1、Designation: E 1523 09Standard 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 () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide acquaints the X-ray photoelectron spectros-copy (XPS) user with the various charge control and
3、chargeshift referencing techniques that are and have been used in theacquisition and interpretation of 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 a
4、ndcharge referencing techniques in XPS and is not necessarilyapplicable to electron-excited systems.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.4 This standard does not purport to address all of thesafety concerns, if
5、any, associated with its use. It is theresponsibility of the user of this standard 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:2E 673 Terminology Relating to Surface AnalysisE
6、902 Practice for Checking the Operating Characteristicsof X-Ray Photoelectron SpectrometersE 1078 Guide for Specimen Preparation and Mounting inSurface AnalysisE 1829 Guide for Handling Specimens Prior to SurfaceAnalysis3. Terminology3.1 DefinitionsSee Terminology E 673 for definitions ofterms used
7、in XPS.3.2 Symbols:BE Binding energy, in eVBEcorrCorrected binding energy, in eVBEmeasMeasured binding energy, in eVBErefReference binding energy, in eVBEmeas, refMeasured Binding energy, in eV, of a reference lineFWHM Full width at half maximum amplitude of a peak in the photoelectronspectrum above
8、 the background, in eVXPS X-ray photoelectron spectroscopyDcorrCorrection energy, to be added to measured binding energies forcharge correction, in eV4. Overview of Charging Effects4.1 For insulating specimen surfaces, the emission of pho-toelectrons following X-ray excitation may result in a tempo-
9、rary (or sometimes persistent) buildup of a positive surfacecharge caused by the photoelectric effect. Its insulating natureprevents the compensation of the charge buildup by means ofelectron conduction from the sample holder. This positivesurface charge changes the surface potential thereby shiftin
10、gthe measured energies of the photoelectron peaks to higherbinding energy. This binding energy shift may reach a nearlysteady-state value of between 2 and 5 eV for spectrometersequipped with nonmonochromatic X-ray sources. The surfacepotential charge and the resulting binding energy shift is,general
11、ly, larger for spectrometers equipped with monochro-matic X-ray sources because of the, generally, lower flux oflow-energy electrons impinging on the specimen surface. Thislower flux arises because focused, monochromatic X-raybeams irradiate only a portion of the specimen and not othernearby surface
12、s (for example, the specimen holder) that aresources of low-energy electrons. The absence of an X-raywindow in many monochromatic X-ray sources (or a greaterdistance of the specimen from the X-ray window) also elimi-nates another source of low-energy electrons.4.2 The amount of induced surface charg
13、e, its distributionacross the specimen surface, and its dependence on experimen-tal conditions are determined by several factors includingspecimen composition, homogeneity, magnitude of surfaceconductivity, total photoionization cross-section, surface to-pography, spatial distribution of the excitin
14、g X-rays, andavailability of neutralizing electrons. Charge buildup is a1This guide is under the jurisdiction 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 M
15、ay 1, 2009. Published June 2009. Originallyapproved in 1993. Last previous edition approved in 2003 as E 1523 03.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the
16、 standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.well-studied (1, 2)3, three dimensional phenomenon that occursalong the sample surface and into the material. The presence ofparticle
17、s 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 is impacted by X-ray radiation.4.3 S
18、everal techniques have been developed for the purposeof controlling 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 7.2.4.4 Seve
19、ral techniques have been developed for the purposeof correcting the binding energy shifts that result from surfacecharging. These corrections are performed after the data hasbeen accumulated and are discussed in 7.3.4.5 The use of the various charge control or charge refer-encing techniques describe
20、d in this guide may depend on theavailable instrument as well as the specimen being analyzed.4.6 Specimens with non-insulating surfaces are those with ahigh enough electron conductivity to dynamically compensatethe electron loss caused by the photoelectric effect; they neitherrequire control of the
21、surface charge buildup nor chargereference corrections. It is important to distinguish the shiftsdue to the temporary charge build caused by the photoelectriceffect from intrinsic charging effects. Intrinsic effects, such asthe accumulation of charge at an interface during film growth,influence the
22、nature of spectra obtained and the BEs measured,but are part of the sample (3). It is also possible that theimpinging of the X-ray changes the charge distribution bymeans of volatilization of certain chemical species or thecreation or charge centers. Such specimens may never achievesteady-state pote
23、ntials. Although artifact to the process ofmeasurement, those changes become part of the sample and arenot necessarily to be corrected or compensated by the methodsdescribed in 7.2 and 7.3.4.7 Major advances in the ability to control sample chargingand to stabilize surface potential were made in the
24、 late 1990sincluding the ability to achieve charge control for small areaanalysis (4). These approaches usually involve the use ofelectron flood guns and some additional methods (ions ormagnetic fields) to control localized surface charge (5, 6). As aresult of these advances it is now possible to co
25、llect highquality reproducible data on many systems. However, theseadvances do not remove all of the challenges for optimizing theconditions for analysis for complex samples or interpreting thedata.4.8 Although changes in surface potential during XPSanalysis and other charging effects are usually vi
26、ewed asproblems to be avoided, such phenomena can be used to extractimportant information about specimens (7-9).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 a
27、nalytical tool.Surface charging acts to shift spectral peaks independent oftheir chemical 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 desi
28、gned toassist in the interpretation of the XPS peak positions, determinesurface chemistry, and allow comparison of spectra of conduct-ing and non-conducting systems of the same element. It isassumed that the spectrometer is generally working properlyfor non-insulating specimens (see Practice E 902).
29、5.2 Although highly reliable methods have now been devel-oped to stabilize surface potentials during XPS analysis ofmost materials (5, 6), no single method has been developed todeal with surface charging in all circumstances (10, 11). Forinsulators, an appropriate choice of any control or referencin
30、gsystem will depend on the nature of the specimen, theinstruments, and the information needed. The appropriate useof charge control and referencing techniques will result in moreconsistent, reproducible data. Researchers are strongly urged toreport both the control and referencing techniques that ha
31、vebeen used, the specific peaks and binding energies used asstandards (if any), and the criteria applied in determiningoptimum results so that the appropriate comparisons may bemade.6. Apparatus6.1 One or more of the charge compensation techniquesmentioned in this guide may be employed in virtually
32、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 under a fine metal mesh (12), can enhanceelectrical contact of t
33、he 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 described here involve charge control (theeffort to control the
34、 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 parameter to consider.Aconstant and relativelyuniformly surfa
35、ce potential provides the conditions needed toobtain reproducible data and optimum peak shape. Correctingthe peak position is accomplished separately using an appro-priate charge referencing technique. In some circumstances,theAuger parameter can provide chemical information withoutthe need to resor
36、t to surface potential corrections.7.2 Avariety of different methods are used to either enhanceconductivity to minimize charge buildup during XPS analysisor to control the surface potential by other methods. Thesemethods employed to control the surface potential in insulating3The boldface numbers in
37、 parentheses refer to a list of references at the end ofthis standard.E1523092specimens are listed in Table 1 in approximate order offrequent use (more frequently used first) and summarizedbelow:7.2.1 Methods for Controlling the Sample Surface Potential:7.2.1.1 Electron Flood Gun (13-16)Use low-ener
38、gy elec-tron flood guns to stabilize the surface potential of insulatorsexamined by XPS (14), in particular when monochromatizedX-rays are employed. Optimum operating conditions, forexample, filament position, electron energy, and electron cur-rent, depend upon the orientation of the electron flood
39、gun withrespect to the specimen and upon the particular design of theelectron flood gun and must, in general, be determined by theuser. Use low-electron energies (usually 10 eV or less) tomaximize the neutralization effect and reduce the number ofelectron bombardment-induced reactions. A metal scree
40、nplaced on or above the specimen can help (17, 18).7.2.1.2 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(19). Such effects appear to be particularly important forfocused X-ray be
41、am systems, where the X-rays strike only arelatively small portion of the specimen. In these circum-stances, the use of a low-energy positive-ion source, in additionto an electron source, helps stabilize (and make more uniform)the surface potential of the specimen. Several commercial XPSnow effectiv
42、ely combine electrons and ions to achieve uniformsurface potentials for many types of insulators.7.2.1.3 Ultraviolet Flood Lamp (20)Ultraviolet radiationcan also produce low-energy electrons (for example, from thespecimen holder) that may be useful in neutralizing specimencharging and stabilizing th
43、e surface potential.7.2.1.4 BiasingApplying a low-voltage bias (-10 to+10 V) to the specimen and observing the changes in thebinding energies of various peaks can be used to learn aboutthe electrical contact of a specimen (or parts of a specimen)with the specimen holder. Peaks in XPS spectrum that s
44、hiftwhen the bias is applied are from conducting regions of thespecimen. Other peaks from insulating regions may not shiftnearly as much or at all and can be interpreted accordingly.This method can sometimes verify that the peaks being usedfor charge referencing (for example, gold 4f or carbon 1s) a
45、rebehaving in the same manner as the peaks of interest from thespecimen (12, 20, 21). For non-uniform or composite (non-conducting or partially conducting) specimens, a variety ofcharge shifts may be observed upon biasing. This may provideuseful information about the sample and indicate a need tomor
46、e carefully connect the specimen to ground or to isolate thesample from ground. Sometimes all data for some specimensare collected with a bias applied (see also 7.4).7.2.1.5 Isolation from GroundFor some materials, ormixtures of materials with different electrical conductivity,differential charging
47、can occur. This phenomenon can be usedto obtain information about the sample (4, 22) and cansometimes be minimized (and a more uniform sample potentialcan be achieved) by isolating the specimen from ground. Insome circumstances an electron flood gun is more effective incontrolling the surface potent
48、ial when the sample is isolatedfrom ground.7.2.2 Methods for Minimizing Charge AccumulationThese methods attempt to stabilize the surface potential byminimizing the charge buildup or potential change by loweringsample resistance to ground or the spectrometer mount.7.2.2.1 Grounding and Enhanced Cond
49、uction PathSurrounding of insulating materials with a conducting materialhas been a common approach to 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).7.2.2.2 Specimen HeatingFor a limited number of speci-mens, heating can increase the electrical conductivity of thespecimen, thus decreasing charging (2).7.3 Binding Energy Reference MethodsAvariety of meth-ods (as listed in Table 2 and described below) have been usedto determine the amount of binding
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