A fixed-focus interferometric confocal measurement system

Abstract

This invention proposes a fixed-focus interferometric spectral confocal measurement system, belonging to the field of ultra-precision measurement. It solves the problems of weakened interference effects and severe signal crosstalk leading to decreased resolution in existing measurement methods. It includes a light source for generating a uniform broadband light source; a beam splitter for dividing the light source into a measurement optical path and a reference optical path; a dispersive component and the surface of the object under test arranged sequentially along the measurement optical path; and an adjustable-focus optical lens assembly and a reference reflection assembly arranged sequentially along the reference optical path. The reference optical path is dispersed to different focal planes by the dispersive component, and the light reflected from the surface of the object under test is collected by the dispersive component and reflected by the beam splitter to a second beam-limiting assembly; the adjustable-focus optical lens assembly ensures that only a single wavelength is in quasi-focus at a given moment; and a spectral demodulation module for calculating the interference fringes generated by the two types of light. It is primarily used for ultra-precision measurement.

Description

Technical Field

[0001] This invention belongs to the field of ultra-precision measurement, and in particular relates to a fixed-focus interferometric spectral confocal measurement system. Background Technology

[0002] Optical microscopes, due to their advantages of being non-contact, non-destructive, and highly precise, have become indispensable tools in the field of industrial measurement. With the rapid development of precision manufacturing, the demand for high-precision inspection is increasing year by year. Spectral confocal sensors are currently the most widely used type of sensor. Compared to laser confocal sensors, spectral confocal sensors use dispersive objectives to focus a broad-spectrum light source to different positions, and achieve rapid, high-precision displacement and thickness measurement without axial scanning through spectral demodulation.

[0003] Although spectral confocal signals do not require axial scanning, the reflected light signals suffer from signal broadening and reduced signal-to-noise ratio compared to traditional laser confocal signals due to the use of multiple wavelengths focused on different object surfaces, making it difficult to achieve nanometer-level resolution.

[0004] To address this issue, existing technologies include signal compression using pupil filtering and improving the signal-to-noise ratio through multi-signal averaging. While these methods have improved measurement resolution to some extent, they still fall short of the requirements for nanometer-level precision detection. Interferometric confocal spectroscopy is a feasible method that achieves nanometer-level resolution by detecting the phase information between the optical signals of the measuring arm and the reference arm. However, existing interferometric confocal spectroscopy methods suffer from weakened interference effects and severe signal crosstalk because the measuring arm returns a narrow-band signal while the reference arm returns a broad-spectrum signal. This limits the actual measurement resolution to the sub-micrometer level. Summary of the Invention

[0005] In view of this, the present invention aims to propose a fixed-focus interferometric spectral confocal measurement system to solve the problems of weakened interference effect and severe signal crosstalk leading to decreased resolution in existing measurement methods.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a fixed-focus interferometric confocal measurement system, comprising:

[0007] The light source is used to generate a uniform broadband light source, and the generated light is obtained as a micron-scale broadband light source after passing through the first beam-limiting component;

[0008] A beam splitter is used to split a micrometer-scale broadband light source into a measurement optical path and a reference optical path. Along the measurement optical path, a dispersive component and the surface of the object under test are arranged sequentially. Along the reference optical path, an adjustable-focus optical lens assembly and a reference reflection assembly are arranged sequentially. The reference optical path is dispersed to different focal planes by the dispersive component. The light reflected from the surface of the object under test is collected by the dispersive component and reflected by the beam splitter to a second beam-limiting assembly. The reference optical path is focused onto the reference reflection assembly by the adjustable-focus optical lens assembly. The light returned from the reference reflection assembly is collected by the adjustable-focus optical lens assembly and then transmitted through the beam splitter to the second beam-limiting assembly. The adjustable-focus optical lens assembly has a variable focal length, ensuring that only a single wavelength is in focus at any given time.

[0009] The spectral demodulation module receives the measurement light and reference light after passing through the second beam-limiting component, and calculates the interference fringes generated by the two types of light.

[0010] Furthermore, the dispersive component is a dispersive objective lens, and the dispersive distance between its maximum wavelength and minimum wavelength is the range of the system.

[0011] Furthermore, the spherical aberration of the dispersive objective lens is corrected so that light focused to all focal points within the range reaches the diffraction limit.

[0012] Furthermore, both the first and second beam-limiting components are pinholes.

[0013] Furthermore, the adjustable focus optical lens assembly has the same wavelength range as the dispersive assembly.

[0014] Furthermore, the adjustable focusing optical lens assembly is a liquid lens or a liquid crystal regulator.

[0015] Furthermore, the adjustable focus optical lens assembly is equipped with an aberration correction lens group to achieve a diffraction-limited spot size when different wavelengths are focused on the reference reflection assembly, thereby suppressing defocus signals.

[0016] Furthermore, the optical path difference introduced by the adjustable focus optical lens assembly and the aberration correction lens group is consistent with the optical path difference introduced by the dispersion assembly.

[0017] Furthermore, the beam-splitting component is a polarization-insensitive beam-splitting prism, which achieves maximum contrast when the ratio of the reflected signal to the reference signal is 1:1.

[0018] Furthermore, the spectral demodulation module is equipped with a signal display module.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] 1. This system uses a focusable optical lens assembly with adjustable focal length, which ensures that the wavelength of the signal light and the reference light are always consistent, effectively shielding the interference of defocused light, stray light, light of other spectral bands and various non-interference light on the interference detection signal; thereby significantly improving the contrast and signal-to-noise ratio of the interference signal, providing a reliable data foundation for subsequent accurate extraction of phase information, and greatly optimizing the axial resolution capability of the system.

[0021] 2. Based on the significant improvement in the system's axial resolution, this system can accurately capture the nanoscale micromorphological changes of the sample under test, breaking through the measurement accuracy limitations of traditional interferometric detection technology and effectively expanding the applicable scenarios and application scope of interferometric morphology detection technology.

[0022] 3. Compared with traditional mechanical scanning interferometric detection systems, the liquid lens introduced in this invention has the technical advantages of fast response speed and large dynamic range, which can significantly improve the system's signal acquisition and data processing efficiency, effectively increase the overall data throughput of the system, and adapt to the needs of high-speed and continuous detection operations.

[0023] 4. This invention achieves focal length control by driving a liquid lens with voltage, eliminating the mechanical scanning motion structure of traditional equipment. This fundamentally avoids the vibration and shaking problems caused by mechanical motion, effectively reducing system measurement errors and significantly improving the operational stability and measurement reliability of the detection device. Attached Figure Description

[0024] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0025] Figure 1 This is a schematic diagram of the structure of a fixed-focus interferometric confocal measurement system according to the present invention;

[0026] Figure 2 This is a schematic diagram of the fixed-focus interferometric confocal measurement system of the present invention when measuring position 1;

[0027] Figure 3 This is a schematic diagram of the fixed-focus interferometric confocal measurement system of the present invention when measuring position 2;

[0028] Figure 4 This is a schematic diagram illustrating the working principle of the adjustable focus optical lens assembly described in this invention.

[0029] Figure 5 This is a schematic diagram of the distribution of a traditional interferometric confocal signal.

[0030] Figure 6 This is a schematic diagram of the signal distribution of this system;

[0031] Figure 7 This is a flowchart of the system's workflow.

[0032] Light source 1; first beam limiting component 2; beam splitting component 3; adjustable focus optical lens component 4; reference reflection component 5; dispersion component 6; surface of the object under test 7; spectral demodulation module 8; signal display module 9; second beam limiting component 10. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other, and the described embodiments are only some embodiments of the present invention, not all embodiments.

[0034] It should be noted that the descriptions of "left," "right," "left side," "right side," "upper part," "lower part," "top," and "bottom" in this invention are defined based on the orientation or positional relationships shown in the accompanying drawings. They are merely for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the described structure must be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0035] In the description of this invention, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0036] Referring to the accompanying drawings, this embodiment of the invention provides a fixed-focus interferometric confocal measurement device for high-precision displacement measurement, which, when used with a transverse scanning stage, can achieve three-dimensional contour measurement.

[0037] like Figure 1 As shown, a fixed-focus interferometric confocal measurement system includes:

[0038] 1. Light source, 2. First beam limiting component, 3. Beam splitting component, 4. Adjustable focus optical lens component, 5. Reference reflection component, 6. Dispersion component, 7. Surface of the object under test, 8. Spectral demodulation module, 9. Signal display module, and 10. Second beam limiting component.

[0039] Specifically, the light source 1, the first beam-limiting component 2, the beam-splitting component 3, and the dispersion component 6 are arranged sequentially from top to bottom. The adjustable focusing optical lens component 4 and the reference reflection component 5 are located on one side of the beam-splitting component 3 and are arranged sequentially from the side closest to the beam-splitting component 3 to the side furthest away. The second beam-limiting component 10 and the spectral demodulation module 8 are located on the other side of the beam-splitting component 3 and are arranged sequentially from the side closest to the beam-splitting component 3 to the side furthest away. The light source 1 is used to generate a uniform broadband light source. It is a broadband light source, and its bandwidth can be determined by a broadband filter. This broadband light source can be composed of a high-power LED, a laser-excited white light source, a laser-excited plasma white light source, or a supercontinuum light source. Both the first beam-limiting component 2 and the second beam-limiting component 10 are pinholes used to obtain a micrometer-sized broadband light source. Other types of components can also be used depending on the specific requirements; any component capable of obtaining a micrometer-sized broadband light source can be used in this application. The pinhole size needs to be controlled within tens of micrometers. Smaller pinhole sizes are more conducive to controlling and improving system resolution, but smaller pinholes also lead to rapid light intensity attenuation. Therefore, a reasonable size needs to be maintained. The pinhole can also be directly replaced by an optical fiber, with a selection made according to the actual situation. The beam-splitting component 3, in conjunction with a polarization-insensitive beam-splitting prism, is responsible for splitting the incident light into a measurement path and a reference path according to a certain ratio. This ratio can be determined by the characteristics of the measured surface material. Maximum contrast can be achieved when the ratio of the reflected signal to the reference signal is 1:1.

[0040] The adjustable-focus optical lens assembly 4 is specifically configured as a liquid lens, located on the reference arm. Its main function is to focus and collect reference light, enabling high-contrast interference between the signal light and reference light entering the spectral demodulation module. The liquid lens has an adjustable focal length, and its focusing function is to focus light of different wavelengths onto the same reference mirror plane, ensuring that only a single wavelength is in focus at any given time, while other wavelengths are out of focus. The liquid lens's applicable wavelength range is the same as the working wavelength range of the dispersive assembly 6, ensuring that when all wavelengths within the working range of the dispersive assembly 6 are focused on the object surface, the liquid lens can focus the same wavelength of light focused on the object surface onto the reference reflection assembly 5. The liquid lens includes an aberration correction lens group, enabling the spot size of different wavelengths focused on the reference mirror to reach the diffraction limit, ensuring suppression of other out-of-focus signals. The reference reflection assembly 5 is specifically configured as a reference plane mirror.

[0041] The dispersive assembly 6, specifically a dispersive objective lens, is located on the measuring arm. It is responsible for dispersing light of different wavelengths onto different focal planes. The dispersive distance between the maximum and minimum wavelengths determines the system's measurement range. The dispersive objective lens needs to have a high magnification, typically 5-10x. A higher magnification allows for a larger pinhole size, increasing the amount of light entering the system and improving the light source utilization rate. On the other hand, the dispersive objective lens needs to correct for spherical aberration, ensuring that light focused at all focal points within the measurement range reaches the diffraction limit and maintaining signal interference contrast. The optical path difference introduced by the liquid lens and its associated aberration correction lens group is consistent with the optical path difference introduced by the dispersive objective lens, ensuring that the reference light and signal light are within the coherence length.

[0042] When using this device for measurement, the light source 1 is first modulated into a micrometer-sized point light source after passing through the first beam-limiting component 2. It is then split into two parts by the beam-splitting component 3. One part—the signal light—is received by the dispersion component 6, dispersed, and then focused onto different focal planes along the axis. The light reflected from the surface 7 of the object under test is also collected by the dispersion component 6, then reflected by the beam-splitting component 3 to the second beam-limiting component 10. The other part—the reference light—is received by the adjustable-focus optical lens component 4 and focused onto the reference reflection component 5. The returned light is also collected by the adjustable-focus optical lens component 4 and passes through the beam-splitting component 3 into the first beam-limiting component 2. The signal light and the reference light enter the spectral demodulation module 8 and interfere to produce fringes. The high-density fringes are calculated by resolving the phase of the fringes. The spectral demodulation module 8 works similarly to a spectrometer; it needs to completely record the high-density fringes formed by the incoming signal light and reference light. The accuracy of its phase calculation of the interference signal directly affects the system's resolution. The signal display module 9 is used to display the interference signal.

[0043] like Figure 2 As shown, when the fixed-focus interferometric spectroscopy confocal measurement system operates at wavelength λ1, the adjustable focusing optical lens assembly 4 automatically micro-focuses wavelength λ1 onto the reference reflection assembly 5, while other wavelengths λ2 are in a defocused state. At this time, the height information of the surface 7 of the object under test can be calculated from the interference signal displayed by the signal display module 8. The signal envelope center can be used to calculate the submicron level height of the object surface, while the fringe phase information can be used to calculate the nanometer level height information.

[0044] like Figure 3 As shown, when the fixed-focus interferometric spectroscopy confocal measurement system operates at wavelength λ2, the adjustable focusing optical lens assembly 4 automatically micro-focuses wavelength λ2 onto the reference reflection assembly 5, while other wavelengths λ1 are in a defocused state. The height information of the surface 7 of the measured object can be calculated from the interference signal displayed by the signal display module 8. At this time, the center of the signal envelope is relative to... Figure 2 Having moved a certain distance, the difference between the two envelope centers can be used to calculate the difference between the current object's height and the previous object's height.

[0045] like Figure 4 As shown, the working modes of liquid lenses include voltage-driven liquid surface curvature deformation type and current-driven thin-film elastic deformation type. The first type is shown here, with a response speed in the millisecond range. By improving the structural design, the viscosity coefficient can be reduced, the response time of the liquid lens can be shortened, and the measurement speed of the system can be improved. The working principle of the liquid lens is to change the curvature of the liquid surface by using different voltages, so that it has different focal lengths, thereby changing the focusing position of different lights. Other devices also have this function, such as liquid crystal controllers, which will not be elaborated on here. Such modules can all be used in this application and are all within the spirit of this invention.

[0046] like Figure 5 As shown, the interference signal generated by the traditional confocal interferometric method has a DC component. These DC components directly reduce the contrast of the fringes and cause the phase of the solution to be unstable and inaccurate. The presence of these DC components means that the signal light and the reference light can only interfere within a certain range, which limits the measurement range of the confocal interferometric method and causes the measurement resolution to remain at the submicron level.

[0047] like Figure 6 As shown, the interference signal generated by the fixed-focus interferometric spectral confocal measurement system proposed in this patent completely suppresses the DC component, and the contrast of the interference signal can be greatly improved. Furthermore, due to the optimized design of the optical path difference between the reference arm and the probe, it has good interference contrast throughout the entire measurement range, making the system's measurable range larger and its resolution higher.

[0048] like Figure 7 The diagram illustrates the workflow of a fixed-focus interferometric spectral confocal measurement system. When the height of the object's surface changes, the center of the light spot entering the spectral demodulation module shifts. The demodulation module first calculates the submicron-level height information H0 of the object's surface, then converts this height information into voltage information and transmits it to the liquid lens driver. The driver adjusts the curvature of the liquid lens according to the input voltage, causing the wavelength currently focused on the object's surface to be focused onto the reference plane mirror via the reference arm. Finally, the interference signal is collected, and the nanometer-level height information H is calculated using the phase information from the interference signal. p In this way, by using coarse positioning plus fine positioning, nanometer-level height information H0+H within a millimeter-level range can be obtained. p .

[0049] This application, by employing a focusable optical lens assembly 4 with adjustable focal length, ensures that the wavelengths of the signal light and the reference light remain consistent, effectively shielding the interference detection signal from defocused light, stray light, light in other spectral bands, and various non-interference light. This significantly improves the contrast and signal-to-noise ratio of the interference signal, providing a reliable data foundation for subsequent accurate extraction of phase information and greatly optimizing the axial resolution of the system. Simultaneously, thanks to the adjustment of the parameters of the focusable optical lens assembly 4 and the dispersive assembly 6, when all wavelengths within the working range of the dispersive objective lens are focused on the object surface, the liquid lens can focus the same wavelength of light focused on the object surface onto the reference plane mirror.

[0050] This precise wavelength matching focusing mechanism completely solves the core technical pain points of focusing offset and optical path matching misalignment of beams of different wavelengths in traditional interferometric detection equipment, achieving a high degree of coordinated and unified precision between the detection optical path and the reference optical path in terms of wavelength, focal length, and optical path accuracy. In traditional detection systems, under the broadband working mode of the dispersive objective lens, there is an inherent deviation in the focusing position of beams of different wavelengths, which can easily cause the reference light and the detection signal light to not correspond precisely, leading to problems such as interference fringe distortion, offset, and blurring, resulting in systematic errors in phase detection and severely restricting the accuracy and stability of precision detection. However, this application, through the coordinated control of dual component parameters, can achieve bidirectional precise adaptation between the object surface detection focusing and the reference plane standard focusing for each characteristic wavelength within the dispersive working bandwidth, eliminating the dispersive matching error caused by broadband detection from the source of the optical path.

[0051] Building upon this foundation, the dynamic focal length adjustment characteristic of the adjustable-focus optical lens assembly 4 can adapt to changes in object distance, differences in the surface morphology of the sample under test, and optical path assembly deviations under different detection conditions, possessing extremely strong adaptive calibration capabilities. When the sample under test exhibits slight undulations, the detection platform experiences minor displacement deviations, or external factors such as ambient temperature and vibration cause slight optical path shifts, the adjustable-focus optical lens assembly can fine-tune the focal length parameters in real time, dynamically correcting the beam focus point and maintaining the same wavelength beam focusing consistency at the detector and reference ends, thus avoiding interference signal attenuation and distortion problems caused by optical path shifts. Compared to traditional fixed-focus optical lens modules, this completely avoids the drawbacks of fixed optical paths being unable to adapt to dynamic detection scenarios and requiring repeated manual calibration of the optical path, significantly reducing equipment debugging difficulty and detection errors caused by environmental interference. Simultaneously, by adjusting it, it ensures that only a single wavelength is in quasi-focus at a certain moment, while other wavelengths are in defocus. This effectively suppresses the influence of defocused light, other spectral bands of light, stray light, and non-interference light on the interference signal, thereby greatly improving the contrast and signal-to-noise ratio of the detected interference signal, which is beneficial for subsequent phase extraction and effectively improves the axial resolution of the system.

[0052] Meanwhile, by leveraging the spectral sorting and adaptation characteristics of the dispersive component 6, the system can accurately distinguish between effective interference wavelengths and ineffective stray wavelengths. Combined with the precise focusing and filtering of the adjustable lens, this further enhances the shielding effect of non-target beams, preventing redundant spectral bands and diffuse reflection stray light from participating in interferometric imaging during broadband operation. This collaborative optimization mechanism allows the system to output clear, high-signal-to-noise ratio, and highly stable interference fringe images even in wide-band, high-precision, and complex detection scenarios. This not only further solidifies the accuracy foundation for subsequent data processing tasks such as phase calculation, topography reconstruction, and displacement detection, but also effectively improves the system's lateral detection uniformity and longitudinal detection resolution.

[0053] In addition, this structural design achieves effective interference utilization across the entire wavelength range in a wide spectral range, overcoming the limitation of traditional equipment that can only rely on a single narrow-band beam for detection. This greatly expands the detection spectral range and applicable scenarios of the equipment. Whether it is the detection of the micro-morphology of the surface of precision optical components, the measurement of microstructure dimensions, or the detection of the thickness of transparent dielectric layers and the precise monitoring of minute deformations, this system can achieve efficient, accurate, and stable automated detection thanks to its precise optical path matching, excellent anti-interference capabilities, and ultra-high resolution. This significantly improves the detection range, detection accuracy, and environmental adaptability of the equipment, effectively making up for the technical shortcomings of traditional interferometric detection equipment in terms of wide spectral adaptability, dynamic operating condition calibration, and resistance to environmental interference, and greatly enhancing the overall detection performance and engineering application value of the equipment.

[0054] The embodiments of the present invention disclosed above are merely illustrative of the invention. These embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention.

Claims

1. A fixed-focus interferometric confocal measurement system, characterized in that, include: The light source (1) is used to generate a uniform broadband light source, and the generated light is obtained as a micron-level broadband light source after passing through the first beam-limiting component (2); The beam splitter (3) is used to split the micron-scale broadband light source into a measurement optical path and a reference optical path. Along the measurement optical path, a dispersive component (6) and the surface of the object to be measured (7) are arranged in sequence. Along the reference optical path, a tunable optical lens assembly (4) and a reference reflection assembly (5) are arranged in sequence. The reference optical path is dispersed to different focal planes by the dispersive component (6). The light reflected from the surface of the object to be measured (7) is collected by the dispersive component (6) and reflected by the beam splitter (3) to the second beam limiting assembly (10). The reference optical path is focused on the reference reflection assembly (5) by the tunable optical lens assembly (4). The light returned by the reference reflection assembly (5) is collected by the tunable optical lens assembly (4) and transmitted through the beam splitter (3) to the second beam limiting assembly (10). The tunable optical lens assembly (4) has a variable focal length so that only a single wavelength is in focus at a certain moment. The spectral demodulation module (8) receives the measurement light and reference light passing through the second beam-limiting component (10) and calculates the interference fringes generated by the two types of light.

2. The fixed-focus interferometric confocal measurement system according to claim 1, characterized in that: The dispersive component (6) is a dispersive objective lens, and the dispersive distance between its maximum wavelength and minimum wavelength is the range of the system.

3. The fixed-focus interferometric confocal measurement system according to claim 2, characterized in that: The spherical aberration of the dispersive objective lens is corrected so that light focused to all focal points within the range reaches the diffraction limit.

4. The fixed-focus interferometric confocal measurement system according to claim 1, characterized in that: Both the first beam limiting component (2) and the second beam limiting component (10) are pinholes.

5. The fixed-focus interferometric confocal measurement system according to claim 1, characterized in that: The adjustable focus optical lens assembly (4) has the same wavelength range as the dispersive assembly (6).

6. The fixed-focus interferometric confocal measurement system according to claim 1, characterized in that: The adjustable focus optical lens assembly (4) is a liquid lens or a liquid crystal regulator.

7. The fixed-focus interferometric confocal measurement system according to claim 1, characterized in that: The adjustable focus optical lens assembly (4) is equipped with an aberration correction lens group to achieve the diffraction limit of the spot size focused on the reference reflection assembly (5) for different wavelengths, and to suppress the defocus signal.

8. The fixed-focus interferometric confocal measurement system according to claim 6, characterized in that: The optical path difference introduced by the adjustable focus optical lens assembly (4) and the aberration correction lens group is consistent with the optical path difference introduced by the dispersive assembly (6).

9. The fixed-focus interferometric confocal measurement system according to claim 1, characterized in that: The beam splitting component (3) is a polarization-insensitive beam splitting prism, which achieves maximum contrast when the ratio of the reflected signal to the reference signal is 1:

1.

10. A fixed-focus interferometric confocal measurement system according to any one of claims 1-9, characterized in that: The spectral demodulation module (8) is equipped with a signal display module (9).

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