In the scanning electron microscope the surface of a solid is depicted with high depth of field. With the help of a wolfram cathode an electron beam is generated, which screens the surface of a sample in the high vacuum. The hereby produced interaction products are detected and provide image information about the surface condition. Via secondary or backscatter-electron contrast different display formats can be chosen. By this evaluations of fracture planes or topographies are possible in high detail. To dissipate a picture in the SEM, the sample has to be conductive. This can happen by vaporizing with an extremely fine gold or graphite layer. Organic samples can also be handled in low vacuum. The GWP owns two scanning electron microscopes of the company JEOL with the following application areas:
» Imaging by secondary or backscatter-electron contrast
» Resolution in the two-digit nm-level (50,000-times)
» Operation with low vacuum (1 to 270 Pa) possible (vaporization of the sample not necessary)
» EDX-element-analysis with silicon-drift-detector
» Large sample chamber: Samples with diameters up to 200 mm, 70 mm height and up to 2 kg weight
Secondary electrons are released by the interaction of the electron beam with the sample. The electrons are directed into a detector by an applied voltage and generate the signal. Shadowing effects create a topographical contrast that produces the image. This mode of detection is used for evaluation and documentation of fracture surfaces, corrosive attack and surface finishes.
With this detector the imaging signal is generated by decelerated and deflected primary electrons. The energy of the back-scattered electrons depends on the density of the sample material. Thus, a material contrast can be generated by e.g. impurities, layer systems or inclusions.
The interaction of the electron beam with the sample material also creates characteristic X-ray radiation. The chemical composition can be determined qualitatively and semiquantitively by analyzing this radiation. For the evaluation of the chemical composition point and integral analyzes are used predominantly. By scanning the sample with the electron beam one can also realize linecans and mappings in false colors. The mappings are particularly interesting for the vivid visualization of the differences in the composition in relation to the structure.
SEM images reproduce their information in grayscale. Certain features can be highlighted with a professional image processing. In this case, the captured images are subsequently colored in order to draw attention to a certain detail and to provide as much information as possible with one single picture. This presentation is widely used for applications in medical technology and pharmacy.