bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-resolved measurements of vibrational relaxation of molecules on surfaces: Hydroxyl groups on silica surfaces
Authors: Casassa M. P. , Heilweil E. J. , Stephenson J. C. , Cavanagh R. R.
Published: July 2002

The vibrational population relaxation rate of the O–H stretching fundamental of hydroxyl groups on SiO2 surfaces was measured directly using picosecond infrared pulses. The vibrational lifetime determined for hydroxyls at the silica–vacuum interface is 204±20 ps. For silica-bound hydroxyls in a saturated atmosphere of CCl4, the lifetime decreases to 159±16 ps. Both lifetimes are many times longer than would be inferred from infrared absorption linewidths.
Development of D2 pellet injectors
Authors: So/rensen H. , Andersen S. A. , Nielsen A. Nordskov , Sass B. , Weisberg K.-V.
Published: July 2002

A versatile extrusion-type pneumatic gun is described. The extrusion nozzle/gun barrel system can easily be exchanged to produce pellets in the diameter range 0.4–2.0 mm. Velocities in the range 0.1–1.35 km/s are obtained by adjusting the propeller gas pressure. It is proposed that this gun type can be used for larger pellets of 3.2 mm diameter as well and to accelerate such pellets by means of a high-pressure discharge.
Far-infrared laser scattering in the ACT-I toroidal device
Authors: Goree J. , Mansfield D. K. , Ono M. , Wong K. L.
Published: July 2002

A far-infrared laser scattering diagnostic has been built for the ACT-I toroidal device. The optical system uses a passively stabilized 447-μm CH3I laser. A polyethylene etalon is the beam splitter. The vacuum windows are plastic (TPX), which we found has the vacuum property Q 6.5×10−9 Torr l/s/cm2. Using paraboloidal and ellipsoidal mirrors for detection optics improves the signal strength and allows a better rf enclosure design for the detector. The diagnostic was tested by scattering from an ion Bernstein wave, a technique which can be used for ion temperature diagnostics.
Application of plasma etching to via hole fabrication in thick GaAs substrates
Authors: Geissberger A. E. , Claytor P. R.
Published: July 2002

Several aspects concerning the production of via holes in thick (200 μm) GaAs wafer have been investigated. Various halogen containing gases (CCl2F2, SiCl4, BCl3, and CF4) and mixtures of these gases with argon have been characterized for etching rate and isotropy: For etch pressure (13 Pa≤P≤40 Pa), rf power densities (0.7 W/cm2≤rf≤1 W/cm2), rf peak voltages (70 V ≤Vrfp≤300 V), total gas flow rates (20 sccm≤F≤70 sccm), substrate temperatures (60 °C≤T≤80 °C), and etch rates (0.3 μm/min≤ER≤3 μm/min) were observed. Several mask materials (SiO, SiN, Ni, and Al) were tried, with SiO, SiN and Ni giving good results. Well defined V-shaped hole profiles, due to crystallographic etching, were obtained with a 40% CCl2F2/60% Ar gas mixture. Finally, the etch rate of this mixture demonstrated Arrhenius behavior over the substrate temperature range, 50 °C≤T≤240 °C, with an apparent activation energy (Ea) of 0.05 eV, thus allowing useful etch rate enhancement at elevated substrate temperatures.
Vacuum outgassing rates of plastics and composites for electrical insulators
Authors: Rosenblum Stephen S.
Published: July 2002

Vacuum outgassing rates have been measured on several plastics and epoxies, some of which were filled with inorganic materials. The materials examined were chosen as possible candidates for the manufacture of vacuum insulators for large accelerators where good vacuum is a necessity. Generally, all the materials had similar vacuum outgassing rates so that in most applications, the choice of the material would be made after thorough examination and comparison of all the material properties including mechanical strength, ease of manufacture, and electrical breakdown strength in vacuum.
Chromium getter studies in the Tokamak Fusion Test Reactor
Authors: Dylla H. F. , LaMarche P. H. , Blanchard W. R. , Budny R. V. , Boody F. P. , Bush C. E. , Groebner R. J. , McCarthy P. J. , Simpkins J. E. , Stratton B. C. , Ulrickson M. A.
Published: July 2002

We have studied the effects of the deposition of thin films (∼0.1 μm) of chromium onto ∼70% of the torus area of the Tokamak Fusion Test Reactor (TFTR). The purpose of these experiments was to test the difference between high surface coverage and high pumping speed gettering schemes with respect to minimizing oxygen impurity generation in high power tokamak discharges. The initial Cr deposition had significant effects on vessel outgassing and subsequent plasma performance: the outgassing of H2O, CO, and CO2 decreased by a factor of 10, oxygen impurity radiation decreased by a factor of 2, the plasma Zeff decreased from 1.3 to 1.1, and the plasma density limit increased by 20%. This improvement correlates with a significant reduction of the edge radiation as the density limit is approached. The effects of the initial and subsequent Cr depositions were relatively long lasting, exhibiting time constants of the order of weeks. We attribute the observed impurity reduction to a modification of the oxide surface on the vessel wall which is apparently a significant impurity source for oxygen.
Chemical cleaning of iron and nickel single crystal surfaces and the removal of wall contaminants in ultrahigh vacuum systems by nitric oxide
Authors: Grunze M. , Strasser G. , Elshazly O.
Published: July 2002

In this note we describe chemical cleaning procedures using exposures of NO, O2, and H2 on iron {112} and nickel {110} single crystal specimens which were subsequently used for ultrahigh vacuum (UHV) gas adsorption studies. The rationale for applying chemical cleaning rather than extensive ion etching cycles is to deplete the bulk of the materials of dissolved impurities more efficiently and to avoid or reduce ion induced damaging of the single crystal surfaces. We also describe our procedure to reduce wall contaminants in UHV systems by flowing nitric oxide through the system during bakeout, and the protection of single crystal specimens against bulk contamination by nitriding the samples with ammonia prior to exposure to air.
Dynamics of gas–surface energy transfer: Inelastic scattering of NO from Ag(111)
Authors: Rettner C. T. , Kimman J. , Fabre F. , Auerbach D. J. , Barker J. A. , Tully J. C.
Published: July 2002

We have determined the velocity distributions of individual quantum states of NO scattering from Ag(111) using molecular-beam techniques to control the incidence energy Ei and angle θi, and detecting individual quantum states of NO at specific scattering angles θf and surface temperatures Ts by two-photon ionization. Here, Ei has been varied between 0.1 and 1.0 eV, θi from 15° to 65°, θf from 0° to 75°, and Ts from 155 to 760 K, with a total of over 1000 such velocity distributions. We report the observation of direct vibrational excitation and of an anticorrelation between translational energy loss to phonons and to rotation. We also show that this system is highly translationally inelastic with up to 55% of Ei being lost to phonons at Ei=1.0 eV and that the final translational energy is rather insensitive to θf. Angular distributions are found to depend on the degree of rotational excitation, but to a much lesser extent than would be expected for direct transfer of normal energy into rotation. Results are discussed in terms of an electronic mechanism for vibrational excitation, trajectory calculations, and simple models.
Pumping of hydrogen during plasma–graphite interaction
Authors: Hsu W. L. , Causey R. A.
Published: July 2002

In a vacuum vessel filled with hydrogen gas and valved off from vacuum pumps, large amounts of hydrogen were removed when a discharge was struck with graphite material cathodes within the vessel. The pumping phenomenon persisted indefinitely and at a constant rate. The amount of hydrogen consumed considerably exceeded the expected hydrogen retention in graphite but can be accounted for, to within a factor of 2, in the hydrogen-rich carbon layers deposited on the surrounding vacuum vessel. This codeposited hydrogen was at least 30 times the amount retained in the graphite electrodes.
The first observation of reflection high-energy electron diffraction intensity oscillations during the growth and sublimation of CdTe
Authors: Arias J. M. , Sullivan G.
Published: July 2002

We report here for the first time the observation of intensity oscillations in the features of the reflection high-energy electron diffraction pattern during the growth and sublimation of CdTe from a (100) surface. Our study shows that both processes follow predominantly a two-dimensional layer-by-layer mechanism for the CdTe compound. The use of these intensity oscillations allows accurate in situ rate measurements for CdTe growth and sublimation as a function of substrate temperature. Intensity oscillations during growth were observed in the temperature range 240–370 °C. The longest lasting and more intense oscillations were seen at about 320–330 °C. Intensity oscillations during sublimation were detected above 360 °C. The activation energy obtained for the sublimation of CdTe was 1.92 eV (44.3 kcal/mol).
Fingerprinting of polymer surfaces with Cf-252 particle desorption mass spectrometry
Authors: Quinones L. , Schweikert E. A.
Published: July 2002

Cf-252 particle desorption mass spectrometry (PDMS) has been applied to a series of industrial polymers. PDMS is a surface technique suited for analysis of polymers since 105 fission fragments impinging on the target are sufficient to obtain a mass spectrum, thus minimizing surface damage and charge-up. Positive and negative ion spectra with masses up to m/z 400 amu were obtained for the following samples: polystyrene, poly(alpha methylstyrene), poly(ethylene terephthalate), poly(methyl methacrylate), and poly(butyl methacrylate). Polymers were studied in the form of solvent recuperated films or as self-standing films. The mass spectra of positive and negative ions showed masses corresponding to different sized monomer fragments. For each polymer, its spectrum can be used as a fingerprint. The mass spectra were compared to those obtained by other mass spectrometric techniques.
Influence of PF6 dopant concentration on the x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy spectra of poly 3-methylthiophene
Authors: Nelson A. J. , Glenis S. , Frank A. J.
Published: July 2002

X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy techniques have been used to characterize electrochemically grown conductive films of poly (3-methylthiophene) in the PF6 doped and undoped states. The PF6 dopant concentration was varied between the undoped and fully doped states. Core-level and valence-level spectra have yielded information on the nature of the polymeric cation and its associated PF6 anion, as well as structural disorder effects in these polymers. Results have revealed that addition of the PF6 counterion causes structural disorder within the polymer and that the PF6 counterion interacts with the highest occupied nonbonding lone-pair orbital, as well as causing shifts in the photoelectric threshold which may indicate the formation of bipolaron bands.
Dense ‘‘diamondlike’’ hydrocarbons as random covalent networks
Authors: Angus John C. , Jansen Frank
Published: July 2002

Dense ‘‘diamondlike’’ hydrocarbon films (a-C:H) are a class of amorphous solids which can be condensed as thin films from nonequilibrium environments by several plasma deposition techniques. Films with hydrogen atom fractions from 0.5 to 0.6 select a structure with an average coordination number close to the theoretical value at which stabilization by bonding and destabilization by strain energy are balanced. This coordination number is achieved by the incorporation of the hydrogen and by the presence of trigonally (sp2) bonded carbon in the carbon skeletal network. Theory predicts a range of hydrogen atom fraction, outside of which fully constrained, random hydrocarbon networks cannot exist. The dense hydrocarbon films show extreme values of physical properties, including hardness and argon diffusivity, which can be related to their unusual structure.
Proposal for a novel gravitational-wave sensor
Authors: Niksch M. , Binnig G.
Published: July 2002

We propose new systems for gravitational-wave detection using antennas of the same kind as used in resonance detectors, but giving up the resonance amplification. This, we believe, is possible by driving the impedance transformation to small masses to an extreme. This requires sensors able to measure displacements of extremely small masses. Tunneling appears to us a most promising sensor for this purpose as it allows measuring the displacement of particles as small as single atoms with a resolution comparable to that of other mechanical detectors. Furthermore, we envisage using a new kind of transformer based on superfluid helium.
A simple model for the Auger electron spectroscopy evaluation of thin-film layer growth systems in which substrate–overgrowth atom mixing occurs
Authors: Ece M. , Vook R. W.
Published: July 2002

Previous studies have shown that the variation of Auger electron spectroscopy (AES) intensities with increasing film thickness can be used to determine the mode of growth. The present report deals with two layer growth cases involving the incorporation of impurity atoms in the overgrowth. If substrate atoms mix with overgrowth atoms, each layer is completed earlier than expected. The change in monolayer thickness, escape length of AES electrons, slope of Auger signal during the growth of each layer, and the ratio of adjacent slopes have been derived. The change in these quantities is calculated for an exponentially decreasing substrate atom concentration in the overgrowth and also for the case in which the concentration of impurities stays constant in the film during the deposition. The work concludes by applying the calculations to two real systems.
Electron cyclotron resonance microwave discharges for etching and thin-film deposition
Authors: Asmussen Jes
Published: July 2002

A recent, important development in low-pressure and low-temperature plasma processing is the microwave electron cyclotron resonance (ECR) discharge. Its lack of electrodes and its ability to create high densities of charged and excited species at low pressures (≲10−4 Torr) make it an attractive processing discharge in etching and thin-film deposition applications. This article reviews the basic physics of ECR discharges and reviews the associated microwave system and applicator technologies. Waveguide and cavity ECR applicators are compared and are described in detail. Several ECR plasma processing reactors are also described. Methods of processing large surfaces are outlined, and typical experimentally measured ECR discharge characteristics are presented.
Strained-layer epitaxy of SnTe on CdTe(110)
Authors: Höchst Hartmut , Niles David W. , Engelhardt Michael A. , Hernández-Calderón Isaac
Published: July 2002

Bulk SnTe is a IV–VI compound which crystallizes in the NaCl structure. However, deposition of SnTe on CdTe(110) can force very thin films of SnTe to grow in the zinc-blende structure, leaving an ‘‘extra’’ electron per atom not involved in tetrahedral bonds. We present an angular-resolved photoemission study of this exotic heterostructure, and show that indeed the first ∼4 monolayers of SnTe do grow in the zinc-blende structure. For films of more than ∼10 layers in thickness, the film relaxes into the more favorable NaCl structure. The valence-and conduction-band offset between NaCl structured SnTe and CdTe(110) are ΔEν=1.01±0.05 eV, and ΔEc=0.37±0.05 eV, respectively. The Fermi level lies at the midpoint of the SnTe band gap.
Chemisorption studies on Cs/Cu(110): Model studies of cesium promoters on copper-based catalysts
Authors: Rodriguez José A. , Clendening William D. , Campbell Joseph M. , Min Wu , Campbell Charles T.
Published: July 2002

The adsorption of CO, CO2, and H2 O, the coadsorption of oxygen and H2 O, and the coadsorption of CO and surface hydroxyl (OHa) on clean and cesium-covered Cu(110) surfaces have been studied with thermal desorption mass spectroscopy, x-ray photoelectron spectroscopy, Auger electron spectroscopy, and work function measurements. We review here results which show that the presence of preadsorbed cesium dramatically affects the adsorption behavior and surface chemistry of CO, CO2, and H2 O on Cu(110). We also present new results which show that surface formate (HCOOa) is not produced in any observable amount upon heating coadsorbed COa and OHa on clean or Cs-dosed Cu(110). The implication of these results to Cs promotion of the catalytic water–gas shift and methanol synthesis reactions over Cu will be discussed.
Carbonate formation and decomposition on KOH/Ag(111)
Authors: Blass P. M. , Zhou X.-L. , White J. M.
Published: July 2002

Using x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, temperature programmed desorption, and isotopic exchange, we have studied the formation of potassium hydroxide on Ag(111) and its conversion to carbonate. KOD forms from D2O and K dosed at 100 K when the surface is warmed to 300 K, and undergoes H/D exchange with H2O at low temperature. KOD(a) may be converted to adsorbed carbonate by exposure to CO2 at 300 K but not at 100 K.
Adsorption and thermal dissociation of disilane (Si2H6) on Si(100)2×1
Authors: Suda Y. , Lubben D. , Motooka T. , Greene J. E.
Published: July 2002

Adsorption and thermal dissociation of Si2H6 on Si(100)2×1 surfaces has been studied using electron energy loss spectroscopy (EELS) and reflection high-energy electron diffraction (RHEED). Surface reactions were followed by recording the intensities and the positions of EELS peaks associated with dangling bonds on the initial 2×1 surface, surface Si–H bonds, and the appearance of dangling bonds due to Si dimer bond rupture together with the relative intensities of the integral and half-order RHEED diffraction spots. The results indicate that with continued disilane exposure, Si2H6 is dissociatively chemisorbed onto dangling bond sites on Si(100) and the initially adsorbed species (probably SiH3) further dissociate to form a mixed (2×1):H monohydride and (1×1)::2H dihydride surface. The mixed surface is then slowly converted to a (1×1)::2H surface through further reactions with Si2H6. While the Si(100)2×1 dangling bonds were saturated at Si2H6 doses of ≂5×1015 cm−2, much higher doses (≂2×1017 cm−2) were required to obtain a dihydride-saturated surface. Saturated (1×1)::2H surfaces were annealed to successively higher temperatures Ta for 15 s each. At Ta=655 K, hydrogen was evolved leading to the reestablishment of a mixed (2×1):H+(1×1)::2H surface. Most of the remaining dihydride was converted to monohydride at temperatures between 705 and 725 K. The 2×1 dangling-bond EELS peak reemerged at Ta=765 K and the clean-surface 2×1 EELS spectrum, signaling the evolution of the remaining H from the new epitaxial Si layer, was obtained after annealing at 955 K. Implications of these results for Si growth by atomic-layer epitaxy is discussed.
Direct observations of enzymes and their complexes by scanning tunneling microscopy
Authors: Elings Virgil B. , Edstrom Ronald D. , Meinke Marilyn H. , Yang Xiuru , Yang Rui , Evans D. Fennell
Published: July 2002

Scanning tunneling microscopy (STM) has been used as a method of studying the relationships between the enzymes of muscle glycogenolysis. In skeletal muscles the activation of phosphorylase b is catalyzed by phosphorylase kinase. This interaction is believed to occur in vivo as part of a multienzyme complex. The molecular structures of phosphorylase b and phosphorylase kinase have been visualized by STM.1 Phosphorylase b can be seen in dimeric and tetrameric forms as well as linear and circular aggregates. Individual molecules of phosphorylase kinase image as planar, bilobate structures with a twofold axis of symmetry and a central depression. STM has also been used to visualize complexes between phosphorylase kinase and its substrate, phosphorylase b.
Composition and structure of boron nitride films deposited by chemical vapor deposition from borazine
Authors: Kouvetakis J. , Patel V. V. , Miller C. W. , Beach D. B.
Published: July 2002

The composition and structure of boron nitride films prepared by thermal and plasma enhanced chemical vapor deposition (CVD) using borazine as a precursor have been studied. Thermal CVD at temperatures between 475 and 550 °C using either a hot-wall or cold-wall reactor results in amorphous boron-rich films of approximate composition BN0.67. Plasma enhanced CVD consistently gives films of 1:1 boron to nitrogen stoichiometry, but the hydrogen content of films deposited below 300 °C is so high that the films react with atmospheric moisture. Optimum conditions for the growth of stoichiometric BN with relatively low hydrogen content were found to be low plasma power, hydrogen–borazine gas mixtures, and a substrate temperature of 550 °C. Films deposited under these conditions are mixtures of poorly crystalline hexagonal and cubic boron nitride.
Hydrogen chemisorption and reaction on GaAs(100)
Authors: Creighton J. R.
Published: July 2002

We have studied the chemisorption of atomic hydrogen and deuterium on gallium-rich (4×6) and arsenic-rich c(2×8) GaAs(100) reconstructed surfaces using temperature programmed desorption (TPD). Molecular hydrogen (specifically D2) desorption from the gallium-rich (4×6) surface exhibits a peak at 480–510 K, depending on the initial coverage. By comparing results from the different reconstructions we conclude that the desorption of molecular hydrogen primarily arises from surface GaHx species. The arsenic-rich c(2×8) surface yields a small amount of molecular hydrogen desorption around 660 K, apparently arising from surface AsHx species. In addition to molecular hydrogen, chemisorbed arsine is detected desorbing from both reconstructions around 330–340 K. This result indicates that atomic hydrogen can etch the GaAs surface. For the maximum exposure studied, ∼15% of the surface arsenic on the arsenic-rich c(2×8) surface is converted to arsine.
Surface force measurements on picometer and piconewton scales
Authors: Bryant P. J. , Kim H. S. , Deeken R. H. , Cheng Y. C.
Published: July 2002

The design of an atomic force spectrometer with force mapping capability is presented and feasibility is demonstrated. The combined technologies of scanning tunneling and atomic force microscopy have been utilized and augmented. Tunnel current control provides the sensitivity for piconewton scale force detection in real space with picometer accuracy. However, the use of a sensitive cantilever beam with a spring constant of the order of 10 N/m or less can be compromised by the precipitous ‘‘lever jump’’ phenomenon during close approach which occurs when the gradient of the attractive potential exceeds the spring constant of the lever. To circumvent this problem a special technique has been developed to control the approach by regulating the probe in a dynamic equilibrium. Preliminary data has indicated the feasibility of this technique.
Growth mechanisms of Si and Ge epitaxial films on the dimer reconstructed Si{100} surface via molecular dynamics
Authors: Srivastava Deepak , Garrison Barbara J.
Published: July 2002

Microscopic and macroscopic growth mechanisms of thin epitaxial films due to Si and Ge deposition on the Si{100}-(2×1) surface are studied via molecular dynamics using Tersoff’s many-body potential. The dimer openings on the reconstructed surface following the Si or Ge deposition are either due to a diffusing adatom induced mechanism or due to a direct insertion of the incoming adatoms into the epitaxial positions. Two distinct layer by layer growth modes of macroscopic epitaxial films are observed; (a) the mode in which the domains of good or bad epitaxial growths remain roughly in the same area of the interface throughout its entire thickness, and (b) the mode in which reconstructions on the buried interfaces can be relieved by long time equilibrations, i.e., the epitaxially deposited film can heal its inner layers during equilibrations. The qualities of the final macroscopic films in these two cases are found to be distinctively different.
Low outgassing and anticorrosive metal surface treatment for ultrahigh vacuum equipment
Authors: Sugiyama Kazuhiko , Ohmi Tadahiro , Morita Mizuho , Nakahara Yoshiyuki , Miki Nobuhiro
Published: July 2002

We report on a stainless steel passivation technology useful for the future

We report on a stainless steel passivation technology useful for the future submicron ultralarge-scale integration (ULSI) manufacturing. In the near future, low pressure processing will become more important. With presently available equipment, outgassing from the inner surface of metal chambers is one of the factors that can hinder ultrahigh vacuum processing. Therefore, absorbed molecular contaminations must be eliminated. We report the development of low outgassing metal surface that is derived from an O2 or F2 passivation process. This process requires an initial metal surface to have mirror finish without Beilby layer. Specifically, by using a 316L stainless surface, under clean conditions, outgas volume of moisture are shown to be reduced by a factor of 102, compared with conventional finished steel surfaces. In addition O2/F2 passivation, though inherently different, give stainless steel surfaces that show excellent corrosion resistance against gases.
Kinetics of dissociative chemisorption of methane and ethane on Pt(110)-(1×2)
Authors: Sun Y.-K. , Weinberg W. H.
Published: July 2002

The initial probability of dissociative chemisorption Pr of methane and ethane on the highly corrugated, reconstructed Pt(110)-(1×2) surface has been measured in a microreactor by counting the number of carbon atoms on the surface following the reaction of methane and ethane on the surface which was held at various constant temperatures between 450 and 900 K during the reaction. Methane dissociatively chemisorbs on the Pt(110)-(1×2) surface with an apparent activation energy of 14.4 kcal/mol and an apparent preexponential factor of 0.6. Ethane chemisorbs dissociatively with an apparent activation energy of 2.8 kcal/mol and an apparent preexponential factor of 4.7×10−3. Kinetic isotope effects were observed for both reactions. The fact that Pr is a strong function of surface temperature implies that the dissociation reactions proceed via a trapping-mediated mechanism. A model based on a trapping-mediated mechanism is used to explain the observed kinetic behavior. Kinetic parameters for C–H bond dissociation of the thermally accommodated methane and ethane are extracted from the model.
Real-time observation of oxygen and hydrogen adsorption on silicon surfaces by scanning tunneling microscopy
Authors: Tokumoto H. , Miki K. , Murakami H. , Bando H. , Ono M. , Kajimura K.
Published: July 2002

The initial stages of the adsorption processes of oxygen and hydrogen on the Si(111)-7×7 surface were studied at room temperature in real time using a ultrahigh vacuum scanning tunneling microscopy (UHV STM). We have found several specific features for each process. (1) the [1̄1̄2] steps are relatively insensitive to oxygen or hydrogen exposure: more resistant to hydrogen than to oxygen. (2) The adsorption proceeds with different chemical reactivities among the adatoms: the adatoms in the faulted half of the 7×7 unit cell are more active than the unfaulted half; the center adatoms in each half are more active and the corner adatoms are the next; the corner holes in the 7×7 unit cell are less active indicating that the 7×7 structure still exists even for fairly high coverages. (3) At initial stages of adsorption, oxygen is adsorbed in the on-top site of the surface adatom forming Si–O and then in the bridge site forming Si–O–Si which causes the disorder in the image, whereas hydrogen is adsorbed in the on-top site forming Si–H.
A dual tunneling-unit scanning tunneling microscope
Authors: Kawakatsu Hideki , Higuchi Toshiro
Published: July 2002

A regular crystalline lattice has a high potential of being used as a well-defined, universal reference scale in the metrological application of scanning tunneling microscopes (STMs). A dual tunneling-unit (DTU) STM, incorporating one XY scanner and two Z scanner with tips was developed to check the feasibility of the ‘‘crystal-reference’’ STM. Basic ideas and problems of substitution measurement and crystal-reference XY positioning table are discussed. With the scope of developing a robust and reliable positioning table, software and circuitry design were carried out on two methods of tip-and-target XY position lock control, and one method of servo tracking of atomic arrays. Promising simulation results were obtained as to the noise tolerance of the controls. A successful application of an impact force driven rough positioning mechanism for coarse tip approach is also introduced. Its performance, operating principles and structure are extremely high vacuum oriented.
Quantification of surface film formation effects in fluorocarbon plasma etching of polysilicon
Authors: Gray David C. , Sawin Herbert H. , Butterbaugh Jeffrey W.
Published: July 2002

A multibeam apparatus has been constructed which allows synthesis of the primary fluxes to a wafer surface during fluorocarbon plasma etching. These species include CF2 as a polymer forming precursor, F as the primary etchant, and Ar+ or CF+x for energetic ion bombardment. The individual fluxes can be adjusted to within an order of magnitude of those encountered in typical plasma etching processes, and can be manipulated independently to ascertain the effects of radical-to-ion flux ratio on fluorocarbon film formation and etching yields. Direct sticking of CF2 radicals on undoped polysilicon films can account for suppression of silicon MONITOR sputtering yields on the order of 70%, but cannot completely stop the net removal of silicon by Ar+ at energies above 150 eV. Studies of the competitive interaction between CF2 and F radicals on undoped silicon surfaces showed up to 30% suppression of Ar+/F etching yields under flux conditions representative of low pressure CF4 reactive ion etching (RIE).
Low-energy electron microscopy of semiconductor surfaces
Authors: Bauer E. , Mundschau M. , Swiech W. , Telieps W.
Published: July 2002

The low-energy electron microscopy work on the structure and dynamics of semiconductor surfaces, in particular of Si(111) and Si(100) is reviewed. The phenomena studied include the (7×7) ↔ (1×1) phase transition of the Si(111) surface, the sublimation of Si from Si(100) and the growth of Si on Si(100). Interactions between steps and between steps and impurities are also discussed as are metal–silicon interactions, illustrated by the formation of CoSi2 by cobalt deposition.
High-resolution x-ray microscopy using an undulator source, photoelectron studies with MAXIMUM
Authors: Capasso C. , Ray-Chaudhuri A. K. , Ng W. , Liang S. , Cole R. K. , Wallace J. , Cerrina F. , Margaritondo G. , Underwood J. H. , Kortright J. B. , Perera R. C. C.
Published: July 2002

We present the first results of high-spatial resolution x-ray imaging studies with an upgraded version of the scanning photoemission multiple application x-ray imaging undulator microscope. The microscope is a multilayer coated Schwarzschild objective that focuses undulator radiation onto the sample. The recent upgrade improved the spatial resolution by a factor six reaching a full width at half maximum value of 0.5 μm. Highly polished mirrors reduced the diffuse background by almost two orders of magnitude and drastically improved the contrast. The improved microscope was used to perform a series of tests on microgrids and reverse Fresnel zone plates. The microscope capability to detect chemical and topological contrast was verified by using patterned metal overlayers on Si and GaAs substrates. Further improvements to increase the flux and the spatial resolution are underway; this includes the installation of a new undulator beamline.
Time-of-flight scattering and recoiling spectrometry
Authors: Rabalais J. Wayne
Published: July 2002

Ion scattering and recoiling spectrometry consists of directing a collimated beam of monoenergetic ions towards a surface and measuring the flux of scattered and recoiled particles from this surface. When the neutral plus ion flux is velocity selected by measuring the flight times from the sample to the detector, the technique is called time-of-flight scattering and recoiling spectrometry (TOF-SARS). TOF-SARS is capable of (1) surface elemental analysis by applying classical mechanics to the velocities of the particles, (2) surface structural analysis by monitoring the angular anisotropies in the particle flux, and (3) ion-surface electron exchange probabilities by analysis of the ion/neutral fractions in the particle flux. Examples of these three areas are presented herein.
Effects of NH3 and N2 source gases and plasma excitation frequencies on the reaction chemistry for Si3N4 thin-film growth by remote plasma-enhanced chemical-vapor deposition
Authors: Theil J. A. , Hattangady S. V. , Lucovsky G.
Published: July 2002

NH3/He and N2/He discharges can produce qualitatively different silicon nitride films when they are used as N-atom sources in the remote plasma-enhanced chemical-vapor deposition process. Radio frequency and microwave excitation at frequencies, 13.56 MHz and 2.54 GHz, respectively, were used to produce significantly different electron temperatures Te, electron energy distribution functions, and electron densities ne. Differences in these plasma parameters were then correlated with important aspects of the silicon nitride reaction chemistry, including the incorporation of hydrogen in SiH and/or Si–NH bonding arrangements.
CO interaction with ultrathin MgO films on a Mo(100) surface studied by infrared reflection–absorption spectroscopy, temperature programmed desorption, and x-ray photoelectron spectroscopy
Authors: He Jian-Wei , Corneille Jason S. , Estrada Cesar A. , Wu Ming-Cheng , Goodman D. Wayne
Published: July 2002

The interaction of CO with MgO ultrathin films grown on a Mo(100) surface is studied using infrared reflection–absorption spectroscopy, temperature programmed thermal desorption (TPD), and x-ray photoelectron spectroscopy. CO adsorbed on 7 ML of MgO shows an infrared band at 2178 cm−1. This blue-shift of the CO stretching frequency relative to that of CO in the gas phase (2143 cm−1) is attributed to electron charge transfer from the CO 5σ orbital to the MgO surface. It is further shown that CO adsorption induces a 0.4 eV shift in the Mg(2p) and O(1s) core levels of the MgO thin films to lower binding energy, consistent with the charge transfer from CO molecules to the MgO thin films. The TPD spectra indicate that CO molecules on the MgO thin films desorb in the 100–180 K sample temperature range. The CO adsorption heat on 7 ML of MgO is deduced to be 9.9 kcal/mol using an isothermal adsorption method.
New technique for oil backstreaming contamination measurements
Authors: Alterovitz S. A. , Speier H. J. , Sieg R. M. , Drotos M. N. , Dunning J. E.
Published: July 2002

Due to the large size and the number of diffusion pumps, space simulation chambers cannot be easily calibrated by the usual test dome method for measuring backstreaming from oil diffusion pumps. In addition, location dependent contamination may be an important parameter of the test. We have measured the backstreaming contamination in the Space Power Facility (SPF) near Sandusky, Ohio, the largest space simulation vacuum test chamber in the U.S.A. We used small size clean silicon wafers as contamination sensors placed at all desired measurement sites. The facility used diffusion pumps with DC 705 oil. The thickness of the contamination oil film was measured using ellipsometry. Since the oil did not wet uniformly the silicon substrate, two analysis models were developed to measure the oil film: (1) continuous, homogeneous film and (2) islands of oil with the islands varying in coverage fraction and height. In both cases, the contamination film refractive index was assumed to be that of DC 705. The second model improved the ellipsometric analysis quality parameter by up to two orders of magnitude, especially for the low coverage cases. Comparison of the two models for our case shows that the continuous film model overestimates the oil volume by <50%. Absolute numbers for backstreaming are in good agreement with published results for diffusion pumps. Good agreement was also found between the ellipsometric results and measurements done by x-ray photoelectron spectroscopy (XPS) and by scanning electron microscopy (SEM) on samples exposed to the same vacuum runs.
Adsorption and desorption kinetics for SiH2Cl2 on Si(111) 7×7
Authors: Coon P. A. , Gupta P. , Wise M. L. , George S. M.
Published: July 2002

The adsorption and desorption kinetics for dichlorosilane (SiH2Cl2) on Si(111) 7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The TPD experiments monitored H2, HCl, and SiCl2 desorption products at 810, 850, and 1000 K, respectively. H2, HCl, and SiCl2 desorption products were also observed in the LITD yield at all surface coverages. In agreement with the TPD studies, linear ramp LITD experiments monitored the disappearance of the H2, HCl, and SiCl2 LITD signals at 810, 850, and 1000 K. LITD measurements determined the initial reactive sticking coefficient (S0) of SiH2Cl2 on Si(111) 7×7 versus surface temperature. The sticking coefficient was S0≊0.36 at temperatures from 150 to 200 K. Above 200 K, the sticking coefficient decreased to S0≊0.10 at 350 K and S0≊0.0025 at 850 K. The temperature-dependent sticking coefficient was consistent with a precursor-mediated adsorption model. As a function of surface coverage following SiH2Cl2 exposure, TPD studies observed that the HCl desorption yield decreased relative to the H2 and SiCl2 desorption yields. These results indicated that when more hydrogen desorbs as H2 at higher coverages, the remaining chlorine is forced to desorb as SiCl2. TPD experiments also determined that the surface chlorine coverage saturated and was independent of surface temperature after large SiH2Cl2 exposures on Si(111) 7×7. Epitaxial growth of silicon by dichlorosilane chemical vapor deposition can be predicted using the measured adsorption and desorption kinetics for dichlorosilane on Si(111) 7×7. When these kinetics are employed together with incident SiH2Cl2 fluxes, very good agreement is observed between predicted and measured silicon growth rates.
Quantitative analysis of ionic solids by secondary neutral mass spectrometry
Authors: Fichtner M. , Goschnick J. , Schmidt U. C. , Schweiker A. , Ache H. F.
Published: July 2002

Using an argon high frequency plasma for excitation and postionization, secondary neutral mass spectrometry was performed to examine the ability of the technique for depth-resolved quantitative analysis of ionic solids. An argon energy Ep=300–500 eV was applied to bombard all alkali halides, some sulfates, Li2SiO3, Li4SiO4, NaNO3, and Na2CO3. The mass spectra obtained were found to be dominated by atomic signals including all elements of the measured salt. Averaged relative sensitivity factors for 20 elements calculated from these signals show close conformity to atomic ionization probabilities. Thus, the majority of sputtered neutrals from salts consists of atoms and these dominate also the whole sputter flux. All measured sensitivity factors lie within a range of 400 being the sensitivity ratio of cesium to fluorine. A moderate matrix dependence of mostly less than 40% is probably due to coemitted thermal and/or nonatomic species. A minor fraction of the neutral sputter flux (<1% total signal intensity) consists of binary molecular clusters, of which the heteroelemental could be useful for compound identification. Erosion rates were determined to be 0.3±0.1 nm/s for five different salts under 0.6 mA/cm2 primary current. The corresponding sputter yields are given.
One GHz leaky SAW velocity of metal layers and bilayers evaporated onto fused quartz
Authors: Walikainen Dale
Published: July 2002

An acoustic microscope operating at 1 GHz was used to determine the surface acoustic wave velocities v of thin film metal layers and metal bilayers deposited onto fused quartz. V(0)’s influence was reduced by gating. This produced a calibrated accuracy of 3%. A program was constructed from the explicitly solved 6×6 theoretical determinant. Single film thicknesses were decided upon by using this theory to produce a v equal to a standard. Since the single film v’s were linear with respect to their thickness, half the single film thicknesses were used for the bilayered films. The velocities for these bilayered films agreed with theory. This experimentally confirms the theoretical technique used here to examine bilayered systems, or a prototype composite interphase. No discrepancy was seen for gold films as others have reported. V(z) seemed insensitive to the formation of intermetallics or CuO. Some annealed and unannealed platinum films did not change the v from that of fused quartz. Two platinum films whose v’s were in agreement with theory peeled off with the tape test.
Ordered Hg0.8Cd0.2Te (111)A and (111)B surfaces: Preparation by annealing in Hg vapor and characterization by low-energy electron diffraction, Auger electron spectroscopy, and electron energy-loss spectroscopy
Authors: Gui John Y. , Stern Donald A. , Hubbard Arthur T.
Published: July 2002

Reported here are studies in which an oriented single-crystal, Hg0.8Cd0.2Te (111), was etched with methanolic bromine solution, ion bombarded with Ar+, and annealed in Hg vapor. Both parallel faces of the crystal, the metal-terminated (111)A surface and the Te-terminated (111)B surface, were characterized after each of the above stages of preparation by Auger electron spectroscopy (AES), electron energy-loss spectroscopy (EELS) and low-energy electron diffraction (LEED). After polishing in air the (111)A and (111)B surfaces were found to be contaminated with C and S-containing substances, and deficient in Cd; the LEED patterns were diffuse. EELS spectra showed a broad C–H stretching band near 2980 cm−1 and another broad band centered at 1300 cm−1 due to C–H bending and C–C stretching of surface hydrocarbons. Etching with bromine solution in air removed S-containing surface contaminants, without significantly changing the LEED and EELS results. Argon ion-bombardment removed the C contamination and increased the Cd content to expected levels, while decreasing the Hg content substantially. LEED showed a (111)A[1×1] pattern with strong diffuse scattering, while the (111)B[1×1] pattern was comparatively weak, indicating that detectable long-range order existed somewhere within the outermost few atomic layers. EELS of both faces displayed a peak at about 140 cm−1 attributable to the excitation of surface phonons. When the ion-bombarded crystal was annealed in Hg vapor, the Hg surface concentration increased. The ratios of metal to Te Auger intensity (IHg/ITe and ICd/ITe ) were larger for the (111)A than for the (111)B surface. The A-face yielded a sharp (2×2) LEED pattern, while the B-face displayed a reproducible, complex, three-fold symmetric pattern indicative of a faceted surface. The annealed surfaces exhibited virtually the same EELS spectra as bombarded surfaces. The annealed surfaces are stable in vacuum, as judged by AES, LEED, and EELS.
Nondestructive evaluation of silicon-on-insulator substrates using x-ray double crystal topography
Authors: Ma David I , Campisi George J. , Qadri Syed B. , Peckerar Martin C.
Published: July 2002

The strain distributions of various silicon-on-insulator (SOI) wafers were studied using Bragg reflection mode double crystal topography analysis. These SOI samples included the separation by implanted oxygen (SIMOX), the zone melt recrystallization (ZMR), and the bond and etch-back silicon-on-insulator. The inherent warpage of each SOI sample was revealed by x-ray double crystal topography. Dislocation loops and surface-terminating defects were also observed on the ZMR sample. The defect density was evaluated by double crystal rocking curve analysis using a symmetric (004) setup. The relative peak intensities and full width at half-maxima were measured. The quality of superficial silicon layers of most SOI wafers was similar to the quality of a single crystal silicon, except the ZMR sample. According to these analyses, SIMOX wafers provide the best single crystal quality and consistent lattice characteristics under various fabrication processes.
Modeling of magnetron etching discharges
Authors: Meyyappan M. , Govindan T. R.
Published: July 2002

Magnetron discharges are currently used in etching silicon, GaAs, and other semiconductor materials since they provide high etch rates and reduced ion bombardment damage to the wafers. A discharge physics model is presented for a 13.56 MHz planar magnetron discharge. The applied magnetic field is uniform and normal to the direction of the electric field. The model consists of the first three moments of the Boltzmann transport equation with appropriate description of elastic and inelastic collision terms. Numerical solutions have been obtained for a chlorine discharge. The study includes a parametric variation of applied bias and magnetic field. The electron density scales linearly with power and modified pressure P[1+(ωc/ν)2]1/2, where ωc is electron cyclotron frequency and ν is elastic collision frequency. The magnetic field lowers the plasma potential, and the potential drop in the glow also becomes significant. In addition, the phase lead of discharge current over the voltage decreases substantially as the magnetic field is increased.
Automating the analysis of Langmuir probe traces: Construction of an algorithm and sensitivity analysis for probe traces from an electron cyclotron resonance plasma source
Authors: Friedmann J. B. , Ritter C. , Bisgaard S. , Shohet J. L.
Published: July 2002

Langmuir probe traces of plasmas are routinely analyzed to determine important characteristics, such as plasma density and electron temperatures. This article describes how a common manual technique to analyze probe data can be automated and how the sensitivity of the results to changes in the operator-specified starting values can be explored. The probe traces, which were taken in an electron cyclotron resonance plasma source, characteristically exhibit two Maxwellian electron populations, one at a bulk (Tebulk≊3–5 eV) and one at a hot (Tehot≊12–20 eV) temperature. The electron current (F) as a function of the sweep voltage (ν) of the probe traces is modeled by: F(ν) = Isate{(1 − p)exp[(Vp − ν)/Tebulk] + p exp[(Vp − ν)/Tehot]}, where Isate is the electron saturation current, p is the ratio of the hot electron current to the total, and Vp is the plasma potential. The algorithm for probe analysis partitions the sweep voltage into three intervals and uses different models and fitting techniques in each interval. The procedure’s sensitivity to different operator inputs is evaluated using factorial experiments.
Elevated temperature oxidation and etching of the Si(111) 7×7 surface observed with scanning tunneling microscopy
Authors: Seiple J. , Pecquet J. , Meng Z. , Pelz J. P.
Published: July 2002

Scanning tunneling microscopy has been used to study the initial oxidation of the Si(111) 7×7 surface over the temperature range 400–600 °C. Oxygen-induced etching of the surface is observed for low O2 pressures (<10−7 Torr) at temperatures between 500 and 600 °C, causing monolayer-deep holes in the surface and the retraction of step edges. The newly exposed surfaces often reconstruct into metastable 5×5 or 9×9 structures, as well as the stable 7×7 structure.