bluuebunny/crossref_metadata_2025 huggingface dataset:
Zone-plate-array lithography in the deep ultraviolet
Authors: Djomehri Ihsan J. , Savas T. A. , Smith Henry I.
Published: July 2002

We describe initial steps we have taken in the development of zone-plate-array lithography (ZPAL), a proposed new paradigm for sub-100 nm lithography. The optimal implementation of ZPAL would employ an undulator emitting soft x rays of 4.5 nm wavelength. However, we have opted to concentrate initially on the major uncertainties associated with ZPAL: (1) fabrication of a large array of zone plates with a center-to-center spacing accuracy finer than the minimum zone width, (2) multiplexing of input beams to the individual zone plates, and (3) coordination of that multiplexing with precision motion of a substrate stage. We fabricated arrays of pure-phase zone plates suitable for the 193 nm output of an ArF laser, and conducted exposure tests using a modified Michelson configuration for focusing the zone plate array on the substrate. We also show the first example of simplistic writing. We show that the micromirror array manufactured by Texas Instruments, for use in projection displays, has dimensions and switching characteristics close to those needed for an ultimate ZPAL system.
Plane-view observation technique of silicon nanowires by transmission electron microscopy
Authors: Tsutsumi Toshiyuki , Suzuki Eiichi , Ishii Kenichi , Kanemaru Seigo , Maeda Tatsuro , Tomizawa Kazutaka
Published: July 2002

We have developed a novel technique of the plane-view transmission electron microscopy (TEM) observation of silicon (Si) nanowires by covering them firmly with a holding layer of Si3N4. The use of the Si3N4 holding layer is shown to be effective to fix the fine structure of the sample without deformation during a focused ion beam (FIB) thinning process, to protect the sample from a high energy FIB, and to give high contrast in the TEM image of the crystalline fine structure. The Si3N4 holding layer is quite suitable, therefore, for plane-view TEM observation of the fine structure such as a Si nanowire. By means of the proposed observation technique, we have successfully observed clear features in the direction of length of narrow (16 nm) Si nanowires fabricated by using SiO2 sidewall masks and the extremely fine (3–5 nm) Si nanowires obtained by the successive self-limiting oxidation. Moreover, the plane-view TEM lattice image of the ultrafine Si nanowire was also observed. The plane-view TEM observation technique described here is useful to analyze various fine nanostructures and contributes to develop nanofabrication technology.
Control of localized access to circuitry through the backside using focused ion beam technology
Authors: Antoniou Nicholas , Thompson Mark , Salen Jesse , Casey David , Goruganthu Rama R. , Ring Rose , Birdsley Jeff , Gilfeather Glen
Published: July 2002

Complex logic chips are almost exclusively assembled in flip chip packaging. This type of assembly complicates traditional debug and circuit modification techniques. Development of new applications and new equipment now enable precise access to the circuitry of flip chip parts. One such application and the equipment developed are being introduced in this article. The technique makes use of optical beam induced current (OBIC) as a way to measure the amount of silicon that is left covering active areas of a flip chip circuit after a trench has been milled in the bulk silicon using a focused ion beam (FIB) system. The apparatus is all contained in one system thus enhancing the throughput of such work. When accessing the circuitry of flip chip parts, it is crucial to be able to locally remove silicon from the backside to within a few microns of the circuitry. This is necessary in order to preserve the integrity of the part and allow access to the circuitry for probe point creation or circuit modification using FIB. OBIC offers a high level of resolution and accuracy in measuring thin layers of bulk silicon in flip chips. In this article we describe the apparatus used, the details of the application, data collected, and a theoretical model developed to confirm the experimental findings.
Demonstration of pattern transfer into sub-100 nm polysilicon line/space features patterned with extreme ultraviolet lithography
Authors: Cardinale G. F. , Henderson C. C. , Goldsmith J. E. M. , Mangat P. J. S. , Cobb J. , Hector S. D.
Published: July 2002

In two separate experiments, we have successfully demonstrated the transfer of dense- and loose-pitch line/space (L/S) photoresist features, patterned with extreme ultraviolet (EUV) lithography, into an underlying hard mask material. In both experiments, a deep-UV photoresist (∼90 nm thick) was spin cast in bilayer format onto a hard mask (50–90 nm thick) and was subsequently exposed to EUV radiation using a 10× reduction EUV exposure system. The EUV reticle was fabricated at Motorola (Tempe, AZ) using a subtractive process with Ta-based absorbers on Mo/Si multilayer mask blanks. In the first set of experiments, following the EUV exposures, the L/S patterns were transferred first into a SiO2 hard mask (60 nm thick) using a reactive ion etch (RIE), and then into polysilicon (350 nm thick) using a triode-coupled plasma RIE etcher at the University of California, Berkeley, microfabrication facilities. The latter etch process, which produced steep (>85°) sidewalls, employed a HBr/Cl chemistry with a large (>10:1) etch selectivity of polysilicon to silicon dioxide. In the second set of experiments, hard mask films of SiON (50 nm thick) and SiO2 (87 nm thick) were used. A RIE was performed at Motorola using a halogen gas chemistry that resulted in a hard mask-to-photoresist etch selectivity >3:1 and sidewall profile angles ⩾85°. Line edge roughness (LER) and linewidth critical dimension (CD) measurements were performed using Sandia’s GORA© CD digital image analysis software. Low LER values (6–9 nm, 3σ, one side) and good CD linearity (better than 10%) were demonstrated for the final pattern-transferred dense polysilicon L/S features from 80 to 175 nm. In addition, pattern transfer (into polysilicon) of loose-pitch (1:2) L/S features with CDs⩾60 nm was demonstrated.
Extreme ultraviolet interferometric measurements of diffraction-limited optics
Authors: Goldberg Kenneth A. , Naulleau Patrick , Bokor Jeffrey
Published: July 2002

At-wavelength interferometric measurements of recently fabricated extreme ultraviolet (EUV) microstepper projection optics have revealed the highest performance for prototype EUV lithographic systems observed to date. The phase-shifting point diffraction interferometer is used to measure and align these two-mirror, multilayer-coated Schwarzschild optical systems designed with a numerical aperture of 0.088 and operating at 13.4 nm wavelength. Root-mean-square wave front error magnitudes as small as 0.60 nm have been achieved, actually exceeding the design tolerance set for these objectives.
Actinic detection of sub-100 nm defects on extreme ultraviolet lithography mask blanks
Authors: Jeong Seongtae , Johnson Lewis , Rekawa Seno , Walton Chris C. , Prisbrey Shon T. , Tejnil Edita , Underwood James H. , Bokor Jeffrey
Published: July 2002

We present recent experimental results from a prototype actinic (operates at the 13 nm extreme ultraviolet wavelength) defect inspection system for extreme ultraviolet lithography mask blanks. The defect sensitivity of the current actinic inspection system is shown to reach 100 nm in experiments with programmed defects. A method to cross register and cross correlate between the actinic inspection system and a commercial visible-light scattering defect inspection system is also demonstrated. Thus, random, native defects identified using the visible-light tool can reliably be found and scanned by our actinic tool. We found that native defects as small as 86 nm (as classified by the visible-light tool) were detectable by the actinic tool. These results demonstrate the capability of this tool for independent defect counting experiments.
Characterization of the manufacturability of ultrathin resist
Authors: Nguyen Khanh B. , Lyons Chris , Schefske Jeff , Pike Chris , Phan Khoi , King Paul , Levinson Harry , Bell Scott , Okoroanyanwu Uzodinma
Published: July 2002

A study was conducted to explore the manufacturability of ultrathin resist by focusing on two key issues, defects and etch resistance. Defects in ultrathin resist were characterized by optical inspection and scanning electron microscopy reviews. A number of representative defect types in the ultrathin resist/hardmask process were identified. With process optimization, defect density in ultrathin resist was reduced to levels that are comparable to that of a baseline 0.5 μm thick resist process on nontopographic wafers. Etch resistance sufficient for patterning metal–oxide–semiconductor transistor gate film stacks was demonstrated for a 100–150 nm thick resist layer.
Critical-dimension control for 100 nm patterns in x-ray lithography
Authors: Tanaka Y. , Iwamoto T. , Fujii K. , Kikuchi Y. , Matsui Y. , Fukuda M. , Morita H.
Published: July 2002

We investigated the printing characteristics of 100 nm line-and-space (L/S) and isolated-line patterns by using TDUR-N908 negative-tone resist, and analyzed the causes of their critical-dimension (CD) variation. For L/S patterns, a positive mask bias improves the dose profile, thereby enlarging the exposure latitude and reducing the linewidth variation of replicated resist patterns. The resist linewidths of L/S and isolated-line patterns at gaps of 10 and 15 μm were within ±10% of the mask linewidth over a wide range of pattern sizes down to 100 nm. The CD variation over the exposure field (24×24 mm) was as small as 8.6 nm (3σ) for L/S and 7.7 nm (3σ) for isolated-line patterns. After the removal of the effects of mask-CD variation and dose nonuniformity from the intrafield CD variation, a systematic CD variation remained for both types of patterns. The cause might be the nonuniformity over the exposure field of the absorber thickness, the x-ray wavelength, and/or the proximity gap.
A holographic phase-shifting interferometer technique to measure in-plane distortion
Authors: Lim Michael H. , Ferrera Juan , Pipe K. P. , Smith Henry I.
Published: July 2002

We describe the use of holographic phase-shifting interferometry (HPSI) to measure in-plane distortion of a substrate in which a shallow grating, produced by interferometric lithography, has been etched. The diffractive metrology inherent in HPSI should enable one to study process-induced distortions down to the level of a few nanometers. We used HPSI to investigate distortion introduced by the anodic bonding of silicon nitride membranes to Pyrex frames in x-ray lithography masks. This was part of an effort to develop an inverted x-ray mask configuration. The HPSI technique gave quantitative measurements of the linear and nonlinear components of distortions in one-dimension. The high levels of distortion found were presumably introduced by the high temperatures used in anodic bonding, combined with the asymmetric manner in which the membranes were brought into contact with the Pyrex frame. The comprehensive, quantitative measurement of distortion provided by HPSI should enable us to modify the mask making process so as to avoid steps that introduce distortion.
Profiling of ultrashallow junctions
Authors: Goeckner M. J. , Felch S. B. , Fang Z. , Oberhofer A. , Chia V. K. F. , Mount G. R. , Poulakos M. , Keenan W. A.
Published: July 2002

Ultrashallow profiles challenge the capabilities of all characterization techniques. In this article, three diagnostic techniques are tested, secondary ion mass spectrometry (SIMS), capacitance–voltage (C–V) profiling and spreading resistance analysis (SRA). SIMS is used to measure the impurity concentration profiles, C–V is used to measure carrier concentration profiles directly and SRA is used to measure resistivity profiles, from which carrier concentrations can be derived. Both SIMS and SRA are calibrated techniques that relate the measured parameter to concentration or resistivity via calibration standards. C–V derives the carrier concentration directly through a mathematical model and calculation. Some of the assumptions, procedures, and limitations of these three techniques for ultrashallow profiles are reviewed and discussed. For this article these diagnostic techniques were used to examine six wafers that had been plasma doped followed by a rapid thermal anneal and three wafers that had been beamline implemented followed by a soak anneal.
Arsenic doped buried plate characterization in deep trenches for a 0.25 μm complementary metal–oxide–semiconductor technology by chemical etching
Authors: Krüger D. , Gaworzewski P. , Kurps R. , Schmidt K. , Luhmann C.
Published: July 2002

We present examples for two-dimensional charge carrier density characterization in advanced semiconductor technologies. The analysis was performed by charge carrier density selective etching in HF:HNO3=1:100–1:200 solutions. We applied a scanning electron microscopy stereo technique for visualization of the two-dimensional etch depth distributions. Different applications of etching techniques are given. Etching of test structures involving sharp boron concentration spikes demonstrates an obtainable lateral resolution in the order of 10 nm. The dependence of the etching rate on charge carrier (dopant) density was analyzed from comparison with reference samples. For etch solutions of HF:HNO3=1:100, we found that etching starts at As densities of about 1×1017 cm−3 for fixed etch times of 10 s. Strongly enhanced etching with a nearly constant etch rate was observed for As densities above 3×1019 cm−3. Application of the stereo scanning electron microscopy method allows two-dimensional characterization of highly B doped source-drain areas in metal–oxide–semiconductor field effect transistors. Furthermore, we performed “n-buried plate” dopant density characterization in a 0.25 μm complementary metal–oxide–semiconductor technology. To now, the application of the scanning electron microscopy stereo technique fails for low As densities of the buried plate near the deep trench. Using focused ion beam techniques at polished cross sections opens a possibility to characterize quasi-three-dimensional dopant distributions on a sub-0.25 μm level, qualitatively.
Nondestructive profile measurements of annealed shallow implants
Authors: Borden P. , Nijmeijer R. , Li J. P. , Bechtler L. , Lingel K.
Published: July 2002

This article demonstrates the capabilities of a new noncontact optical technique for measuring active doping depth of shallow implants. Employing a 2 μm spot size, it provides a fast, nondestructive measurement in dimensions approaching those of individual devices. The method can be used to map micron- and wafer-scale regions, and does not require an edge exclusion zone. Data are presented showing performance on layers varying in depth from 200 to 1200 Å using rapid thermal annealing activated low-energy B11 implants. Center-to-edge diameter scans on 200 mm wafers are presented. In many cases these show edge effects in the outermost centimeter, indicating the importance of minimizing the edge exclusion zone. Measurements are validated through correlation to spreading resistance profiles, and show a depth resolution of better than 3 Å.
Limitations of the calibration curve method for determining dopant profiles from scanning capacitance microscope measurements
Authors: Marchiando J. F. , Kopanski J. J. , Albers J.
Published: July 2002

The calibration or conversion curve method (CCM) refers here to using a database of calculations of the capacitance done for a matrix set of model parameters, such as oxide thickness, uniform dopant density, etc., as one method for interpreting scanning capacitance microscope (SCM) measurements. The method was originally intended to help analyze SCM data for slowly varying dopant profiles, where the condition of quasi charge neutrality could be suitably maintained during SCM measurement. However, when the dopant gradient becomes sufficiently large, the quasi charge neutrality condition becomes less satisfied, and the CCM becomes less accurate (unless the gradient is taken into account in some manner). To help evaluate the limitations of the CCM (without gradient correction) in extracting two-dimensional dopant profiles from simulated SCM data, we analyze two representative ion-implanted steep-gradient dopant profiles currently used in device fabrication. The dopant profiles are derived from Monte Carlo simulation, not from secondary ion mass spectroscopy (SIMS) data. Depending upon the magnitude of the dopant gradient, the CCM results show the dopant profile peak to be both underestimated and displaced. The error in estimating the dopant concentration could be as large as 50% for these representative profiles. The calculations are done for two dimensions.
Low temperature properties of Ba-dispenser cathodes
Authors: Geittner P. , Gärtner G. , Raasch D.
Published: July 2002

The low temperature emission properties of Ba-dispenser cathodes with different metal and metal+oxide top layers were determined as a function of anode voltage and operating time. A “Sc”/Re–I cathode showed room temperature emission, which was nearly four magnitudes larger than for a W–I cathode. At 300 °C, thermionic and field emission were both found superimposed. A Fowler–Nordheim fit derived a remarkable low field emission threshold of 3–4 V/μm. Nevertheless, emission degradation with time was observed due to oxidation of the Ba–O dipoles. At temperatures below 600 °C, oxygen adsorption time constants could be lesser than time constants for desorption of oxygen and resupply the Ba due to oxygen induced desorption, which led to a continuous degradation of emission current. Using oxygen desorption energies and energies required for the Ba resupply (determined from high temperature measurements), we could estimate boundary condition for stable long-term emission. Above 600 °C, required low background pressures in the order of 10−8 mbar are easily obtained. However, at temperatures below 200 °C, stable long-term emission will only be possible at very low total background pressures of at least 10−13 mbar.
Low temperature oxidation and selective etching of chemical vapor deposition a-SiC:H films
Authors: Baklanov M. R. , Van Hove M. , Mannaert G. , Vanhaelemeersch S. , Bender H. , Conard T. , Maex K.
Published: July 2002

Chemical vapor deposition silicon carbide films are considered as a perspective hard mask and stop layer in advanced dry etch technology because of the high chemical and plasma stability. However the a-SiC:H film should be etched away after technological use. In this case the high chemical stability of the a-SiC:H films troubles the solution of this problem. A new approach for the selective removal of the a-SiC:H films is discussed. The basic idea is low-temperature oxidation of the a-SiC:H film and selective removal of the reaction products by wet and/or dry etching. Fourier transform infrared, x-ray photoelectron spectroscopy, and ellipsometry were used for the characterization of the a-SiC:H oxidation and etching of the reaction product. As an example of the practical application, the a-SiC:H films were tested as a dry contact etch stop layer in a 0.18 μm complimentary metal–oxide–semiconductor technology. Results of the electrical evaluation of test structures prepared by this technology are discussed. These results show significant advantage of the new technology compared with the traditional one.
Novolak–diazonaphthoquinone resists: The central role of phenolic strings
Authors: Reiser Arnost , Yan Zhenglin , Han Yu-Kai , Soo Kim Myoung
Published: July 2002

Novolak–diazonaphthoquinone resists are the principal pattern transfer materials of the semiconductor industry, essential tools in building the devices that are making computers possible. In spite of this, there was never a clear understanding of the way Novolak–diazoquinone resists work. In an earlier review [A. Reiser et al., Angew Chem. Int. Ed. Engl. 35, 2428 (1996)] we had presented experiments and ideas that went a long way toward an interpretation of their functional mechanism, yet we had failed in one important respect: we had not recognized the pivotal role of phenolic strings in these systems. Since then we have gained a better understanding of the dissolution process, and the new insights have illuminated and connected seemingly distant phenomena. A coherent and self-consistent picture of dissolution inhibition has emerged. We would like to present it in this article.
Study of AlGaAs/GaAs quantum wells overgrown on in situ Cl2-etched GaAs substrates
Authors: López-López M. , Luyo-Alvarado J. , Meléndez-Lira M. , Cano-Aguilar O. , Megı́a-Garcı́a C. , Ortiz-López J. , Contreras-Puente G. , Ishikawa T.
Published: July 2002

We have studied the properties of in situ Cl2-etched GaAs surfaces and overgrown quantum well (QW) structures as a function of the etching temperature. From reflection high-energy electron diffraction, atomic force microscopy, and Auger electron spectroscopy analysis on Cl2-etched surfaces we found that low etching temperatures (∼50 °C) results in Ga-rich rough surfaces, because at these temperatures the desorption rate of Ga chlorides is much slower than that for As chlorides. At high etching temperatures (∼200 °C) both Ga and As chlorides can be removed, resulting in a more stoichiometric etching which yields a smooth GaAs surface. The optical properties of QW structures overgrown on in situ etched surfaces were compared to those of an ex situ processed sample. For the ex situ processed sample we found degraded photoluminescence (PL) characteristics and a high amount of impurities. The PL properties improved, and the amount of impurities sharply decreased in the in situ sample Cl2 etched at 200 °C. Photoreflectance spectroscopy showed the presence of intense internal electric fields generated by the ex situ processing. The electric field strength was reduced in the in situ Cl2-etched samples.
Photoemission of the SiO2–SiC heterointerface
Authors: O’Brien M. L. , Koitzsch C. , Nemanich R. J.
Published: July 2002

Photoelectron spectroscopy has been performed on SiC surfaces to investigate the valence band characteristics during SiO2 formation. Various stages of the oxide development were investigated. The √3×√3R30° surface is used as the initial surface for the oxidation experiments. The substrates were exposed to a succession of a 30 s oxygen exposure, two 30 s oxygen plasmas, and finally, a plasma-enhanced chemical vapor deposition SiO2 deposition. Ultraviolet photoemission spectroscopy was employed to measure the valence band discontinuity for the oxide on n-type 6H and n-type 4H SiC substrates for each step in the oxidation process. X-ray photoemission spectroscopy was used to confirm the valence band offset. The valence band discontinuity was determined to be 2.0 eV. Furthermore, the location of the valence band maximum of the SiC to the conduction band minimum of the SiO2 is determined to be a constant (∼7.0 eV) between 6H and 4H SiC. Band bending effects are directly measured from ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy. From the UPS measurements of the band bending effects, the interface state density is determined to be ∼5×1012 cm−2.
Optimization of buffer layers for InGaAs/AlGaAs PIN optical modulators grown on GaAs substrates by molecular beam epitaxy
Authors: Katzer D. S. , Rabinovich W. S. , Ikossi-Anastasiou K. , Gilbreath G. C.
Published: July 2002

In this work we compare the effect of the buffer layer on the device quality and surface morphology of strained InGaAs/AlGaAs PIN multiple quantum well (MQW) modulators. We examine GaAs buffer layers and linearly graded InGaAs buffer layers. Our results indicate that for lower indium concentrations in the quantum wells (less than about 23%) better device performance and surface morphology are obtained by growing directly on GaAs. PIN MQWs with indium mole fractions higher than about 24% have better properties when a linearly graded buffer layer is used.
Detecting Energy Emissions from a Rotating Black Hole
Authors: van Putten Maurice H. P. M. , Levinson Amir
Published: July 2002

The rotational energy of a black hole surrounded by a torus is released through several channels. We have determined that a minor fraction of the energy is released in baryon-poor outflows from a differentially rotating open magnetic flux tube, and a major fraction of about η/2 is released in gravitational radiation by the torus with angular velocity η ∼ 0.2 to 0.5 relative to that of the black hole. We associate the energy emitted in baryon-poor outflows with gamma-ray bursts. The remaining fraction is released in torus winds, thermal emissions, and (conceivably) megaelectron-volt neutrino emissions. The emitted gravitational radiation can be detected by gravitational wave experiments and provides a method for identifying Kerr black holes in the Universe.
Rapid Sea-Level Rise Soon from West Antarctic Ice-Sheet Collapse?
Authors: Bentley Charles R.
Published: July 2002

There has been recent concern over whether shrinkage of ice sheets in Antarctica caused by climate change could lead to rapid global sea-level rise. In his Perspective, Bentley discusses the processes at work in ice-sheet behavior and concludes that, in view of the evidence for ice-sheet stability, catastrophic changes are unlikely in the next century or two.
A Spliceosomal Recycling Factor That Reanneals U4 and U6 Small Nuclear Ribonucleoprotein Particles
Authors: Raghunathan Pratima L. , Guthrie Christine
Published: July 2002

The spliceosome removes introns from pre–messenger RNAs by a mechanism that entails extensive remodeling of RNA structure. The most conspicuous rearrangement involves disruption of 24 base pairs between U4 and U6 small nuclear RNAs (snRNAs). Here, the yeast RNA binding protein Prp24 is shown to reanneal these snRNAs. When Prp24 is absent, unpaired U4 and U6 small nuclear ribonucleoprotein particles (snRNPs) accumulate; with time, splicing becomes inhibited. Addition of purified Prp24 protein regenerates duplex U4/U6 snRNPs for new rounds of splicing. The reannealing reaction catalyzed by Prp24 proceeds more efficiently with snRNPs than with deproteinized snRNAs.
Resolution of a Signal Transfer Region from a General Binding Domain in Gβ for Stimulation of Phospholipase C-β2
Authors: Buck Elizabeth , Li Jinrong , Chen Yibang , Weng Gezhi , Scarlata Suzanne , Iyengar Ravi
Published: July 2002

Signaling by guanine nucleotide–binding proteins (G proteins) involves sequential protein-protein interactions. G protein–βγ subunit (Gβγ) interactions with phospholipase C–β2 (PLC-β2) were studied to determine if all Gβ contacts are required for signaling. A peptide encoding Gβ amino acid residues 86 to 105 stimulated PLC-β2. Six residues (96 to 101) within this sequence could transfer signals and thus constitute a core signal transfer region. Another peptide, encoding Gβ amino acid residues 115 to 135, did not substantially stimulate PLC-β2 by itself but inhibited Gβγ stimulation, indicating that residues 115 to 135 constitute a general binding domain. Resolution of signal transfer regions from general binding domains indicates that all protein-protein contacts are not required for signal transfer and that it may be feasible to synthesize agonists and antagonists that regulate intracellular signal flow.
The Robust Australopithecine Face: A Morphogenetic Perspective
Authors: McCollum Melanie A.
Published: July 2002

The robust australopithecines were a side branch of human evolution. They share a number of unique craniodental features that suggest their monophyletic origin. However, virtually all of these traits appear to reflect a singular pattern of nasomaxillary modeling derived from their unusual dental proportions. Therefore, recent cladistic analyses have not resolved the phylogenetic history of these early hominids. Efforts to increase cladistic resolution by defining traits at greater levels of anatomical detail have instead introduced substantial phyletic error.
Identification of an RNA-Protein Bridge Spanning the Ribosomal Subunit Interface
Authors: Culver Gloria M. , Cate Jamie H. , Yusupova G. Zh. , Yusupov Marat M. , Noller Harry F.
Published: July 2002

The 7.8 angstrom crystal structure of the 70
S
ribosome reveals a discrete double-helical bridge (B4) that projects from the 50
S
subunit, making contact with the 30
S
subunit. Preliminary modeling studies localized its contact site, near the bottom of the platform, to the binding site for ribosomal protein S15. Directed hydroxyl radical probing from iron(II) tethered to S15 specifically cleaved nucleotides in the 715 loop of domain II of 23
S
ribosomal RNA, one of the known sites in 23
S
ribosomal RNA that are footprinted by the 30
S
subunit. Reconstitution studies show that protection of the 715 loop, but none of the other 30
S
-dependent protections, is correlated with the presence of S15 in the 30
S
subunit. The 715 loop is specifically protected by binding free S15 to 50
S
subunits. Moreover, the previously determined structure of a homologous stem-loop from U2 small nuclear RNA fits closely to the electron density of the bridge.
Necessity for Afferent Activity to Maintain Eye-Specific Segregation in Ferret Lateral Geniculate Nucleus
Authors: Chapman Barbara
Published: July 2002

In the adult mammal, retinal ganglion cell axon arbors are restricted to eye-specific layers in the lateral geniculate nucleus. Blocking neuronal activity early in development prevents this segregation from occurring. To test whether activity is also required to maintain eye-specific segregation, ganglion cell activity was blocked after segregation was established. This caused desegregation, so that both eyes' axons became concentrated in lamina A, normally occupied only by contralateral afferents. These results show that an activity-dependent process is necessary for maintaining eye-specific segregation and suggest that activity-independent cues may favor lamina A as the target for arborization of afferents from both eyes.
Characterization of Fluoroquinolone Resistance among Veterinary Isolates of Avian
Escherichia coli
Authors: White David G. , Piddock Laura J. V. , Maurer John J. , Zhao Shaohua , Ricci Vito , Thayer Stephan G.
Published: July 2002

Fluoroquinolone-resistant avian
Escherichia coli
isolates from northern Georgia were investigated for
gyrA
and
parC
mutations. All isolates contained a mutation in GyrA replacing Ser83 with Leu; seven isolates also contained mutations replacing Asp87 with either Gly or Tyr. Random amplified polymorphic DNA analysis revealed that quinolone-resistant
E. coli
isolates were genetically diverse.
Characterization of an In Vitro-Selected Amoxicillin-Resistant Strain of
Helicobacter pylori
Authors: DeLoney Cindy R. , Schiller Neal L.
Published: July 2002

An amoxicillin-resistant (Amox
r
) strain of
Helicobacter pylori
was selected for by culturing an amoxicillin-sensitive (Amox
s
) strain in increasingly higher concentrations of amoxicillin, resulting in a 133-fold increase in MIC, from 0.03 to 0.06 μg/ml to 4 to 8 μg/ml. This resistance was stable upon freezing for at least 6 months and conferred cross-resistance to seven other β-lactam antibiotics. β-Lactamase activity was not detected in this Amox
r
strain; however, analysis of the penicillin-binding protein (PBP) profiles generated from isolated bacterial membranes of the Amox
s
parental strain and the Amox
r
strain revealed a significant decrease in labeling of PBP 1 by biotinylated amoxicillin (bio-Amox) in the Amox
r
strain. Comparative binding studies of PBP 1 for several β-lactams demonstrated that PBP 1 in the Amox
r
strain had decreased affinity for mezlocillin but not significantly decreased affinity for penicillin G. In addition, PBP profiles prepared from whole bacterial cells showed decreased labeling of PBP 1 and PBP 2 in the Amox
r
strain at all bio-Amox concentrations tested, suggesting a diffusional barrier to bio-Amox or a possible antibiotic efflux mechanism. Uptake analysis of
14
C-labeled penicillin G showed a significant decrease in uptake of the labeled antibiotic by the Amox
r
strain compared to the Amox
s
strain, which was not affected by pretreatment with carbonyl cyanide
m
-chlorophenylhydrazone, eliminating the possibility of an efflux mechanism in the resistant strain. These results demonstrate that alterations in PBP 1 and in the uptake of β-lactam antibiotics in
H. pylori
can be selected for by prolonged exposure to amoxicillin, resulting in increased resistance to this antibiotic.
Susceptibility to PNU-140690 (Tipranavir) of Human Immunodeficiency Virus Type 1 Isolates Derived from Patients with Multidrug Resistance to Other Protease Inhibitors
Authors: Rusconi Stefano , La Seta Catamancio Simona , Citterio Paola , Kurtagic Semir , Violin Michela , Balotta Claudia , Moroni Mauro , Galli Massimo , d'Arminio-Monforte Antonella
Published: July 2002

In our study we examined the anti-human immunodeficiency virus type 1 (anti-HIV-1) activity of a novel HIV-1 protease inhibitor, PNU-140690 (tipranavir), against patient-derived isolates resistant to multiple other protease inhibitors (PIs). The aim of our experiments was to investigate the genotypes and the in vitro phenotypes of drug resistance of PNU-140690. We carried out drug susceptibility tests with peripheral blood mononuclear cells and a fixed amount of infectious virus (1,000 50% tissue culture infective doses) to determine the 50% inhibitory concentration (IC
50
) and IC
90
, PCR assays for the detection of drug resistance mutations in RNA in plasma, and direct sequencing of PCR products. Phenotypic resistance to PIs was invariably related to genotypic mutations. The substitutions among the amino acid residues of the protease included L10I, K20R, L24I, M36I, N37D, G48V, I54V, L63P, I64V, A71V, V77I, V82A, I84V, and L90M. Isolates from all of the patients had developed a maximal degree of resistance to indinavir, ritonavir, and nelfinavir (IC
50
s, >0.1 μM). We also compared these mutations with the amino acid changes previously described in association with in vivo tipranavir administration. The mutations included the following: I15V, E35D, N37D, R41K, D60E, and A71T. Infections with IIIB, 14aPre, and N70 were inhibited by an average drug IC
90
of 0.18 ± 0.02 μM in multiple experiments. The average mean ± standard error of mean IC
90
for the entire group of multidrug-resistant isolates derived from the mean values for two culture wells with p24 antigen supernatant appeared to be 0.619 ± 0.055 μM (range, 0.31 to 0.86 μM). Tipranavir retained a sustained antiviral activity against PI-MDR clinical isolates and might be useful in combination regimens with other antiretroviral agents for patients who have already failed other PI-containing therapies.
The
Legionella
(
Fluoribacter
)
gormanii
Metallo-β-Lactamase: a New Member of the Highly Divergent Lineage of Molecular-Subclass B3 β-Lactamases
Authors: Boschi Letizia , Mercuri Paola Sandra , Riccio Maria Letizia , Amicosante Gianfranco , Galleni Moreno , Frère Jean-Marie , Rossolini Gian Maria
Published: July 2002

A metallo-β-lactamase determinant was cloned from a genomic library of
Legionella
(
Fluoribacter
)
gormanii
ATCC 33297
T
constructed in the plasmid vector pACYC184 and transformed into
Escherichia coli
DH5α, by screening for clones showing a reduced susceptibility to imipenem. The product of the cloned determinant, named FEZ-1, contains a 30-kDa polypeptide and exhibits an isoelectric pH of 7.6. Sequencing revealed that FEZ-1 is a molecular-class B β-lactamase which shares the closest structural similarity (29.7% of identical residues) with the L1 enzyme of
Stenotrophomonas maltophilia
, being a new member of the highly divergent subclass B3 lineage. All the residues that in L1 are known to be directly or indirectly involved in coordination of the zinc ions were found to be conserved also in FEZ-1, suggesting that the geometry of zinc coordination in the active site of the latter enzyme is identical to that of L1. Unlike L1, however, FEZ-1 appeared to be monomeric in gel permeation chromatography experiments and exhibited a distinctive substrate specificity with a marked preference for cephalosporins and meropenem. The properties of FEZ-1 overall resembled those of a β-lactamase previously purified from the same strain of
L. gormanii
(T. Fujii, K. Sato, K. Miyata, M. Inoue, and S. Mitsuhashi, Antimicrob. Agents Chemother. 29:925–926, 1986) and are as yet unique among class B enzymes, reinforcing the notion that considerable functional heterogeneity can be encountered among members of this class. A system for overexpression of the
bla
FEZ-1
gene in
E. coli
, based on the T7 phage promoter, was also developed.
TEM-72, a New Extended-Spectrum β-Lactamase Detected in
Proteus mirabilis
and
Morganella morganii
in Italy
Authors: Perilli Mariagrazia , Segatore Bernardetta , Rosaria De Massis Maria , Riccio Maria Letizia , Bianchi Ciro , Zollo Alessandro , Rossolini Gian Maria , Amicosante Gianfranco
Published: July 2002

A new natural TEM-2 derivative, named TEM-72, was identified in a
Proteus mirabilis
strain and in a
Morganella morganii
strain isolated in Italy in 1999. Compared to TEM-1, TEM-72 contains the following amino acid substitutions: Q39K, M182T, G238S, and E240K. Kinetic analysis showed that TEM-72 exhibits an extended-spectrum activity, including activity against oxyimino-cephalosporins and aztreonam. Expression of
bla
TEM-72
in
Escherichia coli
was capable of decreasing the host susceptibility to the above drugs.
Use of Saliva Specimens for Monitoring Indinavir Therapy in Human Immunodeficiency Virus-Infected Patients
Authors: Wintergerst Uwe , Kurowski M. , Rolinski B. , Müller M. , Wolf E. , Jaeger H. , Belohradsky B. H.
Published: July 2002

Indinavir concentrations were determined in plasma and saliva over a random period of 4 h. On average, levels in saliva were 70% ± 38% of the corresponding levels in plasma. These findings suggest that saliva might serve as an appropriate specimen for monitoring of plasma indinavir levels in patients treated with indinavir.
Pharmacodynamics of Gemifloxacin against
Streptococcus pneumoniae
in an In Vitro Pharmacokinetic Model of Infection
Authors: MacGowan Alasdair P. , Rogers Chris A. , Holt H. Alan , Wootton Mandy , Bowker Karen E.
Published: July 2002

The pharmacodynamics of gemifloxacin against
Streptococcus pneumoniae
were investigated in a dilutional pharmacodynamic model of infection. Dose fractionation was used to simulate concentrations of gemifloxacin in human serum associated with 640 mg every 48 h (one dose), 320 mg every 24 h (two doses), and 160 mg every 12 h (four doses). Five strains of
S. pneumoniae
for which MICs were 0.016, 0.06, 0.1, 0.16, and 0.24 mg/liter were used to assess the antibacterial effect of gemifloxacin. An inoculum of 10
7
to 10
8
CFU/ml was used, and each experiment was performed at least in triplicate. The pharmacodynamic parameters (area under the concentration-time curve [AUC]/MIC, maximum concentration of drug in serum [
C
max
]/MIC, and the time that the serum drug concentration remains higher than the MIC [
T
> MIC]) were related to antibacterial effect as measured by the area under the bacterial-kill curve from 0 to 48 h (AUBKC
48
) using an inhibitory sigmoid
E
max
model. Weighted least-squares regression was used to predict the effect of the pharmacodynamic parameters on AUBKC
48
, and Cox proportional-hazards regression was used to predict the effect of the three pharmacodynamic parameters on the time needed to kill 99.9% of the starting inoculum (
T
99.9). There was a clear relationship between strain susceptibility and clearance from the model. The simulations (160 mg every 12 h) were associated with slower initial clearance than were the other simulations; in contrast, bacterial regrowth occurred with the 640-mg simulation when MICs were ≥0.1 mg/liter. The percentage coefficient of variance was 19% for AUBKC
48
, and the inhibitory sigmoid
E
max
model best fit the relationship between AUBKC
48
and AUC/MIC.
C
max
/MIC and
T
> MIC fit less well. The maximum response occurred at an AUC/MIC of >300 to 400. In weighted least-squares regression analysis, there was no evidence that
C
max
/MIC was predictive of AUBKC
48
, but both AUC/MIC and
T
> MIC were. A repeat analysis using only data for which the
T
> MIC was >75% and for which hence regrowth was minimized indicated that AUC/MIC alone was predictive of AUBKC
48
. Initial univariate analysis indicated that all three pharmacodynamic parameters were predictive of
T
99.9, but in the multivariate model only
C
max
/MIC reached significance. These data indicate that gemifloxacin is an effective antipneumococcal agent and that AUC/MIC is the best predictor of antibacterial effect as measured by AUBKC
48
. However,
C
max
/MIC is the best predictor of speed of kill, as measured by
T
99.9.
T
> MIC also has a role in determining AUBKC
48
, especially when the dose spacing is considerable. Once-daily dosing seems most suitable for gemifloxacin.
Dosage-Dependent Antifungal Efficacy of V-Echinocandin (LY303366) against Experimental Fluconazole-Resistant Oropharyngeal and Esophageal Candidiasis
Authors: Petraitis Vidmantas , Petraitiene Ruta , Groll Andreas H. , Sein Tin , Schaufele Robert L. , Lyman Caron A. , Francesconi Andrea , Bacher John , Piscitelli Stephen C. , Walsh Thomas J.
Published: July 2002

V-echinocandin (VER-002; LY303366) is a semisynthetic derivative of echinocandin B and a potent inhibitor of fungal (1, 3)-β-
d
-glucan synthase. We studied the antifungal efficacy, the concentrations in saliva and tissue, and the safety of VER-002 at escalating dosages against experimental oropharyngeal and esophageal candidiasis caused by fluconazole-resistant
Candida albicans
in immunocompromised rabbits. Study groups consisted of untreated controls, animals treated with VER-002 at 1, 2.5, and 5 mg/kg of body weight/day intravenously (i.v.), animals treated with fluconazole at 2 mg/kg/day i.v., or animals treated with amphotericin B at 0.3 mg/kg/day. VER-002-treated animals showed a significant dosage-dependent clearance of
C. albicans
from the tongue, oropharynx, esophagus, stomach, and duodenum in comparison to that for untreated controls. VER-002 also was superior to amphotericin B and fluconazole in clearing the organism from all sites studied. These in vivo findings are consistent with the results of in vitro time-kill assays, which demonstrated that VER-002 has concentration-dependent fungicidal activity. Esophageal tissue VER-002 concentrations were dosage proportional and exceeded the MIC at all dosages. Echinocandin concentrations in saliva were greater than or equal to the MICs at all dosages. There was no elevation of serum hepatic transaminase, alkaline phosphatase, bilirubin, potassium, or creatinine levels in VER-002-treated rabbits. In summary, the echinocandin VER-002 was well tolerated, penetrated the esophagus and salivary glands, and demonstrated dosage-dependent antifungal activity against fluconazole-resistant esophageal candidiasis in immunocompromised rabbits.
Pharmacokinetics and Inflammatory Fluid Penetration of Intravenous Daptomycin in Volunteers
Authors: Wise R. , Gee T. , Andrews J. M. , Dvorchik B. , Marshall G.
Published: July 2002

The lipopeptide antimicrobial daptomycin was administered intravenously at a dose of 4 mg/kg of body weight to seven healthy male volunteers. The concentrations of daptomycin in plasma, cantharidin-induced inflammatory fluid, and urine were measured by a microbiological assay. The mean ± standard deviation peak concentrations in plasma and inflammatory fluid were 77.5 ± 8.3 and 27.6 ± 9.5 μg/ml, respectively; the mean terminal elimination half-lives were 7.74 and 13.2 h, respectively. The overall penetration of total drug into the inflammatory fluid (measured by ratio of the area under the concentration-time curve from 0 to 24 h for inflammatory fluid compared with that for plasma) was 68.4%. The mean urinary recovery over 24 h was 59.7%.
International Interlaboratory Quality Control Program for Measurement of Antiretroviral Drugs in Plasma
Authors: Aarnoutse Rob E. , Verweij-van Wissen Corrien P. W. G. M. , van Ewijk-Beneken Kolmer Eleonora W. J. , Wuis Eveline W. , Koopmans Peter P. , Hekster Yechiel A. , Burger David M.
Published: July 2002

An international interlaboratory quality control program for measurement of antiretroviral drugs was initiated. The first round was confined to protease inhibitors and showed large variability in the performance of participating laboratories. The results demonstrate the need for and utility of an ongoing quality control program in this area of bioanalysis.
In Vitro Activities of Tigecycline (GAR-936) against Recently Isolated Clinical Bacteria in Spain
Authors: Betriu Carmen , Rodríguez-Avial Iciar , Sánchez Blas Ali , Gómez María , Álvarez Juan , Picazo Juan J.
Published: July 2002

The antimicrobial activities of tigecycline (GAR-936) were compared with those of other agents against 1,087 strains recently isolated in 12 Spanish medical centers. Tigecycline showed activity against a wide spectrum of aerobic and anaerobic bacteria, including strains such as methicillin-resistant
Staphylococcus aureus
, coagulase-negative staphylococci, penicillin-resistant
Streptococcus pneumoniae
,
Enterococcus faecium
,
Acinetobacter baumannii
, and
Stenotrophomonas maltophilia.
Activities of New Fluoroquinolones, Ketolides, and Other Antimicrobials against Blood Culture Isolates of Viridans Group Streptococci from across Canada, 2000
Authors: Gershon Andrea S. , de Azavedo Joyce C. S. , McGeer Allison , Ostrowska Krystyna I. , Church Deirdre , Hoban Daryl J. , Harding Godfrey K. M. , Weiss Karl , Abbott Lewis , Smaill Fiona , Gourdeau Marie , Murray Gilles , Low Donald E.
Published: July 2002

The rates of nonsusceptibility to penicillin, erythromycin, and clindamycin of 191 blood culture isolates of viridans group streptococci collected from across Canada in 2000 were 36, 42, and 10%, respectively. Although 8% of the strains were resistant to ciprofloxacin (MIC ⩾ 4 μg/ml), the MICs of gemifloxacin, BMS 284756, telithromycin, and ABT 773 at which 90% of the strains were inhibited were 0.06, 0.06, 0.12, and 0.03 μg/ml, respectively.
Pharmacodynamics of Moxifloxacin against
Streptococcus pyogenes
in an In Vitro Kinetic Model
Authors: Odenholt Inga , Löwdin Elisabeth , Gustafsson Ingegerd , Cars Otto
Published: July 2002

The aim of the present study was to investigate the pharmacodynamics of moxifloxacin against strains of
Streptococcus pyogenes
with different susceptibilities to erythromycin by using an in vitro kinetic model simulating human pharmacokinetics of moxifloxacin at oral doses of 400 and 200 mg, respectively. When the different strains of
S. pyogenes
were exposed to the higher dose, the number of bacteria was reduced below the detection limit after 12 h and no regrowth was noted during the following 12 h. At the lower dose there was regrowth of the strains with constitutive and inducible erythromycin resistance of the MLS
B
phenotype. Replication assays of the regrowing bacteria indicated that the failure of moxifloxacin to kill the MLS
B
strains at the lower dose was likely caused by the emergence of preexisting resistant subpopulations. Thus, the present study indicates that the presently used 400-mg dose seems to have an advantage over the lower dose in that the risk for selection of resistant subpopulations is minimized.
Simultaneous Direct Counting of Total and Specific Microbial Cells in Seawater, Using a Deep-Sea Microbe as Target
Authors: Maruyama Akihiko , Sunamura Michinari
Published: July 2002

To rapidly and accurately enumerate total and specific microbes in aquatic samples, fluorescent in situ hybridization was combined with direct counting via direct immobilization of cells on a polymer-coated Nuclepore filter. The technique, named FISH-DC, achieved almost complete recovery of total cells and reproducibility of
Psychrobacter pacificensis
cells of deep-sea origin (error, ≤3%) in a mixed culture and in natural seawater. Target cells immobilized on the filter were also successfully enumerated after stringent 3-cycle hybridization and even after a 16-month preservation at −30°C.