bluuebunny/crossref_metadata_2025 huggingface dataset:
Epitaxial growth of CaF2 on GaAs(100)
Authors: Sinharoy S. , Hoffman R. A. , Rieger J. H. , Farrow R. F. C. , Noreika A. J.
Published: July 2002

Insulating films on the GaAs(100) surface for use as passivating layers or in MIS structures pose a special problem because of a lack of a stable native oxide and also because of difficulties associated with the preparation of a clean, stoichiometric GaAs(lOO) surface in the absence of Ga or As vapor sources. Group II fluorides (CaF2, BaF2, SrF2, and CdF2) are good insulators at or below room temperature, and their cubic fluorite structure makes them good candidates for epitaxy on the cubic GaAs(100) surface. The close structural match between CaF2 and GaAs (3.2% lattice mismatch) makes this combination especially attractive for epitaxy studies. We report the first epitaxial growth of CaF2 on a GaAs(100) substrate that was cleaned under UHV conditions by ion–heat treatment. The depositions were carried out at pressures in the range of 10−9 Torr with a specially designed evaporator supplying the CaF2 molecules. In situ AES and LEED were used for the characterization of the structure and composition of the substrate and the film. The CaF2 films were characterized ex situ by RHEED and optical microscopy. High quality epitaxial films were obtained at substrate temperatures of 500–600 °C.
Fourier transform photoelectron spectroscopy
Authors: Muehlhoff L. , Muehlhoff H.-M.
Published: July 2002

This paper describes a new method of data acquisition and analysis for photoelectron spectroscopy (ESCA/UPS). Rather than sweeping through a preset range of energies at a constant rate accumulating count rates at each energy, nonlinear sweeps are used for each Fourier coefficient of the spectrum. Compared to conventional scan mode, this method offers two advantages: (1) The noise contents of each Fourier coefficient can be predetermined by setting the length of the sweep interval. This is very helpful for the subsequent application of deconvolution techniques because they amplify the contribution of higher order components. Using time efficiently, the noise contents of these components can be reduced more than the noise of lower order contributions. This is not possible for spectra taken by normal means because their noise spectra are flat. (2) Much time can be saved by not recording higher order Fourier coefficients whose contribution to the spectrum is negligible. A data acquisition system for ESCA has been developed to test these advantages. Spectra taken by conventional scan mode and by Fourier transform photoelectron spectroscopy (FTPS) are presented for comparison.
Quantitative surface structural determination using impact collision ion scattering spectroscopy
Authors: Yarmoff Jory A. , Williams R. Stanley
Published: July 2002

Impact collision ion scattering spectroscopy (ICISS), employing 5 keV 6Li+ projectiles, was used to study the surface structure of Cu(110) and Cu(110) (2×1)-O. A new method of ICISS data analysis is presented which utilizes a one-atom Monte Carlo computer simulation to map out the flux of ions behind each atom visible to the incoming ion beam. The results of these calculations for various structural parameters were compared with the experimental data until a best fit was achieved. The results for the clean surface support a model with a 10±5% contraction of the first atomic layer spacing and a surface vibrational amplitude enhancement of 1.5±0.5 times the bulk value. For the (2×1) oxygen-induced reconstruction, a model in which every other [001] row in the surface plane is vacant, the first-to-second layer spacing is expanded 25±10% and the second-to-third layer spacing is contracted 10±5%, was consistent with the experimental results.
Aluminum/polyimide interface formation: An x-ray photoelectron spectroscopy study of selective chemical bonding
Authors: Atanasoska Lj. , Anderson Steven G. , Meyer H. M. , Lin Zhangda , Weaver J. H.
Published: July 2002

High-resolution x-ray photoelectron spectroscopy has been used to study the formation of the aluminum/polyimide (Al/PI) interface at room temperature. Aluminum films up to 80 Å thick were vapor deposited onto cured polyimide in situ. Our results show that Al is chemically active for coverages below ∼20 Å. Preferential aluminum bonding with PI occurs at carbonyl sites, as evidenced by the rapid attenuation of the carbonyl C 1s core level photoemission intensity. In the initial reaction stage (1–2 Å), Al preferentially occupies planar imide rings. These adatoms transfer charge to carbonyl carbon atoms via oxygen and this resonance hybrid state of C–O–Al weakens, but does not break, carbonyl bonds. With increasing Al coverage, the formation of strong Al–O bonds is observed and is attributed to a C–O–Al complex compound. Metallic Al is observed at a nominal coverage of 2 to 5 Å. The Al/PI interface exhibits the attenuation behavior characteristic of cluster growth through both reacted and unreacted regions. Annealing the 80-Å Al/PI interface at 300 °C results in extended reaction with less selective chemical interaction of Al with PI.
Computer program for photoemission data analysis and display
Authors: Mahowald P. H. , Friedman D. J. , Carey G. P. , Bertness K. A. , Yeh J. J.
Published: July 2002

A computer program for photoemission data analysis which operates using the VAX computer or the Macintosh personal computer is described. It includes functions which give a summary of the data such as centroid and area, performs curve fitting for extracting chemical information from core levels, and it prepares figures for publication, as well as many other functions. A brief description of the program and its capabilities is given.
Some improvement work on ConFlat joints and their limit of reliability in a large-size ultrahigh vacuum system
Authors: Unterlerchner W.
Published: July 2002

The 2-km ultrahigh vacuum system of the CERN Intersecting Storage Rings (ISR) was operated in the 10−12 Torr range. During operation with circulating proton beams, it was found that the lifetime and intensity of the beams were deteriorated by a pressure rise effect. The measures to cope with this phenomenon included the installation of additional pumping speed, surface conditioning of the vacuum chambers by glow discharge, and longer bakeouts at 300 °C. The latter put particular stress on the demountable all-metal [ConFlat (CF)] joints. This paper describes the summary of investigations and measurements on the CF joints of the ISR and the subsequent changes in design and material. A recording of vacuum leaks during a period of almost 12 years on the tubular UHV system with more than 6000 CF joints has enabled a statistical study of their reliability to be made. An analysis of these leaks is presented and failure details are illustrated using modern laboratory equipment such as scanning electron microscope and x-ray analysis.
Will tomorrow’s high-vacuum pumps be universal or highly specialized?
Authors: Duval P.
Published: July 2002

With the notable exception of cryopumps, the high-vacuum pumps used today are the same as those used 20 years ago, but with the new requirements of some important users it may be that we are entering a new area in pump evolution. The user demand for industrial types of application can be summarized as follows: increased reliability, reduced hazards when pumping dangerous gases, reduced pollution caused by oil backstreaming, reduced costs in the use of expensive fluids (perfluoropolyether), easy maintenance, and very low level of vibration. These requirements force the pump manufacturers to change the characteristics of their products: (i) rotary pumps with improved tightness, and corrosion proofing, (ii) dry pumps renewal, and (iii) specialized turbomolecular pumps. The high-vacuum pumps we will use three to five years from now will be more or less the same as those used today but will tend to be more specialized and devoted to a specific application. Over the past ten years, there has been little significant invention in the field of high-vacuum production, apart from the breakthrough of the cryopumps (which were more an evolution of something known than a real new product generated by a new concept). Like all industries, the creative activity in the vacuum industry is cyclical but, contrary to the semiconductor industry, for instance, the periods of its cycles are quite long (about 25 years) and we are presently leaving the plateau probably of a period of consolidation, to enter a new period of excitement and blossoming innovation, I hope. Indeed we may consider the early sixties as the last ‘‘regeneration’’ era of the high-vacuum production technology; this period witnessed the birth of ultrahigh vacuum, the direct drive pumps, the ion getter pump, and the turbomolecular pump; it is time now for a new one ! Somebody said that ‘‘...forecasting is something difficult and risky, especially if the forecast concerns the future...’’ but risk is part of life.
Recommended procedure for measuring pumping speeds
Authors: Hablanian M. H.
Published: July 2002

The American Vacuum Society’s tentative standard for measuring pumping speed was first issued in 1963 and published in a revised form in 1971. The newly established Recommended Practices Committee of AVS felt that it was appropriate to review the standard to reconcile it with modern developments in technology and technique. These include among others: new units for pressure measurement, establishment of new pumping methods, recognition of emergence of international standards, and general improvements in high-vacuum practices. The main substantial change involves the positioning of the pressure gauge in the measurement chamber at a distance from the pump entrance equivalent to 0.5 of pump inlet diameter. This will reduce the measured speed values by 10% to 15%, depending on pump size and inlet geometry, but will correspond more closely to the definition of pumping speed in relation to the time of evacuation of a given volume.
An analysis of equations for flow in thin, rectangular channels
Authors: O’Hanlon John F.
Published: July 2002

There are three methods for calculating the gas flow in thin, rectangular channels anywhere in the viscous flow region (incompressible viscous flow, or compressible viscous flow with choked or unchoked exit). Two of these models have been compared to experimental data in a limited range of channel length-to-thickness ratio L/e. However, they have not been compared to experimental data over a wide enough L/e range to establish bounds on their usefulness. We have compared methods of Sasaki and Yasunaga and Kieser and Grundner and observed the former technique to agree with experimental data for the range L/e > 150 for viscous flow, unchoked exit, while the latter is in agreement over the range 8 < L/e < 400, for molecular, transition, and viscous flow (choked or unchoked exit). We examine a new model for flow in choked ducts proposed by Santeler and show that it agrees with measurements in the viscous flow region (thickness-to-mean free path ratio e/λ > 100) with choked exit for all values of L/e except for the largest. A correction to the method of Kieser and Grundner is offered which improves its accuracy for long channels in the high pressure region.
Unified formula describing the impedance dependence of a quartz oscillator on gas pressure
Authors: Kokubun K. , Hirata M. , Ono M. , Murakami H. , Toda Y.
Published: July 2002

The impedance dependence of a tuning-fork-shaped quartz oscillator on gas pressure is theoretically analyzed on the basis of a string-of-beads model of the quartz oscillator. The theoretical results are numerically compared with experimental data. The analysis has been already carried out in molecular flow and viscous flow region. In this paper, the impedance behavior in the intermediate flow region is formulated by making use of the theory of slip effect. Furthermore, by the formal extension of this formula to the molecular flow region, a single formula describing the impedance dependence on gas pressure over the whole pressure region is obtained. This formula is found to be in good agreement with experimental data. However, there is still a slight discrepancy (<20%) between the two in the intermediate flow region. This shows the limit of the slip theory. For this reason, the Millikan’s empirical formula is adopted in the intermediate flow region. The unified formula obtained thusly is in numerical agreement with experimental data for various kinds of gases.
Scanning tunneling microscopy of silver, gold, and aluminum monomers and small clusters on graphite
Authors: Ganz Eric , Sattler Klaus , Clarke John
Published: July 2002

We have deposited small numbers of atoms of silver, gold, and aluminum onto cleaved graphite substrates in ultrahigh vacuum. Using a scanning tunneling microscope, we have observed monomers, groups of monomers, dimers, and a trimer. The adsorption sites and atomic spacings are determined by direct observation.
Growth of epitaxial CaSi2 films on Si(111)
Authors: Morar J. F. , Wittmer M.
Published: July 2002

The growth of single-crystal epitaxial CaSi2 films on Si(111) is described along with a determination of the silicide atomic structure. Films were prepared using standard ultrahigh vacuum evaporation techniques in a silicon molecular beam epitaxy system. Cross-sectional transmission electron micrographs of atomically abrupt step-free interfaces extending well over 500 Å are presented. These films constitute the first example of an epitaxial non-transition-metal silicide. The lack of d electrons and the large difference in electronegativity between Ca and Si provide a new single-crystal metal/silicon interface with significantly different characteristics from the thoroughly studied transition-metal silicides.
Chemical composition of soft vacuum electron beam assisted chemical vapor deposition of silicon nitride/oxynitride films versus substrate temperature
Authors: Zhang B.-R. , Yu Z. , Collins G. J. , Hwang T. , Ritchie W. H.
Published: July 2002

Thin silicon nitride/oxynitride films were deposited on silicon substrates by a soft vacuum electron beam plasma assisted deposition in a triad gas feedstock mixture of silane, ammonia, and nitrogen. Fourier transform infrared spectra, x-ray photoelectron spectra, Auger electron spectra, and secondary ion mass spectra of these thin films were analyzed and compared to determine both the bulk chemical composition and the interfacial properties. Film composition varied from SiO2 to Si3N4 as the substrate temperature (Ts) varied from 50 to 450 °C. Between 150 and 350 °C a stable SiOxNyHz structure formed. The films deposited with NH4–SiH4–N2 mixtures contained large (20%–60%) amounts of N–H bonds, but no detectable Si–H bonds (<0.5%). The total hydrogen content of deposited films decreased by a factor of 10 (to 2%–3%) when only nitrogen and silane were used as source gases, in comparison with that obtained using the triad feedstock mixture.
Effect of magnetic field on plasma characteristics of built-in high-frequency coil type sputtering apparatus
Authors: Yamashita Mutsuo
Published: July 2002

The effects of a perpendicular magnetic field on the plasma characteristics of a high-rate sputtering apparatus with a built-in high-frequency (HF) coil were investigated. When the proper perpendicular magnetic field, between 20 and 30 G, was applied to the plasma region, the plasma density markedly increased and the lowest limit of the operating gas pressure could be reduced from 1.3×10−3 to 2.45×10−4 Torr. The optimum perpendicular magnetic-field strength regularly increased with an increase in the operating HF at pressures of 5×10−4 Torr or less, but was little affected by the other discharge parameters, such as HF power and target voltage. The frequency of the largest harmonic component of the HF voltage, detected from the plasma region, agreed with the electron cyclotron frequency calculated from the optimum magnetic-field strength. Therefore, it is suggested that a resonancelike phenomenon occurred in the plasma region.
The decomposition of [Mn(CO)5]2(μ-SiH2)
Authors: Stauf G. T. , Dowben P. A. , Emrich K. , Hirschwald W. , Boag N. M.
Published: July 2002

The prospect of using chemical vapor deposition to deposit mixed metals and silicides from a single source compound is attractive but largely uninvestigated. Studies of decomposition energies of such compounds are nearly nonexistent. One such compound which has successfully been used to make a silicide coating is [Mn(CO)5 ]2 (μ-SiH2 ). We have used electron impact mass spectroscopy, photoionization mass spectroscopy, and photoabsorption to determine bond energies within this compound. The combination of methods allows a high degree of confidence in the resultant ionization and fragment appearance potentials. Some possible mechanisms of decomposition are discussed. A complete ionic decomposition thermodynamic cycle has been generated, and the results are used to illuminate the coating processes previously observed.
Photodesorption of atoms from metal particles with visible laser light
Authors: Weidenauer R. , Vollmer M. , Hoheisel W. , Schulte U. , Träger F.
Published: July 2002

The interaction of small metal particles on surfaces with low-intensity laser light gives rise to an interesting new photodesorption process. Atoms are ejected after excitation of a surface plasmon oscillation in the metal particles. The effect offers the possibility to manipulate a particle size distribution on the surface in a well-defined way. First results show that even a considerable narrowing of a distribution can be achieved. The experimental results reported here include measurements of the desorption rate as a function of irradiation time for fixed laser wavelength and intensity, and for a specific initial particle size distribution. In addition, a model for the narrowing of the size distribution has been developed. Good agreement with experiment is found.
Plasma polymerization by magnetron glow discharge. II. Effect of magnetic field on chemical characteristics of plasma polymers of tetrafluoroethylene revealed by electron spectroscopy for chemical analysis
Authors: Sato K. , Yeh Y.-S. , Yasuda H.
Published: July 2002

Plasma polymerizations of tetrafluoroethylene (TFE) by magnetron glow discharges were investigated by varying the magnetic field of a magnetron electrode system. Polymer deposition rates at two strategically selected positions in the electrode space and chemical characteristics of deposited films revealed by electron spectroscopy for chemical analysis (ESCA) were studied as functions of a magnetic field. It was found the presence of a magnetic field influences the spatial distribution of deposition rates and chemical characteristics of the deposited polymers. The intensity, width, and the relative location of the intense glow (in the form of a ring) with respect to the electrode surface vary with the magnetic field strength and the discharge voltage. Because the intense glow is highly localized (only in a ring shape adhering close to the electrode surface), the remaining glow discharge, which fills the majority of the interelectrode space, becomes a weak, but uniform field for plasma polymerization. This situation is similar to the tail-flame portion of a rf tubular reactor. In the presence of a magnetic field in the range investigated and at a relatively low level of discharge wattage, plasma polymerization yields films with three-dimensional networks comprised of relatively long segments and lesser amounts of quaternary carbons. The overall characteristics of plasma polymers, however, are similar to those formed without a magnetic field.
Fiber optic sensor for substrate temperature monitoring
Authors: Yeh Yunhae , Lee Chung E. , Atkins Robert A. , Gibler William N. , Taylor Henry F.
Published: July 2002

The use of an optical fiber Fabry–Perot interferometric temperature sensor for monitoring substrate temperature in a rf sputtering system is demonstrated. The sensor head consists of a continuous length of single mode silica fiber which contains two internal mirrors a distance of 1.08 cm apart to form the interferometer cavity. The laser light source for the sensor is located outside the vacuum system and connected to the sensor head via a fiber optic feedthrough. The accuracy of the sensor was 0.05 °C in this experiment, but considerable improvement is possible with better signal averaging techniques. The fiber optic sensor provided substrate temperature information during rf plasma excitation, when a nearby thermocouple ceased to function due to electromagnetic interference.
Biomolecules and surfaces
Authors: Ratner Buddy D. , Castner David G. , Horbett Thomas A. , Lenk Thomas J. , Lewis Kenneth B. , Rapoza Richard J.
Published: July 2002

The majority of the processes that occur in living systems take place at surfaces or interfaces. Biomolecules at surfaces exhibit crystallization, reconstruction, contraction, diffusion in the surface plane, sorption, epitaxy, phase transitions, catalysis, catalytic fouling, electron tunneling, work function shifts, oxidation, and contamination. However, these terms from the jargon of surface science are rarely used to describe the phenomena occurring in living systems. In this review, the terminology and phenomenology that biologists deal with will be placed in the context of contemporary surface science. Methods to maintain the structural integrity of biomolecules under ultrahigh vacuum (UHV) conditions (analogous to efforts by catalysis researchers to relate their UHV studies to high-pressure processes in liquids) will be discussed. Data will be presented on biomolecules at organic and inorganic interfaces obtained by x-ray photoelectron spectroscopy, static secondary ion mass spectrometry, attenuated total reflectance infrared spectroscopy, and scanning tunneling microscopy. The nature and kinetics of protein reorganization at interfaces will be emphasized. Methods to study these geometric reorganizations include classical biochemical elution, angular dependent XPS, and infrared absorption spectroscopy.
An aerosol model of particle generation during pressure reduction
Authors: Wu J. J. , Cooper D. W. , Miller R. J.
Published: July 2002

We have measured particle generation in a vacuum chamber as the pressure was reduced. Particle sampling was performed by using a vacuum-compatible particle detector positioned near the gas outlet. We observed a bimodal frequency distribution of particle sizes, with peaks at 0.5–1 μm and >10 μm. The flux of particles due to gravitation was measured by a second detector placed in the chamber below the gas outlet. Thermocouple measurements showed cooling occurred during pressure reduction. A model of particle evolution includes homogeneous nucleation of water saturated by abrupt cooling associated with pressure reduction, growth by condensation onto preexisting aerosol particles, and particle settling due to gravitation was used to understand the experimental results. Our simulations indicate that the aerosol particles in the chamber initially, predominantly sub-0.5 μm in size, serve as condensation nuclei, giving rise to a population of large particles. The concentration and size distribution of the population of small particles, on the other hand, were extremely sensitive to small changes of the chamber evacuation rates. The submicron-sized peak was likely due to a mix of homogeneous and heterogeneous nucleation of the saturated water vapor in our experiments. The experimental particle settling velocities were much higher than the calculated values. Significant reduction of particle generation in the evacuation of the vacuum chamber was achieved by reducing the pumping speed to lessen the temperature drop, thus preventing the saturation by water vapor. Purging the chamber by dry nitrogen flow before the pressure reduction was performed almost completely eliminated particle generation even at relatively high pumping speeds.
Reverse breakdown in long wavelength lateral collection CdxHg1−xTe diodes
Authors: Elliott C. T. , Gordon N. T. , Hall R. S. , Crimes G.
Published: July 2002

Long wavelength diodes in CdxHg1−xTe show large deviations from ideality in their reverse characteristics. The excess currents are attributed in many published papers on band to band tunneling at high reverse bias and to trap assisted tunneling at low reverse bias. Measurements of photocurrent multiplication, current–voltage characteristics, and noise have been made on long wavelength loophole diodes to determine the breakdown mechanism. This has produced strong evidence that the reverse characteristics of good quality diodes of this type are limited by impact ionization. At higher biases, there is evidence of an additional breakdown mechanism, probably tunneling.
Thermal desorption measurement technique
Authors: O’Hanlon John F.
Published: July 2002

Practical thermal desorption measurements require an ultrahigh vacuum (UHV) system with a simple method of calibrating the residual gas analyzer for a number of gases. The calibration system should be easy to move between systems. We describe how the gas injection method of Kendall has been used to calibrate an UHV system and present example measurements of the carbon dioxide and carbon monoxide evolution during the decomposition of a commercial triple carbonate used in cathode ray tube manufacture. The activation energy of the BaCO3 component of the triple carbonate was found to be 94.7 kcal/mol.
Search for pressure dependence in the sensitivity of several common types of hot-cathode ionization gauges for total pressures down to 10−7 Pa
Authors: Filippelli Albert R. , Dittmann Sharrill
Published: July 2002

Dependence of sensitivity on pressure in He, N2, and H2 has been investigated for a group of 16 hot-cathode ionization gauges, representing both extractor and Bayard–Alpert types, for total pressure as low as 5×10−8 Pa. Absolute sensitivities were determined using a primary high vacuum standard. An independent method, that of measuring the sensitivity ratio of one gauge to another, was also employed. Within a scatter of about ±3%, ±4%, and ±4% respectively, the N2, He, and H2 sensitivity ratio data showed no clearly discernible pressure dependence down to total pressures as small as 5×10−8 Pa. The absolute sensitivity measurements in He and N2 over the range 10−7 to 10−3 Pa in total pressure, also were constant within a scatter of about ±4% and ±3%, respectively. As a consequence of drift in the background component of the total pressure, all the absolute sensitivity measurements at total pressures below 10−7 Pa exhibited an apparent pressure dependence not evident in the sensitivity ratio results. In the case of H2, drift in the pressure persisted at total pressures orders of magnitude larger than the background pressure, and for all the gauges led to an apparent difference of approximately 10% between the H2 sensitivities at 10−8 and 10−5 Pa. Results of further investigation suggest that the apparent pressure dependence in the sensitivities is an artifact produced by the well-known phenomenon of thermal dissociation of H2 at hot filaments and associated processes of H2 pumping and production of other species such as CO and C2H4.
History of helium leak detection
Authors: Nerken Albert
Published: July 2002

The first atomic bomb used uranium isotope 235, which occurs in nature together with uranium-238, from which it has to be separated. The separation was accomplished in 1944–1945 in a huge ‘‘diffusion’’ plant using microporous tubing as the diffusion medium. The process was such that moist ambient air could not be tolerated in the process chambers. Accordingly, all the equipment had to be ‘‘free’’ of leaks. Equipment of the size used in the diffusion plant had never been tested to such an extreme specification. A number of leak detection devices were tried and proved inadequate. Ultimately, a simplified mass spectrometer based on the Nier 60 in. spectrometer tube was chosen for leak detection. It could easily be set to detect helium which was the choice for ‘‘probe’’ gas. From an original sensitivity such that a flow of 10−6 std cm3 of helium could be detected, the current unit has been improved so as to detect a flow of 10−10–10−11 std cm3. Size of the detector has been reduced by almost an order of magnitude and operation is highly automated.
Selective aluminum chemical vapor deposition
Authors: Tsubouchi Kazuo , Masu Kazuya
Published: July 2002

Selective deposition of metal is the key technology for deep-submicron multilevel metallization of future very large-scale integrated circuits. In this article, selective and nonselective deposition of high quality aluminum by low-pressure chemical vapor deposition using dimethylaluminum hydride (DMAH) and hydrogen was discussed. It was found that, by thermal decompsition, aluminum was deposited on the electrically conductive surface and was not deposited on the electrically insulating surface. A 0.3 μm φ deep-submicron via holes with large aspect ratio more than 3 were completely filled by the selective deposition of aluminum. The selectively deposited aluminum was confirmed to be single crystal (100) and (111) aluminum on (111) and (100) silicon, respectively. We have developed a new controllable method of the selective and the nonselective deposition using a plasma excitation technique. We have successfully planarized via holes with the selective and the subsequent nonselective deposition of aluminum. For the selective growth mechanism of aluminum on silicon, we have proposed a surface electrochemical reaction model, in which free electrons on the surface catalytically contribute to the reaction. In this model, a surface terminated hydrogen atom on silicon surface reacts with methyl radical in the DMAH molecule to produce volatile methane. The hydrogen atom of the DMAH molecule remains on the newly deposited aluminum surface as a new terminated hydrogen atom. The terminated hydrogen atom on the surface reacts repeatedly with methyl radical to deposit aluminum.
Measurement of the elemental composition of a-SiC:H films by electron spectroscopy and high energy ion scattering spectrometry
Authors: Loboda M. J. , Baumann S. , Edgell M. J. , Stolt K.
Published: July 2002

Amorphous hydrogenated silicon carbide (a-SiC:H) thin films were deposited at low (175 °C) and medium (600 °C) temperatures using plasma enhanced chemical vapor deposition. By variation of the precursor gases, a broad range of film stoichiometry was obtained. The a-SiC:H films were characterized by electron spectroscopy [x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES)], high energy ion scattering spectrometry [Rutherford backscattering spectrometry (RBS), elastic recoil detection], and ellipsometry. Quantitation of the carbon to silicon ratio was performed using elemental sensitivity factors in electron spectroscopy and compared with the inherently quantitative values obtained by RBS. The discrepancy between XPS and RBS was 5%–10%. The discrepancy between AES and RBS was significantly worse, but could be reduced to the same range as XPS with the use of an appropriate standard material. It was observed that ion beams typically used in sputter depth profiling did not result in significant preferential sputtering in the a-SiC:H films, but those used in ion scattering measurements can modify films with large hydrogen concentrations. Calculations of film thickness from the ion scattering data showed excellent agreement with results obtained from ellipsometry.
H2O adsorption on Ni(s) (111) surfaces: Evidence for a step induced influence on the adsorption geometry
Authors: Benndorf Carsten , Mundt Carolin
Published: July 2002

The adsorption of H2O on stepped Ni(s) (111) surfaces with nominal (221) and (665) orientation was studied with thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), and work function change measurements (ΔΦ). LEED from both clean surfaces confirmed a (1×1) structure attributed to the presence of (111) terraces and (111̄) steps [Ni(221)=3(111) terraces + 1(111̄) steps, Ni(665)=11(111) terraces + 1(111̄) steps]. In TDS experiments on both surfaces desorption states are detected which can be attributed either to terrace or to step sites. The step induced states, which are missing on ‘‘flat’’ Ni(111), are found at higher desorption temperatures compared to the terrace state (denoted as B with Tm=175–178 K) or to the ice multilayer state (denoted as A with Tm=155–160 K). The step induced states are related to the desorption of H2O monomers adsorbed at step sites (denoted as C, Tm=225 K) or to the recombination of H2O dissociation fragments (state D and E at Tm=260 and 325 K, respectively). The ΔΦ measurements demonstrate a significant increase of the effective H2O dipole moment at step sites, an opposite behavior is found at terrace sites. These data demonstrate that at low H2O coverages the steps are first decorated by oriented H2O monomers. At higer coverages a bilayer is expanding from the oriented H2O monomers at step sites onto the (111) terraces. Due to the presence of ‘‘flip-up’’ and ‘‘flop-down’’ H2O dipole orientations the effective dipole moment of the H2O bilayer terrace clusters is lowered.
Photoemission study of the SiO2/Si interface structure of thin oxide film on vicinal Si(100) surface
Authors: Niwano Michio , Katakura Hitoshi , Takeda Yuki , Takakuwa Yuji , Miyamoto Nobuo , Maki Michiyoshi
Published: July 2002

The SiO2/Si interface structure of thin oxide films thermally grown on vicinal Si(100) surfaces with the surface normal inclined at small angles with respect to [100] toward [001], has been investigated using high-resolution core-level photoemission spectroscopy with synchrotron radiation. Photoemission data suggest that as a result of the thermal oxidation Si(111) and (110) facets are favorably generated at the SiO2/Si interface on the vicinal Si(100) surfaces. It is also suggested that an ordered phase of SiO2 is present on the interfacial Si(111) facets. The generation of this phase is found to strongly depend on the inclination angle of the substrate surface to the (100) plane. This dependence is interpreted in terms of the variation in the density of interfacial Si(111) facets with the inclination angle.
Estimating the gas partial pressure due to diffusive outgassing
Authors: Santeler Donald J.
Published: July 2002

The operating pressure of a leak-tight vacuum system is usually limited by the outgassing of materials in the system. A major source of this outgassing is frequently water, carbon oxides, and hydrocarbons which are bound to the surface and which may be partially removed with a relatively low-temperature bakeout. Diffusive outgassing from the interior of various materials may then become the major residual gas source particularly if the system is to be operated at an elevated temperature. Hydrogen outgassing from iron-based metals such as stainless steel, and water outgassing from plastic or polymeric materials are typical examples where diffusive outgassing processes have been demonstrated to be major gas sources. The diffusive transport of gas through materials is governed by the second order differential equation Dδ2C/δX2=δC/δt, known as Fick’s Law [W. Jost, Diffusion in Solids, Liquids, Gases (Academic, New York, 1960); R. M. Barrier, Diffusion In and Through Solids (Cambridge University, Cambridge, 1941); H. S. Carslaw, and J. C. Jaeger, Conduction of Heat in Solids, 2nd ed. (Clarendon, Oxford, 1959)]. Two alternate solutions of this equation are available which provide for simplified approximations for both the short-term and the long-term behavior, as well as for the exact numerical answer. An important aspect of true diffusive outgassing is its excellent predictability as compared to experiment. This permits calculating the outgassing rate following a known time-temperature-environment history. A simple graphical solution is demonstrated to accomplish this for the example of hydrogen outgassing from a thick walled stainless steel vessel operating at an elevated temperature following various bakeout schedules.
Study of the oxygen transport through Ag (110), Ag (poly), and Ag 2.0 Zr
Authors: Outlaw R. A. , Wu D. , Davidson M. R. , Hoflund Gar B.
Published: July 2002

The transport of oxygen through high-purity membranes of Ag (110), Ag (poly), Ag (nano), and Ag 2.0 Zr has been studied by an ultrahigh vacuum permeation method over the temperature range of 400–800 °C. The data show that there are substantial deviations from ordinary diffusion-controlled transport. A surface limitation has been confirmed by glow-discharge studies where the upstream O2 supply has been partially converted to atoms, which, for the same temperature and pressure, gave rise to over an order of magnitude increase in transport flux. Further, the addition of 2.0 wt % Zr to the Ag has provided increased dissociative adsorption rates, which, in turn, increased the transport flux by a factor of 2. It was also observed that below a temperature of 630 °C, the diffusivity exhibits an increase in activation energy of over 4 kcal/mol, which has been attributed to trapping of the atomic oxygen and/or kinetic barriers at the surface and subsurface of the vacuum interface. Above 630 °C, the activation barrier decreases to the accepted value of ∼11 kcal/mol for Ag (poly), consistent with zero concentration at the vacuum interface.
Si incorporation during molecular beam epitaxy growth of GaAs and preferential attachment of Si atoms at misorientation steps
Authors: Däweritz L. , Hagenstein K. , Schützendübe P.
Published: July 2002

The effect of Si doping on the growth process of GaAs on the singular GaAs(001) surface and the attachment kinetics of Si atoms at misorientation steps on vicinal GaAs(001) surfaces have been studied by reflection high-energy electron diffraction (RHEED). During the successive growth of undoped and doped layers on the singular surface, its smoothness degrades due to Si incorporation at concentrations as low as 1016 cm−3. At high concentrations the surface ordering of undoped layers grown before and after the doped layers differs considerably. As evidenced by RHEED intensity recovery curves recorded after growth interruption the surface diffusion processes are hindered by Si atoms or clusters. The behavior of the RHEED intensity during Si deposition on vicinal GaAs(001) surfaces tilted toward the (111)Ga plane is used to analyze the surface migration kinetics with a competition between incorporation of Si atoms at step edges and their islanding on terraces. For a misorientation of 2° and a substrate temperature of 580 °C a self-organized in-plane incorporation of Si atoms in a (3×2) symmetry along the step edges is observed. For Si depositions not exceeding the amount to be attached at the step edges, the GaAs growth can be continued without adverse effects on the growth front.
Influence of different etching mechanisms on the angular dependence of silicon nitride etching
Authors: Barklund A. M. , Blom H.-O.
Published: July 2002

Results from etching experiments where Si3N4 films have been bombarded by argon ions at different angles of incidence, with or without a simultaneous exposure to XeF2 molecules, are presented. The experiments have been performed using a Kaufman-type ion beam source. With these experiments, one obtains the angular dependence of physical sputtering and ion enhanced chemical etching by fluorine. The results are compared with reactive ion beam etching (RIBE) of silicon nitride, silicon dioxide, and polysilicon using CHF3+O2 in order to find the influence of a polymer layer that may be formed on the surface of silicon nitride during etching. Such a polymer layer will strongly affect the angular dependence of the etching process. The formation of the polymer layer, responsible for the variations in the angular dependence, is a result of the flux of polymer precursors, atomic fluorine, and energetic particles to the etched surface. This means that the angular dependence of the RIBE process can be changed by varying the process parameters. This offers a possibility to tailor a process with a specific angular dependence that can be used to etch structures with controlled sidewall profiles.
Quantitative analysis of contaminants in ultrapure gases at the parts-per-trillion level using atmospheric pressure ionization mass spectroscopy
Authors: Siefering Kevin , Berger Henry , Whitlock Walter
Published: July 2002

Atmospheric pressure ionization mass spectroscopy (APIMS) has been demonstrated as a quantitative analysis technique for gas phase contaminants such as moisture, oxygen, carbon dioxide, and methane in ultrapure bulk gases (nitrogen, argon, and hydrogen). Previous analytical techniques typically provide quantitative analysis above 10–100 ppb detection limits. The APIMS technique offers true sub-parts-per-billion capability, with detection limits as low as 1 ppt. The APIMS works by ionizing a small fraction of the matrix gas in a corona discharge plasma. These matrix gas ions then transfer their ionic charge to impurities through collisions. This transfer of charge is usually thermodynamically favored due to the relatively high ionization potential of the matrix gas. The result is preferential ionization of the impurities, leading to a sensitivity increase of as much as 106 over standard electron impact mass spectrometers. The APIMS is calibrated by mixing parts-per-billion and parts-per-trillion level standard gases using a double-dilution blending system of unique design. Nonlinear calibration relationships based on models of plasma reaction kinetics have been developed to successfully account for nonlinearities seen in the calibration data. A selective moisture removal technique has been developed to eliminate interference effects due to unwanted moisture in the plasma reactor during analysis for other contaminants.
Molecular dynamics modeling of microstructure and stresses in sputter-deposited thin films
Authors: Fang C. C. , Prasad V. , Jones F.
Published: July 2002

A molecular dynamics model to study thin film formation processes is presented. In this model, the Lennard-Jones and Moliére potential functions are used to describe the interaction between the atoms, and ions and atoms, respectively, and determine the atomic forces among the atoms. A third-order, nonuniform time step integration scheme is used to determine the new position and velocity of each particle within the film and the substrate. The use of nonuniform time interval results in significant savings in CPU time. The force evaluation is performed in a short cutoff range which requires that the list of neighbor particles be known. A ‘‘position index method’’ which is much more efficient than the previous schemes, is proposed and implemented in order to sort the table of atoms locally. The frictional force originating from the heat bath is represented by using the generalized Langevin equation such that the temperature of the substrate can be kept constant during the film growth. A moveable periodic boundary condition which can allow the film to expand or contract depending on the internal stresses within the film and the substrate, is used in the present molecular dynamics (MD) model. A local stress parameter is defined in order to examine the effects of defects, voids, and gas impurity on local stress distribution. The intrinsic stresses are then calculated from the film structure. This improved MD model can predict the effect of adatom energy, ion bombardment, and gas impurity on microstructure and intrinsic stresses in thin films very successfully.
Accuracy and applicability of the spinning rotor gauge to calibration in gas mixture environments
Authors: Hinkle L. D. , Jacobs R. P.
Published: July 2002

The spinning rotor gas friction gauge (SRG) is being used increasingly for measurement and calibration of gauges in cases where a single gas type environment is not achievable. This may involve gas mixtures composed of molecules with differing mass values and surface interaction characteristics. Since the molecular momentum transfer and hence rate of rotational decay of the spinning rotor depend on molecular mass and accommodation coefficient, the calibration of the pressure indication must reflect the gas mixture ratio. The theoretical expression for the calibration factor is derived and compared with experimental data in the molecular flow regime. Agreement to within 5% is typical with inert gas mixtures. The proper application of this method allows the SRG to accurately calibrate other high vacuum gauges for use with known gas mixtures encountered in processing conditions.
Analysis of outgassing after Joint European Torus discharges under beryllium first wall conditions
Authors: Philipps V. , Ehrenberg J.
Published: July 2002

During the beryllium phase of the Joint European Torus, hydrogen (H2, HD, D2), hydrocarbon (CxHy), and CO outgassing after discharges has been analyzed by means of mass spectroscopy and penning gauges. This analysis has been performed over a period of up to 3000 s after ohmic and additionally heated limiter and X-point discharges. It has been found that the temporal behavior of hydrogenic outgassing could be characterized by a uniform power law t−n with n=0.73±0.1 which is independent of plasma operation parameters. This result is discussed in view of possible transport and retention mechanism in and deuterium release mechanism from plasma facing surfaces. It is concluded that not a single, but several mechanism can explain the release behavior. However, one of the most plausible explanations is accumulation of hydrogen in limiters and walls. Outgassing of hydrocarbon and CO were significantly lower under beryllium wall conditions as compared to the earlier all carbon wall conditions.
Adhesion between polycarbonate substrate and SiO2 film formed from silane and nitrous oxide by plasma-enhanced chemical vapor deposition
Authors: Shinoda Makoto , Nishide Toshikazu , Shichi Yushi
Published: July 2002

We studied the adhesion of a silicon dioxide film, formed from silane and nitrous oxide gas by plasma-enhanced chemical vapor deposition, to a polycarbonate substrate and the bonding between the film and the substrate by x-ray photoelectron spectroscopy. Adhesion of the SiO2 film to the polycarbonate increases with increasing SiH4 flow rate. All SiO2 film squares produced at a SiH4 flow rate of 10 standard cm3 min−1 exhibited good adhesion to the substrate. An analysis of the x-ray photoelectron spectra indicates that more phenyl groups are decomposed as adhesion increases. The bonding between the SiO2 film and the polycarbonate substrate is discussed on the basis of x-ray photoelectron spectroscopy. The Si–O–C bonding between SiO2 and polycarbonate results in good adhesion between the film and the substrate.