M. P. Seah

Elastic Scattering Corrections in AES and XPS. II. Estimating Attenuation Lengths and Conditions Required for their Valid Use in Overlayer/Substrate Experiments†

We examine substrate/overlayer experiments and the equations commonly used to quantify overlayer thicknesses. Comparisons with accurate Monte-Carlo simulations show that using attenuation lengths (rather than inelastic mean free paths) eliminates most of the error due to elastic scattering without increasing the complexity of the quantification. We give attenutation lengths for 27 elements, calculated by the criterion that systematic errors in such quantifications should be minimized. These are therefore the best attenuation length values to use in layerwise quantification. We show that, provided these attenuation length values are used, the error in estimation of the thickness of an overlayer due to elastic scattering can be limited to +(5% +1 A) for an emission angle ≤58° from the surface normal, and +(10%a + 1 A) for an emission angle ≤63° from the surface normal. This accuracy is acceptable for most analytical work. Other methods (such as analytical transport theory) are much more complicated, and achieve a high precision that is often unnecessary in view of other uncertainties typically present in these experiments (such as errors due to surface morphology and diffraction effects). The results presented here, using the full theory, show that the analysts simple straight-line approximation is in fact of adequate accuracy, provided that the correct values of attenuation length are used. Simple semi-empirical equations are presented, which allow the analyst to estimate the attenuation length for electrons of kinetic energy between 50 and 2000 eV, to a standard uncertainty of 6%.

Interface adsorption, embrittlement and fracture in metallurgy: A review

Abstract Interface adsorption or segregation in metallurgy is reviewed with particular reference to the phenomenon of grain boundary adsorption and its effect on important mechanical properties such as temper brittleness in steels. No attempt is made to provide a complete bibliography but rather to put in perspective the significant and properly documented observations relating to segregation. General theories are then developed which first relate segregation to readily available basic parameters and then, in turn, with a new theory, the degree of embrittlement to segregation. In this way predictions may be made and remedial measures proposed for metallurgical problems involving grain boundary or interfacial segregation.

Kinetics of surface segregation

Abstract The kinetics of surface segregation, are analysed and illustrated by Auger electron spectroscopy observations of tin segregation to the free surface of iron. The analysis extends the diffusion approach of McLean (1957) by incorporating the effects of surface evaporation and by relaxing the constraint of a linear relation between the solute concentration at the surface and that in the adjacent bulk layer. Parameters are evaluated for the iron-tin system and the importance of the predictions is demonstrated by Auger electron measurement. It is found that marked deviations from McLeans analysis can occur at surfaces and that incorporation of the modifications presented here can lead to improvements in the accuracy of both the measurement of surface segregation levels and the deduction of solute diffusivity data, from the kinetic observations, by factors as high as 2 and 30, respectively. The method is applicable to solute diffusivities in the temperature range down to 450°C in iron.

Surface segregation and its relation to grain boundary segregation

Abstract Equilibrium surface segregation concentrations have been measured for iron-tin alloys over the temperature range 500° to 850°C using Auger electron spectroscopy. The results are compared with the grain boundary segregation measurements reported earlier for these alloys, over the same temperature range, and also with the surface and grain boundary segregation data deduced from surface and interfacial energy measurements at 1420°C (Seah and Hondros 1973). The observations are all fully compatible with the theoretical predictions which incorporate the appropriate entropy terms. At low temperatures, in dilute solutions, it is found that the surface enrichment is approximately 130 times that at the grain boundary, whereas at 1420°C the ratio is only six. This difference, which will depend on the particular system considered, indicates that careful correlation experiments are required if surface segregation is to be used as a direct guide to grain boundary segregation. A second practical difference is ...

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

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.

Quantitative XPS: I. Analysis of X-ray photoelectron intensities from elemental data in a digital photoelectron database

Abstract An analysis of the correlation of theoretical predictions for photoelectron intensities is made with experimental data from an XPS digital database for 46 solid elements measured using a spectrometer with calibrated intensity and energy scales. This analysis covers single element samples measured for Al and Mg Kα X-rays. The spectral data are for widescans at 1 eV energy intervals with kinetic energies from 200 to 1506 eV using Al X-rays and to 1273 eV using Mg X-rays. In addition are narrow scans around the photoelectron peaks at 0.1 eV energy intervals. All spectra have the instrument intensity/energy response function removed so that the peak areas are proportional to the number of electrons emitted per steradian per incident Kα photon. Correlations are made for the ionisation cross sections of Scofield and the inelastic mean free paths given by the TPP-2M formula. The correlations are excellent, apart from a factor which may be associated with the background removal arising from the use of the Tougaard Universal cross section. These correlations lead directly to pure element relative sensitivity factors suitable for quantitative analysis. General equations are also provided to extract values for a new form of relative sensitivity factor for an average matrix. These average matrix relative sensitivity factors lead to simpler equations involving matrix factors that are effectively unity instead of the traditional values in the range 0.3 to 3.0.

Argon Cluster Ion Beams for Organic Depth Profiling: Results from a VAMAS Interlaboratory Study

The depth profiling of organic materials with argon cluster ion sputtering has recently become widely available with several manufacturers of surface analytical instrumentation producing sources suitable for surface analysis. In this work, we assess the performance of argon cluster sources in an interlaboratory study under the auspices of VAMAS (Versailles Project on Advanced Materials and Standards). The results are compared to a previous study that focused on C(60)(q+) cluster sources using similar reference materials. Four laboratories participated using time-of-flight secondary-ion mass spectrometry for analysis, three of them using argon cluster sputtering sources and one using a C(60)(+) cluster source. The samples used for the study were organic multilayer reference materials consisting of a ∼400-nm-thick Irganox 1010 matrix with ∼1 nm marker layers of Irganox 3114 at depths of ∼50, 100, 200, and 300 nm. In accordance with a previous report, argon cluster sputtering is shown to provide effectively constant sputtering yields through these reference materials. The work additionally demonstrates that molecular secondary ions may be used to monitor the depth profile and depth resolutions approaching a full width at half maximum (fwhm) of 5 nm can be achieved. The participants employed energies of 2.5 and 5 keV for the argon clusters, and both the sputtering yields and depth resolutions are similar to those extrapolated from C(60)(+) cluster sputtering data. In contrast to C(60)(+) cluster sputtering, however, a negligible variation in sputtering yield with depth was observed and the repeatability of the sputtering yields obtained by two participants was better than 1%. We observe that, with argon cluster sputtering, the position of the marker layers may change by up to 3 nm, depending on which secondary ion is used to monitor the material in these layers, which is an effect not previously visible with C(60)(+) cluster sputtering. We also note that electron irradiation, used for charge compensation, can induce molecular damage to areas of the reference samples well beyond the analyzed region that significantly affects molecular secondary-ion intensities in the initial stages of a depth profile in these materials.

Ion detection efficiency in SIMS:: Dependencies on energy, mass and composition for microchannel plates used in mass spectrometry

Abstract The effects of ion energy, mass and composition on the detection efficiency of a microchannel plate (MCP) have been studied in detail, using a time-of-flight (TOF) mass spectrometer. This spectrometer is used for static secondary ion mass spectrometry (static SIMS) although the data are relevant to any ion-detection system. A model is developed that shows how the efficiency falls with increased mass and decreased ion impact energy at the front of the MCP. At an impact energy of 20 keV, the efficiency for the detection of cationised PS oligomers of mass 10,000 amu is approximately 80%, whereas at 5 keV it has fallen to ∼5%. The model is extended to estimate the effect of ion composition on the detection efficiency. It was found that ions with a high hydrogen content have a lower efficiency than those that consist of a cluster of high atomic number atoms. The spread of detection efficiencies arising from both composition and mass may be reduced by increasing the ion impact energy. Therefore, up to a mass of 4000 amu, the spread for ions of 100% observed for 5-keV ion impact energy is reduced to a negligible spread for ions of 20-keV impact energy, where the efficiency is approximately unity, independent of the composition. A simple method is provided to determine the correct voltage to operate the MCP for a given efficiency. This operating voltage should be determined for the highest mass ions in the required range. . Published by Elsevier Science B.V.

Static SIMS: towards unfragmented mass spectra — the G-SIMS procedure

Abstract A study is presented of the effects of the different positive ion beam species: Ar+, Ga+, Xe+, Cs+ and SF5+ and of their energies from 4 to 25 keV, on the fragmentation behaviour in static Secondary Ion Mass Spectrometry (SIMS) spectra for samples of the polymers: polytetrafluoroethylene (PTFE), polystyrene (PS) and polycarbonate (PC). The overall effect of energy is found to be weak over the entire mass spectrum. However, large differences are observed in restricted mass ranges amongst fragmentation groups. The fragmentation is quantified in terms of the partition functions of the fragments from a plasma with effective temperature, Tp. It is found that fragmentation is least for high mass projectiles at low energies, but that the trend is different for polyatomic ions. A methodology is developed, which unifies all of the fragmentation behaviour to a single plot — the Unified Cascade Gradient plot. An equivalence of mass and energy is shown and that the chemistry of the bombarding ion is unimportant. By extrapolation of the data to low Tp, a new spectroscopy, known as gentle-SIMS or G-SIMS is formed. The G-SIMS spectrum is in the static regime. Significant peaks in the G-SIMS spectra are those peaks, which would be emitted from a surface plasma of very low temperature and thus have little post-emission rearrangement or fragmentation. Those peaks are, thus, directly characteristic of the material without rearrangement and provide a direct interpretation and identification. In the tests of the method described, this is supported and indicates that the G-SIMS analysis will be significantly less ambiguous than static SIMS so that interpretation will be possible in the absence of a relevant reference spectrum.

Site competition in surface segregation

Abstract Observations have been made, by Auger electron spectroscopy, of site competition between tin, sulphur and calcium co-segregating to the free surface of otherwise pure iron in the temperature range 600–800°C. The correlations between the segregation levels of the species are explained for sub-monolayer and multilayer tin adsorption, in terms of a simple model. In the absence of calcium the reduction in available adsorption sites for tin is a function of both the tin and sulphur coverages. With calcium present the formation of surface precipitates of calcium compounds is observed. Excellent agreement is obtained between the observed interrelated behaviour of the segregants and the predictions of the model, over a wide range of experimental conditions. A similar behaviour is expected, and must be allowed for, in all studies of segregation thermodynamics and kinetics of surfaces and interfaces exhibiting more than two components.