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.
