Interference exposure light path parallelism adjusting method

Abstract

The invention relates to the technical field of optics, and discloses an interference exposure light path parallelism adjusting method so as to improve the quality of an interference image in application scenes such as component preparation and further improve the yield of prepared products. The method comprises the following steps: deploying an interference exposure light path, wherein the interference exposure light path comprises a pinhole filter and a collimating lens; the pinhole filter is arranged on a displacement table capable of displacing along the direction of an optical axis; the pinhole filter comprises a convergent lens and a pinhole, and the pinhole is located at the focus of the convergent lens. A white screen parallel to the plate glass is used for displaying shearing interference patterns formed by the front plane and the rear plane of the plate glass based on the emergent light beams of the collimating lens, and a camera is used for sequentially collecting images of the shearing interference patterns displayed by the white screen in the displacement process of the displacement table; and selecting the position of the displacement table when the fringe width of the interference fringes in the acquired image is maximum as a target position, so that the collimating lens converts divergent light emitted by the pinhole filter into parallel light.

Description

Technical Field

[0001] The present invention relates to the field of optical technology, and particularly to a method for adjusting the parallelism of an interference exposure optical path. Background Art

[0002] A grating is an important diffractive optical element and is widely used in optical communication, spectrometers, high-power laser systems, etc. With the development of technology, the requirements for the grating diffractive wavefront of optical systems are also getting higher and higher. The parallelism of the exposure optical path is one of the important factors affecting the grating diffractive wavefront. Currently, common methods for detecting the parallelism of the optical path include the Moiré fringe method and the standard grating reference method. Among them, the Moiré fringe method requires the fabrication of a detection grating, resulting in poor timeliness. The low-aberrations reference grating required by the standard grating reference method is difficult to fabricate, the preparation is extremely cumbersome, and an interferometer is needed for repeated detection, which is extremely time-consuming.

[0003] Currently, the prior art can judge whether the parallelism of the split beam has been adjusted by monitoring the diffraction efficiency of the split beam after passing through a volume Bragg grating. However, since the aperture of the volume Bragg grating is smaller than the aperture of the collimating lens, this method can only measure the parallelism of a partial aperture of the split beam and cannot measure the parallelism of the entire surface of the split beam.

[0004] The prior art can also use a spherical interferometer to test and record the aberrations of the double-beam optical path respectively, and complete the regulation of the aberration of the interference exposure field by matching the aberration of one split beam with the aberration of the other split beam. However, the price of a spherical interferometer is relatively expensive, especially for large-aperture spherical interferometers. Summary of the Invention

[0005] The object of the present invention is to disclose a method for adjusting the parallelism of an interference exposure optical path to improve the quality of interference images in application scenarios such as device preparation, thereby improving the yield of the prepared products.

[0006] To achieve the above object, a method disclosed by the present invention includes:

[0007] Step S11, deploy an interference exposure optical path, where the interference exposure optical path includes a pinhole filter and a collimating lens; the pinhole filter is deployed on a displacement stage that can be displaced along the optical axis direction; the pinhole filter includes a converging lens and a pinhole, and the pinhole is located at the focal point of the converging lens.

[0008] Step S12, display a shearing interference pattern formed by the front and rear planes of the flat glass based on the beam emitted from the collimating lens on a white screen parallel to the flat glass, and sequentially collect images of the shearing interference pattern displayed on the white screen by a camera during the displacement of the displacement stage.

[0009] Step S13: Select the position of the displacement stage when the fringe width of the interference fringes in the acquired image is the largest as the target position, so that the collimating lens converts the divergent light emitted from the pinhole filter into parallel light.

[0010] To achieve the above object, the present invention also discloses another method for adjusting the parallelism of an interference exposure optical path, including:

[0011] Step S21: Deploy an interference exposure optical path for preparing a grating based on holographic interferometry. The interference exposure optical path includes a beam splitter that divides a light source into two beams. A first pinhole filter and a first collimating lens are deployed on the optical path of the first split beam; the first pinhole filter is deployed on a first displacement stage that can be displaced along the optical axis direction; the first pinhole filter includes a first converging lens and a first pinhole, and the first pinhole is located at the focal point of the first converging lens; a second pinhole filter and a second collimating lens are deployed on the optical path of the second split beam; the second pinhole filter is deployed on a second displacement stage that can be displaced along the optical axis direction; the second pinhole filter includes a second converging lens and a second pinhole, and the second pinhole is located at the focal point of the second converging lens.

[0012] Step S22: After blocking the second split beam, use a white screen parallel to the flat glass to display the shear interference pattern formed by the front and rear planes of the flat glass based on the light beam emitted from the first collimating lens, and use a camera to sequentially acquire images of the shear interference pattern displayed on the white screen during the displacement process of the first displacement stage. Then select the position of the first displacement stage when the fringe width of the interference fringes in the acquired image is the largest as the target position, so that the first collimating lens converts the divergent light emitted from the first pinhole filter into a first beam of parallel light.

[0013] Step S23: After blocking the first split beam, use the white screen to display the shear interference pattern formed by the front and rear planes of the flat glass based on the light beam emitted from the second collimating lens, and use a camera to sequentially acquire images of the shear interference pattern displayed on the white screen during the displacement process of the second displacement stage. Then select the position of the second displacement stage when the fringe width of the interference fringes in the acquired image is the largest as the target position, so that the second collimating lens converts the divergent light emitted from the first pinhole filter into a second beam of parallel light.

[0014] Step S24: Replace the flat glass with a sample on which grating information is engraved through the exposed interference fringes, and perform an exposure process on the sample with the interference fringes generated by the first beam of parallel light and the second beam of parallel light.

[0015] Preferably, it further includes: Step S25, calculating a similarity index between the shearing interference pattern formed by the first parallel light beam and the shearing interference image formed by the second parallel light beam; and using this similarity index as one of the quantization indexes of the wavefront aberration after the sample preparation.

[0016] Preferably, this similarity index is the product of the luminance similarity, the contrast similarity, and the structural similarity.

[0017] Preferably, in the process of calculating the luminance similarity, the contrast similarity, and the structural similarity, first convert the two shearing interference images with the same image size into grayscale images, and calculate the average grayscale μ of a single pixel in the two grayscale images x and y respectively x 、μ y and the grayscale variance σ x 、σ y , and the covariance σ between the two grayscale images xy ; then calculate respectively according to the following formulas:

[0018] l(x,y) = (2μ x μ y + C1) / (μ 2 x + μ 2 y + C1;

[0019] c(x,y) = (2σ x σ y + C2) / (σ 2 x + σ 2 y + C2);

[0020] s(x,y) = (σ xy + C3) / (σ x σ y + C3);

[0021] wherein, C1, C2, and C3 are constants, l(x,y) is the luminance similarity, c(x,y) is the contrast similarity, and s(x,y) is the structural similarity.

[0022] The present invention creatively adopts the method of flat-shearing interference to detect the beam parallelism, and has the following beneficial effects:

[0023] Compared with the existing method for detecting the optical path parallelism, the method for detecting the optical path parallelism of the present invention is simpler and does not require the use of high-precision devices such as standard gratings; and it can improve the quality of the interference image in application scenarios such as component preparation, thereby improving the yield of the prepared product.

[0024] In shear interference, the interference pattern consists of equally spaced straight fringes. The generated interference pattern is captured by a camera and transmitted to a computer, and then the wavefront curvature radius of the split beam is calculated based on the fringe width of the interference fringes.

[0025] The wavefront curvature radius R L is calculated by the formula: where b is the fringe width of the interference fringes, λ is the laser wavelength, S is the shear amount, i is the incident angle of the light beam on the front surface of the flat glass, n is the refractive index of the flat glass, and h is the thickness of the flat glass.

[0026] When the light beam incident on the front surface of the flat glass is a parallel light, its wavefront is a plane, and the fringe width of the interference fringes formed by the shear interference of the flat glass reaches the maximum, and the corresponding calculated wavefront curvature radius also reaches the maximum. If the distance between the pinhole of the pinhole filter and the focal length of the collimating lens is not equal to the focal length of the collimating lens, at this time the light beam is not a parallel light, its wavefront is a spherical surface, and the corresponding calculated wavefront curvature radius will decrease. Moreover, through the actual measurement of the wavefront curvature radius of the interference image during the displacement of the displacement stage by the interferometer, the conclusion is consistent with the above-mentioned reasoning, ensuring the reliability of the technology of the present invention; however, in the specific applications of different scenarios, there is no need to deploy an interferometer for detecting the wavefront curvature radius to save costs.

[0027] Hereinafter, the present invention will be described in further detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0029] Figure 1 is the optical path diagram of the interference exposure optical path parallelism adjustment method disclosed in the embodiment of the present invention in a specific application scenario. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The following describes the embodiments of the present invention in detail with reference to the accompanying drawings, but the present invention can be implemented in many different ways defined and covered by the claims.

[0031] Embodiment 1

[0032] This embodiment discloses an interference exposure optical path parallelism adjustment method.

[0033] The optical path in the application scenario of this embodiment is as Figure 1As shown in the figure, the components represented by each device index are as follows: 1. Laser; 2. Beam splitter 2; 3. Pinhole filter 1; 4. Mirror 1; 5. Collimating lens 1; 6. Flat glass; 7. Pinhole filter 2; 8. Mirror 2; 9. Collimating lens 2; 10. White screen; 11. Camera; 12. Computer.

[0034] Among them, pinhole filter 1 is composed of lens 1 and pinhole 1, and the distance between lens 1 and pinhole 1 is fixed. Pinhole filter 2 is composed of lens 2 and pinhole 2, and the distance between lens 2 and pinhole 2 is fixed. The apertures and focal lengths of lens 1 and lens 2 are the same, and the apertures of pinhole 1 and pinhole 2 are the same. Pinhole filter 1 is installed on displacement stage 1, and pinhole filter 2 is installed on displacement stage 2. Collimating lens 1 and collimating lens 2 are aspherical lenses with the same focal length and aperture. The front and back surfaces of flat glass 6 are parallel and defect-free. After adjusting the parallelism of split beam 1 and split beam 2, remove flat glass 6, and place the grating substrate to be exposed at the original position of flat glass 6. Split beam 1 and split beam 2 overlap and interfere at the grating substrate to be exposed to form a grating.

[0035] The laser emitted by the laser is divided into two beams of equal intensity after passing through the beam splitter. Its transmitted light is split beam 1, and its reflected light is split beam 2. Split beam 1 becomes divergent light after passing through pinhole filter 1, is reflected by mirror 1, and is collimated by collimating lens 1. Theoretically, if the distance from pinhole 1 of pinhole filter 1 to collimating lens 1 is exactly the focal length of collimating lens 1, split beam 1 will become parallel light after passing through pinhole filter 1 and collimating lens 1. When split beam 1 is transmitted to flat glass 6, the front and back surfaces of flat glass 6 will reflect split beam 1, and the reflected light generated by the front and back surfaces will interfere at white screen 10. This process is called shear interference. The interference pattern is equally spaced straight stripes. The generated interference pattern will be photographed by camera 11 and transmitted to the computer, and then the wavefront curvature radius of split beam 1 will be calculated according to the fringe width of the interference fringes.

[0036] Wavefront curvature radius R L The calculation formula is:

[0037] Among them, b is the fringe width of the interference fringes, λ is the laser wavelength, S is the shear amount, and the calculation formula of S is: i is the incident angle of the light beam on the front surface of the flat glass, n is the refractive index of the flat glass, and h is the thickness of the flat glass.

[0038] When the split beam 1 is a parallel beam of light, its wavefront is a plane, and the width of the interference fringes formed by the shearing interference of the parallel flat glass reaches the maximum. The calculated wavefront curvature radius also reaches the maximum. If the distance between the pinhole 1 of the pinhole filter 1 and the collimating lens 1 is not equal to the focal length of the collimating lens 1, then the split beam 1 is not a parallel beam of light at this time, its wavefront is a spherical surface, and the calculated wavefront curvature radius will decrease. Based on the above principle, it can be judged whether the split beam 1 is a parallel beam of light according to whether the wavefront curvature radius reaches the maximum.

[0039] The parallelism of the split beam 1 and the split beam 2 is adjusted separately. When adjusting the split beam 1, first use a light blocking screen to cover the split beam 2. The displacement stage 1 carries the pinhole filter 1 and moves back and forth along the optical axis. When the calculated wavefront curvature radius of the split beam 1 reaches the maximum, the parallelism of the split beam 1 reaches the best. Fix the displacement stage 1 and record the interference pattern formed by the reflection of the split beam 1 through the parallel flat glass at this time as the interference pattern 1. The incident angles of the split beam 1 and the split beam 2 on the parallel flat glass are opposite. The process of adjusting the parallelism of the split beam 2 is the same as the process of adjusting the parallelism of the split beam 1. When the parallelism of the split beam 2 is adjusted to the best, record the interference pattern formed by the reflection of the split beam 2 through the parallel flat glass at this time as the interference pattern 2.

[0040] In addition to adjusting the parallelism of the split beam 1 and the split beam 2, the optical path parallelism adjustment method described in this embodiment can also qualitatively analyze the diffraction wavefront of the grating formed by the interference exposure of the split beam 1 and the split beam 2.

[0041] Generally, the diffraction wavefront of the grating is determined by two parts. One is the aberration of the holographic exposure system, and the other is the surface shape of the grating glass substrate. In the process of fabricating the grating by holographic interferometry, the transmittance coefficient of the grating can be expressed as: where K and a are constants, d0 = λ0 / (2sinα0), d0 is the grating fringe spacing, and λ0 is the laser wavelength. is called the aberration of the holographic interference exposure system. is the wavefront aberration of the interference arm.

[0042] As can be seen from the above formula, aberration will distort the grating fringes. Therefore, the wavefront aberration of the two interference arms in the interference optical path determines the diffraction wavefront of the grating. After adjusting the parallelism of the split beam 1 and the split beam 2 to the best, the similarity index of the interference pattern 1 and the interference pattern 2 can be calculated. The value range of the similarity index is [0,1]. The similarity index can measure the similarity degree of two pictures. When the similarity index is closer to 1, it means that the interference pattern 1 and the interference pattern 2 are more similar, that is, the wavefronts of the split beam 1 and the split beam 2 are more similar. In this case, the diffraction wavefront of the grating formed by the interference exposure of the split beam 1 and the split beam 2 is better. The smaller the similarity index, the greater the difference between the interference pattern 1 and the interference pattern 2, that is, the greater the difference between the wavefronts of the split beam 1 and the split beam 2. In this case, the diffraction wavefront of the grating formed by the interference exposure of the split beam 1 and the split beam 2 is worse.

[0043] In this embodiment, the similarity index is the product of the brightness similarity, the contrast similarity, and the structure similarity. Preferably, in the process of calculating the brightness similarity, the contrast similarity, and the structure similarity, first convert the shear interference images with the same size of the two images into grayscale images, and calculate the average grayscale μ of a single pixel in the two grayscale images x and y respectively. x 、μ y and the grayscale variance σ x 、σ y , and the covariance σ between the two grayscale images xy ; then calculate respectively according to the following formula:

[0044] l(x,y)=(2μ x μ y +C1) / (μ 2 x +μ 2 y +C1;

[0045] c(x,y)=(2σ x σ y +C2) / (σ 2 x +σ 2 y +C2);

[0046] s(x,y)=(σ xy +C3) / (σ x σ y +C3);

[0047] Among them, C1, C2, and C3 are constants, l(x,y) is the brightness similarity, c(x,y) is the contrast similarity, and s(x,y) is the structure similarity.

[0048] In summary, in the method disclosed in this embodiment, there are essentially two methods. The core content of one of them includes the following steps S11 to S13.

[0049] Step S11: Deploy an interference exposure optical path, which includes a pinhole filter and a collimating lens; the pinhole filter is deployed on a displacement stage that can be displaced along the optical axis direction; the pinhole filter includes a converging lens and a pinhole, and the pinhole is located at the focal point of the converging lens.

[0050] Step S12: Use a white screen parallel to the flat glass to display the shear interference pattern formed by the front and back planes of the flat glass based on the light beam emitted by the collimating lens, and use a camera to sequentially collect images of the shear interference pattern displayed on the white screen during the displacement of the displacement stage.

[0051] Step S13: Select the position of the displacement stage when the fringe width of the interference fringes in the collected image is the largest as the target position, so that the collimating lens converts the divergent light emitted by the pinhole filter into parallel light.

[0052] In this embodiment, the core content of another method for adjusting the parallelism of the interference exposure optical path at least includes the following steps S21 to S24.

[0053] Step S21: Deploy an interference exposure optical path for preparing a grating based on holographic interferometry. The interference exposure optical path includes a beam splitter that divides the light source into two parts. On the optical path of the first split light, a first pinhole filter and a first collimating lens are deployed; the first pinhole filter is deployed on a first displacement stage that can be displaced along the optical axis direction; the first pinhole filter includes a first converging lens and a first pinhole, and the first pinhole is located at the focal point of the first converging lens; on the optical path of the second split light, a second pinhole filter and a second collimating lens are deployed; the second pinhole filter is deployed on a second displacement stage that can be displaced along the optical axis direction; the second pinhole filter includes a second converging lens and a second pinhole, and the second pinhole is located at the focal point of the second converging lens.

[0054] Step S22: After blocking the second split light, use a white screen parallel to the flat glass to display the shear interference pattern formed by the front and back planes of the flat glass based on the light beam emitted by the first collimating lens, and use a camera to sequentially collect images of the shear interference pattern displayed on the white screen during the displacement of the first displacement stage. Then select the position of the first displacement stage when the fringe width of the interference fringes in the collected image is the largest as the target position, so that the first collimating lens converts the divergent light emitted by the first pinhole filter into the first beam of parallel light.

[0055] Step S23: After blocking the first beam splitter, display the shearing interference pattern formed by the front and rear planes of the flat glass on the white screen based on the light beam emitted by the second collimating lens, and use a camera to sequentially collect images of the shearing interference pattern displayed on the white screen during the displacement of the second displacement stage. Then, select the position of the second displacement stage when the fringe width of the interference fringes in the collected images is the largest as the target position, so that the second collimating lens converts the divergent light emitted by the first pinhole filter into a second parallel light beam.

[0056] Step S24: Replace the flat glass with a sample on which grating information is engraved through the exposed interference fringes, and perform an exposure process on the sample with the interference fringes generated by the first parallel light beam and the second parallel light beam.

[0057] Further, after step S24, it further includes: Step S25: Calculate the similarity index between the shearing interference pattern formed by the first parallel light beam and the shearing interference image formed by the second parallel light beam; and use this similarity index as one of the quantization indexes of the wavefront aberration after the sample preparation.

[0058] Thus, the method for adjusting the parallelism of the interference exposure optical path disclosed in this embodiment creatively uses the method of flat shearing interference to detect the parallelism of the light beam. Compared with the existing optical path parallelism detection methods, the optical path parallelism detection method of the present invention is simpler and does not require the use of high-precision devices such as standard gratings; and it can improve the quality of the interference image in application scenarios such as component preparation, thereby improving the yield of the prepared products.

[0059] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for adjusting the parallelism of an interference exposure optical path, characterized in that: include: Step S11, deploying an interference exposure optical path, wherein the interference exposure optical path includes a pinhole filter and a collimating lens; the pinhole filter is deployed on a displacement platform that can be displaced along the optical axis; the pinhole filter includes a converging lens and a pinhole, and the pinhole is located at the focus of the converging lens; Step S12, using a white screen parallel to the flat glass to display the shearing interference patterns formed by the light beam emitted from the collimating lens on the front and rear planes of the flat glass, and using a camera to sequentially capture images of the shearing interference patterns displayed on the white screen during the displacement of the displacement stage; Step S13: selecting the position of the translation stage when the fringe width of the interference fringe in the collected image is the largest as the target position, so that the collimating lens converts the divergent light emitted by the pinhole filter into parallel light.

2. A method for adjusting the parallelism of an interference exposure optical path, characterized in that: include: Step S21, deploying an interference exposure optical path for preparing a grating based on a holographic interference method, wherein the interference exposure optical path includes a beam splitter that splits a light source into two, and deploying a first pinhole filter and a first collimating lens on the optical path of the first split beam; the first pinhole filter is deployed on a first displacement stage that can be displaced along an optical axis; the first pinhole filter includes a first converging lens and a first pinhole, and the first pinhole is located at the focus of the first converging lens; a second pinhole filter and a second collimating lens are deployed on the optical path of the second split beam; the second pinhole filter is deployed on a second displacement stage that can be displaced along the optical axis; the second pinhole filter includes a second converging lens and a second pinhole, and the second pinhole is located at the focus of the second converging lens; Step S22, after shielding the second split beam, using a white screen parallel to the flat glass to display the shearing interference patterns formed by the light beam emitted by the first collimating lens on the front and rear planes of the flat glass, and using a camera to sequentially capture images of the shearing interference patterns displayed on the white screen during the displacement of the first translation stage, and then selecting the position of the first translation stage when the fringe width of the interference fringe in the captured image is the largest as the target position, so that the first collimating lens converts the divergent light emitted by the first pinhole filter into a first beam of parallel light; Step S23, after shielding the first split beam, using the white screen to display the shearing interference patterns formed by the light beam emitted from the second collimating lens on the front and rear planes of the flat glass, and using the camera to sequentially capture images of the shearing interference patterns displayed on the white screen during the displacement of the second translation stage, and then selecting the position of the second translation stage when the fringe width of the interference fringe in the captured image is the largest as the target position, so that the second collimating lens converts the divergent light emitted from the first pinhole filter into a second beam of parallel light; Step S24, replacing the flat glass with a sample having grating information recorded by interference fringes of exposure, and exposing the sample with interference fringes generated by the first beam of parallel light and the second beam of parallel light.

3. The method for adjusting the parallelism of the interference exposure light path according to claim 2, characterized in that: Also includes: Step S25, calculating a similarity index between the shearing interference pattern formed by the first beam of parallel light and the shearing interference image formed by the second beam of parallel light; And the similarity index is used as one of the quantitative indexes of the wavefront aberration after the sample is prepared.

4. The method for adjusting the parallelism of the interference exposure light path according to claim 3, characterized in that: The similarity index is the product of brightness similarity, contrast similarity and structure similarity.

5. The method for adjusting the parallelism of the interference exposure light path according to claim 4, characterized in that: In the process of calculating brightness similarity, contrast similarity and structural similarity, the shear interference images of the two images with the same size are first converted into grayscale images, and the average grayscale μ of a single pixel in the two grayscale images x and y is calculated respectively. x , μ y And the grayscale variance σ x , σ y , and the covariance σ between the two grayscale images xy ; Then calculate according to the following formula: l(x,y)=(2μ x m y +C1) / (μ 2 x +m 2 y +C1; c(x,y)=(2σ x s y +C2) / (σ 2 x +s 2 y +C2); s(x,y)=(σ xy +C3) / (σ x σ y +C3); Among them, C1, C2, C3 are constants, l(x, y) is the brightness similarity, c(x, y) is the contrast similarity, and s(x, y) is the structural similarity.

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