A magnetic lens pole piece structure and a method of assembling the same

Abstract

The application provides a magnetic lens pole shoe structure and an assembling method thereof. The magnetic lens pole shoe structure comprises a pole shoe base and a pole shoe end; the pole shoe base is in a cylindrical structure, and a ring-shaped step part extending along a central axis direction is arranged at one end of the pole shoe base; the ring-shaped step part comprises a first end surface, and the first end surface is a cylindrical surface surrounding the central axis; the pole shoe end comprises a second end surface, and the first end surface and the second end surface are in close contact; and the magnetic permeability of the pole shoe end is greater than that of the pole shoe base. The application solves the problems of the traditional integral pole shoe, such as large machining difficulty, insufficient structural strength of the split pole shoe, and magnetic performance attenuation caused by assembly stress, and greatly improves the overall working performance and use reliability of the magnetic lens under the premise of considering the process feasibility and cost advantage.

Description

Technical Field

[0001] This invention relates to the field of atomic force microscopy, and in particular to a magnetic lens pole piece structure and its assembly method. Background Technology

[0002] The pole pieces in existing electro-optical magnetic lenses are typically manufactured as a single integrated structure. However, due to the complex internal contours of the pole pieces and the high requirements for coaxiality and machining accuracy, integrated pole pieces generally face challenges such as high machining difficulty, long machining cycles, and high manufacturing costs. This is especially true for materials with high saturation permeability, whose high hardness and poor machinability further increase the machining difficulty. Machining errors during the integrated process often lead to the scrapping of the entire component, resulting in significant material cost losses and reduced utilization of high-performance magnetic materials.

[0003] Furthermore, in existing split magnetic pole structures, some designs employ assembly methods such as welding, screw connections, or adhesive bonding. However, for electron optical magnetic lens structures, the stability of the magnetic field distribution and the continuity of the magnetic circuit in the working area of ​​the pole shoe have a significant impact on the electron beam focusing performance. Welding may cause local permeability changes and residual stress in the heat-affected zone, thus affecting the magnetic field stability of the pole shoe's working area; screw connections may lead to local magnetic reluctance discontinuities and uneven magnetic flux distribution; and adhesive bonding may affect the continuity of the magnetic circuit due to the presence of a non-magnetic adhesive layer. In addition, local assembly stress may cause slight deformation or coaxial misalignment in the working magnetic pole region at the pole shoe tip, further affecting the focusing accuracy and magnetic field stability of the electron optical system. Summary of the Invention

[0004] This invention provides a magnetic lens pole shoe structure and its assembly method. By using a split structure and a temperature difference assembly method, it solves the contradiction between the difficulty in processing integral high permeability pole shoes and the insufficient strength of the base, and avoids the problem of magnetic performance degradation caused by assembly stress.

[0005] In a first aspect, embodiments of the present invention provide a magnetic lens pole piece structure, including a pole piece base and a pole piece end; The pole shoe base has a cylindrical structure, and one end of the pole shoe base is provided with an annular stepped part extending along the central axis. The annular stepped portion includes a first end face, which is a cylindrical surface surrounding the central axis; The end of the pole shoe includes a second end contact surface, and the first end contact surface and the second end contact surface are in contact with each other; The permeability at the end of the pole shoe is greater than that at the base of the pole shoe.

[0006] Optionally, the annular stepped portion may also include a third end face and a fourth end face; Both the third and fourth end surfaces are annular planes perpendicular to the central axis, and the third and fourth end surfaces are parallel to each other. The first end face is connected to the third end face and the fourth end face at its two ends along the central axis direction, respectively; The end of the pole shoe also includes a fifth end face and a sixth end face; The third end face and the fifth end face are fitted together, and the fourth end face and the sixth end face are fitted together.

[0007] Optionally, the height of the first end face along the central axis is greater than 5 mm.

[0008] Optionally, the end of the pole shoe away from the pole shoe base is provided with a seventh end face, which is perpendicular to the central axis and is used to withstand the assembly thrust along the central axis.

[0009] Optionally, at the first temperature, the outer diameter of the second end face is larger than the inner diameter of the first end face, and at the second temperature, the outer diameter of the second end face is smaller than the inner diameter of the first end face, so that the second end face and the first end face form an assembly gap at the second temperature.

[0010] Optionally, at the first temperature, the difference between the outer diameter of the second end face and the inner diameter of the first end face is optional, and the material of the pole shoe base is pure iron.

[0011] Optionally, the pole shoe end is made of an iron-nickel alloy with a nickel mass fraction of 80%.

[0012] In a second aspect, embodiments of the present invention also provide a method for assembling a magnetic lens pole shoe structure, used for assembling the magnetic lens pole shoe structure of any one of the first aspects, the assembly method comprising: Cool the end of the pole shoe to a second temperature to create an assembly gap between the second end face and the first end face; Align the cooled pole shoe end and pole shoe base coaxially along the central axis, and apply an axial thrust along the central axis to the pole shoe end until the third end face and the fifth end face are in contact, and the fourth end face and the sixth end face are in contact. This causes the temperature at the tip of the pole shoe to rise back to the first temperature.

[0013] Optionally, a seventh end face is provided at the end of the pole shoe that is away from the pole shoe base. The seventh end face is perpendicular to the central axis and is used to withstand the assembly thrust along the central axis. Align the cooled pole shoe end and pole shoe base coaxially along the central axis, and apply an axial thrust along the central axis to the pole shoe end until the third and fifth end surfaces are in contact, and the fourth and sixth end surfaces are in contact, including: Align the cooled pole shoe end and pole shoe base coaxially along the central axis, and apply an axial thrust along the central axis to the seventh end face until the third and fifth end faces are in contact and the fourth and sixth end faces are in contact.

[0014] This invention employs a split magnetic lens pole shoe structure. The pole shoe base uses a low-permeability material to ensure the conductivity of the main magnetic circuit, effectively reducing processing and manufacturing costs. The pole shoe ends use a high-saturation-permeability iron-nickel alloy material to enhance local magnetic flux carrying capacity, suppress local magnetic saturation, and effectively improve the working accuracy of the magnetic lens. A high-precision, tight fit between the base and ends is achieved through a temperature-induced shrinkage interference fit assembly method, ensuring structural mechanical stability and magnetic circuit continuity. This invention effectively solves the problems of high processing difficulty in traditional integral pole shoes and insufficient structural strength in split pole shoes, as well as magnetic performance attenuation caused by assembly stress. While considering process feasibility and cost advantages, it significantly improves the overall working performance and reliability of the magnetic lens. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of an immersion magnetic lens provided in an embodiment of the present invention; Figure 2 This is a partial structural schematic diagram of a magnetic lens pole shoe structure provided in an embodiment of the present invention; Figure 3 This is a partial structural schematic diagram of another magnetic lens pole shoe structure provided in an embodiment of the present invention; Figure 4 A partial structural schematic diagram of another magnetic lens pole shoe structure provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of the magnetic lens pole shoe structure of a non-immersion magnetic lens provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of the magnetic lens pole piece structure of a converging magnetic lens provided in an embodiment of the present invention; Figure 7 This is a flowchart illustrating a method for assembling a magnetic lens pole shoe structure according to an embodiment of the present invention. Figure 8 This is a flowchart illustrating another method for assembling a magnetic lens pole piece structure according to an embodiment of the present invention. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be fully described below with reference to the accompanying drawings in the embodiments of this invention, through specific implementation methods. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort fall within the protection scope of this invention.

[0017] Figure 1 This is a schematic diagram of the structure of an immersion magnetic lens provided in an embodiment of the present invention; Figure 2 This is a partial structural schematic diagram of a magnetic lens pole piece structure provided in an embodiment of the present invention. Figure 3 This is a partial structural schematic diagram of another magnetic lens pole shoe structure provided in an embodiment of the present invention. Figure 4 This is a partial structural schematic diagram of another magnetic lens pole piece structure provided in an embodiment of the present invention. The magnetic lens pole piece structure provided in this embodiment can be specifically applied in electron microscopes, electron beam exposure machines, electron beam welding equipment, particle accelerators, and other similar applications. It is used to achieve electron beam focusing, deflection control, and high-resolution imaging, and is particularly suitable for field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) objective pole pieces where magnetic field uniformity and imaging quality requirements are extremely high, as well as in the projection lens groups of high-precision electron beam direct writing systems. (Reference) Figures 1-4 The present invention provides a magnetic lens pole shoe structure, including a pole shoe base 10 and a pole shoe end 20; The pole shoe base 10 has a cylindrical structure, and one end of the pole shoe base 10 is provided with an annular step portion 30 extending along the central axis direction y; The annular stepped portion 30 includes a first end face 31, which is a cylindrical surface surrounding the central axis; The end of the pole shoe 20 includes a second end contact surface 21, and the first end contact surface 31 and the second end contact surface 21 are in contact with each other; The magnetic permeability of the pole shoe end 20 is greater than that of the pole shoe base 10.

[0018] The pole piece base 10 can be understood as the main structural component of the magnetic lens. It has a cylindrical structure and provides mechanical support and magnetic flux conduction path for the entire pole piece. The central axis direction y can be understood as the direction of the axis of symmetry along the cylindrical structure of the pole piece base 10 (i.e., the longitudinal extension direction of the pole piece), which is consistent with the propagation direction of the electron beam. The pole piece end 20 can be understood as a functional component located at the end of the pole piece base 10. It is made of a high saturation permeability material (such as permalloy, cobalt-iron alloy, etc.) to improve the magnetic flux carrying capacity of the local area, reduce the local magnetic saturation phenomenon, and thus improve the magnetic field uniformity and focusing accuracy of the lens.

[0019] For details, please refer to Figures 1-4The magnetic lens pole shoe structure consists of two parts: a pole shoe base 10 and a pole shoe end 20. The pole shoe base 10 is a cylindrical structure made of pure iron, with an annular stepped portion 30 extending along the central axis y at one end. This annular stepped portion 30 includes a first end face 31 (a cylindrical surface surrounding the central axis) for contact with a second end face 21 on the pole shoe end 20. The pole shoe end 20 is made of a high-saturation permeability material (such as permalloy), whose permeability is higher than that of the pole shoe base 10. The pole shoe end 20 is assembled into the pole shoe base 10 using a cold-shrink interference fit, ensuring a tight fit between the first end face 31 and the second end face 21, thus achieving mechanical stability and magnetic circuit connectivity for the entire pole shoe. During the operation of the magnetic lens, the magnetic flux generated by the excitation coil (not shown in the figure) is conducted from the pole shoe base 10 through the annular step portion 30 to the pole shoe end 20. Since the magnetic permeability of the pole shoe end 20 is higher than that of the pole shoe base 10, the magnetic flux is concentrated and enhanced in the region of the pole shoe end 20, forming a high gradient and high uniformity focusing magnetic field, thereby achieving high-resolution focusing and precise control of the electron beam.

[0020] This invention employs a split magnetic lens pole shoe structure. The pole shoe base uses a low-permeability material to ensure the conductivity of the main magnetic circuit, effectively reducing processing and manufacturing costs. The pole shoe ends use a high-saturation-permeability iron-nickel alloy material to enhance local magnetic flux carrying capacity, suppress local magnetic saturation, and effectively improve the working accuracy of the magnetic lens. A high-precision, tight fit between the base and ends is achieved through a temperature-induced shrinkage interference fit assembly method, ensuring structural mechanical stability and magnetic circuit continuity. This invention effectively solves the problems of high processing difficulty in traditional integral pole shoes and insufficient structural strength in split pole shoes, as well as magnetic performance attenuation caused by assembly stress. While considering process feasibility and cost advantages, it significantly improves the overall working performance and reliability of the magnetic lens.

[0021] refer to Figure 3 Optionally, the annular stepped portion 30 may also include a third end face 32 and a fourth end face 33; Both the third end face 32 and the fourth end face 33 are annular planes perpendicular to the central axis, and the third end face 32 and the fourth end face 33 are parallel to each other. The first end face 31 is connected to the third end face 32 and the fourth end face 33 at its two ends along the central axis direction y, respectively; The end of the pole shoe 20 also includes a fifth end face 22 and a sixth end face 23; The third end face 32 and the fifth end face 22 are fitted together, and the fourth end face 33 and the sixth end face 23 are fitted together.

[0022] For details, please refer to Figure 2The annular stepped portion 30 further includes a third end face 32 and a fourth end face 33. Both the third end face 32 and the fourth end face 33 are annular planes perpendicular to the central axis and are parallel to each other. The first end face 31 (cylindrical surface) connects to the third end face 32 and the fourth end face 33 at its two ends along the central axis direction y, forming a complete annular positioning groove structure. Correspondingly, the pole shoe end 20 also includes a fifth end face 22 and a sixth end face 23. During assembly, the third end face 32 is in contact with the fifth end face 22, the fourth end face 33 is in contact with the sixth end face 23, and the first end face 31 is in contact with the second end face 21. This precise positioning through three-sided contact further improves the coaxiality and mechanical stability of the assembly between the pole shoe end 20 and the pole shoe base 10, while increasing the magnetic circuit contact area, reducing magnetic resistance, and ensuring efficient and low-loss transmission of magnetic flux between the pole shoe base 10 and the pole shoe end 20.

[0023] refer to Figure 4 Optionally, the height of the first end face 31 along the central axis direction y is greater than 5mm.

[0024] For details, please refer to Figure 3 If the height of the first end contact surface 31 is too small (for example, less than 5mm), the radial fit length between the pole shoe end 20 and the pole shoe base 10 will be insufficient. On the one hand, this will cause the pole shoe end 20 to be prone to tilting or eccentricity during assembly, affecting coaxiality. Those skilled in the art can select a suitable first end contact surface height according to the specific size of the pole shoe and the magnetic field requirements.

[0025] refer to Figure 3 Optionally, the end of the pole shoe end 20 away from the pole shoe base 10 is provided with a seventh end face 24, which is perpendicular to the central axis and is used to withstand the assembly thrust along the central axis direction y.

[0026] For details, please refer to Figure 3 The end of the pole shoe 20 furthest from the pole shoe base 10 has a seventh end face 24, which is perpendicular to the central axis. During the cold-shrink interference fit assembly process, the pole shoe end 20 is first cooled and shrunk, and then pushed into the annular stepped portion 30 of the pole shoe base 10 along the central axis direction y. The seventh end face 24 serves as the direct bearing surface for the assembly thrust, bearing the thrust applied by the pressing tool along the central axis direction y, allowing the pole shoe end 20 to move evenly and smoothly to the predetermined assembly position until the fifth end face 22 and the sixth end face 23 are tightly fitted with the third end face 32 and the fourth end face 33, respectively. This ensures that the assembly thrust is evenly distributed along the axial direction, avoiding tilting or damage to the pole shoe end 20 due to eccentric loading.

[0027] Optionally, at the first temperature, the outer diameter of the second end face 21 is larger than the inner diameter of the first end face 31, and at the second temperature, the outer diameter of the second end face 21 is smaller than the inner diameter of the first end face 31, so that the second end face 21 and the first end face 31 form an assembly gap at the second temperature.

[0028] Specifically, to achieve a cold-shrink interference fit between the pole shoe end 20 and the pole shoe base 10, a temperature difference assembly process is designed based on the thermal expansion and contraction characteristics of the material: at room temperature or a first temperature (e.g., heating the pole shoe base 10), the outer diameter of the second end face 21 of the pole shoe end 20 is designed to be larger than the inner diameter of the first end face 31 of the pole shoe base 10. Alternatively, the pole shoe end 20 is cooled to a second temperature (e.g., cooled to -100°C with liquid nitrogen). Due to the thermal contraction effect of the material, the dimensions of the pole shoe end 20 shrink, making the outer diameter of the second end face 21 smaller than the inner diameter of the first end face 31, thus forming an assembly gap between them. Those skilled in the art can reasonably select the values ​​of the first and second temperatures and design the corresponding dimensional tolerances based on the linear expansion coefficient of the pole shoe material and the required interference amount.

[0029] It should be noted that in the above embodiment, the inner wall (first end contact surface 31) of the pole shoe base 10 and the outer wall (second end contact surface 21) of the pole shoe end 20 are assembled by an interference fit. However, it should be understood that this is only an exemplary fitting method. In actual implementation, for example, the outer wall of the pole shoe base 10 and the inner wall of the pole shoe end 20 can also be fitted with an interference fit. Regardless of whether an "inner wall-outer wall" fit or an "outer wall-inner wall" fit is used, the core is to use temperature difference or press-fitting process to achieve close contact between the two components, ensuring the continuity of the magnetic circuit and the mechanical stability. Those skilled in the art can flexibly choose the inner and outer positional relationship of the mating surfaces according to the specific structural design of the pole shoe, assembly space limitations, and the convenience of processing technology.

[0030] Optionally, at the first temperature, the difference between the outer diameter of the second end face 21 and the inner diameter of the first end face 31 is a preset assembly interference, and the material of the pole shoe base 10 is pure iron (such as electrical pure iron).

[0031] Optionally, the material of the pole shoe end 20 is an iron-nickel alloy with a nickel mass fraction of 80% (such as permalloy).

[0032] Figure 5 This is a schematic diagram of the magnetic lens pole shoe structure of a non-immersion magnetic lens according to an embodiment of the present invention. Figure 6 This is a schematic diagram of the magnetic lens pole piece structure of a converging magnetic lens according to an embodiment of the present invention. (Reference) Figure 1 , Figure 5 and Figure 6 The magnetic lens pole piece structure provided in this application can be applied to different types of magnetic lens designs.

[0033] exist Figure 1 In the immersion magnetic lens shown, the pole shoe end 20 extends close to the sample, and the annular stepped portion 30 of the pole shoe base 10 has a large inner diameter, allowing the magnetic field region through which the electron beam passes to cover the sample surface. The high permeability material (permalloy) of the pole shoe end 20 forms a strong and uniform focusing magnetic field in the region near the sample. The electron beam is focused when passing through this region, and the sample is immersed in the magnetic field, achieving sub-nanometer resolution. The pure iron main magnetic circuit of the pole shoe base 10 guides the magnetic flux generated by the excitation coil to the pole shoe end 20.

[0034] exist Figure 5 In the non-immersion magnetic lens shown, the pole piece 20 maintains a certain distance from the sample, and the electron beam reaches the sample surface only after leaving the magnetic field region of the pole piece 20. The tip geometry of the pole piece 20 is optimized to create a specific gradient distribution of the magnetic field in the space between the pole piece 20 and the sample, achieving focusing of the electron beam and aberration correction. Since the sample is not immersed in the magnetic field, this structure is suitable for magnetic samples sensitive to magnetic fields or applications requiring a large working distance.

[0035] exist Figure 6 In the converging magnetic lens shown, the magnetic flux is highly concentrated in the tip region of the pole shoe end 20, forming an extremely strong axial magnetic field gradient. The electron beam is focused into an extremely small beam spot (nanoscale) within an extremely short focal length.

[0036] Figure 7 This is a flowchart illustrating a method for assembling a magnetic lens pole piece structure according to an embodiment of the present invention. (Refer to...) Figure 7 The present invention also provides a method for assembling a magnetic lens pole piece structure, used to assemble the magnetic lens pole piece structure in the above embodiments. The assembly method includes: S110. Cool the end of the pole shoe to a second temperature to create an assembly gap between the second end face and the first end face.

[0037] The second temperature can be understood as a temperature below room temperature.

[0038] Specifically, the pole shoe end 20 is placed in a cooling medium to lower its temperature to a second temperature (e.g., -100°C). At this temperature, the material of the pole shoe end 20 (e.g., an iron-nickel alloy with 80% nickel mass fraction) shrinks due to thermal contraction, reducing its outer diameter so that the outer diameter of the second end face 21 is smaller than the inner diameter of the first end face 31 of the pole shoe base 10, creating a sufficient fitable gap between them.

[0039] S120. Align the cooled pole shoe end and pole shoe base coaxially along the central axis, and apply an axial thrust along the central axis to the pole shoe end until the third end face and the fifth end face are in contact, and the fourth end face and the sixth end face are in contact.

[0040] Coaxial alignment can be understood as adjusting the central axis of the pole shoe end 20 to coincide with the central axis of the pole shoe base 10; axial thrust can be understood as the assembly pressure applied along the central axis direction y.

[0041] Specifically, the cooled pole shoe end 20 and pole shoe base 10 are coaxially aligned along the central axis on an assembly fixture, so that the second end face 21 is aligned with the entrance of the first end face 31. Then, an axial thrust along the central axis y is applied to the pole shoe end 20 using a pressing tool, smoothly pushing the pole shoe end 20 into the annular step portion 30 of the pole shoe base 10. The thrust is continued until the fifth end face 22 of the pole shoe end 20 is completely fitted with the third end face 32 of the annular step portion 30, and the sixth end face 23 is completely fitted with the fourth end face 33. At this point, the pole shoe end 20 has reached the predetermined axial assembly position.

[0042] S130, raise the temperature at the end of the pole shoe back to the first temperature.

[0043] The first temperature can be understood as a temperature higher than the second temperature, or the initial temperature, such as room temperature of 20℃-25℃.

[0044] Specifically, after axial positioning is completed, the assembled pole shoe assembly is removed from the press-fit fixture and placed in a room temperature environment to stand, or a gentle heating method is used to allow the temperature of the pole shoe end 20 to rise back to the first temperature. As the temperature rises, the material of the pole shoe end 20 undergoes thermal expansion, and its outer diameter gradually increases. The gap between the second end face 21 and the first end face 31 disappears, forming an interference fit.

[0045] The magnetic lens pole shoe assembly method provided in this invention achieves high-precision and high-reliability assembly between the pure iron pole shoe base and the iron-nickel alloy pole shoe end through a cold shrink interference fit process. This avoids assembly methods such as welding, screw connection, or adhesive fixing, and ensures the tightness and low magnetic resistance of the magnetic circuit contact interface.

[0046] Figure 8 This is a flowchart illustrating another method for assembling a magnetic lens pole piece structure according to an embodiment of the present invention. (Refer to...) Figure 8In the above embodiment, the end of the pole shoe end 20 away from the pole shoe base 10 is provided with a seventh end face 24. The seventh end face 24 is perpendicular to the central axis and is used to withstand the assembly thrust along the central axis direction y. "S120, Align the cooled pole shoe end and the pole shoe base coaxially along the central axis, and apply an axial thrust along the central axis direction to the pole shoe end until the third end face and the fifth end face are in contact, and the fourth end face and the sixth end face are in contact." This can be further refined as follows: Align the cooled pole shoe end and pole shoe base coaxially along the central axis, and apply an axial thrust along the central axis to the seventh end face until the third and fifth end faces are in contact and the fourth and sixth end faces are in contact.

[0047] like Figure 8 As shown, another method for assembling a magnetic lens pole piece structure includes the following steps: S210. Cool the end of the pole shoe to a second temperature to create an assembly gap between the second end face and the first end face.

[0048] S221. Align the cooled pole shoe end and pole shoe base coaxially along the central axis, and apply an axial thrust along the central axis to the seventh end face until the third end face and the fifth end face are in contact, and the fourth end face and the sixth end face are in contact.

[0049] Specifically, the cooled pole shoe end 20 and pole shoe base 10 are coaxially aligned along the central axis on an assembly fixture, so that the second end face 21 is aligned with the entrance of the first end face 31. Then, an axial thrust along the central axis y is applied to the seventh end face 24 of the pole shoe end 20 using a pressing tool, smoothly pushing the pole shoe end 20 into the annular step portion 30 of the pole shoe base 10. The thrust is continued until the fifth end face 22 of the pole shoe end 20 is completely fitted with the third end face 32 of the annular step portion 30, and the sixth end face 23 is completely fitted with the fourth end face 33. At this point, the pole shoe end 20 has reached the predetermined axial assembly position.

[0050] S230, raise the temperature at the end of the pole shoe back to the first temperature.

[0051] The above description is merely a preferred embodiment of the present invention and the technical principles employed. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and various obvious changes, readjustments, combinations, and substitutions can be made without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the concept of the present invention, the scope of which is determined by the scope of the appended claims.

Claims

1. A magnetic lens pole shoe structure, characterized in that, Includes the pole shoe base and the pole shoe end; The pole shoe base has a cylindrical structure, and one end of the pole shoe base is provided with an annular stepped portion extending along the central axis. The annular stepped portion includes a first end face, which is a cylindrical surface surrounding the central axis; The end of the pole shoe includes a second end contact surface, and the first end contact surface and the second end contact surface are in contact with each other; The magnetic permeability of the pole shoe end is greater than the magnetic permeability of the pole shoe base.

2. The magnetic lens pole shoe structure according to claim 1, characterized in that, The annular stepped portion further includes a third end face and a fourth end face; Both the third end face and the fourth end face are annular planes perpendicular to the central axis, and the third end face and the fourth end face are parallel to each other. The first end face is connected to the third end face and the fourth end face at its two ends along the central axis direction, respectively; The end of the pole shoe also includes a fifth end face and a sixth end face; The third end face and the fifth end face are in contact with each other, and the fourth end face and the sixth end face are in contact with each other.

3. The magnetic lens pole shoe structure according to claim 1, characterized in that, The height of the first end face along the central axis is greater than 5 mm.

4. The magnetic lens pole shoe structure according to claim 1, characterized in that, The end of the pole shoe away from the pole shoe base is provided with a seventh end face, which is perpendicular to the central axis and is used to withstand the assembly thrust along the central axis.

5. The magnetic lens pole shoe structure according to claim 1, characterized in that, At a first temperature, the outer diameter of the second end face is greater than the inner diameter of the first end face, and at a second temperature, the outer diameter of the second end face is smaller than the inner diameter of the first end face, so that the second end face and the first end face form an assembly gap at the second temperature.

6. The magnetic lens pole shoe structure according to claim 5, characterized in that, At the first temperature, the difference between the outer diameter of the second end face and the inner diameter of the first end face is 0.010 mm to 0.014 mm.

7. The magnetic lens pole shoe structure according to claim 1, characterized in that, The base of the pole shoe is made of pure iron.

8. The magnetic lens pole shoe structure according to claim 1, characterized in that, The material of the pole shoe end is an iron-nickel alloy with a nickel mass fraction of 80%.

9. A method for assembling a magnetic lens pole shoe structure, characterized in that, The assembly method for assembling the magnetic lens pole shoe structure according to any one of claims 1 to 8 includes: Cool the end of the pole shoe to a second temperature to create an assembly gap between the second end face and the first end face; Align the cooled pole shoe end and pole shoe base coaxially along the central axis, and apply an axial thrust along the central axis to the pole shoe end until the third end face and the fifth end face are in contact, and the fourth end face and the sixth end face are in contact. The temperature at the end of the pole shoe is raised back to the first temperature.

10. The assembly method of the magnetic lens pole shoe structure according to claim 9, characterized in that, The end of the pole shoe away from the pole shoe base is provided with a seventh end face, which is perpendicular to the central axis and is used to withstand the assembly thrust along the central axis. The cooled pole shoe end and pole shoe base are coaxially aligned along the central axis. An axial thrust along the central axis is applied to the pole shoe end until the third end face and the fifth end face are in contact, and the fourth end face and the sixth end face are in contact, including: The cooled end of the pole shoe and the base of the pole shoe are aligned coaxially along the central axis. An axial thrust along the central axis is applied to the seventh end face until the third end face and the fifth end face are in contact, and the fourth end face and the sixth end face are in contact.

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