bluuebunny/crossref_metadata_2025 huggingface dataset:
An XPS study of sputtered a-Si,Ge alloys
Authors: Lucovsky G. , Chao S. S. , Tyler J. E. , De Maggio G.
Published: July 2002

The x-ray photoelectron (XPS) spectra of amorphous silicon–germanium alloys (a-Si,Ge) were measured using Al Kα excitation. Homogeneity of the alloy samples was established through measurements of the optical absorption edge, the Raman scattering, and the electron energy loss (EELS) spectra. We have studied in detail the binding energies (BE) of the Si 2 p and Ge 3d core states, and in particular any systematic shifts in their difference BE (Si 2 p)-BE (Ge 3d) as a function of alloy composition. We place an upper limit of 0.4 eV as the chemical shift to larger binding energies of the Si 2 p state between a-Si and an alloy with 80 at.% Ge. This in turn yields a maximum difference in Pauling electronegativities (X) of about 0.1. Our results therefore support the values of X obtained via thermochemical methods, and have added insight into the way other atoms will bond in amorphous alloys containing both Si and Ge.
RHEED streaks and instrument response
Authors: Van Hove J. M. , Pukite P. , Cohen P. I. , Lent C. S.
Published: July 2002

Reflection high-energy electron diffraction (RHEED) is so sensitive to surface morphology that it is difficult to separate the roles of instrument response and surface imperfection in the diffraction. To address this difficulty we have used MBE grown GaAs(001) as a test surface to study the angular dimensions of the diffracted beams. This is important if RHEED is to be useful as a quantitive probe of surface structure for in situ studies of crystal growth. The limitations placed by instrumental uncertainties on the maximum resolvable distance are estimated. Measurements of the angular length of the RHEED steaks versus angle of incidence are presented which show the changes expected from a combination of a uniformly broadened reciprocal lattice rod and an instrument limit due to angular uncertainties. Resolvable distances are obtained with RHEED that are much larger than those typically obtained with low-energy electron diffraction.
Chemically assisted ion beam etching of GaAs, Ti, and Mo
Authors: Chinn J. D. , Fernandez A. , Adesida I. , Wolf E. D.
Published: July 2002

Investigations with argon and chlorinated ion beams have shown that the etch rates of GaAs and Ti are strongly dependent on the flux of molecular and atomic chlorine. Etch rate increases were observed with an increase in the chlorine flux from the background gas composition or from the impinging ion beam. At increased chlorine concentrations in the presence of ion bombardment, wall profiles were altered from an overcut slope to nearly vertical. In the etching of Mo, etch rates were limited by the removal of low volatility chlorides by ion-assisted mechanisms.
Measurement of oxide growth stress on thin iron films
Authors: Abel P. B. , Heuer A. H. , Hoffman R. W.
Published: July 2002

Thin film stress arises from a constrained volume change when a film is attached to a substrate. The Pilling–Bedworth theory for anionic-diffusion-controlled oxidation predicts large compressive oxide stresses. To our knowledge, we report the first dynamic stress measurement of an evaporated 50 to 150 nm Fe film oxidized at 250 to 350 °C under 6.6×10−2 Pa (5×10−4 Torr) O2 in a LN2 trapped, oil-diffusion-pumped vacuum system. Away from interfaces, the Auger MVV transition peaks at 44 and 51 eV are characteristic of Fe3O4. Comparisons of EXAFS results with spectra of iron oxide standards indicate only the presence of Fe3O4. Type 302 stainless steel substrates are end clamped, with the free end connected to a Cahn microbalance. Stresses are calculated from substrate dimensions, film thickness, and the force exerted by the vacuum microbalance necessary to restore the substrate’s free end to its initial position. The stresses measured are large, but contrary to most oxides, are tensile. Typically, a 50 nm film oxidized at 300 °C under 6.6×10−2 Pa flowing O2 developed a maximum tensile stress of about 1010 dynes/cm2.
High-resolution electron energy loss studies of Al and Ge chemisorption on silicon
Authors: Kelly M. K. , Margaritondo G. , Anderson J. , Frankel D. J. , Lapeyre G. J.
Published: July 2002

We used high-resolution electron energy loss spectroscopy to study the formation of Al–Si interfaces. In particular, we confirmed that as-grown monolayers of Al on Si (111) 7×7 have metallic character as suggested by photoemission results on the Schottky barrier formation process. This conclusion, however, cannot be generalized to the case of reconstructed overlayers. Preliminary results on Ge overlayers revealed a metallic character of reconstructed Si–Ge 5×5.
Angle-resolved photoemission from bromine chemisorbed on Ni(100)
Authors: Dowben P. A. , Sakisaka Y. , Rhodin T. N.
Published: July 2002

Molecular bromine dissociatively adsorbs to form a chemisorbed overlayer on Ni(100). The bromine induced 4px, y and 4pz orbitals have been identified at Γ̄ with binding energies of 6.6±0.2 eV and 5.3±0.2 eV below the Fermi energy, respectively. The relative ionization cross-section variations of the two bromine levels 4px and 4pz are observed to differ with changing photon energy.
Sensitivity of hot cathode ionization gages
Authors: Tilford Charles R.
Published: July 2002

As a part of an experimental program of some years duration we have attempted to characterize several hot cathode ionization gage types in the high vacuum range by the uniformity, linearity, and stability of their nitrogen sensitivity. Results for six commonly used types are summarized here. Of the gages tested, the most promising overall performance was obtained from tubulated Bayard–Alpert gages with two tungsten filaments mounted 180° apart about the grid. Conventional triode gages with tungsten filaments came close to this level of performance and have superior high pressure linearity.
An investigation on surface conditions for Si molecular beam epitaxial (MBE) growth
Authors: Xie Y. H. , Wang K. L. , Kao Y. C.
Published: July 2002

Contamination-free Si substrate surfaces are vitally important to the subsequent molecular beam epitaxy (MBE) growth. A number of studies on the major contaminants, i.e., C and O, on Si surfaces have been done, and new cleaning procedures for reducing these contaminants have been proposed. However, to date the detailed behavior of C and O on Si surfaces and their effects on the subsequent Si MBE growths are not well established. In the present work, the behavior of the contaminants on heated Si surfaces were investigated using Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS). Also, the chemical bonding of C on Si surfaces was studied by analyzing the AES line shapes of carbon and comparing them with those known carbon chemical states. For the chemical cleaning procedures used in this study, a silicon oxide protection layer is grown onto the cleaned Si surface prior to sample loading. The property of this silicon oxide layer is studied using ellipsometry measurements. To verify the effectiveness of the cleaning procedures, MBE films grown subsequently are analyzed using Rutherford backscattering (RBS). The resulting channeling yield of 2.5% suggests the nearly perfect crystalline quality of these films. Deep-level transient spectroscopy (DLTS) is used to study the defect in the epilayer, and the defect concentration is determined to be in the 1013 cm−3 range.
Microfabricated TEM sections of amorphous superlattices
Authors: Deckman H. W. , Dunsmuir J. H. , Abeles B.
Published: July 2002

Internal structure of layered amorphous superlattices was studied using transmission electron microscopy. Amorphous semiconductor superlattices were sectioned for TEM study using a microfabrication technique which yields sections with accurately controlled dimensions suitable for quantitative image analysis. Sections were created with a lithographic technique that utilizes a submonolayer of colloidal particles as an etching mask. Images of the sections reveal that amorphous superlattices can be grown without cumulative roughening. The absence of cumulative roughening allows layers to be grown flat and smooth to within 5 Å on a ∼100 Å lateral scale even after deposition of hundreds of layers. In highly localized regions unusual defect structures appear as bifurcations of individual layers.
Formation of submicron oxide widths on aluminum in the presence of keV electron beams and CO2 or N2O
Authors: Pitts J. R. , Massopust T. P. , Czanderna A. W. , Kazmerski L. L.
Published: July 2002

The purpose of this paper is to report the size of oxide islands or line w idths that can be grown as smaller diameter electron beams are used for the ox idation. The implications of the potential lateral resolution available b etween oxide  lines  for electronic materials are discussed. The localize d oxide growth on thin Al films occurs in the presence of .5 to 10 keV ele ctron beams and high vacuum level pressures of carbon dioxide or nitrous o xide. Auger electron spectroscopy shows saturation of the O (KLL) signal and depletion of the Al (LMM) signal after an exposure of 5000 l carbon di oxide with a 2 keV beam of 7 A/sq. cm. The oxide is spatially restricted to the beam impact region and is stable for long periods of time in vacuum . The most plausible mechanism for this oxide growth is dissociation of t he carbon dioxide or the nitrous oxide by the electron beam in the region of impingement on or near the surface. Oxygen atoms thus formed can then react with the Al, and carbon monoxide or nitrogen desorbs. (AIP)
Synchrotron-radiation excited carbon 1s photoemission study of Cr/organic polymer interfaces
Authors: Jordan J. L. , Sanda P. N. , Morar J. F. , Kovac C. A. , Himpsel F. J. , Pollak R. A.
Published: July 2002

The chemical nature of the adhesive interface between Cr and three polymers has been investigated using synchrotron-radiation excited photoemission from the polymer carbon 1s core levels. PMDA–ODA polyimide (PI), an important polymer in the electronics industry, was studied along with two simple polymers, PMPO and PVMK, which contain structural fragments of the complicated PI protomer. The carbon 1s spectra were decomposed into distinct peaks which were assigned to specific chemical species and monitored as a function of Cr coverage. Our results confirm that the carbonyl groups on these polymer surfaces are targets for reactive metals during the initial stages of metallization. This initial reaction stage was followed by gradual formation of an electron-rich, carbidelike, carbon species.
Observation of significant nitrogen–oxygen bond weakening in nitric oxide on Rh(100)
Authors: Villarrubia J. S. , Richter Lee J. , Gurney Bruce A. , Ho W.
Published: July 2002

An EELS spectrometer equipped with a multichannel detector for fast data acquisition was employed to study the interaction of nitric oxide with the Rh(100) surface at temperatures between 90 and 300 K. We resolve vibrational modes at 920 and 1600 cm−1, which we assign to two different NO adsorption states, α1NO and α2NO, respectively. We attribute the low vibrational frequency of α1NO to a much-weakened N–O molecular bond resulting from highly inclined or side-on bonding of the NO. At 90 K and exposures below 0.13 L only α1NO is observed. At higher exposures the α2NO state begins to populate. The α1NO coverage passes through a maximum at 0.25 L whereas the α2NO coverage increases monotonically to saturation. Exposing the NO predosed surface to oxygen results in conversion of α1NO to α2NO. Successive EELS spectra taken during a linear temperature ramp indicate that the two species become unstable, undergoing interconversion and decomposition. The maximum rate of decrease in α1NO coverage is at 163 K, while that for α2NO is at 199 K.
Testing site size requirements in chemisorption: Experiment and theory
Authors: Campbell Charles T. , Paffett Mark T. , Voter Arthur F.
Published: July 2002

‘‘Ensemble requirements’’ in surface chemistry refer to the number and shape of contiguous empty sites necessary for chemisorption of a particular molecule. Ensemble effects can play a major role in directing the course of surface reactions, leading, for example, to dramatic changes in catalytic selectivity when the active metal component is diluted upon alloying with an inert metal. We will review here fundamental surface science studies that have attempted to isolate and probe this site size requirement by diluting active sites on a single crystal surface with an inert metal overlayer. In these studies, initial sticking probability and saturation coverage are measured as a function of diluent metal coverage. We will emphasize recent results in our lab on the interaction of simple molecules (CO, H2, O2) with Cu-, Ag-, and Bi-dosed Pt(111). These show that the group IB metals (Cu, Ag, Au) are poor diluents since they cluster into two-dimensional islands. We find that adsorbed Bi spreads out uniformly over the surface and effectively tests ensemble sizes. Theoretical models based upon Monte Carlo simulations will be summarized which, when compared to such data, allow more accurate determination of ensemble sizes. These models predict uptake curves that deviate strongly from the commonly used (1−θ)A law (A=number of sites in ensemble), which is valid only for an array of isolated ensembles.
An x-ray photoelectron spectroscopy study on ozone treated GaAs surfaces
Authors: Ingrey S. , Lau W. M. , McIntyre N. S.
Published: July 2002

The formation of sacrificial oxides on GaAs is a well established procedure in the preparation of in situ cleaned substrates for epitaxial growth. Oxides formed from ozone exposure promise a more controlled alternative to thermal and air formed oxides. This paper reports on a study of the composition and structures of oxides on GaAs(100) surfaces and, in addition, examines the persistence of carbon-containing species at the oxide surface and oxide–semiconductor interface. X-ray photoelectron spectroscopy (XPS) was used in a high spectral resolution mode, to distinguish between the different oxide phases formed. Surfaces oxidized under ozone for 10–30 min had Ga/As oxide ratios close to unity. By contrast, acid treated, DI water rinsed surfaces exposed to air produce oxides rich in either Ga or As depending on conditions. Heating of the ozone-produced oxide to ≥600 °C results in sequential decomposition of As2O5, As2O3, and Ga2O3. Oxide layers produced by short-term ozone exposures were analyzed by angle-resolved XPS. A 10 min exposure produced an oxide of 2.0 nm thickness. Surface carbon-containing contaminants were removed to a level ≤0.2 monolayer by heating the specimen to 400 °C in vacuum (base pressures of 2×10−8 Pa). Surfaces heated to ≥600 °C to desorb all oxides, also showed no evidence of carbon-containing species and the As/Ga ratio was unity.
TiC as a diffusion barrier between Al and CoSi2
Authors: Appelbaum A. , Murarka S. P.
Published: July 2002

The stability of the reactively sputtered titanium carbide film sandwiched between cobalt disilicide and aluminum has been studied using Rutherford backscattering, Auger electron spectroscopy, x-ray diffraction, Nomarski optical and transmission electron microscopy. The titanium carbide film is an excellent diffusion barrier through heat treatments at temperatures up to 450 °C for an anneal time of 30 min. The barrier breaks down at 500 °C. Co2Al9 and Ti7Al5Si12 compounds were formed when annealed at 550 °C.
X-ray photoelectron spectroscopy observations of argon-ion bombardment effects on phase separated structures such as SiNx alloys or Si/Si3N4 interfaces
Authors: Kubler L. , Haug R. , Hlil E. K. , Bolmont D. , Gewinner G.
Published: July 2002

SiNx layers have been prepared in an ultrahigh vacuum (UHV) system by reactive electron beam evaporation of Si under NH3 ambient. In situ x-ray photoelectron spectroscopy (XPS) measurements show that films formed in this way consist essentially of a Si/Si3N4 mixture on a microscopic scale with the Si and Si3N4 clusters embedded in a subnitride compound. Upon mild Ar-ion bombardment (ion energy ∼1 keV), the analysis of the Si 2p core level lines for various stoichiometries reveals enhanced subnitride local atomic arrangements. This demonstrates the growth, by atomic redistribution, of the intermediate domains connecting the Si-rich and nitride domains. With increasing ion energy it is found that the phase separation progressively disappears and finally, within the probed depth, the whole film may be thought of as an homogeneous random bonding alloy. A similar behavior observed on a planar Si/Si3N4 interface, probably explains, for the main part, the diffuse nature of the interface observed in sputter profiling experiments. Evidence for preferential nitrogen sputtering as well as modifications of the XPS valence band upon ion bombardment is presented: Thus ion etching of initially heterogeneous structures results in drastic changes in film composition and structure. The present finding may account for a more homogeneous growth in plasma deposited films where ion bombardment is used during the growth.
X-ray photoelectron spectroscopy analysis of hexafluorodianhydride–oxydianiline polyimide: Substantiation for substituent effects on aromatic carbon 1s binding energies
Authors: Buchwalter L. P. , Silverman B. D. , Witt L. , Rossi A. R.
Published: July 2002

Hexafluorodianhydride–oxydianiline (HFDA–ODA) polyimide was characterized using x-ray photoelectron spectroscopy (XPS). Elemental composition and polymer stoichiometry as elucidated with XPS are reported. The HFDA–ODA polymer cured at 360 °C was fully imidized giving rise to a single nitrogen 1s peak at 400.8 eV. The carbon 1s data showed an unexpectedly large peak at 285.9 eV. This peak could not be justified only by C–N and C–O electrons, but required the carbon 1s electrons from the HFDA aromatic rings to be included. The small peak at 287.5 eV was assigned to the tertiary carbon 1s electrons of the –C(CF3)2 group. The carbon 1s data interpretation is substantiated by molecular orbital calculations.
Structure and diffracted intensity in a model for irreversible island-forming chemisorption with domain boundaries
Authors: Evans J. W. , Nord R. S.
Published: July 2002

Despite the awareness that island-forming chemisorption is often kinetically limited and intrinsically nonequilibrium, there is little sophisticated analysis of the corresponding island structure or diffracted intensity. Here we analyze a model where species irreversibly and immobilely chemisorb (commensurately) from a precursor source, with distinct rates for island nucleation (chemisorption in an empty region) and growth (chemisorption at island perimeters), the latter rates being larger. Specifically, we consider the formation of one-dimensional double-spaced islands, and two-dimensional checkerboard C(2×2) islands on a square lattice. In both cases (permanent) domain boundaries form between out-of-phase islands. We analyze scaling of the saturation coverage, a characteristic linear island dimension, spatial correlations, etc., with the ratio of growth to nucleation rates. The structure of individual islands, and of the saturation domain boundary ‘‘network’’ are elucidated. The corresponding diffracted intensity exhibits significant interference at superlattice beams, and diminution at integral order beams as saturation is approached.
A comparison of the interaction of CO, N2, and OC with Cu(100)
Authors: Müller W. , Bagus P. S.
Published: July 2002

The cluster model approach has been used to compare the chemisorption properties, including vibrational frequencies (ωe) and dynamic dipole moments (e*), of C- and O-bonded CO and N2 at an on-top site of Cu(100). The binding energies and metal–adsorbate stretch ωe and e* are found to decrease in the order CO>N2>OC, with N2 expected to be bound and OC almost unbound; the metal–adsorbate bond distances and intra-adsorbate stretch e* (including the sign) are in the opposite order, OC>N2>CO. While the C–O stretch frequency for C-bonded CO is strongly reduced with respect to the free molecule, the frequencies for N2 and O-bonded CO are found to be only slightly changed. The sequence in the strength of the interaction is explained by the different amount of metal-to-adsorbate charge transfer which is identified as the driving force behind the chemisorption process. Our theoretical results are compared with experimental data and the differences in the bonding of CO and N2 to transition metals are addressed.
Time-of-flight and surface residence time measurements for ion-enhanced Si–Cl2 reaction products
Authors: Rossen Rebecca A. , Sawin Herbert H.
Published: July 2002

The kinetics of ion-enhanced etching reactions have been investigated by exposing a silicon sample to coincident beams of Cl2 and modulated Ar+ ions. The distribution and time-dependent emission of neutral products were measured by mass spectrometry for a range of Ar+ bombardment energies (125–800 eV) and Cl2/Ar+ flux ratios (20–200). SiCl2 and SiCl4 are the primary etching products, and the emission of SiCl4 is enhanced relative to SiCl2 as the bombardment energy is reduced. A model for SiCl2 production incorporating a surface residence time and an emission temperature greater than that of the sample is consistent with data acquired for three flight path lengths. SiCl4 is emitted with a thermally equilibrated velocity after a residence time that varies from 180 to 300 μs over the range of ion energies and flux ratios. These results suggest that physical sputtering plays only a minor role in the kinetics of ion-enhanced etching.
Quantitative x-ray photoelectron spectroscopy analysis of inorganic mixtures
Authors: Paparazzo E.
Published: July 2002

The accuracy of quantitative results obtained by the first-principles model (FPM) was tested on the basis of simple peak intensity measurements in x-ray photoelectron spectroscopy analysis. We analyzed mixtures of inorganic compounds having a common ion giving rise to two (or more) peaks shifted in binding energy above instrumental resolution. The intensity ratio between the components of the common ion was compared with the FPM atomic ratio in pairs of peaks belonging to the different compounds of the mixture, and then corrected for the appropriate stoichiometric coefficients. Quantitative results were typically affected by relative errors of ±10%. An advantage of this approach is that intensity ratio measurements require no knowledge of photoionization cross sections, nor of factors depending on photoelectron kinetic energy. A possible use of the present approach is suggested for the analysis of mixtures of two given compounds considered in a wide range of compositions. Advantages and disadvantages of the approach with respect to the local standards method are discussed.
X-ray photoelectron spectroscopy peak shape analysis for the extraction of in-depth composition information
Authors: Tougaard Sven
Published: July 2002

The idea of extracting in-depth composition information through an analysis of the x-ray photoelectron spectroscopy (XPS) peak shape was investigated. The peak shape is described by a single parameter D which is the ratio of the peak area to the increase in background signal 30 eV below the peak energy. Based on a first-order calculation, analytic expressions for D were found for homogeneous solids as well as for the situation where impurities are located at a certain depth underneath the solid surface. An evaluation which includes all inelastic scattering events showed that for an exact treatment, higher-order terms are important. Therefore an empirical formula based on a calculation to infinite order gives a more general description. In experimental XPS one has to deal with the problem of background subtraction. This problem was discussed and an approximate description was suggested.
Precision calibration of a static pressure divider by means of a spinning rotor gauge
Authors: Berman A. , Fremerey J. K.
Published: July 2002

A method for precision calibration of a static pressure divider and for characterization of its ultrahigh vacuum (UHV) capabilities is described. The method makes use of the gas expansion technique with a spinning rotor gauge (SRG) for pressure determination. Relative volume calibration is performed with uncertainties below 0.02% at pressures in the upper high vacuum range. The method simultaneously allows for a linearizing correction of the SRG characteristic. In the UHV section of the apparatus, an outgassing rate of 30 pPa m/s, nitrogen equivalent, is determined under seal-off conditions. This figure is derived from an observed 4 μPa/h pressure increase as measured by the SRG. A UHV base pressure of about 1 nPa is derived from the same SRG record by taking into account the estimated pumping speed at the exhaust valve.
Secondary ion yield changes in Si and GaAs due to topography changes during O+2 or Cs+ ion bombardment
Authors: Stevie F. A. , Kahora P. M. , Simons D. S. , Chi P.
Published: July 2002

Changes in secondary ion yields of matrix and dopant species have been correlated with changes in surface topography during O+2 bombardment of Si and GaAs. In Si, profiles were measured in (100) wafers at 6- and 8-keV impact energy. At 6 keV, a yield increase of about 70% occurred for Si+ over a depth range of 2.5 to 3.5 μm, with changes in other species ranging from a decrease of ∼20% for Si+3 to an increase of more than 25% for O+. The development of a rippled surface topography was observed in scanning electron micrographs over the same depth range. Similar effects occurred over a 3–5 μm depth range for 8-keV ions, and in (111) silicon at a depth of 3 to 4 μm for 6-keV ions. No differences were noted between p- and n-type silicon, or implanted and unimplanted silicon. In GaAs, profiles were measured in (100) wafers at 2.5-, 5.5-, and 8-keV impact energies. At 8 keV, a yield increase of about 70% was found for GaO+ in the range 0.6–1.0 μm, with smaller changes for other matrix species. At 5.5 keV, similar effects were observed, but over a depth interval of 0.3 to 0.7 μm. No yield changes were detected at 2.5-keV impact energy. The yield changes at the higher energies were again correlated with the onset of changes in topography. No change in ion yield or surface topography was noted for Cs+ bombardment of Si or GaAs. The topography and ion yield changes are affected by the angle of incidence and, for Si, the oxygen coverage. The results show that the practice of normalizing secondary ion mass spectrometry dopant profiles to a matrix signal must be modified for situations where matrix yield changes occur.
Scanning tunneling microscopy study of conductive ceramics
Authors: Nishikawa Osamu , Tomitori Masahiko
Published: July 2002

The surfaces of composite ceramics of Si3N4 and TiN were studied with the ultrahigh vacuum scanning tunneling microscope (STM). The boundaries of the conductive TiN and nonconductive Si3N4 grains were not shown clearly in the STM images and the maps of dI/ds, the variation of the tunneling current due to the modulated tunneling gap, because the entire ceramic surface was covered by an adsorbed conductive layer. Since irradiation by a pulsed YAG laser beam desorbed most of the adlayer, the surface areas of nonconductive grains were recognized as the highly resistive areas in the dI/ds map. After laser irradiation, surface conduction was resumed in several hours with the recovery of the adlayer and the grain boundaries became unrecognizable again in a few tens of hours even in a vacuum of 10−10 Torr.
Detection of vortex motion in a type-II superconductor by tunneling
Authors: Dittrich R. , Heiden C.
Published: July 2002

A tunnel junction with adjustable barrier width was made using superconducting electrodes, one of which was a niobium tip and the other a niobium sheet. Quasiparticle tunneling characteristics with normal-state resistances of the order of 102 to 105 Ω could be obtained. When operating in the constant current mode a clearly visible modulation of the gap voltage was observed associated with the passing of vortices across the junction. While these observations indicate the possibility of scanning tunneling microscopy of vortices at rest, we report here on local investigations of vortex motion using constant tip position. The gap modulation signal was analyzed using its autocorrelation function.
The effect of excitation frequency on emission spectra of polymerization plasmas
Authors: Lopata E. , Countrywood J.
Published: July 2002

The excitation frequency is known to have an effect on the rate of deposition of polymer thin films produced by a glow discharge process. This has been attributed to changes in the electron energy distribution function (EEDF). Any appreciable change in the EEDF would be expected to be revealed by plasma emission spectroscopy. Our findings are herein reported for deposition plasmas containing C3 H6 and Ar at dc, 40-kHz, and 27.12-MHz excitation frequencies; included are data on film deposition rate and optical clarity which also vary with excitation frequency.
Chemically enhanced ion etching on refractory metal silicides
Authors: O’Brien W. L. , Rhodin T. N. , Rathbun L. C.
Published: July 2002

Mechanisms of chemically enhanced ion etching on TiSi2 and MoSi2 were studied using high-resolution (250 ns) direct time-of-flight (TOF) spectroscopy and steady-state surface techniques (Auger electron spectroscopy and x-ray photoelectron spectroscopy). Argon ion pulses (4 μs, 0.2 mA/cm2) were used in combination with a high-pressure chlorine gas doser (1×10−5 Torr) to study ion etchant product distributions. Ion product TOF distributions were interpreted in terms of the collisional cascade model with corrections for ionization probability. Surface chemical and compositional changes were measured after etching the silicide surfaces. Differences in TOF distributions of the same species from different substrates (e.g., Si from Si, TiSi2 and MoSi2) are discussed on the basis of these chemical and compositional changes. Etching mechanisms for the silicides are discussed by comparison to the etching of the elemental surfaces.
Prospects for a contamination-free ultravacuum facility in low-Earth orbit
Authors: Naumann Robert J.
Published: July 2002

The old concept of using the wake of a spacecraft to obtain an ultrahigh vacuum is revisited. A wakeshield can be configured so that a surface of interest does not subtend any walls that could become contaminated, thus it should be possible to achieve a contamination-free, ultrahigh vacuum capability with infinite pumping speed even in the presence of high heat loads and moderate gas loads. This paper analyzes the conceptual design for a Space Ultravacuum Research Facility (SURF), both in a shuttle-attached mode and as a free flyer. It is shown that even in the shuttle-attached mode, it should be possible to obtain vacuum levels equivalent to 10−9 to 10−10 Torr with O and N2 as the primary constituents. As a free flyer the SURF will be limited primarily by the gas load from the process being performed. For chemical beam epitaxy it is shown that equivalent vacuum levels of 10−14 Torr should be possible at 300 km.
The role of adsorbed gases in metal on metal epitaxy
Authors: Egelhoff W. F. , Steigerwald D. A.
Published: July 2002

It is found that a variety of (deliberately) adsorbed gases influence in some manner the epitaxial growth of metals on metals. The adsorbed gases investigated included molecularly adsorbed CO and H2O and dissociatively adsorbed H, O, N, C, and S. The effects of adsorbed gases have been investigated in metal-on-metal epitaxial systems including Cu/Cu(100), Ni/Ni(100), Cu/Ni(100), Ni/Cu(100), Fe/Cu(100), Cu/Fe(100), Fe/Ag(100), and Fe/Fe(100). Although not all of the possible gas–metal combinations among these systems have been studied, enough have been to make the following generalizations. Around room temperature and above, these gases exhibit a strong tendency to ‘‘float out’’ to the growing surface, hardly reducing the extent of epitaxial ordering. All but the most strongly bound, e.g., C or N, have a strong tendency to float out even during epitaxy at temperatures as low as 100 K. The most strongly bound, e.g., C, N, or O, tend to suppress the agglomeration or interdiffusion (which otherwise occurs) when a monolayer of a transition metal is deposited on a noble metal at room temperature or above, e.g., Fe/Cu(100) or Ni/Cu(100). In some cases, these effects may be useful as a new tool for gaining improved control over epitaxial growth.
Static secondary ion mass spectroscopy study of ethylene and acetylene on Ni: Coverage dependence
Authors: Zhu X.-Y. , Castro M. E. , Akhter S. , White J. M. , Houston J. E.
Published: July 2002

The coverage dependence of the reaction pathways followed by ethylene and acetylene on Ni(111) and Ni(100) has been studied by temperature programmed static secondary ion mass spectroscopy and temperature programmed desorption. The coverage dependences are interpreted in terms of repulsive interactions between adsorbates and site blocking effects on decomposition channels.
Reaction kinetics of surface silicon hydrides
Authors: Greenlief C. Michael , Gates Stephen M. , Holbert Philip A.
Published: July 2002

The chemical vapor deposition (CVD) of Si from SiH4 is a complex process involving both gas phase and surface reactions. During CVD, surface silicon hydrides [SiHx(a)] are formed and ultimately decompose to form a Si film. We have investigated the surface decomposition kinetics of SiH3(a), SiH2(a), and SiH(a), formed following the adsorption of atomic hydrogen on Si(111)-(7×7) and Si(100)-(2×1), using static secondary ion mass spectrometry (SSIMS) and temperature programmed desorption (TPD). The kinetic parameters for the decomposition of SiH3(a) are reported for the first time. The decomposition kinetics of SiH2(a) and SiH(a) as measured by SSIMS agree well with previous TPD and isothermal desorption measurements. Correlations between the surface decomposition of SiHx species and the activation energies measured for Si film growth, as well as comparisons with silicon hydride species formed from the adsorption of silanes will be discussed.
The adsorption of Cs and O2 on a clean GaAs(110) surface under light illumination
Authors: Guo Tailiang
Published: July 2002

The activation process of the GaAs negative electron affinity (NEA) photocathode is now well known. Activation supplemented by light illumination has not yet been reported. In this work we have investigated the effects of while light illumination on the adsorption and desorption of Cs and O2 on the GaAs(110) surface during various stages of activation of the GaAs NEA photocathode. Its effects on photoemission sensitivity, emission stability, and recovery of impaired photoemission have also been observed. It is suggested that the incident photons would produce electron–hole pairs on the GaAs surface. Phonons would be produced in recombination of the electrons and holes thus produced. Based upon the above supposition, all the investigated effects are explained and discussed.
Molecular dynamics simulations of metal adsorbates on metal surfaces: Rh on Ag(100)
Authors: Raeker Todd J. , Sanders David E. , DePristo Andrew E.
Published: July 2002

The mechanisms of thin-metal film growth on metal surfaces are topics of considerable scientific and technological interest. We have studied the way in which a deposited metal adlayer behaves as a function of metal substrate temperature. Molecular dynamics simulations are used where the interaction energy and the corresponding forces are generated from the recently developed corrected effective medium (CEM) method. In previous work, the CEM method has been shown to predict the geometric and energetic properties of clean and adsorbate-covered surfaces accurately. In this paper, we present results for the Rh on Ag(100) surface system. We show that Rh atoms penetrate the Ag(100) surface by exchanging with Ag atoms in the surface layer structure. Furthermore, this phenomena is found to depend on both the initial coverage of Rh atoms and the Ag substrate temperature. As the substrate temperature increases, the number of Rh atoms exchanging with Ag atoms in the surface increases. Energetic and dynamical aspects of this system are used to understand the reasons why and also the way in which the exchange takes place.
Charged particle densities and energy distributions in a multipolar electron cyclotron resonant plasma etching source
Authors: Hopwood J. , Reinhard D. K. , Asmussen J.
Published: July 2002

The discharge downstream from a 9 cm i.d. multipolar electron cyclotron resonant (ECR) plasma was characterized in terms of positive ion density, electron energy distribution functions (EEDFs), ion energies incident on a biased conducting substrate, and silicon etching performance. Ion density in the process chamber was found to decrease exponentially away from the plasma source with a peak density of 6×1011 cm−3 in 1 mTorr argon with 250 W of microwave power. While EEDFs are non-Maxwellian, no evidence of high-energy ECR electrons was observed in the process chamber. The ion flux to a conducting substrate diverges by approximately 15° from normal but the ion incidence energy is easily controllable with a dc bias to the substrate. Despite the divergent ion flux from the ECR source, anisotropic etching of silicon was possible with etch rates of 300 nm/min. Uniform etching of silicon over 7.6 cm (3 in.) diam wafers was attained 8 cm below the plasma source.
Scanning tunneling microscopy study of diffusion, growth, and coarsening of Si on Si(001)
Authors: Mo Y.-W. , Kariotis R. , Swartzentruber B. S. , Webb M. B. , Lagally M. G.
Published: July 2002

The growth, diffusion, and coarsening of Si on Si(001) have been investigated with scanning tunneling microscopy (STM). A diffusion coefficient for Si has been determined. Anisotropy in the island shapes during epitaxy is shown to be principally a growth structure due to an anisotropic accommodation coefficient. Diffusional anisotropy is small. An ordered ‘‘diluted-dimer’’ structure is observed at low coverages and temperatures.
X-ray photoelectron spectroscopy studies of contamination and cleaning of surfaces exposed to a fluorocarbon plasma
Authors: Ermolieff A. , Marthon S. , Bertin F. , Pierre F. , Daviet J. F. , Peccoud L.
Published: July 2002

A reactive ion etching (RIE) with a mixture of fluorocarbon gases is used to etch selectively SiO2 patterns on Si wafer. X-ray photoelectron spectroscopy (XPS) measurements were carried out to determine the walls contamination of the reactor with different cathode coatings like Al, Si, and photoresist. The contamination of different wafers (SiO2, Si with and without resist) following RIE processes was also studied in order to understand the mechanism of the selective etching. Then different cleaning processes, in particularly NF3 and O2 plasmas followed or not by a hydrophilic treatment, were also qualified by XPS. For every sample, the Al(2p) or Si(2p), C(1s), O(1s), and F(1s) photoelectron lines were monitored and concentrations determined as usual by integration of these lines. F(1s) and C(1s) were decomposed into several components corresponding to different chemical bonds. The results show that the contamination of the reactor walls by fluoropolymers is due to the presence of Si or of the photoresist on the cathode; there is no polymer formation with an Al cathode coating. The polymer contamination on the reactor walls is cleaned off by an O2 plasma and the residual fluorine forms a passivative layer with the Al of the reactor walls. During the plasma etching of a SiO2 film, there is formation of C–Si, C–C, O–Si–F, F–Si bonds but it is only when there is no oxygen left that CFx bonds are formed. These polymer layers stop the etching process on silicon. They are cleaned by O2 or NF3 plasma, but the latter removes the polymers without increasing the oxygen, fluorine or nitrogen contamination and appears to be most appropriate for improving the surface quality of silicon after fluorocarbon plasma etching.
The effect of laser reflow on the variation of stress with thermal cycling in aluminum thin films
Authors: Venkatraman R. , Chen S. , Bravman J. C.
Published: July 2002

In order to understand the role of film thickness and grain size on the stress-temperature characteristics of aluminum films, we have compared the stress behavior of hot sputter-deposited aluminum films before and after they were subjected to laser reflow, a process which produces a large grained, hillock-free microstructure. For Al thin films, the σ–T curves commonly reported in the literature and those observed by us exhibit a large asymmetry between the tensile and compressive flow stresses induced on thermal cycling. The effect of the laser reflow is to reduce this asymmetry, i.e., reduce the tensile flow stress levels to values that are comparable to the flow stresses in compression. The magnitudes of stress thus obtained agree well with those predicted by considering a misfit dislocation mechanism by which we have observed stress relaxation to occur in Al films in in situ transmission electron microscopy thermal cycling experiments. The model used to calculate flow stresses takes into account the energies of the dislocations to be laid down at the film-substrate and film-native oxide interfaces. As predicted by the model, the stress in the laser reflowed films is inversely proportional to the film thickness in both tension and compression. While the difference between the stress behavior of reflowed and unreflowed films diminishes as film thickness decreases, a final stage of hardening below 200 °C is always seen for the thinner as-deposited films that is absent in their reflowed counterparts.
Radio frequency plasma etching of Si/SiO2 by Cl2/O2: Improvements resulting from the time modulation of the processing gases
Authors: McNevin S. C.
Published: July 2002

The etching of poly-Si and its selectivity over gate oxide SiO2 were measured for both Cl2 and Cl2/O2 plasmas and the experimental data indicate that modulation of the processing gases increases the Si/SiO2 selectivity while decreasing the unwanted deposition. The best results to date have been Si etch rates of ∼1000 Å/min and Si/SiO2 selectivities of ∼50. It has also been demonstrated that the Si/SiO2 selectivity is strongly dependent on the presence of photoresist. A qualitative model for these results is proposed, which appears to fit these observations. In this model, hydrocarbon fragments from sputtered photoresist reduces the SiO2 to Si, with a corresponding decrease in the Si/SiO2 etching selectivity. This carbonaceous material can be removed by the oxygen before it causes etching damage to the SiO2. Oxygen, unfortunately, can also react with the etching reaction products (e.g., SiCl4) to form SiO2 and the Si oxychlorides (Si2OCl6, Si3OCl8), which deposit on the wafer. The experimental evidence indicates, however, that this unwanted deposition can be minimized by alternating the Cl2 and O2 gases, rather than by running them both at a constant pressure, as is typically done. Of most immediate interest is the fact that this improvement by modulation can be achieved on time scales (≳1 s), which are attainable with the fairly long residence times in present generation hex etchers.
Growth of thin Ni films on the W(110) surface
Authors: Mingde Xu , Smith R. J.
Published: July 2002

The growth of thin Ni films on the W(110) surface was studied to look for any adsorbate-induced structural modifications of the W substrate which might play a role in the modified magnetic and catalytic properties of the Ni film, and because these two materials are excellent candidates for forming metallic superlattices with atomically abrupt interfaces. We performed experiments using high-energy ion backscattering and channeling together with x-ray photoemission spectroscopy and low energy electron diffraction. The initial growth of the Ni film on W(110) at room temperature appears to be in a double-layer mode, which differs significantly from the growth modes reported previously. The double layer grows in a coincidence structure on the W substrate. The film assumes the bulk Ni structure beginning at a coverage of ∼5 ML based on the observation of Ni–Ni shadowing beginning at 7.7 ML for a normally incident ion beam. We observed that the W(110) substrate structure is not altered by the adsorption of Ni atoms, which supports a model for the interface having an atomically abrupt transition at room temperature.
Tip–sample forces in scanning probe microscopy in air and vacuum
Authors: Grigg D. A. , Russell P. E. , Griffith J. E.
Published: July 2002

Forces between a probe and sample have been measured in air and vacuum using a rocking beam force balance sensor capable of simultaneously obtaining both force and tunneling topographs. Force versus distance curve measurements demonstrate the effects of capillary forces in air but not in vacuum. Simultaneous scanning tunneling microscope (STM) imaging and force measurement in air show spatial force variations consistent with repulsive force tunneling. Our results support observations of large repulsive forces in STM occurring from surface contaminants. These results show how forces can affect image formation in both STM and scanning force microscopy.
Photodesorption from copper, beryllium, and thin films
Authors: Foerster C. L. , Halama H. J. , Korn G.
Published: July 2002

Ever increasing circulating currents in electron-positron colliders and light sources demand lower and lower photodesportion (PSD) from the surfaces of their vacuum chambers and their photon absorbers. This is particularly important in compact electron storage rings and B meson factories where photon power of several kW cm−1 is deposited on the surfaces. Given the above factors we have measured PSD from 1 m long bars of (a) solid copper and solid beryllium, and (b), TiN, Au, and C thin films deposited on solid copper bars. Each sample was exposed to about 1023 photons/m with a critical energy of 500 eV at the vacuum ultraviolet ring of the National Synchrotron Light Source. PSD was recorded for two conditions: after a 200 °C bakeout and after an Ar glow discharge cleaning. In addition, we also measured reflected photons, photoelectrons, and desorption as functions of normal, 75, 100, and 125 mrad incident photons.
Influence of polymer formation on the angular dependence of reactive ion beam etching
Authors: Barklund A. M. , Blom H.-O.
Published: July 2002

The decreasing dimensions and increasing complexity of very large scale integrated circuits will also result in a more complex topography. There will be an increasing demand for simulation programs in order to accurately predict the topography during, e.g., dry etching. Such simulation programs need information about the angular dependence of the etch rate of the used etch process. The angular dependence can be measured for every specific etch process and the results can be used as input data for the simulation program. A general knowledge of the angular dependence may contribute substantially to the development of future simulation programs. The angular dependence of the etch rate has been studied for Si3N4 using a Kaufman type ion beam source and CHF3+O2 as processing gas mixture. The experimental results confirm that the formation of a polymer layer plays an important role for the angular dependence of the etch rate in the case of Si3N4. We have found that it is possible to change the angular dependence to a large extent by varying the O2 content in the gas mixture. This is the case because the oxygen will strongly affect the formation of the polymer layer on the etched surface. The angular dependence of physical sputtering with Ar and a more chemical etch with CF4 of such a polymer, deposited from CHF3, has also been studied.