Click here to go back to frontpage

Scanning Electron Microscopy with X-ray micro analysis

Durch Integration einer Elektronenstrahlmikroanalyse sind zudem chemische Analysen möglich. Damit kann eine Vielzahl von Fragestellungen an unterschiedlichsten Materialien beantwortet werden.


Scanning electron microscopy can be used to examine the surface of samples or objects, for example to visualize manufacturing marks, damages or other changes to the surface with a high depth of focus. This results in three-dimensional-like images. In addition, electron microprobe analysis (EMPA) can be used to determine the chemical composition locally in very small volumes, allowing the analysis of individual phases, local inhomogeneity or composite materials. This allows statements to be made about the composition and properties, from which in turn a wide range of information can be derived, e.g. about the genesis of the material. For most applications, however, suitable specimens must be prepared for the chemical analysis (thin or thin sections) to ensure a uniform geometry between the sample surface and the various detectors. This is the only way to achieve reproducible results. The scanning electron microscope (ZEISS EVO 60 MA 25) available at CEZA is equipped with a very large sample chamber (420 mm Ø x 330 mm) and, in addition to the usual high vacuum system, also has the possibility of working in a variable pressure range (10-400 Pa). Thus, objects can be examined in situ without sampling or preparation. The so called VP-mode (variable pressure) allows the examination of non-conductive, gassing or moist samples (textiles, wood, leather etc.) without prior preparation (e.g. vaporization), as the charges of the sample can be compensated by the individual partial pressure in the chamber. For electron microprobe analysis, an energy dispersive Xray spectrometry system (EDS) with a silicon drift detector (SDD) is connected to the SEM. The detector has a guaranteed resolving power of at least 127 eV at Mn-Kα and can process very high pulse rates, allowing e.g. the acquisition of elemental distribution images (mapping) in relatively short measuring times. The modular analysis system (BRUKER Esprit) allows turning the element distributions into a phase image after proper quantitative mapping.

Figure 1: Backscattered electron image of a lead slag from the Celtic oppidum Manching with copper oxide (dark grey) and a silver drop.
Figure 2: Elemental mapping of the slag shown in Fig. 1.


Scanning electron microscopy is a versatile technique that provides morphological images with high depth of focus and numerous analytical information. The method is based on the interaction between electrons and matter. Electrons are generated by an electron gun in the SEM under high vacuum and are finely focused via electromagnetic lenses to form an electron beam that scans the surface of a sample or object in a two-dimensional fashion. The interaction with the atoms of the material under investigation is manifold and leads to the simultaneous release of different signals. Of these, the generation of secondary electrons (SE) and backscattered electrons (BSE) are by far the most important. Secondary electrons have a low energy and originate from the uppermost nanometers of the material. They are detected with a corresponding SE detector and are used for topographic imaging. Since surface inclination, edge effects and shading are significant here, and the electrons originate only from a small depth, secondary electrons are particularly suitable for plastic visualization (so-called topography contrast). Backscattered electrons can also be imaged using a BSE detector, but here the composition of the material is important. The electron beam (consisting of primary electrons) penetrates the sample to varying degrees depending on the material and the number of backscattered electrons also depends on the type of material. The higher the (average) atomic number of the material, the more backscattered electrons reach the detector. By displaying the backscattered electrons as gray scales, the sample can be imaged in the so-called compositional or atomic number contrast. If the backscattered electrons are detected with a special detector system (EBSD detector), interference patterns are also formed, which are caused by the diffraction of the electrons at the crystal lattice of the phases located in the sample. Electron backscattering thus enables, among others, the determination of the crystal structure and its spatial distribution. By integrating an energy dispersive X-ray spectrometer (EDX), the chemical composition can also be determined spatially resolved. The interaction of the electron beam generates material-characteristic X-rays, which are simultaneously detected in the entire energy range with the SD detector. This allows qualitative and quantitative point, line and area analyses within a short time.


Figure 3: Secondary electron image of small beads of pure tin, which have precipitated on the surface of a graphite crucible after laboratory experiments.


All analytical investigations should be carried out on prepared and electrically conductive samples to obtain reproducible results. Scanning electron microscopy and especially electron microprobe analysis are highly spatially resolving methods that were developed for the examination of micro samples and react very sensitively to changes in the surface.

Sample properties

Solid and vacuum-stable, inorganic and organic samples can be analyzed. The samples should be free of water, solvents and other gassing substances. Depending on the problem and the material, tiny particles can be examined directly and without preparation. However, it is often necessary to prepare cross and thin sections and to coat non-conductive samples with an electrically conductive material. With certain restrictions, non-conductive samples can also be examined without sputtering in VP mode, which also applies to moist and gassing materials.