Electron microscopy

Electron microscopy can be divided into static and scanning types. Static types include transmission electron microscopes, reflection electron microscopes, and low-energy electron microscopes
. Scanning types include secondary electron microscopes and Auger electron microscopes.

2.1 Reflection Electron Microscopy (REM)
Transmission electron microscopy uses electrons to penetrate samples and generate diffraction to form images. The image is
the projection of the sample surface in the normal direction and does not carry surface information. Reflection electron microscopy, on the other hand, uses a high-energy electron beam that is almost parallel
to the sample surface and then collects the electron beam reflected from the surface at a small angle to form an image. This is
the biggest difference between reflection and transmission electron microscopy. In addition, since reflection microscopy uses a parallel imaging method, unlike
scanning microscopy, which uses a sequential imaging method; its capture of image changes is basically limited by the time it takes to record the image
. Therefore, reflection microscopy is very helpful for real-time observation of surface dynamic phenomena!

2.2 Low Energy Electron Microscopy (LEEM)
The difference between low energy electron microscopy and reflection electron microscopy is that low energy electron microscopy uses low energy electrons to be
incident vertically on the sample surface. Since low energy electrons incident on the surface of a general sample will be reflected back in large quantities,
they carry strong surface information. Using the diffraction generated by these electrons to form an image is a very good method. However,
there are three bottlenecks in the development of LEEM: (1) It is necessary to separate the incident electrons and the imaging electrons. This problem is currently
solved by using a magnetic mirror with a 60-degree deflection function. (2) The design of the objective lens is also a major problem, because the electrons
must be reduced from tens of keV to several eV when passing through the objective lens. In order to achieve high resolution, the distance between the objective lens and the sample surface
is generally only 2 mm. (3) When using LEEM, all materials and components used must be maintained in an ultra-high vacuum
state.

2.3 Scanning Electron Microscopy (SEM)
SEM mainly consists of two parts. One is the main body that provides and gathers electrons on the specimen and generates information, including the electron
gun, electromagnetic lens, sample chamber and vacuum system. The other is the imaging system that displays the image (see Figure 13).
SEM uses the secondary electrons generated by the sample to form an image. The energy of the secondary electrons is mostly below 50eV
, so these electrons are generated near the surface. Secondary electrons are generated after multiple collisions of incident electrons. They
do not have the intrinsic properties of atoms, but are closely related to the work function of escaping from the surface. The change of the work function of the sample surface
is not only related to the composition elements, but also has a great relationship with the regional structure. This is why the SEM image can reflect the surface
roughness.

Figure 13. Schematic diagram of SEM device

2.4 Scanning Auger Microscopy (SAM)
Scanning Auger Microscopy is a combination of a scanning electron microscope and an Auger electron spectrometer (ASE).
The biggest difference between SAM and SEM is that Auger electrons are used to collect signals instead of secondary electrons. Auger electrons were first
discovered by Pierr Auger of France, so they are named after him. When the inner electrons of an atom are
excited by an external energy source and detach from the atom, the outer electrons of the atom will quickly migrate to the holes of the inner electrons and release
energy. The released energy may be released in the form of X-rays, or the released energy may in turn excite another outer
electron to detach from the atom. The other electron that is excited in the reaction and detaches from the atom and leaves the surface of the specimen is an Auger
electron. This electron also has energy that represents the characteristics of the atom. Therefore, analyzing Auger electrons can also obtain information about the composition of the material
. The escaping electrons generally have a kinetic energy of less than 2keV. When moving in a solid, they can travel a maximum of
5nm without losing energy due to collisions. Therefore, Ogier electrons have the intrinsic properties of the atoms that make up the solid sample and can be used to
determine the type of atoms. In addition, when the sample collects Ogier electrons, only
the Ogier electrons generated in the area about 5nm away from the surface can be detected, so it reflects the atomic composition properties of several layers on the surface of the sample.
Due to the Ogier electron generation rate, the energy of the loaded electrons is greater than 10keV and decays quickly. The energy generally used
is less than 5keV. In addition, the Ogier electron generation rate is much lower than that of secondary electrons, so a stronger light source must be used; but once
a strong light source is used, the degree of electron focusing will be greatly affected, which will cause technical problems in optics. Therefore,
the resolution of SAM is not as good as that of SEM.