bluuebunny/crossref_metadata_2025 huggingface dataset:
Scanning tunneling microscope and electron beam induced luminescence in quantum wires
Authors: Samuelson L. , Gustafsson A. , Lindahl J. , Montelius L. , Pistol M.-E. , Malm J.-O. , Vermeire G. , Demeester P.
Published: July 2002

Quantum wire structures of GaAs/AlGaAs have been grown by metalorganic vapor phase epitaxy in V grooves using pre-etched corrugated substrates and have been characterized by high-resolution transmission electron microscopy. Low-temperature cathodoluminescence (CL) identifies luminescence peaks with the spatial distributions of the different recombinations, achieving a top view spatial resolution of ≊0.2 μm in the CL images. Principally we report how a scanning tunneling microscope (STM) induces local luminescence in the sample structure, and we spectrally resolve STM-induced luminescence for the tip in different positions relative to the wires. We have recorded the luminescence from a single wire and observed band-filling effects resulting from varying levels of excitation into a wire. We have demonstrated the difference between recombination of electron-hole pairs generated in CL and the recombination of injected holes from the STM tip with a thermalized distribution of accumulated electrons in scanning tunneling luminescence.
New optics design methodology using diffraction grating on spherical mirrors for soft x-ray projection lithography
Authors: Fukuda Hiroshi , Terasawa Tsuneo
Published: July 2002

Diffraction gratings on spherical mirrors can be used instead of aspheric mirrors in soft x-ray projection lithography. A method for converting aspheric mirrors to gratings on spherical mirrors is introduced, and used to design optics for soft x-ray projection lithography. Ray tracing through the new grating optics shows resolution capability better than 0.1 μm which is comparable to that of the conventional aspheric mirror optics. To reduce chromatic aberration, however, the spectral width of the system should be less than 0.05 nm for 13 nm wavelength. The gratings typically have spacing of 50–1000 μm, and can be fabricated using a binary process. This approach reduces stringent accuracy requirements of aspheric mirror optics for soft x-ray projection lithography.
Depth profiling of dopants in thin gate oxides in complementary metal– oxide–semiconductor structures by resonance ionization mass spectrometry
Authors: Downey S. W. , Emerson A. B. , Georgiou G. E. , Bevk J. , Kistler R. C. , Moriya N. , Jacobson D. C. , Wise M. L.
Published: July 2002

The thickness of gate dielectrics for complementary metal–oxide–semiconductor (CMOS) devices with 0.1 to 0.35 μm gate dimensions is typically 4 to 7 nm. Besides serving as an insulator, the dielectric, usually SiO2, also acts as a diffusion barrier between the doped polycrystalline silicon gate material and the Si substrate (channel). Dopant diffusion from the poly-gate into the device channel may cause undesirable shifts in the transistor threshold voltage. The determination of dopant diffusion through the gate dielectric into the channel region of the transistor is necessary to optimized device processing. This paper reports a method, resonance ionization mass spectrometry (RIMS), to measure dopant distribution profiles near the gate oxide. The pertinent spectroscopy of sputtered B and P atoms is demonstrated. Matrix effects are shown to be diminished. RIMS depth profiles of CMOS test structures show both dopant penetration into the channel and blockage by the gate oxide depending upon annealing conditions.
Lattice and defect structures of polymerizable diacetylene Langmuir–Blodgett films studied by scanning force microscopy
Authors: Vithana Hemasiri , Johnson David , Shih Raymond , Mann J. Adin , Lando Jerome
Published: July 2002

The scanning force microscope has been used to study the lattice and defect structures of multilayers of the unsaturated fatty acid, 12-8 diacetylene (10,12-pentacosadiynoic acid) in ambient conditions. Films were prepared by the Langmuir–Blodgett technique on ordinary microscope glass and indium tin oxide coated glass. Lattice structures were deduced from the well resolved molecular images and, before polymerization, were found to be nearly centered rectangular with lattice parameters (0.88±0.06) nm and (0.51±0.04) nm. After exposing to ultraviolet radiation for polymerization the lattice structure changed to an oblique lattice with lattice parameters (0.466±0.008) nm and (0.55±0.01) nm. Molecular level defects such as dislocations and grain boundaries were resolved in these films very clearly. Observation of these kinds of defects implies that it is possible to reliably image the real surface molecules under ambient conditions. Polymerization was found to take place in one of the lattice directions and the modulation perpendicular to that direction was more pronounced than along the polymer backbone.
Characterization and modeling of sidewall defects in selective epitaxial growth of silicon
Authors: Bashir R. , Neudeck G. W. , Haw Y. , Kvam E. P.
Published: July 2002

The sidewall defects in selective epitaxial growth (SEG) of silicon were characterized and the nature of these defects was investigated. Electrical characterization of the sidewall defects was performed using diodes fabricated in structures using the SEG of silicon and chemical-mechanical polishing. Diodes were fabricated with various perimeter to area ratios to extract the bulk and sidewall saturation current densities and ideality factors in as-grown SEG diodes and reoxidized sidewall SEG diodes. Transmission electron microscopy was used to show that nitrogen annealing of the sample with the sidewall oxide removed exhibited a dramatic decrease in the sidewall defects as compared to the sample with the sidewall oxide present during the anneal. The generation of the defects was attributed to stress due to the mismatch in thermal expansion coefficients of oxide and silicon and a model describing the generation of these defects was formulated and described.
Distinct imaging of the nucleic acid bases on SrTiO3(100) surface by scanning tunneling microscopy
Authors: Tanaka Hiroyuki , Kawai Tomoji
Published: July 2002

Scanning tunneling microscopy (STM) imaging of four nucleic acid bases, adenine, guanine, thymine, and cytosine, has been performed on the surface of SrTiO3 single-crystal substrates. The images indicate that the bases are deposited as individual molecules in ultrahigh vacuum condition, and can be individually observed by STM. A comparison of the heights and lateral dimensions of the deposited species and also the bias dependence of the heights, enables differentiation between four nucleic acid bases in real space.
Comparison of damage and Si oxidation kinetics resulting from electron cyclotron resonance and distributed electron cyclotron resonance plasma processing
Authors: Hu Y. Z. , Li M. , Wang Y. , Irene E. A. , Hugon M. C. , Varniere F. , Jiang N. , Froment M. , Agius B.
Published: July 2002

Interface damage and oxidation behavior are compared for electron cyclotron resonance plasma oxidation of silicon for two different plasma system configurations: a conventional system where the plasma is generated and extracted normal to the Si wafer surface, and another system where the plasma is made uniform by developing the magnetic field parallel to the substrate using permanent magnets outside the chamber and antennas inside, viz. distributed electron cyclotron resonance (DECR) plasma. Samples were compared using spectroscopic ellipsometry, nuclear reaction analysis, and transmission electron microscopy. Less damage was observed in the DECR plasma configuration, but otherwise the systems were comparable.
Characteristics of δ -doped InGaAs/GaAs pseudomorphic double-quantum-well high electron mobility transistors
Authors: Hsu R. T. , Kao M. J. , Wang J. S. , Hsu W. C.
Published: July 2002

δ-doped InGaAs/GaAs pseudomorphic double-quantum-well high electron mobility transistors grown by low pressure metalorganic chemical vapor deposition are fabricated and demonstrated. The heterostructure with 2 μm gate length and 100 Å channel spacer thickness shows a peak extrinsic transconductance and a maximum saturation current density of 121 mS/mm and 423 mA/mm at 300 K, respectively. Meanwhile, an interesting broad twin-peak transconductance plateau (∼4 V) is observed.
BCl3/N2 dry etching of InP, InAlP, and InGaP
Authors: Ren F. , Lothian J. R. , Kuo J. M. , Hobson W. S. , Lopata J. , Caballero J. A. , Pearton S. J. , Cole M. W.
Published: July 2002

Etch rates for InP, InAlP, and InGaP can be controlled between ∼1500 Å/min and ≳1 μm/min by varying the microwave power, rf power, or N2 composition in BCl3N2 discharges under electron cyclotron resonance conditions. The surface morphology of InGaP is poor at low microwave power (250 W), but becomes very smooth at high powers (1000 W). InP has exactly the opposite dependence on microwave power, while InAlP shows little change in morphology over a wide range of powers. The high ion current under electron cyclotron resonance conditions enables etching of In-based semiconductors without the need for high sample temperatures to enhance desorption of InClX species.
Semiconductor on glass photocathodes as high-performance sources for parallel electron beam lithography
Authors: Schneider J. E. , Baum A. W. , Winograd G. I. , Pease R. F. W. , McCord M. , Spicer W. E. , Costello K. A. , Aebi V. W.
Published: July 2002

The throughput of electron beam lithography has historically been limited by electron–electron interactions that cause blurring at high currents. We present a system configuration for maskless parallel electron beam lithography using a new multiple primary source technology that, by employing widely spaced beams, significantly reduces this problem. The proposed source technology, a negative electron affinity (NEA) photocathode, allows us to generate an array of high brightness, low energy spread, independently modulated beams over a large area. In order to assess the effects of electron–electron interactions in this system, Monte Carlo simulations have been performed. The results of these calculations indicate that this configuration enjoys significant advantages over existing maskless systems. By restricting the area of emission for the individual beamlets to submicron dimensions, the blurring due to statistical electron–electron interactions can be significantly reduced for a given current at the wafer. For example, at 50 kV a total current of more than 2.5 μA can be obtained with less than 10 nm beam blurring. Preliminary experimental results suggest that high brightness emission can be maintained from a NEA photocathode in a demountable vacuum system.
Current characteristics in near field emission scanning tunneling microscopes
Authors: Mesa G. , Sáenz J. J. , García R.
Published: July 2002

The operation of the scanning tunneling microscope (STM) in the near field emission regime provides a direct, noninvasive approach for investigating surfaces at nanometer scale. Here, we present a study of the current characteristics in a near field emission STM. The influence of tip’s geometry is analyzed. We show that the electron field emission from the sample is stable against tip-shape changes due to adsorbate diffusion or atomic rearrangements.
(NH4)2Sx preepitaxial treatment for GaAs chemical beam epitaxy regrowth
Authors: Sik H. , Driad R. , Legay P. , Juhel M. , Harmand J. C. , Launay P. , Alexandre F.
Published: July 2002

This article describes a novel preepitaxial cleaning treatment of GaAs, using a (NH4)2Sx solution. This cleaning treatment is characterized by low carbon and oxygen surface contamination. The treated-surface presents a low temperature (2×1) reconstruction (<500 °C), which allows selective chemical beam epitaxy processes at low temperature. Applying this process to the selective regrowth of the base contact layer of heterojunction bipolar transistors (HBTs), high dc current gains are measured, similar to those obtained with HBTs processed in a conventional way.
Effect of surface pretreatment of submicron contact hole on selective tungsten chemical vapor deposition
Authors: Yeh Wen-Kuan , Chen Mao-Chieh , Lin Mou-Shiung
Published: July 2002

The effects of various predeposition treatments of silicon substrates on selective tungsten chemical vapor deposition were investigated using the silane reduction process. The predeposition treatments include HF or buffered HF wet etch and/or NF3 plasma etch. The experimental results show remarkable differences in the initial nucleation of W and the smoothness of the W surface and the W/Si interface among the various treatments, as revealed by scanning electron microscope inspection and atomic force microscope analysis. Plasma etch leads to a rough W/Si interface while wet treatment with HF dip results in a fairly smooth interface. The experimental results show that better I–V characteristics for the W/n-Si Schottky contact can be obtained by the wet etch treatment prior to the W deposition. The reverse bias junction leakage for the wet etch pretreated W/p+n junction diode is smaller than that of the plasma pretreated diode. In addition, the substrate surface treatments were found to result in Si consumption of various degrees.
Rapid thermal oxidation of Si using a mixture of N2O and O2 at various partial pressure of N2O
Authors: Eftekhari G.
Published: July 2002

To increase the rate of oxidation of Si in N2O, oxygen was added to N2O. The effect of varying the percentage of N2O in the total gas on the quality of the oxide was examined. It was experimentally determined that for a percentage of N2O more than about 50, the quality of the oxide is comparable to the oxide grown using pure N2O. This may suggest that to improve the quality of thermal oxide on Si (especially its hardness against hot carriers) a certain level of nitrogen atoms is sufficient. Increasing the nitrogen content of the oxide beyond that level does not further improve the oxide hardness and only results in smaller oxide growth rate.
Atomic force microscopy reconstruction of G-wire DNAa)
Authors: Vesenka James , Marsh Thomas , Miller Richard , Henderson Eric
Published: July 2002

A fundamental problem in atomic force microscopy (AFM) image interpretation is distinguishing features arising from tip geometry from true molecular detail. In this study, a novel 4-stranded form of DNA (the ‘‘G wire’’) was coadsorbed with 7.6-nm-diam colloidal gold probe calibration standards and examined by AFM. After the probe apices were reconstructed from AFM images of the standards, the artificial broadening of the coadsorbed G-wire DNA was removed, resulting in more reliable image interpretation. Using simple geometric models, a favorable comparison between observed and modeled G-wire cross sections suggests that reconstructions removed about 25% of the tip-broadened AFM image in these studies.
Single electron electronics: Challenge for nanofabrication
Authors: Ahmed Haroon
Published: July 2002

Single electronics has the potential to overcome the limitations of complementary metal oxide semiconductor (CMOS) technology as device dimensions shrink towards the 10 nm scale. The basis of single electronics is the Coulomb blockade of electron transport which occurs when an electron can be localized on an isolated island. Many structures have been devised to demonstrate the Coulomb blockade and single electron transistors (SETs), based on this effect have been constructed. Metallic structures can be fabricated with vertical and lateral island and tunnel barrier structures in which the Coulomb blockade has been demonstrated at temperatures up to 77 K. Semiconductor single electronics has also been demonstrated with GaAs and silicon based structures. Silicon on insulator has the attractive feature that it may be used for conventional CMOS circuits as well as single electronics in the same chip. Several applications of SETs have been demonstrated such as memories and logic circuits. Memories based on charge storage on nanoislands and operating at room temperature will almost certainly have applications in the future. Nanofabrication for SETs requires advances in lithographic techniques well beyond the current state of the art to the sub 10 nm size scale. Electron beam lithography and scanning probe techniques offer the best prospects for the future although some more esoteric techniques based on atomic particle deposition and colloid chemistry may also offer some benefits.
Development of a wafer level technique for monitoring and control of deposition temperature in high-vacuum physical vapor deposition technology
Authors: Wilson R. , Hulseweh T. , Krolikowski W.
Published: July 2002

Deposition temperatures in the range of 500 °C are required for aluminum via fill planarization processes. The control of the deposition temperature is a critical factor in achieving consistent via fill planarization. A wafer level technique was developed and proven to be consistent through the operating temperature range to highlight any deviation in the deposition temperature.
Electronic structure of the nanoscale Al/SiO2/Si system
Authors: Zavodinsky V. G. , Kuyanov I. A.
Published: July 2002

Local-density approximation calculations have been used to study the electronic structure of the Al14/n-fold SiO2 ring/Si15 cluster system for n=3, 4, and 6. The sixfold ring placed between aluminum and silicon has an insulator character with the fundamental band gap of 4 eV and the energetic barrier of 3.5 eV. The fourfold ring has a band gap of 2 eV, however its insulator properties are doubtful because of the very small distance between the Fermi level and the first unoccupied state (0.2 eV). The threefold SiO2 ring has not a fundamental gap at all. Considering the Al14/n-fold SiO2 ring /Si15H̄18 system as a model of the metal-oxide-semiconductor device, we can predict that the thin silica film (d≈7 Å) can be used as an insulator layer. However, if the thickness is less than 5 Å, the insulator properties of silica disappear.
Surface process and interaction of Mo clusters on highly oriented pyrolytic graphite observed by scanning tunneling microscopy
Authors: Xu H. , Ng K. Y. S.
Published: July 2002

Scanning tunneling microscopy was employed to study Mo clustering and interaction with substrate on highly oriented pyrolytic graphite. Mo follows the island growth mode, and the Mo clusters are distributed uniformly on a graphite surface. Thermal effects promote the diffusion of edge atoms and result in smooth edge and shape changes of the clusters. The appearance of triangular Mo clusters reflects the metal-substrate interaction. Cluster diffusion on a graphite surface was observed and the value of the diffusion coefficient for small Mo clusters were estimated to be on the order of 10−19 cm2/s at room temperature. Cluster-induced electron scattering standing waves were observed on the graphite surface.
Electron irradiance of conductive sidewalls: A determining factor for pattern-dependent charging
Authors: Hwang Gyeong S. , Giapis Konstantinos P.
Published: July 2002

Numerical simulations of charging and profile evolution during gate electrode overetching in high density plasmas have been performed to investigate the effect of long conductive sidewalls on profile distortion (notching). The results reveal that the angle of electron irradiance of the conductive portion of the sidewalls affects profoundly the charging potential of the gates. Larger angles, obtained for thinner masks and/or thicker polysilicon, result in reduced gate potentials which, through their influence on the local ion dynamics, cause more severe notching at all lines of the microstructure.
Local work function for Cu(111)–Au surface studied by scanning tunneling microscopy
Authors: Jia J. F. , Inoue K. , Hasegawa Y. , Yang W. S. , Sakurai T.
Published: July 2002

We report results of the measurement of local work function, or apparent barrier height, on the Au/Cu(111) surface using scanning tunneling microscopy (STM). By measuring a response of tunneling current to a change of the tunneling gap distance during scanning, we can obtain a work function image simultaneously with a topographic STM image. In this way, we could successfully observe the difference in local work function due to different elements and atomic structures. Our results show that the mean work function value of the Au overlayer is 7±3% larger than that of the Cu(111) substrate and the value of the work function of the second Au layer is the same as that of the first Au layers within the measurement uncertainty. At the step edges, the work function values are lower than that on the terrace. The width and depth of the low work function trough at Au–Au, and Cu–Au single height steps were also obtained. The measured values are consistent with those measured on vicinal surfaces. This work also suggests that the local work function measurement can be used to identify the element on a surface and that the STM is quite useful to get information on how the work function is related with surface structures and how the work function spatially changes on nanometer scales.
Electric field induced structural change for poly(vinylidene fluoride-co-trifluoroethylene) ultrathin films studied by scanning Maxwell stress microscope
Authors: Kajiyama Tisato , Khuwattanasil Noppadol , Takahara Atsushi
Published: July 2002

Poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF/TrFE), VDF 75 mol %] ultrathin films of 10 nm thickness were prepared on gold coated silicon wafer substrates by a spin-coating method and their aggregation states were investigated by Fourier transform infrared reflection absorption spectroscopic measurement. It was found that P(VDF/TrFE) formed a ferroelectric phase even in a confined state such as an ultrathin film. The electric field was locally imposed on the ultrathin film through a gold coated cantilever tip. The change of dipole moment orientation of P(VDF/TrFE) was evaluated by measuring surface potential change, performed by scanning Maxwell stress microscopy (SMM). The SMM images revealed that the local dipole moment orientation of P(VDF/TrFE) in ultrathin films could be changed by an application of electric field through an atomic force microscope tip.
Multiwafer molecular beam epitaxy for high volume production of GaAs/AlGaAs heterojunction bipolar transistor wafers
Authors: Block T. R. , Wojtowicz M. , Han A. C. , Olson S. R. , Oki A. K. , Streit D. C.
Published: July 2002

Circuits incorporating GaAs/AlGaAs heterojunction bipolar transistors (HBTs) are being increasingly used in commercial applications with volume requirements that can only be met using multiwafer reactors. HBT wafers grown in single and multiwafer molecular beam epitaxy (MBE) reactors are found to be very similar in terms of both material characteristics and resultant device performance. Postgrowth defect levels are reduced by an order of magnitude with the multiwafer system compared to single wafer systems and are similar to those obtained on commercially purchased metal organic chemical vapor deposition wafers. Multiwafer MBE systems are thus shown to be a viable means for the high volume production of HBT wafers.
Photo- and electroluminescence characterization of erbium doped SiGe
Authors: Neufeld E. , Sticht A. , Brunner K. , Riedl H. , Abstreiter G. , Holzbrecher H. , Bay H.
Published: July 2002

We have performed photo- and electroluminescence measurements on erbium doped silicon and SiGe samples containing different amounts of germanium. All samples were grown completely by molecular beam epitaxy. Oxygen was used as a codopant to enhance the luminescence efficiency. The photoluminescence data show a systematic variation of the thermal activation energy obtained from temperature dependent intensity measurements with varying germanium contents. For samples with an alternating Si/SiGe layer structure stronger photoluminescence is observed when the erbium ions are incorporated into the SiGe layers rather than into the silicon layers. We attribute this to the capture of photogenerated carriers in the SiGe layers. Electroluminescence from erbium doped SiGe samples which have been processed as diodes has been observed up to room temperature in both surface and edge emitting geometries.
Planar field emitters fabricated by sulfur-doped boron nitride
Authors: Yokota Yuuko , Tagawa Shigeru , Sugino Takashi
Published: July 2002

Boron nitride (BN) films are grown on sapphire substrates by plasma-assisted chemical vapor deposition. BN films are doped with sulfur (S). The electrical resistivity of the S-doped BN film is reduced to 103 Ω cm, while the electrical resistivity of the undoped BN film is 1011 Ω cm. It is demonstrated that the negative electron affinity appears on the BN surface. Insertion of a GaN layer between the BN film and sapphire leads to a tight adhesion of the BN film. Cathode and anode electrons are formed on the BN film and the sapphire substrate, respectively, by evaporating Ti and Au. An emission current of 1 μA is obtained at an electric-field strength of 16 V/μm for the planar field emitter. An emission current density as high as 0.1 A/cm2 is detected. It is expected that the planar field emitters can be operated at several tens V with a decreased cathode–anode spacing and that the present field emitter structure is applicable to a field-emission panel display.
Microstructure characterization of oxidized nanocrystalline Si:H film by transmission electron microscopy
Authors: Lu Hai , Liu Jianhong , Li Wei , Chen Kunji , Huang Xinfan
Published: July 2002

Oxidized nanocrystalline (nc) Si:H film has been produced by plasma enhanced chemical vapor deposition with a high hydrogen diluted silane and followed by thermal oxidization in a conventional furnace at a relatively low temperature of 800 °C. The resulting films were studied using cross-sectional transmission electron microscopy, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy. It is found that nc-Si dots embedded in an a-SiO2 matrix can be formed by the above method and some of them exhibit the columnar growth morphology. Both the nc-Si:H deposition and the later thermal treatment are revealed to contribute to the film microstructures. It is further suggested that by choosing optimum oxidation time, the size of Si nanoparticles can be well controlled.
Au/Ge-based Ohmic contact to an AlGaAs/InGaAs pseudomorphic high electron mobility transistor with an undoped cap layer
Authors: Lee Jong-Lam , Kim Yi-Tae , Yoo Hyung Mo , Lee Gi Young
Published: July 2002

The effect of existence of undoped GaAs/AlGaAs cap layers on Ohmic contact resistivity in an AlGaAs/InGaAs pseudomorphic high electron mobility transistor was investigated. The Au/Ge/Ni/Au Ohmic contact formed on the undoped GaAs cap layer yields the lowest contact resistivity of 3.8×10−6 Ω cm2. Meanwhile, the contact resistivity increases a little to 5.0×10−6 Ω cm2 for the contacts formed on an n-Al0.23Ga0.77As layer exposed by removing the undoped cap layer. Both contact resistivities are comparable to those obtained using the n+-GaAs cap layer. The good Ohmic contacts obtained independent of removal of the undoped cap layer are due to the formation of the interfacial compounds, Au2Al and Au2Ga, during annealing. The interfacial compounds penetrate deep into the buried InGaAs channel, resulting in direct contacts to the channel.
Application of development-free vapor photolithography in silicon nitride etching
Authors: Hong Xiaoyin , Duan Shengquan , Lu Jianping , Wang Peiqing , Chen Yongqi
Published: July 2002

A new dry etching technique—development-free vapor photolithography—was used to transfer a pattern onto silicon nitride film. Plasma-enhanced chemical vapor deposition silicon nitride film and thin low pressure chemical vapor deposition silicon nitride film can be etched to get a positive pattern. The difference in the etching rate between exposed and unexposed areas was attributed to the difference in the concentration of accelerators that was controlled through a photochemical reaction. The reaction mechanism and other phenomena are discussed.
Study of local stress using stress-absorbing Si diaphragm
Authors: Zheng D. W. , Wang XinHua , Shyu K. , Chang C.-T. , Guo Y. , Sarihan V. , Wen Weijia , Tu K. N.
Published: July 2002

A methodology was developed to study the residual stress distribution of thin-film patterns residing on a silicon wafer. The Si underlying the patterns studied was thinned down so that the deformation caused by residual stress in the microstructure could be detected by a Twyman–Green interferometer. A procedure called “numerical etching” was implemented to simulate the Si etching process, which linked the stress state of the microstructure on a regular wafer to that on a Si diaphragm. An initial stress field on the pattern was assumed, and its effect on the bending of the Si diaphragm beneath was calculated and compared to the measured value. The discrepancy between them was used to modify the initially assumed stress field and repeated until a satisfactory match was achieved. We believe that the approximated stress field sufficiently reflects the real stress distribution in the patterned structure under investigation. The stress distribution in an electroless Ni pad residing on Si for a flip-chip packaging application is used as an example.
Abatement of perfluorocarbons with an inductively coupled plasma reactor
Authors: Liao M. Y. , Wong K. , McVittie J. P. , Saraswat K. C.
Published: July 2002

Preliminary tests using an inductively coupled plasma (ICP) reactor to abate C2F6, CF4, and CHF3 show promising results. In conjunction with this, some of the operating parameters, which need to be considered when trying to obtain maximum abatement efficiency, are explored. Near 100% destruction efficiency (DRE) for the persistently fluorinated compounds (PFCs) mentioned earlier has been achieved with O2 as the additive gas. Similarly, the abatement of CF4 with CH4 and/or CH4/O2 as the additive gas, has shown comparable results. For the non-CF4 PFCs, like C2F6 and CHF3, the formation of CF4 as an abatement by-product was monitored and shown that it can be controlled by varying the operating parameters of the ICP reactor. The trends in the DRE observed while manipulating the ICP operating parameters such as ICP power, residence time, amount/type of additive gas, and Ar dilution, show that there are trade offs which must be taken into account. A quadrupole mass spectrometer was used for analyzing the abatement effluents.
Temperature dependence of neutral and positively charged Si and SiCl etch products during argon-ion-enhanced etching of Si(100) by Cl2
Authors: Materer N. , Goodman Rory S. , Leone Stephen R.
Published: July 2002

The temperature dependence of neutral and positively charged Si and SiCl etch products produced during argon-ion-enhanced etching of Si(100) in a molecular-chlorine background are measured over a temperature range between 323 and 923 K. The neutral Si and SiCl product fluxes, detected by laser ionization mass spectroscopy, undergo a monotonic decrease in intensity with increasing temperature. A model based on the temperature dependence of the molecular chlorine sticking probability successfully explains the observed results. In contrast to the neutral species, the yields of the positively charged etch products, Si+ and SiCl+, exhibit a more complex temperature dependence. The ion yields first decrease with increasing temperature, then increase to a maximum at approximately 800 K. As the temperature is further increased to 923 K, both ionic products rapidly decrease. The initial decrease in ionic products mimics the decrease in the neutral products and is likely to be due to the decreasing sticking coefficient of molecular chlorine. A second mechanism, possibly the changing electronic properties of the silicon substrate with increasing temperature, is required to explain the local maximum in the product yield at 800 K.
Ultra-low-energy ion-implant simulation using computational-efficient molecular dynamics schemes and the local damage accumulation model
Authors: Kang Jeong-Won , Kang E. S. , Son M. S. , Hwang H. J.
Published: July 2002

We have investigated effects of atomic dynamics for ultra-low-energy As and B ion implants using a highly efficient molecular dynamics scheme. We simulated ion implantation by molecular dynamics simulation using the recoil ion approximation method and the local damage accumulation model proposed in the article. The Local damage accumulation probability function consists of deposited energy in a unit cell, implant dose rate, target material, projectile atom, and the history of the recoil event in a cell. The results of simulations agree with the experimental results. The MDRANGE results considering no damage were different from the tail region. Using the local damage accumulation model and the recoil ion approximation method, we simulated dopant two-dimensional profiles and two-dimensional damage profiles.
Incorporation of Si in InAlAs grown by low pressure metal-organic chemical vapor deposition assessed by optical and transport measurements
Authors: Tribuzy C. V. B. , Yavich B. , Souza P. L. , Menchero J. G.
Published: July 2002

We present a study of Si incorporation in InAlAs layers grown lattice matched on InP substrates by low pressure metal-organic chemical vapor deposition. Distinct Si doping levels are achieved for different values of diluted flows of SiH4. Photoluminescence, capacitance–voltage, and Hall measurements are used to characterize the samples. In order to investigate a possible autocompensation effect, the samples were subjected to a heat treatment. Analysis of the photoluminescence spectra before and after annealing, with the help of a fitting procedure, reveals that Si is also incorporated as an acceptor in the sublattice V. The position of the acceptor peak is consistent with our theoretical calculations. Photoluminescence measurements as a function of temperature show the anomalous inverted S-shape behavior. The analysis of the fitted spectra gives support to a carrier localization effect.
Characteristics of InGaP/GaAs delta-doped heterojunction bipolar transistor
Authors: Chen Jing-Yuh , Wang Wei-Chou , Pan Hsi-Jen , Feng Shun-Ching , Yu Kuo-Hui , Cheng Shiou-Ying , Liu Wen-Chau
Published: July 2002

An InGaP/GaAs heterojunction bipolar transistor with a δ-doped sheet structure has been fabricated successfully. High current gain, especially in the very low collector current regime, low collector-emitter offset voltage (ΔVCE) characteristics are obtained. There is an extremely small offset voltage of about 55 meV due to the elimination of the potential spike at the emitter-base heterojunction by a δ-doped sheet and a 50 Å very thin setback layer. On the other hand, the high current gain of 280 also reveals that the degradation of current gain due to the potential spike can be avoided. In addition, the influence of the δ-doped sheet and the undoped setback on device properties is studied.
Resist debris formation and proximity exposure effect in electron beam lithography
Authors: Deshmukh P. R. , Rangra K. J. , Wadhawan O. P.
Published: July 2002

Deliberate formation of unsupported resist microfragments called resist debris (RD) over the exposed pattern areas in electron beam lithography (EBL) is found to predict proximity exposure (PE) effects. Some simulation results and corresponding experimental results on RD formation discussed in the article show that both intra- and interpattern PE effects can be observed and explained from the nature of RD distribution. This gives a different kind of tool to tackle the problem of PE effect in EBL.
Extreme Diversity, Conservation, and Convergence of Spider Silk Fibroin Sequences
Authors: Gatesy John , Hayashi Cheryl , Motriuk Dagmara , Woods Justin , Lewis Randolph
Published: July 2002

Spiders (Araneae) spin high-performance silks from liquid fibroin proteins. Fibroin sequences from basal spider lineages reveal mosaics of amino acid motifs that differ radically from previously described spider silk sequences. The silk fibers of Araneae are constructed from many protein designs. Yet, the repetitive sequences of fibroins from orb-weaving spiders have been maintained, presumably by stabilizing selection, over 125 million years of evolutionary history. The retention of these conserved motifs since the Mesozoic and their convergent evolution in other structural superproteins imply that these sequences are central to understanding the exceptional mechanical properties of orb weaver silks.
Biosynthesis of Complex Polyketides in a Metabolically Engineered Strain of
E. coli
Authors: Pfeifer Blaine A. , Admiraal Suzanne J. , Gramajo Hugo , Cane David E. , Khosla Chaitan
Published: July 2002

The macrocyclic core of the antibiotic erythromycin, 6-deoxyerythronolide B (6dEB), is a complex natural product synthesized by the soil bacterium
Saccharopolyspora erythraea
through the action of a multifunctional polyketide synthase (PKS). The engineering potential of modular PKSs is hampered by the limited capabilities for molecular biological manipulation of organisms (principally actinomycetes) in which complex polyketides have thus far been produced. To address this problem, a derivative of
Escherichia coli
has been genetically engineered. The resulting cellular catalyst converts exogenous propionate into 6dEB with a specific productivity that compares well with a high-producing mutant of
S. erythraea
that has been incrementally enhanced over decades for the industrial production of erythromycin.
Regulation of Longevity and Stress Resistance by Sch9 in Yeast
Authors: Fabrizio Paola , Pozza Fabiola , Pletcher Scott D. , Gendron Christi M. , Longo Valter D.
Published: July 2002

The protein kinase Akt/protein kinase B (PKB) is implicated in insulin signaling in mammals and functions in a pathway that regulates longevity and stress resistance in
Caenorhabditis elegans
. We screened for long-lived mutants in nondividing yeast
Saccharomyces cerevisiae
and identified mutations in adenylate cyclase and
SCH9
, which is homologous to Akt/PKB, that increase resistance to oxidants and extend life-span by up to threefold. Stress-resistance transcription factors Msn2/Msn4 and protein kinase Rim15 were required for this life-span extension. These results indicate that longevity is associated with increased investment in maintenance and show that highly conserved genes play similar roles in life-span regulation in
S. cerevisiae
and higher eukaryotes.
A Silent Slip Event on the Deeper Cascadia Subduction Interface
Authors: Dragert Herb , Wang Kelin , James Thomas S.
Published: July 2002

Continuous Global Positioning System sites in southwestern British Columbia, Canada, and northwestern Washington state, USA, have been moving landward as a result of the locked state of the Cascadia subduction fault offshore. In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion. No seismicity was associated with this event. The sudden displacements are best explained by ∼2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone, a rupture equivalent to an earthquake of moment magnitude 6.7. This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress loading of the megathrust earthquake zone, can occur in discrete pulses.
Reversal of Obesity- and Diet-Induced Insulin Resistance with Salicylates or Targeted Disruption of
Ikkβ
Authors: Yuan Minsheng , Konstantopoulos Nicky , Lee Jongsoon , Hansen Lone , Li Zhi-Wei , Karin Michael , Shoelson Steven E.
Published: July 2002

We show that high doses of salicylates reverse hyperglycemia, hyperinsulinemia, and dyslipidemia in obese rodents by sensitizing insulin signaling. Activation or overexpression of the IκB kinase β (IKKβ) attenuated insulin signaling in cultured cells, whereas IKKβ inhibition reversed insulin resistance. Thus, IKKβ, rather than the cyclooxygenases, appears to be the relevant molecular target. Heterozygous deletion (
Ikkβ
+/−
) protected against the development of insulin resistance during high-fat feeding and in obese

Lep
ob/ob

mice. These findings implicate an inflammatory process in the pathogenesis of insulin resistance in obesity and type 2 diabetes mellitus and identify the IKKβ pathway as a target for insulin sensitization.
Translating the Histone Code
Authors: Jenuwein Thomas , Allis C. David
Published: July 2002

Chromatin, the physiological template of all eukaryotic genetic information, is subject to a diverse array of posttranslational modifications that largely impinge on histone amino termini, thereby regulating access to the underlying DNA. Distinct histone amino-terminal modifications can generate synergistic or antagonistic interaction affinities for chromatin-associated proteins, which in turn dictate dynamic transitions between transcriptionally active or transcriptionally silent chromatin states. The combinatorial nature of histone amino-terminal modifications thus reveals a “histone code” that considerably extends the information potential of the genetic code. We propose that this epigenetic marking system represents a fundamental regulatory mechanism that has an impact on most, if not all, chromatin-templated processes, with far-reaching consequences for cell fate decisions and both normal and pathological development.
SNARE Function Analyzed in Synaptobrevin/VAMP Knockout Mice
Authors: Schoch Susanne , Deák Ferenc , Königstorfer Andreas , Mozhayeva Marina , Sara Yildirim , Südhof Thomas C. , Kavalali Ege T.
Published: July 2002

SNAREs (soluble NSF-attachment protein receptors) are generally acknowledged as central components of membrane fusion reactions, but their precise function has remained enigmatic. Competing hypotheses suggest roles for SNAREs in mediating the specificity of fusion, catalyzing fusion, or actually executing fusion. We generated knockout mice lacking synaptobrevin/VAMP 2, the vesicular SNARE protein responsible for synaptic vesicle fusion in forebrain synapses, to make use of the exquisite temporal resolution of electrophysiology in measuring fusion. In the absence of synaptobrevin 2, spontaneous synaptic vesicle fusion and fusion induced by hypertonic sucrose were decreased ∼10-fold, but fast Ca
2+
-triggered fusion was decreased more than 100-fold. Thus, synaptobrevin 2 may function in catalyzing fusion reactions and stabilizing fusion intermediates but is not absolutely required for synaptic fusion.
A High-Resolution Absolute-Dated Late Pleistocene Monsoon Record from Hulu Cave, China
Authors: Wang Y. J. , Cheng H. , Edwards R. L. , An Z. S. , Wu J. Y. , Shen C.-C. , Dorale J. A.
Published: July 2002

Oxygen isotope records of five stalagmites from Hulu Cave near Nanjing bear a remarkable resemblance to oxygen isotope records from Greenland ice cores, suggesting that East Asian Monsoon intensity changed in concert with Greenland temperature between 11,000 and 75,000 years before the present (yr. B.P.). Between 11,000 and 30,000 yr. B.P., the timing of changes in the monsoon, as established with
230
Th dates, generally agrees with the timing of temperature changes from the Greenland Ice Sheet Project Two (GISP2) core, which supports GISP2's chronology in this interval. Our record links North Atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer East Asian Monsoon originates.
Regulation of Cell Survival by Secreted Proneurotrophins
Authors: Lee Ramee , Kermani Pouneh , Teng Kenneth K. , Hempstead Barbara L.
Published: July 2002

Neurotrophins are growth factors that promote cell survival, differentiation, and cell death. They are synthesized as proforms that can be cleaved intracellularly to release mature, secreted ligands. Although proneurotrophins have been considered inactive precursors, we show here that the proforms of nerve growth factor (NGF) and the proforms of brain derived neurotrophic factor (BDNF) are secreted and cleaved extracellularly by the serine protease plasmin and by selective matrix metalloproteinases (MMPs). ProNGF is a high-affinity ligand for p75
NTR
with high affinity and induced p75
NTR
-dependent apoptosis in cultured neurons with minimal activation of TrkA-mediated differentiation or survival. The biological action of neurotrophins is thus regulated by proteolytic cleavage, with proforms preferentially activating p75
NTR
to mediate apoptosis and mature forms activating Trk receptors to promote survival.
A Conserved Family of Prolyl-4-Hydroxylases That Modify HIF
Authors: Bruick Richard K. , McKnight Steven L.
Published: July 2002

Mammalian cells respond to changes in oxygen availability through a conserved pathway that is regulated by the hypoxia-inducible factor (HIF). The alpha subunit of HIF is targeted for degradation under normoxic conditions by a ubiquitin-ligase complex that recognizes a hydroxylated proline residue in HIF. We identified a conserved family of HIF prolyl hydoxylase (HPH) enzymes that appear to be responsible for this posttranslational modification. In cultured mammalian cells, inappropriate accumulation of HIF caused by forced expression of the HIF-1α subunit under normoxic conditions was attenuated by coexpression of HPH. Suppression of HPH in cultured
Drosophila melanogaster
cells by RNA interference resulted in elevated expression of a hypoxia-inducible gene (
LDH
, encoding lactate dehydrogenase) under normoxic conditions. These findings indicate that HPH is an essential component of the pathway through which cells sense oxygen.
Dynamics of Pleistocene Population Extinctions in Beringian Brown Bears
Authors: Barnes I. , Matheus P. , Shapiro B. , Jensen D. , Cooper A.
Published: July 2002

The climatic and environmental changes associated with the last glaciation (90,000 to 10,000 years before the present; 90 to 10 ka B.P.) are an important example of the effects of global climate change on biological diversity. These effects were particularly marked in Beringia (northeastern Siberia, northwestern North America, and the exposed Bering Strait) during the late Pleistocene. To investigate the evolutionary impact of these events, we studied genetic change in the brown bear,
Ursus arctos
, in eastern Beringia over the past 60,000 years using DNA preserved in permafrost remains. A marked degree of genetic structure is observed in populations throughout this period despite local extinctions, reinvasions, and potential interspecies competition with the short-faced bear,
Arctodus simus
. The major phylogeographic changes occurred 35 to 21 ka B.P., before the glacial maximum, and little change is observed after this time. Late Pleistocene histories of mammalian taxa may be more complex than those that might be inferred from the fossil record or contemporary DNA sequences alone.
A Rational Chemical Synthesis of C
60
Authors: Scott Lawrence T. , Boorum Margaret M. , McMahon Brandon J. , Hagen Stefan , Mack James , Blank Jarred , Wegner Hermann , de Meijere Armin
Published: July 2002

Isolable quantities of C
60
, the smallest stable fullerene, have been synthesized in 12 steps from commercially available starting materials by rational chemical methods. A molecular polycyclic aromatic precursor bearing chlorine substituents at key positions forms C
60
when subjected to flash vacuum pyrolysis at 1100°C. No other fullerenes are formed as by-products. The methods we have developed for the target-specific synthesis of fullerenes, applied here to a synthesis of C
60
, should make possible the directed laboratory preparation of other fullerenes as well, including those not accessible by graphite vaporization.