Showing papers in "Surface and Interface Analysis in 1998"

Determination of the concentration of surface hydroxyl groups on metal oxide films by a quantitative XPS method

Abstract: Quantitative XPS has been used to determine the surface concentration of hydroxyl groups in native air-formed oxide films on metals having low surface areas. A mathematical expression has been derived to give the concentration of surface hydroxyl groups as a function of the intensity ratio of the OH to O2− contributions to the O 1s photopeak. This expression is based on modeling the oxide film on a metal to be a multilayer system consisting of an outermost layer of organic contamination, a layer of chemisorbed water, a surface hydroxylated region of the oxide film and the inner portion of the oxide film. The average values of the experimentally determined concentrations of surface hydroxyl groups are 15, 13, 11, 6 and 8 OH nm−2 for oxide-covered aluminum, chromium, titanium, tantalum and silicon, respectively. X-ray photoelectron spectroscopy depth profiles using argon ion sputtering and variable-angle XPS have been utilized in this work. Surface treatments have employed either ultrasonic cleaning with organic solvents or argon plasma treatment. © 1998 John Wiley & Sons, Ltd.

Accuracy of the non‐destructive surface nanostructure quantification technique based on analysis of the XPS or AES peak shape

Abstract: The accuracy of XPS and AES quantification by peak shape analysis was established from a detailed analysis of a range of model spectra and three sets of experiments. It was found that information on the concentration-depth profile in the surface region up to depths of ∼ 5λ i (where λ i is the inelastic electron mean free path) is primarily contained in the spectral energy region up to ∼100 eV below the peak energy and is essentially completely contained by the energy region up to ∼200 eV below the peak. Analysis of a larger energy range than 100 eV does not add much to the information on the details of the structure in the outermost 5λ i but gives the possibility to determine additional structural parameters that describe the composition at larger depths. The structural parameters that describe the chemical composition of the outermost 5-10λ i of the solid were divided into primary and secondary parameters: the primary parameters are the three most important parameters needed to describe the main characteristics of the distribution of atoms; the secondary parameters are parameters other than the three primary parameters that describe the finer details of the depth distribution of atoms in the outermost 5-10λ i of the surface region. The uncertainty in the determined three primary parameters is typically 5-10%. The uncertainty in the determined secondary parameters is typically?35%. Different models of depth profiles can be distinguished when they differ significantly over a width of more than ∼ 1/3λ i at any depth? 5λ i . The uncertainty in the total determined amounts of atoms within the surface region is ∼5-10% as long as the depths are within the primary probing depth of the method (i.e. <5λ i ). The absolute quantification of a set of samples where the in-depth distribution varies considerably gives a root-mean-square scatter of 15%. This is reduced to ∼10% when elastic scattering effects are modelled by a simple analytical expression.

XPS: binding energy calibration of electron spectrometers 5—re-evaluation of the reference energies

Abstract: The binding energies of the calibration peaks for x-ray photoelectron spectroscopy—Cu 2p3/2, Ag 3d5/2 and Au 4f7/2—have been reassessed based on the traceable data recorded in 1984 using unmonochromated x-rays and an analyser resolution of 0.3 eV. The changes in those energies, for different x-ray sources and analyser resolutions, have been calculated and the results compared with further data. This includes work with monochromatic Al x-rays recorded at high energy resolution, allowing the binding energies to be referred to a new zero value set at the Fermi edge measured for Ag. A consistent set of data is presented for the calibration and assessment of photoelectron spectrometers with energy resolutions in the range 0.2–1.5 eV, when used with unmonochromated Al or Mg x-rays or monochromated Al x-rays. © 1998 John Wiley & Sons, Ltd.

Interfacial interactions of plasma‐polymerized acrylic acid and an oxidized aluminium surface investigated using XPS, FTIR and poly(acrylic acid) as a model compound

Abstract: X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are used to characterize the interface between poly(acrylic acid) (PAA) and an aluminium oxyhydroxide (AlOOH) surface. Reaction of the PAA carboxylic acid functionality with hydroxyl groups at the surface of the oxide to form a monodentate carboxylate complex C(O)OAl was identified by FTIR. The degree of conversion of the acid to the salt at the interface was estimated to be 40% from the XPS analysis of PAA overlayers of varying thickness. An equivalent reaction was identified at the interface between AlOOH and a monolayer of strongly bonded plasma-polymerized acrylic acid (ppAAc) that remained after solvent washing. The resistance of the monolayer of ppAAc to solvent washing was attributed to the formation of interfacial carboxylate complexes with the AlOOH surface. © 1998 John Wiley & Sons, Ltd.

Quantitative analysis of overlapping XPS peaks by spectrum reconstruction: determination of the thickness and composition of thin iron oxide films

Abstract: The composition and thickness of thin iron oxide films on polycrystalline pure iron were evaluated from Fe 2p spectra as measured by x-ray photoelectron spectroscopy. To this end the experimental spectra were reconstructed from reference spectra of the constituents Fe 0 , Fe 2+ and Fe 3+ . The background contributions in the spectra owing to inelastic scattering of signal electrons were calculated from the depth distributions of these constituents and their reference spectra. In the reconstruction procedure the film thickness and the concentrations of Fe 2+ and Fe 3+ in the oxide film were used as fit parameters The values obtained for the oxide film tbickness were compared with thickness values determined from the intensity of the corresponding O 1s spectra and with thickness values resulting from ellipsometric analysis. The sensitivity of the reconstruction procedure witb regard to film thickness and film composition was tested. © 1998 John Wiley & Sons, Ltd.

Effects of surface treatment on the surface chemistry of NiTi alloy for biomedical applications

Abstract: Alloys of NiTi have gained popularity in biomedical applications; however, Ni is known to cause toxic and allergic reactions in the body, and concerns have been expressed regarding the material's biocompatibility. In this study, coupons of equiatomic NiTi were prepared by four methods, namely, mechanically polishing to a mirror finish, electropolishing, chemical etching and argon plasma etching, to produce various levels of roughness, and then examined by atomic force microscopy (AFM), XPS and AES. The resulting surface chemistry depended upon the method of preparation and was found not to be a function of surface roughness. The mechanically polished samples, although having the smoothest surface, showed the highest level of Ni in the near-surface region. The other preparation methods produced surfaces that were predominantly TiO2, with the electropolished surfaces showing the next smoothest surface and the least Ni in the near-surface region. The correlation between method of preparation, surface roughness and surface chemistry may be important in the preparation of NiTi for biomedical applications. © 1998 John Wiley & Sons, Ltd.

Control of the stoichiometry in the deposition of cobalt oxides on SiO2

Abstract: Both CoO and Co3O4 overlayers have been deposited on SiO2 by evaporation from metallic Co and subsequent oxidation with oxygen and a plasma of oxygen. The combined use of ion scattering spectroscopy and XPS shows that both oxides grow in the form of small particles on the surface of SiO2. Ion scattering spectroscopy also shows that the surface of cobalt oxide exposed to a plasma of oxygen is enriched in oxygen ions with respect to the surface of the cobalt oxide formed by exposure to oxygen. The Co 2p spectra corresponding to the deposits obtained by oxidation with O2 are characteristic of CoO, while those corresponding to the deposits obtained after oxidation with a plasma are typical of Co3O4. Moreover, the OCo/Co ratios determined by XPS and factor analysis indicate the formation of CoO stoichiometry in the former case and Co3O4 stoichiometry in the latter. It has also been observed that no shift in either binding energy or modified Auger parameter α′ appears as a function of coverage. This absence of shifts is interpreted as a consequence of the type of screening mechanism that dominates the relaxation of the photoholes in these oxides. © 1998 John Wiley & Sons, Ltd.

Surface excitation parameters of low-energy electrons crossing solid surfaces

Abstract: The surface excitation parameter, which describes the influence of surface excitations by electrons for the vacuum side in electron spectroscopies, has been calculated for electrons of 200-2000 eV energies crossing surfaces of Cu, Ag, Au, Fe, Pd, Ni, MgO and SiO 2 , These calculations were performed for both incident and escaping electrons by the use of dielectric response theory. Spatially varying differential inverse mean free paths for surface excitations as a function of electron distance from the surface were found. The results showed that small differences existed in the surface excitation parameter among different metals but large differences occurred between metals and semiconductors or insulators, Calculated surface excitation parameters were fitted very well to a simple formula, i.e. P. = aE -b , where P. is the surface excitation parameter and E is the electron energy.

XPS Studies of the modification of polystyrene and polyethyleneterephthalate surfaces by oxygen and nitrogen plasmas

Abstract: Polystyrene and polyethyleneterephthalate surfaces were exposed to helium, oxygen/helium and nitrogen/hydrogen plasmas singly and in combination. The treated surfaces were evaluated by water contact-angle measurements and by x-ray photoelectron spectroscopy. It was found that the oxygen and nitrogen tend to graft to common carbon atoms, to form amide groups. The water wettability was found to correlate with the fraction of electronegative atoms incorporated into the surface.

Compositional changes in plasma‐deposited fluorocarbon films during ageing

Abstract: Plasma polymer coatings were deposited from perfluoro-1,3-dimethylcyclohexane (PFDMCH) and their composition and surface properties studied by XPS, grazing-angle Fourier transform infrared spectroscopy and contact angle measurements as a function of time after fabrication as they were stored under ambient conditions for more than 2 years. The spontaneous ambient oxidation of PFDMCH plasma polymers was found to be multi-step process. The rapid initial oxygen uptake, assigned to reaction between carbon-centred radicals incorporated into the coating during deposition and in-diffusing atmospheric O 2 , was similar to that of plasma polymers deposited from hydrocarbon-based monomers (alkanes, alkylamines, alcohols), suggesting that the density of radicals incorporated during deposition was similar. Subsequently, however, the extent of oxidation was much lower for PFDMCH coatings. This can be attributed to the lack of availability of hydrogen abstraction reactions, which are important for radical propagation in hydrocarbon-based plasma polymers. While XPS recorded a continuous incorporation of oxygen for more than 2 years, the air/water contact angles decreased only during the first 2 months and on further storage remained stable. There appeared to be only a small extent of surface restructuring as assessed from the small depth variations of compositions. The surface was enriched in CF 3 groups at all times.

Depth distribution and bonding states of phosphorus implanted in titanium investigated by AES, XPS and SIMS

Abstract: The depth distribution of phosphorus implanted in titanium at 20 keV with doses ranging from 1×1015 to 3×1017 cm−2 was investigated by AES, SIMS and XPS. For small doses (⩽1016 cm−2) Gaussian-like phosphorus profiles have been observed. For doses >1017 cm−2 the depth profiles suggest that a TiP-like surface layer is formed without additional thermal treatment. In the implanted layer a certain amount of oxygen was found. Factor analysis of the spectra obtained during AES and XPS depth profiling proved that the surface region of the implanted samples is composed of Ti metal, surface oxide and Ti phosphide, which is produced at high doses. From P L2,3 VV Auger spectra the existence of a second bonding state of phosphorus is deduced, which is distinguished from TiP by differences in the peak shape and position. This species is distributed mainly in a narrow profile located at a depth near the projected range of the phosphorus ions, i.e. in the region of maximum lattice defects. It is attributed to phosphorus on interstitial sites. The amount of this species reaches saturation for a dose of ∽1017 cm−2. © 1998 John Wiley & Sons, Ltd.

Quantitative AES. VIII: analysis of auger electron intensities from elemental data in a digital auger database

Abstract: An analysis of the correlation of theoretical predictions for Auger electron intensities is made with the experimental data of the high-resolution digital Auger database. This analysis covers single-element samples measured for 5 keV and 10 keV beam energies. The spectral data are for wide scans of 20–2500 eV at 1 eV energy resolution and 1 eV energy intervals, as well as narrow scans at 0.25 eV energy resolution and 0.1 eV energy intervals. All spectra have the instrument intensity/energy response function removed so that peak areas are measured in units of electrons emitted per steradian per incident electron. Correlations are made for calculations of the intensities using the cross-section of Casnati et al. applied to the K, L, M and N shells, the backscattering factor of Shimizu and the inelastic mean free paths of Tanuma et al. The intensities, summed over all peaks relating to initial ionizations in shells of a given principal quantum number, correlate with the theoretical predictions with no fitting parameters, with a mean error of a factor of only 1.04 and a scatter factor of 1.3. Expressions are provided to extend the database to other beam energies and angles of incidence. General equations are also provided to extract new sensitivity factors for the average matrix, which lead to matrix factors that effectively reduce to unity for peaks above 175 eV, instead of the traditional matrix factors that cover a range of 0.1–7.2. These sensitivity factors for the average matrix may be used for peak areas, peak heights or differential peak-to-peak intensities. In the latter two cases, spectral broadening may be necessary to obtain accurate results. © 1998 John Wiley & Sons, Ltd.

XPS: binding energy calibration of electron spectrometers 4—assessment of effects for different x‐ray sources, analyser resolutions, angles of emission and overall uncertainties

Abstract: A detailed analysis is made of the binding energy calibration of X-ray photoelectron spectrometers when using monochromated Al Kα x-rays or unmonochromated Al or Mg Kα x-rays. The binding energies of the peaks for Cu 2p3/2, Ag 3d5/2 and Au 4f7/2, as well as for the Ni Fermi edge, are measured at high resolution using monochromated Al Kα x-rays. The apparent binding energy shifts of the peaks are then calculated for this source, and also for the Al and Mg unmonochromated x-ray sources, using full synthetic Kα x-ray structures, as a function of Gaussian spectrometer energy resolutions in the range 0.2–1.5 eV. For all three x-ray sources, the relative binding energies for the Cu 2p3/2 and Au 4f7/2 peaks are contained within ±0.015 eV but the effects for Ag 3d5/2 are stronger and the containment range must be increased to ±0.026 eV. Further data and calculations are provided for surface core-level shifts and here it is found necessary to restrict emission angles to 56° for all the peak separations to be restricted to the above range of ±0.026 eV. Other instrumental effects may give rise to additional larger or smaller effects. Non-optimized settings for monochromators can show further shifts of up to ±0.2 eV. The uncertainties associated with the above calibration are then analyzed to show how the uncertainty at 95% confidence varies across the binding energy range. Example calculations show that seven repeats of both the Cu 2p3/2 and Au 4f7/2 binding energies may be used to define the peak repeatability and that one or two measurements can then be made for each calibration peak to define the calibration. The precise number of measurements to be used depends on the peak energy repeatability and the required confidence limits for the calibration. In practical situations, however, it is likely that the greatest uncertainty in the binding energy scale arises from the drift in the electronics between calibrations. © 1998 John Wiley & Sons, Ltd.

Depth-resolved analysis by laser-induced breakdown spectrometry at reduced pressure

Abstract: The 581 nm output from a dye laser in a —uence range between 2.86 and 11.47 J cmo2 was used to ablate pure Zn and Fe foils. The average ablation rate (AAR, lm per shot) was calculated for diUerent experimental variables (buUer gas, pressure, laser —uence and focal conditions). Deposition of previously ablated material in the ablation crater results in large variation of the observed AAR values. This eUect was observed in air and argon buUer gases at atmospheric pressure. The situation is largely alleviated at reduced pressure due to free expansion of the ablated material. Under these circumstances the capability of laser-induced plasmas to resolve interfacial structures is improved. The eUect on depth-resolved studies was checked with a commercial Zn-coated steel sample. Due to the Gaussian-like energy distribution of the incident laser beam, the material is ablated to produce a conical crater. This fact ensures that the Zn signal remains for a longer time because the ablated region spreads over the edge gradually. At low pressure the emission peaks are better de—ned and the background becomes —at. However, these conditions produce also the lowest net intensities and some peaks are not detected. An Ar atmosphere produces more intense spectral lines at both pressure levels. Best analytical results were obtained at reduced pressure, with a slight improvement in depth resolution in the presence of Ar. 1998 John Wiley & Sons, Ltd.

Surface characterization of synthetic vulcanized rubber treated with oxygen plasma

Abstract: The surface of a synthetic vulcanized styrene-butadiene rubber (R2) was treated in an oxygen plasma to improve adhesion in joints prepared with a one-component solvent-based polyester-urethane adhesive. The modifications produced on the rubber surface by plasma treatment were assessed using advancing and receding contact angle measurements, x-ray photoelectron spectroscopy, (XPS), infrared-attenuated total reflection spectroscopy and scanning electron microscopy. Adhesion was obtained from T-peel tests of treated R2 rubber/polyurethane adhesive joints. Several experimental variables were considered, such as the radio-frequency power and the length and lifetime of the plasma treatment. The treatment in the oxygen plasma produced a noticeable decrease in contact angle, which can be mainly ascribed to the creation of C-O and C=O moieties on the rubber surface. Advancing and receding contact angles only differed by ∼10°. Depending on the experimental conditions used, some ablation was produced on the surface, which was more noticeable as the length and power of the treatment increased. An adequate performance of adhesive joints was obtained using a power of 50 W and a time for oxygen plasma treatment of <10 min. The changes in the rubber surface remained for 2 h after plasma treatment, as indicated by the variation in peel strength and XPS data. Although improved adhesion was obtained by treating the rubber in an oxygen plasma, the T-peel strength values are not sufficient to assure technical use, probably due to the migration of waxes and zinc stearate to the surface once the treatment was carried out. Finally, sulfur oxidation was produced by the plasma treatment, and for severe conditions solid crystals of a sodium salt of an oxidized sulfur compound (sodium sulphate or an organic sulphate) appeared on the treated rubber surface.

Degradation behaviour of ZnS phosphor powders under different experimental conditions

Abstract: The phosphor powder ZnS:Cu,Al,Au has been investigated extensively as a possible green phosphor for field emission displays (FEDs). Phosphor powders of ZnS:Cu,Al,Au have been subjected to electron bombardment (1–5 keV) at oxygen pressures ranging from 1×10−7 to 2×10−6 Torr and different electron current densities (45–58 mA cm−2). Auger electron spectroscopy (AES) and cathodoluminescence (CL), both excited by the same electron beam, were used to monitor changes in surface composition and luminous efficiency during electron bombardment. It was shown that the degradation rate of the CL intensity is a linear function of oxygen pressure. The interactive constant (a measure of the probability of surface reactions) and the burning parameter (a measure of the probability of CL degradation per incident electron) plotted as a function of electron beam energy follow the same trend as the electron impact ionization cross-section for an atom. © 1998 John Wiley & Sons, Ltd.

Use of aminosilane coupling agent in the synthesis of conducting, hybrid polypyrrole–silica gel particles

Abstract: A new route for the synthesis of conducting polypyrrole–silica composites is described. Silica gel particles were modified by aminopropyltriethoxysilane (APS) and acted as porous inorganic substrates for the in situ synthesis of polypyrrole (PPy). The resulting polypyrrole–silica–APS hybrid materials were characterized by XPS, thermogravimetric analysis, BET and conductivity measurements. It is found that APS yields a substantial increase in the PPy mass loading (from 6 to 11.5 wt.%) but, more importantly, the hybrid particles obtained are conductive and have a negligible static charge in XPS due to their PPy-rich surfaces. In the absence of any APS treatment, polypyrrole is mainly loaded in the pores of the silica gel so these polypyrrole–silica particles are poorly conductive and charge up during XPS analysis. The conductivity threshold (10−2 S cm−1) was obtained for an APS initial concentration of 1% (v/v). In addition, these conducting hybrid particles have a fairly high specific surface area in the 160–180 m2 g−1 range. © 1998 John Wiley & Sons, Ltd.

Practical correction formula for elastic electron scattering effects in attenuation of auger electrons and photoelectrons

Abstract: We define a new parameter to describe the effects of elastic electron scattering in XPS and AES. The parameter is the ratio of emitted intensity from a layer of atoms located at a given depth in a solid calculated from theories that take into account and neglect elastic electron scattering. We have found that the correction parameter can be expressed by a simple formula. This formula is of general validity for typical experimental geometries applied in practical XPS and AES. The formula was determined by fitting an analytical expression to the results of extensive Monte Carlo calculations made under variation in the full relevant range of electron energy, matrix atomic number, depth of origin of emitted electrons and angular emission anisotropy. The formula depends on the inelastic (λi) and the transport (λtr) mean free path for electron scattering. Three assumptions were made in the calculations, namely that the geometry is close to normal emission, that the angle between x-ray source and analyser axis is close to the magic angle (54°) and that the ratio λtr/λi is approximately constant over the analysed depth. However, the result is expected to vary only slightly when these assumptions are not strictly fulfilled. © 1998 John Wiley & Sons, Ltd.

Surface characterization of PVP-sized and oxygen plasma-treated carbon fibers

Abstract: The objectives of this work were to characterize the surface of commercial carbon fibers, focusing on the effect of polymer sizing and the effect of oxygen radiofrequency plasma treatment. The fiber surface composition was determined by x-ray photoelectron spectroscopy (XPS) and the surface topography was examined by scanning transmission electron microscopy (STEM) and atomic force microscopy (AFM). Voltage-contrast XPS distinguished between unsized and sized carbon fibers, by which the former behaved as a conductive material whereas the latter behaved as a mixture of both conductive and non-conductive materials due to a contribution from a polymeric sizing material. The AFM measurements revealed that oxygen plasma treatment for 30 s roughened the unsized fiber surfaces; however, further treatment smoothed the overall topography. Oxygen plasma treatment for 30 s also increased the surface oxygen content. © 1998 John Wiley & Sons, Ltd.

Degradation of poly(vinyl alcohol) thin films during monochromatized XPS: substrate effects and x‐ray intensity dependence

Abstract: Films of poly(vinyl alcohol), ∼2 nm thick, on silicon, copper and gold substrates were allowed to degrade for up to 800 min during monochromatized Al Kα XPS. Relative initial degradation rates of 1: ∼33: ∼41 were measured for the three substrates, consistent with their yield of photoelectrons and secondary electrons The initial rate of degradation was found to vary linearly with the x-ray intensity.

Part 1. N-(m-(3-(trifluoromethyl)diazirine-3-yl)phenyl)-4-maleimido-butyramide (MAD) on silicon, silicon nitride and diamond†

Abstract: Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS) are used to characterize the grafting of the reagent N-(m-(3-(trifluoromethyl)diazirine-3-yl)phenyl)-4-maleimido-butyramide (MAD) to various substrates: silicon, silicon nitride and diamond. MAD carries a maleimide function for thermochemical modification of thiolated molecules and a diazirine function that is lost during light activation (350 nm light). Photoactivation leads to carbene-mediated grafting to solid supports. X-ray photoelectron spectroscopy atomic constituents and chemical shifts, as well as ToF-SIMS molecular peaks and characteristic fragments of the photoimmobilized molecule, are identified. Extended interpretation of surface analysis data suggests that diamond is the substrate with the highest MAD grafting efficiency and that the formation of C-O bonds upon diazirine photoactivation is involved. The difference in grafting extent for the three substrates leads to the conclusion that other reaction sites could be involved but they are not identified. © 1998 John Wiley & Sons, Ltd.

Ta2O5/SiO2 multilayered thin film on Si as a proposed new reference material for SIMS depth profiling

Abstract: A delta oxide multilayered thin film is proposed as a new reference material for SIMS depth profiling. This material is composed of seven thick Ta 2 O 5 layers of 18 nm separated by 1 nm SiO 2 delta layers. The surface of the thin film was very flat and all the interfaces were sharp according to atomic force microscopy (AFM) and transmission electron microscopy (TEM). In SIMS depth profiling, the dynamic range of the Si + ion signal at the SiO 2 layer was large enough to define the depth resolution. The surface topographic development after ion beam sputtering was negligible under various sputtering conditions, as observed by AFM. This was well correlated with the result that there is no significant deterioration of SIMS interface resolution with sputter depth. This reference material can be useful in SIMS depth profiling analysis to optimize the experimental parameters for a better depth resolution and to calibrate the sputtering rate.

The use of high-resolution XPS and ToF-SIMS to investigate segregation phenomena of minor components of a model coil coating formulation

Abstract: The segregation of minor components-a melamine-formaldehyde resin and a poly(acrylic) flow control agent-in a model epoxy resin coil coating applied to hot-dipped galvanized steel has been investigated by high-resolution XPS and time-of-flight SIMS (ToF-SIMS). It is shown that by using the highest resolution monochromated XPS currently available, the C Is spectrum can be peak fitted to account for all eleven carbon functionalities present in the three components of the organic coating. A ToF-SIMS analysis of the melamine-formaldehyde resin has been undertaken and a comprehensive ion fragmentation scheme for the positive ion spectrum of this molecule in the range 0-500 Da is proposed. It is shown, by both surface analytical techniques, that when the flow control agent is excluded from the formulation the surface of the paint film is enriched in the melamine-fornaaldehyde component. On addition of the flow control agent such segregation is only identified by XPS; the ToF-SIMS spectrum resembles that of the flow control agent. This is taken to be an indication of monolayer segregation of this component at the paintlair interface. Such segregation phenomena are shown to be insensitive to substrate surface pretreatment.

Concentration–depth calibration and bombardment‐induced impurity relocation in SIMS depth profiling of shallow through‐oxide implantation distributions: a procedure for eliminating the matrix effect

Abstract: Secondary ion yields are known to be strongly enhanced by the presence of oxygen in the analysed sample. The magnitude of the yield enhancement is often significantly different for impurity and matrix ion species. This kind of SIMS matrix effect severely aggravates concentration calibration in depth profiling through regions of transiently varying oxygen concentration. To eliminate the matrix effect, a procedure has been developed that allows the differences in yield enhancement to be corrected in a quantitative manner. The procedure will ultimately be required to calibrate profiles extending through native surface oxide layers. The calibration exercise was carried out for boron in silicon. The dependence of the B + /Si + sensitivity ratio, R B.Si , on the oxygen content of the sample was explored in situ by implanting 1.9 keV O 2 + ions at O° (normal incidence) into a uniformly B-doped reference sample, followed by sputter profiling through the synthesized oxide with the same beam incident at 75°. All measurements were performed at base pressure. During oxygen build-up after initial sputter cleaning the Si + and SiO + yields increased by factors of 200 and 500, respectively, whereas for B + the yield increased only 40 times. Almost inverse yield changes were observed during oxide removal. Bombardment-induced mixing caused a broadening of the oxide/Si interface and some relocation of B atoms. Under internally consistent assumptions the relatively small boron mixing effect could be separated from the oxygen-induced B + yield enhancement effect. The normalized SiO + signal I SiO + , was used as a measure of the oxygen content of the samples bombarded at the two different impact angles The B + yields and the sensitivity ratios R B.Si (I SiO + ) could be fitted very well by polynomial functions. The polynomials were employed to quantify the depth profiles of 0.5 and 2 keV 11 B implanted in Si test samples covered with 6 nm layers of thermal SiO 2 (i.e. thinner than the synthesized oxide layer that can be produced by the 1.9 keV O 2 + beam at O°). The compositional changes encountered in passing from the thermal oxide into the Si substrate had be taken into account, not only for time-to-depth conversion but also for concentration calibration based on the measured sensitivity ratios. The changes in erosion rate and Si density around the interface were modelled by error functions. Direct evidence is presented that, for accurate calibration, density and sensitivity changes must be treated separately. Even though the through-oxide variations of R B.Si are quite different for O° and 75°, the calibrated 2 keV 11 B profiles derived from measurements at these two vastly different impact angles agree very well, even at the interface. This implies that the large matrix effect occurring in through-oxide profiling at 75° can be eliminated using the new calibration procedure. Minor differences (<10%) between the calibrated 2 keV 11 B profiles from measurements at 0° and 75° can be attributed to differences in bombardment-induced relocation. The mixing effect is particularly severe for the profiles of the very narrow and shallow 0.5 keV 11 B implantation distributions, which turned out to be heavily distorted at depths below 10 nm, both at 0° and 75°. Hence it is mandatory, for reasonably accurate profile measurements, to use O 2 + energies that are significantly (∼50%) lower than the implantation energy, both for normal and oblique incidence of the probing beam.

Evaluation of validity of the depth-dependent correction formula (CF) for elastic electron scattering effects in AES and XPS

Abstract: Elastic electron scattering in XPS and AES vary considerably with depth of origin of emitted electrons. To account for this, we introduced in a recent paper a simple correction factor CF. The function CF is the ratio of emitted peak intensity from a layer of atoms located at a given depth in a solid calculated from theories that take into account and neglect elastic electron scattering. The observed depth dependence of CF is well described by a simple analytical formula that depends only on the inelastic and the transport mean free paths. In the present paper the limits of validity of the formula are determined by comparison to results of extensive Monte Carlo simulations. It is found to be valid for most XPS and AES peaks provided that the angle of emission is AE30A and the angle between x-ray anode and analyser axis is 45‐65A. The procedure for application of CF in practical surface analysis is also discussed. John Wiley & Sons, Ltd. ( 1998 Surf. Interface Anal. 26, 374E384 (1998)

Quantitative AES VII. The ionization cross‐section in AES

Abstract: Inner shell ionization cross-sections are analyzed for use with Auger electron spectroscopy (AES). Correlations of the cross-sections of Gryzinski, Casnati et al., Jakoby et al. and Drawin are made with experimental data from the digital Auger database. Jakoby et al.'s cross-section is realistic for K shell ionizations but can become negative for other shells unless modified, so is of limited use for AES. Results for the ratios of experimental and calculated intensities for electron beam energies of 5 keV and 10 keV, for overpotentials in the range 2–60, are used to rank the effectiveness of the remaining three cross-sections. The experimental ratios show means and scatter standard deviations of 1.01±0.12, 0.99±0.05 and 1.04±0.09 when compared with the predictions for the cross-sections of Gryzinski, Casnati et al. and Drawin, respectively. Casnati et al.'s cross-section provides the best correlation, with the goodness of fit being similar for ionizations in the K, L, M and N shells. Casnati et al.'s cross-section is therefore recommended for future use in AES. The use of Gryzinski's or of Drawin's cross-sections, by comparison, can lead to errors as high as 50% at the low value of 1.5 for the overpotential (i.e. for high-energy Auger electrons). © Crown copyright 1998. Reproduced with the permission of the Controller of Her Majesty's Stationery Office.

Simple model for the XPS analysis of polysaccharide‐coated surfaces

Abstract: A simple model was developed for the interpretation of results stemming from the XPS analysis of polymer surfaces modified by polyethyleneimine adsorption followed by surface coupling of polysaccharides. Theoretical surface compositions were evaluated from the calculated XPS intensity arising from a layered structure composed of a surface oxidized polymer, an intermediate polyethyleneimine layer and a top polysaccharide layer. Results of calculations were compared to analytical data obtained from fixed-angle XPS analysis of polysaccharide-coated surfaces and to the performances of the latter in in vitro cell adhesion experiments. It is shown that the model can correctly predict the relationship between the experimental surface composition and the cell-resistant properties of polysaccharide-coated surfaces. © 1998 John Wiley & Sons, Ltd.

Application of low-energy noble-gas ion scattering to the quantitative surface compositional analysis of binary alloys and metal oxides

Abstract: Low-energy noble-gas ion scattering (LEIS) probes the outermost atomic layer of a material, but a quantitative compositional analysis of this layer is not straightforward. It is demonstrated that quantification by calibration can be done, assuming that ion fractions and shielding effects are the same for the reference sample and sample of interest. These assumptions are critically evaluated and LEIS experiments on binary alloys and metal oxides are presented that can partly verify these assumptions. The LEIS measurements of a Cu–Au alloy and CuO powder as a function of initial energy indicate the absence of matrix effects in the ion fractions after scattering from the metal atoms in these systems. In metal oxides, shielding of surface metal atoms by shadowing/blocking and ion neutralization by neighbouring atoms can significantly influence the quantification of the metal atom concentration and is determined by the local atomic arrangement as illustrated by LEIS experiments of CuO and ZnO samples.

Determination of photoelectron attenuation lengths in thin oxide films on iron surfaces using quantitative XPS and elastic recoil detection

Abstract: Thin oxide films play an important role in the corrosion of metals. Using XPS it is possible in principle to obtain information on the chemical state of near-surface atoms, the stoichiometry of the surface layer and its thickness. A problem is the quantification of XPS spectra, due to the large uncertainty in the value of both electron attenuation lengths as well as sensitivity factors. By applying the Tougaard background subtraction method and comparing only peak intensities of the Fe 2p and O 1s peaks with those of Fe, FeO and Fe2O3 reference samples, we determined the stoichiometry of the oxide layer grown on Fe(100) and Fe(110) at 200°C and room temperature to be Fe0.90±0.05O. We combined XPS with the high-energy ion-beam technique of elastic recoil detection (ERD). By comparing the absolute coverages of oxygen determined by ERD with the intensities of different XPS peaks, we were able to determine the attenuation lengths for kinetic energies of 776 eV (Fe 2p) and 957 eV (O 1s) to be 9.3×1015 and 9.9×1015 Fe0.9O molecules cm−2, respectively. © 1998 John Wiley & Sons, Ltd.

Sputtering-induced surface roughness of polycrystalline Al films and its influence on AES depth profiles

Abstract: Sputtering-induced surface roughness is the main source of degradation of the depth resolution observed during depth profiling of polycrystalline metals. Atomic force microscopy (AFM) images of polycrystalline Al films at different mean sputtered depths are used to calculate both the depth distribution function (DDF) and the angular distribution function (ADF) of the evolving Al grain surfaces. The shape of the DDF changes with increasing mean sputtered depth, which implies the generation of two different roughness stages during sputtering. However, Auger electron spectroscopy (AES) depth profiling and AFM results show a linear increase of roughness vs. mean sputtered depth in the case of evaporated, polycrystalline Al films. A simple model is developed to calculate the AES intensity for a rough surface. The intensity behaviour as a function of the sputtering time depends on the ADF of microplanes and on the sample tilt angle and generally shows a marked decrease for high tilt angles. The sputtering rate distribution is determined using the DDF. A good fit of the AES depth profile of the Al film requires both the calculated intensity behaviour and the convolution using the DDF, which depends on the sputtering time. © 1998 John Wiley & Sons, Ltd.