Electron Microscope Imaging System
1. Objective lens and stigmator
The objective lens is the first imaging lens of the imaging system. The performance of an electron microscope is mainly determined by the optical characteristics of the objective lens. Any imperfections in the objective lens will be further magnified by other lenses in the imaging system. Therefore, the aberration of the objective lens is required to be as small as possible and the magnification is high enough, and the objective lens with strong excitation, short focal length (1.5-3mm) and objective lens diaphragm are usually used to reduce spherical aberration. Because the optical aperture is too small, the resolution of the objective lens will be reduced due to the diffraction effect and the interference of the electron beam will be strong due to the pollution effect of the optical edge, so the optical aperture cannot be too small.
The stigmatizer is an important part to ensure the resolution of the objective lens is improved. It uses a magnetic field with adjustable strength and orientation to correct the axial astigmatism caused by the non-axisymmetric objective lens.
The commonly used electromagnetic stigmatizer is composed of two groups of four pairs of electromagnets evenly installed around the gap between the pole pieces. The two elliptical correcting magnetic fields formed are perpendicular to each other, and by changing the excitation intensity and the direction of the magnetic field of the two sets of electromagnetics—a non-rotationally symmetrical correcting magnetic field can be formed. If its strength is equal to the astigmatism field of the lens, and the directions are perpendicular to each other, it can eliminate the astigmatism.
2. Intermediate mirror and projection mirror
The intermediate mirror is a weak magnetic lens with long focal length and variable magnification. The inner hole of the pole piece is larger, and the magnification is between 0-20. When the magnification is greater than 1, it is used to enlarge the objective mirror; when the magnification is less than 1, it is used to reduce the objective mirror.
The projection mirror is a short focal length strong magnetic lens, using up and down symmetrical small-aperture pole shoes - to obtain a large magnification. The projection mirror has a certain excitation current, which plays the role of magnifying the middle image and projecting it onto the fluorescent screen.
3. Three-stage amplification
The highest magnification of an electron microscope is determined by factors such as the performance of the instrument, the resolution of the photographic base plate (or fluorescent screen), etc. Generally speaking, the secondary magnification imaging system can obtain an electronic image magnified by about 200,000 times. The objective lens magnifies the details of the sample by 100 times and forms the image on the objective mirror, while the intermediate mirror magnifies the image of the objective lens by 20 times and images it on the projection mirror; the projection mirror magnifies the image of the intermediate mirror by 100 times and forms the image on the fluorescent screen. The adjustment of the number of amplified monks can be realized by changing the exciting current of the middle insert. After the excitation current of the middle town is changed, the excitation current of the objective lens should also be changed appropriately to ensure that the mirror plane of the objective lens is blended with the middle mounting surface, so as to obtain a clear image of the focal point.
In the three-stage imaging system, if the excitation current of the object is changed, the image of the objective lens can be made under the middle mount.
The object mirror is a "virtual object", which is formed into a small real image on the object surface A of the projection mirror. The projection mirror is imaged on the fluorescent screen, so that an electronic image magnified by thousands to tens of thousands of times can be obtained. The image distortion produced by this method can be reduced by the mutual compensation of the barrel distortion of the intermediate mirror and the pincushion distortion of the projection mirror.
If the objective lens is closed and the intermediate mirror is used instead of the object town for imaging, the magnification can be reduced to about 100-300 plus, and images with a larger field of view can be obtained.
