High-NA long-working-distance DUV microscope objective

Abstract

The invention discloses a DUV microscope objective with high NA and long working distance. The microscope objective is sequentially composed of a first lens group with negative focal power, a second lens group with positive focal power and a third lens group with positive focal power from an image side to an object side along an optical axis. The first lens group comprises a biconcave lens and a negative meniscus lens, the second lens group contains a diaphragm and is composed of a plurality of positive lenses, and the third lens group is composed of two positive meniscus lenses with concave surfaces facing the object side. By strictly controlling the focal length proportional relation (1 < = f1 / f < = 3, 11.2 < = f2 / f < = 12.2, and 12.7 < = f3 / f < = 13.7) and key structure parameters of the three groups of lenses, and by adopting an aspheric and full single lens structure, the contradiction between a high numerical aperture and a long working distance is cooperatively solved. According to the invention, the excellent performance that NA is greater than or equal to 0.85 and the working distance is greater than or equal to 14.6 mm at the wave band of 248nm is realized, the aberration correction is good, the service life is long, and the optical system is suitable for semiconductor detection and other precise optical fields.

Description

Technical Field

[0001] This invention relates to the field of optical imaging technology, specifically to a high numerical aperture (NA) and long working distance deep ultraviolet (DUV) microscope objective, suitable for industrial applications such as semiconductor defect detection, wafer analysis, laser processing, and precision visual measurement. Background Technology

[0002] As the core component of optical microscopes, microscope objectives are widely used in life sciences, materials analysis, and industrial inspection. With the advancement of semiconductor manufacturing processes towards smaller nodes, higher demands are placed on the resolution of microscopic imaging systems. According to the diffraction limit principle, imaging resolution is directly proportional to wavelength; therefore, microscope objectives operating in the deep ultraviolet band (such as 248 nm) possess higher resolution and have become key equipment for semiconductor inspection and analysis.

[0003] However, existing deep ultraviolet (DUV) microscope objectives have significant performance limitations. On one hand, achieving high numerical aperture (NA) typically involves complex lens structures and shortened working distances, limiting their applicability in applications requiring a certain objective-sample distance, such as laser processing and packaging inspection. For example, the microscope objective disclosed in patent CN119291899A, while achieving an NA of 0.8, has a working distance of only 2.2 mm, which is insufficient to meet the operational space requirements of practical applications. On the other hand, microscope objectives with longer working distances, such as those described in CN119291906A (10 mm), have a numerical aperture of only 0.4, resulting in limited resolution and failing to meet high-precision imaging requirements.

[0004] Therefore, there is a lack of existing microscope objectives that can simultaneously achieve high numerical aperture, long working distance, and excellent aberration correction in the deep ultraviolet band. In addition, cemented lenses are often used in traditional designs to correct aberrations, but cemented materials are prone to aging and decreased transmittance in the ultraviolet band, affecting the lifespan of the objective. Summary of the Invention

[0005] To address the aforementioned issues, this invention proposes a novel optical structure. Through rational optical configuration design, optical power allocation, and aspherical application, it achieves a working distance greater than 14.6 mm while ensuring high NA (≥0.85) and high resolution (≤0.2 μm). Furthermore, it employs a single-lens structure throughout, avoiding the ultraviolet lifetime issues associated with cemented lenses, thereby better meeting the needs of modern semiconductor industry and other high-precision optical inspection fields.

[0006] The technical solution of the present invention is as follows: A high-NA, long-working-distance DUV microscope objective, consisting of a first lens group, a second lens group, and a third lens group sequentially along the optical axis from the image side to the object side, is characterized by: The first lens group has negative optical power and is used to balance field curvature and correct coma. It consists of a biconcave negative lens and a meniscus negative lens arranged sequentially from the image side to the object side. The second lens group has positive optical power and an aperture stop inside it to correct spherical aberration and axial chromatic aberration; The third lens group has positive optical power and is composed of two meniscus positive lenses arranged sequentially from the image side to the object side, with the concave surfaces of both meniscus positive lenses facing the object side. The total focal length of the microscope objective is f. The focal lengths f1 of the first lens group, f2 of the second lens group, and f3 of the third lens group satisfy the following relationships: 1≤|f1 / f|≤3; 11.2≤|f2 / f|≤12.2; 12.7≤|f3 / f|≤13.7.

[0007] Furthermore, the air gap between the first lens group and the second lens group along the optical axis is d, and satisfies the following relationship: 5f ≤ d ≤ 15f.

[0008] Furthermore, the radius of curvature of the object-side surface of the meniscus positive lens closest to the object side in the third lens group is r16, and satisfies the following relationship: 0.01f ≤ 1 / r16 ≤ 0.06f.

[0009] Furthermore, the second lens group consists of four positive lenses arranged sequentially from the image side to the object side, including: a first meniscus lens, a biconvex lens, a second meniscus lens, and a third meniscus lens; the aperture stop is located between the biconvex lens and the second meniscus lens.

[0010] Furthermore, the image-side surface of the first meniscus lens in the second lens group and / or the image-side surface of the first meniscus lens in the third lens group are even-order aspherical surfaces.

[0011] Furthermore, the surface profile of the even-order aspherical surface satisfies the following formula: in, The axial sagitta in the Z-direction of an even-order aspherical surface; The height of an even-order aspherical surface; To fit the curvature of the sphere, it is numerically the reciprocal of the radius of curvature; To fit the conic coefficients; to are the coefficients of the 4th, 6th, 8th, and 10th orders of the aspherical polynomial, respectively.

[0012] Furthermore, the microscope objective has a working wavelength of 248 nm, a numerical aperture (NA) of not less than 0.85, a resolution better than 0.2 μm, and a working distance of not less than 14.6 mm.

[0013] Compared with the prior art, the technical effects of the present invention This invention provides a UV-band microscope objective with a high numerical aperture (NA=0.85), high resolution, and a long working distance greater than 14.6 mm. By rationally allocating lens power, controlling the light path within the lens, and using aspherical lenses, aberrations are reduced. All lenses in this invention are single lenses, avoiding the use of cemented lenses, thereby extending the microscope objective's lifespan in the UV band. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the optical path of the high-NA long-working-distance DUV microscope objective of the present invention; Figure 2 This is a modulation transfer function (MTF) plot of a high-NA, long-working-distance DUV microscope objective; Figure 3 This is an axial aberration map of a high-NA, long-working-distance DUV microscope objective; Figure 4 These are field curvature / distortion diagrams of high-NA, long-working-distance DUV microscope objectives; Figure (a) shows the field curvature curve of the high-NA, long-working-distance DUV microscope objective, and Figure (b) shows the distortion curve of the high-NA, long-working-distance DUV microscope objective. Detailed Implementation

[0015] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Those skilled in the art will understand that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0016] This invention provides a high numerical aperture (NA) microscope objective, particularly suitable for a wavelength of 248 nm, in the deep ultraviolet (DUV) band, with a long working distance. By using a three-component anti-long-distance optical power distribution structure with a "negative-positive-positive" configuration and by strictly controlling the focal length ratio and key structural parameters of each component, the contradiction between high NA and long working distance is resolved in a coordinated manner, and excellent aberration correction is achieved.

[0017] Example 1: like Figure 1 As shown, the optical system of the microscope objective in this embodiment includes a first lens group G1, a second lens group G2 and a third lens group G3 along the optical axis from the image side to the object side, wherein the second lens group G2 is provided with an aperture stop STO.

[0018] The first lens group G1 has negative optical power and includes a first lens L1 and a second lens L2 arranged sequentially along the optical axis. The first lens L1 is a biconcave lens with negative optical power, and the second lens L2 is a meniscus lens with negative optical power, its concave surface facing the image side. The main function of this lens group is to balance the field curvature of the system and correct the coma generated by the subsequent positive optical power group, while also laying the foundation for providing a long working distance for the system.

[0019] The second lens group G2 has positive optical power and includes a third lens L3, a fourth lens L4, a fifth lens L5, and a sixth lens L6 arranged sequentially along the optical axis. Specifically, the third lens L3 is a meniscus lens with positive optical power (concave side facing the image), the fourth lens L4 is a biconvex lens with positive optical power, and the fifth and sixth lenses L6 are both meniscus lenses with positive optical power. The aperture stop STO is positioned between the fourth lens L4 and the fifth lens L5. This lens group is the main converging group of the system, providing most of the positive optical power and undertaking the crucial task of correcting spherical aberration and axial chromatic aberration. The fourth lens L4 (biconvex) contributes positive spherical aberration to compensate for the negative spherical aberration generated by the third lens group.

[0020] The third lens group, G3, has positive optical power, and is arranged along the optical axis as the seventh lens L7 and the eighth lens L8. Both are meniscus lenses with positive optical power, and their concave surfaces face the object side. This object-facing configuration is one of the key features for achieving a high numerical aperture while maintaining a long working distance. This lens group is located close to the sample and is responsible for collecting large-angle light to achieve high NA. Fine control of its surface curvature ultimately balances the working distance and aberrations.

[0021] This embodiment also satisfies the following design conditions: 1. Optical Power Allocation: In this embodiment, the total focal length of the microscope objective is f. Through optimized design, the ratios of the focal lengths f1 of the first lens group, f2 of the second lens group, and f3 of the third lens group to the total focal length f satisfy: |f1 / f|=2.4; |f2 / f|=12.2; |f3 / f|=13.2. By rationally allocating optical power, aberrations in the microscope objective optical system of this embodiment are corrected. The first lens group G1 balances the field curvature of the optical system through its negative optical power and corrects the coma generated by the second lens group G2 and the third lens group G3. This ensures effective division of labor and synergy among the three lens groups in aberration correction and optical path extension.

[0022] 2. Key Spacing Conditions: The air gap d between the first lens group G1 and the second lens group G2 is controlled within the range of the relationship 5f ≤ d ≤ 15f. This allows the system to achieve a working distance of up to 14.648mm while avoiding excessive pressure on aberration (spherical aberration, coma) correction due to too small a gap, or insufficient working distance and redundant system due to too large a gap.

[0023] 3. Object-side surface curvature condition: The curvature radius r16 of the object-side surface (S16 surface) of the eighth lens L8, which is the closest surface to the object side in the third lens group G3, satisfies the relationship 0.01f ≤ 1 / r16 ≤ 0.06f. This ensures that the surface has sufficient curvature to collect large-angle light to achieve high NA, while not being excessively curved to avoid significantly shortening the working distance or introducing spherical aberration that is difficult to correct.

[0024] By setting each lens in the first lens group G1, the second lens group G2, and the third lens group G3 to be a single lens, the problem of low objective lens life in the ultraviolet band caused by using cemented lenses is avoided.

[0025] Table 1 below details the basic parameters of each lens in this embodiment, including: surface type, radius of curvature R, thickness d, material refractive index Nd, and Abbe number Vd. The units for radius of curvature R and thickness d are millimeters.

[0026] In this embodiment, fluorite and fused silica are used as materials to ensure the transmittance of the microscope objective in the ultraviolet band.

[0027] To effectively correct higher-order aberrations, this embodiment introduces two even-order aspherical surfaces at key locations: An image-side view of the third lens L3 located in the second lens group G1.

[0028] Another image-side surface is located on the seventh lens L7 in the third lens group G2. These aspherical surfaces are mainly used to correct coma, spherical aberration, and astigmatism. Their surface shape is defined by the general even-order aspherical formula, and the specific coefficients are shown in Table 2 below.

[0029] Table 2 Table 2 provides the design coefficients for even-order aspherical surfaces of the lenses in the microscope objectives in the embodiments. The specific values ​​can be adjusted according to product requirements and are not intended to limit the embodiments of the present invention. The surface number column in Table 2 has the same meaning as the surface number in Table 1. In the embodiments of the present invention, "E" represents an exponent with a base of 10.

[0030] In this embodiment, the surface of the aspherical lens satisfies the following formula: Where z(h) is the axial sagitta in the Z direction of the aspherical surface; h is the height of the aspherical surface; c is the curvature of the fitted sphere, which is numerically the reciprocal of the radius of curvature; and k is the fitted conic coefficient. to are the coefficients of the 4th, 6th, 8th, and 10th orders of the aspherical polynomial, respectively.

[0031] Optical software simulation verification shows that this embodiment has good imaging performance. Figure 2 The figure shows the MTF curve of an embodiment of the present invention. As shown in the figure, the MTF value is close to the diffraction limit and is greater than 0.3 at 3600 lp / mm, indicating excellent imaging quality.

[0032] Figure 3 This is an axial aberration diagram according to an embodiment of the present invention, where the vertical axis represents the normalized pupil coordinates and the horizontal axis represents the axial aberration, as shown below. Figure 3 As shown, in the embodiment, the distance from the image plane to the point where the ray intersects the optical axis is controlled within ±0.15μm in different regions of the on-axis field of view, which means that the aberrations are well controlled in different fields of view.

[0033] Figure 4 The figures are field curvature / distortion diagrams of an embodiment of the present invention. Figure (a) shows the meridional and sagittal field curvature curves of the microscope objective, both of which are controlled within 0.5 μm. Figure (b) shows the distortion curve of the microscope objective, with a maximum distortion of 0.31%, indicating that the field curvature and distortion have been effectively corrected.

[0034] The design specifications achieved in this embodiment are shown in Table 3:

Claims

1. A high-NA, long-working-distance DUV microscope objective, comprising a first lens group (G1), a second lens group (G2), and a third lens group (G3) sequentially along the optical axis from the image side to the object side, characterized in that: The first lens group (G1) has negative optical power and is used to balance field curvature and correct coma. It consists of a biconcave negative lens (L1) and a meniscus negative lens (L2) arranged sequentially from the image side to the object side. The second lens group (G2) has positive optical power and has an internal aperture stop (STO) for correcting spherical aberration and axial chromatic aberration. The third lens group (G3) has positive optical power and is composed of two meniscus positive lenses (L7, L8) arranged sequentially from the image side to the object side, with the concave surfaces of the two meniscus positive lenses (L7, L8) facing the object side. The total focal length of the microscope objective is f. The focal lengths f1 of the first lens group (G1), f2 of the second lens group (G2), and f3 of the third lens group (G3) satisfy the following relationship: 1≤|f1 / f|≤3. 11.2≤|f2 / f|≤12.2; 12.7≤|f3 / f|≤13.

7.

2. The high-NA, long-working-distance DUV microscope objective according to claim 1, characterized in that: The air gap between the first lens group (G1) and the second lens group (G2) along the optical axis is d, and satisfies the following relationship: 5f ≤ d ≤ 15f.

3. The high-NA, long-working-distance DUV microscope objective according to claim 1, characterized in that: The radius of curvature of the object-side surface of the meniscus positive lens (L8) closest to the object side in the third lens group (G3) is r16, and satisfies the following relationship: 0.01f ≤ 1 / r16 ≤ 0.06f.

4. The high-NA, long-working-distance DUV microscope objective according to claim 1, characterized in that: The second lens group (G2) consists of four positive lenses arranged sequentially from the image side to the object side, including: a first meniscus lens (L3), a biconvex lens (L4), a second meniscus lens (L5), and a third meniscus lens (L6); the aperture stop (STO) is located between the biconvex lens (L4) and the second meniscus lens (L5).

5. The high-NA, long-working-distance DUV microscope objective according to claim 4, characterized in that: The image-side surface of the first meniscus lens (L3) in the second lens group (G2) and / or the image-side surface of the first meniscus lens (L7) in the third lens group (G3) are even-order aspherical surfaces.

6. The high-NA, long-working-distance DUV microscope objective according to claim 5, characterized in that: The surface profile of the even-order aspherical surface satisfies the following formula: in, The axial sagitta in the Z-direction of an even-order aspherical surface; The height of an even-order aspherical surface; To fit the curvature of the sphere, it is numerically the reciprocal of the radius of curvature; To fit the conic coefficients; to are the coefficients of the 4th, 6th, 8th, and 10th orders of the aspherical polynomial, respectively.

7. The high-NA, long-working-distance DUV microscope objective according to claim 1, characterized in that: The microscope objective has a working wavelength of 248 nm, a numerical aperture (NA) of not less than 0.85, a resolution better than 0.2 μm, and a working distance of not less than 14.6 mm.

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