A differential confocal measurement system based on heterodyne illumination and its measurement method

Abstract

The present invention provides a differential confocal measurement system based on heterodyne illumination and a measurement method thereof, belonging to the field of optical detection technology. In the differential confocal measurement system based on heterodyne illumination, the heterodyne illumination unit is used to provide a pre-focus point light source and a post-focus point light source of two different frequencies; the differential confocal unit is arranged in the emission direction of the light beam of the heterodyne illumination unit, and is used to receive the pre-focus light intensity signal and the post-focus light intensity signal of the light beam reflected by the object to be measured, and obtain the height information of the object to be measured; the angle measurement unit is arranged in the emission direction of the light beam of the heterodyne illumination unit, and is used to receive the spot signal formed by the light beam reflected by the object to be measured, and obtain the inclination angle information of the object to be measured. The technical solution provided by the present invention can effectively improve the axial resolution and signal-to-noise ratio of the differential confocal measurement system, make the point spread function of the differential confocal measurement system clearer, and at the same time, an angle measurement unit for measuring angles is added, which can measure the inclination angle of the object to be measured in real time and improve the measurement accuracy.

Description

Technical Field

[0001] The invention belongs to the technical field of optical detection, and in particular relates to a differential confocal measurement system based on heterodyne illumination and a measurement method thereof. Background Art

[0002] As a non-contact, high-precision optical imaging technology, confocal microscopy is widely used in various fields due to its excellent performance in three-dimensional surface topography measurement. Compared with traditional microscopes, confocal microscopy is particularly outstanding in axial resolution, especially in obtaining sample depth information and fine surface contours. By using a confocal pinhole, the system can effectively suppress out-of-focus scattered light, thereby significantly improving the clarity and contrast of the image. Therefore, confocal microscopy has become an indispensable tool in semiconductor manufacturing, materials science, microstructure detection and biomedicine.

[0003] To further improve the axial resolution of confocal microscopy, researchers have proposed a variety of innovative methods. Among them, 4Pi microscopy is one of the classic technologies for improving axial resolution. This technology uses two opposing objective lenses to illuminate the sample and collect signals, so that light interferes simultaneously above and below the sample, thereby greatly improving the axial resolution, even up to 3 to 7 times that of traditional microscopy. This technology is particularly effective in fine three-dimensional imaging and is suitable for detecting tiny structural details. In addition, structured light microscopy can further improve the axial resolution by projecting a stripe grating on the sample and analyzing the movement or change of the grating. Structured light microscopy combines phase recovery technology, which can effectively overcome the limitations of optical scattering in traditional confocal microscopy and is suitable for high-precision imaging of multi-level samples. Imaging technology based on phase recovery is also a cutting-edge method for improving axial resolution. By analyzing the phase changes of the interference image, the axial structure of the sample can be accurately reconstructed, especially in the non-destructive imaging of biological samples. The application of these technologies has made confocal microscopy make significant progress in improving axial resolution, providing stronger support for high-precision three-dimensional imaging.

[0004] Although existing technologies have made significant progress in improving the axial resolution of confocal microscopes, they still have certain limitations. Although the 4Pi microscope can greatly improve the axial resolution, its system structure is complex. Structured light microscopy requires high precision control of the light beam, and because it needs to collect and process multiple frames of images, the imaging speed is slow and is not suitable for real-time imaging. Phase recovery technology can accurately reconstruct the axial structure of the sample, but this method relies on complex computational algorithms and is easily affected by sample surface scattering and noise, resulting in reduced imaging quality and unable to meet modern high-precision measurement requirements. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the prior art and propose a differential confocal measurement system based on heterodyne illumination and a measurement method thereof.

[0006] To achieve the above object, the present invention adopts the following specific technical solutions:

[0007] The present invention provides a differential confocal measurement system based on heterodyne illumination, comprising a heterodyne illumination unit, a differential confocal unit and an angle measurement unit; wherein:

[0008] The heterodyne illumination unit is used to provide a front-focus point light source and a rear-focus point light source of two different frequencies;

[0009] The differential confocal unit is arranged in the emission direction of the light beam of the heterodyne illumination unit, and is used to receive the pre-focus light intensity signal and the post-focus light intensity signal of the light beam reflected by the object to be measured, so as to obtain the height information of the object to be measured;

[0010] The angle measurement unit is arranged in the emission direction of the light beam of the heterodyne illumination unit, and is used to receive the light spot signal formed by the light beam reflected by the object to be measured, and obtain the tilt angle information of the object to be measured.

[0011] Furthermore, the heterodyne illumination unit comprises a dual-frequency laser light source, a first beam splitter, a first plane mirror, a first focusing lens, a first pinhole, a second focusing lens, a third focusing lens, a second pinhole, a fourth focusing lens, a second plane mirror, and a second beam splitter, which are sequentially arranged along the optical path; wherein,

[0012] The dual-frequency laser light source is used to emit laser light beams containing two different frequencies;

[0013] The first beam splitter is used to split light beams of different frequencies;

[0014] The first plane mirror and the second plane mirror are respectively used to change the propagation direction of the light beam;

[0015] The first focusing lens and the second focusing lens are respectively used to focus the light beam;

[0016] The third focusing lens and the fourth focusing lens are respectively used to expand the light beam;

[0017] The first pinhole and the second pinhole are respectively used for spatial filtering to block the out-of-focus light beam to form a pre-focus point light source and a post-focus point light source;

[0018] The second beam splitter is used to combine two light beams with different frequencies.

[0019] Further, the differential confocal unit includes a dichroic mirror, an objective lens, an objective lens driver, a third beam splitter, and a differential assembly which are sequentially arranged along the optical path;

[0020] The differential assembly includes a fourth beam splitter, a front-focus focusing lens, a front-focus pinhole, a front-focus photodetector, a back-focus focusing lens, a back-focus pinhole, and a back-focus photodetector; wherein,

[0021] The dichroic mirror is used to distinguish the excitation light beam and the illumination light beam of the object to be measured;

[0022] The objective lens is used to focus the light beam onto the surface of the object to be measured;

[0023] The objective lens driver is used to move the objective lens along the direction of the light beam, thereby controlling the light beam to be at a focusing position on the object to be measured;

[0024] The third beam splitter and the fourth beam splitter are respectively used to split the light beam;

[0025] The pre-focus focusing lens and the post-focus focusing lens are respectively used to focus the light beam;

[0026] The pre-focus pinhole and the post-focus pinhole are used for spatial filtering to block the out-of-focus light beam;

[0027] The pre-focus photoelectric detector and the post-focus photoelectric detector are used to receive the pre-focus light intensity signal and the post-focus light intensity signal respectively, and then analyze the height information of the object to be measured.

[0028] Further, the angle measurement unit includes a fifth focusing lens and a photodetector sequentially arranged along the optical path;

[0029] The fifth focusing lens is used to focus the light beam;

[0030] The photoelectric detector is used to receive the light spot signal and then analyze the tilt angle information of the object to be measured.

[0031] Furthermore, the measurement is performed by a differential confocal measurement system based on heterodyne illumination, and the measurement method comprises the following steps:

[0032] S1, a pre-focus point light source and a post-focus point light source of different frequencies respectively emit a first measuring beam and a second measuring beam to measure the object to be measured. After being reflected by the object to be measured, the first measuring beam and the second measuring beam carrying the information of the object to be measured are reflected to the angle measurement unit and the differential confocal unit. The first measuring beam and the second measuring beam carrying the information of the object to be measured interfere with each other and form a frequency domain limited light spot;

[0033] S2. The method for obtaining the height information of the object to be measured is:

[0034] Respectively collecting a pre-focus light intensity signal of a frequency-domain limited light spot on a pre-focus photodetector and a post-focus light intensity signal of a frequency-domain limited light spot on a post-focus photodetector, and obtaining a differential confocal response signal curve according to the pre-focus light intensity signal and the post-focus light intensity signal to determine the height information of the object being measured;

[0035] The method for obtaining the tilt angle information of the object to be measured is:

[0036] The light spot signal on the photodetector is collected, the peak position of the light spot signal and the tilt angle are matched one by one, the light spot image signal of the object to be measured on the photodetector is analyzed, and the tilt angle information of the object to be measured is obtained.

[0037] Furthermore, the method for obtaining the height information of the measured object in step S2 is specifically as follows:

[0038] S2-01a, adjusting the defocusing amount of the first pinhole and the second pinhole, collecting the pre-focus light intensity signal of the frequency domain limited light spot on the pre-focus photodetector and the post-focus light intensity signal of the frequency domain limited light spot on the post-focus photodetector;

[0039] S2-02a, subtracting the pre-focus light intensity signal from the post-focus light intensity signal to obtain a differential confocal signal curve;

[0040] S2-03a. Obtain height information of the object to be measured based on the coordinates corresponding to the zero-crossing position of the differential confocal signal curve.

[0041] Furthermore, the method for obtaining the tilt angle information of the object under test in step S2 is specifically as follows:

[0042] S2-01b, processing the light spot signal collected on the photoelectric detector to obtain a frequency domain limited light spot;

[0043] S2-02b, calculating the coordinates of the frequency domain limited light spot when there is no tilt angle, and obtaining the coordinates of the frequency domain limited light spot corresponding to different tilt angles by a calibration method;

[0044] S2-03b, by fitting method, the peak position and tilt angle of the frequency domain limited spot signal are matched one by one;

[0045] S2-04b. Analyze the light spot image signal of the object under test on the photoelectric detector to obtain the tilt angle information of the object under test.

[0046] Furthermore, the correction step of the height information is also included:

[0047] S3, performing tilt correction on the three-dimensional coordinate data of the height information according to the tilt angle information of the measured object and the displacement information of the piezoelectric ceramic displacement stage, and the specific calculation formula is:

[0048] ;in,

[0049] ;

[0050] Wherein, x and y represent the displacement coordinates of the object to be measured driven by the piezoelectric ceramic displacement stage; z represents the height information of the object to be measured; ( ) represents the coordinate after tilt correction; Indicates the height information after tilt angle correction; , Respectively represent the tilt angles of the object under test in the X-axis direction and the Y-axis direction; Indicates that (x, y, z) is rotated around the X axis first , and then rotate around the Y axis The rotation matrix of .

[0051] The present invention can achieve the following technical effects:

[0052] 1. The differential confocal measurement system based on heterodyne illumination and the measurement method thereof provided by the present invention can simultaneously obtain the height information and the tilt angle information of the object to be measured.

[0053] 2. The differential confocal measurement system based on heterodyne illumination and the measurement method thereof provided by the present invention can utilize the tilt angle information to further correct the height information of the object being measured.

[0054] 3. The differential confocal measurement system based on heterodyne illumination and the measurement method thereof provided by the present invention can improve the axial resolution and signal-to-noise ratio of the differential confocal measurement system by increasing the linear interval slope of the differential confocal signal curve.

[0055] 4. The differential confocal measurement system based on heterodyne illumination provided by the present invention does not require a complex dual objective lens structure, thereby reducing system complexity and environmental sensitivity.

[0056] 5. The measurement method of the differential confocal measurement system based on heterodyne illumination provided by the present invention does not require complex image reconstruction algorithms and high-precision grating projection, so it maintains a high imaging speed and is easy to operate while improving the resolution, and takes into account both accuracy and speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 is a schematic structural diagram of a differential confocal measurement system based on heterodyne illumination according to an embodiment of the present invention;

[0058] Figure 2 is a schematic diagram of interference between a front-focus point light source and a back-focus point light source according to an embodiment of the present invention;

[0059] Figure 3 is a flow chart of a measurement method of a differential confocal measurement system based on heterodyne illumination according to an embodiment of the present invention;

[0060] Figure 4 is a schematic diagram of interference intensity distribution according to an axial displacement of an embodiment of the present invention;

[0061] Figure 5 Schematic diagram of differential axial intensity variation of a differential confocal measurement system according to an embodiment of the present invention.

[0062] Reference numerals include:

[0063] 1 dual-frequency laser light source, 2 first beam splitter, 3 first plane mirror, 4-1 first focusing lens, 4-2 second focusing lens, 5-1 first pinhole, 5-2 second pinhole, 6-1 third focusing lens, 6-2 fourth focusing lens, 7 second plane mirror, 8 second beam splitter, 9 dichroic mirror, 10 objective lens, 11 object to be measured, 12 third beam splitter, 13 fifth focusing lens, 14 photoelectric detector, 15 fourth beam splitter, 16-1 pre-focus focusing lens, 17-1 pre-focus pinhole, 18-1 pre-focus photoelectric detector, 16-2 post-focus focusing lens, 17-2 post-focus pinhole, 18-2 post-focus photoelectric detector, 19 piezoelectric ceramic translation stage. DETAILED DESCRIPTION

[0064] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are represented by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, the detailed description thereof will not be repeated.

[0065] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present 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 herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0066] Combine the following Figure 1-Figure 5 The specific working mode of the differential confocal measurement system and the measurement method based on heterodyne illumination according to the embodiment of the present invention is described in detail:

[0067] Embodiment 1:

[0068] The differential confocal measurement system based on heterodyne illumination in the embodiment of the present invention is as follows: Figure 1As shown, it includes a heterodyne illumination unit, a differential confocal unit and an angle measurement unit. The heterodyne illumination unit is used to provide two beams of pre-focus point light sources and post-focus point light sources of different frequencies. The differential confocal unit is arranged in the emission direction of the light beam of the heterodyne illumination unit, and is used to receive the pre-focus light intensity signal and the post-focus light intensity signal of the light beam reflected by the object 11 to obtain the height information of the object 11. The angle measurement unit is arranged in the emission direction of the light beam of the heterodyne illumination unit, and is used to receive the spot signal formed by the light beam reflected by the object 11 to obtain the tilt angle information of the object 11.

[0069] In the embodiment of the present invention, the heterodyne illumination unit comprises a dual-frequency laser light source 1, a first beam splitter 2, a first plane mirror 3, a first focusing lens 4-1, a first pinhole 5-1, a third focusing lens 6-1, a second focusing lens 4-2, a second pinhole 5-2, a fourth focusing lens 6-2, a second plane mirror 7, and a second beam splitter 8, which are sequentially arranged along the optical path. The dual-frequency laser light source 1 is used to emit an orthogonal polarized light beam containing two frequencies; the first beam splitter 2 is used to split two orthogonal polarized light beams of different frequencies; the first plane mirror 3 and the second plane mirror 7 are respectively used to change the propagation direction of the light beam; the first focusing lens 4-1 and the second focusing lens 4-2 are respectively used to focus the light beam; the third focusing lens 6-1 and the fourth focusing lens 6-2 are respectively used to expand the light beam; the first pinhole 5-1 and the second pinhole 5-2 are respectively used for spatial filtering, blocking the defocused light beam, and forming a pre-focus point light source and a post-focus point light source; the second beam splitter 8 is used to combine the two light beams of different frequencies. The dual-frequency laser light source 1 used in the heterodyne illumination unit in the embodiment of the present invention can emit orthogonal polarized light beams. In the embodiment of the present invention, the first beam splitter 2 is a polarization beam splitter, so when the orthogonal polarized light beam is split by the first beam splitter 2, it will be divided into two polarized light beams with different frequencies. In the embodiment of the present invention, considering matching with the dual-frequency laser light source 1 and the first beam splitter 2, the second beam splitter 8 is also a polarization beam splitter. Other methods can be used to form two light beams with different frequencies for measurement according to actual conditions, and the present invention is not limited to this.

[0070] The differential confocal unit in the embodiment of the present invention comprises a dichroic mirror 9, an objective lens 10, an objective lens driver, a third beam splitter 12, and a differential assembly which are sequentially arranged along the optical path. The differential component includes a fourth beam splitter 15, a pre-focus focusing lens 16-1, a pre-focus pinhole 17-1, a pre-focus photodetector 18-1, a post-focus focusing lens 16-2, a post-focus pinhole 17-2, and a post-focus photodetector 18-2; the dichroic mirror 9 is used to distinguish the excitation light beam and the irradiation light beam of the object 11 to be measured; the objective lens 10 is used to focus the light beam onto the surface of the object 11 to be measured; the objective lens driver is used to move the objective lens 10 along the direction of the light beam, thereby controlling the focusing position of the light beam on the object 11 to be measured; the third beam splitter 12 and the fourth beam splitter 15 are respectively used to split the light beam; the pre-focus focusing lens 16-1 and the post-focus focusing lens 16-2 are respectively used to focus the light beam; the pre-focus pinhole 17-1 and the post-focus pinhole 17-2 are used for spatial filtering to block out-of-focus light beams; the pre-focus photodetector 18-1 and the post-focus photodetector 18-2 are respectively used to receive the pre-focus light intensity signal and the post-focus light intensity signal, thereby analyzing the height information of the object 11 to be measured. In the embodiment of the present invention, the pre-focus photodetector 18-1 and the post-focus photodetector 18-2 are both photomultiplier tubes. The object 11 to be measured in the embodiment of the present invention has a fluorescent property, so the dichroic mirror 9 is selected to distinguish the excitation light beam and the irradiation light beam of the object 11 to be measured, so as to ensure that after the object 11 to be measured is irradiated by the pre-focus point light source and the post-focus point light source, the excited fluorescence signal can be received by the photodetector 14, the pre-focus photodetector 18-1, and the post-focus photodetector 18-2, thereby improving the efficiency of receiving effective measurement signals and improving the sensitivity of measurement. The technical solution of the present invention can also be used to measure ordinary objects to be measured 11. When measuring ordinary objects to be measured 11, the photodetector 14, the pre-focus photodetector 18-1, and the post-focus photodetector 18-2 only receive the reflected light beam of the object 11 as the measurement signal.

[0071] The angle measurement unit in the embodiment of the present invention includes a fifth focusing lens 13 and a photodetector 14 arranged in sequence along the optical path; the fifth focusing lens 13 is used to focus the light beam; the photodetector 14 is used to receive the light spot signal and then analyze the tilt angle information of the measured object 11. In the embodiment of the present invention, the photodetector 14 is a planar array CCD camera.

[0072] In the embodiment of the present invention, the dual-frequency laser light source 1 emits an orthogonal polarized light beam containing two frequencies, and the orthogonal polarized light beam is split by the first beam splitter 2. The transmitted light beam after the splitting is the first measuring light beam, and the first measuring light beam is sequentially transmitted to the second beam splitter 8 through the first focusing lens 4-1, the first pinhole 5-1, and the third focusing lens 6-1. The reflected light beam after the splitting by the first beam splitter 2 is the second measuring light beam. After the second measuring light beam is reflected by the first plane mirror 3, it is sequentially transmitted to the second plane mirror 7 through the second focusing lens 4-2, the second pinhole 5-2, and the fourth focusing lens 6-2, and then incident on the second beam splitter 8 after being reflected by the second plane mirror 7. The second beam splitter 8 combines the first measuring light beam and the second measuring light beam, and then incident on the object to be measured 11 through the dichroic mirror 9 and the objective lens 10. The first measuring light beam and the second measuring light beam carrying the information of the object to be measured 11 after being reflected by the object to be measured 11 are reflected by the dichroic mirror 9, and the reflected light beams are incident on the third beam splitter 12. The reflected light beam split by the third beam splitter 12 is received by the photodetector 14 through the fifth focusing lens 13; the transmitted light beam is split again by the fourth beam splitter 15, and the split reflected light beam is received by the pre-focus photodetector 18-1 through the pre-focus focusing lens 16-1 and the pre-focus pinhole 17-1; the split transmitted light beam is received by the post-focus photodetector 18-2 through the post-focus focusing lens 16-2 and the post-focus pinhole 17-2. The first measuring light beam and the second measuring light beam carrying the information of the object 11 interfere with each other at the photodetector 14, the pre-focus photodetector 18-1, and the post-focus photodetector 18-2 to form a frequency domain limited light spot. The tilt angle information of the object 11 is obtained by analyzing the frequency domain limited light spot on the photodetector 14; the height information of the object 11 is obtained by analyzing the pre-focus photodetector 18-1 and the post-focus photodetector 18-2.

[0073] In the embodiment of the present invention, the third focusing lens 6-1 and the fourth focusing lens 6-2 have the same optical parameters, the first pinhole 5-1 is arranged before or after the focus of the third focusing lens 6-1, and the second pinhole 5-2 is arranged before or after the focus of the fourth focusing lens 6-2. Therefore, the light beams transmitted by the first pinhole 5-1 and the second pinhole 5-2 are one divergent and one convergent, but the light beams transmitted by the two pinholes can be either divergent or convergent. The first pinhole 5-1 and the second pinhole 5-2 are guaranteed to be arranged before or after the focus of the corresponding lens, but the first pinhole 5-1 and the second pinhole 5-2 cannot be arranged before and after the focus of the corresponding lens at the same time. The first pinhole 5-1 and the second pinhole 5-2 transmit two different frequencies of the front focus point light source and the back focus point light source, respectively, and the front focus point light source and the back focus point light source respectively send out the first measurement light beam and the second measurement light beam to measure the object 11 to be measured. After being reflected by the object 11, the front focus photodetector 18-1, the back focus photodetector 18-2 and the photodetector 14 respectively receive the signals carrying the information of the object 11 to be measured. The first measuring beam and the second measuring beam carrying the information of the measured object 11 are two axially separated beams of different frequencies. The first measuring beam and the second measuring beam interfere with each other to generate a frequency-domain limited light spot. In the embodiment of the present invention, the light beam emitted by the first measuring beam is a pre-focus beam that is divergent, and the light beam emitted by the second measuring beam is a post-focus beam that is convergent, then they will form images at the near end and far end of the objective lens 10, such as Figure 2 shown.

[0074] In the embodiment of the present invention, the dual-frequency laser light source 1 emits an orthogonal polarized light beam containing two frequencies, wherein when the difference between the two frequencies is small, the interference effect and multi-photon excitation effect of the light beam can significantly enhance the intensity and efficiency of the fluorescence signal excitation of the object 11 to be measured. An appropriately small frequency difference can increase the light intensity of the point light source in the frequency domain limited spot, thereby enhancing the fluorescence signal. In the embodiment of the present invention, the range of a relatively small frequency difference is a frequency difference of several MHz to tens of MHz. A frequency difference that is too large will cause the interference effect to weaken and multi-photon excitation to be difficult to occur, resulting in a decrease in the fluorescence signal intensity and excitation efficiency. However, a frequency difference that is too small and close to zero may cause phase locking, reduce the enhancement effect of the interference effect, and affect the intensity of the fluorescence signal excitation. Therefore, the frequency difference between the two frequencies can be selected according to actual conditions, and the present invention does not limit this.

[0075] The differential confocal measurement system based on heterodyne illumination in the embodiment of the present invention uses a pre-focus point light source and a post-focus point light source of different frequencies, that is, two light beams of different frequencies and at non-focal points as illumination light sources. Since light has electromagnetic properties, it obeys wave theory. According to the superposition principle, at a point in space where two or more light waves overlap, the electric field generated is equal to the vector sum of each component disturbance. Two focal points from the same source but with different frequencies are separated in the axial direction. When the Airy disks of the two foci are separated by a certain distance in the axial direction, there is a small overlapping area between them. Since the frequencies of the two foci are different, the interference of the two spots in the overlapping area will produce a beat frequency phenomenon. The principle is as follows: Figure 2 As shown, A(u)exp(iω1t) represents a complex light field, where A(u) describes the amplitude distribution of the light field at the spatial position u, and exp(iω1t) describes the phase change of the light field over time. B(u)exp(iω1t) represents a complex light field, where B(u) describes the amplitude distribution of the light field at the spatial position u, and exp(iω1t) describes the phase change of the light field over time. The frequency domain limited spot in the embodiment of the present invention is Figure 2 The red overlapping area in the middle. The first measuring beam and the second measuring beam are two axially overlapping beams with different frequencies, forming a frequency domain limited light spot. In the overlapping area, the two beams interfere with each other, and the interference effect makes the effective imaging area more refined in the axial direction. By extracting the frequency domain limited light spot information in the overlapping area for height information calculation, the axial resolution of the differential confocal measurement system can be improved, and an angle measurement unit is added to measure the tilt angle of the object 11 in real time, and the height information is corrected again according to the tilt angle, thereby improving the measurement accuracy.

[0076] Embodiment 2:

[0077] In an embodiment of the present invention, a measurement method of a differential confocal measurement system based on heterodyne illumination is provided, and a differential confocal measurement system based on heterodyne illumination is used to implement measurement, such as Figure 3 As shown, the measurement method includes the following steps:

[0078] S1, a pre-focus point light source and a post-focus point light source of different frequencies respectively emit a first measuring beam and a second measuring beam to measure the object 11 under test. After being reflected by the object 11, the first measuring beam and the second measuring beam carrying information of the object 11 under test are reflected to the angle measurement unit and the differential confocal unit. The first measuring beam and the second measuring beam carrying information of the object 11 under test interfere with each other and form a frequency domain limited light spot.

[0079] In the embodiment of the present invention, when the first measuring beam and the second measuring beam measure the object 11, the three-dimensional light field incident on the object 11 is the sum of the light fields of the two point light sources, the front-focus point light source and the back-focus point light source, which can be expressed as formula (1):

[0080] (1)

[0081] in, Represents the three-dimensional point spread function of the front-focus point light source and the back-focus point light source; represents the light field distribution of the illumination field; +∆z / 2 and -∆z / 2 represent the axial displacement of the pre-focus point light source and the post-focus point light source relative to the focus, respectively; (x0, y0, z0) represent the front focus spatial positions of the third focusing lens 6-1 and the fourth focusing lens 6-2, respectively; r0 represents the position vector of the point light source space; r1 represents the position vector of the space where the object 11 is located; w1 represents the frequency of the light beam passing through the third focusing lens 6-1; w2 represents the frequency of the light beam passing through the fourth focusing lens 6-2; t represents time; j represents the sign representing an imaginary number; i represents the sign representing an imaginary number; h1 represents the point spread function of the illumination field; k represents the wave number of light; z1 represents the axial position of the object 11; M1 represents the magnification; Represents the spatial Dirac function of a point light source.

[0082] The illumination field in the embodiment of the present invention represents that the pre-focus point light source and the post-focus point light source reach the object under test 11 through the heterodyne illumination unit, and the entire process represents the illumination field.

[0083] In the embodiment of the present invention, since the object 11 has fluorescence properties, the illumination wavelength and the emission wavelength of the differential confocal measurement system based on heterodyne illumination are different, so the differential confocal measurement system in the embodiment of the present invention can be considered to be an incoherent system. The light field intensity distribution is used instead in the three-dimensional point spread function equation of the two point light sources. The three-dimensional point spread function of the two point light sources can be expressed as formula (2):

[0084] (2)

[0085] in, It represents the light intensity distribution of the pre-focus photodetector 18-1 or the post-focus photodetector 18-2; U2 represents the light field distribution of the pre-focus photodetector 18-1 or the post-focus photodetector 18-2; r2 represents the spatial position of the pre-focus photodetector 18-1 or the post-focus photodetector 18-2; Δω represents the frequency difference between the two light beams.

[0086] Using normalized optical units, it is expressed as formula (3):

[0087] (3)

[0088] Wherein, u represents the axial normalized coordinate; z represents the axial coordinate; Represents the half numerical aperture angle.

[0089] The fluorescence signal emitted by the object 11 is proportional to the light intensity irradiated on the point. There are three types of fluorescence signals emitted by the object 11: one is excited by two point light sources, and the other is excited by the interference between the two point light sources. The frequency is . Indicates the interference point of the fluorescence signal emitted by the object 11, the pre-focus photodetector 18-1 or the post-focus photodetector 18-2 The total fluorescence signal intensity distribution can be expressed as formula (4):

[0090] (4)

[0091] in, represents the total fluorescence signal intensity distribution of the pre-focus photodetector 18 - 1 or the post-focus photodetector 18 - 2 ; Δu represents the defocus amount.

[0092] S2. The method for obtaining the height information of the object 11 is:

[0093] The pre-focus light intensity signal of the frequency domain limited light spot on the pre-focus photodetector 18-1 and the post-focus light intensity signal of the frequency domain limited light spot on the post-focus photodetector 18-2 are collected respectively, and the differential confocal response signal curve is obtained according to the pre-focus light intensity signal and the post-focus light intensity signal to determine the height information of the object 11 under test.

[0094] S2-01a, adjust the defocus amount Δu of the first pinhole 5-1 and the second pinhole 5-2. As the defocus amount changes, the interference intensity of the first measuring beam and the second measuring beam will also change, such as Figure 4 As shown in the figure, as the defocus amount ∆u increases, the full width at half maximum of the main peak of the interference intensity distribution decreases, and the axial resolution improves. However, the intensity of the first side lobe increases with the increase of ∆u, and finally a double peak phenomenon will appear due to the distance between the two spots being too far. Normalized coordinates, when ∆u=3, the full width at half maximum of its main peak is reduced by 15.31% compared to when ∆u=0. Collect the pre-focus light intensity signal of the frequency domain limited light spot on the pre-focus photodetector 18-1 and the post-focus light intensity signal of the frequency domain limited light spot on the post-focus photodetector 18-2.

[0095] In the embodiment of the present invention, the pre-focus pinhole 17-1 and the post-focus pinhole 17-2 have no axial offset and are infinitesimal, and the light intensity of the pre-focus photodetector 18-1 and the post-focus photodetector 18-2 after interference is expressed as formula (5) and formula (6):

[0096] (5)

[0097] (6)

[0098] Wherein, h2 represents the point spread function of the pre-focus photodetector 18-1 or the post-focus photodetector 18-2; r represents the position vector in space; u d Indicates the defocus amount of the pre-focus photodetector 18-1 or the post-focus photodetector 18-2. The illumination field in the embodiment of the present invention represents a point light source passing through the heterodyne illumination unit to reach the object under test 11, and the entire process represents the illumination field.

[0099] S2-02a, taking the difference between the pre-focus light intensity signal and the post-focus light intensity signal, and obtaining a differential confocal signal curve, which is expressed as formula (7):

[0100] (7)

[0101] like Figure 5 As shown, with the change of the defocus amount Δu, the differential axial intensity of the differential confocal measurement system based on heterodyne illumination will also change. As the defocus amount Δu gradually increases, the slope of the linear interval of the differential confocal signal curve will also gradually increase, so the axial resolution and signal-to-noise ratio of the differential confocal measurement system based on heterodyne illumination are also improved. However, with the further increase of the defocus amount Δu, the slope will become smaller and distorted due to the gradual deviation of the pre-focus point light source and the post-focus point light source, resulting in signal distortion. In practical applications, the defocus amount Δu can be selected according to actual conditions, and the present invention does not limit this.

[0102] S2-03a. Use formula (7) to obtain the coordinates corresponding to the zero-crossing position of the differential confocal signal curve to obtain the height information of the object 11.

[0103] The method for obtaining the tilt angle information of the object 11 is:

[0104] In the embodiment of the present invention, the spot signal on the photodetector 14 is collected, the peak position of the spot signal and the tilt angle are matched one by one, the spot image signal of the object 11 on the photodetector 14 is analyzed, and the tilt angle information of the object 11 is obtained.

[0105] S2-01b, processing the light spot signal collected on the photoelectric detector 14 to obtain a frequency domain limited light spot.

[0106] In the embodiment of the present invention, the photodetector 14 is a CCD array camera. The frequency domain limited light spot area shows a higher gray value on the CCD array camera. The frequency domain limited light spot can be obtained by using an image processing algorithm to process only the high intensity area in the middle of the CCD array camera. A threshold T is set, and the area with an intensity lower than T is removed, and only the high intensity area is retained to obtain the frequency domain limited light spot. For details, see calculation formula (8). In practical applications, the selected photodetector 14 is different, so the method of obtaining the frequency domain limited light spot is also different. The corresponding method can be selected according to the specific situation, and the present invention does not limit this.

[0107] (8)

[0108] in, represents the intensity of the light spot signal collected on the photodetector 14; T represents the threshold value; Indicates the signal intensity of the frequency-domain limited spot.

[0109] S2-02b. Calculate the coordinates of the frequency-domain limited spot when there is no tilt angle , obtain different tilt angles through calibration method , The coordinates of the corresponding frequency-domain limited spot .in, Indicates the tilt angle in the X-axis direction. Indicates the tilt angle in the Y-axis direction.

[0110] In the embodiment of the present invention, step S2-02b calculates the coordinates of the frequency domain limited light spot when there is no tilt angle. The coordinates of the frequency-domain limited light spot corresponding to different tilt angles The weighted center is used for calculation and extraction, and other methods in the prior art may also be used to obtain the coordinates, which is not limited in the present invention. See calculation formula (9) for details.

[0111] (9)

[0112] Among them, x cen The horizontal coordinate of the frequency-domain limited spot; y cen represents the ordinate of the frequency-domain limited light spot, and x and y represent the coordinates of the pixels on the photodetector 14, respectively.

[0113] S2-03b, the offset coordinates and tilt angles of the frequency domain limited spot signal are matched one by one by a fitting method. , ,Will , Offset coordinates with frequency-domain limited spot Polynomial fitting is performed respectively, see calculation formula (10) for details.

[0114] (10)

[0115] in, , represents the coefficient of the kth term of the fitted polynomial; , Represents the kth power of the fitted polynomial x, y; m represents the highest order of the polynomial.

[0116] S2-04b, analyze the spot image signal of the object 11 on the photoelectric detector 14, calculate the coordinates on the photoelectric detector 14 at this time according to formula (9), and compare it with the coordinates of the frequency domain limited spot when there is no tilt angle Perform subtraction to obtain the offset of the object 11 under test at this time , and substitute it into formula (10) to obtain the tilt angle information of the object 11 , .

[0117] S3, correcting the height information according to the tilt angle information of the object 11 and the displacement information of the piezoelectric ceramic displacement stage 19, and the specific calculation is as follows:

[0118] (11)

[0119] in, ;

[0120] Among them, x a ,y a represents the displacement coordinates of the object 11 driven by the piezoelectric ceramic displacement stage 19; z represents the height information of the object 11; ( ) represents the coordinate after tilt correction; Indicates the height information after tilt angle correction; , Respectively represent the tilt angles of the object 11 in the X-axis direction and the Y-axis direction; Indicates that (x a ,y a , z) first rotate around the X axis , and then rotate around the Y axis The rotation matrix of .

[0121] The present invention provides a differential confocal measurement system based on heterodyne illumination and a measurement method thereof. The heterodyne illumination principle is utilized, and the frequency difference of the light beam and the axial offset of the light source are used as key factors for controlling the axial resolution through a heterodyne illumination unit, so that a higher resolution capability can be achieved. Compared with a 4Pi microscope, the present invention does not require a complex dual objective lens structure, thereby reducing the system complexity and environmental sensitivity. Compared with a structured light microscope, the present invention does not require a complex image reconstruction algorithm and a high-precision grating projection, so that a high imaging speed is maintained while improving the resolution. The added angle measurement unit can measure and compensate for the tilt angle of the sample in real time, further improving the measurement accuracy. The differential confocal measurement system based on heterodyne illumination and the measurement method thereof provided by the present invention can improve the axial resolution and signal-to-noise ratio of the differential confocal measurement system by increasing the linear interval slope of the differential confocal signal curve.

[0122] The embodiment of the present invention verifies the effectiveness of the technical solution of the present invention through comparative experiments. A standard plane mirror is used, and the surface accuracy standard PV value is 3.56μm and the RMS value is 0.89μm. The parameters of the differential confocal measurement system based on heterodyne illumination are as follows: the wavelength of the dual-frequency laser light source 1 is 633nm, the beam width of the dual-frequency laser light source 1 is 2.4mm, the focal length of the objective lens 10 is 3.3mm, the focal lengths of the third focusing lens 6-1 and the fourth focusing lens 6-2 are both 45mm, and the sizes of the pre-focus pinhole 17-1 and the post-focus pinhole 17-2 are both 15μm. Figure 5 It can be seen that a reasonable selection of the defocus amount Δu can improve the axial resolution of the differential confocal measurement system. Normalized coordinates, when ∆u=2, the axial resolution of the differential confocal measurement system based on heterodyne illumination provided by the present invention is 73 nm, which is 6% higher than the axial resolution of the measurement system in the prior art. At the same time, the technical solution of the present invention adds a correction step to correct the height information by measuring the tilt angle. After the tilt angle correction, the surface accuracy standard PV value is 3.85μm, the RMS value is 0.94μm, and the accuracy is improved by about 14%. This improvement effectively reduces the error caused by the surface of the object 11 to be measured, significantly improves the measurement accuracy, and further improves the overall performance of the system by optimizing the axial resolution.

[0123] Through experimental comparison, the measurement method provided by the embodiment of the present invention is compared with the traditional measurement method. The comparison results are as follows:

[0124] Comparison Table

[0125] name PV (μm) RMS (μm) Standard plane mirror 3.56 0.89 The present invention measures 3.85 0.94 Traditional solution measurement 3.25 0.81

[0126] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0127] Although the embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and cannot be understood as limiting the present invention. Those skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

[0128] The above specific implementations of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A differential confocal measurement system based on heterodyne illumination, characterized in that: It includes a heterodyne illumination unit, a differential confocal unit and an angle measurement unit; wherein, The heterodyne illumination unit is used to provide a front-focus point light source and a rear-focus point light source of different frequencies; The differential confocal unit is arranged in the emission direction of the light beam of the heterodyne illumination unit, and is used to receive the pre-focus light intensity signal and the post-focus light intensity signal of the light beam reflected by the object to be measured (11), and obtain the height information of the object to be measured (11); The angle measurement unit is arranged in the emission direction of the light beam of the heterodyne illumination unit, and is used to receive a light spot signal formed by the light beam reflected by the object to be measured (11), and obtain the tilt angle information of the object to be measured (11); The heterodyne illumination unit comprises a dual-frequency laser light source (1), a first beam splitter (2), a first plane mirror (3), a first focusing lens (4-1), a first pinhole (5-1), a third focusing lens (6-1), a second focusing lens (4-2), a second pinhole (5-2), a fourth focusing lens (6-2), a second plane mirror (7), and a second beam splitter (8) which are sequentially arranged along an optical path; wherein: The dual-frequency laser light source (1) is used to emit laser light beams containing different frequencies; The first beam splitter (2) is used to split light beams of different frequencies; The first plane mirror (3) and the second plane mirror (7) are respectively used to change the propagation direction of the light beam; The first focusing lens (4-1) and the second focusing lens (4-2) are respectively used to focus the light beam; The first pinhole (5-1) and the second pinhole (5-2) are respectively used for spatial filtering to block the out-of-focus light beam and form a pre-focus point light source and a post-focus point light source; The third focusing lens (6-1) and the fourth focusing lens (6-2) are respectively used to expand the light beam; The second beam splitter (8) is used to combine two light beams with different frequencies.

2. The differential confocal measurement system according to claim 1, characterized in that: The differential confocal unit comprises a dichroic mirror (9), an objective lens (10), an objective lens driver, a third beam splitter (12), and a differential assembly which are sequentially arranged along the optical path; The differential component comprises a fourth beam splitter (15), a pre-focus focusing lens (16-1), a pre-focus pinhole (17-1), a pre-focus photodetector (18-1), a post-focus focusing lens (16-2), a post-focus pinhole (17-2), and a post-focus photodetector (18-2); wherein, The dichroic mirror (9) is used to distinguish the excitation light beam and the illumination light beam of the object to be measured (11); The objective lens (10) is used to focus the light beam onto the surface of the object to be measured (11); The objective lens (10) driver is used to move the objective lens (10) along the direction of the light beam, thereby controlling the focusing position of the light beam on the object to be measured (11); The third beam splitter (12) and the fourth beam splitter (15) are used to split the light beam respectively; The pre-focus focusing lens (16-1) and the post-focus focusing lens (16-2) are respectively used to focus the light beam; The pre-focus pinhole (17-1) and the post-focus pinhole (17-2) are used for spatial filtering to block out-of-focus light beams; The pre-focus photoelectric detector (18-1) and the post-focus photoelectric detector (18-2) are used to receive the pre-focus light intensity signal and the post-focus light intensity signal respectively, and then analyze the height information of the object to be measured (11).

3. The differential confocal measurement system according to claim 2, characterized in that: The angle measurement unit comprises a fifth focusing lens (13), a photodetector (14) and a piezoelectric ceramic displacement stage (19) which are sequentially arranged along the optical path; The fifth focusing lens (13) is used to focus the light beam; The photoelectric detector (14) is used to receive the light spot signal and then analyze the tilt angle information of the object to be measured (11); The piezoelectric ceramic displacement stage (19) is used to move the object to be measured (11).

4. A measurement method using the differential confocal measurement system based on heterodyne illumination as claimed in claim 3, characterized in that: The measurement is realized by a differential confocal measurement system based on heterodyne illumination, and the measurement method comprises the following steps: S1, a pre-focus point light source and a post-focus point light source of different frequencies respectively emit a first measuring light beam and a second measuring light beam to measure the object to be measured (11), after being reflected by the object to be measured (11), the first measuring light beam and the second measuring light beam carrying information of the object to be measured (11) are reflected to the angle measurement unit and the differential confocal unit, the first measuring light beam and the second measuring light beam carrying information of the object to be measured (11) interfere with each other, and form a frequency domain limited light spot; S2、 The method for obtaining the height information of the measured object (11) is: Respectively collecting a pre-focus light intensity signal of a frequency-domain limited light spot on a pre-focus photodetector (18-1) and a post-focus light intensity signal of a frequency-domain limited light spot on a post-focus photodetector (18-2), and obtaining a differential confocal response signal curve based on the pre-focus light intensity signal and the post-focus light intensity signal to determine height information of the object under test (11); The method for obtaining the tilt angle information of the object to be measured (11) is: The light spot signal on the photodetector (14) is collected, the peak position of the light spot signal and the tilt angle are matched one by one, the light spot image signal of the object to be measured (11) on the photodetector (14) is analyzed, and the tilt angle information of the object to be measured (11) is obtained.

5. The measuring method according to claim 4, characterized in that: The method of obtaining the height information in step S2 is specifically as follows: S2-01a, adjusting the defocusing amount of the first pinhole (5-1) and the second pinhole (5-2), collecting the pre-focus light intensity signal of the frequency domain limited light spot on the pre-focus photodetector (18-1), and the post-focus light intensity signal of the frequency domain limited light spot on the post-focus photodetector (18-2); S2-02a, subtracting the pre-focus light intensity signal from the post-focus light intensity signal to obtain a differential confocal signal curve; S2-03a. Obtain the height information of the object (11) to be measured based on the coordinates corresponding to the zero-crossing position of the differential confocal signal curve.

6. The measuring method according to claim 5, characterized in that: The method of tilting the angle information in step S2 is specifically as follows: S2-01b, processing the light spot signal collected on the photoelectric detector (14) to obtain a frequency domain limited light spot; S2-02b, calculating the coordinates of the frequency domain limited light spot when there is no tilt angle, and obtaining the coordinates of the frequency domain limited light spot corresponding to different tilt angles by a calibration method; S2-03b, using a fitting method to make the offset coordinates and tilt angles of the frequency domain limited spot signal correspond one to one; S2-04b, analyzing the light spot image signal of the object to be measured (11) on the photoelectric detector (14) to obtain the tilt angle information of the object to be measured (11).

7. The measuring method according to claim 6, characterized in that: It also includes the correction steps for the height information: S3, correcting the height information according to the tilt angle information and height information of the measured object (11) and the displacement information of the piezoelectric ceramic displacement stage (19), the specific calculation formula is: in, Wherein, x and y represent the displacement coordinates of the object to be measured (11) driven by the piezoelectric ceramic displacement stage (19); z represents the height information of the object to be measured (11); (x′, y′, z′) represent the coordinates after tilt correction; z′ represents the height information after tilt angle correction; θ x ,θ y Respectively represent the tilt angles of the object (11) to be measured in the X-axis direction and the Y-axis direction; R y (θ y )R x (θ x ) means rotating the (x, y, z) coordinates around the X axis by θ x , and then rotate θ around the Y axis y The rotation matrix of .

Images