Auger spectrometer, tutorial and review

APPLIED SPECTROSCOPY REVIEWS, 34(3), 139–158 (1999)

Auger Spectrometers:A Tutorial Review

DAVID H. NARUMAND and KENTON D. CHILDSPhysical Electronics, Inc.6509 Flying Cloud DriveEden Prairie, MN 55344

I. INTRODUCTION ........................................................................... 140

II. THE AUGER PROCESS................................................................ 141A. Auger Spectra .......................................................................... 141B. AES Surface Sensitivity .......................................................... 145C. Auger Quantification ............................................................... 145D. Artifacts ................................................................................... 146

III. ELECTRON SPECTROMETERS.................................................. 147A. Retarding Field Analyzer (RFA) ............................................ 148B. Cylindrical Mirror Analyzer (CMA)....................................... 151C. Concentric Hemispherical Analyzer (CHA) ........................... 151D. Detector Properties and Multi-channel Detection .................. 153

IV. THE SCANNING AUGER MICROSCOPE ................................. 154A. Instrumental Configuration ..................................................... 154B. Instrument Modes.................................................................... 154

BIBLIOGRAPHY............................................................................ 158

139

Copyright 1999 by Marcel Dekker, Inc. www.dekker.com

140 NARUMAND AND CHILDS

I. INTRODUCTION

Auger Electron Spectroscopy (AES) is a fast, “non-destructive” analyti-cal technique used to determine the elemental composition of the top fewatomic layers of a surface or exposed interface in a solid material. AES candetect all elements except hydrogen and helium, and it can provide semi-quantitative information with an average detectability limit of 0.1 to 1 atomicpercent. Newer AES instruments with field emission electron sources providerapid characterization of sample features down to 10 nm in size. The tech-nique is based on the energy analysis and detection of characteristic electronsemitted during relaxation of a core level ionization state in a parent atom.The emission process is commonly referred to as Auger emission, and theresultant characteristic electron as an Auger electron, in honor of the Frenchscientist, Pierre Auger, who first discovered this phenomenon in 1925.

The techniques commonly referred to as Auger electron spectroscopy(AES) and scanning Auger microscopy (SAM) utilize a focused beam ofelectrons directed on the sample surface with sufficient energy to cause thenecessary core level ionization. The ability to focus such a beam of electronsinto an extremely small spot (<10 nm) and the very limited inelastic meanfree path of the emitted Auger electrons (typically 0.4 to 5 nm) are responsi-ble for the unparalleled spatial resolution offered by AES and SAM. Spatialdistributions of elemental or chemical components are mapped across thesurface by electronically scanning the incident electron beam over the sam-ple, and in depth by combining Auger analysis in an alternating fashion withion sputter removal of surface material.

AES and SAM applications are generally limited to inorganic sampleswhere spatially resolved analysis is of primary interest. While electricallyconductive samples are most easily analyzed, proper experimental techniquewill frequently allow for acceptable results from insulating materials. Augeranalysis can provide chemical state identification in inorganic compounds,but the molecular or chemical bond information, which is generally of inter-est in the analysis of organic compounds, is not reflected in the Auger elec-tron emission process. The most common industrial applications fall withinthe metallurgical, advanced materials, electronics, semiconductor and micro-engineering segments.

The analytic technique requires relatively sophisticated (and expensive)instrumentation to realize useful results: Transport of electrons and ions re-quires a high vacuum instrumental environment. Preventing surface contami-nation during the analysis requires an ultra high vacuum (UHV) requirement–system base pressures of (10−10–10−9) Torr are desired. An electron opticalsystem is required to generate and focus the incident electron beam; for mostapplications, it must also be capable of scanning the beam over the sample

AUGER SPECTROMETERS 141

surface. An electron spectrometer is needed to measure the energy distribu-tion of electrons emitted from the sample surface. An ion source is requiredfor in situ sputter cleaning of sample surfaces prior to analysis, and to providea means of surface material removal for composition depth profiling. Finally,modern implementations utilize full computer control of the hardware systemand data acquisition process. Computers also enable the sophisticated numer-ical algorithms used for data display, manipulation and interpretation. Theseindividual components are now commercially available through a variety ofvendors, as are fully integrated systems offering very high performance.

Numerous review articles and comprehensive reference books havebeen written, reflecting the relative maturity of this analytical technique andassociated instrumentation. Several of these are listed in the bibliography andshould be consulted for a more detailed treatment than is possible here.

II. THE AUGER PROCESS

Auger electrons are emitted in the relaxation of an excited ion with aninner shell vacancy. In this process an electron from a higher lying energylevel fills the inner shell vacancy with the simultaneous emission of an Augerelectron. This simultaneous two electron Coulombic rearrangement results ina final state with two vacancies. Auger electron emission is one of two relax-ation mechanisms possible in an excited ion. The other is x-ray fluorescence,in which a photon is emitted. The two relaxation processes for an excited ionare shown in the energy level diagrams of Figure 1. In these diagrams theinitial vacancy occurs in the K shell.

The sum of the Auger yield and the fluorescence yield is unity, sincean excited ion must relax by either Auger electron emission or x-ray emis-sion. Auger electron mission is the more probable decay mechanism for lowenergy transitions, i.e., for low atomic number elements with an initial va-cancy in the K shell and for all elements with initial vacancies in the L or Mshells. By choosing an appropriate Auger transition, all elements (except Hand He) can be detected with high sensitivity.

Auger transitions are typically labeled by the energy levels of the elec-trons involved, using x-ray spectroscopy nomenclature. The first label corre-sponds to the energy level of the initial core hole. The second and third labelsrefer to the initial energy levels of the two electrons involved in the Augertransition. Thus the Auger transition shown in Figure 1(a) is a KLIILIII transi-tion, or simply a KLL transition.

A. Auger Spectra

Figure 2 illustrates the interaction between an incident electron beamand a solid sample. This diagram depicts the emission events that occur,


FIG. 1. Following K shell ionization by interaction with an energetic particle,this schematic represents relaxation via (a) Auger electron emission, and (b) and X-ray fluorescence.

FIG. 2. The interaction between an incident electron beam and a solid sam-ple, showing the analysis volumes for Auger electrons, back-scattered electrons, andx-ray fluorescence.


including Auger electrons, low energy secondary electrons, backscatteredelectrons that have undergone energy loss processes, elastically backscatteredelectrons, and x-ray photons. The energy distribution of emitted electrons,N(E), plotted against kinetic energy, E, constitutes the fundamental AESmeasurement–an example of which is shown in Figure 3(a). The distinctivefeatures of this measured spectrum are:

• Strong intensity at very low energies (<50 eV) owing to near sur-face secondary electron emission.

• A very intense peak at the energy of the incident electron beam dueto elastically backscattered primary electrons (3 keV in the caseillustrated here).

• A broad background signal of moderate intensity, spanning inter-mediate energies, and resulting from multiple, inelastic scatteringevents involving primary or high energy secondary electrons.

• Narrow, relatively low intensity Auger electron peaks riding on topof the broad background.

Because the Auger peaks are of relatively low intensity, and for histori-cal reasons, it is common to differentiate this N(E) spectrum and display(dN(E)/dE) vs. E. This has the result of strongly accentuating the sharplydefined Auger peaks from the background signal.

In Auger electron spectroscopy, elemental identification is determinedby the energy positions of the Auger peaks. The kinetic energy of an Augerelectron is equal to the energy difference of the singly ionized initial stateand the doubly ionized final state. This energy can be estimated by the differ-ence in binding energy of the three energy levels involved in the Augertransition. For an arbitrary ABC transition of an atom of atomic number z,the measured Auger electron energy, referenced to the Fermi level, is giv-en by:

EABC (z) = EA (z)–EB (z)–E*C (z)–ϕs (1)

The spectrometer work function is A, and the binding energies are alsoreferenced to the Fermi level. E* is the binding energy of a level in thepresence of a core hole and is greater than the binding energy of the samelevel in a neutral atom. There are various means of calculating the Augerelectron energies to take into account the electron binding energies in theinitial ion, interactions due to the two-hole final state, and relaxation effects.

Since every element has a unique set of energy levels, emitted Augerelectrons escape from the surface with a kinetic energy characteristic of theparent atom, and each element has a unique set of Auger peaks. The kineticenergies of the most useful Auger peaks are typically between 40 eV and


FIG. 3. (a) N(E) spectrum showing the complete secondary electron energydistribution, including the low energy secondary peak, the elastically back-scatteredpeak, the secondary electron background, and Auger peaks. (b) Differentiated N(E)spectrum, (dN(E)/dE) vs. E.


2500 eV. Elemental identification is typically accomplished by comparisonof the acquired spectrum with a compilation of reference spectra from well-characterized samples.

Variations in chemistry may change binding energies, relaxation ener-gies, and Auger transition probabilities. These effects can modify the energypositions and relative intensities of Auger peaks and their loss structure. Vari-ations in chemistry may also affect the valence band density of states, whichcan affect the measured Auger peak intensities and shapes for those transi-tions involving valence electrons. These dependencies on chemical environ-ment make chemical fingerprinting possible by measuring the precise energyand line shape of the Auger peaks.

B. AES Surface Sensitivity

Electrons emitted in the solid must be transported to the surface inorder to escape the solid and be analyzed. Emitted Auger electrons may expe-rience both elastic and inelastic scattering events by interaction with the elec-trons in the solid. These interactions depend on the energy of the transportedelectron and on the sample electronic structure. The surface sensitivity ofAES arises from the relatively short inelastic mean free path for Auger elec-trons (generally 5 nm or less), which typically have kinetic energies between40 and 2500 eV. Only those Auger electrons that originate within a fewmonolayers of the surface can escape without energy loss. Electrons thatundergo inelastic loss processes before emerging from the sample surfaceform loss structure on the low kinetic energy side of the Auger peaks. Elec-trons that have experienced random and multiple loss processes contribute tothe continuous background.

C. Auger Quantification

In Auger electron spectroscopy, quantification of the observed elementsis determined from the relative intensities of the Auger peaks. The measuredintensity of an arbitrary Auger peak is a complicated function of a largenumber of sample and instrumental factors. These include:

• the number of atoms of that element per unit volume,• the primary electron current,• the Auger transition probability for that element,• the ionization cross section of that element by incident electrons,• the ionization cross section of that element by scattered electrons,• the mean free path of the emitted Auger electron,• the angle between the collected Auger electron and the surface

normal,


• the analyzer acceptance solid angle,• the analyzer transmission function,• the electron detector efficiency, and• the surface roughness.

As a result, elemental quantification from first principles is nearly im-possible using AES. Additionally, accurate quantification using empiricalmeans is difficult in principle, and requires the utmost attention to experimen-tal detail. Reproducible results are achieved with considerably greater ease.

Given the preceding notes of caution, a commonly used approach toquantification involves defining sensitivity factors, Sa, such that, for a mea-sured Auger intensity, Ia, Ia /Sa is a value proportional to the concentration ofelement ‘a’. A set of relative sensitivity factors, which are normalized to areference material, can be tabulated for each beam voltage. For two pureelemental samples, I1 /S1 ≡ I2 /S2, each sensitivity factor being related to thereference sample sensitivity factor by S1 = (I1 /Iref)Sref. A general expressionfor estimating the atomic concentration of any constituent in a sample, Xa,can be written as:

X a = I a /S a

ΣI i /S i

(2)

where the summation is over all observed elements.This expression may be used for all homogeneous samples if the ratio

of the sensitivity factors is matrix independent. The errors inherent in thisapproach involve possible differences between the reference sample and theunknown sample for electron mean free path, backscatter factor, atomic den-sity, surface roughness, chemical environment and Auger transition probabil-ities. This approach minimizes the effect of surface roughness, since it isexpected to decrease the Auger intensities of all constituents by nearly thesame factor. In addition, the determination of sensitivity factors from stan-dard samples close in composition and chemistry to the unknown will mini-mize the contributions of the other errors.

D. Artifacts

There are various sample constraints and potential artifacts that maylimit the applicability of AES for some materials or distort the interpretationof the Auger data:

• Samples must be UHV compatible, and of appropriate size to fitinto an Auger system.

• Insulators may charge-up under the electron beam, causing an en-


ergy shift in the Auger spectrum. In extreme cases, insulators maycharge to the point that nomeaningful spectra can be collected.Wrapping or coating the sample with conductive material may min-imize sample charging. Varying the primary beam energy and tilt-ing the sample to a grazing angle of incidence between the electronbeam and the sample surface may also aid in reduction of chargingaffects.

• The surface composition of some samples may be changed by elec-tron stimulated desorption, adsorption, diffusion, dissociation, oxi-dation, or reduction. These effects may be observed by a time de-pendence of the surface composition, and may be minimized byreducing the primary beam current density and total electron dose.Some materials, such as organics, can be severely decomposed bythe electron beam.

• Differences in sputter yield for elements on the surface, and otherion beam induced damage may change the surface composition dur-ing ion sputtering.

• For some analytical geometries, a rough surface topography mayresult in analytical shadowing of portions of the surface from theelectron beam, the ion beam or the spectrometer.

III. ELECTRON SPECTROMETERS

Measurement of the energy distribution of electrons emitted from thetest sample surface is at the heart of AES and SAM and requires, of course,an electron spectrometer. A spectrometer consists of an electron energy ana-lyzer with an electron detector at its output. All practical electron energyanalyzer concepts are based on retardation or deflection of electrons as theypass through an electric or magnetic field, the degree of retardation/deflectionbeing in some manner proportional to their energy or velocity. A variety ofdevices used for Auger analysis have been reviewed in the literature, and arelisted in the bibliography. This review will be limited to three fundamentaltypes: The retarding field analyzer (RFA) is included because of its relativesimplicity and its historical significance in the development of AES. Thecylindrical mirror analyzer (CMA) and the concentric hemispherical analyzer(CHA) are included because they are now the overwhelming analyzers ofchoice for Auger electron spectroscopy.

Spectrometers for potential use in AES should be compared on thebasis of five general factors: transmission (T), energy resolution (R), detectorperformance, background suppression and geometric considerations. Thetransmission, which represents a measure of the “collection efficiency” of an


analyzer, is defined as the acceptance solid angle of the analyzer, Ω 0 (steradi-ans), normalized to 4π, and modified by a multiplicative factor, (K ≤ 1),which accounts for any electrons which are intercepted somewhere in theirflight through the analyzer:

T ≡ K(Ω 0 /4π) (3)

The energy resolution, generally quoted as a percentage and denotedby (∆E/E), is a measure of the minimum energy difference between twoelectrons which can be detected by a given analyzer. Generally, ∆E is definedas the full width at half maximum (FWHM) of the analyzer response to amonochromatic source of electrons of energy E0.

Geometric considerations take several forms. First, the physical sizeand form of various analyzer concepts differ significantly, and can determinethe possibility of integrating an analyzer into a given instrument configura-tion. Second, topographic structure in the sample surface will affect the per-formance of different analyzer geometries in different ways. In extremecases, physical “line-of-sight” shadowing of the analyzer entrance can makeanalysis of a given surface impossible. More subtly, local (even microscopic)variations in surface topography can give rise to sizable variations in boththe emitted Auger electron intensity and, depending on analyzer geometry,the detected intensity.

Issues regarding detector performance are highlighted below, and it issimply noted that proper attention to mechanical design and material selec-tion is critical to minimize scattering of electrons off internal analyzer sur-faces giving rise to spurious background signal.

A. Retarding Field Analyzer (RFA)

In its basic form, the RFA consists of three concentric, spherical elec-trodes, the two inner most being in the form of grids, and the sample beingpositioned at the common center point—see Figure 4(a). The inner and outerelectrodes are held at ground potential, while a retarding, negative bias isapplied to the central electrode. This arrangement constitutes a high passenergy filter for electrons emitted from the sample, the current collected atthe outer electrode being given by

I(Vr) = ∫EP

VR

N(E)dE (4)

It follows that


FIG. 4. Schematic representations of: (a) the retarding field analyzer (RFA),(b) the cylindrical mirror analyzer (CMA) integrated with a coaxial electron gun, and(c) the concentric hemispherical analyzer (CHA) and associated input optics.

(continued)


FIG. 4. Continued.

N(E) = −S dI(VR)dVR

D (5)

where N(E) is the desired electron energy distribution, VR is the retardinggrid potential and Ep is the energy of the electron beam incident on thesample. While the transmission of this analyzer can be quite high, it suffersfrom poor energy resolution and a poor signal-to-noise ratio relative to theCMA and CHA. The poor signal-to-noise results from the fact that the Augersignal is superimposed upon a large, integrated background signal of secon-dary and backscattered primary beam electrons.

The retarding field analyzer is of considerable historical, if not techni-cal significance, given the critical role it played in launching AES as a practi-cal means of surface analysis. In the 1960’s, surface structure research utiliz-ing low energy electron diffraction (LEED) was quite popular and the thencommon LEED instrument was readily modified to function as an RFA. Thesecond derivative mode of a common LEED system, used in conjunctionwith a swept retard potential on the central electrode, provided a means ofmeasuring (dN(E)/dE). In a plot of (dN(E)/dE) vs. E, the Auger peaks are


strongly accentuated relative to the broad flat background signal and thispresentation format is still widely utilized today.

The widespread availability of such LEED equipment in laboratoriesaround the world allowed a rapid flurry of research to improve AES experi-mental technique and to apply AES in a variety of surface research fields.

B. Cylindrical Mirror Analyzer (CMA)

The shortcomings of the RFA led to the use of the CMA for the analy-sis of Auger electrons. The basic analyzer consists of two concentric cylin-ders—see Figure 4(b). The inner cylinder (radius R1) is at ground potential,while a negative potential applied to the outer cylinder (radius R2) determinesthe pass energy. With the sample point located on the common cylinder axis,the mean acceptance angle of the CMA (α 0) is defined by the position of anentrance slit in the inner cylinder, and the acceptance half angle (∆α) isdefined by its width (wi). An electron detector is positioned behind an aper-ture placed in the focal plane of the CMA. This aperture, in conjunction withthe outer cylinder potential, defines the nominal pass energy. The energyresolution is determined primarily by the chosen value of ∆α and the size ofthe exit aperture (w0).

When the geometry is chosen such that α 0 = 42.31°, second order fo-cusing occurs and electrons originating on axis with an energy of

E 0 =(1.31)qV0

ln(R 2 /R 1)(6)

are imaged back to a point on axis at a distance L = 6.21R1. This configura-tion maximizes the achievable transmission at a given energy resolution. Incommon practice, α 0 = 42.31° and ∆α = ±6° will allow a transmission ofT > 7% and energy resolution of ∆E/E > (0.3–0.6)%.

The CMA has the strong geometric advantage of collecting electronsuniformly around the 360° azimuth, with the exception of “shadows” castalong small azimuthal segments subtended by the necessary mechanical sup-port structures. This characteristic is optimized by integrating the primaryelectron gun with the CMA in a coaxial arrangement, which simplifies align-ment of the analysis point to the axis of the CMA and minimizes topographicshadowing effects for any sample orientation.

C. Concentric Hemispherical Analyzer (CHA)

This analyzer consists of two concentric hemispheres, and is also fre-quently referred to as a spherical capacitor analyzer (SCA) or simply a hemi-


spherical analyzer (HSA)—see Figure 4(c). The inner sphere has radius R 1

and applied potential V1 > 0, while the outer sphere is of radius R 2 with ap-plied potential −V2 < 0; it is useful to define the mean radius as R 0 = (R 1 +R 2)/2. Electrons of energy E0 entering the CHA on its central radius arefocused back to an energy dispersed image at the opposing point, 180°around the CHA, provided that

(qV1) = E 0S2R 0

R 1

− 1D and (qV2) = E 0S2R 0

R 2

− 1D (7)

An entrance aperture at the input to the CHA defines the imaged area,while the potential applied to the inner and outer spheres and an exit aperturedefine the pass energy. The imaging aberrations of the CHA differ with re-spect to in-plane and out-of-plane entrance angles, and so these entrance andexit apertures generally are rectangular slits. Typically, a CHA designed forAuger analysis will have energy resolution of ∆E/E0 = (1–2)%.

The CHA is always used in conjunction with relatively sophisticated,so-called input optics which image electrons emitted from the sample ontothe entrance slit of the CHA. The input optics serve several functions: First,they provide a workable physical arrangement by moving the CHA awayfrom the test sample, thereby allowing necessary placement of other compo-nents of the instrument such as the electron and ion guns. Second, the im-aging properties of these optics are optimized with respect to the aberrationsof the CHA to provide maximum net transmission and energy resolution forthe overall analyzer system. Finally, these optics are generally designed toretard the analyzed electrons to some percentage of their initial energy priorto their entrance into the CHA. The net energy resolution of the system thenbecomes

(∆E /E) = RR(∆E /E 0) (8)

RRT ≡ (E 0 /E) ≤ 1 (9)

where RR is commonly referred to as the retard ratio and E 0 is the analyzerpass energy. As the retard ratio decreases, the net energy resolution obviouslyimproves, but this is usually at the expense of a decrease in overall transmis-sion. For Auger applications, retard ratios of RR ≡ (0.1–0.5) are typicallyused with resulting overall energy resolution of ∆E/E ≡ (0.05–0.5)%. A welldesigned CHA system should be capable of at least ∆α > 12° or T > 1% atan energy resolution of ∆E/E = 0.5%.

The flexibility offered by the input optics is a primary strength of theCHA. By varying the retard ratio the analyst can optimize instrumental setup


for the experiment at hand. Improved energy resolution can be helpful inseparating overlapping Auger peaks, or in detailed examination of subtlepeak shifts or shape changes as a result of chemical bonding. In other caseswhere signal-to-noise is of primary interest, maximizing the transmission atthe expense of decreased energy resolution is desirable. The narrow accep-tance cone defined by the input optics, however, makes the CHA signifi-cantly more prone to the problems associated with sample topography.

D. Detector Properties and Multi-channel Detection

As a final note to this section, the desirable properties of the electrondetector at the output of the spectrometer should be considered: Quantumefficiencies very near unity are generally achievable, taking full advantageof the transmission of the energy analyzer. The so-called ”dead time” of thedetector, i.e. the minimum time between resolvable arrival events, should beas short as possible to allow maximum signal rates. Finally, the detector”dark count” rate (false events) should be minimal so as not to contribute tothe spectral background level.

Recent developments in detector designs have significantly enhancedthe effective transmission of both the CMA and the CHA. The original imple-mentations of these two analyzer concepts utilized the aforementioned energyselection apertures placed in the focal surface of the analyzer, with a singlemultiplier/detector system collecting transmitted electrons. In these configu-rations a channeltron type electron multiplier is most commonly used. Elec-trons of similar, but slightly differing energies are brought to focus at slightlydiffering locations on the focal surface—it is this chromatic aberration orlinear energy dispersion, which allows these devices to function as an energyanalyzer. This offers the possibility of detecting multiple channels of energyresolved spectral data simultaneously, thereby increasing the data acquisitionrate in direct proportion to the number of channels.

Parallel (simultaneous) detection of electrons of several different ener-gies has been implemented by various schemes, all of which equate function-ally to having multiple energy selection apertures spread across the focalsurface, each with a separate channel of electron multiplier/detection elec-tronics. In the case of the CMA, this has been achieved by combining addi-tional optics near the analyzer focal surface with a microchannel plate elec-tron multiplier and discrete anode detector design. In the case of the CHA,various schemes have been applied, the most common being: 1) multiple exitapertures and channeltron electron multipliers; and 2) a microchannel plateelectron multiplier with a discrete anode detector. As many as 16 channelsof parallel data collection have been demonstrated and more are likely in thefuture.


IV. THE SCANNING AUGER MICROSCOPE

A modern, full-featured scanning Auger microscope has evolved intoa very complex instrument, which is commercially available from severalsuppliers. Relatively simple Auger spectrometers might still by assembled byindividual researchers for specific purposes; but even in this case, it is mostlikely that the individual components (electron gun. ion gun, spectrometer,etc.) would be purchased. The purpose of this section is to provide a broadsense of the configuration and capabilities of a complete instrument at thetime of publication (circa 1999), recognizing that instruments continue toundergo change and improvement.

A. Instrumental Configuration

The schematic representation of Figure 5 conveys the principal compo-nents of a complete scanning Auger microscope. These include:

• A complete electron optical column for generating, focusing andscanning the incident electron beam. Typically, beam energy willbe variable over a range of (0–25) keV, beam current over (l–100)nA, and beam diameter varying over (10–100) nanometers.

• A secondary electron detector (SED) for doing simple secondaryelectron imaging of the sample, entirely analogous to what is donein a scanning electron microscope (SEM).

• A CMA or CHA electron spectrometer with multi-channel detection(Figure 5 arbitrarily shows the CHA).

• A scanning argon ion gun for in situ sample cleaning and sputterdepth profiling.

• A UHV analysis chamber (base pressure <5 × 10−10 Torr) with aload locked sample introduction system.

• A sample positioning system (sample stage), typically with fiveaxes of motorized motion . . . X, Y, Z, tilt and rotation.

• Necessary electronics for digital control of all instrument functionsand data acquisition.

• An instrument computer with sophisticated instrument control, dataacquisition, data analysis, and data presentation software.

• A display for live and stored images, spectra and profiles (this maybe incorporated into the display monitor of the instrument com-puter).

B. Instrument Modes

Under control of the system computer, the following operating/acquisi-tion modes are a part of routine Auger analysis:


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1. Scanning Electron Microscope (SEM):

For the purposes of finding, positioning and documenting the desiredfeature for Auger analysis, real time imaging of the sample is achieved bycollecting a secondary electron signal synchronously with the raster of theincident electron beam. Conventionally, this signal is used to modulate a grayscale map on an X–Y display, providing a very distinct “image” of sampletopographic and compositional structure. Figures 6(a,b) show two SEM mi-crographs of a cross sectioned metal matrix composite sample. The compos-ite was composed of a Ti-6%Al-4%V matrix with TiB2/C coated monofila-ment fibers as the reinforcing agent.

FIG. 6. Images of a cross section through a metal matrix composition—thecircular features are SiC monofilaments (with a W core) used as a reinforcing agentin the Ti matrix: (a) Low magnification SEM image showing the overall structure,(b) High magnification SEM image of the fiber/matrix of the reaction zone betweenthe Ti matrix and the TiB2/C-coated SiC fiber, (c) Ti Auger intensity map, and (d)B Auger intensity map.


2. N(E) Survey:

With the incident electron beam either stationary (point mode) or scan-ning across a defined region (area mode), the energy distribution of secon-dary electrons is measured over a broad range by sweeping the pass energyof the electron spectrometer. Generally, this is the first Auger acquisitionperformed on a sample to evaluate its cleanliness and to determine whatelements are present. Figure 3 is an Auger spectrum of relatively clean Cu.

3. N(E) Multiplex:

When one or more Auger features of interest have been identified, theinstrument can be set up to perform multiple spectral acquisitions over rela-tively narrow energy ranges to maximize the signal-to-noise ratio of the mea-surement. Again, these acquisitions can be done in point or area mode.

4. Auger Maps and Linescans:

These modes are used to measure and display the spatial distributionof a particular element or chemical state. In this case, the incident electron

FIG. 7. Sputter depth profile through the B-rich, Ti boride and C coating ona free standing SiC fiber. This is the same fiber type shown in the metal matrixcomposite in Figure 6.


beam is stepped point-by-point across either a one-dimensional (linescan) ora two-dimensional (map) position array on the sample surface. The intensityof a particular Auger spectral feature (e.g. oxygen peak height) is measuredpixel-by-pixel providing a one or two-dimensional “image” of the spectralfeature. Figure 6(b) shows a high magnification SEM micrograph of thecross-sectioned metal matrix composite, and the corresponding Titanium,Figure 6(c), and Boron, Figure 6(d), elemental Auger maps.

5. Sputter Depth Profiling:

Any, or all of the previous acquisition modes can be repeated in analternating manner with periods of ion sputter etching of the sample surface,resulting in a profile of the feature of interest into the near-surface region ofthe sample. In some cases, the ion sputtering process can be such that aslittle as 5A

˚of material is removed each cycle, providing extremely high

depth resolution. In the other extreme, profiling depths of more than severalmicrons may become unacceptably time consuming. Figure 7 shows a sputterdepth profile through a free standing, coated SiC fiber of the same type usedin the metal matrix composite shown in Figure 6.

BIBLIOGRAPHY

G. Gergely, “Commemoration of the 25th anniversary of Auger electron spectro-scopy,” Vacuum 45, 311 (1994).

D. Briggs and M.P. Seah, Practical Surface Analysis, Wiley, New York (1983), 2nd

Ed. Vol. 1 (1990).I.F. Ferguson, Auger Microprobe Analysis, Adam Hilger, Bristol (1989).G.C. Smith, Surface Analysis by Electron Spectroscopy, Plenum Press, New York

(1994).K.D. Childs, B.A. Carlson, L.A. LaVanier, J.F. Moulder, D.F. Paul, W.F. Stickle,

D.G. Watson, Handbook of Auger electron spectroscopy, C.L. Hedberg (ed.),Physical Electronics Inc. (1995).

H. Ibach (ed.), Electron Spectroscopy for Surface Analysis, Springer-Verlag, Berlin(1977).

R.C.G. Leckey, “Recent developments in electron energy analysers,” J. ElectronSpectrosc. Relat. Phenom., 43, 183 (1987).

D. Roy and D. Tremblay, “Design of electron spectrometers,” Rep. Prog. Phys. 53,1621 (1990).

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Auger spectrometer, tutorial and review