Method for reducing electron beam aberration by using multi-pole magnetic field

Abstract

Through deep analysis of a focusing magnetic field, a deflection magnetic field and an electron gun structure, a multi-pole magnetic field system is established, the influence of aberration on an electron beam spot is reduced, the influence of aberration on the shape of the electron beam spot is reduced through structure optimization, the shape formed by an electron beam is closer to a circle, power density distribution of the electron beam is Gaussian distribution, and energy distribution is uniform.

Description

Technical Field

[0001] The invention relates to the technical field of electron beam processing, in particular to a method for reducing electron beam aberration by utilizing a multi-pole magnetic field. Background Art

[0002] For electron beam welding, a good design of the electron gun requires that the smallest possible beam spot be obtained on the workpiece. Analyzing the physical factors that constitute the beam spot size and the influence of the geometric parameters of the electron optical system on the beam spot size is the basis for designing a more complete electron optical system for welding, so as to better form the electron beam focus, ensure the minimum beam spot diameter, increase the energy density of the electron beam, obtain better beam quality, and ultimately improve the quality of the electron beam workpiece. Summary of the invention

[0003] During electron beam processing, electrons disperse and move in different directions due to the strong space charge repulsion between them. The greater the distance the electrons move, the more obvious the dispersion trend of the electron beam is, and the low energy density of the electron beam may fail to meet the working requirements. In order to achieve the high energy density required for electron beam processing, the scattered electrons must be focused.

[0004] In order for the magnetic lens to have good focusing and imaging capabilities, the field distribution must be strictly rotationally symmetric; when the electron beam is within the near-axis range, the radial force on the electron is proportional to the distance from the axis; the electrons leave the cathode at the same initial velocity to ensure that the electron image is clear and similar to the geometric shape of the object. However, in actual electron focusing imaging, the object has a certain size, the object has a certain distance from the axis, or the electron beam has a certain angle, so part of the electron beam will enter the lens range far from the axis. The lens field has different effects on near-axis and far-axis electrons. The far-axis is more strongly deflected and cannot converge with near-axis electrons. The resulting image will not be a dot, but other shapes, which produces geometric aberrations. The diameter of the electron beam increases with the increase of the deflection angle in the deflection system. The deflection amount of the electron beam is also not proportional to the deflection current, which will cause distortion and defocusing. This phenomenon that occurs during the deflection of the electron beam is called deflection aberration. The existence of aberration directly affects the size and shape of the electron beam spot, and further affects the energy density of electron beam processing. Therefore, relevant research and analysis of aberrations must be carried out to provide theoretical support for obtaining the optimal beam focus formed under dynamic focusing current. Figure 1 shown.

[0005] In the scanning control system, as the deflection angle of the electron beam changes, the length of the distance the electron beam travels before reaching the forming platform also changes. Under a fixed focusing current, the focal length of the electron beam remains unchanged. Therefore, when the length of the electron beam path changes, the position of the electron beam focal point may leave the working platform and focus above or below the working platform. In this way, the corresponding electron beam spot is a defocused beam spot. To achieve good focusing of the electron beam in the entire forming area, it is necessary to adjust the focusing current in real time according to the deflection angle and position of the electron beam so that the focusing current of each deflection point is at a set height. This method of adjusting the focus according to the deflection amount is called dynamic focusing.

[0006] Through in-depth analysis of the focusing magnetic field and the deflection magnetic field, a focusing coil with a more uniform magnetic field and a deflection coil with a larger uniform magnetic field range are established to reduce the influence of aberrations on the electron beam spot. The influence of aberrations on the shape of the electron beam spot is reduced through structural optimization. The circular focus spot formed by the electron beam has a Gaussian power density distribution and uniform energy distribution.

[0007] For the electron flight trajectory under the influence of aberration, to form an ideal electron trajectory - Gaussian trajectory, the following ideal conditions must be met at the same time: (1) The required magnetic focusing system must be axially symmetric and the electron trajectory must meet the paraxial condition; (2) The speed of electron generation is uniform and the current and voltage do not fluctuate during the process; (3) Install the electron gun system to achieve high precision and no errors.

[0008] If the above ideal conditions cannot be met at the same time or there are some conditions that do not meet the above ideal conditions, ideal imaging cannot be achieved. At this time, when the electrons are emitted from the object point at a certain speed and direction, the intersection point formed with the Gaussian plane will deviate from the Gaussian image point, resulting in a certain amount of deviation, which is defined as the aberration. According to the cause of the aberration formation, it is divided into: spherical aberration, other geometric aberrations (astigmatism, coma), and chromatic aberration. In the actual electron beam processing operation process, deflection aberration will always exist and is inevitable, but the influence of aberration can be reduced by analyzing the cause of the aberration, thereby improving the performance of the deflection scanning focusing system. The main reason for the spherical aberration is that the object point is on the axis, but the electron beam is very wide, and the refractive index of the edge of the lens field is larger than that of the center. Therefore, the electrons passing through the edge of the field have different intersection points on the axis with the near-axis electrons. The intersection position of the paraxial rays and the side beam on the axis is different, which is a circle with blurred edges. When the electron beam electrons are emitted from the cathode, the speed of each electron is different, and there is a big difference. The focusing strength of electrons at different speeds is also different, which will cause the convergence force between electrons in the electron beam to change. The aberration generated at this time is called chromatic aberration. Usually the central chromatic aberration has the greatest impact. The main reason for coma is that the object point is close to the axis, but the electron beam is wide, and the electron beam is tilted at a certain angle to the axis, the symmetry of the rays is destroyed, and the electrons passing through the edge of the lens will not focus on the near-axis image point.

[0009] Comsol is used to simulate the electron magnetic lens model. By changing the inner diameter of the magnetic lens, the number of turns of the coil, and the focusing current, the uniformity of the magnetic field line distribution of the magnetic lens is analyzed. The reasons for the aberration and geometric distortion of the electron beam under the action of the magnetic field are analyzed. The optimal parameters are obtained through simulation. A new electron beam multi-pole magnetic lens optimization model is proposed and established. The multi-pole magnetic field that can achieve electron beam astigmatism elimination is studied and proposed. Figure 1 It is a quadrupole magnetic field model.

[0010] Using CST to simulate the electron gun model, by changing the inner diameter of the magnetic lens, the number of turns of the coil, and the focusing current, the uniformity of the magnetic field line distribution of the magnetic lens is analyzed, and the causes of aberration and geometric distortion of the electron beam under the action of the magnetic field are analyzed. The optimal parameters are obtained through simulation, and a new electron gun optimization model is proposed and established. Figure 2 This is a model of an electron gun.

[0011] Using the CST electron gun model, the electron beam spot that produces astigmatism is optimized to achieve an ideal Gaussian distribution, and the elliptical beam spot or irregular beam spot is corrected by a multi-pole magnetic field to achieve a perfect circle or a near-perfect circle effect. Figure 3 As shown, the irregular beam spot is optimized to a circular beam spot to achieve the effect of eliminating astigmatism. Description of the drawings:

[0012] Figure 1 It is a quadrupole magnetic field model diagram provided for the example of the present invention.

[0013] Figure 2 It is a model diagram of an electron gun provided for an example of the present invention.

[0014] Figure 3 This is an electron beam spot model diagram provided for an example of the present invention.

Claims

1. The structural design of the electron gun is improved based on the structure of the electron gun.

2. Use COMSOL software to simulate the magnetic field strength of the multipolar magnetic field, and analyze the force conditions of the electron beam in the multipolar magnetic field through calculation.

3. Use CST software to simulate the movement trajectory of the electron beam in the multipolar magnetic field, so as to analyze the function of the multipolar magnetic field.

4. Optimize the multi-pole magnetic field coil to achieve the effect of eliminating the aberration of the electron beam.

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