Transmission Electron Microscope Imaging Principle
The electron beam of a scanning electron microscope does not pass through the sample, but focuses on a small area of the sample as much as possible, and then scans the sample line by line. The incident electrons excite secondary electrons on the surface of the sample. The microscope observes the electrons scattered from each point. The scintillation crystal placed beside the sample receives these secondary electrons, and modulates the electron beam intensity of the picture tube after amplification, thus changing the brightness of the picture tube fluorescent screen. The image is a three-dimensional representation that reflects the surface structure of the specimen. The deflection coil of the cathode ray tube is synchronously scanned with the electron beam on the surface of the sample, so that the fluorescent screen of the cathode ray tube displays the morphology image of the sample surface, which is similar to the working principle of industrial televisions. Due to the fact that electrons in such microscopes do not need to transmit through the sample, the voltage required for electron acceleration does not need to be very high.
The resolution of a scanning electron microscope is mainly determined by the diameter of the electron beam on the surface of the sample. The magnification is the ratio of the scanning amplitude on the cathode ray tube to the scanning amplitude on the sample, which can continuously vary from tens of times to hundreds of thousands of times. Scanning electron microscopy does not require very thin samples; The image has a strong sense of three dimensionality; It can analyze the composition of substances using information such as secondary electrons, absorbed electrons, and X-rays generated by the interaction between electron beams and substances.
The manufacturing of scanning electron microscopes is based on the interaction between electrons and matter. When a high-energy electron beam bombards the surface of a material, the excited region will produce secondary electrons, Auger electrons, characteristic X-rays and continuous spectrum X-rays, backscattered electrons, transmitted electrons, as well as electromagnetic radiation in the visible, ultraviolet, and infrared regions. At the same time, electron hole pairs, lattice vibrations (phonons), and electron oscillations (plasmas) can also be generated.
