Showing papers in "Surface and Interface Analysis in 1991"

Calculations of electron inelastic mean free paths. III. Data for 15 inorganic compounds over the 50–2000 eV range

Abstract: We report calculations of electron inelastic mean free paths (IMFPs) of 50–2000 eV electrons for a group of 14 organic compounds: 26-n-paraffin, adenine, β-carotene, bovine plasma albumin, deoxyribonucleic acid, diphenylhexatriene, guanine, kapton, polyacetylene, poly(butene-1-sulfone), polyethylene, polymethylmethacrylate, polystyrene and poly(2-vinylpyridine). The computed IMFPs for these compounds showed greater similarities in magnitude and in the dependences on electron energy than was found in our previous calculations for groups of elements and inorganic compounds (Papers II and III in this series). Comparison of the IMFPs for the organic compounds with values obtained from our predictive IMFP formula TPP-2 showed systematic differences of ∼40%. These differences are due to the extrapolation of TPP-2 from the regime of mainly high-density elements (from which it had been developed and tested) to the low-density materials such as the organic compounds. We analyzed the IMFP data for the groups of elements and organic compounds together and derived a modified empirical expression for one of the parameters in our predictive IMFP equation. The modified equation, denoted TPP-2M, is believed to be satisfactory for estimating IMFPs in elements, inorganic compounds and organic compounds.

Calculations of electorn inelastic mean free paths. II. Data for 27 elements over the 50–2000 eV range

Abstract: We report calculations of electron inelastic mean free paths (IMFPs) for 50–2000 eV electrons in a group of 27 elements (C, Mg, Al, Si, Ti, V, Cr, Fe, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Ta, W, Re, Os, Ir, Pt, Au and Bi). This work extends our previous calculations (Surf. Interface Anal. 11, 57 (1988)) for the 200–2000 eV range. Substantial variations were found in the shapes of the IMFP versus energy curves from element to element over the 50–2000 eV range and we attribute these variations to the different inelastic scattering properties of each material. Our calculated IMFPs wee fitted to a modified form of the Bethe equation for inelastic electron scattering in matter; this equation has four parameters. These four parameters could be empirically related to several material parameters for our group of elements (atomic weight, bulk density and number of valence electron per atom). IMFPs and those initially calculated was 13%. The modified Bethe equation and our expressions for the four parameters can therefore be used to estimate IMFPs in other materials. The uncertainties in the algorithm used for our IMFP calculation are difficult to estimate but are believed to be largely systematic. Since the same algorithm has been used for calculating IMFPs, our predictive IMFP formula is considered to be particularly useful for predicting the IMFP dependence on energy in the 50–2000 eV range and the material dependence for a given energy.

XPS O 1s binding energies for polymers containing hydroxyl, ether, ketone and ester groups

Abstract: XPS spectra of nine model polymers containing oxygen functional groups are studied. The measured O 1s binding energies were 532.8 eV for ether and alcohol oxgens, 532.2 eV for ketone oxygens and 532.2 and 533.7 eV for the carbonyl- and ether-type oxygens in ester groups, respectively. Comparison with previous experimental and theoretical O 1s binding energies for these groups is also presented. Absolute O Is binding energies of polymers are not in agreement with those published by other groups. However, with the exception of ketone oxygens, binding energy shifts observed for the various functional groups are comparable.

The effect of carbonate contaminations on the XPS O 1s band structure in metal oxides

Abstract: The aim of the paper is to give examples of XPS analysis, showing how O 1s bands can be affected by surface carbonates. A proper peak assignment (e.g. to O− OH− species) needs the preliminary determination of a carbonate contribution. Although the samples examined represent Co and La compounds only, the principal conclusions can be extended also to other oxide systems.

Modification of polypropylene surfaces by flame treatment

Abstract: The changes induced on the surface of polypropylene homopolymer following flame treatment have been studied. Surface compositions were determined using x-ray photoelectron spectroscopy and compared to surface free energies estimated from contact angle measurements. The effect of air-to-gas ratio, total flow rate, contact time with the flame and the distance between the inner cone tip of the flame and the polymer have been investigated. Mild flame treatments were found to be effective in promoting the adhesion of polyurethane paints to the polypropylene. The adhesion between flame-treated polypropylene and the paint film was assessed using a composite butt test and the measured bond strengths were found to be well in excess of those obtained using solvent wiping or chlorinated polyolefin primers.

An X‐ray photoelectron spectroscopic investigation of the selectivity of hydroxyl derivatization reactions

Abstract: The Selectivity of trifluoroacetic anhydride (TFAA), used for the vapor-phase derivatization of hydroxyl groups in multifunctional oxygen-containing polymeric surfaces, has been investigated by XPS studies on model epoxides systems. The reactivity of TFAA towards epoxides observed here is discussed in the context of the large amount of published literature implying high selectivity of this reaction towards hydroxyl groups. Acetyl chloride (AcCl) and heptafluorobutyryl chloride (HFBuCl), used to derivatize hydroxyl groups in the literature, were also examined, with regard to cross-reaction with epoxides. Both AcCl and HFBuCl react with epoxides, forming chloroacetates and perfluorinated chloroacetates, respectively. Moreover, HFBuCl is shown to react with double bonds forming perfluorinated chloroacetates, while AcCl is unreactive towards them. The reactivity of HFBuCl to wards epoxides and unsaturation compromises its use as a derivatization reagent of hydroxyl groups. However, the unique reaction products formed by exposure of hydroxyls and epoxides to AcCl vapor may allow hydroxyl species to be distinguished from epoxides in multifunctional carbon-oxygen polymeric surfaces.

High-resolution monochromated xps of poly(methyl methacrylate)thin films on a conducting substrate

Abstract: Thickness-dependent charging and the effect of vertical differential charging on spectral resolution are demonstrated, and the conditions required for optimum spectral resolution are established Evidence is provided for an interaction between the carboxyl group of PMMA and the substrate, and for an interaction between carboxyl groups of the polymer The latter is manifested as a dependence of the C=O and C−O C 1s chemical shift on polymer tacticity Four different curve-fitting schemes are applied to the high-resolution C 1s data

XPS valence band analysis of plasma–treated polymers

Abstract: The valence bands (VB) of remote oxygen and remote nitrogen plasma-treated polyethylene and polypropylene were studied to determine changes in the basic polymer structure. Each of the polymers showed a characteristic valence band profile, which retained much of its original shape after plasma treatment. This has suggested that even though plasma treatment can cause the incorporation of a high proportion of functional groups, significant segments of the original polymer structure remain unchanged within the x-ray photoelectron spectroscopy (XPS) sampling depth. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) suggests that near the outer surface, the polyethylene structure is modified by branching and cross-linking.

Analytical expression describing the attenuation of auger electrons and photoelectrons in solids

Abstract: The attenuation of Auger electrons and photoelectrons in solids as described by the so-called depth distribution function (DDF), has been studied by means of a Monte Carlo simulation of electron transport in matter. Elastic scattering plays a significant role in the model. Based on the results of a large number of simulations, an empirically derived analytical expression for the DDF is proposed that includes the effects of elastic scattering. As part of this analytical DDF, a new attenuation parameter (AP) is introduced, which being a material constant independent of emission angle and layer thickness, can assume a central role in quantitative AES and XPS. The proposed AP as well as the general validity of the results is discussed. Most importantly, a simple relationship between the most significant quantities governing the transport of electrons in transport of electrons in solids, i.e. the inelastic mean free path (IMFP), the total mean free path (TEMP) and the attenuation parameter, was derived from the results. By a single Monte Carlo simulation, the AP can be determined. The DDF for different experimental geometries, film thicknesses, etc. can then be determined analytically.

Low-energy implantation and sputtering of TiO2 by nitrogen and argon and the electrochemical reoxidation†

Abstract: In this paper we report the investigation of the implantation and radiation damage during sputter profiles of 30 nm TiO2 layers on Ti with 3 keV N2+. The profiles are analysed by normal and angle-resolved XPS measurements. Supplementary electrochemical reoxidants are performed, handling the sample without any contact to air during the whole process. Additional information is obtained by comparison with 4 keV Ar+ and 3 keV air sputter profiles. At the beginning of a 3 keV N2+ sputter profile some TiO2 at the surface is converted to TiNxOy, which is only partially oxidizable by repassivation. A portion of the implanted nitrogen exists as molecular N2. After higher sputter dosed the composition changes drastically. Preferential sputtering reduces the oxygen concentration to an extent to allow TiNxO1−x and TiN formation. The TiNxOy changes its composition since it is now fully reoxidizable, as are all compounds with a Ti oxidation state of less than 4 +. N2 reacts with TiNxO1−x by recoil excitation and no longer exists. The occurrence of TiO, Ti2O3 and Ti3N4 is unlikely, but possible in small concentrations. The products of all electrochemical repassivations are TiO2 and N2. Angle-resolved XPS measurements show Ar- and N2-attenuated surface layers. During Ar and air implantation these layers remain constant owing to an equilibrium between outgassing and implantation of new sputter gas. After nitrogen implantation the N2-attenuated layers are constant in the beginning and increase when the N2 begins to react with TiNxO1−x until the N2 totally disappears. An addition of oxygen to the sputter gas can decrease the surface roughening effect, which is often found after prolonged sputtering, XPS binding energies show that Ar and N2 gas do not occur in gas bubbbbles.

AES: Accurate intensity calibration of electron spectrometers—results of a BCR interlaboratory comparison co-sponsored by the VAMAS SCA TWP

Abstract: An interlaboratory study of the calibration of the intensity scales of Auger electron spectrometers has been completed. Thirty-eight respondees have completed intensity measurements using pre-calibrated reference materials of Cu, Ag and Au in either the direct or differential modes. The reference material calibrations, made at NPL, involve the recording of true spectra under defined conditions using the metrology spectrometer, in which all instrumental contributions to intensity have been accurately clibrated. The ratio of the spectral intensity form the respondee to that derived at NPL gives the intensity–energy response function of the respondee's instrument. In the direct mode this function may be calibrated at 1 eV intervals from 10 to 2500 eV using any one of the reference materials. By using all three materials it is shown that the average repeatability of the calibrations for the 38 respondees is better than 3% over the energy range 10–2500 eV, improving to < 2% over the range 100–1000eV. The mot serious unknown parameter that has given rise to uncertainty in the intensity function in the past has been the channel electron multiplier ditector efficiency. It is shown here that this term behaves as predicted theoretically and, if ignored, can lead to a factor of two error in intensity. With the present calibration the errors should be reduced to 2% for spectrometers in which there is little internal scattering. In the differential mode twelve peaks are used to define the intensity-energy calibration. This calibration is less accurate than that in the direct mode and, at the present time, a one-standard deviation uncertainty of ∼11% is more appropriate. In both modes a loss of intensity owing to multiplier overloading may cause severe errors in the high-energy peaks.

Curve synthesis and optimization procedures for X‐ray photoelectron spectroscopy

Abstract: The selection of functions to represent XPS lineshapes is discussed. An improved Lorentzian/Gaussian product function is proposed for fitting symmetrical XPS peaks, and a further empirical modification is shown to represent asymmetric (metallic) XPS peak profiles very closely. One additional parameter sets the asymmetry at half-height, while a second governs the tail profile. An approach to curve synthesis that rapidly yields good first approximations to complex multicomponent spectra is described. Previous fits to model systems may be incorporated, and background intensity differences between the high-and low-energy sides of each model peak may be retained in the fit. A simple least-squares optimization procedure is described in which divergence is impossible but which is still sufficiently fast for microcomputer implementation. Physically desirable parameter constraints can be applied, and excessively large parameter changes are prevented, even for small components of the fit. The relative precisions of the computed parameters also estimated.

A third‐generation Auger microscope using parallel multispectral data acquisition and analysis

Abstract: The design and construction of an ultrahigh vacuum multi-imaging scanning electron microscope is described. The microscope is designed to contain two field electron emission columns and can acquire simultaneous digital images from a 16-channel electron spectrometer, a four-quadrant back-scattered electron (BSE) detector, an Si(Li) x-ray detector, a SEM detector and the current flowing to ground through the sample. Because there is exact spatial registration between corresponding pixels in each of the images, it is possible to use the image set to make quantitative interpretations of the surface and subsurface chemistry

Experimental and theoretical XPS study of model molecules for poly(methyl methacrylate)

Abstract: High-resolution C is and O is gas-phase X-ray photoelectron spectra of methyl isobutyrate, methyl methacrylate, methyl acrylate, acrylic acid, acetic acid and formic acid have been investigated as model compounds for solid poly(methyl methacrylate) (PMMA). Secondary chemical shifts, variations in peak shapes and intensities for the different core lines were observed. It is shown that asymmetries and differences in line widths of the component peaks should be considered for a correct analysis of the solid polymer spectra. The observed intensities of the different C 1s and O is peaks in the gas-phase spectra do not follow stoichiometric ratios owing to different shake-off/shake-up probabilities. The gas-phase core electron binding energies have been compared with ab initio ΔSCF calculations where the relaxation of the core-hole state was included. Using a group shift argument, the relationship between the gas-phase model molecule shifts and the solid PMMA shifts have been discussed and applied to the analysis of the PMMA spectra.

Surface modification of extended chain polyethylene fibres to improve adhesion to epoxy and unsaturated polyester resins

Abstract: Extended chain polyethylene fibres have been treated in ammonia and oxygen lo‐pressure gas discharges (plasmas) in order to enhance adhesion to epoxy and unsaturated polyester resins, respectively, and thus significantly improve fibre/resin interfacial properties in fibre‐reinforced polymer composites. Ammonia plasma treatment results in the incorporation of amine functional groups onto the fibre suface. The treated fibre surface has been analysed using XPS and spectrophotometric techniques. Extended chain polyethylene/epoxy composites made from ammonia, plasma‐treated fibres show a marked increase in interlaminar shear strength over composites made from untreated, corona‐treated or oxygen plasma‐treated fibres. The increase in fibre/resin adhesion after ammonia plasma treatment is confirmed by SEM observations of fracture surfaces, which show clean interfacial fracture surfaces in composites made from treated fibres. Fibres modified by oxygen plasma treatment contain a significant concentration of carbon‐oxygen functionalities, which contribute to the polarity of the surface and hence increase wet‐out by unsaturated polyester resins. The concentration and nature of carbon‐oxygen species on the fibre surface have been determined by XPS. Pull‐out tests on multifilament yarns embedded in a polyester resin confirm the high fibre/matrix adhesion achieved with the oxygen plasma‐treated fibres compared to corona‐treated or untreated fibres. Tensile properties of the fibres are reduced significantly after prolonged treatment in an oxygen plasma, while in an ammonia plasma the fibre strength is unaffected.

Oxide formation on Fe and Ti thin films and on Fe thin films modified with ultrathin layers of Ti

Abstract: Reactions of O2 and H2O to from thin oxides on Fe, Ti and Ti-modified Fe thin films have been studied by XPS, following O2 and H2O exposure in the range 0-600 L, to form oxides with thicknesses of < 40 A. XPS Iineshape analysis is used, utilizing a special combination of reflection electron energy-loss spectroscopy (REELS) and nonlinear least-squares fitting routines to model the intrinsic and extrinsic energy losses that accompany potoemission for the Ti and Fe 2p lines. During the formation of the thinnest oxides, this approach yields a unique picture of the composition of the oxide, while for the thicker layers, there is little significant difference between this fitting approach and (1) fitting approaches using an integral background approach or (2) direct deconvolution methods using the REELS data. Both Fe and Ti are quite reactive to O2, yielding a surface oxide that is apparently an FeO-dominated FeO/Fe3O4 bilayer on Fe surface oxide that is predominantly TiO2 on the Ti surface. Pure Fe and Ti surfaces are unreactive to H2O. Predosing of thewse surfaces with low levels of O2 (5 L) does not increase the reactivity appreciably toward H2O. Ti dispersed on the Fe surface as an adatom layer, with an equivalent thickness of 3 A, greatly suppresses the reactivity of Fe toward O2, while the Ti is oxidized primarily to TiO2, In contrast to the pure Ti layers, these Ti adatom layers are quite reactive toward H2O, yielding oxides (no evidence for hydroxide) in a uniform distribution of oxidation states (Ti+2, Ti+3 and Ti+4).

Noval detection scheme for the analysis of hydrogen and helium by secondary ion mass spectrometry

Abstract: For the analysis of hydrogen and helium in solids and thin films by secondary ion mass spectrometry, the use of Cs+ primary ions in conjunction with the detection of HCs+ and HeCs+ molecular ions is proposed. For both species, detection limits of the order of 1019 atoms cm−3 are obtained at sputter removal retes of ∼1 nm s−1. A relative depth resolution of ∼2% is demonstrated for hydrogenated amorphous Is layers on single-crystal Si. In these specimens, the HCs+ intensities scale linearly with the hydrogen concentrations cH in the range investigated (cH < 20at.%); hence, a quantitative evaluation of the H content appears possible by means of a single standard.

AES and factor analysis study of cord‐oxidized brass layers and rubber‐to‐brass interface chemical composition

Abstract: Auger electron spectroscopy (AES) and factor analysis (FA) were applied to study the chemical structure of oxidized brass layers and rubber-to-brass interface of tyre cords. The optimum requirements of sample preparation, instrument performance, data acquisition and analysis were determined to ensure accurate quantitative data. The elemental and chemical depth profiles of oxidized brass layers plated on steel cord and rubber-to-brass interface for standard vulcanization technology and after ageing obtained.

Surface composition of pentlandite under flotation‐related conditions

Abstract: X-ray photoelectron spectroscopy (XPS) and electrochemical techniques have been applied to the investigation of the surface oxidation of pentlandite. Photoelectron spectra indicate that the initial reaction on exposure to air is removal of iron from the pentlandite lattice to form a hydrated iron oxide overlayer and leave a metal-deficient pentlandite in addition to a restructured nickel-ion sulphide Further oxidation resulted in some nickel being transferred to the oxide. In dilute acetic acid, the oxide layer was largely soluble

Quantitative aspects of ultraviolet photoemission of adsorbed xenon—a review

Abstract: Ultraviolet photoelectron spectroscopy (UPS) is usually not considered to be a quantitative technique for the analysis of solid surfaces. However, careful examination of the He(I)-induced 5p spectra of adsorbed xenon provides the basis for quantitative description of non-uniform solid surfaces and/or adsorbed layers of xenon themselves. The shape, intensity and electron binding energy of the Xe 5p emission can be associated with the distribution, concentration and local surface potential of particular adsorption sites, respectively. Structural defects, such as steps, edges of evaporated metal islands and pores, can be characterized in this way. A particularly important application of the photoemission of absorbed xenon (PAX) is the determination of the coverage and average island size of metal adsorbates. The early stages of film growth and the thermally activated healing of surface roughness of such deposits can be followed quantitatively by PAX. Interesting quantitative information can also be obtained on the structure of the adsorbed xenon layer itself, such as the critical coverage for the two-dimensional 2D Xe gas ↔ 2D Xe solid phase transition, the nucleation of three-dimensional Xe clusters at defects and the presence and equilibrium topography of adsorbed Xe multilayers. These analytical possibilities of the PAX method are illustrated with applications to the Ru(001) surface, the epitaxially grown Ag film, the bimetallic Ag/Ru(001) system and the trimetallic and the trimetallic AgAu/Ru(001) system.

SIMS profile simulation using delta function distributions

Abstract: SIMS profiles of abrupt dopant distributions in semiconductor materials contain distortions introduced by the measurement process. In quantitative depth profile studies, it is important to determine and remove these errors. We have used the idea that a given input profile may be represented by a series of adjacent delta-doped planes. Given a knowledge of the SIMS profile obtained from one single plane (the resolution function), it is possible to simulate the expected SIMS profile from a given input profile by the convolution with the resolution function. Experimental data have been obtained from special samples of molecular beam epitaxial GaAs containing both delta-doped planes and uniformly doped regions (1-20 nm thick). The dopants investigated were Si, Be and Al. It is shown that the simulated and experimental profiles of the uniformly doped regions show good agreement, thus demonstrating the validity of the method. We also present results of the convolution of an input function having exponential decays on the leading and trailing edges, which show the corrections that should be made to experimentally determined profiles. Additional simulations have been made with Gaussian inputs, which are relevant in dopant diffusion studies. Silicon diffusion lengths as small as 4 atomic planes (1.1 nm) have been deduced

XPS and SSIMS analysis of polymers: The effect of remote nitrogen plasma treatment on polyethylene, poly(ethylene vinyl alcohol) and poly(ethylene terephthalate)

Abstract: A study has been undertaken in which both x-ray photoelectron spectroscopy (XPS) and Fast atom bombardment static secondary ion mass spectrometry (FAB-SSIMS) have been used to study the effects of remote nitrogen plasma treatment on polymers such as linear low-density polyethylene (LLDPE), poly(ethylene vinyl alcohol) (EVOH) and poly(ethylene terephthalate) (PET). For comparison, remote oxygen plasma treatment was also performed on LLDPE. A very rapid uptake of nitrogen was observed for all polymers. Negative FAB-SSIMS indicated CN−, CNO− and C2N fragments on each of the nitrogen plasma-treated polmers. Positive FAB-SSIMS spectra of plasma-treated LLDPE showed relatively high intensity, high mass fragments, thought to originate from additives. These were not observed for the other two polymers. Significant amounts of aromatic-type fragments were observed in the positive FAB-SSIMS spectra of all treated polymers. Surface stability studies have shown that for both nitrogen and oxygen plasma-treaed LLDPE there is a substantial decrease in the surface functionality on exposure to air. This effect was much less prevalent for EVOH and PET.

Auger parameter shifts in the case of the non-local screening mechanism: Applications of the electrostatic model to molecules, solids and adsorbed species†

Abstract: A simple electrostatic model is applied to predict and rationalize the auger parameter shifts in molecules, solids and adsorbed species. The model is valid in the case of the non-local screening mechanism, i.e. when the screening of the core hole is due to the polarization of the nearest-neighbour ligands (no charge transfer from the ligands to localized electronic levels of the atom with the core hole, as occurs with the main peak of the core-ionized heavier 3dn transition metal ions or the light lanthanide ions). To good approximation it is shown that: (1) the Auger parameter shift is a function of the number, distance, electronic polarizability and local symmetry of the first-neighbour ligands of the core-ionized atom; (2) the dipole–dipole interactions can play a key role in determining the extent of the extra-atomic relaxation energy and hence of the Auger parameter shift.

Impurity migration during SIMS depth profiling

Abstract: Among the many processes that affect the accuracy of sputter depth profiling, ion beam-induced segregation (IBIS) may result in impurity relocations over many micrometres. The most devastating IBIS mechanism of distribution disruption is known to become operative when certain impurities in silicon are profiled under oxide-forming measurement conditions. Here, we forward evidence that in this particular manifestation of IBIS the bild-up of an electric field across an insulating surface layer provides an important driving force. Migration towards as well as away from the bombarded surface is observed whenever negative or positive charging conditions are applied. This behaviour has only been observed for elements with a tendency to become singly positively charged ions. In the absence of an electric field, a smaller segregation effect of Group Ia elements away form the surface is shown. Occurrence of this effect for the Group Ia and not for the other elements is indicative of a possible influence of the thermodynamic differences. Further aspects of this field-induced migration are discussed on the basis of extensive experiments in which the role of the concentration of segregating impurity and the mobility of the impurity in the insulating layer is investigated. A justification of the methodology adopted is provided. Earlier attempts to explain this particular form of IBIS are examined critically.

Facts and artefacts in the characterization of Si/SiGe multiayers with SIMS

Abstract: A number of Si/Si1−x Gex heterostructures, grown by molecular beam epitaxy (MBE) and vapour phase epitaxy (VPE), were examined with secondary ion mass spectrometry (SIMS). A variety of experimental conditions was employed, limited to positive secondary ions and moderate to excellent depth resolution. The most elaborate results were obtained for O2+ primary ion bombardment at energies of 2–8 keV. These demonstrate (for x < 0.4): (1) a minimal erosion rate variation with germanium content x; (2) the near-absence of matrix effects, i.e. x dependence, of the secondary ion yields for impurities like B, Ga, Sn, Sb and others; (3) an approximately concentration-independent sensitivity for germanium. Furthermore, a surface-energy-difference-driven, Ge-segregation mechanism operative during MBE growth is discussed. This phenomenon is just one manifestation of the fundamentally different processes during MBE and VPE. Such processes may be responsible for the fact that interfaces in MBE samples, as obtained with SIMS, are loss abrupt than for VPE material. A simple explanation in terms of an inferior substrate-cleaning procedure cannot be ruled out, however.