Spatial Resolution in XPS

The irradiating X-ray beam cannot be focussed upon and scanned across the specimen surface as is possible with an electron beam. Practical methods of small-spot XPS imaging rely on restriction of the source size or the analysed area. By using a focussing crystal monochromator for the X-rays, beam sizes of less than 10 pm may be achieved. This must in turn correspond with the acceptance area and alignment on the sample of the electron spectrometer, which involves the use of an electron lens of low aberration. The practically achievable spatial resolution is rarely better than 100 pm. A spatial resolution value of 200 pm might be regarded as typical, and it must also be remembered that areas of up to several millimetres in diameter can readily be analysed. 

The relatively poor spatial resolution of XPS compared, for example, with electron microscopy techniques such as SAM is more than offset by the benefit of concurrent chemical state identification. 

Detailed and authoritative reviews of experimental techniques, and also of the areas of application of XPS are given by Barr (1994) and Briggs and Seah (1990). The applications of XPS include the study of the following. 

1 The Natural Passivation and Corrosion of Metals and Alloys XPS studies of the air-formed natural passive layer on aluminium surfaces have identified a number of hydroxides as well as alumina (Barr, 1977). The oxidation of pure iron and of stainless steels and other iron alloys have also been extensively 

---

AES as well as XPS can be used for surface imaging, but the spatial resolution of AES is generally better. The reason is that the primary electron beam used for exciting the surface atoms in AES can be well focused on areas as small as a few nanometer. X-rays are more difficult to focus and additional techniques, not discussed here, are required to improve the spatial resolution in XPS, which for most instruments today is in the ten micrometer range. In order to acquire a chemical image an electronic guiding system scans the surface while the AES or XPS intensity of chosen peaks is measured and stored numerically. The chemical map of the surface thus produced... 

In general, XPS is the preferred technique for studying oxidation states and ligand effects. However, there is a good reason why one should be interested in deriving chemical information from AES whereas the spatial resolution of XPS is at best a few micrometers, Auger spectra can be obtained from spots with diameters as small as a few nanometers. It would be extremely interesting to have oxidation state information on the same scale as well ... 

It was only the use of larger metal particles deposited on these supports, up to 1 pm in size, comparable to the spatial resolution of XPS, that enabled researchers to understand the origin of electrochemical promotion and, at the same time, to discover the backspillover mechanism of metal-support interactions [137,138]. 

Historically, XPS instruments have relied upon the energy analyzer to define the analysis area because technology was not available to finely focus the X-ray beam, limiting the spatial resolution of XPS. A commonly used approach to improve the spatial resolution is shown in Figure 14.4, where an aperture is placed in the analyzer s input lens to restrict the analysis area. Only the photoelectrons from a given small area of the sample pass through the aperture and into the analyzer. The practical lower limit for small-area XPS analysis with this approach is 25 pm. 

AES has the following features 10-50 A analysis depth, 100 A spatial resolution, in-situ fracture analysis, large sample size, color maps and line scans. The AES provides depth profiling and formation about grain boundary particles. Like in XPS, there is a chemical shift, but the spectrum, in this case, is more complex because it involves three levels. 

One of the more recent advances in XPS is the development of photoelectron microscopy [ ]. By either focusing the incident x-ray beam, or by using electrostatic lenses to image a small spot on the sample, spatially-resolved XPS has become feasible. The limits to the spatial resolution are currently of the order of 1 pm, but are expected to improve. This teclmique has many teclmological applications. For example, the chemical makeup of micromechanical and microelectronic devices can be monitored on the scale of the device dimensions. 

SALI compares fiivorably with other major surface analytical techniques in terms of sensitivity and spatial resolution. Its major advantj e is the combination of analytical versatility, ease of quantification, and sensitivity. Table 1 compares the analytical characteristics of SALI to four major surfiice spectroscopic techniques.These techniques can also be categorized by the chemical information they provide. Both SALI and SIMS (static mode only) can provide molecular fingerprint information via mass spectra that give mass peaks corresponding to structural units of the molecule, while XPS provides only short-range chemical information. XPS and static SIMS are often used to complement each other since XPS chemical speciation information is semiquantitative however, SALI molecular information can potentially be quantified direedy without correlation with another surface spectroscopic technique. AES and Rutherford Backscattering (RBS) provide primarily elemental information, and therefore yield litde structural informadon. The common detection limit refers to the sensitivity for nearly all elements that these techniques enjoy. 

Like XPS, the application of AES has been very widespread, particularly in the earlier years of its existence more recently, the technique has been applied increasingly to those problem areas that need the high spatial resolution that AES can provide and XPS, currently, cannot. Because data acquisition in AES is faster than in XPS, it is also employed widely in routine quality control by surface analysis of random samples from production lines of for example, integrated circuits. In the semiconductor industry, in particular, SIMS is a competing method. Note that AES and XPS on the one hand and SIMS/SNMS on the other, both in depth-profiling mode, are complementary, the former gaining signal from the sputter-modified surface and the latter from the flux of sputtered particles. 

The primary drawback to the application of XPS in adhesion science is associated with the limited spatial resolution of the technique. This can make it difficult to study processes that are highly localized, such as corrosion, or to accurately characterize certain types of failure surfaces where, for example, the locus of failure may pass back and forth between two phases. 

We shall concern ourselves here with the use of an X-ray probe as a surface analysis technique in X-ray photoelectron spectroscopy (XPS) also known as Electron Spectroscopy for Chemical Analysis (ESCA). High energy photons constitute the XPS probe, which are less damaging than an electron probe, therefore XPS is the favoured technique for the analysis of the surface chemistry of radiation sensitive materials. The X-ray probe has the disadvantage that, unlike an electron beam, it cannot be focussed to permit high spatial resolution imaging of the surface. 

X-ray photoelectron spectroscopy is frequently applied in the fields of catalysis and polymer technology. It has poor spatial resolution, and is generally limited to homogenous samples. Radiation sensitive materials are more appropriate for XPS analysis, as the X-ray beam is less damaging to the specimen surface than the electron beam used in AES, partly due to the lower flux densities that are used. 

Auger electron spectroscopy is preferred over XPS where high spatial resolution is required, although the samples need to be conducting and tolerant to damage from the electron beam. Many oxides readily decompose under electron radiation, and this may give rise to difficulty in spectral interpretation, and this has restricted the application of AES in the field of catalysis. 

AES is similar to XPS in its function, but it has unparalleled high sensitivity and spatial resolution (of approximately 30-50 nm). Both AES and XPS involve the identification of elements by measurement of ejected electron energies. Fig. 2.12... 

Depth profiles of matrix elements on Mn- and Co-perovskite layers of fuel cathodes have been measured by LA-ICP-MS in comparison to other well established surface analytical techniques (e.g., SEM-EDX).118 On perovskite layers at a spatial resolution of 100p.m a depth resolution of 100-200 nm was obtained by LA-ICP-MS. The advantages of LA-ICP-MS in comparison to other surface analytical techniques (such as XPS, AES, SIMS, SNMS, GD-OES, GDMS and SEM-EDX) are the speed, flexibility and relatively low detection limits with an easy calibration procedure. In addition, thick oxide layers can be analyzed directly and no charging effects are observed in the analysis of non-conducting thick layers. 

The electron beam used as a probe in AES can be focused to analyze a very small area on the sample surface (diameter l-50p) 62 172). On the other hand, the spatial resolution that can presently be achieved with XPS is relatively poor since it is very difficult to focus the X-ray beam. Therefore, since AES and XPS techniques exhibit complementary strengths, they are often employed together to achieve an accurate determination of the locus of failure in adhesion systems. 

Consideration of Surface Analysis Concerns. The researchers in this study used a wide range of surface and other tools, taking appropriate advantage of the strengths of the various methods. Previous work had shown that AES and XPS could be used to study chromate films without unreasonable problems and provided a basis for the current study. XPS was used to obtain specific chemical information while AES was used whenever spatial resolution and electron imaging were desired. RBS and electron microprobe work was used to analyze composition structures of thicker layers. 

---