bluuebunny/crossref_metadata_2025 huggingface dataset:
Heteroepitaxy of GaAs on InP by molecular-beam epitaxy
Authors: Harbison J. P. , Lo Y. H. , Abeles J. H. , Deri R. J. , Skromme B. J. , Hwang D. M. , Florez L. T. , Seto M. , Nazar L. , Lee T. P. , Nahory R. E.
Published: July 2002

We report the successful growth of GaAs on InP by molecular-beam epitaxy. We find no significant effect of substrate misorientation on growth morphology or dislocation structure. However the integrated low-temperature photoluminescence from samples grown on 3° off (100) InP is found to be 120×brighter than that from on-axis (100) substrate samples, and waveguides on these misoriented substrates show significantly lower losses than those grown on-axis (9.3 dB/cm vs 16 dB/cm). 1.3-μm gate-length field effect transistors fabricated from this material show excellent dc and microwave properties (transconductance of 120–170 mS/mm, low-field mobility of 3000 cm2/V s, and cutoff frequency of 11 GHz), comparable to companion GaAs on GaAs devices. These results make this material a promising system for combining long-wavelength InP/InGaAsP optoelectronic devices with high-speed GaAs electronic devices on the same chip.
X-point tunneling in AlAs–GaAs–AlAs double barrier heterostructures
Authors: Ting D. Z.-Y. , McGill T. C.
Published: July 2002

The dynamics of X-point tunneling in AlAs–GaAs–AlAs double barrier heterostructures is studied with numerical simulation. The problem is formulated within the framework of the one-band Wannier orbital model which allows for simultaneous descriptions of the Γ-point double barrier profile and the X-point double well profile in this heterostructure. Time dependences of the following processes are illustrated: (i) resonant tunneling of Γ-valley electron packets via the X-point quantum well states localized in the AlAs layers, (ii) nonresonant tunneling of Γ-electron packets (with energy below the AlAs Γ-point barrier) through the X-point continuum states, and (3) resonant tunneling via Γ-X mixed quantum well states localized in both the GaAs Γ-point quantum well and the AlAs X-point double quantum well. In addition, the effects of X-point tunneling on the escape times of electrons localized in the GaAs quantum wells are examined.
Feature size and temperature sensitive process windows for excimer laser planarization of aluminum
Authors: Baseman Robert J.
Published: July 2002

The minimum laser fluences required to planarize interlevel vias in a two-metal-level test structure have been measured as a function of via size and sample temperature. Generally, the minimum fluence required increases as the via size increases, resulting in larger operating process windows for smaller vias. Under some circumstances, however, the planarization dynamics may be dominated by features associated with groups of vias. While required fluences for planarization and sample damage both decrease as the sample temperature increases, the relative insensitivity of the damage threshold generally results in larger process windows at higher temperatures.
Modeling of novel heterojunction tunnel structures
Authors: Ting D. Z.-Y. , Yu E. T. , Collins D. A. , Chow D. H. , McGill T. C.
Published: July 2002

We have implemented a simple model that allows realistic yet rapid simulation of conventional as well as interband resonant tunneling devices. Using this model we have studied GaAs/AlAs asymmetric triple barrier structures and found that coherence between the quasibound states in the two quantum wells should be observable in the I–V characteristics of the devices. We have also examined InAs–GaSb–InAs broken-gap interband tunnel devices and found that, despite the absence of classically forbidden barrier regions, a resonant tunneling process is involved in producing the observed negative differential resistance. Furthermore, we have found that maximum peak current densities should be found in devices with GaSb layer thicknesses corresponding to a single, rather than a multiple transmission resonance peak in the broken-gap region.
Homogeneous nucleation of dislocations in In0.4Ga0.6As/GaAs near critical thickness
Authors: Breen K. R. , Uppal P. N. , Ahearn J. S.
Published: July 2002

Homogeneous nucleation of dislocation loops was observed in In0.4Ga0.6As/GaAs films grown with thicknesses near critical thickness (5 nm) for this system. In films with larger thicknesses the dislocation networks were observed as irregular orthogonal arrays. The irregular arrays presumably form from the dislocation loops that grow and interact as the In0.4Ga0.6As film thickens. These results are in sharp contrast to those obtained on films with x≤0.25. Here long dislocation segments are observed even in films with thicknesses just above critical value and the films with larger thicknesses exhibit a regular orthogonal array of dislocations. We attribute these differences in dislocation morphology between the low and high x-value material to a change in the nucleation mechanism for dislocation formation. This change, in turn, corresponds to an observed change from two-dimensional growth at lower x values to three-dimensional growth at higher values. The three-dimensional growth of In0.4Ga0.6As was confirmed by RHEED observations.
The growth of bismuth and antimony overlayers on InP(110)
Authors: Stephens C. , Zahn D. R. T. , Fives K. , Cimino R. , Braun W. , McGovern I. T.
Published: July 2002

The adsorption of Bi and Sb on clean cleaved InP(110) surfaces has been studied with soft x-ray photoemission spectroscopy at the BESSY synchrotron radiation source in Berlin. Bi and Sb were deposited onto ultrahigh vacuum cleaved InP(110) surfaces at substrate temperatures of 300 K [room temperature (RT)] and 120 K [low temperature (LT)]. In this paper we focus on the evolution of the overlayer emission intensities as a function of coverage. Spectra of the Bi 5d and Sb 4d core level emission lines were taken from the submonolayer regime up to coverages of approximately 60 and 20 monolayers (ML) of Bi and Sb, respectively. The spectra were then deconvoluted using a core level fitting routine. In all cases, good agreement between the experimental and fitted curves was achieved by introducing two chemically shifted components of the Bi 5d and Sb 4d core level emission which are interpreted in terms of Bi(Sb)–substrate and Bi(Sb)–Bi(Sb) bonds. However, for RT deposition, the evolution of the Bi 5d and Sb 4d spectra is significantly different in the coverage range below 2 ML. In the Bi case the adatom–substrate component grows first, and only after about 0.5 ML is the Bi–Bi component observed. The two components have equal intensity at about 2 ML. In contrast, the two components in the Sb 4d spectra grow with equal intensity until 1 ML. The spectra for higher coverages for both systems are dominated by the component assigned to adatom–adatom bonds and the overlayer is found to be semimetallic. The LT spectra do not have the easily resolvable features seen in the RT spectra which may indicate that the growth is much more disordered. However, the overlayers at LT are more laminar. In addition, we present the band bending versus coverage behavior for the Bi/p-InP(110) interface, which is compared to that of Sb/p-InP(110).
A reflection high-energy electron diffraction study of AlAs/GaAs tilted superlattice growth by migration-enhanced epitaxy
Authors: Chalmers S. A. , Gossard A. C. , Petroff P. M. , Kroemer H.
Published: July 2002

We have used reflection high-energy electron diffraction to investigate the influence of substrate temperature, arsenic pressure, and Al composition on the growth of tilted superlattices(TSLs) on atomically stepped surfaces. In the temperature range of 450–675 °C, we found that GaAs and TSLs grow smoother at high temperatures and low arsenic pressures. In the same range, AlAs grows smoother at low temperatures and high arsenic pressures. We also found that raising the AlAs composition of TSLs raises the substrate temperature required for pure step flow growth. We have also found new evidence that a low-temperature, smooth-surface growth regime for AlGaAs exists below 450 °C.
Investigation of low growth temperature AlGaAs and GaAs using metal–insulator–semiconductor diagnostic structures
Authors: Campbell A. C. , Crook G. E. , Rogers T. J. , Streetman B. G.
Published: July 2002

GaAs and Al0.3Ga0.7As layers grown by molecular-beam epitaxy (MBE) at 250 °C and incorporated as the gate insulators in metal–insulator–semiconductor (MIS) structures are examined using capacitance–voltage and current–voltage techniques. Samples which are not annealed, and samples annealed for 10 and 20 min under an arsenic ambient at 600 °C are examined. MIS structures using material grown at low temperatures without annealing are found to be extremely leaky at room temperature due to a high defect concentration. For unannealed samples and samples annealed for 10 min, a deep level trap is found to be the source of free carriers in the low growth temperature layer. Capacitance–voltage profiling can be accomplished on both structures, indicating the Fermi level is weakly pinned in these structures. For samples annealed for 20 min, however, no modulation of the device capacitance is possible, and the layer exhibits extremely high resistivity. This indicates that the Fermi level is pinned strongly in samples annealed for 20 min, possibly due to the precipitation and/or interfacial segregation of metallic arsenic.
Advanced electron cyclotron resonance chemical vapor deposition SiC coatings and x-ray mask membranes
Authors: Shimkunas A. R. , Mauger P. E. , Bourget L. P. , Post R. S. , Smith L. , Davis R. F. , Wells G. M. , Cerrina F. , McIntosh R. B.
Published: July 2002

Recent work in electron cyclotron resonance chemical vapor deposition (ECR-CVD) SiC coating technology has demonstrated the potential for fabricating high-quality SiC membranes for x-ray lithography masks. Stoichiometric SiC coatings were deposited on 100-mm-diam silicon wafers at a susceptor temperature of 930 °C in a single-wafer ECR-CVD system at deposition rates of 200 Å/min with better than ±5% thickness uniformity and ±1% wafer-to-wafer reproducibility. The coatings were deposited under tensile stress, with a wafer-to-wafer stress repeatability of ±15%. The coatings were extremely smooth, and x-ray diffraction (XRD) and transmission electron microscopy (TEM) studies showed that they were amorphous with scattered submicron crystalline inclusions. One- and 2-μm-thick membranes were made in 30 and 50 mm diameters at yields of 80%. A peak transmittance of 65% was measured at 633 nm for 1-μm-thick SiC membranes. The bi-axial elastic modulus, E/(1−ν), of the SiC membranes was (4–6)×1012 dyn/cm2, about three times greater than that of (100) Si membranes. Hydrogen was undetectable in the form of either CH or SiH bonds to within the experimental detection limit of <10 ppm. Out-of-plane distortion could not be detected after an absorbed dose of 9.6 or 29 MJ/cm3 during exposure at the Center for X-ray Lithography beamline at the University of Wisconsin’s Aladdin synchrotron. The membrane transmittance decreased by about 1% after the 29 MJ/cm3 dose. Plans for future work include modifications to the ECR-CVD equipment to increase substrate temperature uniformity, temperature limit, and deposition rate, and to reduce defect density.
Extrinsic p-doped HgCdTe grown by direct alloy growth organometallic epitaxy
Authors: Taskar N. R. , Bhat I. B. , Parat K. K. , Ghandhi S. K. , Scilla G. J.
Published: July 2002

This paper describes the doping behavior of arsenic in HgCdTe, grown by organometallic epitaxy using the direct alloy growth process. It is shown that arsenic readily incorporates into HgCdTe during this growth process, to a doping concentration of 1×1017 cm−3. Secondary-ion mass spectroscopy (SIMS) data clearly establishes the presence of arsenic in these layers. Moreover, excellent stability under annealing conditions (16 h at 270 °C followed by 10 h at 220 °C) indicates the suitability of arsenic as an extrinsic dopant source for HgCdTe. The net acceptor concentration is shown to be linearly proportional to the Hg overpressure. This fact, combined with mobility values which are comparable to those of bulk grown material, indicates that these layers are relatively uncompensated. High mobility values are preserved well into the saturated region of the acceptor concentration versus arsine flow rate characteristic. This is explained by the fact that the doping concentration is limited by surface coverage of the As species, and not by arsenic incorporation into compensating or inactive sites. Evidence for this process is presented by the SIMS data, which shows that the total arsenic concentration is linearly related to the net acceptor concentration over the entire doping range.
Diffusion mechanisms in mercury cadmium telluride
Authors: Stevenson D. A. , Tang M-F. S.
Published: July 2002

The key diffusion quantities for component diffusion in mercury cadmium telluride are reviewed: interdiffusion; intrinsic diffusion; and tracer self-diffusion. The interrelation between these quantities is described as well as their relation to practical diffusion problems. A review of the experimental information for these diffusion quantities is presented in the framework of diffusion theory and diffusion mechanisms. In particular, the interrelation between the fundamental diffusion quantities is used to assign diffusion values. The interpretation of marker motion and Kirkendall effect experiments is presented in the context of theory and experimental diffusion information in this system.
Silicon reaction of TiNx diffusion barriers at high temperatures
Authors: Jiménez M. C. , Fernandez M. , Albella J. M. , Martinez-Duart J. M.
Published: July 2002

TiNx diffusion barriers have been deposited onto n-Si wafers by reactive sputtering in a N2+Ar atmosphere, with a nitrogen content varying in the 0%–25% range. The relative composition of the as-deposited films was examined by Rutherford backscattering spectroscopy (RBS) and Auger electron spectroscopy (AES) analysis yielding a TiNx composition between x=0.8 and x=1.2, respectively. The near stoichiometric samples were thermally annealed at temperatures between 600 and 900 °C in vacuum and N2 atmosphere. Films with composition TiN1.1 and TiN0.9 present very good behavior as barrier layers against the Si diffusion and a very low reactivity with the Si substrate. When the nitrogen content in the as-deposited films decreases, the films lose their barrier properties. For the TiN0.8 samples, Si diffusion through the films is observed after annealing in vacuum at temperatures of 600 °C and above. However, when the heat treatment is carried out in a N2 atmosphere an improvement in the barrier characteristics and reactivity with the Si at the interface was observed.
A multichamber single-wafer chemical vapor deposition reactor and electron cyclotron resonance plasma for flexible integrated circuit manufacturing
Authors: Zhou Zhen-Hong , Yu Fuzhong , Reif Rafael
Published: July 2002

A multichamber single-wafer chemical vapor deposition reactor has been designed and built for ultrahigh vacuum, low temperature, selective silicon epitaxy. The reactor is equipped with in situ monitors for study and control of both predeposition wafer cleaning and epitaxial growth process. A detailed description of the system and its preliminary performance are reported. An argon or helium plasma excited by electron cyclotron resonance (ECR) has been used for predeposition wafer cleaning. Results of the ECR plasma characterization by a Langmuir probe are presented. Furthermore, structural quality of the wafers subjected to plasma cleaning were evaluated by Rutherford backscattering spectroscopy and scanning electron microscopy. Results showed that a damage free substrate can potentially be achieved at a wafer temperature of 500 °C.
Fabrication of sub-100-nm T gates with SiN passivation layer
Authors: Nummila K. , Tong M. , Ketterson A. A. , Adesida I.
Published: July 2002

Low resistance T-shaped gates as small as 60 nm have been fabricated using high resolution electron-beam lithography (EBL). A silicon nitride (SiNx) passivation layer has been used to define the bottom of the T gate and to provide mechanical support for the top of the T gate. A two-step etch was performed to define the gate footprint in the SiNx. First a short wet etch is used to isotropically etch the SiNx to provide a wider top opening following by reactive ion etching (RIE) to transfer the narrow resist pattern anisotropically into the bottom of the SiNx. A bilayer resist lift-off process is then used to determine the top of the T gate and the thickness of the gate metal. End-to-end gate resistances of 450 Ω/mm have been measured for sub-0.1-μm-long gates with a 0.5-μm-wide top and with 250-nm-thick metallization. The resistance can easily be further decreased by increasing the metal thickness and/or by widening the top of the T gate. Gate capacitances Cgs and Cgd measured on GaAs metal–semiconductor field effect transistors (MESFETs) with a SiNx layer showed a slight increase in capacitance compared to the devices without the SiNx layer. Slightly higher extrinsic transconductances gm and unity current-gain cut-off frequencies fT were achieved for the devices with the SiN layer. Intrinsic unity current-gain cut-off frequencies fT of up to 102 GHz were measured. The fabrication of GaAs MESFETs with T gates down to 60 nm is demonstrated with the SiNx passivation layer process.
Direct nanometer scale patterning of SiO2 with electron-beam irradiation
Authors: Allee D. R. , Umbach C. P. , Broers A. N.
Published: July 2002

Nanometer scale patterns have been fabricated in SiO2 by direct electron-beam exposure. Two techniques have been developed to eliminate the surface contamination and enable the subsequent development of the patterns in HF based wet etches: (1) exposing the oxide through a sacrificial layer (previously reported) and (2) O2 reactive ion etching (RIE). The latter approach eliminates the need for a sacrificial layer and improves resolution by reducing the forward scattering of the beam. To determine the resolution of this process, patterns were fabricated with both 50- and 300-kV electrons in thin SiO2 membrane samples and imaged in transmission. Transmission imaging avoids the resolution limit of secondary electron micrographs set by the lateral range of secondary electrons. At 300 keV with a line dose of 7.5 μC/cm, arrays of lines with a period down to 15 nm were achieved as opposed to the 21-nm period previously reported using a sacrificial layer and secondary electron imaging of bulk substrates. A better understanding has also been obtained of the profiles of the patterns in SiO2.
Effect of many-body corrections on intersubband optical transitions in GaAs–AlxGa1−x As multiple quantum wells
Authors: Jogai B.
Published: July 2002

A self-consistent calculation of the energy levels of n-doped quantum wells (MQW) is reported. The eigenstates of the system are calculated from a four-band k.p Hamiltonian. The Hartree and exchange–correlation components of the Coulomb interaction are included self-consistently. The Hartree component is included by solving the Poisson equation; the exchange–correlation interaction is incorporated within the density–functional scheme. Also included are the vertex and depolarization effects which modify the intersubband transition energies. All of these many-body corrections are shown to strongly influence the ground to excited state transition within the conduction band. Further, it is revealed that contrary to previous claims, the many-body effects in these structures are insensitive to temperature. Consequently, such effects cannot account for the observed blue shift of the principal infrared peak as a function of decreasing temperature.
Remote plasma enhanced chemical vapor deposition of GaP with in situ generation of phosphine precursors
Authors: Choi S. W. , Lucovsky G. , Bachmann K. J.
Published: July 2002

Thin homoepitaxial films of gallium phosphide (GaP) have been grown by remote plasma enhanced chemical vapor deposition utilizing in situ generated phosphine precursors. The GaP forming reaction is kinetically controlled with an activation energy of 0.65 eV. The increase of the growth rate with increasing radio frequency (rf) power between 20 and 100 W is due to the combined effects of increasingly complete excitation and the spatial extension of the glow discharge toward the substrate, however, the saturation of the growth rate at even higher rf power indicates the saturation of the generation rate of phosphine precursors at this condition. Slight interdiffusion of P into Si and Si into GaP is indicated from GaP/Si heterostructures grown under similar conditions as the GaP homojunctions.
Polycapillary collimator for point source proximity x-ray lithography
Authors: Vartanian M. , Youngman R. , Gibson D. , Drumheller J. , Frankel R.
Published: July 2002

A full-scale prototype collimator suitable for point source proximity x-ray lithography has been constructed using polycapillary fiber technology. On the basis of single-fiber calibrations, the optic when tested is expected to deliver ≂0.24 mJ/cm2 for a J/2π sr source pulse at 〈λ〉=14 Å. Simulations predict that after 40 cm the beam will be uniform to within 4%(3σ) over a 20×20 mm2 field. The local divergence of the beam is designed to be 5.0±5% mrad full width at half-maximum for optimum processing latitude. Because the divergent beam from a point source projects out-of-plane mask-wafer gap variations into lateral printing errors, collimation will be required if overlay requirements for future sub-0.25 μm devices are to be achieved. The predicted throughput advantage for this collimator compared to an uncollimated source removed to a sufficient distance to meet the requirements for first-generation ultra-large-scale integration is a factor of ≂5, assuming a 3σ=1 μm gap error.
Light scattering studies of ZnSe/GaAs heterostructures
Authors: Talaat H. , Elissa L. , Negm S. , Burstein E. , Yeganeh M. S. , Yodh A. G.
Published: July 2002

Raman studies using the allowed and the forbidden longitudinal optical (LO) Fröhlich interactions are employed to investigate the heterostructure of ZnSe/GaAs as a counterpart to three-photon mixing or second harmonic generation studies. Our samples are an undoped ZnSe/GaAs (001) heterojunction with a ZnSe overlayer varying from 50 to 5000 Å. The scattering intensities for the allowed and the forbidden LO interactions at both resonant and nonresonant incident frequencies for ZnSe are investigated. The nonresonant forbidden LO phonon shows the effect previously observed for the pseudomorphic region of ZnSe thickness as well as for thicker films. The allowed LO intensities show a clear interference effect similar to recent interference effects observed in second harmonic generation for identical ZnSe/GaAs heterostructures. At resonant incident frequency in ZnSe, forbidden LO phonon scattering shows strong enhancement and is an order of magnitude larger than the allowed scattering due to wave-vector-dependent scattering in ZnSe.
Plasma passivation of etch-induced surface damage on GaAs
Authors: Ko K. K. , Pang S. W.
Published: July 2002

Various plasma passivation techniques for removal of etch-induced damage on GaAs have been studied. It was found that a Cl2 plasma generated with an electron cyclotron resonance source can efficiently remove dry etch-induced damage with minimal etching of the GaAs. Complete recovery of the electrical characteristics of both the Schottky diodes and unalloyed transmission lines was found with a 30 s Cl2 plasma passivation at 25 °C. The chlorine reactive species used for passivation were generated with 50 W microwave power at 2 mTorr without any rf power applied at the stage. The Cl2 passivated surface was thermally stable up to 450 °C. Similar recovery was also observed for diodes passivated with a N2 plasma. Compared to Cl2, however, N2 plasma passivation requires a higher temperature (350 °C) and higher microwave power (500 W). Capacitance–voltage measurements show that the presence of H2 in the plasma during passivation results in dopant depletion near the surface, but the dopants can be reactivated after annealing at temperature ≥450 °C for 3 min. Plasma passivation with H2S was found to result in a partial recovery of the electrical characteristics for the etched diodes and transmission lines. Annealing at 300 °C is also required after H2S plasma passivation to desorb the excess S on the GaAs surface. Changes in the defect density as a function of the conditions used for passivation have been correlated to Schottky diode characteristics.
Reducing electron energy dispersion of nonformed metal–insulator–metal electron emitters using the near-threshold drive method
Authors: Suzuki Mutsumi , Kusunoki Toshiaki , Shinada Hiroyuki , Yaguchi Tomio
Published: July 2002

The energy distribution of electrons emitted from an Al/Al2O3/Au metal–insulator–metal (MIM) electron emitter is measured. The thickness of the insulator is 5.5 nm. The energy distribution becomes narrower as the operating voltage Vd decreases since the low energy tail of the distribution is cut off by the potential barrier of the surface work function φ of the emitter. When the emitter is operated in the nonformed state, ΔE, the full width at half-maximum of the distribution, is 0.32 eV for Vd=5.0 V, which is slightly above φ of Au (4.7 eV). As Vd increases, the high-energy tail of the distribution broadens whereas the shape of the low-energy tail remains unchanged. For a formed MIM emitter, ΔE becomes broader by 0.15–0.2 eV more than ΔE of a nonformed emitter at each Vd; thus, operation in the nonformed state is essential to obtain good monochromaticity. The spatial distribution of the work function in the emitter surface is also measured by the retarding potential method. The variation of φ, which limits the ultimately attainable monochromaticity with the near-threshold drive method, is measured as 0.05 eV. It is estimated that ΔE less than 0.2 eV could be attained by the near-threshold drive if the insulator thickness and Vd are further reduced.
Proximity effect correction for nanolithography
Authors: Rau Richard , McClellan James H. , Drabik Timothy J.
Published: July 2002

The resolution of electron-beam lithography is limited by the proximity effect, which is due to the scattering of incident electrons. This scattering leads to exposure of neighboring areas and, therefore, to pattern degradations. It can be modeled by convolving the incident dosage distribution with a point spread function and the subsequent developing process by a pointwise nonlinear function. This article presents an iterative algorithm that exploits this overall model to compute corrections of the proximity effect in the nanometer range. A convex error function is derived that satisfies all physical constraints and the problem is set up as a convex, constrained, and nonlinear minimization problem. The correction algorithm is a hybrid of the conjugate gradient and gradient projection algorithms. Its performance for a test pattern is evaluated and then a detailed analysis of the effect of inaccuracies of the model parameters and of the necessary quantization of the corrected incident dosage distribution is presented.
Optical proximity correction for intermediate-pitch features using sub-resolution scattering bars
Authors: Fung Chen J. , Laidig Tom , Wampler Kurt E. , Caldwell Roger
Published: July 2002

Using our original scattering bar implementation as a starting point, we identify two major discontinuities in the CD versus pitch curve. The first discontinuity occurs at the pitch where two standard scattering bars will no longer fit between features at the proper optical distance and must be replaced by a single shared bar. The second discontinuity occurs at the pitch where a single shared bar no longer fits between features at the proper optical distance, and must be deleted. We propose several techniques that can be used in the pitch range around these discontinuities that allow us to decompose them into a finer series of smaller discontinuities, in effect smoothing out the curve. We introduce the concepts of “placement elasticity” and “optical weight modulation,” and two new classes of scattering bars: thin scattering bars and binary halftone scattering bars. We combine all of these elements in a hybrid optical proximity correction strategy, recommending actual dimensions for i-line, KrF (248 nm) and ArF (193 nm) reticles. We report both simulated and actual results for an i-line embodiment of this approach, demonstrating a greater than 50% improvement in overlapped depth-of-focus for both on-axis and off-axis illumination.
Epitaxial staircase structure for the calibration of electrical characterization techniques
Authors: Clarysse T. , Caymax M. , De Wolf P. , Trenkler T. , Vandervorst W. , McMurray J. S. , Kim J. , Williams C. C. , Clark J. G. , Neubauer G.
Published: July 2002

Frequently electrical characterization techniques [such as the spreading resistance probe (SRP)], rely on the availability of a set of well-calibrated, homogeneously doped Si samples to establish the calibration curves (and parameters) necessary for the conversion of resistance measurements into carrier profiles. Although ideally such a calibration should be verified daily, in practice, time considerations limit the daily verification to one (or a few) calibration samples. To remedy this situation a special multilayer Si structure has been grown consisting of a decreasing B-doped staircase containing seven flat 4–5 μm thick calibration layers doped from 1020/cm3 down to 1015/cm3 separated by slightly (factor 2–3) higher doped 1–2 μm thick interface layers. The latter are included to facilitate the SRP calibrations as the SRP correction factor within the calibration layers now becomes very close to one. Since presently, a calibration curve can be generated quickly from a single measurement, daily measurements over a period of several months clearly indicate concentration-dependent drifts of the SRP-calibration curve. In addition to the calibration purposes we will demonstrate that this sample also can be used for the direct comparison of SRP, nano-SRP, scanning capacitance microscopy (SCM), and selective etching, etc. in terms of their dynamic range, quantification properties, and sensitivity.
Oxidation properties of silicon dots on silicon oxide investigated using energy filtering transmission electron microscopy
Authors: Single C. , Zhou F. , Heidemeyer H. , Prins F. E. , Kern D. P. , Plies E.
Published: July 2002

We used a side view transmission electron microscopy (TEM) technique which allows us to study the oxidation process of Si dots on SiO2 systematically for different dot sizes, oxidation times, and temperatures. Using energy filtering TEM (EFTEM) an excellent contrast between Si and SiO2 is achieved, independent from the crystal orientation. Si dots on SiO2 with initial diameters from 10 to 60 nm were oxidized with different oxidation times at 850 °C. The resulting shapes of the SiO2 and the embedded Si cores were determined from the EFTEM micrographs. A strong retardation of the oxidation process compared to planar oxidation as well as a self-limiting effect for long oxidation times are reported. Furthermore a pattern dependent oxidation is observed, depending on the aspect ratios of the dots.
Effect of short-time helicon-wave excited N2–Ar plasma treatment on the interface characteristic of GaAs
Authors: Wada Satoshi , Kanazawa Keisuke , Okamoto Nariaki , Ikoma Hideaki
Published: July 2002

The short-time helicon-wave excited N2–Ar plasma treatments of n-type GaAs (100) substrates were performed in order to investigate the effect of these treatments on the interface characteristics of GaAs and to explore a possibility of a robust surface passivation of GaAs. X-ray photoelectron spectroscopic measurements indicated formation of GaN at the insulative-layer GaAs interface. The current-density–voltage (J–V) characteristics of the Schottky or tunnel metal–insulator–semiconductor diodes were measured before and after the plasma treatment. The analysis of these data suggested that the “intrinsic” surface Fermi level pinning near the midgap of the GaAs forbidden band was partially removed and the Fermi level was newly pinned at EC−0.2–0.3 eV (EC: the conduction band edge) after the plasma treatment. This is probably due to generation of high-density plasma-induced donor-like interface states having the energy level (or levels) in this energy region. The reverse leakage current increased with this plasma treatment. However, it decreased after the post-thermal annealing at moderate temperatures (200–500 °C) in N2 ambient. The thermal and air-exposure stability were substantially improved with the plasma treatment. The J–V characteristics did not show any deterioration after air exposure for three months for the plasma-treated samples, whereas these greatly degraded with air exposure for three weeks for the untreated ones.
Use of an SF5+ polyatomic primary ion beam for ultrashallow depth profiling on an ion microscope secondary ion mass spectroscopy instrument
Authors: Gillen Greg , Walker Marlon , Thompson Phillip , Bennett Joe
Published: July 2002

A magnetic sector secondary ion mass spectrometry (SIMS) instrument has been fitted with a modified hot filament duoplasmatron ion source for generation of SF5+ primary ion beams for SIMS depth profiling applications. The SF5+ primary ion beam has been evaluated by depth profiling of several low energy boron ion implants, boron delta-doped structures and a Ni/Cr metal multilayer depth profiling standard reference material. Using 3.0 keV impact SF5+ bombardment at a 52° impact angle with oxygen flooding gives a trailing edge decay length (1/e) for the boron implants and delta-doped layers of 1.3 nm. Under the same conditions, O2+ bombardment gives a trailing edge decay length (1/e) of 2.3 nm. The use of the SF5+ beam without oxygen flooding gives a substantial increase in decay length that is related to the formation of ripples as determined by atomic force microscopy. In the case of the Ni/Cr reference material, a significant reduction in sputter-induced topography is observed with SF5+ bombardment.
Activation Volumes for Solid-Solid Transformations in Nanocrystals
Authors: Jacobs Keren , Zaziski David , Scher Erik C. , Herhold Amy B. , Paul Alivisatos A.
Published: July 2002

The transition between four- and six-coordinate structures in CdSe nanocrystals displays simple transition kinetics as compared with the extended solid, and we determined activation volumes from the pressure dependence of the relaxation times. Our measurements indicate that the transformation takes place by a nucleation mechanism and place strong constraints on the type of microscopic motions that lead to the transformation. The type of analysis presented here is difficult for extended solids, which transform by complicated kinetics and involve ill-defined domain volumes. Solids patterned on the nanoscale may prove to be powerful models for the general study of structural transitions in small systems, as well as in extended solids.
The Reluctant Man of the Year
Authors: Cohen Jon
Published: July 2002

When David Ho learned that
Time
magazine was considering naming him “Man of the Year,” he prayed that another candidate would win.
Spatially and Functionally Distinct Ca
2+
Stores in Sarcoplasmic and Endoplasmic Reticulum
Authors: Golovina Vera A. , Blaustein Mordecai P.
Published: July 2002

The organization of calcium (Ca
2+
) stores in the sarcoplasmic and endoplasmic reticulum (S-ER) is poorly understood. The dynamics of the storage and release of calcium in the S-ER of intact, cultured astrocytes and arterial myocytes were studied with high-resolution imaging methods. The S-ER appeared to be a continuous tubular network; nevertheless, calcium stores in the S-ER were organized into small, spatially distinct compartments that functioned as discrete units. Cyclopiazonic acid (an inhibitor of the calcium pump in the S-ER membrane) and caffeine or ryanodine unloaded different, spatially separate compartments. Heterogeneity of calcium stores was also revealed in cells activated by physiological agonists. These results suggest that cells can generate spatially and temporally distinct calcium signals to control individual calcium-dependent processes.
Shipboard Geophysical Indications of Asymmetry and Melt Production Beneath the East Pacific Rise Near the MELT Experiment
Authors: Scheirer Daniel S. , Forsyth Donald W. , Cormier Marie-Hélène , Macdonald Ken C.
Published: July 2002

Near the Mantle Electromagnetic and Tomography (MELT) Experiment, seamounts form and off-axis lava flows occur in a zone that extends farther to the west of the East Pacific Rise than to the east, indicating a broad, asymmetric region of melt production. More seamounts, slower subsidence, and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west. Variations in axial ridge shape, axial magma chamber continuity, off-axis volcanism, and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17°15′S than beneath 15°55′S. Recent volcanism occurs above mantle with the lowest seismic velocities.
Zinc Fingers in
Caenorhabditis elegans
: Finding Families and Probing Pathways
Authors: Clarke Neil D. , Berg Jeremy M.
Published: July 2002

More than 3 percent of the protein sequences inferred from the
Caenorhabditis elegans
genome contain sequence motifs characteristic of zinc-binding structural domains, and of these more than half are believed to be sequence-specific DNA-binding proteins. The distribution of these zinc-binding domains among the genomes of various organisms offers insights into the role of zinc-binding proteins in evolution. In addition, the complete genome sequence of
C. elegans
provides an opportunity to analyze, and perhaps predict, pathways of transcriptional regulation.
A Sense of the End
Authors: Gasser Susan M.
Published: July 2002

How a cell distinguishes a double-strand break from the end of a chromosome has long fascinated cell biologists. It was thought that the protection of chromosomal ends required either a telomere-specific complex or the looping back of the 3′ TG-rich overhang to anneal with a homologous double-stranded repeat. These models must now accommodate the findings that complexes involved in nonhomologous end joining play important roles in normal telomere length maintenance, and that subtelomeric chromatin changes in response to the DNA damage checkpoint. A hypothetical chromatin assembly checkpoint may help to explain why telomeres and the double-strand break repair machinery share essential components.
Taming Superacids: Stabilization of the Fullerene Cations HC
60
+
and C
60
·+
Authors: Reed Christopher A. , Kim Kee-Chan , Bolskar Robert D. , Mueller Leonard J.
Published: July 2002

A new superacid, H(CB
11
H
6
X
6
) (where X = chlorine or bromine), whose conjugate base is the exceptionally inert CB
11
H
6
X
6
–
carborane anion, separates Brønsted acidity from oxidizing capacity and anion nucleophilicity in a manner not previously achieved. Reaction of this superacid with C
60
gives HC
60
+
as a stable ion in solution and in the solid state. In a separate experiment, an oxidant was developed such that the long-sought C
60
·+
ion can be synthesized in solution. The preparation of these two fullerene carbocations is a notable departure from the prevalent chemistry of C
60
, which is dominated by the formation of anions or the addition of nucleophiles. The H(CB
11
H
6
X
6
) superacid overcomes the major limitations of presently known superacids and has potentially wide application.
Selection and Characterization of Human Immunodeficiency Virus Type 1 Variants Resistant to the (+) and (−) Enantiomers of 2′-Deoxy-3′-Oxa-4′-Thio-5-Fluorocytidine
Authors: Richard Nathalie , Salomon Horacio , Rando Robert , Mansour Tarek , Bowlin Terry L. , Wainberg Mark A.
Published: July 2002

Human immunodeficiency virus (HIV) type 1 (HIV-1) variants were selected for resistance to the (+) and (−) enantiomers of a novel nucleoside analogue, 2′-deoxy-3′-oxa-4′-thio-5-fluorocytidine (dOTFC), by use of the infectious molecular clone HIV HXB2D and the human T-cell line MT-4. The dOTFC-resistant variants that were selected were 10-fold less sensitive than wild-type virus, and cloning and sequencing of the complete reverse transcriptase (RT)-coding region identified the mutation M184V. Studies with mutated recombinant HXB2D virus confirmed the importance of the M184V mutation in conferring resistance to (−)dOTFC in MT-4 cells, although no difference in sensitivity was observed in primary cells. The M184V substitution also displayed decreased susceptibility to (+)dOTFC. Selection with (+)dOTFC also produced variants which were 10-fold more resistant than the wild type, and a novel mutation, D67G, was identified following cloning and sequencing of the RT genes. The D67G mutation was introduced into HXB2D by site-directed mutagenesis, and the data obtained confirmed the importance of this mutation in conferring resistance to both (+)dOTFC and (−)dOTFC. Mutated recombinant molecular clone HXB2D-D67G was further selected with (+)dOTFC, and three of six clones sequenced contained both the D67G and M184V mutations, while the other three of the six clones contained only the D67G mutation. Clinical isolates of HIV-1 which are (−) 2′-deoxy-3′-thiacytidine-resistant also displayed resistance to both (+)dOTFC and (−)dOTFC.
Inhibitors of Sterol Biosynthesis and Amphotericin B Reduce the Viability of
Pneumocystis carinii
f. sp.
carinii
Authors: Kaneshiro Edna S. , Collins Margaret S. , Cushion Melanie T.
Published: July 2002

Pneumocystis carinii
synthesizes sterols with a double bond at C-7 of the sterol nucleus and an alkyl group with one or two carbons at C-24 of the side chain. Also, some human-derived
Pneumocystis carinii
f. sp.
hominis
strains contain lanosterol derivatives with an alkyl group at C-24. These unique sterols have not been found in other pathogens of mammalian lungs. Thus,
P. carinii
may have important differences in its susceptibility to drugs known to block reactions in ergosterol biosynthesis in other fungi. In the present study, inhibitors of 3-hydroxy-3-methyglutaryl coenzyme A reductase, squalene synthase, squalene epoxidase, squalene epoxide-lanosterol cyclase, lanosterol demethylase, Δ
8
to Δ
7
isomerase, and
S
-adenosylmethionine:sterol methyltransferase were tested for their effects on
P. carinii
viability as determined by quantitation of cellular ATP levels in a population of organisms. Compounds within each category varied in inhibitory effect; the most effective included drugs targeted at squalene synthase, squalene epoxide-lanosterol cyclase, and Δ
8
to Δ
7
isomerase. Some drugs that are potent against ergosterol-synthesizing fungi had little effect against
P. carinii
, suggesting that substrates and/or enzymes in
P. carinii
sterol biosynthetic reactions are distinct. Amphotericin B is ineffective in clearing
P. carinii
infections at clinical doses; however, this drug apparently binds to sterols and causes permeability changes in
P. carinii
membranes, since it reduced cellular ATP levels in a dose-dependent fashion.
Efficacy of FK463, a (1,3)-β-
d
-Glucan Synthase Inhibitor, in Disseminated Azole-Resistant
Candida albicans
Infection in Mice
Authors: Maesaki Shigefumi , Hossain Mohammad Ashraf , Miyazaki Yoshitsugu , Tomono Kazunori , Tashiro Takayoshi , Kohno Shigeru
Published: July 2002

The efficacy of FK463, a new (1,3)-β-
d
-glucan synthase inhibitor, against azole-resistant
Candida albicans
strains has been studied. The MIC of FK463 was lower than those of azoles and amphotericin B against
CDR1
-expressing C26 and
CaMDR
-expressing C40 strains. All mice treated with FK463 (1 mg/kg) survived disseminated murine candidiasis. The fungal burden in the kidney after 6 days was markedly reduced after therapy with FK463 and amphotericin B sodium deoxycholate, and plasma (1,3)-β-
d
-glucan concentration was found to be lower in FK463-treated mice. In our study, FK463 was found to be a potent antifungal agent against disseminated infection with azole-resistant
C. albicans
.
In Vitro Activities of Terbinafine against
Aspergillus
Species in Comparison with Those of Itraconazole and Amphotericin B
Authors: Moore Caroline B. , Walls Caroline M. , Denning David W.
Published: July 2002

Compared with the in vitro activities of itraconazole (geometric mean MIC [GM], 0.56 μg/ml) and amphotericin B (GM, 0.66 μg/ml), the in vitro activity of terbinafine was inferior against
Aspergillus fumigatus
(GM, 19.03 μg/ml) (
P
< 0.05) and superior against
A. flavus
(GM, 0.10 μg/ml),
A. terreus
(GM, 0.16 μg/ml), and
A. niger
(GM, 0.19 μg/ml). Clinical correlation is required, as trailing endpoints are problematic.
In Vitro Activities of Ertapenem (MK-0826) against Clinical Bacterial Isolates from 11 North American Medical Centers
Authors: Fuchs Peter C. , Barry Arthur L. , Brown Steven D.
Published: July 2002

This study compared the in vitro activities of the new long-half-life carbapenem ertapenem (also known as MK-0826 and L-749,345) with those of imipenem, amoxicillin-clavulanate, and ciprofloxacin against 5,558 recent clinical isolates from 11 North American medical centers. We confirmed the greater activity of ertapenem than of imipenem against the
Enterobacteriaceae
and the greater activity of imipenem against pseudomonads and gram-positive bacteria.
Variants Other than Aspartic Acid at Codon 69 of the Human Immunodeficiency Virus Type 1 Reverse Transcriptase Gene Affect Susceptibility to Nucleoside Analogs
Authors: Winters Mark A. , Merigan Thomas C.
Published: July 2002

The T69D mutation in the human immunodeficiency virus type 1 reverse transcriptase (RT) gene has been associated with reduced susceptibility to dideoxycytosine (ddC); however, several other mutations at codon 69 have been observed in antiretroviral drug-treated patients. The Stanford HIV RT and Protease Sequence Database was interrogated and showed that 23% of patients treated with nucleoside RT inhibitors (NRTI) had mutations at codon 69. These variants included T69N, -S, -A, -G, -E, -I, and -K mutations that were present in patients treated with NRTI but not in drug-naive patients. Treatment history information showed that a substantial percentage of these codon 69 changes occurred in patients administered non-ddC-containing regimens. Different and specific patterns of other RT gene mutations were associated with the various codon 69 mutations. Drug susceptibility assays showed that viral constructs containing codon 69 variants could have reduced susceptibility to ddC and other RT inhibitors. These results suggest that the T69D mutation is not the only codon 69 variant associated with drug resistance and that ddC is not the only drug affected.
Characterization of Growth and Acid Formation in a
Bacillus subtilis
Pyruvate Kinase Mutant
Authors: Fry B. , Zhu T. , Domach M. M. , Koepsel R. R. , Phalakornkule C. , Ataai M. M.
Published: July 2002

Based on measurements and theoretical analyses, we identified deletion of pyruvate kinase (PYK) activity as a possible route for elimination of acid formation in
Bacillus subtilis
cultures grown on glucose minimal media. Evidence consistent with the attenuation of PYK flux has come from metabolic flux calculations, metabolic pool and enzymatic activity measurements, and a series of nuclear magnetic resonance experiments, all suggesting a nearly complete inhibition of PYK activity for glucose-citrate fed cultures in which the amount of acid formation was nearly zero. In this paper, we report the construction and characterization of a
pyk
mutant of
B. subtilis
. Our results demonstrate an almost complete elimination of acid production in cultures of the
pyk
mutant in glucose minimal medium. The substantial reduction in acid production is accompanied by increased CO
2
production and a reduced rate of growth. Metabolic analysis indicated a dramatic increase in intracellular pools of phosphoenolpyruvate (PEP) and glucose-6-P in the
pyk
mutant. The high concentrations of PEP and glucose-6-P could explain the decreased growth rate of the mutant. The substantial accumulation of PEP does not occur in
Escherichia coli pyk
mutants. The very high concentration of PEP which accumulates in the
B. subtilis pyk
mutant could be exploited for production of various aromatics.