ISO/TC 201/SC 7 - Electron spectroscopies

This document provides a description of methods by which the coating thickness and chemical composition of "core-shell" nanoparticles (including some variant and non-ideal morphologies) can be determined using electron spectroscopy techniques. It identifies the assumptions, challenges, and uncertainties associated with each method. It also describes protocols and issues for the general analysis of nanoparticle samples using electron spectroscopies, specifically in relation to their importance for measurements of coating thicknesses. This document focuses on the use of electron spectroscopy techniques, specifically X-ray photoelectron spectroscopy, Auger electron spectroscopy, and synchrotron-based methods. These cannot provide all of the information necessary for accurate analysis and therefore some additional analytical methods are outlined in the context of their ability to aid in the interpretation of electron spectroscopy data.

This document specifies the minimum amount of information spectroscopy to be reported with the analytical results to describe the methods of charge control and charge correction in measurements of core-level binding energies for insulating specimens by X‑ray photoelectron. It also provides methods for charge control and for charge correction in the measurement of binding energies.

This document provides an introduction to (and some examples of) the types of information that can be obtained about nanostructured materials using surface-analysis tools (Clause 5). Of equal importance, both general issues or challenges associated with characterizing nanostructured materials and the specific opportunities or challenges associated with individual methods are identified (Clause 6). As the size of objects or components of materials approaches a few nanometres, the distinctions among "bulk", "surface" and "particle" analysis blur. Although some general issues relevant to characterization of nanostructured materials are identified, this document focuses on issues specifically relevant to surface chemical analysis of nanostructured materials. A variety of analytical and characterization methods will be mentioned, but this report focuses on methods that are in the domain of ISO/TC 201 including Auger Electron Spectroscopy, X‑ray photoelectron spectroscopy, secondary ion mass spectrometry, and scanning probe microscopy. Some types of measurements of nanoparticle surface properties such as surface potential that are often made in a solution are not discussed in this Report. Although they have many similar aspects, characterization of nanometre-thick films or a uniform collection of nanometre-sized particles present different characterization challenges. Examples of methods applicable to both thin films and to particles or nano-sized objects are presented. Properties that can be determined include: the presence of contamination, the thickness of coatings, and the chemical nature of the surface before and after processing. In addition to identifying the types of information that can be obtained, the document summarizes general and technique-specific Issues that must be considered before or during analysis. These include: identification of needed information, stability and probe effects, environmental effects, specimen-handling issues, and data interpretation. Surface characterization is an important subset of several analysis needs for nanostructured materials. The broader characterization needs for nanomaterials are within the scope of ISO/TC 229 and this document has been coordinated with experts of TC 229 Joint Working Group (JWG) 3. This introduction to information available about nanomaterials using a specific set of surface-analysis methods cannot by its very nature be fully complete. However, important opportunities, concepts and issues have been identified and many references provided to allow the topics to be examined in greater depth as required.

This document is intended to aid the operators of X-ray photoelectron spectrometers in their analysis of typical samples. It takes the operator through the analysis from the handling of the sample and the calibration and setting-up of the spectrometer to the acquisition of wide and narrow scans and also gives advice on quantification and on preparation of the final report.

This document specifies the necessary information required in a report of analytical results based on measurements of the intensities of peaks in Auger electron and X-ray photoelectron spectra. Information on methods for the measurement of peak intensities and on uncertainties of derived peak areas is also provided.

This document is designed to allow the user to assess, on a regular basis, several key parameters of an X‑ray photoelectron spectrometer. It is not intended to provide an exhaustive performance check, but instead provides a rapid set of tests that can be conducted frequently. Aspects of instrument behaviour covered by this document include the vacuum, measurements of spectra of conductive or non-conductive test specimens and the current state of the X‑ray source. Other important aspects of the instrument performance (e.g. lateral resolution) fall outside the scope of this document. The document is intended for use with commercial X‑ray photoelectron spectrometers equipped with a monochromated Al Kα X‑ray source or with an unmonochromated Al or Mg Kα X‑ray source.

This document specifies several methods for measuring the oxide thickness at the surfaces of (100) and (111) silicon wafers as an equivalent thickness of silicon dioxide when measured using X-ray photoelectron spectroscopy. It is only applicable to flat, polished samples and for instruments that incorporate an Al or Mg X-ray source, a sample stage that permits defined photoelectron emission angles and a spectrometer with an input lens that can be restricted to less than a 6° cone semi-angle. For thermal oxides in the range 1 nm to 8 nm thickness, using the best method described in this document, uncertainties, at a 95 % confidence level, could typically be around 2 % and around 1 % at optimum. A simpler method is also given with slightly poorer, but often adequate, uncertainties.

ISO 19668:2017 specifies a procedure by which elemental detection limits in X-ray photoelectron spectroscopy (XPS) can be estimated from data for a particular sample in common analytical situations and reported. This document is applicable to homogeneous materials and is not applicable if the depth distribution of elements is inhomogeneous within the information depth of the technique.

ISO 15470:2017 describes the way in which specific aspects of the performance of an X-ray photoelectron spectrometer are described.

ISO 15471:2016 specifies the requirements for the description of specific aspects of the performance of an Auger electron spectrometer.

ISO 17973:2016 specifies a method for calibrating the kinetic energy scales of Auger electron spectrometers with an uncertainty of 3 eV, for general analytical use in identifying elements at surfaces. In addition, it specifies a method for establishing a calibration schedule. It is applicable to instruments used in either direct or differential mode, where the resolution is less than or equal to 0,5 % and the modulation amplitude for the differential mode, if used, is 2 eV peak-to-peak. It is applicable to those spectrometers equipped with an inert gas ion gun or other method for sample cleaning and with an electron gun capable of operating at 4 keV or higher beam energy.

ISO/TR 18394:2016 provides guidelines for identifying chemical effects in X-ray or electron-excited Auger-electron spectra and for using these effects in chemical characterization.

ISO 18554:2016 provides a simple procedure for identifying, estimating and correcting for unintended degradation in the elemental composition or chemical state of a material which occurs as a result of X-radiation during the time that a specimen material is exposed to the X-rays used in X-ray photoelectron spectroscopy (XPS). ISO 18554:2016 does not address comparisons between different types of material nor does it address the mechanisms, depth, or chemical nature of the degradation that occurs. The correction procedure proposed is only valid if the changes are caused by the X-rays and result in less than a 30 % reduction or increase in intensity of a chosen photoelectron peak from the sample material.

ISO 19830:2015 Standard is to define how peak fitting and the results of peak fitting in X-ray photoelectron spectroscopy shall be reported. It is applicable to the fitting of a single spectrum or to a set of related spectra, as might be acquired, for example, during a depth profile measurement. This International Standard provides a list of those parameters which shall be reported if either reproducible peak fitting is to be achieved or a number of spectra are to be fitted and the fitted spectra compared. This International Standard does not provide instructions for peak fitting nor the procedures which should be adopted.

ISO 18118:2015 gives guidance on the measurement and use of experimentally determined relative sensitivity factors for the quantitative analysis of homogeneous materials by Auger electron spectroscopy and X-ray photoelectron spectroscopy.

ISO 13424:2013 specifies the minimum amount of information required in reports of analyses of thin films on a substrate by XPS. These analyses involve measurement of the chemical composition and thickness of homogeneous thin films, and measurement of the chemical composition as a function of depth of inhomogeneous thin films by angle-resolved XPS, XPS sputter-depth profiling, peak-shape analysis, and variable photon energy XPS.

ISO/TR 19319:2013 describes: functions and their relevance to lateral resolution: point spread function (PSF), line spread function (LSF), edge spread function (ESF), modulation transfer function (MTF) and contrast transfer function (CTF); experimental methods for the determination of lateral resolution and parameters related to lateral resolution: imaging of a narrow stripe, sharp edge and square-wave gratings; physical factors affecting lateral resolution, analysis area and sample area viewed by the analyser in Auger electron spectroscopy and X-ray photoelectron spectroscopy.

ISO 15472:2010 specifies a method for calibrating the binding-energy scales of X‑ray photoelectron spectrometers, for general analytical purposes, using unmonochromated Al or Mg X‑rays or monochromated Al X‑rays. It is only applicable to instruments which incorporate an ion gun for sputter cleaning. It further specifies a method to establish a calibration schedule, to test for the binding-energy scale linearity at one intermediate energy, to confirm the uncertainty of the scale calibration at one low and one high binding-energy value, to correct for small drifts of that scale and to define the expanded uncertainty of the calibration of the binding-energy scale for a confidence level of 95 %. This uncertainty includes contributions for behaviours observed in interlaboratory studies but does not cover all of the defects that could occur. ISO 15472 is not applicable to instruments with binding-energy scale errors that are significantly non-linear with energy, to instruments operated in the constant retardation ratio mode at retardation ratios less than 10, to instruments with a spectrometer resolution worse than 1,5 eV, or to instruments requiring tolerance limits of ±0,03 eV or less. It does not provide a full calibration check, which would confirm the energy measured at each addressable point on the energy scale and which would have to be performed in accordance with the manufacturer's recommended procedures.

ISO 29081:2010 specifies the minimum amount of information required for describing the methods of charge control in measurements of Auger electron transitions from insulating specimens by electron-stimulated Auger electron spectroscopy and to be reported with the analytical results. Information is provided in Annex A on methods that have been found useful for charge control prior to or during AES analysis. This annex also contains a table summarizing the methods or approaches, ordered by simplicity of approach. Some methods will be applicable to most instruments, others require special hardware, others might involve remounting the specimen or changing it. A similar International Standard has been published for X‑ray photoelectron spectroscopy (ISO 19318).

ISO/TR 18392:2005 gives guidance for determining backgrounds in X-ray photoelectron spectra. The methods of background determination described are applicable for evaluation of spectra of photoelectrons and Auger electrons excited by X-rays from solid surfaces.

ISO 24237:2005 specifies a method for evaluating the repeatability and constancy of the intensity scale of X-ray photoelectron spectrometers, for general analytical purposes, using unmonochromated Al or Mg X-rays or monochromated Al X-rays. It is only applicable to instruments that incorporate an ion gun for sputter cleaning. It is not intended to be a calibration of the intensity/energy response function. That calibration may be made by the instrument manufacturer or other organization. The present procedure provides data to evaluate and confirm the accuracy with which the intensity/energy response function remains constant with instrument usage. Guidance is given on some of the instrument settings that may affect this constancy.

ISO 24236:2005 specifies a method for evaluating the constancy and repeatability of the intensity scale of Auger electron spectrometers, for general analytical purposes, using an electron gun with a beam energy of 2 keV or greater. It is only applicable to instruments that incorporate an ion gun for sputter cleaning. It is not intended to be a calibration of the intensity/energy response function. That calibration may be made by the instrument manufacturer or other organization. The present procedure provides data to evaluate and confirm the accuracy with which the intensity/energy response function remains constant with instrument usage. Guidance is given on some of the instrumental settings that may affect this constancy.

ISO 21270:2004 specifies two methods for determining the maximum count rate for an acceptable limit of divergence from linearity of the intensity scale of Auger and X-ray photoelectron spectrometers. It also includes methods to correct for intensity non-linearities so that a higher maximum count rate can be employed for those spectrometers for which the relevant correction equations have been shown to be valid.

ISO 17974:2002 specifies a method for calibrating the kinetic energy scales of Auger electron spectrometers used for elemental and chemical state analysis at surfaces. It also specifies a calibration schedule for testing the kinetic energy scale linearity at one intermediate energy, for confirming the uncertainty of the scale calibration at one low and one high kinetic energy value, for correcting for small drifts of that scale and defining the expanded uncertainty of the calibration of the kinetic energy scale for a confidence level of 95 % (with this uncertainty including contributions for behaviours observed in interlaboratory studies but not covering all possible defects). It is applicable only to those instruments incorporating an ion gun for sputter cleaning. It is not applicable to instruments with kinetic energy scale errors significantly non-linear with energy, those operated at relative resolutions poorer than 0,2 % in the constant delta E/E mode or 1,5 eV in the constant delta E mode, those requiring tolerance limits of plus or minus 0,05 eV or less, nor those with an electron gun that cannot be operated in the energy range 5 keV to 10 keV. It does not provide a full calibration check for confirming the energy measured at each addressable point on the energy scale, this being performed according to the manufacturer's recommendations.

ISO/TR 14187 provides an introduction to (and some examples of) the types of information that can be obtained about nanostructured materials using surface-analysis tools. Of equal importance, both general issues or challenges associated with characterising nanostructured materials and the specific opportunities or challenges associated with individual methods are identified. As the size of objects or components of materials approaches a few nanometres, the distinctions among "bulk", "surface" and "particle" analysis blur. Although some general issues relevant to characterisation of nanostructured materials are identified, this Technical Report focuses on issues specifically relevant to surface chemical analysis of nanostructured materials. A variety of analytical and characterisation methods will be mentioned, but this report focuses on methods that are in the domain of ISO/TC 201 including auger electron spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and scanning probe microscopy. Some types of measurements of nanoparticle surface properties such as surface potential that are often made in a solution are not discussed in this Report. Although they have many similar aspects, characterisation of nanometre-thick films or a uniform collection of nanometre-sized particles present different characterisation challenges. Examples of methods applicable to both thin films and to particles or nano-sized objects are presented. Properties that can be determined include: the presence of contamination, the thickness of coatings, and the chemical nature of the surface before and after processing. In addition to identifying the types of information that can be obtained, the Technical Report summarises general and technique-specific Issues that must be considered before or during analysis. These include: identification of needed information, stability and probe effects, environmental effects, specimen-handling issues, and data interpretation. This introduction to information available about nanomaterials using a specific set of surface-analysis methods cannot by its very nature be fully complete. However, important opportunities, concepts and issues have been identified and many references provided to allow the topics to be examined in greater depth as required.

ISO 14701:2011 specifies several methods for measuring the oxide thickness at the surfaces of (100) and (111) silicon wafers as an equivalent thickness of silicon dioxide when measured using X-ray photoelectron spectroscopy. It is only applicable to flat, polished specimens and for instruments that incorporate an Al or Mg X-ray source, a specimen stage that permits defined photoelectron emission angles and a spectrometer with an input lens that can be restricted to less than a 6° cone semi-angle. For thermal oxides in the range 1 nm to 8 nm thickness, using the best method described in the standard, uncertainties, at a 95 % confidence level, could typically be around 2 % and around 1 % at optimum. A simpler method is also given with slightly poorer, but often adequate, uncertainties.

ISO 10810:2010 is intended to aid the operators of X‑ray photoelectron spectrometers in their analysis of typical samples. It takes the operator through the analysis from the handling of the sample and the calibration and setting-up of the spectrometer to the acquisition of wide and narrow scans and also gives advice on quantification and on preparation of the final report.

ISO/TR 18394:2006 provides guidelines for identifying chemical effects in X-ray or electron-excited Auger-electron spectra and for using these effects in chemical characterization.

ISO 20903:2006 specifies the necessary information required in a report of analytical results based on measurements of the intensities of peaks in Auger electron and X-ray photoelectron spectra. Information on methods for the measurement of peak intensities and on uncertainties of derived peak areas is also provided.

ISO 18118:2004 gives guidance on the measurement and use of experimentally determined relative sensitivity factors for the quantitative analysis of homogeneous materials by Auger electron spectroscopy and X-ray photoelectron spectroscopy.

ISO 19318:2004 specifies the minimum amount of information describing the methods of charge control and charge correction in measurements of core-level binding energies for insulating specimens by X-ray photoelectron spectroscopy that shall be reported with the analytical results. Information is also provided on methods that have been found useful for charge control and for charge correction in the measurement of binding energies.

ISO 15471:2004 describes the way in which specific aspects of the performance of an Auger electron spectrometer shall be described.

ISO 15470:2004 describes the way in which specific aspects of the performance of an X-ray photoelectron spectrometer shall be described.

ISO/TR 19319:2003 provides information for measuring (1) the lateral resolution, (2) the analysis area, and (3) the sample area viewed by the analyser in Auger electron spectroscopy and X-ray photoelectron spectroscopy.

ISO 17973:2002 specifies a method for calibrating the kinetic energy scales of Auger electron spectrometers with an uncertainty of 3 eV, for general analytical use in identifying elements at surfaces. In addition, it specifies a method for establishing a calibration schedule. It is applicable to instruments used in either direct or differential mode, where the resolution is less than or equal to 0,5 % and the modulation amplitude for the differential mode, if used, is 2 eV peak-to-peak. It is applicable to those spectrometers equipped with an inert gas ion gun or other method for sample cleaning and with an electron gun capable of operating at 4 keV or higher beam energy.