Analytical chemistry of surfaces. Part II. Electron spectroscopy

David M. Hercules, and Shirley H. Hercules. J. Chem. Educ. , 1984, 61 (6), p 483 ... and Joseph A. Gardella, Jr. Energy & Fuels 1996 10 (1), 141-148...
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Analytical Chemistry of Surfaces Part II. Electron Spectroscopy David M. Hercules and Shirley H. Hercules University of Pittsburgh, Pittsburgh, PA 15260 The first part of this article ( I ) dealt with some general aspects of surface analysis. Now we will look a t twoof the four principal surface techniques: X-ray photoelectron spectroscopy (ESCA) and Auger electron spectroscopy (AES). We will consider some fundamental asDects of each techniaue. . . importnnt features of instrumentation, and sume ~.xarnplesof how ESCA and AES have been a .~.~ l i to e danalvtical surface problems. Fundamentals The fundamental processes important for ESCAand AES are summarized in Table 1. The primary process is an ionization phenomenon brought about by either a photon or an electron. For photoionization, an electron is produced having discrete kinetic energy (process A, Table 1). This electron is the one which is measured in ESCA. The kinetic energyof the electron is essentially the difference between the energy of the incident photon and the hinding energy of the electron. Electron ionization of an atom produces both an excited ion and a second electron. However, because of electron-electron interactions, discrete electron energies are not observed. Thus, when using electron ionization, one cannot observe ESCA electrons. Two secondary procesess are important. Tahle 1C shows relaxation of an excited ion by photon emission. This results in X-ray fluorescence. A competitive process with X-ray fluorescence is Auger electron emission, shown in Tahle ID. Here an excited ion relaxes by emission of a second electron to form a doubly ionized atom. I t is important to note that photoionization can produce either an ESCA electron or an Auger electron, while electron ionization can produce only Auger electrons. The kinetic energy of the Auger electron does not depend on photon energy, whereas the ESCA photoelectron does. Figure 1 summarizes the relationships between ESCA, X-ray, and Auger processes for photoionization of a 1s electron. On the left is the ESCA process; the photon ejects a 1s electron from the atom. In the center of the diagram, an electron drops from the 2p orbital to fill the 1s hole, and a photon is emitted. This results in Ka X-ray emission. On the right-hand side, a 2s electron drops to fill the 1s hole, simultaneously expelling a 2p electron. This is called the KLL Auaer Drocess. Again. " . note that Auaer and X-rav emission are compeiitive processes. In generalrfor low energy processes

Table 1. Fundamental Physical Processes In Electron Spectroscopy Primary Processss2 A. Photoionization A+' e- (discreet energy-ESCA) A hv E (kinetic) = E (photon) E (binding) 8. EleCbon loniation A e; A+' 2e- (not discreet energy) Secondary Pmcesses 6.Photon Emission h"' (X-ray) A+* - A f D. Auger Electron Emlssion A*' A++ e- (discreet energy-Auger) E (Auger) does not depend on photon energy

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ESCA

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X- Roy ENERGETICS

ESCA: T, = hv- EB(ls) X-Roy: h v ' r EB(ls)- EB(2p) Auger: T, =EB(ls)- EB(2s)- EB1(2p) Figure 1. Diagram of ESCA. X-ray, and Auger processes (3.

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(1000 eV) the Aueer effect will dominate while for hieher energy processes (16,000 eV) X-ray emission will domrnate. Firmre 2 shows a block diamam of an electron spectrometer k for eyther ESCA or Auger m~asurements(2).~ h spectrometer consists of an ionizing source, a sample, an electron enerm analyzer, a detector, and a read-out &stem. ~raditionall;;, ESCA instruments have used X-rays as ionizing sources and Auger spectrometers, electron guns.~orESCA measurements using an X-ray source, the electron energy analyzer is usually capahle of high resolution because high r&olution is important for obtaining chemical information. When an electron gun source is used, it is usually coupled with a lower resolution analyzer and chemical information inherent in the Auger lines is lost. Generally, the signal-to-noiseratio that can he achieved with an electron gun source is higher than that which can be ohtained with an X-rav source hv about an order of maenitude. Electron spectrometers must operate under a vacuum of 10-%r~ or lower, 10-'" torr is ideal. At pressures higher than 10-6 torr, the electrons would be scattered en route from the sample to the detector. The residual magnetic field in the vicinity of the sample and analyzer must he reduced below gauss. Stray fields can cause perturbation of the electron paths and thus must he eliminated. Conventional vacuum systems readily allow pumping in the torr range, with some effort lo-" torr can he achieved. For an electrostatic analyzer, magnetic shielding materials are effective for reducing stray magnetic fields to an acceptable level.

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molecules).It is sensitive to all elements exceot H and He. The specificity of ESCA is very good, namely, there is little systematic overlap lines between elements, although . of spectral . a t times overlaps do occur (Pt 4f and Al 2p, Hg 4f and Si ESCA is a quantitative technique. I t is possible to estimate the composition of a surface without calibration to within 30% (relative) of its real value. In well-calibrated systems, the relativestandard deviation of a measurement is &allg f 5% or hetter. The main difficulties with ESCA are that 11 has ti" lateral resolution across a surface cx-).I hecause the diameter of the X-ray heam is usually several millimeters. It is not very effective to couole ESCA with ion etrhina to obtain denth profiles (2 resoiution) because the data acquisition rate is slower than for other techniaues. This is because of a ooorer signal-to-noise ratio relativeto techniques like AES. o n the other hand. bv varvine the take-off anele or usine ohotoelectrons having differinckinetic energies,-it is possigie to obtain some information about depth distributions over the first 100

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Figures 4 and 5 show how ESCA can he used to determine oxidation states on catalysts. Figure 4 shows the Mo 3d spectra of a Mo/A1203catalyst reduced for various times in hydrogen a t 500°C (4). Note the significant change which occurs from the spectrum at the top (fresh catalyst) to that which has been treated for 200 min. fiy deconvolthion of the spectral envelopes in Figure4 it is possihle to ohtain the typeofoxidation

ESCA

We will now consider how ESCA is used for the analytical chemistry of surfaces. Figure 3 shows a typical broad-scan ESCA spectrum, that of a Ni/W/AhO3 catalyst, using AlK, X-rays for ionizing radiation (3).Note the presence of nickel, tungsten, aluminum, and oxygen peaks in the spectrum. In the insert a t the upper left, under higher amplification, one sees the Ni 2p line as well as the Ni LMM Auger lines. Also a carbon is line is observed at 285 eV; this is a ubiquitous carbon line which shows up in most electron spectra. Its origin is carbonaceous material present in the atmosphere. From Figure 3 it is evident that ESCA can be applied as a simple qualitative tool to rstahlish the presence or absence of elements on a surface. Table 2 summarizes the analytical characteristics of ESCA. Generally ESCA electrons have kinetic energies of a p roxi mately 100-1500 eV with nominal escape depths of 20 single most valuable attribute of ESCA is its ability to provide information about chemical bonding (for example, oxidation states of elements or structural information about organic

8: he

Figure 2. Block diagram of electron spectrometer (2).

Table 2. Analvtlcal Characteristics of ESCA

Energy Range

Kinetic Energies: 100-1500 eV Escape Depth: 20 A Peak Location: *0.1 eV Chemical information Oxidation State Organic Sbucture Bonding Inlormation Elemental Sensitivity Elements: 2 > 2 Specificity:very gwd Sensilivily Variations: SOX QuantitativeAnalysis Absolute: *30% Relative: *5% Detection Limh: 0.1% monolayer Matrix Effects:some Other Aspecb Vacuum: 10-6-10-'0 torr Depth Profiling Capabiliy: yes, slow x-y Resolution; none Speed: slow; typical run is 30 min Sample Deshuctian: none in 95% of lhe samples 484

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I 1000

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Binding Energy l e v ) Figure 3. ESCA spectrum of a NIIWIAi203catalyst (3).

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state distribution shown in Figure 5 (5).The change in the Mo oxidation state as a function of reduction time can clearly be seen. Initially the molybdenum, is in the +6 state. Initial reduction leads to rapid loss of the +6 state and a rapid rise of the +5 state. After about 40 min all Mo +6 disappears and the curve for +5 goes through a maximum. After about 600 min, the mixture of oxidation states reaches a steady state (60% +4 and 40% +5) and remains constant. This type of information is extremely valuable in correlating catalyst performance with the nature of surface species. ESCA also can he used to follow differential rates of reactions of two species on a surface. For example, ESCA can detect a difference in rates of sulfiding of Ni and W species on the surface of a NiIWlA1203 catalyst when reacted with an H2/H2S mixture (6). The peak corresponding to sulfided nickel (854 eV) is almost fully developed after 1.6 h of sulfiding, while the sulfided tungsten peak (31.5 eV) is not completelv developed even after 10 h of sulfiding. Some unsulfided tungsten remains after long sulfiding times (37.5 eV) whereas none uf the unsulfided nirkel (857.5 eV) can he seen. Figure 6 shows how ESCA can be used quantitatively (7). A plot of NiIAI ESCA intensity for a series of nickel-containing catalysts is shown versus their hulk percent. Note that the curve is linear hetween 0 and 17 weight percent Ni hut that the curve shows a break a t -17% Ni and the slope increases. From 0 to 17%. the nickel binds to the alumina lattice in a highly dispersed fashion. At percentages higher than 17% crystallites of NiO begin to form on the catalyst surface. So ESCA shows two linear regions and thus detects the change in the nature of the nickel species on the catalyst.

Although Figure 6 shows two nicely linear regions, com~licationscan arise with catalvst surfacts. A correlation has intensity ratio for carbeen found between the H ~ I C E S C A hon-supported Rh ratalvsts and the Rh crvstallitr size measured dy'x-ray diffraction (8).All data points were measured for catalvsts containing the same percentage Rh but treated at differkt temperat&es to produce diffgrent sizes of Rh partirles. As the Hh particles herome larger, the Hh/C ratio measured by FSCA derreases.'l'his is consistent with the interpretation that more of the Kh dispersed over the surface in small partirlescan beseen by the FSCA spectrometer than for larre particles. Not all photoelectruns eiected from Rh atoms can escape from the iarge particles, whereas most can escape from the small particles. ESCA has been used to study the separation of a polymer from a metal surface ( I ) . The insert of Figure 7 shows a diagram of a polymer-metal interface (2). An important question is how the polymer separates from the metal. Does a clean separation occur at the interface. or does tearing occur within the polymer and/or metal lavers as indicated iythe dashed lines in Fieure Itb)? Hecause the ~ o l v m econrained r silicon. it was easy to answer this question c9). Figure 7 shows the ESCA spectra of the polymer-aluminum system. In spectrum A, lines 3 and 5 correspond to aluminum and line 2 is from phosphorus contamination introduced by the cleaning process. Spectrum B is that of the polymer before bonding to aluminum. Lines 1and 4 correspond to the lines of silicon. Spectrum C shows the aluminum after removal of the polymer layer. Because of the presence of all 5 lines, it was concluded that tearing occurred within the polymer layer. Similarly, the polymer layer which had been removed from the aluminum gave spectrum B, i.e., i t showed no presence of aluminum. Thus, one can conclude the tearing was restricted within the polymer layer. ESCA can also be used to monitor reactions at polymer surfaces (10). In Figure 8(a) one sees only C 1s and F 1s lines from the surface of Teflon@(top spectrum). When the Teflon"

Reduction Time l m i n ) Figure 5. MO oxidation states as a function of reduction time (5). Reductionat 500°C in HZ: 15% Moos on Ai2O3; A = MO(+~);x = MO(+~):o = Mo(+4).

Nickel Content l w g l % ) Figure 4. Mo 3dESCA spectra of a Mo/Ai203 catalyst (15% MoOs/AI,Os) as a function of reduction time (4).

Figure 6. ESCA intensity ratio versus bulk nickel content for Ni/Al&a catalysts

(7).

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is etched in NaINH3, the fluorine line disappears, the carhon shifts to lower values and a significant oxygen signal appears (middle spectrum). These are characteristic of carhoxylic acids. When the etched surface is treated in chlorox, some of the non-fluorinated polymer formed by the NaINH3 treatment is dissolved, and the high energy carbon line reappears along with the fluorine line (bottom). Figure 8(h) shows that one can also determine something about the products of a surface-etching reaction (10). The top of Figure 8(h) again shows the NaINH3 etched Teflon* surface. The middle spectrum shows the Teflon control treated with Brz and CC14. Note, no oxygen or hromine peaks appear. The bottom spectrum shows the etched, treated Teflon@; notice the appearance of a hromine line. This reaction is characteristic of olefins and indicates the NaINHa etch produces olefins as well as carhoxylic acids, Auger lines appearing in an ESCA spectrum can also he used to advantage. This is illustrated by the effects on copper spectra caused by annealing a Cu-Ni alloy (11). Figure 9 shows chanees of a C u m i allov as " in the Cu LMM Aueer .. snectrum . a function annealing time. Notice that initially Cu is present as CuzO (peak at 917 eV). During the annealing process the ~

Auger spectrum changes to one characteristic of elemental copper (peak at 920 eV). This is consistent with a reaction durine annealine: CuvO - Ni 2Cu NiO. If one monitors the rzative inwnsities of Cu and Ni as a function of annealing time. the Cu intensitv decreases as a function of time while the ~i concentrationincreases. By plotting the square of the atomic concentration of Ni as a function of annealine time. it can he shown that the reaction is diffusion controlGd (4):

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Binding Energy Figure 8. ESCA study of polymer surface reactions (10);(a)heatmentwith Nal NH3,(b) treatment with Br./CC14.

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KINETIC ENERGY

Polymer Metal Figure 7. (a) ESCA spectra of an aluminum-polymer system (9). A, clean Al suface befae bonding to polymer: 8, bulk poiymer; C. A1 surtace a h sbipplng of the polymer. Lines: 1 = Si 2s. 2 = P Zp, 3 = A1 2s. 4 = Si 2p. 5 = A1 2p. (b) diagram of polymermetal interface

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Journal of Chemical Education

Kinetic energy.

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Figure 9. Changes in Auger specba oi a CulNi alloy induced by annealing at 250°C ( l I).

Auger Spectroscopy

AES can he used to obtain both qualitative and quantitative elemental surface compositions. Figure 10 shows the Auger spectrum of a Ni-Fe-Cr-B alloy (3).Note that lines are clearly seen for each of the major components. Also notice that S, Si, and C are present as impurities and that a large oxygen peak is observed indicating that the surface is heavily oxidized. Note that AES spectra are different from ESCA spectra; AES spectra are plotted as derivatives while ESCA spectra are integrals (compare Figs. 3 and 10). The reason for this is

that Auger spectra, caused by electron ionization, lie on top of a very high, scattered electron hackground. Consider the energy distribution of electrons from a silver foil bombarded with 1000 eV electrons, as shown in Figure 11(12). The total enerev distribution of electrons. N W . shows little structure ~~exce;;t when amplified, N ( E ) x 10. The weak peak a t -350 eV corresoonds to the Aueer electrons from silver. Needless LO say, it r'epresents a smaii peak on a very large hackgruund. Differentiation, d N E u d E : ' , removes the barkgruund and clearly shows the main Ag 41NN Auger peaks, s m w weaker . of EK.4 soectra peaks and the O KLL ~ e a k1)ifierrntiitton is not necessary because the primary photon beam produces a lower electron background, Table 3 summarizes the analytical characteristics of Auger electron spectroscopy. The energy range is somewhat broader than for ESCA, hut chemical information is marginal due to the use of low resolution analyzers (except for some rare cases like distinguishing Si and $302). The electron escape depth is about the same as for ESCA. Elemental sensitivity and quantitative capabilities are similar to ESCA, although detection limits for most elements are slightly lower for AES than for ESCA. One great advantage of AES over ESCA is that an electron beam can be focused (0.5 fi or better) and thus lateral (x-v) . . . resolution with AES is oossihle. Use as an Anger microprohe provides element maps of surfaces. Also, br;ttae the signal-tn-noise ratio in AES is hight:r than for ESCA, a spectrum can he recorded in >I shurtt,r period of time. Comhinine AES with ion etrhine is an efiective wav. to . ~rovide deptcprofiles of materials; ihis was discussed in Part I (I). Commercial Auger spectrometers frequently have multiplexing devices that allows AES intensities of six to eight elements to he monitored simultaneouslv. The use of Auger spectrosropy for depth profiling is illustrated in Fimre 12, which shows the ion etrhinr! of a nirhrome film on silic& (13).These curves were ohtainedhy monitoring ~

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Figure 10. Auger spectrum of a Ni/Fe/Cu/B alloy (3).

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Figure 11. Energy distributions. YEJ,of elemons from a silver target ( 13.

Table 3. Analytical Characteristics of AES Energy Range Kinetic Energies: 50-2500 eV Escape Depth: 20 A Peak Locations: i t eV Chemical informallon: Marginal Elemental Sensitivity 1 Elements: 2 > 2 Specificity: gaod Sensitivity Variations: 50X Quantitative Analysis Absolute: i 3 0 % Relative: i 5 % Detection Limit: 0.05% monolayer Matrix Effects: some Other Aspects Vacuum: 10-8-10-'9 t a n Depth Profiling Capability: yes, rapid, muniplex X-y Resolution: 0.5 Elemental Mapping as e Minoproba Speed: fast, most saectra take minutes: elemental mapping in mlnutes Sample destruction; hequent: particularly bad for organics

ETCHING TiME,MINUTES

ETCHING TIME.MlNUTES Figure 12. Depth profiles of nichrome films on silicon ( 13); (a) before heating, (b) alter heating at 450°C tor 30 s.

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F i q m I3 Scanning Auger images of an inclusion in a ferrous alloy ( 16)

ventional microprohe hecause of its hetter spatial resolution and surface sensitivity. A sample of a ferrous alloy was sertioned, polished, and sputtered with an ion beam to remove surface contamination introduced by the polishinp process. Figure 13 shows the SAM images ohtained from the sample. Figure 13a is a secondary electron image of the sample showing a -2 pm inclusion (1) within a -6 pm particle (2). The Auger element maps of Figure I3b, c, and d show that t,he larger inclusion is sulfurrich (h) and that the smaller inclusion contains manganese (d). The surrounding material is primarily Fe (c). One prohlem with SAM element maps is that they give only relative concentrations and no indication of t,he ahsolute signal level. Thus, Figure 13h shows t,hat t,he surrounding area is iron-rich relative to the inclusion hut does not necessarilv indicate that iron is lacking in the inclusion. To obtain a better measure of the relative concentrations of t,he three elements, a line scan was recorded across the center of the inclusion as shown in Figure 14. This indicates that the core of the inclusion ( x = 4.00 urn) is essentiallv a Mn-S precipitate and is part of a larger inclusion containing Fe, Mn, and S. One side of the inclusion ( x = 3.00 um) consists mostly of Fe and S. Thus, one can say that the inclusion is heterogeneous and consists of a mixture of iron and manganese sulfides. Combined ESCA-AES Studies

Figure 14. Line scansafa ferrous allay acrossthe inclusion of Figure 19 of Ref. ( 1 6 ) The inclusion is hom xranging horn 2.5 to 7.5 p Changes in species mcur

a x = 2.5.3.5.4.8and7.5p.

peak-to-peak Auger intensities for each of the elements indicated. ~ o t that, e before heating, both Cr and Ni aresegregated near the silicon interface and that they have low concentrations a t the surface. Oxygen shows its majnr concentration in the first 50 A of the surface, and a t the metal-silicon interface. The interface is found to he ahout 150 A below the surface, i.e., the nichrome layer is 150 A thick. After heating a t 450' for 30 s, the thickness of the film has not changed significantly, hut the chromium has diffused toward the surface, and oxygen has diffused inward. In fart, the first 75 A of the layer corresponds almost entirely to chromium oxide. Nickel still is oresent lareelv as elemental nickel hut is now segregated a t the interface between t,he chromium oxide and the silicon. The increased oxveen . . content, a t the metal-silicon interface st,ill exists. An excellent example of how Auger spectrosropy, using the scanning Auger rnicroproh~(SAM), can he used to characterize surface segregation comes from a study of inclusions in metal alloys (14). The SAM offers advantages over the con488

Journal of Chemical Education

Frequently it is necessary to cornhine two or more surface technioues for a oarticular studv. This allows the oositive attributes of each technique to he applied to a prohlem. One frequent comhination is to use the chemical shift information ohtainahle by ESCA coupled with the depth profiling capahility of AES. An example of how t,hese can he used follows. A combined ESCA-AES study has heen carried out to elucidate the chemical effects on the surface of solder when it is corroded by reactive gases (15).Figure 15 shows the Sn and P h ESCA lines of solder before and after meltine. The lines at the top correspond mostly to SnO2 and Ph02, respectively. After melting, however, metal peaks are evident a t lower binding energies. The smaller peak represented hy a dashed line around 485 eV for Sn is due to the metal, and almost the entire lead spectrum is that of the metal. (The dotted line a t -138 eV is the oxide.) Thus, even a simple operation like melting can change the composition of the surface! Figure lfi shows an ESCA study of the effect of oxygen exposure on the PhISn and the Olmetal (i.e., P h Sn) ratios in solder as well as in P h and Sn foils. Note that the PhISn ratio drops significantly on oxygen exposure (diamonds) as the O/(Ph Sn) ratio increases: This shows that as the alloy is oxidized, the more reactive metal (Sn) tends to concentrate a t the surface. T o corroborate this result, an AES depth profile was run on a sample of solder that had heen exposed to oxygen. Figure 17 shows the results. Note that the O h e t a l ratio drops quickly on sputterina.. (open . circles) while the PhISn ratio first intread and then decreases, apprnaching the hulk value (1.0). Fignre 1R shows changes in the chemical states of solder which are induced by corrosion with NO2 as they are measured

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by ESCA. The top of the diagram corresponds to the oxidation states of Ph, the bottom to oxidation states of Sn as a function of oxidation time in 150 ppm NO? in Nz. Note that for Sn, the oxidation process is simple: Sn SnOz during the entire time of reaction. For Ph, however, PhOz is the initial product which rises to maximum concentration a t -60 min (open circles) and then d r o ~ as s PhO becomes the dominant species. At longer rrarlion iimes PhO remains the dominant s&it!s. These d;na weeest that iniriallv a thin lavrr of SnOl and P h 0 is ~ formed on%e surface. As tbe Sn diff;ses outward, the film becomes oxygen poor forming some SnO. T h e following reaction then becomes important (AH' = -56.7 kcallmole) SnO + P b O 2 SnOz + PbO as oxidation continues, a steady-state film is established consisting primarily of SnOz and PbO.

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(13) PHI Applications Note 7801, PhysicalElemonies Industries, Ine.. Eden Prairie. MN. 1974. 114) PHI A ~ l i u r t i o n s Note 1'343, Physical Elfftronies Indusfriea, Inc, Eden Prairie, MN. 1979. (IS) Okamoto.Y..Csrter, W. L a n d Hcrculos,D.M..Appl. Speelrose.33.287 (1979).

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Acknowledgment

We would like to express appreciation to the National Science Foundation for support of some of the work referenced in this paper. We would also like to thank Physical Electronics Industries Inc. for their permission to use some of the examples described.

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Llterature CHed (11 Hercules,D. M.. and HereulosS. H., J. CmM. Eouc. 61.401 (1984). (21 Herrulea, D. M.,"Surfaee Characterization UsingElwtrnn Smtmscom."in"ChsrH I . AeademicPreas. acforizatinnof Metal and Polymer L~rfsces."(Editor: Lee, I.. New Ynrk, 1977, Vnl. I , p. 899. (31 Hercdes. D.M.,"ESCA and Auger Swefmmpy."ACS Tape Course C-44. American Chemical Society. Waahington,DC. 1979. (41 Patterson,T.A..Carwr,.l.C.,Iayden.D.E.,andHerml~.D.M.,J. Phys Chem.80, 1700 i197fil. (51 Zing& D. S.. Ph.D. Thesis, University ofPittsburgh, 1971:see also Zn i g&,' D. Makourky,..I E.. Tischer. R. E., Brown, F.R., and Hercules, D. M., J. Phys. Chom. 84.

O2 E x p o s e d l

Figure 16. Effect of oxygen exposure an W I S n and Olmetal ratios of solder ( 17). Results obtained by AES (2 keV beam). 0 = PbISn: A = O/(Pb Sn): 0 = OISn for Sn foil: O = OIPb for Pb foil.

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W. N.. J , iblol., 46, 295'(197