ISO TR 18392-2005 Surface chemical analysis - X-ray photoelectron spectroscopy - Procedures for determining backgrounds《表面化学分析 X射线光电子光谱学 背景测定程序》.pdf

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1、 Reference number ISO/TR 18392:2005(E) ISO 2005TECHNICAL REPORT ISO/TR 18392 First edition 2005-12-01 Surface chemical analysis X-ray photoelectron spectroscopy Procedures for determining backgrounds Analyse chimique des surfaces Spectroscopie de photolectrons X Protocoles pour dterminer les fonds c

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5、e address given below. ISO 2005 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below o

6、r ISOs member body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2005 All rights reservedISO/TR 18392:2005(E) ISO 2005 All rights reserved ii

7、i Contents Page Foreword iv Introduction v 1 Scope. 1 2 Terms and definitions. 1 3 Symbols and abbreviated terms . 1 4 Types of background in XPS . 1 5 Removal of X-ray satellites from electron spectra 2 6 Estimation and removal of inelastic electron scattering from electron spectra 2 6.1 General In

8、formation 2 6.2 Procedures to account for inelastic electron scattering 2 6.2.1 Introduction . 2 6.2.2 Estimation of the linear background and its removal.3 6.2.3 Integral background removal 3 6.2.4 Removal based on the electron inelastic-scattering cross-section 4 6.3 Procedures accounting for both

9、 inelastic and elastic scattering 5 6.4 Less commonly used procedures. 5 6.5 Role of surface and core-hole effects in background determination 6 6.6 Determining the background for inhomogeneous materials . 6 7 Comparisons of procedures for removing effects of inelastic electron scattering from elect

10、ron spectra 7 Bibliography . 8 ISO/TR 18392:2005(E) iv ISO 2005 All rights reservedForeword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out throu

11、gh ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collabora

12、tes closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standar

13、ds. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. In exceptional circumstances, when a technical committee has collecte

14、d data of a different kind from that which is normally published as an International Standard (“state of the art”, for example), it may decide by a simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature and does not have

15、to be reviewed until the data it provides are considered to be no longer valid or useful. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO/TR 1839

16、2 was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee SC 5, Auger electron spectroscopy. ISO/TR 18392:2005(E) ISO 2005 All rights reserved v Introduction This Technical Report gives guidance for determining backgrounds in X-ray photoelectron spectra. The methods o

17、f background determination described in this report are applicable for quantitative evaluation of spectra of photoelectrons and Auger electrons excited by X-rays from solid surfaces and surface nanostructures. The use of background determination in X-ray photoelectron spectroscopy (XPS) has develope

18、d from the need (i) for accurate quantitative information on chemical composition (including in-depth composition) of surface/interface layers and nanostructures, (ii) for unambiguous identification of chemical states of surface species and (iii) for extracting electronic-structure information from

19、photoelectron spectra excited from solids. It is therefore necessary to separate the intrinsic part of a spectrum, which is associated with the photoionization or photoexcitation process by the X-radiation of interest in XPS or the Auger-electron decay process and which is needed for further analysi

20、s, from other parts of the spectrum (the background) appearing due to other processes. There are widely used procedures available for background subtraction in XPS that are reviewed in detail in References 1 to 4. Here, the most common procedures and their use are summarized, including methods (i) c

21、ommonly available in commercial software systems, (ii) available and used in more advanced laboratories and (iii) used in individual laboratories to develop understanding of the processes reflected in the XPS spectra. TECHNICAL REPORT ISO/TR 18392:2005(E) ISO 2005 All rights reserved 1 Surface chemi

22、cal analysis X-ray photoelectron spectroscopy Procedures for determining backgrounds 1 Scope This Technical Report gives guidance for determining backgrounds in X-ray photoelectron spectra. The methods of background determination described in this report are applicable for evaluation of spectra of p

23、hotoelectrons and Auger electrons excited by X-rays from solid surfaces. 2 Terms and definitions For the purposes of this document, the terms and definitions given in ISO 181155apply. 3 Symbols and abbreviated terms AES Auger electron spectroscopy PIA Partial intensity analysis QUASES TMQuantitative

24、 analysis of surfaces by electron spectroscopy REELS Reflection electron energy loss spectroscopy XPS X-ray photoelectron spectroscopy 4 Types of background in XPS The electrons produced by X-ray irradiation of surfaces are either photoelectrons (as a result of the primary photoionization process) o

25、r Auger electrons (as a result of the secondary, core-hole decay process). Contributions to the measured spectra (i.e., electron energy distributions) from electrons scattered inelastically in the sample, from the secondary electron cascade, and in the case of excitation by non-monochromatic X-ray i

26、rradiation from photoelectrons induced by X-ray satellites and by bremsstrahlung radiation constitute the background. It is usually not necessary in practical XPS to determine the secondary-electron cascade background at low energies. In this Technical Report, a description of methods for removing X

27、-ray satellites is given in Clause 5 and for removing inelastic electron scattering in Clause 6. A brief comparison is given in Clause 7 of the effectiveness of procedures for removing the effects of electron inelastic scattering from electron spectra. NOTE 1 In some cases, the intensity of the intr

28、insic part of a spectrum is distributed among features attributable to the “no-loss” main peak and to various electronic excitations associated with the creation of the core hole. The latter intrinsic contributions are sometimes denoted as the “intrinsic background”. The identification of the intrin

29、sic loss features and measurement of their intensities can be important for quantitative applications of XPS. NOTE 2 Time-varying fluctuations of the analytical signal due to sources of noise5will lead to uncertainty in the signal intensity. Intensity contributions due to noise are not included in t

30、he types of background discussed in this Technical Report. ISO/TR 18392:2005(E) 2 ISO 2005 All rights reserved5 Removal of X-ray satellites from electron spectra For XPS with non-monochromated X-ray sources, a fixed satellite structure is associated with the exciting main X-ray line (often Al or Mg

31、K radiation). These X-ray satellites lead to corresponding features in the XPS spectra. For selected photoelectron peaks measured with Al or Mg X-ray sources, intensity is removed from higher-kinetic-energy channels corresponding to the energy differences between the K 3,4 , K , etc., X-ray satellit

32、e positions and the K 1,2main peak and the corresponding intensity ratios6to remove the satellite contributions in the given spectral region. In such a way, scaled photoelectron peaks corresponding to the peaks excited by the X-ray satellites are subtracted. This subtraction process can be applied i

33、n turn to remove satellite peaks associated with other photoelectron peaks. The subtraction process may also erroneously remove an equivalent intensity from Auger peaks present in the spectrum if these are mistakenly identified as photoelectron peaks. 6 Estimation and removal of inelastic electron s

34、cattering from electron spectra 6.1 General Information Various procedures have been developed for separating the part of intrinsic origin in the measured photo-excited electron spectra from the contributions due to electrons that are inelastically scattered in the sample1-4 . These procedures (incl

35、uding those described in Clause 5) are usually applied to XPS data following data acquisition and require digital-data acquisition and handling capability. Prior to application of a procedure for removing the inelastic electron scattering, a measured spectrum normally should be corrected for the spe

36、ctrometer response function7, 8in cases where the distortion of the spectral shape due to instrumental effects is not negligible. To remove the effect of inelastic electron scattering in the spectrum, two different strategies can be followed: either to remove (subtract) the contribution attributable

37、 to electron inelastic scattering from the spectrum, or to include a background component in the model function being used to fit the spectrum. The electron-scattering contribution can be considered either as a background for the whole spectrum or as a sum of tail contributions9, 10from individual p

38、hotoelectron peaks. In the case of background removal/subtraction, the parameters of the background components are fixed and, after creating such a background, can be subtracted from the measured spectrum. In the case of background fitting, some or all of the parameters of the background components

39、are allowed to vary in the fitting process. NOTE The methods described here for removing the contributions of electron inelastic scattering from a spectrum may not be used for some specific applications of XPS (i.e., total reflection XPS or Auger-photoelectron coincidence spectroscopy) without furth

40、er consideration. 6.2 Procedures to account for inelastic electron scattering 6.2.1 Introduction Even in the case of very thin samples, a considerable fraction of the electrons in a spectrum have been inelastically scattered; therefore, the estimation of the background for inelastic electron scatter

41、ing is very important for quantitative applications. Common procedures for removing the effects of inelastic scattering are briefly described. ISO/TR 18392:2005(E) ISO 2005 All rights reserved 3 6.2.2 Estimation of the linear background and its removal In this widely used method, two arbitrarily cho

42、sen points in the spectrum are selected and joined by a straight line2as an approximation of the true background. These points are generally chosen such that the peak is positioned between them. The intensity values at the chosen points may be the values at the corresponding energies or the average

43、value over a small energy interval around the chosen points. Figure 1 illustrates a linear background for a Cu 2p 3/2XPS spectrum11 . This is the most popular method for insulators, where the straight line is horizontal. This approach is used for polymers with great success for peak fitting. However

44、, the use of the linear background in the case of peaks of transition metals (e.g., Fe 2p) leads to significant systematic errors in estimating the peak areas. Key X binding energy (eV) Y intensity (arbitrary units) Figure 1 Example of linear background and its subtraction (The XPS spectrum used her

45、e is copper 2p 3/2 . The upper curve shows the measured spectrum and the linear background. The lower curve is the spectrum after subtraction of the background.) 6.2.3 Integral background removal This (widely used) method, proposed by Shirley12, 13 , employs a mathematical algorithm to approximate t

46、he inelastic scattering of electrons as they escape from the solid. The algorithm is based on the assumption that the background is proportional to the area of the peak above the background at higher kinetic energies. This method has been modified to optimize the required iterations14 , to provide f

47、or a sloping inelastic background15 , to provide for a background based upon the shape of the loss spectrum from an elastically backscattered electron16 , and to include a band gap for insulators2 . Figure 2 shows the Shirley or integral background for the Cu 2p 3/2XPS spectrum given in Figure 111an

48、d the spectrum after this background has been subtracted. It should be emphasized that the correct use of this method requires the application of the valid algorithm12and the proper iteration limit14 . ISO/TR 18392:2005(E) 4 ISO 2005 All rights reservedKey X binding energy (eV) Y intensity (arbitrar

49、y units) Figure 2 Example of integral background and its subtraction (The XPS spectrum used is copper 2p 3/2 . The upper curve shows the measured spectrum and the integral background. The lower curve is the spectrum after subtraction of the background.) 6.2.4 Removal based on the electron inelastic-scattering cross-section This (often used) method, proposed by Tougaard4 , uses an algorithm that is based on a description of the inelastic-scattering processes taking place in the sample. Th