Depth selective information

Mossbauer Spectral Analysis and Analog Measurements. Mossbauer spectra were obtained in the temperature range between 200 and 270 K and in two different energy windows (14.4 and 6.4 keV), which provide depth selective information about a sample [346]. To compensate for low counting statistics due to limited integration time, all available spectra were summed for the integrations on the undisturbed and brushed surface, respectively. In addition to kamacite (a-(Fe,Ni)) ( 85%) and small amounts of ferric oxide (see Fig. 8.38), all spectra exhibit features indicative for an additional mineral phase. Based on analog measurements... 

XPS is among the most frequently used techniques in catalysis. It yields information on the elemental composition, the oxidation state of the elements and in favorable cases on the dispersion of one phase over another. When working with flat layered samples, depth-selective information is obtained by varying the angle between sample surface and the analyzer. Several excellent books on XPS are available [5,8,17-20], In this section we first describe briefly the theory behind XPS, then the instrumentation, and finally we illustrate the type of information that XPS offers about catalysts and model systems. 

Since the excitation depth can be selected by varying the electron-beam energy, depth-resolved information can be obtained. 

There are a vast number of possible recipes to derive information-theoretical indices. In-depth discussions of selected information-theoretical indices, also referred to as information content indices (ICI), appear elsewhere [17-19]. 

Q.) is based on the ejection of the recoiled particles out of the sample in the forward direction by an energetic heavy ion beam. The measured energy spectra of these recoiled atoms can be related to their concentration profiles. The use of range foil in front of the energy detector to permit selective absorption of the various recoils introduces a few limitations in the application of the technique, e.g. deterioration of the energy resolution and hence the depth resolution, the limitation on the accessible depth in the depth profile information, etc. Indeed, the practical utility of the experimental set-up is enormously reduced in the region where overlapping spectra of various atoms are difficult to separate. 

MEIS, by contrast, is frequently used to achieve depth profiling information with close to monolayer resolution. The energy of the incident ion beam is in the order of 100 keV. At such energies, the shadow cone radius is relatively small and incident ions are able to channel hundreds of nanometres into the bulk of a crystalline lattice. It is possible, with careful sample alignment, to selectively illuminate a given number of surface layers (see below), in which case one may achieve layer by layer compositional information as a function of depth. 

SACX was selected as it represented the best value and the lowest total cost of ownership due to better yields and wetting, lower dross rates and improved thermal fatigue resistance. It is clear that a comprehensive experiment design, in-depth and informed analysis conducted by a knowledgeable multi-function team is required to make this selection. However, this investment in pre-production process design for lead-free PCB assemblies will pay dividends in designing a process that delivers the lowest total cost of ownership throughout the life cycle of an electronic device. 

Figure 8.5 shows the LEIS spectra of ZnAl204 and ZnO as a characteristic example of a multicomponent system analyzed by this technique [Brongersma and Jacobs, 1994]. Since only the surface peaks of A1 and O were detected for ZnAl204, the Zn atoms must be located in the subsurface layers. The onset of the tail agrees between the spectra, indicating that Zn is present in the second and deeper layers. This example illustrates the strength of the LEIS technique, in that characteristic peaks from different elements can be used to selectively analyze the atomic composition of the topmost surface. In addition, the shape of the tails could provide information on the in-depth distribution of the elements. 

The data chain of the collected atoms can be converted to a one-dimensional composition-depth profile. The depth profile shows an average concentration of solute within the aperture, and there is always a possibility that the chemical information from the selected area is a convolution of more than one phase, as indicated diagrammatically in Figure 1.5, which represents the analysis of a FIM specimen containing second phase particles and also an interface across which there is a change of composition. 

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