Introduction to Imaging Principles of Transmission Electron Microscopy
The structure of the transmission electron microscope consists of two parts: the main part is the illumination system, the imaging system and the observation studio; the auxiliary part is the vacuum system and the electrical system.
1. Lighting system
The system is divided into two parts: electron gun and condenser. An electron gun consists of a filament (cathode), a grid, and an anode. The heating filament emits a beam of electrons. When a voltage is applied to the anode, the electrons are accelerated. The potential difference between anode and cathode is the total accelerating voltage. Accelerated electrons with energy are ejected from holes in the anode plate. The energy of the emitted electron beam is related to the accelerating voltage, and the grid plays the role of controlling the shape of the electron beam. The electron beam has a certain divergence angle. After adjusting the condenser lens, a parallel electron beam with a small or even zero divergence angle can be seen. The current density (beam current) of the electron beam can be adjusted by adjusting the current of the condenser lens.
The size of the area on the sample that needs to be illuminated is related to the magnification. The higher the magnification, the smaller the illuminated area. Therefore, a finer electron beam is required to irradiate the sample. The beam spot size of the electron beam directly emitted by the electron gun is larger and the coherence is also poor. In order to utilize these electrons more effectively and obtain illumination electron beams with high brightness and good coherence to meet the needs of transmission electron microscopes at different magnifications, the electron beams emitted by the electron gun need to be further converged to provide different beam spots. size. , approximately parallel illumination beams. This task is usually accomplished by two electromagnetic lenses called condensers. In the figure, C1 and C2 represent the first condenser and the second condenser, respectively. C1 usually remains the same, and its role is to set the intersection of the electron guns to reduce the size of the image by more than an order of magnitude. In addition, a beam tilt device is installed in the illumination system, which can easily tilt the electron beam in the range of 2° to 3° to illuminate the sample at different tilt angles.
2. Imaging system
The system includes electronic optical elements such as sample chamber, objective lens, intermediate mirror, contrast diaphragm, diffraction diaphragm, projection lens, etc. The sample chamber has a mechanism to ensure that the vacuum of the main body is not damaged during frequent sample changes. The sample can be moved in the X and Y directions in order to find the position to be observed. The parallel electron beam obtained by the converging lens irradiates the sample and carries information reflecting the characteristics of the sample after passing through the sample. The electronic image is formed under the action of the objective lens and the contrast diaphragm, and then enlarged by the intermediate mirror and the projection lens. The final electronic image is obtained on a fluorescent screen.
The illumination system provides a coherent illuminating electron beam, which carries the structural information of the sample after passing through the sample and propagates in different directions (for example, when there is a crystal face group satisfying the Bragg equation, 2 angles may be generated in the direction intersecting the incident beam diffracted beam). Objectives will come from different parts of the sample with the same direction of propagation. Electrons converge into a single spot on the back focal plane, and electrons traveling in different directions form different spots accordingly. A direct beam of zero scattering angle converges at the focal point of the objective, forming a central spot. In this way, a diffraction pattern is formed on the back focal plane of the objective. On the image plane of the objective, these electron beams recombine for coherent imaging. By adjusting the lens current of the intermediate lens, the object plane of the intermediate lens and the rear focal plane of the objective lens are coincident, which can be displayed on the fluorescent screen. The diffraction pattern obtained above can make the object plane of the intermediate lens coincide with the image plane of the objective lens, thereby obtaining a microscopic image. Through the cooperation of the two intermediate mirrors, the length and magnification of the camera can be adjusted within a larger range.
3. Observation studio
The electronic image is reflected on the fluorescent screen. The fluorescent light is proportional to the electron beam current. Use an electronic dry plate instead of a fluorescent screen to take pictures. The photosensitive ability of the dry plate is related to its wavelength.
4. Vacuum system
The vacuum system consists of mechanical pump, oil diffusion pump, ion pump, vacuum measuring instrument and vacuum pipeline. Its function is to remove the gas in the lens barrel, so that the vacuum degree of the lens barrel must reach at least 10-5 Torr, and the best vacuum degree can reach 10-9-10-10 Torr. If the vacuum is low, collisions between electrons and gas molecules can cause scattering and affect contrast. It will also cause high voltage ionization between the electron grid and the anode, causing inter-electrode discharge. Residual gases can also corrode the filament and contaminate the sample.
5. Power control system
The instability of accelerating voltage and lens magnetic current can cause serious chromatic aberration and reduce the resolution of electron microscope. Therefore, the stability of accelerating voltage and lens current is an important criterion to measure the performance of electron microscope. The TEM circuit is mainly composed of the following parts: high voltage DC power supply, lens excitation power supply, deflection coil power supply, electron gun filament heating power supply, vacuum system control circuit, vacuum pump power supply, camera drive device and automatic exposure circuit.
In addition, many high performance electron microscopes are equipped with scanning accessories, energy spectroscopy, electron energy loss spectroscopy.
