Laser scanning line confocal surface defect detection apparatus and method based on differential interference contrast

Abstract

The application discloses a kind of laser scanning line confocal surface defect detection equipment and method based on differential interference contrast, and the equipment mainly includes laser, first mirror, second mirror, beam expander, polarizer, beam splitter, Wollaston prism, third mirror, beam scanning device, F-theta telecentric field lens, fourth mirror, cylindrical ellipsoid mirror, linear optical fiber bundle, optical filter, photomultiplier, measured element, polarizer, lens, photodetector, photodiode.The application can detect the surface defect of measured sample at high speed and high precision, and identify the concave-convex type of defect according to differential interference contrast signal, and has optical tomography capability, and can exclude the influence of back scattering light of transparent optical element.

Description

Technical Field

[0001] This invention belongs to the field of optical inspection technology, specifically relating to a laser scanning line confocal surface defect detection device and method based on differential interference contrast. Background Technology

[0002] With continuous technological innovation and the urgent need for highly integrated and miniaturized intelligent devices in production and daily life, precision optics and semiconductor components are playing an increasingly important role, widely used in emerging strategic industries such as intelligent manufacturing, large-scale integrated circuits, consumer electronics, precision medicine, aerospace, and military equipment. Surface micro- and nano-defects are crucial indicators for evaluating the quality of precision optics and semiconductor components. Defects not only affect the key performance aspects of components in terms of optics, mechanics, thermodynamics, and electrochemistry, but for high-energy and high-load systems, defects can even damage components and paralyze the entire system. Therefore, the requirements for detecting surface micro- and nano-defects in precision optics and semiconductor components are becoming increasingly stringent, making high speed, high sensitivity, high precision, and automation ever more urgent.

[0003] In recent years, scholars at home and abroad have conducted in-depth research on surface defect detection and have proposed a variety of detection methods.

[0004] Topographic-based detection methods utilize specific high-resolution microscopy instruments to measure the microscopic morphology of optical component surfaces, thereby locating defects. While atomic force microscopy and scanning electron microscopy offer extremely high detection accuracy, they both cause secondary damage to the component surface and have very low detection efficiency, failing to meet the demands of automated detection.

[0005] Traditional line confocal microscopy uses a line beam to illuminate the sample in one dimension. The imaging beam returned from the sample is received by a linear CCD camera, which converts the light signal into an electrical signal. An image acquisition card then converts the electrical signal from the CCD into an image. It follows a strict object-image conjugation principle, where the numerical aperture (NA) and the linear field of view (FOV) are mutually constrained. To improve the efficiency of collecting scattered light from defects, the NA must be increased, but this limits the FOV, resulting in a linear FOV on the order of millimeters, thus limiting detection efficiency. Furthermore, traditional line confocal microscopy only has confocal filtering capability in the width direction of the linear field of view; it loses its filtering characteristics in the length direction, causing crosstalk and increased noise in the scattered light, thereby reducing detection sensitivity. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, this invention provides a laser scanning line confocal surface defect detection device and method based on differential interference contrast, aiming to achieve high-speed and high-precision detection of optical element surfaces, identify and classify defect types, and simultaneously achieve optical tomography capability to eliminate the influence of backscattered light from transparent optical elements.

[0007] To achieve the above-mentioned objectives, the invention adopts the following technical solution:

[0008] The present invention discloses a laser scanning line confocal surface defect detection device based on differential interference contrast, characterized in that a laser is provided for emitting a laser beam, and a beam expander is provided behind the laser. A first reflecting mirror and a second reflecting mirror are sequentially provided between the beam expander and the laser. A beam scanning device is provided behind the beam expander for converting the laser beam into a line beam. A polarizer, a beam splitter, a Wollaston prism, and a third reflecting mirror are sequentially provided between the beam expander and the beam scanning device. An analyzer, a lens, and a photodetector are sequentially provided behind the beam splitter.

[0009] A photodiode and an F-theta telecentric field mirror are arranged sequentially behind the beam scanning device; the F-theta telecentric field mirror converts the line beam into a converging scanning laser beam, which is then reflected by the fourth reflecting mirror and incident perpendicularly onto the surface of the component under test.

[0010] After the light incident on the surface of the component under test returns along the original optical path, it passes through the beam splitter and then sequentially through the analyzer, the lens, and the photodetector.

[0011] A cylindrical ellipsoidal mirror is provided above the surface of the component under test. The inner surface of the cylindrical ellipsoidal mirror is coated with a highly reflective material, and notches are provided at its bottom and top. The front focal line of the cylindrical ellipsoidal mirror coincides with the position where the laser beam converges on the surface of the sample under test.

[0012] A linear fiber bundle is provided above the cylindrical ellipsoidal mirror, and the entrance of the linear fiber bundle coincides with the back focal line of the cylindrical ellipsoidal mirror. A photomultiplier tube is provided at the exit of the linear fiber bundle, and a filter is provided at the bottom opening of the photomultiplier tube; wherein, "back" refers to the direction of laser beam irradiation.

[0013] The laser scanning line confocal surface defect detection device based on differential interference contrast described in this invention is also characterized in that: a sample moving device is provided below the tested element to control the movement of the tested element.

[0014] The beam scanning device is a polygonal scanning mirror, an acousto-optic modulator (AOM), or a galvanometer.

[0015] The notches above and below the cylindrical ellipsoidal mirror are rectangular, and the length of the rectangular opening is greater than the laser line scanning length.

[0016] The instantaneous scanning center of the beam scanning device is located at the aperture stop of the F-theta telecentric field mirror.

[0017] The present invention provides a method for detecting and identifying confocal surface defects on laser scanning lines based on differential interference contrast. The method is characterized by its application in a laser scanning line confocal surface defect detection device and is performed according to the following steps:

[0018] Step 1: The laser emits a laser beam of a specified wavelength, which passes through the first reflector and the second reflector in sequence to adjust the propagation direction of the light path so that it enters the center of the beam expander.

[0019] The beam expander enlarges the diameter of the beam, which is then incident on the polarizer to cause linear polarization. The beam then passes through the beam splitter and the Wollaston prism, where the Wollaston prism decomposes the beam into p-beams and s-beams with orthogonal polarization directions. After the beam is adjusted by the third reflecting mirror, it is then incident on the beam scanning device.

[0020] Step 2: Under the action of the driving circuit, the beam scanning device scans the incident light into a line beam at high speed. The line beam is then focused by the F-theta telecentric field mirror, reflected by the fourth reflecting mirror, and incident perpendicularly onto the surface of the device under test for line scanning. When the device under test is scanned by the laser beam line for each line, the sample moving device drives the device under test to move synchronously in a direction perpendicular to the line scan. At the same time, the photodiode generates a line trigger signal to adjust the speed of the sample moving device to achieve full coverage scanning of the sample surface. The p-light and s-light spots on the surface of the device under test are offset by a certain distance of micrometers along the line scan direction.

[0021] Step 3: If there is a defect on the surface of the component under test, the laser beam scanned on the component under test will generate scattered light when it hits the defect surface. According to the principle of linear confocal, the cylindrical ellipsoidal mirror will only reflect the scattered light from the defect on the surface of the component under test to its own back focal line. After being transmitted by the linear fiber bundle on the back focal line and processed by the filter, it will be received by the photomultiplier tube and converted into a scattered electrical signal. The scattered electrical signal will then be processed by the computer to separate the scattered light from the surface of the component under test and the back scattered light.

[0022] If there are no defects on the surface of the component being tested, the laser beam scanning on the component will not produce scattered light, and the computer will not receive the scattered electrical signal.

[0023] Step 4: The reflected light from the surface of the component under test returns along the original path. If there are no defects on the surface of the component under test, the optical path difference between the p-beam and the s-beam is zero; if there are protrusions or pits on the surface of the component under test, an optical path difference will be generated between the p-beam and the s-beam.

[0024] The p-beam and s-beam are combined after passing through the Wollaston prism, then incident on the analyzer through a beam splitter to cause interference. After being focused by a lens, they are finally received by a photodetector. Based on the changing characteristics of the interference signal in the online scanning direction, the computer uses the interference signal to identify the type of defect concavity and convexity based on the interference differential contrast.

[0025] Compared with the prior art, the beneficial technical effects of the present invention are reflected in the following aspects:

[0026] 1. This invention innovatively proposes a linear confocal technique using a cylindrical ellipsoidal mirror structure. In traditional linear confocal microscopes, the numerical aperture and linear field of view are mutually constrained. To improve the collection efficiency of defect-scattered light, the numerical aperture must be increased, but this limits the field of view, resulting in a linear field of view only on the order of millimeters. This invention utilizes the reflection of light from one focal line of the cylindrical ellipsoidal mirror to another focal line to achieve linear confocality, thereby breaking through the technical bottleneck of the mutual constraint between the numerical aperture and linear field of view in traditional linear confocal microscopes, and significantly improving the linear field of view.

[0027] 2. This invention proposes a novel high-speed scanning technology for laser line confocal dark-field scattering. Traditional line confocal microscopes only have confocal filtering capability in the width direction of the line field of view, losing filtering characteristics in the length direction of the line field of view, resulting in crosstalk and increased noise in the scattered light, thereby reducing detection sensitivity. This invention enables the line confocal cylindrical ellipsoidal mirror to maintain complete confocal performance at every moment of laser scanning, overcoming the problems of crosstalk between adjacent scanning positions, decreased confocal performance, and increased background noise in traditional line confocal microscopes.

[0028] 3. This invention applies the principle of differential interference contrast to a line scanning device. Based on the changing characteristics of the interference signal in the line scanning direction, it achieves defect convexity / concave type identification based on differential interference contrast.

[0029] 4. This invention can achieve high-speed and high-precision detection of the surface of optical elements, and has optical tomography capability. It reflects only the scattered light from the front focal line of the cylindrical elliptical mirror to its own rear focal line, thereby eliminating the influence of the back scattered light of the transparent optical element. Attached Figure Description

[0030] Figure 1 This is an optical schematic diagram of the laser scanning line confocal surface defect detection device and method based on differential interference contrast according to the present invention;

[0031] Figure 2This is a schematic diagram illustrating the principle of defect concavity / convexity type identification based on interference differential contrast in this invention.

[0032] Figure 3 This is a diagram illustrating the process of calculating the phase profile from the interference light intensity of the uneven defects obtained from the photodetector in this invention.

[0033] Figure 4 This is a cross-sectional view of the cylindrical ellipsoidal mirror of the present invention;

[0034] Figure 5 A detailed view of the linear fiber optic bundle inlet;

[0035] Figure 6 A detailed view of the linear fiber bundle outlet;

[0036] The following are the labels in the diagram: 1. Laser, 2. First reflecting mirror, 3. Second reflecting mirror, 4. Beam expander, 5. Polarizer, 6. Beam splitter, 7. Wollaston prism, 8. Third reflecting mirror, 9. Beam scanning device, 10. F-theta telecentric field mirror, 11. Fourth reflecting mirror, 12. Cylindrical ellipsoidal mirror, 13. Linear fiber bundle, 14. Filter, 15. Photomultiplier tube, 16. Component under test, 17. Analyzer, 18. Lens, 19. Photodetector, 20. Photodiode. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0038] In this embodiment, a laser scanning line confocal surface defect detection device based on differential interference contrast is described, such as... Figure 1 As shown, it includes:

[0039] Laser 1 is used to emit a laser beam of a specified wavelength;

[0040] The first reflecting mirror 2 and the second reflecting mirror 3 are used to adjust the position of the laser beam so that it enters the beam expander 4;

[0041] Beam expander 4 is used to increase the diameter of the laser beam;

[0042] Polarizer 5 is used to adjust the polarization state of the laser beam;

[0043] Beam splitter 6 is used to split the incident light into two beams;

[0044] Wollaston Prism 7 is used to decompose the incident beam into p-beams and s-beams with orthogonal polarization directions, and to combine the reflected p-beams and s-beams.

[0045] The third reflecting mirror 8 is used to adjust the direction of the laser beam so that it enters the beam scanning device 9;

[0046] Beam scanning device 9 is used to convert incident light into a line beam;

[0047] Photodiode 20 is used to generate a row trigger signal. When scanning a row, photodiode 20 converts the optical signal into an electrical signal;

[0048] The F-theta telecentric field lens 10 is used to focus a line beam, eliminate aberrations and distortions, and improve the quality of the line beam.

[0049] The fourth reflecting mirror 11 is used to reflect the scanning laser beam onto the surface of the element under test 16;

[0050] The cylindrical ellipsoid 12 is located above the surface of the measured element 16. It has a cavity structure to collect scattered light. It has rectangular notches at the bottom and top, and the length of the rectangular opening is greater than the scanning length. The cross-section of the cylindrical ellipsoid is a partially elliptical profile, and the whole is cylindrical. The cylindrical ellipsoid has two focal lines at the front and back.

[0051] A linear fiber bundle 13 is positioned above the cylindrical ellipsoidal mirror 12, with its entrance coinciding with the back focal line of the cylindrical ellipsoidal mirror, used to guide the scattered light to the photomultiplier tube 15.

[0052] Filter 14 is used to filter out stray light outside the specified wavelength to improve detection accuracy;

[0053] A photomultiplier tube 15 is positioned above the linear fiber bundle 13 to collect the intensity of scattered light;

[0054] The component under test, 16, is an optical component;

[0055] The analyzer 17 is used to make the vibration directions of the p-beam and the s-beam coincide so that they interfere.

[0056] Lens 18 is used to focus the light beam onto photodetector 19;

[0057] Photodetector 19 is used to receive interference signals;

[0058] like Figure 2The diagram illustrates the principle of defect concavity / convexity type identification based on interference differential contrast according to the present invention. The incident light is converted into 45° linearly polarized light by polarizer 5, and then incident on Wollaston prism 7 through beam splitter 6. Wollaston prism 7 decomposes the incident beam into p-beams and s-beams with orthogonal polarization directions (p-beams are shown as dashed lines in the diagram, and s-beams as solid lines in the diagram). The two beams form a small angle and are then reflected by third mirror 8 and beam scanning device 9. They are then perpendicularly incident on the surface of the tested element 16 through F-theta telecentric field mirror 10 and fourth mirror 11. The two beams, being very close together, illuminate the sample. The minute concavity / convexity at every point on the sample surface in the field of view causes an optical path difference between the two beams. The returning p-beams and s-beams re-merge through Wollaston prism 7 and then pass through analyzer 17, causing the vibration directions of the two beams to align and interfere. Based on the changing characteristics of the interference signal in the linear scanning direction, defect concavity / convexity type identification based on interference differential contrast is achieved.

[0059] like Figure 3 The diagram illustrates the process of calculating the phase profile from the interference light intensity of uneven defects obtained from a photodetector, as per this invention. If the surface of the measured component is free of defects, the interference light intensity received by the photodetector will not change; however, if pits or bumps appear during the scanning process, the interference light intensity will change, and the change in interference light intensity will differ depending on the type of defect. Based on the light intensity curves of the p-ray and s-ray differential interference, the phase gradient phi is obtained through the relationship between interference light intensity and phase, I = cos(phi). Integrating the phase gradient phi yields the phase profile, from which the defect type can be determined.

[0060] like Figure 4 The image shows a cross-sectional view of a cylindrical ellipsoidal mirror. The internal contour of the cylindrical ellipsoidal mirror is an ellipse. The back focal line of the cylindrical ellipsoidal mirror is a certain distance from itself. The focused laser line overlaps with the front focal line of the cylindrical ellipsoidal mirror. By placing the surface of the optical element on the front focal line, the cylindrical ellipsoidal mirror reflects only the scattered light from the surface defects of the measured element onto its own back focal line. The focusing angle of the cylindrical ellipsoidal mirror is determined by its upper and lower edges. The working distance is limited by the distance from the lower edge to the front focal line.

[0061] like Figure 5 The image shows a detailed view of the inlet of a linear fiber bundle. This linear fiber bundle contains several polymethyl methacrylate (PMMA) fibers. The inlet is a linear fiber array.

[0062] like Figure 6 The image shows a detailed view of the linear fiber bundle outlet. The outlet is a circular fiber array.

[0063] In this embodiment, laser 1 emits a beam of light of a specified wavelength. The propagation direction of the light path is adjusted by the first reflecting mirror 2 and the second reflecting mirror 3, so that it enters the center of the beam expander 4. Then, the beam is linearly polarized by the polarizer 5, and after passing through the beam splitter 6, it is incident on the Wollaston prism 7. The Wollaston prism 7 decomposes the incident beam into p-beams and s-beams with orthogonal polarization directions. The two beams form a small angle and are then incident on the beam scanning device 9 after passing through the third reflecting mirror 8. The beam scanning device 9 converts the incident light into a line beam. The F-theta telecentric field mirror 10 focuses the line beam and incident perpendicularly on the surface of the device under test 16 through the fourth reflecting mirror 11 for line scanning. The spots of p-beams and s-beams on the surface of the device under test are offset by a small distance along the line scanning direction. If there are protrusions or pits on the surface of the device under test, the scattered light generated by the defects is reflected by the cylindrical ellipsoidal mirror 12 and then guided by the linear fiber bundle 13 to the filter 14. Finally, it is received by the photomultiplier tube 15, realizing the dark field scattering detection of defects. The reflected light from the defect returns along its original path. The uneven shape of the defect causes an optical path difference between the p-beam and the s-beam. The p-beam and s-beam return to the Wollaston prism 7 and merge, then are reflected by the beam splitter 6 to the analyzer 17, where they interfere. The interference light is received by the photodetector 19. Based on the changing characteristics of the interference signal in the online scanning direction, the type of defect unevenness is identified based on the interference differential contrast.

[0064] In this embodiment, a laser scanning line confocal surface defect detection method based on differential interference contrast is performed according to the following steps:

[0065] Step 1: Laser 1 emits a laser beam of a specified wavelength, which passes through the first reflector 2 and the second reflector 3 in sequence to adjust the propagation direction of the light path so that it enters the center of the beam expander 4.

[0066] After the beam expander 4 expands the diameter of the beam, it is incident on the polarizer 5, causing the beam to be linearly polarized. Then, it passes through the beam splitter 6 and the Wollaston prism 7, where the Wollaston prism 7 decomposes the beam into p-beams and s-beams with orthogonal polarization directions. After the beam is adjusted by the third reflecting mirror 8, it is incident on the beam scanning device 9.

[0067] Step 2: Under the action of the driving circuit, the beam scanning device 9 scans the incident light into a line beam at high speed. The line beam is then focused by the F-theta telecentric field mirror 10 and reflected by the fourth reflecting mirror 11, and then incident perpendicularly onto the surface of the device under test 16 for line scanning. When the device under test 16 is scanned by the laser beam line for each line, the sample moving device drives the device under test 16 to move synchronously in the direction perpendicular to the line scan. At the same time, the photodiode 20 generates a line trigger signal to adjust the speed of the sample moving device to achieve full coverage scanning of the sample surface. The p-light and s-light spots on the surface of the device under test 16 are offset by a distance of a few micrometers along the line scan direction.

[0068] Step 3: If there is a defect on the surface of the component under test 16, the laser beam scanning on the component under test 16 will generate scattered light when it is incident on the defect surface. According to the principle of linear confocal, the cylindrical ellipsoidal mirror 12 will only reflect the scattered light from the defect on the surface of the component under test 16 to its own back focal line. After being transmitted by the linear fiber bundle 13 on the back focal line and processed by the filter 14, it will be received by the photomultiplier tube 15 and converted into a scattered electrical signal. The scattered electrical signal will then be processed by the computer to separate the scattered light from the surface of the component under test 16 and the back scattered light.

[0069] If there are no defects on the surface of the component under test 16, the laser beam scanning on the component under test 16 will not produce scattered light, and the computer will not receive the scattered electrical signal.

[0070] Step 4: The reflected light from the surface of the tested component 16 returns along the original path. If there are no uneven defects on the surface of the tested component 16, the optical path difference between the p-beam and the s-beam is zero; if there are protrusions or pits on the surface of the tested component 16, an optical path difference will be generated between the p-beam and the s-beam.

[0071] The p-beam and s-beam are combined after passing through the Wollaston prism 7, then incident on the analyzer 17 through the beam splitter 6 to interfere, and then focused by the lens 18 before being received by the photodetector 19. Thus, based on the changing characteristics of the interference signal in the online scanning direction, the computer is used to identify the type of defect concavity and convexity of the interference signal based on the interference differential contrast.

Claims

1. A laser scanning line confocal surface defect detection device based on differential interference contrast, characterized in that, A laser (1) is provided for emitting a laser beam, and a beam expander (4) is provided behind the laser (1). A first reflector (2) and a second reflector (3) are provided in sequence between the beam expander (4) and the laser (1). A beam scanning device (9) is provided behind the beam expander (4) for converting the laser beam into a line beam. A polarizer (5), a beam splitter (6), a Wollaston prism (7), and a third reflector (8) are provided in sequence between the beam expander (4) and the beam scanning device (9). An analyzer (17), a lens (18), and a photodetector (19) are provided in sequence behind the beam splitter (6). A photodiode (20) and an F-theta telecentric field mirror (10) are arranged in sequence behind the beam scanning device (9); the F-theta telecentric field mirror (10) converts the line beam into a converging scanning laser beam and, after being reflected by the fourth reflecting mirror (11), is incident perpendicularly onto the surface of the element under test (16). After the light incident on the surface of the component under test (16) returns along the original optical path, it passes through the beam splitter (6) and then sequentially through the analyzer (17), the lens (18) and the photodetector (19). A cylindrical ellipsoidal mirror (12) is provided above the surface of the tested element (16). The inner surface of the cylindrical ellipsoidal mirror (12) is coated with a highly reflective material, and there are notches at its bottom and top. The front focal line of the cylindrical ellipsoidal mirror (12) coincides with the position where the laser beam converges on the surface of the tested element (16). A linear fiber bundle (13) is provided above the cylindrical ellipsoidal mirror (12), and the entrance of the linear fiber bundle (13) coincides with the back focal line of the cylindrical ellipsoidal mirror (12). A photomultiplier tube (15) is provided at the exit of the linear fiber bundle (13), and a filter (14) is provided at the bottom opening of the photomultiplier tube (15). Hereinafter, "back" refers to the direction of laser beam irradiation. A sample moving device is provided below the measured element (16) to control the movement of the measured element (16). The beam scanning device (9) is a polygonal scanning mirror, an acousto-optic modulator (AOM), or a galvanometer. The notches above and below the cylindrical ellipsoidal mirror (12) are rectangular, and the length of the rectangular opening is greater than the laser line scanning length. The instantaneous scanning center of the beam scanning device (9) is located at the aperture stop of the F-theta telecentric field mirror (10).

2. A method for detecting and identifying confocal surface defects on laser scanning lines based on differential interference contrast, characterized in that, It is applied in the laser scanning line confocal surface defect detection equipment as described in claim 1, and is performed according to the following steps: Step 1: The laser (1) emits a laser beam of a specified wavelength, which passes through the first reflector (2) and the second reflector (3) in sequence to adjust the propagation direction of the light path so that it enters the center of the beam expander (4). After the beam expander (4) expands the diameter of the beam, it is incident on the polarizer (5) to make the beam linearly polarized. Then, it passes through the beam splitter (6) and the Wollaston prism (7). The Wollaston prism (7) decomposes the beam into p-light and s-light with orthogonal polarization directions. After the beam is adjusted by the third mirror (8), it is incident on the beam scanning device (9). Step 2: The beam scanning device (9) scans the incident light into a line beam at high speed under the action of the driving circuit. The line beam is then focused by the F-theta telecentric field mirror (10), reflected by the fourth reflecting mirror (11), and incident perpendicularly onto the surface of the test element (16) for line scanning. When the test element (16) is scanned by the laser beam line for each line, the sample moving device drives the test element (16) to move synchronously in the direction perpendicular to the line scan. At the same time, the photodiode (20) generates a line trigger signal to adjust the speed of the sample moving device to achieve full coverage scanning of the sample surface. The p-light and s-light spots on the surface of the test element (16) are offset by a certain distance of micrometers along the line scan direction. Step 3: If there is a defect on the surface of the tested element (16), the laser beam scanned on the tested element (16) will generate scattered light when it is incident on the defect surface. According to the principle of linear confocal, the cylindrical ellipsoidal mirror (12) will only reflect the scattered light from the defect on the surface of the tested element (16) to its own back focal line. After being transmitted by the linear fiber bundle (13) on the back focal line and processed by the filter (14), it will be received by the photomultiplier tube (15) and converted into a scattered electrical signal. The scattered electrical signal will then be processed by the computer to separate the scattered light from the surface of the tested element (16) and the back scattered light. If there are no defects on the surface of the component under test (16), the laser beam scanned on the component under test (16) will not produce scattered light, and the computer will not receive the scattered electrical signal. Step 4: The reflected light from the surface of the tested component (16) returns along the original path. If there are no uneven defects on the surface of the tested component (16), the optical path difference between the p-beam and the s-beam is zero. If there are protrusions or pits on the surface of the tested component (16), an optical path difference will be generated between the p-beam and the s-beam. The p-beam and s-beam are combined after passing through the Wollaston prism (7), and then incident on the analyzer (17) through the beam splitter (6) to cause interference. After being focused by the lens (18), they are finally received by the photodetector (19). Thus, based on the changing characteristics of the online scanning direction of the interference signal, the computer is used to identify the defect concavity and convexity type of the interference signal based on the interference differential contrast.

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