S (2p)photoelectron
'ititat; and r PhcL,electron Spectroscopies Kenneth W. Nebesny, Brian L. Maschhoff, and Neal R. Armstrong
LaboratoN for Electron Soecboscoov and SurfaceAnalysis and department' of Chemistry The University of Arizona Tucson. A 2 85721 Surface analysis using electron spectroscopies is now a well-established and continuously expanding area. With appropriate data treatment procedures, both Auger electron spectroscopy (AES, not to he confused with atomic emission spectroscopy) and Xray photoelectron spectroscopy (XPS or ESCA) can routinely supply reliable
\
with binding energies of less than loo0 eV. The kinetic energy of the electron escaping the solid and successfully traversing the analyzer to the detector is specific to the element of origin and is further varied by changes in the oxidation state of that element. Shifts in spectral lines are sizable in both AES and XPS, hut more easily interpreted for XPS. Molecular information about the solid can generally he obtained together with its elemental makeup and an estimate of concentration. Since the development of the first modern commercial surface spectrometers in the late 1960s and early 1970s, there has been a steady refinement and improvement in the instrumentation for surface electron spectroscopies (see (c) Ultrehighvw x-ray source
'
hv
qualitative and semiquantitative characterization of the near-surface region (top 1-100 A) of most solids. A schematic of the spectroscopic processes is shown in Figure 1 (using the S(2p) photoemission (XPS) and S (LMM) Auger emission as examples). Both XPS and AES typically involve ionization of core and valence electrons 0003-2700/89/0361-469A/$01.50/0 @ 1989 American Chemical Society
Gardella's forthcoming article in the CHEMISMay 1 issue of ANALYTICAL TRY [I]). These include better vacuum and sample handling, improved excitation source designs (multiple-energy X-ray sources with higher photon fluxes (XPS) and brighter, higher spatial resolution electron beams (AES)), high-resolution and high-throughput
Flgure 1. Schematic of the spectro-
scopic emission processes: (a) X-ray photoelectron process for sulfur, (b) Auger process for sulfur, and (c) the near-surface electron emission process.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7. APRIL 1. 1989
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INS7RUMENTATlON efficiency electron kinetic energy analyzers, new detector designs (multichannel versions are an attractive, developing alternative), and, of course, better computer control and data-processing packages. Multiple-technique surface analytical systems, whose price tags exceed several hundred thousand dollars, are the norm these days for most “routine” surface electron spectrometers. Most instrument vendors have also recognized that there is a growing countertrend in the demand for singletechnique instruments, sometimes attached to materials processing systems, that reflects the general philosophy of taking analysis to the sample rather than vice versa. As predicted in David Hercules’ 1978 article (Z), AES and XPS are the preeminent surface analytical techniques because of their widespread availability. Secondary ion mass spectroscopy (SIMS) is considerably more sensitive but is more difficult to quantitate. Electron diffraction techniques such as low-energy electron diffraction (LEED) can provide surface structural information. The wider availability of synchrotron radiation sources has provided options for analysis of other forms of local surface structure through the use of extended X-ray absorption fine structure (EXAFS) and related spectroscopies ( 3 , 4 ) . Along with the changes in instrumentation, there has been an increasing interest in understanding (and removing) the limitations to the quantitative analytical capabilities of surface analytical methods in general and surface electron spectroscopies in particular (5-10). Surface electron spectroscopies are not trace analysis techniques; 0.1-1% concentration ranges are typical detection limits for most elements. Even in these concentration ranges, AES and XPS are not considered as quantitative as other analytical techniques used for materials characterization. The electron microprobe combined with X-ray fluorescence is one example of a “quantitative” materials characterization technique. The electron microprobe, however, is not a true surface analysis technique, primarily because the sampling depth extends up to micrometers versus 1-1000 A for AES and XPS. Both sample matrix and instrumental problems described in this article for AES and XPS have often been considered too formidable for reliable quantitation. Today, surface analysis techniques are used to characterize new technology materials whose dimensions are small (length, width, and depth in the range of 100 A-1 rm). It is foreseeable that surface analytical information will
he used to define device specifications and to differentiate competing technologies. Reliable quantitative information from these surface analytical techniques is therefore becoming essential. Recognition of this fact has led to numerous publications and to several new conferences on quantitative surface analysis. The Topical Conference on Quantitative Surface Analysis, held biennially (1985,1987,1989), generally precedes the American Vacuum Society National Symposium. A larger topical conference on quantitative surface analysis is held yearly in Europe. Several detailed reviews describe some of the factors that must be considered to obtain quantitative compositional information from electron spectroscopies (5-10). Quantitative relationships in AES and XPS
We first consider the problems in quantitation for AES. The detected current (IA)(generally the peak area or a measured parameter proportioned to that area) in an electron beam excited Auger emission signal from a solid is given hy the formalism of Seah (6),
1
-z/XM,EA~ose dz
(1)
where 10represents the electron beam excitation source current (amps.cm-2), U A ( E ~ represents ) the electron impact ionization cross section (at primary beam energy E,) leading to Auger electron emission ( 1 0 , 1 + I M ( E ~ ,repre~) sents the additional population of Auger electrons produced at energy E Aby backscattering of primary source electrons and other secondary electrons; rM is the backscattering coefficient examined at angle (Y to the surface normal ( Z Z ) , T(E*) represents the transmission efficiency of the analyzer a t E A (the kinetic energy of the detected Anger electron), and D(E*)represents the detector efficiency. The integral term contains the concentration of the analyte ( N A )modified hy the exponential decay term, which takes into account that the Auger emission signal decays as the distance z below the surface plane increases (with decay constant A, the electron escape depth at energy EA)(13-15). The angle of analysis, 0, to the surface normal is included, and it is clear that as 8 increases, the surface sensitivity also increases. For € =I90°, and assuming that all of the detected Auger emission oc-
470 A * ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1. 1989
curs within the depth z = 5X, most Auger spectra in the kinetic energy range from 50 to 1500 eV arise from within 1 to 100 A of the solid-vacuum interface. The principal uncertainties in Equation 1 arise from V A ( E J , r M , and XM,E~, assuming that the instrumentally dictated parameters, T(E*)andD(a,), are known or can be determined in a straightforward manner. For adsorbates (A) at suhmonolayer to monolayer levels on well-ordered surfaces (Figure Za), Schoeffel and Hubbard showed several years ago that it is possible to fully account for the uncertainties in Equation 1and to calculate absolute surface coverages from the measured Auger emission current (16). This method, or related approaches, is extensively used to quantitate surface coverages of a wide variety of ordered and disordered adsorbates on well-ordered surfaces. Such data are routinely correlated with surface coverages determined from electrochemical (voltammetric) data, thermal desorption mass spectrometry, and low-energy electron diffraction. The experimenter is greatly aided here by the fact that scattering of Auger electrons (described below) that arises from the analyte adsorbate layer is minimal. Adsorbates on well-ordered surfaces at submonolayer to monolayer coverages generally are the easiest systems to quantitate absolutelv using surface electron spectroscopies (either AES or XPS). The next easiest case is when the solid can be considered homogeneous within the analysis depth (ca. 5X) for all the elements (e.g., A and B) of interest (Figure Zb). The uncertainties in Equation 1 can be partially cancelled by considering relative atomic ratios (or the atomic percentage) of a pair of elements
If T and D are known accurately, modifications can subsequently be made to the spectral intensities ( I A I A , ) and Eauation 2 reduces to
-
If pure element standards are available for A, B, and all other elements under
consideration, then an atomic percentage of A or B can be computed from I*/le, IA-/IB- (the Auger current ratio from the pure element standards) and which corrects a “matrix factor,” pm, for differences in the backscattering coefficients and in the atom densities between the standards and the unknowns (5,6,17). Unfortunately, pure element (vacuum-compatible) standards are difficult, or even impossible, to obtain for many solids of interest. An alternative, recommended whenever possible, is to use standards that are near the suspected composition (and have the same matrix) of the unknown. The relative atomic ratio or atomic percentage of the unknown ( N d N B ) , k is computed from the measured Auger signal ratios for the unknown and the sMdard(IA/IB),,”k and ( I A ~ I B )respectively~~~, and from the stoichiometry of the standard, (NA/NB).td, according to
x (N$N,),
= (IA/IB),,”k
(NA&),k
(IBIIA)atd X
(4)
or, for atomic percentage (XA)calculations, XA(uok) =
Iilatdl
IA(unk)
1
p x - x IihkI
IA(std)
,
(5)
r Electrons from
XACW
Another option is the correction of the measured intensity ratios, using sensitivity factors for each element, that are calculated, measured, or estimated from commercially available tables (18).The best approach is to measure these sensitivity factors so that they are specifically correct for the analyzer, operating conditions, and sample matrices that are of interest. There is still considerable risk in the use of sensitivity factors obtained from other instruments and laboratories. For X-ray photoelectron spectroscopy and X-ray excited Auger electron emission, there are similar considerations for the quantitation of composition of homogeneous solids. The range of kinetic energies of the photoelectrons emitted from the solid is similar to Auger spectroscopy,90 the sampling depths are comparable. Backscattering does not enter into the production of photoelectrons, hut there are more critical angular dependencies involving the angle of the source to the sample normal and the takeoff angle of analysis (6).A relationship similar to Equation 2 can be written for relative intensityratios, which includes small corrections for angular asymmetries (L) (7),
m w
/”
.
.
Electrons f m
*-substrate,
scattered in overlayer
.
.,.,....
.
Figure 2. Electron emission events In the near-surface region. (a) A W eladmn M photosladmn e m ~ h m lram monolayer W e S On the awlace. (b) scanasd u nonaoattsred eledm! m W n . and (0)eI&m mmed from the ovalaya u smlned from the 8ubsmte and man 6calmr.d In the w(xIayw.
, “A(E,) %EA) IA/IB= -x -X I
~ B B ( E & XM(K~)
LA/LBx NA/NB
(6)
where most of the terms have the definitions assigned above and a is now the photoionization probability (values tabulated from Scofield’s calculations or Wagner’s sensitivity tables generally are used) (19, 20). Equations 4 and 5 are once again more convenient if standards are available. Another factor of importance in both AES and XPS is that the original pho-
toionization event does not always lead to a single spectrwcopic peak. The kinetic energy of the detected photoelectron is dependent on the energy difference between the initial state (before photoemission) and final state (after photoemission)of the atom and its surroundings. Photoionization of electrons from p, d, and f orbitals always leads to two peaks (because of spin orbit coupling inthe finalstate). Relative intensities (R) are predicted by the ratio of the quantum numbers 6 = 1 s) for those orbitals, R = (2j l)/(Zj’ 1). If the peaks are sufficiently resolved
+
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ANALYTICAL CHEMISTRY, VOL. 81, NO. 7, APRIL 1, 1989
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0 5 eV apart), only one is used in q h i titative analysis; otherwise, the collective area of both peaks should be used. The S ( ~ P I / Zlines . ~ / ~(unresolved) ) and the Sn(Sd,/z,,n) lines (resolved) in Figure 4h on p. 476 A illustrate examples of this phenomenon. Photoelectrons may also gain or lose energy at the site of photoemission through shake-off or shake-up processes (common for some transition metal compounds). From Eguations 2,3, and 6, it is clear that for homogeneous materials where no standards exist, reasonable quantitation might still result if good estimates can be made for the variables in those relationships. However, the analyst must be assured of four things. First, the probability of ionization of the Auger or photoelectron or a corresponding relative yield factor must be available or calculable, and reasonably accurate. Second, good descriptions are needed to quantitate electron backscattering processes in solids. Third, the electron transport (energy loss) processes in solids that determine the escape depth must be well described. Finally, the analyzer transmission efficiency and the detedor efficiency must be quantitatively described over the entire kinetic energy range of interest. Determinate (unidirectional) errors in any of these parameters can be expected. These errors will prevent the analyst from determining absolute concentrations of the analyte, but relative atomic ratio or atomic percentage calculations will generally cancel most of these errors. Factors that affect peak shapes h
AES and XPS Now the question becomes, “How does the analyst obtain accurate peak areas in both AES and XPS, with which to proceed toward quantitation?” This critical question, if answered properly, can lead to good quantitative characterization of many solid surfaces (2128). Unlike many atomic emission spectroscopies where the line shapes are relatively simple, the detected peaks in either photoelectron or Auger spectroscopy of solids can often be obscured by spectral background contributions from a large number of other electron emission events (described below) in the near-surface region of the sample, making peak area determination difficult. Some of these are shown schematically in Figure 2. Both Auger and pbotoelectrons may lose energy a t the emission site-an intrinsic energy loss process seen mainly in free electron metals-because of the relaxation of conducting electrons accompanying photoemission. It is also possible that
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472A
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7. APRIL 1. 1989
they lose energy away from the emission site, before escaping the solid-an extrinsic energy loss process. Thia energy loss is attributable to energy exchange between the emitted electron and the population of conducting electrons in the near-surface region of the solid (21). This last process includes the so-called plasmon loss processes that actually lead to new peaks, displaced up to 40 eV below the kinetic energy of the parent Auger or photoelectron peak (most pronounced for metal systems). All of these energy losses are the factors that limit the sampling depth of XPS or AES in solids (13). The spectral background in AES arises from the following: electrons orieinallv a t the kinetic energy of the so&e (tkically 2ooo-5OOO eV) that have been scattered from surface and suhsurface atoms and lost energy in the process, electrons arising from Auger emission events of higher kinetic energy than for the element of interest, asymmetry in the spectral line shape (to the low kinetic energy side of the peak) because of the aforementioned “intrinsic” energy loss processes a t the point of origin of the Auger electron in the solid, and “extrinsic” energy loss processes that arise from scattering events occurring between the suhsurface point of origin of the electron and the solidvacuum interface (29,30). For the fust two processes, the population of scattered electrons builds exponentially to lower kinetic energies, resulting in a “secondary cascade” on which the Auger electrons are superimposed (30). The analytically relevant data may easily be less than 1% of the total secondary emission from the sample. This fact is one of the principal reasons why the detection limit for most elements using this technique is not better than 1 part per thousand. Because Auger events produce an electron whose kinetic energy is independent of the source energy, the spectral properties of the electron beam or Xray source that create the Auger electron do not enter into the fmal spectrum. The resolution and transmission efficiency of the analyzer and the energy-dependent response of the detector, however, do play a role in determining the spectral shapes and relative intensities for AES data. These complications are so extreme that for years AES was exclusively conducted wing electrostatic analyzers whose pass energies were modulated, and Auger spectra were only presented in first-derivative form, after lock-in amplifier demodulation. The pitfalls of this approach to
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quantitation are well documented (6IO,22,23). For XPS, the spectral background is still a problem but is generally lower in intensity. The principal origins of this background are electrons from photoemission events of higher kinetic energy than the one of interest, which contribute to the cascade of secondary electrons; Bremsstrahlung emission from the X-ray source, which creates a continuum of photoelectron emieaion from the sample (largely missing when monochromatic X-ray sources are used); and the same extrinsic energy losses described above for Auger electrons. In XPS, the asymmetry of the Xray source can contribute slightly to the finalspectral line shape (a few percent asymmetry is introduced into the peak shapes) and can he accounted for ifit is considered important (31,32).As in AES, the resolution and transmission efficiency of the analyzer play a role in the signal-to-backgroundratios as well as the spectral line shapes and relative intensities of each spectral peak of interest.
tensities of derivative peaks, can be directly measured. This often requires the use of much smaller beam currents (80 as not to saturate the electron multiplier detector) and, as a bonus, electron beam damage problems in AES can be made less severe. Methods for correct peak area measurements in AES and XPS are summarized in the panels of Figure 4, where the major Auger and photoelectron transitions for a well-characterized material, SnSz, are taken through a series of modifications to the final, corrected (ready-for-quantitation) form (28). SnSz is one member of a family of transition metal dichalcogenides (Table I has more examples) that appear to provide good standards with which to test the various AES and XF'S data treatment procedures now under development. These materials are van der Waals solids and layered structures and are readily cleaved to produce homogeneous, stoichiometric, chemically inert (and therefore almost always con-
Determhatimot the analykally relevant Au@ercn photoelectnm Intensity
In AES, the practice of presenting and analyzing spectral data in the first-derivative mode is still widespread for qualitative and semiquantitative analysis. Examples of this are shown in Figure 3 for Auger spectra of Sn in a SnOz thin film and as a component in an indium-tin oxide thin film. (Similar and more detailed examples are availablein Reference 22.) The peak-to-peak intensity of the Sn, 0, and In Auger signals is related to the actual peak areas for these spectra, but it also contains contributions attributable to the secondary electron background, separate energy loea peaks, and the analyzer transmission efficiency and detector efficiency functions. In addition, the Auger signal in the spectrum of the indium-tin oxide thin film appears to he well resolved-an artifact of the mcdulation/demodulation process that produced these derivative mode peaks. The actual Sn concentration is less than is apparent from the peak-topeak intensity in that spectrum (22). Substantial line shape changes are observed in these spectra as the oxidation states of the metals change, making the use of these derivative mode data in Equation 2 or 3 even less desirAll of the recently proable (6,22,33). duced commercial spectrometers acquire data for both AES and XPS using electron-countingtechniques. For electron beam excited Auger spectra, this means that peak areas, rather than in474A
I
I
1M)
2w I
-
Figure 3. Auger spectra of Sn in (a) indium-tin oxide thin film.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
taminant-free) surfaces (34-36). The Sn(MNN) and S(LMM) Auger transitions cover a wide kinetic energy range and are distorted by all of the spectrometer and sample-related energy loea processes described above. Step 1in Figure 4a calls for the removal of the kinetic energy dependence of analyzer transmission efficiency. This would seem to be a trivial step at first, but it has proved to be complicated in many instances. The present generation of bemispherical electron energy analyzers, with their associated electrostatic collection lenses ( I ) , can generally operate in a constant pass energy mode (AE= constant, normally used for XF'S) or a constant relative resolution mode (AE/ E = constant, normally used for AES) that yields different analyzer transmission efficiencies as a function of electron kinetic energy (5-10). It is necessary to know exactly bow this transmission efficiency varies in order to properly correct the data in that first
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step. In the constant AE mode, the image of electron emission from the Sam. ple must completely fill the entrance slit of the analyzer at all energies to ensure a constant transmission efficiency function. Many surface analysts are still not aware of this problem. The situation is further complicated because many instrument manufacturers find it difficult to fully characterize their analyzer systems, especially in the constant pass energy mode of operation. Needed improvements in this critical area are anticipated. Alternatively, during operation in the AEIE = constant mode for both A E S and XPS, the analyzer transmission efficiencygenerally is directly proportional to the kinetic energy of the detected electron over a wide kinetic energy range and is most useful for quantitative purposes. The spectral intensities need only be divided by kinetic energy to obtain the corrected form. Upon correction for transmission efficiency, the data are left superimposed on a secondary electron w a d e arising principally from electron sources at higher kinetic energies. Step 2 removes the contributions of bigber kinetic energy electrons to the cascade background by a simple linearization of the log-log form of the data and subtraction (22, 30). At this point the Auger spectrum represents the actual peak of interest, all of the peaks caused by discrete energy losses, and the large background attributable to Auger electrons, which lost appreciable energy on the way out of the solid and no longer appear as a peak of any sort (see Figure 2). In Figure 4b we pick up the two XPS peaks after having already carried out Steps 1 and 2 (not shown, although similar manipulations are necessary). How to proceed at this point becomes a question of the level of accuracy desired by the analyst versus the computational time and effort expended to obtain that accuracy. The simplest option is to use either a straight line or a polynomial fitted function to describe the background (37, 38). Neither of these approaches gives a physically realistic result, and they may introduce considerable error in the final spectrum. An alternate approach is to assume that the background beneath the peak a t any kinetic energy is simply proportional to the scaled sum of intensities from spectral data at higher kinetic energies (integral background method) (39).This approach is simple, more accurate than the first methods, and available in virtually all of the software packages supplied with XPS systems. However, it still leaves some energy 476A
1 1
I
;BP 5
KiWc energy (ev)
Kinetic enetgl (ev)
Binding energy .(ev) .
.
.:
., BWng energy (ev)
',,,.
Flaure 4. Methods for correclion of peak area measurements in (a)AES and (bjXPS.
loss contributions in the spectrum that should be removed, and it may overcorrect for energy losses near the major spectral peaks (21,25,26). More sophisticated approaches involve deconvolution methods that correct the background, remove energy loss peaks, and can also enhance resolution and reduce noise (23,24,31,32). A separate experiment is necessary, as shown in Step 3, Figure 4. An electron beam is reflected off of the sample at the kinetic energy of the spectral peak of interest to create an electron energy loss spectrum (EELS). The energy of this electron beam is too low to cause Auger emission in the vicinity of either the AES or XPS peaks. One obtains a large peak a t EO,represented by electrons e l a s t i d y scattered (without energy l a ) from the sample, and resolved peaks and a background arising from electrons originally at Eo, which penetrated the subsurface region, were scattered with loss of energy and then escaped the solid. The portion of the spectrum away from the peak at EO can be used to model the energy losses experienced by Auger or X-ray photoelectrons generated in this same subsurface region. The EELS data also experience the same analyzer broadening as the XPS or AES spectra. Thus we often call the EELS data a sample instrument re-
ANALYTICAL CHEMISTRY, VOL. 61. NO. 7, APRIL 1, 1989
sponse function (SIRF), because most of the information not of interest for quantitation, arising from both the sample and the instrument, is contained in these data. The data seen a t Step 3 of Figure 4 can be described to a first approximation as a convolution of the SIRF and the desired spectrum. Iterative or Fourier domain deconvolution procedures can then be applied, although the Fourier domain method is often favored for its speed, flexibility, and noise suppression characteristics (24). One can take advantage of the fact that convolved functions in a data (time or energy) domain (i.e., the data of Figure 4, Step 4, (g(x)) and the EELS (f(x)) can be deconvolved by Fourier transformation of both functions,
5
g(x) €4 f(x) G(X) x F(X) (7) followed by division of the data function with the EELS function in the Fourier domain and then an inverse transform back to the data domain (Equations 7,8, and 9).
This type of approach is similar to what one can use to treat most forms of
spectroscopic data (UV-vis, FT-IR, NMR,etc.), mainly for resolution enhancement (40-42). In this case, however, we measure the instrument response (similar in some respects to the slit function in optical spectroscopies) rather than guess or model it. The final forms of the Auger and XPS data are shown in Figure 4, Step 5. Resolution enhancement is slight for the Auger data because the Auger emission process inherently leads to broad spectral peaks. Certain precautions must be taken with the EELS data when deconvolving XPS spectra, because the fwhm of the electron beam may be comparableto the fwhm of a photoelectron peak. The asymmetry in energy of the electron beam may also need to be accounted for, because its shape will be reflected in the fmal form of the XPS data. These corrections can be readily incorporated into the data reduction scheme just described. For quantitation purposes, however, the data are now in a form whereby reliable peak areas can be measured.
Detwmlnationof relatlve atomic
raUos OT atomlc percentages h homogeneoussolids After each spectral peak has been treated in one of the ways described above, the computation of the atomic percentage of a particular element becomes straightforward, provided one of the above equations (Equations 2,3, or 6) can be used. Table I shows how good the agreement can be between expected compositions and those actually determined for many metal oxides, metal sulfides, and sulfur oxyanion salts. These may seem a t first like trivial analytical situations, but determinations like these have actually been a challenge for the surface analysis community. To date, little trace &e., part-perthousand) analysis has been attempted using any of the data treatment procedures described. Many real-world surface analytical problems, however, involve high elemental concentrations, albeit in a small near-surface region. To date, concentration determinations have been difficult when the spectral peaks are closely spaced (less than 5 eV) and background adjustments are difficult, when the spectral peaks are widely spaced in kinetic energy (i.e., greater than 500 eV) 80 that inaecuracies in the estimates for Tand D are not canceled completely, when the sample has a large contaminant overlayer whose thickness is unknown, or when the sample is not truly homogeneous throughout the sampling depth. One very interesting and relevant case, not involving homogeneous sol47781
ids, is the characterization of a solid with a thin overlayer. The overlayer thickness is typically greater than monolayer but less than the total sampling depth of the electron spectroscopic technique (e.g., a thin oxide layer over a metal or semiconductor, Figure Zc) (6-10,21). As device dimensions shrink in materials used in microelectronics, information storage, and other technologies, this problem is encountered with increasing frequency. One generally wishes to obtain an estimate of the overlayer thickness, the concentration of all elements within the overlayer and their oxidation states, and the concentration and oxidation states of all elements in the substrate material. AES can be used in this type of analysis, but XPS, with its higher spectral resolution, is ultimately more useful if the overlayer contains the mme elements as the substrate, but in different oxidation states (as in oxide overlayers). The direct deconvolution approaches described above are not always applicable to t h i kind of characterization. A combination of deconvolution and spectral fitting is more useful to fully analyze the XPS spectra from the overlayer and the substrate (21, 26). Careful attention to the shape of the background for a spectral peak from the substrate material can provide useful information about the overlayer thickness and about whether the overlayer is continuous or has formed in isolated islands (4345). This kind of overlayer analysis, combined with sputteddepth profiing techniques, is complementary to Rutherford back-
scattering spectrometry, which has a lower thickness limit for characterization of most overlayers of approximately 100 A (46).
Fuwe Pmepec(s Will surface spectroscopies ever lend themselves to absolute concentration determinations? Probably not in the near future. However, the questions to be answered by the surface analyst rarely require that degree of rigor. We probably can look forward to a wider variety of "routine" matrix correction factors and iterative approaches to concentration determinations, as were developed early on for X-ray fluorescence analysis of solids (17,47,48).Because of the extremely small sampling depths involved, the analyst will always have to be attentive to factors that are ignored by the user of spectroscopic probes with larger sampling depths. An encouraging sign is the continued interest in this area (e.g., research carried out hy the National Institute of Standards and Technology, formerly the National Bureau of Standards, and activities of the ASTM E-42 committee as well as research being done a t the National Physical Lahoratories in the United Kingdom and in other organizations in Europe). Using well-characterized materials against which to test data treatment methodologies, we expect that the capabilities for quantitative analysis of most vacuum-compatible solids with Auger spectroscopy and XPS will rapidly improve. Much of the research reported here w m supported by grants fmm the National Science Foundation, Department of Energ-Sandia National Labora-
Table 1. Relative atcnnk ratios from AES and XPS Examination of materlalswtthknownqion Expected
Yaterlab (1.cmique)
.lOlchld*
Metal a i m , o x i k , and sulhr oxyanlon salts TI02 (AES) om = 2.0 OlSi = 2.0 SlOn (AES) 01s = 4.0 01s = 3.0 01s = 1.5 OIS = 2.0
ANALYTICAL CHEMISTRY. VOL. 61, NO. 7, APRIL 1, 1989
D.tm1n.d .loichtotn.hy
1.9 2.08
Ref.
23 27 24 24 24 24 28 28 28 28 28
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Delivered PerformanceThe PHI Advantage Ofcourse,a multi.technique system can onlybe as good the techno~ogyof its elements.And thatkwhere the PHladvantage comes in. The 5500 combinesthe delivered Sometimes One techniquejust can't provide performanceof PHFs ESCA,SIMS and Auger enough answen to difficult surface-related technologiPj into one system dedicated to problems. With the 5500 from PHI, the analyticalneeds. capability to employ several crucial surface Our high performance ESCA series, the analysis techniques is available in one foundation for the 5500, delivers the versatile package. quantitative chemicalanswers you need. By adding the worlds highest resolution x-ray monochromator. optimal spectral quality is made possible. For excellent detection sensitivityin the ppm-ppb atomic mnge, the 5500 can be expanded
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with the 5500, PHI provides Solutions Rrkin.tlmer, 6509 F
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!s,and the O p t i d Data Storage Center and the Materiala Characterizetion Ragram at the Universityof Arizona. MaoyofthemetsldiehaleosenidsaamplesusedinourstudiDaweresgiftfr~m B l u e Parliinaon, Du Pont
Reterences
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(1) Cardella, J. Anal. Chem., in press. (2) Hercules, D. M. Anal. Chem. 1978.50,
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13) Fay, M. J.; Proctor, A,; Hoffman, D. P.; Hwc@es, D. M. Anal. Chem. 1988, SO, 1YZ5 A.
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Abma, H. D.; White, J. H.; Albarelli, M. J.; Bommarito, G. M.; Bedzyk, M. J.; M. J. Phys. Chem. 1988. 92,
14)
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15) Powell, C. J.; %ah, M. P. Surf. Interace Anal. 1986.9, 79. (6f %ah. M. P. In Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D.; %ah, M. P., E&.; John Wiley and Sons: New York, 1983, pp. 181-216. (I) %ah, M. P. J. Vac. Sci. Teehnol. A, 1985,3,1330. (8) %ah, M. P. Vacuum 1986.36,399. 19) Powell, C. J. Surf. Interface Anal. 1988, 11,103. (10) Grant, J. T.; Williams, P.; Fine, J.; Powell, C. J. Surf. Interface Anal., in
press.
Send foryour FREE 8page Condensed Catalw today!
Whether you're involved in ground-breaking research, product development, or QC, you can count on electrochemistryfor precise answers-and you can depend on EG&G PARC for the right combination of instruments, technical support and applications know-how. Gain the Power of a ComputerAssisted Laboratory Streamline your compound characterizations, analytical measurements, biological investigations, process development, mechanistic studies, and more. The Model 270 Electrochemical Analysis System is fast, precise, and easy to use. Select Standard or Expert mode, and make use of more than 20 basic electrochemical techniques. Plus, you can cost-effectively expand into corrosion, impedance, and customized elecimchemical measurements.
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(11) Gryeinaki, M. Phys. Rev. 1965, 138, A366. (12) Icbimura, S.; Shimuza, R. Surf. Sei. 1981,112,386,1985,124, L49. (13) Penn, D. J. Electron Spectroscopyand Related Phenomena 1976,9,29. (14) Tanuma, S.;Powell, C. J.; Penn, D. sur Scr 1981 192, m 9 . 115) face kAnal. ah,'M 1979, P.IDencb, . I , 2. W. A. Surf.Inter-
Detect a Wide Variety of Compounds The Model 3848 Voltammetric Analyzer is extremely versatfieenabling you to automatically determine trace concentrations of ions, metals, and inorganics. Accuracy and repeatability are assured through low noise electronics and microprocessor-controlled operation.
(16) Schoeffel, J. A.; Hubbard, A. T.Anal. Chem. 1917.49.2330. 117) Hall, P. M.; Morabito, J. M. Surf. Sei. 1979,83,391. (18) Davis, L. E.; MacDonald, N. C.; Palm-
burg, P. W.; Reach, G. E.; Weber, P. E. Handbook of Auger Electron Spectroscoy ; Perkin-Elmer Corp.: Eden Prairie, 1976. (19) Scofield, J. H. J. Electron Spectmscop y and Related Phenomena 1976,8,129. (20) Wagner. C. D. In Quantitatiue Sur ace Analysis of MateriaLp; McIntye,, N! S., Ed.; Amencan Society for Teaand Materials: Philadelphia, PA, 1978;ASTM STP 643, p. 31. 121) Masebboff, B. L.; Nebesny, K. W.; Zavadil, K. R.; Fordemwalt, J. W.; Armstrong, N. R. Spectrochim. Acta, Port B 1988,438,536. (22) Burrell. M. C.; Kaller, R. C.; Arnstrong, N. R. Anal. Chem. 1982,54,2511. (23) Burrell, M. C.; Armstrong, N. R.Appl. Surf. Sci. 1973,17,53. (24) Nebem ,K W , Armstrong, N. R. J. Electron &ec&osf,y and Related Phenomena 1986,37,355. (25) Maschhoff, B. L.; Zavadil, K. R.; Nebesny, K. w.; Armstrong, N. R J. Vac. Sei. Technol. 1988, A6,907.
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Analyze the Most Complex Compound The Model loo, LCEC detector offers femtogram sensitivity providing state-of-the-art technology for your catecholamineanalyses. Pulse mode lets you detect compounds such as carbohydrates. Your methods development is streamlined with the prescan feature.
APPLIED RESEARCH ~
(26) Maschhoff, B. L., Pb.D. Dieaertation, University of Arizona. 1988. 127) Burrow, B. J.; Armstrong. N. R.; Bun-
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ker, B.; Quinn, R. K.; Salmi, D. Appl. Surf. Sei. 19&1,20,161. 128) Nebeany, K. W.; Armstrong, N. R., submitted for publication in J. Vae. Sei. Technol. 129) Tougaard, S.; Sigmund, P. Phys. Reu. B 1982,25,4452. 130) Sickafua, E. N. Surf. Sei. 1980, 100,
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Circb 42 for literahrs. Ciccb 43 to have a representative call. ANALYTICAL CHEMISTRY, VOL. 61. NO. 7, APRIL 1, 1989
13iiHam,D. D.; DeKoven, B. M.
surf.
~
Interface AM^. 1987,10,63. (32). Koenig, M. F.;Grant, J. T. J. Electron Spectroscopy and Reloted Phenomena
1984,33,9. (33) Seah. M. P.; Hollmurn, M. W. J . Election Spectroscopyand Related Phenomena 1981.42.255. (34) Stickney.J. J.:Rosaam.S. D.;Schardt.
B. C.; Solomon,T.; Hubbard, A. T.; Parkinson, B. A. Surf. Sei. 1984.136.15. (35) Sarid. D.; Henson, T. D.; Armstrong. N. R.; Bell. L. S. Appl. Phys. Lett. 1988.
52,2252. (36) Tributaeh. H.Structure and Bonding; 1982.49, 128. (37) Proctor, A.; Hercules, D. H. Appl. Spectiox. 1984.30.505. (38) Shewood! P.M.A. In Practical Surface Analysts; Big@, D.; Seah. M. P.. Eds.; John Wiley and Sans: New York, 1983, pp. 445-76. (39) Shirley. D. A. Phys. Rev. B 1972. 5,
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Brigham. E. 0. The Fast Fourier Translorm; Prentice Hall New Jersey,
(40)
1974. (41) Horlick. G. Anal. Chem. 1972,44943. (42) Lam, R. B.: Wiebolt, R. C.; Isenhour,
T.L.A~l.Chem.I98l,53,887A. S. Surf. Interf. Anal. 1986.
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(44) Tougaard, S. J. Vac. Sei. Technol. 198'7. A5,1230,1275. (45) Tougaard. S. Appl. Surf. Sei.. in preen. (46) Chu. W. K.; Mayer. J. W.; Nicolet.
M.-A. Backscattering Spectrometry; Aeademic Press:New York. 1974. (47) Beamen, D. R.: Isai. J. A. Electron Beom Microanalysis; American Society for Testing and Materials: Philadelphia, 1972; ASTM STP 506. (48) Heinrich, K.F.J. AnaLChern. 1972.44, 350.
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Kenneth W. Nebesny (left J rrwicrd his R.A. degrw in chemistry from Qurrns
College, City Uniuersity of N e u York, and his Ph.11. in analytical chemistry from the Uniuersity of Arizona in 1984. Currently he is the research specialist in surface science in the Department of Chemistry, Uniuersity of Arizona. His research interests include quantitative surface analysis, surface crystallography, and new methods for deposition of nouel thin-film materials. Brian L. Maschhoff (center) receiued his B.S. degree in chemistry from the Uniuersity of New Mexico in I981 and his Ph.D. in analytical chemistry from the Uniuersity of Arizona in 1988. I n addition, he spent two years at Los Alamos National Laboratories. He is now a postdoctoral fellow a t Rutgers Uniuersity. Neal R. Armstrong (right) receiued his Ph.D. from the Uniuersity of New Mexico in 1974 after completinggraduate research at Sandia National Laboratories. Following a postdoctoral fellowship a t Ohio State Uniuersity and a facultyappointment a t Michigan State Uniuersity, he joined the faculty at the Uniuersity of Arizona, where he is professor and head of chemistry. His interests are in the areas of surface chemistries and electrochemistries of actiue metals, deuelopment of quantitative surface analysis methodologies, and the deuelopment and characterization of new molecular electronic materials for photoelectrochemical, chemical sensor, and nonlinear optical applications.
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