1、Designation: E1523 09E1523 15Standard Guide toCharge Control and Charge Referencing Techniques inX-Ray Photoelectron Spectroscopy1This standard is issued under the fixed designation E1523; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revi
2、sion, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide acquaints the X-ray photoelectron spectroscopy (XPS) user with the various charge control
3、 and charge shiftreferencing techniques that are and have been used in the acquisition and interpretation of XPS data from surfaces of insulatingspecimens and provides information needed for reporting the methods used to customers or in the literature.1.2 This guide is intended to apply to charge co
4、ntrol and charge referencing techniques in XPS and is not necessarily applicableto electron-excited systems.1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.4 This standard does not purport to address all of the safety co
5、ncerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E673 Terminology Relating to Surface Ana
6、lysis (Withdrawn 2012)3E902 Practice for Checking the Operating Characteristics of X-Ray Photoelectron Spectrometers (Withdrawn 2011)3E1078 Guide for Specimen Preparation and Mounting in Surface AnalysisE1829 Guide for Handling Specimens Prior to Surface Analysis3. Terminology3.1 DefinitionsSee Term
7、inology E673 for definitions of terms used in XPS.3.2 Symbols:BE Binding energy, in eVBEcorr Corrected binding energy, in eVBEmeas Measured binding energy, in eVBEref Reference binding energy, in eVBEmeas, ref Measured Binding energy, in eV, of a reference lineFWHM Full width at half maximum amplitu
8、de of a peak in thephotoelectron spectrum above the background, in eVXPS X-ray photoelectron spectroscopycorr Correction energy, to be added to measured bindingenergies for charge correction, in eV4. Overview of Charging Effects4.1 For insulating specimen surfaces, the emission of photoelectrons fol
9、lowing X-ray excitation may result in a temporary (orsometimes persistent) buildup of a positive surface charge caused by the photoelectric effect. Its insulating nature prevents thecompensation of the charge buildup by means of electron conduction from the sample holder. This positive surface charg
10、e changesthe surface potential thereby shifting the measured energies of the photoelectron peaks to higher binding energy. This binding1 This guide is under the jurisdiction of ASTM Committee E42 on Surface Analysis and is the direct responsibility of Subcommittee E42.03 on Auger ElectronSpectroscop
11、y and X-Ray Photoelectron Spectroscopy.Current edition approved May 1, 2009June 1, 2015. Published June 2009June 2015. Originally approved in 1993. Last previous edition approved in 20032009 asE1523 03.E1523 09. DOI: 10.1520/E1523-09.10.1520/E1523-15.2 For referencedASTM standards, visit theASTM web
12、site, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.3 The last approved version of this historical standard is referenced on www.astm.org.This document is not an
13、ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In
14、 all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1energy shift may reach a nearly steady-state value of between 2 and 5 eV
15、 for spectrometers equipped with nonmonochromaticX-ray sources. The surface potential charge and the resulting binding energy shift is, generally, larger for spectrometers equippedwith monochromatic X-ray sources because of the, generally, lower flux of low-energy electrons impinging on the specimen
16、surface. This lower flux arises because focused, monochromatic X-ray beams irradiate only a portion of the specimen and not othernearby surfaces (for example, the specimen holder) that are sources of low-energy electrons. The absence of an X-ray window inmany monochromatic X-ray sources (or a greate
17、r distance of the specimen from the X-ray window) also eliminates another sourceof low-energy electrons.4.2 The amount of induced surface charge, its distribution across the specimen surface, and its dependence on experimentalconditions are determined by several factors including specimen compositio
18、n, homogeneity, magnitude of surface conductivity,total photoionization cross-section, surface topography, spatial distribution of the exciting X-rays, and availability of neutralizingelectrons. Charge buildup is a well-studied (1, 2)4, three dimensional phenomenon that occurs along the sample surfa
19、ce and intothe material. The presence of particles on or different phases in the specimen surface may result in an uneven distribution of chargeacross the surface, a phenomenon known as differential charging. Charge buildup may also occur at phase boundaries or interfaceregions within the depth of t
20、he sample that is impacted by X-ray radiation.4.3 Several techniques have been developed for the purpose of controlling charge buildup and the subsequent changes in surfacepotential in order to obtain meaningful and reproducible data from insulating specimens. These techniques are employed duringthe
21、 data acquisition and are discussed in 7.2.4.4 Several techniques have been developed for the purpose of correcting the binding energy shifts that result from surfacecharging. These corrections are performed after the data has been accumulated and are discussed in 7.3.4.5 The use of the various char
22、ge control or charge referencing techniques described in this guide may depend on the availableinstrument as well as the specimen being analyzed.4.6 Specimens with non-insulating surfaces are those with a high enough electron conductivity to dynamically compensate theelectron loss caused by the phot
23、oelectric effect; they neither require control of the surface charge buildup nor charge referencecorrections. It is important to distinguish the shifts due to the temporary charge build caused by the photoelectric effect fromintrinsic charging effects. Intrinsic effects, such as the accumulation of
24、charge at an interface during film growth, influence thenature of spectra obtained and the BEs measured, but are part of the sample (3). It is also possible that the impinging of the X-raychanges the charge distribution by means of volatilization of certain chemical species or the creation or charge
25、 centers. Suchspecimens may never achieve steady-state potentials.Although artifact to the process of measurement, those changes become partof the sample and are not necessarily to be corrected or compensated by the methods described in 7.2 and 7.3.4.7 Major advances in the ability to control sample
26、 charging and to stabilize surface potential were made in the late 1990sincluding the ability to achieve charge control for small area analysis (4). These approaches usually involve the use of electronflood guns and some additional methods (ions or magnetic fields) to control localized surface charg
27、e (5, 6). As a result of theseadvances it is now possible to collect high quality reproducible data on many systems. However, these advances do not removeall of the challenges for optimizing the conditions for analysis for complex samples or interpreting the data.4.8 Although changes in surface pote
28、ntial during XPS analysis and other charging effects are usually viewed as problems to beavoided, such phenomena can be used to extract important information about specimens (7-9).5. Significance and Use5.1 The acquisition of chemical information from variations in the energy position of peaks in th
29、e XPS spectrum is of primaryinterest in the use of XPS as a surface analytical tool. Surface charging acts to shift spectral peaks independent of their chemicalrelationship to other elements on the same surface. The desire to eliminate the influence of surface charging on the peak positionsand peak
30、shapes has resulted in the development of several empirical methods designed to assist in the interpretation of the XPSpeak positions, determine surface chemistry, and allow comparison of spectra of conducting and non-conducting systems of thesame element. It is assumed that the spectrometer is gene
31、rally working properly for non-insulating specimens (see Practice E902).5.2 Although highly reliable methods have now been developed to stabilize surface potentials during XPS analysis of mostmaterials (5, 6), no single method has been developed to deal with surface charging in all circumstances (10
32、, 11). For insulators,an appropriate choice of any control or referencing system will depend on the nature of the specimen, the instruments, and theinformation needed. The appropriate use of charge control and referencing techniques will result in more consistent, reproducibledata. Researchers are s
33、trongly urged to report both the control and referencing techniques that have been used, the specific peaksand binding energies used as standards (if any), and the criteria applied in determining optimum results so that the appropriatecomparisons may be made.4 The boldface numbers in parentheses ref
34、er to a list of references at the end of this standard.E1523 1526. Apparatus6.1 One or more of the charge compensation techniques mentioned in this guide may be employed in virtually any XPSspectrometer.6.2 Some of the techniques outlined require special accessory apparatus, such as electron flood s
35、ources or a source forevaporative deposition.6.3 Certain specimen mounting procedures, such as mounting the specimen under a fine metal mesh (12), can enhance electricalcontact of the specimen with the specimen holder, or reduce the amount of surface charge buildup. This and other methods ofspecimen
36、 mounting to reduce static charge are described in detail in Guide E1078 and Guide E1829.7. Procedures7.1 The methods described here involve charge control (the effort to control the buildup of charge at a surface or to minimizeits effect), charge referencing (the effort to determine a reliable bind
37、ing energy despite buildup of charge), or some combinationof the two. For charge control, peak shape is the most important parameter to consider.Aconstant and relatively uniformly surfacepotential provides the conditions needed to obtain reproducible data and optimum peak shape. Correcting the peak
38、position isaccomplished separately using an appropriate charge referencing technique. In some circumstances, the Auger parameter canprovide chemical information without the need to resort to surface potential corrections.7.2 A variety of different methods areis used to either enhance conductivity to
39、 minimize charge buildup during XPS analysisor to control the surface potential by other methods. These methods employed to control the surface potential in insulatingspecimens are listed in Table 1 in approximate order of frequent frequency of use (more frequently used first) and summarizedbelow:7.
40、2.1 Methods for Controlling the Sample Surface Potential:7.2.1.1 Electron Flood Gun (13-16)Use of low-energy electron flood guns to stabilize the surface potential of insulatorsexamined by XPS (14), in particular when monochromatized X-rays are employed. Optimum operating conditions, for example,fil
41、ament position, electron energy, and electron current, depend upon the orientation of the electron flood gun with respect to thespecimen and upon the particular design of the electron flood gun and must, in general, be determined by the user. Use low-electronenergies (usually 10 eV or less) to maxim
42、ize the neutralization effect and reduce the number of electron bombardment-inducedreactions. A metal screen placed on or above the specimen can help (17, 18).7.2.1.2 Low Energy Ion SourceRecent work indicates that portions of an insulator surface can be negatively charged, evenwhen some areas expos
43、ed to X-rays are charged positively (19). Such effects appear to be particularly important for focused X-raybeam systems, where the X-rays strike only a relatively small portion of the specimen. In these circumstances, the use of alow-energy positive-ion source, in addition to an electron source, he
44、lps stabilize (and make more uniform) the surface potentialof the specimen. Several commercial XPS now effectively combine electrons and ions to achieve uniform surface potentials formany types of insulators.7.2.1.3 Ultraviolet Flood Lamp (20)Ultraviolet radiation can also produce low-energy electro
45、ns (for example, from thespecimen holder) that may be useful in neutralizing specimen charging and stabilizing the surface potential.7.2.1.4 BiasingApplying a low-voltage bias (-10 to +10 V) to the specimen and observing the changes in the binding energiesof various peaks can be used to learn about
46、the electrical contact of a specimen (or parts of a specimen) with the specimen holder.Peaks in the XPS spectrum that shift when the bias is applied are from conducting regions of the specimen. Other peaks frominsulating regions may not shift nearly as much or at all and can be interpreted according
47、ly. This method can sometimes verifythat the peaks being used for charge referencing (for example, gold 4f or carbon 1s) are behaving in the same manner as the peaksof interest from the specimen (12, 20, 21). For non-uniform or composite (non-conducting or partially conducting) specimens, avariety o
48、f charge shifts may be observed upon biasing. This may provide useful information about the sample and indicate a needto more carefully connect the specimen to ground or to isolate the sample from ground. Sometimes all data for some specimensare collected with a bias applied (see also 7.4).TABLE 1 M
49、ethods Used to Stabilize or Control Surface PotentialDuring XPS AnalysisApproach/Method SectionApproach/Method SectionControlling the Sample Surface Potential 7.2.1Electron Flood Gun 7.2.1.1Low Energy Ion Source 7.2.1.2Ultraviolet Flood Lamp 7.2.1.3Biasing 7.2.1.4Isolation from Ground 7.2.1.5Minimizing Charge Accumulation 7.2.2Grounding and Enhanced Conduction Path 7.2.2.1Specimen Heating 7.2.2.2E1523 1537.2.1.5 Isolation from GroundFor some materials, or mixtures of materials with different electrical conductivity, differentialcharging can occur. This phenomenon can be
