bluuebunny/crossref_metadata_2025 huggingface dataset:
Solid-phase-epitaxial growth and formation of metastable alloys in ion implanted silicon
Authors: Narayan J. , Holland O. W. , Appleton B. R.
Published: July 2002

Transmission electron microscopy (cross-section and plan-view) and ion backscattering techniques have been combined to study the details of solid-phase-epitaxial (SPE) growth in Sb+, In+, Bi+, Ga+, and As+ implanted silicon after furnace annealing in the temperature range 450 to 650 °C. The ion implanted amorphous layer grew ‘‘defect-free’’ in 〈001〉 orientations and the crystalline–amorphous (c–a) interface during growth contained undulations ∼5 Å over the intervals of 200–500 Å. During SPE growth in 〈111〉 orientations, the c–a interface was atomically smooth initially, but eventually became nonplanar due to the formation of twins. From SPE growth rates at different temperatures, the activation energy associated with the growth was determined to be 2.6±0.3 eV. The dopant concentrations in defect-free SPE grown layers were found to exceed equilibrium solid solubility limits by as much as a factor of 560 in the Si–Bi system. The absolute maximum concentrations, corresponding to the intersections of free-energy versus composition curves of amorphous and crystalline silicon, were calculated and the results were compared with the observed concentrations. The observed concentrations, which were found to depend upon the amorphous state or free energy of as-implanted silicon, approached the calculated limits in the case of the Si–Sb and Si–As systems. However, for the Si–In, Si–Ga, and Si–Bi systems, interfacial segregation and solute redistribution during SPE growth prevented the maximum achievable concentrations. The concentrations of dopants in excess of the solubility limits could be precipitated out completely only after annealing treatments at relatively high temperatures (≥1050 °C).
Extrinsic effects in reflection high-energy electron diffraction patterns from MBE GaAs
Authors: Pukite P. R. , Van Hove J. M. , Cohen P. I.
Published: July 2002

One uncontrolled parameter of MBE growth on GaAs is the orientation of the substrate. Typical material is oriented to within 1/2 ° of a low-index bulk plane so that the resulting surface consists of a staircase with random terrace lengths and step heights. These steps are important since at room temperature they can orient epitaxial layers and at high temperatures the terrace lengths can be comparable to diffusion distances. Ordered step arrays can also be an important factor in the formation of the characteristic streaked RHEED pattern. We have measured the profiles of these streaks from different wafers with nominally GaAs(001) surfaces and have observed striking differences which we ascribe to ordered staircase steps due to crystal misorientation. Misorientations as small as 1 mrad have been found. Surfaces in which the misorientation from the (001) was about 6.5 mrad were studied extensively and a reciprocal lattice was constructed from a map of the diffracted intensity vs scattering angles. On surfaces misoriented by 1°, RHEED oscillations were found to be much weaker than on the 6.5 mrad surface. Differences in the oscillations when the beam was directed first down and then perpendicular to the staircase direction were also observed, but these cannot yet be separated from differences due to temperature and flux ratios.
Electronic and chemical analysis of fluoride interface structures at subnanometer spatial resolution
Authors: Scheinfein M. , Isaacson M.
Published: July 2002

Using a finely focused beam of 100 keV electrons of less than 0.5 nm diam we have been able to analyze changes in the transmission electron energy loss spectra (EELS) at subnanometer spatial resolution as the electron probe is moved across various fluoride interface structures. In addition, using the method of dose resolved EELS we have been able to determine the radiation chemistry of CaF2 beam exposure.
Chemical sputtering of silicon by F+, Cl+, and Br+ ions: Reactive spot model for reactive ion etching
Authors: Tachi Shin’ichi , Okudaira Sadayuki
Published: July 2002

Chemical sputtering yields of silicon resulting from the incidence of isotopically pure 19F+, 35Cl+, and 81Br+ ion beams were measured in the 100–3000 eV energy range. It is found for these cases that the chemical sputtering yields saturate in the high energy range of more than 600 eV. The saturated yields (atoms/ion) are 0.18 for F+/Si, 0.45 for Cl+/Si, and 0.23 for Br+/Si. The yields increased with increasing ion energy in the 100–500 eV region with different slopes. The steepest slope is observed for the case of Br+/Si, and the smallest is the F+/Si case. This difference in the slope indicates that the chemical reaction between Br+ and Si cannot be activated without high acceleration of Br+, and the same occurs for Cl+/Si. On the other hand the F+ ion is assumed to react with Si with a relatively high probability even when the energy is low. On the basis of these results, a reactive spot model is proposed for the ion-assisted chemical etching process. A generalized method to get anisotropic profiles is also presented for reactive ion etching of Si with fluorine-, chlorine-, and bromine-containing gases.
Reaction of silicon with chlorine and ultraviolet laser induced chemical etching mechanisms
Authors: Sesselmann W. , Hudeczek E. , Bachmann F.
Published: July 2002

The reaction of 〈100〉Si with Cl2 and the excimer laser induced chemical etching process using 308 and 248 nm radiation have been investigated. Our results show that even at high Cl2 pressures, only a thin passivating chlorinated surface layer is built up which impedes further reaction. Pulsed excimer laser etching at high energy fluences is dominated by thermal evaporation. At lower energy fluences the melt depth decreases and good etch profiles with excellent spatial resolution are observed. At the lowest laser fluences nonthermal, wavelength dependent etching occurs. The laser etching at 308 nm is based on locally enhanced surface chlorination due to effective photodissociation of Cl2 in the gas phase. Photoinduced desorption, most likely due to nonradiative electron-hole recombination, leads to etching. Conversely, at 248 nm, negligible gas phase photodissociation occurs and the surface reaction of Cl2 molecules limits the etch rate. However, the higher photon energy (5.0 eV for 248 nm) leads to efficient photodesorption of surface atoms and molecules by direct bond breaking which can give rise to higher etch rates than at 308 nm. Depending on the actual conditions, etching at 308 or 248 nm is more efficient. High etch rates can be achieved by combining the effective photodesorption process at 248 nm with a high degree of surface chlorination due to generation of chlorine gas phase radicals in a microwave discharge. In addition, the spatial resolution capability for direct pattern transfer by excimer laser induced chemical etching has been analyzed as a function of energy fluence, gas pressure, and gas phase radical concentration.
Selective etching of GaAs and Al0.30Ga0.70As with citric acid/hydrogen peroxide solutions
Authors: Juang C. , Kuhn K. J. , Darling R. B.
Published: July 2002

A volumetric 10:1 ratio of citric acid (50% by weight) and hydrogen peroxide (30%) is shown to be a better selective etchant of GaAs/Al0.30Ga0.70As systems than the more commonly used ammonium-hydroxide/hydrogen peroxide solutions in terms of the selectivity of the etching rate, the smoothness of the etched surface, and the abruptness of the etch-defined edge. In addition, the citric acid/hydrogen peroxide solution does not attack Shipley 1400 series positive photoresists. This simple and reproducible selective etching process has been applied to prepare GaAs optical devices for transmission measurements (‘‘doughnut’’ samples).
PTBSS: A high resolution single component aqueous base soluble chemically amplified resist
Authors: Novembre A. E. , Tai W. W. , Kometani J. M. , Hanson J. E. , Nalamasu O. , Taylor G. N. , Reichmanis E. , Thompson L. F. , Tomes D. N.
Published: July 2002

Poly(4-tert-butoxycarbonyloxystyrene-co-sulfur dioxide) PTBSS has been found to act as a sensitive x-ray (λ=1.4 nm) and weakly sensitive deep ultraviolet (λ=248 nm) chemically amplified, aqueous base soluble positive acting resist. Improvements in the lithographic sensitivity can be achieved by optimizing the post-exposure baking (PEB) conditions. For example, increasing the PEB temperature from 115 °C to 145 °C improved the x-ray sensitivity of a 2.6:1 4-tert-butoxycarbonyloxystyrene (TBS): sulfur dioxide (SO2) resist fivefold. A secondary gain in sensitivity was achieved when the PEB time was extended from the standard 2.5 min to 4.0 min. The sensitivity was also shown to be dependent on the time elapsed from exposure to post-exposure bake. Increasing this time from 1 to 30 min increased the required x-ray exposure energy by 60% arylmethyl model compounds of arylmethyl sulfone have provided information on how the deep-UV sensitivity of PTBSS can be improved from a value of 1800 to 200 mJ/cm.2 Additionally, these model compounds were shown to function as effective acid generating and/or dissolution inhibitor species when added to PTBSS.
Underwater scanning tunneling microscopy of organic and biological molecules
Authors: Mou Jianxun , Sun Wenjun , Yan Junjue , Yang W. S. , Liu Cheng , Zhai Zhonghe , Xu Qiang , Xie Youchang
Published: July 2002

With a collection of images taken with the scanning tunneling microscope from the samples of several different organic and biological molecules adsorbed on graphite and/or gold substrates and covered with a very thin layer of 50% glycerol–water solution we show that covering such samples with the solution could facilitate imaging of them and improve the quality of images thus collected, and very often images with atomic resolution could be obtained.
Dendritic aggregation of gold particles on graphite surface
Authors: Nishitani Ryusuke , Kasuya Atsuo , Kubota Sakae , Nishina Yuichiro
Published: July 2002

Early stages of gold-film formation on graphite by gold vapor deposition have been studied by means of scanning tunneling microscope (STM). Several kinds of microclusters of about 2 nm in diameter, as nuclei for film formation, are resolved to an atomic scale. Fine particles of 5–10 nm in diameter grow at nucleation sites. These particles aggregate and coalesce with each other to form large islands. The growth pattern depends on the substrate temperature. Dendritic islands are observed at a substrate temperature of around 80 °C
Photovoltage on silicon surfaces measured by scanning tunneling microscopy
Authors: Kuk Y. , Becker R. S. , Silverman P. J. , Kochanski G. P.
Published: July 2002

Surface bias voltages induced on a scanning tunnel microscope (STM) junction illuminated with laser radiation are spatially resolved for silicon surfaces. Surface photovoltages of ∼0.3 V for Si(111)-(7×7) and <0.1 V for Si(001)-(2×1) are observed with large reductions in the vicinity of surface/subsurface defects associated with midgap states. These reductions, attributed to a variation in the recombination rate, have typical surface screening lengths that are less than those found in the bulk. A small, atomically varying signal of 3–5 mV is also observed and is due to spatial variations in rectification efficiency rather than photovoltage.
Plasmaless etching of silicon using chlorine trifluoride
Authors: Saito Yoji , Yamaoka Osamu , Yoshida Akira
Published: July 2002

Silicon etching using ClF3 gas has been carried out at various substrate temperatures from −20 to 120 °C. The etching properties (etch rate, pressure effect, and surface morphology) depend remarkably on the temperature. Based on the mass spectroscopic measurements, SiF4 is specified as a main product. The minor product SiF2 is also found, and the ratio of the signal intensity of SiF2 to that of SiF4 decreases with temperature. From Auger electron analyses on the etched surface, the density of fluorine atoms on the surface decreases with increasing the temperature. It is concluded that the density of the fluorosilyl species greatly influences the etching reaction process.
Tunneling and 1/f noise currents in HgCdTe photodiodes
Authors: Nemirovsky Y. , Unikovsky A.
Published: July 2002

The 1/f noise currents and the dc dark current–voltage characteristics are measured over a wide range of reverse bias voltages (0≤Vd≤−1.5 V) and operating temperatures (30–120 K). The diodes are fabricated by ion implanting boron (n+) on bulk p-type material with x≊0.22. Native anodic sulfide in combination with deposited ZnS is used for surface passivation. The dc dark currents of the photodiodes are modeled to extract the tunneling currents from the measured dark currents. The modeling takes into consideration diffusion, generation–recombination, and the two types of tunneling currents (trap assisted tunneling and band to band tunneling). The measurements demonstrate that the dominant mechanism that produces 1/f noise in HgCdTe photodiodes is tunneling, in particular trap assisted tunneling. The correlation between the 1/f noise currents and the dc tunneling currents is given by In=α(It)β(f )−1/2, where It is the tunneling current. The empirical factors β and α are approximately β≊0.5 and α≊1×10−6 for a wide range of temperatures and reverse bias voltages, where the dominant tunneling mechanism is associated with trap assisted tunneling.
Stress and microstructure of sputter-deposited thin films: Molecular dynamics simulations and experiment
Authors: Fang C. C. , Jones F. , Kola R. R. , Celler G. K. , Prasad V.
Published: July 2002

Sputter-deposited tungsten (W) thin films exhibit high intrinsic tensile and compressive stresses. When used as the absorber for x-ray lithographic masks, the stress induced in-plane and out-of-plane distortions produce significant very large scale integrated circuit pattern distortions. In this article, the origin of intrinsic stress and its abrupt transition from the tensile to the compressive state have been investigated theoretically as well as experimentally. A physical understanding of this transition may lead to better designs of sputter deposition processes used to create thin film x-ray mask absorbers. Using a two-dimensional molecular dynamics (MD) model for W, the microstructure and stresses of sputter-deposited films are calculated as a function of various deposition parameters and compared with the experimental data obtained by employing a variety of thin film deposition and characterization techniques. The MD simulations demonstrate that the transition from highly tensile to highly compressive stress depends on the ability of the process to create tightly packed gas impurity atoms in very dense tungsten films, a state achieved only at sufficiently high levels of Ar ion bombardment and adatom energies. The variations in film stress as a function of sputtering pressure using Ar and Xe as background gases are also simulated. Both experiments and simulations show that the pressure at the stress transition decreases when the background gas is changed from Ar to Xe. The simulation results for a wide range of substrate bias further demonstrate that the stresses in sputter-deposited thin films can be controlled successfully by the use of ion energy.
Localized thinning of GaAs/GaAlAs nanostructures by a combined scanning electron micrograph/focus ion beam system for high-quality cross-sectional transmission electron microscopy samples
Authors: Assayag G. Ben , Vieu C. , Gierak J. , Chaabane H. , Pepin A. , Henoc P.
Published: July 2002

A new versatile procedure combining conventional lithography and focused ion beam coupled to scanning electron microscopy to micromachine high-quality cross-sectional transmission electron microscopy (TEM) samples is proposed. Electron transparent areas are generated with a high degree of localization, within 0.1 μm, over distances of several millimeters in GaAs/GaAlAs heterostructures. TEM observations demonstrate that no defects are introduced during the thinning process and different conditions of illumination are achievable.
Emission characteristics of metal–oxide–semiconductor electron tunneling cathode
Authors: Yokoo Kuniyoshi , Tanaka Hiroshi , Sato Shinji , Murota Junichi , Ono Shoichi
Published: July 2002

We have fabricated a metal–oxide–semiconductor (MOS) electron tunneling cathode with ultrathin SiO2 and examined the emission characteristics. We found that the emission occurred from an entire gate area by electron tunneling through the potential barrier in the MOS diode and the emission current was 0.7% of the total current flowing through the diode. The emission was also found to be nearly independent of pressure.
Atomic force microscopy and friction force microscopy of chemically modified surfaces
Authors: Fujihira Masamichi , Morita Yasuhiro
Published: July 2002

We describe the study of a chemically modified quartz surface by atomic force microscopy and friction force microscopy. The chemically modified surface was covered with hydrocarbon (HC) and fluorocarbon (FC) in different parts. The region selective surface chemical modification with two different silanization reagents was successfully accomplished by the following procedure: exposed quartz surfaces of a quartz plate covered with microlithographically prepared patterns of chromium film were first modified with one of the silanization reagents, then the patterns of chromium film were etched, and finally, newly exposed quartz surfaces were modified with the other silanization reagent. The chemical modification with such etching allowed patterning with a submicrometer scale. Since the location of the surfaces covered with two different chemical species was known in the chemically modified surfaces, assignment of the measured frictions to the individual surfaces was definite. The relative values of the difference in friction between HC and FC regions were roughly 1:4.
Thermal and acid-catalyzed deprotection kinetics in candidate deep ultraviolet resist materials
Authors: Wallraff G. , Hutchinson J. , Hinsberg W. , Houle F. , Seidel P. , Johnson R. , Oldham W.
Published: July 2002

Deep ultraviolet (UV) chemically amplified (CA) resists are leading candidates for semiconductor lithography manufacturing in the sub-half-micron regime. In this article, we describe in situ, high data rate, accurate measurements of the chemical kinetics that occur in CA resists during the post-exposure bake. The thermal and acid-catalyzed deprotection of two candidate deep-UV resist materials, poly(p-t-butoxycarbonyloxystyrene) (PTBOCST) and poly(t-butylmethacrylate) (PTBMA), was characterized. The thermal deprotection of PTBOCST and PTBMA showed auto-accelerated behavior as the reaction proceeds, while the acid-catalyzed deprotection displayed inhibition as extent of conversion increased. We propose models for the thermal and acid-catalyzed deprotection and extracted rate coefficients using a stochastic kinetics simulator. Excellent agreement between the model and experimental data was obtained.
High aspect ratio aligned multilayer microstructure fabrication
Authors: Lee K. Y. , Rishton S. A. , Chang T. H. P.
Published: July 2002

This paper presents a technique for fabricating precisely aligned high aspect ratio multilayer microstructures based on anodic bonding of silicon and PyrexTM glass. The process involves stacking alternate silicon substrates containing patterns in two dimensions with thin Pyrex glass spacers to form tall three-dimensional structures. The methods used for accurately aligning the layers by optical microscopy and for bonding the layers anodically are described. The application of this technique for building fully functional microlenses for electron-beam microcolumns is presented.
Scanning tunneling microscopy of silver and gold metal liquidlike films
Authors: Smith K. W. , Rigby S. J. , Turner R. J. , Walmsley D. G. , McGarvey J. J.
Published: July 2002

Metal liquidlike films (MELLFs) are formed when an aqueous colloidal metal suspension is in contact with an immiscible organic liquid phase. That the major constituent of the films is metal is evident from their metallic luster and high reflectivity. They can easily flow, support hydrodynamic waves, and generally do not tear upon insertion of objects through them. They thus exhibit the rheological properties of liquids. They do not, however, conduct. These films have been lifted onto an atomically flat gold substrate and examined with a scanning tunneling microscope. The very high vertical resolution of this instrument provides a unique insight into the shape of the deposited metal particles. On silver MELLFs two types of features are consistently observed: long, slablike features with flat faces of width ∼500 Å, and spheroid features of similar lateral dimension, some of which also exhibit flat facets (smooth to within 10 Å). Only the latter type of feature has been observed on gold MELLFs, but the lateral dimensions are smaller, approximately 200 Å. The dimensions of the spheroidal particles are confirmed by both transmission electron microscopy and atomic force microscopy.
Fabrication and characterization of single-electron transistors and traps
Authors: Ji L. , Dresselhaus P. D. , Han Siyuan , Lin K. , Zheng W. , Lukens J. E.
Published: July 2002

Al/AlOx/Al tunnel-junction-based prototype single-electron devices have been fabricated using e-beam lithography with bilayer resist, and two-angle shadow evaporation. A single-electron transistor is shown to operate at 4.2 K. Single-electron traps consisting of an array of seven junctions, coupled to a cross-type single-electron transistor for monitoring the change of trapped charges, have been also made and demonstrated long-term trapping of single charges. We describe the fabrication process and discuss the device characteristics, noise, yield, uniformity, and long-term stability.
Benign making of sharp tips for STM and FIM: Pt, Ir, Au, Pd, and Rh
Authors: Nam A. J. , Teren A. , Lusby T. A. , Melmed A. J.
Published: July 2002

Sharp tips for various modern microscopies, such as field-ion microscopy (FIM) and scanning tunneling microscopy (STM), can be prepared by electropolishing in solutions which are relatively innocuous for the environment as well as the researcher, compared to the often hazardous solutions still in widespread use. We have made measurements of polishing times as a function of solution and voltage parameters and we report conditions for electropolishing sharp tips of Pt, Ir, Au, Pd, and Rh using relatively benign solutions.
Fabrication of nanostructures using scanning probe microscopes
Authors: Hosaka S. , Hosoki S. , Hasegawa T. , Koyanagi H. , Shintani T. , Miyamoto M.
Published: July 2002

We present nanostructure fabrication techniques using field evaporation and local heating in scanning probe microscopes, especially the scanning tunneling microscope (STM), atomic force microscope (AFM), and the scanning near-field optical microscope (SNOM). The detachment of sulfur atoms from the surface of cleaved MoS2 and atomic scale fabrication were demonstrated with a field evaporation in STM. Field evaporation in AFM forms nanometer-sized gold dots on a SiO2/Si substrate. Local heating with SNOM changes a phase of the GeSbTe recording film from amorphous to crystalline, and forms high reflectivity domains 60 nm in diameter. Moreover, we discuss these applications to a semiconductor process and data storage.
Nanofabrication technology by gas cluster ion beams
Authors: Matsuo Jiro , Toyoda Noriaki , Yamada Isao
Published: July 2002

Reactive gas cluster ion beam etching, which has many advantages for plasma etching, is proposed. The anisotropic etching of Si is realized with a reactive cluster ion beam. Various kinds of cluster ion beams, from gaseous materials such as Ar, O2, N2, SF6, N2O, and CO2, can be generated by expanding them through a Laval nozzle into a high vacuum. The etching rate of W and Si bombarded by SF6 cluster ions was quite high as the result of reactive sputtering. Surprisingly, the sputtering yield reaches 2200 Si atoms/ion, when Si is sputtered by SF6 cluster ions, with an average size of 2000. This yield is two orders of magnitude larger than the value reported for reactive sputtering using monomer ions. 2.2 μm of Si was etched by SF6 cluster ions, with an energy of 25 keV, at a dose of 5×1015 ions/cm2. Due to the low ion dose, the charge induced damage is reduced. The selectivity of Si to SiO2 increases with decreasing incident energy of cluster ions and finally infinite selectivity can be achieved at 5 keV. This energy corresponds to 2.5 eV per SF6 molecule, when the cluster size is 2000. A 0.5 μm hole was etched by a SF6 ion cluster ion beam and anisotropic etching was demonstrated.
Persistence pays off: Sir Charles Oatley and the scanning electron microscope
Authors: Everhart T. E.
Published: July 2002

Shortly after World War II, Sir Charles Oatley initiated research at the Cambridge University engineering laboratories on what has evolved into the modern scanning electron microscope. While much of the research was actually conducted by research students under Oatley’s supervision, he continually provided ideas, resources, and encouragement. He then was instrumental in having this instrument commercialized. His students often continued in the field for some time, making contributions both to the instrument and to its applications that led to improved performance and wider acceptance. This article attempts to capture some of the accomplishments of Sir Charles Oatley as seen by those who worked closely with him. The author believes that Sir Charles deserves the title: ‘‘Father of the Modern Scanning Electron Microscope.’’
High selectivity plasma etching of silicon dioxide with a dual frequency 27/2 MHz capacitive radio frequency discharge
Authors: Tsai W. , Mueller G. , Lindquist R. , Frazier B. , Vahedi V.
Published: July 2002

A 27 MHz/2 MHz Ar/CO/CFx rf capacitive discharge was used to selectively etch submicron SiO2/Si, SiO2/TiSi2 and SiO2/TiN structures. The borophosphosilicate glass etch rate (∼1.0 μm/min) was observed to decrease with bottom electrode temperature, whereas etch selectivity to TiSi2 (≳50:1) was found to increase with temperature, with a photoresist selectivity ≳50:1. Plasma emission of the dual frequency discharge indicated the presence of an abundant concentration of the CF2 radical (249 nm) as compared to a lower frequency (400 kHz) rf discharge with low etch selectivity (<20:1). Langmuir probe measurement indicated an enhancement of ion density of an order of magnitude (∼5×1010 cm−3) as compared to the low frequency (400 kHz) rf discharge. Particle-in-cell simulation was used to calculate plasma density as a function of excitation frequency. Plasma density scales with the square of the source frequency while it remains relatively independent of the substrate frequency.
Electron field emission from chemical vapor deposited diamond
Authors: Zhu W. , Kochanski G. P. , Jin S. , Seibles L.
Published: July 2002

Diamond films and islands have been synthesized with varying defect densities and dopant levels by chemical vapor deposition for field emission display applications. Vacuum field emission measurements show that the electric field required to induce electron emission from diamond can be significantly reduced when the samples are properly grown or postgrowth processed so that they contain a substantial amount of structural defects. The defective diamond is characterized by a broadened peak at 1332 cm−1 in Raman spectroscopy with a full width at half-maximum in the range of 7–11 cm−1. These materials require a turn-on field (for a current density of 0.01 mA/cm2) as low as 15 V/μm, and a threshold field (for a current density of 10 mA/cm2) as low as 30 V/μm for electron emission. These field values are almost an order of magnitude lower than those required for high quality, p-type semiconducting diamond. Detailed numerical analysis of the emission current-voltage data from multiple diamond tips with varying geometric and electronic properties is presented, and the mechanisms responsible for such low voltage electron emission from defective diamond are discussed.
Optical monitoring of the development of InAs quantum dots on GaAs(001) by reflectance anisotropy spectroscopy
Authors: Steimetz E. , Zettler J.-T. , Richter W. , Westwood D. I. , Woolf D. A. , Sobiesierski Z.
Published: July 2002

Reflectance anisotropy spectroscopy (RAS) in combination with reflection high-energy electron diffraction (RHEED) was used to study in situ the initial steps of molecular beam epitaxial growth of InAs on GaAs(001). Due to the large lattice mismatch InAs is known to grow in Stranski–Krastanov mode leading to the formation of quantum dots after the transition from two- to three-dimensional growth mode. In this article the precise determination of the growth mode transition and the subsequent development of the islands have been of particular interest. During the growth of the two-dimensional InAs layer, the RHEED-pattern changed from the c(4×4) of the clean GaAs to a (1×3) surface reconstruction. Accordingly, the RAS-spectra, taken every 0.2 ML, indicate changes of the As-dimer configuration. At 1.8 ML (spotty RHEED-pattern) a saturation of the intensity of the dimer related RAS-signal around 2.6 eV was found. The relaxation of the InAs layer and the formation of the quantum dots was followed by time-resolved RAS at 2.6 and 4 eV. It is shown here, that the time constant of this process, the thickness of the InAs wetting layer and the equilibrium morphology of the islands are strongly temperature dependent. The remaining equilibrium InAs wetting layer thickness at the surface was estimated to be about 1 ML (0.8 ML at 625 K and 1.2 ML at 725 K).
Materials’ properties measurements: Choosing the optimal scanning probe microscope configuration
Authors: Burnham N. A. , Gremaud G. , Kulik A. J. , Gallo P.-J. , Oulevey F.
Published: July 2002

Rheological models are used to represent different scanning probe microscope configurations. The solutions for their static and dynamic behavior are found and used to analyze which scanning probe microscope configuration is best for a given application. We find that modulating the sample at high frequencies results in the best microscope behavior for measuring the stiffness of rigid materials, and that by modulating the tip at low frequencies and detecting the motion of the tip itself (not its position relative to the tip holder) should be best for studying compliant materials in liquids.
Identification and visualization of questionable regions in atomic force microscope images
Authors: Leung Edward C. W. , Markiewicz Peter , Goh M. Cynthia
Published: July 2002

An algorithm for the identification of areas that do not necessarily represent the true sample surface in atomic force microscope images is presented. These areas describe regions of the surface which might not have made contact with the probe tip during a raster scan, giving data which should be deemed questionable. Through the identification of these questionable data points, a more accurate picture of the sample can be obtained. The procedure is applied to several atomic force microscope images for the improvement of sample images and for obtaining tip information. While the algorithm is applicable to all such images, its sensitivity to noise reflects shortcomings in the assumption made in deconvolution.
Copper dry etching with precise wafer-temperature control using Cl2 gas as a single reactant
Authors: Miyazaki H. , Takeda K. , Sakuma N. , Kondo S. , Homma Y. , Hinode K.
Published: July 2002

Cu lines 0.25 μm wide are successfully patterned by high-temperature dry etching without sidewall protection of the deposited films. Use of Cl2 gas as a single reactant rather than a mixture of gaseous chlorine compounds is the key to this anisotropic etching. This reactant suppresses the side etching of Cu patterns without creating a residue when the wafer temperature is precisely controlled between 230 and 270 °C. In our fundamental examination by simulation, we found that the thermal reaction rate reaches a minimum at about 200 °C, and this minimum is much lower than the ion-assisted reaction rate in the actual patterning. This peculiar result is due to the transition of the rate-limiting steps which is caused by the depletion of the CuClx layer on the Cu surface. That is, the formation of Cu3Cl3 appears to be a rate-limiting step at temperatures above 200 °C, while the dissociative adsorption of Cl2 is a rate-limiting step at lower temperatures. The anisotropy in the actual patterning is attributed to the intrinsic suppression of Cu3Cl3 formation at temperatures ranging from 230 to 270 °C.
Auger voltage contrast imaging for the delineation of two-dimensional junctions in cross-sectioned metal–oxide–semiconductor devices
Authors: Werner W. S. M. , Lakatha H. , Smith H. E. , LeTarte L. , Ambrose V. , Baker J.
Published: July 2002

A new two-dimensional junction delineation technique named Auger voltage contrast (AVC) is described. AVC is a mapping tool that indicates the dopant type of each point of the cross-sectioned surface by measuring the Auger electron energy shifts that mirror the workfunction differences caused by the built-in junction, i.e., caused by Fermi level differences between n- and p-doped silicon. AVC makes use of the dual capabilities of field emission Auger electron spectroscopy instrumentation for spectroscopy at each pixel and for high spatial resolution. A theoretical examination is made of the important practical aspects of this previously observed Auger energy line shift, in particular the effect of surface states on the Fermi level position and the effect of beam-induced electron-hole pair creation and transport. AVC results obtained from cross-sectioned metal–oxide–semiconductor structures are shown to have good agreement with both theory and with one-dimensional secondary ion mass spectroscopy and spreading resistance probe profiles from the same sample. A fast and efficient algorithm has been devised which fits the Auger Si LVV spectrum to that of a reference spectrum obtained from a region of known doping, so that it is possible to make a calculation of the energy shift at each pixel with resolution on the order of 50 meV and in a way that speeds analysis and minimizes storage size.
Depletion mode GaAs metal–oxide–semiconductor field effect transistors with Ga2O3(Gd2O3) as the gate oxide
Authors: Hong M. , Ren F. , Kuo J. M. , Hobson W. S. , Kwo J. , Mannaerts J. P. , Lothian J. R. , Chen Y. K.
Published: July 2002

We have successfully fabricated depletion mode GaAs metal–oxide–semiconductor field effect transistors using Ga2O3(Gd2O3) as the gate oxide and an oxygen implant isolation technique. Growth of the device structure including the deposition of Ga2O3(Gd2O3) was performed in a multichamber molecular beam epitaxy system. A 1 μm×100 μm device shows excellent dc and microwave characteristics with low output conductance. Complete pinchoff at Vg=−2.5 V and operation in the accumulation mode of up to Vg=2.5 V were measured. The maximum transconductance was 100 mS/mm, with a high drain current density of 315 mA/mm. Microwave testing yielded a fT of 14 GHz and a fmax of 35 GHz.
Development of texture in interconnect thin film stacks
Authors: Knorr D. B. , Merchant S. M. , Biberger M. A.
Published: July 2002

Interconnect metallization, used for the fabrication of silicon (Si) semiconductor devices, utilizes sputtered stacks often composed of thin (⩽100 nm) titanium (Ti), titanium nitride (TiN), or Ti/TiN underlayer(s), covered by aluminum (Al) alloy conductors 400–1000 nm in thickness. The texture (preferred orientation) in such films is an important consideration for both processing and reliability. Two aspects of texture are addressed in this study: (1) the effect of processing parameters on Ti and Ti/TiN underlayer texture development, and (2) the effect of underlayer texture on the texture of the Al alloy subsequently deposited. An increase in the substrate temperature causes a transition in Ti texture from 〈0002〉 to 〈101̄1〉. When TiN is reactively sputtered on Ti, the Ti〈0002〉 induces a TiN〈111〉, while the Ti〈101̄1〉 correlates with the formation of TiN〈311〉. Under the deposition conditions for the Al alloys that were studied, the presence of only a titanium underlayer sharpens the Al〈111〉 texture but does not introduce a crystallographic texture component other than the Al〈111〉. In contrast, the textured TiN in a Ti/TiN underlayer stack promotes texture inheritance, where TiN〈111〉→Al〈111〉 and TiN〈311〉→Al〈311〉 are orientation correlations indicated by the data. The orientation of a film in a stack depends on the surface energy of the top layer and on the interfacial energy between layers. Atomic bonding in the underlayer strongly affects the minimization of the interfacial energy and determines whether local, grain scale lattice matching between the underlayer and the top layer is operative.
Cleaning of CHF3 plasma-etched SiO2/SiN/Cu via structures using a hydrogen plasma, an oxygen plasma, and hexafluoroacetylacetone vapors
Authors: Ueno Kazuyoshi , Donnelly Vincent M. , Tsuchiya Yasuaki
Published: July 2002

Cleaning processes for CHF3 reactive ion etched Cu vias, consisting of exposure to a hydrogen plasma, an oxygen plasma, and hexafluoroacetylacetone [H(hfac)] vapors have been investigated. After each step in the cleaning process, the dielectric surface and the Cu surface of via structures were analyzed by in situ by angle-resolved x-ray photoelectron spectroscopy. A hydrogen plasma was effective in removing carbon and fluorine deposits on all of the surfaces, and CuO and Cu2O on the Cu surface at the via bottom. It was not effective, however, in removing the Cu deposited on the dielectric surfaces. An oxygen plasma is effective in removing all the carbon and some fluorine deposits. Cu deposits on the dielectric surfaces were not removed, however, and the Cu surface was oxidized. Exposure to H(hfac) vapors reduced some of the Cu deposits on the dielectric, however Cu diffusion into SiO2 possibly occurred during this exposure at the elevated temperature of 200 °C. CuO and Cu2O were removed by the H(hfac) exposure above 150 °C. A three-step cleaning sequence was devised which consists of a brief oxygen-plasma exposure, a dilute-HF solution dip, followed by exposure to H(hfac) vapors. The cleaning sequence is effective in obtaining a clean dielectric surface and an oxide-free Cu surface at the via bottom that allows low contact resistances of 5.8–6.8×10−11 Ω cm2.
Simulation of Coulomb interactions in electron beam lithography systems—A comparison of theoretical models
Authors: Stickel W.
Published: July 2002

The Loeffler effect (trajectory displacement) has been calculated for a magnetic lens doublet electron-optic system considered to represent the basic configuration for next generation lithography. The results of two analytic and two numeric (Monte Carlo) models are compared and presented graphically as well as by analytic approximations, revealing significant quantitative differences within the parameter space considered relevant. Less severe discrepancies in the trends appear to be useful as a guide for system design, but experimental verification is needed to determine the validity of any of the approaches compared.
Experimental results of the stochastic Coulomb interaction in ion projection lithography
Authors: de Jager P. W. H. , Derksen G. , Mertens B. , Cekan E. , Lammer G. , Vonach H. , Buschbeck H. , Zeininger M. , Horner C. , Löschner H. , Stengl G. , Bleeker A. J. , Benschop J. , Shi F. , Volland B. , Hudek P. , Heerlein H. , Rangelow I. W. , Kaesmaier R.
Published: July 2002

Throughput and resolution are connected in ion and electron projection lithography (IPL and EPL) because of the space charge and Coulomb interaction between the particles in the beam. Due to the lack of experimental data it was not possible to estimate this effect accurately. Therefore an experiment setup has been developed which has the most significant parameters close to planned IPL exposure tools. These parameters are the linear particle density and the crossover shape and size. The stochastic Coulomb interaction blur, depending on the total beam current, has been measured in about 100 settings of the beam current, beam energy, and crossover shape. The results show that the stochastic Coulomb interaction blur scales to the power of 0.587±0.101 (1σ) of the linear particle density in a system with a uniform crossover of 400 μm. To decrease the current density in crossover IPL systems can have an aberrated crossover. In case of this type of crossover of 670 μm the current dependency is 0.820±0.072 (1σ). With an aberrated crossover the stochastic Coulomb interaction is not uniform over the exposure field; it decreases towards the edges of the exposure field and plateaus near its center. This reflects the shape of the crossover. The experimental results can be used to make predictions of the allowable current in planned IPL exposure tools. The process development tool can have 1.00±0.25 (1σ) μA He+ ion current at 100 nm resolution. The allowable H+ ion current in the beta tool at 100 nm resolution is 9.09±2.14 (1σ) μA and 3.69±0.81 (1σ) μA at 50 nm. This corresponds to a throughput of 40.1±0.4 (1σ) wafers per hour (300 mm) at 100 nm and 26.1±2.0 (1σ) wafers per hour (300 nm) at 50 nm.
Point contact current–voltage measurements on individual organic semiconductor grains by conducting probe atomic force microscopy
Authors: Kelley Tommie W. , Frisbie C. Daniel
Published: July 2002

Conducting probe atomic force microscopy (CP-AFM) was used to make point contact current–voltage (I–V) measurements on individual microscopic grains of the organic semiconductor sexithiophene (6T). The 6T grains ranged from 1 to 6 molecules (2–14 nm) in thickness, 1–2 μm in length and width, and were deposited by thermal evaporation onto SiO2 substrates previously patterned with 200 nm wide Au wires. Au-coated AFM probes were used to image the substrates in air to identify individual 6T grains which grew in contact with a wire. The same probes were used to record the I–V characteristics of single grains. Analysis of the differential resistance as a function of probe wire separation yielded typical grain resistivities of 100 Ω cm and contact resistances of ∼100 MΩ. Over the 0–3 V range probed, the shape of the I–V curves can be attributed to a combination of the nonlinear I–V characteristics of the Au-6T junctions and the ohmic response of the grain. In general, we have shown that CP-AFM is a reliable method for correlating electrical transport properties with microscopic morphology in organic semiconductors.
Virus-Assisted Mapping of Neural Inputs to a Feeding Center in the Hypothalamus
Authors: DeFalco Jeff , Tomishima Mark , Liu Hongyan , Zhao Connie , Cai XiaoLi , Marth Jamey D. , Enquist Lynn , Friedman Jeffrey M.
Published: July 2002

We report the development of a pseudorabies virus that can be used for retrograde tracing from selected neurons. This virus encodes a green fluorescent protein marker and replicates only in neurons that express the Cre recombinase and in neurons in synaptic contact with the originally infected cells. The virus was injected into the arcuate nucleus of mice that express Cre only in those neurons that express neuropeptide Y or the leptin receptor. Sectioning of the brains revealed that these neurons receive inputs from neurons in other regions of the hypothalamus, as well as the amygdala, cortex, and other brain regions. These data suggest that higher cortical centers modulate leptin signaling in the hypothalamus. This method of neural tracing may prove useful in studies of other complex neural circuits.
Recovery of Infectious Ebola Virus from Complementary DNA: RNA Editing of the GP Gene and Viral Cytotoxicity
Authors: Volchkov Viktor E. , Volchkova Valentina A. , Mühlberger Elke , Kolesnikova Larissa V. , Weik Michael , Dolnik Olga , Klenk Hans-Dieter
Published: July 2002

To study the mechanisms underlying the high pathogenicity of Ebola virus, we have established a system that allows the recovery of infectious virus from cloned cDNA and thus permits genetic manipulation. We created a mutant in which the editing site of the gene encoding envelope glycoprotein (GP) was eliminated. This mutant no longer expressed the nonstructural glycoprotein sGP. Synthesis of GP increased, but most of it accumulated in the endoplasmic reticulum as immature precursor. The mutant was significantly more cytotoxic than wild-type virus, indicating that cytotoxicity caused by GP is down-regulated by the virus through transcriptional RNA editing and expression of sGP.
A Protein Antibiotic in the Phage Qβ Virion: Diversity in Lysis Targets
Authors: Bernhardt Thomas G. , Wang Ing-Nang , Struck Douglas K. , Young Ryland
Published: July 2002

A
2
, a capsid protein of RNA phage Qβ, is also responsible for host lysis. A
2
blocked synthesis of murein precursors in vivo by inhibiting MurA, the catalyst of the committed step of murein biosynthesis. An A
2
-resistance mutation mapped to an exposed surface near the substrate-binding cleft of MurA. Moreover, purified Qβ virions inhibited wild-type MurA, but not the mutant MurA, in vitro. Thus, the two small phages characterized for their lysis strategy, Qβ and the small DNA phage φX174, effect host lysis by targeting different enzymes in the multistep, universally conserved pathway of cell wall biosynthesis.
Real-Space Imaging of Nucleation and Growth in Colloidal Crystallization
Authors: Gasser U. , Weeks Eric R. , Schofield Andrew , Pusey P. N. , Weitz D. A.
Published: July 2002

Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.
Genes Lost and Genes Found: Evolution of Bacterial Pathogenesis and Symbiosis
Authors: Ochman Howard , Moran Nancy A.
Published: July 2002

Traditionally, evolutionary biologists have viewed mutations within individual genes as the major source of phenotypic variation leading to adaptation through natural selection, and ultimately generating diversity among species. Although such processes must contribute to the initial development of gene functions and their subsequent fine-tuning, changes in genome repertoire, occurring through gene acquisition and deletion, are the major events underlying the emergence and evolution of bacterial pathogens and symbionts. Furthermore, pathogens and symbionts depend on similar mechanisms for interacting with hosts and show parallel trends in genome evolution.
Superconductivity in 4 Angstrom Single-Walled Carbon Nanotubes
Authors: Tang Z. K. , Zhang Lingyun , Wang N. , Zhang X. X. , Wen G. H. , Li G. D. , Wang J. N. , Chan C. T. , Sheng Ping
Published: July 2002

Investigation of the magnetic and transport properties of single-walled small-diameter carbon nanotubes embedded in a zeolite matrix revealed that at temperatures below 20 kelvin, 4 angstrom tubes exhibit superconducting behavior manifest as an anisotropic Meissner effect, with a superconducting gap and fluctuation supercurrent. The measured superconducting characteristics display smooth temperature variations owing to one-dimensional fluctuations, with a mean-field superconducting transition temperature of 15 kelvin. Statistical mechanic calculations based on the Ginzburg-Landau free-energy functional yield predictions that are in excellent agreement with the experiments.