Electron Microscopy

Electron interaction with the surface

 Suppose emitted electrons beam, from the electron source, as shown in Figure 1, collide to the sample surface. In this collision, according to the energy of the beam and type of collided particle, it is possible that observe various particles or beam, which each one is characteristic of the different surface properties, and as a result, with use of different detectors can be used each of them to interpret the different characteristics of surface atoms.
According to figure 1, the secondary backscattering, Auger electron, and X-Ray, constitute the most common particles or radiation emission resulting from the electron beam collision to the sample surface, which we will review them in following.


                             
Figure 1, the interaction between high-energy electrons with solid sample. 

1. The electron beam. 2. The scape range [of Auger electrons 3. The escape range of secondary electrons 4. X-ray range. 5. The exit range of backscattering electrons
 






The secondary electrons:
These electrons are obtained based on the inelastic collision between initial incident electrons with conductive band electrons, and sometimes, with the electrons in the valence band. The emitted electrons, which are called secondary electrons, have the energy between 5 and 10 eV. Considering to their low energy, these electrons are collected easily with a positive voltage 300-100 V, at the front of the detector of the type PMT. Because it usually is collected more than 50 percent of these electrons, thus is created a three-dimensional image of the sample with high the field depth.

Backscattered Electrons

Backscattered Electrons
These electrons are emitted based on elastic collision of incident electrons with the atoms nucleus of the sample surface and any material that its atoms have more protons, causes to rise these backing electrons.
Because, the collisions are elastic, there will be no change in the energy of the scattered electrons. The scattering angle also can be from zero to 180 degrees. Because these electrons have high energy, the secondary electrons are not easily collectable.
The most common detector, for this purpose is the Surface barrier detector, which is placed on the sample and below objective lens, and electrons colliding with it are identified. these detectors are made of semiconductor materials. A semiconductor material has a filled valence band and an empty conductive band, when backscattering electrons collide to this detector, electrons move from the valence band to the conductive band, and electrons in this band, travel freely or return to the the valence band.
X-Ray
We know that, when high-energy electrons collide to a metal sample, cause the production of x-ray radiation. The energy of X-Ray is associated with difference electron energy levels of the collided atoms and since these levels and their variations are known for each substance with detecting output beam can be identified some features of atoms on the surface. Among these characteristics, it can be noted to determination of the elements amount, in the sample or doing quantitative analysis.
In electron microscopes, a technique that has been formed based on the use of output beams, like X-ray, in order to identify substances called Energy dispersive spectrometry (EDS) technique, and is placed usually on the scanning electron microscope as one of the lateral equipment.
Auger electrons:
When the electrons collide to atoms sample, it is possible that an electron be removed, from one of the atomic level, and _ leave a hole. In this case, for which atom returns _ to stable state. According to figure 2, an electron moves from a higher level, _ toward the hole, and therefore, is released, a lot of energy, which may occur, three modes, depending on the difference in energy levels:

    
                                       

Figure 2: the production of Auger electron

 
If the emitted electron be from the external circuit, the emitted energy is low, and is emitted as a photon. If the level holes be insider, the released energy become greater, and in this case X-Ray emit, which is discussed above. Sometimes, additional energy is transferred to one of the inner electrons, and makes it emits.
Said to these emitted electrons Auger electrons. Their detecting is called the detector AES, and offers important information of material including its type and concentration. Analysis of Auger electrons is applicable according to their very low energy only in certain circumstances, such as high vacuum.
For better understanding, we consider the energy of two state, X-Ray emitting and Auger electrons for titanium. The energy of emitted x-ray due to electrons transition from the level 4M two level 2 L is equal to:

                                                                           E_X=E_L2-E_M4=457.8 ev                                                                   
Its corresponding for the Auger electrons, their energy is equal to: 
                                                                         E_Auger=E_L2-E_M4=423 ev    


In Figure 3, the X-ray production process and Auger electrons, is shown, as comparative.

                    
Figure 3: X-ray and electron Auger formation process
 

Sometimes, as shown in Figure 4, the electrons energy and the type of sample materials as well as layer thickness is in such a way that the electron beam passes through the thin sample and in this case more particles being emitted, such as elastic passing of scattering electron and inelastic passing of scattering electron.
                           


Figure 4: The interaction between high-energy electrons, with solid sample, in analysis of STEM or TEM, 1- incident electrons. 2. The secondary electrons. 3. Backscattering electrons. 4. Heat. 5. The elastic scattering electrons. 6. Passing electrons. 7. Inelastic scattering electrons. 8. The thin sample. 9.. 10 X-Ray.