1、 Electronic Optical System: The Core of Focusing and Scanning
The electron optical system is the core component of scanning electron microscopy (SEM), responsible for generating, accelerating, focusing, and controlling the scanning of electron beams on the surface of the sample. Its core components include:
Electron gun: As an electron source, it generates free electrons through thermal or field emission. Thermal emission electron guns (such as tungsten filaments) have low cost but limited resolution; Field emission electron guns (such as lanthanum hexaboride or cold field emission) have high brightness, small beam spot, and resolution of up to 1nm, making them suitable for high-precision imaging.
Electromagnetic lens: focuses and accelerates an electron beam through an electromagnetic field. Two stage electromagnetic lenses converge the electron beam into a beam spot with a diameter of several nanometers, ensuring high-resolution imaging.
Scanning coil: Control the electron beam to perform grating scanning on the surface of the sample, synchronize the position of the electron beam on the fluorescent screen of the cathode ray tube, and achieve point by point correspondence between the surface features of the sample and the image.
2、 Signal detection principle: multidimensional information acquisition
When the electron beam interacts with the sample, multiple signals are excited, and the detector captures these signals to achieve sample morphology and composition analysis:
Secondary Electron (SE): The incident electron excites the outer layer electrons of the sample atoms to escape, with low energy (0-50eV), only coming from a depth of 5-10nm on the surface, highly sensitive to surface morphology, and used for high-resolution surface imaging.
Backscattered Electrons (BSE): The incident electrons are reflected after scattering by the atomic nucleus of the sample, and the yield increases with the increase of atomic number. It can display the contrast of atomic number and is used for qualitative analysis of composition.
Characteristic X-rays: Electromagnetic radiation generated when the incident electrons excite the inner layer electrons of the sample atoms to transition. The energy or wavelength is directly related to the type of element, and element qualitative and quantitative detection is achieved through energy dispersive spectroscopy (EDS) analysis.
3、 System collaboration and imaging mechanism
When the electron beam scans the sample surface, the detector synchronously captures the signal and converts it into an electrical signal. After amplification, it modulates the brightness of the fluorescent screen of the picture tube to form an image corresponding to the characteristics of the sample surface. Field emission SEM further enhances resolution and stability through high brightness electron sources and optimized vacuum systems, making it suitable for surface microstructure analysis of multiple materials such as semiconductors, metals, and geological minerals.