AOI automatic focusing system and method suitable for low amplification factor

Abstract

The invention discloses an AOI automatic focusing system and method suitable for a low amplification factor. The system comprises an incident illumination light path and a reflection collection light path. The incident illumination light path comprises a collimating lens, a first focusing lens, a field diaphragm, a grating mask, a projection lens, a first spectroscope and a focusing objective lens which are sequentially arranged along the emergent direction of the light source; the reflection collection light path comprises a focusing objective lens, a first spectroscope, a second spectroscope, a second focusing lens, a first camera, a third focusing lens and a second camera, and the second spectroscope is located at the reflected light output end of the first spectroscope; the second focusing lens and the first camera are sequentially arranged along the reflected light output end of the second spectroscope; the third focusing lens and the second camera are sequentially arranged along the emergent light output end of the second spectroscope. The optical path is simple in structure, easy to integrate, low in requirements for optical path devices and cameras, low in cost and suitable for long-period height change environments and low-magnification amplification working conditions.

Description

Technical Field

[0001] The present invention relates to the field of optical technology, and in particular to an AOI auto-focusing system and method suitable for low magnification. Background Art

[0002] In the process of semiconductor wafer manufacturing, defects such as grooves, particles, and scratches are inevitable, which have a significant impact on the yield of chips. In order to improve the yield of chips and reduce production costs, it is necessary to perform defect detection on wafers. Since the unit modules on the surface of wafers are tiny, wafer defect detection usually uses an optical microscope system to collect images of the wafer surface and analyze the images to obtain information such as the type, size, and location of the defects.

[0003] Optical microscopy requires an objective lens with a certain magnification to obtain the image of the wafer surface by scanning or positioning imaging. Usually, a vacuum is used to adsorb the wafer onto the stage. The commonly used objective lens has a certain depth of focus, at the millimeter or even micron level; due to the mechanical flatness of the stage, wafer warping and thickness non-uniformity, changes in the pattern structure and material on the wafer surface, mechanical deformation caused by ambient temperature, mechanical vibration, posture changes caused by movement and other influences, the image may be out of focus; in order to obtain a clear microscopic image and ensure the quality of the collected image data and the sensitivity of defect detection, it is necessary to keep the wafer surface in the focal plane or focal depth range of the objective lens: this is autofocus. Autofocus needs to meet the following characteristics

[0004] 1. Sufficiently long working range: The defocus distance between two adjacent sampling points must be smaller than the working range of autofocus, otherwise, an out-of-focus image will be output; so a sufficiently long working range can ensure good detection performance and equipment capacity.

[0005] 2. Excellent working accuracy and response speed: ensure that the wafer changes slightly near the focal plane of the objective lens and within the focal depth range to ensure image clarity and consistency.

[0006] 3. Low cost: Since the post-process is very sensitive to equipment costs, the autofocus solution should have a low cost to achieve wide application.

[0007] 4. Easy integration: The autofocus solution should be easy to integrate into the optical system and can also be used independently as a separate module.

[0008] An autofocus system that meets the above requirements can use a low NA microscope objective to obtain a larger field of view and ensure the efficiency of the inspection process. The autofocus system can detect the distance change between the wafer surface and the objective in real time, and promptly feed it back to the motion control system of the motion stage to drive the wafer stage to compensate for the error distance, so as to always obtain a clear wafer surface image.

[0009] Many types of autofocus systems have been developed. There are three main types: active ranging, contrast detection, and phase detection.

[0010] Active ranging is to determine the distance (object distance) between the wafer surface and the objective lens by emitting a beam of energy (such as laser or infrared) and measuring its return time delay or other parameters. This method has the advantages of high speed and high accuracy, but the active ranging method usually requires a dedicated sensor to implement, which is costly. And when the chip process is getting smaller and smaller, and the wafer surface process is becoming more and more complex, the measurement results of the ranging sensor will fluctuate due to the influence of the wafer surface pattern, material or process step difference, which will affect the focusing effect, thereby affecting the image quality and detection performance.

[0011] Confocal detection determines the focal position by analyzing the contrast (or clarity) of the image, that is, scanning different imaging planes along the optical axis of the objective lens to obtain multiple groups of sample images, and relying on the image contrast (or clarity) evaluation algorithm to determine the defocus amount. As the lens gradually focuses, the image contrast (or clarity) will become higher and higher, and the image will become clearer and clearer; while as the lens gradually defocuses, the image contrast (or clarity) will become lower and lower, and the image will become more and more blurred. Confocal detection technology has the advantages of being simple and easy to implement and widely applicable in focusing, but it requires the collection of many photos, the amount of data is large, and the speed is very slow. This affects the production capacity of the equipment and does not meet the high-speed requirements of semiconductor defect detection.

[0012] Phase detection achieves focusing by comparing the phase difference of light passing through different positions of the lens. Phase detection technology has the advantages of high accuracy, fast speed and wide application range in focusing.

[0013] Patent CN112415735A discloses a real-time autofocus system for a microscope, which can be understood as a type of phase detection, but the modules are more complicated.

[0014] Patent CN117170082A discloses an autofocus system based on the principle of image-square differential parallel confocal detection, which can be understood as a type of phase detection. The optical path is designed to be coaxial, and it is difficult to calculate the deviation.

[0015] Patents: CN117097984A, CN117170081A, CN117174036A, CN117528234A, CN117555123A, etc. can be understood as a type of contrast detection, which requires multiple focusing and is cumbersome to use. When facing different wafers, multiple measurements and calibrations are required, and the stability is poor.

[0016] Patents: CN117850174A, CN118330868A, etc. can be understood as a type of active ranging. When facing wafers with more complex surfaces, the measurement results will fluctuate, which will affect the image quality.

[0017] In order to overcome the above technical defects in the field of semiconductor back-end packaging AOI inspection, it is necessary to develop an autofocus system with a sufficiently long working range, low cost and easy integration to detect the distance between the objective lens and the wafer, so as to cope with long-period height changes and realize a wide range of equipment applications. Summary of the invention

[0018] The present invention provides an AOI automatic focusing system and method suitable for low magnification, so as to solve the deficiencies of the prior art.

[0019] A first aspect of the present invention provides an AOI autofocus system suitable for low magnification, comprising: an incident illumination light path, a reflected collection light path;

[0020] The incident illumination light path comprises a light source, a collimating lens, a first focusing lens, a field stop, a grating mask, a projection lens, a first beam splitter and a focusing objective lens, wherein the collimating lens, the first focusing lens, the field stop, the grating mask, the projection lens, the first beam splitter and the focusing objective lens are sequentially arranged in a straight line along the projection direction of the light source, wherein the focusing objective lens is located at the output end of the first beam splitter output light, and the output end of the focusing objective lens light path is aligned with the wafer to be measured and forms a first plane imaging on the wafer to be measured;

[0021] The reflected light collection path includes a focusing objective lens, a first beam splitter, a second beam splitter, a second focusing lens, a first camera, a third focusing lens, and a second camera, wherein the second beam splitter is located at the reflected light output end of the first beam splitter;

[0022] The second focusing lens and the first camera are arranged in sequence along the reflected light output end of the second beam splitter, and the first plane image formed on the wafer to be tested is formed into a second plane image through the focusing objective lens, the first beam splitter, the second beam splitter, and the second focusing lens and is collected by the first camera;

[0023] The third focusing lens and the second camera are arranged in sequence along the output end of the second beam splitter. The first plane image formed on the wafer to be tested is formed into a third plane image through the focusing objective lens, the first beam splitter, the second beam splitter, and the third focusing lens and is collected by the second camera.

[0024] The second camera collects the grating-shaped third plane imaging, and measures the defocus distance of the surface of the wafer to be measured through an image processing algorithm.

[0025] Furthermore, a first rectangular through hole and a second rectangular through hole are provided on the field aperture. The first rectangular through hole is used for tilting the grating mask and forming a grating image on the surface of the wafer to be measured; the second rectangular through hole is the first camera illumination area.

[0026] Furthermore, the first rectangular through hole is smaller than the second rectangular through hole, and the shape of the second rectangular through hole matches the target surface of the first camera.

[0027] Furthermore, the grating mask is based on transparent glass and plated with an opaque chrome layer grating structure, which is tiltedly arranged in the first rectangular through hole provided on the field stop, and the center of the field stop and the grating mask are in the same plane.

[0028] Furthermore, the light source is a main light source or an independent light source of the detection device.

[0029] Furthermore, the light source is a direct surface light source or other light sources that have been homogenized.

[0030] Furthermore, the first camera is a TD I line scan camera or an area array camera, and the second camera is an area array camera.

[0031] Furthermore, the wafer is a wafer after back-end packaging and is placed on a stage; the stage receives a signal fed back by an autofocus system to perform position adjustment with micron-level precision.

[0032] A second aspect of the present invention provides a focusing method for an AOI autofocus system with a low magnification as described above, comprising the following steps:

[0033] The light projected by the light source passes through the collimating lens, the first focusing lens, the field stop, the grating mask, the projection lens, the first beam splitter and the focusing objective lens to form an incident illumination light path;

[0034] The center parts of the field stop and the grating mask are in the same plane and are placed in the common focal plane between the first focusing lens and the projection lens. The light source passes through the collimating lens and the first focusing lens, the field stop and the grating mask, and then through the projection lens, the first beam splitter and the focusing objective lens to form a first plane image on the wafer to be tested.

[0035] The first plane image on the wafer to be tested is processed by the focusing objective lens, the first beam splitter, the second beam splitter, the second focusing lens and the first camera to form a second plane image, and the formed second plane image will be collected by the first camera for use by the AOI system;

[0036] The first plane image on the wafer to be tested will be processed by the focusing objective lens, the first beam splitter, the second beam splitter, the third focusing lens and the second camera to form a third plane image. The third plane image formed is collected by the second camera. After image processing, the defocus distance of the surface of the wafer to be tested is measured, and the surface of the wafer to be tested is adjusted to return to the positive focal plane.

[0037] Further, the third plane imaging is processed to measure the defocus distance of the surface of the wafer to be tested, and the surface of the wafer to be tested is adjusted to return to the positive focal plane, specifically: the positive focal position of the wafer plane to be tested is taken as the standard position, the grating stripes are arranged in the X-axis direction, and the direction perpendicular to the X-axis is the Y-axis. At the standard position, the Y-axis coordinate of the clearest part is obtained through the image processing algorithm, and it is marked as the zero point; wherein the image processing algorithm is a gradient algorithm, a grayscale variance algorithm, and a frequency domain analysis algorithm;

[0038] When the height of the wafer plane to be tested changes, that is, when it is out of focus, the clearest part of the third plane imaging will be translated in the Y-axis direction and will deviate from the zero point. The defocus distance of the wafer to be tested is calculated based on the offset distance and direction between the coordinates and the zero point.

[0039] According to the defocus distance of the wafer to be tested, the distance between the focusing objective lens and the plane of the wafer to be tested is adjusted so that the surface of the wafer to be tested returns to the positive focal plane.

[0040] The beneficial effects of the present invention are:

[0041] The present invention mainly forms a first plane image on the wafer to be tested through a specially constructed grating mask process, and forms a plane image on the surface of the second camera through a reflection collection light path. After image processing, the defocus distance of the surface of the wafer to be tested is measured, and the surface of the wafer to be tested is adjusted to return to the positive focal plane. At the same time, the first camera receives the light emitted from the second rectangular through hole of the field of view diaphragm, collects the image of the wafer surface and inputs it into the AO I system for detecting defects on the wafer surface. The entire device is compatible with two systems to realize the co-optical axis of the dual systems. The two systems are the AO I wafer defect detection system and the autofocus system.

[0042] The optical path structure of the present invention is simple, easy to integrate, has low requirements on optical path devices and cameras, is low in cost, and is suitable for long-period height change environments and low-magnification magnification conditions. It has fast focus search speed, strong anti-scattered light interference ability, higher signal-to-noise ratio, and reduced matching requirements on the optical system of the auto-focusing optical system and the optical system of the equipment scanning imaging system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0044] Figure 1 It is a schematic diagram of the optical path structure of the present invention.

[0045] Figure 2 It is a schematic diagram of the grating mask structure of the present invention.

[0046] Figure 3 It is a schematic diagram of the field stop structure of the present invention. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0048] The technical solutions provided by various embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0049] like Figure 1 As shown, the present invention proposes an AOI autofocus system suitable for low magnification, comprising: an incident illumination light path, a reflection collection light path;

[0050] The incident illumination light path includes a light source 1, a collimating lens 2, a first focusing lens 3, a field stop 4, a grating mask 5, a projection lens 6, a first beam splitter 7 and a focusing objective lens 8. The collimating lens 2, the first focusing lens 3, the field stop 4, the grating mask 5, the projection lens 6, the first beam splitter 7 and the focusing objective lens 8 are sequentially arranged in a straight line along the projection direction of the light source 1, wherein the focusing objective lens 8 is located at the output end of the outgoing light of the first beam splitter 7, and the output end of the light path of the focusing objective lens 8 is aligned with the wafer to be measured and forms a first plane imaging on the wafer to be measured;

[0051] The reflection and collection optical path includes a focusing objective lens 8, a first beam splitter 7, a second beam splitter 15, a second focusing lens 9, a first camera 10, a third focusing lens 11, and a second camera 12. The second beam splitter 15 is located at the reflected light output end of the first beam splitter 7;

[0052] The second focusing lens 9 and the first camera 10 are arranged in sequence along the reflected light output end of the second beam splitter 15, and the first plane image formed on the wafer to be tested is formed by the focusing objective lens 8, the first beam splitter 7, the second beam splitter 15, and the second focusing lens 9 to form a second plane image which is collected by the first camera 10;

[0053] The third focusing lens 11 and the second camera 12 are arranged in sequence along the light output end of the second beam splitter 15. The first plane image formed on the wafer to be tested is formed by the focusing objective lens 8, the first beam splitter 7, the second beam splitter 15, and the third focusing lens 11 to form a third plane image which is collected by the second camera 12.

[0054] The second camera 12 collects the grating-shaped third plane image, and measures the defocus distance of the surface of the wafer to be measured through an image processing algorithm.

[0055] like Figure 3 As shown, a first rectangular through hole and a second rectangular through hole are arranged on the field aperture 4. The first rectangular through hole is used to place the grating mask in an inclined manner and form a grating image on the surface of the wafer to be measured; the second rectangular through hole is the illumination area of ​​the first camera. The first rectangular through hole is smaller than the second rectangular through hole, and the shape of the second rectangular through hole matches the target surface of the first camera.

[0056] Among them, the first camera 10 can be a line scan camera or an array camera, and the second camera 12 can only be an array camera. The camera sampling frequency range is 20-200Hz. The first camera 10 is used to receive the light emitted from the second rectangular through hole of the field of view aperture 4 and collect the imaging of the wafer surface. It is a part of the AOI system. The AOI system is used to detect defects on the surface of the wafer. Its receiving camera can be a line scan camera or an array camera. The second camera 12 is used to receive the light emitted from the first rectangular through hole of the field of view aperture 4 and collect the grating imaging placed in the first rectangular through hole. It is a part of the autofocus system. Since the algorithm of the present invention is aimed at the clarity of the image, the second camera 12 can only be an array camera.

[0057] like Figure 2 The figure shows the structure of the grating mask 5. The grating mask 5 is based on transparent glass and plated with an opaque chrome grating structure, which is tiltedly arranged in the first rectangular through hole of the field stop 4, and the center of the field stop and the grating mask are in the same plane.

[0058] The light source 1 is the main light source or an independent light source of the detection device, and is a direct surface light source or a light source after other light sources are homogenized.

[0059] like Figure 1 As shown, the optical output end of the focusing objective lens 8 is aligned with the wafer to be tested, and the wafer is a wafer after back-end packaging and is placed on the stage; the stage receives the signal fed back by the autofocus system to perform position adjustment with micron-level precision.

[0060] A second aspect of the present invention provides a focusing method for an AOI autofocus system with a low magnification as described above, comprising the following steps:

[0061] The light projected by the light source 1 passes through the collimating lens 2, the first focusing lens 3, the field stop 4, the grating mask 5, the projection lens 6, the first beam splitter 7 and the focusing objective lens 8 to form an incident illumination light path 13;

[0062] The center parts of the field stop 4 and the grating mask 5 are in the same plane and are placed in the common focal plane between the first focusing lens 3 and the projection lens 6. The light source 1 passes through the collimating lens 2 and the first focusing lens 3, the field stop 4 and the grating mask 5, and then forms a first plane image on the wafer to be tested via the projection lens 6, the first beam splitter 7 and the focusing objective lens 8. The light spot of the first plane image is affected by the field stop 4 and the grating mask 5. The grating mask 5 is placed in the through hole 1, and the shape of the through hole 2 matches the target surface of the TDI line scan camera or the area array camera. Such a design can further reduce the interference of scattered light and enhance the signal-to-noise ratio.

[0063] The first plane image on the wafer to be tested is processed by the focusing objective lens 8, the first beam splitter 7, the second beam splitter 15, the second focusing lens 9 and the first camera 10 to form a second plane image. The formed second plane image will be collected by the first camera 10 for use by the AOI system;

[0064] The first plane image on the wafer to be tested will be processed by the focusing objective lens 8, the first beam splitter 7, the second beam splitter 15, the third focusing lens 11 and the second camera 12 to form a third plane image. The formed third plane image is collected by the second camera 12. After image processing, the third plane image measures the defocus distance of the surface of the wafer to be tested, and adjusts the surface of the wafer to be tested to return to the positive focal plane.

[0065] After image processing, the third plane imaging measures the defocus distance of the wafer surface to be tested, and adjusts the wafer surface to return to the positive focal plane. The principle is: when the wafer to be tested is in focus, the center of the grating is in focus, and its first plane imaging on the wafer surface to be tested is the clearest. Along the grating stripe direction, the farther away from the edge of the grating, the more it deviates from the focus, and the more blurred the first plane imaging on the wafer surface. The characteristics of the first plane imaging finally formed are: the image in the middle of the grating is the clearest, and the image becomes more blurred towards the sides.

[0066] The first plane image formed by the grating mask 5 will form a third plane image on the surface of the second camera 12 through the reflection collection light path. The plane focus position of the wafer to be tested is taken as the standard position, the arrangement direction of the grating stripes is the X-axis, and the direction perpendicular to the X-axis is the Y-axis. At the standard position, a simple and fast image processing algorithm (such as gradient algorithm, grayscale variance algorithm and frequency domain analysis algorithm, etc.) is used to obtain the Y-axis coordinate of the clearest part and mark it as the zero point. Assuming that the wafer surface is in focus, the second camera 12 captures the image, and a grating-shaped image can be obtained, with the arrangement direction of the grating stripes as the X-axis and the direction perpendicular to the X-axis as the Y-axis. The characteristics of the grating image are: the image at the center is clear, and the image becomes more blurred as it moves toward the Y-axis. For the grating image, the edge clarity characteristics (such as contrast, etc.) of the grating stripes of the clear center image can be judged to finally determine the clearest position, record the Y-axis coordinate and set it as the zero point.

[0067] When the height of the wafer plane to be tested changes, that is, when it is out of focus, the clearest part of the third plane imaging will be translated in the Y-axis direction and will deviate from the zero point. According to the offset distance and direction between the coordinates and the zero point, the defocus distance of the wafer to be tested can be obtained through simple data processing. According to the measured defocus distance, the distance between the focusing lens 8 and the wafer plane to be tested is adjusted so that the surface of the wafer to be tested returns to the positive focal plane.

[0068] The above description is only an embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of the claims of the present invention.

Claims

1. An AOI autofocus system suitable for low magnification, characterized in that: include: Incident illumination light path, reflected collection light path; The incident illumination light path comprises a light source, a collimating lens, a first focusing lens, a field stop, a grating mask, a projection lens, a first beam splitter and a focusing objective lens, wherein the collimating lens, the first focusing lens, the field stop, the grating mask, the projection lens, the first beam splitter and the focusing objective lens are sequentially arranged in a straight line along the projection direction of the light source, wherein the focusing objective lens is located at the output end of the first beam splitter output light, and the output end of the focusing objective lens light path is aligned with the wafer to be measured and forms a first plane imaging on the wafer to be measured; The reflected light collection path includes a focusing objective lens, a first beam splitter, a second beam splitter, a second focusing lens, a first camera, a third focusing lens, and a second camera, wherein the second beam splitter is located at the reflected light output end of the first beam splitter; The second focusing lens and the first camera are arranged in sequence along the reflected light output end of the second beam splitter, and the first plane image formed on the wafer to be tested is formed into a second plane image through the focusing objective lens, the first beam splitter, the second beam splitter, and the second focusing lens and is collected by the first camera; The third focusing lens and the second camera are arranged in sequence along the output end of the second beam splitter, and the first plane image formed on the wafer to be tested is formed into a third plane image through the focusing objective lens, the first beam splitter, the second beam splitter, and the third focusing lens, and is collected by the second camera; The second camera collects the grating-shaped third plane imaging, and measures the defocus distance of the surface of the wafer to be measured through an image processing algorithm.

2. The AOI autofocus system suitable for low magnification according to claim 1, characterized in that: The field aperture is provided with a first rectangular through hole and a second rectangular through hole. The first rectangular through hole is used for tilting the grating mask and forming a grating image on the surface of the wafer to be measured; the second rectangular through hole is the first camera illumination area.

3. The AOI autofocus system suitable for low magnification according to claim 2, characterized in that: The first rectangular through hole is smaller than the second rectangular through hole, and the shape of the second rectangular through hole matches the target surface of the first camera.

4. The AOI autofocus system suitable for low magnification according to claim 1, characterized in that: The grating mask is based on transparent glass and plated with an opaque chrome layer grating structure. It is tiltedly arranged in the first rectangular through hole on the field stop, and the center of the field stop and the grating mask are in the same plane.

5. The AOI autofocus system suitable for low magnification according to claim 1, characterized in that: The light source is a main light source of the detection device or an independent light source.

6. The AOI autofocus system suitable for low magnification according to claim 1 or 5, characterized in that: The light source is a direct surface light source or a light source after other light sources have been homogenized.

7. The AOI autofocus system suitable for low magnification according to claim 1, characterized in that: The first camera is a TDI line scan camera or an area array camera, and the second camera is an area array camera.

8. The AOI autofocus system suitable for low magnification according to claim 1, characterized in that: The wafer is a wafer after back-end packaging and is placed on a stage; the stage receives a signal fed back by an autofocus system to perform position adjustment with micron-level precision.

9. A focusing method for an AOI autofocus system with low magnification as claimed in any one of claims 1 to 8, characterized in that: The following steps are involved: The light projected by the light source passes through the collimating lens, the first focusing lens, the field stop, the grating mask, the projection lens, the first beam splitter and the focusing objective lens to form an incident illumination light path; The center parts of the field stop and the grating mask are in the same plane and are placed in the common focal plane between the first focusing lens and the projection lens. The light source passes through the collimating lens and the first focusing lens, the field stop and the grating mask, and then through the projection lens, the first beam splitter and the focusing objective lens to form a first plane image on the wafer to be tested. The first plane image on the wafer to be tested is processed by the focusing objective lens, the first beam splitter, the second beam splitter, the second focusing lens and the first camera to form a second plane image, and the formed second plane image will be collected by the first camera for use by the AOI system; The first plane image on the wafer to be tested will be processed by the focusing objective lens, the first beam splitter, the second beam splitter, the third focusing lens and the second camera to form a third plane image. The third plane image formed is collected by the second camera. After image processing, the defocus distance of the surface of the wafer to be tested is measured, and the surface of the wafer to be tested is adjusted to return to the positive focal plane.

10. The AOI auto-focus method applicable to low magnification according to claim 9, characterized in that: The third plane imaging measures the defocus distance of the surface of the wafer to be tested through image processing, and adjusts the surface of the wafer to be tested to return to the positive focal plane, specifically: the positive focal position of the wafer plane to be tested is taken as the standard position, the grating stripes are arranged in the X-axis direction, and the direction perpendicular to the X-axis is the Y-axis. At the standard position, the Y-axis coordinate of the clearest part is obtained through the image processing algorithm, and it is marked as the zero point; wherein, the image processing algorithm is a gradient algorithm, a grayscale variance algorithm, and a frequency domain analysis algorithm; When the height of the wafer plane to be tested changes, that is, when it is out of focus, the clearest part of the third plane imaging will be translated in the Y-axis direction and will deviate from the zero point. The defocus distance of the wafer to be tested is calculated based on the offset distance and direction between the coordinates and the zero point. According to the defocus distance of the wafer to be tested, the distance between the focusing objective lens and the plane of the wafer to be tested is adjusted so that the surface of the wafer to be tested returns to the positive focal plane.

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