High-image-quality ultraviolet objective lens based on reflection symmetric structure

Abstract

The invention discloses a high-image-quality ultraviolet objective lens based on a reflection symmetric structure, and particularly relates to the technical field of high-precision optical lenses, the high-image-quality ultraviolet objective lens comprises a ninety-degree prism and a protection window, a first lens is arranged on one side of the ninety-degree prism, a second lens is arranged on one side of the first lens, a third lens is arranged on one side of the second lens, and the protection window is arranged on the ninety-degree prism. A fourth lens is arranged on one side of the third lens, a fifth lens is arranged on one side of the fourth lens, and a sixth lens is arranged on one side of the fifth lens; the objective lens adopts a reflection symmetrical structure, incident and emergent rays are symmetrical to the axis, and the same lens is imaged in an up-down symmetrical position, so that the system magnification is constantly 1x, distortion can be compensated to be below 2 * 10 <-5 >%, the lens material has high transmittance and low absorption in an ultraviolet band, the focal plane temperature drift is reduced, accurate exposure is ensured, and the chip manufacturing yield is improved. And meanwhile, the modular design and the high-precision interface are convenient to assemble and maintain, and cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of high-precision optical lenses, and more specifically, to a high-image-quality ultraviolet objective lens based on a reflective symmetric structure. Background Art

[0002] In today's semiconductor manufacturing, optical detection and many other high-precision optical application fields, the performance requirements for UV objective lenses are becoming increasingly stringent. As chip manufacturing technology moves towards smaller processes, the lithography process poses extremely high challenges to the imaging quality and focal plane stability of UV objective lenses.

[0003] Traditional high-quality ultraviolet objective lenses have many limitations in practical applications. On the one hand, during the manufacturing process, due to the limitations of the processing technology, the curvature of the lens is difficult to reach the precise value of the theoretical design. A slight curvature deviation will cause a change in the light propagation path, which will in turn cause a large distortion, seriously affecting the accuracy of imaging. For example, in the photolithography process, image distortion may cause deviations in the chip circuit pattern, reducing the chip yield. On the other hand, under different working environments, temperature changes will cause changes in the refractive index of the lens material and changes in the air distance in the lens barrel, resulting in drift of the optimal focal plane position, that is, the focal plane temperature drift problem. This makes it difficult to maintain a stable imaging focus during long-term exposure, affecting the exposure quality. In addition, in some common optical path systems that require simultaneous alignment and exposure, it is difficult for traditional objective lenses to take into account the optical performance requirements of different bands. For example, in the ultraviolet exposure band and the visible light alignment band, different optical elements are usually required to achieve their respective functions, which not only increases the complexity and cost of the system, but may also introduce additional alignment errors and reduce the overall accuracy of the system. At the same time, with the increase of electromagnetic interference sources in the working environment, external electromagnetic interference will affect the performance of the optical components inside the objective lens, resulting in a decrease in imaging quality. Traditional objective lenses often lack effective electromagnetic shielding measures in design. Therefore, we propose a high-image quality ultraviolet objective lens based on a reflective symmetric structure to solve the above problems. Summary of the invention

[0004] In order to overcome the above-mentioned defects of the prior art, the embodiment of the present invention provides a high-quality ultraviolet objective lens based on a reflective symmetric structure. The objective lens adopts a reflective symmetric structure, the incident and outgoing light rays are symmetrical about the axis, and the same lens forms images at symmetrical positions above and below, so that the system magnification is constant at 1x and can compensate for distortion up to 2×10 -5 % or less, the lens material has high transmittance and low absorption in the ultraviolet band, which reduces focal plane temperature drift, ensures accurate exposure, and improves chip manufacturing yield. At the same time, modular design and high-precision interface facilitate assembly and maintenance, reducing costs.

[0005] To achieve the above object, the present invention provides the following technical solution: A high-image-quality ultraviolet objective lens based on a reflection-symmetric structure, comprising a ninety-degree prism and a protective window. On one side of the ninety-degree prism, there is a lens one. On one side of the lens one, there is a lens two. On one side of the lens two, there is a lens three. On one side of the lens three, there is a lens four. On one side of the lens four, there is a lens five. On one side of the lens five, there is a lens six; The lens one, lens two, and lens four are positive-focal-length converging lenses. The focal length of the lens one is set as f1, and the focal length range is 1.44 < f1 / f < 1.72. The focal length of the lens two is set as f2, and the focal length range is 2.21 < f2 / f < 2.46. The focal length of the lens four is set as f4, and the focal length range is 1.05 < f4 / f < 1.4; The lens three and lens five are set as negative-focal-length diverging lenses. The focal length of the lens three is set as f3, and the focal length range is -1.25 < f3 / f < -1.09. The focal length of the lens five is set as f5, and the focal length range is -6.21 < f5 / f < -6.04; The lens six is set as a concave reflecting lens, with a radius of R6, and the radius range is 1.41 < |R6 / f1| < 1.75; Ultraviolet light enters the optical system from above, sequentially passes through a set of protective windows above, is reflected by the upper surface of the ninety-degree prism and enters the objective lens, then sequentially passes through the lens one, lens two, lens three, lens four, and lens five and is incident on the lens six. The light is reflected backward at the lens six and then sequentially enters the lens five, lens four, lens three, lens two, and lens one, and finally is reflected by the lower surface of the ninety-degree prism and enters another set of protective windows below.

[0006] In a preferred embodiment, the objective lens adopts a reflection-symmetric structure, such that the incident and outgoing light rays are symmetric about the axis of the objective lens and pass through the upper and lower symmetric positions of the same lens. When the curvature of the lens has a deviation of less than 2% from the designed value, the distortion can be compensated to less than 2×10 -5 % by adjusting the relative positions of the objective lens and the ninety-degree prism, and the lens four and lens five.

[0007] In a preferred embodiment, the objective lens adopts a reflection-symmetric structure, such that the incident and outgoing light rays are symmetric about the axis of the objective lens and pass through the upper and lower symmetric positions of the same lens, thereby keeping the magnification of the objective lens system constant at 1x.

[0008] In a preferred embodiment, the objective lens operates in the ultraviolet band and has a transmittance of 99% in the aligned working band through coating.

[0009] In a preferred embodiment, the materials of the lens one, lens two, lens three, lens four, lens five, and lens six are selected to meet the requirements of high transmittance, low absorption, and optical stability in the ultraviolet band.

[0010] In a preferred embodiment, the overall structure of the objective lens system adopts a modular design, and the ninety-degree prism, lens one, lens two, lens three, lens four, lens five and lens six are assembled and adjusted as modules, wherein the connection between the modules adopts a high-precision mechanical interface.

[0011] In a preferred embodiment, a shielding cover is provided outside the objective lens system. The shielding cover is made of a metal alloy material capable of shielding electromagnetic interference, and its electromagnetic shielding effect within the electromagnetic frequency range of 100kHz-1GHz is 60dB; the thickness of the shielding cover is 2 mm, and a sealing structure with a splicing gap of 0.08 mm is adopted.

[0012] In a preferred embodiment, the edges of the lens 1, lens 2, lens 3, lens 4, lens 5 and lens 6 are chamfered, the chamfer angle is within the range of 45°±5°, the chamfer width is 0.2 mm, the chamfer surface roughness is 0.04 μm, and the non-optical surface of the lens is coated with an antistatic coating with a thickness of 5 μm and a surface resistance of 10 6 -10 9 ohm range.

[0013] Technical effects and advantages of the present invention: 1. To achieve low distortion and high magnification stability, a reflective symmetrical structure is adopted. The incident and outgoing light rays are symmetrical to the axis of the objective lens and pass through the upper and lower symmetrical positions of the same lens. When the curvature of the lens deviates from the design value by less than 2%, the distortion can be compensated to 2×10 -5 % or less, and the system magnification remains constant at 1x. This enables accurate and minimally distorted images to be obtained in high-precision imaging applications, while ensuring the consistency and accuracy of the imaging results. It is suitable for fields such as semiconductor lithography and precision measurement that require extremely high dimensional accuracy, improving imaging quality and measurement accuracy; it has high UV band transmittance and low focal plane temperature drift characteristics, and the lens material meets the requirements of high transmittance and low absorption in the UV band, reducing light energy loss and improving UV light utilization. At the same time, the reasonable selection of the thermal expansion coefficient of the lens material and the compensation effect of the reflective symmetric structure on temperature changes can effectively reduce focal plane temperature drift. In the UV exposure system, the stable focal plane position ensures accurate exposure, improves exposure consistency and accuracy, and improves the yield rate of processes such as chip manufacturing.

[0014] Second, reduce the requirements for lens processing accuracy. Traditional objective lenses require high-precision lens curvature to control distortion, while the reflective symmetric structure of the present invention combined with the adjustment mechanism reduces the stringent requirements for lens processing accuracy. Even if the lens curvature deviates, low distortion can still be achieved through adjustment, reducing manufacturing costs, improving the feasibility and efficiency of lens processing, and shortening the production cycle; the objective lens system adopts a modular design, and the 90-degree prism and lens group can be assembled and adjusted independently. The high-precision mechanical interface between modules has high positioning accuracy and small relative displacement tolerance. This design is conducive to the separate processing and testing of each module during the assembly process, improving assembly efficiency and quality, while facilitating equipment maintenance and repair, and reducing maintenance costs and difficulty.

[0015] 3. Good compatibility with the alignment system in the same optical path. The objective lens works in the ultraviolet band and has a transmittance of 99% in the alignment working band, and can work in the same optical path with the alignment system. In semiconductor manufacturing, the alignment error of the dual alignment system is avoided and the alignment accuracy is improved. The exposure and alignment functions are realized through one objective lens, which simplifies the optical path structure, reduces costs, and improves the overall accuracy and efficiency of the system; electromagnetic shielding and protection measures enhance reliability, and the external shielding cover effectively shields 100kHz-1GHz electromagnetic interference, with a shielding efficiency of not less than 60dB, to prevent external electromagnetic interference from affecting the performance of internal optical components. The chamfering treatment and antistatic coating of the lens edge reduce the problems of edge scattering and electrostatic adsorption of dust, further improve the stability and reliability of the objective lens, extend the service life of the objective lens, and ensure its stable operation in complex working environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of light radiation of the high-image-quality ultraviolet objective lens of the present invention.

[0017] In the figure: 1. 90-degree prism; 2. Protective window; 3. Lens one; 4. Lens two; 5. Lens three; 6. Lens four; 7. Lens five; 8. Lens six. DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] Refer to the instruction manual Figure 1, a high-image-quality ultraviolet objective lens based on a reflection symmetry structure, comprising a ninety-degree prism 1 and a protective window 2. On one side of the ninety-degree prism 1, there is a lens 1 3. On one side of the lens 1 3, there is a lens 2 4. On one side of the lens 2 4, there is a lens 3 5. On one side of the lens 3 5, there is a lens 4 6. On one side of the lens 4 6, there is a lens 5 7. On one side of the lens 5 7, there is a lens 6 8; Lens 1 3, lens 2 4, and lens 4 6 are positive focal length converging lenses. The focal length of lens 1 3 is set to f1, and the focal length range is 1.44 < f1 / f < 1.72. The focal length of lens 2 4 is set to f2, and the focal length range is 2.21 < f2 / f < 2.46. The focal length of lens 4 6 is set to f4, and the focal length range is 1.05 < f4 / f < 1.4; Lens 3 5 and lens 5 7 are set as negative focal length diverging lenses. The focal length of lens 3 5 is set to f3, and the focal length range is -1.25 < f3 / f < -1.09. The focal length of lens 5 7 is set to f5, and the focal length range is -6.21 < f5 / f < -6.04; Lens 6 8 is set as a concave reflecting lens, with a radius of R6, and the radius range is 1.41 < |R6 / f1| < 1.75; Ultraviolet light enters the optical system from above and successively passes through a set of upper protective windows 2, is reflected by the upper surface of the 90-degree prism 1 and enters the objective lens, then successively transmits through lens 1 3, lens 2 4, lens 3 5, lens 4 6 and lens 5 7 and then is incident on lens 6 8. The light is reversed at lens 6 8 and then successively enters lens 5 7, lens 4 6, lens 3 5, lens 2 4 and lens 1 3 again. Finally, it is reflected by the lower surface of the 90-degree prism 1 and enters another set of lower protective windows 2; The 90-degree prism 1 is made of K9 glass with stable optical properties, good transmittance and low dispersion characteristics in the ultraviolet band. The protective windows 2 are made of fused quartz material that can maintain high transmittance in the ultraviolet band and protect internal components; Lenses 1 3, 2 4 and 4 6 are selected as positive focal length converging lenses and made of H-ZF7 glass. Assuming f is 100 mm, the focal length f1 of lens 1 3 is processed to about 150 mm through precision optical processing to meet 1.44 < f1 / f < 1.72, the focal length f2 of lens 2 4 is processed to about 230 mm to meet 2.21 < f2 / f < 2.46, and the focal length f4 of lens 4 6 is processed to about 120 mm in the range of 1.05 < f4 / f < 1.4; Lenses 3 5 and 5 7 are selected as negative focal length diverging lenses and made of F2 glass, so that the focal length f3 of lens 3 5 is between -1.25 < f3 / f < -1.09, and the focal length f5 of lens 5 7 is between -6.21 < f5 / f < -6.04; Lens 6 8 is a concave mirror, and the substrate is made of glass-ceramics with a low coefficient of thermal expansion and coated with an aluminum film. If f1 is 150 mm, then R6 is controlled between 210 - 260 mm according to 1.41 < |R6 / f1| < 1.75. During installation, the 90-degree prism 1 is placed at the start of the optical path, and each lens is successively installed on one side of it and fixed with a high-precision lens barrel or positioning structure to ensure that the optical axes of each lens coincide with the system optical axis and have an appropriate interval. An optical experimental platform is built to simulate the path of ultraviolet light passing through the upper protective window 2, being reflected by the upper surface of the 90-degree prism 1 and entering the objective lens, then successively passing through each lens and being incident on lens 6 8, being reflected and then passing through each lens again, and finally being reflected by the lower surface of the 90-degree prism 1 and entering the lower protective window 2. Equipment such as a power meter and a beam analyzer is used to measure the energy loss and beam quality of the light at each component to verify the accuracy of the optical path.

[0020] The objective lens adopts a reflection-symmetric structure, so that the incident and outgoing light rays are symmetric about the axis of the objective lens and pass through the upper and lower symmetric positions of the same lens. When the curvature of the lens deviates from the design value by less than 2%, the distortion can be compensated to 2×10 by adjusting the relative positions of the objective lens, the 90-degree prism 1, lens 4 6 and lens 5 7 -5% or less; during assembly, high-precision optical alignment equipment such as laser interferometer and autocollimator are used to accurately measure and record the initial position and angle of each lens and the ninety-degree prism 1 to ensure that the incident and outgoing light rays are symmetrical to the axis of the objective lens and pass through the initial state of the upper and lower symmetrical positions of the same lens. When the high-precision lens curvature measuring instrument detects that the deviation of the lens curvature from the design value is less than 2%, a computer-controlled fine-tuning mechanism with a displacement accuracy of 0.001mm is used to first adjust the relative position of the objective lens and the ninety-degree prism 1, and a high-precision distortion measuring instrument is used to monitor the distortion in real time. If the requirements are not met, the positions of lens four 6 and lens five 7 are fine-tuned until the distortion is compensated to 2×10 -5 % or less. After the adjustment is completed, the objective lens is subjected to environmental simulation tests of different temperatures and humidity to verify that the reflective symmetrical structure and the adjusted state can continue to maintain the low distortion effect in the actual use environment.

[0021] The objective lens adopts a reflective symmetrical structure, so that the incident and outgoing light rays are symmetrical about the axis of the objective lens and pass through the upper and lower symmetrical positions of the same lens, thereby keeping the magnification of the objective lens system constant at 1x; after completing the adjustment of claim 2, use a high-precision optical alignment device to check again whether the incident and outgoing light rays are strictly symmetrical about the axis of the objective lens and pass through the upper and lower symmetrical positions of the same lens. If there is a deviation, fine-tune it, and use a high-precision magnification measuring instrument based on digital image correlation technology to monitor the magnification of the objective lens system in real time under different light incident angles and working distances. If the magnification deviates from 1x, fine-tune the lens spacing and angle through a lens spacing adjustment gasket with a thickness accuracy of 0.001mm and an angle adjustment mechanism with an angle accuracy of 0.001°. If the magnification is slightly larger, increase the lens spacing, and if it is slightly smaller, reduce it. After each adjustment, re-measure the magnification until the magnification of the objective lens system remains constant at 1x, and then test it under different light intensities, wavelength ranges and other working conditions to verify the stability of the magnification in actual complex scenes.

[0022] The objective lens works in the ultraviolet band, and has a transmittance of 99% in the alignment working band through coating. In order to meet the requirements that the objective lens works in the ultraviolet band and has a transmittance of 99% in the alignment working band, a multi-layer film system is plated on the surface of each lens by ion beam assisted deposition technology. The bottom anti-reflection film is made of MgF2 material. The anti-reflection effect is formed in the ultraviolet band by accurately controlling the coating thickness. The middle layer is a special functional film composed of a variety of dielectric materials for the alignment working band. The film structure and thickness are optimized to achieve high transmittance. The outer protective film is made of SiO2 material to enhance wear resistance and chemical stability. During the coating process, a spectrometer is used to monitor the transmittance in real time, and the coating rate, ion beam energy and other parameters are adjusted according to the monitoring data. For example, when the transmittance in the alignment working band does not reach 99%, the thickness of a dielectric film in the middle layer is adjusted. At the same time, the temperature, humidity and vacuum degree of the coating environment are strictly controlled. After the coating is completed, the surface roughness of the film layer is detected by an atomic force microscope, the adhesion is tested by the cross-cutting method, and the lens is subjected to high temperature, high humidity and other environmental simulation tests to detect the stability of the film layer and the change in transmittance.

[0023] The material selection of lens 13, lens 24, lens 35, lens 46, lens 57 and lens 68 meets the requirements of high transmittance, low absorption and optical stability in the ultraviolet band; for the materials of lens 13 to lens 68, in addition to considering the requirements of high transmittance, low absorption and optical stability in the ultraviolet band, the transmittance in the 200-400nm band is measured on an ultraviolet spectrometer and is required to be not less than 90%, the thermal expansion coefficient is measured with a thermal analysis device to ensure that temperature changes have little effect on optical properties, a long-term ultraviolet irradiation test is carried out using a UV aging test chamber to observe changes in optical properties, and after screening out materials that meet the requirements, high-precision grinding and polishing processes are used in lens processing, and the grinding pressure, speed, and polishing liquid and polishing pad selection are controlled to avoid the introduction of impurities and stress, and the stress is detected with a polarizing stress meter after processing. If the stress is too large, annealing treatment is performed to ensure that the optical properties of the material are not affected.

[0024] The overall structure of the objective lens system adopts a modular design. The ninety-degree prism 1, lens one 3, lens two 4, lens three 5, lens four 6, lens five 7 and lens six 8 are assembled and adjusted as modules. The modules are connected by a high-precision mechanical interface. The positioning accuracy error of the interface does not exceed ±0.01 mm. After the modules are assembled, the tolerance of the relative displacement is controlled within ±0.005 mm. The objective lens system is designed as a modular structure. The ninety-degree prism 1 is an independent module, and the lens group is another independent module. During manufacturing, the ninety-degree prism 1 is processed with high precision to ensure the angle accuracy and surface flatness. Each lens of the lens group is processed and tested separately to ensure that the optical performance meets the requirements. The modules are connected by a fixed The positioning pins and positioning holes have a matching structure, and the positioning pin diameter accuracy is controlled at ±0.005mm, and the positioning hole aperture accuracy is controlled at ±0.008mm to ensure that the interface positioning accuracy error does not exceed ±0.01mm. During assembly, first use a high-precision assembly fixture to install the lens group module in the lens barrel to ensure the accurate relative position of each lens, and then assemble the ninety-degree prism module with the lens group module through the positioning pins and positioning holes. Use a high-precision displacement sensor to monitor the relative displacement to ensure that the tolerance after assembly is controlled within ±0.005mm. After assembly, the image quality is tested using a resolution test board and an imaging system, the focal length accuracy is detected using a focal length measuring instrument, and disassembly and reassembly tests are performed to verify the convenience and reliability of the modular design.

[0025] A shielding cover is provided outside the objective system. The shielding cover is made of a metal alloy material capable of shielding electromagnetic interference, and its electromagnetic shielding effect is 60dB within the electromagnetic frequency range of 100kHz-1GHz; the thickness of the shielding cover is 2 mm, and a sealing structure with a splicing gap of 0.08 mm is adopted; the aluminum alloy 6061 material with good electromagnetic shielding performance is used to make the external shielding cover of the objective system, and the thickness is controlled at 2-3 mm. It is processed into an adaptive shape through precision sheet metal processing, and laser welding is adopted at the splicing part to ensure that the gap is 0.08 mm to form a sealing structure, and the surface of the shielding cover is subjected to conductive oxidation treatment to enhance the electromagnetic shielding performance. The electromagnetic shielding effectiveness test equipment is used to test within the electromagnetic frequency range of 100kHz-1GHz to ensure that the shielding effectiveness is not less than 60dB. After passing the test, the shielding cover is installed outside the objective system and ensures good grounding. The grounding is connected to the grounding terminal of the equipment using a low-resistance wire and the grounding resistance does not exceed 0.1Ω. After installation, the entire objective system is subjected to an electromagnetic compatibility test to detect whether the optical performance of the objective is affected under different electromagnetic interference environments.

[0026] The edges of lens 1 3, lens 2 4, lens 3 5, lens 4 6, lens 5 7 and lens 6 8 are chamfered, the chamfer angle is within the range of 45°±5°, the chamfer width is 0.2 mm, the chamfer surface roughness is 0.04 μm, and the non-optical surface of the lens is coated with an antistatic coating with a thickness of 5 μm and a surface resistance of 10 6-10 9 ohm range; CNC edge grinding machine is used to chamfer the edges of lens 13 to lens 68, and the chamfer angle is set within the range of 45°±5° and the width does not exceed 0.2 mm. After edge grinding, high-precision polishing is performed to make the surface roughness less than 0.05 microns. The quality of chamfering is tested by optical microscope and surface roughness meter. 5 microns thick organic polymer antistatic coating is applied on the non-optical surface of the lens by spraying process. During coating, the thickness tester is used for real-time monitoring to ensure uniformity. After curing, the surface resistance tester is used to measure to ensure that the surface resistance is within 10 6 -10 9 In the ohm range, the coated lenses are tested in different humidity environments to detect surface resistance stability and antistatic effect.

[0027] Finally, a few points should be explained: First, in the description of this application, it should be noted that, unless otherwise specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, which may refer to mechanical connection or electrical connection, or internal communication between two components, or direct connection. "upper", "lower", "left", "right", etc. are only used to indicate relative positional relationships. When the absolute position of the object being described changes, the relative positional relationship may change; Secondly: In the drawings of the embodiments disclosed in the present invention, only the structures related to the embodiments disclosed in the present invention are involved, and other structures can refer to the general design. In the absence of conflict, the same embodiment and different embodiments of the present invention can be combined with each other; Finally: The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A high-quality ultraviolet objective lens based on a reflective symmetric structure, comprising a ninety-degree prism (1) and a protective window (2), characterized in that: On one side of the 90-degree prism (1), there is a first lens (3). On one side of the first lens (3), there is a second lens (4). On one side of the second lens (4), there is a third lens (5). On one side of the third lens (5), there is a fourth lens (6). On one side of the fourth lens (6), there is a fifth lens (7). On one side of the fifth lens (7), there is a sixth lens (8). The first lens (3), the second lens (4), and the fourth lens (6) are positive focal length converging lenses. The focal length of the first lens (3) is set as f1, and the focal length range is 1.44 < f1 / f < 1.

72. The focal length of the second lens (4) is set as f2, and the focal length range is 2.21 < f2 / f < 2.

46. The focal length of the fourth lens (6) is set as f4, and the focal length range is 1.05 < f4 / f < 1.

4. The third lens (5) and the fifth lens (7) are negative focal length diverging lenses. The focal length of the third lens (5) is set as f3, and the focal length range is -1.25 < f3 / f < -1.

09. The focal length of the fifth lens (7) is set as f5, and the focal length range is -6.21 < f5 / f < -6.

04. The sixth lens (8) is a concave reflecting lens with a radius of R6, and the radius range is 1.41 < |R6 / f1| < 1.

75. Ultraviolet light enters the optical system from above, passes through the upper set of protective windows (2) in sequence, is reflected by the upper surface of the 90-degree prism (1) and enters the objective lens, then passes through the first lens (3), the second lens (4), the third lens (5), the fourth lens (6), and the fifth lens (7) in sequence and is incident on the sixth lens (8). The light is reflected backward at the sixth lens (8) and then enters the fifth lens (7), the fourth lens (6), the third lens (5), the second lens (4), and the first lens (3) in sequence, and finally is reflected by the lower surface of the 90-degree prism (1) and enters the lower set of protective windows (2).

2. The high-quality ultraviolet objective lens based on a reflection symmetric structure according to claim 1, characterized in that: The objective lens adopts a reflective symmetrical structure, so that the incident and outgoing light rays are symmetrical about the axis of the objective lens and pass through the upper and lower symmetrical positions of the same lens. When the curvature of the lens deviates from the design value by less than 2%, the distortion can be compensated to 2×10 by adjusting the relative positions of the objective lens and the ninety-degree prism (1), the lens four (6) and the lens five (7). -5 %the following.

3. The high-quality ultraviolet objective lens based on a reflection symmetric structure according to claim 2, characterized in that: The objective lens adopts a reflection-symmetric structure, so that the incident and outgoing light is symmetric about the axis of the objective lens and passes through the upper and lower symmetric positions of the same lens, thereby keeping the magnification of the objective lens system constant at 1x.

4. The high-quality ultraviolet objective lens based on a reflection symmetric structure according to claim 1, characterized in that: The objective lens operates in the ultraviolet band and has a transmittance of 99% in the aligned working band through coating.

5. The high-quality ultraviolet objective lens based on a reflection symmetric structure according to claim 1, characterized in that: The materials of the first lens (3), the second lens (4), the third lens (5), the fourth lens (6), the fifth lens (7), and the sixth lens (8) are selected to meet the requirements of high transmittance, low absorption, and optical stability in the ultraviolet band.

6. The high-quality ultraviolet objective lens based on a reflection symmetric structure according to claim 1, characterized in that: The overall structure of the objective lens system adopts a modular design. The 90-degree prism (1), the first lens (3), the second lens (4), the third lens (5), the fourth lens (6), the fifth lens (7), and the sixth lens (8) are assembled and adjusted as modules. Among them, the connection between the modules uses a high-precision mechanical interface.

7. The high-quality ultraviolet objective lens based on a reflection symmetric structure according to claim 1, characterized in that: The outside of the objective lens system is provided with a shielding cover. The shielding cover is made of a metal alloy material that can shield electromagnetic interference, and its electromagnetic shielding effect in the electromagnetic frequency range of 100 kHz - 1 GHz is 60 dB. The thickness of the shielding cover is 2 mm, and it adopts a sealed structure with a splicing gap of 0.08 mm.

8. The high-quality ultraviolet objective lens based on a reflection symmetric structure according to claim 1, characterized in that: The edges of the lens 1 (3), lens 2 (4), lens 3 (5), lens 4 (6), lens 5 (7) and lens 6 (8) are chamfered, the chamfer angle is within the range of 45°±5°, the chamfer width is 0.2 mm, the chamfer surface roughness is 0.04 μm, and the non-optical surface of the lens is coated with an antistatic coating with a thickness of 5 μm and a surface resistance of 10 6 -10 9 ohm range.

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