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Recent Developments in XPS Instrumentation enhancements have helped increase the power of XPS, but quantitation remains difficult Although X-ray photoelectron spectroscopy (XPS) is at a point in its history at which a state of technological maturity has been reached, progress continues to be made on a number of aspects of this important surface analysis technique. Recent advances have included improved control over the depth sensitivity of XPS analyses, enhancements in XPS detection and computer systems, and higher spatial resolution. Current progress and prospects for XPS were reviewed at the 1986 Eastern Analytical Symposium by Cedric J. Powell, chief of the surface science division at the National Bureau of Standards. Powell spoke at a session on "Modern Methods of Thin Film and Surface Analysis" that was chaired by Richard J. Colton of the Naval Research Laboratory. XPS is a surface technique that involves excitation by X-rays (frequently Al or Mg K« radiation) and subsequent emission of photoelectrons from core levels of atoms in the near-surface region of a sample. "By measuring the kinetic energy of these photo-emitted electrons with an electron energy ana-
800
lyzer," explained Powell, "and knowing the characteristic energy of the incident X-ray, we can determine the binding energy of the particular core electrons, and, from tabulated data, identify the element." XPS also is referred to as ESCA (electron spectroscopy for chemical analysis), a term introduced by Kai Siegbahn of the University of Uppsala, who won a 1981 Nobel Prize for his pioneering work in the field of electron spectroscopy (Siegbahn, K., Science 1982, 2J7[4555], 111-21). With characteristic X-rays from Al or Mg anodes, all elements in the periodic table except hydrogen and helium can be detected by XPS. But the real strength of the technique lies in its ability to provide information on the chemical environment of analyte atoms by measuring small chemical shifts that appear in elemental peaks (Figure 1). An additional strong point for XPS, according to Powell, is that "compared to other techniques using incident electrons or ions, damage caused by incident radiation is very small, though not insignificant for some materials." Although commercial XPS instruments have by now attained a high de-
600 400 200 Binding energy (keV)
gree of sophistication, considerable progress continues to be made on instrumentation improvements. For example, the application of more efficient X-ray monochromators has improved spectral resolution (particularly important for measurement of chemical shifts) and signal-to-background ratios. In addition, the advent of synchrotron radiation sources has facilitated tuning of the incident X-ray energy and consequently has improved control
important requirement... is to be able to look at the surface material with appropriate spatial resolution for the . . . specimen and problem.· % over surface sensitivity, the extent to which a surface technique is sensitive to analytes in the near-surface region. Traditionally, somewhat coarser control over surface sensitivity has been based on the selection of different anode materials in conventional X-ray sources. Greater control over surface sensitiv-
290 280 Binding energy (keV)
Figure 1. Grazing-angle XPS of a Kevlar fiber. (a) Survey scan and (b) carbon region, in which the split peak indicates the presence of two species in different chemical environments. (Adapted from Batich, CD.; Holloway, P.H.; Kosinski, M.A., CHEMTECH, 1986, pp. 494-99.)
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Figure 2. Schematic of the types of het erogeneities that may occur in speci mens for surface analysis. Shown are (a) surface grains that appear in vari ous patterns; (b) a submonolayer film resulting from the segregation of atoms from the bulk, con tamination from the vacuum ambient, or deliber ate deposition; (c) a thin film deposited on a sub strate; and (d) a surface with a composition that varies as a function of depth. Other important as pects of surface heterogeneity involve the possi ble presence of roughness and defects.
ity is afforded by the relatively recent addition of instruments that permit the user to conveniently vary the elec tron takeoff angle. As photoelectron takeoff trajectories approach more grazing angles, there is an increased emphasis on near-surface regions of the material. This geometrical approach is attractive because it doesn't require
transport of the experiment to a syn chrotron light source. In addition, electron energy analyz ers are now being marketed with more efficient lens systems that make it easi er to optimize the instrument for dif ferent operating conditions. The com mercial availability of instruments with parallel detectors has helped in crease the overall speed and efficiency of XPS analysis, and a wider range of modules for specimen processing, such as reaction chambers, is now available. As is the case for nearly all classes of analytical instruments, another recent improvement is that computer systems now provide more elegant and more powerful facilities for experimental setup and control, data reduction, and graphic data presentation.
practical materials. The repeatability (precision) of measurements may be good—10% or better—but statements of accuracy may be difficult or impos sible." There are several reasons for these difficulties, one of which involves the problem of specimen complexity. As shown in Figure 2, multiple surface phases may be present, and unless their signal contributions are untangled, a meaningful analysis cannot be ob tained. Variations in surface topogra phy, roughness, and crystallinity also may throw a monkey wrench into quantitation efforts. In addition, questions concerning the calibration of the intensity scale of
" More There have also been recent improvements in knowledge . . . (is) the lateral surface reso lution of XPS instru required in order ments. Higher lateral that surface analyses resolution makes it pos sible to analyze smaller of known accuracy areas of a surface, which is particularly impor can be made tant when specimens routinely." are known to be hetero geneous. Figure 2 shows some of the different types of heterogeneities that may be ten contribute to quantitation difficul encountered in real specimen surfaces. ties. "How do we make proper intensity "The problem comes up of how one can measurements?" asked Powell. " I t get meaningful information on the dif sounds simple, but it isn't." The accu ferent surface phases that may exist," rate measurement of intensities de said Powell. "An important require mands reliable estimation of the back ment here is to be able to look at the ground intensity under a peak of inter surface material with appropriate spa est and correction for the effects of the tial resolution for the particular type of inelastic scattering of secondary elec specimen and problem." trons as they exit the surface. "The background due to bremsstrahlung Recent enhancements in the lateral resolution of XPS instruments have photoemission and the secondary elec been achieved at the X-ray input stage, tron cascade can usually be determined where sharp-focusing X-ray monoby analytical extrapolation using vari chromators are used, and at the photoous mathematical functions," ex electron emission stage, where highplained Powell, "but the deconvolution resolution electron optical configura correction for inelastic scattering in the tions are used to select smaller vicinity of a peak is complex and not specimen areas. The present capability always possible." of commercial XPS instruments ranges Another quantitation problem con from approximately 50 to 250 μπι. This cerns correction of elemental sensitiv resolution is crude compared to some ity factors for matrix effects, specifical other surface techniques—lateral reso ly the dependence of the electron at lutions of 0.05-0.1 μπι have been dem tenuation length on type of material onstrated for Auger electron spectros and electron energy. These depen copy and secondary ion mass spec dences generally are not well known, trometry—but it is still useful for a although convenient, approximate for number of important applications. mulae are available as guides. Accord XPS has long proved its mettle as a ing to Powell, "It is clear that more powerful qualitative technique, but knowledge of basic processes, better Powell points out that quantitation has methodology, and additional reference not been its strong suit: "It is still not data and materials are required in or usually possible for the average investi der that surface analyses of known ac gator to make credible claims for the curacy can be made routinely." accuracy of surface analyses made on Stu Borman
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