Auger maps

The results shown in Figure 6 above are an example of this mode of analysis, but include additional information on the chemical states of the Si. The third most frequently used mode of analysis is the Auger mapping mode, in which an Auger peak of a particular element is monitored while the primary electron beam is raster scanned over an area. This mode determines the spatial distribution, across the surface, of the element of interest, rather than in depth, as depth profiling does. Of course, the second and third modes can be combined to produce a three-dimensional spatial distribution of the element. The fourth operational mode is just a subset of the third mode a line scan of the primary beam is done across a region of interest, instead of rastering over an area. 

This is illustrated in Eig. 2.30 [2.167]. The surface was that of a fractured compact of SiC to which horon and carhon had been added to aid the sintering process. The aim of the analysis was to establish the uniformity of distribution of the additives and the presence or absence of impurities. The Auger maps show not only very non-uniform distribution of boron (Eig. 2.30a) but also strong correlation of boron with sodium (Eig. 2.30c), and weaker correlation of boron with potassium (Eig. 2.30b). Point analyses for points A and B marked on the images reveal the presence of sulfur and cal-... 

Figure 7. Secondary electron micrograph and carbon (KLL) map of untreated catalyst A. photomicrograph obtained at 8kV, O.lnA primary beam B. corresponding carbon (KLL) Auger map obtained in (peak-background)/ background mode, at 8kv, lOnA primary beam.

Intergranular Corrosion. In order to identify the species present in the narrow crevices of intergranular corrosion paths in metallic materials, which may be only 1-2 pm wide, the use of an Auger map using SAM has been successful. 

In SAM the electron beam can be focussed to provide a spatial resolution of < 12 nm, and areas as small as a few micrometers square can be scanned, providing compositional information on heterogeneous samples. For example, the energy resolution is sufficient to distinguish the spectrum of elemental silicon from that of silicon in the form of its oxide, so that a contaminated area on a semiconductor device could be identified by overlaying the Auger maps of the two forms of silicon obtained from such a specimen. 

Figure 5. Iron foil after carbon precipitation at 620°C. Key a, laminar carbon film, region A, and carbon free regions C b, carbon precipitated regions, facetted carbon free regions, and intermediate stepped zone and c, carbon Auger map of area in b, (KLL 271 eV).

The Auger mapping was performed on a VG ESCALAB 200 using an electron gun, which allows 2000 A lateral resolution. The electron beam, of energy of lOkeV (inducing a specimen current of 20 nA), was rastered to produce 128 x 128 pixel maps, which were recorded on an IBM 486 computer. 

The Auger maps were corrected for topographical effects using the (peak-background)/background algorithm. 

In a subsequent study, Ertl et al reported on the overall surface composition of a reduced catalyst which had a bulk composition similar to catalyst 1 in the present study. The surface composition of the two samples is, however, vastly different. In particular, the iron content is lower than the iron content of the present sample, in the unreduced state. The levels of potassium and aluminum are, on the other hand, much larger. It was pointed out that the surface composition as determined was consistent with earlier chemisorption experiments, and led to the conclusion that ca 40% of the accessible surface should be covered with potassium oxide. If the potassium and aluminum oxides were indeed present, as suggested, in the form of surface films, then the bulk of the catalyst would consist predominantly of metallic iron. If a sampling depth of 2-4 nm for XPS is assumed, then the analysis presented in Ref. 19 would represent a catalyst which is largely covered by a multilayer of promoter oxides. Consequently, a very much smaller fraction than 60% of the surface would consist of free iron. In accordance with the SEM and Auger map images the activation procedure of Ertl et led to a surface of elemental iron, which is covered almost entirely by a layer of promoter oxides that contain occasional crystals of embedded ternary oxides. 

Figure 3.1. Auger maps from an industrial ammonia synthesis catalyst after reduction, showing the lateral distribution of Fe, K, Al, and Ca at the surface.

Figure 12. Auger maps ([peak-background]/background) recorded from the joint (in the area shown in Fig, 11) after 2 min Ar etching, for the following elements (a) Pb, (b) 0, (c) Sn and (d) Cl. (Reproduced from Ref. 8. by permission of Elsevier Science, Amsterdam.)...

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