1. Structural differences
It is mainly reflected in the different positions of the samples in the electron beam optical path. The sample of TEM is in the middle of the electron beam, the electron source emits electrons above the sample, after passing through the condenser, and then penetrating the sample, a follow-up electromagnetic lens continues to amplify the electron beam, and the epiphysis is projected on the fluorescent screen; the sample of SEM is in the electron beam. At the end, the electron beam emitted by the electric source above the sample is reduced by several stages of electromagnetic lenses and reaches the sample. Of course, the structure of the subsequent signal detection side processing system will also be different, but there is no substantial difference in terms of basic physical principles.
2. Basic working principle
Transmission electron microscope: When the electron beam passes through the sample, it will scatter with the atoms in the sample. The electrons passing through a certain point on the sample at the same time are in different directions. This point on the sample is between 1-2 times the focal length of the objective lens. The electrons are re-converged after being magnified by the objective lens, forming a magnified real image of the point, which is the same as the imaging principle of the convex lens. There is a contrast formation mechanism here, and the theory is not discussed in depth, but it can be imagined that if the interior of the sample is absolutely uniform, without grain boundaries, and without atomic lattice structure, then the magnified image will not have any contrast. This kind of substance does not exist, so there is a reason for this kind of instrument to exist. Scanning electron microscope: The electron beam reaches the sample, excites the secondary electrons in the sample, and the secondary electrons are received by the detector, through signal processing and modulating the light emission of a pixel on the display, because the diameter of the electron beam spot is nanoscale, and the pixel of the display is 100 Above a micron, the light emitted by this 100-micron-and-above pixel represents the light emitted by the region on the sample that is excited by the electron beam. Amplification of this object point on the sample is achieved. If the electron beam is raster scanned in an area of the sample, the brightness of the pixels of the display can be modulated one by one from the geometric arrangement, and the magnified imaging of this sample area can be realized.
3. Requirements for samples
(1) Scanning electron microscope
SEM sample preparation has no special requirements on the thickness of the sample, and can use methods such as cutting, grinding, polishing or cleavage to present a specific section, thereby transforming it into an observable surface. If such a surface is directly observed, only surface processing damage can be seen. Generally, different chemical solutions must be used for preferential etching to produce a contrast that is conducive to observation. However, corrosion will cause the sample to lose part of the true state of the original structure, and at the same time introduce some artificial interference.
(2) Transmission electron microscope
Since the quality of the microscopic image obtained by TEM strongly depends on the thickness of the sample, the observation part of the sample should be very thin. For example, the TEM sample of a memory device can only have a thickness of 10-100 nm, which brings great difficulties to the TEM sample preparation. difficulty. In the process of sample preparation, the yield of manual grinding or mechanical control for beginners is not high, and the sample will be scrapped once it is excessively ground. Another problem in TEM sample preparation is the positioning of observation points. General sample preparation can only obtain a thin observation range of the order of 10 mm. When precise positioning and analysis are required, the target often falls outside the observation range. At present, the ideal solution is to use focused ion beam etching (FIB).
