Wafer defect detection system based on dual-channel optical imaging

Abstract

The invention belongs to the technical field of optical precision measurement, and particularly relates to a wafer defect detection system based on dual-channel optical imaging. According to the invention, dark field scattering and diffraction phase microscopy are combined as two optical imaging channels; optical paths of the two imaging modes are compatible and synchronized; through optical system calibration and optimization, high-precision acquisition and alignment of multi-modal images are realized; calibrating the sensitivity and resolution of the system by using a standard sample to ensure that the detection performance meets a preset requirement; carrying out dual-channel synchronous imaging on the wafer sample to be detected, and respectively obtaining a dark field scattering image and a diffraction phase image; carrying out conjoint analysis on the multi-modal data through an image algorithm, extracting size, depth and structure information of defects, and generating a high-precision defect detection report; the system specifically comprises a dark field scattering imaging module, a diffraction phase microscopic imaging module and an image processing unit. According to the invention, the defect of a single imaging mode can be effectively overcome, and the detection precision and robustness of the system are remarkably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of optical precision measurement, and in particular relates to a wafer defect detection system. Background Art

[0002] With the rapid development of the semiconductor industry, the size of integrated circuit chips has been continuously miniaturized in accordance with Moore's Law, and the process nodes have gradually moved towards the nanometer level. This technological advancement has not only significantly improved the performance of semiconductor devices, but also put forward higher requirements for defect detection technology. Wafer defects, including particle contamination, scratches, residues, etc., may not only affect the electrical performance of the device, but may also cause device failure and even reduce the yield of the entire production line. Therefore, accurate and reliable defect detection technology occupies a vital position in semiconductor manufacturing and has become a key link in ensuring product quality and production efficiency.

[0003] Optical inspection technology has become the mainstream method for wafer defect detection due to its advantages of non-contact, high-speed, and large-area detection. Traditional brightfield optical inspection technology mainly detects surface defects through the reflection or transmission characteristics of light. However, with the continuous reduction of process nodes, the defect size gradually approaches or even falls below the wavelength scale of light, making brightfield inspection technology face significant bottlenecks in resolution and sensitivity. Specifically, brightfield inspection has difficulty in effectively distinguishing tiny defects from background noise, especially in low contrast or complex backgrounds, and the detection effect is severely limited.

[0004] To overcome the above problems, dark field detection technology has gradually become the core method of wafer defect detection due to its high sensitivity to tiny defects. Unlike bright field detection, dark field technology can significantly enhance the signal response of sub-wavelength defects by collecting scattered light signals caused by defects, thereby achieving efficient detection of defects such as tiny particles and scratches. In recent years, dark field scattering detection technology has been widely used in wafer surface defect detection, especially in the field of semiconductor measurement equipment. For example, KLA, a leading semiconductor measurement equipment company, has developed the Puma series and Surfscan SP systems using dark field optical technology, which has greatly improved the detection capabilities of wafer defects and provided advanced detection solutions for global semiconductor manufacturers.

[0005] However, although dark field scattering detection technology performs well in the detection of tiny defects, it still faces some technical bottlenecks in high-precision detection. First, the defect signal intensity of dark field scattering detection is closely related to the irradiation direction of the light source. For defects with specific directionality, detection may be difficult due to weak scattered light. Secondly, in low contrast or complex backgrounds, the interference between background light and scattered light may lead to reduced detection accuracy. In addition, dark field technology lacks the ability to obtain defect depth information, making it difficult to accurately characterize defects with greater depth. These limitations limit the application potential of dark field technology in high-end semiconductor manufacturing.

[0006] Patent CN116359249A discloses a TDI-based line scanning dark field scattering wafer surface defect detection device and method, the device includes an illumination light source, a beam shaping component, a motion control component, a microscope imaging component, a CMOS image sensor and a computer. Through the principle of line scanning and dark field scattering, the scattered light of the wafer surface defects can be effectively collected, and the CMOS image sensor is used for rapid imaging, and then the image enhancement and defect extraction are performed by the computer. However, the limited depth of field of the microscope system, the increased edge field aberration caused by the large field of view, and the instability caused by mechanical vibration may still lead to signal annihilation in the edge area of ​​the image, affecting the detection of deep sub-wavelength defects.

[0007] Patent CN118883578A discloses an automatic optical inspection system and defect detection method, which can quickly detect tiny defects on the surface of semiconductor wafers through multi-angle light source illumination and high-resolution camera imaging. The system can adjust the light source angle and imaging parameters according to different defect types to improve detection accuracy. However, the detection accuracy of this technology is still limited when dealing with low-contrast defects, and it lacks the ability to obtain defect depth information.

[0008] Patent CN109991233A proposes an optical detection device and method, which realizes high-precision detection by controlling the light sources to emit detection beams at different relative positions of multiple light sources and the sample to be detected, and the camera collects images under different light sources. However, this technology mainly relies on a single imaging mode, and the detection accuracy of small defects in low contrast or complex backgrounds still needs to be improved. In addition, this technology lacks the ability to obtain defect depth information, making it difficult to accurately characterize defects with greater depth. Summary of the invention

[0009] The purpose of the present invention is to propose a wafer defect detection system based on dual-channel optical imaging with high detection accuracy and strong anti-interference ability to meet the needs of advanced process nodes for high-precision defect detection.

[0010] The wafer defect detection system based on dual-channel optical imaging provided by the present invention combines dark field scattering and diffraction phase microscopy technology as two optical imaging channels; the optical paths of the two optical imaging channels are compatible and synchronized; high-precision acquisition and alignment of multimodal images are achieved through optical system calibration and optimization; the sensitivity and resolution of the system are calibrated using standard samples to ensure that the detection performance meets the preset requirements; dual-channel synchronous imaging is performed on the wafer sample to be tested to obtain a dark field scattering image and a diffraction phase image respectively; multimodal data is jointly analyzed through an image algorithm to extract the size, depth and structural information of the defect, and a high-precision defect detection report is generated; specifically comprising: a dark field scattering imaging module, a diffraction phase microscopy imaging module, and an image processing unit; wherein:

[0011] The dark field scattering imaging module is used to collect scattered light signals of wafer surface defects;

[0012] The diffraction phase microscopy imaging module is used to obtain the phase information and depth structure of the defect;

[0013] The image processing unit is used to perform fusion analysis on the collected multi-modal images and extract the size, depth and structural information of the defects.

[0014] Further:

[0015] The dark field scattering imaging module comprises: an oblique incident light source, an imaging unit, and a high-sensitivity detector; wherein:

[0016] An inclined incident light source is used to provide a light field with non-vertical illumination;

[0017] An imaging unit, used for collecting light signals scattered from the sample surface and imaging;

[0018] Highly sensitive detector, used to receive the scattered light signal from the defect and generate a dark field scattered light image.

[0019] The diffraction phase microscopy imaging module includes: a coherent light source, an imaging unit, a phase analysis unit, and a high-sensitivity detector; wherein:

[0020] A coherent light source, used for providing a coherent light field required for phase imaging;

[0021] An imaging unit, used for collecting light signals reflected from the sample surface and forming an image;

[0022] A phase analysis unit, used to analyze the phase information of the defect and generate a diffraction phase image;

[0023] High-sensitivity detector: used to record the interference image.

[0024] Through optical path design and calibration, the optical path compatibility and synchronization of the dark field scattering imaging module and the diffraction phase microscopy imaging module are ensured (the wavelengths of the two bands are separated by a dichroic mirror and a shared microscope objective lens).

[0025] The image processing unit includes: an image registration module, a phase demodulation module of the carrier frequency interference pattern; wherein:

[0026] An image registration module for spatially aligning the dark-field scattering image with the diffraction phase image;

[0027] The phase demodulation module of the carrier frequency interferogram is used to perform phase recovery processing on the diffraction phase image, so as to extract more detailed characteristic information of the sample.

[0028] Furthermore, the oblique incident light source of the dark field scattering imaging module is based on the dark field scattering detection technology, the principle of which is:

[0029] By irradiating the wafer surface with oblique incident light and utilizing the scattering effect of tiny surface defects or irregularities on the incident light, scattered light signals at a specific angle are collected to achieve defect imaging. The optical path design of the oblique incident light source ensures that background light does not participate in imaging, and only the scattered light from the defects is received by the detector, thereby significantly improving image contrast and achieving high-sensitivity detection of tiny defects.

[0030] The scattering of particles on the wafer surface is a complex problem and it is difficult to get a strict solution. But it can be qualitatively expressed as:

[0031]

[0032] In the formula, I pis the scattering intensity of surface particles, I0 is the incident light intensity, d is the particle diameter, and λ is the incident wavelength (reference: Germer T A.Angular dependence and polarization of out-of-plane optical scattering from particulate contamination, subsurface defects, and surface microroughness [J]. Applied Optics, 1997, 36 (33): 8798-8805). According to this formula, there is a proportional relationship between the scattering intensity of surface particles and the sixth power of the particle diameter and the fourth power of the incident wavelength. Therefore, in order to improve the sensitivity of the scattered signal, in the present invention, in order to meet the system's requirements for light source wavelength, adjustability, spot stability and other optical properties, a semiconductor laser with a central wavelength of 405nm is selected as the light source. The laser has a circular or elliptical beam with a collimated output, an output power of about 50mW, an integrated heat dissipation module, and good power stability and anti-interference performance.

[0033] Further,

[0034] The light beam is first emitted by a 455nm wavelength laser, and then expanded and collimated after passing through a beam expansion system consisting of a polarizer, lenses L1 and L2 (there is a pinhole between lenses L1 and L2 to improve the spatial coherence of the light beam). Figure 1 As shown, two dichroic mirrors are installed between lenses L2 and L5. Dichroic mirror 1 can reflect light with a wavelength of 455nm, while dichroic mirror 2 allows light with a wavelength of 455nm to pass through. The purpose is to separate light beams of different wavelengths, thereby achieving directional transmission of light beams, so that light with a wavelength of 455nm can be accurately irradiated onto lens L5) and focused at the focal point of the objective lens (such as Figure 1 As shown, there is a reflector between L5 and the objective lens to fold the light path). The focused light beam enters the microscope system through the objective lens, becomes parallel light, and is incident vertically on the sample surface. The microscope system consists of a microscope objective lens and a matching tube lens L7 (as shown in FIG. Figure 1 As shown, there is a dichroic mirror 3 between the objective lens and the tube lens L7. The dichroic mirror 3 can reflect light of 455nm wavelength in order to separate light beams of different wavelengths. The sample is imaged after passing through the aforementioned optical system. The light beam is focused onto the grating, where diffraction side beams in multiple directions are generated, thereby achieving spatial replication of the light beam. Next, the first-order diffracted light is interfered with the zero-order light through a spatial filter, and the interference image is recorded on the camera CMOS2 (as shown in FIG. Figure 1As shown, there are two mirrors 3 and 4 between the spatial filter and the camera CMOS2, the purpose of which is also to fold the optical path), so as to effectively extract the phase information of the sample and perform imaging. In order to prevent the fringes at the carrier frequency interference pattern from being too dense and affecting the phase demodulation, a 4f system consisting of lenses L8 and L9 is inserted between the image plane of the microscope system and the camera;

[0035] Based on this, we only need to consider the light field of the 0th order diffraction light and the 1st order diffraction light after passing through the grating:

[0036] U(x,y)=U o (x,y)+U1(x,y)exp(iβx), (2)

[0037] Where U0 is the complex amplitude of the 0th order diffracted light, U1 is the complex amplitude of the 1st order light and is multiplied by the exponential phase tilt factor with the diffraction angle, and i represents an imaginary number. The magnitude of the 1st order diffraction angle is Where Λ is the grating constant.

[0038] Since passing through the 4f system composed of lenses L8 and L9 is equivalent to performing two Fourier transforms, its spatial coordinates are flipped in both the x and y directions. For the convenience of representation, let:

[0039]

[0040] Among them, M 4f is the magnification of the 4f system, A0 is the intensity of the 0th order light, A1 is the intensity of the 1st order light, and α is the normalization factor. The interference intensity captured at the camera target surface is:

[0041]

[0042] Where U c (x, y) is the light field at the camera, U c * (x, y) is its conjugate. The interference pattern I can be restored by phase adjustment method c (x', y') and remove the carrier frequency β. φ1 is a constant and can be removed by setting the background height to 0. The result φ0(x', y') contains information such as the surface morphology or object structure of the sample.

[0043] Furthermore, the principle of the dual-channel optical imaging wafer defect detection system is as follows:

[0044] The present invention integrates dark field scattering imaging and diffraction phase microscopy into the same optical system. The two complement each other to achieve a comprehensive characterization of wafer defects. Dark field scattering imaging is suitable for detecting sub-wavelength-scale surface particles and cracks by enhancing the signal response of tiny defects. The scattering intensity of dark field scattering surface particles is proportional to the sixth power of the particle diameter and the fourth power of the incident wavelength. For interference detection, the phase change caused by the surface ridge height can be approximately expressed as It can be seen that in terms of depth detection, phase information has higher sensitivity in the vertical direction than scattering intensity information. Therefore, diffraction phase microscopy can reveal the three-dimensional structure and depth distribution of defects through precise phase information. The combination of the two technologies achieves complementary advantages in spatial resolution and depth information.

[0045] Furthermore, the image registration module includes the following working contents:

[0046] Load the initial image and perform geometric transformation operations based on the shooting features of the image;

[0047] Select the region of interest (ROI) in the dark-field scattering image;

[0048] SIFT algorithm is used to extract feature points in diffraction phase microscopy images and dark field scattering images.

[0049] Matching is done by the nearest neighbor matching method and the matching points are screened by the ratio test;

[0050] Calculate the geometric transformation parameters (affine transformation) between images and accurately align the images;

[0051] Display the registration results.

[0052] Furthermore, the phase demodulation module of the carrier frequency interference pattern has the following principle:

[0053] In the off-axis interference optical system, based on the principle of double-beam interference, the interference pattern can be expressed as

[0054]

[0055] Where a(x,y) is the interference pattern background intensity, γ(x,y) is the amplitude modulation factor, is the phase information. x and ω yis the carrier frequency component. In the off-axis interference pattern with carrier frequency components, the interference fringes are highly dense due to the large carrier frequency introduced by the phase, and the phase changes at different positions will cause the fringes to deform. The required phase information is hidden in these dense fringes. Overall, the fringes caused by the carrier frequency are dense, while the background intensity and amplitude modulation factor change slowly. Therefore, assuming that the carrier frequency components in the x and y directions are much larger than the background light intensity, amplitude modulation factor and phase gradient, it can be approximately considered that the following conditions hold:

[0056]

[0057] The first-order and second-order partial derivatives of the interference pattern in the y direction are calculated as follows:

[0058]

[0059] Then the wrapped phase of the object light wave is:

[0060]

[0061] Where unwrap represents the unwrapping function. Off-axis interference makes the interference fringes carrying phase information dense through the angle between the object light wave and the reference light, thereby effectively separating the fringe phase information from the background intensity and amplitude modulation factor. Therefore, even a single interferogram with carrier frequency can recover the phase information.

[0062] Furthermore, the principle of the image fusion analysis is as follows:

[0063] Since dark field scattering imaging and diffraction phase microscopy obtain sample information from different physical angles, they each have advantages and limitations when providing images. Therefore, after completing the aforementioned image processing module, it is necessary to comprehensively extract all information about sample defects by comparing and analyzing the dark field scattering image and the diffraction phase image. This comparative analysis process can combine the advantages of the two images, supplement the deficiencies of a single image mode, and improve the accuracy and robustness of defect detection.

[0064] Compared with the prior art, the present invention has at least the following beneficial effects:

[0065] (1) The present invention combines multi-modal images of the same detection position with signal processing algorithms to give full play to the high sensitivity of dark field scattering in the detection of tiny defects and the accuracy of diffraction phase microscopy in the characterization of structural information, thereby achieving effective complementarity between the two and improving detection accuracy and reliability;

[0066] (2) The diffraction phase microscopy technique in the examples of the present invention uses bright field illumination, which overcomes the problem of dark field scattering's dependence on the contrast between background light and scattered light, and improves the detection accuracy. The sensitive detection degree of dark field scattering at a small scale can also serve as a supplement to the diffraction phase microscopy technique;

[0067] (3) In the examples of the present invention, the off-axis interference design effectively suppresses the noise caused by optical components or environmental vibrations, enhances the stability and robustness of the system, and meets the needs of high-precision and high-reliability detection in wafer manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 It is a structural diagram of the dual-channel optical imaging wafer defect detection system of the present invention.

[0069] Figure 2 It is a flowchart of the algorithm of the image registration module of the present invention.

[0070] Figure 3 This is an example of registration of pictures taken by the optical system of the present invention using a resolution plate.

[0071] Figure 4 It is a flowchart of the algorithm of the phase demodulation module of the carrier frequency interference diagram of the present invention.

[0072] Figure 5 This is the test result of using 10 μm polystyrene microspheres in the diffraction phase microscopy module in the system of the present invention.

[0073] Figure 6 This is the system test result of the dark field scattering module in the system of the present invention using 10 μm polystyrene microspheres.

[0074] Figure 7 This is the test result of 1μm polystyrene microsphere dual-channel system.

[0075] Figure 8 This is the test result of 300nm polystyrene microsphere dual-channel system.

[0076] Fig. 9 This is the test result of the dual-channel system of 200nm polystyrene microspheres.

[0077] Fig.10 This is the test result of 150nm polystyrene microsphere dual-channel system.

[0078] Fig.11 This is the test result of 60nm polystyrene microsphere dual-channel system. DETAILED DESCRIPTION

[0079] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0080] This embodiment provides a dual-channel optical imaging wafer defect detection system based on dark field scattering and diffraction phase microscopy, such as Figure 1 As shown, the system is built on an optical air floating table and is integrated with two optical detection systems, in which the purple light beam represents the optical path of the dark field scattering system, and the blue light beam represents the optical path of the diffraction phase microscopy system.

[0081] The light source of the dark field scattering system optical path is a 405nm wavelength laser, which first passes through the Galileo telescope system composed of lenses L3 and L4 to expand the beam, and then passes through the dichroic mirror 1. The dichroic mirror 1 is a short-wave pass, which can pass the light of 405nm wavelength and reflect the light of 455nm wavelength. After the dichroic mirror 1, a dichroic mirror 2 is set. The dichroic mirror 2 is a long-wave pass, which allows the light of 455nm wavelength to pass through and reflects the light of 405nm wavelength at the same time, thereby realizing the separation of two beams of light with different wavelengths. The light of 405nm wavelength is reflected after passing through the dichroic mirror 2, and is obliquely incident on the sample through an adjustable angle reflector 1, and the sample surface is scattered. The sample is placed on the sample stage, which is a displacement stage that can control the displacement of the sample. The scattered light is then received by the objective lens, passes through the reflector 2, and then passes through the dichroic mirror 3. The dichroic mirror 3 is a short-wave pass, which can pass the light of 405nm wavelength and reflect the light of 455nm wavelength. Finally, the scattered light is imaged onto the camera CMOS1 through the tube lens L6 that matches the objective lens to achieve image acquisition.

[0082] The optical path of the diffraction phase microscope system uses a 455nm wavelength laser as the light source, and is expanded and collimated through a polarizer and a beam expansion system composed of double-cemented lenses L1 and L2. A pinhole is also added between lenses L1 and L2 to improve the spatial coherence of the beam. The expanded beam is first reflected by dichroic mirror 1, then passes through dichroic mirror 2, and then focuses to the focal point of the objective lens through lens L5. In this process, the optical path is folded using mirror 2. The focused beam enters the microscope system through the objective lens, becomes parallel light, and is vertically incident on the sample surface. After the objective lens collects the reflected light, it passes through mirror 2 and then through dichroic mirror 3 to the tube lens L7 that matches the objective lens. The grating is placed at the focal point of tube lens L7, which can generate diffraction side beams in multiple directions, thereby realizing the spatial replication of the beam. Then, these diffracted beams pass through lens L8, and its focal position is aligned with the grating. Through the spatial filter, only the first-order diffracted light and the zero-order light are retained, and then through lens L9. Lenses L8 and L9 together form a 4f system to ensure that the light beam is properly amplified and spatially filtered. The first-order diffracted light and the zero-order light interfere here, and the interference image is recorded on the camera CMOS2. In order to further fold the light path, two reflectors 3 and 4 are also set between the spatial filter and the camera CMOS2 to ensure the optimization of the light path and the imaging effect.

[0083] The dual-channel optical imaging wafer defect detection system can perform dark field scattering imaging and diffraction phase microscopy imaging on the same position of the same wafer. After the images collected by CMOS1 and CMOS2 are uploaded to the computer, the images are further processed and analyzed. Specifically, it includes:

[0084] S1, build a dark field scattering imaging module to collect scattered light signals of wafer surface defects;

[0085] S2, build a diffraction phase microscopy imaging module to obtain the phase information and depth structure of the defect;

[0086] S3, writes an image processing unit to process and analyze the collected images, extract the size, depth and structural characteristics of the defects, and also realizes the registration of the images obtained by the two optical paths to ensure the spatial alignment of the images.

[0087] Specifically:

[0088] The dark field scattering imaging module in S1 includes:

[0089] The oblique incident light source is composed of a 405nm wavelength laser, a lens L3, a lens L4, a dichroic mirror 1, a dichroic mirror 2, and a reflector 1: it is used to provide a non-vertical irradiation light field;

[0090] The imaging unit, consisting of a bright-field and dark-field objective lens and its matching tube lens L6, collects light signals scattered from the sample surface and forms an image. In the dark-field imaging mode, the objective lens collects scattered light at a specific angle, thereby effectively excluding the light directly reflected from the sample surface and enhancing the visibility of the defect area.

[0091] High-sensitivity detector (CMOS1): used to receive dark-field scattering images.

[0092] The diffraction phase microscopy module in S2 includes:

[0093] The coherent light source is composed of a 455nm laser, a polarizer, a lens L1, a pinhole, a lens L2, a dichroic mirror 1, a dichroic mirror 2, and a lens L5, wherein the function of the reflector 2 is to fold the light path: to provide the coherent light field required for phase imaging;

[0094] The imaging unit is composed of a bright-field and dark-field objective lens, a matching tube lens L7, and a dichroic mirror 3: it is used to collect light signals reflected from the sample surface and form an image.

[0095] The phase analysis unit is composed of a grating, a lens L8, a spatial filter, and a lens L9, wherein the function of the reflector 3 and the reflector 4 is to fold the optical path: to analyze the phase information of the defect and generate a diffraction phase image.

[0096] High-sensitivity detector (CMOS2): used to record the interference image.

[0097] Further, through optical path design and calibration, the optical path compatibility and synchronization of the dark field scattering imaging module and the diffraction phase microscopy imaging module are ensured (the two light wavelengths are separated by a dichroic mirror and share one objective lens).

[0098] The image processing unit in S3 includes:

[0099] An image registration module for spatially aligning the dark-field scattering image with the diffraction phase image;

[0100] The phase demodulation module of the carrier frequency interferogram is used to perform phase recovery processing on the diffraction phase image, so as to extract more detailed characteristic information of the sample.

[0101] Since there is a significant difference in the field of view between the dark-field scattering light path and the diffraction phase microscopy system, in the dual-channel optical system, the dark-field scattering light path is mainly used to roughly locate the preliminary position and morphological characteristics of the sample defects, and the diffraction phase microscopy system can accurately locate and further characterize the detailed structure and phase information of the target defects, thereby completing the in-depth analysis and high-resolution imaging of the defects. At the same time, the dark-field scattering imaging system has higher sensitivity in lateral resolution than the diffraction phase microscopy system, and can effectively detect smaller-scale surface defects. This is because dark-field scattering imaging can significantly enhance the detectability of tiny scattering signals, thereby improving the ability to detect tiny defects.

[0102] S31, in order to ensure the accuracy and consistency of the images in the two imaging modes, sufficient image calibration and comparison must be performed before imaging. To this end, an image registration module is designed for calibration to ensure that the images under different optical paths can be accurately matched in spatial scale. The flowchart is as follows Figure 2As shown in Figure 1. First, the diffraction phase microscopy image presents a mirror image relationship with the dark field scattering image in the up, down, left and right directions. If no proper flip correction is performed, direct matching will result in an error in the correspondence of the feature points. Therefore, before the matching operation, the diffraction phase microscopy image needs to be flipped horizontally and vertically to ensure that its direction is consistent with the dark field scattering image. In addition, since the dark field scattering image (low magnification) has a large field of view and a wide coverage area, while the diffraction phase microscopy image (high magnification) has a small field of view and only covers a local area, directly performing feature matching on a global scale will significantly increase the computational complexity and may reduce the matching accuracy. Therefore, in the registration process, a preliminary region of interest (ROI) should be manually or automatically selected in the dark field scattering image, which should correspond to the field of view of the diffraction phase microscopy image. Within this selected ROI, the feature point matching method can be used for precise registration to improve the matching accuracy and computational efficiency. In the selection of specific methods for image registration, based on the characteristics of the optical path and the characteristics of the dual-channel image, the following methods are selected and operated in sequence:

[0103] (1) In the feature extraction stage, the SIFT algorithm is used to extract feature points from the dark field scattering image and the diffraction phase microscopy image respectively. The SIFT algorithm has excellent scale invariance and can adapt to images of different resolutions, ensuring that feature points can be stably extracted at different scales of the image. In addition, the SIFT algorithm is highly robust to illumination changes and noise and can extract stable feature points under complex conditions, so it performs particularly well in image matching. Due to its high computational efficiency, the SIFT algorithm is particularly suitable for real-time image processing tasks.

[0104] (2) In the feature matching stage, matching point pairs are screened through the nearest neighbor matching method and ratio test. Nearest neighbor matching is an intuitive and computationally efficient matching method that is suitable for large-scale data processing and provides preliminary matching point pairs for subsequent screening and optimization. There may be multiple similar feature points in the image, resulting in matching errors, and the ratio test can effectively reduce the incidence of mismatching. The ratio test is highly robust to changes in illumination, noise interference, and geometric deformation, and can adapt to a variety of complex application scenarios.

[0105] (3) In the estimation stage of geometric transformation, the affine transformation model is used to estimate the geometric relationship between images based on matching point pairs. The affine transformation model can accurately describe geometric deformations such as rotation, scaling, translation, and shearing between images, and is suitable for alignment between different fields of view. Through the affine transformation model, the geometric relationship between the two images can be accurately estimated and corrected, providing a reliable basis for subsequent image fusion and further processing. Finally, the diffraction phase microscopy image is transformed into the coordinate system of the dark field scattering image to achieve image alignment. Through this calibration process, errors caused by optical path configuration, imaging magnification, and field of view differences can be eliminated, thereby improving the accuracy and reliability of the final detection results. The system was tested using the USAF-1951 resolution plate (positive test target), and the results were as follows: Figure 3 As shown. Figure 3 In the four pictures, Figure 3 (a) The dark field scattering system actually takes pictures and performs image registration and positioning. Figure 3 (b) is the actual picture taken by diffraction phase microscopy; Figure 3 (c) Figure 3 (d) is the image registration result of the dual-channel imaging optical path. From the result, it can be seen that the two images have achieved good registration.

[0106] S32, the image taken by the diffraction phase microscopy system is processed by the phase demodulation module of the carrier frequency interferogram for phase recovery. In the frequency domain, dense interference fringes correspond to the high-frequency part, while information such as background intensity is located in the low-frequency part. Through the appropriate frequency domain filter, the background interference can be effectively removed and the phase information can be extracted. The spectrum of the carrier frequency interferogram consists of three parts: the 0-level fundamental frequency, the +1-level high frequency (containing the phase information of the object light wave) and its -1-level conjugate term. The +1-level part is along the k-th phase in the spectrum. x and k y Direction deviation ω x and ω y. The +1 level part is extracted through a spatial filter, and the complex amplitude is restored through the inverse Fourier transform (IFT), and the phase is further restored using the inverse tangent function. However, as a global operation, the Fourier transform performs poorly in processing local signal feature changes, especially signals with large frequency changes. In the carrier frequency interference pattern, the fringes in the area with a large phase gradient are dense, while the fringes in the area with a small phase gradient are sparse, so the algorithm is required to have strong local analysis capabilities. To this end, the fractional Fourier transform (FrFT) provides an effective solution. Unlike the traditional Fourier transform, the fractional Fourier transform achieves multi-scale local analysis by adjusting the transform order, thereby enhancing the ability to capture local features of the signal. Its flexible parameter control and time-frequency domain switching characteristics enable it to more accurately restore the local phase information of the interference pattern with large frequency changes, which is better than the Fourier transform. The fractional Fourier transform is an "order" transformation of the Fourier transform, and the "angle" of the transformation is controlled by introducing the parameter α. Its transformation formula is as follows:

[0107]

[0108] Among them, K α (x,x') is the kernel of fractional Fourier transform. The complete process of diffraction domain defect detection algorithm based on fractional Fourier transform is as follows: Figure 4 As shown in the figure, first, the interference patterns of the reference wafer and the wafer to be tested are synchronously separated by double exposure interferometry measurement technology, and the WFF algorithm is used to reconstruct the complex amplitude of the interference fringes, which effectively suppresses the speckle noise and improves the reconstruction accuracy. On this basis, the complex amplitude field of the reference and tested samples is calculated by multi-distance diffraction propagation, and a complex amplitude sequence containing different axial position information is constructed to enhance the response characteristics of the defect in the space-frequency domain. In order to improve the phase sensitivity of the defect area, the algorithm converts the phase disturbance caused by the microscopic defect into an amplitude modulation signal by performing point-by-point complex conjugate multiplication operations on the complex amplitude field of the tested sample and the reference sample. Subsequently, the modulated complex amplitude field is extracted in the fractional domain using FrFT, and the directional enhancement of the phase gradient characteristics of sub-wavelength defects is achieved by optimizing the transformation order and fractional domain parameters.

[0109] In order to evaluate the system's ability to detect defects of different sizes, wafer samples containing polystyrene microspheres of different sizes were prepared to simulate granular defects on unpatterned wafers. The concentration of the microsphere suspension used was 2.5%. Taking 1μm particle size microspheres as an example, 1ml of suspension contained about 40 billion microspheres.

[0110] To avoid microsphere aggregation, the suspension was diluted 500 times with ultrapure water and then evenly dispersed on a 2-inch single-sided polished silicon wafer through a spray bottle. The roughness of the silicon wafer was less than 0.5nm, the flatness TIR was less than 3μm, and the warpage TTV was less than 10μm. All sample preparation and testing were carried out in a clean room.

[0111] First, the sample was tested using a diffraction phase microscope system to evaluate the feasibility of the algorithm. The test results of the 10μm particle size are as follows: Figure 5 shown. Figure 5 (a) is the original interference pattern recorded, which is adjusted to have the best contrast of the interference pattern fringes; Figure 5 (b) for Figure 5 (a) Calibration image after flipping; Figure 5 (c) is the Fourier spectrum corresponding to the interference pattern. The three lobes in the spectrum are completely separated, which meets the phase recovery condition of the carrier frequency interference pattern, verifying that the constructed diffraction phase microscopy system has the ability to detect the phase in the vertical direction of the sample; Figure 5 (d) is the phase result of the microsphere sample restored using the fractional Fourier method. Figure 5 (a) It can be seen that there are background noise and diffraction rings in the interference pattern, while in Figure 5 The noise effect in the original image is almost invisible in the phase recovery result in (d), and the phase result and its lateral width are consistent with the characteristics of a 10μm microsphere.

[0112] At the same time, comparing the results taken by the dark field scattering system, such as Figure 6 As shown. Figure 5 and Figure 6 Comparing the two images, it can be seen that it is feasible to accurately align the images obtained by the dark field scattering system and the diffraction phase microscopy system through image registration technology. Specifically, in the dark field scattering image with a larger field of view, the defect is first initially located, and then, combined with the diffraction phase microscopy image, the detailed morphological information of the defect can be further obtained.

[0113] Subsequently, the defects of smaller sizes were further detected, and gradually reduced to the sub-wavelength scale, to verify the detection capability of the system in high-precision defect identification. Figures 7 to 11As shown in the figure, microsphere samples of 1um, 300nm, 200nm, and 150nm sizes were tested respectively. Among them, each picture (a) represents the original image taken by the scattering imaging system and positioned by the image registration algorithm; (b) represents the original image taken by the diffraction phase microscopy system; (c) and (d) are the dark field scattering system image and the diffraction phase microscopy image after registration respectively; (e) and (f) are the stereogram and plane graph of the phase recovery results of the microsphere sample respectively. As can be seen from the figure, the analysis of the test results of samples of different sizes shows that the system has the ability to detect subwavelength scale defects. Specifically, for polystyrene microspheres with larger sizes (especially above 200nm), the diffraction phase microscopy system shows better performance in defect detection, and can more clearly present the size, morphology and other structural characteristics of the defects. In contrast, the dark field scattering system has weak imaging clarity and particle resolution in this size range, and can only provide information on the existence of defects, but cannot accurately characterize their specific morphological characteristics. Therefore, compared with a single dark-field scattering system, the dual-channel detection system combined with diffraction phase microscopy significantly improves the detection accuracy and characterization capabilities.

[0114] However, as the defect size further decreases, the imaging contrast and phase recovery accuracy of the diffraction phase microscopy system are gradually disturbed by noise, especially when the defect size drops to 60nm, this effect is particularly obvious. In this case, the dark field scattering system can still maintain good imaging contrast and effectively detect the presence of tiny particles with its high sensitivity to tiny defects. When the phase recovery results of the diffraction phase microscopy system are interfered by noise and it is difficult to distinguish the defect characteristics, the dark field scattering system can still provide clear detection results, providing strong support for the reliability of defect identification.

Claims

1. A wafer defect detection system based on dual-channel optical imaging, characterized in that: Combine dark field scattering and diffraction phase microscopy techniques as two optical imaging channels; the optical paths of the two optical imaging channels are compatible and synchronized; and high-precision acquisition and alignment of multimodal images are achieved through optical system calibration and optimization; The sensitivity and resolution of the system are calibrated using standard samples to ensure that the detection performance meets the preset requirements; dual-channel synchronous imaging is performed on the wafer sample to obtain dark field scattering images and diffraction phase images respectively; multi-modal data are jointly analyzed through image algorithms to extract the size, depth and structural information of defects, and generate high-precision defect detection reports; specifically including: dark field scattering imaging module, diffraction phase microscopy imaging module, image processing unit; among which: The dark field scattering imaging module is used to collect scattered light signals of wafer surface defects; The diffraction phase microscopy imaging module is used to obtain the phase information and depth structure of the defect; The image processing unit is used to process and analyze the collected images, extract the size, depth and structural characteristics of the defects, and also realize the registration of the images obtained by the two optical paths to ensure the alignment of the images in space.

2. The wafer defect detection system based on dual-channel optical imaging according to claim 1, characterized in that: The dark field scattering imaging module comprises: an oblique incident light source, an imaging unit, and a high-sensitivity detector; wherein: An inclined incident light source is used to provide a light field with non-vertical illumination; Imaging unit: used to collect light signals scattered from the sample surface and form an image; High-sensitivity detector for receiving dark-field scattering images; The diffraction phase microscopy imaging module includes: a coherent light source, an imaging unit, a phase analysis unit, and a high-sensitivity detector; wherein: A coherent light source, used for providing a coherent light field required for phase imaging; An imaging unit, which collects light signals reflected from the sample surface and forms an image; A phase analysis unit, used to analyze the phase information of the defect and generate a diffraction phase image; Highly sensitive detectors for recording interference images; And through optical path design and calibration, ensure the optical path compatibility and synchronization of the dark field scattering imaging module and the diffraction phase microscopy imaging module; The image processing unit includes: an image registration module, a phase demodulation module of the carrier frequency interference pattern; wherein: An image registration module for spatially aligning the dark-field scattering image with the diffraction phase image; The phase demodulation module of the carrier frequency interferogram is used to perform phase recovery processing on the diffraction phase image, so as to extract more detailed characteristic information of the sample.

3. The wafer defect detection system based on dual-channel optical imaging according to claim 2, characterized in that: The oblique incident light source of the dark field scattering imaging module is based on the dark field scattering detection technology. It illuminates the wafer surface with oblique incident light, utilizes the scattering effect of tiny surface defects or irregularities on the incident light, and collects scattered light signals at a specific angle to achieve defect imaging. The optical path design of the oblique incident light source makes the background light not participate in the imaging, and only the scattered light of the defects is received by the detector, thereby significantly improving the image contrast and achieving high-sensitivity detection of tiny defects. A semiconductor laser with a central wavelength of 405nm is selected as the light source; the laser has a collimated circular or elliptical beam output, an output power of about 50mW, an integrated heat dissipation module, and good power stability and anti-interference performance.

4. The wafer defect detection system based on dual-channel optical imaging according to claim 3 is characterized in that: In its optical path: The light beam is first emitted by a 455nm laser, and after passing through a beam expansion system consisting of a polarizer, double-cemented lenses L1 and L2, it is expanded and collimated. A pinhole is also added between lenses L1 and L2 to improve the spatial coherence of the light beam; the expanded light beam passes through lens L5 and is focused at the focal point of the objective lens. During this period, two dichroic mirrors are installed between lenses L2 and L5. Dichroic mirror 1 is used to reflect light with a wavelength of 455nm, while dichroic mirror 2 allows light with a wavelength of 455nm to pass through, so that light beams of different wavelengths are separated and directional transmission of the light beam is achieved, so that light with a wavelength of 455nm can be accurately irradiated onto lens L5; there is also a reflector between lens L5 and the objective lens to fold the light path; the focused light beam enters the microscope system through the objective lens, becomes parallel light, and is vertically incident on the sample surface; the microscope system is composed of a microscope objective lens and a matching tube lens L7, and a dichroic mirror 3 is also provided between the objective lens and the tube lens L7, and the dichroic mirror 3 is used to reflect light with a wavelength of 455nm to achieve light beam separation of different wavelengths; the sample is imaged after passing through the aforementioned optical system; the light beam is focused on the grating, and diffraction side beams in multiple directions are generated at the grating, thereby achieving spatial replication of the light beam; then the first-order diffraction light and the zero-order light are interfered by the spatial filter, and the interference image is recorded on the camera CMOS2; wherein, two reflectors 3 and 4 are also provided between the spatial filter and the camera CMOS2 to realize folded light path, so as to effectively extract the phase information of the sample and perform imaging; a 4f system composed of lenses L8 and L9 is inserted between the image plane of the microscope system and the camera; Based on this, consider the light field of the 0th order diffraction light and the 1st order diffraction light after passing through the grating: U(x,y)=U o (x,y)+U1(x,y)exp(iβx),(1) Wherein, U0 is the complex amplitude of the 0th order diffracted light, U1 is the complex amplitude of the 1st order light and is multiplied by the exponential phase tilt factor with the diffraction angle, i is an imaginary unit, and the magnitude of the 1st order diffraction angle is β=2π / Λ, where Λ is the grating constant; After passing through the 4f system composed of lenses L8 and L9, it is equivalent to performing two Fourier transforms. Its spatial coordinates are flipped in both the x and y directions. For the convenience of representation, let: Among them, M 4f is the magnification of the 4f system, A0 is the intensity of the 0th-order light, A1 is the intensity of the 1st-order light, and α is the normalization factor; the interference intensity captured at the camera target surface is: Where U c (x, y) is the light field at the camera, U c * (x, y) is its conjugate; the interference pattern I is restored by phase adjustment c (x', y'), and remove the carrier frequency β; φ1 is a constant; by setting the background height to 0 and removing it, the result φ0(x', y') implies the surface morphology or object structure information of the sample.

5. The wafer defect detection system based on dual-channel optical imaging according to claim 2, characterized in that: The image registration module includes the following working contents: Load the initial image and perform geometric transformation operations based on the shooting features of the image; Extract feature points from diffraction phase microscopy images and match them in dark field scattering images; Select the region of interest in the dark-field scattering image for further processing; Calculate the geometric transformation parameters between images and accurately align the images; Display the registration results and verify the accuracy.

6. The wafer defect detection system based on dual-channel optical imaging according to claim 2, characterized in that: In the phase demodulation module of the carrier frequency interference pattern, for the off-axis interference optical system, based on the principle of double-beam interference, the interference pattern is expressed as Where a(x,y) is the interference pattern background intensity, γ(x,y) is the amplitude modulation factor, is the phase information; ω x and ω y is the carrier frequency component; In the off-axis interferogram with carrier frequency components, the interference fringes appear highly dense due to the large carrier frequency introduced by the phase, and the phase changes at different positions will cause the fringes to deform; The required phase information is hidden in these dense fringes; from the overall point of view, the fringes caused by the carrier frequency are dense, while the background intensity and amplitude modulation factor change slowly; therefore, assuming that the carrier frequency components in the x and y directions are much larger than the background light intensity, amplitude modulation factor and phase gradient, the following conditions hold: The first-order and second-order partial derivatives of the interference pattern in the y direction are calculated as follows: Then the wrapped phase of the object light wave is: Among them, unwrap represents the unwrapping function; off-axis interference makes the interference fringes carrying phase information dense through the angle between the object light wave and the reference light, thereby effectively separating the fringe phase information from the background intensity and amplitude modulation factor; therefore, even a single interference pattern with carrier frequency can restore the phase information.

7. The wafer defect detection system based on dual-channel optical imaging according to claim 2, characterized in that: Since dark-field scattering imaging and diffraction phase microscopy obtain sample information from different physical angles, after completing the aforementioned image processing module, all information about sample defects can be comprehensively extracted by comparing and analyzing the dark-field scattering image and the diffraction phase image. This comparative analysis process can combine the advantages of the two images, supplement the shortcomings of a single image mode, and improve the accuracy and robustness of defect detection.

8. The wafer defect detection system based on dual-channel optical imaging according to claim 2, characterized in that: It is an integration of two optical detection systems, where the purple light beam represents the optical path of the dark field scattering system, and the blue light beam represents the optical path of the diffraction phase microscopy system; The light source of the dark field scattering system optical path is a 405nm wavelength laser, which first passes through the Galileo telescope system composed of lenses L3 and L4 for beam expansion, and then passes through dichroic mirror 1; the dichroic mirror is short-wave pass, which can pass light with a wavelength of 405nm and reflect light with a wavelength of 455nm; after dichroic mirror 1, dichroic mirror 2 is set, which is long-wave pass, allowing light with a wavelength of 455nm to pass through and reflecting light with a wavelength of 405nm, thereby realizing the separation of two beams of light with different wavelengths; The light with a wavelength of 405nm is reflected after passing through the dichroic mirror 2, and is incident obliquely on the sample through a reflector 1 with an adjustable angle, and scattering occurs on the sample surface; the sample is placed on the sample stage, which is a displacement stage that can control the displacement of the sample; the scattered light is then received by the objective lens, passes through the reflector 2, and then passes through the dichroic mirror 3, which is a short-wave pass that can pass the light with a wavelength of 405nm and reflect the light with a wavelength of 455nm; finally, the scattered light is imaged onto the camera CMOS1 through the tube lens L6 that matches the objective lens, realizing image acquisition; The optical path of the diffraction phase microscopy system uses a 455nm wavelength laser as the light source, and passes through a polarizer and a beam expansion system composed of double-cemented lenses L1 and L2 for beam expansion and collimation; a pinhole is also added between lenses L1 and L2 to improve the spatial coherence of the light beam; the expanded light beam is first reflected by dichroic mirror 1, then passes through dichroic mirror 2, and then passes through lens L5 to focus on the objective lens; in this process, the light path is folded using reflector 2; the focused light beam enters the microscope system through the objective lens, becomes parallel light, and is vertically incident on the sample surface; after the objective lens collects the reflected light, it passes through reflector 2 and then through dichroic mirror 3 to reflect to tube lens L7 that matches the objective lens; the grating is placed at the focus of L7, which can generate diffraction side beams in multiple directions, thereby realizing the spatial replication of the light beam; then, these diffracted light beams pass through lens L8, and its focal position is aligned with the grating; Through the spatial filter, only the first-order diffraction light and the zero-order light are retained, and then pass through lens L9. Lenses L8 and L9 together form a 4f system to ensure that the light beam is properly amplified and spatially filtered; the first-order diffraction light and the zero-order light interfere here, and the interference image is recorded on the camera CMOS2; in order to further fold the light path, two reflectors 3 and 4 are also set between the spatial filter and the camera CMOS2 to ensure the optimization of the light path and the imaging effect.

Images