Metallic reflection grating based on external cavity frequency selection of narrow linewidth laser and manufacturing method thereof

Abstract

The present application relates to the metal reflection grating based on the external cavity frequency selection of narrow linewidth laser and a manufacturing method thereof, and the metal reflection grating comprises the following specific steps: S1, preparing and pretreating a substrate; S2, performing electron beam lithography treatment on the substrate in S1; S3, performing dry etching treatment on the substrate in S2; S4, performing metal layer deposition treatment on the surface of the substrate in S3 to obtain a grating.The present application has the beneficial effects of simple structure and manufacturing method, the metal reflection grating can suppress the diffraction efficiency of 0th order light, concentrate energy on ±1st order, improve the signal contrast ratio and signal-to-noise ratio of the system, and improve the design to concentrate the diffraction energy on any desired order to realize different applications.

Description

Technical Field

[0001] This invention relates to the field of reflective grating manufacturing technology, specifically to a metal reflective grating based on frequency selection of the external cavity of a narrow-linewidth laser and its fabrication method. Background Technology

[0002] With the rapid iteration of 3D NAND storage technology, the performance requirements of light sources for wafer morphology measurement in production lines continue to increase. 635nm ultra-high precision tunable narrow linewidth lasers have become the core key components of measurement instruments, and their performance directly determines the measurement accuracy and production line yield.

[0003] Currently, publicly available patents related to similar metal reflective gratings both domestically and internationally exhibit significant differences in technical approaches, performance indicators, and industrial applicability. These patents primarily focus on the development of dedicated gratings for the 635nm band, typically employing classic blazed grating topologies with a line density covering 2400 lines / mm to 3600 lines / mm. They utilize a gold-based, titanium-tungsten composite multilayer film design to achieve coordinated amplitude and phase modulation, suppressing zero-order and ±1-order emission. However, both processes require high manufacturing costs and precision.

[0004] In summary, there are significant differences in structural design, manufacturing process, performance indicators and industrial adaptability among similar metal reflective grating patents at home and abroad. Foreign patents are technically mature but have poor adaptability and high costs, while domestic patents, although they meet the needs of domestic production, have insufficient core performance and low degree of industrialization.

[0005] To address the aforementioned issues, this invention proposes a metal reflective grating based on an ultra-high precision tunable narrow linewidth laser external cavity frequency selection device and its fabrication method, relying on the foundation of precision laser technology.

[0006] Therefore, we need to develop a metal reflective grating that combines high performance, low cost, and full self-adaptability, and design a simpler nanostructure and a simpler processing technology, so as to perfectly meet the core requirements of ultra-high precision tunable narrow linewidth lasers used in 3D NAND production lines. Summary of the Invention

[0007] This invention addresses the technical problems existing in the prior art by providing a metal reflective grating based on the frequency selection of the external cavity of a narrow linewidth laser and its fabrication method.

[0008] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: The fabrication method of a metal reflective grating based on the external cavity frequency selection of a narrow linewidth laser includes the following specific steps: S1, the substrate is prepared and pretreated; S2, electron beam lithography is performed on the substrate in S1; S3, dry etching is performed on the substrate in S2; S4, perform metal layer deposition on the surface of the substrate in S3 to obtain a grating.

[0009] The beneficial effects of the present invention are as follows: the method for fabricating the metal reflective grating includes the following specific steps: S1, preparing and pre-treating the substrate; S2, performing electron beam lithography on the substrate in S1; S3, performing dry etching on the substrate in S2; S4, performing metal layer deposition on the surface of the substrate in S3 to obtain the grating.

[0010] The structure and manufacturing method of this invention are simple. The metal reflective grating can suppress the diffraction efficiency of the 0th order light and concentrate the energy to the ±1st order, which can improve the system signal contrast and signal-to-noise ratio. At the same time, it can improve the design and concentrate the diffraction energy to any desired order to achieve different applications.

[0011] Based on the above technical solution, the present invention can be further improved as follows.

[0012] Furthermore, step S1 includes the following specific steps: S11 uses high-purity fused silica substrate, cut to a specified size; S12, single-sided polishing of fused silica substrate to control surface shape accuracy to λ / 10, where λ=632.8nm and surface roughness Ra<0.5nm; S13, the fused silica substrate from S12 is first immersed in SC-1 cleaning solution for 10-15 minutes to remove surface organic matter and particles; then immersed in SC-2 cleaning solution for 10-15 minutes to remove surface metal ions; finally rinsed repeatedly with deionized water 3-5 times and dried with nitrogen to obtain the fused silica substrate. S14. Place the dried fused silica substrate into an oven and dry it at 80~100℃ for 20-40 minutes to completely remove residual moisture from the surface. Cool it to room temperature before use.

[0013] The advantages of adopting the above-mentioned further solution are that the solution is simple and reasonable in design. By polishing the fused silica substrate and simultaneously performing ultrasonic cleaning and drying, metal ions on the surface of the fused silica substrate are removed, ensuring that the fused silica substrate is free of cracks and chipping, thereby ensuring that the surface cleanliness of the fused silica substrate meets the requirements of photolithography.

[0014] Furthermore, step S2 includes the following specific steps: S21, Photoresist coating, which is to use spin coating to uniformly coat high-resolution electron beam photoresist onto a clean fused silica substrate surface, controlling the spin coating speed to 3500~5500 r / min, the spin coating time to 40~60 s, and finally controlling the photoresist thickness to 300~600 nm. S22, pre-baking, which involves placing the fused silica substrate coated with electron beam photoresist into a hot plate and soft-baking at 180~200℃ for 10~15 minutes to completely remove the solvent from the photoresist. S23, Electron Beam Exposure: The fused silica substrate is placed in an electron beam lithography apparatus. The electron beam acceleration voltage is set to 10~20kV, and the beam spot size is ≤5nm. Based on the design requirements of 2400 lines / mm and a period of 416.7nm, a pre-drawn grating pattern file is imported, and the electron beam scanning speed and dose are precisely controlled at 50~100μC / cm. 2 Selective exposure of the photoresist ensures the formation of periodic exposure areas and meets the linewidth uniformity standard; S24, Development: The exposed fused silica substrate is placed in the corresponding developer and developed at room temperature for 60-90 seconds to remove the photoresist in the exposed area and form a periodic electron beam photoresist grating mask; after development, it is rinsed with isopropanol to terminate the development reaction and dried with nitrogen. S25, post-baking, which involves placing the developed fused silica substrate onto a hot plate and hard-baking at 120~140℃ for 1~2 minutes.

[0015] The advantages of adopting the above-mentioned further solution are that the solution is simple and reasonable. After the photoresist is coated on the fused silica substrate, a pre-baking treatment is performed to completely remove the solvent in the photoresist, enhance the adhesion between the photoresist and the substrate, and improve the photoresist's resistance to electron beam damage, thus avoiding the phenomenon of delamination and cracking of the photoresist layer during subsequent exposure. At the same time, electron beam lithography is used to selectively expose the photoresist to ensure the formation of periodic exposure areas and meet the linewidth uniformity standard; In addition, post-baking further enhances the mechanical strength and etching resistance of the photoresist mask, providing stable mask support for subsequent dry etching.

[0016] Furthermore, step S3 includes the following specific steps: S31, Etching process debugging, that is, selecting an inductively coupled plasma (ICP) etching machine, introducing a CF4 / Ar mixed etching gas, wherein CF4:Ar=1:2~1:3, optimizing process parameters: RF power 300~500W, chamber pressure 1~5 Pa, etching temperature room temperature~50℃; S32, Inductively Coupled Plasma Etching, involves placing the qualified fused silica substrate into the etching machine and etching according to the calibrated parameters. The goal is to accurately transfer the pattern of the photoresist mask into the fused silica substrate to form a rectangular trench structure. The trench depth is controlled between 100 and 300 nm, and the sidewall verticality is >85°. S33, photoresist removal: After etching, the fused silica substrate is removed and placed in an oxygen plasma ashing instrument. Oxygen is introduced at a power of 200-300W for 5-10 minutes to remove the residual photoresist mask. Then, it is ultrasonically cleaned with acetone and ethanol for 5-10 minutes in sequence to thoroughly remove organic residues. After rinsing with deionized water, it is dried with nitrogen. S34, Groove shape detection, that is, using AFM and SEM to re-inspect the groove depth, sidewall angle, and period uniformity of the fused silica grating.

[0017] The advantages of adopting the above-mentioned further solution are that the solution is simple and the design is reasonable. The above steps are used to process and form grating grooves on the substrate, and ensure that all parameters of the fused silica grating, such as groove depth, sidewall angle, and period uniformity, meet the design requirements. If they do not meet the requirements, the photolithography and etching process must be repeated.

[0018] Furthermore, step S4 includes the following specific steps: S41, Pre-treatment before coating: The fused silica grating that has passed etching is subjected to another ultra-clean cleaning, ultrasonically cleaned with deionized water for 5 minutes, and then dried with nitrogen; then placed in a vacuum coating machine and evacuated to 1×10⁻⁶. -5 Below Pa; S42,Cr adhesion layer deposition, that is, using magnetron sputtering process, using a chromium target as the target material, controlling the sputtering power to 100~200W, the deposition rate to 0.1~0.2nm / s, to deposit a 10~20nm thick Cr layer on the grating surface; during the deposition process, the substrate temperature is controlled to room temperature~100℃; S43, Au reflective layer deposition, that is, after the Cr layer is deposited, there is no need to remove the substrate. Continue to use the magnetron sputtering process, using a gold target as the target material, controlling the sputtering power to 200~300W, the deposition rate to 0.3~0.5nm / s, and depositing a 100-300nm thick Au layer on the Cr layer. S44, film layer inspection, which involves using an ellipsometer to measure the thickness of the Cr and Au layers to ensure a thickness error of <±5nm; using a scanning electron microscope to measure the film layer density to ensure there are no pinholes or detachment defects; and using a spectrophotometer to measure the reflectance of the target wavelength range, i.e., visible light to near-infrared light, to ensure a reflectance >95%.

[0019] The advantages of adopting the above-mentioned further scheme are that it is simple, reasonably designed, deposits Cr and Au layers on the substrate, and ensures that the Cr and Au layers meet the relevant performance requirements.

[0020] Furthermore, it also includes S5, post-processing and safety testing, specifically including the following steps: S51, edge post-processing, is to chamfer and polish the edges of the grating to remove burrs and residual film generated during etching and coating. S52, geometric parameter detection, that is, using AFM and SEM to re-detect the period, groove depth, line width, and sidewall angle of the grating to ensure that all geometric parameters meet the design requirements, with a period error of ±0.5nm; S53, Optical performance testing, which involves using a spectrophotometer to test the 0th / -1st order diffraction efficiency of the grating to ensure that the diffraction efficiency meets the design standards; using an interferometer to test the wavefront aberration; and using a stray light tester to test the stray light level to ensure that the optical performance meets the standards. S54, Environmental stability test, which involves high and low temperature cycling test on the grating, -40℃~85℃, 8-12 cycles, and damp heat test, temperature 30℃-50℃, humidity 80%-90%, for 20-30 hours, to verify the adhesion of the metal film and the stability of the grating structure.

[0021] The advantages of adopting the above-mentioned further solution are that the solution is simple and reasonably designed. The edge of the grating is chamfered and polished to remove burrs and residual film generated during the etching and coating process, so as to avoid edge defects affecting the overall performance of the grating. After the treatment, an ultra-clean cleaning is performed again to remove edge debris. Meanwhile, the geometric parameters, optical performance, and environmental stability of the grating were tested to ensure that all geometric parameters and optical performance of the grating met the standards, and the adhesion of the metal film and the stability of the grating structure were verified, with no problems such as film peeling or structural deformation.

[0022] Furthermore, it also includes S6, which uses a dicing machine or laser cutter to cut the grating to the required size.

[0023] The advantages of adopting the above-mentioned further solution are that the structure is simple and the design is reasonable. The grating is cut by a dicing machine or a laser cutting machine to obtain the required grating size.

[0024] The present invention also relates to a metal reflective grating based on the external cavity frequency selection of a narrow linewidth laser, comprising: a transparent substrate, wherein a metal layer is etched on the substrate, the refractive index of the metal layer being higher than that of the substrate; the metal layer having a grating-like structure.

[0025] The advantages of adopting the above-mentioned further scheme are that the structure is simple and the design is reasonable. By etching a metal layer on the substrate to form a metal reflection grating, the metal reflection grating can suppress the diffraction efficiency of the 0th order light and concentrate the energy to the ±1st order, thereby improving the system signal contrast and signal-to-noise ratio.

[0026] Furthermore, the metal layer includes a transition layer and a gold layer, the transition layer being distributed between the gold layer and the substrate, and the gold layer having a grating-like structure.

[0027] The advantages of adopting the above-mentioned further scheme are that the structure is simple and the design is reasonable. The metal layer includes a transition layer and a gold layer. The function of the transition layer is to make the gold layer bond more firmly to the quartz substrate.

[0028] Furthermore, the transition layer is a chromium layer with a thickness of 40-60 nm; the gold layer has a thickness of 100-300 nm.

[0029] The advantages of adopting the above-mentioned further scheme are that the structure is simple and the design is reasonable. The chromium layer is a transition layer between the quartz substrate and the gold layer. The role of the chromium layer is to make the gold layer bond more firmly with the quartz substrate. Meanwhile, the thickness of the chromium and gold layers is designed to meet the corresponding structural performance requirements. Attached Figure Description

[0030] Figure 1 This is a flowchart illustrating the fabrication process of the metal reflective grating in this invention; Figure 2 This is a schematic diagram of the structure of the metal reflective grating in this invention; Figure 3 This is the optical path diagram of the metal reflective grating in the laser external cavity frequency selection system of this invention; Figure 4 This is a graph showing the effect of the Fourier series truncation order on the diffraction efficiency of the -1st order in this invention. Figure 5 This refers to the -1st order diffraction efficiency when the depth and width change simultaneously at a wavelength of 630nm in this invention. Figure 6 This refers to the -1st order diffraction efficiency when the depth and width change simultaneously at a wavelength of 635nm in this invention. Figure 7 This refers to the -1st order diffraction efficiency when the depth and width change simultaneously at a wavelength of 640nm in this invention.

[0031] The attached diagram lists the components represented by each number as follows: 1. Substrate; 2. Metal layer; 21. Transition layer; 22. Gold layer. Detailed Implementation

[0032] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0033] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the stated features. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0034] In the description of this application, it should also be noted that, unless otherwise expressly specified and limited, the terms "set up," "install," "connect," and "link" 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 mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this technology based on the specific circumstances.

[0035] In the description of this application, spatial relation terms such as "below," "under," "below," "below," "above," "over," etc., are used herein to describe the relationship between one element or feature shown in the figures and other elements or features. It should be understood that, in addition to the orientation shown in the figures, spatial relation terms also include different orientations of the device in use and operation. For example, if the device in the figures is flipped, an element or feature described as "below" or "under" or "below" of other elements or features will be oriented "above" other elements or features. Therefore, the exemplary terms "below" and "under" can include both upper and lower orientations. Furthermore, the device may also include other orientations (e.g., rotated 90 degrees or other orientations), and the spatial descriptive terms used herein are interpreted accordingly.

[0036] In the description of this application, the term "for example" is used to mean "used as an example, illustration, or description." Any embodiment described as "for example" in this application is not necessarily to be construed as being more preferred or advantageous than other embodiments. The following description is provided to enable any person skilled in the art to make and use the invention. Details are set forth in the following description for purposes of explanation. It should be understood that those skilled in the art will recognize that the invention can be made without using these specific details. In other instances, well-known structures and processes will not be described in detail to avoid obscuring the description of the invention with unnecessary detail. Therefore, the invention is not intended to be limited to the embodiments shown, but is consistent with the broadest scope of the principles and features disclosed in this application.

[0037] Example 1 like Figures 1 to 7 As shown, this embodiment provides a method for fabricating a metal reflective grating based on the external cavity frequency selection of a narrow linewidth laser, including the following specific steps: S1, the substrate is prepared and pretreated; S2, electron beam lithography is performed on the substrate in S1; S3, dry etching is performed on the substrate in S2; S4, perform metal layer deposition on the surface of the substrate in S3 to obtain a grating.

[0038] The fabrication method of the metal reflective grating includes the following specific steps: S1, preparing and pre-treating the substrate; S2, performing electron beam lithography on the substrate in S1; S3, performing dry etching on the substrate in S2; S4, performing metal layer deposition on the surface of the substrate in S3 to obtain the grating.

[0039] The structure and manufacturing method of this embodiment are simple. The metal reflective grating can suppress the diffraction efficiency of the 0th order light and concentrate the energy to the ±1st order, which can improve the system signal contrast and signal-to-noise ratio. At the same time, it can improve the design and concentrate the diffraction energy to any desired order to achieve different applications.

[0040] Example 2 Based on Example 1, in this example, step S1 includes the following specific steps: S11 uses high-purity fused silica substrate, cut to a set size, such as 25mm×25mm or 50mm×50mm, to ensure that the substrate is free of cracks and chipping. S12, the fused silica substrate is polished on one side to control the surface shape accuracy to λ / 10, where λ=632.8nm, the surface roughness Ra<0.5nm, and there are no defects such as scratches, pits, or stains; S13 uses the RCA standard cleaning process (SC-1+SC-2) combined with ultrasonic cleaning. The specific process is as follows: The fused silica substrate in S12 is first immersed in SC-1 cleaning solution (ammonia:hydrogen peroxide:deionized water = 1:1:5, temperature 70~80℃) for 10~15 minutes to remove surface organic matter and particles; then it is immersed in SC-2 cleaning solution (hydrochloric acid:hydrogen peroxide:deionized water = 1:1:6, temperature 70~80℃) for 10~15 minutes to remove surface metal ions; finally, it is rinsed repeatedly with deionized water 3~5 times and dried with nitrogen to obtain the fused silica substrate; The specific cleaning steps mentioned above include: First, place the single-sided polished quartz substrate material in a petri dish, ultrasonically clean it with alcohol, rinse it with deionized water, and dry it with a nitrogen gun; second, ultrasonically clean it with acetone, rinse it with deionized water, and dry it; third, ultrasonically clean it with glass melt, wipe it with degreased cotton, rinse it with deionized water, and dry it; finally, ultrasonically clean it with deionized water, rinse it with deionized water, and dry it. S14. Place the dried fused silica substrate into an oven and dry it at 80~100℃ for 20-40 minutes to completely remove residual moisture from the surface. Cool it to room temperature before use.

[0041] The solution is simple and reasonably designed. By polishing the fused silica substrate and simultaneously performing ultrasonic cleaning and drying, metal ions on the surface of the fused silica substrate are removed, ensuring that the fused silica substrate is free of cracks and chipping, thereby ensuring that the surface cleanliness of the fused silica substrate meets the requirements of photolithography.

[0042] Example 3 Based on the above embodiments, in this embodiment, step S2 includes the following specific steps: S21, Photoresist Coating, which is to use spin coating to uniformly coat high-resolution electron beam photoresist (such as PMMA 950k) onto a clean fused silica substrate surface. The spin coating speed is controlled at 3500~5500 r / min and the spin coating time is 40~60s. The final photoresist thickness is controlled at 300~600nm (to adapt to the electron beam lithography resolution and match the subsequent etching depth requirements). Among them, PMMA 950K is a polymethyl methacrylate (PMMA) with a molecular weight of 950,000, which is widely used as a positive electron beam photoresist and has important applications in micro-nano fabrication, semiconductor manufacturing, MEMS and nanoelectronic devices.

[0043] S22, pre-baking, involves placing the fused silica substrate coated with electron beam photoresist into a hot plate and soft-baking it at 180~200℃ for 10~15 minutes to completely remove the solvent in the photoresist, enhance the adhesion between the photoresist and the substrate, and improve the photoresist's resistance to electron beam damage, thus avoiding delamination and cracking of the photoresist layer during subsequent exposure. S23, Electron Beam Exposure: The fused silica substrate is placed in an electron beam lithography apparatus. The electron beam acceleration voltage is set to 10~20kV, and the beam spot size is ≤5nm. Based on the design requirements of 2400 lines / mm and a period of 416.7nm, a pre-drawn grating pattern file is imported, and the electron beam scanning speed and dose are precisely controlled at 50~100μC / cm. 2 Selective exposure of the photoresist ensures the formation of periodic exposure areas and meets the linewidth uniformity standard; S24, Development: The exposed fused silica substrate is placed in the corresponding developer and developed at room temperature for 60-90 seconds to remove the photoresist in the exposed area and form a periodic electron beam photoresist grating mask. After development, it is rinsed with isopropanol to terminate the development reaction and dried with nitrogen to avoid insufficient development leading to blurred stripes or over-development leading to linewidth distortion. S25, post-bake, which involves placing the developed fused silica substrate into a hot plate and hard-bake at 120~140℃ for 1~2 minutes to further enhance the mechanical strength and etching resistance of the photoresist mask, providing stable mask support for subsequent dry etching.

[0044] The solution is simple and reasonably designed. After the photoresist is coated on the fused silica substrate, a pre-baking process is performed to completely remove the solvent in the photoresist, enhance the adhesion between the photoresist and the substrate, and improve the photoresist's resistance to electron beam damage, thus avoiding delamination and cracking of the photoresist layer during subsequent exposure. At the same time, electron beam lithography is used to selectively expose the photoresist to ensure the formation of periodic exposure areas and meet the linewidth uniformity standard; In addition, post-baking further enhances the mechanical strength and etching resistance of the photoresist mask, providing stable mask support for subsequent dry etching.

[0045] Example 4 Based on the above embodiments, in this embodiment, step S3 includes the following specific steps: S31, Etching process debugging, namely, using an inductively coupled plasma (ICP) etching machine, introducing a CF4 / Ar mixed etching gas, where CF4:Ar=1:2~1:3, optimizing process parameters: RF power 300~500W, chamber pressure 1~5 Pa, etching temperature room temperature~50℃, ensuring high anisotropy and high selectivity (fused silica / photoresist >3:1) in the etching process, reducing side etching; S32, Inductively Coupled Plasma Etching, involves placing a qualified fused silica substrate into an etching machine and etching according to pre-calibrated parameters. The goal is to precisely transfer the pattern of the photoresist mask into the fused silica substrate to form a rectangular trench structure. The trench depth is controlled between 100 and 300 nm, and the sidewall perpendicularity is >85°. The etching depth is monitored in real time during the etching process to avoid etching too deep or too shallow. In this process, after spin coating of photoresist, electronic exposure is used to perform patterned photolithography and then splicing to achieve the required large-area fabrication of 18mm×6mm. S33, photoresist removal: After etching, the fused silica substrate is removed and placed in an oxygen plasma ashing instrument. Oxygen is introduced at a power of 200-300W for 5-10 minutes to remove the residual photoresist mask. Then, it is ultrasonically cleaned with acetone and ethanol for 5-10 minutes in sequence to thoroughly remove organic residues. After rinsing with deionized water, it is dried with nitrogen. S34, Groove shape inspection, that is, using AFM (atomic force microscope) and SEM (scanning electron microscope) to re-inspect the groove depth, sidewall angle and period uniformity of the fused silica grating to confirm that all parameters meet the design requirements. If they do not meet the requirements, the photolithography and etching process must be repeated.

[0046] The solution is simple and reasonably designed. The above steps are used to process and form grating grooves on the substrate. It is ensured that all parameters of the fused silica grating, such as groove depth, sidewall angle, and period uniformity, meet the design requirements. If they do not meet the requirements, the photolithography and etching process must be repeated.

[0047] Example 5 Based on the above embodiments, in this embodiment, step S4 includes the following specific steps: S41, Pre-treatment before coating: The fused silica grating that has passed etching is subjected to another ultra-clean cleaning, ultrasonically cleaned with deionized water for 5 minutes, and then dried with nitrogen; then placed in a vacuum coating machine and evacuated to 1×10⁻⁶. -5 To avoid defects such as oxidation and pinholes during the coating process, the Pa level is kept below 100 Pa. S42,Cr adhesion layer deposition, that is, using magnetron sputtering process, using a chromium target as the target material, controlling the sputtering power to 100~200W, the deposition rate to 0.1~0.2nm / s, to deposit a 10~20nm thick Cr layer on the grating surface; during the deposition process, the substrate temperature is controlled to room temperature~100℃ to avoid deformation of the grating structure due to high temperature; S43, Au reflective layer deposition: After the Cr layer is deposited, there is no need to remove the substrate. Continue with magnetron sputtering using a gold target, controlling the sputtering power at 200~300W and the deposition rate at 0.3~0.5nm / s to deposit a 100-300nm thick Au layer on the Cr layer (thickness greater than the skin depth of the incident light to ensure high reflectivity); maintain a vacuum environment throughout the process to ensure that the Au layer is uniform, dense, and free of pinholes. In this process, a chromium metal layer is sputtered onto the substrate surface using a magnetron sputtering method, and then a gold layer is deposited on the chromium layer using a magnetron sputtering method. S44, film layer inspection, which involves using an ellipsometer to measure the thickness of the Cr and Au layers to ensure a thickness error of <±5nm; using a scanning electron microscope to measure the film layer density to ensure there are no pinholes or detachment defects; and using a spectrophotometer to measure the reflectance of the target wavelength range, i.e., visible light to near-infrared light, to ensure a reflectance >95%.

[0048] The scheme is simple and well-designed, depositing Cr and Au layers on the substrate while ensuring that the Cr and Au layers meet the relevant performance requirements.

[0049] Example 6 Based on the above embodiments, this embodiment also includes S5, post-processing and security detection, which specifically includes the following steps: S51, edge post-processing, is to chamfer and polish the edges of the grating to remove burrs and residual film generated during etching and coating, so as to avoid edge defects affecting the overall performance of the grating; after processing, ultra-clean cleaning is performed again to remove edge debris. S52, geometric parameter detection, that is, using AFM and SEM to re-detect the period, groove depth, line width, and sidewall angle of the grating to ensure that all geometric parameters meet the design requirements, with a period error of ±0.5nm; S53, Optical performance testing, which involves using a spectrophotometer to test the 0th / -1st order diffraction efficiency of the grating to ensure that the diffraction efficiency meets the design standards; using an interferometer to test the wavefront aberration; and using a stray light tester to test the stray light level to ensure that the optical performance meets the standards. S54, Environmental stability test, which involves high and low temperature cycling test on the grating, -40℃~85℃, 8-12 cycles, and damp heat test, temperature 30℃-50℃, humidity 80%-90%, for 20-30 hours, to verify the adhesion of the metal film and the stability of the grating structure.

[0050] The solution is simple and reasonably designed. It involves chamfering and polishing the edges of the grating to remove burrs and residual film generated during etching and coating, thus preventing edge defects from affecting the overall performance of the grating. After processing, it is then subjected to ultra-clean cleaning to remove edge debris. Meanwhile, the geometric parameters, optical performance, and environmental stability of the grating were tested to ensure that all geometric parameters and optical performance of the grating met the standards, and the adhesion of the metal film and the stability of the grating structure were verified, with no problems such as film peeling or structural deformation.

[0051] Example 7 Based on the above embodiments, this embodiment also includes S6, which involves using a dicing machine or a laser cutter to cut the grating into the required size.

[0052] The scheme has a simple structure and reasonable design. It uses a dicing machine or laser cutting machine to cut the grating to obtain the required grating size.

[0053] Example 8 Based on the above embodiments, this embodiment also provides a metal reflective grating based on the external cavity frequency selection of a narrow linewidth laser, comprising: a transparent substrate 1, on which a metal layer 2 is etched, the refractive index of the metal layer 2 being higher than that of the substrate 1; the metal layer 2 having a grating-like structure.

[0054] The scheme has a simple structure and reasonable design. By etching a metal layer on the substrate to form a metal reflective grating, the metal reflective grating can suppress the diffraction efficiency of the 0th order light and concentrate the energy to the ±1st order, thereby improving the system signal contrast and signal-to-noise ratio.

[0055] Preferably, in this embodiment, the substrate 1 is made of a transparent material with a low refractive index, such as fused silica.

[0056] The metal layer 2 is composed of a metal material with a higher refractive index than the substrate.

[0057] In addition, the operating wavelength of this metal reflective grating is 630~640nm, with a center wavelength of 635nm; the diffraction efficiency is ≥65% for the -1st order diffraction efficiency.

[0058] Preferably, in this embodiment, the overall size of the metal reflective grating is 18mm×6mm×6mm; the grating is a one-dimensional grating with a line density of 2400 lines / mm, material (grating teeth: pure gold, grooves: air), incident angle of 83.6°, and polarization state (TM).

[0059] In addition, the grating layer of the metal reflective grating is configured with a period of 416.67 nm, a groove width of s = 300 nm, a groove depth of d = 160 nm, a scribe line error of <0.8 nm, and a power incident on the metal reflective grating of less than 20 mW.

[0060] Based on the above scheme, the core principle of applying the metal reflective grating to the external cavity frequency selection of a narrow linewidth laser provided in this embodiment is as follows: using a semiconductor gain chip as the gain medium and a metal reflective grating as the external cavity frequency selection feedback element, an external cavity resonant structure is formed by a plane mirror and a diffraction grating: the metal reflective grating acts as a frequency selection device to achieve wavelength tuning and linewidth narrowing. The laser is fed back through the external cavity mirror, and the diffracted light returns to the grating after being reflected by the mirror. By adjusting the mirror to rotate around a fixed point, continuous tuning of the output wavelength can be achieved.

[0061] Example 9 Based on the above embodiments, in this embodiment, the metal layer 2 includes a transition layer 21 and a gold layer 22. The transition layer 21 is distributed between the gold layer 22 and the substrate 1, and the gold layer 22 has a grating-like structure.

[0062] The scheme has a simple structure and a reasonable design. The metal layer 2 includes a transition layer 21 and a gold layer 22. The function of the transition layer 21 is to make the gold layer 22 bond more firmly with the fused silica substrate.

[0063] Example 10 Based on the above embodiments, in this embodiment, the transition layer 21 is a chromium layer with a thickness of 40-60 nm; the gold layer 22 has a thickness of 100-300 nm.

[0064] The scheme has a simple structure and a reasonable design. The chromium layer is a transition layer between the quartz substrate and the gold layer. The role of the chromium layer is to make the gold layer bond more firmly to the quartz substrate. Meanwhile, the thickness of the chromium and gold layers is designed to meet the corresponding structural performance requirements.

[0065] Preferably, in this embodiment, the thickness of the chromium layer is 50 nm.

[0066] In addition, the thickness of the gold layer 22 is preferably 200 nm.

[0067] This invention provides a metal reflective grating based on the external cavity frequency selection of a narrow linewidth laser and its fabrication method. The fabrication method includes the following specific steps: S1, the substrate is prepared and pretreated, specifically including the following steps: S11 uses high-purity fused silica substrate, cut to a specified size; S12, single-sided polishing treatment of fused silica substrate, controlling surface shape accuracy to λ / 10 (λ=632.8nm), surface roughness Ra<0.5nm; S13, the fused silica substrate from S12 is first immersed in SC-1 cleaning solution for 10-15 minutes to remove surface organic matter and particles; then immersed in SC-2 cleaning solution for 10-15 minutes to remove surface metal ions; finally rinsed repeatedly with deionized water 3-5 times and dried with nitrogen to obtain the fused silica substrate. S14. Place the dried fused silica substrate into an oven and dry it at 80~100℃ for 20-40 minutes to completely remove residual moisture from the surface. Cool it to room temperature before use. S2, performing electron beam lithography on the substrate in S1, specifically including the following steps: S21, Photoresist coating, which is to use spin coating to uniformly coat high-resolution electron beam photoresist onto a clean fused silica substrate surface, controlling the spin coating speed to 3500~5500 r / min, the spin coating time to 40~60 s, and finally controlling the photoresist thickness to 300~600 nm. S22, pre-baking, which involves placing the fused silica substrate coated with electron beam photoresist into a hot plate and soft-baking at 180~200℃ for 10~15 minutes to completely remove the solvent from the photoresist. S23, Electron Beam Exposure: The fused silica substrate is placed in an electron beam lithography apparatus. The electron beam acceleration voltage is set to 10~20kV, and the beam spot size is ≤5nm. Based on the design requirement of 2400 lines / mm (period 416.7nm), a pre-drawn raster pattern file is imported, and the electron beam scanning speed and dose are precisely controlled at 50~100μC / cm. 2 Selective exposure of the photoresist ensures the formation of periodic exposure areas and meets the linewidth uniformity standard; S24, Development: The exposed fused silica substrate is placed in the corresponding developer and developed at room temperature for 60-90 seconds to remove the photoresist in the exposed area and form a periodic electron beam photoresist grating mask; after development, it is rinsed with isopropanol to terminate the development reaction and dried with nitrogen. S25, post-baking, that is, placing the developed fused silica substrate into a hot plate and hard baking at 120~140℃ for 1~2 minutes; S3, dry etching is performed on the substrate in S2, specifically including the following steps: S31, Etching process debugging, that is, selecting an inductively coupled plasma (ICP) etching machine, introducing a CF4 / Ar mixed etching gas (CF4:Ar=1:2~1:3), and optimizing process parameters: RF power 300~500W, chamber pressure 1~5 Pa, etching temperature room temperature~50℃; S32, ICP etching, involves placing the qualified fused silica substrate into the etching machine and etching according to the calibrated parameters. The goal is to accurately transfer the pattern of the photoresist mask into the fused silica substrate to form a rectangular trench structure. The trench depth is controlled between 100 and 300 nm, and the sidewall verticality is >85°. S33, photoresist removal: After etching, the fused silica substrate is removed and placed in an oxygen plasma ashing instrument. Oxygen is introduced at a power of 200-300W for 5-10 minutes to remove the residual photoresist mask. Then, it is ultrasonically cleaned with acetone and ethanol for 5-10 minutes in sequence to thoroughly remove organic residues. After rinsing with deionized water, it is dried with nitrogen. S34, Groove shape detection, that is, using AFM and SEM to re-inspect the groove depth, sidewall angle, and period uniformity of the fused silica grating; S4, perform metal layer deposition on the surface of the substrate in S3 to obtain the grating, specifically including the following steps: S41, Pre-treatment before coating: The fused silica grating that has passed etching is subjected to another ultra-clean cleaning, ultrasonically cleaned with deionized water for 5 minutes, and then dried with nitrogen; then placed in a vacuum coating machine and evacuated to 1×10⁻⁶. -5 Below Pa; S42,Cr adhesion layer deposition, that is, using magnetron sputtering process, using a chromium target as the target material, controlling the sputtering power to 100~200W, the deposition rate to 0.1~0.2nm / s, to deposit a 10~20nm thick Cr layer on the grating surface; during the deposition process, the substrate temperature is controlled to room temperature~100℃; S43, Au reflective layer deposition, that is, after the Cr layer is deposited, there is no need to remove the substrate. Continue to use the magnetron sputtering process, using a gold target as the target material, controlling the sputtering power to 200~300W, the deposition rate to 0.3~0.5nm / s, and depositing a 100-300nm thick Au layer on the Cr layer. S44, Film layer inspection, which involves using an ellipsometer to measure the thickness of the Cr and Au layers, ensuring a thickness error of <±5nm; using a scanning electron microscope to measure the film layer density, ensuring the absence of pinholes and flaking defects; and using a spectrophotometer to measure the reflectance in the target wavelength range, i.e., visible light to near-infrared, ensuring a reflectance >95%. S5, post-processing and safety inspection, specifically includes the following steps: S51, edge post-processing, is to chamfer and polish the edges of the grating to remove burrs and residual film generated during etching and coating. S52, geometric parameter detection, that is, using AFM and SEM to re-detect the period, groove depth, line width, and sidewall angle of the grating to ensure that all geometric parameters meet the design requirements, with a period error of ±0.5nm; S53, Optical performance testing, which involves using a spectrophotometer to test the 0th / -1st order diffraction efficiency of the grating to ensure that the diffraction efficiency meets the design standards; using an interferometer to test the wavefront aberration; and using a stray light tester to test the stray light level to ensure that the optical performance meets the standards. S54, environmental stability test, which involves high and low temperature cycling test of the grating, -40℃~85℃, 8-12 cycles, and simultaneously damp heat test, temperature 30℃-50℃, humidity 80%-90%, for 20-30 hours, to verify the adhesion of the metal film and the stability of the grating structure. S6, the grating can be cut to the required size using a dicing machine or laser cutter.

[0068] This invention provides a metal reflective grating based on the external cavity frequency selection of a narrow linewidth laser and its fabrication method. The structure and fabrication method are simple. The metal reflective grating can suppress the diffraction efficiency of the 0th order light and concentrate the energy to the ±1st order, which can improve the system signal contrast and signal-to-noise ratio. At the same time, it can improve the design and concentrate the diffraction energy to any desired order to achieve different applications.

[0069] This invention provides a metal reflective grating based on the external cavity frequency selection of a narrow linewidth laser and its fabrication method. The purpose is to overcome the shortcomings of the prior art and improve the problems of poor control accuracy, poor etching depth, poor process control or high cost of the current reflective gratings. The metal reflective grating designed and fabricated using this method can compress the efficiency of the 0th order light to less than 1%, and greatly improve the efficiency of the ±1st order light.

[0070] This invention provides a metal reflective grating based on the external cavity frequency selection of a narrow linewidth laser and its fabrication method, which has the following beneficial effects: (1) Since the grating is made of two different materials, the substrate material and the metal layer material, and the refractive indexes of the two are different, the main function of the grating is to suppress the zero order and realize ±1 order diffraction. It is used in conjunction with the plane mirror to realize the system function described in the background.

[0071] (2) Since the etching depth of the grating steps is at the level of hundreds of nanometers, and the present invention uses photoresist with good steepness, the steepness of the sidewall of the etching steps can be precisely controlled.

[0072] (3) The grating structure of the present invention is simple, the manufacturing method can be quantitatively controlled, and the manufacturing cost is low, which provides conditions for mass production in the later stage.

[0073] The design of a metal reflective grating, as indicated by the specifications, requires a periodic electromagnetic structure. This structure spatially and periodically modulates the amplitude, phase, or polarization state of the incident electromagnetic wave to achieve functions such as beam splitting, deflection, and filtering. Metal reflective gratings possess excellent conductivity and optical reflection properties, giving them irreplaceable advantages in polarization control and high-reflectivity diffraction devices in the infrared to visible light band. However, because the diffraction characteristics of this grating are highly sensitive to structural parameters (groove width and depth), and the diffraction behavior under oblique incidence and specific polarization states requires strict consideration of electromagnetic coupling effects, traditional scalar diffraction theory can no longer meet the design accuracy requirements.

[0074] Therefore, this invention employs the rigorous coupled-wave analysis (RCWA) method for full-vector electromagnetic simulation. Through parameter scanning optimization of the system, the optimal structural parameters are finally determined, providing theoretical support for the actual fabrication of the device.

[0075] For the conditions of "small period, deep trench, metallic medium, and large angle of incidence" in this design, RCWA can accurately capture the reflection, transmission and multi-level coupling effects of electromagnetic waves at the metal-air interface. The accuracy of the calculation results is much higher than that of scalar diffraction theory, which is the core of this design.

[0076] Polarization state is one of the key factors affecting the diffraction efficiency of a grating. The electric field vector of TM polarization (p polarization) is parallel to the incident plane, and its electromagnetic response in a metal grating differs significantly from that of TE polarization (s polarization, where the electric field is perpendicular to the incident plane). The optimization objective of this design focuses on TM polarization, which requires adjusting structural parameters to ensure that the -1st order diffraction meets the phase-matching condition, while suppressing the optical power loss of other orders and maximizing the energy distribution of the target order.

[0077] As stated in the design specifications, according to the grating equation, its diffraction orders only exist at the 0th and -1st orders, and its diffraction mechanism mainly includes: S15-1, Specular reflection of incident light on the surface of the grating teeth: Part of the incident light is directly reflected by the surface of the gold teeth, forming 0th order diffraction light; S15-2, Multiple reflections and coupling of incident light entering the slot: After the incident light is refracted into the air slot, it undergoes multiple reflections at the upper and lower interfaces (air-gold) of the slot, resulting in a phase difference between the reflected light. S15-3, Multi-order coupling effect: The reflected light inside the slot and the reflected light from the surface of the grating teeth interfere in space, and the order of the grating equations forms diffracted light; In S15-4, the depth d determines the phase accumulation of the reflected light in the groove, and the width s determines the proportion of the light field participating in the coupling. Therefore, the precise matching of d and s is the key to maximizing the -1st order diffraction efficiency.

[0078] The core of grating diffraction is satisfying the phase-matching condition, meaning the propagation directions of each order of diffracted light must satisfy the grating equations. For oblique incidence, the equations for a reflection grating are as follows:

[0079] in The refractive index of the reflecting medium, The refractive index of the incident medium is... The incident wavelength, For the grating period, Let m be the incident angle and m be the diffraction order. This corresponds to the diffraction angle of the diffraction order.

[0080] Diffraction efficiency is a core indicator for evaluating grating performance. For metallic gratings, due to the high absorption characteristics of metals, the diffraction efficiency must consider the reflection diffraction efficiency (the diffraction efficiency formed after the incident light is reflected by the grating). Its calculation formula is as follows:

[0081] in: The m-th order diffraction efficiency Let be the power of the m-th order diffracted light. Let be the total power of the incident light. Due to the high absorption of pure gold, the transmission diffraction efficiency is negligible; therefore, this design focuses only on the reflection diffraction efficiency. The optimization objective is to maximize the (-1st order reflection diffraction efficiency) while ensuring stability in the wavelength range of 630nm to 640nm.

[0082] TM polarization diffraction of a metal reflective grating is a complex process involving the interaction between electromagnetic waves and a periodic metal-air interface. Its essence lies in the multi-field coupling and interference effects of the incident light field, the induced current field on the metal surface, and the standing wave light field within the groove. Based on the parameters of this design, the diffraction mechanism can be developed from three core stages: light field coupling, phase accumulation, and interference filtering. These stages work synergistically to ultimately determine the -1st order diffraction efficiency, as detailed below: S18-1, Initial coupling between incident light field and metal surface By design, when incident light is obliquely incident on the grating surface at a large angle of 0~90°, it first interacts with the periodically distributed pure gold grating teeth to form initial optical field coupling. This process directly determines the proportion of the optical field participating in subsequent diffraction and the degree of polarization state retention. (1) Decomposition of optical field components: The electric field vector of the TM polarized incident light is parallel to the incident surface and can be decomposed into a component along the grating period direction (X-axis) and a component perpendicular to the grating surface direction (Z-axis); the magnetic field vector is perpendicular to the incident surface (Y-axis direction). Among them, the X and Z components are the key components for exciting the surface current and surface plasma of the metal, and thus dominate the longitudinal propagation of the optical field in the groove.

[0083] (2) Induced current on metal surface: Pure gold is a good conductor. Its free electrons undergo collective oscillation under the drive of the incident light component, forming a periodic induced current. This induced current will radiate new electromagnetic waves, becoming one of the important sources of diffracted light. Its radiation direction is determined by the grating period and the current oscillation frequency.

[0084] (3) Light field reflection and transmission distribution: Due to the high reflectivity (reflectivity > 95%) of pure gold in the 630~640nm wavelength range, most of the incident light is reflected by the surface of the grating teeth (forming the initial reflected light field), and only a very small amount of light (penetration depth 5~10nm) is absorbed by the metal; at the same time, some of the incident light is refracted into the air groove (transmitted light field), and its refraction angle satisfies Snell's law: It propagates almost parallel to the tank wall.

[0085] S18-2, Multiple reflections and phase accumulation of the light field within the slot The transmitted light field entering the air slot is not emitted directly, but is reflected multiple times between the upper interface (air-gold tooth top) and the lower interface (air-gold substrate) of the slot, forming a standing wave light field. The core of this process is the phase accumulation effect, which is directly determined by the depth d.

[0086] (1) Reflection process and light field superposition: The light field in the slot first propagates to the bottom of the slot and is reflected (under TM polarization, the metal surface reflectance coefficient and the phase change of the reflected light π); the reflected light propagates upward to the top of the slot (the top surface of the metal tooth) and is reflected again (also accompanied by the phase change π); after multiple reflections, a standing wave is formed in the slot along the z-axis direction, and the positions of the nodes and antinodes of the standing wave are determined by the depth d and the wavelength λ.

[0087] (2) Phase difference calculation: The optical path difference between two adjacent reflected beams (e.g., the first reflected beam and the third reflected beam) is... ( (where is the propagation angle of the light field within the slot), and the corresponding phase difference is . (Both reflections produce a π phase shift, with a total shift of 2π, which is equivalent to no additional phase difference).

[0088] S18-3, Interference Sorting and Energy Distribution in Multi-Order Diffraction The periodic structure of the grating causes all reflected light to interfere in space. Only the diffraction orders that satisfy the grating equation can form stable propagating light, while other orders are suppressed due to destructive interference.

[0089] This design maximizes the -1st order diffraction efficiency of TM polarization through a process of parameter definition, model building, parameter scanning simulation, result analysis, and optimal parameter determination. The specific steps are as follows: Based on the design requirements, all fixed and variable parameters are defined: S19-1, fixed parameters: set the overall size of the grating to 18mm×6mm×6mm; line density 2400lines / mm; material (grating teeth: pure gold, groove: air); incident angle 0~90° scanning; polarization state (TM); wavelength (630nm, 635nm, 640nm). S19-2, variable parameters: groove width s (50nm~350nm), groove depth d (100nm~300nm); S19-3, Sweep step size: To ensure simulation accuracy and efficiency, the sweep step size of s is set to 5nm (61 sampling points in total), and the sweep step size of d is set to 5nm (41 sampling points in total). The total number of sweep combinations is 61×41=2501 sets, and 2501 sets of simulations are completed under each wavelength.

[0090] The RCWA model was established. Since the computational accuracy of the RCWA method is affected by the Fourier number truncation order, a Fourier series truncation order parameter scan needs to be performed beforehand to determine its impact on the simulation results in order to balance simulation accuracy and speed. The truncation order was set from 5 to 350, and the -1st order diffraction efficiency distribution curve was plotted as follows. Figure 4As shown, the numerical calculation results tend to be stable when the truncation order is greater than 225. Therefore, this design selects the Fourier series truncation order of 200 as the electromagnetic calculation parameter.

[0091] A two-dimensional full-factor scan was employed, meaning that simulations were performed one by one for all combinations of groove width s and groove depth d. The -1st order diffraction efficiency was calculated for each combination at three wavelengths: 630 nm, 635 nm, and 640 nm. The resulting diffraction efficiency distribution is shown in the figure below. Figures 5 to 7 As can be seen, the region with a diffraction efficiency greater than 45% in the -1st order has been marked with a red box in the figure.

[0092] The design results, obtained through full vector simulation with 2501 sets of parameters, show the variation trend of the -1st order diffraction efficiency with groove width s and groove depth d at three wavelengths: 630nm, 635nm, and 640nm. Figures 5 to 7 As can be seen, the curved clouds with a diffraction efficiency greater than 45% at the -1st order under the three wavelengths are distributed in the upper right middle part and the lower left part of the figure. To ensure that the selected parameters are easy to process and manufacture, the optimal parameters are selected only from the upper right middle part of the figure. The final selected optimal parameters are: incident medium air (n=1.0), incident angle θ=83.6° (relative to the normal), polarization state TM; grating layer settings: period 416.67nm, groove width s=300nm, groove depth d=160nm; as the final optimal parameters, the corresponding -1st order diffraction efficiency at the TM polarization of the 630nm~640nm wavelength; substrate settings: 200nm pure gold substrate (absorbs all transmitted light, no transmitted diffraction order), as shown in Table 1 below. In summary, the simulation of this design meets the project requirements.

[0093] Table 1. Diffraction efficiency of the -1st order under the project wavelength requirements.

[0094] While embodiments or examples of this disclosure have been described with reference to the accompanying drawings, it should be understood that the above embodiments are merely exemplary embodiments or examples, and the scope of the invention is not limited by these embodiments or examples, but only by the granted claims and their equivalents. Various elements in the embodiments or examples may be omitted or replaced by their equivalents. Furthermore, the steps may be performed in a different order than that described in this disclosure. Further, various elements in the embodiments or examples may be combined in various ways. Importantly, as the technology evolves, many elements described herein can be replaced by equivalents that appear after this disclosure.

Claims

1. A method for fabricating a metal reflective grating based on frequency selection in the external cavity of a narrow-linewidth laser, characterized in that, The specific steps include the following: S1, the substrate is prepared and pretreated; S2, electron beam lithography is performed on the substrate in S1; S3, dry etching is performed on the substrate in S2; S4, perform metal layer deposition on the surface of the substrate in S3 to obtain a grating.

2. The method for fabricating a metal reflective grating based on frequency selection of a narrow-linewidth laser external cavity according to claim 1, characterized in that, S1 includes the following specific steps: S11 uses high-purity fused silica substrate, cut to a specified size; S12, single-sided polishing of fused silica substrate to control surface shape accuracy to λ / 10, where λ=632.8nm and surface roughness Ra<0.5nm; S13, the fused silica substrate from S12 is first immersed in SC-1 cleaning solution for 10-15 minutes to remove surface organic matter and particles; then immersed in SC-2 cleaning solution for 10-15 minutes to remove surface metal ions; finally rinsed repeatedly with deionized water 3-5 times and dried with nitrogen to obtain the fused silica substrate. S14. Place the dried fused silica substrate into an oven and dry it at 80~100℃ for 20-40 minutes to completely remove residual moisture from the surface. Cool it to room temperature before use.

3. The method for fabricating a metal reflective grating based on frequency selection of a narrow-linewidth laser external cavity according to claim 1, characterized in that, S2 includes the following specific steps: S21, Photoresist coating, which is to use spin coating to uniformly coat high-resolution electron beam photoresist onto a clean fused silica substrate surface, controlling the spin coating speed to 3500~5500 r / min, the spin coating time to 40~60 s, and finally controlling the photoresist thickness to 300~600 nm. S22, pre-baking, which involves placing the fused silica substrate coated with electron beam photoresist into a hot plate and soft-baking at 180~200℃ for 10~15 minutes to completely remove the solvent from the photoresist. S23, Electron Beam Exposure: The fused silica substrate is placed in an electron beam lithography apparatus. The electron beam acceleration voltage is set to 10~20kV, and the beam spot size is ≤5nm. Based on the design requirements of 2400 lines / mm and a period of 416.7nm, a pre-drawn grating pattern file is imported, and the electron beam scanning speed and dose are precisely controlled at 50~100μC / cm. 2 Selective exposure of the photoresist ensures the formation of periodic exposure areas and meets the linewidth uniformity standard; S24, Development: The exposed fused silica substrate is placed in the corresponding developer and developed at room temperature for 60-90 seconds to remove the photoresist in the exposed area and form a periodic electron beam photoresist grating mask. After development, rinse with isopropanol to stop the development reaction, and then dry with nitrogen. S25, post-baking, which involves placing the developed fused silica substrate onto a hot plate and hard-baking at 120~140℃ for 1~2 minutes.

4. The method for fabricating a metal reflective grating based on the frequency selection of a narrow-linewidth laser external cavity according to claim 1, characterized in that, S3 includes the following specific steps: S31, Etching process debugging, that is, selecting an inductively coupled plasma etching machine, introducing a CF4 / Ar mixed etching gas, wherein CF4:Ar=1:2~1:3, optimizing process parameters: RF power 300~500W, chamber pressure 1~5 Pa, etching temperature room temperature~50℃; S32, Inductively Coupled Plasma Etching, involves placing the qualified fused silica substrate into the etching machine and etching according to the calibrated parameters. The goal is to accurately transfer the pattern of the photoresist mask into the fused silica substrate to form a rectangular trench structure. The trench depth is controlled between 100 and 300 nm, and the sidewall verticality is >85°. S33, photoresist removal: After etching, the fused silica substrate is removed and placed in an oxygen plasma ashing instrument. Oxygen is introduced at a power of 200-300W for 5-10 minutes to remove the residual photoresist mask. Then, it is ultrasonically cleaned with acetone and ethanol for 5-10 minutes in sequence to thoroughly remove organic residues. After rinsing with deionized water, it is dried with nitrogen. S34, Groove shape detection, that is, using AFM and SEM to re-inspect the groove depth, sidewall angle, and period uniformity of the fused silica grating.

5. The method for fabricating a metal reflective grating based on frequency selection of a narrow-linewidth laser external cavity according to claim 1, characterized in that, S4 includes the following specific steps: S41, Pre-treatment before coating: The fused silica grating that has passed etching is subjected to another ultra-clean cleaning, ultrasonically cleaned with deionized water for 5 minutes, and then dried with nitrogen; then placed in a vacuum coating machine and evacuated to 1×10⁻⁶. -5 Below Pa; S42,Cr adhesion layer deposition, that is, using magnetron sputtering process, using a chromium target as the target material, controlling the sputtering power to 100~200W, the deposition rate to 0.1~0.2nm / s, to deposit a 10~20nm thick Cr layer on the grating surface; During the deposition process, the substrate temperature was controlled to be between room temperature and 100℃. S43, Au reflective layer deposition, that is, after the Cr layer is deposited, there is no need to remove the substrate. Continue to use the magnetron sputtering process, using a gold target as the target material, controlling the sputtering power to 200~300W, the deposition rate to 0.3~0.5nm / s, and depositing a 100-300nm thick Au layer on the Cr layer. S44, film layer inspection, which involves using an ellipsometer to measure the thickness of the Cr and Au layers to ensure a thickness error of <±5nm; using a scanning electron microscope to measure the film layer density to ensure there are no pinholes or detachment defects; and using a spectrophotometer to measure the reflectance of the target wavelength range, i.e., visible light to near-infrared light, to ensure a reflectance >95%.

6. The method for fabricating a metal reflective grating based on the frequency selection of an external cavity of a narrow-linewidth laser according to any one of claims 1-5, characterized in that, It also includes S5, post-processing and safety testing, specifically including the following steps: S51, edge post-processing, is to chamfer and polish the edges of the grating to remove burrs and residual film generated during etching and coating. S52, geometric parameter detection, that is, using AFM and SEM to re-detect the period, groove depth, line width, and sidewall angle of the grating to ensure that all geometric parameters meet the design requirements, with a period error of ±0.5nm; S53, Optical performance testing, which involves using a spectrophotometer to test the 0th / -1st order diffraction efficiency of the grating to ensure that the diffraction efficiency meets the design standards; using an interferometer to test the wavefront aberration; and using a stray light tester to test the stray light level to ensure that the optical performance meets the standards. S54, Environmental stability test, which involves high and low temperature cycling test on the grating, -40℃~85℃, 8-12 cycles, and damp heat test, temperature 30℃-50℃, humidity 80%-90%, for 20-30 hours, to verify the adhesion of the metal film and the stability of the grating structure.

7. The method for fabricating a metal reflective grating based on frequency selection of a narrow-linewidth laser external cavity according to claim 6, characterized in that, It also includes S6, which uses a dicing machine or laser cutter to cut the grating to the required size.

8. A metal reflective grating based on frequency selection of an external cavity in a narrow-linewidth laser, characterized in that, include: A transparent substrate (1) has a metal layer (2) etched on it, the refractive index of the metal layer (2) being higher than that of the substrate (1); the metal layer (2) has a grating-like structure.

9. The metal reflective grating based on the frequency selection of a narrow-linewidth laser external cavity according to claim 8, characterized in that, The metal layer (2) includes a transition layer (21) and a gold layer (22). The transition layer (21) is distributed between the gold layer (22) and the substrate (1). The gold layer (22) has a grating structure.

10. The metal reflective grating based on the frequency selection of a narrow-linewidth laser external cavity according to claim 9, characterized in that, The transition layer (21) is a chromium layer with a thickness of 40-60 nm; the gold layer (22) has a thickness of 100-300 nm.

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