1、Designation: E 1523 03Standard Guide toCharge Control and Charge Referencing Techniques inX-Ray Photoelectron Spectroscopy1This standard is issued under the fixed designation E 1523; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide acquaints the XPS user with the variouscharge control and charge shift referencing techniques
3、that areand have been used in the acquisition and interpretation ofX-ray photoelectron spectroscopy (XPS) data from surfaces ofinsulating specimens and provides information needed forreporting the methods used to customers or in the literature.1.2 This guide is intended to apply to charge control an
4、dcharge referencing techniques in XPS and is not necessarilyapplicable to electron-excited systems.1.3 SI units are standard unless otherwise noted.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this st
5、andard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 673 Terminology Relating to Surface Analysis2E 902 Practice for Checking the Operating Characteristicsof X-Ray Photoelectro
6、n Spectrometers2E 1078 Guide for Specimen Handling in Auger ElectronSpectroscopy, X-Ray Photoelectron Spectroscopy, andSecondary Ion Mass Spectrometry2E 1829 Guide for Specimen Preparation and Mounting inSurface Analysis23. Terminology3.1 Definitions:3.1.1 See Terminology E 673 for definitions of te
7、rms used inX-ray photoelectron spectroscopy.3.1.2 SymbolsBE Binding energy, in eVBEcorrCorrected binding energy, in eVBEmeasMeasured binding energy, in eVBErefReference binding energy, ineVBEmeas,refMeasured Binding energy, ineV, of a reference lineFWHM Full width at half maximumamplitude of a peak
8、in the pho-toelectron spectrum above thebackground, in eVXPS X-ray photoelectron spectros-copyDcorrCorrection energy, to be addedto measured binding energiesfor charge correction, in eV4. Overview of Charging Effects4.1 For insulating specimen surfaces, the emission of pho-toelectrons following X-ra
9、y excitation may result in a buildupof a positive surface charge. This positive surface chargechanges the surface potential thereby shifting the measuredenergies of the photoelectron peaks to higher binding energy.This binding energy shift may reach a nearly steady-state valueof between 2 and 5 eV f
10、or spectrometers equipped withnonmonochromatic X-ray sources. The surface potentialcharge and the resulting binding energy shift is, generally,larger for spectrometers equipped with monochromatic X-raysources because of the, generally, lower flux of low-energyelectrons impinging on the specimen surf
11、ace. This lower fluxarises because focused, monochromatic X-ray beams irradiateonly a portion of the specimen and not other nearby surfaces(for example, the specimen holder) that are sources of low-energy electrons. The absence of an X-ray window in manymonochromatic X-ray sources (or a greater dist
12、ance of thespecimen from the X-ray window) also eliminates anothersource of low-energy electrons.4.2 The amount of induced surface charge, its distributionacross the specimen surface, and its dependence on experimen-tal conditions are determined by several factors includingspecimen composition, homo
13、geneity, magnitude of surfaceconductivity, total photoionization cross-section, surface to-pography, spacial distribution of the exciting X-rays, andavailability of neutralizing electrons. Charge buildup is awell-studied (1, 2)3, three dimensional phenomenon that occurs1This guide is under the juris
14、diction of ASTM Committee E42 on SurfaceAnalysis and is the direct responsibility of Subcommittee E42.03 on Auger ElectronSpectroscopy and X-ray Photoelectron Spectroscopy.Current edition approved May 10, 2003. Published July 2003. Originallyapproved in 1993. Last previous edition approved in 1997 a
15、s E 1523 97.2Annual Book of ASTM Standards, Vol 03.06.3The boldface numbers given in parentheses refer to a list of references at theend of the text.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.along the sample surface and into th
16、e material. The presence ofparticles on or different phases in the specimen surface mayresult in an uneven distribution of charge across the surface, aphenomenon known as differential charging. Charge buildupmay also occur at phase boundaries or interface regions withinthe depth of the sample that i
17、s impacted by x-ray radiation.Some specimens undergo time-dependent changes in the levelof charging because of electron, X-ray, or thermal damage orbecause of volatilization. Such specimens may never achievesteady-state potentials.4.3 Several techniques have been developed for the purposeof controll
18、ing charge buildup and the subsequent changes insurface potential in order to obtain meaningful and reproduc-ible data from insulating specimens. These techniques areemployed during the data acquisition and are discussed in 7.1.4.4 Several techniques have been developed for the purposeof correcting
19、the binding energy shifts that result from surfacecharging. These corrections are performed after the data hasbeen accumulated and are discussed in 7.2.4.5 The use of the various charge control or charge refer-encing techniques described in this guide may depend on theavailable instrument as well as
20、 the specimen being analyzed.5. Significance and Use5.1 The acquisition of chemical information from variationsin the energy position of peaks in the XPS spectrum is ofprimary interest in the use of XPS as a surface analytical tool.Surface charging acts to shift spectral peaks independent oftheir ch
21、emical relationship to other elements on the samesurface. The desire to eliminate the influence of surfacecharging on the peak positions and peak shapes has resulted inthe development of several empirical methods designed toassist in the interpretation of the XPS peak positions, determinesurface che
22、mistry, and allow comparison of spectra of conduct-ing and nonconducting systems of the same element. It isassumed that the spectrometer is generally working properlyfor non-insulating specimens (see Practice E 902).5.2 No ideal method has been developed to deal withsurface charging (3, 4). For insu
23、lators, an appropriate choice ofany control or referencing system will depend on the nature ofthe specimen, the instruments, and the information needed. Theappropriate use of charge control and referencing techniqueswill result in more consistent, reproducible data. Researchersare strongly urged to
24、report both the control and referencingtechniques that have been used, the specific peaks and bindingenergies used as standards (if any), and the criteria applied indetermining optimum results so that the appropriate compari-sons may be made.6. Apparatus6.1 One or more of the charge compensation tec
25、hniquesmentioned in this guide may be employed in virtually any XPSspectrometer.6.2 Some of the techniques outlined require special acces-sory apparatus, such as electron flood sources or a source forevaporative deposition.6.3 Certain specimen mounting procedures, such as mount-ing the specimen unde
26、r a fine metal mesh (5), can enhanceelectrical contact of the specimen with the specimen holder, orreduce the amount of surface charge buildup. This and othermethods of specimen mounting to reduce static charge aredescribed in detail in Guide E 1078 and Guide E 1829 .7. Procedures7.1 The methods des
27、cribed here involve charge control (theeffort to control the buildup of charge at a surface or tominimize its effect), charge referencing (the effort to determinea reliable binding energy despite buildup of charge), or somecombination of the two. For charge control, peak shape is themost important p
28、arameter to consider. Correcting the peakposition is accomplished separately using an appropriatecharge referencing technique. In some circumstances the Augerparameter can provide chemical information without the needto resort to surface potential corrections.7.1.1 Methods to Control Surface Potenti
29、al:7.1.1.1 Electron Flood Gun (6-9)Use low-energy electronflood guns to stabilize the static charging of insulators exam-ined by XPS (7), in particular when monochromatized X-raysare employed. Optimum operating conditions, for example,filament position, electron energy, and electron current, dependu
30、pon the orientation of the electron flood gun with respect tothe specimen and upon the particular design of the electronflood gun and must, in general, be determined by the user. Uselow-electron energies (usually 10 eV or less) to maximize theneutralization effect and reduce the number of electronbo
31、mbardment-induced reactions. A metal screen placed on orabove the specimen can help (10,11).7.1.1.2 Ultraviolet Flood Lamp (12)Ultraviolet radiationcan also produce low-energy electrons (for example, from thespecimen holder) that may be useful in neutralizing specimencharging.7.1.1.3 Specimen Heatin
32、gFor a limited number of speci-mens, heating can increase the electrical conductivity of thespecimen, thus decreasing charging (2).7.1.1.4 Electrical Connection7.1.1.4.1 Grounding and enhanced conduction pathSurrounding of insulating materials with a conducting materialhas been a common approach to
33、minimizing the charge buildup on samples. This can mean masking a solid sample with aconducting aperture, grid or foil or mounting particles on aconducting foil or tape (2).7.1.1.4.2 Isolation from groundFor some materials, ormixtures of materials with different electrical conductivity,differential
34、charging can occur. This phenomenon can be usedto obtain information about the sample (13, 14) and cansometimes be minimized (and a more uniform sample potentialcan be achieved) by isolating the specimen from ground.7.1.1.5 BiasingApplying a low-voltage bias (10 to +10V) to the specimen and observin
35、g the changes in the bindingenergies of various peaks can be used to learn about theelectrical contact of a specimen (or parts of a specimen) withthe specimen holder. Peaks in XPS spectrum that shift whenthe bias is applied are from conducting regions of the speci-men. Other peaks from insulating re
36、gions may not shift nearlyas much or at all and can be interpreted accordingly. Thismethod can sometimes verify that the peaks being used forcharge referencing (for example, Au 4f or C 1s) are behavingin the same manner as the peaks of interest from the specimenE1523032(5,12,15). For nonuniform or c
37、omposite (nonconducting orpartially conducting) specimens, a variety of charge shifts maybe observed upon biasing. This may provide useful informationabout the sample and indicate a need to more carefully connectthe specimen to ground or to isolate the sample from ground.Sometimes all data for some
38、specimens are collected with abias applied (see also Section 7.3).7.1.1.6 Low Energy Ion SourceRecent work indicates thatportions of an insulator surface can be negatively charged, evenwhen some areas exposed to X-rays are charged positively(16). Such effects appear to be particularly important forf
39、ocused X-ray beam systems, where the X-rays strike only arelatively small portion of the specimen. In these circum-stances the use of a low-eneergy positive-ion source, inaddition to an electron source, may help stabilize (and makemore uniform) the surface potential of the specimen.7.2 Binding Energ
40、y Reference Methods:A variety of methods are often used to determine the amountof binding energy shift resulting from surface charging. Eachof these methods is based on the assumption that differentialcharging (along the surface or within the sample) is not presentto a significant degree. If signifi
41、cant differential charging isfound to occur or thought to be present, it may be necessary toalter the method of charge control.7.2.1 Adventitious Carbon Referencing (5,6,12,17-21)Unless specimens are prepared for analysis under carefullycontrolled atmospheres, the surface, generally, is coated byadv
42、entitious contaminants. Once introduced into the spectrom-eter, further specimen contamination can occur by the adsorp-tion of residual gases, especially in instruments with oildiffusion pumps. These contamination layers can be used forreferencing purposes if it is assumed that they truly reflect th
43、esteady-state static charge exhibited by the specimen surfaceand that they contain an element with a peak of known bindingenergy. Carbon is most commonly detected in adventitiouslayers, and photoelectrons from the C 1s transition are thosemost often adopted as a reference.7.2.1.1 A binding energy of
44、 284.8 eV is often used for the C1s level of this contamination and the difference between themeasured position in the energy spectrum and the referencevalue, above, is the amount of surface potential shift caused bycharging. This reference energy is based on the assumption thatthe carbon is in the
45、form of a hydrocarbon or graphite and thatother carbon species are either not present or can be distin-guished from this peak.7.2.1.2 A significant disadvantage of this method lies in theuncertainty of the true nature of the carbon and the appropriatereference values which have a wide range as repor
46、ted in theliterature (6,18,19) that ranges from 284.6 to 285.2 eV for theC 1s electrons. Therefore, it is recommended that if adventi-tious carbon is to be used for referencing, the reference bindingenergy should be determined on the users own spectrometer.Ideally, this measurement should be carried
47、 out on a substratesimilar in its chemical and physical properties to the material tobe analyzed and covered by only a thin, uniform contaminationlayer (that is, of the order of a monolayer).7.2.1.3 Care must be taken where adventitious hydrocarboncan be chemically transformed, as, for example, by a
48、 stronglyoxidizing specimen (19). With less than one monolayer cover-age of adventitious carbon, the C 1s binding energy sometimesdecreases (20). The carbon binding energy may also shift as aconsequence of ion sputtering; evidence has been found forcarbon of lower binding energy, possibly graphite o
49、r, morelikely, carbon in domains approaching atomic dimensions (12).One method for distinguishing the presence of more than onetype of carbon is to monitor the FWHM of the C 1s photo-electron peak. Abnormally broad peaks suggest the presence ofmore than one type of carbon or differential charge. Broadenedcarbon 1s peaks may result from the presence of more than onetype of carbon or differential charging. Despite the limitationsand uncertainties associated with the use of adventitious carbonfor static-charge referencing, it is the most convenient andcommonly applied technique.
