Introduction to the composition principle of electron microscope
An electron microscope consists of a lens barrel, a vacuum system and a power cabinet. The lens barrel mainly consists of electron gun, electron lens, sample holder, fluorescent screen and camera mechanism, which are usually assembled into a column from top to bottom; The vacuum system consists of a mechanical vacuum pump, a diffusion pump and a vacuum valve, and is connected with the lens barrel through an air extraction pipeline; The power cabinet is composed of high voltage generator, excitation current stabilizer and various adjustment control units.
The electron lens is the most important part in the lens barrel of the electron microscope. It uses a spatial electric field or magnetic field symmetrical to the axis of the lens barrel to bend the electron trajectory to the axis to form focus, which is similar to that of the glass convex lens to focus the light beam, so it is called the electron lens. Most modern electron microscopes use electromagnetic lenses, and the strong magnetic field generated by a very stable DC excitation current passing through a coil with pole shoes focuses the electrons.
An electron gun is a component consisting of a tungsten filament hot cathode, a grid and a cathode. It can emit and form an electron beam with uniform speed, so the stability of accelerating voltage is required to be not less than one ten thousandth.
Electron microscope can be divided into transmission electron microscope, scanning electron microscope, reflection electron microscope and emission electron microscope according to structure and use. Transmission electron microscope (TEM) is often used to observe the fine material structure that can't be distinguished by ordinary microscope. Scanning electron microscope (SEM) is mainly used to observe the morphology of solid surface, and can also be combined with X-ray diffractometer or electron energy spectrometer. The electron micro is formed by the scattering of electron beam by atoms of the sample. In the thin or low-density part of the sample, the electron beam scatters less, so more electrons pass through the objective diaphragm and participate in imaging, which makes them appear brighter in the image. On the contrary, the thicker or denser part of the sample appears darker in the image. If the sample is too thick or too dense, the contrast of the image will deteriorate and even be damaged or destroyed by absorbing the energy of the electron beam.
The top of the lens barrel of the transmission electron microscope is an electron gun. The electrons are emitted by the tungsten filament hot cathode and focused by the first and second condenser lenses. After the electron beam passes through the sample, it is imaged on the intermediate mirror by the objective lens, and then amplified step by step by the intermediate mirror and projection mirror, and imaged on the fluorescent screen or photographic plate.
The magnification of the intermediate mirror can be continuously changed from several tens times to several hundred thousand times by adjusting the excitation current. By changing the focal length of the intermediate mirror, the electron microscopic image and electron diffraction image can be obtained on the tiny part of the same sample. In order to study the thick metal slice samples, an ultra-high voltage electron microscope with an accelerating voltage of 3500 kV was developed by the Electron Optics Laboratory in Dulos, France.
The electron beam of scanning electron microscope does not pass through the sample, but only scans on the surface of the sample to excite secondary electrons. The scintillation crystal placed next to the sample receives these secondary electrons and modulates the electron beam intensity of the picture tube after amplification, thus changing the brightness on the screen of the picture tube. The deflection coil of the picture tube keeps synchronous scanning with the electron beam on the sample surface, so that the fluorescent screen of the picture tube shows the morphological image of the sample surface, which is similar to the working principle of industrial TV sets.
The resolution of scanning electron microscope mainly depends on the diameter of electron beam on the sample surface. The magnification is the ratio of the scanning amplitude on the picture tube to the scanning amplitude on the sample, which can be continuously changed from dozens to hundreds of thousands of times. Scanning electron microscope does not need very thin samples; The image has a strong three-dimensional sense; The composition of the substance can be analyzed by using the information of secondary electrons, absorbed electrons and X-rays generated by the interaction between the electron beam and the substance.
The electron gun and condenser of the scanning electron microscope are almost the same as those of the transmission electron microscope, but in order to make the electron beam thinner, an objective lens and an astigmatic diffuser are added under the condenser, and two sets of scanning coils perpendicular to each other are also installed in the objective lens. A sample table which can move, rotate and tilt is installed in the sample chamber under the objective lens.
