bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanoscale lithography on Langmuir–Blodgett films of behinic acid
Authors: Garnaes J. , Bjo/rnholm T. , Zasadzinski J. A. N.
Published: July 2002

By increasing the force when scanning with an atomic force microscope, it was possible to scratch holes in a monolayer of behinic acid. Patterns with a line width of 50 to 90 nm were ‘‘written’’ with an applied force of ≊70 nN within a few minutes. The measured depth was consistent with the height of a monolayer of behinic acid with tilted chains (≊2 nm). Ten hours later, the pattern was imaged again and no change was found when an applied force of ≊10 nN was used. Contrary to other monolayers, which often have hole defects, the behinic acid monolayer was free of such defects and seemed, in particular, well suited for nanoscale lithography. Based on several different images, a model of how the tip removes the monolayer was formulated.
Inelastic processes in time dependent tunneling in a scanning tunneling microscope junction
Authors: Miskovsky N. M. , Park Sookyung H. , Cutler P. H. , Sullivan T. E.
Published: July 2002

Experiments on frequency mixing and tunneling time measurements have stirred renewed interest in the problem of tunneling through time dependent barriers in a vacuum diode configuration such as the scanning tunneling microscope (STM) junction. In the present work we have formulated a theory of photoassisted emission for a planar bimetallic junction with both a static dc voltage and a time dependent bias of angular frequency ω. An expression for the current is derived using kinetic theory and the transfer Hamiltonian method. The wave functions are determined from time dependent perturbation theory and are calculated recursively to infinite order. Although the current is initially calculated within the free-electron model, this kinetic formalism allows for the inclusion of band-structure effects through the density of states factors. The photoassisted inelastic tunneling current was calculated as a function of frequency, fields, barrier parameters, and material asymmetry. The results are applied to an analysis of the tunneling time experiments of Nguyen et al. [H. Q. Nguyen et al., IEEE Trans. Electron Devices 36, 2671 (1989)]. The conclusions impose some constraints but do not limit the experimental procedure used for extracting tunneling times in an irradiated STM junction.
Microscopic theory of scanning tunneling microscope for finite electric field and current
Authors: Hirose Kenji , Tsukada Masaru
Published: July 2002

A new method of obtaining a self-consistent surface electronic structure for a finite electric field and current within the local density functional formalism is presented. In the method, two different chemical potentials are assigned to the left and right electrodes, and the scattering waves are calculated by the step-by-step recursion matrix method. This method is applied to the Na bimetallic planar junction. Effective potential, electric current, and charge density distribution are presented for given values of the separation and the bias voltage.
Growth and characterization of GaN on sapphire (0001) using plasma-assisted ionized source beam epitaxy
Authors: Kim K. , Yoo M. C. , Shim K. H. , Verdeyen J. T.
Published: July 2002

A new approach to expitaxial GaN growth using the plasma-assisted ionized source beam epitaxy (PAISBE) is described. To facilitate the growth of GaN, an accelerated partially ionized Ga source beam was used along with an atomic nitrogen beam from an rf-discharge nitrogen plasma. To control the amount of N+2 ions leaving the discharge tube (thus participating in the GaN growth), a grid was installed at the exit end of the plasma tube. The growth parameters under control were the power and frequency of the rf discharge, the energy and fraction of ionization of the Ga beam, the Ga and N fluxes, and the substrate temperature. The crystal quality and surface morphology of the PAISBE-grown GaN films on sapphire (0001) were analyzed using reflection high-energy electron diffraction, x-ray diffraction, and scanning electron microscopy. The best films were obtained with a partially ionized Ga beam at an rf power of 140 W, which exhibited a full width at half maximum of 25 min at the (0002) diffraction peak for a 0.4-μm-thick film. The buffer layer was a GaN film grown at 400 °C. Results illustrating the effects of various control parameters and, in particular, those demonstrating the advantages of PAISBE are presented.
Interface characterization in an InP/InGaAs resonant tunneling diode by scanning tunneling microscopy
Authors: Skala S. L. , Wu W. , Tucker J. R. , Lyding J. W. , Seabaugh A. , Beam E. A. , Jovanovic D.
Published: July 2002

The interfaces of a lattice-matched InP/InGaAs double-barrier resonant tunneling diode are studied by scanning tunneling microscopy. The interfaces have been mapped with atomic resolution for a length of over 4000 Å, and scanning tunneling microscopy images show the roughness to be very asymmetric with the inverted (InP on InGaAs) interface being considerably rougher than the normal interface. Roughness wave vectors determined from Fourier analysis are well fitted by a Lorentzian function and allow determination of roughness amplitudes and correlation lengths for the different interfaces. A chemical asymmetry between the interfaces is also observed in certain images which exhibit enhanced and reduced state densities at the normal and inverted interfaces, respectively. Termination of the growth surface by differing column V species during interface formation most likely affects the local state density.
Study of H− beams for ion-projection lithography
Authors: Chen C.-H. , Guharay S. K. , Reiser M. , Riordon J. , Orloff J. , Melngailis J.
Published: July 2002

The desired properties of ion sources used in ion-projection lithography are high brightness and low energy spread. Due to a very stringent demand in many other applications, the state of the art of negative hydrogen ion (H−) beams has advanced significantly in recent years. H− ion beams with the transverse ion temperature of about 0.3 eV have been obtained from surface plasma sources in pulsed mode operation [V. G. Dudnikov and G. E. Derevyankin, AIP Conf. Proc. 287, 239 (1994)]; a design for a cw source to achieve a comparable, or even lower energy spread and high brightness has been reported [S. K. Guharay, M. Reiser, and V. G. Dudnikov, Rev. Sci. Instrum. 65, 1745 (1994)]. The energy spread of H+ ion beams now in use is in the range of about 6–10 eV. This renders the H− ion beams an attractive alternative to the H+ ion beams for ion-projection lithography. No data of H− ion exposure of resists are available, and we have carried out experiments illuminating PMMA resists with various doses of H− ions and H+ ions at 24 keV. Within experimental accuracy the exposure characteristics (e.g., sensitivity and contrast) are identical for both H− and H+ ions.
Adaptive metrology: An economical strategy for judging the acceptability of a mask pattern
Authors: Wang Weidong , Ye Jun , Owen Art B. , Berglund Carl N. , Pease Roger Fabian W.
Published: July 2002

Size and placement of all features in a mask used for semiconductor integrated circuit manufacture must be within specified ranges for the mask to be acceptable. However the decision on whether the mask meets these specifications is usually based on a limited set of measurements; less than 1 in 106 of all possible sites. Conventional variance analysis on such measurements is inadequate and often misleading because mask pattern errors have significant systematic components. Here we describe a more efficient metrology approach for determining whether a mask meets specifications, ‘‘adaptive metrology,’’ where the results obtained as the measurements proceed are used to decide how many, and where, more measurements are needed. The process is continued until there is sufficient information to make a decision whether or not the mask is acceptable. In this way the systematic component of the mask errors can be exploited either to increase the accuracy of decisions or to shorten the average measurement time for a given decision accuracy.
Local modification of n-Si(100) surface in aqueous solutions under anodic and cathodic potential polarization with an in situ scanning tunneling microscope
Authors: Ye J. H. , Pérez-Murano F. , Barniol N. , Abadal G. , Aymerich X.
Published: July 2002

Local modification of H-terminated n-Si(100) surface in an aqueous hydrofluoric acid (HF) solution was carried out under anodic and cathodic potential polarization with an in situ scanning tunneling microscope (STM). Two different modifications in the HF solution were obtained. At cathodic polarized potential, H-terminated silicon surface is attacked by HF at the defects of silicon surface under an electric field between the tip and silicon surface. However, at anodic polarized potential (by applying positive potential pulses), the nanostructures produced are probably due to the formation of silicon oxide. The apparent depth of the nanostructures, as observed with the STM, decreases with time because of the dissolution of silicon oxide in the solution. Effects of the tunneling current, the potential of silicon surface, and pulse amplitude show that the formation of silicon oxide is dependent on the electric field and local electrochemical oxidation of silicon surface at anodic potential.
A diazoquinone positive photoresist for x-ray lithography
Authors: Aviram Ari , Bucca Daniel , Seeger David
Published: July 2002

Diazoquinone photoresists are very reliable materials for photolithography. They have been used extensively by the electronic industry because of their reliability, stability, and ease of operation; and a large body of information has been gathered regarding their application and use. Therefore, it would be natural to extend their use to the field of x-ray lithography. Unfortunately, the response of these resists to x-ray radiation is poor, and requires 1.2 J/cm2. Such a dose is too high for practical applications. Amplified resists have been used successfully in x-ray lithography, but this came with the usual environmental sensitivity of the amplified resists. Here we report the modification of diazoquinone resists for x-ray lithography. This was accomplished by substituting hydrogen atoms in the photoactive compound (PAC) by iodine. The attachment was carried out in two ways. In the first approach, we used iodine containing moieties to bridge several conventional diazonaphthoquinone sulfonates. Some of these bridging groups contained one iodine atom, while others contained more. We were able to determine the relationship between the number of iodine atoms in the PAC to the x-ray lithography sensitivity. The second approach was to synthesize a new diazonaphthoquinone where one of the hydrogen atoms on the naphthalene ring was substituted with an iodine atom. This modification enabled us to understand the importance of the proximity effect of the iodine to the diazoquinone group. It was found that placing one iodine atom next to the diazoquinone group produces an improvement that is equivalent to placing three iodine atoms on the bridge. The described modifications lead to considerable improvement in the sensitivity of the resist. This article describes the synthetic methods employed and the corresponding lithographic results.
Thermal analysis of an x-ray mask membrane in a plasma environment
Authors: Laudon M. F. , Thole K. A. , Engelstad R. L. , Resnick D. J. , Cummings K. D. , Dauksher W. J.
Published: July 2002

Quantifying the temperatures of a membrane while the mask is in a plasma environment is essential in controlling absorber stress due to deposition, and both etch rate and feature profile due to etching. Temperature gradients across the membrane during deposition lead to nonuniform stress across the absorber resulting in large distortions during pattern transfer [W. Dauksher et al., J. Vac. Sci. Technol. B 13, 3103 (1995)]. This article presents a procedure to obtain the steady state temperature profile of a mask/membrane while in a plasma environment (deposition or etch) subjected to different cooling configurations. Membrane heat fluxes and heat transfer coefficients were determined using a thermal transient technique which compares analytical solutions to experimental results. The steady state temperature profile of the membrane was then obtained by using these fluxes and heat transfer coefficients in a three-dimensional finite element model. The analysis procedure was demonstrated on a mask subjected to no helium backside cooling and a mask subjected to flowing helium backside cooling. Good agreement was obtained between the finite element solution, analytical solution, and the experimental results.
Processing control for 0.25 μm x-ray exposures of commercially available resists: The potential for adaptive control
Authors: Taylor James W. , Gamsky Chris , Rhyner Steve , Howes Glenn , Dentinger Paul , Nelson Carla , Yang Cheng , Reilly Michael
Published: July 2002

The processing of chemically amplified resists to yield 0.25 μm structures requires control of a number of key parameters. Some of these may be at the limits of the control range or at the limits of achievable uniformity. Where it may not be possible to improve the control or the uniformity, it may be possible to use adaptive control to compensate. The focus of this article is to explore what is needed for adaptive control with two commercial x-ray resists, Shipley SAL 605 (negative tone) and Hoechst AZ PF-514 (positive tone) and to explore the feasibility of adaptive control with the SAL 605 resist. Statistical optimization has been used previously to provide robust process latitude by focusing on the preexposure bake temperature and time, the x-ray dose, and the postexposure bake temperature and time. To determine the required control and the slope of the linewidth change with the change in each variable, a statistical model of the response surface was constructed. The statistical results and new work with developer normality point out the major contributions from the wafer baking conditions using a track vacuum hot plate. The different slopes of the variations with linewidth suggest that whole wafer compensation may be possible for certain parameters, and that field-to-field variation arising from hot plate uniformity might be compensated for the SAL 605 resist with another. Preliminary results with Fourier transform infrared spectroscopy will be presented that support these approaches and suggest that further efforts to provide in-line measurements might be justified by correlating the infrared cross-linking signal with the processing parameter that yields the correct linewidth.
Calculation of local density of states at an atomically sharp Si tip
Authors: Huang Z.-H. , Cutler P. H. , Miskovsky N. M. , Sullivan T. E.
Published: July 2002

The local density of states (LDOS) at an atomically sharp Si tip is calculated using a tight-binding model. A pyramidal shaped cluster of ten layers is constructed to represent the tip. The top six layers are allowed to relax while the bottom four are fixed to simulate the bulk. The forces are calculated and used to optimize the atomic geometry. It is found that the LDOS of the topmost atom is significantly different than that in the bulk, and that it changes little with an applied bias. However, the topmost atom does not have any strongly localized occupied state, though there exists more localized p-type states above the highest occupied states. This suggests that the discrete character of the structure plays a significant role in determining the LDOS for an atomic size tip.
Accurate secondary ion mass spectrometry analysis of shallow doping profiles in Si based on the internal standard method
Authors: Liu G. L. , Uchida H. , Aikawa I. , Kuroda S. , Hirashita N.
Published: July 2002

An accurate quantitative analysis of shallow dopant profiling has been investigated by secondary ion mass spectrometry (SIMS) with an internal standard method. In this method, the internal standard layer is formed by polycrystalline silicon (poly-Si) deposition and followed by standard ion implantation on the Si substrate to be measured. The reproducibility of the measurements for the dopant profiles in the substrate is found to be less than ±5%. A chemical absolute quantitative analysis of implanted dosage is also proposed. The dopant profiles in Si substrates obtained by the internal standard SIMS method, calibrated by the chemical analysis, agree well with the results of capacitance–voltage measurements for metal–oxide–semiconductor diodes. The practical usefulness of the internal standard SIMS method for quantitative dopant profiling is demonstrated by analyzing typical doping profiles in device fabrication processes.
Feature contrast in dose-equalization schemes used for electron-beam proximity control
Authors: Peckerar Martin , Marrian Christie , Perkins F. Keith
Published: July 2002

Dose equalization by the GHOST technique fails to provide homogeneous contrast when the two-Gaussian model does not adequately portray the beam-energy dose distribution. This leads to an undesirable pattern dependence to the contrast obtained. The degree to which this breakdown occurs is shown in computer simulation. The most severe breakdown occurs in enclosed (annular) structures where the effect of ‘‘higher order’’ Gaussian deposition peaks are concentrated. Unanticipated background fluctuations of about 10% of the nominal exposure dose are obtained. As a result of this study, test structures are proposed which can extend the utility of dose equalization.
Schottky barrier tuning at Al/GaAs(100) junctions
Authors: Berthod C. , Bardi J. , Binggeli N. , Baldereschi A.
Published: July 2002

Using an ab initio pseudopotential approach, we have investigated the electronic structure of ideal Al/GaAs(100) and Al/Ga1−xAlxAs(100) junctions, and the change of the corresponding Schottky barrier height versus the alloy composition x and in the presence of ultrathin group-IV atom interlayers. We find large changes in the Schottky barrier height which agree well with the experimental data. In order to interpret the observed trends we have analyzed the charge density response to chemical substitutions near the junction. This allowed us to extend to metal/semiconductor interfaces a microscopic linear-response theory approach previously employed to interpret band-offset trends at semiconductor heterojunctions.
Facet generation during molecular beam epitaxy of GaAs/AlGaAs multilayers on GaAs (001) patterned substrates
Authors: Takebe T. , Fujii M. , Yamamoto T. , Fujita K. , Watanabe T.
Published: July 2002

Extra facet generation during molecular beam epitaxy of GaAs/AlGaAs multilayers on (001) GaAs substrates patterned with stripes running in the [1̄10], [110], and [100] directions and having various slopes, designated as ‘‘[1̄10] stripe,’’ ‘‘[110] stripe,’’ and ‘‘[100] stripe,’’ respectively, has systematically been investigated for the first time. It has been confirmed that extra (111)A and (114)A facets are generated on the [1̄10] stripes, extra (1̄11)B and (1̄13)B facets on the [110] stripes, and extra (031), (011), (045), and (013) facets on the [100] stripes, depending on the intersection angle θ of the sidewall and the substrate plane. Among them, the (114)A, (1̄13)B, and (013) facets are important because they persist over a wide range of θ and, therefore play a detrimental role in forming flat and uniform sidewall layers on the (001) patterned substrates. It has been made clear that no extra facets are generated on the sidewalls and flat and uniform layers maintaining the initial as-etched stripe patterns can be grown for the [1̄10] stripes with 10°≥θ, the [110] stripes with 24°≥θ≥20°, and the [100] stripes with 19°≥θ. Extra (111)A-related facets have developed on the (111)A-related intersection of the equivalent [100] and [010] stripes, while no extra facets have developed on the (1̄11)B-related intersection. The facet generation behavior has been compared between the (001) patterned substrates and previously studied (111)A patterned substrates.
Atomic resolution imaging of InP(110) surface observed with ultrahigh vacuum atomic force microscope in noncontact mode
Authors: Sugawara Y. , Ohta M. , Ueyama H. , Morita S. , Osaka F. , Ohkouchi S. , Suzuki M. , Mishima S.
Published: July 2002

True atomic resolution imaging of the cleaved semi-insulating InP(110) surface was demonstrated using an ultrahigh vacuum atomic force microscope (AFM) in noncontact mode. The force gradient acting on the tip was detected by the frequency modulation method. The rectangular lattice could be clearly observed. The image contrast suddenly changed during the scan, which suggests that the noncontact AFM imaging is performed under the condition of nearly monoatomic tip–sample force interaction. Atomic defects have been clearly and reproducibly observed. These results suggest that noncontact AFM has the potential for true atomic-scale lateral resolution and is quite effective for atomic surface structure analysis in real space.
Ga-induced restructuring of Si(112) and Si(337)
Authors: Baski A. A. , Whitman L. J.
Published: July 2002

The adsorption of gallium on Si(112) and Si(337) has been studied with scanning tunneling microscopy. When clean, these two high-index surfaces are unstable and facet to other orientations: (112) to (111)- and (337)-like nanofacets, and (337) to the stable (5 5 12) plane. When Ga is adsorbed onto each of these surfaces and annealed, the top surface layers undergo substantial rearrangements, exposing Ga-reconstructed (112) planes in both cases.
Thermal desorption spectroscopy and molecular beam time-of-flight studies of silicon wafer ultraviolet/ozone cleaning
Authors: Yamaguchi K. , Uematsu Y. , Ikoma Y. , Watanabe F. , Motooka T. , Igarashi T.
Published: July 2002

Ultraviolet (UV)/ozone cleaning is a promising way of decontaminating silicon wafers for industrial very large scale integrated processing. We have investigated the cleaning process in high-vacuum conditions utilizing a pulsed supersonic valve and an excimer lamp. With this setup, we are able to supply atomic oxygen onto Si surfaces with and without the simultaneous UV light exposure. We have discovered that thermal desorption spectroscopy analysis of the surface adsorbed species shows marked differences in the wafers processed under different conditions in high vacuum. We have confirmed that UV irradiation is a very essential factor in the UV/ozone cleaning process. In addition, we have carried out time-resolved studies of species coming off the wafer surfaces during UV/ozone cleaning, and found that the main by-products are CO, CO2, and H2O. The hydrocarbon removal reaction is not oxygen-supply limited.
Printing halftone photographic images on diamond by focused silicon ion implantation
Authors: Erickson L. E. , Champion H. G. , Fraser J. W. , Hussey R. , Schmuki P. , Porco C.
Published: July 2002

A process for creating archival records on a very long-lived durable material is demonstrated. Optical and scanning electron microscope viewable photographs were printed into the surface of a chemical vapor deposition diamond wafer by silicon ion implantation. The damage caused by the implant converts the transparent diamond to visible (black) forms of carbon. The photographs were printed using both halftone and gray-scale encoding. The halftone encoding was accomplished by implanting an area proportional to the desired optical density within the 1 μm square pixel at a fixed area dose. This photograph may optionally be “fixed” by annealing the sample at 1000 °C. This transforms the amorphous carbon to graphite. For the gray-scale encoded image, an 800 nm square is implanted with an ion dose proportional to the optical density of each pixel.
0.1 μm WSiN-gate fabrication of GaAs metal-semiconductor field effect transistors using electron cyclotron resonance ion stream etching with SF6–CF4–SiF4–O2
Authors: Jin Yoshito , Takahashi Chiharu , Nishimura Kazumi , Ono Toshiro , Matsuo Seitaro
Published: July 2002

We investigate etching characteristics of WSiN gates using electron cyclotron resonance ion stream etching with a SF6–CF4–SiF4–O2 gas mixture, and we fabricate 0.1 μm WSiN gates for ultrahigh speed GaAs metal-semiconductor field effect transistors (MESFETs). A vertical WSiN gate etched pattern is obtained as a result of CF4 addition. Moreover, uniform etching over the entire wafer can be attained with high selectivity between the WSiN and the GaAs and with accurate control of the gate length. A current-gain cutoff frequency (fT) of 131.4 GHz with a 3σ value of 5.0 GHz in whole wafer has been obtained for 0.1 μm gate GaAs MESFETs.
Selective SiO2/Si3N4 etching in magnetized inductively coupled C4F8 plasma
Authors: Lee Ho-Jun , Kim Joong Kyun , Kim Jung Hun , Whang Ki-Woong , Kim Jeong Ho , Joo Jung Hoon
Published: July 2002

Characteristics of selective SiO2 etching over Si3N4 in a magnetized inductively coupled high density C4F8 plasma have been investigated as a function of process parameters. The SiO2 etch rate increases with both external dc magnetic field (0–15 G) and rf discharge power at a pressure of 1.2 mTorr. For Si3N4, the etch rate also increases with rf power at unmagnetized and weakly magnetized (6 G) plasma conditions, but it decreases with discharge power when the magnetic field is higher than 12 G. At the substrate bias voltage of −100 V, the lowest etch rate of Si3N4 is obtained at two different process conditions namely low power, unmagnetized and high power, magnetized discharges. As the pressure increases, selectivity is degraded severely due to the rapid increase in the Si3N4etch rate. Resulting selectivities varied from approximately 1:1 to 40:1 without altering the feed gas chemistry. Measurement of fluorine density and CF, CF2 flux incident on the substrate reveal that the improvement of SiO2/Si3N4 etch selectivity is correlated to the rate of change of both CF and CF2 radicals being greater than that of fluorine. It is also shown that the high ion current at sufficiently low fluorine density is important for high selectivity. These results imply that the high density low pressure plasma source is indeed important for the selective etching of SiO2/Si3N4.
X-ray scattering analysis of interface roughness and diffusion
Authors: Baribeau J.-M.
Published: July 2002

Examples of applications of x-ray scattering techniques to the characterization of Si/Si1−xGex heterostructures on (001) Si grown by either molecular beam epitaxy (MBE) or ultrahigh vacuum chemical vapor deposition (UHV-CVD) are presented. Triple-axis high-resolution x-ray diffraction is used to investigate morphological instabilities in Si/Si1−xGex multilayers grown at high temperature. Intensity mapping of highly asymmetric Bragg reflections reveals the presence of undulations along [100] directions. The amplitude and wavelength of the undulations can be adjusted by appropriate choice of growth conditions and structure geometry, which offers interesting prospects for the design of Si1−xGex quantum wires and dots. Specular reflectivity and diffuse scattering is used to estimate the interface roughness and its characteristic length scale and correlation of Si/Si1−xGex heterostructures. Interfaces in superlattices are generally narrow (0.3–0.5 nm), highly correlated vertically, and influenced by growth conditions. Heterostructures grown by MBE or UHV-CVD have comparable interface widths, but structures grown by MBE have a longer autocorrelation length (0.5 μm versus 30 nm). Diffraction and grazing angle specular reflectivity is also used to study diffusion processes in Si/Si1−xGex heterostructures upon thermal annealing. The analysis reveals an enhancement of the diffusion in the early stage of annealing that is not due to strain, but may be linked to grown-in, nonequilibrium point defects. The large difference in the diffusivity of Ge in Si and Ge also causes an anisotropy in the diffusion, which leads to the broadening of Si1−xGex quantum wells upon annealing.
Cyclotron resonance measurements of Si/SiGe two-dimensional electron gases with differing strain
Authors: Griffin N. , Arnone D. D. , Paul D. J. , Pepper M. , Robbins D. J. , Churchill A. C. , Fernández J. M.
Published: July 2002

Far-infrared cyclotron resonance measurements have been used to investigate the effective mass in the strained silicon channels of modulation-doped, two-dimensional electron gases grown on relaxed Si1−xGex. By using a range of Ge fractions x, the effect of strain was investigated. Consistent results were obtained when the resonance positions were fitted to a model for zero-dimensional confinement, yielding m*≈0.196 me for most samples. The use of this formula was justified by invoking electron localization due to a disorder potential. The observed confinement effect was strongest in two samples where the Si channel was partially relaxed, suggesting this to be a possible mechanism. Qualitatively different results were obtained for a sample with a high background concentration of donor impurities, indicating that the type of disorder present can affect the nature of the resonances.
Effects of active-channel thickness on submicron GaAs metal semiconductor field-effect transistor characteristics
Authors: Ahmed M. M.
Published: July 2002

In small signal GaAs metal semiconductor field-effect transistor (MESFET) fabrication technology where a gate recess is required, the optimum value of residual channel thickness is one of the most crucial parameters for high transconductance operation of the device. The effects of active channel thickness on device characteristics have been investigated. In a different strategy to that of previously reported work, in this study we have fixed gate length Lg at 200 nm and varied the channel thickness, a from 100 to 40 nm. This means that the shape of the depletion layer, which is a function of gate length, was largely fixed for all the devices. The variation in the device characteristics depended on the change in the active channel thickness, which modifies the field distribution inside the channel. It was found that, for optimum doping concentration, the value of transconductance increases in a parabolic fashion by decreasing the aspect ratio (Lg/a) of the device, and a highest value of transconductance is observed at Lg/a≈2. This aspect ratio is 2–3 times lower than the conventional reported values.
Blurring effect analysis of an x-ray mask for synchrotron radiation lithography
Authors: Kim Il Yong , Kwak Byung Man , Jeon Young Jin , Choi Sang Soo
Published: July 2002

During the process of synchrotron radiation lithography, blurring occurs largely because the mask and patterns are distorted under transient thermal loads. An analysis model of blurring has been proposed in this article. Irradiated energy is calculated by summing the moving x-ray power incident to a wafer. The blur is caused mainly by two sources: displacement of the pattern and pattern width change. A transient analysis has been made. In a sample calculation, the distortion of a printed pattern after the process was 1.2 nm while the maximum in-plane distortion during the process was 3.5 nm and width change −3.5 nm for a 1 Hz scanning frequency. Comparison between distortions without considering blurring effect and distortions considering blurring effect was offered. The effect of scanning frequency on printed pattern distortion and width change was also presented.
Apparent tunnel barrier heights of PtIr–Au interfaces in relation to the Au surface composition
Authors: Boyer Lionel , Noël Sophie , Houzé Frédéric
Published: July 2002

In the literature, the phenomenon of barrier height lowering in tunneling experiments is mainly assigned to the presence of contaminants at the interface. In this article we present the results of barrier height measurements made between a PtIr tip and various types of gold surfaces. Vacuum deposited and galvanic gold samples are investigated, as received, after ultraviolet (UV)/ozone treatment and covered with a fluid used to lubricate electrical contacts. It is found that the value of the barrier height depends both on the gold deposition process and on the treatment. Results of surface analyses performed on the nonlubricated samples are also reported. They show that the galvanic gold is more contaminated by carbon than the vapor deposited gold. The effects of the ultraviolet (UV)/ozone treatment are then described. It is observed that for nonlubricated surfaces, the higher the oxygen content, the higher the mean barrier height, and that, for each kind of gold, the lubricated surface exhibits the largest barrier height. All these various results are discussed with reference to the results of the surface analyses and to articles found in the literature.
Effects of sintering on Au/Ti/GaAs Schottky barrier submicron metal-semiconductor field-effect transistors characteristics
Authors: Ahmed M. M.
Published: July 2002

This investigation reports on the effects of sintering (25 °C up to 450 °C) on Au/Ti/GaAs Schottky barrier submicron metal-semiconductor field-effect transistors with an interfacial layer. In short channel devices the presence of an interfacial layer at the metal-semiconductor junction is inevitable. This reduces the transconductance and gives rise to the nonideal I–V characteristics. The adverse effects of the interfacial layer can be reduced significantly by sintering the finished devices at 350 °C in H2/N2 ambient for 30 s. This enhances the Schottky response of the device and an improvement up to 40% in the device extrinsic transconductance has been achieved.
Electrophysical characteristics and low-energy cathodoluminescence of vacuum fluorescent display and field emission display screens
Authors: Bukesov S. A. , Nikishin N. V. , Dmitrienko A. O. , Shmakov S. L. , Kim J. M.
Published: July 2002

Special methods are proposed to estimate the conductivity of a thin phosphor layer deposited onto the screen of vacuum fluorescent display and field emission display. Some phosphor compositions have been studied with the aid of these methods. The conductivity of screens has been found to be subject to the voltage applied to a display and to alter significantly under the influence of electron bombardment. Electron-induced conduction is considered as the main cause of the observed conductivity. A correlation between some features of cathodoluminescence and electron-induced conduction has been revealed. Recommendations on how to calculate the optimal mode of display operation are given.
Equilibrium shape of CoSi2 hut clusters on Si(100)
Authors: Brongersma S. H. , Castell M. R. , Perovic D. D. , Zinke-Allmang M.
Published: July 2002

CoSi 2 clusters of constant height on a Si(100) surface grow in a square shape at first, but at a critical size a shape transition to clusters with large aspect ratios occurs. With each cluster connected to an implanted layer of cobalt by a thermally induced defect that serves as a diffusion channel, clusters can grow independently with a continuous supply of cobalt. When the cobalt supply is limited, clusters grow up to a specific volume and then have time to adjust their height, assuming their minimum-energy shape. Although calculations indicate that this should be a square pyramid, experiments indicate that a more elongated cluster corresponds to equilibrium.
Multilayer oxidation of AlAs by thermal and electron beam induced decomposition of H2O in ultrahigh vacuum
Authors: Yi S. I. , Mitchell W. J. , Chung C.-H. , Hu E. L. , Weinberg W. H.
Published: July 2002

The oxidation of AlAs by the decomposition of water has been investigated in ultrahigh vacuum using high-resolution electron energy loss spectroscopy, Auger electron spectroscopy, and temperature-programmed desorption. Significant oxidation of the AlAs near-surface region was observed after a single adsorption/anneal cycle in which multilayers (>10 monolayers) of water, adsorbed at 100 K, were annealed to above room temperature. Sputter profiling shows that repeated cycles of multilayer water exposure at 100 K followed by annealing results in a surface oxide that is at least 20–30 Å thick. The extent of surface oxidation, as measured by Auger electron spectroscopy, is not affected by annealing to 700 K. However, at temperatures in excess of 800 K, diffusion of subsurface AlAs to the oxide surface was observed. Moreover, it was found that irradiating a partially oxidized AlAs surface with a 3 keV electron beam for sufficiently long times would significantly increase the extent of surface oxidation.
Focused ion beam etching of resist/Ni multilayer films and applications to metal island structure formation
Authors: Nakayama Masayoshi , Wakaya Fujio , Yanagisawa Junichi , Gamo Kenji
Published: July 2002

A new method to fabricate single-electron tunneling structures with magnetic materials using focused ion beam (FIB) sputtering and lithography techniques is proposed. By using this method, a small metal island with a size comparable to or smaller than the FIB diameter, connected with source and drain electrodes via tunnel junctions, can be fabricated. In the present article, some important parameters in this method, such as sputtering yields of the photoresist (AZ 1350) and nickel (Ni) layers, were measured and cross-sectional images of the grooves delineated on a AZ/Ni/SiO2 substrate, which is the basic structure in this method, were observed. At present, a 0.4-μm-wide groove is obtained. However, this width is limited not by a fundamental process but by noise in the scan signal and stage vibration.
Proposals for exact-point transmission-electron microscopy using focused ion beam specimen-preparation technique
Authors: Ishitani T. , Taniguchi Y. , Isakozawa S. , Koike H. , Yaguchi T. , Matsumoto H. , Kamino T.
Published: July 2002

A focused ion beam (FIB) has been actively applied for preparation of about 0.1-μm-thick specimens for transmission electron microscopes (TEMs). For device failure analyses, however, it is mostly difficult to prepare the exact-point TEM specimens. The reason is that the failures are mostly beneath the surface and their exact locations are unknown. Then, even step-by-step FIB cross sectioning may sputter away the failures in the TEM specimen preparation. In the present study, we review two proposals for exact-point TEM microscopy using FIB specimen-preparation technique: (1) high-voltage scanning electron microscopes (HV-SEMs) imaging in TEM and (2) energy filtering TEM (EF-TEM) imaging. The HV-SEM imaging provides information on not only the sample surface but also the inner structure up to about 1 μm deep. The EF-TEM imaging is applicable even for 0.5-μm-thick specimens at 100 kV in the accelerating voltage, in contrast with about 0.1-μm-thick specimens for conventional TEM imaging. Preliminary experiments have supported that either proposal presumably improves the yield of the exact-point TEM inspection.
Low brightness and high emittance electron gun for a reducing image projection system
Authors: Nakasuji Mamoru , Shimizu Hiroyasu
Published: July 2002

An electron gun for an accelerating voltage of 100 kV to be operated under space charge limited condition is designed, and its emittance and brightness are calculated. To obtain low brightness, a Pierce-type electron gun with a Wehnelt electrode and a control anode is adopted. Choosing a low value of the ratio between anode and cathode curvatures reduces the electric field strength at the cathode. A maximum emittance of 1.8 mm mrad and a minimum brightness of 350 A/(cm2 sr) are obtained under the above conditions with a cathode diameter of 1.66 mm. The radius of the Wehnelt aperture and its axial position are optimized. The effects of deviations of the shapes of the Wehnelt and control electrodes from concentric sphere are studied. It is found that in the space between the anode and the control anode the electrodes must be spherical up to an angle of at least 55°, whereas in the space between the Wehnelt electrode and the control anode the electrodes must be spherical up to an angle of at least 21°.
Inductively coupled plasma damage in GaN Schottky diodes
Authors: Cao X. A. , Zhang A. P. , Dang G. T. , Cho H. , Ren F. , Pearton S. J. , Shul R. J. , Zhang L. , Hickman R. , Van Hove J. M.
Published: July 2002

The effects of H2 or N2 plasma exposure on the current–voltage characteristics of GaN Schottky diodes were examined as a function of source power and rf chuck power. Under all conditions there was a strong reduction in diode reverse breakdown voltage and an increase in forward and reverse currents. The results are consistent with creation of a thin (⩽600 Å) n-type conducting surface region after ion bombardment of the GaN surface. Much of the degradation in diode quality can be recovered by annealing in N2 at 750 °C.
Plasma assisted two stage selenization process for the preparation of low resistivity ZnSe films
Authors: Jeyakumar R. , Chadda G. K. , Lakshmikumar S. T. , Rastogi A. C.
Published: July 2002

Preparation of low resistivity, ZnSe films using a plasma assisted two stage selenization process is reported. The presence of N2 plasma enhances the crystallinity of the films. The films are optically transparent and have a band gap of 2.65 eV which remains unchanged by the plasma enhancement process. Plasma grown ZnSe films exhibit lower resistivities possibly due to doping by the incorporation of N2 ions. ZnSe films formed at low reactor pressures (<1.5 mbar) in the presence of N2 plasma showed a significant lowering of the resistivity to 10 Ω cm. XPS analysis shows a chemical shift of 1.6 eV for the Se 3d line of the plasma selenized sample instead of 0.9 eV for the nonplasma sample.
DX centers in Al0.37Ga0.63As/GaAs and In0.34Al0.66As/In0.36Ga0.64As heterostructures
Authors: Wieder H. H. , Sari Huseyin
Published: July 2002

Electrical and galvanomagnetic measurements, made on modulation δ-doped-(Si) Al0.37Ga0.63As/GaAs and In0.34Al0.66As/In0.36Ga0.64As heterostructures, fabricated into modulation doped field effect transistor-like gated Hall bars, were used to determine their DX center energies and densities in the normal and persistent photoconductive mode. Self-consistent Poisson/Schrödinger simulations of the gate voltage dependence and of the temperature dependence of the charge transport parameters of these heterostructures provide the position of the Fermi levels in the barrier layers, EFB, relative to the conduction band minima and the electron densities in their quantum wells. The energy, EDX=0.14 eV of Al0.37Ga0.63As determined from the temperature independent equilibrium position of EFB is consistent with the average of the three lowest DX center energies of this alloy. It is also consistent with that determined by others, on epitaxial Al0.37Ga0.63As layers, using conventional Hall measurement, deep level transient spectroscopic and hydrostatic pressure measurements. Strain relaxed In0.34Al0.66As/In0.36Ga0.64As heterostructures, grown on GaAs substrates, were used for similar measurements and simulations. These yield the energy of the DX centers in In0.34Al0.66As, relative to its Γ-valley minimum, EDX=0.18 eV. This value in conjunction with the previously determined DX center energies of AlAs and that of In0.52Al0.48As, above its conduction band edge, EDX(x) is correlated with, but not identical to, the composition dependence of the L band of InxAl1−xAs.
Node-connection/quantum phase-shifting mask: Path to below 0.3 μm pitch, proximity effect free, random interconnects and memory patterning
Authors: Fukuda Hiroshi
Published: July 2002

New design concepts of the alternating phase-shifting masks (PSMs) are proposed. They enable alternating PSMs to be applied to random patterns that have the least design restrictions and reduced proximity effects. The original design patterns are decomposed into several subpatterns using geometrical operations, so that each subpatterns can be achieved by alternating type PSMs. The possibility of patterning a below 0.3 μm pitch random interconnect with conventional deep ultraviolet tools is shown.
Theory of Coulomb scattering in particle beams using Markov’s method
Authors: Groves T. R.
Published: July 2002

An analytic theory of Coulomb scattering in particle beams is derived. It rests on the assumption that the N-body interaction can be well approximated by the vector superposition of all individual two-body interactions on a single, randomly chosen particle. This is the only assumption or approximation needed. A direct mathematical correspondence exists with the general problem of random flights, which was solved by Markov. The theoretical prediction agrees with Monte Carlo simulation to within a few percent for the lateral trajectory displacement for a particular case. The computation time required for the analytical result is at least an order of magnitude smaller than that required for Monte Carlo simulation.
Quantum mechanical tunneling through a biased double-cascaded barrier
Authors: Mohaidat J. M. , Bitar Riyad N.
Published: July 2002

We provide a detailed study of the leakage tunneling current through two cascaded-barrier structures with different potentials sandwiched between a metal and substrate electrodes. The current–voltage (I–V) characteristics were obtained by solving the time-dependent Schrödinger equation. The structure Al/Ta2O5/SiO2/n+–Si is taken as an example. The effects of the SiO2 barrier thickness and the gate biasing polarity are studied. We found that the effect of the SiO2 barrier thickness, along with positive gate biasing was to slightly reduce the leakage current value. However, the effect of the SiO2 layer thickness, along with negative gate biasing, was to significantly change the I–V characteristics. The computed I–V characteristics were plotted according to the Fowler–Nordheim tunneling expression to check on its adequacy for cascaded potentials. Such plots will also be valuable in evaluating experimental findings. Results are compared with recently published experimental results on Ta2O5 thin films on Si substrates.
Plasma polymerized methylsilane. II. Performance for 248 nm lithography
Authors: Monget C. , Joubert O.
Published: July 2002

Some of the major limitations of top surface imaging schemes are now well documented: critical dimension (CD) control across the wafer can be a serious issue as well as line edge roughness. A primary focus of our work has been to investigate the performance of the 248 nm bilevel negative tone approach of the chemical vapor deposition photoresist process based on the plasma polymerization of methylsilane. In this article, CD control data within a field and across the wafer are presented. CD control is shown to be very strongly dependent on the uniformity of the development step. The best results are obtained when using straight chlorine for the plasma etch development step.
The Great Success of a "Foul Book"
Authors: Hull David L.
Published: July 2002

Victorian Sensation
The Extraordinary Publication, Reception, and Secret Authorship of
Vestiges of the Natural History of Creation
. James A. Secord. University of Chicago Press, Chicago, 2001. 644 pp. $35, £22.50. ISBN 0-226-74410-8.

Secord offers a comprehensive analysis of how Robert Chamber's anonymously published (1844) theory of evolution was read and how it became a sensational best-seller.
Seismic Evidence for Hotspot-Induced Buoyant Flow Beneath the Reykjanes Ridge
Authors: Gaherty James B.
Published: July 2002

Volcanic hotspots and mid-ocean ridge spreading centers are the surface expressions of upwelling in Earth's mantle convection system, and their interaction provides unique information on upwelling dynamics. I investigated the influence of the Iceland hotspot on the adjacent mid-Atlantic spreading center using phase-delay times of seismic surface waves, which show anomalous polarization anisotropy—a delay-time discrepancy between waves with different polarizations. This anisotropy implies that the hotspot induces buoyancy-driven upwelling in the mantle beneath the ridge.