Fixed focus optical system

Abstract

The invention discloses a fixed-focus optical system, and relates to the technical field of optics, and the fixed-focus optical system comprises an image sensor, a lens group, a diaphragm, a beam splitter, a plane reflector, at least one curved reflector and a wafer which are sequentially arranged from an image plane to an object plane. The fixed-focus optical system further comprises a light source which is arranged corresponding to one side, facing the first reflector, of the beam splitter and is used for emitting illumination light. Wherein the image sensor, the lens group, the diaphragm, the beam splitter and the first reflector form a first optical axis, the beam splitter and the first optical axis are arranged at an included angle, the plane reflector and the first optical axis are arranged at an included angle, and the at least one curved reflector is arranged between the plane reflector and a wafer and used for reflecting illumination light passing through the plane reflector to the wafer. The coaxial hot spot phenomenon is effectively eliminated, and the compactness of the system and the image uniformity are improved.

Description

Technical Field

[0001] This invention relates to the field of optical technology, and in particular to a fixed-focus optical system. Background Technology

[0002] In semiconductor manufacturing, two-dimensional barcodes / characters are typically etched or marked on the wafer surface for production tracking. Barcode reading systems usually need to quickly and reliably identify the code information across multiple processes, including photolithography, etching, and cleaning. Because wafer surfaces have high reflectivity (especially bare silicon or metal films), the uniformity of imaging illumination and stray light control directly affect the accuracy of the barcode reading.

[0003] Currently, the industry generally adopts coaxial refractive illumination optical solutions, such as COGNEX's In-Sight1740 series: the illumination light passes through a beam splitter, such as a beam splitter prism or beam splitter, and illuminates the wafer surface along the optical axis of the imaging optical path, and returns to the imaging device through the same optical axis, but it is prone to generating significant coaxial hot spots.

[0004] like Figure 1 As shown, a coaxial hot spot refers to the residual reflection of illumination light on the surface of the front lens group before it reaches the wafer, returning along the original optical axis to the imaging plane, causing an abnormal increase in brightness in the central area. This optical axis return can cause local pixel saturation, weakening the grayscale contrast of the marked area and affecting the stability of the recognition algorithm. The hot spot effect is particularly pronounced under conditions of high-power LEDs or low divergence angle light sources.

[0005] To suppress coaxial hot spots, existing technologies often employ methods such as adding diffusers in front of the light source or reducing the reflectivity of the lens surface coating. However, these solutions cannot fundamentally prevent hot spots caused by reflections from the front lens surface, so this type of hot spot problem is almost unavoidable. Summary of the Invention

[0006] The main objective of this invention is to propose a fixed-focus optical system that aims to eliminate the coaxial hot spot phenomenon in wafer reading systems.

[0007] To achieve the above objectives, the present invention proposes a fixed-focus optical system, which includes an image sensor, a lens group, an aperture, a beam splitter, a plane mirror, at least one curved mirror, and a wafer arranged sequentially from the image plane to the object plane. The fixed-focus optical system also includes a light source, which is arranged on the side of the beam splitter facing the first reflector to emit illumination light; The image sensor, lens group, aperture, beam splitter, and first reflector form a first optical axis. The beam splitter is set at an angle to the first optical axis, and the planar reflector is set at an angle to the first optical axis. At least one curved reflector is disposed between the planar reflector and the wafer to reflect the illumination light passing through the planar reflector to the wafer.

[0008] In one embodiment, at least one curved mirror includes a first curved mirror and a second curved mirror arranged sequentially from the image plane to the object plane; Wherein, a second optical axis is formed between the first curved reflector and the plane reflector, and a third optical axis is formed between the first curved reflector and the second curved reflector. The second optical axis and the first optical axis are set at an angle, and the third optical axis is set at an angle to the second optical axis.

[0009] In one embodiment, the angle between the second optical axis and the first optical axis is A, the angle between the line containing the first curved mirror and the first optical axis is B, the angle between the line containing the third optical axis and the first optical axis is C, and the angle between the line containing the second curved mirror and the first optical axis is D. Each of these angles satisfies the following: 85°≤A≤95°, 20°≤B≤30°, 35°≤C≤45°, 20°≤D≤30°.

[0010] In one embodiment, the first curved mirror has a negative optical power and a focal length of f100, and the second curved mirror has a positive optical power and a focal length of f200, wherein the focal lengths satisfy the following: 120mm≤|f100|≤130mm, 100mm≤|f200|≤110mm.

[0011] In one embodiment, the first curved mirror and the second curved mirror are aspherical mirrors.

[0012] In one embodiment, the beam splitter makes an angle E with the first optical axis, where E = 45°; and / or, The beam splitter splits the illumination light in a 50:50 ratio.

[0013] In one embodiment, the distance from the light source to the beam splitter is the same as the distance from the aperture to the beam splitter; and / or, The emission direction of the light source normal forms a 90° angle with the first optical axis, and the divergence angle of the light source is F, where 20°≤F≤120°.

[0014] In one embodiment, the lens group includes a first lens, a second lens, a third lens, a fourth lens, and a fifth lens arranged sequentially from the image plane to the object plane, wherein the first lens has a negative optical power, the second lens has a positive optical power, the third lens has a positive optical power, the fourth lens has a negative optical power, and the fifth lens has a positive optical power.

[0015] In one embodiment, the focal length of the first lens is f1, the focal length of the second lens is f2, the focal length of the third lens is f3, the focal length of the fourth lens is f4, and the focal length of the fifth lens is f5, wherein each focal length satisfies: 8mm≤|f1|≤12mm, 24mm≤|f2|≤28mm, 22mm≤|f3|≤26mm, 22mm≤|f4|≤24mm, 20mm≤|f5|≤22mm.

[0016] In one embodiment, the fourth lens and the fifth lens are cemented together; and / or, The first lens, the second lens, the third lens, the fourth lens, and the fifth lens are glass spherical lenses.

[0017] In the technical solution of this invention, illumination light is emitted from a light source, split by a beam splitter, and then reaches a plane mirror. The plane mirror reflects the illumination light to a curved mirror, which then reflects it to the wafer, thereby illuminating the wafer and acquiring the identification code on it. The light then sequentially passes through the curved mirror, plane mirror, beam splitter, aperture, and lens group before finally reaching the image sensor. Specifically, the illumination light emitted from the light source is reflected by the beam splitter and enters the plane mirror, which deflects the light towards the curved mirror. The curved mirror, through its curvature, focuses and projects the illumination light onto the wafer surface, forming a uniform illumination area. After acquiring the identification code on the wafer, the light sequentially passes through the curved mirror, plane mirror, beam splitter, aperture, and lens group before finally reaching the image sensor. The image sensor converts the optical image formed by the lens group into an electrical signal for subsequent image processing and barcode recognition. Because the off-axis and tilted arrangement of the plane and curved mirrors prevents residual reflected light from returning to the image sensor along the original optical axis, the conditions for coaxial hot spots are completely eliminated. Through this technical solution, the present invention effectively eliminates the coaxial hot spot phenomenon and avoids pixel saturation in the central region. Furthermore, the multiple reflections using the plane and curved mirrors significantly reduce the length along the first optical axis, improving the system's compactness. Moreover, the use of aspherical mirrors enhances image uniformity, making the grayscale contrast of the marked area on the wafer surface clearer, thereby improving the accuracy and stability of the code reading. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0019] Figure 1 This addresses the hot spot generated on the image plane by current coaxial illumination refractive solutions of similar products. Figure 2 This is a schematic diagram of the structure of an embodiment of the fixed-focus optical system provided by the present invention; Figure 3 for Figure 2 A schematic diagram of the structure of a fixed-focus optical system that conceals light rays; Figure 4 for Figure 2 Object-side MTF image quality of a fixed-focus optical system; Figure 5 for Figure 2 A uniform illumination field generated on the wafer surface by a fixed-focus optical system; Figure 6 for Figure 2 Hot spots generated on the image plane by a fixed-focus optical system.

[0020] Explanation of icon numbers: 100. Fixed-focus optical system; 1. Image sensor; 2. Lens group; 3. Aperture; 4. Beam splitter; 5. Plane mirror; 6. First curved mirror; 7. Second curved mirror; 8. Wafer; 9. Light source.

[0021] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0024] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the use of "and / or" or "and / or" throughout the text implies three parallel solutions. For example, "A and / or B" includes solution A, solution B, or a solution that simultaneously satisfies A and B. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0025] In existing technologies, two-dimensional barcodes are typically etched or marked on the wafer surface during semiconductor manufacturing for production tracking. The barcode reading system needs to quickly identify the barcode information across multiple processes. Due to the high reflectivity of the wafer surface, existing coaxial refractive lighting solutions are prone to generating coaxial hot spots, leading to abnormally high brightness in the central area. Figure 1 The hot spots generated on the image plane by coaxial illumination refractive solutions in current similar products affect the stability of the recognition algorithm. Existing solutions add diffusers or reduce lens reflectivity, but cannot completely eliminate the hot spot problem caused by reflections from the front lens surface.

[0026] To address the above problems, this invention proposes a fixed-focus optical system 100.

[0027] Please see Figure 2 In one embodiment of the present invention, the fixed-focus optical system 100 includes an image sensor 1, a lens group 2, an aperture 3, a beam splitter 4, a plane mirror 5, at least one curved mirror, and a wafer 8 arranged sequentially from the image plane to the object plane. The system also includes a light source 9, positioned on the side of the beam splitter 4 facing the first mirror, for emitting illumination light. The image sensor 1, lens group 2, aperture 3, beam splitter 4, and first mirror form a first optical axis. The beam splitter 4 is arranged at an angle to the first optical axis, the plane mirror 5 is also arranged at an angle to the first optical axis, and at least one curved mirror is disposed between the plane mirror 5 and the wafer 8 to reflect the illumination light passing through the plane mirror 5 back to the wafer 8.

[0028] Among them, the beam splitter 4 refers to a transparent optical element with beam-splitting function, which can be implemented using a glass substrate coated with a beam-splitting film. It can reflect a portion of the light and transmit another portion. It forms a non-perpendicular angle with the first optical axis, causing the incident light to deflect. The plane mirror 5 refers to an optical reflective element with a flat surface, which can be implemented using an aluminum-plated or silver-plated glass lens. Its angle setting is used to adjust the transmission direction of the illumination light. The curved mirror refers to an optical reflective element with curvature, which can be implemented using a spherical or aspherical mirror. It guides the illumination light to the surface of the wafer 8 through reflection and focusing characteristics. The light source 9 refers to the light-emitting device used to provide the illumination beam, which can be implemented using LEDs or laser diodes. Its position corresponds to the direction of the reflective surface of the beam splitter 4 to form a lateral illumination path. Specifically, to improve the illumination effect, the light source 9 can be selected as a single LED or an LED array or other stable near-infrared light source 9 with a wavelength range of 800nm~920nm. The planar reflector 5 and the curved reflector can be selected as having a substrate material of 6061-T6 and a near-infrared reflective film, with a reflectivity of greater than 99.5% for light with a wavelength between 800nm ​​and 920nm.

[0029] In the technical solution of this invention, illumination light is emitted from light source 9, split by beam splitter 4, and reaches plane mirror 5. Plane mirror 5 reflects the illumination light to curved mirror, and then to wafer 8, thereby illuminating and acquiring the identification code on wafer 8. The light then sequentially passes through curved mirror, plane mirror 5, beam splitter, aperture 3, and lens group 2, finally reaching image sensor 1. Specifically, the illumination light emitted from light source 9 is reflected by beam splitter 4 and enters plane mirror 5, which deflects the light towards the curved mirror. The curved mirror focuses and projects the illumination light onto the surface of wafer 8 through its curvature characteristics, forming a uniform illumination area. After acquiring the identification code on wafer 8, the light sequentially passes through curved mirror, plane mirror 5, beam splitter, aperture 3, and lens group 2, finally reaching image sensor 1. Image sensor 1 converts the optical image formed by lens group 2 into an electrical signal for subsequent image processing and barcode recognition. Because the off-axis and tilted arrangement of the plane and curved mirrors prevents residual reflected light from returning to the image sensor along the original optical axis, the conditions for coaxial hot spots are completely isolated. Through this technical solution, the present invention effectively eliminates the coaxial hot spot phenomenon and avoids pixel saturation in the central region. Furthermore, the multiple reflections by the plane and curved mirrors significantly reduce the length along the first optical axis, improving the system's compactness. The separation of the illumination and imaging optical paths enhances image uniformity, making the grayscale contrast of the marked area on the wafer 8 surface clearer, thereby improving the recognition accuracy and stability of the code reading system.

[0030] Compared to existing technologies, this invention employs an off-axis and tilted arrangement of planar and curved mirrors in the illumination optical path, causing residual reflected light to deviate geometrically from the imaging optical axis, thus preventing it from returning to the image sensor along the original optical axis. This structure not only effectively cuts off the light energy return path caused by reflection from the front lens in traditional coaxial illumination but also avoids the aggregation of high-brightness spots on the sensor, fundamentally eliminating the conditions for coaxial hot spots. Compared to traditional solutions that rely on diffusers or low-reflection coatings, this invention achieves physical shielding of hot spots through a non-coaxial optical path, improving the system's anti-stray light capability while ensuring the stability of illumination efficiency and imaging contrast.

[0031] In one embodiment of the present invention, at least one curved mirror includes a first curved mirror 6 and a second curved mirror 7 arranged sequentially from the image plane to the object plane. A second optical axis is formed between the first curved mirror 6 and the plane mirror 5, and a third optical axis is formed between the first curved mirror 6 and the second curved mirror 7. The second optical axis and the first optical axis are arranged at an angle, and the third optical axis is arranged at an angle to the second optical axis.

[0032] The first curved mirror 6 is an optical element with a curved surface, which can be implemented using a concave or convex mirror. It is used to change the direction of light propagation and form a second optical axis. The second curved mirror 7 is an optical element that cooperates with the first curved mirror 6 to form a third optical axis. It can be implemented using a mirror with the opposite curvature to the first curved mirror 6. It is used to adjust the angle of the light path for a second time. The second optical axis is the axis formed by the light path between the plane mirror 5 and the first curved mirror 6. It can be achieved by adjusting the tilt angle of the plane mirror 5 to deviate the illumination light from the first optical axis. The third optical axis is the axis formed by the light path between the first curved mirror 6 and the second curved mirror 7. It can be achieved by setting the relative positions of the two curved mirrors to form a multi-stage deflection path.

[0033] Specifically, the illumination light, after being reflected by the beam splitter 4, propagates along the first optical axis to the plane mirror 5, where it is deflected into the second optical axis direction. The first curved mirror 6 receives the beam from the second optical axis direction and reflects it to the third optical axis direction. The second curved mirror 7 then reflects the beam from the third optical axis direction onto the surface of the wafer 8. The two optical axis deflections create spatial separation between the illumination light and the imaging optical path, preventing the illumination light from returning to the image sensor 1 along the original optical axis. The non-coaxial arrangement of the second and first optical axes eliminates coaxial hot spots caused by reflections from the front lens surface. The angled arrangement of the third and second optical axes further expands the optical path folding space, achieving uniform illumination while maintaining the telecentric optical path characteristics.

[0034] Compared to existing technologies, in traditional coaxial refractive illumination schemes, the illumination light and imaging light share the same optical axis, which easily leads to reflected light from the lens surface directly returning to the imaging device, generating noticeable hot spots. This invention constructs a non-coaxial optical path structure through two-stage optical axis deflection, physically severing the coaxial reflection path. Compared to existing solutions that rely on diffusers or low-reflection coatings to only reduce hot spot intensity, this invention, through innovative optical path structure, completely eliminates the fundamental conditions for hot spot formation.

[0035] Through the above technical solution, this invention effectively suppresses the coaxial hot spot phenomenon caused by the high reflectivity of the wafer 8 surface, avoiding abnormal brightness increases in the imaging center area. The illumination light, after multi-stage optical axis deflection, uniformly covers the wafer 8 surface, significantly improving the grayscale contrast of the two-dimensional barcode / character, ensuring stable identification of marked information by the reading system during photolithography, etching, and other processes. The optical path folding design achieves large-angle deflection within a limited space, meeting the requirements of semiconductor manufacturing equipment for compact optical systems.

[0036] In one embodiment of the present invention, the angle between the second optical axis and the first optical axis is A, the angle between the line containing the first curved mirror 6 and the first optical axis is B, the angle between the line containing the third optical axis and the first optical axis is C, and the angle between the line containing the second curved mirror 7 and the first optical axis is D. Each angle satisfies: 85°≤A≤95°, 20°≤B≤30°, 35°≤C≤45°, and 20°≤D≤30°.

[0037] The angle A between the second optical axis and the first optical axis refers to the tilt angle between the optical path axis from the beam splitter 4 to the first curved reflector 6 and the imaging optical path axis. This can be achieved using a mechanical positioning structure of the reflector mounting bracket. This angle range ensures that the illumination optical path and the imaging optical path are spatially orthogonal. The angle B between the line containing the first curved reflector 6 and the first optical axis refers to the tilt angle of the reflector surface normal relative to the imaging optical axis. This can be achieved by adjusting the rotation angle of the reflector support mechanism. This angle range causes the illumination light to deviate from its original optical axis direction after reflection. The angle C between the line containing the third optical axis and the first optical axis refers to the tilt angle of the optical path axis from the second curved reflector 7 to the wafer 8 relative to the imaging optical axis. This can be achieved by adjusting the curvature center position of the curved reflector. This angle range further changes the direction of light propagation. The angle D between the line containing the second curved reflector 7 and the first optical axis refers to the tilt angle of the reflector surface normal relative to the imaging optical axis. This can be achieved using a multi-degree-of-freedom adjustment platform for angle calibration. This angle range ensures that the illumination light ultimately covers the surface of the wafer 8 via an asymmetric path.

[0038] Specifically, by controlling the angle A between the second optical axis and the first optical axis to be close to 90°, stray light reflected from the beam splitter 4 cannot return to the image sensor 1 along the original optical axis, thus eliminating the coaxial hot spot generation path. The angle B between the first curved mirror 6 and the angle D between the second curved mirror 7 form complementary tilt angles, causing the illumination light path and the imaging light path after two curved surface reflections to be spatially separated. The angle C of the third optical axis serves as an intermediate transition angle, adjusting the optical path deflection amplitude to prevent residual reflected light from coupling with the imaging lens group. The combined constraints of these angles ensure that the illumination light undergoes multiple asymmetric deflections before reaching the wafer 8, blocking the possibility of stray light reflected from the lens surface returning along the original path.

[0039] In one embodiment of the present invention, a fixed-focus optical system 100 is provided, wherein the optical power of the first curved reflector 6 is negative and the focal length is f100, the optical power of the second curved reflector 7 is positive and the focal length is f200, and the focal lengths satisfy: 120mm≤|f100|≤130mm, 100mm≤|f200|≤110mm.

[0040] As can be understood, optical power equals the difference between the convergence of the image-side beam and the convergence of the object-side beam; it characterizes the ability of an optical system to deflect light. The larger the absolute value of optical power, the stronger the bending ability of light; the smaller the absolute value, the weaker the bending ability. When optical power is positive, the refraction of light is converging; when optical power is negative, the refraction of light is diverging. Optical power can be used to characterize a single refractive surface of a lens (i.e., one surface of the lens), a single lens, or a system formed by multiple lenses (i.e., lens group 2).

[0041] In this context, a negative optical power means the mirror diverges the incident light, which can be achieved using a concave mirror to reduce the probability of reflected light returning along the original optical axis. A positive optical power means the mirror converges the incident light, which can be achieved using a convex mirror to compensate for the diverged beam energy distribution. The absolute value range of the focal length is limited to 120-130mm and 100-110mm, which can be achieved by adjusting the radius of curvature of the mirror to ensure a balance between divergence and convergence.

[0042] Specifically, the negative optical power of the first curved mirror 6 causes the illumination light incident on the wafer 8 to diverge, reducing the probability of reflected light from the mirror surface returning to the image sensor 1 along the original optical axis, thereby reducing light intensity concentration in the central area. The positive optical power of the second curved mirror 7 converges and compensates for the diverging beam, avoiding light energy loss or image blurring due to excessive divergence. By controlling the absolute value of the focal length of the first mirror in the range of 120-130mm, and simultaneously limiting the absolute value of the focal length of the second mirror in the range of 100-110mm, an optimal ratio of divergence and compensation is achieved. This effectively suppresses the hot spot effect caused by coaxial backlighting while maintaining uniform beam coverage on the surface of the wafer 8.

[0043] Compared to existing technologies, traditional solutions, by adding diffusers or reducing lens reflectivity, can only partially reduce the hot spot effect, but cannot eliminate the brightness anomaly in the central region caused by optical axis reversion. This solution, through a combination of negative and positive optical power mirrors and specific focal length matching, fundamentally changes the beam propagation path, causing the reflected light from the front lens to deviate from the original optical axis direction, while maintaining the stability of the imaging optical path, thus achieving synergistic optimization of hot spot suppression and image quality.

[0044] Through the above technical solution, the present invention effectively reduces the coaxial hot spot phenomenon caused by the high reflectivity of the wafer 8 surface, makes the intensity distribution of the illumination light received by the image sensor 1 more uniform, improves the imaging contrast of the two-dimensional barcode / character, and thus improves the recognition accuracy and stability of the code reading system in the semiconductor manufacturing process.

[0045] In one embodiment of the present invention, the first curved surface mirror 6 and the second curved surface mirror 7 are aspherical mirrors.

[0046] Understandably, aspherical lenses are characterized by a continuous change in curvature from the center to the periphery. Unlike spherical lenses, which have a constant curvature from the center to the periphery, aspherical lenses have better curvature radius characteristics, which have the advantages of improving distortion aberrations and astigmatism. This design can precisely control the incident angle and reflection path of light by adjusting the surface parameters, thereby suppressing stray light reflection caused by spherical aberrations.

[0047] Specifically, in one embodiment of the present invention, the surface shape of the aspherical lens in the fixed-focus optical system should satisfy the following equation:

[0048] Where Z is the surface height of the aspheric surface at the radial position y; C is the curvature (C=1 / y, y is the radius of curvature); k is the conic constant; A, B, C, D, E, F, G... represent the aspheric coefficients of the fourth, sixth, eighth, tenth, twelfth, fourteenth, sixteenth... orders, respectively.

[0049] More specifically, in one embodiment of the present invention, the coefficients of even-order terms for each aspherical surface are shown in Table 1 below.

[0050] Table 1

[0051] Specifically, the curved shape of the aspherical mirror is optimized so that after the illumination beam is deflected by the plane mirror 5, it forms a predetermined optical path through continuous reflection by the first curved mirror 6 and the second curved mirror 7. The off-axis characteristic of the aspherical mirror eliminates the inherent field curvature and coma of the spherical mirror, preventing unexpected deflection of light during reflection. At the same time, the curvature distribution of the aspherical mirror can guide residual reflected light outside the optical path, reducing the intensity of stray light returning along the original optical axis, thereby preventing the formation of a bright hot spot in the center of the imaging area.

[0052] In one embodiment of the present invention, the beam splitter 4 has an angle of 45 degrees with the first optical axis, and the beam splitter 4 splits the illumination light in a ratio of 50:50.

[0053] The angle between the beam splitter 4 and the first optical axis refers to the tilt angle of the plane of the beam splitter 4 relative to the optical axis direction. Specifically, a precision mechanical fixture can be used to fix the beam splitter 4 so that its normal direction forms a 45-degree angle with the optical axis. This angle setting ensures that the incident light and reflected light form orthogonal optical paths, blocking the return path of stray light reflected from the surface of the front lens group. The 50:50 beam splitting ratio means that the transmittance and reflectance of the beam splitter 4 for incident light are equal. This can be achieved using a dielectric film beam splitter 4 or a metal film beam splitter 4. This ratio balances the energy of the illumination light projected onto the wafer 8 and the energy of the reflected light received by the imaging optical path, preventing excessive reflected light from entering the imaging sensor.

[0054] Specifically, when the beam splitter 4 is set at a 45-degree angle, the illumination light emitted by the light source 9 is reflected by the beam splitter 4 and propagates along the direction perpendicular to the first optical axis, while the imaging light reflected by the wafer 8 passes through the beam splitter 4 and returns along the original optical axis. This orthogonal optical path structure causes stray light reflected from the surface of the front lens to deviate from the imaging optical axis and fail to reach the image sensor 1. The 50:50 beam splitting ratio ensures a balanced energy distribution between the illumination light and the imaging light, guaranteeing sufficient illumination intensity on the surface of the wafer 8 while preventing the sensor from receiving excessive reflected light, which could lead to overexposure in the central area. The two techniques work together to eliminate coaxial hot spots from both the physical path and light intensity control dimensions through geometric optical path isolation and dynamic energy balance.

[0055] Through the above technical solution, the present invention effectively eliminates the coaxial hot spot phenomenon during imaging on the surface of wafer 8, and avoids the decrease in grayscale contrast caused by overexposure in the central area of ​​the image. The orthogonal optical path structure suppresses stray light interference reflected from the surface of the front lens group, and the symmetrical beam splitting ratio ensures the energy balance between the illumination and imaging optical paths, thereby improving the imaging uniformity and recognition stability of barcodes or characters on the surface of wafer 8.

[0056] In one embodiment of the present invention, the distance from the light source 9 to the beam splitter 4 is the same as the distance from the aperture 3 to the beam splitter 4, the normal emission direction of the light source 9 forms a 90-degree angle with the first optical axis, and the divergence angle of the light source 9 is controlled within the range of 20 degrees to 120 degrees.

[0057] The fact that the distance from light source 9 to beam splitter 4 is the same as the distance from aperture 3 to beam splitter 4 means that their spatial positions are symmetrical with respect to beam splitter 4. This can be achieved using equidistant mounting brackets. This symmetrical layout ensures that the propagation path lengths of the two beams in the medium of beam splitter 4 are equal. The fact that the normal emission direction of light source 9 forms a 90-degree angle with the first optical axis means that the emission direction of light source 9 is orthogonal to the imaging optical axis. This can be achieved by adjusting the mounting angle of light source 9. This orthogonal structure ensures that the illumination light, after being reflected by beam splitter 4, completely deviates from the imaging optical axis. The divergence angle of light source 9, ranging from 20 degrees to 120 degrees, refers to the range of beam divergence half-angles. This can be achieved by selecting LED modules with different divergence angles. This angle range balances the illumination coverage area and stray light suppression requirements.

[0058] Specifically, the symmetrical arrangement of the light source 9 and the aperture 3 relative to the beam splitter 4 eliminates the optical path difference between the two beams within the beam splitter 4, thus allowing chromatic aberration and coma to be synchronously compensated in the lens group 2. The orthogonal optical path structure ensures that the illumination light, after reflection by the beam splitter 4, completely deviates from the imaging optical axis propagation path, preventing reflected light from the front lens surface from returning to the image sensor 1 along the original optical axis. A lower limit of 20 degrees for the divergence angle ensures that the illumination light covers the effective area of ​​the wafer 8 surface, while an upper limit of 120 degrees prevents excessively large divergence angles from causing invalid light rays to enter the optical path and create stray interference.

[0059] Compared to existing technologies, traditional solutions reduce hot spots by adding diffusers or optimizing coatings, but cannot eliminate axial backlighting caused by reflections from the front lens surface. This solution fundamentally blocks the coaxial reflection path through an orthogonal optical path structure, and combines this with a symmetrical layout to eliminate aberrations caused by optical path differences, achieving hot spot suppression without the need for additional optical components.

[0060] Through the above technical solutions, this invention effectively eliminates the abnormal increase in brightness in the central region caused by coaxial hot spots, improving the uniformity and contrast of wafer 8-surface marking imaging. The orthogonal optical path structure avoids coaxial propagation of illumination and imaging light, the symmetrical layout reduces the difficulty of aberration compensation, and the divergence angle control ensures effective illumination area coverage while reducing interference from ineffective optical paths, thereby improving the recognition accuracy and stability of the code reading system.

[0061] In one embodiment of the present invention, a lens group 2 includes five lenses arranged sequentially from the image plane to the object plane, with the optical powers of the five lenses being negative, positive, positive, negative, and positive, respectively.

[0062] The first lens has a negative optical power, meaning it has a diverging effect on the incident beam. This can be achieved using a concave lens to control the divergence angle of the incident beam and prevent excessive convergence from the front lens group, thus avoiding reflection. The second lens has a positive optical power, meaning it has a converging effect on the incident beam. This can be achieved using a convex lens to compensate for field curvature distortion introduced by the first lens and maintain optical path compactness. The third lens has a positive optical power, meaning it further enhances the converging effect. This can be achieved using a biconvex lens to balance aberrations introduced by the front lens group. The fourth lens has a negative optical power, meaning it has a diverging effect. This can be achieved using a meniscus concave lens to form an achromatic pair with the fifth lens. The fifth lens has a positive optical power, meaning it has a final converging effect. This can be achieved using a plano-convex lens to correct axial chromatic aberration and reduce the probability of reflected light returning from the lens surface.

[0063] Specifically, an optical path control structure is formed by alternating combinations of positive and negative optical powers. The first lens with negative optical power reduces the convergence of the incident beam, decreasing the intensity of stray light reflected from the front lens group. The combination of positive optical powers of the second and third lenses compensates for field curvature distortion while shortening the overall length of the optical system through the converging effect of the intermediate lens group 2. The negative optical power of the fourth lens and the positive optical power of the fifth lens form an achromatic structure, suppressing chromatic aberration and altering the propagation path of reflected light, causing residual reflected light to deviate from the original optical axis. The cemented design of the fourth and fifth lenses eliminates the air interface between the two lenses, directly reducing the number of secondary reflection surfaces. The selection of glass spherical lenses ensures a symmetrical distribution of curvature on the surfaces of each lens, avoiding stray light accumulation caused by asymmetrical reflection.

[0064] Through the above technical solutions, this invention effectively suppresses the abnormal brightness in the central region caused by coaxial hot spots, improving the grayscale contrast of wafer 8-surface marking imaging. By dispersing the propagation direction of residual reflected light, local pixel saturation on the imaging surface is avoided. Cemented lenses are used to reduce interface reflection, minimizing stray light interference with imaging uniformity. The alternating optical power arrangement structure design ensures imaging quality while achieving an asymmetrical distribution of reflection paths, fundamentally blocking the formation conditions of coaxial hot spots.

[0065] In one embodiment of the present invention, the focal length of the first lens is f1, the focal length of the second lens is f2, the focal length of the third lens is f3, the focal length of the fourth lens is f4, and the focal length of the fifth lens is f5. Each focal length satisfies the following: 8mm≤|f1|≤12mm, 24mm≤|f2|≤28mm, 22mm≤|f3|≤26mm, 22mm≤|f4|≤24mm, and 20mm≤|f5|≤22mm.

[0066] The first lens has a negative optical power, meaning its focal length is between 8-12mm. This can be achieved using a meniscus lens with its concave side facing the image plane, used for initial divergence of incident light. The second lens has a positive optical power, meaning its focal length is between 24-28mm. This can be achieved using a biconvex spherical lens, used for initial convergence of diverging light. The third lens has a positive optical power, meaning its focal length is between 22-26mm. This can be achieved using a plano-convex lens, used to maintain the converging trend. The fourth lens has a negative optical power, meaning its focal length is between 22-24mm. This can be achieved using a meniscus lens with its concave side facing the object plane, used to generate secondary divergence. The fifth lens has a positive optical power, meaning its focal length is between 20-22mm. This can be achieved using a biconvex spherical lens, used to complete the final image convergence.

[0067] Specifically, a diverging-converging optical path structure is formed by combining two lens groups with alternating positive and negative optical powers. The negative optical power of the first lens causes the incident light to diverge, reducing the reflection intensity of the front lens surface; the positive optical power of the second lens converges the diverging light; the third lens maintains the converging trend to control aberrations; the negative optical power of the fourth lens generates secondary divergence, changing the propagation direction of the reflected light; and the positive optical power of the fifth lens reconverges the light onto the image plane.

[0068] In some specific embodiments, the fourth and fifth lenses can be combined using a cemented joint to reduce interface reflection by eliminating air gaps. Each lens can be made of spherical glass, and a specified focal length range can be achieved by adjusting the radius of curvature and center thickness. For example, the radius of curvature of the concave surface of the first lens can be set to 15-18 mm, and the center thickness can be controlled at 2-3 mm.

[0069] Compared to existing technologies, traditional coaxial refractive schemes employ a continuous positive optical power lens group 2, where reflected light returns along the original optical axis to form a hot spot. This scheme introduces two divergence processes, particularly the negative optical power configuration of the fourth lens, which shifts the reflected light path, fundamentally disrupting the conditions for hot spot formation. In contrast, existing technologies that optimize diffusers or coatings can only reduce reflection intensity but cannot change the direction of the light path.

[0070] Through the above technical solution, the present invention effectively suppresses the coaxial hot spot phenomenon caused by the reflection of the front lens surface, avoids the pixel saturation problem caused by the abnormal increase in brightness in the imaging center area, and maintains the imaging resolution and contrast of the optical system, so that the grayscale information of the marked area on the wafer 8 surface can be accurately captured.

[0071] In one embodiment of the present invention, the fourth lens and the fifth lens are cemented together, and the first lens, the second lens, the third lens, the fourth lens and the fifth lens are glass spherical lenses.

[0072] Among them, bonding refers to the process of gluing two lenses together to form a composite lens structure using optical adhesive, specifically epoxy resin or UV-curing adhesive. This structure can eliminate air gaps between the two lenses, thereby reducing interface reflection. A glass spherical lens is a lens made of optical glass material with a spherical surface curvature, specifically made of BK7 or F2 grade glass. The spherical curvature has rotational symmetry, making it easy to control during processing.

[0073] Specifically, after the fourth and fifth lenses are cemented together, the air-glass interface that originally existed between the two lenses is eliminated, thereby reducing stray light paths caused by Fresnel reflection. When the illumination light passes through lens group 2, the reflectivity of the cemented interface can be reduced to less than one-quarter of its original value compared to the air interface. At the same time, glass material has better optical uniformity than resin material in the visible light band, and the spherical curvature processing accuracy can be controlled within λ / 4, making the energy distribution of the light spots formed by residual reflected light on the surface of each lens more symmetrical and avoiding high-intensity focusing at the center of the image plane.

[0074] Specifically, in one embodiment of the present invention, the parameters of each mirror are shown in Table 2 below. It should be noted that the interval refers to the distance from the image plane to the object plane in the direction from the image plane to the next plane. For example, the thickness of the image plane of the first lens refers to the thickness of the first lens, while the thickness of the object plane of the second lens refers to the interval between the first lens and the second lens.

[0075] Table 2

[0076] Through the above technical solution, the present invention effectively suppresses the coaxial hot spot phenomenon caused by residual reflection on the lens surface, thereby improving the uniformity of the grayscale distribution of the two-dimensional barcode on the wafer surface, avoiding misjudgment of the recognition algorithm caused by local pixel saturation, and realizing highly stable barcode reading operation in the semiconductor manufacturing process.

[0077] in, Figure 4 The object-side MTF image quality map of the present invention has an imaging quality close to the diffraction limit, which can ensure high-precision wafer reading conditions; Figure 5 This invention generates a uniform illumination field on the wafer surface. Figure 6 This invention demonstrates that no hot spots are generated on the image plane, essentially eliminating the adverse effects of coaxial hot spots on wafer surface reading.

[0078] The above description is merely an exemplary embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention specification and drawings under the technical concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A fixed-focus optical system, characterized in that, The fixed-focus optical system includes an image sensor, a lens group, an aperture, a beam splitter, a plane mirror, at least one curved mirror, and a wafer arranged sequentially from the image plane to the object plane. The fixed-focus optical system also includes a light source, which is disposed on the side of the beam splitter facing the first reflector, for emitting illumination light; The image sensor, lens group, aperture, beam splitter, and first reflector form a first optical axis. The beam splitter is set at an angle to the first optical axis, and the planar reflector is set at an angle to the first optical axis. At least one curved reflector is disposed between the planar reflector and the wafer to reflect the illumination light passing through the planar reflector to the wafer.

2. The fixed-focus optical system as described in claim 1, characterized in that, The at least one curved mirror includes a first curved mirror and a second curved mirror arranged sequentially from the image plane to the object plane; Wherein, a second optical axis is formed between the first curved reflector and the plane reflector, and a third optical axis is formed between the first curved reflector and the second curved reflector. The second optical axis and the first optical axis are set at an angle, and the third optical axis is set at an angle to the second optical axis.

3. The fixed-focus optical system as described in claim 2, characterized in that, The angle between the second optical axis and the first optical axis is A, the angle between the line containing the first curved mirror and the first optical axis is B, the angle between the line containing the third optical axis and the first optical axis is C, and the angle between the line containing the second curved mirror and the first optical axis is D. Each of these angles satisfies the following: 85°≤A≤95°, 20°≤B≤30°, 35°≤C≤45°, 20°≤D≤30°.

4. The fixed-focus optical system as described in claim 2, characterized in that, The first curved reflector has a negative optical power and a focal length of f100, and the second curved reflector has a positive optical power and a focal length of f200, wherein the focal lengths satisfy the following: 120mm≤|f100|≤130mm, 100mm≤|f200|≤110mm.

5. The fixed-focus optical system as described in claim 2, characterized in that, The first curved surface mirror and the second curved surface mirror are aspherical mirrors.

6. The fixed-focus optical system as described in claim 1, characterized in that, The angle between the beam splitter and the first optical axis is E, where E = 45°; and / or, The beam splitter splits the illumination light in a 50:50 ratio.

7. The fixed-focus optical system as described in claim 1, characterized in that, The distance from the light source to the beam splitter is the same as the distance from the aperture to the beam splitter; and / or, The emission direction of the light source normal forms a 90° angle with the first optical axis, and the divergence angle of the light source is F, where 20°≤F≤120°.

8. The fixed-focus optical system as described in claim 1, characterized in that, The lens group includes a first lens, a second lens, a third lens, a fourth lens, and a fifth lens arranged sequentially from the image plane to the object plane. The first lens has a negative optical power, the second lens has a positive optical power, the third lens has a positive optical power, the fourth lens has a negative optical power, and the fifth lens has a positive optical power.

9. The fixed-focus optical system as described in claim 8, characterized in that, The first lens has a focal length of f1, the second lens has a focal length of f2, the third lens has a focal length of f3, the fourth lens has a focal length of f4, and the fifth lens has a focal length of f5. The focal lengths satisfy the following: 8mm≤|f1|≤12mm, 24mm≤|f2|≤28mm, 22mm≤|f3|≤26mm, 22mm≤|f4|≤24mm, 20mm≤|f5|≤22mm.

10. The fixed-focus optical system as described in claim 8, characterized in that, The fourth lens and the fifth lens are cemented together; and / or, The first lens, the second lens, the third lens, the fourth lens, and the fifth lens are glass spherical lenses.

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