bluuebunny/crossref_metadata_2025 huggingface dataset:
Process and device technologies for 1 Gbit dynamic random-access memory cells
Authors: Kaga Toru , Ohkura Makoto , Murai Fumio , Yokoyama Natsuki , Takeda Eiji
Published: July 2002

This article discusses the technological issues involved with continuing the miniaturization of dynamic random-access memory cells into the gigabit era. Ever-smaller giga-generation dynamic random-access memory cells require three-dimensional high-charge density capacitors with high-ε insulating films, leading to the need for further improvements in lithographic resolution for ever-smaller, higher aspect ratio memory cells, and planarization technologies for reducing the memory-cell height. This article demonstrates two technologies for meeting these two requirements: high acceleration energy electron-beam lithography and KrF excimer-laser phase-shift photolithography, and plate-wiring merge technology. Metal–insulator–metal 1.6 nm Ta2O5 CROWN capacitors and single Si3N4 spacer OSELO isolation technology for an experimental 1 Gbit dynamic random-access memory chip are also discussed.
Molecular-beam epitaxial growth of GaxIn1−xP–GaAs (x∼0.5) double heterojunction laser diodes using solid phosphorus and arsenic valved cracking cells
Authors: Baillargeon J. N. , Cho A. Y.
Published: July 2002

Growth of GaxIn1−xP–GaAs (x∼0.5) double heterojunction injection lasers was achieved with molecular-beam epitaxy (MBE) employing phosphorus and arsenic valved cells. White phosphorus at 25 °C, generated in situ from amorphous red, provided the P4 vapor necessary for alloy growth. Photoluminescence performed on unintentionally doped epitaxial GaxIn1−xP (x=0.395–0.55) on GaAs showed 300 K spectral full-width-at-half-maximum between 35–40 meV and were as narrow as 9 meV at 77 K. Broad area (60×450 μm2) stripe geometry laser diodes with 0.1 μm active regions had threshold current densities on the order of 1.5 kA/cm2. Threshold current densities for diodes with 0.25 μm active region widths were about 2 kA/cm2. The data show optical devices and compounds grown by MBE with solid phosphorus and arsenic valved cracking cells are comparable to that realized by other techniques which use AsH3 and PH3 sources.
Comparative study of the interface roughness of Ag/Au and Cu/Au multilayers with scanning tunneling microscopy and x-ray diffraction
Authors: Heyvaert I. , Temst K. , Van Haesendonck C. , Bruynseraede Y.
Published: July 2002

The interface roughness is a very important parameter determining the performances of multilayered structures. Ag/Au and Cu/Au textured multilayers have been produced by thermal evaporation in an ultrahigh vacuum chamber. After deposition of the Au, Ag, or Cu layer, the surface roughness at different sampling lengths was probed in situ by scanning tunneling microscopy. After completion of the multilayers, the samples were taken from the ultrahigh vacuum chamber and measured in an x-ray-diffraction unit. Low-angle as well as high-angle specular x-ray-diffraction curves were analyzed using a refinement program which takes into account layer thickness fluctuations, interface roughness, and crystalline disorder. The average interface roughness, obtained from fitting the x-ray-diffraction profiles, is then compared to the roughness of the film growth front, which was probed with the scanning tunneling microscope. The roughness obtained from the x-ray-diffraction profiles corresponds to a lateral sampling scale which is in good agreement with the average grain size within the layers, indicating that x rays scatter coherently over a volume which is relatively small. Our work also shows the complementarity of the scanning tunneling microscopy and x-ray-diffraction technique to probe interface roughness of multilayers.
Tapping mode atomic force microscopy observation of self-affine fractal roughness in electrochemically roughened silver electrode surfaces
Authors: Otsuka I. , Iwasaki T.
Published: July 2002

We have investigated recrystallization of an electrochemically roughened silver electrode surface in Ar-saturated water, using cyclic voltammetry (CV), Auger electron spectroscopy (AES), and tapping mode atomic force microscopy (TMAFM). Comparison of CVs before and after 92 h relaxation in water indicates that surface flattening occurs at room temperature when the roughened Ag is in contact with water. An AES spectrum of the Ag surface before the relaxation shows that chlorine still remains on the surface, probably in the form of AgCl. We determined the roughness exponents (α) for the roughened Ag surfaces before and after the relaxation from the slopes of the linear regions observed in plots of log (the interface width) vs log (length scale) calculated from TMAFM data. At larger length scales than the average Ag nodule size, αs were 0.50±0.02 and 0.64±0.02 for the roughened surfaces before and after the relaxation, respectively. These results suggest that the scaling theory may be applicable to the recrystallization process at the submicron level.
Experimental evaluation of a 20×20 mm footprint microcolumn
Authors: Kratschmer E. , Kim H. S. , Thomson M. G. R. , Lee K. Y. , Rishton S. A. , Yu M. L. , Zolgharnain S. , Hussey B. W. , Chang T. H. P.
Published: July 2002

A miniaturized 1 kV electron beam column with a 20×20 mm square footprint for application in arrayed lithography was developed. The actual beam forming optics measured from the electron emitter to the last electrode in the beam focusing Einzel lens is only 3.5 mm in length. The electron source is a miniaturized, high brightness (120 μA/sr), low heating power (<1.5 W) Zr/O/W Schottky field emitter that provides a stable beam current with <1%/h current fluctuations. A custom designed, ultralow profile (0.8 mm high) annular microchannel plate (MCP) detector is fitted into the working distance, which can be varied between 1 and 5 mm, between the Einzel lens and the sample. The MCP provides a high gain, up to 3×104, detector for secondary and backscattered electron detection from solid samples. The beam is scanned over the sample using a prelens double octupole deflector for large field size, ≥100 μm, at low distortions and low deflection aberrations. Using a computer controlled digital pattern generator, patterns with less than 100 nm linewidth were exposed in polymethylmethacrylate resist and reactively ion etched into an underlying tungsten layer.
Thermal imaging of thin films by scanning thermal microscope
Authors: Oesterschulze E. , Stopka M. , Ackermann L. , Scholz W. , Werner S.
Published: July 2002

In macroscopic photothermal measurement techniques inhomogeneous thermal waves are used to probe thermal properties of materials on a macroscopic scale. The same principle has been adapted to a high-resolution scanning thermal microscope. Heating the thermal probe periodically results in an amplitude and a phase signal which can be referred to the dynamical thermal behavior of the sample. This measurement mode allows the investigation of the thermal diffusivity of samples in contrast to experiments known from static scanning thermal microscopy which are related to the thermal conductivity. The photothermal scanning thermal microscope technique was used to investigate grains of thin polycrystalline diamond films. For topography measurements the thermal probes were additionally employed for scanning tunneling microscopy.
Ballistic-electron emission microscopy studies of charge trapping in SiO2
Authors: Kaczer B. , Pelz J. P.
Published: July 2002

We have used ballistic-electron emission microscopy (BEEM) to locally inject and microscopically characterize trapped charge in metal-oxide-semiconductor structures made with Pt metal electrodes, moderately thick (∼25 nm) SiO2 films and p-type Si substrates. Electrons injected into the oxide conduction band produce a local suppression in the transmitted BEEM current across the oxide. This suppression depends on the voltage Vox applied across the oxide during injection, indicating that its source is within the oxide film, and does not simply result from hot-electron damage at the metal-oxide interface. This suppression is accompanied by a local increase in the BEEM I-V curve threshold voltage, consistent with a build-up of trapped BEEM electrons in the oxide. For a given trapped charge density we find that the threshold varies systematically with Vox applied during I-V curve measurement. We use these variations to make estimates of the trapped charge density and approximate depth in the oxide film, and find them consistent with previous macroscopic observations. We further study the lateral extent of the suppression and the lifetime of this effect. We also measure the height of the Pt-SiO2 barrier and observe its lowering due to the image force effect.
Effect of interface defect formation on carrier diffusion and luminescence in In0.2Ga0.8As/AlxGa1−xAs quantum wells
Authors: Rich D. H. , Rammohan K. , Lin H. T. , Tang Y. , Meshkinpour M. , Goorsky M. S.
Published: July 2002

We have examined the influence of strain relaxation on the excitonic recombination and diffusion in In0.2Ga0.8As/AlxGa1−xAs quantum-well (QW) samples designed for high-electron-mobility transistors, using spectrally and spatially resolved polarized cathodoluminescence (CL). Six molecular-beam epitaxial grown samples, with varying channel thicknesses ranging from 75 to 300 Å, were examined at various temperatures between 87 and 300 K. An increase in misfit dislocation density occurred with increasing channel thicknesses and resulted in changes in the dark line defect (DLD) density, polarization anisotropy, QW excitonic luminescence energy, and luminescence activation energy, as observed in CL. The influence of misfit dislocations on the ambipolar diffusion of excess carriers in a direction parallel to the dislocation line, in varying proximity to the DLDs, was examined with a CL-based diffusion experiment. The temperature dependence of the CL imaging was examined, enabling a study of the spatial variation of the activation energies associated with thermal quenching of the GaAs/Al0.25Ga0.75As multiple QW and In0.2Ga0.8As QW luminescence. The CL intensity exhibits an Arrenhius-type dependence on temperature and is controlled by thermally activated nonradiative recombination. The activation energies for both the In0.2Ga0.8As QW and Al0.25Ga0.75As MQW luminescence are found to vary spatially in close proximity to the misfit dislocations. We have utilized a new approach to obtain 2D images of the activation energies. The influence of the strain relaxation on the polarization and energy of the In0.2Ga0.8As QW excitonic luminescence was examined with linearly polarized CL and CL wavelength imaging. A strain-induced modification of the luminescence energy and an increase in the polarization anisotropy was measured near DLDs. Thus, we find that certain DLDs exhibit significant polarization and energy variations in their optical properties, in addition to their more familiar nonradiative behavior.
Tunable Schottky barriers and the nature of Si interface layers in Al/GaAs(001) diodes
Authors: Sorba L. , Paggel J. J. , Franciosi A.
Published: July 2002

Silicon layers grown by molecular beam epitaxy in the interface region of Al/n-GaAs(001) Schottky diodes have been shown to tune the Schottky barrier height in the 0.3–1.1 eV range, provided that high enough excess fluxes of As or Al are employed during Si deposition. We studied the incorporation of As and Al in the interface layer for Si growth temperatures of 300 and 400 °C, and for interlayer thicknesses in the 10–150 Å range. Following deposition under an excess As flux we found an As atomic concentration cAs=0.4–0.5 at 300 °C and cAs=0.2 at 400 °C. The excess Al flux yielded an Al atomic concentration cAl=0.7–0.8 at 300 °C, and cAl=0.6–0.7 at 400 °C. Correspondingly, the lattice structure of the interlayers appears dramatically different from that of Si. We conclude that the 10–100 Å interface layers used by some authors to change the barrier height bear little resemblance to degenerate bulk Si, so that several of the mechanisms proposed to explain Schottky barrier tuning should be reevaluated.
Computer simulation of the field emission from multilayer cathodes
Authors: Kryuchenko Yu. V. , Litovchenko V. G.
Published: July 2002

Computer analysis of the field emission from multilayered cathodes of Si–SiO2–Si [quantum well (QW)]–SiO2 for different system parameters (the thicknesses of the SiO2 insulator and QW layers, carrier concentration in the substrate, crystal orientation, etc.) has shown that a well pronounced peak on the Fowler–Nordheim dependence appears. The origin of this peak and its location on the electric field axis is discussed. It is shown that they are caused by the resonance tunneling of electrons through the quantum levels in the quantum well.
Nonlinear analysis of the I–V characteristics in Ti/Si and TiSi2/Si Schottky diodes
Authors: Pérez-Rigueiro J. , Jiménez C. , Pérez-Casero R. , Martínez-Duart J. M.
Published: July 2002

Ti/Si and TiSi2/Si diodes have been prepared by direct current (dc) sputtering and rapid thermal processing. Their I–V characteristics from 77 K up to 300 K have been analyzed using a nonlinear least squares method to fit the experimental data. All the experimental data have been fitted using two exponentially dependent currents and a series resistance. Two parameters are fitted for each current: a saturation current and an exponential parameter. As is discussed in the article, one current can be assigned to the thermoionic current, since the exponential parameter fits well to the theoretical value q/kT and therefore a temperature independent Schottky barrier height can be calculated. The second current, which yields an exponential parameter independent of temperature, cannot be assigned to a mechanism of direct tunneling through the barrier. As a result, we have proposed a trap-enhanced tunneling mechanism to explain this current.
Scanning tunneling microscopy imaging and manipulation of DNA oligomer adsorbed on Cu(111) surfaces by a pulse injection method
Authors: Tanaka Hiroyuki , Kawai Tomoji
Published: July 2002

We have developed a pulse injection method of DNA oligomer aqueous solution onto Cu(111) surfaces as a new sample-preparation technique for scanning tunneling microscopy (STM) studies of biomolecules. The STM images revealed that the surface of Cu(111) retains its atomic flatness even after the injection of DNA-containing solution and that the deposition of intact DNA oligomers without aggregation is possible. The observed internal structure of deposited DNA suggests the promising possibility of sequencing DNA by means of STM. In order to examine the possibility of manipulation of DNA, translational positioning of DNA oligomer has been performed by a lateral pushing of the DNA with scanning tip.
Contamination reduction in low voltage electron-beam microscopy for dimensional metrology
Authors: Bruenger W. H. , Kleinschmidt H. , Hässler-Grohne W. , Bosse H.
Published: July 2002

Contamination of structures to be analyzed in an electron microscope is a severe problem for dimensional metrology applications, especially for low voltage electron microscopy. Two methods for contamination reduction which do not use a temperature variation of the sample or its environment have been investigated in this article. First, injection of inert and reactive gases into the area of electron-beam impact to create a locally confined area of increased pressure which reduces the contamination growth by a factor of 2. Second, prescanning the surroundings of the area is intended for metrology measurements. This method effectively immobilizes contaminants on the specimen surface if the delay time between immobilization and measurement is short enough (approximately 40 ms), to avoid the replenishment of contaminants. A combination of the two techniques is recommended for a damage free low voltage electron-beam metrology.
Minimization of suboxide transition regions at Si–SiO2 interfaces by 900 °C rapid thermal annealing
Authors: Lucovsky G. , Banerjee A. , Hinds B. , Claflin B. , Koh K. , Yang H.
Published: July 2002

Transitions regions at Si–SiO2 interfaces contain excess suboxide bonding arrangements which contribute to interface roughness and also can give rise to electronically active defects. This article provides insights into the origin and temperature stability of these suboxide bonding arrangements by studying different interface formation processes, e.g., rapid thermal oxidation and plasma-assisted oxidation, and then subjecting these interfaces to rapid thermal annealing (RTA). The interfacial bonding chemistry has been studied before and after the RTA by Auger electron spectroscopy and it has been demonstrated that interfacial transition regions with suboxide bonding are a direct result of thermal and plasma-assisted oxidation at temperatures up to at least 800 °C, and that the excess suboxide bonding in interfacial transition regions is significantly reduced following a 30 s, 900 °C RTA. The kinetics of this interfacial annealing process are essentially the same as observed for the RTA-induced separation of homogeneous suboxide thin films (SiOx, x<2) into silicon nanocrystals and stoichiometric SiO2.
Physical and electrical properties in metal-oxide-Si capacitors with various gate electrodes and gate oxides
Authors: Chang-Liao K.-S. , Chen L.-C.
Published: July 2002

The physical and electrical properties in metal-oxide-Si (MOS) capacitors with the gate electrodes deposited using poly-Si or amorphous-Si (a-Si) and with the gate oxide grown in O2 or N2O have been investigated. The differences of a gate oxide grown in N2O with a conventional furnace and an oxide film annealed in N2O by a rapid thermal process (RTP) were also studied. The analysis of physical properties included the thickness variations of oxide films, the shrink ratios of gate electrode films, the nitrogen and hydrogen concentrations in oxides, and the SiO2/Si interfacial strain. The measurement of electrical properties in MOS capacitors included the interface trap density (Dit), the charge-to-breakdown, and the hot electron and radiation induced ▵Dit and flatband voltage shifts. To improve the electrical reliability of MOS devices with ultrathin gate oxides, an oxynitride should be introduced although some properties of it are slightly inferior to those of conventional oxides. The combination of a gate electrode deposited using a-Si and a gate oxide annealed in N2O using RTP is shown to exhibit excellent charge-to-breakdown performance and to reduce hot electron and radiation induced interface traps. This improvement can be explained using a mechanism based on the release of compressive stress in the oxide and the relaxation of SiO2/Si interfacial strain, which could be qualitatively analyzed using infrared spectra.
Contacts on Si1−x−yGexCy alloys: Electrical properties and thermal stability
Authors: Aubry-Fortuna V. , Barthula M. , Perrossier J.-L. , Meyer F. , Demuth V. , Strunk H. P. , Chaix-Pluchery O.
Published: July 2002

In this work, we have investigated the reactions between Zr and SiGeC alloys using rapid thermal annealing. The interactions of the metal films with the Si1−x−yGexCy alloys have been investigated by using sheet resistance measurements, Rutherford backscattering (RBS), energy dispersive spectroscopy (EDS), and x-ray diffraction. The morphologies of surfaces and interfaces were examined using atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM). The analyses indicated that the C49–Zr(Si1−zGez)2 phase is the final phase of the reaction. The ternary alloy even annealed at temperature as high as 800 °C indicates the same Ge index x as in the as-deposited Si1−xGex layer. We did not observe any Ge segregation after annealing. The results indicate that Zr may be a good candidate for contacts on IV–IV alloys in terms of thermal stability. In addition, we have studied the electrical properties of Schottky contacts to SiGeC alloys. We have shown that the dependence of the SBH on the metal work function is less pronounced for the alloys than for pure Si. The addition of Ge in the binary alloys always leads to a decrease of the SBH to p type. The incorporation of C results in a large increase of the SBHs and provides the possibility to get field effect on p-type IV–IV alloys by using Schottky contacts.
Analysis of electron emission degradation in silicon field emitter arrays
Authors: Song Yoon-Ho , Lee Jin Ho , Kang Seung-Youl , Lee Yong-Il , Cho Kyoung Ik , Yoo Hyung Joun
Published: July 2002

Degradation of electron emission characteristics in single-crystalline and polycrystalline silicon field emitter arrays and its mechanism have been studied. A critical biasing time tc at which the emission current started to be degraded dominantly was observed at a pressure of 5×10−7 Torr. The tc is shortened as the emission current increases. Also, the emission current repeatedly measured within tc with a complete relaxation after each measurement would not be degraded even though the total biasing time exceeded the critical time. The experimental results indicate that the degradation in silicon field emitters mainly originates from thermal instability of the silicon tips, especially a decrease in the effective emitting area rather than changes in the work function and geometrical factor.
Effects of carbon on boron diffusion in SiGe: Principles and impact on bipolar devices
Authors: Osten H. J. , Heinemann B. , Knoll D. , Lippert G. , Rücker H.
Published: July 2002

We show that the incorporation of low carbon concentration (<1020 cm−3) within the SiGe region of SiGe heterobipolar transistors (HBT) can significantly suppress boron outdiffusion caused by later processing steps. We were able to obtain fT/fmax ⩾50 GHz for a simple SiGe:C transistor with a box-shaped Ge profile. Comparing the high frequency performance of molecular beam epitaxy grown SiGe:C HBTs with identical SiGe HBTs, we found an increase in fT and fmax by a factor of more than 2. The static characteristics for SiGe:C HBTs demonstrate that the transistors should be suitable for circuit applications. Process margins for SiGe HBT technology are shown to be relaxed due to C incorporation. The dramatic reduction of B diffusion in C-rich Si under conditions of point defect equilibrium is attributed to a reduction of the concentration of interstitials available for the B diffusion.
Compliant substrate technology: Status and prospects
Authors: Brown April S.
Published: July 2002

Compliant substrates offer a new approach to strain management in lattice-mismatched structures. The role of the compliant substrate is to reduce the strain in a mismatched overlayer by sharing the strain via deformation of the substrate, or by nucleating and confining defects in the substrate. This can be accomplished by using a thin, “free-standing” substrate. Current efforts are primarily focused on the specification, design, and fabrication of an “ideal” compliant substrate. Key processing issues include the etching of the substrate and the bonding of the substrate to a mechanical handle wafer. Dramatically reduced dislocation densities, as well as reduced relaxation rates, have been observed for the growth of mismatched overlayers on compliant substrates.
Focused-ion-beam-assisted etching of diamond in XeF2
Authors: Taniguchi Jun , Ohno Naoto , Takeda Shuuichi , Miyamoto Iwao , Komuro Masanori
Published: July 2002

Ga focused-ion-beam (FIB)-assisted etching of single-crystal diamond and thin film diamond in XeF2 was studied. The etch yield in FIB-assisted etching of diamond in XeF2 is enhanced some six times over the physical sputtering yield. In the crystal orientation dependence of the etch yield in FIB-assisted etching, the (100) face produced the highest etch yield of the three faces—(100), (110), and (111). Thin film diamond produces the lowest etch yield. A diamond field emitter with a tip radius of less than 100 nm was obtained using Ga FIB spot exposure.
Gate recessing optimization of GaAs/Al0.22Ga0.78As heterojunction field effect transistor using citric acid/hydrogen peroxide/ammonium hydroxide for power applications
Authors: Hue X. , Boudart B. , Crosnier Y.
Published: July 2002

The etching characteristics of GaAs and Al0.22Ga0.78As in citric acid/hydrogen peroxide/ammonium hydroxide etching solution were studied. The selectivity of GaAs to Al0.22Ga0.78As was as high as 200 at 20 °C (510 at 0 °C) by optimizing the pH and citric acid/hydrogen peroxide ratio. To our knowledge, this is the best result. The etch rate of GaAs is 1000 Å/min and permits a good control of the etched depth. The effects of the etching solution temperature and the doping level of GaAs on the selectivity were determined. The solution stability study over the time proves that the process is very reliable. This wet etching was applied to the gate recess etching of a heterojunction field effect transistor (HFET). Observations and measurements by electron beam microscopy are presented for short gate lengths. They show a very good surface morphology and that the gate recess width is independent of the gate length. It only depends on the etching time. The breakdown voltage value can be adjusted by the gate recess width. The possibilities of this wet etching are consequently demonstrated to be particularly appropriate to the realization of millimeter power HFETs.
Reduction of line edge roughness in the top surface imaging process
Authors: Mori Shigeyasu , Morisawa Taku , Matsuzawa Nobuyuki , Kaimoto Yuko , Endo Masayuki , Matsuo Takahiro , Kuhara Koichi , Sasago Masaru
Published: July 2002

This article presents a novel top surface imaging (TSI) process that is highly sensitive and reduces line edge roughness (LER). We found that LER and residue decreased when we used a chemically amplified (CA) resist, consisting of a base polymer with a high molecular weight and a photo-acid generator producing an acid with a high molecular weight. The top thin silylated layer of the exposed region on the CA resist causes the LER. A breakthrough step in the dry-development process improves the silylation contrast between the exposed and unexposed regions. We then apply a novel step in the dry-development process which involves a predry development bake at a temperature above the glass transition temperature of the silylated polymer. This step is effective in rolling and smoothing the edge of silylated layer by thermal flow. By applying the predry-development bake step above the glass transition temperature, we were able to reduce the LER to less than 6 nm. We have demonstrated a novel TSI process for achieving highly sensitive and improved LER.
Ionized titanium deposition into high aspect ratio vias and trenches
Authors: Zhong G. , Hopwood J.
Published: July 2002

The deposition of titanium into high aspect ratio vias and trenches is investigated using ionized physical vapor deposition (I-PVD). Sputtered titanium atoms are ionized by a high density, inductively coupled plasma of argon at 10 and 30 mTorr. The Ti+is then collimated by the plasma sheath and directionally deposited into vias and trenches ∼1 μm in width. The ability of I-PVD to deposit titanium at the bottom of narrow, deep vias and trenches is characterized by cross sectional scanning electron micrographs. The bottom coverage of 3:1 aspect ratio vias increases from 45% to 75% as the argon pressure and plasma density increase. The percentage of titanium flux that is ionized by I-PVD is extracted from analysis of bottom coverage data and falls between 50% and 85%. A method of extracting the effective transverse temperature of Ti+ is also developed. Transverse temperatures increase from 0.13 to 0.18 eV as argon pressure and radio frequency power are increased.
Investigating the process latitude of a low temperature metalorganic chemical vapor deposition TiNitride process
Authors: Bulger J. M. , Whelan C. S. , Dumont Andrew , Kuhn Markus , Clark Joseph
Published: July 2002

A design of experiments was used to characterize a metalorganic chemical vapor deposition (MOCVD) TiN production process and hardware. The factors evaluated included pedestal temperature, tetrakis diethylamido titanium (TDEAT) flow, reactor pressure, ammonia flow, and nitrogen purge flow. Responses include density, resistivity, deposition rate, thickness uniformity, step coverage, stress, resistance stability, and particles. All factors studied but the nitrogen purge had some first order effect on the responses investigated. Temperature has the most significant effect on the overall film quality and step coverage. Increasing temperature increases the density and lowers the resistivity of the film while reducing the step coverage. Pressure is a dominant factor and effects all the responses studied. Increasing the pressure serves to improve the film quality and step coverage. Ammonia used at high rates increases the density of the film while reducing the deposition rate and thickness nonuniformity. The TDEAT flow primarily controls the deposition rate of the system; too high a flow can result in an increase in the resistivity and a drop in film density. Characterization of the CVD TiN process indicated that for the factor ranges studied the center point process is manufacturable and controllable. All of the responses were found to be within acceptable ranges.
Emission characteristics of Spindt-type field emitter arrays in oxygen ambient
Authors: Gotoh Y. , Utsumi K. , Nagao M. , Tsuji H. , Ishikawa J. , Nakatani T. , Sakashita T. , Betsui K.
Published: July 2002

We have examined endurance of Spindt-type field emitter arrays (FEAs) against oxygen ambient. In this study, the endurance of four different emitter materials, Ni, Pt, Au, and Mo was investigated. The electron emission characteristics of FEAs with 1 to 1600 tips were measured in ultrahigh vacuum (UHV) and in oxygen ambient. For all the FEAs, the emission current decreased with an increase in oxygen pressure. Some of the emitters were destructed during the emission test in oxygen ambient. The detailed analyses of the emission characteristics were performed by calculating the noise power and observing the emission pattern on the phosphor screen. The noise power increased with an increase in oxygen pressure. Emission pattern on the phosphor screen showed enlarged emission area, as compared with the pattern obtained in UHV. The present results indicated that even the Pt emitters were affected by the ambient oxygen. The elimination of oxygen is the essential subject for the development of field emission display.
Nanometer-scale Si-selective epitaxial growth using an ultrathin SiO2 mask
Authors: Miyata Noriyuki , Watanabe Heiji , Ichikawa Masakazu
Published: July 2002

Si-selective epitaxial growth (Si-SEG) with Si2H6 gas is performed on clean Si(001)-2×1 open linear windows in an ultrathin SiO2 mask formed by electron-beam-induced selective thermal decomposition (EB-STD) to fabricate a Si nanowire. We demonstrate that Si-SEG is possible in a 15-nm-wide Si(001)-2×1 open linear window formed by EB-STD. However, the width of the Si wire increases with Si growth, because the oxide mask decomposes thermally at the oxide/Si(001)2×1 boundary. An ultrathin oxide layer grown at higher temperature is effective to suppress the widening of the Si wire, even if the oxide thickness is not changed.
Field emission properties of diamondlike carbon films deposited by ion beam assisted deposition
Authors: Wang Jing , Li Wen-Zhi , Li Heng-De
Published: July 2002

The ion beam sputtering technique was employed to prepare diamondlike carbon (DLC) films at nearly room temperature. Simultaneous ion beam bombardment during film growth was also conducted in order to study the bombardment effects. Raman spectroscopy was used to evaluate the structure property. The sp3 fraction was found to strongly depend on the bombarding ion energy, giving a highly sp3 bonded DLC at ion beam bombarding energies around 800 eV. The emission characteristics of these amorphous DLC films were compared. Field emission from DLC deposited with higher bombarding energies exhibit enhanced emission properties. High electron emission current, about 370 μA over a 1 cm2 cathode area, was obtained with an electric field of 18.5 V/μm. The mechanism for electron emission from DLC films is also discussed.
Thermal compensation of x-ray mask distortions
Authors: Feldman M.
Published: July 2002

Image placement accuracy is widely regarded as one of the factors limiting the application of x-ray lithography to very large scale integration. Image placement errors may arise during mask fabrication, during exposure, or with time and use. Consequently, substantial effort has gone into reducing patterning errors, controlling stress, and selecting optimal mask materials. Alternatively, heat loads may be intentionally applied to compensate known horizontal and vertical distortions in the x-ray mask. In this procedure, a send-ahead wafer is used to determine overlay throughout the exposure field, and the heat loading is tailored during subsequent exposures to correct any errors. The process is simplified for storage ring exposures because proper overlay needs to be maintained, at any given moment of time, only along the line being exposed by the x-ray beam. A wide range of corrections may be made, either to reduce mask distortions or to match distortions already present on the wafer. Experiments were performed projecting 0.31 W of heat in the form of light onto x-ray masks. Displacements of ±5 nm/W/μm of air gap were observed on an old AT&T boron nitride membrane mask, ±7 nm/W/μm of air gap on an IBM silicon membrane mask, and ±11 nm/W/μm of air gap on an IBM silicon carbide mask. This sensitivity should prove useful for correcting distortion with the 10 μm gaps expected in the near future.
Destructive abatement of CF4 and C2F6 via a plasma induced reaction with CaO
Authors: Delattre James L. , Friedman Todd L. , Stacy Angelica M.
Published: July 2002

The plasma induced reactions of CF4 and C2F6 with CaO powder were examined in an effort to develop new methodologies for perfluorocarbon abatement. The reaction of CaO with CF4 (10 sccm flow), activated by an inductively coupled plasma (500 W and 100 mTorr) resulted in the destruction of >70% of the CF4. Under the same conditions >80% of C2F6 was destroyed. For both reactions, the major solid product is polycrystalline CaF2, an environmentally benign mineral with commercial value. The major gaseous products are CO and CO2. These reaction products are attractive alternatives to fluorine-containing byproducts such as F2(g), COF2(g), and HF(aq) generated by traditional perfluorocarbon abatement techniques.
Preparation of macroscopic two-dimensional ordered array of indium nanodots
Authors: Chen S. H. , Fei G. T. , Cui P. , Cheng G. S. , Zhu Y. , Zhu X. G. , Zeng Z. Y. , Zhang L. D.
Published: July 2002

A macroscopic two-dimensional highly ordered array of indium nanodots was achieved using a very simple method of vacuum evaporation on anodic aluminum. The ordered indium nanodots, seated on small, shallow holes of an Al template, are arranged in a rhombic pattern with a typical period of about 100 nm. The formation mechanism of the indium nanodots is discussed.
Analysis of electron standing waves in a vacuum gap of scanning tunneling microscopy: Measurement of band bending through energy shifts of electron standing wave
Authors: Suganuma Y. , Tomitori M.
Published: July 2002

Numerical simulation for the electron standing wave excited between a scanning tunneling microscopy (STM) tip and a sample in a field emission regime has been performed using a one-dimensional potential including a tip shape effect. It can qualitatively trace experimental results of the differential conductance (dI/dV) spectra with oscillatory peaks, which originate from the standing wave excitation. Furthermore, a band bending effect on a semiconductor surface has been evaluated including a multiple image potential in addition to a potential with a tip shape effect. By fabricating tips with a regular shape, the strength of electric field between tip and sample can be estimated from tip displacement while taking the dI/dV spectra. A band bending effect on dI/dV spectra has been evaluated to explain a parallel shift of the peaks in the spectra by the amount of band bending. We have experimentally demonstrated the shift with light irradiation to change the band bending to be flat; at least 0.4 eV upward band bending for n-type Si(001) 1 Ω cm was estimated from the peak shift. This spectroscopic method is promising to probe the electric field in the vacuum gap between tip and sample in the STM.
Frictional properties of titanium carbide, titanium nitride, and vanadium carbide: Measurement of a compositional dependence with atomic force microscopy
Authors: Lee Seunghwan , El-bjeirami Oussama , Perry Scott S. , Didziulis Stephen V. , Frantz Peter , Radhakrishnan Gouri
Published: July 2002

The frictional properties of TiC(100), TiN(100), and VC(100) surfaces have been investigated on the microscopic scale by atomic force microscopy. In this work, friction measurements were performed under controlled ambient conditions to emulate the use of these materials as hard coatings. A variety of tip materials, silicon nitride, titanium carbide, titanium nitride, and tungsten carbide, were used to investigate the correlation between surface composition and frictional properties of the carbide and nitride substrates. The surface compositions of both clean and air-exposed samples were characterized by x-ray photoelectron spectroscopy to complete the composition/friction correlation. In these studies, the TiC(100) substrate exhibited the lowest frictional response of the substrate samples, regardless of tip composition. The friction measurements on TiN(100) and VC(100) exhibited a strong dependence on counterface composition and were consistently higher than those of TiC. In addition to the compositional dependence, the influence of humid conditions on the frictional properties of these carbide and nitride substrates has been investigated. The measured friction of each of these samples increased monotonically as a function of relative humidity. The magnitude of these increases has been correlated with the substrate surface free energies as probed by water contact angle measurements.
Effect of nitrogen doping on field emission characteristics of patterned diamond-like carbon films prepared by pulsed laser deposition
Authors: Shin I. H. , Lee T. D.
Published: July 2002

Effect of nitrogen doping on field emission characteristics of patterned diamond-like carbon (DLC) films was studied. The patterned DLC films were fabricated by the method reported previously [I. H. Shin and T. D. Lee, J. Vac. Sci. Technol. B 17, 690 (1999)]. Nitrogen doping in the DLC film was carried out by introducing N2 gas into the vacuum chamber during deposition. Level of doping was controlled by varying N2 flow rate up to 50 sccm. Nitrogen content in the films almost linearly increased with increasing N2 flow rate. Higher emission current density of 0.3–0.4 mA/cm2 was observed for the films with 6 at. % N than the undoped films, but the emission current density decreased with further increase of N contents. Some changes in CN bonding characteristics with increasing N contents were observed. The CN bonding characteristics which seem to affect the electron emission properties of these films were studied by Raman spectroscopy, x-ray photoemission spectroscopy and Fourier transform infrared spectroscopy. Electrical resistivity and optical band gap measurements showed consistence with the above analysis.
Neuroimaging Databases
Authors:   The Governing Council of the Organization for Human Brain Mapping (OHBM)
Published: July 2002

These are comments written by the Governing Council of the Organization for Human Brain Mapping (OHBM), the primary international organization dedicated to neuroimaging research. The purpose of these comments is to identify and frame issues concerning data sharing within the neuroimaging community. Data sharing has become an important issue in most fields of science. The neuroimaging community is no exception, and it clearly perceives potential benefits in such efforts, as have been realized in other fields such as genomics. At the same time, such efforts can be costly (both in time and expense), and there are important factors that differentiate brain imaging from other fields and that pose specific challenges to the generation of useful neuroimaging databases. These include the rapid pace of change in brain imaging technologies; the complexity of the variables that must be specified to meaningfully interpret the results (such as the method of image acquisition, behavioral design, and subject characteristics); and concerns about participant confidentiality. These issues are outlined with the goal of framing and promoting a public discussion of the benefits and risks of data sharing, which can inform the field of neuroimaging as well as others that face similar challenges.
Origin of the Hard X-ray Emission from the Galactic Plane
Authors: Ebisawa Ken , Maeda Yoshitomo , Kaneda Hidehiro , Yamauchi Shigeo
Published: July 2002

The Galactic plane is a strong emitter of hard x-rays (2 to 10 kiloelectron volts), and the emission forms a narrow continuous ridge. The currently known hard x-ray sources are far too few to explain the ridge x-ray emission, and the fundamental question of whether the ridge emission is ultimately resolved into numerous dimmer discrete sources or truly diffuse emission has not yet been settled. In order to obtain a decisive answer, using the Chandra X-ray Observatory, we carried out the deepest hard x-ray survey of a Galactic plane region that is devoid of known x-ray point sources. We detected at least 36 new hard x-ray point sources in addition to strong diffuse emission within a 17′ by 17′ field of view. The surface density of the point sources is comparable to that at high Galactic latitudes after the effects of Galactic absorption are considered. Therefore, most of these point sources are probably extragalactic, presumably active galaxies seen through the Galactic disk. The Galactic ridge hard x-ray emission is diffuse, which indicates omnipresence within the Galactic plane of a hot plasma, the energy density of which is more than one order of magnitude higher than any other substance in the interstellar space.
Stage-Specific Transcription of Distinct Repertoires of a Multigene Family During
Plasmodium
Life Cycle
Authors: Preiser P. R. , Khan S. , Costa F. T. M. , Jarra W. , Belnoue E. , Ogun S. , Holder A. A. , Voza T. , Landau I. , Snounou G. , Rénia L.
Published: July 2002

Members of a multigene family in the rodent malaria parasite
Plasmodium yoelii yoelii
code for 235-kilodalton proteins (Py235) that are located in the merozoite apical complex, are implicated in virulence, and may determine red blood cell specificity. We show that distinct subsets of
py235
genes are expressed in sporozoites and hepatic and erythrocytic stages. Antibodies to Py235 inhibited sporozoite invasion of hepatocytes. The switch in expression profile occurred immediately after transition from one stage to another. The results suggest that this differential expression is driven by strong biological requirements and provide evidence that hepatic and erythrocytic merozoites differ.
The Mantle Flow Field Beneath Western North America
Authors: Silver P. G. , Holt W. E.
Published: July 2002

Although motions at the surface of tectonic plates are well determined, the accompanying horizontal mantle flow is not. We have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western North America. We find that the mantle velocity is 5.5 ± 1.5 centimeters per year due east in a hot spot reference frame, nearly opposite to the direction of North American plate motion (west-southwest). The flow is only weakly coupled to the motion of the surface plate, producing a small drag force. This flow field is probably due to heterogeneity in mantle density associated with the former Farallon oceanic plate beneath North America.
The Seismic 8° Discontinuity and Partial Melting in Continental Mantle
Authors: Thybo H. , Perchuć E.
Published: July 2002

Strong, scattered reflections beyond 8 degrees (8°) offset are characteristic features of all high-resolution seismic sections from the continents. The reflections identify a low-velocity zone below approximately 100 kilometers depth beneath generally stratified mantle. This zone may be caused by partial melting, globally initiated at equal depth in the continental mantle. Solid state is again attained at the Lehmann discontinuity in cold, stable areas, whereas the zone extends to near the 400-kilometer discontinuity in hot, tectonically active areas. Thus, the depth to the Lehmann discontinuity may be an indicator of the thermal state of the continental mantle.
Vacuum Squeezing of Solids: Macroscopic Quantum States Driven by Light Pulses
Authors: Garrett G. A. , Rojo A. G. , Sood A. K. , Whitaker J. F. , Merlin R.
Published: July 2002

Femtosecond laser pulses and coherent two-phonon Raman scattering were used to excite KTaO
3
into a squeezed state, nearly periodic in time, in which the variance of the atomic displacements dips below the standard quantum limit for half of a cycle. This nonclassical state involves a continuum of transverse acoustic modes that leads to oscillations in the refractive index associated with the frequency of a van Hove singularity in the phonon density of states.
A High-Temperature Electrical Conduction Mechanism in the Lower Mantle Phase (Mg,Fe)

1−
x

O
Authors: Dobson David P. , Richmond Nicola C. , Brodholt John P.
Published: July 2002

Measurements of electrical conductivity at high pressure and temperature were taken on the lower mantle phase magnesiowüstite with varying Fe
3+
content. Although previous measurements at atmospheric pressure suggest Fe
2+
-Fe
3+
hopping (small polaron) as the dominant conductivity mechanism, the present experiments show a change in charge transport mechanism with temperature. The lower temperature measurements are consistent with small polaron conduction, but at higher temperatures, which are more applicable to the lower mantle, a large polaron mechanism is suggested. Because these mechanisms have different temperature and compositional dependencies, this transition has important implications for extrapolation to mantle conditions.
Report Slams Japanese Program
Authors: Normile Dennis
Published: July 2002

TOKYO
—Japan's rock-solid faith in predicting earthquakes is about to get a shock from a draft report that says the research program has failed to warn the population about impending earthquakes and has overstated its ability to develop accurate forecasts. The program has been on the defensive since the deadly Kobe earthquake in 1995.
Maintenance of Human T Cell Anergy: Blocking of IL-2 Gene Transcription by Activated Rap1
Authors: Boussiotis Vassiliki A. , Freeman Gordon J. , Berezovskaya Alla , Barber Dwayne L. , Nadler Lee M.
Published: July 2002

In the absence of costimulation, T cells activated through their antigen receptor become unresponsive (anergic) and do not transcribe the gene encoding interleukin-2 (IL-2) when restimulated with antigen. Anergic alloantigen-specific human T cells contained phosphorylated Cbl that coimmunoprecipitated with Fyn. The adapter protein CrkL was associated with both phosphorylated Cbl and the guanidine nucleotide–releasing factor C3G, which catalyzes guanosine triphosphate (GTP) exchange on Rap1. Active Rap1 (GTP-bound form) was present in anergic cells. Forced expression of low amounts of Rap1-GTP in Jurkat T cells recapitulated the anergic defect and blocked T cell antigen receptor (TCR)– and CD28-mediated IL-2 gene transcription. Therefore, Rap1 functions as a negative regulator of TCR-mediated IL-2 gene transcription and may be responsible for the specific defect in IL-2 production in T cell anergy.