A random metal mesh, and a method of making and using the same

Abstract

The application relates to the technical field of electromagnetic shielding materials, and particularly discloses a random metal mesh, a preparation method and application thereof. The roughness, period and width of a photoresist mesh line are precisely controlled through a laser direct writing technology, a stress induction mechanism of high-temperature baking-rapid cooling is combined, the difference in thermal expansion coefficients of the photoresist and a metal layer is utilized to drive the metal film to break along stress concentration points, and uniform random cracks with line edges extending inward are formed. Meanwhile, through the synergy of a developing process and ultraviolet flood exposure, the photoresist at the bottom of the cracks is completely removed, the prepared metal mesh has the advantages of preventing high-order diffraction stray light and high electromagnetic shielding performance, the process has strong repeatability, is suitable for industrial mass production, has a wide application prospect, and can meet the demand for high-stability and high-performance random metal meshes in different scenes.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic shielding materials technology, and in particular to a random metal mesh grid, its preparation method, and its application. Background Technology

[0002] With the rapid development of modern military technology, weaponry not only requires optical windows to possess excellent wear and thermal shock resistance to withstand harsh environments, but also excellent electromagnetic shielding performance to prevent external electromagnetic waves such as cosmic rays, radar waves, and radio waves from interfering with internal components or preventing internal electromagnetic signals from leaking out and becoming detection sources. Common electromagnetic shielding technologies for optical windows include transparent conductive oxide coating technology, metal mesh thin film technology, and subwavelength microstructure dielectric thin film technology. Comparatively, metal mesh thin film technology is more mature and has long been mass-produced. However, with technological advancements, the drawbacks of periodic metal meshes have become apparent: these meshes cannot avoid stray light effects caused by higher-order diffraction, thus affecting the imaging sensitivity and accuracy of infrared detection systems. Therefore, research on metal mesh technology has shifted towards the application of random metal meshes, especially the fabrication of random metal meshes on curved optical windows, which is a promising development trend due to its ability to balance environmental adaptability, electromagnetic shielding, and imaging quality.

[0003] Currently, there are two main categories of methods for fabricating random metal mesh gratings: The first is the photomask lithography method: This involves designing random metal mesh patterns using simulation software and fabricating a photomask, then using this fixed photomask to fabricate the metal mesh. To ensure resolution and shielding performance, this method requires a tight bond between the photomask and the photoresist layer, but this process can damage the photomask, necessitating periodic replacement. Furthermore, from a technical compatibility perspective, this method is only suitable for fabricating planar metal mesh gratings, failing to meet the requirements of curved optical windows and hindering efficient processing of large-area substrates. The second category is the mask slurry drying method: This method allows the mask slurry to naturally dry and crack, forming a mesh template, and then fabricating the random metal mesh grating. However, this method cannot effectively control the density and randomness of the cracks, making it difficult to accurately obtain the desired random or periodic metal mesh grating structure. It also suffers from low processing efficiency for large areas, failing to meet the comprehensive requirements of military equipment for high precision, high reliability, and large-scale application of optical windows.

[0004] Therefore, it is necessary to develop a method for fabricating a random metal mesh structure that can meet the requirements of efficient processing of large-area substrates and adaptation to curved optical windows, while also ensuring the electromagnetic shielding performance and imaging quality of the optical window. Summary of the Invention

[0005] To address the challenges of existing random mesh grating fabrication methods in achieving efficient processing of large-area substrates, adapting to curved optical windows, and ensuring electromagnetic shielding performance and imaging quality of optical windows, this invention provides a random metal mesh grating, its fabrication method, and its applications.

[0006] To solve the above-mentioned technical problems, the first aspect of the present invention provides a method for preparing a random metal mesh grid, comprising the following steps: S1, a photoresist layer is formed on the substrate surface; S2, using laser direct writing technology to photolithographically lithographically lithographically develop the photoresist on the substrate surface to form rough photoresist grid lines on the substrate surface; wherein, the roughness of the photoresist grid lines is 100nm~200nm, the period is 1mm~3mm, and the line width is 5μm~30μm; S3, deposit a metal layer on the substrate surface after photolithography, then heat the substrate to a first preset temperature to soften the photoresist, hold it for a first preset time, then cool it to a second preset temperature, and cool it at a low temperature for a second preset time to form a random mesh mask pattern on the substrate surface. S4, expose and develop the substrate with the random grid mask pattern on its surface using ultraviolet light to obtain the random grid pattern; S5. A thin metal film is deposited on the surface of the developed substrate and then peeled off to obtain a random metal mesh.

[0007] Compared to existing technologies, the random metal mesh fabrication method provided by this invention increases the roughness of the photoresist mesh lines by controlling the conditions of laser direct writing technology. Compared to smooth lines (when the roughness is small), the surface of rough lines has more stress concentration points, providing more initial sites for subsequent crack initiation and avoiding the problem that smooth lines are difficult to crack due to uniform stress distribution. At the same time, by controlling the period and line width of the photoresist mesh lines, the basic structural dimensions of the mesh are pre-set, fundamentally solving the technical defects of existing mask liquid drying cracking methods that rely on natural drying processes and cannot actively control the crack density and randomness of the mesh. This provides a stable foundation for the uniform generation of subsequent random cracks, thereby ensuring the consistency and stability of the final random mesh structure. Furthermore, by utilizing the difference in thermal expansion coefficients between the photoresist and the upper vapor-deposited metal layer, stress is induced through a "high-temperature baking-rapid cooling" process: high-temperature baking softens the photoresist and causes slight flow, while the upper metal layer, due to its different thermal expansion coefficient, does not undergo significant deformation, resulting in significant interfacial stress between the two. This interfacial stress exerts a continuous tensile force on the upper metal film, causing cracks to form in the metal film. Subsequently, rapid cooling to the set temperature further intensifies the stress change, causing the stress to be released along the concentration points of the rough surface of the photoresist lines, ultimately forming random cracks extending inward from the edge of the lines.

[0008] This invention, through the synergistic effect of roughness control and stress control, can stably generate random cracks that meet the requirements. It avoids the problem of high-order diffraction stray light in periodic grids and avoids the performance differences caused by process fluctuations in random structures in traditional methods. While meeting the comprehensive requirements of imaging accuracy and electromagnetic shielding, it significantly improves the repeatability and mass production potential of the process, making it more suitable for industrial production needs.

[0009] Specifically, S1 includes the following steps: cleaning the substrate, then spin-coating photoresist onto the substrate surface, baking, and forming a photoresist layer on the substrate surface.

[0010] Specifically, the substrate is ultrasonically cleaned with acetone and alcohol sequentially for 20 to 30 minutes, and then dried with nitrogen to obtain a clean substrate.

[0011] It should be noted that the method for preparing the random metal mesh described in this invention is applicable to the preparation of planar or curved optical windows, and the substrate can be either a flat substrate or a curved substrate.

[0012] For example, the substrate may be zinc sulfide, sapphire, spinel, diamond, zinc selenide, infrared germanium, or infrared silicon.

[0013] Furthermore, in S1, the coating thickness of the photoresist is 1μm~10μm.

[0014] Specifically, the photoresist is AZ4562 photoresist or AZ4620 photoresist. The present invention can select conventional photoresists in the art, and the present invention does not make any special limitation on the specific photoresist model.

[0015] Furthermore, in S1, the spin coating speed is 1000rpm~3000rpm, and the spin coating time is 59s~61s.

[0016] Furthermore, in S1, the baking temperature is 90℃~110℃, and the baking time is 2min~4min.

[0017] Furthermore, in S2, the motor speed loop gain of the laser direct writing head is 150Hz~180Hz, the laser direct writing power is 40mW~80mW, and the scanning rate is 1mm / s~10mm / s.

[0018] The optimized motor speed loop gain for laser direct writing enables the laser direct writing head to generate high-frequency, small-amplitude vibrations, thereby producing photoresist grid lines with relatively large roughness. This lays a precise line foundation for the subsequent stress-induced cracking to form uniform random cracks. The optimized power and scanning rate ensure the uniformity of line width and the clarity of the contour.

[0019] Furthermore, in S2, the developing solution used is a potassium hydroxide solution or sodium hydroxide solution with a mass concentration of 1.5% to 4%, and the developing time is 10s to 30s.

[0020] The optimized development conditions can effectively dissolve the unexposed photoresist to avoid residue, and prevent excessive corrosion that could cause line distortion, thus ensuring the accuracy and roughness of the grid lines and laying the foundation for subsequent processes.

[0021] Furthermore, in S3, the metal layer is a Cr metal layer.

[0022] Furthermore, in S3, the thickness of the metal layer is 50nm~150nm.

[0023] Furthermore, in S3, the evaporation temperature is 10℃~60℃.

[0024] Furthermore, in S3, the first preset temperature is 90℃~120℃, and the first preset time is 5min~30min.

[0025] Furthermore, in S3, the second preset temperature is 0℃~10℃, and the second preset time is 5min~30min.

[0026] By adjusting the high-temperature baking temperature (first preset temperature) and the cooling temperature (second preset temperature), the number, distribution period, and linewidth of cracks can be precisely adjusted, ultimately obtaining a random structure with uniform crack distribution and high linewidth consistency. This solves the problems of uncontrollable and poor uniformity of random mesh structures in traditional methods, laying the foundation for the subsequent preparation of high-quality random metal meshes.

[0027] Furthermore, in S4, the exposure dose of the ultraviolet light generalized exposure is 20mJ~40mJ.

[0028] The photoresist portion of the random grid mask pattern formed in S3 is further chemically cured by ultraviolet light exposure. After exposure, the photoresist area that needs to be retained in the mask pattern (corresponding to the skeleton of the subsequent metal grid) will undergo a cross-linking reaction due to photosensitivity, which enhances the structural stability and prevents the photoresist layer from dissolving and deforming during subsequent development due to uncured photoresist, thus ensuring the accuracy of the outline of the random grid pattern.

[0029] Furthermore, in S4, the developing solution used is a potassium hydroxide solution or sodium hydroxide solution with a mass concentration of 1.5% to 3%, and the developing time is 10s to 30s.

[0030] Further, in S5, the metal thin film includes a transition metal film layer and a main metal film layer; wherein, the material of the transition metal film layer is Cr, Ni or Ti, and the film layer thickness is 10nm~30nm; the material of the main metal film layer is Cu, Au, Ag or Al, and the film layer thickness is 100nm~500nm.

[0031] Furthermore, in S5, the evaporation temperature is 100℃~200℃.

[0032] Specifically, in S5, after the vapor deposition is completed, the substrate is immersed in an organic solvent for 10 to 30 minutes, and then ultrasonically peeled off. After peeling, the sample is cleaned with alcohol and deionized water in sequence, and finally dried with nitrogen to obtain a random metal mesh.

[0033] For example, the organic solvent mentioned above can be acetone or N-methylpyrrolidone.

[0034] Secondly, the present invention provides a random metal mesh grid, which is prepared by the method for preparing random metal mesh grids described in any one of the above claims.

[0035] Thirdly, the present invention provides an electromagnetic shielding optical window, the electromagnetic shielding optical window comprising the aforementioned random metal mesh.

[0036] The method for fabricating random metal mesh grids provided by this invention precisely controls the roughness, period, and width of the photoresist mesh grid lines using laser direct writing technology. Combined with a stress-induced mechanism of high-temperature baking and rapid cooling, the difference in thermal expansion coefficients between the photoresist and the metal layer drives the metal film to crack along stress concentration points, forming uniform random cracks extending inward from the line edges. Simultaneously, the clarity and stability of the random mesh grid pattern are further ensured through the synergistic effects of development and ultraviolet permeation. The fabricated metal mesh grid possesses the advantages of both protection against higher-order diffraction stray light and high electromagnetic shielding performance. Furthermore, the process is highly repeatable and suitable for industrial mass production, showing broad application prospects in the military field and meeting the demand for high-stability, high-performance random metal mesh grids in various scenarios. Attached Figure Description

[0037] Figure 1 This is a flowchart illustrating the fabrication of a random metal mesh grid according to an embodiment of the present invention; Figure 2 An image of the random metal mesh grid prepared in Embodiment 1 of the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0039] To better illustrate the present invention, further examples are provided below.

[0040] Example 1 This embodiment provides a method for preparing a random metal mesh grid, including the following steps: S1. The sapphire substrate of the flat window is ultrasonically cleaned with acetone and alcohol for 20 minutes in sequence, and then blown clean with a nitrogen gun to obtain a clean flat window substrate. S2, spin-coat AZ4562 photoresist onto the clean flat window substrate, spin-coat speed is 1500 rpm, spin-coat time is 60 s, spin-coat photoresist thickness is about 8 μm, then place the flat window in a clean oven and bake at 110℃ for 4 min to form a photoresist layer on the surface of the flat window. S3. Place the flat panel window on the fixture of the laser direct writing equipment, adjust the motor speed loop gain of the laser direct writing head to 180Hz to generate high-frequency small floating vibration of the writing head, control the laser direct writing power to 70mW, and the scanning rate to 1mm / s. Then perform laser direct writing lithography and development to form rough photoresist grid lines on the photoresist layer. The photoresist line period is 2mm and the line width is 10μm. The developer is a 3% sodium hydroxide solution, and the development time is 20s. S4. Place the flat window into the evaporation coating machine for metal evaporation. The evaporation metal is Cr, the metal layer thickness is 50nm, and the evaporation temperature is 30℃. S5. Place the flat window in a clean oven and bake at 110°C for 10 minutes. After baking, immediately remove it and place it in a low-temperature environment test chamber at 0°C for 10 minutes. The stress generated by the metal layer will cause cracks to appear from the edge defects of the line with high roughness and extend and spread inward to form a random grid mask pattern. S6. Place the flat plate window on the exposure machine for general exposure at an exposure dose of 35mJ, and then develop it using a 3% sodium hydroxide solution for a development time of 15s to obtain a random grid pattern. S7. Place the flat window into the evaporation coating machine for metal evaporation. The evaporated metal layers are Cr and Au. The thickness of the Cr layer is 10nm and the thickness of the Au layer is 200nm. The evaporation temperature is 200℃. S8. Immerse the plate window in N-methylpyrrolidone for 10 min, then perform ultrasonic peeling for 5 min. Clean the sample with alcohol and deionized water in sequence, and finally purge with a nitrogen gun to obtain a random metal mesh sample, which is denoted as optical window I.

[0041] Example 2 This embodiment provides a method for preparing a random metal mesh grid, including the following steps: S1. The curved window zinc sulfide substrate is ultrasonically cleaned with acetone and alcohol for 20 minutes in sequence, and then blown clean with a nitrogen gun to obtain a clean curved window substrate. S2, spin-coat AZ4620 photoresist onto the clean curved window substrate, spin-coat speed is 2500 rpm, spin-coat time is 60 s, spin-coat photoresist thickness is about 5 μm, then place the flat window in a clean oven and bake at 100℃ for 3 min to form a photoresist layer on the surface of the curved window. S3. Place the curved window on the fixture of the laser direct writing equipment, adjust the motor speed loop gain of the laser direct writing head to 150Hz to generate high-frequency small floating vibration of the writing head, control the laser direct writing power to 50mW, and the scanning rate to 4mm / s. Then perform laser direct writing lithography and development to form rough photoresist grid lines on the photoresist layer. The photoresist line period is 1mm and the line width is 5μm. The developer is a 2% sodium hydroxide solution, and the development time is 30s. S4. Place the curved window into the evaporation coating machine for metal evaporation. The evaporation metal is Cr, the metal layer thickness is 80nm, and the evaporation temperature is 30℃. S5. Place the curved window in a clean oven and bake at 100℃ for 30 minutes. After baking, take it out immediately and place it in a low temperature environment test chamber at 10℃ for 30 minutes. The stress generated by the metal layer will cause cracks to appear from the edge defects of the line with high roughness and extend and spread inward to form a random grid mask pattern. S6. Place the curved window on the exposure machine for general exposure with an exposure dose of 30mJ, and then develop it with a 2% sodium hydroxide solution for 30s to obtain a random grid pattern. S7. The curved window is placed in an evaporation coating machine for metal evaporation. The evaporated metal layers are Cr and Au, with a Cr layer thickness of 10nm and an Au layer thickness of 200nm. The evaporation temperature is 200℃. S8. Immerse the curved window in acetone for 10 minutes, then perform ultrasonic peeling for 5 minutes. Clean the sample with alcohol and deionized water in sequence, and finally purge with a nitrogen gun to obtain a random metal mesh sample, which is denoted as optical window II.

[0042] Example 3 This embodiment provides a method for preparing a random metal mesh grid, including the following steps: S1. The curved window sapphire substrate is ultrasonically cleaned with acetone and alcohol for 20 minutes in sequence, and then blown clean with a nitrogen gun to obtain a clean curved window substrate. S2, spin-coat AZ4620 photoresist onto the clean curved window substrate, spin-coat speed is 3000 rpm, spin-coat time is 60s, spin-coat photoresist thickness is about 3μm, then place the flat window in a clean oven and bake at 90℃ for 2min to form a photoresist layer on the surface of the curved window. S3. Place the curved window on the fixture of the laser direct writing equipment, adjust the motor speed loop gain of the laser direct writing head to 160Hz to generate high-frequency small floating vibration of the writing head, control the laser direct writing power to 80mW, and the scanning rate to 10mm / s. Then perform laser direct writing lithography and development to form rough photoresist grid lines on the photoresist layer. The photoresist line period is 3mm and the line width is 30μm. The developer is a 4% sodium hydroxide solution, and the development time is 10s. S4. Place the curved window into the evaporation coating machine for metal evaporation. The evaporation metal is Cr, the metal layer thickness is 150nm, and the evaporation temperature is 60℃. S5. Place the curved window in a clean oven and bake at 120°C for 5 minutes. After baking, immediately remove it and place it in a low-temperature environment test chamber at 0°C for 5 minutes. The stress generated by the metal layer will cause cracks to appear from the edge defects of the line with high roughness and extend and spread inward to form a random grid mask pattern. S6. Place the curved window on the exposure machine for general exposure with an exposure dose of 20mJ, and then develop it with a 3% sodium hydroxide solution for 10s to obtain a random grid pattern. S7. The curved window is placed in an evaporation coating machine for metal evaporation. The evaporated metal layers are Cr and Au, with a Cr layer thickness of 10nm and an Au layer thickness of 200nm. The evaporation temperature is 200℃. S8. Immerse the curved window in acetone for 10 minutes, then perform ultrasonic peeling for 5 minutes. Clean the sample with alcohol and deionized water in sequence, and finally purge with a nitrogen gun to obtain a random metal mesh sample, which is denoted as optical window III.

[0043] Comparative Example 1 This comparative example provides a method for preparing a random metal mesh grid. The only difference from Example 2 is that the gain of the motor speed loop of the laser direct writing head in step S3 is adjusted from 150Hz to 120Hz. The rest is exactly the same and will not be described again here. The obtained random metal mesh grid sample is denoted as optical window pair I.

[0044] Comparative Example 2 This comparative example provides a method for preparing a random metal mesh grid. The only difference from Example 2 is that the baking temperature in step S5 is adjusted from 100°C to 70°C. The rest is exactly the same. The obtained random metal mesh grid sample will not be described again here. It is referred to as optical window pair II.

[0045] Comparative Example 3 This comparative example provides a method for preparing a random metal mesh grid. The only difference from Example 2 is that the high-temperature baking step in step S5 is omitted. The remaining steps are exactly the same, and the specific steps are as follows: S1. The curved window zinc sulfide substrate is ultrasonically cleaned with acetone and alcohol for 20 minutes in sequence, and then blown clean with a nitrogen gun to obtain a clean curved window substrate. S2, spin-coat AZ4620 photoresist onto the clean curved window substrate, spin-coat speed is 2500 rpm, spin-coat time is 60 s, spin-coat photoresist thickness is about 5 μm, then place the flat window in a clean oven and bake at 100℃ for 3 min to form a photoresist layer on the surface of the curved window. S3. Place the curved window on the fixture of the laser direct writing equipment, adjust the motor speed loop gain of the laser direct writing head to 150Hz to generate high-frequency small floating vibration of the writing head, control the laser direct writing power to 50mW, and the scanning rate to 4mm / s. Then perform laser direct writing lithography and development to form rough photoresist grid lines on the photoresist layer. The photoresist line period is 1mm and the line width is 5μm. The developer is a 2% sodium hydroxide solution, and the development time is 30s. S4. Place the curved window into the evaporation coating machine for metal evaporation. The evaporation metal is Cr, the metal layer thickness is 80nm, and the evaporation temperature is 30℃. S5. Place the curved window in a low-temperature environment test chamber at 10℃ and cool for 30 minutes. S6. Place the curved window on the exposure machine for general exposure with an exposure dose of 30mJ, and then develop it with a 2% sodium hydroxide solution for 30s to obtain a random grid pattern. S7. The curved window is placed in an evaporation coating machine for metal evaporation. The evaporated metal layers are Cr and Au, with a Cr layer thickness of 10nm and an Au layer thickness of 200nm. The evaporation temperature is 200℃. S8. Immerse the curved window in acetone for 10 minutes, then perform ultrasonic peeling for 5 minutes. Clean the sample with alcohol and deionized water in sequence, and finally purge with a nitrogen gun to obtain a random metal mesh sample, which is denoted as optical window pair III.

[0046] Comparative Example 4 This comparative example provides a method for preparing a random metal mesh, which differs from Example 2 only in that the overexposure and development in step S6 are omitted; the rest are exactly the same. The specific steps are as follows: S1. The curved window zinc sulfide substrate is ultrasonically cleaned with acetone and alcohol for 20 minutes in sequence, and then blown clean with a nitrogen gun to obtain a clean curved window substrate. S2, spin-coat AZ4620 photoresist onto the clean curved window substrate, spin-coat speed is 2500 rpm, spin-coat time is 60 s, spin-coat photoresist thickness is about 5 μm, then place the flat window in a clean oven and bake at 100℃ for 3 min to form a photoresist layer on the surface of the curved window. S3. Place the curved window on the fixture of the laser direct writing equipment, adjust the motor speed loop gain of the laser direct writing head to 150Hz to generate high-frequency small floating vibration of the writing head, control the laser direct writing power to 50mW, and the scanning rate to 4mm / s. Then perform laser direct writing lithography and development to form rough photoresist grid lines on the photoresist layer. The photoresist line period is 1mm and the line width is 5μm. The developer is a 2% sodium hydroxide solution, and the development time is 30s. S4. Place the curved window into the evaporation coating machine for metal evaporation. The evaporation metal is Cr, the metal layer thickness is 80nm, and the evaporation temperature is 30℃. S5. Place the curved window in a clean oven and bake at 100℃ for 30 minutes. After baking, take it out immediately and place it in a low temperature environment test chamber at 10℃ for 30 minutes. The stress generated by the metal layer will cause cracks to appear from the edge defects of the line with high roughness and extend and spread inward to form a random grid mask pattern. S6. Place the curved window into the evaporation coating machine for metal evaporation. The evaporated metal layers are Cr and Au. The thickness of the Cr layer is 10nm and the thickness of the Au layer is 200nm. The evaporation temperature is 200℃. S7. Immerse the curved window in acetone for 10 minutes, then perform ultrasonic peeling for 5 minutes. Clean the sample with alcohol and deionized water in sequence, and finally purge with a nitrogen gun to obtain a random metal mesh sample, which is denoted as optical window pair IV.

[0047] Comparative Example 5 This comparative example provides a method for preparing a random metal mesh grid, the specific steps of which are as follows: S1. The curved window zinc sulfide substrate is ultrasonically cleaned with acetone and alcohol for 20 minutes in sequence, and then blown clean with a nitrogen gun to obtain a clean curved window substrate. S2, a 2μm thick layer of water-based acrylic resin crackle paint is spin-coated onto the clean curved window substrate surface using the spin coating method. The surface is then placed in a constant temperature and humidity chamber at 20~25℃ and 50%~60% for 0.5h, where the crackle paint automatically dries and cracks to form a crack template. S3. The sample is placed in an evaporation coating machine for metal evaporation. The evaporation metals are Cr and Au. The thickness of the Cr film is 10 nm and the thickness of the Au film is 200 nm. The evaporation temperature is 200 °C. S4. Soak the sample in acetone for 10 minutes, then sonicate for 5 minutes. Clean the sample with alcohol and deionized water by sonication in sequence, and then purge with a nitrogen gun to obtain a random metal mesh sample, which is denoted as optical window pair V.

[0048] Performance testing The metal mesh electromagnetic shielding optical windows prepared in Examples 1 to 3 and Comparative Examples 1 to 5 were subjected to yield testing (based on 1000 pieces). The optical transmittance was tested using a Fourier transform infrared spectrometer, and the electromagnetic shielding performance was tested according to the GJB 8820-2015 standard. The specific results are shown in Table 1.

[0049] Table 1 Optical window performance

[0050] The results show that, in the fabrication process of the random metal mesh grid provided in the embodiments of the present invention, the motor speed loop gain of the laser direct writing process, high-temperature baking-low-temperature cooling, and generalized exposure are the key process parameters that determine the performance of the electromagnetic shielding optical window of the random metal mesh grid.

[0051] In Comparative Example 1, reducing the motor speed loop gain in the laser direct writing process leads to increased surface smoothness and reduced roughness of the photoresist grid lines formed by photolithography. This results in a lack of sufficient stress concentration points during subsequent stress induction, a reduction in the number of crack initiations, and uneven distribution of cracks. Ultimately, the grid's random performance is insufficient, making it impossible to effectively avoid higher-order diffraction stray light.

[0052] In Comparative Example 2, lowering the high-temperature baking temperature prevents the photoresist from softening sufficiently and producing slight flow. Consequently, significant interfacial stress cannot be formed between the photoresist and the upper metal layer, resulting in a lack of sustained tensile force on the metal film. The metal film exhibits no directional cracking path, only sporadic cracks, making it impossible to form effective electrical connections or construct a uniform random mesh mask pattern.

[0053] Comparative Example 3 omits the high-temperature baking step and directly enters the low-temperature cooling stage. The photoresist remains rigid and there is no interfacial stress between it and the metal layer due to deformation differences. The metal film has no driving force for cracking and cannot form random cracks at all. After subsequent development, only periodic photoresist lines can be obtained, and random metal grids cannot be prepared.

[0054] In Comparative Example 4, the generalized exposure was omitted, and the photoresist below the crack was not fully exposed. After development, there was still photoresist residue at the crack, which meant that after the conductive metal was deposited, selective stripping could not be achieved at the crack. Only periodic grid lines could be obtained after stripping, and random metal grids could not be prepared.

[0055] Comparative Example 5 uses crack-resistant paint as the crack material. The crack morphology of this material is extremely sensitive to temperature and coating thickness. It is difficult to form a metal mesh sample with uniform crack degree and distribution and uniform line width. Moreover, it is very easy for the crack to be incomplete (some areas have no cracks), resulting in a blind spot in the mesh. In addition, the crack-resistant paint has too strong adhesion to the substrate, which can easily cause damage to the substrate or breakage of the mesh structure when the sample is peeled off, resulting in a low yield.

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a random metal mesh, characterized in that, Includes the following steps: S1, a photoresist layer is formed on the substrate surface; S2, using laser direct writing technology to photolithographically lithographically lithographically develop the photoresist on the substrate surface to form rough photoresist grid lines on the substrate surface; wherein, the roughness of the photoresist grid lines is 100nm~200nm, the period is 1mm~3mm, and the line width is 5μm~30μm; S3, deposit a metal layer on the substrate surface after photolithography, then heat the substrate to a first preset temperature to soften the photoresist, hold it for a first preset time, then cool it to a second preset temperature, and cool it at a low temperature for a second preset time to form a random mesh mask pattern on the substrate surface. S4, expose and develop the substrate with the random grid mask pattern on its surface using ultraviolet light to obtain the random grid pattern; S5, deposit a metal thin film on the developed substrate surface, peel it off, and obtain a random metal mesh. In S2, the motor speed loop gain of the laser direct writing head is 150Hz~180Hz, the laser direct writing power is 40mW~80mW, and the scanning rate is 1mm / s~10mm / s. In S3, the first preset temperature is 90℃~120℃, and the first preset time is 5min~30min; In S3, the second preset temperature is 0℃~10℃, and the second preset time is 5min~30min.

2. The method for preparing a random metal mesh grid as described in claim 1, characterized in that, S1 specifically includes the following steps: cleaning the substrate, then spin-coating photoresist onto the substrate surface, baking, and forming a photoresist layer on the substrate surface.

3. The method for preparing a random metal mesh grid as described in claim 2, characterized in that, In S1, the coating thickness of the photoresist is 1 μm to 10 μm; and / or In S1, the spin coating speed is 1000 rpm to 3000 rpm, and the spin coating time is 59 s to 61 s; and / or In S1, the baking temperature is 90℃~110℃, and the baking time is 2min~4min.

4. The method for preparing a random metal mesh grid as described in claim 1, characterized in that, In S2, the developing solution used is a potassium hydroxide solution or sodium hydroxide solution with a mass concentration of 1.5% to 4%, and the developing time is 10s to 30s.

5. The method for preparing a random metal mesh grid as described in claim 1, characterized in that, In S3, the metal layer is a Cr metal layer; and / or In S3, the thickness of the metal layer is 50nm~150nm; and / or In S3, the evaporation temperature is 10℃~60℃.

6. The method for preparing a random metal mesh grid as described in claim 1, characterized in that, In S4, the exposure dose of the ultraviolet light flooding is 20mJ~40mJ; and / or In step S4, the developing solution used is a potassium hydroxide solution or sodium hydroxide solution with a mass concentration of 1.5% to 3%, and the developing time is 10s to 30s.

7. The method for preparing a random metal mesh grid as described in claim 1, characterized in that, In S5, the metal thin film includes a transition metal film layer and a main metal film layer; wherein the material of the transition metal film layer is Cr, Ni, or Ti, and the film thickness is 10 nm to 30 nm; the material of the main metal film layer is Cu, Au, Ag, or Al, and the film thickness is 100 nm to 500 nm; and / or In S5, the evaporation temperature is 100℃~200℃.

8. A random metal mesh, characterized in that, It is prepared by the method for preparing random metal mesh according to any one of claims 1 to 7.

9. An electromagnetically shielded optical window, characterized in that, The electromagnetic shielding optical window includes the random metal mesh as described in claim 8.

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