How does a scanning electron microscope work? What are the advantages?
1: Scanning Electron Microscope
Since the transmission electron microscope is imaged by TE, it is required that the thickness of the sample must be within the size range that the electron beam can penetrate. To this end, it is necessary to transform large-sized samples to an acceptable level for transmission electron microscopy through various cumbersome sample preparation methods.
Whether it can directly use the material properties of the sample surface material for microscopic imaging has become the goal pursued by scientists.
After hard work, this idea has become a reality ----- scanning electron microscope (ScanningElectronicMicroscopy, SEM).
SEM is an electronic optical instrument that uses a very fine electron beam to scan the surface of the sample to be observed, and collects a series of electronic information generated by the interaction between the electron beam and the sample, which is transformed and amplified to form an image. It is a useful tool for studying three-dimensional surface structure.
Its working principle is:
In the high-vacuum lens barrel, the electron beam generated by the electron gun is focused into a thin beam by the electron converging lens, and is scanned and bombarded point by point on the sample surface to generate a series of electronic information (secondary electrons, back-reflected electrons, transmitted electrons, absorption Electronics, etc.), various electronic signals are received by the detector, amplified by the electronic amplifier, and then input to the picture tube controlled by the picture tube grid.
When the focused electron beam scans the surface of the sample, due to the different physical and chemical properties, surface potential, elemental composition and concave-convex shape of the surface of different parts of the sample, the electronic information excited by the electron beam is different, resulting in the electron beam of the picture tube The intensity also changes continuously, and finally an image corresponding to the surface structure of the sample can be obtained on the fluorescent screen of the kinescope. Depending on the electronic signal received by the detector, the backscattered electron image, secondary electron image, absorption electron image, etc. of the sample can be obtained respectively.
As described above, a scanning electron microscope mostly has the following modules: electron optical system module, high voltage module, vacuum system module, micro signal detection module, control module, micro stage control module, etc.
Two: the advantages of scanning electron microscopy
1. Magnification
Since the size of the fluorescent screen of the scanning electron microscope is fixed, the change of magnification is realized by changing the scanning amplitude of the electron beam on the surface of the sample.
If the current of the scanning coil is reduced, the scanning range of the electron beam on the sample will be reduced and the magnification will be increased. The adjustment is very convenient, and it can be continuously adjusted from 20 times to about 200,000 times.
2. Resolution
Resolution is the main performance index of SEM.
The resolution is determined by the diameter of the incident electron beam and the type of modulation signal:
The smaller the electron beam diameter, the higher the resolution.
Different physical signals used for imaging have different resolutions.
For example, SE and BE electrons have different emission ranges on the surface of the sample, and their resolutions are different. Generally, the resolution of SE is about 5-10 nm, and that of BE is about 50-200 nm.
3. Depth of field
It refers to a range of capabilities that a lens can simultaneously focus and image on various parts of a sample with unevenness.
The final lens of the scanning electron microscope adopts a small aperture angle and a long focal length, so a large depth of field can be obtained, which is 100-500 times larger than that of a general optical microscope and 10 times larger than that of a transmission electron microscope.
Large depth of field, strong three-dimensional sense, and realistic shape are the outstanding features of SEM.
Specimens for SEM are divided into two categories:
1 is a sample with good conductivity, which can generally maintain its original shape and can be observed in an electron microscope without or with a little cleaning;
2. Non-conductive samples, or samples that lose water, outgas, shrink and deform in vacuum, need to be properly treated before they can be observed.
