Showing papers in "Surface and Interface Analysis in 1988"

Calculations of electron inelastic mean free paths for 31 materials

Abstract: We present new calculations of electron inelastic mean free paths (IMFPs) for 200–2000 eV electrons in 27 elements (C, Mg, Al, Si, Ti, V, Cr, Fe, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au and Bi) and four compounds (LiF, SiO2, ZnS and Al2O3). These calculations are based on an algorithm due to Penn which makes use of experimental optical data (to represent the dependence of the inelastic scattering probability on energy loss) and the theoretical Lindhard dielectric function (to represent the dependence of the scattering probability on momentum transfer). Our calculated IMFPs were fitted to the Bethe equation for inelastic electron scattering in matter; the two parameters in the Bethe equation were then empirically related to several material constants. The resulting general IMFP formula is believed to be useful for predicting the IMFP dependence on electron energy for a given material and the material-dependence for a given energy. The new formula also appears to be a reasonable but more approximate guide to electron attenuation lengths.

Quantitative analysis of the inelastic background in surface electron spectroscopy

Abstract: Recent progress, in the quantitative analysis of the distortion of surface electron spectra, resulting from inelastic electron scattering, is reviewed. A physical model for the problem is formulated and based on this, basic deconvolution formulae are found. In the formulae, detailed information on the differential inelastic electron scattering cross-section K is essential. Methods to determine,K are studied and developed. One method is based on a dielectric response calculation. Furthermore, it is found that for the noble and transition metals, K is sufficiently similar that a simple ‘universal’ cross-section can be defined. In addition, methods for experimental determination of cross-sections are found and in particular a formula is derived which allows the extraction of direct quantitative cross-section information through a simple analysis of REELS spectra. The importance of proper background correction is investigated for model electron spectra. Experimental XPS spectra are also studied and it is found that for both the simple and the noble metals, the primary excitation spectra extend 2–3 inelastic scattering events, i.e. ∼50 eV, below the peak energy. Beyond this energy, the spectra are of zero intensity over a wide energy range. Besides, experimental spectra are analysed on the basis of experimentally determined cross-sections and the results are discussed, in relation to the analysis based on theoretical cross-sections. The importance of defining simple procedures for routine analysis of spectra is stressed and it is demonstrated that the proposed ‘universal’ cross-section provides a very simple method for approximate spectral analysis. Finally, some important issues to be considered in the near future are pointed out.

An x‐ray photoelectron spectroscopic study of some chromium–oxygen systems

Abstract: A series of pure and mixed chromium standards has been analysed by x-ray photoelectron spectroscopy in order to obtain a sufficiently homogeneous and self-consistent set of spectral parameters to be used in the resolution of complex spectra. Additional information was obtained about the stability of the various species under Ar ion bombardment. The results relevant to potassium chromate and dichromate, chromous acid, chromium (III) oxide, hydroxide and nitrate are described and compared critically with those in the published literature.

Advances in charge neutralization for XPS measurements of nonconducting materials

Abstract: The build-up of a positive charge during the XPS measurement of nonconducting materials can severely affect the spectral resolution leading to line broadening and time-dependent energy shifts. The use of a low kinetic energy electron flood gun in conjunction with a metal screen placed in close proximity to the sample surface permits the establishment of a uniform surface potential. The advantages of this approach are demonstrated with spectral data taken from a variety of organic materials. Spectral line widths of 0.9–1.3 eV are routinely obtainable for the C(1s) core level photoelctron signals. Further, the reproducibility obtained on line positions (±0.1 eV) allows the use of a calibration procedure to study the relative line positions between different samples. This combined with better resolution allows a more complete assignment of spectral features including some nearest neighbor effects. A shift between aromatic and aliphatic C(1s) lines is observed.

Comparison of electron attenuation lengths and escape depths with inelastic mean free paths

Abstract: It is shown that calculations of electron attenuation lengths and escape depths would require modification of definitions proposed in the literature, since these definition are not universally applicable. Exemplary experimental arrangements are indicated for which more general definitions are necessary, and such definitions are proposed. A Monte Carlo program has been developed with which it is possible to estimate attenuation lengths and escape depths. A comparison of these parameters is made for photoelectrons arising from s levels.

Analysis of polymer surfaces by SIMS. 12. On the fragmentation of acrylic and methacrylic homopolymers and the interpretation of their positive and negative ion spectra

Abstract: The positive and negative ion spectra from homologous series of alkyl methacrylate, hydroxyalkyl methacrylate and alkyl acrylate polymers; poly (methacrylic acid) and poly(methacrylic anhydride) are reported. A systematic interpretation of the spectra can be accomplished on the basis of fragmentation pathways consistent with the known effects of other ionizing radiations. The negative ion spectra, previously little studied, are a powerful probe of structure for this class of polymer.

Ignition mechanism of the titanium–boron pyrotechnic mixture

Abstract: Titanium–boron pyrotechnic reactions are essentially gasless, are very exothermic, and are known to initiate only at extremely high temperatures. The reactants are stable in normal laboratory environments and require no special sample handling, such as inert storage. These factors make the titanium–boron mixture ideal for one-shot thermal heat source applications. Mound has been investigating energetic material ignition properties for a number of years. Pyrotechnic mixtures of TiHx/KClO4 have revealed that the surface composition of the titanium fuel was TiO2 and its presence on the fuel's surface controls the TiHx + KClO4 reaction. In the present study the surface chemistry of titanium and of boron have been examined before ignition. To understand the effect of temperature on the reactants and the mixture, titanium powder, boron powder, and blends were analyzed at ambient and elevated temperatures. XPS, TG and DTA results presented will show that the oxide on boron is the controlling factor in the ignition mechanism of the titanium–boron pyrotechnic reaction.

Investigation of a passive film on an ironchromium alloy by AES and XPS

Abstract: Passive films formed on a Fe24Cr alloy in 0.5 M H2SO4 at 0.7 V are characterized by AES depth profiling and XPS. Obtained results and evaluation procedures are critically compared. It is found that correction for electron escape depth significantly improves the accuracy of AES profile data. The use of low energy peaks in AES allows metallic species to be distinguished from oxidized species and profiles with good depth resolution to be obtained. Quantification is impeded by peak overlap. XPS results depend significantly on the background subtraction and peak deconvolution procedure employed. From the different data it is concluded that the ratio Cr/Cr + Fe in the passive film studied is approximately 0.9 compared to 0.26 for the bulk alloy. The film thickness is 1.5 ± 0.3 nm. Within the precision of present XPS data the metal composition of the layers adjacent to the film is the same as that of the bulk.

XPS studies of the thermal behaviour of passivated Zircaloy‐4 surfaces

Abstract: The composition and the chemical states of components of Zircaloy-4 (zirconium alloy) surfaces were studied in the temperature range between room temperature and 500°C. Each sample was kept at constant temperature (25, 100, 200, 300, 400, 500°C) for up to 16 hours. The changes of composition and chemical states of the Zircaloy-4 surface during heating were monitored by x-ray photoelectron spectroscopy (XPS). Originally, the components form well-defined layers elucidated by angle-resolved x-ray photoelectron spectroscopy (ARXPS). In contrast to depth profiling using ion sputtering, ARXPS is non-destrutive. However, it is applicable for layers of up to a few nanometres thickness only. The experiments showed a decomposition of the ZrO2 coverage above 200°C accompanied by oxygen diffusion into the bulk. These processes lead to the reduction of ZrO2 to metallic zirconium on the surface at 300°C and higher temperatures. The oxygen diffusion into the bulk was indicated by AES depth profiles. The layered structure observed up to a heating temperature of 200°C could not be seen at higher temperatures. After Zr metal appears at the surface during the heating process, a reaction with the adsorbed hydrocarbons takes place, leading to the formation of zirconium carbide. Though the depth resolution of an AES depth profile does not permit identification of layers with thicknesses in the nanometre region, the temperature-dependent behaviour of oxyen is reflected by its AES profiles, showing features in accordance with the results from ARXPS, especially with respect to the fact that well-defined layers vanish above 200°C.

Esca and reels characterization of electrically conductive polyimide obtained by ion bombardment in the keV range

Abstract: Polymide films were bombarded with Ar+ at 150 keV at various doses from 5 × 1012 to 2 × 1017 ions cm−2. Ion bombardment was found to produce a drastic decrease of the electrical resistivity of the polymide from about 1016 to 3 × 10−3 Ω cm, the effect being dependent on the ion dose. The chemical structure of the conductive films obtained was characterized by means of ESCA and REELS techniques. The modification of the original polymer seems to proceed at low ion doses (up to 5 × 1014 ions cm−2) by means of the progressive elimination of the carbonyl groups and the related destruction of the imidic rings, while at high doses (from 5 × 1015 ions cm−2) the carbonization of the polyimide occurs with the production of an amorphous carbon still containing significant amount of residual N and O atoms.

Interpretation of the fragmentation patterns in static SIMS analysis of polymers. Part I. Simple aliphatic hydrocarbons

Abstract: A number of simple aliphatic hydrocarbon polymers have been studied by static SIMS. The low-mass positive spectra reflect structural differences in the polymeric structure, which result in characteristic fingerprint spectra. Unsaturation and differences in branching lead to very distinct spectral features which show promise for the application of SIMS as an analytical technique in polymer-related areas. An attempt is made to interpret the spectra in terms of molecular structure, fragmentation mechanisms and ion stabilities. The chemical nature of ion beam damage in PE and PP was studied to some extent.

Selective oxidation and chemical state of Al and Ti in (Ti, Al)n coatings

Abstract: (Ti, Al)N hard coatings are a promising alternative to TiN owing to an improved resistance to oxidation. In order to study the oxidation process in more detail, (Ti, Al)N coatings reactively sputter-deposited on high speed steel substrates from a target composed of Ti:Al = 50:50 (at%) were annealed in air and in vacuum at temperatures between 500 and 800 °C. Auger electron spectroscopy (AES) sputter depth profiling as well as X-ray photoelectron spectroscopy (XPS) studies indicate a high stability of the Ti-N bond, whereas Al has considerable mobility in the mixed nitride. The chemical shift of the Al 2p XPS and LVV AES peaks allow a distinction between oxide, nitride and metallic states of Al. Preferential sputterin during depth profiling with 3 keV Ar+ ions resulted in a relative depletion of Al in the surface layer. It is concluded, that the Al in (Ti, Al)N is only weakly bonded to nitrogen as compared to Ti.

XPS and IR(ATR) analysis of Pd oxide films obtained by electrochemical methods

Abstract: Palladium oxide was detected by XPS and IR (Attenuated Total Reflection) analysis of the surface of a Pd electrode which was polarized in the medium 1 M KOH. This oxide set a peak of Pd 3d5/2 shifted 3.5 eV with respect to the metal which is a much higher value than those reported and obtained for PdO and PdO2. An IR absorption band of 535 cm−1 was found to be different from those reported in the literature of PdO and PdO2. The presence of this oxide is in agreement with results obtained using electrochemical methods where the existence of PdO3 has been assumed.

XPS analysis of oxides

Abstract: The principal scope of this work is to explore the capacity of XPS spectroscopy in the characterization of oxides through consideration of O 1s spectra. Several examples of oxides, analysed both separately and in the form of mechanical mixtures, are reviewed. The quantitative aspects involved in the XPS analysis of oxide mixtures were studied by use of an approach based on intensity ratio measurements of the single O 1s components. Results obtained in the mixtures are utilized for establishing the presence, or otherwise, of a single oxidic phase at real interfaces of iron-alumina systems, metallurgical coatings, and glasses.

Ion bombardment effects on the near-surface composition during sputter profiling

Abstract: The alteration of the near-surface composition that has often been observed in ion bombarded alloys and compounds results from the dynamic interplay of several processes. In addition to displacement mixing leading to a randomization of atomic locations, which is dominant at relatively low temperatures, and preferential loss of alloying elements by sputtering, many thermally-activated processes, including radiation-enhanced diffusion, radiation-induced segregation and Gibbsian adsorption, also play important roles. The relative significance of each process in the evolution of the sputtered flux composition and of the compositional profile of the target depends on the type of alloys, temperature and ion characteristics. Although a good semi-quantitative understanding already exists for most of the individual processes, the number of systematic investigations of alloy sputtering that consider all of these processes and their potential interactions is still rather limited. A fundamental understanding of the synergistic effects of these processes is, however, essential for quantitative analysis of multicomponent-material surfaces, since sputtering is routinely applied in conjunction with various analytical techniques to depth-profile the composition. The purpose of this paper is to characterize the processes in simple physical terms and to point out under what conditions they may become important for compositional changes in near-surface regions of sputtered alloys, based on selected results of recent model calculations and experimental studies.

Edge effects and image contrast in scanning Auger microscopy: A theory/experiment comparison

Abstract: The contribution of surface roughness to the artefacts present in images and line-scans from Scanning Auger Microscopes have been explored by Monte Carlo modelling and by experiment. The samples chosen for these studies are metal lines with sharp rectangular cross-sections deposited upon silicon substrates. The Monte Carlo simulation reveals shadowing effects due to the analyser/sample geometry and the obstruction due to the height of the overlayer line; forward scattering effects through the sidewall of the overlayer which lead to edge enhancements in the overlayer and substrate Auger signals; back-scattering effects which show up as an apparent broadening of the overlayer lines and substrate signal peaks appearing in the otherwise forbidden shadowed region. All of these effects are visible in the experimental results obtained for Si substrates with Al and with Au lines upon them. The experimental edge enhancement artefact is compared with the results of the simulation and good agreement is obtained. The difficulties which arise in the separation of topographical and chemical contrast are discussed and some suggestions for removal or reduction of the magnitude of these effects are outlined.

Adhesion of polymers to metals: A review of the results obtained studying a model system

Abstract: Following a rapid overview of the various mechanisms involved in the process of polymer adhesion to metals, modelling is attempted in order to describe, at molecular scale, the metal—polymer interface in its technological reality. In particular, the concept of an active site is explained and the role of these sites in bonding mechanisms is illustrated. The model is then verified in laboratory using experimental and theoretical techniques appropriate for the problem posed. Electrochemistry in organic environment is used to impart to the surfaces a homogeneous acid or basic character (i.e. electron acceptor or donor) and to set up an intense anisotropic electrical field allowing nonrandom simulation of the fields induced by the roughness of the potential of the surface of an industrial object. It is thus demonstrated that the establishment of a strong metal—polymer bond results from so-called Lewis acid—base reactions activated by the interfacial electrical field. Finally, the ageing study of the metal—polymer system thus obtained shows that the interface is stable relative to atmospheric oxygen and that vacuum pyrolysis converts the polymer into a carbon fibre strongly bonded to the metal.

A quantitative surface analytical study of electrochemically-formed copper oxides by XPS and X-ray-induced Auger spectroscopy

Abstract: Electrochemically partially-oxidized Cu specimens are transferred from the electrolyte (pH 13 and 14) within a closed system without air contact to the UHV and examined with XPS especially by interpreting the x-ray-induced Auger signal L3MM. The signal was fitted with standard spectra of clean Cu and Cu2O using an approximation by the Gauss/Lorentz curves. The oxide thickness calculated from the intensity ratio of the Cu2O and Cu signal with an appropriate value for the escape depth of the electrons agrees well with the evaluation of the electrochemical data. Angular dependent measurements permit a reliable examination of Cu2O of less than 0.5 monolayers. The water content of the oxide layer deduced from a quantitative evaluation of the O 1s XPS signal decreases with increasing thickness of Cu2O approaching less than 10% for ca. 10 monolayers. The small amount of hydroxide is located at the outmost layer. The growth of an outer CuO film leads to an increase of the hydroxide content to ca. 30%. These studies show that the Cu L3MM Auger signal may be used to evaluate quantitatively the amount of Cu2O.

XPS characterization of ultrafine Si3N4 powders

Abstract: The mechanical properties of Si3N4 based ceramic bodies are largely controlled by the phases formed on the grain boundaries during sintering. For this reason determination of the state and the chemical composition of the surface of the starting Si3N4 powder is of prime theoretial and practical importance, especially in the cases when advanced ultrafine powders are used. In this work an ultrafine Si3N4 powder, obtained by high temperature plasma nitridation of silicon, has been characterized by XPS. Oxygen, carbon and also a small amount of potassium were detected as surface impurities. The Si 2p and Si (KLL) spectral lines could be decomposed into two components corresponding to silicon in Si3N4 and SiO2 phases. From angle-resolved and Ar-ion depth-profiling experiments a layer model, consisting of a Si3N4 core covered by a relatively thick SiO2 layer and a carbon contaminant overlayer, could be elucidated.

SiO2 in 6:3 (stishovite) and 4:2 Co‐ordination—Characterization by core level spectroscopy (XPS/XAES)

Abstract: A comprehensive XPS/XAES investigation of the dense 6:3 co-ordinated SiO2 polymorph stishovite in comparison with different SiO4/2 samples and Si was carried out for the first time By evaluation of initial state and final state shift contributions, effective charges and extra-atomic relaxation energies were determined and discussed in the context of the electronic structure for different co-ordination numbers The reduced binding energies of Si and O lines for stishovite are due to the higher extra-atomic relaxation energy in comparison to SiO4/2, which correlates with the higher dielectric constant and the lower gap energy in SiO6/3 The effective charges are only slightly higher in stishovite than eg in quartz

The influence of Y on the structure and growth mechanism of alumina scales

Abstract: Small additions of yttrium to NiAl result in a decreasing oxygen transport through the Al2O3 scale on AiAl at 1270 K, as has been shown by nuclear reaction analysis (18O(p, α)15N). A higher concentration, e.g. 0.5 wt%, has the opposite effect. Yttrium segregates to the grain boundaries in the alloy, and an enhanced Y concentration has been observed by means of EPMA in the oxide scale above the grain boundaries of the alloy. Transition aluminium oxides are shown to be formed in the initial stage at 1170 K with (glancing angle) x-ray diffraction. Only α-Al2O3 is formed after oxidation for several minutes at higher temperatures (1470 K). The rate of transformation of transition Al2O3 to the stable α-Al2O3 modification depends on the oxidation temperature. The transition is shown to be retarded by the presence of Y. The primary transport processes in the Al2O3 scales are short-circuit diffusion of oxygen and/or aluminium. At 1170 K, while transition oxides are still present, Al transport predominates. At higher temperatures, when the oxide has the α-Al2O3 structure, oxygen transport is enhanced. This would also explain the retardation of oxygen transport by the addition of Y, since Y was found to prevent the formation of the most stable form of Al2O3.

Quantitation of coverages on rough surfaces by XPS: An overview

Abstract: Selected methods for the XPS quantitation of surface coverages are summarized with an emphasis on non-planar samples with overlayers, e.g. powders of high surface area. Equations have been modified for uniform terminology and are presented based on a photoelectron spectrometer with an E−1 transmission function. Approaches which require a minimum of information from independent analytical techniques are given special consideration. Quantitative schemes discussed include calibration curves, surface sensitivity factors, exponential attenuation of bulk signals by overlayers, catalyst models, and the use of subshell twins or peak shape distortions related to photoelectron energy losses.

The surface analysis of a plasma modified contact lens surface by SSIMS

Abstract: The oxygen glow discharge plasma modification of a contact lens material has been studied using SSIMS. SSIMS provides evidence at the molecular level for the conversion of surface organosiloxane species to inorganic silica functionalities with treatment time. These findings shown an excellent correlation with a dramatic increase in surface hydrophilicity, as derived from surface energy studies, on plasma treatment. This study further illustrates the potenial of SSIMS in the determination of the molecular changes at the surface during the plasma treatment process.

Oxidation of NiCr and NiCrFe alloys at room temperature

Abstract: The primary stages of the oxidation of NiCr23 and of NiCr21Fe12 and the final oxide layers were studied at room temperature and oxygen pressures between 10−6 Pa and 10−4 Pa using AES and XPS. The composition of the surface during oxygen exposure was monitored by continuous recording of the Auger transitions M23 VV and L3M23V of the metallic components and the KL23L23 transition of oxygen. The low energy M23VV transitions are especially indicative of the present chemical state which is additionally characterized by the 2p photoelectron spectra. After different oxygen exposures, the thickness of the oxide layers was determined by angle resolved AES, XPS results, and by sputter depth profiling which also gives the elemental in depth distribution. It is concluded that the initial preferential oxidation of Cr is followed by a surface enrichment and oxidation of the remaining elements with lower affinity to oxygen, i.e. Ni in NiCr and both Ni and Fe in NiCrFe.

VAMAS Surface chemical analysis standard data transfer format with skeleton decoding programs

Abstract: In surface analysis today many commercial instruments are operated through a computer. This computer is also used for processing the captured data, using routines from a built-in set of options for peak synthesis, peak deconvolution, background subtraction, peak area measurement, quantification in various levels of sophistication, mapping, depth profile presentation, smoothing, differentiation and a host of other functions. However, many analysts wish to process their data on another computer in their own particular way using programs written to their specification and under their full control. They need to encode the data in the data-capture computer into a form suitable for transmission then decode it into the form required in the receiving computer. A standard format for the transferred data would clearly lead to economies in the number of programs required to effect the encoding and decoding. To meet the above requirement, as a result of a series of iterative discussions over the last three years, first within the UKESCA Users Group and then with the VAMAS National Representatives and users and manufacturers within the VAMAS Community as well as the ASTM Committee E-42.11, we have produced a Surface Chemical Analysis Standard Data Transfer Format defined in a standard metalanguage. This Format is intended to be used by those wishing to transfer data from computer to computer via parallel interfaces or via serial interfaces over direct wire, telephone line, local area network or other communications link. It is suitable for AES, EDX, FABMS, ISS, SIMS, SNMS, UPS, XPS, XRF and similar analytical methods. It covers elemental maps, depth profiles and sequences of data resulting from a variety of experiments. Thus the application of the Format is very general.