Objective lens

Abstract

The invention relates to the technical field of optics and relates to an objective lens. The four lens groups are arranged to form a double-telecentric optical system, and optimization and correction of spherical aberration, coma, chromatic aberration, astigmatism, telecentricity and distortion of incident laser beams are achieved through the four lens groups. According to the invention, ultraviolet wavelength high-resolution imaging can be satisfied, high light collection efficiency can be ensured, and weak ultraviolet imaging and a large imaging field of view can be realized.

Description

Technical Field

[0001] The invention relates to the technical field of optics, and particularly to an objective lens. Background Art

[0002] Ultraviolet light sources are often used in semiconductor measurement or detection applications. According to the Rayleigh criterion, the limiting resolution of the lens is σ=0.61λ / NA; where λ represents the wavelength and NA represents the numerical aperture of the lens. From the above formula, it can be seen that compared with visible light, under the same numerical aperture, using short-wavelength ultraviolet light imaging can distinguish smaller details and achieve higher resolution.

[0003] Currently, industrial lenses are generally designed and optimized for visible light, and these lenses may not perform well for ultraviolet imaging. With the development of semiconductor measurement and detection technology, the requirements for lens imaging performance have become increasingly higher, requiring not only high resolution and large target imaging, but also high magnification requirements. Since there are fewer optical materials with high transmittance in the ultraviolet band, it is difficult to correct the chromatic aberration of ultraviolet lenses and improve the imaging quality. In addition, compared to general microscope lenses, industrial lenses usually require higher object distances. All of these increase the difficulty of designing ultraviolet lenses. Summary of the invention

[0004] In view of this, an object of the present invention is to provide a high magnification double telecentric optical system with high resolution, large target surface imaging, and high object distance.

[0005] In the first aspect, an embodiment of the present invention provides an objective lens, comprising: a first lens group arranged in sequence from the object side to the image side, for controlling the divergence angle and the bending angle of the incident light beam to reduce the spherical aberration and coma of the incident light beam; a second lens group, for correcting the chromatic aberration of the incident light beam by using a positive and negative lens combination with a dispersion difference; a third lens group, which is a double-cemented negative lens group structure, for adjusting the bending of the incident light beam relative to the upper part of the optical axis and correcting the astigmatism of the incident light beam; a fourth lens group, which is a combination of convex lenses and concave lenses, for optimizing the telecentricity of the incident light beam and correcting distortion; the first lens group and the second lens group form a front lens group, the third lens group and the fourth lens group form a rear lens group, and apertures are arranged on the rear focal plane of the front lens group and the front focal plane of the rear lens group to form a double telecentric lens structure.

[0006] In some specific implementations, the first lens group includes a first positive lens group with positive optical power, which is mainly composed of a first lens, a second lens, and a third lens with successively decreasing degrees of bending of the light incident surface. The first lens and the second lens are concave mirrors, the third lens is a plano-convex lens, and the first positive lens group is used to bend a light beam incident at a large angle.

[0007] In some specific implementations, the first lens group also includes a second positive lens group with positive optical power, which is mainly composed of a double convex lens and a meniscus lens. The second positive lens group is used to smoothly bend light and control spherical aberration and coma.

[0008] In some specific implementations, the second mirror group includes two double-cemented lens groups, the first double-cemented lens group is provided with a positive meniscus lens and a negative meniscus lens in sequence along the incident direction of the light beam, and the second double-cemented lens group is provided with a biconcave lens and a biconvex lens in sequence along the incident direction of the light beam; and the dispersion coefficient of the positive meniscus lens is greater than that of the negative meniscus lens, and the dispersion coefficient of the biconcave lens is less than that of the biconvex lens.

[0009] In some specific implementations, the dispersion coefficients of the positive meniscus lens, the negative meniscus lens, the biconcave lens and the biconvex lens are respectively in the following ranges: [58, 62], [33, 37], [28, 32], [62, 64].

[0010] In some specific implementations, the third lens group is provided with a positive meniscus lens and a double concave lens in sequence along the incident direction of the light beam, and the concave surface of the positive meniscus lens is the light incident surface.

[0011] In some specific implementations, the fourth lens group is provided with a positive meniscus lens and a plano-convex lens in sequence along the incident direction of the light beam, and the concave surface of the positive meniscus lens is the light incident surface.

[0012] In some specific implementations, the total length of the objective lens is less than or equal to 720 mm, the object side numerical aperture is greater than or equal to 0.35, and the magnification is 6X.

[0013] In some specific implementations, the object side working distance of the objective lens is greater than 80 mm, and the diameter of the object side field of view is 45 mm.

[0014] In some specific implementations, the object-side resolution of the objective lens is greater than or equal to 0.83 um.

[0015] The embodiments of the present invention bring the following beneficial effects: The embodiment of the present invention provides an objective lens, which forms a double telecentric optical system by arranging four lens groups, and optimizes and corrects the spherical aberration, coma, chromatic aberration, astigmatism, telecentricity and distortion of the incident laser beam through the four lens groups. It can meet the requirements of ultraviolet wavelength high-resolution imaging and ensure high light collection efficiency, and can realize weak ultraviolet light imaging and a large imaging field of view.

[0016] Other features and advantages of the present disclosure will be set forth in the following description, or some features and advantages may be inferred or unambiguously determined from the description, or may be learned by implementing the above-mentioned technology of the present disclosure.

[0017] In order to make the above-mentioned objectives, features and advantages of the present disclosure more obvious and easy to understand, preferred embodiments are specifically cited below and described in detail with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 A schematic diagram of the structure of an objective lens provided by an embodiment of the present invention; Figure 2 A schematic diagram of the MTF of the objective lens provided in an embodiment of the present invention; Figure 3 The point diagram of the objective lens provided by the embodiment of the present invention.

[0020] Icon: O-object side; ST-aperture; I-image side; G1-first mirror group; G2-second mirror group; G3-third mirror group; G4-fourth mirror group; G11-first positive lens group; G12-second positive lens group; 1-first lens; 2-second lens; 3-third lens; 4-fourth lens; 5-fifth lens; 6-sixth lens; 7-seventh lens; 8-eighth lens; 9-ninth lens; 10-tenth lens; 11-eleventh lens; 12-twelfth lens; 13-thirteenth lens; 14-filter device. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are 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.

[0022] The objective lens provided in the embodiment of the present application is applied to an optical inspection and measurement scene to provide a light source for the object to be inspected and measured. Among them, the optical inspection and measurement scene can be mainly a semiconductor product, especially a wafer optical inspection and measurement scene. That is, in this embodiment, the object to be inspected and measured is a wafer. Among them, the light source is a laser light source, that is, a laser beam is projected onto the wafer.

[0023] Among them, in the existing technology, with the development of semiconductor measurement and detection technology, the requirements for lens imaging performance have become increasingly higher, which not only need to meet the requirements of high resolution and large target surface imaging, but also need to meet the requirements of high magnification. Due to the small number of optical materials with high transmittance in the ultraviolet band, it is difficult to correct the chromatic aberration of ultraviolet lenses and improve the imaging quality. In addition, compared with general microscope lenses, the requirements for the objective lens of industrial lenses usually need to be improved, which increases the difficulty of designing ultraviolet lenses.

[0024] Therefore, in view of the above background technology, the embodiments of the present application provide an objective lens that can achieve the above lens requirements in optical inspection and measurement scenarios.

[0025] For details, see Figure 1 The present invention provides a structural diagram of a projection objective lens of a first embodiment. The total length of the projection objective lens of this embodiment does not exceed 720 mm, is suitable for the ultraviolet spectrum range, has an object numerical aperture greater than or equal to 0.35, a magnification of 6X, an object working distance greater than 80 mm, and a diameter of an object field of view of 45 mm.

[0026] In this embodiment, the objective lens is provided with a first lens group G1, a second lens group G2, a third lens group G3 and a fourth lens group G4 in sequence from the incident direction of the light beam; wherein the first lens group G1 and the second lens group G2 constitute a front lens group for reducing the spherical aberration and chromatic aberration of the incident light beam; the third lens group G3 and the fourth lens group G4 constitute a rear lens group for correcting the spherical aberration, coma, astigmatism and telecentricity of the incident light beam as well as optimizing and correcting the distortion.

[0027] Specifically, the first lens group G1 controls the divergence angle and bending angle of the incident light beam to achieve control of spherical aberration and coma, and the second lens group G2 corrects the chromatic aberration of the incident light beam by using a combination of positive and negative lenses with different dispersions. The third lens group G3 corrects astigmatism by adjusting the relative position of the incident light beam relative to the optical axis. Specifically, the third lens group G3 adjusts the incident light beam to bend relative to the upper part of the optical axis so that the astigmatism is corrected; the fourth lens group G4 optimizes the telecentricity of the incident light beam and corrects the distortion through a combination of a convex lens and a concave lens.

[0028] In this embodiment, a stop ST is provided on the rear focal plane of the front lens group and the front focal plane of the rear lens group, so that the overall objective lens system forms a double telecentric lens structure.

[0029] In this embodiment, the first lens group is composed of two positive lens groups, namely, a first positive lens group G11 and a second positive lens group G12. The first positive lens group G11 is used to bend a large-angle incident light beam, and the second positive lens group G12 is used to gently bend the light beam, thereby controlling spherical aberration and coma.

[0030] The first positive lens group G11 is a positive focal power lens group, which is composed of three lenses, namely: a first lens 1, a second lens 2 and a third lens 3. The bending degrees of the incident light surfaces of the three lenses along the optical axis direction decrease in sequence. Specifically, the first lens 1 and the second lens 2 are concave mirrors, and the third lens 3 is a plano-convex lens. The concave and convex directions of the above lenses are determined according to the corresponding incident light surface curvature radius and the light exit surface curvature radius.

[0031] Specifically, if the curvature radius of the light incident surface and the curvature radius of the light exiting surface of the first lens 1 are negative values, then for the concave surface of the first lens 1 facing the object side, the curvature radius of the light incident surface and the curvature radius of the light exiting surface of the second lens 2 are negative values, then for the concave surface of the second lens 2 facing the object side, the curvature radius of the light incident surface of the third lens 3 is infinite, and the curvature radius of the light exiting surface of the third lens 3 is negative, then for the convex surface of the third lens facing the image side. In addition, if the absolute values ​​of the curvature radii of the first lens 1, the second lens 2, and the curvature radii of the light incident surfaces of the third lens 3 increase in sequence, it means that the bending degrees of the light incident surfaces of the first lens 1, the second lens 2, and the third lens 3 decrease in sequence.

[0032] In this embodiment, by adjusting and setting the curvature radii of the first lens 1, the second lens 2 and the third lens 3, the first lens 1, the second lens 2 and the third lens 3 are adjusted to adjust the bending degree of the incident light beam, thereby bending the incident light beam at a large angle.

[0033] The second positive lens group G12 is also a positive power lens group, which is composed of two lenses, namely: a fourth lens 4 and a fifth lens 5.

[0034] Specifically, the fourth lens 4 is a biconvex lens, and the fifth lens 5 is a meniscus lens. The meniscus lens is a positive meniscus lens, and its concave surface is arranged toward the image side. In this embodiment, the second positive lens group G12 is used to gently bend the light, thereby controlling the spherical aberration and coma of the incident light beam.

[0035] In this embodiment, the second lens group G2 includes two double-cemented lens groups, the first double-cemented lens group is sequentially provided with a sixth lens 6 and a seventh lens 7 along the incident direction of the incident light beam, wherein the sixth lens 6 is a positive meniscus lens, and the seventh lens 7 is a negative meniscus lens; the second double-cemented lens group is sequentially provided with an eighth lens 8 and a ninth lens 9 along the incident direction of the incident light beam, wherein the eighth lens 8 is a biconcave lens, and the ninth lens 9 is a biconvex lens.

[0036] Among them, the refractive index and dispersion coefficient of the positive meniscus lens, negative meniscus lens, biconcave lens and biconvex lens in this embodiment are different.

[0037] Specifically, the dispersion coefficient is greater than that of the negative meniscus lens, the dispersion coefficient of the double concave lens is smaller than that of the double convex lens, and the positive meniscus lens, the negative meniscus lens, the double concave lens and the double convex lens have different refractive indices, and the refractive index of the positive meniscus lens is smaller than that of the negative meniscus lens, the refractive index of the negative meniscus lens is greater than that of the double concave lens, and the refractive index of the double concave lens is smaller than that of the negative meniscus lens.

[0038] Specifically, the dispersion coefficient of the positive meniscus lens is any value in [58, 62], the dispersion coefficient of the negative meniscus lens is any value in [33.37], the dispersion coefficient of the biconcave lens is any value in [28, 32], and the dispersion coefficient of the biconvex lens is any value in [62, 64]. The refractive index of the positive meniscus lens is any value in [1.62, 1.66], the refractive index of the negative meniscus lens is any value in [1.73, 1.77], the refractive index of the biconcave lens is any value in [1.70, 1.74], and the refractive index of the biconvex lens is any value in [1.60, 1.64].

[0039] It can be seen from the above that in this embodiment, by combining convex and concave lenses with different refractive indices and dispersion rates, the chromatic aberration of the incident light beam can be reduced.

[0040] In this embodiment, the third lens group G3 is a double-cemented negative lens group structure, and a tenth lens 10 and an eleventh lens 11 are sequentially arranged along the incident direction of the incident light beam, wherein the tenth lens 10 is a positive meniscus lens, and the eleventh lens 11 is a double concave lens. The concave surface of the positive meniscus lens is the light incident surface.

[0041] The dispersion coefficient of the tenth lens 10 is the same as that of the eleventh lens 11, but the refractive index thereof is different, and the refractive index of the tenth lens 10 is greater than that of the eleventh lens 11. By setting it in this way, the bending tendency of the incident light beam can be adjusted, and the incident light beam is adjusted to be bent relative to the upper part of the optical axis, thereby correcting the astigmatism of the incident light beam.

[0042] Specifically, the refractive index of the tenth lens is any value in the range of [1.63, 1.67], and the refractive index of the eleventh lens is any value in the range of [1.49, 1.53].

[0043] In this embodiment, the fourth lens group G4 is a combination lens of a convex lens and a concave lens, and a twelfth lens 12 and a thirteenth lens 13 are sequentially arranged along the incident direction of the incident light beam.

[0044] The twelfth lens 12 is a concave lens, specifically a positive meniscus lens, whose concave surface is arranged at the light incident surface; the thirteenth lens 13 is a convex lens, specifically a plano-convex lens, whose convex surface is arranged at the light incident surface. Moreover, in this embodiment, the refractive index of the twelfth lens 12 is less than the refractive index of the thirteenth lens 13.

[0045] Specifically, in this embodiment, the refractive index of the twelfth lens 12 is any one of [1.82, 1.86], and the refractive index of the thirteenth lens 13 is any one of [1.48, 1.52].

[0046] Since the projection objective has a double telecentric structure, the chief light of each field of view on the object side is incident on the front surface of the objective approximately parallel to the optical axis; the chief light of each field of view on the image side is emitted approximately parallel to the optical axis and converges on the image plane.

[0047] Table 1 shows the specific parameters of the objective lens of the above embodiment. A positive radius R indicates that the center of curvature is on the side close to the object, and a negative radius R indicates that the center of curvature is on the side close to the image. The units of the radius R and thickness d are both in millimeters. Among them, "O" in the serial number represents the object, "I" represents the image, "ST" represents the aperture, "a" represents the mirror surface on the side close to the object, and "b" represents the mirror surface on the side close to the image. Taking "11a" as an example, it represents the mirror surface on the side close to the object of the eleventh lens.

[0048] Table 1. Parameters of the objective lens in the embodiment serial number Face type Radius R Thickness d Refractive Index / Abbe Number O STANDARD unlimited 86.925 1a STANDARD -118.600 6.500 1.65;55.89 1b STANDARD -70.060 0.200 2a STANDARD -173.650 5.500 1.62;63.39 2b STANDARD -101.650 0.200 3a STANDARD unlimited 5.200 1.52;64.17 3b STANDARD -241.670 18.000 4a STANDARD 223.810 25.000 1.49;70.41 4b STANDARD -505.260 30.000 5a STANDARD 113.280 25.000 1.55;63.46 5b STANDARD 519.330 0.900 6a STANDARD 66.940 10.100 1.64;60.10 6b STANDARD 281.270 23.200 1.75;34.95 7a STANDARD 281.270 23.200 1.75;34.95 7b STANDARD 41.950 6.800 8a STANDARD -312.600 21.850 1.72;29.51 8b STANDARD 46.150 25.000 1.62;63.39 9a STANDARD 46.150 25.000 1.62;63.39 9b STANDARD -163.180 30.000 ST STANDARD unlimited 30.000 10a STANDARD -69.270 25.000 1.65;33.85 10b STANDARD -36.780 25.000 1.51;61.04 11a STANDARD -36.780 25.000 1.51;61.04 11b STANDARD 68.440 31.100 12a STANDARD -109.290 25.000 1.84;43.13 12b STANDARD -88.780 30.700 13a STANDARD 115.410 25.000 1.50;66.95 13b STANDARD unlimited 108.700 I STANDARD unlimited 0.000 For this embodiment, an objective lens structure corresponding to an implementable mode is also proposed. In this implementation mode, a filter device is arranged between the thirteenth lens and the image side to achieve filtering processing of the incident light beam.

[0049] Specifically, the optical filter device may be any one of an optical attenuation sheet, an optical filter, and a dustproof sheet having a similar refractive index and thickness to the thirteenth lens.

[0050] For the above objective lens structure, the first lens to the thirteenth lens are spherical lenses, and in order to further reduce the chromatic aberration of the objective lens, the dispersion coefficients corresponding to the lenses in the above objective lens should meet the following conditions: 54≤v1≤58, 62≤v2≤66, 62≤v3≤66, 68≤v4≤72, 62≤v5≤66, 58≤v6≤62, 33≤v7≤37, 28≤v8≤32, 62≤v9≤64, 62≤v10≤66, 62≤v11≤66, 40≤v12≤44, 65≤v13≤69, 62≤v14≤66. Among them, v1, v2, v3, v4, v5, v6, v7, v8, v9, v10, v11, v12, v13 are dispersion coefficients of the first lens to the thirteenth lens, respectively, and v14 is dispersion coefficient of the filter device.

[0051] In addition, the refractive indices corresponding to the lenses in the above-mentioned objective lens should satisfy the following conditions: 1.63≤n1≤1.67, 1.60≤n2≤1.64, 1.50≤n3≤1.54, 1.48≤n4≤1.52, 1.53≤n5≤1.57, 1.62≤n6≤1.66, 1.73≤n7≤1.77, 1.70≤n8≤1.74, 1.60≤n9≤1.64, 1.63≤n10≤1.67, 1.49≤n11≤1.53, 1.82≤n12≤1.86, 1.48≤n13≤1.52, and 1.50≤n14≤1.54. Among them, n1, n2, n3, n4, n5, n6, n7, n8, n9, n10, n11, n12, and n13 are reflectivity representations of the first lens to the thirteenth lens respectively, and n14 is the reflectivity representation of the filter device.

[0052] See also Figure 2 , is a schematic diagram of the MTF of the objective lens in the above embodiment. It can be seen from the figure that for the objective lens in this embodiment at high spatial frequencies, for example, when the spatial frequency is 100lp / mm, the contrast is still above 40%, which can fully illustrate that the imaging effect of the objective lens in this embodiment is still very good when the target surface is large.

[0053] See also Figure 3, is the point diagram of the objective lens in the above embodiment, and the figure contains five different spot diagrams, wherein the five figures respectively show the point diagrams corresponding to the spots formed by the object under different field of view ranges. The different field of view ranges refer to the different offsets of the object relative to the detection surface of the sensor, for example, the image plane: the center of the object on the 0.000mm surface is concentric with the center of the detection surface, and the image plane: the center offset of the 5.625mm surface is 5.625mm. The radius of the Airy disk in the figure is 3.702μm, and the RMS radii corresponding to the above five different fields of view are 2.772μm, 3.194μm, 3.799μm, 4.076μm and 4.856μm respectively; the corresponding GEO halves are 4.692μm, 7.590μm, 12.669μm, 16.832μm and 20.898μm respectively.

[0054] The objective lens provided in this embodiment can realize large target surface imaging. The object side numerical aperture of the objective lens is 0.35, so that the objective lens can accept light with a wider range of incident angles, thereby having a higher light receiving ability and focusing ability, so that the objective lens has a higher resolution and better imaging quality, and the object side resolution can reach 0.83um. In addition, under the premise of ensuring a large target surface, the objective lens can achieve a relatively high object distance, so that the object distance is above 80mm, and there is sufficient space for installation between the objective lens and the object surface; and, for the large target surface optimization design of this embodiment, the maximum imaging target surface diameter of the objective lens reaches 45mm, and it can still maintain a high imaging quality.

[0055] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An objective lens, characterized in that: The following are arranged in sequence from the object side to the image side: The first mirror group is used to control the divergence angle and the bending angle of the incident light beam to reduce the spherical aberration and coma aberration of the incident light beam; The second mirror group corrects the chromatic aberration of the incident light beam by using a combination of positive and negative lenses with different dispersion; The third lens group is a double-cemented negative lens group structure, which is used to adjust the bending of the incident light beam relative to the upper part of the optical axis and correct the astigmatism of the incident light beam; The fourth lens group is a combination of convex and concave lenses, which is used to optimize the telecentricity of the incident light beam and correct distortion; The first lens group and the second lens group form a front lens group, the third lens group and the fourth lens group form a rear lens group, and apertures are arranged on the rear focal plane of the front lens group and the front focal plane of the rear lens group to form a double telecentric lens structure.

2. The objective lens according to claim 1, characterized in that The first lens group includes a first positive lens group with positive optical power, which is mainly composed of a first lens, a second lens and a third lens with decreasing degrees of bending of the light incident surface. The first lens and the second lens are concave mirrors, and the third lens is a plano-convex lens. The first positive lens group is used to bend a light beam incident at a large angle.

3. The objective lens according to claim 2, characterized in that The first lens group also includes a second positive lens group with positive optical power, which is mainly composed of a double convex lens and a meniscus lens. The second positive lens group is used for gently bending light and controlling spherical aberration and coma.

4. The objective lens according to claim 1, characterized in that The second lens group includes two double-cemented lens groups, the first double-cemented lens group is provided with a positive meniscus lens and a negative meniscus lens in sequence along the incident direction of the light beam, and the second double-cemented lens group is provided with a biconcave lens and a biconvex lens in sequence along the incident direction of the light beam; and the dispersion coefficient of the positive meniscus lens is greater than that of the negative meniscus lens, and the dispersion coefficient of the biconcave lens is less than that of the biconvex lens.

5. The objective lens according to claim 4, characterized in that The dispersion coefficients of the positive meniscus lens, the negative meniscus lens, the biconcave lens and the biconvex lens are respectively in the following ranges: [58, 62], [33, 37], [28, 32], [62, 64].

6. The objective lens according to claim 1, characterized in that The third lens group is provided with a positive meniscus lens and a double concave lens in sequence along the incident direction of the light beam, and the concave surface of the positive meniscus lens is the light incident surface.

7. The objective lens according to claim 1, characterized in that The fourth lens group is provided with a positive meniscus lens and a plano-convex lens in sequence along the incident direction of the light beam, and the concave surface of the positive meniscus lens is the light incident surface.

8. The objective lens according to claim 1, characterized in that The total length of the objective lens is less than or equal to 720 mm, the object side numerical aperture is greater than or equal to 0.35, and the magnification is 6X.

9. The objective lens according to claim 1, characterized in that The object side working distance of the objective lens is greater than 80 mm, and the diameter of the object side field of view is 45 mm.

10. The objective lens according to claim 1, characterized in that The object-side resolution of the objective lens is greater than or equal to 0.83 um.

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