23.5 · Best Practices for Analysis of Rough Bulk Samples

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background, making it more difficult to discern the position of the bremsstrahlung maximum. Nevertheless, the general shape of the EDS bremsstrahlung continuum is a powerful indicator of geometric effects that modify X-ray production, both bremsstrahlung and characteristic X-rays.

.  Figure 23.13 shows this effect for ideal flat polished K411 glass and several shards. The deviation in spectral shape from the ideal case is readily apparent, and analysis of spectra with such severe deviations in shape results in large deviations in the relative accuracy of the normalized quantitative results.

23.5\ Best Practices for Analysis of Rough Bulk Samples

The optimum approach to the analysis of a rough specimen is obviously to prepare a polished flat surface, but the analyst may be confronted with a situation where no physical modification of the as-received specimen is permitted. That is, the rough surface itself is the object of interest, so that grinding

and polishing would modify or destroy the material that is actually necessary for the final result. How should the analyst proceed in such a case?

The analysis of rough surfaces is inevitably going to be compromised compared to analysis of the ideal flat polished specimen. The analyst must seek to obtain the best possible result under the circumstances, so the analytical strategy must be carefully considered. Electrons that backscatter off rough surfaces are likely to produce remote excitation of X-rays from material(s) that are likely to differ from the location where the beam is striking, as shown schematically in .  Fig. 23.14. It may be thought that the collimator on the EDS will restrict the view of the EDS to just the region directly excited by the incident beam. This is not the case. The collimator typically permits acceptance of X-rays with at least 50 % efficiency from an area that is 5 mm in diameter or larger. The exact transmission response depends on the particular EDS detector and its collimator, but the region of transmission can be readily determined by mapping a uniform target, for example, an aluminum SEM mounting stub, at the lowest magnification setting (maximum sized scan

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. Fig. 23.15  X-ray mapping experiment to determine extent of collimator acceptance. Large scale low magnification map (3 × 2.5 mm) of an aluminum stub. The graph shows the intensity measured at the center of a series of such maps recorded at different working distances

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field) of the SEM, as shown in .  Fig. 23.15. The aluminum stub was mapped with the specimen plane located at the manufacturer’s specified ideal working distance for this SEM/EDS. The Al intensity map encoded with a pseudocolor scale shows that the transmission varies from maximum (100 %) along the top of the image to values in the range 60–70 % at the bottom. The top-to-bottom asymmetry in this map reveals that the collimator on this EDS system is actually misaligned, since with proper orientation the maximum of the collimator transmission should be at the image center (coincident with the optic axis). The transmission function of the collimator as a function of vertical distance along the SEM optical axis can be determined by repeating the mapping at different working distances. The graph in .  Fig. 23.15 shows the intensity measured at the center of each map, revealing a decrease of approximately 40 % as the working distance was increased from 10 to 20 mm. This collimator thus allows high transmission from a large volume of space, with dimensions of at least

3 × 2.5 × 10 mm, so that any X-rays produced in this volume with a line-of-sight to the EDS detector will contribute to the measured spectrum.

Optimizing the EDS spectrum measured from a rough, irregularly shaped surface requires careful consideration of the selection of the location on the specimen to be measured.

The analyst must be aware of the location of the EDS relative to the measured location to avoid the situation illustrated in

.  Fig. 23.16a, where the beam location leads to an X-ray path that must pass through the bulk of the specimen to reach the EDS, leading to extremely high absorption. Ideally, using a specimen stage with several rotation axes, a rotation about a vertical axis will bring the feature of interest to directly face the EDS, thus minimizing the absorption, as shown in

.  Fig. 23.16b. A further rotation about a horizontal axis places the feature perpendicular to beam to minimize backscattering and remote X-ray excitation (.  Fig. 23.16c). Note that although backscattering is minimized by establishing normal beam incidence (effectively a zero tilt angle), the backscattered electrons are broadly emitted with a cosine distribution so that while the majority are emitted at high angles there still remains a small but significant fraction emitted at low angles to the surface that may strike nearby features and excite the surrounding materials, contributing to the spectrum measured at the beam impact position.