Microsphere-assisted differential interferometric confocal displacement measurement system and measurement method

Abstract

The present invention relates to the field of optical detection, and in particular to a differential interferometric confocal displacement measurement system and a measurement method based on microsphere assistance, wherein the measurement system comprises a laser light source, a test platform, an optical detection structure, a beam splitting structure, a first polarization beam splitting structure, a second polarization beam splitting structure, a detection unit and a signal processing unit; the test platform is used to place a sample, and transparent microspheres are evenly attached to the surface of the sample, and the transparent microspheres are used to increase the magnification of the sample during imaging; the linearly polarized light output by the laser light source is sequentially passed through the optical detection structure, the beam splitting structure, the first polarization beam splitting structure, and the second polarization beam splitting structure to generate four-way interference signals with phases of 0°, 90°, 180°, and 270°, which are received by the detection unit and transmitted to the signal processing unit after photoelectric conversion to calculate the surface height of the sample. The present invention can improve the lateral resolution of the measurement system and enhance the imaging quality through the transparent microspheres attached to the surface of the sample.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical detection, and in particular relates to a microsphere-assisted differential interference confocal displacement measurement system and a measurement method. Background Art

[0002] In modern precision measurement technology, the acquisition of three-dimensional surface topography is particularly important and is widely used in the manufacturing and testing of microelectronic components and the analysis of tissue structure in the biomedical field. Although traditional three-dimensional measurement methods, such as stylus profilometers, can provide a certain degree of measurement accuracy, they often damage the sample due to the contact measurement method, especially when measuring complex surfaces or soft materials, it is difficult to obtain accurate results. Therefore, non-contact optical measurement technology has gradually become a research hotspot.

[0003] As an important non-contact measurement tool, confocal microscopy has become the preferred solution for high-precision surface profile measurement due to its excellent performance in high axial resolution and high lateral resolution. By limiting the focal range of the detection beam, confocal microscopy can effectively reduce the interference of background light, thereby significantly improving the signal-to-noise ratio of the image and achieving high-precision imaging of the sample. Although confocal microscopy has outstanding performance in resolution, its imaging speed is limited by mechanical scanning, especially in fast measurement scenarios, this problem significantly affects the measurement efficiency.

[0004] In order to overcome this limitation, differential confocal microscopy came into being. Differential confocal microscopy improves depth resolution by detecting light beams of different focal lengths separately, using signal differences. This not only maintains the high-resolution advantage of traditional confocal microscopy, but also speeds up the measurement speed to a certain extent, expanding its potential in high-precision and high-speed measurement applications. However, the improvement in lateral resolution of interferometric differential confocal microscopy is still limited, and the lateral resolution is still mainly limited by the numerical aperture of the optical system and the size of the focus. Although some improvement measures, such as the use of more advanced optical components and more precise scanning platforms, can improve the lateral resolution to a certain extent, these methods are often accompanied by higher technical difficulties and costs. Summary of the invention

[0005] In view of this, the present invention aims to provide a microsphere-assisted differential interferometric confocal displacement measurement system and measurement method to solve the problem of difficulty and high cost in improving the lateral resolution of traditional differential confocal microscopes.

[0006] To achieve the above object, the technical solution created by the present invention is implemented as follows:

[0007] A microsphere-assisted differential interferometric confocal displacement measurement system comprises a laser light source, a test platform, an optical detection structure, a beam splitting structure, a first polarization splitting structure, a second polarization splitting structure, a pre-focus detection unit, a post-focus detection unit and a signal processing unit;

[0008] The test platform is used to place samples, and transparent microspheres are evenly attached to the surface of the samples, and the transparent microspheres are used to increase the magnification of the samples during imaging;

[0009] The laser light source is used to output linearly polarized light;

[0010] The optical detection structure includes a first polarization beam splitter prism, a reference plane mirror, an objective lens and an objective lens driver. The objective lens driver is used to drive the objective lens to move axially and transversely. The linear polarized light is converted into S polarized light and P polarized light by the first polarization beam splitter prism. The S polarized light is used as the reference light and then reflected by the reference plane mirror and then returned to the first polarization beam splitter prism. The P polarized light is used as the measurement light and then passes through the objective lens and a transparent microsphere to be incident on the sample, and then reflected by the sample and then returned to the first polarization beam splitter prism.

[0011] The beam splitting structure is used to convert the combined S-polarized light and P-polarized light into circularly polarized light respectively, and each beam of circularly polarized light is divided into two beams;

[0012] The first polarization splitting structure is used to split two circularly polarized light beams to generate two interference signals with phases of 0° and 180°;

[0013] The pre-focus detection unit is used to receive two interference signals with phases of 0° and 180°, and transmit them to the signal processing unit after photoelectric conversion;

[0014] The second polarization splitting structure is used to split the other two circularly polarized light beams to generate two interference signals with phases of 90° and 270°;

[0015] The post-focus detection unit is used to receive two interference signals with phases of 90° and 270°, and transmit them to the signal processing unit after photoelectric conversion;

[0016] The signal processing unit is used to calculate four interference signals with phases of 0°, 90°, 180°, and 270° to obtain the surface height of the sample.

[0017] Furthermore, the beam splitting structure includes a first 1 / 4 wave plate and a beam splitter. The combined S polarized light and P polarized light are respectively converted into left-handed circularly polarized light and right-left-handed circularly polarized light by the first 1 / 4 wave plate. The left-handed circularly polarized light is split into a first left-handed circularly polarized light and a second left-handed circularly polarized light by the beam splitter. The right-left-handed circularly polarized light is split into a first right-left-handed circularly polarized light and a second right-left-handed circularly polarized light by the beam splitter. The first left-handed circularly polarized light and the first right-left-handed circularly polarized light are incident on the first polarization splitting structure, and the second left-handed circularly polarized light and the second right-left-handed circularly polarized light are incident on the second polarization splitting structure.

[0018] Furthermore, the first polarization splitting structure is a second polarization splitting prism, and the first left-handed circularly polarized light beam and the first right-left circularly polarized light beam are respectively converted into S polarized light and P polarized light by the second polarization splitting prism, and the two S polarized light beams and the two P polarized light beams generate two interference signals with phases of 0° and 180°.

[0019] Furthermore, the pre-focus detection unit includes a first focusing lens, a first pinhole, a first detector, a second focusing lens, a second pinhole and a second detector. A beam of S polarized light and a beam of P polarized light are focused by the first focusing lens and pass through the first pinhole to be received by the first detector, and an interference signal with a phase of 0° is generated after photoelectric conversion. Another beam of S polarized light and another beam of P polarized light are focused by the second focusing lens and pass through the second pinhole to be received by the second detector, and an interference signal with a phase of 180° is generated after photoelectric conversion.

[0020] Furthermore, the second polarization splitting structure includes a second 1 / 4 wave plate and a third polarization splitting prism. The second left-handed circularly polarized light and the second right-left circularly polarized light are converted into linearly polarized light by the second 1 / 4 wave plate. The two beams of linear polarized light are respectively converted into S-polarized light and P-polarized light by the third polarization splitting prism. The two beams of S-polarized light and the two beams of P-polarized light generate two interference signals with phases of 90° and 270°.

[0021] Furthermore, the post-focus detection unit includes a third focusing lens, a third pinhole, a third detector, a fourth focusing lens, a fourth pinhole and a fourth detector. A beam of S polarized light and a beam of P polarized light are focused by the third focusing lens and pass through the third pinhole and are received by the third detector. After photoelectric conversion, an interference signal with a phase of 90° is generated. Another beam of S polarized light and another beam of P polarized light are focused by the fourth focusing lens and pass through the fourth pinhole and are received by the fourth detector. After photoelectric conversion, an interference signal with a phase of 270° is generated.

[0022] Furthermore, the signal processing unit includes a phase extraction module, a phase unwrapping module and a surface height calculation module. The phase extraction module extracts the wrapped phase according to the following formula: :

[0023]

[0024] in, and are the post-focus interferometric modulation intensity and the pre-focus interferometric modulation intensity, respectively. , , , are the light intensities of the interference fringes in the four interference images with phases of 0°, 90°, 180°, and 270° respectively;

[0025] The phase unwrapping module wraps the phase according to the following formula Unwrap and obtain the unwrapped phase :

[0026]

[0027]

[0028] in, and are the gradients of the wrapping phase along the transverse and longitudinal directions of the sample, is the Laplace operator, Wrap phase Center at point The phase value of Wrap phase Center at point The phase value of

[0029] The surface height calculation module calculates the surface height of the sample by the following formula :

[0030]

[0031] in, is the wavelength at the corresponding position of the sample.

[0032] A microsphere-assisted differential interferometry confocal displacement measurement system is implemented using the above-mentioned microsphere-assisted differential interferometry confocal displacement measurement system, characterized in that it includes the following steps:

[0033] S1: The linearly polarized light output by the laser light source is converted into S-polarized light and P-polarized light by the first polarization beam splitter prism. The S-polarized light is used as the reference light and then reflected by the reference plane mirror and returned to the first polarization beam splitter prism. The P-polarized light is used as the measurement light and then incident on the sample through the objective lens and the transparent microsphere, and then reflected by the sample and returned to the first polarization beam splitter prism. The combined S-polarized light and P-polarized light are converted into circularly polarized light by the beam splitting structure and each circularly polarized light is divided into two beams. The two circularly polarized light beams are split by the first polarization beam splitter structure to generate two interference signals with phases of 0° and 180°, which are received by the pre-focus detection unit. The other two circularly polarized light beams are split by the second polarization beam splitter structure to generate two interference signals with phases of 90° and 270°, which are received by the post-focus detection unit.

[0034] S2: The pre-focus detection unit and the post-focus detection unit convert the received interference signal into an electrical signal and transmit it to the signal processing unit. The signal processing unit calculates the four interference signals with phases of 0°, 90°, 180°, and 270° to obtain the surface height of the sample;

[0035] S3: Control the objective lens driver to drive the objective lens to move horizontally to perform horizontal scanning on the sample, and obtain a two-dimensional image of the sample by image stitching;

[0036] S4: Control the objective lens driver to drive the objective lens to move axially to perform axial scanning on the sample, vertically superimpose the two-dimensional image of the transverse scanning, and obtain a three-dimensional image of the sample.

[0037] Furthermore, the process of calculating the sample surface height by the signal processing unit according to the four-way interference signal is as follows:

[0038] S201: Extracting the wrapped phase based on four interference images:

[0039]

[0040] in, and are the post-focus interferometric modulation intensity and the pre-focus interferometric modulation intensity, respectively. , , , are the light intensities of the interference fringes in the four interference images with phases of 0°, 90°, 180°, and 270°, respectively. is the extracted parcel phase;

[0041] S202: Extracting the package phase Unwrap and obtain the unwrapped phase :

[0042]

[0043]

[0044] in, and are the gradients of the wrapping phase along the transverse and longitudinal directions of the sample, is the Laplace operator, Wrap phase Center at point The phase value of Wrap phase Center at point The phase value of

[0045] S203: Convert the unwrapped phase to the surface height of the sample:

[0046]

[0047] in, is the surface height of the sample, is the wavelength at the corresponding position of the sample.

[0048] Furthermore, after obtaining the two-dimensional image of the sample, the following steps are also included:

[0049] S310: using a Gaussian filtering algorithm to filter noise from the two-dimensional image of the sample;

[0050] S320: using a plane three-point correction method to perform tilt correction on the two-dimensional image; wherein step S320 specifically includes the following steps:

[0051] S321: Select three coordinate points on the two-dimensional image , , , substitute into the plane equation to get the plane normal vector coefficient , , , and then according to the normal vector coefficient of the plane , , Fitting to get the fitting plane:

[0052]

[0053]

[0054] in, , , is the coordinate of any point on the fitting plane;

[0055] S322: Bring the coordinate points of the four corners in the two-dimensional image into the fitting plane, find the minimum value of the surface height among the four coordinate points, and use the plane where the minimum value is located as the standard plane;

[0056] S323: Subtract the height difference from the surface height of the sample calculated in step S203 to obtain a corrected height value.

[0057] Compared with the prior art, the invention can achieve the following beneficial effects:

[0058] 1. Attach transparent microspheres to the surface of the sample, and use the microspheres to increase the magnification of the sample, thereby improving the lateral resolution of the differential interferometric confocal displacement measurement system. This solution is simple to implement and has low cost.

[0059] 2. Gaussian filtering can effectively filter out Gaussian noise in two-dimensional images, facilitating subsequent image correction.

[0060] 3. Use the plane three-point correction method to perform tilt correction on the height of the sample in the two-dimensional image to improve the measurement accuracy.

[0061] 4. Using polarized light for differential interferometry detection can suppress background noise and improve spatial resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The drawings constituting part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation on the present invention. In the drawings:

[0063] Figure 1 It is a structural schematic diagram of a microsphere-assisted differential interferometric confocal displacement measurement system according to an embodiment of the present invention.

[0064] Description of reference numerals:

[0065] 1. Laser light source; 2. First polarization beam splitter; 3. Reference plane mirror; 4. Objective lens; 5. Transparent microsphere; 6. Test platform; 7. First 1 / 4 wave plate; 8. Beam splitter; 9. Second polarization beam splitter; 10. First focusing lens; 11. First pinhole; 12. First detector; 13. Second focusing lens; 14. Second pinhole; 15. Second detector; 16. Second 1 / 4 wave plate; 17. Third polarization beam splitter; 18. Third focusing lens; 19. Third pinhole; 20. Third detector; 21. Fourth focusing lens; 22. Fourth pinhole; 23. Fourth detector; 23. Sample 24. DETAILED DESCRIPTION

[0066] In order to make the purpose, technical solution and advantages of the invention more clear, the invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described here are only used to explain the invention and do not constitute a limitation of the invention.

[0067] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0068] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0069] In the description of the invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the invention can be understood according to specific circumstances.

[0070] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments.

[0071] like Figure 1 As shown, the microsphere-assisted differential interferometric confocal displacement measurement system provided by the embodiment of the present invention includes a laser light source 1, a test platform 6, an optical detection structure, a beam splitting structure, a first polarization splitting structure, a second polarization splitting structure, a pre-focus detection unit, a post-focus detection unit and a signal processing unit.

[0072] The test platform 6 is used to carry and place the sample 24. The surface of the sample 24 is evenly attached with transparent microspheres 5. The transparent microspheres 5 are used to increase the magnification of the sample 24 during imaging. Before the test, the sample 24 is placed in a liquid emulsion, and the surface of the liquid emulsion is covered with transparent microspheres 5. After the liquid emulsion evaporates, the transparent microspheres 5 are evenly attached to the surface of the sample 24. The transparent microspheres 5 are made of transparent materials such as silicon dioxide and barium titanate.

[0073] According to the theory of geometric optics, the magnification M of the transparent microsphere 5 can be expressed as:

[0074]

[0075] In the formula, It is the ratio of the refractive index of the internal and external environments of the transparent microsphere.

[0076] For example, the refractive index of silicon dioxide is The refractive index of air is 1.46. is 1, Substituting into the above formula, we get that the magnification of the silica microspheres is about 2.7 times.

[0077] The laser light source 1 is used to output a single-wavelength linearly polarized laser, for example, a laser with a wavelength of 633 nm.

[0078] The optical detection structure includes a first polarization beam splitter prism 2, a reference plane mirror 3, an objective lens 4 and an objective lens driver. The objective lens driver is used to drive the objective lens 4 to move axially and transversely to achieve transverse scanning and axial scanning of the sample 24; the first polarization beam splitter prism 2 is used to split linear polarized light into vertical S polarized light and P polarized light.

[0079] The beam splitting structure includes a first 1 / 4 wave plate 7 (fast axis 45°) and a beam splitter 8. The first 1 / 4 wave plate 7 is used to convert linearly polarized light into circularly polarized light, and the beam splitter 8 is used to split the circularly polarized light into two beams.

[0080] The first polarization splitting structure is a second polarization splitting prism 9, which is used to convert circularly polarized light into vertical S polarized light and P polarized light.

[0081] The pre-focus detection unit includes a first focusing lens 10, a first pinhole 11, a first detector 12, a second focusing lens 13, a second pinhole 14 and a second detector 15. The first focusing lens 10 and the second focusing lens 13 are used to focus the light beam; the first pinhole 11 and the second pinhole 14 are used to filter stray light and provide imaging quality; the first detector 12 is used to receive an interference image with a phase of 0°, and output an interference signal with a phase of 0° after photoelectric conversion; the second detector 15 is used to receive an interference image with a phase of 180°, and output an interference signal with a phase of 180° after photoelectric conversion.

[0082] The second polarization splitting structure includes a second quarter wave plate 16 (fast axis 45°) and a third polarization splitting prism 17. The second quarter wave plate 16 is used to convert circularly polarized light into linearly polarized light. The third polarization splitting prism 17 is used to split the linearly polarized light into S polarized light and P polarized light.

[0083] The post-focus detection unit includes a third focusing lens 18, a third pinhole 19, a third detector 20, a fourth focusing lens 21, a fourth pinhole 22 and a fourth detector 23; the third focusing lens 18 and the fourth focusing lens 21 are used to achieve focusing of the light beam; the third pinhole 19 and the fourth pinhole 22 are used to filter stray light and provide imaging quality; the third detector 20 is used to receive an interference image with a phase of 90°, and output an interference signal with a phase of 90° after photoelectric conversion; the fourth detector 23 is used to receive an interference image with a phase of 270°, and output an interference signal with a phase of 270° after photoelectric conversion.

[0084] Defocusing is achieved by moving the positions of the first detector 12 , the second detector 15 , the third detector 20 , and the fourth detector 23 .

[0085] The signal processing unit is used to calculate the four-way interference signals with phases of 0°, 90°, 180°, and 270° to obtain the surface height of the sample 24. The signal processing unit includes a phase extraction module, a phase unwrapping module, and a surface height calculation module; wherein,

[0086] The phase extraction module extracts the wrapped phase according to the following formula :

[0087]

[0088] in, and are the post-focus interferometric modulation intensity and the pre-focus interferometric modulation intensity, respectively. , , , are the light intensities of the interference fringes in the four interference images with phases of 0°, 90°, 180°, and 270° respectively;

[0089] The phase unwrapping module unwraps the phase according to the following formula :

[0090] The phase unwrapping module wraps the phase according to the following formula Unwrap and obtain the unwrapped phase :

[0091]

[0092]

[0093] in, and are the gradients of the wrapping phase along the transverse and longitudinal directions of the sample, is the Laplace operator, Wrap phase Center at point The phase value of Wrap phase Center at point The phase value of

[0094] The surface height calculation module calculates the surface height of the sample 24 by the following formula: :

[0095]

[0096] in, is the wavelength at the corresponding position of sample 24.

[0097] The working principle of the differential interferometric confocal displacement measurement system based on the assistance of microspheres is as follows: the linearly polarized light output by the laser light source 1 is converted into S polarized light and P polarized light by the first polarization beam splitter prism 2; the S polarized light is used as the reference light and then reflected by the reference plane mirror 3 and then returned to the first polarization beam splitter prism 2; the P polarized light is used as the measurement light and then incident on the sample 24 by the objective lens 4 and the transparent microsphere 5; and then reflected by the sample 24 and then returned to the first polarization beam splitter prism 2; the P polarized light and the S polarized light are combined and then passed through the first 1 / 4 wave plate 7 to form left circularly polarized light and right left circularly polarized light; the left circularly polarized light is divided into a first left circularly polarized light and a second left circularly polarized light by the beam splitter 8; the right left circularly polarized light is divided into a first right left circularly polarized light and a second right left circularly polarized light by the beam splitter 8; the first left circularly polarized light and the first right left circularly polarized light are respectively converted into S polarized light and P polarized light by the second polarization beam splitter prism 9; a beam of S polarized light and a beam of P polarized light are focused by the first focusing lens 10 After being focused and passing through the first pinhole 11, the first detector 12 receives the S polarized light and generates an interference signal with a phase of 0° after photoelectric conversion. The other S polarized light and the other P polarized light are focused by the second focusing lens 13 and pass through the second pinhole 14 and are received by the second detector 15. After being photoelectric conversion, an interference signal with a phase of 180° is generated. The second left-handed circularly polarized light and the second right-left circularly polarized light are converted into linearly polarized light by the second 1 / 4 wave plate 16. The two linearly polarized lights are converted into S polarized light and P polarized light respectively by the third polarization beam splitter prism 17. The S polarized light and the P polarized light are focused by the third focusing lens 18 and pass through the third pinhole 19 and are received by the third detector 20. After being photoelectric conversion, an interference signal with a phase of 90° is generated. The other S polarized light and the other P polarized light are focused by the fourth focusing lens 21 and pass through the fourth pinhole 22 and are received by the fourth detector 23. After being photoelectric conversion, an interference signal with a phase of 270° is generated.

[0098] The above content describes in detail the microsphere-assisted differential interferometric confocal displacement measurement system provided by an embodiment of the present invention. Corresponding to the measurement system, an embodiment of the present invention also provides a measurement method implemented by using the measurement system.

[0099] The microsphere-assisted differential interferometric confocal displacement measurement method provided in the embodiment of the present invention comprises the following steps:

[0100] S1: The linearly polarized light output by the laser light source is converted into S-polarized light and P-polarized light by the first polarization beam splitter prism. The S-polarized light is used as the reference light and then reflected by the reference plane mirror and returned to the first polarization beam splitter prism. The P-polarized light is used as the measurement light and then enters the sample through the objective lens and the transparent microsphere, and then returns to the first polarization beam splitter prism after being reflected by the sample. The combined S-polarized light and P-polarized light are converted into circularly polarized light by the beam splitting structure and each circularly polarized light is divided into two beams. The two circularly polarized light beams are split by the first polarization beam splitter structure to generate two interference signals with phases of 0° and 180°, which are received by the pre-focus detection unit. The other two circularly polarized light beams are split by the second polarization beam splitter structure to generate two interference signals with phases of 90° and 270°, which are received by the post-focus detection unit.

[0101] S2: The pre-focus detection unit and the post-focus detection unit convert the received interference signal into an electrical signal and transmit it to the signal processing unit. The signal processing unit calculates the four interference signals with phases of 0°, 90°, 180°, and 270° to obtain the surface height of the sample.

[0102] S3: Control the objective lens driver to drive the objective lens to move laterally to perform transverse scanning on the sample, and obtain a two-dimensional image of the sample by image stitching.

[0103] Due to the imaging magnification effect of the transparent microspheres, the single field of view of the measurement system is limited. In one scanning process, only a partial lateral image can be obtained. It is necessary to control the objective lens driver to drive the objective lens to move horizontally to achieve lateral scanning, take multiple lateral field images and stitch them together to obtain four complete lateral field interference images, that is, to obtain four interference images with phases of 0°, 90°, 180°, and 270°. , , , .

[0104] The process of the signal processing unit calculating the sample surface height based on the four-way interference signal is as follows:

[0105] S201: Extracting the wrapped phase based on four interference images:

[0106]

[0107] in, and are the post-focus interferometric modulation intensity and the pre-focus interferometric modulation intensity, respectively. , , , are the light intensities of the interference fringes in the four interference images with phases of 0°, 90°, 180°, and 270°, respectively. is the extracted parcel phase.

[0108] S202: Extracting the package phase Unwrap and obtain the unwrapped phase :

[0109]

[0110]

[0111] in, and are the gradients of the wrapping phase along the transverse and longitudinal directions of the sample, is the Laplace operator, Wrap phase Center at point The phase value of Wrap phase Center at point The phase value of .

[0112] Since the extracted phase is usually a "wrapped" phase, that is, between −π and 𝜋, it needs to be expanded to the actual height range through a phase unwrapping algorithm. This can be done using the least squares phase unwrapping algorithm to obtain the unwrapped phase .

[0113] S203: Convert the unwrapped phase into the surface height of the sample. The relationship between the interference phase and the surface height is:

[0114]

[0115] in, is the surface height of the sample at different locations, is the wavelength at the corresponding position of the sample.

[0116] After image stitching, two-dimensional imaging of the sample in the lateral direction can be achieved.

[0117] The sample may be tilted due to reasons such as the tilt of the measuring platform, resulting in errors in the surface height measurement of the sample, affecting the measurement accuracy. The measured surface height needs to be tilt corrected. Before the tilt correction, the two-dimensional image of the sample needs to be noise filtered.

[0118] The present invention adopts Gaussian filtering algorithm to filter noise from two-dimensional images. Gaussian filtering belongs to linear smoothing filtering and is suitable for filtering Gaussian noise. After Gaussian filtering, the pixel value of each point is obtained by weighted average of the gray value of the point itself and the gray values ​​of other pixels in its neighborhood, and the weight coefficient of the weighted average is obtained by sampling and normalizing the two-dimensional discrete Gaussian function.

[0119] The zero-mean discrete Gaussian function of the Gaussian template is:

[0120]

[0121] In the formula, represents the signal value after filtering, is the standard deviation, which determines the smoothness of the image after Gaussian filtering.

[0122] The steps of Gaussian filtering are:

[0123] i. Calculate the filter template value according to the two-dimensional discrete Gaussian function;

[0124] ii. Perform boundary expansion, scan the pixels in the image one by one, perform convolution operation on the pixel values ​​of each element in its neighborhood with the Gaussian template, and assign the obtained convolution value to the pixel corresponding to the current point in the target image;

[0125] iii. Until all pixels of the image are processed.

[0126] After Gaussian filtering of the two-dimensional image, the plane three-point correction method is used to correct the surface height, which specifically includes the following steps:

[0127] First, select three coordinate points on the two-dimensional image , , , substituting into the plane equation we get:

[0128]

[0129] in, is the surface height of the sample at different locations .

[0130] The normal vector coefficient of the plane is obtained by the above plane equation , , , and then according to the normal vector coefficient of the plane , , Fitting plane in, , , are the coordinates of any point on the fitted plane.

[0131] Secondly, bring the coordinate points of the four corners of the two-dimensional image into the fitting plane. For a 256*256 two-dimensional image, the coordinate points of the four corners are (0, 0), (255, 0), (255, 255), and (0, 255). Find the minimum value of the surface height among the four coordinate points, and use the plane where the minimum value is located as the standard plane.

[0132] Finally, the height difference is obtained by subtracting the fitting plane from the standard plane, and then the surface height of the sample calculated in step S203 is calculated. Subtract the altitude difference to get the corrected altitude value.

[0133] S4: Control the objective lens driver to drive the objective lens to move axially to perform axial scanning on the sample, vertically superimpose the two-dimensional image of the transverse scanning, and obtain a three-dimensional image of the sample.

[0134] After the two-dimensional image is corrected, a high-precision lateral wide-field image is obtained, and then the objective lens driver is controlled to move the objective lens to the next section, and steps S1 to S3 are repeated to obtain a two-dimensional image of the next section that is horizontally scanned. By axially scanning the sample, the two-dimensional images of the horizontal scans of different sections are vertically superimposed to finally obtain a three-dimensional image of the sample.

[0135] The present invention uses polarized light for differential interference detection, which has the following advantages:

[0136] 1) Polarized light can effectively suppress background noise, especially noise from scattered, reflected or ambient light sources. By appropriate polarization selection and filtering, the interference of noise can be significantly reduced and the accuracy of the signal can be enhanced.

[0137] 2) By selecting and controlling polarized light, higher spatial resolution can be achieved in differential interferometry. Especially in high-resolution imaging or small displacement measurements, polarized light helps provide more detailed interference patterns, thereby improving spatial resolution.

[0138] 3) The application of polarized light can make interferometry more flexible. By adjusting the polarization state, it can adapt to different optical elements or measurement environments, thereby maintaining efficient measurement performance.

[0139] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0140] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A microsphere-assisted differential interferometric confocal displacement measurement system, characterized in that: It includes a laser light source, a test platform, an optical detection structure, a beam splitting structure, a first polarization splitting structure, a second polarization splitting structure, a pre-focus detection unit, a post-focus detection unit and a signal processing unit; The test platform is used to place samples, and transparent microspheres are evenly attached to the surface of the samples, and the transparent microspheres are used to increase the magnification of the samples during imaging; The laser light source is used to output linearly polarized light; The optical detection structure includes a first polarization beam splitter prism, a reference plane mirror, an objective lens and an objective lens driver. The objective lens driver is used to drive the objective lens to move axially and transversely. The linear polarized light is converted into S polarized light and P polarized light by the first polarization beam splitter prism. The S polarized light is used as the reference light and then reflected by the reference plane mirror and then returned to the first polarization beam splitter prism. The P polarized light is used as the measurement light and then passes through the objective lens and a transparent microsphere to be incident on the sample, and then reflected by the sample and then returned to the first polarization beam splitter prism. The beam splitting structure is used to convert the combined S-polarized light and P-polarized light into circularly polarized light respectively, and each beam of circularly polarized light is divided into two beams; The first polarization splitting structure is used to split two circularly polarized light beams to generate two interference signals with phases of 0° and 180°; The pre-focus detection unit is used to receive two interference signals with phases of 0° and 180°, and transmit them to the signal processing unit after photoelectric conversion; The second polarization splitting structure is used to split the other two circularly polarized light beams to generate two interference signals with phases of 90° and 270°; The post-focus detection unit is used to receive two interference signals with phases of 90° and 270°, and transmit them to the signal processing unit after photoelectric conversion; The signal processing unit is used to calculate the four-way interference signals with phases of 0°, 90°, 180°, and 270° to obtain the surface height of the sample; The signal processing unit includes a phase extraction module, a phase unwrapping module and a surface height calculation module. The phase extraction module extracts the wrapped phase according to the following formula: : in, and are the post-focus interferometric modulation intensity and the pre-focus interferometric modulation intensity, respectively. , , , are the light intensities of the interference fringes in the four interference images with phases of 0°, 90°, 180°, and 270° respectively; The phase unwrapping module wraps the phase according to the following formula Unwrap and obtain the unwrapped phase : in, and are the gradients of the wrapping phase along the transverse and longitudinal directions of the sample, is the Laplace operator, Wrap phase Center at point The phase value of Wrap phase Center at point The phase value of The surface height calculation module calculates the surface height of the sample by the following formula : in, is the wavelength at the corresponding position of the sample.

2. The microsphere-assisted differential interferometric confocal displacement measurement system according to claim 1, characterized in that: The beam splitting structure includes a first 1 / 4 wave plate and a beam splitter. The combined S polarized light and P polarized light are respectively converted into left-handed circularly polarized light and right-left-handed circularly polarized light by the first 1 / 4 wave plate. The left-handed circularly polarized light is split into a first left-handed circularly polarized light and a second left-handed circularly polarized light by the beam splitter. The right-left-handed circularly polarized light is split into a first right-left-handed circularly polarized light and a second right-left-handed circularly polarized light by the beam splitter. The first left-handed circularly polarized light and the first right-left-handed circularly polarized light are incident on the first polarization splitting structure, and the second left-handed circularly polarized light and the second right-left-handed circularly polarized light are incident on the second polarization splitting structure.

3. The microsphere-assisted differential interferometric confocal displacement measurement system according to claim 2, characterized in that: The first polarization splitting structure is a second polarization splitting prism. The first left-handed circularly polarized light beam and the first right-left circularly polarized light beam are respectively converted into S polarized light and P polarized light by the second polarization splitting prism. The two S polarized light beams and the two P polarized light beams generate two interference signals with phases of 0° and 180°.

4. The microsphere-assisted differential interferometric confocal displacement measurement system according to claim 3, characterized in that: The pre-focus detection unit includes a first focusing lens, a first pinhole, a first detector, a second focusing lens, a second pinhole and a second detector. A beam of S polarized light and a beam of P polarized light are focused by the first focusing lens and pass through the first pinhole to be received by the first detector, and an interference signal with a phase of 0° is generated after photoelectric conversion. Another beam of S polarized light and another beam of P polarized light are focused by the second focusing lens and pass through the second pinhole to be received by the second detector, and an interference signal with a phase of 180° is generated after photoelectric conversion.

5. The microsphere-assisted differential interferometric confocal displacement measurement system according to claim 2, characterized in that: The second polarization splitting structure includes a second 1 / 4 wave plate and a third polarization splitting prism. The second left-handed circularly polarized light and the second right-left circularly polarized light are converted into linearly polarized light by the second 1 / 4 wave plate. The two beams of linearly polarized light are respectively converted into S-polarized light and P-polarized light by the third polarization splitting prism. The two beams of S-polarized light and the two beams of P-polarized light generate two interference signals with phases of 90° and 270°.

6. The microsphere-assisted differential interferometric confocal displacement measurement system according to claim 5, characterized in that: The post-focus detection unit includes a third focusing lens, a third pinhole, a third detector, a fourth focusing lens, a fourth pinhole and a fourth detector. A beam of S polarized light and a beam of P polarized light are focused by the third focusing lens and pass through the third pinhole to be received by the third detector, and an interference signal with a phase of 90° is generated after photoelectric conversion. Another beam of S polarized light and another beam of P polarized light are focused by the fourth focusing lens and pass through the fourth pinhole to be received by the fourth detector, and an interference signal with a phase of 270° is generated after photoelectric conversion.

7. A microsphere-assisted differential interferometry confocal displacement measurement method, implemented using the microsphere-assisted differential interferometry confocal displacement measurement system according to any one of claims 1 to 6, characterized in that: The steps include: S1: The linearly polarized light output by the laser light source is converted into S-polarized light and P-polarized light by the first polarization beam splitter prism. The S-polarized light is used as the reference light and then reflected by the reference plane mirror and returned to the first polarization beam splitter prism. The P-polarized light is used as the measurement light and then incident on the sample through the objective lens and the transparent microsphere, and then reflected by the sample and returned to the first polarization beam splitter prism. The combined S-polarized light and P-polarized light are converted into circularly polarized light by the beam splitting structure and each circularly polarized light is divided into two beams. The two circularly polarized light beams are split by the first polarization beam splitter structure to generate two interference signals with phases of 0° and 180°, which are received by the pre-focus detection unit. The other two circularly polarized light beams are split by the second polarization beam splitter structure to generate two interference signals with phases of 90° and 270°, which are received by the post-focus detection unit. S2: The pre-focus detection unit and the post-focus detection unit convert the received interference signal into an electrical signal and transmit it to the signal processing unit. The signal processing unit calculates the four interference signals with phases of 0°, 90°, 180°, and 270° to obtain the surface height of the sample; S3: Control the objective lens driver to drive the objective lens to move horizontally to perform horizontal scanning on the sample, and obtain a two-dimensional image of the sample by image stitching; S4: Control the objective lens driver to drive the objective lens to move axially to perform axial scanning on the sample, vertically superimpose the two-dimensional image of the transverse scanning, and obtain a three-dimensional image of the sample.

8. The microsphere-assisted differential interferometry confocal displacement measurement method according to claim 7, characterized in that: The process of the signal processing unit calculating the sample surface height based on the four-way interference signal is as follows: S201: Extracting the wrapped phase based on four interference images: in, and are the post-focus interferometric modulation intensity and the pre-focus interferometric modulation intensity, respectively. , , , are the light intensities of the interference fringes in the four interference images with phases of 0°, 90°, 180°, and 270°, respectively. is the extracted parcel phase; S202: Extracting the package phase Unwrap and obtain the unwrapped phase : in, and are the gradients of the wrapping phase along the transverse and longitudinal directions of the sample, is the Laplace operator, Wrap phase Center at point The phase value of Wrap phase Center at point The phase value of S203: Convert the unwrapped phase to the surface height of the sample : in, is the wavelength at the corresponding position of the sample.

9. The microsphere-assisted differential interferometric confocal displacement measurement method according to claim 8, characterized in that: After obtaining the two-dimensional image of the sample, the following steps are also included: S310: using a Gaussian filtering algorithm to filter noise from the two-dimensional image of the sample; S320: using a plane three-point correction method to perform tilt correction on the two-dimensional image; wherein step S320 specifically includes the following steps: S321: Select three coordinate points on the two-dimensional image , , , substitute into the plane equation to get the plane normal vector coefficient , , , and then according to the normal vector coefficient of the plane , , Fitting to get the fitting plane: in, , , is the coordinate of any point on the fitting plane; S322: Bring the coordinate points of the four corners in the two-dimensional image into the fitting plane, find the minimum value of the surface height among the four coordinate points, and use the plane where the minimum value is located as the standard plane; S323: Subtract the fitting plane from the standard plane to obtain a height difference, and then subtract the height difference from the surface height of the sample calculated in step S203 to obtain a corrected height value.

Images