bluuebunny/crossref_metadata_2025 huggingface dataset:
Reactive ion etching of GaAs through wafer via holes using Cl2 and SiCl4 gases: A comprehensive statistical approach
Authors: Camacho A. , Morgan D. V.
Published: July 2002

The reactive ion etching behavior of GaAs in Cl2 and SiCl4 gases is investigated with regard to the suitability of this process for etching through wafer via holes using a photoresist mask. The criteria used are etch rate, profile (wall angle), selectivity, undercut, and smoothness of the etch. The experimental points were chosen using a D-optimal statistical design. The experimental results have been evaluated by multiple stepwise regression, eliminating insignificant terms from a quadratic model of the controlled parameters (mask bake time, power, pressure, chlorine flow, and etch time). Empirical models of the dc bias and each etch criterion are obtained. The highest etch rates, at over 3 μm/min, are obtained in high chlorine concentrations at high dc bias values (high power and low pressure) for short etch times. This method yielded an optimized process for etching metallizable via holes.
Ultrahigh vacuum atomic force microscope with sample cleaving mechanism
Authors: Ohta M. , Sugawara Y. , Morita S. , Nagaoka H. , Mishima S. , Okada T.
Published: July 2002

Our newly constructed ultrahigh vacuum atomic force microscope (UHV AFM) with a sample cleaving mechanism is described. The cantilever is scanned by a tube scanner and its deflection is detected by the fiber-optic interferometer method. Our AFM has dual-axes piezoelectric micropositioners. The x-axis micropositioner is used for sample cleavage, which forces the sample to press a block for cleavage. The z-axis micropositioner is used for coarse positioning of the cantilever to the sample. All the components of the fiber-optic interferometer, except for the optical fiber, are located outside the chamber, so that a very compact and simple UHV AFM design is possible. The performance of the instrument is discussed with images of both graphite and GaAs (110) surface obtained in UHV.
Reactive ion etching of Ta–Si–N diffusion barriers in CF4+O2
Authors: McLane G. F. , Casas L. , Reid J. S. , Kolawa E. , Nicolet M.-A.
Published: July 2002

Ta36Si14N50 amorphous layers were reactive ion etched in CF4+O2 plasmas. The etch depth was determined as a function of gas composition, pressure, and cathode power. Adding small amounts of O2 to CF4 increased the etch rates up to approximately 15% O2 concentration, with etch rates then decreasing with further addition of O2. Etch rates increased with both pressure and power. Etching proceeded only after an initial delay time which depended upon gas composition and power. The delay is probably caused by a surface native oxide which must be removed before etching can commence. The presence of a surface oxide was observed from Auger electron spectroscopy intensity depth profile measurements and is estimated to be 2 nm thick. Under optimal conditions, the etch rate of Ta36Si14N50 is about seven times higher than for SiO2, thus providing a high degree of selectivity for integrated circuit processing.
Silicon interlayer based surface passivation of near-surface quantum wells
Authors: Kodama Satoshi , Koyanagi Satoshi , Hashizume Tamotsu , Hasegawa Hideki
Published: July 2002

In view of the urgent necessity to establish a suitable passivation technology applicable to compound semiconductor quantum structure surfaces, the latest version of the silicon interlayer based passivation process was applied to passivation of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As near-surface quantum wells (QWs). The process utilizes an ultrathin molecular beam epitaxy silicon/ultrathin photo-enhanced chemical vapor deposition (photo-CVD) silicon nitride double layer as the interface control layer together with a main passivation dielectric of thick photo-CVD SiO2 layer. The effectiveness of passivation was studied by comparing the photoluminescence (PL) intensities of passivated samples with those of unpassivated QWs that showed exponential decrease with reduction of surface-to-well distance. A complete recovery of PL intensity was achieved by passivation with a maximum recovery factor larger than 103, consistent with reduced interface state densities in low 1010 cm−2 eV−1 range recently realized on In0.53Ga0.47As metal–insulator–semiconductor capacitors using the same technique.
Electrochemical formation of Se atomic layers on Au(100)
Authors: Lister T. E. , Huang B. M. , Herrick R. D. , Stickney J. L.
Published: July 2002

Studies of the electrodeposition of atomic layers of Se have been prompted by their importance in the formation of II–VI compound semiconductors by electrochemical atomic layer epitaxy (ECALE). ECALE is a method where a compound is formed by the sequential, alternated, electrodeposition of atomic layers of the constituent elements making up a compound. It has been shown that the structure of the first atomic layer is generally the most critical. The present article describes three different methods for forming that first atomic layer. Studies of the structures resulting from those methods have revealed four distinctly different Se structures, all of which can be considered atomic layers, as none is more than a single-Se atom thick. Graphs of coverage versus potential have been obtained for each of the methods, and show distinct plateaus over as much as 0.15 V, indicating the stability of some of the structures. The sequence of structures first involves the formation of a 1/4 coverage (2×2), followed by a (2×√10) at 1/3 coverage, and then a c(2×2) at 1/2 coverage. In addition, Se8 rings were observed to form, much the same as the S8 rings previously observed for S adsorbed on Au surfaces. At coverages near 0.7, these rings begin to coalesce into a nearly complete monolayer of Se atoms, but with systematic absences, which roughly correspond to the initial holes in the Se8 rings. This structure appears to have an average unit cell which can be described as a (3×√10).
Nanostructuring of gold electrodes for immunosensing applications
Authors: Musil Christian R. , Jeggle Daniel , Lehmann Hans W. , Scandella Loris , Gobrecht Jens , Döbeli Max
Published: July 2002

The fundamental characteristic of nanotechnology is the fabrication of structures with molecular dimensions. In connection with a novel immunosensor, a specifically tailored molecular environment is required which combines immobilized biochemical recognition with electrochemistry. These demands are fulfilled by the creation of nanometer-size electrodes in a thin film (∼30 nm) of gold consisting of openings commensurate with the size of the biomolecule (∼10 nm). We report on three complementary and advanced fabrication techniques used to produce closely packed distributions of these features: direct focused ion beam milling, lift-off using polystyrene latex nanospheres as a masking material, and resist exposure by individual ionization tracks produced during high energy heavy ion (∼0.1 MeV/amu) irradiation. The latter two processes are stochastic in nature and exploit the fact that no precise arrangement of the openings is necessary for this application. Fabrication results were analyzed by scanning electron and scanning force microscopy. The three methods are compared with respect to opening size, definition, and packing density and process throughput and control. While nanosphere lift-off represents a relatively simple and reproducible structuring technique, heavy ion irradiation has the potential for achieving the smallest openings (∼15 nm). Ultimately, the choice of nanostructuring method is dictated by the electrode’s performance in the sensor.
Study of large area high density magnetic dot arrays fabricated using synchrotron radiation based x-ray lithography
Authors: Rousseaux F. , Decanini D. , Carcenac F. , Cambril E. , Ravet M. F. , Chappert C. , Bardou N. , Bartenlian B. , Veillet P.
Published: July 2002

Large area arrays of dots have been patterned in Au/Co/Au(111) sandwiches with ultrathin Co layers (0.6 to 2 nm) and a perpendicular easy magnetization axis. Dot dimensions down to 0.2 μm have been achieved using x-ray lithography, with either positive resist and direct ion beam etching or a lift-off process with aluminum mask. Both processes have been tested against the damages they induce to the fragile structure of the samples. The magneto-optical effects and magnetization reversal processes in the arrays have been characterized versus Co thickness, dot dimension, and lattice aspect ratio. For high quality samples, the domain walls propagation mechanism that drives magnetization reversal in as-grown films is drastically modified in dot arrays, leading to a large increase of the coercive field with dot diameter reduction, together with changes in the shape of the hysteresis loops.
GaAs growth on (111)B substrates by molecular-beam epitaxy: A study of the first stages of growth on ultraviolet–ozone prepared surfaces
Authors: García B. J. , Fontaine C. , Muñoz Yagüe A.
Published: July 2002

UV–ozone substrate preparation and molecular-beam epitaxial growth conditions of GaAs on (111)B GaAs substrates are reported. Changes in the reflection high-energy electron diffraction pattern (RHEED) during the first stages of growth reflect the planarization process over an initially rough surface. New evidence of this process is reported, based on the intensity oscillations of the RHEED specular beam, which highlights a rise of the measured growth rate in the first steps of growth during surface flatness recovery.
Characterization of a single-layer quantum wire structure grown directly on a submicron grating
Authors: Gustafsson A. , Samuelson L. , Hessman D. , Malm J.-O. , Vermeire G. , Demeester P.
Published: July 2002

A single AlGaAs/GaAs quantum well (QW)/quantum wire (QWR) structure was grown by metalorganic vapor phase epitaxy on a submicron period grating of V grooves. High resolution transmission electron microscopy studies of the sample identifies three regions of the QW; between the grooves, approximately 3.5 nm thick and oriented along (100), on the sidewalls of the V groove slightly thinner and oriented along {111}. At the bottom of the groove an approximately 70 nm wide crescent shaped region forms a QWR. In addition, a vertical quantum well (VQW) extends from the bottom of each V groove in the GaAs substrate to the surface. The luminescence spectra of the sample are dominated by a peak originating in the QW, with additional peaks of the QWR, the VQW and the AlGaAs barrier. The striped nature of the sample is revealed in the top view cathodoluminescence (CL) images of all four peaks. In side view CL images, the QWR emission appears spot like, whereas the emission of the VQW is elongated in the direction perpendicular to the surface. Photoluminescence excitation spectroscopy reveals that the main source for excitation in the QWR comes from the VQW, even though a small contribution comes from the QW.
Calculation of etching profile in the photolithographic process on As2S3 thin films
Authors: Mamedov S. , Kisliuk A.
Published: July 2002

The model of the formation etching profile in the photolithographic process on the As2S3 thin films has been proposed. Using experimental dependence of dissolution rate versus exposure the evolution of the etching profile was calculated. The model allows one to predict the etching profile and simulates the process.
Effect of dislocations in strained Si/SiGe on electron mobility
Authors: Ismail K.
Published: July 2002

In this article the presence of threading and misfit dislocations in strained modulation-doped Si/SiGe heterostructures and their effect on electron mobility are investigated. It is found that the low-temperature electron mobility is sensitive to threading dislocations when their density exceeds 3×108 cm−2, and decreases by two orders of magnitude when the threading dislocation density is 1×1011 cm−2. The room-temperature mobility is reduced under the same conditions by 10% and 50%, respectively. Misfit segments in the graded Ge-content buffer limit the mobility when the Si channel is 0.4 μm or less away from that buffer. Misfit dislocations at the Si-channel bottom interface very strongly scatter the electrons in the channel once a continuous network of misfit segments is created.
New insights into the kinetics of the stress-driven two-dimensional to three-dimensional transition
Authors: Chen K. M. , Jesson D. E. , Pennycook S. J. , Thundat T. , Warmack R. J.
Published: July 2002

We have systematically investigated the morphological evolution of Ge0.5Si0.5 strained films during postdeposition annealing. The changes of the surface structure are found to follow the kinetic route of strain relaxation at different stages. A number of interesting features are revealed, which include the existence of an energy barrier to the two-dimensional/three-dimensional (2D/3D) transition, and a self-limiting effect in the growth kinetics of strained 3D islands. We demonstrate that the annealing approach provides a new way to grow coherent islands with uniform size.
High index orientation effects of strained self-assembled InGaAs quantum dots
Authors: Lubyshev D. I. , González-Borrero P. P. , Marega E. , Petitprez E. , Basmaji P.
Published: July 2002

Optical characterization of strained InGaAs/GaAs quantum dots grown by molecular beam epitaxy on (001) and (n11)B, where n=1, 2, 3, 5, and 7 orientations is reported in this work. Quantum dot photoluminescence emission shows remarkable orientation effects, presented in peak shape, full width at half-maximum, and integrated intensity. Quantum dots grown on the (711)B plane demonstrate high quantum efficiency: integrated photoluminescence ratio between quantum dots and quantum well is about 10. Our results indicate an enhancement of the quantum dots onset thermal quenching energy by a factor of 2.5 for all orientations. Activation energy for thermal stimulated electron–hole emission in quantum dots is 2–5 times higher than in quantum wells. Photoluminescence polarization measurements show strong in-plane dependence caused by the quantum dots’ structural anisotropy.
Surface characterization of sidewall protection film on GaAs steep via holes etched by magnetron ion etching
Authors: Takano H. , Sumitani K. , Matsuoka H. , Sato K. , Ishihara O. , Tsubouchi N.
Published: July 2002

We studied the magnetron ion etching of GaAs in SiCl4/Cl2 discharges and sidewall passivation effect which closely relates to the anisotropic dry etching. The effects of a variety of process parameters on the final via hole profiles and morphologies were also examined. Furthermore, in order to determine the surface chemistry of the residues on GaAs sidewall in magnetron plasmas, surface characterization was undertaken using scanning electron microscopy, transmission electron microscopy combined with an energy dispersive x-ray spectrometer, and x-ray photoelectron spectroscopy. It was confirmed that the residues, namely the sidewall protection film, consists of double layers. The aluminum content of the underlayer is four times greater than that of the upper layer. The aluminum mainly results from sputtering of the alumina cathode covers. The formation of the sidewall protection film was identified as the key factor in controlling the via hole profile for GaAs device fabrication.
Field emission properties of ta-C films with nitrogen doping
Authors: Park Kyu Chang , Moon Jong Hyun , Chung Suk Jae , Jung Jae Hoon , Ju Byeong Kwon , Oh Myung Hwan , Milne W. I. , Han Min Koo , Jang Jin
Published: July 2002

We have studied the electron emission characteristics of tetrahedral amorphous carbon (ta-C) films with different nitrogen content. With increasing N content in ta-C, the room-temperature conductivity as well as the emission current decreases and then increases, resulting in minima. The Fermi level shifts toward the conduction band and, thus, the work function decreases by N doping in ta-C, however, the emission currents of doped ta-C films are less than those of undoped ta-C.
In-rich 4×2 reconstruction in novel planar growth of InAs on GaAs(001)
Authors: Xue Q. K. , Hasegawa Y. , Ogino T. , Kiyama H. , Sakurai T.
Published: July 2002

Molecular beam epitaxy growth of lattice mismatched InAs/GaAs(001) system is studied by in situ scanning tunneling microscopy (STM) and reflection high energy electron diffraction. We found that deposition of submonolayer (∼0.6 ML) In on the GaAs(001)-As-rich 2×4 surface could result in a new well-ordered 4×2 reconstruction, and that if the growing front maintains this reconstruction, the multilayer InAs will grow two-dimensionally and usually observed 3D islanding is completely suppressed. Atomic structures for the 4×2 reconstruction are discussed on the basis of voltage-dependent STM image. A “domain wall” structure, representing a new type of surface strain relief mechanism in the novel growth will also be discussed.
Accuracy of thin film stress measurements with c-Si microbeams fabricated by dry etching
Authors: Boutry M. , Bosseboeuf A. , Bourouina T. , Grandchamp J. P. , Dufour-Gergam E. , Gilles J. P.
Published: July 2002

Thin Al, Cr, and W films were deposited on monocrystalline (100) Si microbeams fabricated by a SCREAM-like dry etching process. Curvature measurements and three-dimensional finite element method mechanical simulations of the composite microbeams were combined to evaluate the residual mechanical stress in the metallic films. Factors limiting the measurement accuracy are analyzed. It is shown that the measurement accuracy is largely dependent on microloading effects during the Si isotropic etching step necessary to release the microbeams from the substrate.
Across-wafer nonuniformity of long throw sputter deposition
Authors: Mayo A. A. , Hamaguchi S. , Joo J. H. , Rossnagel S. M.
Published: July 2002

Physical sputter deposition has been used at longer-than-normal cathode-to-sample distances for semi-directional deposition within high aspect ratio features. “Long throw” sputter deposition can be advantageous over other means of directional sputtering, such as collimated sputter deposition, because of the absence of collimators and related problems. However, due to the finite target size and sample geometry, an asymmetry is observed at the wafer edge with a thicker deposit on the inward-facing walls of trench and via structures compared with the outward-facing walls. We have used numerical simulation as well as metal sputter deposition experiments to characterize this asymmetry, which is typically 2–3:1 at the wafer edge. We also discuss how ionized sputter deposition would alter the deposition profile in the edge region.
Deposition of silicon nitride by low-pressure electron cyclotron resonance plasma enhanced chemical vapor deposition in N2/Ar/SiH4
Authors: Moshkalyov S. A. , Diniz J. A. , Swart J. W. , Tatsch P. J. , Machida M.
Published: July 2002

Plasma deposition of silicon nitride on silicon substrates in a microwave electron cyclotron resonance N2/Ar/SiH4 discharge was studied as a function of gas pressure (1–5 mTorr), gas composition, and discharge power (250–1000 W). The dependencies of deposition parameters on discharge characteristics obtained at 1 mTorr appear to be essentially different from those at higher pressures. Optical emission spectroscopy was used for plasma characterization. A high degree of ionization and dissociation of gas molecules was observed under present plasma conditions. It is shown that the contribution of ionized species to film deposition is comparable with that of neutral ones under high power and low pressure conditions. The best quality of films was obtained at a moderate rather than the highest available dissociation degree of silane.
Direct nano-printing on Al substrate using a SiC mold
Authors: Pang S. W. , Tamamura T. , Nakao M. , Ozawa A. , Masuda H.
Published: July 2002

Nanostructures in Al were generated by printing with hard SiC molds. This nano-printing technology replaces the lithography and the etching or deposition processes to produce patterns directly in metal. Dots, short lines, and long lines were formed in the SiC molds by electron beam lithography and reactive ion etching. High aspect ratio features as small as 40 nm with depth up to 840 nm were patterned in the SiC molds. By pressing the SiC mold onto the Al substrate at room temperature, nanostructures in the SiC mold were reproduced accurately and uniformly in Al. Large arrays of nanostructures down to 40 nm were printed in Al with similar results for dots, short lines, and long lines. Using atomic force microscopy to analyze the cross sections of the SiC molds and printed Al nanostructures, depth dependence on feature size was observed. This nano-printing technology simplifies the processes for generating nanostructures with high throughput and high uniformity.
Structure of InAs/AlSb/InAs resonant tunneling diode interfaces
Authors: Nosho B. Z. , Weinberg W. H. , Zinck J. J. , Shanabrook B. V. , Bennett B. R. , Whitman L. J.
Published: July 2002

We have used in situ plan-view scanning tunneling microscopy to study the surfaces and interfaces within an InAs/AlSb/InAs resonant tunneling diodelike structure grown by molecular beam epitaxy. The nanometer and atomic-scale morphologies of the surfaces have been characterized following a number of different growth procedures. When InAs(001)-(2×4) is exposed to Sb2 a bilayer surface is produced, with 1 monolayer (ML) deep (3 Å) vacancy islands covering approximately 25% of the surface. Both layers exhibit a (1×3)-like reconstruction characteristic of an InSb-like surface terminated with >1 ML Sb, indicating that there is a significant amount of Sb on the surface. When 5 ML of AlSb is deposited on an Sb-terminated InAs surface, the number of layers observed on each terrace increases to three. Growth of an additional 22 ML of InAs onto the AlSb layer, followed by a 30 s interrupt under Sb2, further increases the number of surface layers observed. The root-mean-square roughness is found to increase at each subsequent interface; however, on all the surfaces the roughness is ⩽2 Å. The surface roughness is attributed to a combination of factors, including reconstruction-related stoichiometry differences, kinetically limited diffusion during growth, and lattice-mismatch strain. Possible methods to reduce the roughness are discussed.
Oscillating contrast in room-temperature scanning tunneling microscope images of localized charges in III–V semiconductor cleavage surfaces
Authors: Domke C. , Heinrich M. , Ebert Ph. , Urban K.
Published: July 2002

Positively and negatively charged defects and dopant atoms in n-doped GaAs(110) surfaces give rise, at room temperature, in occupied-state scanning tunneling microscope images to a bright elevation, which is surrounded by a dark depression ring. This oscillating contrast is not observed in empty-state images. A similar effect is found on n-doped GaP(110) surfaces. A simulation of the contrast induced by localized charges on (110) surfaces of III–V semiconductors suggests that the oscillation in room-temperature scanning tunneling microscope images can be explained as the image of the local potential change (screened Coulomb potential) induced by the presence of the charge.
Graphite-based x-ray masks for deep and ultradeep x-ray lithography
Authors: Coane Philip , Giasolli Robert , De Carlo Francesco , Mancini Derrick C. , Desta Yohannes , Göttert Jost
Published: July 2002

The cost-effective fabrication of high-aspect-ratio microstructures using x rays largely depends on the availability and quality of x-ray masks. The architecture of a mask is mainly determined by the photon energy of the synchrotron source, the x-ray flux, and the thickness of the resist. Typically, the mask membrane is made from a low-atomic-number material and can either be a frame-supported, several microns thin membrane (carbon, silicon carbide, silicon nitride, or silicon) or a bulk material (beryllium) with a thickness of up to 1 mm. The absorber pattern is formed from high atomic number materials such as gold, tungsten, or tantalum, and the final pattern geometry can be defined either with additive (electroplating) or subtractive (etching, milling) processes. One approach that is designed to reduce cost and turn-around time is the fabrication of x-ray masks using graphite sheet stock for the mask membrane. Rigid graphite offers unique properties, such as moderate x-ray transmission, relatively low cost, and the ability to be used with either subtractive or additive processes. This article will report details on the lithographic fabrication of graphite x-ray masks using intermediate x-ray masks for pattern transfer. First experimental results will be presented for synchrotron x-ray exposures performed at the Center for Advanced Microstructures and Devices synchrotron in Baton Rouge and the Advanced Photon Source at Argonne National Laboratory.
New line of high voltage high current pulse generators for plasma-based ion implantation
Authors: Maulat O. , Roche M. , Le Coeur F. , Lesaint O. , Arnal Y. , Pelletier J.
Published: July 2002

The two general specifications required for plasma-based ion implantation are low pressure, large size plasmas and high voltage high current pulse generators. In addition, pulses with rise and fall times of the order of the inverse ion plasma frequency and with much longer durations than those of the inverse ion plasma frequency are most often required. To fulfill these requirements, a new type of high voltage generator using a pulse transformer has been developed. A “mettglass”® magnetic core is used as step-up pulse transformer. Voltage at the primary is provided by transistor switches which can achieve rise and fall times of less than 1 μs and maximum pulse currents of 100 A. The primary of the transformer consists of 96 turns wired in parallel and the secondary of 96 turns wired in series. The performances reported with this pulse generator were obtained on a test resistor and then on a substrate immersed in a plasma.
Novel antireflective layer using polysilane for deep ultraviolet lithography
Authors: Sato Y. , Shiobara E. , Miyoshi S. , Asano M. , Matsuyama H. , Onishi Y. , Nakano Y. , Hayase S.
Published: July 2002

A bottom antireflective layer (ARL) is essential for deep ultraviolet (UV) lithography. We have just developed a polysilane antireflective layer (PSARL) process. PSARL can act as an antireflective layer due to the Si–Si bond which has absorption in the deep UV region, and it can be etched with high selectivity to resist under etch conditions that use a chlorine plasma which is a suitable inorganic silicon etchant. Similar to the conventional organic ARL process, PSARL can be simply spin coated onto the substrate, and removed with an asher using an O2/CF4 gas mixture. The partially cross-linked diphenylsilane copolymer was evaluated with a view to its application as an ARL material and the results obtained are presented in this article.
Finite element analysis of SCALPEL wafer heating
Authors: Kim Byungkyu , Engelstad Roxann L. , Lovell Edward G. , Stanton Stuart T. , Liddle J. Alexander , Gallatin Gregg M.
Published: July 2002

A high-throughput scattering with angular limitation projection electron beam lithography (SCALPEL) tool will typically deliver up to 1.5×10−4 J to an area of 250 μm×250 μm over a time of 200 μs, corresponding to a power input of 0.75 W. This heat deposition occurs in the upper 60 μm of a wafer creating local thermal strain at the time of image formation, and depends on mask and tool conditions and specific boundary conditions. Initial modeling results indicate expansion-induced peak pattern placement errors of ∼1 nm on a local scale (i.e., a 3 mm effective field size), several nanometers in a chip area, and a few microns overall. Since the use of a mainly predictive correction algorithm is anticipated, it is vital to conduct a rigorous investigation of the heat transfer and strain formation. Results are presented of such an analysis, illustrating the heating response for the fundamental building blocks of the SCALPEL writing strategy.
Low-permittivity nanocomposite materials using sculptured thin film technology
Authors: Venugopal Vijayakumar C. , Lakhtakia Akhlesh , Messier Russell , Kucera John-Paul
Published: July 2002

The feasibility of using sculptured thin film technology to engineer multiphase low-permittivity (also called low-k) materials for microelectronic and electronic packaging applications is established via a numerical study. The chosen films are theoretically modeled as multiphase, uniaxial, dielectric nanocomposite materials and their anisotropic relative permittivity tensors are estimated using the Bruggeman formalism.
Effectiveness of Ti, TiN, Ta, TaN, and W2N as barriers for the integration of low-k dielectric hydrogen silsesquioxane
Authors: Zeng Yuxiao , Russell Stephen W. , McKerrow Andrew J. , Chen Linghui , Alford T. L.
Published: July 2002

The interactions between low-k (dielectric constant) material hydrogen silsesquioxane (HSQ) and barrier layers, Ti, Ta, physical-vapor deposited (PVD), and chemical-vapor deposited (CVD) TiN, PVD TaN, and CVD W2N, have been investigated by using sheet-resistance measurement, x-ray diffraction, transmission electron microscopy, Rutherford backscattering spectrometry, elastic resonance scattering, and forward recoil spectrometry. The conventionally used dielectric PETEOS [plasma-enhanced chemical vapor deposition tetraethylorthosilicate (TEOS)] was also studied as a control. The results show that none of these barriers except Ti can react with HSQ and PETEOS at elevated temperatures. However, significant outdiffusion of hydrogen due to the degradation of the HSQ films upon annealing exists for all barrier/HSQ structures, and exhibits a strong barrier dependence. Metal barriers Ti and Ta, and CVD TiN can retard the H outdiffusion, whereas PVD TaN and PVD TiN induce the H outdiffusion from the HSQ films. The microstructural and structural considerations and a proposed “nitride effect” mechanism can give some reasonable interpretations to these observations. Upon annealing, the structural and/or compositional changes of these barrier films give rise to corresponding sheet resistance variations. The phase transformation of the Ta film on HSQ is completely inhibited because of the possible role of diffusing H atoms, as compared to the phase transformation occurring in the Ta film on PETEOS.
Fabrication and characterization of nanoresonating devices for mass detection
Authors: Davis Z. J. , Abadal G. , Kuhn O. , Hansen O. , Grey F. , Boisen A.
Published: July 2002

We report on a novel fabrication process and preliminary characterization of a nanomechanical resonating device, which is to be used for mass detection. The fabrication of the device is based on laser lithography on Al coated SiO2/p++Si/SiO2/Si structures, followed by dry and wet etching. We have fabricated highly doped polysilicon free-hanging cantilevers and anchored drivers for lateral cantilever vibration, where the motion of the cantilever is parallel to the substrate. The cantilevers are actuated electrically by applying an ac voltage between the cantilever and driver. The laterally vibrating cantilever structures are approximately 30–50 μm in length, 1.8 μm in height, and 500 nm in width. The characterization of the resonators was performed by direct observation of the cantilever through an optical microscope. An electrical measuring technique is also presented and discussed. Typical values of resonant frequency and quality factor, at 1 atm, are approximately 500 kHz and 50, respectively. Moreover, a dependence of the resonant frequency on the applied dc voltage between the cantilever and driver has been found.
Modeling the hysteresis of a scanning probe microscope
Authors: Dirscherl Kai , Garnæs Jørgen , Nielsen Lars , Jøgensen Jan Friis , Sørensen Mads Peter
Published: July 2002

Most scanning probe microscopes use piezoelectric actuators in open loop configurations. Therefore a major problem related to these instruments is the image distortion due to the hysteresis effect of the piezo. In order to eliminate the distortions, cost effective software control based on a model for hysteresis can be applied to the scanner. We describe a new rate-independent model for the hysteresis of a piezo scanner. Two reference standards were used to determine the accuracy of the model; a one-dimensional grating with a period of 3.0 μm and a two-dimensional grating with 200 nm pitch. The structures were scanned for different scan ranges varying from 5 V peak to peak to 440 V peak to peak, so that 99% of the scanners’ full motion range was covered. A least-squares fit of the experiments to the hysteresis model provided standard deviations per scan range of around 0.2%. This represents an uncertainty of 1 pixel. Since our model is based on a differential equation, it is flexible even to simulate arbitrary experimental conditions such as a sudden change in the offset.
Investigation of the mechanism for Ti/Al ohmic contact on etched n-GaN surfaces
Authors: Sheu J. K. , Su Y. K. , Chi G. C. , Jou M. J. , Liu C. C. , Chang C. M. , Hung W. C. , Bow J. S. , Yu Y. C.
Published: July 2002

In this study, the mechanism for ohmic contact of Ti/Al bilayer formation on as-grown, etched and postetch annealed GaN surfaces were investigated. A nonalloyed Ti/Al ohmic contact to etched GaN surface, with postetch annealing prior to metal deposition, was obtained. The specific contact resistance of 2.3×10−4 Ω cm2 was obtained. The nonalloyed ohmic contact may be attributed to the postetch annealing which generates nitrogen vacancies that result in a heavily n-type surface thereby forming a tunneling junction. On the other hand, the nonalloyed Ti/Al contact on as-grown and as-etched GaN surfaces exhibits non-ohmic behavior. After alloying at 500 °C for 5 min, Ti/Al contacts on as-grown, as-etched and postetched annealing GaN surfaces have specific contact resistances around 9.8×10−5, 1×10−4, and 7.2×10−5 Ω cm2, respectively. Nonalloyed Ti/Al ohmic contacts would be especially useful for fabricating high breakdown, recessed-gate field effect transistors on GaN since the moderate postannealing condition converts only the near surface layer to heavily n type.
Supercritical resist dryer
Authors: Namatsu Hideo , Yamazaki Kenji , Kurihara Kenji
Published: July 2002

We have developed a supercritical resist dryer in which resist patterns are developed and successively dried with a supercritical fluid. The usage of supercritical fluid in pattern drying is most effective for preventing pattern collapse, which is a significant problem in fine patterning of less than 100 nm critical dimensions. The key to performing ideal supercritical resist drying is the prevention of water contamination due to moisture in the dryer chamber. This is because the moisture dissolved in the supercritical fluid causes pattern deformation which results from film swelling. As a technique to prevent water contamination, we have devised pressure-controlled supercritical drying where a supercritical fluid is sent pressurized to a Teflon coated chamber at near the critical point. The supercritical resist dryer made on the basis of this technique is an essential apparatus for next-generation lithography.
Geometry effects arising from anodization of field emitters
Authors: Seidl A. , Takai M. , Hosono A. , Yura S. , Okuda S.
Published: July 2002

It has been shown by previous work [M. Takai et al., J. Vac. Sci. Technol. B 13, 441 (1995)] that field emission of silicon emitters can be enhanced by anodization forming a layer of porous silicon. Typically, an increase of the emission current of a factor of 10 could be measured. The effect of enhanced emission is usually attributed to nanometer sized silicon rods on the surface, leading to local peaks of electrical field strength [E. C. Boswell et al., J. Vac. Sci. Technol. B 14, 1895 (1996)]. The possibility of a sharpening of the tip has also been discussed. The theoretical part of the above-mentioned investigations has been qualitative in nature. In this work, a quantitative discussion of these effects is performed. For this purpose, a finite element (FEM) model has been constructed on the basis of the test structures. Since the performance of FEM degrades in the vicinity of singularities, such as sharp tips analytical solutions have been employed in order to check the accuracy of the numerical solution and to discuss the behavior of the electrical field close to “rough” surfaces.
Temperature dependence of exciton localization in Zn1−xCdxSe quantum wells
Authors: Díaz-Arencibia P. , Hernández-Calderón I. , Hernández-Ramírez L. M. , Tamargo M. C.
Published: July 2002

We report the results of the investigation of the temperature dependence of the spontaneous emission of localized excitons in Zn1−xCdxSe quantum wells (QWs). Two main peaks, which show a strong change in relative intensities with temperature, dominate the spectra. The presence in the spectra of biexcitons and bound excitons was ruled out after the corresponding analyses. Calculation of the fundamental transitions of the QWs suggested that the peaks are due to thickness fluctuations of one and two monomolecular layers. The successful analysis of the spectra in terms of a simple two-level model indicated that localization of excitons due to QW thickness fluctuations and exciton migration are basic processes which have noticeable influence in the spontaneous emission of Zn1−xCdxSe QWs.
Surface migration of group V atoms in GaAsP grown on GaAs channeled substrates by molecular beam epitaxy
Authors: Tatsuoka Y. , Kamimoto H. , Kitada T. , Shimomura S. , Hiyamizu S.
Published: July 2002

Surface migration of group V atoms (As and P) during molecular beam epitaxy was investigated for the first time by measuring lateral profiles of phosphorus content in GaAsP layers grown on (100) GaAs channeled substrates (CSs) with 12–16 μm wide (n11)A side-slope facet region (n=3, 4, and 5) using As4 and P2 beams. The P content (x) of the GaAs1−xPx layer on the (411)A side-slope region was 43% smaller (x=0.11) than that on the (411)A flat substrates, while P contents of GaAsP layers on (311)A and (511)A side-slope regions were larger than those of GaAsP layers on a corresponding flat substrate. The observed lateral P content profile was well reproduced by simulation based on a conventional surface migration model. The migration lengths of As atoms, LAs, were determined as LAs(411)A=18 μm, LAs(100)=28 μm, LAs(511)A=33 μm, and LAs(311)A=35 μm, by comparing the observed and simulated profiles.
High performance InP high electron mobility transistors by valved phosphorus cracker
Authors: Chin T. P. , Chen Y. C. , Barsky M. , Wojtowicz M. , Grundbacher R. , Lai R. , Streit D. C. , Block T. R.
Published: July 2002

InP-based high electron mobility transistors (HEMTs) are grown by solid source molecular beam epitaxy with a valved phosphorus cracker. An InP/InGaAs composite channel improves both the channel mobility and the breakdown voltage for W-band power applications. The optimization of the InP/InGaAs interface quality is found critical to the device performance. A 1.5 V increase in breakdown voltage is observed in a 0.15 μm T-gate device. A two-stage power amplifier circuit delivered 316 mW output power with 17 dB linear gain at 94 GHz. A pseudomorphic HEMT structure with 75% InGaAs channel is used for extremely high frequency (up to 200 GHz) applications. InP etch-stop layers are used to improve the threshold voltage uniformity, which is closely related to the gate-recess depth uniformity. By inserting this InP etch-stop layer in the InAlAs barrier layer, the threshold voltage (Vth) standard deviation across a 3-in. wafer is reduced from 50 to 27 mV. An amplifier with 10 dB gain at 190 GHz is fabricated.
Brushite (CaHPO4⋅2H2O) by XPS
Authors: Chusuei Charles C. , Goodman D. Wayne
Published: July 2002

Dibasic calcium phosphate dihydrate (CaHPO4⋅2H2O, DCPD) is the least thermodynamically stable calcium phosphate phase according to Ostwald’s step rule and is an important biomineral involved in the growth of hydroxyapatite [Ca10(PO4)6(OH)2, HAP], the principal component of bone and teeth. DCPD is one of the first calcium phosphate phases that form at the onset of nucleation and growth of HAP crystals. X-ray photoelectron spectra (XPS) of the Ca 2p, P 2p, and O 1s lines from DCPD are presented. Since DCPD is sensitive to phase transformation, its identity was verified by x-ray diffraction prior to XPS scans.
Tropical Origins for Recent North Atlantic Climate Change
Authors: Hoerling Martin P. , Hurrell James W. , Xu Taiyi
Published: July 2002

Evidence is presented that North Atlantic climate change since 1950 is linked to a progressive warming of tropical sea surface temperatures, especially over the Indian and Pacific Oceans. The ocean changes alter the pattern and magnitude of tropical rainfall and atmospheric heating, the atmospheric response to which includes the spatial structure of the North Atlantic Oscillation (NAO). The slow, tropical ocean warming has thus forced a commensurate trend toward one extreme phase of the NAO during the past half-century.
Switching Repulsion to Attraction: Changing Responses to Slit During Transition in Mesoderm Migration
Authors: Kramer Sunita G. , Kidd Thomas , Simpson Julie H. , Goodman Corey S.
Published: July 2002

Slit is secreted by cells at the midline of the central nervous system, where it binds to Roundabout (Robo) receptors and functions as a potent repellent. We found that migrating mesodermal cells in vivo respond to Slit as both an attractant and a repellent and that Robo receptors are required for both functions. Mesoderm cells expressing Robo receptors initially migrate away from Slit at the midline. A few hours after migration, these same cells change their behavior and require Robo to extend toward Slit-expressing muscle attachment sites. Thus, Slit functions as a chemoattractant to provide specificity for muscle patterning.
Recruitment of HAT Complexes by Direct Activator Interactions with the ATM-Related Tra1 Subunit
Authors: Brown Christine E. , Howe LeAnn , Sousa Kyle , Alley Stephen C. , Carrozza Michael J. , Tan Song , Workman Jerry L.
Published: July 2002

Promoter-specific recruitment of histone acetyltransferase activity is often critical for transcriptional activation. We present a detailed study of the interaction between the histone acetyltransferase complexes SAGA and NuA4, and transcription activators. We demonstrate by affinity chromatography and photo–cross-linking label transfer that acidic activators directly interact with Tra1p, a shared subunit of SAGA and NuA4. Mutations within the COOH-terminus of Tra1p disrupted its interaction with activators and resulted in gene-specific transcriptional defects that correlated with lowered promoter-specific histone acetylation. These data demonstrate that the essential Tra1 protein serves as a common target for activators in both SAGA and NuA4 acetyltransferases.
Direct Measurement of the Preferred Sense of NO Rotation After Collision with Argon
Authors: Lorenz K. Thomas , Chandler David W. , Barr James W. , Chen Wenwu , Barnes George L. , Cline Joseph I.
Published: July 2002

The preferred sense of product molecule rotation (clockwise or counterclockwise) in a bimolecular collision system has been measured. Rotationally inelastic collisions of nitric oxide (NO) molecules with Ar atoms were studied by combining crossed molecular beams, circularly polarized resonant multiphoton ionization probing, and velocity-mapped ion imaging detection. The observed sense of NO product rotation varies with deflection angle and is a strong function of the NO final rotational state. The largest preferences for sense of rotation are observed at the highest kinematically allowed product rotational states; for lower rotational states, the variation with deflection angle becomes oscillatory. Quantum calculations on the most recently reported NO-Ar potential give good agreement with the observed oscillation patterns in the sense of rotation.
Role of Cell-Specific SpoIIIE Assembly in Polarity of DNA Transfer
Authors: Sharp Marc D. , Pogliano Kit
Published: July 2002

SpoIIIE mediates postseptational chromosome partitioning in
Bacillus subtilis
, but the mechanism controlling the direction of DNA transfer remains obscure. Here, we demonstrated that SpoIIIE acts as a DNA exporter: When SpoIIIE was synthesized in the larger of the two cells necessary for sporulation, the mother cell, DNA was translocated into the smaller forespore; however, when it was synthesized in the forespore, DNA was translocated into the mother cell. Furthermore, the DNA-tracking domain of SpoIIIE inhibited SpoIIIE complex assembly in the forespore. Thus, during sporulation, chromosome partitioning is controlled by the preferential assembly of SpoIIIE in one daughter cell.
Superconductor Flux Pinning and Grain Boundary Control
Authors: Larbalestier David
Published: July 2002

Ten years ago, a new class of high-transition temperature superconductors was discovered. This led to much excitement about potential technologies, but this potential has not been realized. In his Perspective, Larbalestier discusses recent results published in
Science
by Yang and Lieber (
p. 1836
) and by Norton
et al
. (
p. 755
), which may offer ways to improve the ability of superconductors to carry high currents.
Conditional Manipulation of Sex Ratios by Ant Workers: A Test of Kin Selection Theory
Authors: Sundström Liselotte , Chapuisat Michel , Keller Laurent
Published: July 2002

Variable queen mating frequencies provide a unique opportunity to study the resolution of worker-queen conflict over sex ratio in social Hymenoptera, because the conflict is maximal in colonies headed by a singly mated queen and is weak or nonexistent in colonies headed by a multiply mated queen. In the wood ant
Formica exsecta
, workers in colonies with a singly mated queen, but not those in colonies with a multiply mated queen, altered the sex ratio of queen-laid eggs by eliminating males to preferentially raise queens. By this conditional response to queen mating frequency, workers enhance their inclusive fitness.
Photolysis of the Carbon Monoxide Complex of Myoglobin: Nanosecond Time-Resolved Crystallography
Authors: Šrajer Vukica , Teng Tsu-yi , Ursby Thomas , Pradervand Claude , Ren Zhong , Adachi Shin-ichi , Schildkamp Wilfried , Bourgeois Dominique , Wulff Michael , Moffat Keith
Published: July 2002

The biological activity of macromolecules is accompanied by rapid structural changes. The photosensitivity of the carbon monoxide complex of myoglobin was used at the European Synchrotron Radiation Facility to obtain pulsed, Laue x-ray diffraction data with nanosecond time resolution during the process of heme and protein relaxation after carbon monoxide photodissociation and during rebinding. These time-resolved experiments reveal the structures of myoglobin photoproducts, provide a structural foundation to spectroscopic results and molecular dynamics calculations, and demonstrate that time-resolved macromolecular crystallography can elucidate the structural bases of biochemical mechanisms on the nanosecond time scale.
The Year of the Dendrite
Authors: Sejnowski Terrence J.
Published: July 2002

Octopus-like neurons, the cells of the brain, typically have many input “arms”—the dendrites, which receive signals from other neurons—and one output arm, the axon. Two new reports in this week's
Science
show that the dendrites process information in a manner even more complex than expected. In his Perspective, Sejnowski explains how the observations of Magee and Johnston (
p. 209
) and Markram
et al.
(
p. 213
) on backpropagating action potentials illuminate the mechanism of Hebbian learning, the synaptic strengthening that occurs when a receiving neuron is active at the same time that a signal comes in.