Wide-spectrum high numerical aperture flat-field microscopic objective for wafer defect detection

Abstract

The application discloses a wide-spectrum high-numerical-aperture flat-field microscopic objective for wafer defect detection, and relates to microscopic optical imaging and semiconductor wafer optical detection technology. The application sequentially arranges an external aperture diaphragm, a first lens group with positive focal length, a second lens group with negative focal length and a third lens group with positive focal length from a collimation side to a sample side. The first lens group is matched with high and low dispersion materials to suppress primary axial chromatic aberration, the second lens group is arranged with negative, negative, positive, positive and negative focal lengths to simultaneously correct field curvature, distortion and magnification chromatic aberration, the third lens group is matched with a three-lens cemented lens and a rear single lens to construct a high-numerical-aperture converging light path, and a lens barrel light shielding structure is used to effectively suppress non-effective stray light. The application can realize 1.42 mm large field imaging on the sample side in a wide-spectrum range of 650-950 nm, and the numerical aperture on the sample side can reach 0.54, so that the application is suitable for high-precision optical detection of semiconductor wafer micro defects.

Description

Technical Field

[0001] This invention relates to microscopic optical imaging and semiconductor wafer optical inspection technology, specifically to a broadband high numerical aperture flat-field microscope objective for wafer defect detection. Background Technology

[0002] Wafer surface defect inspection typically requires balancing resolution, single field-of-view coverage, and image consistency within a limited scanning time. Bright-field inspection mode primarily acquires specular and pattern reflection information from the wafer surface; dark-field inspection mode mainly captures scattered light from particles, scratches, residues, and edge structures. The objective lens must receive effective light at a large angle while controlling field curvature, astigmatism, and chromatic aberration at the field of view edges.

[0003] When using the same objective lens in the 650–950 nm wavelength range, the residual axial chromatic aberration and magnification chromatic aberration of ordinary visible light microscope objectives are directly reflected in the focusing position and the edge of the field of view. Dark-field signals are inherently weak, and surface reflections from multiple lenses, reflections from the inner walls of the microscope tube, and light from ineffective apertures further enhance the background. The actual challenge encountered in research and development is not simply increasing the numerical aperture, but rather simultaneously controlling chromatic aberration, field curvature, distortion, and stray light within a large sample field of view, while ensuring that the objective lens can be matched with the subsequent tube and aperture positions.

[0004] Existing microscope objectives cannot simultaneously meet the above multiple constraints and are not suitable for high-speed defect detection scenarios that integrate bright and dark fields on wafers. Summary of the Invention

[0005] This invention aims to solve several design challenges of existing visible light microscope objectives: First, when using a shared objective in the 650–950 nm wavelength range, residual axial chromatic aberration and magnification chromatic aberration directly affect the focusing position and field of view edges, degrading the imaging effect at the field of view edges; Second, in dark-field imaging, defect scattering signals are weak, and stray light generated by reflections from the lens surface, the inner wall of the microscope tube, and non-effective aperture light raises the imaging background; Third, the design challenge lies not in simply increasing the numerical aperture, but in simultaneously achieving effective control of chromatic aberration, field curvature, and distortion within a large sample field of view; Fourth, the objective structure is difficult to meet the system assembly requirements of matching the position of the rear end tube and aperture.

[0006] To address the aforementioned technical problems, this invention provides a broadband microscope objective that differs from conventional visible light plan-field objective structures. Specifically, it is a broadband high numerical aperture plan-field microscope objective for wafer defect detection. This objective employs an external aperture stop on the collimating side, three sets of optical power zones, and a combination of 14 spherical lenses. It can image a 1.42 mm diameter field of view on the sample side within the 650–950 nm wavelength range, while controlling the numerical aperture on the sample side to approximately 0.54.

[0007] The specific technical solution adopted in this invention is as follows: This invention proposes a broadband high numerical aperture flat-field microscope objective for wafer defect detection, which consists of an external aperture stop, a first lens group with positive optical power, a second lens group with negative optical power, a third lens group with positive optical power, and the sample surface arranged sequentially from the collimation side to the sample side.

[0008] The aperture stop has a half-aperture of 4.0 mm to 4.5 mm and an entrance pupil diameter of 8.0 mm to 9.0 mm; the axial distance between the aperture stop and the first optical surface of the first lens is 36 mm to 44 mm, and the ratio of the axial distance to the absolute value of the effective focal length of the microscope objective is 5.5 to 6.5.

[0009] Furthermore, the center of the aperture stop of the present invention is located on the optical axis of the first lens group with positive optical power, and the axial air gap between the aperture stop and the first lens group is 40 mm. The axial air gap is used to achieve aperture matching and to cut off non-imaging light rays that exceed the effective aperture before they enter the first lens group.

[0010] Furthermore, the positive optical power first lens group of the present invention includes four spherical lenses arranged with air gaps along the optical axis, which are arranged sequentially from the collimation side to the sample side as a first lens, a second lens, a third lens, and a fourth lens; the optical powers of the first lens, the second lens, the third lens, and the fourth lens are positive, negative, positive, and positive, respectively, and air gaps are provided between adjacent lenses.

[0011] Furthermore, the first lens is used to achieve initial beam convergence and aperture compression; the second lens is used to compensate for axial chromatic aberration generated by the first lens group; the third lens is used to converge the edge field of view beam; and the fourth lens corrects the residual spherical aberration of the first lens group through the curvature combination of its two side surfaces, and matches the exit beam aperture with the incident aperture of the subsequent second lens group.

[0012] Furthermore, the negative optical power second lens group of the present invention is an aberration correction group, comprising five spherical lenses arranged with air gaps along the optical axis, namely the fifth lens, the sixth lens, the seventh lens, the eighth lens, and the ninth lens in sequence from the collimation side to the sample side; the optical powers of the fifth lens, the sixth lens, the seventh lens, the eighth lens, and the ninth lens are negative, negative, positive, positive, and negative in sequence, and air gaps are provided between adjacent lenses.

[0013] Furthermore, the radius of curvature of the first surface of the fifth lens is 9.2 mm to 11.2 mm, and the radius of curvature of the second surface is 5.3 mm to 6.4 mm; The radius of curvature of the surface of the sixth lens facing the collimation side is -5.7 mm to -4.6 mm; The seventh lens is a positive optical power compensation lens disposed in the middle of the second lens group; The eighth lens is used in conjunction with the low-dispersion material lenses placed before and after it to correct the magnification chromatic aberration of the microscope objective. The surface curvature radius of the ninth lens facing the collimation side is -31.7 mm to -25.9 mm, and the ninth lens is the negative power lens at the end of the second lens group.

[0014] Furthermore, the second lens group is provided with an annular extinction groove around its periphery, and the surface of the annular extinction groove is provided with an extinction layer, which is used to intercept and absorb non-imaging light formed by reflection from the large curvature surface of the lens, thereby reducing the influence of stray light on the wafer defect detection image.

[0015] Furthermore, the positive optical power third lens group of the present invention includes a tenth lens, an eleventh lens, a twelfth lens, a thirteenth lens, and a fourteenth lens; Along the collimation side to the sample side, the tenth lens, the eleventh lens, and the twelfth lens constitute a cemented triplet lens; Following the cemented triplet lens, two independent single lenses, the thirteenth and fourteenth lenses, are sequentially arranged. There are air gaps between the cemented triplet lens and the thirteenth lens, and between the thirteenth lens and the fourteenth lens.

[0016] Furthermore, the third lens group of the present invention transforms the high numerical aperture beam on the sample side into a beam shape adapted to the aberration correction of the second lens group, and suppresses residual chromatic aberration by utilizing the refractive index and dispersion difference of different optical materials at the cemented interface.

[0017] Furthermore, the microscope objective of the present invention images a 1.42 mm diameter field of view on the sample side in the 650–950 nm wavelength band, and controls the numerical aperture on the sample side to be 0.54.

[0018] The beneficial effects of this invention are as follows: 1. In this invention, the aperture stop is positioned on the collimating side of the first lens group and maintained at an axial distance of 40 mm from the first lens. This structure defines the effective aperture before the light beam enters the objective lens body, facilitating pupil matching with the tube lens or relay optical path, while reducing the amount of light exceeding the effective aperture entering multiple lenses.

[0019] 2. The first lens group of this invention adopts a combination of materials and optical power of "high refractive index positive lens - low dispersion negative lens - high refractive index positive lens - positive lens". Among them, the low dispersion negative lens is located between the two high refractive index positive lenses, mainly responsible for primary axial chromatic aberration compensation, and there is no need to add a separate set of visible light achromatic cemented doublet lenses.

[0020] 3. The second lens group of this invention adopts a five-piece air-spaced structure of negative-negative-positive-positive-negative. The front and rear negative lenses change the edge light height, and the middle positive lens restores the beam convergence state, so that field curvature, astigmatism, distortion, and magnification chromatic aberration can be respectively distributed to different surfaces for correction.

[0021] 4. The light-shielding groove near the second lens group, the lens edge extinction, and the collimation side aperture together form a stray light control path.

[0022] 5. The third lens group of this invention employs a cemented triple lens and two sample-side single lenses. The cemented triple lens utilizes the refractive index and dispersion difference of the three types of glass to control residual chromatic aberration, and the two single lenses close to the sample form a sample-side numerical aperture of 0.5375.

[0023] 6. This invention performs joint optimization at five wavelengths: 0.65, 0.725, 0.8, 0.875, and 0.95 μm. Simulation summary plots show that the imaging curves of each wavelength remain close across multiple field points, which can reduce the amount of refocusing required when switching between multiple wavelengths.

[0024] 7. The broadband high numerical aperture plan-field microscope objective proposed in this invention is fabricated using a spherical surface. Except for the cemented triplet lens in the third lens group, the other lenses are spaced by air gaps, which facilitates adjustment of the axial spacing using spacers and also facilitates disassembly and assembly adjustment.

[0025] Specifically, this invention relates to an infinity-corrected flat-field microscope objective for collecting reflected and scattered light from wafer surfaces. The objective employs a fully refractive spherical structure, is designed for a wavelength range of 650–950 nm, has a sample-side numerical aperture of 0.54, and can be used in conjunction with bright-field illumination, dark-field illumination, and multi-wavelength detection optical paths. Attached Figure Description

[0026] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0027] Figure 1 This is the axial optical path diagram of the optical system of the present invention in the prescription direction (prescription direction: collimation side → sample side); Figure 2 This is a three-dimensional layout diagram of the optical system in the Zemax simulation results of this invention; Figure 3 This is a list of optical design parameters for the optical system of the present invention; Figure 4This is the ray fan pattern from the Zemax simulation results of this invention; Figure 5 This is a graph of the modulation transfer function (MTF) of polychromatic light diffraction in the Zemax simulation results of this invention. Figure 6 This is a field curvature distortion diagram from the Zemax simulation results of this invention; Figure 7 This is a point array diagram of the Zemax simulation results of this invention; Figure 8 This is a three-dimensional wavefront function plot from the Zemax simulation results of this invention. Detailed Implementation

[0028] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

[0029] Implementation Method 1, in conjunction with the following Figure 1 This invention provides a detailed description of a broadband high numerical aperture flat-field microscope objective for wafer defect detection, so that those skilled in the art can clearly reproduce the technical solution of this invention.

[0030] The present invention proposes a broadband high numerical aperture flat-field microscope objective for wafer defect detection, which consists of an external aperture stop, a first lens group with positive optical power, a second lens group with negative optical power, a third lens group with positive optical power, and the sample surface arranged sequentially from the collimation side to the sample side.

[0031] To maintain consistency with the original Zemax optical simulation formulation parameters, the optical structures in this embodiment are described in order from the collimation side to the sample side. This arrangement is opposite to the actual light propagation direction detected by the device. In actual operation, the wafer is located on the sample surface P. The reflected or scattered light from the wafer first enters the third lens group G3, which is close to the sample surface, then passes through the second lens group G2 and the first lens group G1, and finally exits from the collimation side and enters the tube lens.

[0032] The 29th surface in the formulation is the sample surface; the 1st surface is the aperture stop. The "image-side numerical aperture of 0.5375" given in the formulation corresponds to the sample-side numerical aperture when used in reverse. The maximum image height of 0.708 mm given in the formulation corresponds to the sample field of view radius, and the sample-side full field of view diameter is approximately 1.417 mm.

[0033] Furthermore, in one specific embodiment, the overall structure of the broadband high numerical aperture flat-field microscope objective proposed in this invention will be described. The microscope objective includes an aperture stop S, a first lens group G1, a second lens group G2, a third lens group G3, and a sample surface P. Arranged sequentially from the collimation side to the sample side, these components are: aperture stop S, first lens group G1, second lens group G2, third lens group G3, and sample surface P. All imaging surfaces are spherical.

[0034] The system comprises four lenses: the first lens group G1, which has positive optical power and consists of four spherical lenses spaced apart by air; the second lens group G2, which has negative optical power and consists of five spherical lenses spaced apart by air; and the third lens group G3, which has positive optical power and consists of five lenses, with the first three forming a cemented triplet and the last two being single lenses spaced apart by air. The entire system has a total of 14 lenses.

[0035] like Figure 1 As shown in the figure, the left side represents the collimation side and aperture stop, while the right side represents the sample side. The actual direction of light propagation is opposite to the direction modeled in the figure.

[0036] Specifically: (1) The specific design of the aperture stop and collimation side air section is as follows: The aperture stop S is located outside the collimating side of the first lens group G1. The half-aperture of the aperture stop is 4.0 mm to 4.5 mm, and the corresponding entrance pupil diameter is 8.0 mm to 9.0 mm. The axial distance between the aperture stop and the first optical surface of the first lens is 36 mm to 44 mm, and the ratio of the axial distance to the absolute value of the effective focal length of the microscope objective is 5.5 to 6.5.

[0037] Furthermore, the center of the aperture stop of the present invention is located on the optical axis of the first lens group with positive optical power, and the axial air gap between the aperture stop and the first lens group is 36 mm to 44 mm, preferably 38 mm to 42 mm. The axial air gap is used to achieve aperture matching and to cut off non-imaging light rays exceeding the effective aperture before they enter the first lens group. In this embodiment, the axial air gap is 40 mm.

[0038] Preferably, the aperture stop half-diameter is 4.25 mm, corresponding to an entrance pupil diameter of 8.5 mm. The axial distance from the aperture stop to the first surface of the first lens in the first lens group is 40 mm. The ratio of this distance to the absolute value of the effective focal length of the objective lens is 6.0.

[0039] In practical applications, the aperture stop can be installed in a separate aperture mount at the front of the lens barrel, or it can be placed in the interface assembly that connects to the lens barrel or relay optical path. The aperture mount is limited by annular steps, and the center of the aperture stop coincides with the optical axis of the first lens group. The 40mm air section behind the aperture stop is used to establish aperture matching and to cut off ineffective large-angle light rays before entering the lens group.

[0040] (2) The specific design of the first lens group G1 is as follows: The first lens group G1 includes a first lens L1, a second lens L2, a third lens L3, and a fourth lens L4. Along the collimation side to the sample side, the optical powers of the first, second, third, and fourth lenses are positive, negative, positive, and positive, respectively, and air gaps are provided between adjacent lenses.

[0041] The first lens L1 is made of H-LAF2 lanthanide glass and is responsible for the first beam convergence and aperture compression.

[0042] The second lens L2, made of H-FK61 low-dispersion fluorine crown glass, is placed between two positive power lenses to compensate for the axial chromatic aberration generated by the first set of lenses.

[0043] The third lens L3 uses H-LAK7A high-refractive-index glass to further reduce the beam at the edge of the field of view.

[0044] The fourth lens L4 is made of H-LAF3B glass. The curvature of its two side surfaces is used to correct the residual spherical aberration of the first lens group and to match the exit beam aperture with the incident aperture of the subsequent second lens group.

[0045] An air gap is provided between the end of the first lens group and the second lens group. The 10th surface of the prescription is an auxiliary surface located in the air, used for modeling and grouping, and does not represent an individual lens.

[0046] (3) The specific design of the second lens group G2 is as follows: The second lens group G2 is the main aberration correction group, including the fifth lens L5, the sixth lens L6, the seventh lens L7, the eighth lens L8, and the ninth lens L9. Along the collimation side to the sample side, the optical powers of the fifth, sixth, seventh, eighth, and ninth lenses are negative, negative, positive, positive, and negative, respectively, and air gaps are provided between adjacent lenses.

[0047] The radius of curvature of the first surface of the fifth lens is 9.2 mm to 11.2 mm, and the radius of curvature of the second surface is 5.3 mm to 6.4 mm. The radius of curvature of the surface of the sixth lens facing the collimation side is -5.7 mm to -4.6 mm; The seventh lens is a positive optical power compensation lens disposed in the middle of the second lens group; The eighth lens is used in conjunction with the low-dispersion material lenses placed before and after it to correct the magnification chromatic aberration of the microscope objective. The surface curvature radius of the ninth lens facing the collimation side is -31.7 mm to -25.9 mm, and the ninth lens is the negative power lens at the end of the second lens group.

[0048] Preferably, the radius of curvature of the first surface of the fifth lens is 9.7 mm to 10.7 mm, and the radius of curvature of the second surface is 5.6 mm to 6.1 mm; the radius of curvature of the surface of the sixth lens facing the collimation side is -5.4 mm to -4.9 mm; and the radius of curvature of the surface of the ninth lens facing the collimation side is -30.3 mm to -27.4 mm.

[0049] In this embodiment, the fifth lens L5 uses H-ZLAF series glass, with a first surface curvature radius of 10.173 mm and a second surface curvature radius of 5.852 mm.

[0050] The sixth lens, L6, uses H-LAF50B glass, and its collimation side curvature radius is -5.152mm, making it the negative lens with the larger curvature in the second group.

[0051] The seventh lens, L7, uses H-FK95N low-dispersion glass as an intermediate positive power compensation lens.

[0052] The eighth lens, L8, uses ZF7L high-dispersion glass to work with the front and rear low-dispersion materials to control chromatic aberration at magnification.

[0053] The ninth lens, L9, is made of H-ZF88 high-dispersion glass with a collimation side curvature radius of -28.831mm, and serves as the negative lens at the end of the second group of lenses.

[0054] The second lens group forms a local beam expander and reconvergence through the front and rear negative lenses and the two middle positive lenses. This group mainly controls the field curvature, astigmatism, distortion, and chromatic aberration at the edge of the field of view. A ring-shaped light-shielding groove can be set near the second lens group in the lens barrel. The inner diameter of the light-shielding groove is made up with a margin according to the effective beam envelope at each position to avoid blocking the effective light.

[0055] (4) The specific design of the third lens group G3 is as follows: The third lens group G3 is located on the sample side and includes the tenth lens L10, the eleventh lens L11, the twelfth lens L12, the thirteenth lens L13, and the fourteenth lens L14. This group as a whole has positive optical power.

[0056] The tenth lens L10, the eleventh lens L11, and the twelfth lens L12 are made of H-ZLAF91 heavy lanthanum flint glass, H-ZPK7 heavy phosphorus crown glass, and H-LAK4L lanthanum crown glass, respectively. The three lenses are cemented together at surfaces 22 and 23 to form a cemented triplet lens. Following the cemented triplet lens are H-ZF88 single lens L13 and H-ZLAF90 single lens L14, respectively. There are air gaps between the two single lenses and between them and the cemented triplet lens.

[0057] The third lens group is located closest to the sample surface and is responsible for receiving large-angle reflected and scattered light from the sample side. L14 is closest to the sample surface, with an axial distance of 0.708 mm from its last surface to the sample surface. The third lens group converts the high numerical aperture beam from the sample side into a beam shape suitable for correction by the second lens group, while controlling the residual chromatic aberration by utilizing the refractive index and dispersion difference of the cemented interface material.

[0058] (5) The design of the lens barrel, positioning, and light-shielding structure is as follows: The individual lenses in the first and second lens groups are positioned using lens barrel steps and metal spacers, respectively. The thickness of the spacers is used to define the air gap; the inner hole of the spacer should be larger than the effective beam aperture at the corresponding position, with allowance for adjustment. The cemented triplet lens in the third lens group is installed as a single unit, with the remaining two individual lenses positioned separately.

[0059] The outer circumference of the lens, the inner wall of the lens barrel, and the non-light-transmitting surfaces of the spacers can be treated with exfoliation. An annular light-blocking groove near the second lens group is used to absorb ineffective light reflected from surfaces with high curvature. The position and inner diameter of the light-blocking groove should be based on actual non-sequential stray light analysis results, avoiding settings based solely on mechanical experience.

[0060] Each lens group can be assembled using segmented lens barrels. The first, second, and third lens groups should be individually centered before final assembly. During final assembly, the sample-side optical axis of the third lens group should be used as a reference to check for eccentricity and tilt in each group. Specific assembly and adjustment tolerances need to be determined separately based on Zemax tolerance analysis.

[0061] Furthermore, in this embodiment, the working principle of the broadband high numerical aperture flat-field microscope objective proposed in this invention will be explained. During actual testing, the wafer is located on sample surface P. Reflected light from bright-field illumination or scattered light from dark-field illumination enters the fourteenth lens L14 from the sample side. The third lens group first receives large-angle light and completes the first stage of convergence and chromatic aberration compensation through a cemented triple lens and two single lenses.

[0062] The beam then enters the second lens group. The negative-negative-positive-positive-negative arrangement of the second lens group alters the propagation height and angle of the edge field rays relative to the on-axis rays, used to balance field curvature, astigmatism, distortion, and magnification chromatic aberration.

[0063] The beam, corrected by the second lens group, enters the first lens group. The first lens group further compresses axial chromatic aberration by utilizing the dispersion difference between the H-FK61 and the adjacent high-refractive-index glass, and converts the beam into a collimated output. An aperture stop located on the collimating side defines the effective aperture of the system and matches it with the entrance pupil of the subsequent tube mirror. The tube mirror then images the collimated beam onto the image sensor.

[0064] This objective lens itself is not limited to bright-field or dark-field illumination. The illumination light can be coaxial reflection, annular oblique incidence, or other external structures. The function of the objective lens is to collect the light returned from the wafer and form a uniform output beam at infinity.

[0065] Furthermore, in one embodiment, the following design may also be adopted; The aperture stop can be adjusted between the first and second lens groups, or between the second and third lens groups. After adjustment, the pupil position, distortion, and astigmatism need to be re-optimized.

[0066] The first lens group can replace part of the air-spaced single lens with a cemented achromatic group, or use a low-order aspherical surface on one of the lenses to reduce the number of lenses.

[0067] The second lens group can split a large-curvature negative lens into two smaller-curvature negative lenses, or reduce it to two negative lenses and one positive compensation lens.

[0068] The third lens group can adjust the bonding method or add a protective window on the sample side. After the protective window is added, spherical aberration must be recalibrated according to its thickness and refractive index.

[0069] Low-dispersion and high-dispersion glasses can be interchanged between materials with similar refractive indices, Abbe numbers, and relative partial dispersions.

[0070] Lens positioning can be achieved using independent spacers, stepped lens barrels, elastic pressure rings, or segmented lens barrels; stray light control can be achieved using light-shielding grooves, independent light-shielding plates, blackening of lens edges, and matte coating on the lens barrel.

[0071] The fixed aperture stop can be replaced with an adjustable aperture stop, or a filter or beam splitter can be added to the collimation side.

[0072] In summary, this invention addresses several key optical design challenges: first, by configuring an external aperture stop to establish an aperture position that matches the tube mirror; second, by controlling primary axial chromatic aberration through a combination of high and low dispersion materials in the first lens group; third, by correcting field curvature, distortion, and magnification chromatic aberration through a negative-negative-positive-positive-negative structure in the second lens group; fourth, by forming a high numerical aperture sample-side converging beam through a cemented triple lens in the third group and subsequent single lenses; and fifth, by setting a positioning and light-shielding structure in the tube mirror that matches the beam aperture, facilitating assembly and reducing ineffective light entering the subsequent imaging channel.

[0073] Implementation Method Two: Combination Figures 2 to 8 This embodiment describes a joint optimization performed at five wavelengths: 0.65, 0.725, 0.8, 0.875, and 0.95 μm. 1. Parameter design is shown in Table 1: Table 1

[0074] 2. The optical formulation is shown in Table 2, where "Thickness / Gap" represents the axial distance from the current surface to the next surface; "Medium" represents the medium following the current surface. Surfaces 22 and 23 are the bonding interface. Surface 10 is the auxiliary modeling surface in the air.

[0075] Table 2

[0076] 3. Assembly as follows: Each lens in the first lens group G1 and the second lens group G2 has its air gap controlled by spacers. The spacers and the lens barrel steps together complete the axial positioning. In the third lens group, the tenth lens L10, the eleventh lens L11, and the twelfth lens L12 are first glued together to form a triplet lens, and then installed as a whole into the lens barrel; the thirteenth lens L13 and the fourteenth lens L14 are positioned by spacers.

[0077] The adhesive thickness, type, and curing process of the cemented triplet lens should be determined based on the transmittance (0.65–0.95 μm), refractive index matching, and environmental reliability. The aperture stop can be integrated with the objective lens barrel or located within the lens tube assembly. Regardless of the method used, the axial distance and coaxial relationship between the aperture stop and the first surface of L1 should remain constant.

[0079] 4. The work process is as follows: (1) The wafer return light enters the fourteenth lens L14 and the thirteenth lens L13 from the sample surface P. The two lenses closer to the sample side receive large-angle light and introduce the beam into the cemented triple lens.

[0080] (2) Cemented triplet lenses L10 to L12 utilize the refractive index and dispersion difference of the three materials to compensate for chromatic aberration and spherical aberration of high numerical aperture beams.

[0081] (3) The light beam enters the second lens group G2. The ninth lens L9 and the sixth lens L6, etc., negative lenses, work together with the seventh lens L7 and the eighth lens L8 positive lenses to adjust the propagation height of the light rays in the edge field of view and control field curvature, astigmatism, distortion and magnification chromatic aberration.

[0082] (4) The beam enters the first lens group G1. The low dispersion material of the second lens L2, in conjunction with the adjacent high refractive index positive lens, compensates for the remaining axial chromatic aberration and converts the beam into an approximately parallel output.

[0083] (5) The collimating side aperture stop S defines the effective aperture. The output light enters the tube mirror and forms an image on the sensor.

[0084] 5. Simulation Setup and Results Explanation This implementation uses Zemax Optical Studio 19.4 for optical design. Optimized wavelengths are 0.650, 0.725, 0.800, 0.875, and 0.950 μm; the field of view includes an on-axis field of view and multiple off-axis fields of view, with a maximum half-field angle of 6.0676°.

[0085] Figure 2 The diagram shows a three-dimensional layout of the optical system in this embodiment. The diagram illustrates the lens assemblies arranged sequentially along the optical axis, the optical path propagation path, and the direction of the 650nm to 950nm multicolor rays. The scale marks the axial dimensions of the system, visually showing the overall optical structure of the objective lens, the lens arrangement, and the relationship between the telecentric layout of the optical path, which is used to explain the optical configuration of this objective lens.

[0086] Figure 3 The table shown is a list of optical design parameters for the optical system in this embodiment; where EFL is the effective focal length, BFL is the back focal length, the total length is the total length of the objective lens optical system, and the working F / # is the system's working aperture number; the image height characterizes the system's maximum field of view, the entrance pupil diameter is 8.5mm, and the entrance pupil position, exit pupil diameter, and exit pupil position are used to characterize the telecentric optical path layout; the field of view data lists the coordinates of multiple characteristic field of view of the system; the wavelength parameters correspond to the working spectral range of 650nm to 950nm, with each wavelength having equal weight, indicating that the system needs to simultaneously achieve aberration correction within this wide wavelength range.

[0087] Figure 4 The diagram shows ray fan plots, illustrating the Ex and Ey ray aberration curves for five different fields of view. These curves correspond to working wavelengths of 650nm, 725nm, 800nm, 875nm, and 950nm, respectively. The maximum scaling range for lateral aberration is ±5μm, which characterizes the system's transverse aberration correction level across the entire field of view and the wide spectral range of 650–950nm, reflecting the objective lens's suppression of chromatic aberration, coma, and astigmatism at the edge of the field of view.

[0088] Figure 5The figure shows the modulation transfer function (MTF) curves of polychromatic light diffraction, corresponding to the working wavelength range of 650nm to 950nm. It shows the MTF characteristics in the meridional and sagittal directions under multiple characteristic fields of view. The horizontal axis is the spatial frequency, and the vertical axis is the OTF magnitude. As can be seen from the figure, the meridional and sagittal MTF curves of the entire field of view are closely matched, maintaining high imaging transfer capability in a wide spatial frequency range. This indicates that the objective lens has excellent astigmatism correction in a wide spectrum of 650-950nm and a wide field of view, with stable resolution and image consistency, and is suitable for imaging the detection of bright-field and dark-field defects on wafer surfaces.

[0089] Figure 6 The image shows the field curvature and distortion curves. The left side shows the field curvature curve, and the right side shows the F-Tan (Theta) distortion curve, corresponding to working wavelengths of 650nm, 725nm, 800nm, 875nm, and 950nm, respectively. The field curvature results show that the meridional and sagittal field curvature values ​​are extremely small, and the field curvature is effectively suppressed under the wide spectrum of the system. The maximum distortion across the entire field of view is only 0.3580%. These results indicate that the objective lens can simultaneously control field curvature and distortion in the wide band of 650–950nm and across the entire field of view, ensuring the geometric fidelity of wafer images and meeting the measurement and imaging consistency requirements for large field-of-view wafer surface defect detection.

[0090] Figure 7 The diagram shows the distribution of light spots at wavelengths of 650 nm, 725 nm, 800 nm, 875 nm, and 950 nm under five characteristic fields of view, in micrometers. The outer dashed line represents the reference boundary of the diffraction Airy disk. The RMS radius of the light spots in each field of view is close to the diffraction limit. The results indicate that the objective lens effectively corrects chromatic aberration, spherical aberration, and coma across a wide spectral range of 650–950 nm and the entire field of view. It exhibits good spot convergence and can ensure imaging resolution of defects such as tiny particles and scratches on wafers, making it suitable for both bright-field and dark-field defect detection.

[0091] Figure 8 The figure shows a three-dimensional wavefront function plot, indicating a wavefront peak-to-valley difference of 0.0327 Hz and a wavefront RMS value of only 0.0099 Hz, far below 1 / 14 Hz and close to the ideal diffraction limit. This result demonstrates extremely low residual wavefront aberration at the objective lens's central field of view, resulting in excellent optical imaging quality. This is beneficial for improving the signal-to-noise ratio of weak scattering signals in dark fields, meeting the requirements for high-precision imaging detection of minute defects in wafers.

[0092] As can be seen from the above figures, the curves of the five wavelengths remain close in each field of view, the field curvature and distortion curves do not show obvious abrupt changes, the edge field of view spots do not spread over a large area, and the relative illumination remains stable in the field of view.

[0093] The above description of the technical solution provided by the present invention through several specific embodiments is intended to highlight the advantages and benefits of the technical solution provided by the present invention. However, the above-described specific embodiments are not intended to limit the present invention. Any reasonable modifications and improvements to the present invention, reasonable combinations of implementation methods and equivalent substitutions based on the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A broadband high numerical aperture flat-field microscope objective for wafer defect detection, characterized in that, An external aperture stop, a first lens group with positive optical power, a second lens group with negative optical power, and a third lens group with positive optical power are arranged sequentially from the collimation side to the sample side, along with the sample surface.

2. The broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 1, characterized in that, The aperture stop is located outside the collimation side of the first lens group. The half-aperture of the aperture stop is 4.0 mm to 4.5 mm, and the entrance pupil diameter is 8.0 mm to 9.0 mm. The axial distance from the aperture stop to the first surface of the first lens in the first lens group is 36 mm to 44 mm, and the ratio of the axial distance to the absolute value of the effective focal length of the objective lens is 5.5 to 6.

5.

3. A broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 2, characterized in that, The center of the aperture stop is located on the optical axis of the first lens group with positive optical power. The axial air gap between the aperture stop and the first lens group is 40 mm. The axial air gap is used to achieve aperture matching and to cut off non-imaging light rays that exceed the effective aperture before they enter the first lens group.

4. A broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 1, characterized in that, The positive optical power first lens group includes four spherical lenses arranged with air gaps along the optical axis, which are arranged sequentially from the collimation side to the sample side as the first lens, second lens, third lens and fourth lens; the optical powers of the first lens, second lens, third lens and fourth lens are positive, negative, positive and positive respectively, and air gaps are provided between adjacent lenses.

5. A broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 4, characterized in that, The first lens is used to achieve initial beam convergence and aperture compression; the second lens is used to compensate for axial chromatic aberration generated by the first lens group; the third lens is used to converge the edge field of view beam; the fourth lens corrects the residual spherical aberration of the first lens group by combining the curvature of its two side surfaces, and makes the exit beam aperture match the incident aperture of the subsequent second lens group.

6. A broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 1, characterized in that, The negative optical power second lens group is an aberration correction group, which includes five spherical lenses arranged with air gaps along the optical axis. The lenses are arranged in sequence from the collimation side to the sample side as the fifth lens, sixth lens, seventh lens, eighth lens, and ninth lens. The optical powers of the fifth lens, sixth lens, seventh lens, eighth lens, and ninth lens are negative, negative, positive, positive, and negative, respectively, and air gaps are provided between adjacent lenses.

7. A broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 6, characterized in that, The radius of curvature of the first surface of the fifth lens is 9.2 mm to 11.2 mm, and the radius of curvature of the second surface is 5.3 mm to 6.4 mm. The radius of curvature of the surface of the sixth lens facing the collimation side is -5.7 mm to -4.6 mm; The seventh lens is a positive optical power compensation lens disposed in the middle of the second lens group; The eighth lens is used in conjunction with the low-dispersion material lenses placed before and after it to correct the magnification chromatic aberration of the microscope objective. The surface curvature radius of the ninth lens facing the collimation side is -31.7 mm to -25.9 mm, and the ninth lens is the negative power lens at the end of the second lens group; The second lens group has an annular extinction groove around its periphery, and the surface of the annular extinction groove is provided with an extinction layer, which is used to intercept and absorb non-imaging light formed by reflection from the large curvature surface of the lens, thereby reducing the influence of stray light on the wafer defect detection image.

8. A broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 1, characterized in that, The positive optical power third lens group includes the tenth lens, the eleventh lens, the twelfth lens, the thirteenth lens, and the fourteenth lens; Along the collimation side to the sample side, the tenth lens, the eleventh lens, and the twelfth lens constitute a cemented triplet lens; Following the cemented triplet lens, two independent single lenses, the thirteenth and fourteenth lenses, are sequentially arranged. There are air gaps between the cemented triplet lens and the thirteenth lens, and between the thirteenth lens and the fourteenth lens.

9. A broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 8, characterized in that, The third lens group converts the high numerical aperture beam on the sample side into a beam shape adapted to the aberration correction of the second lens group, and suppresses residual chromatic aberration by utilizing the refractive index and dispersion difference of different optical materials at the cemented interface.

10. A broadband high numerical aperture flat-field microscope objective for wafer defect detection according to claim 1, characterized in that, The microscope objective images a 1.42 mm diameter field of view on the sample side within the 650–950 nm wavelength band, and controls the numerical aperture on the sample side to be 0.54.

Images