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p p
Analytical Chemistry of Surfaces Part I. General Aspects David M. Hercules a n d Shirley H. Hercules University of Pinsburgh. Pittsburgh, PA 15260 A surface is generally considered to he the boundary layer of one phase a t its interface with another. The surfaces most frequently encountered in chemistry are a t solid-gas or liquid-solid interfaces. 1Lnally the surface is considered as being part of the sdid. The surface is usually considered to he more than one atomic laver deen. . A .eeneral definition is that a surface is a region of nonuniform atomic potentials. Although this is an excellent fundamental definition. it is not ~racticallv useful, because it does not relate to measurements that are readilv. nerformed on surfaces. . Generally, surfaces are not uniform nor do they represent discrete, well-defined layers. I t is clear that the outermost layer of atoms will be different from the bulk, and in most cases there will be a continuous transition between the two. This has prompted some workers to refer to the outermost laver of atoms as a surface, the transition layer as the selvedge, and the remainder as the bulk. For surface spectroscopy, it is best to adopt an operational definition. Ours will be that the surface region is that volume of the solid which a measurement technique samples. Therefore when one looks a t the same material with different techniques, he may in fact he sampling different "surfaces." -. ~ . Although the field of surface science has been reviewed reneatedlv and often. little has been written about the analytical chemistry of sirfaces. Some general articles (1-6) have been written, and a t least three hooks exist (7-91, but there seems to he a lack of a brief, comprehensive overview. Thus the present set of articles. ~~~~~~~
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Surface Characterization Modern mrthcrls nf surface analysis stand in vonlrnst to the so-called classical methuds. 'l'hp difftwnces l~etweenthem arr summarized in Table 1.Classical methods include adsorption isotherms, which aive surface areas and pore-size distrihutions, measureme& of surface r o ~ ~ h n e s s , ~ h o t o e l e cwork tric function, ellipsometry for thickness measurements, photoelectric work function, ellipsometry for thickness measurements, reflectivity, and microscopy to obtain surface topography. The information obtained by the classical methods can he characterized as "descriptive." This means that they provide an index to qualitative surface features such as roughness, topography, etc., but little quantitative or "chemical" infor~~~~
Spectroscopic Techniques The way in which spectroscopic techniques for surfaces function is shown in Figure 1. A beam is incident on the surface and penetrates to some depth within the surface layer. A second beam exits from the surface and ultimatelv is analyzed by a spectrometer. The beams in question may he photons. electrons or ions. It will be instructive to see that hv varying the nature of the beams in and out of the surface, one can generate a larae - number of surface analvtical techniques. Tahle 2 shows combinations of beams-in and beams-out for various possibilities involving photons, electrons, and ions. T h e techniques marked with double asterisks in Tahle 2 are those which will be discussed in detail in this paper. Those Table 1. Comparison of "Classical" and "Modern" Methods for Surtace Characterization Modern Spectroscopic Methods Provide
Ciassicial Methods Adsorption Isotherms
Elemhtal Analysis Chemicallnformation Oxidation State Functional @oups QuantitaveAnalysis Elemental Ratios Oxidation State Ratios Dishibution Lateral
Surface Areas Pore Size Distributions Surface Roughness Photoelectric Work Function Ellipsometry M~CIOSCOPY
Reflectivity
In10 Bulk
~
Modern mrthods of surface snnlysis are spectr~sri~pi(.. Th~y ran nrovidt. d~emicalinformatim which the classical methods cannot. For example, one can obtain elemental analyses, information about oxidation states and organic functional . groups, quantitative analyses either as elemental ratios or oxidation state ratios, and distributions of materials either across the surface o r from the surface inward tqward the bulk. The ability to acquire this type of information ahout surfaces vlaces surface characterization within the realm of the analytical chemist. Analytical problems pose four questions: What elements are present? How much of each element is present? What f o r k or forms of the element are present? ~
What are the relative prr(rnti1ges of thr differrnt sl~eciwof that element? It is onls rerrntls that such information could he acquired ahout species on surfaces. Quantitation of surface spectroscopic methods is still a matter of research and debate. Nevertheless, the ability to acquire chemical information about surfaces has placed surface analysis in the forefront of modern analytical chemistry.
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This paper is the first of three parts, Parts I1 and Ill to appear in the followingtwo Issues of THIS JOURNAL. 402
Journal of Chemical Education
' \
Photons Electrons Ions Figure 1. Relationship of probe and analyzed COPY.
beams in
surface spectros-
Table 2.
Techniques Generated by Combinations of Photon, Electron, and Ion Beams
, BEAM IN
PHOTONS
Infrared Spectroscopy (FTIR)' Raman Spectroscopy (LRS)' x n a y Fluorescence (XRF) Extended X-Ray Absorption Fine Structure (EXAFS)'
Elechon Microprobe' ELECTRONS Appearance-Potential Spectroscopy (APS) Cathodoluminascence
IONS
Ion Microprobe: X-Rays (IMXA) lan-induced X-Rays
>
BEAM OUT ELECTRONS
PHOTONS
IONS
X-Ray Photoelectmn Spectroscopy (ESCA)" Laser Mass Spectrometry (LAMMA) UV-PhotoelectronSpectroscopy (UPS)' Praton-Induced Auger Electrons
Auger Electron Spectroscopy (AESJ" Low-Energy Electron Diffraction(LEED)' Electron Microscopy' Electron-Impact spectroscopy (EIS) Ion Nelmalization Spearoscopy (INS)
lan-Induced Auger Electrons
Electron-Induced ion Desorption (EIID)
Secondary-Ion Mass Spectramefry (SIMS)" Low-Energy Ion Scanering Spectroscopy (ISS)" High Energy Ion Scaitering Spectroscopy (RBS)'
lon-Micmnrabe: Ions llMMAI discusred in detail here, but imponant techniques nonmhelers O i s c u ~ s dindetail inmis paper.
.Not
marked with single asterisks also have considerable utility, but space will not permit an exhaustive discussion. One quickly learns that surface spectroscopy is rivaled only by the federal government for the use of acronyms. Thus, with apologies, these acronyms are included in the table. Photons-in and vhotons-out generate the well-known t t hniques ~ of infrared and Rnnian iprctrosrnpy. Heri~uarof the ad\.nnces in infrored brought a t r u t hy the use ~~I'F'ourier Transform method, IFTIRJ and thr ~nrreasedu t ~ l t r \of Raman spectroscopy by use of lasers (LRS), FTIR and ~ R S are hecomine- imwrtant modern surface analvtical techninues. . Surface-enhanced Raman spectroscopy is a particularly excitine field. Another technioue that is iust beeinnine to emerge as a valuahlr surfare t w l is extended X-r>iyalisorpti~mfine structuri*.EXAFS. This invol\,es nnnlvsis nt the fin? atrt~cture on X-ra; absorption bands and permits determination of structural information. Using photons in and electrons out generates the very valuable techniques of X-ray photoelectron spectroscopy (ESCA) and UV photoelectron spectroscopy (UPS). UPS can he very valuahle for surface sprctroicopy esprrially when using synrhrotron r~~diation. However. UI'S will not he treatrd hrrr. ESC.A will he discussed in detail in Part 11. Elertrms h a w hten used for dwades for gimerating photuna in the electron microprobe. Similarly, usin:! 1:lertrons to rxtite or her (.Ircfrons pencrates Auger electron spectruxopy (AES);scat tering uf'elrctror~sgenerates low-energy ele:tron diiirartiutt ILFKI)) and electron microscopy. Augt.r clwtrun spectroscopy hai proven 01 great utility and will he discussed further tP;~rt11). I.uw-cnerrv electrun d~ffrartionvruvidrs structural information but &n be applied only to highly ordered surfaces such as single crvstals. " The main use of ions in surface spectroscopy involves the ions-in. ions-out mode. This zenerates two imvortant techniques t o be discussed in p a r t 111: secondary ion mass spectrometry (SIMS) and low-energy ion-scattering spectroscopy (ISS). The scattering of high-energy ions (Rutherford backscattering) is also of great utility. It is one of the few techniques which gives ahsolute film thickness without prior calibration. When probing a surface with any beam, one must he concerned with the effect of the probe beam on the surface. Some effects of probes on surfaces are given in Table 3. Photons are the least destructive. For the soft-X-ray photons used in ESCA, in greater than 95%of the cases no decomposition of the surface occurs. Infrared photons are even less surface disruptive. Electrons are more destructive and the nature of the effect varies from insulators to conductors. Electron beams are
particularIy destructive for organic materials. In addition to chemical effects, electron beams cause serious sample charging for many insulators, to the extent that frequently Auger spectra of insulators cannot be obtained. Ion beams, on the other hand, cause sputtering of all materials. Their effectiveness for surface analysis relies on the fact that signals can be obtained in ISS and SIMS under conditions where the net sputtering from the surface is small during the time of an analysis ("static" conditions). Experimental Parameters A number of ex~erimental~ a r a m e t e r sare i m ~ o r t a n for t surface spectroscopy. These parameters impose limitations or comdications on surface analvses. Thwe reviewed here will be sampling depth, sample charging, and surface contamination. Referring to Figure 1, the beam-in and heam-out combination will be surface sensitive only if some phenomenon limits the penetration depth of the incident beam or the escape depth of the departing beam (or both). Table 4 gives typical penetration (escape) depths of photons, electrons, and ions in the energy range used for surface analysis. Note that -1 keV electrons and ions have penetration depths of 10-20 A, while -1 keV photons have penetration depths of about 104 A. Thus one can see that either the beam-in or the beam-out must involve electrons or ions. A photon-in, photon-out Table 3.
Photons Least Destructive
Effect of Probes on Sudaces
IR < UV < VUV 95% OK
< X-Rays
Electrons
More Destructive
insulator^ > Semiconductors > Cond~ctors Charging Bad for Organic Materials
Ion Beams
Cause Spunering
All Materials Spunersd
Table 4. Penetration Depths ol Particles
Particle
Enera" feVl
Deoths iAl
photon Electron
1000 1000 1000
10.000
10"s
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technique will not normally be surfwr sensitive. Rrferring to 'l'ablr 2, nutt, that tht.ftmr techniques t11 hediscussed in this paper are forms of spectroscopy in which either electrons or ions are analyzed. The only surface-sensitive techniques involving photon emission are infrared and Raman spectrosCOPY. The escave . denth . of a particle such as an electron or ion will vary as a function of its-kinetic energy. A plot of the escape denth of electrons versus kinetic enerm -- is shown in Figure - 2. Notice that very-low-energy electrons have large escape depths as do high-energy electrons. Electrons having energies of about 100 eV have minimum escape depths. Therefore spectroscopy done using high-energy or low-mergy rlectnms will not necrssarily hr stlrfi~crsensitive. In AES, and ESCA, the rlwtnms mt*aiuredare usually in the I00 I000 eV range and thus have escape depths on the order of 10-20 A. If one is bomharding a surface with charged particles or having charged particles emitted from a surface, charging of the sample may occur. This is a significant problem for all soectroscovic techniaues involvine electrons or ions. Table S'outlines &me of the.difficulties caused by sample charging. For example, charging can distort spectra, shift the location of peaks, or cause movement of species on the surface. The extent of the vrohlem deoends on the nature of the samvle. For insulator8, charging effects can be quite severe, whilk in conductors they are virtually nonexistent. The question arises as to how one compensates for sample charging. Fortunately, the surface conductivity of most samples is much greater than the hulk conductivity. Therefore, even though samples tend t o build up charge on the surface, frequently there is sufficient surface conduction that the charge builds quickly to a steady state value and does not change. In ESCA there are frequently stray electrons in the vicinity of the sample which also help to reduce sample charging. Because of the above two features, i t is usually possible to calibrate surface charging using a calibration material that tracks surface charee. This assumes that " charging across the surface is uniform. Another way to compensate for surface charging is to use an electron flood gun. If a surface tends to build up a positive charge, a stream of low energy (thermal) electrons can be used to neutralize the pos-
hive charge. A major difficulty arises u,hen the sample builds up a negative chnryr (as in Auger sprctruscopy oiinsulators) where neither flood guns nor caliGation is uniformly effective. Contamination of a surface can also he a problem. A surface as seen by a spectrometer is shown in Figure 3. Notice that the samole has three lavers: a contamination laver., the sttrfnce laye;, and the hulk material. The contamination layer and the surface laver are normallv samoled hv the snectrometer. The contaminkion layer is u & l l y composed of'species adsorhed from the atmosphere by handling, or from the vacuum system. Even in what is normally regarded as high vacuum, the lifetime of a clean surface can he relatively short
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where 7 is the lifetime of the surface and P is the pressure in torr. Thus one can see that even a t torr, a clean surface will become contaminated in only 1s. Equation (1) is calculated assuming a sticking probability of unity and represents the time required to form 1% of a monolayer. Table 6 gives the lifetime of clean surfaces calculated for the formation of 0.01 and 1monolayer a t three different pressures. One can see that even a t torr, exposure t o a high vacuum system for 1h will approximate monolayer coverage. Thus all surface experiments for which it is desired to have no significant contamination from the vacuum system must be carried out a t pressures below 10-9 torr. Quantitative Asoects
The concentration distribution of a measured species as a function of depth can seriously affect quantitative surface measurements, as illustrated in Figure 4. The top of Figure 4 shows an adsorbed layer in which the adsorhed atoms or molecules are distributed as shown on the left; a t less than monolayer coverage some ofthe substrate is exposed. Profile 1is the concentration profile of the system a t the left. Note that the concentration of the adsorhed material droos abruptly to zero as one crosses theinterface. In such a sit";tion it is possible to express surface concentrations in meaningful units, for example atoms per cmr or fractional monolayers. ~ which The center part of Figure 4 shows t w distributions correspond to a thin layer, several atomic layers thick. Concentration Profile 2 is for a well-defined thin layer separated cleanly a t the substrate-layer interface. Here again it is possible to express concentrations in meaningful units, for ex-
(
Contomination La er Surface Layer Sampled
1-40;
I I0
100
1000
I0.m
KINETIC ENERGY lev) Figure 2. Relationshipbetween electon escape depth in solids and kinetic energy (10).
Table 5. Dlfficuliles Caused bv SamDle Charaincl
Distortion of Spectra Shins of Peak Location Movement on Surface Extent of Problem insulators >> Semiconductors > Conductors Compensation Surlace Conductivity Stray Electrons Calibration FloodGun
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Figure 3. Model fwa real surlace ( 10).
Table 6.
llme Required for Formation of a Monolayer (Sticking Probabllity of Unity)
Pressure (torr) lo+
Time (0 = .Ol)
Time (0 = 1)
0.1 s 10s
3s Ih 10 h
20 min
ample, pg/cm2. In Profile 3 the concentration of the species varies continuously from the outermost surface layer into the bulk. For this situation it is not possible to express the surface concentration in meaningful units. Quantitation of the results will depend on the sampling depth of the surface technique used and the form of the distrihution profile. When one has a multilayered system, three components or more, even more complications can arise. In Profile 4 one has components A and B in a wafered effect overlaying the bulk C. In Profile 5, however, component B is located at the interface between A and C. Here again, we must emphasize the importance of the sampling depth of the technique used. Further complicating measurements is the fact that the sampling depth may not he the same for each component of h an electron. which is the sama surface. The escaDe d e ~ t of pling depth for thatparti'cular component of the sample giving rise to the electron. is determined hv the electron kinetic energy. The kinetic energy of an escaping electron in turn is a function of the hinding energy, which, of course, will he different for each component. The effect of sampling depth on quantitative surface analyses is shown in Figure 5. Consider a material composed of A and B which is richer in component A at the surface than it is in the bulk; the depth distrihution of A is a simple exponential. In Figure 5(a) the concentration of component A is 100%at the surface and 0%in the hulk; in Figure 5(h) 50%at the surface and 0%in the bulk; and in Figure 5(c) 100%at the surface and 50%in the bulk. The vertical lines in each set of figures correspond to sampling depths of 4,10,20, and 26 A.
For Figure 5(a),if the sampling depths for components A and Bare both 4 A, the measured ratio of AD willhe 5.0. Similarly, if the sampling depth of each component is 10 A, the ratio will be 2.0, and if both are 20 A it will be 1.0. If A has a much greater sampling depth than B (26versus4 A), the measured ratio will he 16.9 and if it is much less than B (4 versus 26 A) it will be 0.25. Similar results are shown in Figure 5(h). Of particular interest is Figure 5(c), where, if A has a sampling
100
Adsorbed
Layer Rotio
- - -
60
4.0
4.0
0.71
Thin Layer
4
Multi - Layered Systems
10
Rotio
2 0 2 6 kA 10.0 20.0 26.0 4.0
80
10.0 20.0 4.0 26.0
5.0 3.1 54.2 0.54
60
C(%) 40 20
'0
I0
20
30
40
x(A)
Figure 4. Three situations impoRant in quantitative surface analysis (9).
Figure 5. Concenb-attm profiles fwthree hypometical materials (9). B is G% bulk comoonent. X. and X. are escaDe . deoths . for ohotoelecbons fmm atoms A and B.~espectweI~. Numbem at me rop (4. 10.20.26r carespMd to e ecuon escape depth in A Ratiosgwen areatomcrat 0s measured oy ESCA tor aglven sel of X values.
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depth of 26.0 A and B 4.0 A, the ratio will be 54, whereas, if A is 4.0 A and B is 26 A the measured ratio of AIB will be 0.54. Figure 5 illustrates two very interesting things. First, i t shows dramatically the effect of sampling depth on surface analyses. Second, i t shows that if onecando measurements a t different sampling depths one can derive information about the d e ~ t distribution h of a material in a samole. ~ u o i h e way r to derive information about {he distribution of species from the surface into the bulk is to vary the angle of take-off between the sample and the spectrometer. In Figure 6 the dotted line represents the incident beam and the solid line with the arrow, the path taken by the emitted particle eoineto the s~ectrometer.If one has an electron emitted at the interface hr;ween the surface layer (dashed region, amd the hulk, the effective pathlrnmh for a 20" rake-off analtt from the surface will be m&h than for 65'. The effective escape depth is shown by the length of the arrow passing through the dashed region. Thus the signals from the surface will be enhanced at small angles relative to the surface and the signals from the bulk will be decreased. This can be calculated from the equation shown in the middle of Figure 6. Note that the angle 0 is the angle the exit beam makes relative to the surface. The relative intensities of signals from the surface layer and the bulk for the two situations are shown by the solid
lines at the bottom. Thus. a small take-off anele relative to the surface greatly enhancea the signals riom species in the outer laver and derreases the relative sianal intensitv of thr bulk. - ~ o t hangular distribution anduvariable sampling depth measurements can measure distributions only if they occur in the top -100 A of the sample. Frequently depth distrihutions occur to distances much greater than this. Under such circumstances the method of choice is to do depth profiling by etching a sample with an ion beam. In this technique a beam of energetic ions bombards the surface and slowly erodes the surface during the bomhardment process. This is illustrated in Figure 7. In practice it is possible to etch a t rates lower than 10 A/min, up to rates as fast as 100 Als. Thus ion bombardment represents an effective technique for "drilling" through a sample surface. Figure 8 shows how ion etching and Auger spectroscopy can be combined for depth profiling (I11. A narrow electron beam is centered in a much wider ion beam and the Aueer sienal induced by the electron beam is monitored conti~uous&as the ion beam etches through the surface. In Part I1 we shall see examples of how this combination can be used to depth profile of a variety of materials. One of the problems in using ion beams is that calihration of ion etching rates is quite difficult. Further, the ion etching rate varies from one substrate to another, increasing difficulties with calihration. A system such as that shown in Figure 8 produces a depth profile which is a plot of intensity (concentration) of a material as a function of etching time (depth). However, the observed depth profile of a thin layer is actually a convolution of three functions as shown in Figure 9. The observed depth profile (4) is a combination of the true d e ~ t distribution h (1).the sampling depth of. the rnrasurina'technique used (z;, a n d t h e width of the scrambled laver caused b\. the ion beam 13,. What this says is that, even if one has a discrete interface'between
Normal
Surface
/&10n
Bulk: I = Kexp (-a/sin6') Surface: I = K/sin 0
Relative Intensities
1
Figure 6 . Effect of take-off angle on electron escape depms in solids. Top Spectrometer angles 65' and 20' relative to lhe surlace. Midd1le:lnteensities of bulk and surface signals as a function of We-off angle, 8. Bottom: Relative intensities of surface-tc-bulk for 8 = 65' and 8 = 20".
Beam
Area Bein; Analyzed
Figure 8. Combinationof iowtching and Auger spectroscopyfwdepm profiling ( 11).
Surface
Figure 9. Fundiona mntrlbutlngman obsewdepm. Rofileofa sharp imerface. (1) Achlal cancentration profile (CAIof material A. ( 2 ) Sampling depth of probe beam. (3) Scrambling width of the ion beam. (4) Observed depth profile. The convolved function is: Figure 7. Idealized reaction of spunering ions wiih surface atoms (10).
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Journal of Chemical Education
41)
= f(C~)fexp(-a/sin9).exp(-a82)
two layers, because of the sampling depth of a measuring technique (such as AES) and the fact that the ion beam has a tendency to scramble the atoms over a given region (-20A), the depth profile ohsewed cannot he used to sharply define an interface. So far, our discussion has stressed interactions in the sample rather than the instrumental aspects of surface spectroscopy. Table 7 outlines some instrumental factors which are important for surface analvsis. We will consider these for each technique when it is dkcussed, but certain ones merit general consideration. Thus resolution. sensitivitv, and detection ' limits will he dealt with in somedetail. Resolution
Spectroscopic resolution generally limits the ability of a technique to provide qualitative chemical information. Limitations on intensity measurements largely determine the quantitative capability. Resolution is defined as the ability of a technique to determine the location of two closely spaced neaks in the nresence of each other. This is dictated hv the width of the spectral band relative to the spacing between peaks. Spectral bandwidths are usually expressed as the full width a t half maximum height (FWHM). For example, if two peaks of equal intensity having FWHM's of 1 eV are separated by 0.8 eV, it is not possible to resolve them by inspection of the resultant snectrum. If. on the other hand. with the same sew aration (0.8 eV) and FWHM'S of 0.2 eV, two peaks are easiiy discernahle. Frequently in surface analytical techniques the situation arises where overlapping spectral bands limit the acquisition of useful chemical information. This is particularly true in ESCA. In such situations deconvolution techniques are used to extract the pertinent spectral information. Deconvolution usually is done with the aid of an analog or digital computer. If the FWHM's and the shapes of the peaks are known, it is possible to deconvolute a spectral envelope into its components even though to the eye it may appear as a single spectral A
~
-
~
~~~
~
~
-
~
a~
~
AEb= 1.5eV Intensity = 1 ' 1
AEd:O 8eV FWHM= i OeV Intensity = l i I2:l with," doublets)
Table 7. Instrumental Factors ImDortant for Surface Anaksls Q~~ntilatlve Capabilities Relative Absolute Spectroscopic Resolution Limits Chemical Information
Signal-10-Noise Ratio Sensitivity and Detection Limit Universal Applicability Sampling Depth Lateral x-y Resolution
band. Caution must be exercised when doing deconvolutions, narticularlv for noisv. spectra and when workina- with weak . hands, because it is easy to introduce artifacts into the results. However, deconvolution of spectral data has been very useful for surface analysis. Consider the sets of spectral bands shown in Figure 10. The two sets at the top represent deconvolutions of spectral bands separated by 1.5 eV, having FWHM's varying from 0.6 to 2.0 eV. The lower two sets of spectra represent deconvolution of two sets of doublets, with the doublet components (AEJ spaced 0.8 and 3.0 eV apart. = 1.5 eV. intensitv = The snectra at the u m e r rieht 1:8) re&esent an inte;&ting-set.~his is the situation one would encounter for one atom of an element chemicallv shifted from a large number of others by a small amount;for kxamp~e C - 0 in the oresence of a larae number of C-C bonds. For resolving these spectra. FWHM = 0.6 there is no However, it becomes an increasinalv difficult problem as the F.WHM goes to 2. In fact even us& sophisticated deconvolution techniques extracting the small peak from the total spectrum poses an exceedingly difficult problem for FWHM = 2. Thus one can see clearly from this figure how the relationship between peak spacing and peak width can limit acquisition of chemical information. Sensitivity and Detection Llmlt
Sensitivitv and detection limits are important for the quantitative application of any spectroscopic technique. Increasing source intensity will result in improving data only to AEb= I 5 e V Intensity = 1 8
AEd=30eV FWHM = I OeV ntens8ty = 1 i ( i 4 i w~thlndoublets)
Figure 10. Deconvolution of spectral envelopes typical of ESCA spectra ( I n ,
Volume 61
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407.
a point, even if increased source intensity does not induce sample decomposition. The reason for this is background. An increase in signal by using a higher source intensity is accompanied by an increase in background. This will cause increased fluctuations in spurious signals that enter a spectrum, which constitute noise. I t is the signal-to-noise ratio that ultimately determines how much quantitative information can be extracted from a spectrum, and increasing both signal and noise to the same extent eains one nothine. Figure 11 shows a typical ESCA spectrum with superimoosed backmound noise. The sienal intensitv for the oeak will be the difference between thepeak and thk background: Ip - Ig. Thus the signal to hackground ratio is (Ip - Ig)lIg. Although the signal-to-background ratio is important in a technique, more important is the signal-to-noise ratio which is defined as (Ip- I B ) I ~ R Mnoise. S For most spectra, as I p approaches IB,RMS noise can be approximated as 6or the signal to noise ratio is I p - I g l a Another important factor is the relationship between measured intensities and concentrations. For most surface techniques the concentration of a species is linearly proportional to the net intensity, or (Ip - I g ) = k c . The sensitivity of a spectroscopic technique is defined as the slope of a plot of intensity versus concentration, i.e., the calibration curve; two are shown in Figure 12. Expressed mathematically, sensitivity = dlldC and can be approximated as AUp - 1g)l AC -. . The detection limit is the minimum amount of a material detectable using a given spectral line. This represents the concentration for which I p - IBis twice the RMS noise or I,, - I g = 2 6. The most sensitive spectral line will not necessarily show the lowest detection limit. In Figure 12 one sees two lines. Although line 1has the higher sensitivity (greater slope), it occurs where the background is higher as indicated by IB(1). Although line 2 (dashed line) has a smaller slope, it has a lower background IB(2). Thus line 2 will show the lower detection limit, although line 1 shows the higher sensitivity. Most spectroscopic techniques utilize signal enhancement procedures to improve signal to noise ratio. This is frequently done usine multiole scans (comnuter controlled) or multi. . channel teehniquks which increase the signal-to-noise ratio for a eiven observation time. This t w e of sienal enhancement becomes particularly important when one ishealing with weak signals, such as from fractional monolayers on a surface. General Applicability . Of primary importance to the analytical chemist is the general applicability of a technique. An assessment of applicability includes such parameters as restrictions on the nature ~
Kinetic Noise
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of samples, the type of information which can be acquired, the detection limit of the technique, not to mention its cost. The latter is a non-trivial factor in surface analysis since most of the spectroscopic instrumentation is complex and expensive. We will now consider some of the important questions which should be asked of a surface technique to assess its analytical anolicabilitv. We will mention some nertinent ooints for the tkhniques discussed in detail in parts-11 and 111.'~eshallseek to evaluate each of the maior surface techniaues accordine" to the criteria set forth here. Of particular importance is any limitation a technique may impose on the type of sample that can be run. T o be analytically useful, a wide range of samples including insulators and conductors should be useahle. Many techniques impose serious limitations on the nature of the sample. For example LEED requires single crystals and field ionization techniques require samples that can be fabricated to a very sharp point. The energy range and escape depth of a technique are coupled. Generally techniques involving higher energy particles have deeper escape depths and thus become less surface sensitive. A corollarv reauirement on oeak enereies is the ability to locate a peak; how precisely can the pea"k be measured? Chemical information is obtainable hv accuratelv mrasuriny pwk lowtions and sepinrnllng peaks from closely udincent rmes. Thui the imoortant auesrion uf rhe tX'HM reiative to the magnitude of shift giving meaningful chemical information arises. Elemental sensitivity is very important. The number of elements to which a technique is applicable directly affects its desirability for surface analysis. Also, specificity is an important effect, namely, are there any systematic interferences between elements. Variation in sensitivity from the least sensitive to the most sensitive elements is important. The most desirable technique will have uniform sensitivity, preferably high. How effectively can one use a technique for auantitative analvsis? This is imoortant both from the s'tandpoint of ah'olute analyses (without standards), and relative analyses. Detection limits are important and generally the matter of concern is what fractionof a monolayer can give a sienal eaual to 2 J R M S noise. Matrix effects occur frequenrly in surface rernniques and rhus nlujt be ronsidered. Other imovrtant tacturs arise as wrll. \That kind of varuum is necessar;, feasible, or desirable, to "se a technique? Is i t possible to couple a particular technique with depth-profiling capability? If so, does it lend itself readily to multiplexing? What are the lateral ( x , y ) resolution capabilities for mapping
a
F g ~ r e12. Ca oratton c m e for two specsa I nes. 11, Greater s ope b.1 nigner oacdgrabna. (21 louer s ope 0.1 lower oachgrabno.
as a microprobe? How rapidly can a spectrum be obtained? This is particularly important when one considers the prohlems of residual gas adsorption on clean surfaces. Is the technique relatively gentle with surfaces or is it destructive? All of the above questions are very important and should be answered in the evaluation of any surface technique when used by the analytical chemist. In the subsequent papers we shall discuss the four spectroscopic techniques for surface analvsis: ESCA. AES. ISS. and SIMS. Each of the factors showk in 7 k ~ e and 7 mentloned a h w e will be ronsidrred with rward to how ihev affect u s e d a terhniuue fur 110th a u ~ l i t a tive and quantitative surface analysis. Acknowledgment
This set of three papers grew out of three invited lectures on surface analysis (D.M.H.): the Otto M. Smith Lectures at Oklahoma State University (1977), the Society of Manufacturing Engineers Substrate Conditioning Conference (19781, and the Graduate Society for the Advancement of Physical Science Lectures at the University of South Carolina (1979).
We are most appreciative to those responsible for the invitations. Giving these lectures wve awareness of the lack of short. general arrirles that could serve as an introduction to modern surlaceanalvsis. \Ye would like to thank the National Scienrr Foundation for supporting some of the work discussed here. Literature Cited (1) Betferidge,D., Anolyal, 99,994 (1974). (2) Cheng, K. L.. and Prather,J. W., CRC. Crit. Re". Anal Cham, 5.37 (1975). (3) Powell,C. J.,Amar.Lob., 10 141, 17 (19781. 14) Barr,T L.,Amer.Lob., 10 jlll.68 (1978);10 [12],40(1878). (61 Euans,C.A..Anol. Chm~,47,866A (19751. (6) Hercule~,D.M..Anal.Chem.,50,734A (1978). (7) Kane. P. F.,and Larrabee,G. R., "Charsctarizatian ofSolid Surface8,"Plenum
York, 1974. (8) Czanderna. A. W., (Editor), "Methods of Surface AnalyGs," Elsevier. New York,
.".". ,a,=
19) Mclntye, N. S., (Editor),'"QuantitativeSurface Analysis of Materials,"A.S.T.M. Special Publiestion. STP643ASTM, 1916Race Street,Philadelphia, 1978. (10) Hercules, D. M., 'Surface Characterization Using Electron SpectroscopyIESCAY in
Lee. L. H., (Editor) "Charseterizafionof Metll and Polymer Surfaces: 1:Acadernic Press, New York. 1977, p. 399. (11)Hereules, D. M., "ESCA snd Auger Spedmscapy,"A.C.S. Tape Coursp No. C-M. American Chemical Society, Washington, DC, 1979.
Volume 61 Number 5 May 1984
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