Double-telecentric optical system for focusing and leveling and focusing and leveling system

Abstract

The invention relates to a double telecentric optical system for focusing and leveling and a focusing and leveling system. 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 relative to the aperture diaphragm, the image space numerical aperture (NA) of the system is 0.1, the image space resolution is superior to 1.25 m, and the image space resolution is superior to 1.25 m. The back focal length of the system is longer than 220 mm, and the included angle between the principal ray of the system and the optical axis is 0.015 degrees. The invention further discloses a focusing and leveling system comprising the double telecentric optical system.

Description

Technical Field

[0001] This invention relates to the field of semiconductors, and more specifically to a dual telecentric optical system and a focusing and leveling system for focusing and leveling. Background Technology

[0002] The focusing and leveling system is a crucial subsystem in a lithography machine. Its function is to ensure that the wafer plane within the exposure field of view is within the depth of focus of the projection lens. As the core equipment in semiconductor manufacturing, the performance of the lithography machine directly determines the feature size and integration density of integrated circuits. During the lithography process, the projection lens precisely projects the pattern on the photomask onto the wafer surface coated with photoresist. Since the depth of focus of the projection lens is limited, typically only on the order of micrometers or even submicrometers, it is essential to ensure that the wafer surface is precisely within the depth of focus of the projection lens at the moment of exposure. Any slight defocusing or tilting can lead to pattern distortion, linewidth variations, or even circuit malfunction, causing irreversible losses. The focusing and leveling system is designed precisely for this purpose. It monitors the position and orientation of the wafer surface relative to the focal plane of the projection lens in real time and performs dynamic compensation through a precision motion platform, making it one of the key links in ensuring the yield of the lithography process.

[0003] As technology advances, the resolution requirements for lithography are increasing, leading to smaller depths of focus for projection lenses. Consequently, the resolution requirements for focusing and leveling are also rising. Some existing technologies employ dual telecentric lenses, but their image-side numerical aperture (NA) is generally low, typically only 0.03, resulting in low resolution. A low NA also reduces the system's light-gathering efficiency, weakening the light signal intensity received by the sensor. This, in turn, affects the measurement signal-to-noise ratio and response speed, potentially causing lag in focusing and leveling response in high-speed wafer exposure scenarios, thus impacting lithography yield.

[0004] Furthermore, existing telecentric systems also have shortcomings in terms of field of view coverage and structural compatibility. Some systems sacrifice field of view range to improve resolution, resulting in their inability to fully cover the exposure field of view of the lithography machine (typically a rectangular area of ​​26 mm × 33 mm). This necessitates multiple wafer stage movements for stitching measurements, which not only reduces lithography efficiency but may also introduce additional measurement deviations due to stitching errors. Simultaneously, some systems have a short backstop (the distance from the last lens of the optical system to the image plane), limiting the installation space for peripheral components such as sensors and adjustment mechanisms, and increasing the difficulty of integrating the focusing and leveling system with the overall structure of the lithography machine.

[0005] In view of this, it is desirable to provide an improved dual telecentric optical system for focusing and leveling. Summary of the Invention

[0006] The following provides a brief overview of one or more aspects to offer a basic understanding of them. This overview is not an exhaustive summary of all conceived aspects, nor is it intended to identify the key or decisive elements of all aspects, nor to define 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 an introduction to the more detailed description that follows.

[0007] The present invention provides a dual telecentric optical system for focusing and leveling, comprising: an object plane, a first group of lenses, an aperture stop, a second group of lenses, and an image plane arranged sequentially along the optical axis, wherein the first group of lenses and the second group of lenses are arranged symmetrically about the aperture stop, and wherein the image-side numerical aperture (NA) of the system is 0.1 and the image-side resolution is better than 1.25µm.

[0008] In some embodiments, the system has a magnification of 1 and a target field of view of a circle with a diameter of 44 mm.

[0009] In some embodiments, the back intercept of the system is longer than 220 mm.

[0010] In some embodiments, the angle between the principal ray and the optical axis of the system is 0.015°.

[0011] In some embodiments, the distance from the object plane to the image plane is between 940 and 944 mm, and the distance from the first lens in the first group of lenses to the last lens in the second group of lenses is between 484 and 486 mm.

[0012] In some embodiments, the first set of lenses includes six lenses, the second set of lenses includes six lenses, and for each lens in the first set of lenses, the lens in the second set of lenses arranged symmetrically to that lens has the same parameters as that lens.

[0013] In some embodiments, the first group of lenses includes a first lens, a second lens, a third lens, a fourth lens, a fifth lens, and a sixth lens, wherein the first lens is a plano-convex lens, the second lens is a plano-convex lens, the third lens is a biconcave lens, the fourth lens is a biconvex lens, the fifth lens is a plano-convex lens, and the sixth lens is a biconcave lens.

[0014] In some embodiments, the focal length of the first lens is between 345 and 355 mm, the focal length of the second lens is between 375 and 385 mm, the focal length of the third lens is between -85 and -75 mm, the focal length of the fourth lens is between 85 and 95 mm, the focal length of the fifth lens is between 135 and 145 mm, and the focal length of the sixth lens is between -85 and -75 mm.

[0015] In some embodiments, the thickness of the first lens is between 13 and 15 mm, the thickness of the second lens is between 13 and 15 mm, the thickness of the third lens is between 13 and 15 mm, the thickness of the fourth lens is between 13 and 15 mm, the thickness of the fifth lens is between 7 and 9 mm, and the thickness of the sixth lens is between 2 and 4 mm.

[0016] 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 1.6 and 1.7, the refractive index of the third lens is between 1.6 and 1.7, the refractive index of the fourth lens is between 1.5 and 1.6, the refractive index of the fifth lens is between 1.6 and 1.7, and the refractive index of the sixth lens is between 1.6 and 1.7.

[0017] In some embodiments, the Abbe number of the first lens is between 70 and 75, the Abbe number of the second lens is between 55 and 60, the Abbe number of the third lens is between 35 and 40, the Abbe number of the fourth lens is between 70 and 75, the Abbe number of the fifth lens is between 60 and 65, and the Abbe number of the sixth lens is between 55 and 60.

[0018] In some embodiments, the maximum field curvature of the system is 100 µm.

[0019] In some embodiments, the distortion of the system is less than 0.02%.

[0020] The present invention also provides a focusing and leveling system for a photolithography apparatus, comprising: 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, wherein the first telecentric element and the second telecentric element are identical and both include the aforementioned telecentric optical system. Attached Figure Description

[0021] The features, essence, and advantages of the invention will become more apparent when understood in conjunction with the accompanying drawings, which provide a detailed description. In the drawings, the same reference numerals are consistently used. It should be noted that the described drawings are schematic and non-limiting. Some elements in the drawings may be enlarged and are not drawn to scale for illustrative purposes.

[0022] Figure 1 A schematic diagram of the overall structure of a typical focusing and leveling system is shown.

[0023] Figure 2 The overall optical path structure of the dual telecentric optical system of the present invention is shown.

[0024] Figure 3 The partial optical path structure of the dual telecentric optical system of the present invention is shown.

[0025] Figure 4 The field curvature and distortion curves of the dual telecentric optical system of the present invention are shown.

[0026] Figure 5 The MTF curve of the dual telecentric optical system of the present invention is shown.

[0027] Figure 6 A dot diagram of the dual telecentric optical system of the present invention is shown. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the described exemplary embodiments. However, it will be apparent to those 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 have not been described in detail to avoid unnecessarily obscuring the concepts of this disclosure. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Furthermore, the various aspects described in the embodiments can be combined arbitrarily without conflict.

[0029] The focusing and leveling system is a crucial subsystem in a lithography machine. Its core function is to ensure that the wafer plane within the exposure field of view is always precisely within the depth of focus of the projection lens. This precision directly determines the quality of pattern transfer on the wafer surface during lithography and is a key guarantee for obtaining high-resolution, high-fidelity integrated circuit patterns. As semiconductor technology nodes continue to shrink and the requirements for lithography resolution increase, the depth of focus of the projection lens is also decreasing dramatically. This poses unprecedented challenges to the measurement accuracy and stability of the focusing and leveling system.

[0030] Currently, most mainstream technologies for focusing and leveling systems are based on optical triangulation. Figure 1A schematic diagram of a typical focusing and leveling system employing optical triangulation is shown. The system mainly includes: a projection lens (element 1), a wafer (element 2), projection marks (element 3), a first telecentric lens (element 4, also referred to herein as a "telecentric optical system"), a second telecentric lens (element 5, which may be the same as element 4), and a sensor surface (element 6). The projection lens, as the core optical element of the lithography machine, is responsible for accurately projecting the integrated circuit pattern on the photomask onto the wafer surface. The wafer (element 2) is typically a silicon wafer coated with photoresist, placed on a wafer stage capable of six degrees of freedom (X, Y, Z, θ, ωx, ωy). The stage's motion accuracy can reach the nanometer level, used to adjust the wafer's spatial position based on the measurement results of the focusing and leveling system. The projection marks (element 3) are typically high-precision patterns etched onto a quartz substrate. Common mark types include crosshairs, rectangles, and sine stripes, with dimensional accuracy reaching the sub-micrometer level, serving as a reference for focusing and leveling measurements. The first dual telecentric lens (element 4) and the second dual telecentric lens (element 5) are the core imaging elements of the system, responsible for achieving high-precision imaging of the projected mark between the wafer surface and the sensor surface. The sensor surface (element 6) typically employs a high-resolution CCD (charge-coupled device) or CMOS (complementary metal-oxide-semiconductor) image sensor with a pixel size of 2~5μm. It is used to acquire the image of the projected mark and convert the image signal into an electrical signal for transmission to the processing unit.

[0031] The focusing and leveling system typically operates in two core steps: The first step is marker projection. The light emitted from the projection marker (element 3) (usually monochromatic light matching the exposure wavelength, such as 193nm deep ultraviolet light) is refracted by the first dual telecentric lens (element 4) to form a beam parallel to the optical axis, precisely imaged onto the surface of the wafer (element 2), creating a reference marker image perfectly identical to the projected marker. Due to the telecentric nature of the first dual telecentric lens, even with slight tilting of the wafer surface, the size and position of the marker image will not shift, ensuring the stability of the reference marker. The second step is marker detection. After reflection, the reference marker image on the wafer surface returns along the original optical path to the second dual telecentric lens (element 5). After refraction by the second dual telecentric lens, it is again imaged onto the sensor surface (element 6) as parallel light. When the wafer defocuses or tilts due to wafer stage vibration, temperature changes, or surface morphology fluctuations, the reference marker image on the wafer surface undergoes a slight positional change, which is precisely transmitted to the sensor surface by the second dual telecentric lens. The image processing unit analyzes the positional offset of the marked image on the sensor surface (such as the X and Y direction displacement of the crosshair center), and combines this with the magnification and geometric relationship of the optical system to calculate the defocusing amount (Z direction displacement) and tilt angle (ωx and ωy direction rotation) of the wafer relative to the focal plane of the projection lens. The calculation results are then fed back to the wafer stage control system. Based on the feedback signal, the wafer stage control system drives the corresponding motors to adjust the spatial position of the wafer until the wafer plane returns to the focal depth range of the projection lens, thereby achieving closed-loop focusing and leveling control.

[0032] In existing technologies, telecentric optical systems are widely used in measurement systems due to their unique telecentric characteristics. The main feature of a telecentric optical system is that its entrance pupil and / or exit pupil are located at infinity, which makes its principal ray parallel to the optical axis in the object and / or image sides. This ensures that the image size remains unchanged even when the object moves slightly along the optical axis within the depth of field. This characteristic is crucial for high-precision dimensional measurement and position detection because it eliminates the influence of axial position fluctuations on the measurement results. However, in pursuit of high telecentricity (i.e., a very small angle between the principal ray and the optical axis), the optical structure design of existing telecentric objectives is often significantly limited, resulting in generally low image-side numerical aperture (NA). For example, some telecentric objectives have an image-side NA of only 0.03. Numerical NA is defined as NA = n sinθ, where n is the refractive index of the image-side medium, typically n≈1 for air, and θ is half the image-side aperture angle. According to the Rayleigh criterion, the theoretical resolution of an optical system... (i.e., the smallest detail size that the system can resolve) is inversely proportional to NA ( Where k is the process factor, (where NA is the wavelength of light). A lower NA directly limits the theoretical resolution upper limit of the system, making it difficult to meet the higher requirements for focusing and leveling resolution in advanced photolithography processes (e.g., on the order of hundreds of nanometers or even tens of nanometers). Furthermore, a lower NA also affects the system's light-gathering ability (luminous flux and light intensity). (Proportional to the signal-to-noise ratio), which may lead to a decrease in the signal-to-noise ratio, posing challenges in high-speed measurements or low-light environments, and affecting measurement speed and reliability.

[0033] Therefore, there is an urgent need in the field for a novel dual telecentric optical system that can significantly improve image-side numerical aperture and resolution while maintaining excellent telecentric characteristics, in order to meet the stringent performance requirements of focusing and leveling systems imposed by next-generation lithography technologies.

[0034] In view of the above-mentioned defects in the prior art, the present invention proposes an improved dual telecentric optical system for focusing and leveling. Figure 2 The overall 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, an aperture stop, a second group of lenses, and an image plane. The first and second groups of lenses are precisely symmetrically arranged about the aperture stop. This symmetrical design is fundamental to ensuring the system exhibits telecentric characteristics in both the object and image sides.

[0035] One of the core innovations of this invention lies in the successful increase of the image-side numerical aperture (NA) to 0.1 μm through meticulous optical design, while maintaining high telecentricity (an extremely small angle between the system's principal ray and the optical axis, e.g., 0.015°) and reasonable physical dimensions. This represents a significant performance leap compared to existing systems with an NA of 0.03. The increased NA directly leads to a leap in resolution. This system achieves an image-side resolution better than 1.25 µm, easily meeting focusing and leveling requirements at the hundred-nanometer level or even higher. This is crucial for addressing the ever-shrinking depth of focus in current and future lithography machines. The high NA also results in higher luminous flux, which is beneficial for improving the signal-to-noise ratio and measurement speed.

[0036] In some embodiments, the magnification of system 200 is 1, which maintains a 1:1 object-image relationship, meaning the size of the mark on the object surface is exactly the same as the size of the mark on the image surface. The core advantage of this design is the simplification of subsequent image processing and position calculation algorithms: the image processing unit does not need to scale the size of the mark on the image surface; it can directly infer the displacement of the object surface (wafer surface) from the positional offset of the mark on the image surface. For example, if the mark image detected on the sensor surface shifts by 1 µm in the X direction, it can be directly determined that the mark image on the wafer surface has also shifted by 1 µm in the X direction. Combined with the geometric relationship of the optical system, the tilt angle or defocus of the wafer can be quickly calculated. This direct correspondence not only reduces the complexity of data processing but also avoids calculation errors introduced by size scaling, improving the response speed of the focusing and leveling system. In high-speed lithography scenarios, the improved response speed can effectively reduce the lag time of wafer stage adjustment, ensuring that the wafer remains within the depth of focus range throughout the exposure process.

[0037] Furthermore, the target field of view of System 200 is a circle with a diameter of 44 mm. The target field of view refers to the maximum area that the optical system can image. In this invention, it specifically refers to the wafer surface area that the dual telecentric optical system can clearly image. The exposure field of view of current mainstream lithography machines is usually a rectangular area of ​​26 mm × 33 mm, with a diagonal length of approximately 42 mm. The 44 mm diameter field of view of this system can completely cover this rectangular exposure field of view, and even leaves a margin of about 2 mm. This design ensures that in a single measurement, the focusing and leveling system can obtain the wafer position information within the entire exposure area without the need for field stitching through wafer stage movement. In addition, the large field of view design also enables the system to simultaneously detect multiple reference marks on the wafer surface (e.g., arranging 4 marks within the exposure field of view). By measuring multiple marks simultaneously, the stability and accuracy of wafer position measurement can be further improved, reducing the risk of measurement failure due to contamination or damage to a single mark.

[0038] In some embodiments, the back focal length (distance from the last optical element of the optical system to the image plane) of system 200 is greater than 220 mm. Back focal length is a crucial structural parameter of the optical system, directly determining the installation space for peripheral components such as sensors, optical filters, and mechanical adjustment mechanisms. This invention designs the back focal length to be greater than 220 mm, resulting in significant structural compatibility advantages. High-resolution CCD or CMOS sensors typically have large dimensions (e.g., a sensor with a pixel size of 2 μm and a resolution of 2048×2048 has a diagonal length of approximately 5.8 mm; including the sensor housing and circuit board, the total size can reach 20 mm × 20 mm). A back focal length of 220 mm is sufficient to accommodate the sensor and its associated circuitry without spatial interference with the lens group. Furthermore, to reduce the impact of stray light on image quality, focusing and leveling systems typically require the installation of optical filters (such as bandpass filters to filter non-operating wavelengths) in front of the sensor. The extended back focal length provides ample space for filter installation and also allows for the installation of aperture stops or light shields to further suppress stray light. In addition, mechanical adjustment mechanisms (such as translation stages for fine-tuning the image plane position and fine-tuning screws for adjusting lens spacing) also require a certain amount of installation space. The longer back focal length allows for flexible arrangement of these mechanisms and facilitates debugging and maintenance by operators. Furthermore, the extended back focal length also provides room for future system upgrades. For example, if a higher resolution sensor or new optical components (such as polarizers) need to be replaced in the future, there is no need for a large-scale modification of the overall system structure; components can simply be replaced or added within the existing back focal length space, significantly improving the system's scalability.

[0039] In some embodiments, the principal ray of system 200 has an angle of 0.015° with the optical axis. The angle between the principal ray and the optical axis is a core indicator for evaluating the telecentricity of a telecentric optical system. The smaller the angle, the higher the telecentricity and the stronger the system's resistance to axial displacement interference.

[0040] In some embodiments, the distance from the object plane to the image plane is between 940 and 944 mm, from the first lens in the first group of lenses ( Figure 2 From lens L1 in the first group of lenses to the last lens in the second group of lenses ( Figure 2 The distance between the lens L12 in the system is between 484 and 486 mm. This ensures the relative compactness of the system while achieving high performance (high NA, large field of view, high telecentricity), which is beneficial for layout within the limited space of the lithography machine.

[0041] Figure 2 The image shows 12 lenses, with lenses L1 to L6 belonging to the first group and lenses L7 to L12 belonging to the second group. The lens numbering order corresponds to the direction of light propagation.

[0042] For each lens in the first group of lenses, the lens in the second group that is symmetrically arranged with that lens has the same parameters. For example, for lens L2 in the first group of lenses, the lens in the second group that is symmetrically arranged with it is lens L11, and lens L2 and lens L11 have the same parameters. This strict symmetry simplifies the design, manufacturing, and assembly process and ensures the consistency of optical performance on both sides of the image.

[0043] from Figure 2 As can be seen, lens L1 is a plano-convex lens, lens L2 is a plano-convex lens, lens L3 is a biconcave lens, lens L4 is a biconvex lens, lens L5 is a plano-convex lens, and lens L6 is a biconcave lens. Lenses L7 to L12 are of the same type as lenses L1 to L6.

[0044] The focal length, thickness, and material information of lenses L1 to L6 are shown in Table 1. The parameters of lenses L7 to L12 can be derived based on symmetry.

[0045] Table 1: Lens Information

[0046] Abbe number is an important parameter of optical materials, which reflects the degree of dispersion of the material for light of different wavelengths. The larger the Abbe number, the smaller the dispersion and the better the image quality.

[0047] As shown in Table 1, lenses L1 and L2 are both plano-convex lenses with relatively long positive focal lengths. They form the front of the first lens group, primarily responsible for collecting light rays from the object surface and performing initial convergence, while also laying the foundation for subsequent aberration correction. Their relatively large radius of curvature helps control higher-order aberrations (such as higher-order spherical aberration and coma). Lens L3 is a biconcave lens with a negative focal length. Its introduction is mainly used to compensate for the positive spherical aberration and positive field curvature generated by the preceding positive lens, while also working with subsequent lenses to control the Petzval Sum of the system and reduce field curvature. Lens L4 is a biconvex lens with a medium positive focal length (approximately 90 mm). It acts as a "relay," further converging light rays and working in conjunction with other lenses to correct astigmatism and coma. Lens L5 is a plano-convex lens with a positive focal length (approximately 140 mm). It is located near the aperture stop and plays a crucial role in controlling the principal ray angle and ensuring telecentrism. Lens L6 is a biconcave lens with a negative focal length (approximately -80 mm) and a relatively thin thickness (2~4 mm). As the last lens in the first group of lenses, it is located immediately next to the aperture stop. Its negative optical power helps balance the overall optical power of the system and plays an important role in fine-tuning the image-side telecentrism and the final correction of certain residual aberrations (such as distortion). This alternating and combined pattern of positive-positive-negative-positive-positive-negative optical power, through optimized design, effectively controls various monochromatic aberrations (spherical aberration, coma, astigmatism, field curvature) and chromatic aberration of the system.

[0048] The choice of lens material (refractive index and Abbe number) is crucial for aberration correction, especially chromatic aberration correction. In this invention, lenses L1 and L4 are made of materials with Abbe numbers between 70 and 75 (refractive index approximately 1.5 to 1.6). A high Abbe number means that the material has less dispersion for different wavelengths of light, and thus introduces less chromatic aberration. Placing them in the position of positive lenses with high optical power helps suppress axial chromatic aberration and magnification chromatic aberration. Axial chromatic aberration produced by positive lenses is positive (the focal length of longer wavelength light is longer than that of shorter wavelength light), while positive lenses made of high Abbe number materials produce less axial chromatic aberration. Therefore, the high Abbe number design of lenses L1 and L4 can reduce the generation of axial chromatic aberration at its source. At the same time, positive lenses made of high Abbe number materials also produce less magnification chromatic aberration, which helps to ensure that the magnification of different wavelengths of light remains consistent and suppresses magnification chromatic aberration.

[0049] Lenses L2, L3, L5, and L6 are made of materials with relatively low Abbe numbers (between 35 and 65) and refractive indices of approximately 1.6 to 1.7. These materials are typically used in conjunction with lenses of specific shapes and powers to precisely correct chromatic aberration caused by positive lenses. For example, lenses L3 and L6, with negative power, use materials with medium to low Abbe numbers, which can be effectively combined with the preceding positive lenses to achieve chromatic aberration balance. Through the appropriate combination of high and low Abbe number materials, this system achieves excellent apochromatic effect, ensuring that the imaging positions of different wavelengths of light are basically consistent within the designed wavelength range, thereby guaranteeing image sharpness and color fidelity. This is particularly important for focusing and leveling systems that rely on specific illumination wavelengths.

[0050] To more clearly demonstrate the optical path of the dual telecentric optical system of the present invention, Figure 3 A partial optical path structure of the dual telecentric optical system of the present invention is shown. Specifically, Figure 3 The optical path structure from lens L1 to lens L6 is shown. The ray tracing from lens L1 to lens L6 (i.e., the first group of lenses and the aperture stop) is specifically depicted. The propagation paths of the principal ray (typically representing the ray emanating from the center of the field of view and passing through the center of the aperture stop) and the peripheral rays (representing the rays at the maximum aperture angle) are clearly visible in the figure. Furthermore, it can be seen that after passing through the first group of lenses, the principal ray travels towards the aperture stop in a direction nearly parallel to the optical axis, demonstrating the object-side telecentric characteristic.

[0051] in addition, Figure 3Furthermore, it can be seen that the refraction angles of light rays passing through the surfaces of each lens are all within a reasonable range, with no obvious excessive refraction or light spillage. This indicates that the lens parameters (focal length, radius of curvature, and thickness) are reasonable and can effectively control the direction of light propagation. At the same time, the propagation path of light rays inside the lens is smooth, without obvious intersections or overlaps, avoiding increased aberrations caused by light ray intersections.

[0052] Because the system is strictly symmetrical about the aperture stop, the optical path structure from lens L7 to lens L12 (the second group of lenses) is... Figure 3 The structure shown is mirror-symmetric. This symmetry ensures that after passing through the second lens group, the principal ray also reaches the image plane in a direction parallel to the optical axis, thus achieving image-side telecentricity. The entire optical path is smooth and symmetrical, with well-controlled aberrations, directly demonstrating high imaging performance.

[0053] Figure 4 The field curvature and distortion curves of the dual telecentric optical system of this invention are shown. Field curvature refers to the phenomenon that the optimal imaging surface of a planar object is not a plane but a curved surface after passing through the optical system. In focusing and leveling systems, if the field curvature is too large, it will cause the center of the image plane to be sharp while the edges are blurry (or vice versa), making it impossible for the sensor to simultaneously acquire a sharp marker image across the entire field of view, thus affecting the consistency of position measurement. Field curvature is generally divided into meridional field curvature (T) and sagittal field curvature (S): meridional field curvature corresponds to the imaging surface of light rays in the meridional plane (the plane containing the optical axis), and sagittal field curvature corresponds to the imaging surface of light rays in the sagittal plane (the plane perpendicular to the meridional plane). The difference between the two is astigmatism. The smaller the astigmatism, the better the imaging consistency of the system in different directions.

[0054] Figure 4 The field curvature curve on the left shows the system's field curvature values ​​in both the meridional and sagittal directions. The curve shows that the maximum field curvature is approximately 100 µm across the entire 44 mm diameter field of view. This value is very small for such a large field of view and high NA system, indicating a very flat image plane. This is beneficial for obtaining uniform and consistent high-resolution images across the entire measurement field of view, and is crucial for ensuring consistency in focusing and leveling measurements on large-area wafers.

[0055] Distortion is another key geometric aberration, manifested as the image's geometry not being perfectly similar to the object's. This means that straight lines on the object appear as curves after passing through the optical system, or there are differences in magnification at different positions. In focusing and leveling systems, distortion can cause shape distortion of the marked image; for example, the horizontal and vertical lines of a crosshair mark may become curved, or the mark size in edge areas may be stretched / compressed, thus introducing position measurement errors. If the distortion rate is 0.1%, the mark position deviation at the edge of a 44 mm diameter field of view can reach 0.044 mm (44 mm × 0.1%), far exceeding the nanometer-level measurement accuracy requirements.

[0056] Figure 4 The distortion curve on the right shows that the distortion of this system is less than 0.02% across the entire field of view. Such an extremely low distortion level means that the system has virtually no observable image shape distortion. This is an ideal characteristic for focusing and leveling applications that require precise measurement of marker position and shape, as it avoids geometric errors introduced by the optical system itself, ensuring that the measurement results accurately reflect the wafer's pose changes. Taking a crosshair marker in a practical application as an example (10 μm line width, 1 mm length), the maximum shape deviation caused by a 0.02% distortion rate is only 0.2 μm (1 mm × 0.02%), far smaller than the sensor's pixel size (2 μm). The sensor cannot distinguish this tiny deviation at all; therefore, the geometry of the marker image can be considered completely consistent with the object surface marker. This extremely low distortion level fundamentally avoids geometric errors introduced by the optical system, ensuring the accuracy of position measurement and providing a key guarantee for the nanometer-level precision of the focusing and leveling system.

[0057] Figure 5 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 of the imaging quality of an optical system, reflecting its ability to transmit sinusoidal grating signals of different spatial frequencies. A higher MTF value (closer to 1) indicates a stronger ability to maintain contrast at that spatial frequency, resulting in better image sharpness and detail resolution. The MTF curve is typically compared to the diffraction-limited MTF to assess the level of aberration correction of the system. Figure 5 The horizontal axis represents the spatial frequency (unit: lp / mm, i.e., line pairs per millimeter), ranging from 0 to 420 lp / mm, covering the spatial frequencies corresponding to the marking details commonly used in focusing and leveling systems (usually 100-400 lp / mm); the vertical axis represents the MTF value (unitless), ranging from 0 to 1.

[0058] Figure 6 A dot diagram of the dual telecentric optical system of the present invention is shown.

[0059] A dot plot is a visual tool for evaluating the aberration correction effectiveness of an optical system. It simulates the convergence of a large number of light rays (usually hundreds to thousands) originating from a point on the object plane, passing through the optical system, and appearing as a light spot on the image plane. The smaller the spot size and the more concentrated the energy, the smaller the aberrations of the system and the closer the image quality is to the ideal state (diffraction limit). In focusing and leveling systems, the quality of the dot plot directly affects the sharpness of the marker image: if the light spot is too diffuse, the marker image will become blurry with indistinct edges, leading to increased position measurement errors; if the light spot energy is concentrated, the marker image edges will be sharp, and the centroid positioning accuracy will be high. Figure 6 The diagram shows the system's dot plots at different field-of-view locations. The actual root-mean-square (RMS) radius and Airy disk radius are also labeled. The Airy disk radius is the theoretical minimum spot size determined by the diffraction effect of light, representing the diffraction limit of the optical system. If the RMS radius of the actual spot is smaller than the Airy disk radius, it indicates that the system's aberrations have been adequately corrected, and the image quality is close to the diffraction limit.

[0060] from Figure 6 As can be clearly seen, the RMS radius of the light spot is smaller than the Airy disk radius in all fields of view. This indicates that the aberrations (including spherical aberration, coma, astigmatism, etc.) of this system are well corrected, and the energy of the actual imaging light spot is highly concentrated, approaching the diffraction limit. Such imaging quality ensures that the light spot signal received on the sensor surface is sharp and clear, which is very beneficial for subsequent image processing, centroid positioning, and accurate measurement, providing reliable optical assurance for high-precision focusing and leveling.

[0061] 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 systems shown are similar in structure, but the core difference is that the focusing and leveling system of the present invention uses the dual telecentric optical system disclosed in the present invention, which has excellent performance such as high NA and high resolution, as its core imaging element.

[0062] The dual telecentric optical system of the present invention has the following advantages: High resolution and high NA: The image-square NA reaches 0.1, and the image-square resolution is better than 1.25µm, which enables the system to detect tiny changes in the marker image caused by extremely small defocusing or tilting on the wafer surface (such as position shift or blurring), thereby achieving extremely high focusing and leveling sensitivity and accuracy.

[0063] High telecentricity: The angle between the principal ray and the optical axis is only 0.015°, ensuring that even if the wafer experiences slight axial movement within the depth of field, the imaging position marked on the sensor surface will not shift, avoiding measurement errors. This helps to accurately distinguish between the actual defocus amount and changes in wafer surface morphology (such as steps, patterns).

[0064] Low distortion: distortion is less than 0.02%, which ensures the true and reliable geometry of the marked image and improves the accuracy of localization and shape recognition based on image analysis.

[0065] Large field of view: The 44 mm diameter circular field of view completely covers the typical exposure field of a lithography machine, enabling full-field focusing and leveling, improving production efficiency and the consistency of pattern quality within the exposure field.

[0066] Long backstop: The backstop is greater than 220 mm, which provides ample space for the installation and adjustment of the sensor and related mechanical structures and electronic circuits, reducing the difficulty and complexity of system integration.

[0067] Excellent image quality: small field curvature (maximum field curvature of 100 µm), good MTF performance, and a dot plot close to the diffraction limit ensure clear imaging and high contrast throughout the field of view, providing high-quality image signals for high-precision measurements.

[0068] Compactness and symmetry: While achieving high performance, the overall length is reasonably controlled, the structure is symmetrical, which facilitates design, manufacturing and assembly, and some lenses are interchangeable, reducing costs.

[0069] 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."

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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. The system has an image-side numerical aperture (NA) of 0.1 and an image-side resolution better than 1.25 µm. Wherein, the back intercept of the system is greater than 220 mm, and The angle between the principal ray of the system and the optical axis is 0.015°.

2. The system according to claim 1, characterized in that, The system has a magnification of 1 and the target field of view is a circle with a diameter of 44 mm.

3. The system according to claim 1, characterized in that, The distance from the object plane to the image plane is between 940 and 944 mm, and the distance from the first lens in the first group of lenses to the last lens in the second group of lenses is between 484 and 486 mm.

4. The system according to claim 1, characterized in that, The first group of lenses includes six lenses, the second group of lenses includes six lenses, and for each lens in the first group of lenses, the lens in the second group of lenses that is symmetrically arranged with respect to that lens has the same parameters as that lens.

5. The system according to claim 4, characterized in that, The first group of lenses includes a first lens, a second lens, a third lens, a fourth lens, a fifth lens, and a sixth lens, wherein the first lens is a plano-convex lens, the second lens is a plano-convex lens, the third lens is a biconcave lens, the fourth lens is a biconvex lens, the fifth lens is a plano-convex lens, and the sixth lens is a biconcave lens.

6. The system according to claim 5, characterized in that, The focal length of the first lens is between 345 and 355 mm, the focal length of the second lens is between 375 and 385 mm, the focal length of the third lens is between -85 and -75 mm, the focal length of the fourth lens is between 85 and 95 mm, the focal length of the fifth lens is between 135 and 145 mm, and the focal length of the sixth lens is between -85 and -75 mm.

7. The system according to claim 5, characterized in that, The thickness of the first lens is between 13 and 15 mm, the thickness of the second lens is between 13 and 15 mm, the thickness of the third lens is between 13 and 15 mm, the thickness of the fourth lens is between 13 and 15 mm, the thickness of the fifth lens is between 7 and 9 mm, and the thickness of the sixth lens is between 2 and 4 mm.

8. 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 1.6 and 1.7, the third lens has a refractive index between 1.6 and 1.7, the fourth lens has a refractive index between 1.5 and 1.6, the fifth lens has a refractive index between 1.6 and 1.7, and the sixth lens has a refractive index between 1.6 and 1.

7.

9. The system according to claim 5, characterized in that, The Abbe number of the first lens is between 70 and 75, the Abbe number of the second lens is between 55 and 60, the Abbe number of the third lens is between 35 and 40, the Abbe number of the fourth lens is between 70 and 75, the Abbe number of the fifth lens is between 60 and 65, and the Abbe number of the sixth lens is between 55 and 60.

10. The system according to claim 1, characterized in that, The maximum field curvature of the system is 100 µm.

11. The system according to claim 1, characterized in that, The distortion of the system is less than 0.02%.

12. 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; A sensor surface is disposed 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 11.

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