The principles of SEM and Microanalysis
The scanning electron microscope (SEM) enables the investigation of specimens with a resolution down to the nanometer scale. Here an electron beam is generated by an electron cathode and the electromagnetic lenses of the column and finally swept across the surface of a sample (Fig. 1). The path of the beam describes a raster which is correlated to a raster of gray level pixels on a screen. As a consequence the magnification is simply computed by the ratio of the image width of the output medium divided by the field width of the scanned area.
The main signals which are generated by the interaction of the primary electrons (PE) of the electron beam and the specimen´s bulk are secondary electrons (SE) and backscattered electrons (BSE) and furthermore X rays. They come from an interaction volume (Fig. 2) in the specimen which differs in diameter according to different energies of the primary electrons (typically between 200 eV and 30 keV). The SE come from a small layer on the surface and yield the best resolution, which can be realized with a scanning electron microscope. The well known topographical contrast delivers micrographs which resemble on conventional light optical images.
The BSE come from deeper regions of the investigated material thus giving a lower resolution. The typical compositional contrast gives material specific information since the signal is brighter for regions of a higher middle atomic number of the investigated area. As a byproduct of the image giving signals X-rays are produced. They result from ionization processes of inner shells of the atom leading to electromagnetic radiation. The characteristic X-rays give information about the chemical composition of the material. The methods energy dispersive X-ray spectroscopy (EDS) and wavelength dispersive X-ray spectroscopy (WDS) enable the detection of chemical elements from Boron to Uranium in a qualitative and even quantitative manner. They differ in the enegy resolution (the energy values are correlated to the line energy of a special chemical element) in the processing and in time of measurement.
In the conventional scanning electron microscope, which operates in high vacuum, the specimen has to be electrically conductive or has to be coated with a conductive layer (e.g. Carbon, Gold etc.). In the environmental scanning electron microscope (ESEM) two further vacuum states lead to new possibilities. The low vacuum mode allows the imaging of non conductive specimens (polymers, biological samples…) and the ESEM mode even enables the investigation of wet samples by cooling the sample. This offers a wide field of in situ investigations which are realized with additional equipment like tensile stage, heating stage (up to 1500°C) and Peltier cooling stage.
3D information can be gained in the low vacuum mode of the ESEM via a built in ultramicrotome. The method of serial blockface scanning electron microscopy (SBFSEM) is realized by the commercial product 3ViewTM from Gatan. The method of EBSD (electron backscatter diffraction) enables the identification of the crystal phase, of microstructures and the determination of the orientation of individual crystallites. By scanning across selected areas of the specimen one obtains a full microstructural record of these areas. This enables e.g. the investigation of grain size distributions and texture analysis.