Showing papers in "Surface and Interface Analysis in 1986"

Data compilations: their use to improve measurement certainty in surface analysis by aes and xps

Abstract: Data compliations are an integral and vital part of the development of the measurement scheme for surface analysis. They have three main functions that will be considered in this survey: (i) the provision of numerical values for use in calculations of quantitative surface analysis, (ii) the provision of unbiased data for the development of theory, and (iii) the establishment of a benchmark to judge the value of current research. A brief review will be made of practical data relating to AES and XPS instruments; electron guns, x-ray sources, spectrometers and detectors. Next, data compilations of experimental measurements of relative sensitivity factors, attenuation lengths and backscattering factors will be contrasted. Finally, theoretically derived data banks of cross sections, inelastic mean free paths, asymmetry parameters, etc will be considered and conclusions drawn of the need for further work.

Practical surface analysis: state of the art and recent developments in AES, XPS, ISS and SIMS

Abstract: This review considers the well established and most frequently applied surface analysis techniques: AES, XPS, ISS and SIMS in a comparative manner, confronting fundamental principles and present instrumental performance with the requests of the scientist. The primary goal of surface analysis is the determination of the elemental composition of the outermost atomic layer of a solid. In addition, characterization of binding states, of homogeneity and of structural details will often be required. Whereas the information gathered generally contains also contributions from deeper layers, the inherent high in depth resolution of the techniques allows thin film analysis with high precision. Thus, surface analysis includes the analysis of interfaces and thin films by depth profiling. Recent progress has led to improvements e.g. in sensitivity, spectral resolution and data acquisition and handling. With the trend to high spatial resolution, surface analysis is becoming a special branch of microlocal analysis. Quantification is of primary interest. However, the variety of approaches and their dependence on instrumental parameters hamper interlaboratory comparisons, pointing out the need of standardized methods and correction procedures, of well characterized test samples and of accurate data banks. Advance in precise quantification will depend on future progress in our understanding of matrix effects and beam-sample interactions.

A comparison of positive and negative ion static SIMS spectra of polymer surfaces

Abstract: Negative ion static SIMS spectra are presented for several polymers whose positive ion spectra have been well characterized previously. The negative cluster ions present in the spectra of poly-methyl methacrylate, poly-hydroxy ethylmethacrylate and poly-ethylene terephthalate provide detailed information on polymer side chain and back-bone structure complementing and clarifying the information previously only obtainable from positive ion emission. The detection and optimization of structurally informative negative cluster ions is discussed in relation to experimental parameters and the role of surface potential.

The characterization of heterogeneous catalysts by XPS based on geometrical probability 1: Monometallic catalysts

Abstract: Geometrical probability can be used as a basis for the quantification of XPS-signals stemming from random samples such as, for instance, supported heterogeneous catalysts. From a study of three ideal cases, namely extended layers of constant thickness, equally sized spheres and hemispheres on a randomly oriented support, it has been established that the effects of angular and layer thickness averaging are interconnected in such a way that a general model can be derived. It has been found that signal ratios as measured by XPS for convex particles are determined by the surface/volume ratios of the supported phases, nearly independent of particle shape.

Quantitative Non-destructive In-depth Composition Information From XPS

Abstract: The XPS peak shape from inhomogeneous transition metals is studied theoretically under variation of the path length R travelled by the photoelectron in the solid, the XPS peak energy Er, and the particular metal M. The variation in peak shape is characterized by a single parameter D = AplB which is the ratio of the peak area to the increase in background signal 30 eV below the peak energy. It is found that D is a strong function of R, a moderate function of Eo and to first order independent of M. This gives theoretical support to the idea suggested previously of using D for fast non-destructive extraction of in-depth composition information. To a good approximation we have D(R,-E') =(425+Ee)xR-1'r+r'4xr0 oEo, with Eo in eV, R in A and D(R,E') in eV. The practical use of this function in the analysis of experimental data is discussed,

Auger electron spectroscopy depth profiling during sample rotation

Abstract: There are many factors that affect depth resolution of AES compositional depth profiles; quite often, it is degraded mostly by the sample surface roughness. We differentiate between the initial roughness of the sample surface and the roughness induced by ion sputtering during depth profiling. The influence of the ion beam incidence angle and of the ion energy on the depth resolution of multilayer Cr/Ni thin film structures was initially investigated on the samples that were stationary during AES depth profiling. The sputtering induced surface topography was investigated by transmission electron microscopy. In order to diminish topographical effects, we applied sample rotation during AES depth profiling. The sample holder of a commercial AES instrument was equipped with a mechanism that enabled sample rotation (1 rpm) during the simultaneous ion sputtering and AES analysis. The influence of sample rotation on depth resolution was studied on multilayer Cr/Ni thin film structures deposited on smooth substrate and on SiO2/Si samples. In all the cases studied, sample rotation during depth profiling improved depth resolution. This improvement was in some cases surprisingly large, whereas in other cases it was less substantial, depending on the type of sample and its original surface topography, on the ion beam incidence angle, on the ion beam energy, and on the sputer depth. The main reasons for improved depth resolution resulting from sample rotation during depth profiling are the decrease of the angular dependence of the sputtering yield for variously oriented microplanes with respect to the ion beam direction, and the decrease of the roughness induced during ion sputtering.

A model for determining the composition of layer structured samples using XPS electron take‐off angle experiments

Abstract: La region superficielle d'un solide est traitee comme une structure multicouche. Etude de la structure d'un film de stearate de cadmium depose sur une surface de silicium oxyde

Determination of depth resolution from measured sputtering profiles of multilayer structures: Equations and approximations

Abstract: Etablissement d'equations permettant la determination de la resolution a partir de mesures des profils de pulverisation; discussion des possibilites d'application et des erreurs introduites en fonction du rapport entre la resolution et l'epaisseur des couches elementaires

Quantitative AES depth profiling of very thin overlayers

Abstract: A model calculation is proposed which quantitatively evaluates measured AES sputtering profiles of very thin overlayers in terms of the original elemental depth distribution. The model is based on the sequential layer sputtering model, including the escape depth of the Auger electrons. A proposal for the incorporation of a site dependent sputtering yield and of preferential sputtering effects is presented and the limitations and capabilities are discussed. Applications to experimental data are shown for oxide films on tantalum, for an oxidefilm on a NiCr20 alloy and for a passive film on FeCr18Ni9.

Quantitative surface analysis by glow discharge optical spectrometry

Abstract: In an analysis by glow discharge optical spectrometry the relation between the intensity of an optical emission line of an element and its concentration on the surface is exploited for quantitative determination of variations of composition in galvanized coatings and for an estimation of the enrichments on the surface of annealed extra mild steel sheets.

The role of electron spectroscopy in corrosion science

Abstract: The two principal electronspectroscopies – photoelectron and Auger electron – now represent the mature methods of surface analysis and both have been used to further the study of corrosion almost since their inception. This review will examine those areas in which significant developments have been made by their use and will highlight their continuing role in post-corrosion failure analysis. Specific topics which will be covered include: (1) aqueous corrosion: the formation and composition of passive layers, their interaction with surrounding solutions, and their failure by pitting and stress corrosion; (2) coated surfaces: pretreatment for coating, locus of failure and disbonding under chemical or electrochemical attack; and (3) high temperature oxidation: segregation to the surface, the formation of stable layers and interlayer reaction, and the composition of barrier layers and of spalling interfaces.

The relationship between electron and ion induced secondary electron imaging: a review with new experimental observations

Abstract: The phenomenalogical and theoretical aspects of secondary electron emission due to ion bombardment in the keV energy range has been reviewed and a comparision of this process with secondary electron emission due to electron bombardment has been made. The similarities and differences between the contrasts in the secondary electron images of test specimens studied with both scanning microfocused Ga+ beams and electron beams have been explained by the mechanisms of the secondary electron emission processes. From ion induced secondary electron images information on the topography, material and crystallographic nature of specimens can be obtained with high surface sensitivity. Differences in surface potential on different areas of a specimen has also been shown to result in voltage contrast effects.

Electron channelling induced effects in grain boundary AES measurements

Abstract: The influence of incident electron beam angle on the Auger peak height and peak height ratios was studied for clean and surface enriched iron single crystal samples and for grain boundary segregation on polycrystalline samples. The AES measurements were supplemented by observing electron channelling patterns (ECP) for the same samples within a SEM. Auger electron peaks for segregated species Aseg are normalized with respect to the iron substrate Auger peak, AFe, at 651 eV. A considerable change of the Auger peak height ratio Aseg/AFe was observed for surface segregation single crystal planes depending on the incident electron beam angle. The main contribution of the observed changes comes from variations of the iron substrate Auger peak AFe and is attributed to electron channelling effects. The same is true for AES measurements on grain boundaries of polycrystalline samples, if the incident electron beam is nearly parallel to a low index crystallographic pole. Some recommendations are given for performing quantitative AES measurements on grain boundary surfaces.

Analysis of polymer surfaces by SIMS 6—detection of cyclic oligomer on the surface of poly(ethylene terephthalate) film

Abstract: The dominant oligomeric species in poly(ethylene terephthalate) (PET) is cyclic trimer. Comparison of the SIMS spectra of pure polymer and cyclic trimer shows a markedly more intense peak at 577 daltons in the positive ion spectrum from the trimer, since this is the mass of the quasi-molecular ion. This difference can be used to semi-quantitatively analyse for cyclic trimer which has migrated to the surface of PET film.

The shape of the background in AES: nonlinear features in log N(E) v. log E

Abstract: By replotting emitted electron spectra, N(E), as log N(E) v. log E, parts of the background which obey a power law can be identified as linear segments. The importance of this linearity for Auger electron spectroscopy (AES), and the conditions under which it can be observed, are discussed. A simple expression is presented which describes the essential shape of the spectral background for analysis energies in the range 50–2000 eV and primary energies between 3 and 20 keV. It has been suggested elsewhere that such simple expressions are inadequate because ionization of inner shells by backscattered and secondary electrons can introduce broad low intensity features which disrupt the linearity of log N(E) v. log E. In the present work numerically calculated spectral shapes are compared with experimental AES spectra from various specimens including Al and Si. It is shown that some broad features which are often observed in EN(E) (constant spectrometer resolving power) spectra arise solely as a consequence of replotting power law backgrounds in this form and cannot be adduced as evidence of the ionization mechanism. Furthermore, subsurface electron sources can cause nonlinearities in logarithmic spectra which should not be ascribed to the ionization mechanism. It is concluded from a comparison of calculated spectra with those observed experimentally that the ionization mechanism causes only small deviations from linear behaviour which should not prevent the fitting of power law backgrounds to experimental AES data.

Observation of ion nitriding on Fe-Cr, Fe-Ni and Ni-Cr alloy surfaces in a glow discharge plasma

Abstract: For FeCr, FeNi and NiCr alloys, iron nitriding in a glow discharge plasma was investigated by optical emission spectroscopy and Auger electron spectroscopy. The composition change in depth was provided by both techniques and, in addition, Auger nitrogen-metal interatomic transitions give information about chemical environments surrounding N atoms. Chromium-enrichment in the nitrided layers was observed from the variation both in the nitrogen-KLL Auger lineshapes and in the depth profiles. The chemical composition in the layers was strongly affected by the alloyed chromium having a strong affinity with N in the chromium-containing alloys.

Analysis of the Auger KLL spectra of carbon by the pattern recognition method

Abstract: The carbon KLL spectra were analysed using the pattern recognition methods. The standard spectra were recorded for (i) surface tungsten carbide, (ii) bulk tungsten carbide, (iii) graphite and (iv) amorphous carbon. The set of these spectra was a basis for the process of recognition. The careful analysis of the set of standard spectra made possible construction of several procedures (classifiers) for recognizing the chemical form of carbon corresponding to a given spectrum. The KLL spectra of carbonaceous residue growing on nickel foil were analysed using all the developed classifiers. At the early stages of growth the carbidic form of carbon was recognized, followed by the graphitic form at further stages. This result agrees with observations reported in the literature.

XPS study on the thermal degradation of poly‐N,N′/4,4′‐diphenylether/pyromellitimide

Abstract: Thin foil samples of the title polymer preheated in vacuo up to different temperatures with the highest one reaching 1200 K were analysed by XPS. On heat treatment the surface content of oxygen decreased and that of carbon increased monotonously, while that of nitrogen passed over a local maximum at c. 800 K. Ether-type oxygens proved to be more heat resistant than carbonyl type ones. From the change of the N 1s peak the probable mechanism of decomposition up to c. 800 K was inferred to be a decarboxylation to give CHN type structures. At higher temperatures decarbonylation of the imide groups appeared to become predominant, resulting in the formation of amide functionalities. The development of a conductive (polyaromatic) structure was deduced from the gradual decrease in the charge shift with the increase of the temperature of pretreatment.

Probing polymer surface and interface dynamics

Abstract: Polymeric materials generally exhibit various molecular motions and relaxations. Such relaxation processes, which include the glass transition temperature, have significant effects on physical and mechanical behavior. Polymer molecules and segments at surfaces and interfaces also exhibit motions and relaxations. In air or vacuum, such motions ‘permit’ the surface to restructure to minimize the surface free energy. In aqueous solution, the polymer surface restructures and reorients to optimally interact with the aqueous solvent, thereby minimizing the interfacial free energy. XPS and related high vacuum techniques probe the vacuum-equilibrated surface. The best way to probe the polymer-liquid interface is via dynamic contact angle or wetting methods. A number of issues and concerns are discussed: (1) the size or hierarchy of structures: (2) the time course of surface dynamic processes; (3) theory, modeling, and simulation of surface dynamics; and (4) experimental methods.

Study of interfaces in oxidized Fe/Si system by XPS and XAES: Use of the Auger parameter

Abstract: Auger Parameters for a variety of silicon compounds have been measured by a combination of XPS using both AIKα ZrLα characteristic x-radiation, and of XAES using Bremsstrahlung radiation. The Parameters so derived have then been used to study the chemical states of silicon at and near interfaces in the oxide film formed by air-and CO2-oxidation of Fe/Si alloys. Supportive evidence for the interpretations is provided by the widths and shapes of the silicon KL2,3 L2,3 Auger peaks.

The use of an electron flood gun when adopting monochromatic AgLα radiation for the XPS analysis of insulators

Abstract: To combat the problem of specimen charging when analysing insulators with a high energy monochromatic AgLα x-ray source for XPS in the VG ESCA3, the application and use of a low energy electron flood gun is investigated. It is found that individual specimens can require widely differing ‘optimum’ conditions for the satisfactory management of surface charging. However, the use of an accelerating voltage of at least 6 V, together with an emission current of at least 0.25 mA, appears to be satisfactory for the majority of specimens. It is concluded that an electron flood gun, when properly set-up, can play an important role in gaining chemical state information from non-conducting materials using XPS, though it is noted that the use of secondary binding energy referencing may still be necessary.

On the use of markers and tracers to establish the growth mechanism of alumina scales during high temperature oxidation

Abstract: Various ‘marker methods’ have been applied to study the oxidation mechanism of NiAl and Ni-15Cr-24Al-.3Y from 1170 K to 1370 K. Experiments were performed using noble metal markers. The behaviour of both vacuum deposited noble metals as well as implanted noble metals was studied. Also 18O tracers were used. RBS, 18O(p, α)15N resonance spectrometry and SIMS analysis were performed to investigate the alumina scale composition and the position of the marker after oxidation of alloys. The results show that (at 1170 K) NiAl oxidizes mainly by outward diffusion of Al through the oxide scale. It is also shown that noble metal markers may give misleading results. The oxidation mechanism of Ni-15Cr-24Al-.3Y is much more complex. The oxide scale formed on this alloy contains Ni, Cr and Y-rich oxide regions. These regions contribute to the apparent overall oxidation mechanism of the alloy.

Surface-analysis and the Complexity of Real Catalysts

Abstract: This paper is written for specialists of surface techniques not perfectly familiar with catalysis, as well as for scientists working in the area of heterogeneous catalysis. It is aimed at suggesting the problems of characterization that surface analysis is faced with, because of the complexity of real catalysts. We first present the various typical ‘architectures’ to which catalysts may correspond. We shall then illustrate typical problems, and their solutions, by selecting examples in the field of hydrodesulfurization (HDS) catalysts. We shall show how 10 out of the 12 theories proposed for explaining catalytic activity were eliminated thanks to surface techniques. HDS catalysts are remarkable in several respects, especially because they undergo many ‘metamorphoses’ (activation, modifications during catalytic work, ageing, regeneration) from the time of their preparation to that of their death by irreversible deactivation. They therefore provide a large body of illustrative examples concerning the characterization of several ‘architectures’. In spite of the achievements of surface techniques in clarifying many features of HDS catalysts during their life, investigators are still faced with unsolved problems; many such problems are related to the very fine scale (of the order of 1 nm or less) of the details of importance for catalysis. This might suggest directions for new improvements or the development of new surface techniques. As a conclusion, we shall recall that, because complete characterization needs the joint use of several techniques, a methodology of study had to be developed, now exists, and still continues to improve. The main lines of this general methodology are not restricted to catalysts, but can be used for the study of all complex highly dispersed solids.

SIMS analysis of AlxGa1–xas/gaas layered structures grown by metal‐organic vapour phase Epitaxy

Abstract: For AlxGa1-xAs/GaAs layered structures, grown by MOVPE, abrupt interfaces in the order of one atomic distance were observed. These structures are well suited to serve as test samples for studying the influence of bombardment conditions on the quality of SIMS depth profiles. The results of such a study are presented and it is shown that depth profiles can be obtained with a depth resolution of about 25 A. It was found that beyond the interface secondary ion signals decrease exponentially; from the variation of the slope of this exponential decay as a function of primary ion energy it can be concluded that depth resolution is determined by a purely collisional mixing process. From this type of data we can derive a minimal layer thickness in AlxGa1-xAs/GaAs quantum well structures for which quantitative SIMS analysis of the Al content is possible.

Investigation of surface contamination on silicon wafers with SIMS

Abstract: Characterization of the surface of silicon wafers is necessary for control and development of cleaning procedures. An analytical approach for investigation of surface contaminants with SIMS is proposed. The major problem is the presence of the contaminants only in the native oxide (thickness ∼1.5 nm). This means that analysis has to be performed in a surface zone smaller than the implantation depth of the primary ions. Chemical matrix effects and a changing erosion rate are encountered owing to the native oxide and the dynamic process of primary ion implantation and sputtering. Owing to the necessity of analysing extremely thin surface layers very slow sputtering has to be applied (1–10 monolayers per min.). As a consequence the secondary ion intensities are low and hydrocarbon species interfere with the elements to be analysed. For the identification of the secondary ion signals the evaluation of isotopic ratios determined with low mass resolution (M/ΔM ∼ 350) is combined with measurement of high resolution mass spectra.

Convolutional smoothing algorithms in electron spectroscopy

Abstract: Multiple application of the convolutional smoothing algorithm of Savitsky and Golay to electron spectra leads to an undesirable distortion of the lineshape The use of a single convolution of the data with a suitably-chosen Gaussian function gives superior results with a large saving in computation The distortion introduced by this procedure merely simulates the effect of widening the spectrometer slits