bluuebunny/crossref_metadata_2025 huggingface dataset:
Charge detector realization for AlGaAs/GaAs quantum-dot cellular automata
Authors: Bazán G. , Orlov A. O. , Snider G. L. , Bernstein G. H.
Published: July 2002

We perform measurements on an AlGaAs/GaAs double-quantum-dot structure, where dots are separated by an opaque barrier and each dot conductance is measured independently and simultaneously. We measure the Coulomb blockade oscillations (CBOs) for each dot when the structure is configured for one and two dots. When configured as a single-dot device, we sweep the backgate and observe different CBO periods for each dot measured independently, implying dots of different sizes. When the device is configured for two dots, we observe strongly modulated CBOs in the larger dot while CBOs in the smaller dot exhibit almost no influence due to the changing charge of the larger dot. From this experiment, we have realized a charge detection scheme where we observe strong coupling in the detector signal in addition to the detector exhibiting minimal effect on the dot being measured. For an implementation of quantum-dot cellular automata (QCA), (1) cells must couple capacitively and (2) one must be able to detect electron occupation of a quantum dot within a cell. With this investigation, we demonstrate these two key components required for QCA in AlGaAs/GaAs materials.
Sharpening Si field emitter tips by dry etching and low temperature plasma oxidation
Authors: Rakhshandehroo M. R. , Pang S. W.
Published: July 2002

Dry etching and low temperature plasma oxidation were used to sharpen Si field emitter tips. Si emitters with tip radii of 67 nm were formed by dry etching in a Cl2 plasma at 5 mTorr. An electron cyclotron resonance (ECR) source was used to generate the plasma and erosion of a SiO2 mask was employed to control the Si emitter geometry. The Si emitters were sharpened by dry etching in the Cl2 plasma generated at 0.7 and 5 mTorr after the SiO2 mask was completely eroded. After etching the emitters for 7 min at 0.7 mTorr, the tip radius decreased from 67 to 22 nm. The sharpening effect could be attributed to enhanced sputtering yield at sloped surfaces. At higher pressure of 5 mTorr, the sharpening effect was reduced and the tip radius only decreased from 67 to 62 nm. Optical emission spectroscopy and mass spectrometry were used to monitor the etching and sharpening of Si emitter tips. Sharp increases in the Si emission signal at 288.1 nm and mass spectrometric signal for 63SiCl+ were observed once the SiO2 mask was completely eroded and the sharpening of Si emitters had started. Plasma oxidation was also used to sharpen the emitter tips. With the stage at room temperature, up to 257 nm of plasma oxide was grown in an O2 plasma generated at 0.3 mTorr by the ECR source. After oxide removal, the emitters tip radius was found to decrease from 67 to 8 nm. In comparison, the same amount of oxide grown by thermal dry oxidation at 950 and 1100 °C only reduced the tip radius to 32 and 50 nm, respectively. Sharper emitter tips and faster oxidation rate can be obtained by plasma oxidation.
A proximity ion beam lithography process for high density nanostructures
Authors: Wolfe John C. , Pendharkar Sandeep V. , Ruchhoeft Paul , Sen Sudipto , Morgan Mark D. , Horne W. E. , Tiberio R. C. , Randall John N.
Published: July 2002

Image contrast in proximity ion beam lithography is limited by scattered ions which enter the opaque regions of the mask and exit through the sidewalls of the mask windows. The scattering angles are widely distributed resulting in a ‘‘proximity effect’’ whose range is on the order of the mask-to-wafer gap. This problem becomes more severe with increasing pattern density and sets the resolution limit for high density patterns such as interdigital transducers. The only way to counteract this effect is to limit the ion range to a fraction of the mask thickness so that the scattered ions can be recaptured by adjacent sidewalls. This article explores the dependence of image contrast on resolution, pattern density, and beam energy in proximity ion beam lithography. Patterns with feature sizes in the range from 20 to 50 nm and 0.4 μm pitch have been printed with a linewidth change of only 3 nm for a 10% change in dose.
Ionized physical vapor deposition of Cu for high aspect ratio damascene trench fill applications
Authors: Nichols C. A. , Rossnagel S. M. , Hamaguchi S.
Published: July 2002

The ionized physical vapor deposition technique is used to fill high aspect ratio trenches with copper. This technique allows directional filling of embedded features, known as damascene, by sputtering metal atoms into a high density plasma. Large metal-atom ionized-flux fractions are achievable (≊85%) leading to high directionality of deposition at the biased substrate. In this article, we report quantitative measurements of fill directionality of Cu using an inductively coupled plasma (ICP) high density source. Copper is deposited into fairly aggressive (depth/width ≤1.5) damascene trenches. Metal ion flux fractions are estimated from direct measurement of the trench step coverage and compared to simulation. Estimates of the Cu+/Ar+ density ratios are also made to understand the influence of applied ICP power and Cu atom density (magnetron power) on fill directionality. It is found that at high magnetron powers (high copper atom densities) the plasma becomes ‘‘copper rich,’’ where the flux of copper ions exceeds that of the argon ions. At low magnetron power and high ICP power, we find the trench fill to be highly directional. As magnetron power is increased, directionality suffers due to cooling of the plasma by higher copper atom flux.
Pseudomorphic high-electron-mobility transistors with low-temperature-grown GaAs buffers
Authors: Rogers T. J. , Nichols K. B. , Kopp W. F. , Smith F. W. , Actis R.
Published: July 2002

Low-temperature-grown GaAs buffers are incorporated into pseudomorphic high-electron-mobility transistors in order to improve channel breakdown. X-ray diffraction measurements are used to determine the reproducibility of arsenic incorporation into low-temperature-grown GaAs layers. Hall measurement results from pseudomorphic high-electron-mobility transistors structures show little reduction in channel electron mobility and sheet density when the spacer between the low-temperature-grown buffer and the channel is greater than ∼90 nm. Pseudomorphic high-electron-mobility transistors devices using the improved buffer have shown a 45% increase in channel breakdown voltage, a 12% increase in rf power output, and an 8% increase in power-added efficiency at 20 GHz when contrasted with the conventional-buffered pseudomorphic high-electron-mobility transistors.
Physical limits on gigascale integration
Authors: Meindl James D.
Published: July 2002

Future opportunities for gigascale integration (GSI) will be governed by a hierarchy of theoretical and practical limits whose levels can be codified as (1) fundamental, (2) material, (3) device, (4) circuit, and (5) system. Theoretical system limits indicate that in rank order the most serious technological problems impeding progress towards GSI are related to (1) interconnections, (2) heat removal, and (3) transistors. Practical limits on the number of transistors per chip can be extended only if the manufacturing cost per function of GSI continues to decline, which will require, among other advances, enormous progress in semiconductor characterization.
Practical perspective of shallow junction analysis
Authors: Heimbrook L. A. , Baiocchi F. A. , Bittner T. C. , Geva M. , Luftman H. S. , Nakahara S.
Published: July 2002

This paper will provide an industrial perspective comparing the semiconductor industry’s requirements for characterization of ultrashallow junctions versus the available methods for practical analysis. The continued shrinking of integrated circuit features, resulting in more complex functions per chip, place increased demands on the analytical laboratory. The challenge is to balance the need for timely customer response with leading edge analysis of the highest possible precision and accuracy. The technical trends, focusing primarily on the current and future analysis needs of silicon technology, will be discussed from both the customer’s and analyst’s point of view. The current tools for characterization of ultrashallow dopant profiles and the associated general nanotechnology will be described with emphasis on current measurement limitations and future analysis needs from a practical perspective. The gap between the semiconductor industry’s analysis needs and the actual information which can currently be provided on shallow junctions will be summarized.
Effects of electric potentials on surface forces in electrolyte solutions
Authors: Arai Toyoko , Fujihira Masamichi
Published: July 2002

Atomic force microscopy was employed to analyze the electric double layer on an Au film electrode deposited on mica. An Au-covered or a bare Si3N4 tip was used as a probe. An electrolyte solution was aqueous NaOH of pH 10.9. The electrode and the tip potentials were controlled electrochemically by a potentiostat. Repulsive forces were always observed regardless of the polarity of the potential, i.e., the positive and the negative side of the point of zero charge. The result indicates that the strongly adsorbed OH− anions overcompensated the positive charge at the Au surface. The potential of the outer Helmholtz plane at an electrode potential was estimated from the force–distance curves.
Atomic force microscopy stress sensors for studies in liquids
Authors: O’Shea S. J. , Welland M. E. , Brunt T. A. , Ramadan A. R. , Rayment T.
Published: July 2002

Simple, sensitive, and fast sensors can be constructed from standard atomic force microscopy cantilevers. For use in liquid environments, it is preferable to use changes in surface stress as the measurement basis of the sensor. We have constructed such a sensor in an electrochemical cell so that simultaneous cyclic voltammograms and strain measurements can be made. This is demonstrated with examples of underpotential deposition of Pb on Au(111) and electrocapillary effects in KCl.
Scanning force microscopy of polyolefinic rubbers in homopolypropylene matrices
Authors: Labardi M. , Allegrini M. , Marchetti E. , Sgarzi P.
Published: July 2002

Multiphase polyolefins are important materials for industrial applications because of the range of mechanical properties obtainable as a function of nature and composition of the constitutive polymers. We have investigated the phase dispersion of crystalline and amorphous components by atomic force microscopy in the force modulation mode. This technique is able to detect the presence of the olefinic components by means of their elasticity difference. Data concerning a number of different blends are presented and compared to TEM micrographs, where constrast can be obtained exclusively by sample treatment with heavy elements.
Initial stages of In adsorption on Si(111) 7×7
Authors: Lin X. F. , Mai H. A. , Chizhov I. , Willis R. F.
Published: July 2002

Scanning tunneling microscopy and scanning tunneling spectroscopy have been used to study the initial stages of room-temperature deposition of In on a Si(111) 7×7 surface. Individual In atoms are observed residing atop the substrate Si adatoms and rest atoms at T1 sites, showing different geometric configurations. An analysis of the In atomic structures in the dual-bias scanning tunneling microscopy images leads us to conclude that the In half-filled pz-like sp3 hybrids covalently bond to the Si partially filled pz-like sp3 hybridized orbitals and the In px,y-like sp3 hybrids overlap with these on the nearby metal atoms, also showing the covalent bonding. This is also supported by the scanning tunneling spectroscopy spectrum energy measurements about the Fermi level where the covalent bonding between the In and Si saturates the Si intrinsic metallic surface states, producing a surface band gap ∼1.7 eV.
Growth mechanism of planar-type GaAs nanowhiskers
Authors: Haraguchi K. , Hiruma K. , Hosomi K. , Shirai M. , Katsuyama T.
Published: July 2002

The mechanism for the lateral growth of ultrathin GaAs whiskers is discussed in connection with the vapor–liquid–solid growth model. The observed growth rate shows that the migration of the source material plays an essential role in such whisker growth.
Second harmonic spectroscopy of Si(001) surfaces: Sensitivity to surface hydrogen and doping, and applications to kinetic measurements
Authors: Xu Z. , Hu X. F. , Lim D. , Ekerdt J. G. , Downer M. C.
Published: July 2002

We report a spectroscopic second harmonic (SH) study of Si(001) surfaces under both ultrahigh vacuum and epitaxial growth conditions which includes in situ azimuthal rotation of the sample. The results show a strong influence of surface hydrogen termination and bulk doping concentration on the SH spectrum and azimuthal anisotropy. We propose a qualitative model in which a combination of bulk and surface electric-field induced SH polarizations, which augments the usual surface dipole and bulk quadrupole SH polarizations, are primarily responsible for the sensitivity to surface hydrogen and bulk doping. We also report an in situ real-time SH study of disilane adsorption onto and hydrogen desorption from Si(001) surfaces during epitaxial growth. The abilities to detect bulk doping concentration and monitor the growth rate in real time make SH spectroscopy promising as a growth sensor.
Subnanometer analysis of molecular beam epitaxy grown ternary arsenides
Authors: Seaford M. L. , Wu W. , Eyink K. G. , Tomich D. H. , Tucker J. R. , Eastman L. F.
Published: July 2002

InGaAs/InAlAs modulation-doped field-effect transistors (MODFETs) grown by molecular beam epitaxy on InP have been characterized using Hall and cross-sectional scanning tunneling microscopy (XSTM). Spacer layer to quantum well interface XSTM images will be presented showing cluster sizes ranging from 20 to 50 Å for normal growth conditions. The extent of the clustering experimentally found is greater than that predicted by theory for phase separation due to kinetics or bulk thermodynamics. The extent of clustering was not found to be a function of growth temperature while the morphology and cluster orientation was a function of growth temperature. At normal growth temperatures, the clusters tended to self align into binary corridors. Control (and removal) of these binary corridors allowed the growth and fabrication of MODFETs with Schottky barrier heights greater than 1 eV.
Control in sub-100 nm lithography in SAL-601
Authors: Dobisz Elizabeth A. , Marrian Christie R. K.
Published: July 2002

This article focuses on e-beam nanolithography in SAL-601, a negative tone, chemically amplified resist from the Shipley Corporation. It examines the role of exposure conditions and the postexposure bake (PEB) on the ability to define features of critical dimensions of 100 nm and below. Lithography was performed with a 50 kV e beam and 140 nm thick resist, where forward scattering of the e beam is expected to be minimal. Common PEB conditions of 105 °C for 1, 3, and 10 min and 110 °C for 1 min were investigated. Although the different PEB conditions produced a factor of up to 2.4× increase in sensitivity, no change was observed in the contrast, 5.8±0.1, or the minimum linewidths, ∼60 nm. The measured line spread function forward Gaussian widths were characterized by standard deviations, σ=27–33 nm, depending on PEB conditions. These are compared to σ of 19 nm for PMMA and the expected σ of 11 nm for the incident probe (σ=10 nm) convoluted with a Monte Carlo code generated point spread function. The measured backscatter coefficient was 0.51, in agreement with our Monte Carlo simulation results. In the analysis of dose latitude to form critical dimension gaps (±10% precision) between a series of pads of differing sizes, the different PEB conditions produced no observed difference. The feature size, however, had a large effect on the ability to form the critical dimension gap. Furthermore, the dose latitude was much smaller than calculated by integration of Monte Carlo derived line spread functions. A semiempirical model is introduced in which the forward Gaussian width of a measured line spread function (LSF) in developed resist is convoluted with the Monte Carlo code LSF. The calculated exposure profiles based on the semiempirical LSF show much better agreement with the experiment than those based on the expected, σ=11 nm, forward Gaussian width.
Minimum critical defects in extreme-ultraviolet lithography masks
Authors: Lin Yun , Bokor Jeffrey
Published: July 2002

We have performed aerial image simulations to characterize critical defects on extreme-ultraviolet lithography masks based on their interactions with typical mask patterns. Instead of the conventional critical dimension criteria, an exposure–defocus process window allowing a 10% critical dimension variation was used as the criteria for defining the critical defects. We also carried out aerial image simulations of isolated defects to obtain information for future reticle blank defect inspection needs.
Fabrication and behavior of nanoscale field emission structures
Authors: Driskill-Smith A. A. G. , Hasko D. G. , Ahmed H.
Published: July 2002

Novel field emission devices with dimensions an order of magnitude less than the elastic mean free path of electrons in air have been fabricated and tested at atmospheric pressure. It is well known that the operating voltage of field emission devices may be decreased by reducing the radius of the emitter tip and the distance between the emitter tip and extractor electrode. We have developed and tested a nanoscale-tip field emission system consisting of multiple emitter tips with radii of about 1 nm within an extractor aperture 50 nm in diameter. All nanopillar tips were less than 30 nm from the extractor electrode in both the horizontal and vertical directions, so field-emitted electrons traveled ballistically from the nanopillar tips to the extractor electrode even at atmospheric pressure. The extractor turn-on voltage was approximately 7 V; field-emitted currents of about 10 nA were collected typically at an extractor voltage of 9 V.
Gate technology for 70 nm metal–oxide–semiconductor field-effect transistors with ultrathin (<2 nm) oxides
Authors: Tennant D. , Klemens F. , Sorsch T. , Baumann F. , Timp G. , Layadi N. , Kornblit A. , Sapjeta B. J. , Rosamilia J. , Boone T. , Weir B. , Silverman P.
Published: July 2002

Results are described for a gate level technology module developed to produce metal–oxide–semiconductor transistors with physical gate lengths of 70 nm and below. Lithography is performed by direct write e-beam lithography (EBL) using a thermal field-emission EBL system in SAL 601 resist. Critical dimension (CD) control, as measured by several methods, is found to depend not only on dose control but also on writing parameters such as pixel spacing. The pattern transfer using a silicon dioxide hard mask is shown to exhibit a trade-off between anisotropy and selectivity. Transmission electron microscopy cross sections reveal that two atomic layers are removed even when the gate oxide stopping layer is completely intact. We report results for gate lengths down to 60 nm with edge roughness on the order of 5 nm, within the acceptable limits for threshold requirements, while stopping the etch process on oxides as thin as 1.2 nm.
Ultrahigh depth resolution secondary ion mass spectrometry with sub-keV grazing O2+ beams
Authors: Alkemade P. F. A. , Jiang Z. X. , Visser C. C. G. , Radelaar S. , Arnoldbik W. M.
Published: July 2002

Compositional analysis of interfaces in semiconductor materials grown with the most modern equipment requires a substantial improvement of the depth resolution of secondary ion mass spectrometry (SIMS). The lowering of the impact energy to improve depth resolution is limited on most magnetic-sector instruments to ∼1.5 keV. In this work it is shown that in the VG IX70S magnetic-sector instrument a reduction of the impact energy to 600 eV is possible. The reduction is achieved by use of a deceleration electrode in the primary beam line, allowing for independent variation of the energy and the incidence angle θ (50°<θ<80°). The best depth resolution obtained—for a shallow Ge δ layer with a 600 eV θ=80° O2+ beam—was 1.6 nm full width at half-maximum and an exponential decay length of 0.65 nm; about three times better than achievable on any other magnetic-sector SIMS instrument. In addition, a very shallow (∼3–4 nm) Ge δ layer has been analyzed by high-resolution Rutherford backscattering and SIMS. There is good agreement between both techniques.
Fabrication and emission characteristics of GaAs tip and wedge-shaped field emitter arrays by wet etching
Authors: Ducroquet F. , Kropfeld P. , Yaradou O. , Vanoverschelde A.
Published: July 2002

GaAs tip and wedge-shaped field emitter arrays were fabricated by a wet etching technique on a (100) GaAs substrate. Two chemical solutions whose compositions were chosen for their isotropy features were investigated: HCl:H2O2:H2O (40:4:1) and HF:HNO3:H2O (1:1:2). When progressively polarized and after a working period of stabilization, the GaAs tip or wedge arrays are found to supply a stable emission current, with fluctuations not exceeding 3% over some hours. The analysis of the surface before emission shows the presence of a thin oxide layer which is suggested to act as a passivation layer.
Hafnium carbide films and film-coated field emission cathodes
Authors: Mackie W. A. , Xie Tianbao , Blackwood J. E. , Williams S. C. , Davis P. R.
Published: July 2002

We have previously reported on field emission improvements in turn-on voltages and emission stability using ZrC films as coatings on Si and Mo single emitters and emitter arrays. However, during our emission studies of bulk carbides, HfC was found to be slightly superior. We now report on work in progress investigating HfC films and HfC film coated field emission cathodes. Uses for arrays of these field emission cathodes range from video displays to microwave applications. This article deals with physical vapor deposition of HfC, absolute work function measurements, and electron emission properties of these film surfaces. This work demonstrates improvements by using HfC films over films of ZrC and an associated surface work function lowering of more than 1 eV in some instances compared to the clean surface.
Fabrication and characterization of mesoscopic superconductor–semiconductor hybrid structures
Authors: Huber R. , Rahman F. , Thornton T. J. , Norman A. , Stradling R. A.
Published: July 2002

We have deposited superconducting niobium films on high mobility InAs:GaSb quantum wells. The films are patterned with gaps containing up to four niobium lines. The size of the lines and the spacing between them is less than the electron mean free path in the quantum well and ballistic effects are therefore expected to play a role in the current transport. Differential resistance measurements show evidence of supercurrents of up to 10 μA in each of the hybrid structures as well as enhanced conductivity due to Andreev reflection. The supercurrents are suppressed by magnetic fields of order 50 mT. At higher fields, the resistance minimum associated with Andreev reflection evolves into a resistance maximum for fields such that the cyclotron diameter is smaller than the gaps between the niobium. A detailed explanation of the fabrication process is also presented.
Properties of CeO2 thin films deposited on Si(100) and Si(111) substrates by radio frequency-magnetron sputtering
Authors: Jang S. H. , Jung D. , Roh Y.
Published: July 2002

CeO 2 thin films were grown on Si(100) and Si(111) substrates by radio frequency-magnetron sputtering. The growth temperature and the substrate orientation have significant influences on the preferred orientations of deposited CeO2 films. X-ray diffractometry and transmission electron microscopy analyses showed that CeO2 on Si(111) has a better preferred orientation in the direction of the substrate orientation than CeO2 on Si(100). CeO2 films deposited on Si(111) substrates maintain a preferred orientation better than CeO2 films on Si(100), when they are subjected to annealing at 900 °C in O2 atmosphere for 30 min. Rutherford backscattering spectra taken of CeO2/Si before and after annealing showed that CeO2 has strong thermal stability.
Stabilizing the surface morphology of Si1−x−yGexCy/Si heterostructures grown by molecular beam epitaxy through the use of a silicon-carbide source
Authors: Croke E. T. , Vajo J. J. , Hunter A. T. , Ahn C. C. , Chandrasekhar D. , Laursen T. , Smith David J. , Mayer J. W.
Published: July 2002

Si 1−x−y Ge x C y / Si superlattices were grown by solid-source molecular beam epitaxy using silicon carbide as a source of C. Samples consisting of alternating layers of nominally 25 nm Si1−x−yGexCy and 35 nm Si for 10 periods were characterized by high-resolution x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry to determine strain, thickness, and composition. C resonance backscattering and secondary ion mass spectrometries were used to measure the total C concentration in the Si1−x−yGexCy layers, allowing for an accurate determination of the substitutional C fraction to be made as a function of growth rate for fixed Ge and substitutional C compositions. For C concentrations close to 1%, high-quality layers were obtained without the use of Sb-surfactant mediation. These samples were found to be structurally perfect to a level consistent with cross-sectional TEM (<107 defects/cm2) and showed considerably improved homogeneity as compared with similar structures grown using graphite as the source for C. For higher Ge and C concentrations, Sb-surfactant mediation was found to be required to stabilize the surface morphology. The maximum value of substitutional C concentration, above which excessive generation of stacking fault defects caused polycrystalline and/or amorphous growth, was found to be approximately 2.4% in samples containing between 25 and 30% Ge. The fraction of substitutional C was found to decrease from roughly 60% by a factor of 0.86 as the Si1−x−yGexCy growth rate increased from 0.1 to 1.0 nm/s.
Metal/GaN Schottky barriers characterized by ballistic-electron-emission microscopy and spectroscopy
Authors: Bell L. D. , Smith R. P. , McDermott B. T. , Gertner E. R. , Pittman R. , Pierson R. L. , Sullivan G. J.
Published: July 2002

Ballistic-electron-emission microscopy (BEEM) and spectroscopy have been used to characterize the Pd/GaN and Au/GaN interfaces. BEEM spectra yield a Schottky barrier height for Au/GaN of ∼1.05 eV that agrees well with the highest values measured by conventional methods. For both Pd and Au, a second threshold is observed in the spectra at about 0.2–0.3 V above the first threshold. Imaging of these metal/GaN interfaces reveals transmission in nearly all areas, although the magnitude is small and spatially varies. Attempts to perform BEEM measurements on other GaN material have resulted in no detectable transmission in any areas, even at voltages as high as 3.5 V.
Native extended defects in Zn1−yCdySe/InxGa1−xAs heterostructures
Authors: Müller B. , Heun S. , Lantier R. , Rubini S. , Paggel J. J. , Sorba L. , Bonanni A. , Lazzarino M. , Bonanni B. , Franciosi A. , Napolitani E. , Romanato F. , Drigo A. , Bonard J.-M. , Ganière J.-D. , Lazzarini L. , Salviati G.
Published: July 2002

Lattice-matched Zn1−yCdySe/InxGa1−xAs heterojunctions can be fabricated by molecular beam epitaxy on GaAs(001) 2×4 surfaces in a wide range of compositions provided that a suitable strain relaxation profile is achieved within the ternary III–V buffer layer. We focus here on the structural properties of the resulting II–VI/III–V heterostructures and discuss the distribution of native defects, including misfit and threading dislocations, stacking faults, and surface corrugations.
Microfabrication and characterization of gridded polycrystalline silicon field emitter devices
Authors: Huq S. E. , Huang M. , Wilshaw P. R. , Prewett P. D.
Published: July 2002

Uniform arrays of sharp polysilicon field emitters have been fabricated in gridded configuration using state of the art microfabrication techniques including high resolution electron beam lithography and plasma dry etching. Emission currents up to 2.5 μA/tip have been obtained at a 90 V grid-bias. Preliminary lifetime measurements (14 h continuous operation) have been carried out under ultrahigh vacuum conditions.
Optically induced mask critical dimension error magnification in 248 nm lithography
Authors: Randall John N. , Tritchkov Alexander
Published: July 2002

One form of optical proximity effect that further complicates lithography is the unexpected response of the printed image to small perturbations [critical dimension (CD) errors] in the reticle. In this way mask CD errors are actually magnified (they are reduced by less than the reduction factor of the optics) during the optical transfer to the wafer. This effect will require even tighter specifications for mask CD control when the error magnification factor is significantly above unity. The effect is particularly pronounced for tight pitches of small features, but can also impact the printing of small isolated lines. Both resist and optical nonideal responses are involved in this mask error factor (MEF). This article discussed the optical effects that produce the MEF. This article will show where the MEF due to optical effects can be ignored and where they cannot when using 248 nm lithography with a high numerical aperture (NA) tool. We will demonstrate how the NA, partial coherence, and variations in focus can effect the mask error magnification factor. We will also show that resolution enhancement techniques can be used to reduce the mask error magnification factor. In particular, Levenson phase shift masks show particularly low mask error magnification factors for small lines. For some applications it should be possible to design the mask so that the mask error magnification factor of the smallest features is significantly below unity. This would allow loosened reticle CD specifications and/or better CD control of the lithography process.
Direct patterning of single electron tunneling transistors by high resolution electron beam lithography on highly doped molecular beam epitaxy grown silicon films
Authors: Koester T. , Goldschmidtboeing F. , Hadam B. , Stein J. , Altmeyer S. , Spangenberg B. , Kurz H. , Neumann R. , Brunner K. , Abstreiter G.
Published: July 2002

We present a fabrication method for a single electron tunneling transistor (SETT) in silicon. The process is based on bonded and etched back silicon on insulator material with a 40 nm thick highly n-doped Si layer grown by molecular beam epitaxy. The nanometer structure of the SETT is defined by electron beam lithography in combination with a two-layer resist system. The pattern is transferred by anisotropic reactive ion etching. The devices are passivated by low temperature remote plasma enhanced chemical vapor deposition of high quality silicondioxide. An extended region of low conductivity is observed even at T=130 K in the current–voltage characteristics, outside of which a strong Coulomb staircase is visible. The Coulomb blockade is significantly affected by the applied gate voltage. Coulomb oscillations of the blockade width with gate potential are observed.
Critical issues in 157 nm lithography
Authors: Bloomstein T. M. , Rothschild M. , Kunz R. R. , Hardy D. E. , Goodman R. B. , Palmacci S. T.
Published: July 2002

Projection lithography at 157 nm is a candidate technology for the 100–70 nm generations, and possibly beyond. It would provide an evolutionary extension to the current primary photolithographic processes and components: excimer lasers, refractive optics, and transmissive masks. This article presents data on the transmission of optical materials at 157 nm, the performance of optical coatings, the issues that must be faced by photomasks, and the considerations related to engineering resists at this wavelength.
Electron field emission from a patterned diamondlike carbon flat cathode
Authors: Mao D. S. , Zhao J. , Li W. , Ren C. X. , Wang X. , Liu X. H. , Zhou J. Y. , Fan Z. , Zhu Y. K. , Li Q. , Xu J. F.
Published: July 2002

A 300-nm-thick diamondlike carbon (DLC) film was deposited on to a heavily doped n-type Si (111) wafer by filtered arc deposition. A flat thin film cylindrical emitter array was then fabricated by reactive-ion etching the DLC film. Its electron field emission properties were studied using a simple diode structure. A field emission current of 0.1 μA was detected under an electric field of 5.2 V/μm. As large as 64.1 mA/cm2 of field emission current density was achieved after an activation process under a 39 V/μm field. An image formed on the anode screen showed that electron field emission from the DLC flat emitter array is rather uniform. The field emission behavior is also consistent with Fowler–Nordheim theory. Hydrogen plasma surface treatments were found to enhance the field emission properties.
Schottky enhancement of contacts to n-GaAs via the exchange mechanism using NiAlxGa1−x as a metallization
Authors: Chen C.-P. , Lin C.-F. , Swenson D. , Kao C. R. , Jan C.-H. , Chang Y. A.
Published: July 2002

Based on the thermodynamic/kinetic model of the exchange mechanism, the ternary intermetallic compound NiAlxGa1−x (where 0<x⩽1) was identified as a metallization that may be used to fabricate Schottky enhanced contacts to n-GaAs. Experimental phase equilibrium studies of the quaternary Al-Ga-Ni-As system, in conjunction with diffusion data available in the literature, indicated that the phase NiAlxGa1−x fulfills the thermodynamic and kinetic requirements necessary for participation in an exchange reaction with GaAs. Contacts to n-GaAs were fabricated by sputter deposition of NiAlxGa1−x metallizations, with compositions corresponding to x=0.00, 0.25, 0.50, 0.75 and 1.00. These contacts were subjected to rapid thermal processing, and analyzed using cross-sectional high resolution transmission electron microscopy and I-V characterization. Electron microscopy and concomitant electron dispersive spectroscopic analysis indicated that a very thin (2.5 nm) interfacial region of AlxGa1−xAs was formed in annealed contacts for which x>0.00, in accordance with the exchange mechanism model. Schottky barrier enhancement was also observed in all annealed contacts for which x>0.00. The degree of Schottky barrier enhancement was shown to be dependent upon the initial composition of the metallization, again in accordance with the prediction of the exchange mechanism model. Schottky barrier heights as high as 0.96 eV were obtained under the optimum annealing conditions of 400 °C for 1 min. However, these experimentally determined Schottky barrier heights were somewhat smaller than the values that were anticipated based upon the exchange mechanism model. Potential reasons for these discrepancies were discussed. Overall, it was demonstrated that the thermodynamic/kinetic model of the exchange mechanism is a powerful tool for identifying metallizations that may be used to enhance the Schottky barriers of contacts to n-GaAs.
Transfer-matrix quantum-mechanical theory of electronic field emission from nanotips
Authors: Mayer A. , Vigneron J.-P.
Published: July 2002

The ability of the transfer-matrix methodology to efficiently simulate quantum-mechanical field emission from nanotips is demonstrated. This general methodology, well suited to the peculiar geometry of field emission systems where electrons tunnel from an emitter to a collector, is limited in use when numerical instabilities are not controlled. In this article, the method is extended to deal with representations implying rectangular transfer matrices and to provide results with an explicit accuracy evaluation. The usefulness of the layer addition procedure for improving the accuracy and the range of validity of the method is then clearly established. This accuracy evaluation extension enables computations with optimal accuracy. For the specific field emission situation, this methodology is applied with an adequate wave-function representation that takes advantage of a possible n-fold symmetry axis. Field emission computations are presented. The electronic source is represented by a pyramidal protrusion that consists of four atomic layers. A circular aperture is placed on the collector, facing the tip. The modifications in the resulting diffraction figures, induced by successive removal of the constitutive layers of the tip, reveal the necessity of a monoatomic termination for applications in the Fresnel projection microscope.
Nonlithographic technique for the production of large area high density gridded field emission sources
Authors: Holland E. R. , Harrison M. T. , Huang M. , Wilshaw P. R.
Published: July 2002

We report a novel method for the production of high density arrays of gridded microtip field emission electron sources. The microporous structure of Al2O3, formed by anodization of aluminum, is exploited to provide a high density (5×108 cm−2) of sites for emitter fabrication without the need for individual lithographic patterning of each emitter. The small size of the resultant emitters enables operation with only moderate extraction voltage (20–50 V). Tests of groups of ≈200 000 emitters have demonstrated emission current densities suitable for display applications.
Growth of a near-atomic protrusion on molybdenum field emitter tips under argon ion bombardment
Authors: Okuyama F. , Sugie H. , Sato M.
Published: July 2002

Ar + ion bombardment of a molybdenum field emitter is shown to grow a single protrusion with a near-atomic dimension at the very tip of the emitter. This kind of protrusion is generated from a thick amorphous layer covering the monocrystalline emitter tip, and lengthens toward the incoming ions. The protrusion apex is roughened, resulting in atomic projections on its surface. This surface phenomenon may be developed into a technique to prepare metal tips with atomically small dimensions.
Electric force microscopy of induced charges and surface potentials in GaN modified by light and strain
Authors: Bridger P. M. , Bandić Z. Z. , Piquette E. C. , McGill T. C.
Published: July 2002

We have studied molecular beam epitaxy grown GaN films using electric force microscopy to detect sub-1 μm regions of electric field gradient and surface potential variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variation in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy, and can be additionally modified by externally applied strain and illumination. The electron force microscopy signal was found to be a function of the applied tip bias, showed reversal under externally applied strain, and was sensitive to above band gap illumination.
Application of 4-methyl-1-acetoxycalix[6]arene resist to complementary metal–oxide–semiconductor gate processing
Authors: Rooks Michael J. , Aviram Ari
Published: July 2002

Gate lengths from 10 to 30 nm are beyond the reproducible limits of chemically amplified negative resists. Gate stack etching, the most challenging step, requires a negative, nonmetallic resist capable of withstanding enhanced etching with fluorine plasmas. This paper describes a new negative resist, 4-methyl-1-acetoxycalix[6]arene (MAC) which has demonstrated 10 nm scale resolution and etch resistance matching that of novolaks. Because increased etch resistance is required to allow reasonable resist aspect ratios, this work addresses techniques for increasing the etch resistance of MAC resist. The major disadvantage of MAC is its very low sensitivity; to address this we present a technique for combining the use of UVN with MAC.
Nonmonotonic behavior of the scanning capacitance microscope for large dynamic range samples
Authors: Stephenson Robert , Verhulst Anne , De Wolf Peter , Caymax Matty , Vandervorst Wilfried
Published: July 2002

The phenomenon of contrast reversal in scanning capacitance microscopy (SCM) imaging will be discussed, taking into account the implications for samples which contain both p-type and n-type dopants. Experiments show that a monotonic change in SCM output versus dopant concentration for large dynamic range samples (1014–1020 cm−3) is dependent on the applied dc bias. Incorrect adjustment of this parameter can lead to contrast reversal in the SCM images causing a problem for conversion algorithms and dopant quantification. Simulation results demonstrating this feature will be presented. In addition, the appearance of shifting bands which are common in voltage-dependent measurements of p-n junctions complicates the data interpretation and extraction of length measurements within an image, such as the effective electrical channel length in the case of transistor characterization. The significance of these problems with respect to the SCM response curve will be discussed.
Electrical properties of modulation-doped InxAl1−xAs/InyGa1−yAs structures on GaAs and InP substrates with 0.2⩽x, y⩽0.8
Authors: Cai W. Z. , Wang Z. M. , Miller D. L.
Published: July 2002

We have investigated the compositional dependence of surface morphology and electronic properties of modulation-doped InxAl1−xAs/InyGa1−yAs structures (0.2⩽x,y⩽0.8) on strain-relaxed buffers on GaAs and InP substrates. We have found that modulation-doped In0.70Al0.30As/In0.70Ga0.30As on InP shows a two-dimensional electron-gas mobility of 1.3×104 cm2/V s (300 K) and 9.3×104 cm2/V s (77 K) at a sheet carrier concentration of 2.0×1012 cm−2. We suggest that this material system has a potential for device applications if a high-quality Schottky barrier layer can be fabricated.
Harnessing Genomics and Biotechnology to Improve Global Health Equity
Authors: Singer Peter A. , Daar Abdallah S.
Published: July 2002

With decisive and timely action, genome-related biotechnology can be harnessed to improve global health equity. In June 2002 in Kananaskis, Canada, leaders of the G8 industrial nations will develop an action plan to support implementation of the New African Initiative. By extending their discussion of health issues raised in the New African Initiative to include genomics, G8 leaders could signal their intention to increase global health equity by preventing a health genomics divide from developing. There are already some early and growing examples of genome-related biotechnology being applied successfully to health problems in developing countries. But how can genomics be systematically harnessed to benefit health in developing countries? We propose a five-point strategy, including research, capacity strengthening, consensus building, public engagement, and an investment fund.
Regulation of Receptor Fate by Ubiquitination of Activated β
2
-Adrenergic Receptor and β-Arrestin
Authors: Shenoy Sudha K. , McDonald Patricia H. , Kohout Trudy A. , Lefkowitz Robert J.
Published: July 2002

Although trafficking and degradation of several membrane proteins are regulated by ubiquitination catalyzed by E3 ubiquitin ligases, there has been little evidence connecting ubiquitination with regulation of mammalian G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptor (GPCR) function. Agonist stimulation of endogenous or transfected β
2
-adrenergic receptors (β
2
ARs) led to rapid ubiquitination of both the receptors and the receptor regulatory protein, β-arrestin. Moreover, proteasome inhibitors reduced receptor internalization and degradation, thus implicating a role for the ubiquitination machinery in the trafficking of the β
2
AR. Receptor ubiquitination required β-arrestin, which bound to the E3 ubiquitin ligase Mdm2. Abrogation of β-arrestin ubiquitination, either by expression in Mdm2-null cells or by dominant-negative forms of Mdm2 lacking E3 ligase activity, inhibited receptor internalization with marginal effects on receptor degradation. However, a β
2
AR mutant lacking lysine residues, which was not ubiquitinated, was internalized normally but was degraded ineffectively. These findings delineate an adapter role of β-arrestin in mediating the ubiquitination of the β
2
AR and indicate that ubiquitination of the receptor and of β-arrestin have distinct and obligatory roles in the trafficking and degradation of this prototypic GPCR.
Seismic Evidence of an Extended Magmatic Sill Under Mt. Vesuvius
Authors: Auger Emmanuel , Gasparini Paolo , Virieux Jean , Zollo Aldo
Published: July 2002

Mt. Vesuvius is a small volcano associated with an elevated risk. Seismic data were used to better define its magmatic system. We found evidence of an extended (at least 400 square kilometers) low-velocity layer at about 8-kilometer depth. The inferred
S
-wave (∼0.6 to 1.0 kilometer per second) and
P
-wave velocities (∼2.0 kilometer per second) as well as other evidence indicate an extended sill with magma interspersed in a solid matrix.
Development of Spontaneous Airway Changes Consistent with Human Asthma in Mice Lacking T-bet
Authors: Finotto Susetta , Neurath Markus F. , Glickman Jonathan N. , Qin Shixin , Lehr Hans A. , Green Francis H. Y. , Ackerman Kate , Haley Kathleen , Galle Peter R. , Szabo Susanne J. , Drazen Jeffrey M. , De Sanctis George T. , Glimcher Laurie H.
Published: July 2002

Human asthma is associated with airway infiltration by T helper 2 (T
H
2) lymphocytes. We observed reduced expression of the T
H
1 transcription factor, T-bet, in T cells from airways of patients with asthma compared with that in T cells from airways of nonasthmatic patients, suggesting that loss of T-bet might be associated with asthma. Mice with a targeted deletion of the T-bet gene and severe combined immunodeficient mice receiving CD4
+
cells from T-bet knockout mice spontaneously demonstrated multiple physiological and inflammatory features characteristic of asthma. Thus, T-bet deficiency, in the absence of allergen exposure, induces a murine phenotype reminiscent of both acute and chronic human asthma.
Chiral Amplification of Oligopeptides in Two-Dimensional Crystalline Self-Assemblies on Water
Authors: Zepik Helmut , Shavit Edna , Tang Mao , Jensen Torben R. , Kjaer Kristian , Bolbach Gérard , Leiserowitz Leslie , Weissbuch Isabelle , Lahav Meir
Published: July 2002

Differences in the two-dimensional packing arrangements of racemic and enantiomeric crystalline self-assemblies on the water surface of amphiphilic activated analogs of lysine and glutamic acid have been used to prepare oligopeptides of homochiral sequence and oligopeptides of single handedness from chiral nonracemic mixtures. The crystalline structures on the water surface were determined by grazing incidence x-ray diffraction and the diastereomeric composition of the oligopeptides by matrix-assisted laser desorption time-of-flight mass spectrometry with enantio-labeling. These results suggest that reactivity of ordered clusters at interfaces might have played a role in the generation of early homochiral biopolymers.
Unveiling Mechanisms of Collective Behavior
Authors: Pepper John W. , Hoelzer Guy
Published: July 2002

Self-Organization in Biological Systems
. Scott Camazine, Jean-Louis Deneubourg, Nigel R. Franks, James Sneyd, Guy Theraulaz, and Eric Bonabeau. Princeton University Press, Princeton, NJ, 2001. 546 pp. $65, £45. ISBN 0-691-01200-3.

This contribution to the Princeton Studies in Complexity series presents a set of case studies to demonstrate how models can be used to generate and test hypotheses about mechanisms of self-organization that lead to group behaviors.