residues of the σ subunit provide critical contacts with the DNA phosphate backbone and play a role in directing the melted DNA template strand into the RNA polymerase active site. The structure explains how holoenzyme recognizes promoters containing variably spaced –10 and –35 elements and provides the basis for models of the closed and open promoter complexes.
Influence of Gene Action Across Different Time Scales on Behavior
Authors: Ben-Shahar Y. , Robichon A. , Sokolowski M. B. , Robinson G. E.
Published: July 2002

Genes can affect natural behavioral variation in different ways. Allelic variation causes alternative behavioral phenotypes, whereas changes in gene expression can influence the initiation of behavior at different ages. We show that the age-related transition by honey bees from hive work to foraging is associated with an increase in the expression of theforaging(for) gene, which encodes a guanosine 3′,5′-monophosphate (cGMP)–dependent protein kinase (PKG). cGMP treatment elevated PKG activity and caused foraging behavior. Previous research showed that allelic differences in PKG expression result in twoDrosophilaforaging variants. The same gene can thus exert different types of influence on a behavior.