Parallel scanning detection device and method for nanoscale defects on surface of large-aperture optical element

Abstract

A kind of large aperture optical element surface nanoscale defect parallel scanning detection device, comprising: single frequency ultraviolet laser, for generating detection laser beam;Daman beam splitter grating, for splitting the laser beam emitted by single frequency ultraviolet laser, generates the detection sub-beam with uniform intensity distribution;Multi-focal point spatial filtering system, after focusing each detection sub-beam, carries out spatial filtering, filters out high-frequency stray light, forms the detection sub-beam of '' clean '';Scanning detection system is used to scan and record surface defects and its spatial distribution to the optical element to be detected.By controlling the scanning speed of scanning mirror, CCD camera turntable, the fast detection of large aperture optical element surface defect is realized.The present application adopts ultraviolet coherent laser light source and combines parallel beam scanning technology, solves the high resolution fast detection problem of nanoscale defect on the surface of large aperture optical element, and provides technical support for the processing and detection of super-smooth large aperture optical element.

Description

Technical Field

[0001] This invention relates to the field of surface defect detection for large-aperture optical elements, and particularly to the surface detection of large-aperture off-axis parabolic mirrors, aspherical lenses, and other optical elements used in the fabrication of diffraction gratings using holographic interference exposure. Background Technology

[0002] Large-aperture diffraction gratings are core components of systems such as petawatt ultra-intense and ultra-short laser devices, high-energy spectral combined laser weapons, and high-precision displacement measurement workpiece stages. Their fabrication process mainly includes photoresist coating, interference exposure, development, etching, and coating replication. Among these, interference exposure is a crucial step in diffraction grating fabrication. The basic principle is to use two large-aperture parallel ultraviolet laser beams to interfere on the surface of a grating substrate coated with photoresist. The resulting interference fringe pattern is recorded inside the photoresist, forming a latent image of the photoresist grating. Subsequent development and etching remove the exposed photoresist, ultimately forming a photoresist grating mask that serves as the master for subsequent etching or coating replication processes.

[0003] The quality of the exposure field is a key factor determining the quality of photoresist grating masks. Any stray light in the light field will be recorded inside the photoresist, forming defects. Furthermore, since the laser source used for exposure is a coherent ultraviolet light source, the interference between stray light and the main light field is amplified, creating interference noise that severely affects the quality of the photoresist grating mask. Moreover, with the development of high-power laser technology, increasingly stringent requirements are being placed on the surface defects and aperture of diffraction gratings. Currently, the mainstream technique for fabricating large-aperture diffraction gratings (meter-scale and above) is to use a beam-expanding collimating optical system to expand the narrow ultraviolet beam emitted by the laser into a large-aperture beam (meter-scale or above), and then use two large-aperture beams for interference exposure. In this technique, large-aperture off-axis parabolic mirrors and lenses are the core optical components of the exposure system, and their surface defects are a key factor affecting the collimated interference light field. Therefore, high-precision inspection of the surfaces of large-aperture off-axis parabolic mirrors and lens elements is an important step in guiding the ultra-smooth processing of large-aperture optical elements, constructing large-aperture holographic interferometry exposure systems, and developing large-aperture high-quality diffraction gratings.

[0004] Currently, methods for detecting surface defects in optical components, both domestically and internationally, can be broadly categorized into imaging methods and energy methods. Visual inspection, grazing incidence, and filtered imaging are examples of imaging methods, while energy methods primarily include scattering energy analysis and spectral analysis. Due to limitations in detection accuracy, these methods can only detect defects much larger than the incident light wavelength, such as scratches, pits, and edge breaks. The corresponding detection equipment is also relatively mature. However, for large-aperture optical components used in holographic interferometry, the surface defect size must be controlled at the nanometer scale, which the aforementioned methods cannot meet. Scanning tunneling microscopes and atomic force microscopes offer high resolution, enabling the detection of defects at the nanometer or even sub-nanometer scale. They can directly observe the shape of defects and even derive the three-dimensional morphology of the defect surface. However, these methods have a very small field of view, typically at the micrometer level, making it impossible to measure the macroscopic features of defects. Furthermore, their measurement speed is slow and maintenance costs are high, making them unsuitable for rapid, full-aperture detection of large-aperture optical components. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a parallel scanning detection device and method for nanoscale defects on the surface of large-aperture optical elements. By using a coherent ultraviolet laser source combined with a Damman beam splitter grating and a scanning mirror, the rapid detection of surface defects of large-aperture optical elements is achieved. Due to the use of a coherent ultraviolet laser source, the scattered light caused by the defect is enhanced by diffraction interference, which "magnifies" the defect and improves the resolution of defect detection. Furthermore, the use of Damman beam splitter parallel scanning greatly improves the defect detection efficiency.

[0006] The technical solution of the present invention is as follows:

[0007] A parallel scanning detection device for nanoscale defects on the surface of large-aperture optical elements, characterized in that it includes: a single-frequency ultraviolet laser for generating a laser beam to detect the surface of the optical elements;

[0008] Damman beam splitters are used to split the laser beam emitted by a single-frequency ultraviolet laser to generate a detection sub-beam with uniform intensity distribution.

[0009] The multi-focus spatial filtering system focuses each detection sub-beam after it has been split by the Damman beam splitter and then performs spatial filtering to remove high-frequency stray light and form a "clean" detection sub-beam.

[0010] The scanning inspection system is used to scan and record surface defects and their spatial distribution on optical components under inspection. Rapid detection of surface defects on large-aperture optical components is achieved by controlling the scanning speed of the scanning mirror and CCD camera turntable.

[0011] The method for parallel scanning detection using the aforementioned parallel scanning detection device for nanoscale defects on the surface of large-aperture optical elements is characterized by comprising the following steps:

[0012] ① Based on the size of the component being tested, design and select appropriate parameters such as the number of beam splitters N of the Damman beam splitter, the focal length and effective aperture of the focusing objective, the pinhole size of the multifocal spatial filter, and the effective light transmission aperture of the scanning mirror;

[0013] ② Install the Damman beam splitter at the front end of the single-frequency ultraviolet laser so that the laser beam output from the single-frequency ultraviolet laser is perpendicularly incident to the center of the Damman beam splitter, and after splitting, forms a 1×N laser beam array along the vertical Z direction.

[0014] ③ Install the focusing objective and multifocal spatial filter, adjust the spatial position of the multifocal spatial filter so that the pinhole is located on the back focal plane of the focusing objective and the 1×N sub-beam focus after focusing passes through the pinhole, thus completing the spatial filtering "purification" of each detection sub-beam laser.

[0015] ④ Install the scanning mirror on the first rotating stage, and then install the optical element to be tested, the imaging objective lens, the CCD camera, and the first rotating stage in sequence;

[0016] ⑤ Adjust the first rotating stage so that the sub-beam laser array illuminates the effective edge position of the optical element being tested, and control the computer to record the current position of the rotating stage;

[0017] ⑥ Fix the spatial position of the optical element being tested, adjust the positions of the imaging objective and the CCD camera to make them coaxial, and control the computer to record the current position of the first rotating stage;

[0018] ⑦ Use computer-controlled rotary table to achieve angular velocity The optical element under test is scanned, while the rotary table is controlled to move at an angular velocity of 2. The system rotates and tracks the intensity and position of the light reflected by the optical element being tested. By analyzing the modulation data of the intensity by the surface defects of the optical element using software, the surface defect information can be obtained, thus completing the surface defect detection of the optical element.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects:

[0020] 1. This invention uses ultraviolet high coherence laser as the detection light source, which solves the problem that traditional large-aperture visible light and infrared interferometers have insufficient lateral and longitudinal resolution and cannot distinguish nanoscale defects on the surface of optical components. It greatly improves the detection resolution of nanoscale defects on the surface of optical components and solves the problem of detecting surface defects of large-aperture ultra-smooth optical components in the field of holographic interferometry.

[0021] 2. This invention uses a Damman beam splitter combined with parallel scanning technology to fill the problem of rapid detection of nanoscale defects on the surface of meter-scale large-aperture, ultra-heavy optical components, and greatly improves the efficiency of surface defect detection.

[0022] 3. The entire device has a simple structure and is easy to construct. In addition to using an imaging system to perform quantitative analysis of defects, it can also use visual observation of diffraction interference light fields to quickly determine the nature of defects. By designing a mechanical structure, it can realize online detection of surface defects of large-aperture optical elements, and has strong versatility. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of an embodiment of the surface defect scanning and detection device for large-aperture optical elements of the present invention.

[0024] In the figure: 1-Single-frequency ultraviolet laser, 2-Damman beam splitter grating, 3-Focusing objective lens, 4-Multifocal spatial filter, 5-Scanning mirror, 6-First rotating stage, 7-Optical element to be tested, 8-First imaging objective lens, 9-Second imaging objective lens, 10-Third imaging objective lens, 11-First CCD camera, 12-Second CCD camera, 13-Third CCD camera, 14-Second rotating stage, 15-Third rotating stage, 16-Fourth rotating stage, 17-Rotating stage controller, 18-Computer.

[0025] Figure 2 It is a simulation calculation of the diffraction light intensity distribution of scratch defects with different widths and depths.

[0026] Figure 3 It is a simulation calculation of the diffraction intensity distribution of annular defects at different depths.

[0027] Figure 4 It is a simulation calculation of the diffraction intensity distribution of surface defects irradiated by laser light sources of different wavelengths.

[0028] Figure 5 It is a simulation calculation of the effect of light sources with different spatial coherence widths on the intensity distribution of diffracted light from defects. Detailed Implementation

[0029] The present invention will be further described below with reference to embodiments and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0030] A parallel scanning detection device for nanoscale defects on the surface of large-aperture optical components includes: a single-frequency ultraviolet laser 1 for generating a detection laser beam; a Damman beam splitter grating 2 for splitting the laser beam emitted by the single-frequency ultraviolet laser to generate detection sub-beams with uniform intensity distribution; a multi-focus spatial filtering system, including a focusing objective lens 3 and a multi-focus spatial filter 4, for focusing each detection sub-beam and performing spatial filtering to remove high-frequency stray light and form a "clean" detection sub-beam; and a scanning detection system, including a scanning mirror 5 and a first rotating stage 6, for redirecting the detection beam to scan the optical component under test; and rapid detection of surface defects on large-aperture optical components is achieved by controlling the scanning speed of the scanning mirror and the CCD camera turntable.

[0031] Figure 1 This invention relates to a parallel scanning detection device for nanoscale defects on the surface of large-aperture optical elements, comprising: a single-frequency ultraviolet laser 1, wherein in this embodiment a single-frequency fiber laser with a wavelength of 390nm and a linewidth of less than 50kHz is used to output an ultraviolet laser beam; a Damman beam splitter 2, wherein in this embodiment a 1×3 quartz transmission beam splitter grating is used to split the incident 390nm wavelength laser into three detection sub-beams; a multifocal spatial filtering system, wherein the numerical aperture NA of the focusing objective lens 3 is 0.1, and the multifocal spatial filter 4 is a slit with a width of 50 micrometers processed on a stainless steel sheet with a thickness of 0.05mm; and in this embodiment a defect scanning system is constructed using a first rotating stage 6 connected to the input terminals of a rotating stage controller 17 and a computer 18 and a scanning mirror 5, for performing two-dimensional scanning measurements on the optical element 7 to be inspected. The first imaging objective lens 8, the second imaging objective lens 9, and the third imaging objective lens 10, together with the first CCD camera 11, the second CCD camera 12, the third CCD camera 13, the second rotating stage 14, the third rotating stage 15, and the fourth rotating stage 16, constitute a receiving system for receiving and recording the intensity distribution data of the light beam reflected by the optical element to be tested 7.

[0032] The method for parallel scanning detection using the aforementioned parallel scanning detection device for nanoscale defects on the surface of large-aperture optical elements includes the following steps:

[0033] ① Based on the size of the optical element 7 to be tested, design and select appropriate parameters such as the number of beam splitters N of the Damman beam splitter grating 2, the focal length and effective aperture of the focusing objective lens 3, the pinhole size of the multifocal spatial filter 4, and the effective light transmission aperture of the scanning mirror 5.

[0034] ② Install the Damman beam splitter grating 2 at the front end of the single-frequency ultraviolet laser, so that the laser beam output from the single-frequency ultraviolet laser 1 is perpendicularly incident to the center position of the Damman beam splitter grating 2, and after beam splitting, a 1×N laser beam array is formed along the vertical Z direction.

[0035] ③ Install the focusing objective lens 3 and the multifocal spatial filter 4, adjust the spatial position of the multifocal spatial filter 4 so that the pinhole is located on the back focal plane of the focusing objective lens 3 and the focused 1×N sub-beam focal point passes through the pinhole to complete the spatial filtering "purification" of each detection sub-beam laser.

[0036] ④ Install the scanning mirror 5 on the first rotating stage 6, and install the optical element to be tested 7, the first imaging objective lens 8, the second imaging objective lens 9, the third imaging objective lens 10, the first CCD camera 11, the second CCD camera 12, the third CCD camera 13, the second rotating stage 14, the third rotating stage 15, and the fourth rotating stage 16 in sequence.

[0037] ⑤ Adjust the second rotating stage 14, the third rotating stage 15, and the fourth rotating stage 16 so that the sub-beam laser array illuminates the effective edge position of the optical element being tested, and the control computer 18 records the current position of the rotating stage;

[0038] ⑥ Fix the spatial position of the optical element 7 to be tested, and adjust the positions of the first imaging objective lens 8, the second imaging objective lens 9, the third imaging objective lens 10, the first CCD camera 11, the second CCD camera 12, and the third CCD camera 13 to make them coaxial, and control the computer 18 to record the current position of the first rotating stage.

[0039] ⑦ Use computer 18 to control the rotary table at angular velocity The optical element 7 to be inspected is scanned, and at the same time the second rotary stage 14, the third rotary stage 15, and the fourth rotary stage 16 are controlled to move at an angular velocity of 2. The light intensity and position information reflected by the optical element 7 under test are recorded by rotating and tracking. The surface defect information can be obtained by analyzing the modulation data of the intensity by the surface defects of the optical element using software, thus completing the surface defect detection of the optical element.

[0040] Figure 2 This describes the intensity distribution of diffracted light under 390nm laser irradiation, assuming a surface defect of 1mm width and depths of 1nm and 3nm. Figure 2 As can be seen, the deeper the defect, the more obvious the modulation effect of the defect on the intensity of the diffracted light, which also means that the defect is easier to detect. Figure 3 The simulation calculates the effect of a ring-shaped nanodefect with a radius of 7 mm, a width of 1 mm, and depths of 1 nm and 3 nm on the intensity distribution of diffracted light at a wavelength of 390 nm. The results can be seen from the figure. Figure 2 Similarly, the depth of the defect has a significant impact on the modulation of the optical field; the greater the depth, the stronger the modulation effect, and the easier the defect is to detect. Figure 4The figure shows the far-field distribution of diffraction intensity when the defect is irradiated with lasers of different wavelengths, with a width of 2 mm and a depth of 1 nm. As can be seen from the figure, the modulation effect of the defect on the far-field intensity becomes more obvious as the irradiation wavelength gets shorter and shorter, and the intensity distribution curve shows more oscillation peaks. This also means that the defect is easier to distinguish and detect. Figure 5 The simulation calculated the far-field diffraction intensity distribution when a defect with a width of 2 mm and a depth of 1 nm was irradiated with light sources of different spatial coherence widths. It can be seen that the better the spatial coherence of the irradiation source, the more obvious the oscillation of the diffraction intensity, which means that the defect is easier to distinguish.

[0041] In summary, this invention employs an ultraviolet coherent light source in conjunction with multi-channel parallel scanning detection, which can significantly improve the detection capability and efficiency of nanoscale micro-defects on the surface of large-aperture optical components. In particular, it provides a very simple and efficient technical means for the precision processing of ultra-large-aperture, ultra-smooth optical components with a diameter of meters or more.

Claims

1. A parallel scanning detection device for nanoscale defects on the surface of large-aperture optical elements, comprising: A single-frequency ultraviolet laser is used to generate a coherent ultraviolet laser source for detecting the surface of optical components. The Damman beam splitter is used to split the ultraviolet coherent laser source into a detection sub-beam with uniform intensity distribution. A multi-focus spatial filtering system is used to focus and spatially filter the detection sub-beam, filter out high-frequency stray light, and form the detection sub-beam. A scanning inspection system is used to scan and record surface defects and their spatial distribution on optical components to be inspected. The multifocal spatial filtering system is characterized by including a focusing objective (3) and a multifocal spatial filter (4), and the scanning detection system includes a first rotating stage (6), a scanning mirror (5) mounted on the first rotating stage (6), an imaging objective group consisting of N imaging objectives, a CCD camera group consisting of N CCD cameras, a rotating stage group consisting of N rotating stages for placing the N CCD cameras, and a rotating stage controller (17) and a computer (18); the rotating stage controller (17) is connected to the N rotating stages and the computer (18) respectively. The Damman beam splitter (2) is installed at the front end of the single-frequency ultraviolet laser (1) so that the laser beam output by the single-frequency ultraviolet laser (1) is perpendicularly incident to the center position of the Damman beam splitter (2) and split into a 1×N sub-laser beam array; along the transmission direction of the 1×N laser beam are the focusing objective (3), the multi-focus spatial filter (4), the scanning mirror (5), the optical element to be tested, the imaging objective group and the CCD camera group; the spatial position of the multi-focus spatial filter (4) is adjusted to ensure that the pinhole of the multi-focus spatial filter (4) is located at the back focal plane of the focusing objective (3) and the focus of the 1×N sub-beams passes through the pinhole; Adjust the first rotating stage (6) so that the 1×N sub-laser beam array illuminates the effective edge position of the optical element (7) to be tested, and use the computer (18) to record the current position of the first rotating stage (6); Adjust the positions of the imaging objective lens group and the CCD camera group to make them coaxial, and use a computer (18) to record the current positions of the N rotating stages; The computer (18) controls the first rotary table (6) to move at an angular velocity The optical element (7) to be tested is scanned, and at the same time, N rotary stages are controlled to move at an angular velocity of 2. Rotate and track the light intensity and position information reflected by the optical element to be tested (7). By analyzing the modulation data of the intensity of the surface defects of the optical element, the surface defect information can be obtained and the surface defect detection of the optical element can be completed.

2. The parallel scanning detection device for nanoscale defects on the surface of large-aperture optical elements according to claim 1, characterized in that, The single-frequency ultraviolet laser is a narrow-linewidth continuous coherent laser with a wavelength range of 200nm-450nm.

3. The parallel scanning detection device for nanoscale defects on the surface of large-aperture optical elements according to claim 1 or 2, characterized in that, The Damman beam splitter grating is a one-dimensional 1×N beam splitter grating, where N is the number of beam splitting paths.

4. A detection method using the parallel scanning detection device for nanoscale defects on the surface of a large-aperture optical element as described in any one of claims 1-3, characterized in that, Includes the following steps: ① Based on the size of the element to be tested, determine the number of beam splitters N of the Damman beam splitter grating (2), the focal length and effective aperture of the focusing objective (3), the pinhole size of the multifocal space filter (4), and the effective light transmission aperture of the scanning mirror (5); ② Install the Damman beam splitter (2) at the front end of the single-frequency ultraviolet laser (1) so that the laser beam output by the single-frequency ultraviolet laser (1) is perpendicularly incident to the center of the Damman beam splitter (2) and forms a 1×N laser beam array along the vertical Z direction after beam splitting. ③ Install the focusing objective (3) and the multi-focus spatial filter (4), adjust the spatial position of the multi-focus spatial filter (4) so ​​that the pinhole is located on the back focal plane of the focusing objective (3) and the 1×N sub-beam focus after focusing passes through the pinhole to complete the spatial filtering of each detection sub-beam laser; ④ Install the scanning mirror (5) on the first rotating stage (6), and install the optical element to be tested (7), the first imaging objective (8), the second imaging objective (9), the third imaging objective (10), the first CCD camera (11), the second CCD camera (12), the third CCD camera (13), the second rotating stage (14), the third rotating stage (15), and the fourth rotating stage (16) in sequence. ⑤ Adjust the first rotating stage (6) so that the sub-beam laser array illuminates the effective edge position of the optical element (7) to be tested, and the control computer (18) records the current position of the first rotating stage (6); ⑥ Fix the spatial position of the optical element (7) to be tested, and adjust the positions of the first imaging objective (8), the second imaging objective (9), the third imaging objective (10) and the first CCD camera (11), the second CCD camera (12) and the third CCD camera (13) to make them coaxial. The control computer (18) records the current positions of the second rotating stage (14), the third rotating stage (15) and the fourth rotating stage (16); ⑦ Using a computer (18) to control the first rotary table (6) at an angular velocity The optical element (7) to be tested is scanned, and at the same time the second rotary stage (14), the third rotary stage (15), and the fourth rotary stage (16) are controlled to move at an angular velocity of 2. Rotate and track the light intensity and position information reflected by the optical element to be tested (7). The surface defect information can be obtained by analyzing the modulation data of the surface defects of the optical element on the intensity using software, and the surface defect detection of the optical element can be completed.

Images