Scanning Electron Microscopy
In this type of microscope, as shown in Figure 1, the electron beam is emitted from the electron gun, which is located at the top of the microscope, and after passing a column direction, in the vacuum chamber, are focused on the sample surface, by electromagnetic fields.
Figure 1: A schematic representation of an SEM.
1. The electron gun. 2. Magnetic focusing lenses. 3. The primary electrons. 4. The secondary electrons. 5 samples. 6. The sample holder. 7. Magnetic coils. 8. Detector. 9. Scanning current. 10-PMT. 11- imaging. 12. View images.
In SEM, the sample is bombarded with one narrow electron beam, with 100 angstroms diameter. Because of electron beam hitting to the sample, the matter electrons are excited and when returning to their original orbit, as noted, may be emitted from the sample surface, in the shape of secondary electrons, X-Ray, Backscattering electron and Auger electrons. By which can be detected various properties of material such as chemical composition, surface topography, crystallographic, magnetic and electric properties.
Brightness and sharpness of each point of SEM image depends on the intensity of returned electrons from the sample surface, which it also is dependent specifically on the local quality of the sample surface, and thus can be achieved criteria of surface ups and downs. The first laboratory sample of this microscope built in 1938 by Von Ardenne and the first commercially sample of that was built also in 1965 that was formed based on the electron scanning with mechanism electronic.
But today, this type of microscope with new technologies, in fact, are able to give valuable information to researchers.
Figure 2: scanning electron microscopy, manufactured by Joel company
The obtained image by a Optical microscope. The obtained photo by an SEM microscope.
Figure 3: The distinction between obtained images by optical microscope and SEM microscope.
Sample preparation:
Since, to forming image in SEM, use of returning emitted particle process, the sample preparation is done with particular sensitivity. Metal surfaces because of the nature of conductivity don't need to special preparation, while for non-conductive samples is covered on the surface using the sputtering technique, with thin layer of one metal like gold.
The reason is that, in non-conductive surfaces, the colliding surface with electron beam becomes charged locally and as a result, is changed the path of the returned electrons and therefore, can't be achieved a clear image of the sample surface. Powder samples, should be distributed on a conductive layer, such as aluminum and dry thoroughly.
In addition, samples must be free from liquids with high vapor pressure such as: water, organic cleaning solutions and the remained oily layer. Why, for example, water comes in the form of vapor in vacuum condition. Figure 4 shows an example of obtained images in different sizes from micro-SiO2 particles, which are obtained by SEM microscope.
Figure 4: Various taken images from SiO2 micro-particles, using SEM microscope.