bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermomechanical analysis of failure of metal field emitters
Authors: Ancona M. G.
Published: July 2002

An analysis of the thermomechanics of metal (chiefly molybdenum) field emitters under normal operating conditions is given. The focus is on various ‘‘intrinsic’’ phenomena and the possibilities of triggering catastrophic breakdown. The heating is found to be predominantly Nottingham in origin (as opposed to ohmic) and is not appreciable unless the currents are well beyond the normal operating regime, e.g., as might occur under arcing conditions. As a result, the increases in temperature in operating tips are generally minimal unless the heat sinking to the substrate is weak, e.g., in ultrasharp tips, in tips on long, narrow posts, and/or in tips with severely degraded thermal conductivity. Apart from these exceptions, the small temperature excursions mean that no thermal runaway, melting, thermal desorption, or other direct thermal effects occur. Thermal stresses are of course also minimal; however, Maxwell stresses can be large under normal operating conditions, approaching the yield strength under high bias conditions.
Stain-etched porous silicon visible light emitting diodes
Authors: Xu J. , Steckl A. J.
Published: July 2002

Visible light emitting diodes are fabricated from p-type porous silicon (PoSi) thin films (∼200 nm) using indium tin oxide (ITO)/PoSi n-p heterojunction structures. Uniform PoSi thin layers were produced by pure chemical etching (stain etching) of B-doped single crystalline Si in a HF:HNO3-based solution. Electroluminescence (EL), with a spectrum similar to that of the photoluminescence, was observed from the diodes under forward bias only. The diodes show improved I–V characteristics with an ideality factor of 2.1. An EL onset bias as low as 3 mA/cm2 was measured.
Observation of Coulomb staircase and negative differential resistance at room temperature by scanning tunneling microscopy
Authors: Radojkovic P. , Schwartzkopff M. , Enachescu M. , Stefanov E. , Hartmann E. , Koch F.
Published: July 2002

Nanometer-sized Au dots have been fabricated on p-type Si(111) substrates by field-induced transfer of tip material in a scanning tunneling microscope, thus forming a nanometer Schottky contact. Positioning the scanning tunneling microscope tip on a very tiny dot (10–15 nm in diameter), equidistant steps with a spacing of several 100 mV and regions with negative differential resistance are observed in the current–voltage characteristic. Numerical simulations confirm that the determinant capacitance between tip and Au dot falls in the 10−19 F range, implying that the origin of the monitored step structure can be interpreted to arise from single-electron tunnel effects (Coulomb staircase). As the nanometer Schottky contact forms a not well-insulating configuration, the development of negative differential resistance might be explained by a leaking of excess carriers from the dot to the immediate surroundings.
Step coverage comparison of Ti/TiN deposited by collimated and uncollimated physical vapor deposition techniques
Authors: Wang Shi-Qing , Schlueter James , Gondran Carolyn , Boden Ted
Published: July 2002

The step coverage and grain structure of Ti/TiN films sputtered with and without a collimator in a contact/via hole structure were compared using transmission electron microscopy, microelectron diffraction, and Auger electron spectroscopy with field emission microanalysis capability on cross-section samples. The holes have widths ranging from 0.40 to 0.75 μm and aspect ratios from 2.3 to 1.4, respectively. It was found that collimated Ti/TiN films demonstrate reasonably uniform coverage along the sidewall and at the bottom of holes. Collimated Ti bottom coverage is sufficient to assure good contact resistance. In contrast, uncollimated Ti/TiN films demonstrate an overhang on the top sidewall of holes and show decreased coverage when approaching the bottom corner along the sidewall or along the bottom of holes. Film property differences at different regions of a hole using either collimated or uncollimated deposition techniques will also be discussed.
Mold-assisted nanolithography: A process for reliable pattern replication
Authors: Haisma Jan , Verheijen Martin , van den Heuvel Kees , van den Berg Jan
Published: July 2002

A process for reproducibly and reliably realizing thin-layer patterning having details with dimensions of 100 nm or even less is described. This process has been called mold lithography. It is a two-step process: First, a photopolymerization-replication step is carried out, after which pattern transfer is realized through, e.g., wet or dry etching into the substrate material. We performed a number of elementary experiments to evaluate this process. Processing conditions are given and the obtained results are discussed. The strengths of this process are its simplicity and low cost while maintaining compatibility with (standard) semiconductor-technology processing.
Polysilicon gate etching in high density plasmas. IV. Comparison of photoresist and oxide masked polysilicon etching-thickness determination of gate oxide layers using x-ray photoelectron spectroscopy
Authors: Bell F. H. , Joubert O.
Published: July 2002

The characteristics of oxide and photoresist masked polysilicon trench etching has been studied by real-time HeNe laser ellipsometry and quasi in situ x-ray photoelectron spectroscopy (XPS). Poly-Si films on SiO2-covered Si (100) substrates were masked either with a 1-μm-thick photoresist or a 200-nm-thick oxide hard mask. The 200-mm-diam wafers were etched downstream in a helicon high density plasma source using a chlorine-based gas chemistry. When using an oxide hard mask instead of a photoresist mask, the selectivity of polysilicon over oxide was improved by a factor greater than 3. A new approach to the surface characterization of semiconductor submicron structures by XPS is presented. Photoelectron signals originating from the gate oxide film and the underlying silicon substrate were measured in regular arrays of trenches. The ratio between the SiO2 peak area of the gate oxide film and the Si 2p peak area of the silicon substrate was correlated with the thickness of the SiO2 film. The thickness determined was obtained by calibrating peak area ratios with oxide thickness measurements using spectroscopic ellipsometry. Consequently, the gate oxide thicknesses derived from the area ratios were calculated in patterned areas after etching of photoresist and oxide hard masked polysilicon features as a function of the aspect ratio of the features and mask coverage on the wafer. It was found that the gate oxide consumption is enhanced in high aspect ratio features masked with photoresist as well as in areas with high photoresist coverage; the carbon coverage on the gate oxide film was found to scale with the increased gate oxide etching in the small features. Similar effects were not observed with the oxide masked sample.
High-quality GaN and AlN grown by gas-source molecular beam epitaxy using ammonia as the nitrogen source
Authors: Yang Z. , Li L. K. , Wang W. I.
Published: July 2002

High-quality GaN and AlN have been grown by gas-source molecular beam epitaxy (MBE) using ammonia as the nitrogen source. A growth rate as high as 1 μm/h, which is an order-of-magnitude higher than previously reported for the growth of GaN by molecular beam epitaxy, has been achieved. Strong reflection high-energy electron diffraction intensity oscillations have been observed during the growth, making in situ thickness monitor and control as thin as one monolayer possible. Undoped GaN with an electron density as low as 2×1017 cm−3 and Mg-doped p-type GaN with a room temperature hole density of 4×1017 cm−3 were obtained. Low-temperature photoluminescence of as-grown materials was dominated by band-edge emissions, indicative of high-quality materials which are promising for applications to blue light emitters and high-temperature electronic devices.
Field emission from diamond coated molybdenum field emitters
Authors: Choi W. B. , Liu J. , McClure M. T. , Myers A. F. , Zhirnov V. V. , Cuomo J. J. , Hren J. J.
Published: July 2002

Diamond deposition onto single Mo field emitters was accomplished by two methods: microwave plasma chemical vapor deposition and a dielectrophoresis of diamond powder. Observation by transmission electron microscopy and scanning electron microscopy revealed a significant amount of deposition at the tips. The field emission characteristics were measured before and after diamond deposition on the same emitters. Field emission from diamond coated emitters yielded significant increases in emission current and lower Fowler–Nordheim slopes. We discuss a possible mechanism to explain current enhancement that depends primarily upon the Mo-diamond interface.
Laser assisted focused-ion-beam-induced deposition of copper
Authors: Funatsu Jun. , Thompson Carl V. , Melngailis John , Walpole James N.
Published: July 2002

Focused-ion-beam-induced deposition of a low resistivity Cu film has been produced using local heating of the substrate with a semiconductor laser. A spot of order 70 μm diameter is irradiated using suitable optics to couple the 1.3 W, 977 nm laser radiation to the sample. The resistivity of the deposited Cu drops from 150–200 μΩ cm with no laser power to 3 μΩ cm at 1.2 W, when the temperature in the irradiated areas is 120 °C. The metal film deposits only where the ion beam is incident.
Nanocluster formation by spin coating: Quantitative atomic force microscopy and Rutherford backscattering spectrometry analysis
Authors: Partridge A. , Toussaint S. L. G. , Flipse C. F. J. , van IJzendoorn L. J. , van den Oetelaar L. C. A.
Published: July 2002

A recently developed spin coating method has been employed to produce a homogeneous distribution of nanometer-sized metal clusters onto a flat oxidic support. The particle size and distribution, and the total amount of material deposited has been studied by comparing the results of atomic force microscopy (AFM), Rutherford backscattering spectroscopy (RBS), and the appropriate hydrodynamic deposition equations. It is shown that the AFM is capable of producing a three-dimensional image of the surface which enables the particle number density and particle heights to be accurately determined. However, it is clear that as a result of tip convolution effects the particle diameter cannot be accurately determined. Using a hemispherical particle model the amount of material deposited during spin coating can be calculated from the AFM images. This calculation is shown to be accurate to approximately 50% in comparison with the results obtained from RBS. In contrast, it is shown that for a copper acetate precursor the predictions of the hydrodynamic equations are accurate to 2%. In the light of these results an assessment is made of the utility of AFM in the investigation of model catalyst systems and fundamental metal cluster studies.
Microfabrication of near-field optical probes
Authors: Ruiter A. G. T. , Moers M. H. P. , van Hulst N. F. , de Boer M.
Published: July 2002

Near-field optical microscopy generally uses a tapered optical fiber, which is metal coated, to form a sub-wavelength sized light source. Here, a technique for the fabrication of a new type of probe is described. The new design is based on atomic force microscope probes and consists of a silicon nitride cantilever with a solid transparent conical tip. The probes are made using micromechanical techniques, which allow batch fabrication of the probes. A near-field scanning optical microscope system was built to test the probes. This system features force detection by a beam deflection technique and subsequent force feedback together with a conventional optical microscope. A major advantage of the apparatus is the ease at which images are obtained. Results on a test sample show that an optical resolution of 300 nm can be obtained together with a simultaneous height image.
Scanning electron microscope matching and calibration for critical dimensional metrology
Authors: Marchman Herschel
Published: July 2002

The mean value of a measurement set with precision σ has an offset relative to an accepted standard. Calibration of an instrument involves subtracting the offset between the mean value and the reference. Unfortunately, imaging characteristics for most types of microscopes are not constant. Slight changes in feature wall angle, edge and surface roughness, or material properties can dramatically affect the precision and accuracy offset of the metrology instrument. Of course it would be impossible to fabricate a reference standard for every type of sample, as this would require an a priori knowledge of the feature characteristics as well as an infinite number of reference artifacts. Typically the process engineer will use in-house or “golden standards” to track the repeatability of a measurement tool. Unfortunately, variations in the features to be measured will cause the accuracy offset to change in ways that are unknown to the engineer. Furthermore, the offsets of multiple tools in a set will not only change with respect to the reference but also relatively to each other. It is the minimization of this relative offset between the various measurement tools that is referred to as matching. It should be noted that one could use a member in the nonreference set of tools, but then the question naturally arises of which one is most correct. It is desirable for a reference tool to provide the complete topography of a structure, as well as the ability to reduce the effects of sample variation (i.e., edge roughness) through spatial averaging. In this article, a method for using atomic force microscopes to provide reference values for matching of on-line scanning electron microscopes will be described.
Surface morphology and quantum dot self-assembly in growth of strained-layer semiconducting films
Authors: Khor K. E. , Das Sarma S.
Published: July 2002

The energetics of island growth on thin semiconducting films deposited on lattice-mismatched substrates is discussed in this article. Hut clusters similar to those proposed by Mo et al. [Phys. Rev. Lett. 65, 1020 (1990)] for Ge/Si(001), with (10n), (n⩾3), or (11n), (n⩾1), side facets will be analyzed. Results show that hut clusters are the energetically favorable structures during early growth, with side facets of rebonded (105) planes; at later times, larger islands with (11n)-like facets become favorable. It is found that islands nucleate with critical nuclei of about 1 atom and grow two dimensionally until they reach a critical size sc, when it is favorable for the islands to become three dimensional. There is an effective barrier at the transition from two dimensional to three dimensional growth. Beyond the barrier, there is an immediate energy gain which can be large, on the order of 5–10 meV/atom for the highly mismatched system of InAs/GaAs. It is suggested that these results are the underlying reason for the uniformity seen in self-assembled quantum dots in highly mismatched heteroepitaxy.
Controlled formation of organic layers on semiconductor surfaces
Authors: Hovis Jennifer S. , Lee Seung , Liu Hongbing , Hamers Robert J.
Published: July 2002

It has been shown that well-defined, ordered organic layers can be formed on the silicon (100) surface. This is achieved through the interaction of unsaturated C=C bonds with the oriented dimers of the reconstructed Si(100)-(2×1) surface. In this article, we present an investigation of the structure and chemical bonding of organic films prepared using different organic precursors. Data were obtained using scanning tunneling microscopy, Fourier-transform infrared spectroscopy, and x-ray photoelectron spectroscopy. The molecules investigated are cyclopentene, 3-pyrroline, and norbornadiene, representing prototypical cyclic, heterocyclic, and bicyclic unsaturated organic molecules, respectively. Each molecule has at least one unsaturated C=C bond.
Evolution mechanism of nearly pinning-free platinum/n-type indium phosphide interface with a high Schottky barrier height by in situ electrochemical process
Authors: Hasegawa Hideki , Sato Taketomo , Hashizume Tamotsu
Published: July 2002

Recently, unusually high Schottky barrier heights (SBHs) have been realized by our group on n-type InP and related materials by an in situ electrochemical deposition of Pt. In an attempt to understand the underlying mechanism of the SBH enhancement, this article investigates in detail the evolution process of the metal (Pt, Ni, Co, and Ag)-InP interface during the in situ electrochemical process, using current–voltage, capacitance voltage, deep level transient spectroscopy, x-ray photoelectron spectroscopy, Raman, atomic force microscope, and scanning electron microscope measurements. Pt deposition by the electrochemical process realized an oxide-free, defect-free, stress-free, and nearly pinning-free interface, whereas Pt deposition by conventional electron beam evaporation and sputtering processes as well as Ag, Ni, and Co deposition by the electrochemical process gave rise to stressed and pinned interfaces. The observed large process dependence of SBH can be explained by none of the metal induced gap state model, the unified defect model, and the effective workfunction model. It is explained here by the disorder induced gap state model.
Quantification of topographic structure by scanning probe microscopy
Authors: Kiely James D. , Bonnell Dawn A.
Published: July 2002

Several mathematical approaches for quantifying the three-dimensional topographical structure from scanning probe microscopy images are evaluated. Variational, i.e., scale-dependent, roughness based on root-mean-square roughness, Fourier deconvolution, and the two-dimensional autocovariance function are compared for surfaces with widely varying character in order to develop criteria for accurate quantification. Thermally evaporated gold, a calibration grid, polycrystalline Si3N4, and silicon fracture surfaces serve as models for these techniques. The role of image artifacts on each approach is detailed.
Electrical transport properties of silicon delta-doped Al0.30Ga0.70As samples showing suppression of the DX center features
Authors: Corrêa F. J. A. , de Oliveira A. G. , da Silva M. I. N. , Ribeiro G. M. , Sampaio J. F.
Published: July 2002

Photoconductivity and photo-Hall density measurements using an infrared light emitting diode as the light source were carried out on single silicon delta-doped Al0.30Ga0.70As samples as a function of temperature. The samples were grown by molecular beam epitaxy at 530 °C and 600 °C. We have studied the effect of etching the cap layer on the electrical transport properties. An observed persistent photoconductivity effect is explained using a model of parallel conduction in two nearby spatially separated channels. We will present evidence that the DX center is not active for nearly ideal delta-doped samples. We have proposed that the DX-center level related to the conduction-band minimum is strongly dependent on the silicon delta-doping density and on the growth conditions.
Fabrication of surface nanostructures by scanning tunneling microscope induced decomposition of SiH4 and SiH2Cl2
Authors: Rauscher H. , Behrendt F. , Behm R. J.
Published: July 2002

Nanostructures with lateral dimensions below 10 nm are fabricated on Si(111)7×7 surfaces by scanning tunneling microscope induced dissociation of the precursor gases SiH4 and SiH2Cl2. The molecules are activated between the tunnel tip and the surface, where highly reactive species are produced by the interaction of molecules with free electrons in the tunnel gap. The molecular fragments accumulate directly below the tip and build up the nanostructure. Information on the deposition mechanism that is effective at the onset of deposition is obtained by careful evaluation of the deposition voltage threshold regime.
Integration of unit processes in a shallow trench isolation module for a 0.25 μm complementary metal–oxide semiconductor technology
Authors: Chatterjee A. , Ali I. , Joyner K. , Mercer D. , Kuehne J. , Mason M. , Esquivel A. , Rogers D. , O’Brien S. , Mei P. , Murtaza S. , Kwok S. P. , Taylor K. , Nag S. , Hames G. , Hanratty M. , Marchman H. , Ashburn S. , Chen I.-C.
Published: July 2002

This article presents a study of the issues in integrating the pattern, fill, planarization, and surface cleanup processes to design a shallow trench isolation (STI) flow suitable for 0.25 μm complementary metal–oxide semiconductor technologies. Technological choices and their effects on the characteristics of the STI technology are discussed. Experimental data are presented to illustrate how process choices at various stages of the STI flow are made to optimize the STI structure.
Deep ultraviolet resists AZ DX-561 and AZ DX-1300P applied for electron beam and masked ion beam lithography
Authors: Hudek P. , Kostic I. , Belov M. , Rangelow I. W. , Shi F. , Pawlowski G. , Spiess W. , Buschbeck H. , Cekan E. , Eder S. , Löschner H.
Published: July 2002

This article reports work on two positive-tone deep ultraviolet chemically amplified resist (CAR) systems from HOECHST AG that were applied in two different lithographic processes: (i) the AZ DX-561 resist in serial working direct-write scanning electron-beam lithography, and (ii) AZ DX-1300P resist in a novel parallel working masked ion beam lithography. The main parameters of both CARs under the optimized processing conditions for both lithographic processes were generally determined. The contrast of these resists was found to be >10. The sensitivity of AZ DX-561 was about 12 μC/cm2 for 30 keV electrons. For AZ DX-1300P, a sensitivity of 9 μC/cm2 was found for 30 keV electrons and 0.3 μC/cm2 for 80 keV He+ ions. Both resists showed excellent performance for subsequent reactive ion etching.
Atomic investigation of individual apexes of diamond emitters by a scanning atom probe
Authors: Nishikawa Osamu , Sekine Takahiro , Ohtani Yoshikatsu , Maeda Kiyoshi , Numada Yoshihiro , Watanabe Masafumi
Published: July 2002

Diamonds grown by chemical vapor deposition and high-pressure high-temperature processes were mass analyzed by a newly developed scanning atom probe. The mass analysis of the diamonds revealed that the diamonds contain a large amount of hydrogen not only in the surface layer but also in the deep underlying layers and that carbon atoms form the clusters, C3, C5, C8, and C16. The clustering carbon atoms could be strongly bound in the diamond structure. The detection of a large number of neutral particles indicates that most cluster ions were decomposed into a positive ion and a neutral particle immediately after field evaporation. The interesting finding is that most carbon atoms are field ionized as multi-ionized ions such as C2+, C3+, and C4+ and/or CH2+, CH3+, and CH4+ with the mass peak of H+, while the major mass peaks are C+ and/or CH+ when no hydrogen peak is found. This may suggest that the binding state of carbon atoms in the diamonds varies with the binding state with hydrogen atoms.
Deviations from the Fowler–Nordheim theory and peculiarities of field electron emission from small-scale objects
Authors: Fursey G. N. , Glazanov D. V.
Published: July 2002

Some reasons that require modifying the Fowler–Nordheim theory of field emission, especially for nanoscale emitters, are described. One such reason is that the microtip apex curvature radius can become comparable or even less than the width of the potential barrier on the metal–vacuum boundary. Therefore, the electric field near the surface is not homogeneous. Such deviation of the barrier shape from the traditional plane model and of the interaction between the electron and image charge must have an influence upon the emission characteristics. The possible approaches to this problem are reviewed. The results of the numerical solution of Schrödinger equation for electron tunneling, that allows us to calculate the emission current density and energy distribution of emitted electrons for arbitrary barrier shape, are presented. The problem of comparing the obtained results with the prediction of conventional Fowler–Nordheim theory is discussed.
Dynamics of localized excitons in InGaN/GaN quantum wells
Authors: Yu Hongbin , Htoon H. , deLozanne Alex , Shih C. K. , Grudowski P. A. , Dupuis R. D. , Zeng K. , Mair R. , Lin J. Y. , Jiang H. X.
Published: July 2002

By using a “nanoscale aperture” method, we studied the dynamics of localized excitons in active InGaN layers in an InGaN/GaN multiple quantum well (MQW). Local photoluminescence (PL) from local excitation shows very different characteristics from far-field luminescence. PL spectra from these nanoapertures are, in general, blueshifted relative to the spectra acquired in the unmasked regions. The spectra vary from aperture to aperture, reflecting the spatial fluctuation of the In composition in the MQW. In addition, one typically observes spectra with clearly resolved multiple peaks from the small apertures, which may be attributed to compositional and size fluctuations over a length scale longer than the exciton diameter but smaller than the aperture size. The excitation intensity dependence of PL indicates a band-filling effect in the multiple-peak structure. Finally, time-resolved studies reveal a very interesting spectral weight shifting between the higher-energy peaks and the lower-energy peaks.
Fabrication of high-density nanostructures by electron beam lithography
Authors: Dial O. , Cheng C. C. , Scherer A.
Published: July 2002

We demonstrate a fabrication method to define high-density, uniform nanostructures by electron beam lithography at conventional beam voltages (<40 kV). Here we optimize the exposure and development conditions needed to generate such nanostructure arrays using polymethylmethacrylate as positive resist and isopropyl alcohol as a developer. Arrays of 12 nm dots with 25 nm period and 20 nm lines with 40 nm period were fabricated to show the resolution of this optimized process.
Thermally induced interface degradation in (100) and (111) Si/SiO2 analyzed by electron spin resonance
Authors: Stesmans A. , Afanas’ev V. V.
Published: July 2002

A comparative electron spin resonance (ESR) study was performed on thermal (111) Si/SiO2 and (100) Si/SiO2 of the vacuum postoxidation-induced interface degradation in terms of interfacial trivalent Si dangling bond creation (ESR-active Pb, Pb0, and Pb1 defects). In (111) Si/SiO2, the degradation mechanism was isolated as pronounced permanent Pb (∘Si≡Si3) creation from ∼640 °C onward in densities Nc monotonically increasing with anneal temperature; at ∼1100 °C, about 1.1×1013 Pbs cm−2 are created in addition to the as-oxidized state value No∼4.9×1012 cm−2. The (100) Si/SiO2 interface is found to be much less vulnerable. Only electrically harmless Pb1’s are additionally created, the density reaching Nc(Pb1)∼4.4×1012 cm−2 at ∼1100 °C. By contrast, the density of the electrically adverse Pb0 trap tends to decrease. Together with the recently established electrical irrelevance of Pb1, the results add to provide a fundamental reason for the preference of the (100) Si face in devicing.
Mechanism of efficient and stable surface-emitting cold cathode based on porous polycrystalline silicon films
Authors: Komoda Takuya , Sheng Xia , Koshida Nobuyoshi
Published: July 2002

It is demonstrated that a porous polysilicon (PPS) diode with a structure of Au/PPS/n-type Si operates as an efficient stable surface-emitting cold cathode. 1.5 μm of an nondoped polysilicon layer is formed on an n-type (100) silicon wafer and anodized in a solution of HF (50%): ethanol=1:1 at a current density of 10 mA/cm2 for 30 s under illumination by a 500 W tungsten lamp from a distance of 20 cm. Subsequently, a PPS layer is oxidized in a rapid thermal oxidation furnace for 1 h at a temperature of 700 °C. A semitransparent thin Au film (about 10 nm thick) is deposited onto the PPS layer as a positive electrode and an ohmic contact is formed at the back side of the silicon wafer as a negative electrode. When a positive bias is applied to the Au electrode in vacuum, the diode uniformly emits electrons. No electron emission is observed in the negatively biased region. Emission current is about 10−4 A/cm2 at a 20 V bias. It is further demonstrated that electrons are quasiballistically emitted from a PPS diode due to a significantly reduced electron scattering in the PPS layer. As a result, the diode can emit fluctuation-free stable electron emission. The simplified model of emission and energy distribution of electrons are proposed and it can explain the experimental results.
Patterning of octadecylsiloxane self-assembled monolayers on Si(100) using Ar(3P0,2) atoms
Authors: Hill S. B. , Haich C. A. , Dunning F. B. , Walters G. K. , McClelland J. J. , Celotta R. J. , Craighead H. G. , Han J. , Tanenbaum D. M.
Published: July 2002

We report the use of metastable Ar(3P0,2) atoms and a physical mask to pattern octadecylsiloxane self-assembled monolayers grown directly on silicon surfaces. The damage to the monolayer is confirmed using lateral force microscopy, changes in hydrophilicity, and x-ray photoelectron spectroscopy analysis. Metastable atom exposures sufficient to uniformly damage the monolayer should allow pattern transfer to the underlying Si(100) substrate following chemical and plasma etching. With optical manipulation of the incident metastable atoms, this technique could provide the basis for massively parallel nanoscale fabrication on silicon.
Growth of high performance InGaAs/InP doped channel heterojunction field effect transistor with a strained GaInP Schottky barrier enhancement layer by gas source molecular beam epitaxy
Authors: Kuo H. C. , Moser B. G. , Hsia H. , Tang Z. , Feng M. , Stillman G. E. , Lin C. H. , Chen H.
Published: July 2002

In this article, we present a systematic investigation of the Schottky characteristics of metal/Ga0.2In0.8P/InP. It is found that barrier heights of 0.6–0.7 eV are achieved for Schottky junctions formed on Ga0.2In0.8P/InP. Fermi level pinning occurs at the surface of the wet-etched Ga0.2In0.8P prior to metallization. No degradation in the Schottky characteristics is observed at temperatures as high as 250 °C. InGaAs/InP doped channel heterojunction field-effect transistors (HFETs) with a Ga0.2In0.8P Schottky barrier enhancement layer (SBEL) were grown and fabricated. The 0.25 μm gate-length devices show excellent dc and rf performance, with an ft of 117 GHz and an fmax of 168 GHz. These results suggest that GaInP is a promising material as a gate SBEL for InP based HFET applications.
High resolution reactive ion etching of GaN and etch-induced effects
Authors: Cheung R. , Reeves R. J. , Rong B. , Brown S. A. , Fakkeldij E. J. M. , van der Drift E. , Kamp M.
Published: July 2002

We have developed a process using electron beam lithography and reactive ion etching for the high resolution pattern transfer of GaN. 150 nm dots have been fabricated in GaN successfully. Photoluminescence, scanning electron microscopy, and x-ray photoelectron spectroscopy have been employed to compare the damage inflicted on the GaN surfaces after SF6 and Ar plasma exposures. Near-band-edge luminescence analysis indicates the existence of a higher concentration of donors on the top 100 nm of the GaN surface after Ar as supposed to SF6 plasma exposure. An order of magnitude decrease in the ratio of the yellow to the band-edge luminescence intensity is found in the samples subjected to lower ion energies. Formation of pits is observed on the substrate surfaces after plasma treatment. Nitrogen deficient surfaces limited to the top few monolayers, as well as defect propagation down to 100 nm, exist in our plasma exposed GaN samples.
Dry oxidation resistance of ultrathin nitride films: Ordered and amorphous silicon nitride on Si(111)
Authors: Wallace Robert M. , Wei Yi
Published: July 2002

The integration of high-κ dielectrics, such as Ta2O5, in metal–insulator–semiconductor structures requires the careful control of interfacial silicon-oxide growth to preserve electrical properties such as capacitance. This has led to the introduction of silicon nitride films as oxidation resistant barriers between the dielectric and silicon. In this study, we examine the oxidation resistance of ultrathin (<2 nm) nitride films produced from NH3 exposure in ultrahigh vacuum are examined with in situ scanning tunneling microscopy and x-ray photoelectron spectroscopy. We find that ordered nitrides, grown at 950–1000 °C, are resistant to oxidation up to 600 °C for subatmospheric oxygen exposures. Amorphous nitrides, grown at 600 °C, exhibit similar oxidation resistance behavior.
Experimental and analytical study of seed layer resistance for copper damascene electroplating
Authors: Broadbent E. K. , McInerney E. J. , Gochberg L. A. , Jackson R. L.
Published: July 2002

Copper electroplating for integrated circuit damascene applications typically relies on a thin, conductive seed layer to promote nucleation of feature fill and acceptable overall thickness distribution during deposition. Important to the charge-transfer exchange process occurring at the wafer surface is the potential drop within the seed layer during the early stages of plating. The effective damascene surface area, seed layer coverage, and overall resistance of thin Cu films are examined via modeling and experimental measurements. The current-carrying capability of seed-metallized trenches is calculated in terms of an effective trench resistivity and used to compare physical vapor deposition (PVD) methods with chemical vapor deposition (CVD). For a nominal 2:1 aspect ratio trench, the modeled CVD seed resistance was found to be 5× lower than that of ionized PVD and about 2.8× lower than that of ionized PVD with resputter. Wafer-level resistance probing is introduced as a useful means to assess global differences in seed layer resistance on actual patterned substrates. Initial measurements on low-density patterns confirm that PVD seed films exhibit larger increases (+24% to 78%) in resistance than CVD films (+11%) when compared to their respective planar counterparts. Results indicate that seed resistance levels on patterned damascene substrates can be significantly larger than on planar monitor wafers—an important implication for the Cu plating manufacturing process.
Highly selective dry etching of III nitrides using an inductively coupled Cl2/Ar/O2 plasma
Authors: Lee Ji-Myon , Chang Ki-Myung , Lee In-Hwan , Park Seong-Ju
Published: July 2002

The selective etch characteristics of GaN, AlxGa1−xN, and InxGa1−xN have been examined in an inductively coupled plasma reactor using Cl2/Ar/O2 as the etchant gas. Etch rates and selectivities were strongly influenced by the flow rate of oxygen as well as the plasma parameters. Etch rates as high as 5500 Å/min were obtained for GaN, 1850 Å/min for Al0.05Ga0.95N, 420 Å/min for Al0.1Ga0.9N, and 2589 Å/min for In0.12Ga0.88N. Moreover, the etch selectivities of GaN and the In0.12Ga0.88N over the Al0.1Ga0.9N were as high as 24 and 32, respectively. These are the highest values ever reported for an AlGaN film with a relatively low Al composition (x=0.1). An x-ray photoelectron spectroscopy analysis of the etched surface showed that an Al–O bond was formed on the AlGaN surface during the Cl2/Ar/O2 plasma etching and the high selectivity thus obtained could be attributed to the etch-resistant aluminum oxide layer. This oxide layer could be easily etched off by a HF-based solution during the mask removal process.
Impact Event at the Permian-Triassic Boundary: Evidence from Extraterrestrial Noble Gases in Fullerenes
Authors: Becker Luann , Poreda Robert J. , Hunt Andrew G. , Bunch Theodore E. , Rampino Michael
Published: July 2002

The Permian-Triassic boundary (PTB) event, which occurred about 251.4 million years ago, is marked by the most severe mass extinction in the geologic record. Recent studies of some PTB sites indicate that the extinctions occurred very abruptly, consistent with a catastrophic, possibly extraterrestrial, cause. Fullerenes (C
60
to C
200
) from sediments at the PTB contain trapped helium and argon with isotope ratios similar to the planetary component of carbonaceous chondrites. These data imply that an impact event (asteroidal or cometary) accompanied the extinction, as was the case for the Cretaceous-Tertiary extinction event about 65 million years ago.
A Role for Thrombin Receptor Signaling in Endothelial Cells During Embryonic Development
Authors: Griffin Courtney T. , Srinivasan Yoga , Zheng Yao-Wu , Huang Wei , Coughlin Shaun R.
Published: July 2002

The coagulation protease thrombin triggers fibrin formation, platelet activation, and other cellular responses at sites of tissue injury. We report a role for PAR1, a protease-activated G protein–coupled receptor for thrombin, in embryonic development. Approximately half of
Par1
–/–
mouse embryos died at midgestation with bleeding from multiple sites. PAR1 is expressed in endothelial cells, and a PAR1 transgene driven by an endothelial-specific promoter prevented death of
Par1
–/–
embryos. Our results suggest that the coagulation cascade and PAR1 modulate endothelial cell function in developing blood vessels and that thrombin's actions on endothelial cells—rather than on platelets, mesenchymal cells, or fibrinogen—contribute to vascular development and hemostasis in the mouse embryo.
Requirement of Tissue-Selective TBP-Associated Factor TAF
II
105 in Ovarian Development
Authors: Freiman Richard N. , Albright Shane R. , Zheng Shuang , Sha William C. , Hammer Robert E. , Tjian Robert
Published: July 2002

Transcription factor TFIID, composed of TBP and TAF
II
subunits, is a central component of the RNA polymerase II machinery. Here, we report that the tissue-selective TAF
II
105 subunit of TFIID is essential for proper development and function of the mouse ovary. Female mice lacking TAF
II
105 are viable but infertile because of a defect in folliculogenesis correlating with restricted expression of TAF
II
105 in the granulosa cells of the ovarian follicle. Gene expression profiling has uncovered a defective inhibin-activin signaling pathway in TAF
II
105-deficient ovaries. Together, these studies suggest that TAF
II
105 mediates the transcription of a subset of genes required for proper folliculogenesis in the ovary and establishes TAF
II
105 as a cell type–specific component of the mammalian transcriptional machinery.
Identification of a Major Gene Regulating Complex Social Behavior
Authors: Krieger Michael J. B. , Ross Kenneth G.
Published: July 2002

Colony queen number, a major feature of social organization in fire ants, is associated with worker genotypes at the gene
Gp-9
. We sequenced
Gp-9
and found that it encodes a pheromone-binding protein, a crucial molecular component in chemical recognition of conspecifics. This suggests that differences in worker
Gp-9
genotypes between social forms may cause differences in workers' abilities to recognize queens and regulate their numbers. Analyses of sequence evolution indicate that regulation of social organization by
Gp-9
is conserved in South American fire ant species exhibiting social polymorphism and suggest that positive selection has driven the divergence between the alleles associated with alternate social organizations. This study demonstrates that single genes of major effect can underlie the expression of complex behaviors important in social evolution.
A Role for Interaction of the RNA Polymerase Flap Domain with the σ Subunit in Promoter Recognition
Authors: Kuznedelov Konstantin , Minakhin Leonid , Niedziela-Majka Anita , Dove Simon L. , Rogulja Dragana , Nickels Bryce E. , Hochschild Ann , Heyduk Tomasz , Severinov Konstantin
Published: July 2002

In bacteria, promoter recognition depends on the RNA polymerase σ subunit, which combines with the catalytically proficient RNA polymerase core to form the holoenzyme. The major class of bacterial promoters is defined by two conserved elements (the –10 and –35 elements, which are 10 and 35 nucleotides upstream of the initiation point, respectively) that are contacted by σ in the holoenzyme. We show that recognition of promoters of this class depends on the “flexible flap” domain of the RNA polymerase β subunit. The flap interacts with conserved region 4 of σ and triggers a conformational change that moves region 4 into the correct position for interaction with the –35 element. Because the flexible flap is evolutionarily conserved, this domain may facilitate promoter recognition by specificity factors in eukaryotes as well.
Evidence for Nuclear Emissions During Acoustic Cavitation
Authors: Taleyarkhan R. P. , West C. D. , Cho J. S. , Lahey R. T. , Nigmatulin R. I. , Block R. C.
Published: July 2002

In cavitation experiments with deuterated acetone, tritium decay activity above background levels was detected. In addition, evidence for neutron emission near 2.5 million electron volts was also observed, as would be expected for deuterium-deuterium fusion. Control experiments with normal acetone did not result in tritium activity or neutron emissions. Hydrodynamic shock code simulations supported the observed data and indicated highly compressed, hot (10
6
to 10
7
kelvin) bubble implosion conditions, as required for nuclear fusion reactions.
Visual Pigment Gene Structure and the Severity of Color Vision Defects
Authors: Neitz Jay , Neitz Maureen , Kainz Pamela M.
Published: July 2002

Rearrangements of the visual pigment genes are associated with defective color vision and with differences between types of red-green color blindness. Among individuals within the most common category of defective color vision, deuteranomaly, there is a large variation in the severity of color vision loss. An examination of specific photopigment gene sites responsible for tuning photopigment absorption spectra revealed differences that predict these variations in the color defect. The results indicate that the severity of the defect in deuteranomalous color vision depends on the degree of similarity among the residual photopigments that serve vision in the color-anomalous eye.
Does Leptin Contribute to Diabetes Caused by Obesity?
Authors: Taylor Simeon I. , Barr Valarie , Reitman Marc
Published: July 2002

One of the consequences of obesity is an increased risk of non-insulin-dependent diabetes. Cohen
et al
. present evidence in this week's issue (
p. 1185
) that this outcome may be mediated at least in part by leptin, a recently discovered drug that causes satiety. In their Perspective, Taylor
et al
. discuss this finding in the context of what is already known about insulin action and regulation of insulin resistance.
Participation of Presenilin 2 in Apoptosis: Enhanced Basal Activity Conferred by an Alzheimer Mutation
Authors: Wolozin Benjamin , Iwasaki Katsunori , Vito Pasquale , Ganjei J. Kelly , Lacanà Emanuela , Sunderland Trey , Zhao Boyu , Kusiak John W. , Wasco Wilma , D'Adamio Luciano
Published: July 2002

Overexpression of the familial Alzheimer's disease gene
Presenilin 2
(
PS2
) in nerve growth factor-differentiated PC12 cells increased apoptosis induced by trophic factor withdrawal or β-amyloid. Transfection of antisense
PS2
conferred protection against apoptosis induced by trophic withdrawal in nerve growth factor-differentiated or amyloid precursor protein-expressing PC12 cells. The apoptotic cell death induced by PS2 protein was sensitive to pertussis toxin, suggesting that heterotrimeric GTP-binding proteins are involved. A
PS2
mutation associated with familial Alzheimer's disease was found to generate a molecule with enhanced basal apoptotic activity. This gain of function might accelerate the process of neurodegeneration that occurs in Alzheimer's disease, leading to the earlier age of onset characteristic of familial Alzheimer's disease.
Associative Learning Disrupted by Impaired G
s
Signaling in
Drosophila
Mushroom Bodies
Authors: Connolly John B. , Roberts Ian J. H. , Armstrong J. Douglas , Kaiser Kim , Forte Michael , Tully Tim , O'Kane Cahir J.
Published: July 2002

Disruptions in mushroom body (MB) or central complex (CC) brain structures impair
Drosophila
associative olfactory learning. Perturbations in adenosine 3′,5′ monophosphate signaling also disrupt learning. To integrate these observations, expression of a constitutively activated stimulatory heterotrimeric guanosine triphosphate-binding protein α subunit (Gα
s
*) was targeted to these brain structures. The ability to associate odors with electroshock was abolished when Gα
s
* was targeted to MB, but not CC, structures, whereas sensorimotor responses to these stimuli remained normal. Expression of Gα
s
* did not affect gross MB morphology, and wild-type Gα
s
expression did not affect learning. Thus, olfactory learning depends on regulated G
s
signaling in
Drosophila
MBs.
Orientation Maps of Subjective Contours in Visual Cortex
Authors: Sheth Bhavin R. , Sharma Jitendra , Rao S. Chenchal , Sur Mriganka
Published: July 2002

Responses to subjective contours in visual cortical areas V1 and V2 in adult cats were investigated by optical imaging of intrinsic signals and single-unit recording. Both V1 and V2 contain maps of the orientation of subjective gratings that have their basis in specific kinds of neuronal responses to subjective orientations. A greater proportion of neurons in V2 than in V1 show a robust response to subjective edges. Through the use of subjective stimuli in which the orientation of the luminance component is invariant, an unmasked V1 response to subjective edges alone can be demonstrated. The data indicate that the processing of subjective contours begins as early as V1 and continues progressively in higher cortical areas.
Synapse Elimination--For Nerves It's All or Nothing
Authors: Frank Eric
Published: July 2002

At birth, several motoneurons supply neural signals to each muscle fiber, but 2 weeks later all but the input of a single neuron is gone. This process of synapse elimination has now been carefully described in electrophysiological terms in this week's issue in a report by Colman
et al
. (
p. 356
). In his Perspective, Frank explains how the new data fit with what we already know about the changes occurring during synapse elimination and predicts that molecular understanding of this key process will come soon.
Paradoxical Improvement of Impulse Conduction in Cardiac Tissue by Partial Cellular Uncoupling
Authors: Rohr Stephan , Kucera Jan P. , Fast Vladimir G. , Kléber André G.
Published: July 2002

Generally, impulse propagation in cardiac tissue is assumed to be impaired by a reduction of intercellular electrical coupling or by the presence of structural discontinuities. Contrary to this notion, the spatially uniform reduction of electrical coupling induced successful conduction in discontinuous cardiac tissue structures exhibiting unidirectional conduction block. This seemingly paradoxical finding can be explained by a nonsymmetric effect of uncoupling on the current source and the current sink in the preparations used. It suggests that partial cellular uncoupling might prevent the initiation of cardiac arrhythmias that are dependent on the presence of unidirectional conduction block.
Block in Anaphase Chromosome Separation Caused by a Telomerase Template Mutation
Authors: Kirk Karen E. , Harmon Brian P. , Reichardt Isabel K. , Sedat John W. , Blackburn Elizabeth H.
Published: July 2002

Telomeres are essential for chromosome stability, but their functions at specific cell-cycle stages are unknown. Telomeres are now shown to have a role in chromosome separation during mitosis. In telomeric DNA mutants of
Tetrahymena thermophila
, created by expression of a telomerase RNA with an altered template sequence, division of the germline nucleus was severely delayed or blocked in anaphase. The mutant chromatids failed to separate completely at the midzone, becoming stretched to up to twice their normal length. These results suggest a physical block in mutant telomere separation.