Showing papers in "Surface and Interface Analysis in 1985"

On the role of Gibbsian segregation in causing preferential sputtering

Abstract: It is well known that preferential sputtering with binary alloys correlates significantly with chemical binding, but only the sense and not the magnitude of the effect can be understood in these terms. It is argued here that the correlation is, in fact, an indirect one due to bombardment-induced Gibbsian (or similar) segregation. The preferentially removed component is characterized by a composition profile consisting of a one atom-layer-thick spike at the surface; this is the depth from which most sputtered atoms originated so the spike must have near-bulk composition. There is then a severely depleted subsurface region in accordance with Gibbsian segregation equilibrium (or an equivalent effect) and a final return to bulk composition. The reason for the marked correlation with chemical binding is that segregation is governed significantly more often by binding than by the alternatives of size, surface chemistry, an interstitial flux, or long-range ordering.

The energy dependence of electron attenuation lengths

Abstract: An analysis has been made of electron attenuation length data for nine materials in terms of the Bethe theory for inelastic scattering in matter. It was found that the Bethe equation adequately described the energy dependence of the data in all materials over the typical range 100–1500 eV. The Bethe equation appears to be superior to the empirical relation proposed by Wagner, Davis, and Riggs (1980).

Calculations of electron inelastic mean free paths from experimental optical data

Abstract: Etude pour C, Mg, Al, Al 2 O 3 , Cu, Ag, Au et Bi. Les calculs necessitent la connaissance de la variation du transfert de moment avec la section efficace de diffusion differentielle que l'on obtient a partir des calculs de Penn

Energy calibration secondary standards for X-ray photoelectron spectrometers

Abstract: The selection of elementary calibration standards for X-ray photoelectron spectrometers is discussed; it is suggested that Zn and Ga may be more satisfactory routine single-element standards than Cu, Ag or Au. Binding energies for the 2p3/2 and 3d photoelectron peaks from elemental Zn, Ga (and Ge) and for the most intense LMM Auger peaks from Zn and Ga are reported and relevant energy separations compared with published Lα X-ray energies. The instrumental voltage scale was calibrated to a precision of 0.0035% (one standard deviation, SD) by reference to published data for Cu and Au and the SD of the measured peak energies were generally ⩽0.03 eV. Taking residual voltage scaling uncertainties into account, the largest energy separations in the data for Zn and Ga should be accurate to 0.05 eV (one SD).

Quantitative surface analysis by XPS: A comparison among different quantitative approaches

Abstract: Analyse quantitative de surface par spectrometrie photoelectron RX. Comparaison des rapports de surface atomique obtenus par deux methodes quantitatives de conversion de l'intensite de pic photoelectronique. Resultats rapportes pour plusieurs mineraux

Ionicity of metallic oxide surfaces on metals as observed by Auger (XPS) spectroscopy

Abstract: Etude de l'ionicite et de la force des sites de Lewis sur les surfaces d'oxydes metalliques

Electron and ion beam degradation effects in AES analysis of silicon nitride thin films

Abstract: Silicon nitride films are currently investigated by AES combined with ion profiling techniques for their stoichiometry and oxygen content. During this analysis, ion beam and primary electron effects were observed. The effect of argon ion bombardment is the preferential sputtering of nitrogen, forming “covalent” silicon at the surface layer (AES peak at 91 eV). The electron beam irradiation results in a decrease of the covalent silicon peak, either by an electron beam annealing effect in the bulk of the silicon nitride film, or by an ionization enhanced surface diffusion process of the silicon (electromigration). By the electron beam annealing, nitrogen species are liberated in the bulk of the silicon nitride film and migrate towards the surface where they react with the covalent silicon. The ionization enhanced diffusion originates from local charging of the surface, induced by the electron beam.

Calibration of argon ion sputter rates using nuclear microanalysis and Auger spectroscopy

Abstract: Auger electron spectroscopy (AES), in conjunction with argon ion sputtering, is frequently used to obtain composition versus depth profiles. Thin film thicknesses may be estimated from such profiles if the argon ion sputtering rate of a particular material is known. For this work, a series of oxide standards was prepared and calibrated with nuclear microanalysis techniques. These standards were then used to calibrate the composition versus depth profiles obtained using AES.

Study of emission and sputtering yields in some alloys and oxide by glow discharge optical spectrometry: Quantification of analysis

Abstract: In an analysis by glow discharge optical spectrometry (GDOS) the relation between the intensity of spectral emission of an element and its concentration at the surface of the sample is fundamental for interpreting the results. The different factors which occur in this relation are systematically investigated for different materials such as metallic alloys, oxidized matrices or matrices submitted to ionic implantation. The measurements show that the emission yield of an element is independent of the matrix although particular phenomena can affect the apparent experimental values. The simplification which results in the interpretation of experiments in GDOS is evident and a few examples of applications are presented.

Surface characterization of chemically modified (trimethylsily) silicas by 29Si solid state NMR, XPS, and IR photoacoustic spectroscopy

Abstract: The surface reactivity of hydroxyl groups on silica with trimethylchlorosilane (TMCS) is examined using XPS, FTIR-PAS, and solid state NMR. All techniques give responses give responses which can be correlated with increasing TMS surface coverages on Lichrosorb Si 60 silica and provide detection limits of ⩽10% of a TMS monolayer. Solid state NMR is particularly suited to examining reactivity differences between geminal and single silanols. Geminal hydroxyl groups are found to be much more reactive. At higher TMS coverages approaching steric limitations (0.5 TMS monolayer), nearly 100% of the geminal sites are reacted as opposed to only 20% of the single sites. The reactive subset of single silanols appears to include hydrogen-bonded (vicinal) groups. A mechanism is postulated to explain the apparent enhanced reactivity of geminal and vicinal silanols.

Surface studies of keratin fibers and related model compounds using ESCA. I—intermediate oxidation products of the model compound 1‐cystine and their hydrolytical behaviour

Abstract: The intermediate oxidation products of 1-cystine, namely cystine S-monoxide, cystine S,S-dioxide and cysteine sulphinic acid have been synthesized according to the procedures of other workers. Attempts to prepare cystine trioxide and cystine tetroxide have also been carried out. The stability of cystine S-monoxide, cystine S,S-dioxide and cysteine sulphinic acid in aqueous solutions both under mild hydrolytical conditions (2 h, 25 °C) at different pH values and under the conditions of acid hydrolysis (6 N HCl, 105 °C, 1–20 h, 24 h) have been studied. Cystine, its oxidation products and the disproportionation products are identified by ESCA (C 1s, O 1s, N 1s, S 2p). Both cysteine sulphinic acid and cysteic acid exist mainly as Zwitterions due to the fact that in the carbon spectra of each of them an additional peak shifted to about 1.4 eV (relative to the hydrocarbon peak) has been observed, which is attributed to carbon bond to protonated nitrogen. In addition, only in the cases of cysteine sulphinic acid and cysteic acid an increasing of the nitrogen binding energy of about 1 eV compared with those of cystine and its corresponding oxides has been found indicating of protonated nitrogen. Both the monoxide and the dioxide are relatively unstable in aqueous solutions (2 h, 25 °C, pH 1–10). However the monoxide is more stable than the dioxide under these conditions. Cysteine sulphinic acid is stable under this mild hydrolytical condition. Even in 6 N HCl at 105 °C for 1–20 h in vacuo cysteine sulphinic acid is moderately stable. After 24 h it cannot be detected at all. The results achieved from these studies are discussed in detail in conjunction with previous results reported by other investigators concerning the hydrolytical behaviour of cystine intermediate oxidation products.

A multilayer model for GaAs oxides formed at room temperature in air as deduced from an XPS analysis

Abstract: An XPS study of the oxides formed on GaAs at ambient temperature and in ambient atmosphere is reported. Special attention has been paid to the interpretation of the Ga 3d and As 3d energy levels. By varying the angle between energy analysed electrons and the surface of the sample the depth distribution of the various oxides has been deduced. A multilayer model deduced from XPS analysis and the known reactivity of GaAs and the various oxides is proposed.

Si ? O compound formation by oxygen ion implantation into silicon

Abstract: High dose oxygen ion implantation into silicon at 30 keV was performed to produce understoichiometric and stoichiometric surface oxide layers of ∼160 nm thickness. The oxygen depth profile and oxide stoichiometry was determined by RBS and XPS. SiO compound formation was found by IR spectroscopy and XPS in the unannealed samples as well as after target heating, furnace or flash lamp annealing. As implanted understoichiometric layers consist of random bonding like SiOx (O < x < 2), demixing into Si and SiO2 after annealing. Unannealed stoichiometric layers are bond strained SiO2. The activation energies of demixing and of the annealing of bond strains are determined to 0.19 and 0.13 eV, respectively. The removing of bond strains occurs at temperatures ≥800 °C in a time shorter than 1 s. The SiO2/Si transition region of unannealed stoichiometric layers consists of SiOx with an extent of about 10 nm. After annealing this extent diminishes to 0.8–1 nm in consequence of oxidation by excess oxygen from the overstoichiometric oxide region. This thickness is comparable with that of thermal oxide.

Correlation between CdS photoanodic behaviour and electrode chemical modifications: An X-ray photoelectron spectroscopic study

Abstract: The photosensitivity increase of CdS films as a function of the illumination time, is correlated to chemical changes of the electrode surface, analysed by XPS. It is shown that the formation of Cd(OH)2 or CdO is responsible for the photosensitivity change of the illuminated electrode in contact with H2O/NaOH alone. When thiocresol or p-mercaptobenzoic acid is added as reductant, a layer of organic compounds is formed on the electrode during the photoelectrochemical process. However, the XPS and photocurrent spectra indicate that this layer does not prevent the occurrence of Cd(OH)2 or CdO responsible for the photosensitivity changes of the CdS film.

XPS study of ion‐induced changes on the surface of an organosilicon model polymer

Abstract: Changes in composition and bonding induced by bombardment with argon or nitrogen ions in a commercial poly-methyl-phenyl-cyanopropyl-siloxane fluid were examined by XPS. The actual surface composition depended on the ion energy, the etch time and the nature of the gas applied. It was found that phenyl groups were damaged seriously, reaction of the cyano groups took place with the formation of CN type bonds and the polymer degraded with elimination of fragments containing mostly C at the first stage and mainly O and Si at advanced stages. In addition nitrogen was found to build into the polymer surface when bombarding by nitrogen ions. Irreversible changes in the spectra of the fluid polymer were regarded as a sign of crosslinking.

Ion induced surface compositional changes of α brass; a comparative study by AES and sputtered thermal ion mass spectrometry. Application to quantitative AES

Abstract: Surface concentrations and sputtered-off atom concentrations of α brass are measured during ion bombardment, respectively by Auger spectroscopy and by a new quantitative sputtered thermal ion mass spectrometry method A new formulation of the problem of surface composition change is proposed which leads to a definition of matrix effect coefficients, sputter elemental yields and a sputter correction factor μ, connecting surface concentrations to sputtered-off atom concentrations Implications in quantitative analysis by Auger spectroscopy or any mass spectrometry technique are discussed

High spatial resolution Auger linescans across heterogeneous chemical edges by Monte Carlo calculation

Abstract: Previous calculations have shown that Auger yields increase at chemical edge/free space interfaces. In this paper we consider the edge formed by a well defined thin film on a substrate of different chemical composition. Two systems have been investigated: Au and Si and Al on Si. Linescans are presented for a number of different electron beam configurations and show significant features attributable to an edge effect. These effects are explained in terms of the different layer materials and their corresponding backscatter coefficients.

Depth profiling on a sputtered bevel of sub‐degree slope angle

Abstract: A new method is described whereby a surface can be bevel-sectioned by means of sputtering at a slope angle of any desired sharpness. In contrast with other proposals of sputter-bevelling this method produces an elevated bevel section. Measurements on a shallow diffusion profile of boron in a silicon wafer using this method show the profile reproduction to be very satisfactory. Distorting effects on the projection of the depth profile on the sputtered bevel are discussed. Scanning Auger and ion microprobes are compared for their suitability for reproducing a depth profile from its projection on a sputtered bevel.

Stabilization of charge on electrically insulating surfaces during SIMS experiments—experimental and theoretical studies of the specimen isolation method

Abstract: Recent experiments have shown that secondary ion mass spectrometric (SIMS) measurements of insulator surfaces can be greatly facilitated by the use of a charged aperture located immediately above the analysis area (the ‘specimen isolation technique’). This allows the surface potential within the aperture area to be stabilized. Charging is stabilized when the potential difference between the surface and the aperture becomes large enough that excessive charge, in the form of secondary electrons, is drained away from the surface to the aperture. The potential difference generated on such insulating surfaces can be measured experimentally by varying the voltages applied to the electrostatic analyzer to measure the secondary ion intensity as a function of kinetic energy. Such measurements have been carried out on a Cameca IMS-3F instrument for a range of aperture dimensions and for different primary and secondary ions. The surface potential can be stabilized at a potential readily measurable by the electrostatic analyzer for both O− and Cs+ ion beams. The mechanism for this stabilization has been analyzed through calculations of two dimensional contours for the region around the specimen holder. For surfaces with a very high charging potential (>600 V), potential wells which can trap ions of low kinetic energy develop near the surface. Element and oxide secondary ion kinetic energy distribution curves, taken under specimen isolation conditions, which differ from those obtained under normal non-charging SIMS conditions, are evidence for such ion trapping.

Effect of gas phase composition on chromium segregation in NiOCr2O3 solid solutions

Abstract: The SIMS method was applied to determine the near-surface segregation profiles of Cr in Cr-doped NiO single crystals annealed at 1353 K in air and in argon. The decrease of oxygen partial pressure in the gas atmosphere favours surface segregation of Cr. Furthermore, the enrichment factor increases when the Cr bulk concentration decreases. The present research indicates a dynamic state of the surface layer of oxide crystals which may vary as a result of changes in gas composition.

Characterization of silica-supported uranium-molybdenum oxide catalysts

Abstract: The techniques of X-ray diffraction, infrared and electron spin resonance spectroscopies, kinetics of reduction by H2, and XPS were used to investigate the chemical species present at the interface in silica-supported uranium–molybdenum oxide catalysts. In this catalyst series, with constant MoO3 loading (16.7 wt%) and Mo/(Mo + U) atomic ratios ranging between 1 and 0, it was found that the oxidic forms of Mo and U were not well dispersed, MoO3 crystallites being detected on the catalysts with a high Mo/(Mo + U) atomic ratio. In the less-uranium-rich catalysts [Mo/(Mo + U) = 0.89] both MoO3 crystallites and a crystalline ‘uranyl molybdate’ phase were present. This catalyst exhibited the highest reduction degree by H2 and the highest ESR signal intensity of Mo5+ ions when reduced under very mild conditions. The information obtained with these techniques was used to explain the promotional effect of U on the MoO3/SiO2 base catalyst for the selective oxidation of isobutene.

Use of a fast atom beam in ion microscopy (FABIM) for analysis of poorly conducting materials

Abstract: A Cameca IMS3f ion microscope was modified to allow the analysis of poorly conducting materials. A neutralization chamber within the primary ion column constitutes a fast atom beam (FAB) source. The insertion of a thin metallic diaphragm between the sample and the sample holder minimizes the charge build-up. With these simple and cheap modifications, it is possible to change instantaneously from ion to neutral beam excitation for the same analysed area. The characteristics of the primary neutral beam were evaluated and the analytical possibilities using positive and negative secondary ions were investigated on various poorly conducting materials (mineral, multilayered glass, organic). The unique advantages of combining FAB and Ion Microscopy (FABIM) are illustrated: imaging capability, low detection limit, mass spectra at low and high resolution and depth profiles on very small areas. Suggestions are proposed to improve the performance of this preliminary FABIM design.

Facet development and its influence on depth resolution during sputtering of Si

Abstract: A model is developed, assuming mutation of facet lengths to larger values, to explain the experimental observations of the change in the population distribution of the lengths of facets developed during ion bombardment sputtering erosion of Si. The ensuing Gaussian facet length distribution, with mean length increasing linearly with ion fluence, is then used to show that the depth resolution δz/z0 of interface broadening first increases as z and then tends towards a constant value with increasing z0.

Quantitative analysis of complex Auger spectra by least‐squares fitting with prefiltering of spectra

Abstract: The ‘filter-fit’ method, which was originally developed for the analysis of energy dispersive x-ray data, has been applied to the quantitative analysis of Auger spectra for the first time. It is a digital peak deconvolution technique in the presence of the continuum background using least-squares fitting calculations, including the prefiltering of spectra. To evaluate this method, composite-known spectra digitally composed of Cr, O and Ni standard spectra were prepared. For the deconvolution of Cr and O peaks under the influence of the Ni peak tail, the present method showed remarkable advantage over the conventional least-squares fitting without prefiltering. As a digital filter for prefiltering, derivative filters of different orders and ‘top-hat’ filter were examined. In derivative filters, it was found that there is an optimum order in the balance of advantage of suppressing the background and disadvantage of losing peak intensity. The ‘top-hat’ filter is a very practical filter in both background suppression and noise attenuation.

The effect of surface topography evolution on sputter profiling depth resolution in Si

Abstract: Experimental studies of the development of surface topography on Si surfaces during Ar+ ion bombardment and sputter erosion indicates that the Si becomes continuously prismatically facetted and that facet dimensions increase linearly with mean eroded depth. These observations lead to an analytic model of the achievable depth resolution δz/z0 in sputter erosion which is independent, for deep profiles of z0.

Beam brightness modulation (BBM) method as applied to Auger electron spectroscopy

Abstract: The principle of the beam brightness modulation (BBM) method, as well as such advanced characteristics as high speed response, wide dynamic range of primary beam currents, selective signal detection, and signal-to-noise superiority in N(E) spectra, are reported by comparing with both the conventional analyzer energy modulation and the pulse counting methods. Some device-related technologies for the BBM method are also described as well as advantages of Auger electron spectroscopy (AES) using N(E) spectra obtained by the BBM method.

Quantitative analysis of impurities in HgCdTe using secondary ion mass spectrometry

Abstract: SIMS has been used to analyze impurities in epitaxial layers of HgCdTe because of its sensitivity to trace impurities and because it makes it possible to obtain impurity concentration profiles through the thickness of the epitaxial layer. The impurity profiles are quantified using relative sensitivity factors calculated from ion implantation profiles obtained on standard reference samples. Unintentional impurities, such as Cu, and low-level deliberately added impurities, such as As, In, and Ag, have been analyzed. Specialized techniques such as molecular ion spectrometry (As analysis with TeAs−) and secondary ion acceleration voltage offset (Ag analysis) have been applied to improve sensitivity and decrease background signals. The use of these techniques has allowed the establishment of detection limits of less than 0.1 parts per billion atomic (3 × 1015 cm−3). The error in the analyses is, in general, estimated to be less than ±50%. Results from relative sensitivity factor measurements and impurity profiles are presented, and detection limits and accuracy of impurity analysis in HgCdTe using SIMS are discussed.

Electrochemical polymerization of pyrrole on carbon fibres surface

Abstract: The electrochemical polymerization of pyrrole using carbon fibres as substrates gives strongly adhesive films which improve the fibres' mechanical properties.