The effect of carbonaceous contamination on the investigation of the surface oxide on silicon by angle-resolved XPS

The carbon-contaminated native oxide layer on a standard silicon wafer was investigated by angle-resolved signal ratio X-ray photoelectron spectroscopy (AR/SR/XPS). The results, based on intensity measurements of C_1s , O_1s , Si _2p ⁴⁺ and Si^elemental _2p showed the carbon to be ingested into the oxide to a mean depth of 0.4 nm, and the oxide to consist of a fully oxidized layer ( 1 nm) on top, followed by a suboxidic layer (0.8 nm). The conclusions are that the depth location of the carbonaceous contamination is of cardinal importance for the correct interpretation of the oxidic data, and that for well studied systems routine measurements at two take-off angles suffice for quantitative results.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Methodic aspects of XPS depth profiling for investigations of fuel cell components

The focus of this work is to obtain detailed information about the physical composition of the individual fuel cell components in order to optimize them and increase cell durability. An approach for the ex‐situ characterization of fuel cell components, such as gas diffusion media or catalyst layers is by using XPS. In this work, we address methodic aspects of depth profiling of direct methanol fuel cell components (microporous layers and catalyst layers) using adhesive tape peel‐off with subsequent XPS analysis. By using this approach, we have successfully demonstrated cross‐over of Ru ions and possibly Pt ions from the anodic catalyst to the cathodic catalyst layer and even to the microporous layer. Moreover, our experimental results allow determining where the dissolved catalyst ions redeposit within the membrane electrode assembly. Finally, our findings suggest that cross‐over of Pt and Ru ions, observed under different operation conditions, are uncoupled. Copyright © 2014 John Wiley & Sons, Ltd.     

Non-destructive depth profile analysis using synchrotron radiation excited XPS

We have used synchrotron radiation as excitation source in an X-ray photoelectron spectroscopy (XPS) experiment to analyse surface-near element depth profiles non-dectructively. By tuning the photon energy one can vary the kinetic energy of the photoelectrons and in turn the information depth of the measurement. To quantify the sample geometry (e.g. layer thicknesses) model calculations similar as for angle-resolved XPS (ARXPS) measurements are necessary. We have successfully applied this technique to several samples. We will show how to calculate the relative intensities of the peaks, using photoionization cross sections and an experimentally determined analyzer transmission function and the procedure to quantify the geometry for a model sample: natively oxidized Ta covered by carbon contamination. At Sn-doped indium oxide samples we found a sub-monolayer of segregated Sn at the surface which was expected from previous investigations.     

XPS depth profiling investigations on La2Zr2O7 layers prepared by chemical solution deposition

La₂Zr₂O₇ (LZO) layers have been recently investigated as potential buffer layers for superconducting YBa₂Cu₃O_7–x coated conductors deposited on Ni tapes. Chemical solution deposition was used for LZO layer preparation. X-ray photoelectron spectroscopy (XPS) depth profiling is demonstrated to be an important method for layer characterization in addition to X-ray diffraction techniques. XPS measurements revealed layers that are homogeneous in depth, very smooth, and have no significant impurities. A slight difference to the nominal La:Zr stoichimetry is discussed in combination with structural defects that are suspected from spectral changes during ion sputtering.     

Error surface calculations on the parameters defining oxygen depth profile models fitted to ARXPS data obtained from polystyrene exposed to an oxygen/helium plasma

Error surfaces are calculated for the fitting of concentration–depth profiles to angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) data. The shapes of the error surfaces indicate that model parameters related to composition (especially at the very surface of the sample) are well constrained by the data, whereas parameters related to depth have a less significant impact on the fit. It is then shown that certain parameters in the different depth profile models employed are highly correlated and that the different models convey essentially the same information in different ways. Finally, a compromise profile definition is proposed for the fitting of constrained but flexible depth profiles to ARXPS data. Copyright © 2003 John Wiley & Sons, Ltd.     

Energy resolved XPS depth profile of (IrO2, RuO2, Sb2O5, SnO2) electrocatalyst powder to reveal core‐shell nanoparticle structure

Synchrotron‐based energy resolved XPS was used to characterize the structure of IrO₂? RuO₂‐coated Sb₂O₅? SnO₂ nanoparticles. Samples were heat treated at 300, 350, 400, 450 and 500 °C after chloride Ir and Ru precursors were added to Sb₂O₅? SnO₂. Photoelectron kinetic energies of 100, 350 and 1400 eV were employed to obtain an indication of the depth of elemental distributions and chemical shifts. It was shown that the electrocatalyst consists of a core of Sb₂O₅? SnO₂ enriched with Sb₂O₅ towards the surface, with a shell of IrO₂? RuO₂ deposited on this core, and an outer layer of Sb₂O₅? SnO₂ over this shell. No significant chemical interaction occurs between IrO₂? RuO₂ and Sb₂O₅? SnO₂. The energy resolved XPS depth profile technique is effective for studying core‐shell materials. Copyright © 2010 John Wiley & Sons, Ltd.     

Surface morphology and amide concentration depth profile of aminolyzed poly(ethylene terephthalate) films

Surface of biaxially oriented poly(ethylene terephthalate) films was chemically modified by exposure to ethylenediamine (EDA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA) for different treatment times. Variable angle attenuated total reflection Fourier transform infrared (ATR‐FTIR) spectroscopy was used in conjunction with weight loss measurements, scanning electron microscopy (SEM), and atomic force microscopy to establish the surface modification and to draw the depth profile of the newly created species, with emphasis on amide group. A clear differentiation was found between the effects of the three amines studied: EDA produces the highest amidation degree but, because of its deep penetration into the film, leads to delamination of rather thick layers, TETA reacts at and near surface and develops surface cracks without delamination, and TEPA is the mildest reactant, generating amide groups on the surface without visible deterioration of the sample. It was proved that the amide II absorption band became weaker with increasing analyzed depth, with a pronounced heterogeneity near the surface. SEM micrographs showed the development of cracks onto the surface at longer aminolysis time, which allowed a better understanding of ATR‐FTIR observations. Assuming an exponential decay for the depth profile spectrally obtained, the surface concentration of amide groups and the decay constant were determined for the amines and reaction times used. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Quantitative surface analysis of Fe?Ni alloy films by XPS,AES and SIMS

A comparison of quantitative surface analyses of Fe? Ni alloy thin films by various methods has been proposed as a pilot study by the Surface Analysis Working Group of the Consultative Committee for Amount of Substance (CCQM). To test the suitability of Fe? Ni for this purpose, alloy films with different compositions were grown on Si(100) wafers by ion‐beam sputter deposition and the compositions were certified by an isotope dilution method using inductively coupled plasma‐mass spectrometry. The alloy compositions measured with X‐ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) using sensitivity factors determined from pure Fe and Ni metal films agreed with the certified mean values to better than 2%. The alloy compositions quantified by secondary ion mass spectrometry (SIMS) with a C₆₀ ion source agreed to better than 4% with the certified compositions if one of the alloys was used to establish the relative sensitivity factors (RSFs). These results indicate that the quantification of the Fe? Ni alloy is a good method for a CCQM pilot study because matrix effects and ion‐sputtering effects are small for these analytical methods. Copyright © 2007 John Wiley & Sons, Ltd.     

Characterization of surface modifications of stainless steel samples after electron and ion bombardment by SEM, AES, and XPS

The interaction of inert or reactive gas plasmas with the surface of stainless steel has been investigated with the aim, to modify the surface and hence to reduce the outgassing rate of the material, an important factor for the production of an ultrahigh vacuum. The plasma treatments investigated may be an alternative to the common used in situ baking. The samples have been exposed to electrons, argon and oxygen ions either in a DC glow discharge or in a microwave discharge. The DC glow discharge in Ar/O₂, the most effective plasma treatment reduces the outgassing rate by a factor of 10. After this treatment the surfaces of the samples have been investigated with respect to the topography and the chemical composition (depth profile) by Secondary Electron Microscopy (SEM), Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS), respectively. The surface modifications resulting from the different treatments of the samples have been correlated to the outgassing rate.     

Quantification of AES depth profiling data of polycrystalline Al films with Gaussian and non‐Gaussian surface height distributions

Sputtering‐induced roughness is the main distortional factor on the depth resolution of measured depth profiles, in particular, for sputtering of polycrystalline metals. Frequently, the surface height distribution of the sputtering‐induced roughness exhibits an asymmetrical feature. In such a case, a non‐Gaussian height distribution function (HDF) has to be applied for the quantification of a measured depth profile. By replacing the usually applied Gaussian HDF with that of an asymmetrical triangle in the Mixing‐Roughness‐Information depth model, measured Auger electron spectroscopy depth profiling data of the interface of polycrystalline Al films on Si are perfectly fitted. The asymmetric triangle height distributions obtained from the best fit are a reasonable approximation of the height distributions measured by atomic force microscopy. Copyright © 2013 John Wiley & Sons, Ltd.     

Concepts for chemical state analysis at constant probing depth by lab-based XPS/HAXPES combining soft and hard X-ray sources

The greater information depth provided in hard X-ray photoelectron spectroscopy (HAXPES) enables nondestructive analyses of the chemistry and electronic structure of buried interfaces. Moreover, for industrially relevant elements like Al, Si, and Ti, the combined access to the Al 1s, Si 1s, or Ti 1s photoelectron line and its associated Al KLL, Si KLL, or Ti KLL Auger transition, as required for local chemical state analysis on the basis of the Auger parameter, is only possible with hard X-rays. Until now, such photoemission studies were only possible at synchrotron facilities. Recently, however, the first commercial XPS/HAXPES systems, equipped with both soft and hard X-ray sources, have entered the market, providing unique opportunities for monitoring the local chemical state of all constituent ions in functional oxides at different probing depths, in a routine laboratory environment. Bulk-sensitive shallow core levels can be excited using either the hard or soft X-ray source, whereas more surface-sensitive deep core-level photoelectron lines and associated Auger transitions can be measured using the hard X-ray source. As demonstrated for thin Al₂O₃, SiO₂, and TiO₂ films, the local chemical state of the constituting ions in the oxide may even be probed at near-constant probing depth by careful selection of sets of photoelectron and Auger lines, as excited with the combined soft and hard X-ray sources. We highlight the potential of lab-based HAXPES for the research on functional oxides and also discuss relevant technical details regarding the calibration of the kinetic binding energy scale.     

Nitrogen depth distribution, interface and structure analysis of SiNx layers produced by low-energy ion implantation

Thin silicon nitride (SiN _x ) layers with the stoichiometric N/Si ratio of 1.33 in the maximum of the concentration depth distributions of nitrogen were produced by implanting 10 keV¹⁵N ₂ ⁺ in 100 silicon at room temperature under high vacuum conditions. The depth distribution of the implanted isotope was measured by resonance nuclear reaction analysis (NRA), whereas the layer structure of the implanted region and the geometrical thickness of the layers were characterised by high resolution transmission electron microscopy (TEM). SiN _x layers with a thickness of about 30 nm were determined by NRA. Channeling Rutherford backscattering spectrometry was used to determine the disorder in the silicon substrate. Sharp interfaces of a few nanometers between the highly disordered implanted region and the crystalline structure of the substrate thickness were observed by TEM. The high thermal stability of SiN _x layers with N/Si ratios from under to over stoichiometric could be shown by electron beam rapid thermal annealing (1100 °C for 15 s, ramping up and down 5 °C/s) and NRA.     

Optimization of the depth resolution for profiling SiO2/SiC interfaces by dual-beam TOF-SIMS combined with etching

To examine precise depth profiles at the interface of SiO₂/SiC, a high resolution that can detect slight discrepancies in the distribution is needed. In this study, an experimental method to achieve a high resolution of less than 1 nm was developed by using dual-beam time-of-flight secondary ion mass spectrometry (TOF-SIMS). The analysis was preceded by the following three steps: (1) determination of the optimal analytical conditions of the analysis beam (Bi⁺) and sputtering beam (Cs⁺), (2) verification of the etching methods to thin the SiO₂ layer, and (3) confirmation of the benefits of the low-energy sputtering beam directed toward SiO₂/SiC samples. By using the secondary ion intensity peak-to-valley ratio of BN⁻ and BO⁻ of a sample with delta-doped boron multilayers, the appropriate Bi⁺/Cs⁺ condition for a high depth resolution was determined for each energy level of the sputtering beam. Upon verification of the etching methods to thin the SiO₂ layer, slight discrepancies were found between samples that were obtained with different etching methods. The difference in the roughness values of the etched surfaces was proactively utilized for the performance confirmation of the low-energy sputtering beam by means of precise observation of the profiles at the SiO₂/SiC interface. The use of a Cs beam with a low energy between 0.25 and 0.5 keV enabled the detection of slight discrepancies in the roughness of less than 1 nm between samples. The aforementioned method has the potential to accurately detect discrepancies in the intrinsic distribution at the SiO₂/SiC interface among samples.     

Oxide and nitride protective layers on stainless steel studied by AES,WDS and XPS

Protective surface layers on AISI 321 stainless steel were prepared by thermal treatments at two different temperatures in air and two controlled atmospheres. Different oxide and/or nitride layers were formed. Surface morphology of the layers was investigated by scanning electron microscopy (SEM). Auger electron spectroscopy (AES) depth profiling of the samples was performed. Since depth profiling suggested layer thicknesses of the order of hundreds of nanometres, an attempt was made to obtain some fast, averaged information about the layer compositions using wavelength dispersive spectroscopy (WDS) at two different beam energies to obtain probing depths best suited to the layer thickness. X‐ray photoelectron spectroscopy (XPS) profiling of one layer was also performed to obtain information about the chemical states of the elements inside the layer. The analysed samples showed considerable differences with respect to their surface morphology, oxide/nitride layer thicknesses, compositions and layer–metal interface thickness. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

Accurate measurement of sputtered depth for ion sputtering rates and yields: the mesh replica method

For the accurate measurements of crater depths, ion sputtering rates and ion sputtering yields in studies of sputter‐depth profiling using Auger electron spectroscopy (AES) or X‐ray photoelectron spectroscopy (XPS), a proposed mesh replica method has been evaluated. In this method, during ion sputtering, grids of between 50 and 400 mesh (per inch) are placed on the sample to retain unsputtered regions of the original surface to be used as reference. This enables a more accurate measurement of the depth to be made using a stylus profilometer close to the analytical region. The closer‐pitch meshes were thought to offer the prospect of measurements of higher accuracy. Calculations show that sputter deposits from the mesh sides may limit the mesh numbers used to 100 or those of a wider pitch for both stationary and rotated samples. A correlation with published data for stationary samples and new data for rotated samples confirms the calculations. In practice, it is difficult, without a special holder, to have intimate contact between the grid and sample. Such a holder is described. Further calculations concerning the shadowed profiles at the grid bar regions show that the grids may lift off the sample surface by 4–16 µm. This leads to non‐vertical crater walls in each mesh aperture. This effect, however, does not change the above conclusion on the mesh sizes to be used. In this range, the spurious appearance of Auger electrons emitted from the grid material is calculated to be less than 1%. This conclusion applies to the meshes evaluated here, which range in thickness from 13 to 29 µm. Thinner meshes may lead to the applicability of proportionately closer meshed grids in sputter‐profiling applications. Copyright © 2006 John Wiley & Sons, Ltd. The contribution of Martin P. Seah of the National Physical Laboratory is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.     

Characterization of thin alumina films prepared by metal-organic chemical vapour deposition (MOCVD) by high resolution SEM, (AR)XPS and AES depth profiling

Thin alumina films deposited by metal-organic chemical vapour deposition (MOCVD) on AISI 304 substrate have been analyzed using the combination of Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES) and Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS). Both the surface and the alumina/substrate interface region have been analyzed in terms of chemical composition and elemental distribution. Only OH-groups (bounded as AlO(OH):boehmite) have been found as an impurity in the surface region of the oxide film. No carbon was detected. Due to higher temperature deposition, the concentration of OH-groups decreased. After annealing, the oxide/substrate interface changes as a result of chromium penetration into the alumina matrix. Carbon impurities have been detected on both delaminated and annealed alumina film surfaces. Also small amounts of sulfate groups as well as Ca and C impurities have been found on delaminated alumina film after prolonged high-temperature annealing.     

AES depth profiling with a tilted sample in front of a cylindrical mirror analyzer: quantification and profile shape changes according to an extension of the conventional MRI model

The influence of the tilt angle of a sample in front of a cylindrical mirror analyzer (CMA) on AES depth profiles is calculated with the conventional mixing‐roughness‐information (MRI) depth model and an extended MRI model. While the conventional model works with an average electron escape depth value, the extended model takes into account the intensity from different segments along the azimuth angle corresponding to different escape depth values before summing up for the total, measured intensity. The deviation between both approaches is generally less than 4%, even for the worst case at 47.7° tilt angle. The shape of the profile is slightly different for both approaches. Because, for a CMA with coaxial gun, the sample tilt angle varies as the electron beam incidence angle, the influence of the latter has to be additionally taken into account for quantification of AES. In reasonable agreement with experimental results it is shown that above 45° the Auger peak intensity of Cu (914 eV) increases up to about a factor of two for an incidence angle of 85°. Copyright © 2006 John Wiley & Sons, Ltd.     

A guide for the meaningful surface analysis of wood by XPS and ToF-SIMS

The surface analysis of wood and wood products is becoming increasingly important for reasons ranging from the investigation of molecular constituents through to the optimization of industrial processes. As with any natural product, wood analysis is not straightforward, and this review aims to provide guidance for the successful surface analysis of wood by XPS and ToF-SIMS. Through example experiments, three themes are addressed relevant to obtaining meaningful results: considerations related to heterogeneity in the composition of wood (e.g., growth rings); the impact of the chemical removal of minor wood components known as extractives, and whether such a process is necessary; and the potential for misleading or erroneous results as a result of contamination occurring during sample preparation. In addition to discussing successful sample preparation approaches, the important role to be played by MVA in surface analysis is emphasized, particularly in the analysis of ToF-SIMS data. Examples of ToF-SIMS/MVA are provided that highlight the identification of contamination in sample preparation, the quantification of wood composition in terms of cellulose and lignin, and the indication of age of softwood samples. Through consideration of the complexities that influence wood surface analysis, the design and interpretation of consequential experiments become easier and more accurate.     

Application of the chromatographic step profile method for determination of water film pressure and surface free energy of quartz

Summary Investigation of water adsorption by the step profile method (Glueckauf method) was carried out with the help of a modified gas chromatography equipped with thermal-conductivity detector. On the basis of the adsorption isotherm obtained, the water film pressure and the polar component of the surface free energy of quartz were calculated. The calculated value of the polar component of the surface free energy of quartz agrees with analogous values obtained by other methods.