A device and method for detecting surface fine defects based on dark field scattering time of flight

Abstract

This invention belongs to the field of optical detection technology, specifically a device and method for detecting minute surface defects based on dark-field scattering time-of-flight. The device includes two lasers, each preceded by an electro-optic modulator to control the time-domain waveform of the laser. The time-modulated beam illuminates the surface under test and is scattered by the minute defects. The scattered light then passes through a single-path optical microscope system and enters a photodetector to acquire the signal. This invention utilizes at least two or more optical paths to illuminate the object and detects the time difference between the flight times of the scattered light from the surface defects under multi-path illumination to accurately detect the surface defects. By analyzing the time dimension information of the scattered light, this invention avoids discussing the complex propagation process of the beam from the defect to the detector, locking the characteristic information of the defect into the linear propagation path from the light source to the surface defect, thus improving the detection capability of the dark-field scattering detection system.

Description

Technical Field

[0001] This invention belongs to the field of optical inspection, specifically relating to a device and method for detecting minute surface defects based on dark field scattering time-of-flight. Background Technology

[0002] Surface defect detection methods play a crucial role in quality assessment within the precision manufacturing field. Optical detection methods, in particular, largely rely on the modulation of incident light by subtle structural anomalies in the surface defect region, altering some properties of the outgoing light. These differences in beam variation are then used to detect surface defects. One classic method in surface optical inspection is dark-field scattering, primarily used for particle and defect detection on mirrored or transparent surfaces. It involves illuminating the precision surface at a grazing incidence angle to acquire an image. Compared to traditional bright-field illumination systems, dark-field illumination systems capture images from near the surface normal. In this case, reflected light from smooth surfaces does not enter the detector, but light scattered from particles or defects may enter the sensor. This results in extremely high image contrast, with bright signals only at particles or defects, while the background signal from mirrors is almost zero. Similarly, for transparent surfaces, the large incident angle creates a high-contrast image between the transparent (non-scattering region) and opaque (scattering region).

[0003] Thanks to its simple principle, ease of integration, and high detection sensitivity, dark-field scattering-based detection technology possesses unparalleled advantages over other methods, thus attracting widespread attention from researchers both domestically and internationally. However, current mainstream dark-field scattering detection techniques still rely on the spatial distribution of scattered light intensity to identify defect types and sizes. One common approach involves using electromagnetic simulations to obtain far-field diffraction intensity distributions for known defect types, forming a database, and then indirectly comparing it with images obtained from actual experiments to achieve the goal of identifying defect types and sizes. Summary of the Invention

[0004] The purpose of this invention is to provide a device and method for detecting minute surface defects based on dark-field scattering time-of-flight, enabling direct detection of surface defects. By analyzing the time dimension information of scattered light energy, it avoids discussing the complex propagation process of the beam from the defect to the detector, locking the characteristic information of the defect into the linear propagation path from the light source to the surface defect. This invention differs from traditional beam time-of-flight measurement devices such as lidar, which primarily measure the time it takes for the beam energy to propagate from the light source plane to the object and then to the detector; essentially, it is an absolute distance measurement.

[0005] The basic idea of ​​this invention is to introduce multiple light beams and calculate the difference in flight time between each beam to solve for the height difference of minute defects in different spatial regions of the surface. Based on Huygens' diffraction law, regardless of the angle at which an object is radiated, every point on the object's surface can be considered a new wave source. The flight time consumed by this new wave source to propagate to a fixed spatial point is independent of the incident state of the light source. Therefore, the main object of this invention is to measure the difference in flight time between two light beams, essentially a relative distance measurement. The technical solution of this invention is as follows:

[0006] A surface micro-defect detection device based on dark field scattering time-of-flight includes two lasers, an electro-optic modulator, a single-path optical microscopy system, and a photodetector. An electro-optic modulator is placed in front of each laser to control the time-domain waveform of the laser. The longitudinally positioned surface under test is illuminated by two time-domain modulated optical paths. The signal then passes through the single-path optical microscopy system and enters the photodetector to acquire the signal. The height difference Δz of the defect is obtained based on the time difference between the scattering times of the light from the surface micro-defect under the two optical paths, thus accurately detecting the surface micro-defect. The photodetector is a single-point detector. During detection, the surface under test is moved or the single-path optical microscopy system is scanned. During movement, the surface under test or the single-path optical microscopy system moves in the depth direction.

[0007] In this invention, when using a moving surface scanning method, the height difference Δz of the defect is obtained based on the time difference between the flight times of the scattered light from the minute defects on the surface under two optical path illuminations.

[0008]

[0009] Where Δt a , Δt b Δt is the time of flight measured by the photodetector before the surface under test moves. ′a , Δt ′b α1 and α2 are the incident angles, which are the angles between the equiphase surface of the time-domain modulated light source and the macroscopic normal of the surface under test (generally set as the Z-axis of the world coordinate system).

[0010] In this invention, when using a scanning method with a moving single-path optical microscopy system, the height difference Δz of the defect is obtained based on the time difference between the flight times of the scattered light from the minute surface defects under two optical path illuminations:

[0011]

[0012] Where Δx is the displacement of the single-path fiber optic microscopy system along the displacement direction, and Δt a , Δt b Δt is the time of flight measured by the photodetector before the surface under test moves. ′a , Δt ′bα1 and α2 are the incident angles, representing the time of flight measured after the surface under test has moved.

[0013] The present invention also provides a detection method based on the above-mentioned surface minute defect detection device, wherein the detection is performed by moving the surface to be tested or by scanning with a single-channel optical microscopy system to acquire the signal of the photodetector.

[0014] In this invention, there can be more than three lasers. By solving two of the three or more optical paths in groups, multiple height differences can be obtained. Further optimization can yield a more precise result.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] This invention avoids discussing the complex propagation process of the beam from the defect to the detector by analyzing the temporal dimension of scattered light, thus focusing the defect's characteristic information on the linear propagation path from the light source to the surface defect, thereby improving the detection capability of the dark-field scattering detection system. This invention can further analyze the height difference data of the defect to evaluate the characteristics of subtle surface defects. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a surface micro-defect detection device based on dark field scattering time-of-flight.

[0018] Figure 2 This is a schematic diagram of the time-of-flight difference measurement of scattered light under multi-angle illumination.

[0019] Figure 3 This is a schematic diagram of the time-of-flight difference measurement of defective scattered light using a moving sample scanning method.

[0020] Figure 4 This is a schematic diagram of the time-of-flight difference measurement of defective scattered light using a moving optical system.

[0021] The numbers in the figure are: 1, 5 - laser, 2, 6 - electro-optic modulator, 3 - photodetector, 8 - surface under test, 9 - single-channel optical microscopy system, 4, 7 - moving area. Detailed Implementation

[0022] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0023] A surface micro-defect detection device based on dark field scattering time-of-flight is designed. It uses at least two or more optical paths to illuminate the object and accurately detects surface micro-defects by detecting the time difference between the flight times of the scattered light from surface micro-defects under multi-optical path illumination. It includes two lasers, with an electro-optic modulator set in front of each laser to control the time-domain waveform of the laser. The time-domain modulated beam illuminates the surface to be tested and is scattered by the micro-defects. Then, it enters the photodetector through a single-path optical microscope system to acquire the signal.

[0024] Please refer to the following first. Figure 1 , Figure 1 This is a schematic diagram of the surface micro-defect detection device based on dark field scattering time-of-flight according to the present invention. The detection device includes two lasers 1 and 5, with electro-optic modulators 2 and 6 placed in front of each laser to control the time-domain waveform of the laser. The time-domain modulated beam illuminates the surface 8 to be tested and is scattered by the micro-defects, and then enters the photodetector 3 through a single-path optical microscopy system 9 to acquire the signal.

[0025] like Figure 2 As shown, beam paths one-two and three-four are respectively Figure 1 The diagram illustrates the principle of two beams. According to Huygens' law of diffraction, regardless of the angle at which an object is radiated, every point on the object's surface can be considered a new wave source. The time it takes for a new wave source to propagate to the same spatial region is independent of the incident state of the light source. Figure 2 The flight time of the middle beam path one is The flight time of beam path three is This represents the flight time of the beam energy from the light source plane to the surface under test; the flight times corresponding to beam path two and beam path four are equal. This represents the flight time of the light beam from the object to the detector. Therefore, the difference in flight time observed by the detector is the time Δt it takes for the light beam energy to travel from the light source plane to the surface under test and then to the detector. a With Δt b The difference satisfies:

[0026]

[0027] in This design aims to link the flight time of the light beam from the source to the first point of contact with the object with the flight time observed by the detector, thereby avoiding discussions of complex interactions such as multiple scattering of the light beam on the object's surface.

[0028] Since a single-point detector is used to collect scattered light signals, and the single-point detector lacks spatial resolution, this invention provides two scanning methods: moving the surface under test and moving the optical system. The moving areas are as follows: Figure 1 As shown in regions 7 and 4. The two scanning methods are described below:

[0029] (1) Directly move the sample

[0030] like Figure 3 As stated above, when the surface under test is free of defects, the time difference between the detected and observed spatial regions should be zero, because when the sample is moved, the time of flight of the beam energy from the light source plane to each spatial point on the surface under test is uniform and provided by an equivalent optical path of the beam path.

[0031] When defects exist on the surface under test, the time-of-flight difference observed by the detector in different spatial regions is mainly provided by the difference between beam paths three and four, while beam paths two and five are always equivalent. According to... Figure 3 The geometric relationships shown in the dashed diagram are denoted as follows: the flight times corresponding to beam paths three and four are respectively... The difference between the two satisfies:

[0032]

[0033] In the formula, α is the angle between the light source plane and the macroscopic normal to the surface under test, generally called the angle of incidence; c is the speed of light. In actual experiments, only beam paths three and five, or the sum of beam paths four and five, can be obtained, i.e., Δt. a and Δt′ a Therefore, at least one additional beam is required. Let t be the flight time obtained after introducing the second beam. b and Δt′ b Combining equations (1) and (2), we have:

[0034]

[0035] Formula (3) clarifies the conversion relationship between the height difference of the defects on the surface under test and the measurement flight time before and after moving the sample under two-beam illumination. When the surface under test is defect-free, i.e., Δz = 0, the time difference between different spatial points should also be 0; when the surface under test is defective, the relationship between the time difference and the defect height difference is given by formula (3). Even if the sample is placed at a certain tilt, for a defect-free surface, the time difference between different spatial points should be a constant value, i.e., the time difference has a first derivative with respect to the direction of sample displacement and should tend to a constant.

[0036] (2). Mobile optical system

[0037] like Figure 4 As stated above, when the surface under test is defect-free, the time difference between the observed spatial regions before and after the moving detector is no longer zero, but rather a function of the incident angle α of the light source and the moving distance Δx of the detector:

[0038]

[0039] Similarly, by introducing two beams of illumination and moving the detector back and forth, the flight time Δt can be obtained. a , Δt′ a , Δt b , Δt′ b Combining formulas (1) and (4), we have:

[0040]

[0041] When defects exist on the surface under test, the time-of-flight difference between the different spatial regions observed before and after moving the detector is mainly provided by the difference between beam paths three and four. However, compared to directly moving the sample, the spatiotemporal reference plane of the beam does not change, and beam paths two and five are still equivalent. According to Figure 4 From the geometric relationship shown in the dashed box, we can see that Δx and Δx′ satisfy:

[0042] Δx ′ =(Δx+Δz tanΔ)cosΔ#(6)

[0043] And angle satisfy:

[0044] Δ-β=γ#(7)

[0045] Combining formulas (6) and (7), the flight times corresponding to beam path three and beam path four can be derived. The difference satisfies:

[0046]

[0047] After introducing the second beam, combining formulas (1) and (6), we have:

[0048]

[0049] Formula (7) clarifies the functional relationship between the time difference of the scattered light and the ratio of the displacement of the defect height along the displacement direction. Unlike the moving sample scanning method, when there are no defects on the surface to be measured, the time difference between different spatial points is not zero, but a value related to the incident angle of the light source and the displacement, which can be calculated by formula (5).

Claims

1. A device for detecting minute surface defects based on dark field scattering time-of-flight, characterized in that, It includes two lasers, an electro-optic modulator, a single-path optical microscope system, and a photodetector; An electro-optic modulator is placed in front of each laser to control the time-domain waveform of the laser. Two time-modulated optical paths illuminate the longitudinally positioned surface under test. The signal is then acquired by a photodetector through a single-path optical microscope system. The height difference Δz of the defect is obtained by calculating the time difference between the flight times of the scattered light from the surface's minute defects under the illumination of the two optical paths, thus accurately detecting minute surface defects. The photodetector is a single-point detector. During detection, the surface under test is moved or the single-path optical microscope system is scanned. When moving, the surface under test or the single-path optical microscope system moves in the depth direction.

2. The surface minute defect detection device according to claim 1, characterized in that, When using a moving surface scanning method, the height difference Δz of the defect is obtained from the time difference between the flight times of the scattered light from the minute defects on the surface under two optical path illuminations. Where Δt a , Δt b Δt is the time of flight measured by the photodetector before the surface under test moves. ′a , Δt ′b α1 and α2 are the incident angles, representing the angle between the equiphase surface of the time-domain modulated light source and the macroscopic normal of the surface under test, i.e., the Z-axis of the world coordinate system.

3. The surface minute defect detection device according to claim 1, characterized in that, When using a scanning method with a moving single-path optical microscopy system, the height difference Δz of the defect is obtained based on the time difference between the flight times of the scattered light from the minute surface defects under two-path illumination: Where Δx is the displacement of the single-path fiber optic microscopy system along the displacement direction, and Δt a , Δt b Δt is the time of flight measured by the photodetector before the surface under test moves. ′a , Δt ′b α1 and α2 are the incident angles, representing the angle between the equiphase surface of the time-domain modulated light source and the macroscopic normal of the surface under test, i.e., the Z-axis of the world coordinate system.

4. A detection method based on the surface minute defect detection device according to claim 1, characterized in that, During detection, the signal from the photodetector is acquired by moving the surface to be tested or by scanning with a single-channel optical microscopy system. When using a moving surface scanning method, the height difference Δz of the defect is obtained from the time difference between the flight times of the scattered light from the minute defects on the surface under two optical path illuminations. Where Δt a , Δt b Δt is the time of flight measured by the photodetector before the surface under test moves. ′a , Δt ′b α1 and α2 are the incident angles, representing the angle between the equiphase surface of the time-domain modulation of the light source and the macroscopic normal of the surface under test, i.e., the Z-axis of the world coordinate system. When using a scanning method with a moving single-path optical microscopy system, the height difference Δz of the defect is obtained based on the time difference between the flight times of the scattered light from the minute surface defects under two-path illumination: Where Δx is the displacement of the single-path fiber optic microscopy system along the displacement direction, and Δt a , Δt b Δt is the time of flight measured by the photodetector before the surface under test moves. ′a , Δt ′b α1 and α2 are the incident angles, representing the angle between the equiphase surface of the time-domain modulated light source and the macroscopic normal of the surface under test, i.e., the Z-axis of the world coordinate system.

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