A 3D Topography Extraction Method and System Based on Phase-Sensitive Bright and Dark Field OCT

Abstract

This invention belongs to the field of morphology detection, and particularly relates to a method and system for three-dimensional morphology extraction based on phase-sensitive bright-dark field OCT. The method includes: S1: using a broadband scanning light source module to illuminate the sample under test with uniform line field illumination; S2: simultaneously acquiring the bright-field interference signal and dark-field interference signal of the sample under test through an off-axis holographic bright-dark field synchronous receiving optical path; S3: receiving the bright-field interference signal and dark-field interference signal through a dual-channel spectrometer to form an OCT interference spectrum signal, and performing optical coherence tomography (OCT) processing on the OCT interference spectrum signal to extract phase information highly correlated with the sample under test; S4: performing phase unwrapping processing on the extracted phase information; S5: converting the unwrapped continuous phase into height difference information, thereby reconstructing the nanoscale three-dimensional morphology of the sample surface. This invention can achieve nanoscale three-dimensional morphology extraction under millimeter-level field of view.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of topography detection, and particularly relates to a three-dimensional topography extraction method and system based on phase-sensitive bright-dark field OCT. BACKGROUND

[0002] In the fields of optical precision manufacturing, micro-electro-mechanical systems and electronic industry, the three-dimensional topography of the micro surface of a component has a decisive influence on its performance and reliability. In recent years, with the continuous improvement of the detection precision requirement in the industry, the measurement of surface topography has gradually moved from the micron level to the nanometer and even sub-nanometer scale.

[0003] At present, the measurement methods of nanoscale surface three-dimensional topography mainly include contact type and non-contact type. The contact type method such as a stylus profilometer has the advantages of wide measurement range and good repeatability, but it has the risk of damaging the sample surface and is difficult to meet the detection needs of high precision or easily damaged materials. The non-contact method includes non-optical method and optical method. In the non-optical method, a scanning tunneling microscope is only suitable for conductor or semiconductor materials and needs to work in a vacuum environment; an atomic force microscope can detect conductors and non-conductors, but the imaging range is limited and is easily affected by the performance of the probe. In the optical measurement method, the chromatic confocal spectroscopy has high axial resolution, but the lateral resolution is limited, the cost is high and it is sensitive to environmental vibration; the interference method has high sensitivity, but its performance is easily affected by the environment, and there is a constraint relationship between the axial measurement range and the system stability.

[0004] Optical coherence tomography (OCT) as a non-contact, high-resolution three-dimensional imaging technology, its axial resolution is usually 1 µm-10 µm, and it is widely used in biomedical and industrial detection. However, the traditional OCT technology still faces many challenges in industrial applications: first, it has poor environmental adaptability, and the vibration and temperature fluctuation in the industrial field will introduce significant noise, reducing the detection precision and signal-to-noise ratio; second, there is an inherent contradiction between detection speed and resolution, and the scanning speed in high-resolution mode is difficult to meet the high-speed detection demand of the production line; finally, the traditional OCT has insufficient detection capability for high-reflectivity surfaces (such as metals), and the stray light generated by strong reflection will mask the deep and micro-topography signals, resulting in blurred imaging. Especially in the measurement of surface three-dimensional topography, the traditional OCT technology has bottlenecks such as low throughput, limited measurement range and axial resolution usually only at the micron level, which is difficult to meet the task of large dynamic range, high precision and high throughput detection of complex curved surfaces.

[0005] Therefore, there is an urgent need in the industry for a surface three-dimensional topography extraction technology that can operate stably in an industrial environment, has a large field of view, nanoscale precision and high detection efficiency. SUMMARY

[0006] To solve the above-mentioned limitations of the traditional OCT technology in surface three-dimensional topography measurement, break through the detection bottleneck of complex curved surface large dynamic range, high precision and high throughput, the application provides a three-dimensional topography extraction method and system based on phase-sensitive bright-dark field OCT, by constructing a bright-dark field OCT synchronous interference detection mechanism, combining a phase-sensitive OCT algorithm and a phase unwrapping algorithm, finally realizing accurate extraction of the nanoscale three-dimensional topography of the sample surface.

[0007] To achieve the above-mentioned purpose, the technical scheme created by the application is implemented as follows:

[0008] A three-dimensional topography extraction system based on phase-sensitive bright-dark field OCT, comprising a wide-band scanning light source module, an off-axis holographic bright-dark field synchronous receiving light path and a double-channel spectrometer; wherein,

[0009] The wide-band scanning light source module is used to provide uniform line field illumination for the measured sample;

[0010] The off-axis holographic bright-dark field synchronous receiving light path is used to synchronously collect bright field interference signals and dark field interference signals of the measured sample; the bright-dark field synchronous receiving light path comprises a first beam splitter, a bright field interference module configured as a common-path interference structure and a dark field interference module configured as an off-axis angular diversity receiving structure; wherein, the first beam splitter is used to divide the light beam output by the wide-band scanning light source module into a bright field detection light beam and a dark field detection light beam; the bright field interference module is used to capture scattered light scattered by the surface of the measured sample within the aperture angle of the bright field detection light beam, to form a bright field interference signal; the dark field interference module is used to capture scattered light scattered by the surface of the measured sample within the aperture angle of the dark field detection light beam, to form a dark field interference signal;

[0011] The double-channel spectrometer comprises a phase extraction unit, wherein, and a phase conversion unit; wherein, the phase extraction unit is used to perform phase-sensitive optical coherence tomography processing on the bright field interference signal and the dark field interference signal respectively, to extract phase information of the measured sample; the phase unwrapping unit is used to unwrap the phase information of the measured sample; the phase conversion unit is used to convert the unwrapped continuous phase into height difference information, so as to restore the nanoscale three-dimensional topography of the surface of the measured sample.

[0012] Further, the bright field interference module and the dark field interference module respectively comprise a second beam splitter, a reference arm light path and a fiber type sample arm control component; wherein,

[0013] The bright field detection light beam is divided into a bright field measurement light beam and a bright field reference light beam by the second beam splitter of the bright field interference module, the bright field measurement light beam is incident to the fiber type sample arm control component of the bright field interference module, and the bright field reference light beam is incident to the reference arm light path of the bright field interference module;

[0014] The dark field detection light beam is split into a dark field measurement light beam and a dark field reference light beam by a second beam splitter of the dark field interference module, the dark field measurement light beam is incident to a fiber type sample arm regulating assembly of the dark field interference module, and the dark field reference light beam is incident to a reference arm optical path of the dark field interference module.

[0015] Further, the reference arm optical path of the bright field interference module and the reference arm optical path of the dark field interference module respectively comprise a polarization controller, a collimating lens, a focusing lens and a reference plane mirror; wherein,

[0016] The bright field reference light beam is regulated by the polarization controller of the bright field interference module in sequence, collimated by the collimating lens, convergently incident to the reference plane mirror of the bright field interference module by the focusing lens, and returns to the second beam splitter of the bright field interference module by the reflection of the reference plane mirror.

[0017] The dark field reference light beam is regulated by the polarization controller of the dark field interference module in sequence, collimated by the collimating lens, convergently incident to the reference plane mirror of the dark field interference module by the focusing lens, and returns to the second beam splitter of the dark field interference module by the reflection of the reference plane mirror.

[0018] Further, the off-axis holographic bright-dark field synchronous receiving optical path further comprises a sample arm optical path shared by the bright field interference module and the dark field interference module; wherein,

[0019] The sample arm optical path comprises a bright field collimating lens, a dark field collimating lens, a reflecting mirror, a two-dimensional galvanometer and a scanning field mirror; the bright field measurement light beam is collimated by the bright field collimating lens and cooperatively projected to the surface of the measured sample by the two-dimensional galvanometer and the scanning field mirror, so as to realize two-dimensional scanning of the bright field measurement light beam on the surface of the measured sample.

[0020] The dark field measurement light beam is collimated by the dark field collimating lens, reflected by the reflecting mirror, and cooperatively projected to the surface of the measured sample by the two-dimensional galvanometer and the scanning field mirror, so as to realize two-dimensional scanning of the dark field measurement light beam on the surface of the measured sample.

[0021] The reference arm optical path of the dark field interference module is arranged at a preset off-axis angle with the sample arm optical path, so as to realize off-axis angle diversity interference.

[0022] Further, the three-dimensional topography extraction system based on the phase-sensitive bright-dark field OCT further comprises a fiber array for coupling the bright field interference signal and the dark field interference signal into the spectrometer.

[0023] A three-dimensional topography extraction method based on the phase-sensitive bright-dark field OCT, which is realized by using the three-dimensional topography extraction system based on the phase-sensitive bright-dark field OCT, and comprises the following steps:

[0024] S1: using a wide spectrum scanning light source module to perform uniform line field illumination on the measured sample;

[0025] S2: synchronously collecting the bright-field interference signal and the dark-field interference signal of the measured sample through an off-axis holographic bright-dark field synchronous receiving light path;

[0026] S3: forming an OCT interference spectrum signal through a double-channel spectrometer receiving the bright-field interference signal and the dark-field interference signal, and performing optical coherence tomography processing on the OCT interference spectrum signal to extract phase information related to the height of the measured sample;

[0027] S4: performing phase unwrapping processing on the extracted phase information;

[0028] S5: converting the unwrapped continuous phase into height difference information, so as to restore the nanoscale three-dimensional morphology of the surface of the measured sample.

[0029] Further, step S3 specifically includes the following steps:

[0030] S31: performing Fourier transform on the OCT interference spectrum signal to obtain complex amplitude information of the OCT interference spectrum signal;

[0031] S32: performing one-dimensional inverse Fourier transform on the complex amplitude information of the OCT interference spectrum signal to obtain complex amplitude information in the depth domain;

[0032] S33: extracting amplitude information and phase information from the complex amplitude information in the depth domain.

[0033] Further, the formula of phase unwrapping in step S4 is as follows:

[0034] ;

[0035] ;

[0036] ;

[0037] wherein, represents an unwrapped phase, represents a folded phase, represents a translation function value, represents a phase difference, represents a translation function value at a position.

[0038] Further, step S5 specifically includes the following steps:

[0039] S51: performing difference calculation on phase values at adjacent positions to obtain a phase difference map, and performing smoothing processing on the phase difference map by using a Gaussian denoising filter;

[0040] S52: performing two-dimensional integration on the denoised phase difference map to obtain a phase sensitive change map of the surface of the measured sample.

[0041] S53: convert the phase information of the phase sensitive change map into height difference information of different position points, and obtain the nanoscale three-dimensional topography of the surface of the measured sample.

[0042] Using a standard sample with a known thickness, the actual phase of the standard sample is measured, and the difference between the actual phase and the theoretical phase of the standard sample is obtained, and the dispersion coefficient is obtained by polynomial fitting, and the original phase is compensated in the subsequent measurement of the measured sample.

[0043] Further, after converting the unwrapped continuous phase into height difference information, the following steps are further included:

[0044] A fixed mirror is arranged in the sample arm light path, and the environmental phase change is monitored in real time as the phase drift, and the phase drift is subtracted from the compensated phase to realize phase drift compensation.

[0045] Compared with the prior art, the present application can achieve the following beneficial effects:

[0046] 1. The present application uses the phase information of the interference signal, and combines phase-sensitive demodulation and large dynamic range phase unwrapping algorithm, which breaks through the limitation of the traditional OCT technology with axial resolution of only micrometer level, and realizes the extraction of nanoscale three-dimensional topography.

[0047] 2. The detection flux and efficiency are significantly improved: by adopting high uniformity line field illumination mode and efficient scanning strategy, the present application can perform fast scanning in a millimeter level field of view (such as 14mm*14mm). The actual measurement scanning time is only 3.54 seconds, and the scanning efficiency is nearly 4 times higher than that of the traditional OCT in a smaller field of view (7mm*7mm), which effectively solves the contradiction between high resolution and high scanning speed, and meets the high speed demand of industrial online detection.

[0048] 3. The unique off-axis holographic bright-dark field synchronous receiving light path design can simultaneously capture the bright field interference signal representing the macroscopic fluctuation of the sample surface and the dark field interference signal representing the small defects of the sample surface. This mechanism effectively solves the problem of interference phase demodulation caused by diffuse scattering on high reflective surfaces and inclined microstructure surfaces, so that clear and complete three-dimensional topography reconstruction of complex curved surfaces can be realized.

[0049] 4. The present application is a non-contact optical measurement method, which will not cause damage to the sample surface. At the same time, its principle does not depend on the electrical conductivity of the material, so it can be widely used in nanoscale topography detection of conductors, semiconductors and non-conductors, and its application range is much wider than that of scanning tunneling microscope (STM) and other limited technologies.

[0050] 5. By introducing a system calibration and optimization algorithm, the dispersion phase error and real-time phase drift are actively compensated, which significantly reduces the influence of external interference such as environmental vibration, temperature fluctuation, etc. on the measurement accuracy, and improves the stability and reliability of the system in the non-ideal industrial field environment. BRIEF DESCRIPTION OF DRAWINGS

[0051] The accompanying drawings, which form a part of this specification, are included to provide a further understanding of the application, and are incorporated in and constitute a part of this specification. The embodiments of the application, and their

[0052] Figure 1 Structure schematic diagram of the three-dimensional topography extraction system based on phase-sensitive bright-dark field OCT according to the embodiments of the application;

[0053] Figure 2 Flowchart of the three-dimensional topography extraction method based on phase-sensitive bright-dark field OCT according to the embodiments of the application;

[0054] Figure 3 Phase unwrapping result of the surface of the measured sample after being processed by the phase-sensitive algorithm according to the embodiments of the application; wherein the measured sample is a 1-yuan coin;

[0055] Figure 4 Nanometer-level three-dimensional topography obtained after converting the unwrapped continuous phase into height difference information according to the embodiments of the application.

[0056] BRIEF DESCRIPTION OF DRAWINGS

[0057] Broadband light source 1, first collimating lens 2, first beam splitter 3, second beam splitter 4, first converging lens 5, first polarization controller 6, second collimating lens 7, second converging lens 8, first reference plane mirror 9, third converging lens 10, first fiber jumper 11, second polarization controller 12, first plane mirror 13, third beam splitter 14, fourth converging lens 15, third polarization controller 16, third collimating lens 17, fifth converging lens 18, second reference plane mirror 19, sixth converging lens 20, second fiber jumper 21, fourth polarization controller 22, fourth collimating lens 23, fifth collimating lens 24, second plane mirror 25, X-axis galvanometer 26, Y-axis galvanometer 27, scanning field lens 28, seventh converging lens 29, eighth converging lens 30, fiber array 31, double-channel spectrometer 32, measured sample 33. DETAILED DESCRIPTION

[0058] In order to make the purposes, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not constitute a limitation on the present application.

[0059] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0060] In the description of the present application, it should be understood that the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only used to facilitate the description of the present application and simplify the description, and therefore cannot be understood as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second" and the like are only used for description purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined with "first", "second" and the like can explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise specified and limited, the terms "assemble", "connect", "connect" should be understood broadly, for example, it can be a fixed connection, or a detachable connection, or an integral connection; it can be a mechanical connection, or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or the communication between the two elements inside. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0061] In the description of the present application, it should be noted that unless otherwise specified and limited, the terms "assemble", "connect", "connect" should be understood broadly, for example, it can be a fixed connection, or a detachable connection, or an integral connection; it can be a mechanical connection, or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or the communication between the two elements inside. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0062] The present application will be described in detail below with reference to the drawings and embodiments.

[0063] As Figure 1As shown, the embodiment of the present application provides a three-dimensional topography extraction system based on phase-sensitive bright-dark field OCT, which comprises a wide-band scanning light source module, an off-axis holographic bright-dark field synchronous receiving light path, and a double-channel spectrometer 32; wherein the wide-band scanning light source module is used to provide uniform line field illumination for the measured sample 33; the off-axis holographic bright-dark field synchronous receiving light path is used to synchronously collect bright field interference signals and dark field interference signals of the measured sample 33; and the double-channel spectrometer 32 is used to receive the bright field interference signals and the dark field interference signals, and the bright field interference signals and the dark field interference signals constitute an OCT interference spectrum signal; and the double-channel spectrometer 32 adopts a phase-sensitive OCT algorithm combined with a large dynamic range phase unwrapping method to realize the reconstruction of the three-dimensional topography of the measured sample 33 surface at a nanometer level under a millimeter level field of view.

[0064] The wide-band scanning light source module comprises a wide-band light source 1 and a first collimating lens 2. In view of the problem of low flux existing in the traditional OCT technology, and the technical bottleneck of complex scattering characteristics of the microstructure surface and weak defect signals leading to great detection difficulty, the wide-band light source 1 is adopted, and the first collimating lens 2 is used to expand the wide-band light source 1 into high-uniformity line field illumination. The wide-band scanning light source module has good coherence characteristics, and can provide a stable light signal basis for subsequent interference detection.

[0065] The off-axis holographic bright-dark field synchronous receiving light path comprises a first beam splitter 3, a bright field interference module configured as a common-path interference structure, and a dark field interference module configured as an off-axis angle diversity receiving structure; wherein the first beam splitter 3 is used to divide the light beam output by the wide-band scanning light source module into a bright field detection light beam and a dark field detection light beam; the bright field interference module is used to capture the scattered light scattered by the measured sample 33 surface within the aperture angle of the bright field detection light beam, to form a bright field interference signal; and the dark field interference module is used to capture the scattered light scattered by the measured sample 33 surface within the aperture angle of the dark field detection light beam, to form a dark field interference signal.

[0066] The bright field interference module comprises a second beam splitter 4, a bright field reference arm light path, and a bright field fiber type sample arm control component. The bright field detection light beam is divided into a bright field measurement light beam and a bright field reference light beam by the second beam splitter 4, the bright field measurement light beam is incident to the bright field fiber type sample arm control component, and the bright field reference light beam is incident to the bright field reference arm light path. The bright field reference arm light path comprises a first converging lens 5, a first polarization controller 6, a second collimating lens 7, a second converging lens 8, and a first reference plane mirror 9. The bright field fiber type sample arm control component comprises a third converging lens 10, a first fiber jumper 11, and a second polarization controller 12.

[0067] The dark field interference module comprises a first plane mirror 13, a third beam splitter 14, a dark field reference arm light path and a dark field fiber type sample arm control assembly. The dark field detection light beam is reflected by the first plane mirror 13 to the third beam splitter 14, and then is divided into a dark field measurement light beam and a dark field reference light beam by the third beam splitter 14. The dark field measurement light beam is incident to the dark field fiber type sample arm control assembly, and the dark field reference light beam is incident to the dark field reference arm light path. The dark field reference arm light path comprises a fourth converging lens 15, a third polarization controller 16, a third collimating lens 17, a fifth converging lens 18 and a second reference plane mirror 19. The dark field fiber type sample arm control assembly comprises a sixth converging lens 20, a second fiber jumper 21 and a fourth polarization controller 22.

[0068] The off-axis holographic bright-dark field synchronous receiving light path further comprises a sample arm light path shared by the bright field interference module and the dark field interference module, that is, the bright field interference module and the dark field interference module share a set of sample arm light paths. The sample arm light path comprises a fourth collimating lens 23, a fifth collimating lens 24, a second plane mirror 25, an X-axis galvanometer 26, a Y-axis galvanometer 27 and a scanning field lens 28. The X-axis galvanometer 26 and the Y-axis galvanometer 27 constitute a two-dimensional galvanometer.

[0069] The off-axis holographic bright-dark field synchronous receiving light path further comprises a seventh converging lens 29, an eighth converging lens 30 and a fiber array 31.

[0070] The working principle of the off-axis holographic bright-dark field synchronous receiving light path is as follows: the light beam emitted by the broadband light source 1 is collimated by the first collimating lens 2 and then incident to the first beam splitter 3, and is divided into a bright field detection light beam and a dark field detection light beam by the first beam splitter 3. The bright field detection light beam is divided into a bright field measurement light beam and a bright field reference light beam by the second beam splitter 4. The bright field reference light beam is incident to the surface of the first reference plane mirror 9 in sequence through the convergence of the first converging lens 5, the polarization control of the first polarization controller 6, the collimation of the second collimating lens 7, and the convergence of the second converging lens 8, and returns to the second beam splitter 4 through the first reference plane mirror 9. The bright field measurement light beam is coupled into the first optical fiber jumper 11 through the third converging lens 10, and is transmitted to the fourth collimating lens 23 after the polarization control of the second polarization controller 12. The bright field measurement light beam is projected onto the surface of the measured sample 33 through the collimation of the fourth collimating lens 23 and the cooperation of the two-dimensional galvanometer and the scanning field mirror 28, so as to realize the two-dimensional scanning of the bright field measurement light beam on the surface of the measured sample 33. The scattered light beam (referred to as a bright field scattered light beam) scattered by the surface of the measured sample 33 and within the aperture angle of the bright field detection light beam returns to the second beam splitter 4, and the bright field scattered light beam and the bright field reference light beam interfere to form a bright field interference signal. The bright field interference signal is coupled into the optical fiber array 31 through the seventh converging lens 29, and is incident to the dual-channel spectrometer 32 through the optical fiber array 31. The dark field detection light beam is reflected to the third beam splitter 14 by the first plane mirror 13, and is divided into a dark field measurement light beam and a dark field reference light beam by the third beam splitter 14. The dark field reference light beam is incident to the surface of the second reference plane mirror 19 in sequence through the convergence of the fourth converging lens 15, the polarization control of the third polarization controller 16, the collimation of the third collimating lens 17, and the convergence of the fifth converging lens 18, and returns to the third beam splitter 14 through the second reference plane mirror 19. The dark field measurement light beam is coupled into the second optical fiber jumper 21 through the sixth converging lens 20, and is transmitted to the fifth collimating lens 24 after the polarization control of the fourth polarization controller 22. The dark field measurement light beam is projected onto the surface of the measured sample 33 through the collimation of the fifth collimating lens 24, the reflection of the second plane mirror 25, and the cooperation of the two-dimensional galvanometer and the scanning field mirror 28, so as to realize the two-dimensional scanning of the dark field measurement light beam on the surface of the measured sample 33. The scattered light beam (referred to as a dark field scattered light beam) scattered by the surface of the measured sample 33 and within the aperture angle of the dark field detection light beam returns to the third beam splitter 14, and the dark field scattered light beam and the dark field reference light beam interfere to form a dark field interference signal. The dark field interference signal is coupled into the optical fiber array 31 through the eighth converging lens 30, and is incident to the dual-channel spectrometer 32 through the optical fiber array 31.

[0071] The second plane mirror 25 is arranged to make the dark field measurement light beam deviate from the bright field measurement light beam, so that the dark field scattered light beam returns to the third beam splitter 14 and the bright field scattered light beam returns to the second beam splitter 4.

[0072] The bright-field detection light beam realizes bright-field imaging, and the dark-field detection light beam realizes dark-field imaging. Since the aperture angles of the bright-field detection light beam and the dark-field detection light beam are different in spatial positions, the bright-field imaging and the dark-field imaging collect different areas of scattered light. The bright-field imaging collects scattered light on the surface of the measured sample 33 within the aperture angle of the illumination area (the center of the aperture angle is along the normal direction), which can reflect the overall characteristics of the macroscopic undulation of the surface of the measured sample 33 and avoid the interference of stray light on the overall surface profile signal. The dark-field imaging collects scattered light on the surface of the measured sample 33 within the aperture angle of the dark-field area outside the illumination area, which can reflect the micro defects (such as nanoscale scratches and depressions) of the surface of the measured sample 33 and solve the problem of insufficient characterization of local defects in the traditional OCT technology.

[0073] The bright-field imaging adopts a common-path interference design, the paths of the bright-field reference light beam and the bright-field scattered light beam are very close and almost completely share one optical path, so that the bright-field imaging is not sensitive to environmental vibration and thermal drift and has very high phase stability.

[0074] The dark-field imaging adopts an off-axis angle diversity reception design, the dark-field reference light beam is introduced with a certain off-axis angle, and only the dark-field scattered light beam with a specific scattering angle interferes with the dark-field reference light beam to capture light scattered in all directions by the micro defects, particles or rough structures on the surface of the measured sample 33.

[0075] In order to realize the off-axis angle diversity reception, the second reference plane mirror 19 is tilted by a small angle θ relative to the optical axis (i.e. the central propagation direction of the light beam), and the angle θ is the off-axis angle.

[0076] For a strong light-reflecting surface such as a metal surface, the bright-field interference signal will be oversaturated and produce stray light. The dark-field interference signal is not affected by the strong specular reflection, and the micro defects on the surface can be clearly seen.

[0077] When measuring a tilted structure, the light reflected by the tilted surface of the structure will run away, resulting in bright-field imaging. However, the tilted surface scatters light to other directions, which is captured by the dark-field imaging with the off-axis design, solving the problem of phase demodulation.

[0078] The dual-channel spectrometer 32 includes a phase extraction unit, a phase unwrapping unit and a phase conversion unit. The phase extraction unit is configured to perform phase-sensitive optical coherence tomography processing on the OCT interference spectrum signal to extract phase information of the measured sample 33. The phase unwrapping unit is configured to unwrap the phase information of the measured sample 33. The phase conversion unit is configured to convert the unwrapped continuous phase into height difference information, so as to restore the nanoscale three-dimensional topography of the surface of the measured sample 33.

[0079] The above describes the phase-sensitive bright-dark field OCT-based three-dimensional topography extraction system provided by the embodiments of the present application in detail, and the embodiments of the present application also provide a phase-sensitive bright-dark field OCT-based three-dimensional topography extraction method, which is implemented by using the phase-sensitive bright-dark field OCT-based three-dimensional topography extraction system.

[0080] As shown in Figure 2 , the phase-sensitive bright-dark field OCT-based three-dimensional topography extraction method comprises the following steps:

[0081] S1: using a wide-band scanning light source module to perform uniform line field illumination on the measured sample.

[0082] S2: synchronously collecting bright field interference signals and dark field interference signals of the measured sample by an off-axis holographic bright-dark field synchronous receiving light path.

[0083] S3: receiving the bright field interference signals and the dark field interference signals by a dual-channel spectrometer to form an OCT interference spectral signal, and performing optical coherence tomography processing on the OCT interference spectral signal to extract phase information related to the height of the measured sample.

[0084] The optical coherence tomography processing is implemented by using a phase-sensitive OCT algorithm, and specifically comprises the following steps:

[0085] S31: performing Fourier transform on the OCT interference spectral signal to obtain complex amplitude information of the OCT interference spectral signal;

[0086] After the Fourier transform, the complex amplitude information of the OCT interference spectral signal is expressed as:

[0087]

[0088] wherein k represents a wave number, which is an independent variable of the Fourier transform; represents a direct current background signal of a reference arm light path (a dark field reference arm light path or a bright field reference arm light path); represents a direct current background signal of a sample arm light path (a dark field sample arm light path or a bright field sample arm light path); represents an alternating current signal generated by interference; represents a phase factor; and z represents an optical path difference between a certain scattering point (a bright field scattering light beam or a dark field scattering light beam) in the measured sample and a reference plane mirror of the reference arm light path (the dark field reference arm light path or the bright field reference arm light path).

[0089] ​The complex amplitude information of the interference signal is generated in the interference process of the reference light and the measurement light, and is a complex number description of the spectral domain interference light field. Because the scattered / reflected light of different depths of the measured sample participates in the interference at the same time, the spectral domain interference complex amplitude information is a superimposed mixture of all depth information of the measured sample, which carries the light field amplitude and phase information of each layer structure of the measured sample from shallow to deep, but these information are overlapped in the spectral domain and cannot be directly distinguished from different depths.

[0090] S32: One-dimensional inverse Fourier transform is performed on the complex amplitude information of the OCT interference spectrum signal to obtain the complex amplitude information in the depth domain;

[0091] After one-dimensional inverse Fourier transform, the complex amplitude information in the depth domain is expressed as:

[0092] ;

[0093] wherein, represents the complex amplitude of the reference beam returned from the reference arm optical path is the complex conjugate of the complex amplitude of the reference beam returned from the reference arm optical path; represents the complex amplitude of the scattered light returned from the sample arm optical path; represents the depth-related integral variable, and the complex amplitude at the depth z is finally obtained by integration; represents the integral operation, which is used to sum all the wave numbers (i.e. all the spectral components).

[0094] S33: Extracting the amplitude information and the phase information from the complex amplitude information in the depth domain, respectively.

[0095] The complex amplitude information in the depth domain is obtained by performing one-dimensional inverse Fourier transform on the complex amplitude information of the OCT interference spectrum signal, and the complex amplitude information in the depth domain contains the amplitude information (correlated with the scattering / reflection intensity) and the phase information (correlated with the optical path difference and the sample topography / refractive index) at the depth.

[0096] S4: Phase unwrapping processing is performed on the extracted phase information.

[0097] This step is used to realize the phase demodulation of the dynamic range and eliminate the phase wrapping on the phase difference map.

[0098] ;

[0099] wherein, represents the unwrapped phase, represents the folded phase, represents the translation function value. The phase unwrapping process mainly solves integer multiple of the translation function value The phase difference is expressed by the formula:

[0100] .

[0101] The specific unwrapping algorithm refers to the translation of phase, and the specific phase unwrapping translation criterion is through the formula:

[0102] .

[0103] wherein, is the translation function value of the previous point , that is, the accumulated 2π integer multiple offset.

[0104] When , the folded phase of the current point has a positive direction of sharp change (more than ) relative to the previous point, which is usually caused by phase wrapping. In order to make the unwrapped phase continuous, the phase of the current point needs to be reduced by 2 , that is, let , so that the unwrapped phase will be continuous with the previous point.

[0105] When , the same translation value as the previous point is maintained, that is, no additional 2 offset is added. This is applicable to the phase difference in the range . In this case, the unwrapped phase of the current point does not need to be adjusted by 2π offset, so is the same as .

[0106] When , a 2π offset is added to the current phase value, that is, a 2π is added to the current phase value. This is applicable to the phase difference . At this time, the folded phase has a sharp change in the negative direction (may be due to phase wrapping) relative to , and by increasing , the unwrapped phase becomes continuous.

[0107] S5: converting the continuous phase after unwrapping into height difference information, thereby restoring the nanoscale three-dimensional topography of the measured sample surface.

[0108] The phase values of adjacent positions are calculated by difference calculation to obtain a phase difference map, and a Gaussian denoising filter is used to smooth the phase signal, filter out high-frequency noise, and retain the phase changes related to the nanoscale topographic features. Finally, the de-noised phase difference map is two-dimensionally integrated, i.e. the phase difference values at each point are integrated in space in the x and y directions, to obtain a phase sensitive change map of the surface of the measured sample (such as Figure 3 ). In addition, the obtained phase information can be converted into height difference information of different position points, and finally a nanoscale three-dimensional topography of the surface of the measured sample is obtained (such as Figure 4 ).

[0109] The height difference conversion formula is:

[0110] ;

[0111] wherein, represents the nanoscale height information calculated from the phase , represents the wave number of the center wavelength of the light source .

[0112] After converting the unwrapped continuous phase into height difference information, the following steps are included:

[0113] A standard sample with a known thickness is used to measure the actual phase of the standard sample, and the actual phase is subtracted from the theoretical phase of the standard sample. The dispersion coefficient is obtained by polynomial fitting, and is compensated in the subsequent measurement of the measured sample 33.

[0114] ;

[0115] wherein, and represent the constant term and the phase offset caused by different group velocities, respectively. These two terms do not cause dispersion broadening. The latter term is a high-order dispersion term, and the polynomial coefficients other than are the dispersion coefficients that need to be compensated. The dispersion coefficients that need to be compensated are subtracted from the original phase term to achieve reverse compensation.

[0116] After dispersion compensation of the phase, the following steps are included:

[0117] A fixed mirror is placed at the focal plane of the last focusing lens of the sample arm, and the collected interference signal is adjusted to be optimal by adjusting the holder. The fixed mirror is used as a reference to monitor the environmental phase change in real time as a phase drift, and the phase drift is subtracted from the reverse compensated phase to achieve phase drift compensation.

[0118] ​Through the above steps, the application can realize measurement in a millimeter-level field of view range, meet the detection requirement of large-size samples, the root mean square value is 21.08 nm, the PV value is 66.84 nm, and the extraction precision of nanometer-level morphology is achieved; the time for scanning a 14mm*14mm area is 3.54s, and the scanning efficiency is 55.37mm 2 / s, and the time for the traditional OCT to scan a 7mm*7mm area is 3.6s, and the scanning efficiency is only 13.6mm 2 / s. It can be seen that the efficiency of the application is improved by nearly 4 times compared with the traditional OCT, and the nanometer-level surface shape detection of complex curved surfaces of various materials such as conductors and non-conductors can be realized, and the detection result after three-dimensional morphology reduction can directly output parameters such as defect depth, and meet the requirement of micro-surface performance evaluation in industrial quality inspection.

[0119] It should be understood that the various forms of the flow shown above can be used to reorder, add or delete steps. For example, the steps recorded in the present disclosure can be executed in parallel, sequentially or in different orders, as long as the desired results of the technical solutions of the present disclosure can be achieved, and the present disclosure does not limit this.

[0120] The above specific embodiments do not constitute a limitation on the protection scope of the present application. Those skilled in the art should understand that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent replacement and improvement within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A phase-sensitive bright-dark field OCT based three-dimensional topography extraction system, characterized in that, The device comprises a wide-spectrum scanning light source module, an off-axis holographic bright-dark field synchronous receiving light path and a double-channel spectrometer. The wide-spectrum scanning light source module is used for providing uniform linear field illumination for the measured sample. The off-axis holographic bright-dark field synchronous receiving light path is used for synchronously collecting bright field interference signals and dark field interference signals of the measured sample. The double-channel spectrometer is used for receiving the bright field interference signals and the dark field interference signals to form OCT interference spectral signals.

2. The phase sensitive bright-dark field OCT based three-dimensional topography extraction system of claim 1, wherein, The bright field interference module and the dark field interference module each comprise a second beam splitter, a reference arm light path and a fiber type sample arm control assembly. The bright field detection light beam is divided into a bright field measurement light beam and a bright field reference light beam by the second beam splitter of the bright field interference module. The dark field detection light beam is divided into a dark field measurement light beam and a dark field reference light beam by the second beam splitter of the dark field interference module.

3. The phase sensitive bright-dark field OCT based three-dimensional topography extraction system of claim 2, wherein, The reference arm light path of the bright field interference module and the reference arm light path of the dark field interference module each comprise a polarization controller, a collimating lens, a focusing lens and a reference plane mirror. The bright field reference light beam is sequentially subjected to regulation by the polarization controller, collimation by the collimating lens and convergent incidence by the focusing lens of the bright field interference module, and is reflected by the reference plane mirror to return to the second beam splitter of the bright field interference module. The dark field reference light beam is sequentially subjected to regulation by the polarization controller, collimation by the collimating lens and convergent incidence by the focusing lens of the dark field interference module, and is reflected by the reference plane mirror to return to the second beam splitter of the dark field interference module.

4. The phase sensitive bright-dark field OCT based three-dimensional topography extraction system of claim 2, wherein, The off-axis holographic bright-dark field synchronous receiving light path further comprises a sample arm light path shared by the bright field interference module and the dark field interference module. The sample arm light path comprises a bright field collimating lens, a dark field collimating lens, a reflecting mirror, a two-dimensional galvanometer and a scanning field mirror; the bright field measurement light beam is projected onto the surface of the measured sample through the collimation of the bright field collimating lens and the cooperation of the two-dimensional galvanometer and the scanning field mirror, so as to realize the two-dimensional scanning of the bright field measurement light beam on the surface of the measured sample; The dark field measurement light beam is projected onto the surface of the measured sample through the collimation of the dark field collimating lens, the reflection of the reflecting mirror and the cooperation of the two-dimensional galvanometer and the scanning field mirror, so as to realize the two-dimensional scanning of the dark field measurement light beam on the surface of the measured sample; The reference arm light path of the dark field interference module is arranged at a preset off-axis angle with the sample arm light path, so as to realize off-axis angle diversity interference.

5. A method for extracting three-dimensional topography based on phase-sensitive bright-dark field OCT, implemented by using the three-dimensional topography extraction system based on phase-sensitive bright-dark field OCT according to any one of claims 1-4, characterized in that, The method comprises the following steps: S1: using a wide-spectrum scanning light source module to perform uniformity line field illumination on the measured sample; S2: synchronously collecting the bright field interference signal and the dark field interference signal of the measured sample through the off-axis holographic bright-dark field synchronous receiving light path; S3: receiving the bright field interference signal and the dark field interference signal through a double-channel spectrometer to form an OCT interference spectrum signal, and performing optical coherence tomography processing on the OCT interference spectrum signal to extract phase information related to the height of the measured sample; S4: performing phase unwrapping processing on the extracted phase information; S5: converting the unwrapped continuous phase into height difference information, so as to restore the nanoscale three-dimensional topography of the surface of the measured sample.

6. The phase sensitive bright-dark field OCT based three-dimensional topography extraction method of claim 5, wherein, Step S3 specifically comprises the following steps: S31: performing Fourier transform on the OCT interference spectrum signal to obtain complex amplitude information of the OCT interference spectrum signal; S32: performing one-dimensional inverse Fourier transform on the complex amplitude information of the OCT interference spectrum signal to obtain complex amplitude information in the depth domain; S33: extracting amplitude information and phase information from the complex amplitude information in the depth domain.

7. The phase sensitive bright-dark field OCT based three-dimensional topography extraction method of claim 5, wherein, The formula for phase unwrapping in step S4 is as follows: ； ； ； wherein, denotes the unwrapped phase, denotes the folded phase, denotes the translation function value, denotes the phase difference, denotes the translation function value of the position.

8. The phase sensitive bright-dark field OCT based three-dimensional topography extraction method of claim 5, wherein, Step S5 specifically comprises the following steps: S51: performing difference calculation on the phase values of adjacent positions to obtain a phase difference graph, and performing smoothing processing on the phase difference graph by using a Gaussian denoising filter; S52: performing two-dimensional integration on the denoised phase difference graph to obtain a phase sensitive change graph of the surface of the measured sample; S53: converting the phase information of the phase sensitive change graph into height difference information of different position points to obtain the nanoscale three-dimensional topography of the surface of the measured sample. 9.The phase-sensitive bright-dark field OCT based three-dimensional topography extraction method of claim 5, wherein, After converting the unwrapped continuous phase into height difference information, the following steps are further included: A standard sample with a known thickness is used to measure the actual phase of the standard sample, and the actual phase is subtracted from the theoretical phase of the standard sample, a dispersion coefficient is obtained through polynomial fitting, and the original phase is inversely compensated in subsequent measurement of the measured sample.

10. The phase sensitive bright-dark field OCT based three-dimensional topography extraction method of claim 9, wherein, After dispersion compensation of the phase, the following steps are further included: A fixed reflecting mirror is arranged in the sample arm light path to monitor the environmental phase change in real time as a phase drift, and the phase drift is subtracted from the inversely compensated phase to realize phase drift compensation.

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