Interlaboratory study comparing analyses of simulated angle‐resolved X‐ray photoelectron spectroscopy data

An interlaboratory study has been conducted to determine the following: (i) the similarities and differences of film thicknesses and composition profiles obtained from analyses of simulated angle‐resolved X‐ray photoelectron spectroscopy (ARXPS) data by different analysts using different algorithms for data analysis, and (ii) the effects of two assumptions commonly made in data‐analysis algorithms for ARXPS on derived film thicknesses and composition profiles. The analyzed data were generated by the National Institute of Standards and Technology Database for the Simulation of Electron Spectra for Surface Analysis, (SESSA) which provides a simple way to study the influence of the aforementioned effects on compositional depth profile reconstruction. Sets of simulated ARXPS data were produced for thin films of SiO₂, SiON, HfO₂, and HfON of varying thicknesses on a Si substrate. For some HfON films, the N concentration varied with depth. Eleven groups participated in the round robin study. The majority (eight) employed a commercial ARXPS instrument in which the angular distribution is measured for a fixed sample geometry, in contrast to conventional ARXPS in which the sample is tilted for angular variation. The average deviations between the reported average depth, film thickness, and amount of material typically varied between 20% and 30% but were considerably larger, between 30% and 80%, for some cases. The average errors were generally larger for simulations that included elastic scattering and the finite analyzer‐acceptance angle (realistic conditions) than those for simulations that neglected elastic scattering and the finite analyzer‐acceptance angle (simplified conditions). The retrieved N depth profiles were quantitatively different from the true depth profiles and showed substantial variability among the group of members who used the same instrument and analysis software. Copyright © 2014 John Wiley & Sons, Ltd.     

Study of the degradation of plasma‐oxidized polystyrene by time‐ and angle‐resolved x‐ray photoelectron spectroscopy

Angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) measurements were made, in repeated sequences employing Al and Mg x‐ray sources alternately, on a polystyrene sample that had been exposed to an oxygen plasma. It was observed that oxygen was lost from the sample over a period of 5 h and 40 min. The ARXPS data sets were corrected for the time displacement between consecutive measurements at different photoemission angles and fitted with three simple models in order to extract oxygen concentration–depth profiles, consistent with the data, as a function of time. The oxygen depth profiles were found to evolve in a consistent manner, indicating both a loss of average oxygen content and thickness in the ‘oxidized polymer layer’. Copyright © 2003 John Wiley & Sons, Ltd.     

ARXPS investigation of diffusion as a limiting process in the incorporation of adatoms into polystyrene surfaces treated by nitrogen plasmas

Polystyrene films were exposed to nitrogen plasmas for periods up to 4 min. ARXPS measurements revealed the presence of oxygen and nitrogen in the surface because of the plasma treatment. The depth profiles of these adatoms were determined by fitting a Thomas–Windle model profile to the data. As found previously, the best‐fit depth profiles resembled a step function, consistent with case II diffusion. However, the depth of penetration of the adatom species into the polymer surface was not found to vary with plasma duration, which is inconsistent with Case II diffusion, and provides evidence that diffusion processes do not limit the loading of adatom species into the surface during nitrogen plasma treatment. A possible reason for the generation of erroneous step function depth profiles is discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

On the choice of the residual norm function in the interpretation of ARXPS data by regularized fitting

Angle‐resolved X‐ray photoelectron spectroscopy (ARXPS) data taken on a polystyrene film exposed to a nitrogen plasma are interpreted by the fitting of regularized depth profiles. Three ways of measuring the goodness of fit are compared—the χ² statistic with variances drawn from the raw spectra, the χ² statistic with variances drawn from the concentration figures obtained from the data analysis, and a simple sum of the squared differences (ssd) that does not require variances to be calculated. It is shown that for these data, the depth profiles obtained using an objective method for the choice of the regularization parameter are essentially identical irrespective of whether or how the variances are introduced into the calculation. Copyright © 2010 John Wiley & Sons, Ltd.     

Nondestructive in‐depth composition profile of oxy‐hydroxide nanolayers on iron surfaces from ARXPS measurement

In this work the maximum entropy method (MEM) is applied, for the first time, to angle‐resolved X‐ray photoelectron spectroscopy (ARXPS) data from oxy‐hydroxide films on iron surfaces. This nondestructively derives information on the in‐depth distribution of the composition and chemical state. An MEM algorithm was created and first tested on the simulated data. The reconstructed composition depth profiles agreed very well with the theoretical ones up to 5% Gaussian noise added to the data. The same algorithm was then applied to ARXPS data from iron samples to investigate the in‐depth variations in the composition and chemical state of the nanosized oxy‐hydroxide film naturally grown on the iron surface. The resulting surface film presents a complex multilayer structure with concentration gradients. The effect of air exposure on the structure was also investigated. Copyright © 2006 John Wiley & Sons, Ltd.     

Depth profiling by ARXPS in surface analysis

A method of nondestructive depth profiling in near surface regions of solids is described. Models have been discussed from which algorithms for evaluation of measured data are obtained. The algorithms, based on standard profiles with free parameters, have been adjusted to the data resulting from angle resolved XPS (ARXPS) by means of least squares fits. Depth profile analyses and segregation studies were performed on Pt–Ni and Fe–S specimens.     

Depth profiling by ARXPS in surface analysis

A method of nondestructive depth profiling in near surface regions of solids is described. Models have been discussed from which algorithms for evaluation of measured data are obtained. The algorithms, based on standard profiles with free parameters, have been adjusted to the data resulting from angle resolved XPS (ARXPS) by means of least squares fits. Depth profile analyses and segregation studies were performed on Pt-Ni and Fe-S specimens.     

A statistical approach to delta layer depth profiling

We present a simple statistical model describing the removal and relocation of material during a sputter depth profiling experiment. All input parameters are determined from low‐fluence molecular dynamics simulations, making the model de facto parameter free. The model can be used to extrapolate data from the molecular dynamics simulations to projectile fluences relevant to sputter depth profiling experiments. As a result, the erosion of the surface is calculated in terms of fluence‐dependent filling factors of different sample layers. Using input data determined for the 20‐keV C₆₀ cluster bombardment of silicon, it is found that a steady‐state erosion profile is reached after removal of approximately 20 monolayer equivalents of material. Plotting the contribution of particles from a specific layer to the instantaneous sputtered flux, one can directly determine the delta layer response function predicted from such a model. It is shown that this function can be parameterized by the semiempirical Dowsett response function, and the resulting fitting parameters are compared with published depth profile data. The model is then used to study the role of different processes influencing the observed depth resolution. We find that the statistical nature of the sputtering process suffices to explain many features of experimentally measured delta layer depth profiles. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface structure of cationic surfactant solutions investigated by angular resolved X‐ray photoelectron spectroscopy with calibrated transmission function

The main goal of the present work is to investigate the surface structure of cationic surfactant solutions by angular resolved X‐ray photoelectron spectroscopy (ARXPS) after calibrating transmission function of the spectrometer. We have estimated the transmission function of the ARXPS spectrometer, and with it, we investigated solution of tetrabutylammonium iodide in a nonaqueous polar solvent. By genetic algorithm, the fractional concentration‐depth profiles of constituents were reconstructed. These depth profiles evidence the enrichment of surfactant and the depletion of solvent in the surface regime and verify the separate distributions of oppositely charged surfactant ions. From those profiles we draw a conclusion that there is a preferred orientation for cation present at the surface, and the possibility for cation to take this orientation is related to its surface amount. Copyright © 2010 John Wiley & Sons, Ltd.     

Report on the 47th IUVSTA Workshop ‘Angle‐Resolved XPS: the current status and future prospects for angle‐resolved XPS of nano and subnano films’

A summary of the workshop entitled ‘Angle‐Resolved XPS: The Current Status and Future Prospects for Angle‐resolved XPS of Nano and Subnano Films’ is given, which was held at the Riviera Maya, Mexico, 26–30 March 2007, under the main sponsorship of the International Union for Vacuum Science, Technique and Applications (IUVSTA). Angle‐resolved X‐ray photoelectron spectroscopy (ARXPS) can provide detailed chemical as well as depth profile information about the near‐surface composition of materials and thin films. This workshop was held to review the present status and level of understanding of Angle‐resolved XPS, and to stimulate discussions leading to a deeper understanding of current problems and new solutions. The main goal of the workshop was to find better ways to perform experiments and, very importantly, better ways to extract information from the experimental data. This report contains summaries of presentations and discussions that were held in sessions entitled ‘Basics and Present Limits of ARXPS’, the Analysis of ARXPS Data, Applications of ARXPS, Equipment for ARXPS, and Future Developments in ARXPS'. There were 33 participants at the workshop. Copyright © 2009 John Wiley & Sons, Ltd.     

Investigation of an iterative matrix method for the inversion of ARXPS data

Mathematical simulations were used to evaluate an iterative, regularized matrix approach to the recovery of composition depth profiles from ARXPS data containing 1, 3, 5, 7 or 9% noise. The iterative inversion was intended to remove unphysical negative concentration values from the depth profile and to provide some degree of profile sharpening at layer interfaces. It was found that while this procedure worked effectively on data containing just 1% noise, its efficacy diminished as the level of noise increased. Copyright © 2015 John Wiley & Sons, Ltd.     

Application of angle-resolved X-ray photon electron spectroscopy for interface and layer growth studies demonstrated on Ti/Ta-based films deposited on SiO2

X-ray photoelectron spectroscopy (XPS) is one of the main methods for elemental and bonding-state characterization in the near-surface region. Applying it as angle-resolved XPS (ARXPS), it is promising for non-destructive depth profiling in the nanometre-thickness range. The challenges for the application of ARXPS are discussed for the early film growth of Ti and Ta on SiO₂ produced by in situ magnetron sputtering. The measurements showed several phase-formation processes at the interfaces, which were used as the basis for the ARXPS data analysis and resulted in different interlayer growth information for Ti and Ta. From a systematic ARXPS data interpretation in combination with simulations, limits of the method are critically discussed.     

XPS for non-destructive depth profiling and 3D imaging of surface nanostructures

Depth profiling of nanostructures is of high importance both technologically and fundamentally. Therefore, many different methods have been developed for determination of the depth distribution of atoms, for example ion beam (e.g. O₂⁺, Ar⁺) sputtering, low-damage C₆₀ cluster ion sputtering for depth profiling of organic materials, water droplet cluster ion beam depth profiling, ion-probing techniques (Rutherford backscattering spectroscopy (RBS), secondary-ion mass spectroscopy (SIMS) and glow-discharge optical emission spectroscopy (GDOES)), X-ray microanalysis using the electron probe variation technique combined with Monte Carlo calculations, angle-resolved XPS (ARXPS), and X-ray photoelectron spectroscopy (XPS) peak-shape analysis. Each of the depth profiling techniques has its own advantages and disadvantages. However, in many cases, non-destructive techniques are preferred; these include ARXPS and XPS peak-shape analysis. The former together with parallel factor analysis is suitable for giving an overall understanding of chemistry and morphology with depth. It works very well for flat surfaces but it fails for rough or nanostructured surfaces because of the shadowing effect. In the latter method shadowing effects can be avoided because only a single spectrum is used in the analysis and this may be taken at near normal emission angle. It is a rather robust means of determining atom depth distributions on the nanoscale both for large-area XPS analysis and for imaging. We critically discuss some of the techniques mentioned above and show that both ARXPS imaging and, particularly, XPS peak-shape analysis for 3D imaging of nanostructures are very promising techniques and open a gateway for visualizing nanostructures.     

Comparison of two parameter choice methods for the extraction of depth profiles by means of a regularized multipoint profile model from ARXPS data obtained on a three‐component system

Polystyrene films were exposed to nitrogen plasmas for periods up to 8 min. Angle‐resolved X‐ray photoelectron spectroscopy measurements revealed the presence of oxygen and nitrogen in the surface due to the plasma treatment. The depth profiles of these adatoms were determined by fitting a regularized multipoint linear segment model to the data. A regularization parameter chosen such that the chi‐square statistic of the fit to the data was equal to the number of independent data points gave a more intuitive result than a parameter chosen according to the L‐curve criterion. Although the shape of the nitrogen depth profile was observed to vary as a function of the plasma duration, the oxygen depth profiles were nearly identical. Copyright © 2009 John Wiley & Sons, Ltd.     

Quantitative ARXPS investigation of systems with ultrathin aluminium oxide layers

Angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) is a non‐destructive method to investigate the near‐surface structure of specimens with a flat surface. For interpretation of the electron intensities emitted from different depth regions, model calculations are necessary. Based on an earlier algorithm we have developed a program for ARXPS studies of thin multilayers. In our model calculation the sample structure is treated as consisting of several layers (one to three) on the substrate, whereas the top layer can be incomplete. Emitted electrons are assumed to be attenuated exponentially in the layers. Different atomic volumes, electron attenuation lengths (including consideration of elastic scattering) and assumptions on stoichiometry are taken into account for the particular layers. As an application of our model calculations we present a study of a set of Al samples that were oxidized by different methods, i.e. natural and plasma oxidation (plasma obtained by electron cyclotron resonance). The oxide layers produced by plasma oxidation were protected by a 2 nm thick Co film, before exposing the samples to the air. Additionally, in order to check our results of the ARXPS model calculation, x‐ray reflectometry (XRR) analysis was used. Copyright © 2004 John Wiley & Sons, Ltd.     

Fick's law of diffusion depth profiles applied to the degradation of oxidized polystyrene during ARXPS analysis

Angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) measurements were made, using Al Kα and Mg Kα radiation alternately, on a polystyrene sample that had been exposed to a helium plasma. It was observed that oxygen was introduced into the sample surface by the plasma treatment, and that some of it was lost over a period of 5 h under x‐ray irradiation in the vacuum of the spectrometer. Laplace transforms of Fick's law of diffusion profiles were derived and applied to the data. The ARXPS results obtained in this study are consistent with a sample history in which the oxidation of the polymer surface resulting from exposure to plasma is controlled by a diffusion process, whereas the loss of oxygen during exposure to x‐rays is principally controlled by a first‐order reaction such as the liberation of oxygen (presumably as CO₂) from carbon–oxygen groups by the action of radicals created by the ionizing radiation. Copyright © 2005 John Wiley & Sons, Ltd.     

Ultra-Shallow Chemical Characterization of Organic Thin Films Deposited by Plasma and Vacuum-Ultraviolet, Using Angle- and Excitation Energy-Resolved XPS

Nitrogen (N)-rich organic thin films were deposited using both low-pressure plasma- and vacuum-ultraviolet-based techniques, from mixtures of ammonia (NH₃) and ethylene (C₂H₄). These films were investigated using angle-resolved and excitation energy resolved X-ray photoelectron spectroscopy (ARXPS and ERXPS, respectively) in order to determine their sub-surface chemical profiles. These two techniques enable one to tune the ??XPS 95%?? information depth, z _95%, by varying either the angle or the excitation energy. Using a combination of both techniques, z _95% can be varied continuously from 0.7 to 11 nm. The surface-near chemistry is investigated using both high-resolution C 1s spectra and elemental concentrations derived from elemental peak intensities. Results show that while laboratory XPS, and even ARXPS, suggest homogenous surface chemistries, the novel combination of ARXPS and ERXPS points to the existence of a compositional profile in the extreme outer surface layer. Our conclusions are supported by simulations using SESSA software.     

Backscattering effect in quantitative AES sputter depth profiling of multilayers

AES sputter depth profiles of multilayers with constituents of very different backscattering factors show characteristic distortions in the shape of the intensity–depth profiles. These distortions are quantified by introducing an extension of the local effective backscattering factor concept developed in an earlier paper in the mixing‐roughness‐information depth (MRI) model for profile quantification. The extension is based on a linear superposition of two newly defined parameters, the effective backscattering factors for each interface that are diminished with distance from the respective interface by another characteristic parameter, the mean effective backscattering decay length. As shown for a Ni/C multilayer structure of six alternating layers of Ni (38 nm) and C (25 nm) on a Si substrate, AES intensity depth profiles calculated with the presented modification of the MRI model, yield an excellent agreement with the measured profile after some adjustment of the initial mean effective backscattering decay lengths and, sometimes, after a slight change of the backscattering factors given by the Ichimura–Shimizu relations. The backscattering effect is studied as a function of the single layer thickness. A critical layer thickness can be determined, below which the backscattering influence becomes negligible for typical AES depth profiling results. Copyright © 2007 John Wiley & Sons, Ltd.     

ToF‐SIMS depth profiling of a complex polymeric coating employing a C60 sputter source

A complex poly(vinylidene difluoride) (PVdF)/poly(methyl methacrylate) (PMMA)‐based coil coating formulation has been investigated using time‐of‐flight SIMS (ToF‐SIMS). Employing a Bi₃⁺ analysis source and a Buckminsterfullerene (C₆₀) sputter source, depth profiles were obtained through the polymeric materials in the outer few nanometres of the PVdF topcoat. These investigations demonstrate that the PVdF coating's air/coating interface is composed principally of the flow agent included in the formulation. Elemental depth profiles obtained in the negative ion mode demonstrate variations in the carbon, oxygen and fluorine concentrations within the coating with respect to depth. All three elemental depth profiles suggest that the PVdF coating bulk possesses a constant material composition. The oxygen depth profile reveals the presence of a very thin oxygen‐rich sub‐surface layer in the PVdF coating, observed within the first second of the sputter/etch profile. Retrospectively, extracted mass spectra (from the elemental depth profile raw data set) of the PVdF coating sub‐surface and bulk layers indicates this oxygen‐rich sub‐surface layer results from segregation of the acrylic co‐polymers in the formulation towards the PVdF coating air/coating interface. Molecular depth profiles obtained in both the positive and negative secondary ion modes provide supporting evidence to that of the elemental depth profiles. The molecular depth profiles confirm the presence of a sub‐surface layer rich in the acrylic co‐polymers indicating segregation of the co‐polymers towards the PVdF topcoats air‐coating surface. The molecular depth profiles also confirm that the PVdF component of the topcoat is distributed throughout the coating but is present at a lower concentration at the air‐coating interface and in the sub‐surface regions of the coating, than in the coating bulk. Copyright © 2007 John Wiley & Sons, Ltd.     

Quantitative analysis of angle‐resolved XPS spectra recorded in parallel data acquisition mode

The effects of anisotropy of the photoionization cross‐section and elastic scattering of photoelectrons in solids are investigated for angle‐resolved XPS spectra (ARXPS) recorded from α–Al₂O₃ substrate in parallel data acquisition mode. It is shown that for quantitative analysis of ARXPS spectra recorded in parallel data acquisition mode it is essential to account for the anisotropies of the photoionization cross‐sections of the detected photoelectrons for the concerned elements in the solid due to variation of the angle between the incident x‐rays and the detected photoelectrons. Neglecting the effect of elastic scattering only leads to minor errors in quantitative analysis of the ARXPS spectra. By adopting experimentally determined values for the relative sensitivity factors of the concerned photoelectrons in the solid as a function of the detection angle, cumbersome corrections for the effects of anisotropy of the photoionization cross‐section and elastic scattering can be avoided. Copyright © 2004 John Wiley & Sons, Ltd.