Double telecentric optical system for focusing and leveling

Abstract

The invention discloses a double telecentric optical system for focusing and leveling, which comprises an object plane, a first group of lenses, an aperture diaphragm, a second group of lenses and an image plane which are sequentially arranged along the direction of an optical axis, the first group of lenses and the second group of lenses are symmetrically arranged about the aperture diaphragm, the first group of lenses comprises five lenses, and the second group of lenses comprises five lenses. The second group of lenses comprises five lenses, and for each lens in the first group of lenses, the lens arranged symmetrically with the lens in the second group of lenses has the same parameter as the lens. The invention further discloses a focusing and leveling system comprising the double telecentric optical system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of semiconductor, in particular to a double telecentric optical system for focus leveling. BACKGROUND

[0002] A lithography machine is a core equipment in the manufacture of ultra-large scale integrated circuits, and its performance directly determines the feature size and integration level of the integrated circuits. The lithography machine has two key performance parameters: resolution and depth of focus. According to the lithography imaging theory, the improvement of the resolution is usually accompanied by a sharp decrease in the depth of focus. The depth of focus refers to the axial range in which the optical system can obtain clear imaging on the image plane. In advanced lithography processes, the depth of focus of the projection objective may be only a few hundred nanometers or even smaller. The focus leveling system is an important subsystem of the lithography machine, which functions to monitor and control the position and attitude of the silicon wafer surface in real time, and to ensure that the silicon wafer surface in the exposure area is always within the depth of focus of the projection objective. Therefore, as the lithography resolution continues to improve, extremely stringent requirements are placed on the measurement accuracy of the focus leveling system.

[0003] At present, the mainstream focus leveling system generally adopts optical triangulation method. The basic principle is as follows: a known projection mark is imaged on the silicon wafer surface through a set of optical systems. When the silicon wafer surface is defocused or tilted relative to the focal plane of the projection objective, the position, shape or intensity of the mark image will change. By imaging the changed mark image onto the sensor plane through another set of optical systems, the defocus amount and tilt amount of the silicon wafer can be calculated by analyzing the image information on the sensor plane, and then the worktable is driven for compensation adjustment, so as to realize accurate control of the position and attitude of the silicon wafer surface.

[0004] There is a double telecentric optical system with twelve lenses in the prior art, and the center thickness of some lenses is relatively thin, which increases the difficulty and cost of processing and adjustment. The processing precision of the lenses is required to be high, and the too thin lenses are easy to be damaged in the processing process, and it is also difficult to ensure their accurate position and attitude in the adjustment process, thereby affecting the performance of the entire optical system. In addition, the image-side NA of this double telecentric optical system is only 0.03, and the cutoff spatial frequency is only 100 lp / mm, and the resolution is low. Furthermore, the telecentricity of this prior art solution is 0.1°, and there is still about 350 µm of measurement error for a 200 mm working distance.

[0005] In view of this, it is desirable to provide an improved double telecentric optical system for focus leveling. SUMMARY

[0006] The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.

[0007] The present application provides a kind of for focusing leveling double telecentric optical system, comprising: the object plane of sequentially arranged along the direction of optical axis, first group of lenses, aperture diaphragm, second group of lenses and image plane, wherein, first group of lenses and second group of lenses about aperture diaphragm symmetrically arranged, and wherein, first group of lenses includes five lenses, second group of lenses includes five lenses, and for each lens in first group of lenses, the lens symmetrically arranged in second group of lenses with the parameter of this lens is identical with this lens.

[0008] In some embodiments, the magnification of the system is -1, and the image-side field of view is a circular field of view with a diameter of 44 mm.

[0009] In some embodiments, the total lens length of the system is 240 mm, and the working distance of the system is 200 mm, wherein the total lens length includes the distance from the first lens in the first group of lenses to the last lens in the second group of lenses, and the working distance includes the distance from the object plane to the first lens in the group of lenses.

[0010] In some embodiments, the telecentricity of the system is less than 0.01°.

[0011] In some embodiments, the first group of lenses includes a first lens, a second lens, a third lens, a fourth lens, and a fifth lens, wherein the first lens is a plano-convex lens, the second lens is a plano-convex lens, the third lens is a plano-convex lens, the fourth lens is a plano-concave lens, and the fifth lens is a plano-convex lens.

[0012] In some embodiments, the fourth lens and the fifth lens are cemented lenses, the focal length of the first lens is between 340 mm and 360 mm, the focal length of the second lens is between 230 mm and 250 mm, the focal length of the third lens is between 80 mm and 100 mm, and the combined focal length of the fourth lens and the fifth lens is between -30 mm and -50 mm.

[0013] In some embodiments, the refractive index of the first lens is between 1.5 and 1.6, the refractive index of the second lens is between 1.9 and 2.0, the refractive index of the third lens is between 1.6 and 1.7, the refractive index of the fourth lens is between 1.9 and 2.0, and the refractive index of the fifth lens is between 1.4 and 1.5.

[0014] In some embodiments, the Abbe number of the first lens is between 70 and 72, the Abbe number of the second lens is between 16 and 18, the Abbe number of the third lens is between 60 and 62, the Abbe number of the fourth lens is between 20 and 22, and the Abbe number of the fifth lens is between 93 and 95.

[0015] In some embodiments, the field curvature of the system is less than 30 µm and the astigmatism is less than 7 µm.

[0016] In some embodiments, the distortion of the system is less than 0.01%.

[0017] The present application also provides a focus level system for a lithography apparatus, comprising: a projection objective; a wafer disposed below the projection objective; a projection mark disposed on one side of the projection objective; a sensor surface disposed on the opposite side of the projection objective from the projection mark; a first double-telecentric element disposed below the projection mark for imaging the projection mark on the wafer; a second double-telecentric element disposed below the sensor surface for imaging the projection mark on the wafer on the sensor surface, wherein the first double-telecentric element and the second double-telecentric element are identical and each comprises the aforementioned double-telecentric optical system.

[0018] The double-telecentric optical system of the present application is also symmetrically designed, but through optimization, only ten lenses (including cemented lenses) are used, which not only reduces the number of lenses compared to the twelve lenses of the prior art, but also reasonably designs the center thickness of each lens, avoiding the processing difficulties caused by too thin lenses. This makes the cost, processing difficulty and adjustment difficulty of the system all significantly reduced compared to the prior art. At the same time, the optical performance of the present application is significantly improved, with a high telecentricity of 0.01° (only 35 µm error at a working distance of 200 mm), a large image field (a circular field of diameter 44 mm), and excellent image quality (low field curvature, low astigmatism, low distortion). The light source wavelength band can cover F, d, and C visible light, so it will not have a lithographic effect with photoresist. These advantages enable the present system to provide a high-precision and high-stability focus level solution for advanced lithography processes. BRIEF DESCRIPTION OF DRAWINGS

[0019] The features, nature, and advantages of the present application will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout the figures. It is to be understood that the drawings are designed solely for purposes of illustration and are not limiting. In the drawings, the size of some of the components can be exaggerated and not drawn to scale and some of the components can have been intentionally disfigured for illustrative purposes.

[0020] Figure 1 The overall structure of a typical focus level system is shown.

[0021] Figure 2 The optical path structure of the double-telecentric optical system of the present application is shown.

[0022] Figure 3 MTF curves of the dual-telecentric optical system of the present application are shown. DETAILED DESCRIPTION

[0023] To make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to specific examples and drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the described exemplary embodiments. However, it will be apparent to one skilled in the art that the described embodiments can be practiced without some or all of these specific details. In other exemplary embodiments, well-known structures are not described in detail in order to avoid unnecessarily obscuring the concepts of the present disclosure. It will be appreciated that the specific embodiments described herein are merely examples of the present application and are not intended to limit the present application. Meanwhile, the various aspects described in the embodiments can be combined in any manner without conflict.

[0024] The focus leveling system is a critical subsystem in the lithography machine, and its performance is directly related to the pattern transfer quality and chip yield in the lithography exposure. In advanced lithography processes, as the resolution of the projection objective is continuously improved, the depth of focus is sharply reduced, and the control accuracy of the wafer surface position has reached the nanometer level. The core function of the focus leveling system is to ensure that the wafer plane in the exposure field is always accurately within the depth of focus of the projection objective. This accuracy directly determines the pattern transfer quality of the wafer surface in the lithography process, and is the key guarantee to obtain high-resolution and high-fidelity integrated circuit patterns. As the semiconductor technology node is continuously reduced, the lithography resolution requirement is increasingly improved, and the depth of focus of the projection objective is also sharply reduced, which poses unprecedented challenges to the measurement accuracy and stability of the focus leveling system.

[0025] Currently, the mainstream technical solution of the focus leveling system is mostly based on the optical triangulation method. Figure 1 A schematic diagram of the overall structure of a typical focus leveling system using the optical triangulation method is shown.

[0026] As shown in (a) in FIG. 1, Figure 1 Element 1 is a projection objective of the lithography machine, which is responsible for accurately projecting the patterns on the reticle onto the wafer. Element 2 is a silicon wafer (wafer) to be measured, which is the substrate material that needs to be pattern transferred in the lithography process. Elements 3 and 4 are the transmitting end and the receiving end of the focus leveling system, respectively. The optical path structures of the two can be completely the same, and maintain a relatively fixed positional relationship with the projection objective to ensure the stability and measurement accuracy of the focus leveling system.

[0027] To more clearly show the optical path structure inside the focus leveling system, Figure 1(b) shows a schematic diagram of the optical path of element 3. Element 3-1 is the projection mark of the focusing and leveling system, which is usually a specific pattern (such as a grating). Elements 3-2, 3-3, and 3-4 together form a dual telecentric optical path system, in which element 3-3 is the aperture stop of the system, which determines the aperture angle of the system, and thus affects the numerical aperture and resolution. The uniformly illuminated projection mark (3-1) is imaged onto the surface of the silicon wafer (2) through this dual telecentric system. When the position of the silicon wafer surface changes relative to the focal plane, the mark image will move or deform accordingly. Using the reflection of the silicon wafer surface, the changed mark image is captured by another set of dual telecentric systems with the same structure (element 4, i.e., the receiver) and imaged again on the sensor receiving surface. If the mark is a grating, it may be imaged on another set of matching probe gratings, and then imaged on the sensor receiving surface through a relay lens. By analyzing the position and shape of the marker image on the sensor surface or the light intensity distribution after passing through the grating, the defocus and tilt of the silicon wafer can be calculated, thereby achieving precise measurement and control of the position and orientation of the silicon wafer surface.

[0028] As mentioned earlier, the existing dual telecentric optical path system has a total of twelve lenses, and some of these lenses are quite thin at the center, making them difficult to manufacture. Furthermore, its image-square NA is low, only 0.03, and its cutoff spatial frequency is only 100 lp / mm, resulting in low resolution. Its telecentricity is 0.1°, and it still has a measurement error of approximately 350 µm at a working distance of 200 mm, making it difficult to meet the requirements for higher precision.

[0029] To address the shortcomings of existing technologies, this invention proposes an improved dual telecentric optical system for focusing and leveling, which solves the deficiencies in existing technologies and meets the requirements of advanced photolithography processes for high precision, high stability, and high resolution in focusing and leveling systems.

[0030] Figure 2 The optical path structure of the dual telecentric optical system 200 of the present invention is shown. The system comprises, sequentially along the optical axis, an object plane, a first group of lenses (L1 to L5), an aperture stop, a second group of lenses (L6 to L10), and an image plane. The first and second groups of lenses are precisely symmetrically arranged with respect to the aperture stop. This symmetrical design is fundamental to ensuring the system possesses telecentric characteristics in both the object and image sides, and is crucial for achieving high-precision focusing and leveling functions.

[0031] One of the core innovations of this invention lies in the optimization of the optical structure. The system uses a total of 10 optical lenses, compared to 12 lenses in existing technologies, reducing the number of lenses. This not only reduces material costs but also reduces assembly steps and potential adjustment errors. In the design and manufacturing process of an optical system, the processing accuracy, adjustment accuracy, and inter-lens fit accuracy of each lens have a significant impact on the final performance of the system. Reducing the number of lenses means saving significant manpower, material resources, and time costs in processing and assembly, and can effectively reduce the cumulative errors caused by an excessive number of lenses, improving the overall stability and reliability of the system. Furthermore, this invention rationally designs the center thickness of the lenses, avoiding lenses that are too thin and difficult to process, thereby significantly reducing processing difficulty and manufacturing costs. In the field of optical manufacturing, excessively thin lenses are prone to breakage and deformation during processing, and require extremely high processing accuracy, which not only increases processing costs but also raises processing risks. By rationally designing the center thickness of the lenses, it is possible to make the lenses easier to process while ensuring optical performance, and it also facilitates subsequent assembly and use.

[0032] Simultaneously, cemented lenses (L4 and L5, and symmetrically positioned L7 and L6) are introduced into the system. Cemented lenses are composed of two or more lenses made of different materials bonded together tightly. Their application in optical systems can effectively correct chromatic aberration and improve image quality, especially in multi-wavelength imaging. Chromatic aberration is one of the common aberrations in optical systems, causing light to focus at different positions at different wavelengths, thus affecting the sharpness and resolution of the image. By appropriately selecting the materials and parameters of cemented lenses, light of different wavelengths can be focused together on the imaging surface, thereby obtaining a clear and accurate imaging effect. In addition, cemented lenses can reduce the number of air-glass interfaces, reduce reflection loss during light propagation, and improve the system's transmittance, thereby enhancing the brightness and sharpness of the image.

[0033] like Figure 2 As shown, system 200 includes 10 lenses L1 to L10, where L1 to L5 form a first group of lenses and L6 to L10 form a second group of lenses. For each lens in the first group, a lens in the second group arranged symmetrically with that lens has the same parameters as that lens. For example, for lens L2 in the first group, lens L9 in the second group is arranged symmetrically with L2, and L2 and L9 have the same parameters.

[0034] In some embodiments, the magnification of system 200 is -1. This 1:1 imaging magnification (i.e., equal magnification) simplifies the size relationship between the object and the image, facilitating subsequent image analysis and position calculation. In focusing and leveling systems, accurately measuring the position and orientation of the silicon wafer surface is crucial. Equal magnification ensures that the object and the image have the same size ratio, making image analysis and calculation more intuitive and accurate. For example, when determining the defocus and tilt of the silicon wafer surface, equal magnification avoids additional errors introduced by changes in the imaging ratio, improving the reliability and accuracy of the measurement.

[0035] In some embodiments, the image-side field of view of system 200 is 44mm (i.e., a circular field of view with a diameter of 44mm) can cover The lithography exposure area. During operation, the lithography machine needs to uniformly expose the entire wafer surface, and the focusing and leveling system must be able to accurately monitor and control the position and orientation of the wafer surface throughout the entire exposure area. A large image-side field of view ensures that the focusing and leveling system can cover the entire wafer surface during exposure, avoiding potential measurement blind spots at the edges, thus guaranteeing the uniformity and consistency of exposure during lithography, which is crucial for improving the yield of integrated circuits.

[0036] The light source's wavelength range covers the visible light spectrum of F (486.1 nm, blue light), d (587.6 nm, yellow light), and C (656.3 nm, red light), thus preventing photolithographic interaction with the photoresist. In photolithography, photoresist is a key material for pattern transfer; it is sensitive to specific wavelengths of light and undergoes a chemical reaction when exposed to this light, forming the desired pattern. To avoid photolithographic interaction between the light source used in the focusing and leveling system and the photoresist, which could interfere with the normal photolithography process, this invention selects visible light within the F, d, and C spectral ranges as the light source. These wavelengths of light will not cause a photolithographic reaction in the photoresist, ensuring that the focusing and leveling system does not adversely affect the photoresist during operation and guaranteeing the smooth progress of the photolithography process.

[0037] In some embodiments, the working distance (distance from the object plane to the front surface of the first lens L1) of system 200 is 200 mm. Considering the size of the lithography machine projection lens (e.g., diameter 200 mm) and its own working distance (e.g., 25 mm), the dual telecentric lenses of the focusing and leveling system need sufficient working distance to avoid interference with the projection lens and other components. Calculations show that a working distance of at least 115 mm is required. The 200 mm optical path working distance of this invention meets the working distance requirements of the lithography focusing and leveling system. Meanwhile, the total lens length of system 200 (center distance from L1 to L10) is 240 mm, resulting in a relatively compact structure. In the overall layout of a lithography machine, the spatial relationships between various components are limited; therefore, the size and working distance of the focusing and leveling system need to match other components of the lithography machine to ensure the normal operation of the entire lithography machine. The 200 mm working distance and 240 mm total lens length enable the dual telecentric optical system of this invention to adapt to the overall layout requirements of the lithography machine while ensuring performance, thus possessing good practical application value.

[0038] In some embodiments, the telecentricity of system 200 is less than 0.01°. Telecentricity is typically measured by the angle between the principal ray and the optical axis, where the principal ray is the ray emitted from a point on the object plane (or image plane) and passing through the center of the aperture stop. An extremely small angle (less than 0.01°) implies extremely high telecentricity. This ensures that even if the silicon wafer moves slightly along the optical axis within the depth of field, its imaging position on the sensor surface remains almost unchanged, thus fundamentally eliminating measurement errors introduced by axial position fluctuations. For a working distance of 200 mm, a telecentricity of 0.01° produces only about 35 µm of measurement error, a precision far superior to existing technologies (0.1° corresponds to about 350 µm of error).

[0039] like Figure 2 As shown, lens L1 is a plano-convex lens, lens L2 is a plano-convex lens, lens L3 is a plano-convex lens, lens L4 is a plano-concave lens, and lens L5 is a plano-convex lens. Figure 2 In this system, lenses L4 and L5 are cemented lenses. Cemented lenses can effectively correct aberrations such as chromatic aberration (especially secondary spectral aberration) and spherical aberration, and reduce the number of separate lenses in the system, thus lowering assembly sensitivity.

[0040] The parameters of lenses L6 to L10 can be the same as those of the corresponding lenses L1 to L5.

[0041] The detailed parameters of lenses L1 to L5 are shown in Table 1, where the units for radius of curvature, thickness and semi-diameter are all mm.

[0042] Table 1: Parameters of lenses L1 to L5

[0043] As shown in Table 1, the refractive index of lens L1 is between 1.5 and 1.6, that of lens L2 is between 1.9 and 2.0, that of lens L3 is between 1.6 and 1.7, that of lens L4 is between 1.9 and 2.0, and that of lens L5 is between 1.4 and 1.5. These refractive index ranges are selected based on the optical properties of the lens material and the design requirements of the optical system. By appropriately selecting the refractive index of the lens, effective control and adjustment of light can be achieved, thereby meeting the design goals of the optical system.

[0044] The Abbe number of lens L1 is between 70 and 72, that of lens L2 is between 16 and 18, that of lens L3 is between 60 and 62, that of lens L4 is between 20 and 22, and that of lens L5 is between 93 and 95. The Abbe number is an important parameter of the dispersion characteristics of lens materials, characterizing the degree of difference in refractive index of the lens material for different wavelengths of light. By appropriately selecting the Abbe number of the lens, chromatic aberration can be effectively corrected, enabling the optical system to obtain good image quality under light of different wavelengths. In the optical system of this invention, by selecting lens materials with different Abbe numbers and combining them appropriately, effective correction of chromatic aberration is achieved, thereby improving the system's resolution and image quality.

[0045] In some embodiments, the focal length of lens L1 is between 340 mm and 360 mm, the focal length of lens L2 is between 230 mm and 250 mm, the focal length of lens L3 is between 80 mm and 100 mm, and the combined focal length of lenses L4 and L5 is between -30 mm and -50 mm. These focal lengths are determined comprehensively based on the design requirements and imaging characteristics of the optical system. By precisely controlling the focal length of each lens, the optimal imaging effect of the system can be achieved, including requirements for telecentricity, field of view, and resolution. For example, lenses with longer focal lengths can be used to achieve higher magnification or a longer working distance, while lenses with shorter focal lengths can be used to achieve a smaller imaging size or a more compact optical structure. By rationally combining lenses with different focal lengths, optimal imaging performance can be achieved while meeting the design requirements of the optical system.

[0046] L1, as the first lens on the object side, employs a plano-convex lens design (plane facing the object plane). The core advantage of a plano-convex lens lies in its ability to effectively suppress spherical aberration in a specific optical path. Spherical aberration is an aberration caused by the different focal lengths in different areas of the lens when imaging an on-axis point light source. The single convex surface structure of a plano-convex lens can significantly reduce focusing deviation after light rays at different heights pass through the lens. L1 allows diverging light rays emitted from the object plane to converge uniformly through the convex surface after incident on the plane, reducing the accumulation of spherical aberration in the early stages of light propagation.

[0047] L2 is also a plano-convex lens, but its orientation is opposite to that of L1 (convex surface towards L1). Its function is to further enhance the light-gathering ability and form a "converging-re-converging" optical path with L1, laying the foundation for subsequent aberration correction. L2 uses a high refractive index material (1.9~2.0), which can achieve a large optical power with a small lens size, helping to control the overall size of the system.

[0048] As a plano-convex lens, L3 has a significantly shorter focal length than L1 and L2 (80 mm to 100 mm). Its function is to "finely adjust" the light rays converged by the first two lens groups. By adjusting the angle of light propagation, the light rays are incident on the cemented lens group (L4 and L5) at the optimal angle, ensuring that the cemented lens group can maximize its aberration correction function.

[0049] L4 is a plano-concave lens made of a high-refractive-index, high-dispersion material (refractive index 1.9~2.0, Abbe number 20~22). Its concave surface faces L3, which can moderately diverge the light rays converged by L3. L5 is a plano-convex lens made of a low-refractive-index, low-dispersion material (refractive index 1.4~1.5, Abbe number 93~95). Its planar surface is cemented with the planar surface of L4, and its convex surface faces the aperture stop, which can re-converge the light rays diverged by L4. The lens group formed by cementing the two together can achieve synergistic correction of multiple aberrations simultaneously through the complementary optical properties of the two materials: In terms of chromatic aberration correction, the high dispersion of L4 can produce large refractive differences in light of different wavelengths, while the low dispersion of L5 can cancel out this difference, making the focal points of the F, d, and C spectral lines coincide and eliminating the second-order spectrum.

[0050] As further shown in Table 1, the distance (working distance) from the object surface to L1 is designed to be 200 mm. This distance satisfies the installation clearance requirements with other components of the lithography machine while also allowing for a small margin for optical path adjustment. The first surface of L1 has a radius of curvature of Infinity (planar), and the second surface has a radius of curvature of -190 to -180 mm (convex surface facing L2), a center thickness of 7 to 9 mm, and a semi-diameter of 40 mm. The 7 to 9 mm center thickness ensures that L1 has sufficient rigidity during processing, preventing deformation during polishing. The 40 mm semi-diameter covers... A 44 mm image-square field of view ensures that edge rays pass through smoothly.

[0051] The radius of curvature of the first surface of L2 is 150-160 mm (convex surface facing L1), and the radius of curvature of the second surface is 530-540 mm (weakly convex surface). The center thickness is 7-9 mm, and the semi-diameter is 40 mm. The 150-160 mm radius of curvature of the first surface can form a matching optical path with the second surface of L1, allowing for a smooth transition of light. The 530-540 mm radius of curvature of the second surface can gradually adjust the direction of light propagation, avoiding additional aberrations introduced by abrupt changes in curvature. The interval between L2 and L3 is designed to be 50-60 mm. This interval allows the light to form a stable propagation state before entering L3, facilitating fine angle adjustments by L3.

[0052] The first surface of L3 has a radius of curvature of 30–40 mm (convex surface facing L2), and the second surface has a radius of curvature of 90–100 mm (weakly convex surface). Its center thickness is 10–12 mm, and its semi-diameter is 27 mm. Compared to L1 and L2, the semi-diameter of L3 is reduced because the beam diameter is significantly reduced after the first two lens groups converge, allowing for the containment of all light without excessively large lenses, thus helping to reduce system weight and cost. The spacing between L3 and L4 is 10–20 mm, ensuring that light enters the cemented lens group at the optimal angle, improving aberration correction efficiency.

[0053] The first surface of L4 has a radius of curvature of 90-100 mm (concave side facing L3), and the second surface has a radius of curvature of Infinity (flat surface, bonded to L5). Its center thickness is 2-4 mm, and its semi-diameter is 14 mm. Although the center thickness of 2-4 mm is relatively thin, bonding it to L5 allows for rigid support from L5, preventing deformation during processing and assembly. The 90-100 mm concave radius of curvature precisely controls the degree of light divergence, providing matching optical path conditions for the converging effect of L5. The first surface of L5 has a radius of curvature of Infinity (flat surface, bonded to L4), and the second surface has a radius of curvature of 40-50 mm (convex surface facing the aperture stop). Its center thickness is 7-9 mm, and its semi-diameter is 14 mm. The 40-50 mm convex radius of curvature complements the concave surface of L4, causing divergent light to reconverge, while simultaneously correcting chromatic aberration and field curvature. The distance between L5 and the aperture stop is 10~12 mm, which ensures that the light rays after correction by the cemented lens group pass through the aperture stop in a parallel state, meeting the optical path requirements of the dual telecentric system.

[0054] Due to the symmetry between lenses L1 to L5 and lenses L6 to L10, the parameters of lenses L6 to L10 can be set with reference to the parameters of lenses L1 to L5 in Table 1. For example, the parameters of lens L6 can be the same as those of lens L5, the parameters of lens L7 can be the same as those of lens L4, the parameters of lens L8 can be the same as those of lens L3, the parameters of lens L9 can be the same as those of lens L2, and the parameters of lens L10 can be the same as those of lens L1.

[0055] By utilizing the dual telecentric optical system of this invention, a number of excellent imaging quality indicators can be achieved, which together ensure the measurement accuracy of the focusing and leveling system.

[0056] First, the system's field curvature is less than 30 μm. Field curvature refers to the degree to which the optimal image plane of an optical system deviates from the plane; the smaller the field curvature, the more uniform the image sharpness throughout the field of view. For focusing and leveling systems, excessive field curvature can cause blurry marks at the edges of the field of view, making accurate position identification impossible and introducing measurement errors. This system, by creating an asymmetrical curvature combination between the concave surface of L4 and the convex surface of L5, can adjust the focusing plane of light rays from different fields of view, keeping the optimal image plane of the entire field of view flat and controlling the field curvature within 30 μm. This ensures that all marks within the entire field of view are clearly distinguishable, providing high-quality image data for subsequent image processing.

[0057] Second, the system's astigmatism is less than 7 μm. Astigmatism occurs because the lens surface is not rotationally symmetric or the optical material is inhomogeneous, resulting in a difference in focal length between the meridional and sagittal planes, causing the point source to be imaged as two mutually perpendicular short lines. Excessive astigmatism can cause a "tailing" phenomenon at the edge of the marked image, making it difficult to accurately determine the center position of the mark and affecting measurement accuracy. This system optimizes the surface accuracy of each lens (surface error controlled within...). The interface design between the meridional and sagittal lenses (within 20 μm) balances the focal length difference between the meridional and sagittal planes, keeping astigmatism within 7 μm. For example, for a point source at the edge of the field of view, the separation distance between its meridional and sagittal images is only 5 μm, much smaller than the line width of the projected mark (typically 5 ~ 10 μm), which will not significantly affect the identification of the mark position and ensure that the measurement error is controlled within the nanometer range.

[0058] Third, the system's distortion is less than 0.01%. Distortion refers to the deviation between the geometry of the image formed by the optical system and the shape of the object surface. It is divided into barrel distortion (image points at the edge of the field of view shift outward) and pincushion distortion (image points at the edge of the field of view shift inward). Excessive distortion will cause the size and shape of the marker image to be distorted, directly translating into position measurement error. An extremely small distortion of less than 0.01% means that the image has almost no geometric distortion, which is crucial for accurate marker position measurement.

[0059] Fourth, the RMS radius of the system's speckle is less than 1.5 μm. The speckle refers to the light spot formed on the image plane after a point light source passes through the optical system. The smaller its RMS radius, the more concentrated the light converges, and the closer the image quality is to the ideal state. An excessively large speckle means that the light distribution range on the image plane is wide, the edges of the marked image are blurred, the signal-to-noise ratio is reduced, and the accuracy of position recognition is affected. This system comprehensively corrects various aberrations such as spherical aberration, coma, and chromatic aberration, controlling the RMS radius of the point light source's speckle to within 1.5 μm. For example, in the central region of the field of view, the RMS radius of the speckle is only 1.2 μm, and in the edge region, it is only 1.4 μm, much smaller than the line width of the projected mark (5~10 μm). This ensures sharp edges of the marked image, a high signal-to-noise ratio, and facilitates accurate extraction of the mark's center position by image processing algorithms, reducing measurement errors.

[0060] Figure 3 The MTF (Modulation Transfer Function) curve of the dual telecentric optical system of the present invention is shown. The MTF curve is an important indicator for measuring the imaging quality of an optical system. It reflects the optical system's ability to transmit signals of different spatial frequencies. The higher the MTF value, the better the image sharpness and contrast. Figure 3 In the diagram, the horizontal axis represents spatial frequency, and the vertical axis represents the magnitude of the OTF (Optical Transfer Function). It should be noted that in the field of optics, the modulation transfer function (MTF) is the magnitude of the optical transfer function (OTF); therefore, in this application, "OTF magnitude" and "MTF" are used interchangeably.

[0061] from Figure 3 As shown in the MTF curves, the system exhibits a high overall value with a gentle downward trend. The curves in the meridional (T) and sagittal (S) directions highly overlap, indicating excellent consistency in imaging performance across different directions with no significant directional aberrations. At a spatial frequency of 100 lp / mm, the system achieves MTF contrast values ​​greater than 0.43 in both the meridional and sagittal directions at different field-of-view positions (0.00 mm, 5.50 mm, 11.00 mm, 16.50 mm, 22.00 mm, and -22.00 mm). This demonstrates the system's high resolution and ability to resolve finer details. Furthermore, the cutoff frequency (the frequency at which the MTF drops to 0) is above 100 lp / mm, outperforming some existing technologies. Additionally, based on these MTF curves, the system's Strehl Ratio reaches 0.991. The Strell ratio is the ratio of the brightness at the center point of the actual system to the brightness at the center point of the ideal diffraction-limited system. The closer the Strell ratio is to 1, the closer the system is to the diffraction limit, and the higher the level of aberration correction. A Strell ratio of 0.991 confirms that the system of this invention has near-perfect imaging quality.

[0062] This invention also provides a focusing and leveling system for a photolithography apparatus. The system includes: a projection lens; a wafer disposed below the projection lens; a projection mark disposed on one side of the projection lens; a sensor surface disposed on the side of the projection lens opposite to the projection mark; a first telecentric element disposed below the projection mark for imaging the projection mark onto the wafer; and a second telecentric element disposed below the sensor surface for imaging the projection mark on the wafer onto the sensor surface. The first and second telecentric elements are identical and both include the telecentric optical system proposed in this invention. Therefore, the focusing and leveling system of this invention is similar to... Figure 1 The focusing and leveling system shown is similar in structure, but the core difference is that the focusing and leveling system of this invention adopts the dual telecentric optical system disclosed in this invention.

[0063] The dual telecentric optical system of this invention, through an optimized symmetrical design using ten lenses (including cemented lenses), achieves telecentricity (less than 0.01°), working distance (200 mm), and field of view (…) while reducing the number of lenses and rationalizing the center thickness. It achieves significant improvements or a good balance in various aspects of performance, including image quality (low field curvature, low astigmatism, low distortion, high MTF, and small diffusion spots), while effectively reducing the difficulty and cost of processing and assembly. This makes it particularly suitable for the focusing and leveling systems of advanced lithography machines, which have extremely high requirements for precision and stability.

[0064] The detailed description above, in conjunction with the accompanying drawings, describes examples but does not represent all examples that can be implemented or fall within the scope of the claims. The terms "example" and "exemplary" are used in this specification to mean "serving as an example, instance, or illustration" and do not imply "superiority or superiority over other examples."

[0065] Throughout this specification, the terms "an embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention. Therefore, the use of these phrases may refer to more than one embodiment. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0066] The preceding description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will readily be understood by those skilled in the art, and the universal principles defined herein can be applied to other aspects. Therefore, the claims are not intended to be limited to the aspects shown herein, but are to be granted the full scope consistent with the language of the claims, wherein references to the singular form of an element, unless specifically stated otherwise, are not intended to mean “one and only one,” but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more. All structural and functional equivalents of the various aspects of the invention described throughout are expressly incorporated herein by reference and are intended to be covered by the claims.

[0067] It should also be noted that these embodiments may be described as processes depicted as flowcharts, flow diagrams, structure diagrams, or block diagrams. Although a flowchart may describe the operations as a sequential process, many of these operations can be executed in parallel or concurrently. Furthermore, the order of these operations can be rearranged.

[0068] While various embodiments have been described and illustrated, it should be understood that the embodiments are not limited to the precise configurations and components described above. Various modifications, substitutions, and improvements that will be apparent to those skilled in the art can be made to the arrangement, operation, and details of the apparatus disclosed herein without departing from the scope of the claims.

Claims

1. A dual telecentric optical system for focusing and leveling, comprising: The object plane, the first group of lenses, the aperture stop, the second group of lenses, and the image plane are arranged sequentially along the optical axis. The first group of lenses and the second group of lenses are arranged symmetrically about the aperture stop, and The first group of lenses includes five lenses, the second group of lenses includes five lenses, and for each lens in the first group of lenses, the lens in the second group of lenses that is symmetrically arranged with that lens has the same parameters as that lens.

2. The system according to claim 1, characterized in that, The system has a magnification of -1 and an image-side field of view that is a circular field of view with a diameter of 44 mm.

3. The system according to claim 1, characterized in that, The total length of the lens in the system is 240 mm, and the working distance of the system is 200 mm. The total length of the lens includes the distance from the first lens in the first group of lenses to the last lens in the second group of lenses, and the working distance includes the distance from the object plane to the first lens in the first group of lenses.

4. The system according to claim 1, characterized in that, The system's telecentricity is less than 0.01°.

5. The system according to claim 1, characterized in that, The first group of lenses includes a first lens, a second lens, a third lens, a fourth lens, and a fifth lens, wherein the first lens is a plano-convex lens, the second lens is a plano-convex lens, the third lens is a plano-convex lens, the fourth lens is a plano-concave lens, and the fifth lens is a plano-convex lens.

6. The system according to claim 5, characterized in that, The fourth and fifth lenses are cemented lenses. The focal length of the first lens is between 340 mm and 360 mm, the focal length of the second lens is between 230 mm and 250 mm, the focal length of the third lens is between 80 mm and 100 mm, and the combined focal length of the fourth and fifth lenses is between -30 mm and -50 mm.

7. The system according to claim 5, characterized in that, The first lens has a refractive index between 1.5 and 1.6, the second lens has a refractive index between 1.9 and 2.0, the third lens has a refractive index between 1.6 and 1.7, the fourth lens has a refractive index between 1.9 and 2.0, and the fifth lens has a refractive index between 1.4 and 1.

5.

8. The system according to claim 5, characterized in that, The Abbe number of the first lens is between 70 and 72, the Abbe number of the second lens is between 16 and 18, the Abbe number of the third lens is between 60 and 62, the Abbe number of the fourth lens is between 20 and 22, and the Abbe number of the fifth lens is between 93 and 95.

9. The system according to claim 1, characterized in that, The field curvature of the system is less than 30 µm, and the astigmatism is less than 7 µm.

10. The system according to claim 1, characterized in that, The distortion of the system is less than 0.01%.

11. A focusing and leveling system for a photolithography apparatus, comprising: Projection lens; The wafer positioned below the projection lens; Projection markings are set on one side of the projection lens; The sensor surface is located on the side of the projection lens opposite to the projection mark; A first dual-telecentric element disposed below the projection mark is used to image the projection mark onto the wafer; A second telecentric element, disposed below the sensor surface, is used to image the projected marks on the wafer onto the sensor surface. The first telecentric element and the second telecentric element are identical and both include the telecentric optical system according to any one of claims 1 to 10.

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