A method for detecting micro-cracks on a mirror surface

Abstract

This invention discloses a method for detecting microcrack defects on a mirror surface. First, the height information of the surface being measured is pre-scanned and converted into corresponding angle information. Referring to the reflection characteristics of the mirror, a specially customized, angle-adjustable illumination system is used to project incident light at different angles. This light has high collimation and strong light-gathering ability, thereby enhancing the reflected light from the background area and suppressing the reflected light from the microcrack area. The light reflected from the surface enters a custom-designed imaging system, which can only receive reflected light within a certain fixed angle range. At this point, the light is filtered by the fixed light of the optical imaging system, which further increases the grayscale contrast between the microcrack defect features and the background, achieving a double effect of expanding the boundary of the micro-features, and achieving a microcrack defect detection accuracy of 1 / 5 to 1 / 10 of a pixel.

Description

Technical Field

[0001] This invention belongs to the field of crack defect detection technology, specifically relating to a method for detecting microcrack defects on a mirror surface. Background Technology

[0002] During AOI inspection, tiny microcracks on the surface of mirror-reflective objects are often difficult to identify. Missing these defects can lead to varying degrees of loss across different industries. For example, in semiconductor chip manufacturing, mirror cracks can gradually extend due to changes in temperature or stress, ultimately causing product failure.

[0003] Chip surface cracks often manifest as cracks in the silicon substrate on the back side of the wafer. These tiny microcracks are typically around 2µm in size, making them difficult to detect quickly with the naked eye, and even with a high-powered microscope, they are not easily distinguishable. Furthermore, due to the thin and brittle nature of wafers, they are highly susceptible to breakage during subsequent processing, leading to incalculable losses. To improve chip yield, these microcracks are usually rejected during the screening process, thereby reducing costs. Therefore, how to accurately and efficiently detect these microcracks has become a pressing issue for industries such as semiconductor manufacturing processes.

[0004] To detect microcracks on the surface of grains, current technologies can be divided into two categories: non-optical and optical non-destructive testing. Non-optical methods typically involve ultrasonic testing, which relies on the influence of cracks on the amplitude of the ultrasonic waveform and offers good results. However, ultrasonic testing involves immersing the chip in liquid, making it susceptible to contamination, and its testing efficiency is low. Optical non-destructive testing typically employs front-reflective or back-transmittance methods using infrared or near-infrared wavelengths. Front-reflective methods use infrared light perpendicularly illuminating the surface of the object being tested, causing reflection and achieving infrared optical imaging. However, front-reflective methods often suffer from resolution limitations in a single detection, resulting in slow detection speeds. Back-transmittance methods place the photosensitive sensor and illumination source above and below the object being tested. The photosensitive sensor receives infrared or near-infrared light transmitted through the object to detect microcracks. However, back-transmittance methods require high transmittance from the chip being tested and are generally only used for inspecting bare silicon wafers during the grinding process; improvements are urgently needed. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for detecting microcrack defects on mirror surfaces.

[0006] The present invention adopts the following technical solution:

[0007] A method for detecting microcrack defects on a mirror surface is implemented based on a microcrack defect detection device. The microcrack defect detection device includes an illuminator for projecting light, a detection stage for placing the object to be tested, a camera unit set above the detection stage, an imaging system set between the camera unit and the detection stage, an illumination system that reflects light to the imaging system, and a spectral confocal sensor for collecting height information of the mirror surface. The light spot emitted by the spectral confocal sensor is projected onto the surface of the object to be tested through the illumination system and the imaging system in sequence. The light emitted by the illuminator is reflected by the illumination system to the imaging system, then projected onto the mirror surface of the object to be tested, and then reflected back to the imaging system through the mirror surface, reaching the camera unit through the imaging system.

[0008] Its detection method includes the following steps:

[0009] The first step is to activate the spectral confocal sensor. The light spot passes through the illumination system and imaging system in sequence to reach the surface of the object being measured. Then, the detection stage is moved along the preset scanning trajectory to pre-scan and record the height information of the surface of the object being measured, thereby obtaining multiple spectral data points.

[0010] The second step is to traverse multiple spectral data points within the imaging field of view to check if any spectral data points are lost. Then, fit a linear segment of one variable to the spectral data points of the regions before and after the data point loss, calculate the slope, and convert it into the corresponding angle information.

[0011] The third step involves adjusting the illuminator based on the angle information obtained in the second step to project light rays at different angles. These light rays pass through the illumination system and imaging system sequentially to reach the surface of the object being measured. Lines at different angles are projected onto the corresponding surfaces at different angles, reflecting light rays at a uniform specific angle. The imaging system then absorbs this specific angle of light, which reaches the target surface of the camera unit to complete the imaging process.

[0012] Preferably, in the first step, the method for confirming the preset scanning trajectory is as follows: Based on the size of the imaging field of view, the surface of the object to be measured is divided into n imaging fields of view. The multiple imaging fields of view are divided into multiple imaging field of view groups along the Y direction. Each imaging field of view group includes several imaging fields of view arranged along the X direction. Then, the starting point M is 1 mm to the left of the uppermost imaging field of view on the left, and the starting point M is located in the middle position of the height direction of the field of view. The stopping point N is 3 mm to the right of the tail imaging field of the imaging field of view group. The detection platform is controlled to move the object to be measured from point M to point N along the X direction. Then, the displacement platform moves the object to be measured along the Y direction to complete the row change. This process is repeated to form the preset scanning trajectory and complete the pre-scan recording of the height information of the spectral data points in the multiple imaging field of view groups.

[0013] Preferably, the detection platform moves at a speed of 20 mm / s along the X direction and at a speed of 20 mm / s along the Y direction.

[0014] Preferably, in the second step, when there is a loss of spectral data points, the two spectral data points near the blank area formed by the loss of spectral data points are marked as points B and C, respectively. The spectral data points entering the imaging field of view are designated as points A, and the spectral data points leaving the imaging field of view are designated as points D. Then, multiple spectral data points between points A and B, and multiple spectral data points between points C and D are fitted with linear segments of one variable to obtain the two angle information α and β, respectively.

[0015] Preferably, in the third step, the illuminator is adjusted to project incident light rays α and β at different angles based on the obtained α and β angle information. The incident light rays α and β pass through the illumination system and imaging system in sequence to reach the surface of the object under test, and then are reflected back to the imaging system through the mirror surface. The imaging system reaches the camera unit, and the corresponding imaging grayscale contrast is used to obtain two images that are superimposed and displayed. The two incident light irradiation areas and the middle micro-crack area are retained, which is the final test effect image within the imaging field of view. By analogy, the side view effect images of other imaging fields are obtained. Then, all the test effect images are stitched together to obtain the complete test image of the surface of the object under test.

[0016] Preferably, the sampling frequency of the spectral confocal sensor is 40 kHz and the sampling interval is 0.5 μm.

[0017] Preferably, the lighting system is a Kohler lighting system, including a first lens located above the illuminator and a first beam splitter disposed above the first lens, the first beam splitter being disposed at 45°.

[0018] Preferably, the imaging system is a telecentric imaging system, including a second beam splitter positioned above the detection stage and a telecentric lens positioned above the second beam splitter. The second beam splitter is positioned at 45° and parallel to the first beam splitter. The light spot emitted by the spectral confocal sensor passes through the first beam splitter to the second beam splitter and is projected onto the surface of the object being measured. The light emitted by the illuminator passes through the first lens, is reflected by the first beam splitter to the second beam splitter, is projected onto the surface of the object being measured, is then reflected by the second beam splitter through the surface of the object, and finally reaches the camera unit via the telecentric lens.

[0019] Preferably, the camera unit is a CMOS camera.

[0020] Preferably, the microcrack defect detection device further includes an angle adjustment mechanism for adjusting the illumination angle of the illuminator and a moving mechanism for moving the detection platform.

[0021] As can be seen from the above description of the present invention, compared with the prior art, the beneficial effects of the present invention are as follows: The present invention is an optical non-destructive method for microcrack defect detection, which is more efficient and does not damage or contaminate the product compared with ultrasonic contact detection, thus saving costs; compared with common front-light reflection and back-light transmission optical non-destructive testing, its application range is wider and its detection accuracy is higher; it can detect microcracks at low resolution (14um / pixel), and the detection accuracy can reach about 1 / 5-1 / 10pixel.

[0022] For mirror surfaces formed by microcracks at different angles, the height information of the measured object surface is first pre-scanned and converted into corresponding angle information. Referring to the reflection characteristics of the mirror, a specially customized adjustable illumination system is used to project incident light at different angles. This light has high collimation and strong light collection, which enhances the reflected light in the background area and suppresses the reflected light in the microcrack area. The light reflected from the object surface enters a custom-designed telecentric imaging system, which can only receive reflected light within a certain fixed angle range. At this time, after being filtered by the fixed light of the optical imaging system, the highly collimated visible light projected on the mirror surface can further increase the grayscale contrast between the microcrack defect features and the background, achieving the effect of doubly expanding the boundary of the small features, and achieving a microcrack defect detection accuracy of 1 / 5 to 1 / 10 of a pixel. Attached Figure Description

[0023] Figure 1 Schematic diagram of a microcrack defect detection device;

[0024] Figure 2 This is a schematic diagram of diffuse reflection;

[0025] Figure 3 This is a schematic diagram of the illumination and imaging field of view;

[0026] Figure 4 This is a schematic diagram of the microcrack detection principle.

[0027] Figure 5 This is a schematic diagram of the imaging system;

[0028] Figure 6 This is a field-of-view segmentation diagram of the surface of the object under test.

[0029] Figure 7 This is a schematic diagram of the preset scanning trajectory displacement;

[0030] Figure 8 A schematic diagram of scanning to form a single imaging field of view with microcracks;

[0031] Figure 9 A schematic diagram of incident ray α and incident ray β;

[0032] Figure 10This is a schematic diagram of the grayscale images formed by incident ray α and incident ray β.

[0033] Figure 11 This is a schematic diagram of a microcrack test.

[0034] Figure 12 A schematic diagram of microcracks taken for a high-resolution module (0.35um / pixel);

[0035] Figure 13 A schematic diagram of a microcrack taken with a standard resolution module (2µm / pixel);

[0036] Figure 14 A schematic diagram of the microcracks (14µm / pixel) for this application;

[0037] In the figure, (1) illuminator, (2) lighting system, (3) angle adjustment mechanism, (4) spectral confocal sensor, (5) detection stage, (6) surface of the object being measured, (7) imaging system, (8) photographic unit, ① first beam splitter, ② telecentric lens, ③ second beam splitter. Detailed Implementation

[0038] The present invention will be further described below through specific embodiments.

[0039] Reference Figure 1 As shown, a microcrack defect detection device for detecting microcracks on a mirror surface includes an illuminator (1) for projecting light, a detection platform (5) for placing the object to be tested, a camera unit (8) positioned above the detection platform, an imaging system (7) positioned between the camera unit (8) and the detection platform (5), an illumination system (2) for reflecting light to the imaging system (7), a spectral confocal sensor (4) for collecting height information of the mirror surface, an angle adjustment mechanism (3) for adjusting the illuminator's illumination angle, and a moving mechanism for moving the detection platform (5). The angle adjustment mechanism (3) and the moving mechanism are commonly used devices in the mechanical field, enabling adjustment of the illuminator's illumination angle and movement of the detection platform (5) in the X and Y directions. Their specific structures will not be further elaborated here. Specifically, the spectral confocal sensor (4) has a sampling frequency of 40kHz and a sampling interval of 0.5µm; therefore, a microcrack typically lacks 4-5 spectral data points. Furthermore, the camera unit (8) uses a CMOS camera with the following parameters: 5M... 2448*2048; illuminator resolution 0.1°.

[0040] The lighting system (2) adopts the Kohler lighting system, including a first lens located above the illuminator (1) and a first beam splitter ① set above the first lens, wherein the first beam splitter ① is set at 45°.

[0041] The imaging system (7) employs a telecentric imaging system, which can only receive reflected light within a fixed angular range, as referenced. Figure 4 , Figure 5 At this time, the high-collimation visible light projected onto the surface of the object being tested can further increase the contrast between the microcrack defect features and the background grayscale by filtering the fixed light of the optical imaging system, and finally show the contrast between the bright and dark grayscale, thus completing the microcrack detection. Specifically, the imaging system (7) includes a second beam splitter ③ set above the detection stage (5) and a telecentric lens ② set above the second beam splitter ③. The second beam splitter ③ is set at 45° and is parallel to the first beam splitter ①. The light spot emitted by the spectral confocal sensor (4) passes through the first beam splitter ① to the second beam splitter ③ and is projected onto the surface of the object being tested. The light emitted by the illuminator (1) passes through the first lens and is reflected by the first beam splitter ① to the second beam splitter ③ and projected onto the surface of the object being tested. It is then reflected by the second beam splitter ③ through the surface of the object being tested and reaches the camera unit (8) through the telecentric lens ②. Furthermore, the telecentric lens ② has a telecentric < 0.001° and a Magnification of 0.32X.

[0042] A method for detecting microcrack defects on a mirror surface, based on the aforementioned microcrack defect detection device, includes the following steps:

[0043] The first step is to activate the spectral confocal sensor. The light spot passes through the illumination system and imaging system in sequence to reach the surface of the object being measured. Then, the detection stage is moved along the preset scanning trajectory to pre-scan and record the height information of the surface of the object being measured, thereby obtaining multiple spectral data points.

[0044] The second step is to traverse multiple spectral data points within the imaging field of view to check if any spectral data points are lost. Then, fit a linear segment of one variable to the spectral data points of the regions before and after the data point loss, calculate the slope, and convert it into the corresponding angle information.

[0045] The third step involves adjusting the illuminator based on the angle information obtained in the second step to project light rays at different angles. These light rays pass through the illumination system and imaging system sequentially to reach the surface of the object being measured. Lines at different angles are projected onto the corresponding surfaces at different angles, reflecting light rays at a uniform specific angle. The imaging system then absorbs this specific angle of light, which reaches the target surface of the camera unit to complete the imaging process.

[0046] In the first step, the method for confirming the preset scanning trajectory is as follows: Determine the maximum imaging field of view based on the illumination field of view, and then divide the object surface under test into n imaging fields of view according to the size of the imaging field of view, such as... Figure 6As shown, multiple imaging fields of view are divided into multiple imaging field of view groups along the Y direction. Each imaging field of view group includes several imaging fields of view arranged along the X direction. Then, the starting point M is set 1 mm to the left of the uppermost imaging field of view on the left, and the starting point M is located in the middle of the height direction of the field of view. The stopping point N is set 3 mm to the right of the last imaging field of view in the imaging field of view group. The detection platform is controlled to move the object under test along the X direction from point M to point N. Then, the displacement platform moves the object under test along the Y direction to complete the row change. Figure 7 As shown, by analogy, a preset scanning trajectory is formed to complete the pre-scanning recording of the height information of spectral data points in multiple imaging field groups; specifically, the detection platform moves at a speed of 20 mm / s along the X direction and at a speed of 20 mm / s along the Y direction.

[0047] In the second step, when spectral data points are lost and all are located within the same imaging field of view, refer to... Figure 8 As shown, two spectral data points near the blank area formed by the loss of spectral data points are marked as points B and C, respectively. The spectral data point entering the imaging field of view is designated as point A, and the spectral data point leaving the imaging field of view is designated as point D. Then, multiple spectral data points between points A and B, and multiple spectral data points between points C and D are fitted with linear segments of one variable to obtain the angle information α and β, respectively. When there are missing spectral data points located in two adjacent imaging fields of view, two spectral data points near the blank area formed by the loss of spectral data points are marked as points B and C, respectively. The spectral data point entering the imaging field of view where point B is located is designated as point A, and the spectral data point leaving the imaging field of view where point C is located is designated as point D. Then, multiple spectral data points between points A and B, and multiple spectral data points between points C and D are fitted with linear segments of one variable to obtain the angle information α and β, respectively.

[0048] In the third step, when there are missing spectral data points that are all located within the same imaging field of view, refer to... Figures 9 to 11As shown, based on the obtained α and β angle information, the illuminator is adjusted to project incident light rays α and β at different angles. The incident light rays α and β sequentially pass through the illumination system and imaging system to reach the surface of the object under test, and are then reflected back to the imaging system by the mirror surface. The imaging system then reaches the camera unit. Corresponding to the imaging grayscale contrast, two images are superimposed and displayed, retaining the areas illuminated by the two incident light rays and the central micro-crack area, which is the final test result image within the imaging field of view. When spectral data points are lost and located within adjacent imaging fields of view, the obtained α and β angles are used to determine the final result. The information adjustment illuminator projects incident light rays α and β at different angles. The incident light rays α and β pass through the illumination system and imaging system in sequence to reach the surface of the object under test. They are then reflected back to the imaging system by the mirror surface and reach the camera unit through the imaging system. Based on the corresponding image grayscale contrast, the two incident light irradiation areas and the side crack area are retained respectively. Then, the side crack areas of the two images are stitched together to obtain the complete crack area. In this way, side view effect images of other imaging fields are obtained. Then, all test effect images are stitched together to obtain the complete test image of the surface of the object under test.

[0049] The detection principle of this application is as follows: Because the location of microcracks on the mirror surface is not fixed and can occur anywhere on the surface, the surface tilt and protrusion caused by the microcracks vary in tilt angle depending on the degree of fracture of the microcracks. Conventional diffusely reflected visible light has a dispersed angle. When light shines on the mirror surface of the grain, it reaches the camera target surface via a conventional imaging system. Due to the uniform dispersion of light reflection, such as… Figure 2 As shown, even with the presence of microcracks, the grayscale contrast between the cracks and the detection background is low, and the defect features are not obvious. However, when a specially customized, angle-adjustable Kohler illumination system is used in conjunction with a telecentric imaging system, where the illumination field of view uniformly covers the entire imaging field of view, as shown... Figure 3 As shown; for mirror surfaces formed by microcracks at different angles, the illumination system can project highly collimated light rays at corresponding angles, which are then reflected back to the imaging system at a specific angle, and finally reach the camera target surface. The principle of microcrack detection is as follows: Figure 4 As shown, the grayscale contrast between the crack defect features and the detection background is obvious at this time, achieving the effect of doubly expanding the boundary of small features, and achieving a detection accuracy of 1 / 5 to 1 / 10 of a pixel for microcrack defects.

[0050] Based on this application and existing conventional imaging systems, the same microcrack defect was photographed, and the resulting images are shown in the reference image. Figures 12 to 14 As shown, through Figure 14 and Figure 12 , Figure 13 As can be seen from the comparison, the present invention can detect microcracks at low resolution (14um / pixel), and the detection accuracy can reach about 1 / 5 to 1 / 10 pixel.

[0051] This invention is an optical non-destructive testing method for microcrack defects. Compared with ultrasonic contact testing, it is more efficient, does not damage or contaminate the product, and saves costs. Compared with common front-light reflection and back-light transmission optical non-destructive testing, it has a wider range of applications and higher detection accuracy. It can detect microcracks at low resolution (14um / pixel) with a detection accuracy of about 1 / 5 to 1 / 10 pixel.

[0052] The above description is merely a preferred embodiment of the present invention and should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made in accordance with the scope of the patent application and the contents of the specification of the present invention should still fall within the scope of the patent of the present invention.

Claims

1. A method for detecting microcrack defects on a mirror surface, characterized in that: This is based on a microcrack defect detection device, which includes an illuminator for projecting light, a detection stage for placing the object under test, a camera unit positioned above the detection stage, an imaging system positioned between the camera unit and the detection stage, an illumination system that reflects light to the imaging system, a spectral confocal sensor for collecting height information of the mirror surface, and an angle adjustment mechanism for adjusting the illumination angle of the illuminator. The illumination system uses a Kohler illumination system, and the imaging system uses a telecentric imaging system. This system can only receive reflected light within a certain fixed angle range. The light spot emitted by the spectral confocal sensor is projected onto the surface of the object under test through the illumination system and the imaging system in sequence. The light emitted by the illuminator is reflected by the illumination system to the imaging system, then projected onto the mirror surface of the object under test, and then reflected back to the imaging system through the mirror surface, reaching the camera unit through the imaging system. Its detection method includes the following steps: The first step is to activate the spectral confocal sensor. The light spot passes through the illumination system and imaging system in sequence to reach the surface of the object being measured. Then, the detection stage is moved along the preset scanning trajectory to pre-scan and record the height information of the surface of the object being measured, thereby obtaining multiple spectral data points. The second step is to traverse multiple spectral data points within the imaging field of view to check if any spectral data points are lost. Then, fit a linear segment of one variable to the spectral data points of the regions before and after the data point loss, calculate the slope, and convert it into the corresponding angle information. The third step involves adjusting the illuminator based on the angle information obtained in the second step to project light rays at different angles. These rays pass through the illumination system and the imaging system in sequence to reach the surface of the object being measured. Linear projections at different angles are projected onto the object surface at the corresponding angles, reflecting the reflected light rays within a fixed angle range that the telecentric imaging system can accept. The imaging system then absorbs the reflected light rays within this specific angle range, which then reach the target surface of the camera unit to complete the imaging.

2. The method for detecting microcrack defects on a mirror surface according to claim 1, characterized in that: In the first step, the method for confirming the preset scanning trajectory is as follows: Based on the size of the imaging field of view, the surface of the object to be measured is divided into n imaging fields of view. The multiple imaging fields of view are divided into multiple imaging field of view groups along the Y direction. Each imaging field of view group includes several imaging fields of view arranged along the X direction. Then, the starting point M is 1 mm to the left of the uppermost imaging field of view on the left, and the starting point M is located in the middle of the height direction of the field of view. The stopping point N is 3 mm to the right of the tail imaging field of the imaging field of view group. The detection platform is controlled to move the object to be measured from point M to point N along the X direction. Then, the displacement platform moves the object to be measured along the Y direction to complete the row change. This process is repeated to form the preset scanning trajectory and complete the pre-scan recording of the height information of the spectral data points in multiple imaging field of view groups.

3. The method for detecting microcrack defects on a mirror surface according to claim 2, characterized in that: The detection platform moves at a speed of 20 mm / s along the X direction and at a speed of 20 mm / s along the Y direction.

4. The method for detecting microcrack defects on a mirror surface according to claim 2, characterized in that: In the second step, when there are missing spectral data points, the two spectral data points near the blank area formed by the missing spectral data points are marked as points B and C, respectively. The spectral data points that enter the imaging field of view are designated as points A, and the spectral data points that leave the imaging field of view are designated as points D. Then, multiple spectral data points between points A and B, and multiple spectral data points between points C and D are fitted with linear segments of one variable to obtain the two angle information α and β, respectively.

5. The method for detecting microcrack defects on a mirror surface according to claim 4, characterized in that: In the third step, the illuminator is adjusted to project incident light rays α and β at different angles based on the obtained α and β angle information. The incident light rays α and β pass through the illumination system and imaging system in sequence to reach the surface of the object under test, and then are reflected back to the imaging system through the mirror surface. The imaging system then reaches the camera unit. Based on the corresponding imaging grayscale contrast, two images are superimposed and displayed, retaining the two incident light irradiation areas and the middle micro-crack area, which is the final test effect image within the imaging field of view. By analogy, side view effect images of other imaging fields are obtained. Then, all the test effect images are stitched together to obtain the complete test image of the surface of the object under test.

6. The method for detecting microcrack defects on a mirror surface according to claim 1, characterized in that: The spectral confocal sensor has a sampling frequency of 40 kHz and a sampling interval of 0.5 μm.

7. The method for detecting microcrack defects on a mirror surface according to claim 1, characterized in that: The lighting system adopts a Kohler lighting system, including a first lens located above the illuminator and a first beam splitter disposed above the first lens, the first beam splitter being disposed at 45°.

8. The method for detecting microcrack defects on a mirror surface according to claim 7, characterized in that: The imaging system employs a telecentric imaging system, comprising a second beam splitter positioned above the detection stage and a telecentric lens positioned above the second beam splitter. The second beam splitter is positioned at a 45° angle and parallel to the first beam splitter. The light spot emitted by the spectral confocal sensor passes through the first beam splitter to the second beam splitter and is projected onto the surface of the object being measured. The light emitted by the illuminator passes through the first lens, is reflected by the first beam splitter to the second beam splitter, is projected onto the surface of the object being measured, is then reflected by the second beam splitter through the mirror surface, and finally reaches the camera unit via the telecentric lens.

9. The method for detecting microcrack defects on a mirror surface according to claim 1, characterized in that: The camera unit uses a CMOS camera.

10. The method for detecting microcrack defects on a mirror surface according to claim 1, characterized in that: The microcrack defect detection device also includes an angle adjustment mechanism for adjusting the illumination angle of the illuminator and a moving mechanism for moving the detection platform.

Images