XP Area

When designing an XP area, all pieces of electrical equipment and instrumentation are specified for the class, group, and division, as discussed previously. All pieces of equipment and instrumentation within an area must be appropriately specified and installed. The overall classification is only as good as the piece of equipment in an area with the lowest classification. 

Develop a list of steps needed to convert a common kitchen into an XP area. 

Fig. 4.27. Integrated O Is and N Is XPS areas for four different samples, a native oxide on the Si3N4 membrane, a low-power (SOW) plasma oxidation, a high-power (250 W) plasma oxidation, and a 0.4- xm thick wet-oxidized Si wafer (Si02). The inset table gives the Auger parameter for all four samples. See text for further...

Figure 14.35. Plot of IR peak area vs. difference in XPS area. [Adapted, by permission, from Ohwaki T, Ishida H,J. Ad/iesion, 52, Nos. 1-4, 1995, 167-86.]...

The Fp correction factor for each enthalpy interval depends both on the assumed value of Xp and the temperatures of the interval on the composite curves. It is possible to modify the simple area target formula to obtain the resulting increased overall area A etwork for a network of 1-2 exchangers ... 

Subscript i identifies species, and J is a dummy index all summations are over all species. Note that Xp however, when i = J, then Xu = = 1. In these equations / (a relative molecular volume) and (a relative molecular surface area) are pure-species parameters. The influence of temperature on g enters through the interaction parameters Xp of Eq. (4-261), which are temperature dependent ... 

An XPS spectrometer schematic is shown in Figure 7. The X-ray source is usually an Al- or Mg-coated anode struck by electrons from a high voltage (10—15 kV) Alka or Mgka radiation lines produced at energies of 1486.6 eV and 1256.6 eV, with line widths of about 1 eV. The X rays flood a large area (-I cm ). The beam s spot size can be improved to about lOO-jim diameter by focusing the electron beam... 

More detailed discussions of XPS can be found in references 4-12, which encompass some of the major reference texts in this area. 

Modern XPS spectrometers employ a lens system on the input to the CHA, which has the effect of transferring an image of the analyzed area on the sample surface to the entrance slit of the analyzer. The detector system on the output of the CHA consists of several single channeltrons or a channel plate. Such a spectrometer is illustrated schematically in Fig. 2.6. 

A principal disadvantage of conventional XPS was lack of spatial resolution the spectral information came from an analyzed area of several square millimeters and was, therefore, an average of the compositional and chemical analysis of that area. Many technological samples are, on the other hand, inhomogeneous on a scale much smaller than that of conventional XPS analysis, and obtaining chemical information on the same scale as the inhomogeneities would be very desirable. 

XPS has been used in almost every area in which the properties of surfaces are important. The most prominent areas can be deduced from conferences on surface analysis, especially from ECASIA, which is held every two years. These areas are adhesion, biomaterials, catalysis, ceramics and glasses, corrosion, environmental problems, magnetic materials, metals, micro- and optoelectronics, nanomaterials, polymers and composite materials, superconductors, thin films and coatings, and tribology and wear. The contributions to these conferences are also representative of actual surface-analytical problems and studies [2.33 a,b]. A few examples from the areas mentioned above are given below more comprehensive discussions of the applications of XPS are given elsewhere [1.1,1.3-1.9, 2.34—2.39]. 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

Corrosion products formed as thin layers on metal surfaces in either aqueous or gaseous environments, and the nature and stability of passive and protective films on metals and alloys, have also been major areas of XPS application. XPS has been used in two ways, one in which materials corroded or passivated in the natural environment are analyzed, and another in which well-characterized, usually pure metal surfaces are studied after exposure to controlled conditions. 

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

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. 

As discussed earlier, the XPS technique is quantitative. If A, is the area under the peak in the spectrum that is characteristic of element i, then the atomic concentration of element i on the surface of the sample is given by the expression ... 

Fig.. 86. Photograph of the metal initiation zone of a lap joint preptired from hot-dipped galvanized. steel substrates showing the six locations where small-area XPS spectra were acquired. Reproduced by permission of John Wiley and Sons from Ref 411.

However, the spatial resolution of AES is mueh greater than that of XPS and ean approaeh approximately 25 nm. This makes AES a powerful technique for constructing high-resolution maps showing the distribution of chemical species across a surface. Because of the small analysis area, it is an easy matter to combine AES with inert gas sputtering to construct depth profiles showing the distribution of chemical species as a function of distance away from the surface and into the bulk of the solid. Quantitative analysis can be done using sensitivity factors and an equation similar to Eq. 17. 

For a plot of xp = 0.750, slope = 0.7875, read xyy at the equilibrium line for each theoretical tray and plot similar to Figure 8-38. Then determine the area under the curve between the selected x and the product x. Then ... 

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