A polishing method of a bismuth gallium selenide (BGSe) crystal element for infrared nonlinear frequency conversion

Abstract

The application discloses a polishing method of a selenium gallium barium (BGSe) crystal element for infrared nonlinear frequency conversion, which comprises the following steps: sequentially performing crystal orientation on the selenium gallium barium (BGSe) crystal element; cutting a blank; embedding and wrapping; grinding; chemical polishing; physical rough polishing; physical fine polishing; and high-precision physical polishing by using a diamond polishing liquid, wherein the granularity is gradually reduced from 0.5 microns to 0.25 microns, and the rotating speed of a polishing disc is 1000-1500 r / min, so that an ultrasmooth light-transmitting surface crystal element is obtained. The method can effectively reduce the stress of the crystal during polishing, avoid problems such as cracking and surface scratches of the crystal element during polishing, improve the polishing quality, and realize the ultrasmooth polishing of the light-transmitting surface of the BGSe crystal element.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical processing, in particular to a polishing method of a selenium-gallium-barium crystal element for infrared nonlinear frequency conversion. BACKGROUND

[0002] BaGa4Se7 (selenium-gallium-barium, abbreviated as BGSe) is a new type of infrared nonlinear crystal. It has the advantages of wide light transmission band (0.47 μm-18 μm), high damage threshold (570 MW / cm 2 , 1.06 μm) and suitable birefringence (0.07-0.11). BGSe crystal devices can output 3-5 μm and 8-12 μm mid-long wave infrared laser through laser nonlinear frequency conversion technologies such as optical parametric generation (OPG) and optical parametric amplification (OPA), and have important applications in the fields of environmental monitoring, medical diagnosis, infrared remote sensing, etc.

[0003] However, the BGSe crystal has low hardness and is easy to crack. The BGSe crystal element processed by the traditional polishing process has the problems of high roughness of the light transmission surface, and many surface and subsurface defects, which affect the laser damage threshold of the crystal and further limit the efficient output of mid-long wave infrared laser. SUMMARY

[0004] In view of the above problems, the inventors have made explorations in the early stage and proposed a polishing method for nonlinear frequency conversion elements considering the hardness and chemical corrosion characteristics of the BGSe crystal.

[0005] The present application aims to provide a polishing method for a selenium-gallium-barium (BGSe) crystal element for infrared nonlinear frequency conversion, which first etches the surface of the crystal element blank with a suitable chemical reagent, and then uses physical polishing and high-precision physical polishing to achieve super-smooth polishing of the light transmission surface of the crystal element.

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

[0007] 1. A polishing method for a selenium-gallium-barium (BGSe) crystal element for infrared nonlinear frequency conversion, comprising the following steps:

[0008] (1) Crystal orientation: using an X-ray single crystal orientation instrument to orient and determine the crystal face direction of the grown crystal rod;

[0009] (2) Blank cutting: cutting the crystal element blank by using a diamond wire cutting machine;

[0010] (3) Inlay wrapping: wrapping the BGSe element blank with K9 glass by cold inlaying to prevent stress cracking and deformation of the crystal element during polishing;

[0011] (4) grinding: grinding the end face of the inlaid BGSe crystal element on a grinding disc to make the surface scratches evenly distributed, the granularity of the diamond powder in the grinding liquid is 10-14 μm, the rotating speed of the grinding disc ranges from 2000 to 3000 r / min;

[0012] (5) chemical polishing: preparing a corrosion liquid, the corrosion temperature is 25-30℃, the corrosion time is 30-60 minutes, after the corrosion is completed, a cerium oxide (Ce2O3) grinding liquid is used for rough grinding, the granularity of the cerium oxide (Ce2O3) is 5-10 μm, the surface residues are removed, and the rotating speed of the polishing disc ranges from 2000 to 3000 r / min;

[0013] (6) physical rough polishing: using a cerium oxide (Ce2O3) suspension liquid for physical rough polishing, the rotating speed of the polishing disc ranges from 1500 to 2000 r / min, the granularity of the cerium oxide in the cerium oxide (Ce2O3) suspension liquid is 1-5 μm;

[0014] (7) physical fine polishing: using a diamond polishing liquid for high-precision physical polishing, the granularity is gradually reduced from 0.5 μm to 0.25 μm, the rotating speed of the polishing disc ranges from 1000 to 1500 r / min, and an ultra-smooth light-transmitting surface crystal element is obtained.

[0015] Further, the application provides a polishing method of a selenium-gallium-barium (BGSe) crystal element for infrared nonlinear frequency conversion.

[0016] 1. Crystal orientation: using an X-ray single crystal orientation instrument to orient and determine the crystal face direction of the grown crystal rod;

[0017] 2. Blank cutting: designing a crystal element cutting mode according to the nonlinear frequency conversion requirement, and cutting a crystal element blank by using a diamond wire cutting machine;

[0018] 3. Inlaid wrapping: wrapping the BGSe element blank by K9 glass through cold inlaying to prevent stress cracking and deformation of the crystal element in the polishing process;

[0019] 4. Grinding: grinding the end face of the inlaid BGSe crystal element on a grinding disc to make the surface scratches evenly distributed, the granularity of the diamond powder in the grinding liquid is 10-14 μm (the concentration of the diamond powder is 0.5 wt%), the rotating speed of the grinding disc ranges from 2000 to 3000 r / min, and the grinding time is about 10-15 minutes;

[0020] 5. Chemical polishing. The etching solution is prepared with the following formula: H2O2 (30wt%): Ce(NH4)2(NO3)6 (25wt%): H2O = 1ml: 1.5ml: (10-15)ml. The etching temperature is 25-30℃, and the etching time is 30-60 minutes. After etching, the surface residue is removed by rough polishing with 5-10μm cerium oxide (Ce2O3) abrasive (Ce2O3 abrasive concentration 2-3wt%) at a polishing disc speed of 2000-3000r / min for about 30 minutes.

[0021] 6. Physical rough polishing. The surface is uniformly distributed with pits and scratches by physical rough polishing with 5-1μm cerium oxide (Ce2O3) suspension (Ce2O3 polishing powder concentration 1-2wt%) at a polishing disc speed of 1500-2000r / min for about 60 minutes.

[0022] 7. Physical fine polishing. The super-smooth and light-transmitting crystal component is obtained by high-precision physical fine polishing with diamond polishing solution (diamond polishing powder concentration 0.2%-0.3wt%) at a polishing disc speed of 1000-1500r / min for about 2-4 hours, and the particle size is gradually reduced from 0.5μm to 0.25μm.

[0023] Further, the selenium-gallium-barium (BGSe) crystal is a BaGa4Se7 single crystal.

[0024] Further, in step (4), the grinding time is 10-15 minutes, for example, 10 minutes, 11 minutes, 12 minutes, 13 minutes, 14 minutes or 15 minutes.

[0025] Further, in step (4), the diamond powder concentration in the grinding solution is 0.5-1wt%.

[0026] Further, in step (5), the etching solution is prepared by mixing H2O2 aqueous solution, Ce(NH4)2(NO3)6 aqueous solution and deionized water uniformly.

[0027] Further, in step (5), the etching solution is prepared by mixing 30wt% H2O2 aqueous solution, 25wt% Ce(NH4)2(NO3)6 aqueous solution and deionized water uniformly at the following ratio: H2O2 (30wt%) aqueous solution: Ce(NH4)2(NO3)6 (25wt%) aqueous solution: deionized water = 1ml: 1.5-2ml: 10-15ml.

[0028] Further, in step (5), the concentration of cerium oxide in the cerium oxide polishing solution is 2wt%-3wt%, for example, the concentration of cerium oxide is 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt%, 2.5wt%, 2.6wt%, 2.7wt%, 2.8wt%, 2.9wt% or 3wt%.

[0029] Further, in step (6), the concentration of cerium oxide in the cerium oxide suspension is 1wt%-2wt%, for example, the concentration of cerium oxide is 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt% or 2wt%.

[0030] Further, in step (7), the particle size is gradually reduced from 0.5μm, 0.25μm, and the polishing disc rotation speed is 1000-1500r / min, including: using the first diamond polishing solution for high-precision physical polishing, the concentration of diamond polishing powder in the first diamond polishing solution is 0.2-0.3wt%, the particle size of diamond polishing powder is 0.5μm, the rotation speed of the polishing disc is 1000-1500r / min, and the fine polishing time is 1-2 hours; then using the second diamond polishing solution for high-precision physical polishing, the concentration of diamond polishing powder in the second diamond polishing solution is 0.2-0.3wt%, the particle size of diamond polishing powder is 0.25μm, and the rotation speed of the polishing disc is 1000-1500r / min.

[0031] The polishing method of the selenium gallium barium BGSe crystal element for infrared nonlinear frequency conversion has the following effects:

[0032] The present application realizes the super-smooth polishing of the BGSe crystal element, and the polishing method screens and adjusts the etching solution, the grinding liquid and the polishing liquid, explores the polishing process parameters, and effectively reduces the crystal stress during polishing by using chemical reagents to etch the surface of the crystal element blank and soften the surface polishing layer, avoids the problems of cracking and surface scratches of the crystal element during polishing, improves the polishing quality, and realizes the super-smooth polishing of the light transmission surface of the BGSe crystal element. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 BGSe crystal element after polishing;

[0034] Figure 2 Atomic force microscope photograph of the polished surface of the method;

[0035] Figure 3 Atomic force microscope photograph of the conventional polished surface. DETAILED DESCRIPTION

[0036] The application will be described in detail below with reference to the drawings and specific embodiments. However, the following embodiments are only used to explain the application, and the protection scope of the application should include the entire content of the claims, and through the description of the following embodiments, a person skilled in the art can fully realize the entire content of the claims of the application.

[0037] Example 1

[0038] A polishing method of a selenium gallium barium (BGSe) crystal element (BGSe crystal femtosecond optical parametric OPO frequency conversion element) for infrared nonlinear frequency conversion, the polishing method comprising the following steps: (1) using an X-ray single crystal orientation instrument, determining the positions of each crystal surface of the grown BGSe crystal; (2) using a diamond wire cutting machine (diamond wire diameter 100 μm), cutting a crystal element blank, the size of the blank is about 10×10×3 mm 3 , the cutting angle θ=0°, (3) using K9 glass to cold inlay wrap the BGSe element blank on four sides; (4) rough grinding the end surface (i.e. 10×10 mm 2 light passing surface) of the inlaid BGSe crystal element on a grinding disc (Yujing 6B), the granularity of diamond powder in the grinding liquid is 12 μm, the grinding disc rotation speed is in the range of 2500 r / min, and the grinding time is about 10 minutes; wherein the grinding liquid is a diamond water dispersion liquid, the concentration of diamond powder in the diamond dispersion liquid is 0.5 wt%, and the grinding liquid drop adding method is manual titration, so that the grinding liquid immerses the end surface of the crystal element on the grinding disc. 2 (5) preparing an etching liquid: uniformly mixing a 30 wt% H2O2 aqueous solution, a 25 wt% Ce(NH4)2(NO3)6 aqueous solution and deionized water according to the following proportions H2O2 (30 wt%) aqueous solution: Ce(NH4)2(NO3)6 (25 wt%) aqueous solution: deionized water = 1 ml: 1.5 ml: 13 ml, to obtain the etching liquid; immersing the 10×10 mm 2a light-transmitting surface; wherein the cerium oxide (Ce2O3) polishing solution is a Ce2O3 aqueous dispersion, and in the cerium oxide (Ce2O3) polishing solution, the Ce2O3 polishing powder has a particle size of 7 μm and a concentration of 2 wt%; (6) then performing physical rough polishing using a cerium oxide (Ce2O3) suspension, the polishing disc rotating at 1500 r / min, the rough polishing time being about 60 minutes, and the cerium oxide (Ce2O3) suspension being added dropwise manually so as to immerse 10 x 10 mm 2 a light-transmitting surface; the cerium oxide (Ce2O3) suspension is a Ce2O3 aqueous suspension, and in the cerium oxide (Ce2O3) suspension, the Ce2O3 polishing powder has a particle size of 3 μm and a concentration of 1.5 wt%; (7) then performing high-precision physical polishing using a first diamond polishing solution, the first diamond polishing solution having a diamond polishing powder concentration of 0.2 wt% and a diamond polishing powder particle size of 0.5 μm, the polishing disc rotating at 1200 r / min, the fine polishing time being about 1.5 hours, and the first diamond polishing solution being added dropwise manually so as to immerse 10 x 10 mm 2 a light-transmitting surface. Then, high-precision physical polishing is performed using a second diamond polishing solution, the second diamond polishing solution having a diamond polishing powder concentration of 0.2 wt% and a diamond polishing powder particle size of 0.25 μm, the polishing disc rotating at 1200 r / min, the fine polishing time being about 1.5 hours, and the second diamond polishing solution being added dropwise manually so as to immerse 10 x 10 mm 2 a light-transmitting surface. As shown in Figure 1 the roughness (RMS roughness) is 0.5 nm. The atomic force micrograph of the polished crystal element is shown in Figure 2 the roughness (RMS roughness) is 0.5 nm. The atomic force micrograph of the polished crystal element is shown in Figure 3 the conventional polished crystal surface (roughness 1.2 nm) is smooth.

[0039] Example 2

[0040] A polishing method for a selenium gallium barium (BGSe) crystal element (BGSe crystal femtosecond optical parametric OPO frequency conversion element) for infrared nonlinear frequency conversion, the polishing method comprising the following steps: (1) determining the positions of the crystal faces of the grown BGSe crystal using an X-ray single crystal orientation instrument; (2) cutting a crystal element blank having a size of about 10 x 10 x 3 mm 3 using a diamond wire cutting machine (diamond wire diameter 100 μm), the cutting angle θ = 0°, (3) K9 glass is used to cold inlay and wrap the BGSe element blank on four sides; 2 The end surface (10x10 mm) of the inlaid BGSe crystal element is coarsely ground on a grinding disc (Yujing 6B) with a diamond powder granularity of 10 μm, a grinding disc rotation speed of 2000 r / min, and a grinding time of about 10 minutes. The grinding liquid is a diamond water dispersion liquid, and the diamond powder concentration in the diamond dispersion liquid is 0.5 wt%. (5) An etching liquid is prepared by mixing 30 wt% H2O2 aqueous solution, 25 wt% Ce(NH4)2(NO3)6 aqueous solution, and deionized water in the following proportions: H2O2 (30 wt%) aqueous solution: Ce(NH4)2(NO3)6 (25 wt%) aqueous solution: deionized water = 1 ml: 1.5 ml: 10 ml. The 10x10 mm 2 light-passing surface to be polished after step (4) is soaked in the etching liquid, the etching temperature is 30°C, and the etching time is 60 minutes. After etching, a Ce2O3 grinding liquid is used for coarse grinding to remove surface residues, the grinding disc rotation speed is 2000 r / min, and the grinding time is about 30 minutes. The Ce2O3 grinding liquid is a Ce2O3 water dispersion liquid, the Ce2O3 grinding powder granularity is 5 μm, and the Ce2O3 grinding powder concentration in the Ce2O3 grinding liquid is 2 wt%. (6) Then, a Ce2O3 suspension liquid is used for physical coarse polishing, the polishing disc rotation speed is 1500 r / min, and the coarse polishing time is about 60 minutes. The Ce2O3 suspension liquid is a Ce2O3 water suspension liquid, the Ce2O3 polishing powder granularity is 1 μm, and the Ce2O3 polishing powder concentration is 1 wt%. (7) First diamond polishing liquid is used for high-precision physical polishing, the first diamond polishing liquid has a granularity of 0.5 μm and a diamond polishing powder concentration of 0.2 wt%, the polishing disc rotation speed is 1200 r / min, and the fine polishing time is about 2 hours. Second diamond polishing liquid is used for high-precision physical polishing, the second diamond polishing liquid has a diamond polishing powder concentration of 0.2 wt% and a diamond polishing powder granularity of 0.25 μm, the polishing disc rotation speed is 1000 r / min, and the fine polishing time is about 2 hours. An ultra-smooth light-passing surface BGSe crystal element is obtained, as shown in (b) of FIG. 1. Figure 1 The roughness of the light-passing surface of the crystal element is basically the same as in example 1 Figure 1 (a), and the roughness is 0.5 nm.

[0041] Example 3

[0042] A polishing method of a selenium gallium barium (BGSe) crystal element (BGSe crystal femtosecond optical parametric OPO frequency conversion element) for infrared nonlinear frequency conversion, the polishing method comprising the following steps: (1) determining the positions of each crystal surface of the grown BGSe crystal by using an X-ray single crystal orienter; (2) cutting a crystal element blank by using a diamond wire cutting machine (diamond wire diameter 100 μm), the size of the blank being about 10×10×3 mm 3 , the cutting angle θ=0°, (3) wrapping the BGSe element blank with K9 glass around four sides by cold setting; (4) coarsely grinding the end surface (i.e. 10×10 mm 2 light passing surface) of the BGSe crystal element after setting by using a grinding disc (Yujing 6B), the granularity of the diamond powder in the grinding liquid being 14 μm, the grinding disc rotating at a speed of 3000 r / min, and the grinding time being about 10 minutes; wherein the grinding liquid is a diamond water dispersion liquid, the concentration of the diamond powder in the diamond dispersion liquid being 0.5 wt.%. The method of dropping the grinding liquid is manual titration, so that the grinding liquid is immersed in the 10×10 mm 2The light transmitting surface is prepared. (5) The etching solution is prepared by mixing 30wt% H2O2 aqueous solution, 25wt% Ce(NH4)2(NO3)6 aqueous solution and deionized water in the following ratio: H2O2 (30wt%) aqueous solution: Ce(NH4)2(NO3)6 (25wt%) aqueous solution: deionized water = 1ml: 1.5ml: 15ml. The light transmitting surface treated in step (4) is immersed in the etching solution, and the etching temperature is 25°C and the etching time is 30 minutes. After etching, the surface is coarsely polished with cerium oxide (Ce2O3) polishing liquid, and the polishing time is about 30 minutes. The surface is cleaned to remove the residues, and the polishing disc rotates at 3000r / min. The cerium oxide (Ce2O3) polishing liquid is a Ce2O3 water dispersion liquid. In the cerium oxide (Ce2O3) polishing liquid, the particle size of the Ce2O3 polishing powder is 10μm, and the concentration of the Ce2O3 polishing powder is 3wt%. (6) Then, the surface is physically coarsely polished with cerium oxide (Ce2O3) suspension liquid, and the polishing disc rotates at 2000r / min. The coarsely polishing time is about 60 minutes. The cerium oxide (Ce2O3) suspension liquid is a Ce2O3 water suspension liquid. In the cerium oxide (Ce2O3) suspension liquid, the concentration of the Ce2O3 polishing powder is 2wt%, and the particle size of the Ce2O3 polishing powder is 5μm. (7) The surface is further physically polished with the first diamond polishing liquid. In the first diamond polishing liquid, the concentration of the diamond polishing powder is 0.3wt%, the particle size of the diamond polishing powder is 0.5μm, the polishing disc rotates at 1200r / min, and the fine polishing time is about 1 hour. Then, the surface is further physically polished with the second diamond polishing liquid. In the second diamond polishing liquid, the concentration of the diamond polishing powder is 0.3wt%, the particle size of the diamond polishing powder is 0.25μm, the polishing disc rotates at 1500r / min, and the fine polishing time is about 1 hour. Thus, a super-smooth light transmitting surface of the BGSe crystal element is obtained, as shown in FIG. 1(c). Figure 1 The roughness of the light transmitting surface of the crystal element is basically the same as that of the crystal element in Example 1(a), and the roughness is 0.5nm. Figure 1

[0043] Comparative Example 1

[0044] A polishing method of a selenium gallium barium (BGSe) crystal element (BGSe crystal femtosecond optical parametric OPO frequency conversion element) for infrared nonlinear frequency conversion, the polishing method comprising the following steps: (1) determining the positions of the crystal faces of the grown BGSe crystal by using an X-ray single crystal orientation instrument; (2) cutting a crystal element blank by using a diamond wire cutting machine (the diameter of the diamond wire is 100μm), and the size of the blank is about 10×10×3mm 3 , the cutting angle is θ=0°, (3) wrapping the BGSe element blank with K9 glass by cold inlaying; (4) coarsely polishing the end surface (i.e. 10×10mm​2 The rough grinding is performed on a grinding disc (Yujing 6B) on the light passing surface, the diamond powder particle size of the grinding liquid is 14 μm, the grinding disc rotation speed is in the range of 3000 r / min, and the grinding time is about 10 minutes; wherein the grinding liquid is a diamond water dispersion liquid, the diamond powder concentration in the diamond dispersion liquid is 0.5 wt%. The drop method of the grinding liquid is manual titration, and the grinding liquid is immersed on the 10×10 mm 2 The light passing surface. (5) Then, the physical rough polishing is performed by using a cerium oxide (Ce2O3) suspension liquid, the polishing disc rotation speed is 2000 r / min, and the rough polishing time is about 60 minutes; the cerium oxide (Ce2O3) suspension liquid is a cerium oxide (Ce2O3) water suspension liquid, the Ce2O3 polishing powder concentration in the cerium oxide (Ce2O3) suspension liquid is 2 wt%, and the Ce2O3 polishing powder particle size is 5 μm. (6) The end surface (i.e. 10×10 mm 2 The light passing surface. The high-precision physical polishing is performed on the BGSe crystal element end surface (i.e. 10×10 mm 2 The light passing surface. The roughness of the processed crystal surface is as shown in Figure 3 The roughness of the polished crystal surface is 1.2 nm.

[0045] The part of the present application which is not described in detail belongs to the known technology of the person skilled in the art. The above-described embodiments are only used to describe the preferred embodiments of the present application, and the preferred embodiments do not describe all the details and do not limit the present application to the specific embodiments. Without departing from the design spirit of the present application, the person skilled in the art can make various modifications and improvements to the technical solutions of the present application, which should all fall within the protection scope of the present application defined by the claims.

Claims

1. A polishing method for a barium gallium selenide (BGSe) crystal element used for infrared nonlinear frequency conversion, characterized in that, Includes the following steps: (1) Crystal orientation: X-ray single crystal orientation instrument is used to orient and determine the crystal plane orientation of the grown crystal rod; (2) Blank cutting: Use a diamond wire cutting machine to cut out crystal element blanks; (3) Embedding and wrapping: K9 glass is used to cold embed and wrap the BGSe component blank to prevent stress cracking and deformation of the crystal component during polishing; (4) Grinding: Grind the end face of the embedded BGSe crystal element on the grinding disc to make the surface scratches evenly distributed. The diamond powder in the grinding fluid has a particle size of 10-14μm and the grinding disc rotation speed range is 2000-3000r / min. (5) Chemical polishing: Prepare the etching solution; the etching temperature is 25-30℃ and the etching time is 30-60 minutes; after the etching is completed, use cerium oxide (Ce2O3) polishing solution for coarse grinding. The particle size of cerium oxide (Ce2O3) is 5-10μm. Remove the residue on the etched surface. The polishing disc speed range is 2000-3000r / min. (6) Physical rough polishing: Physical rough polishing is performed using a cerium oxide (Ce2O3) suspension, with the polishing disc speed ranging from 1500 to 2000 r / min; the particle size of cerium oxide in the cerium oxide (Ce2O3) suspension is 1-5 μm; (7) Physical polishing: High-precision physical polishing is performed using diamond polishing fluid, with particle size gradually decreasing from 0.5μm to 0.25μm, and the polishing disc rotation speed range is 1000~1500r / min, to obtain crystal elements with ultra-smooth light transmission surface.

2. The method according to claim 1, characterized in that, The described barium gallium selenide (BGSe) crystal is a BaGa4Se7 single crystal.

3. The method according to claim 1, characterized in that, In step (4), the grinding time is 10 to 15 minutes.

4. The method according to claim 1, characterized in that, In step (4), the concentration of diamond powder in the polishing slurry is 0.5wt% to 1wt%.

5. The method according to claim 1, characterized in that, In step (5), the corrosion solution is prepared by the following steps: mixing H2O2 aqueous solution, Ce(NH4)2(NO3)6 aqueous solution and deionized water evenly to obtain the corrosion solution.

6. The method according to claim 1, characterized in that, In step (5), a 30wt% H2O2 aqueous solution, a 25wt% Ce(NH4)2(NO3)6 aqueous solution, and deionized water are mixed evenly in the following ratio: H2O2 (30wt%) aqueous solution: Ce(NH4)2(NO3)6 (25wt%) aqueous solution: deionized water = 1ml: 1.5-2ml: 10-15ml to obtain the corrosion solution.

7. The method according to claim 1, characterized in that, In step (5), the concentration of cerium oxide in the cerium oxide polishing slurry is 2wt% to 3wt%.

8. The method according to claim 1, characterized in that, In step (6), the concentration of cerium oxide in the cerium oxide (Ce2O3) suspension is 1wt% to 2wt%.

9. The method according to claim 1, characterized in that, In step (7), the high-precision physical polishing using diamond polishing slurry, with particle size decreasing progressively from 0.5μm to 0.25μm and polishing disc rotation speed ranging from 1000 to 1500 r / min, includes: high-precision physical polishing using a first diamond polishing slurry, wherein the diamond polishing powder concentration in the first diamond polishing slurry is 0.2 to 0.3 wt%, the diamond polishing powder particle size is 0.5μm, the polishing disc rotation speed is 1000 to 1500 r / min, and the fine polishing time is 1 to 2 hours; and then high-precision physical polishing using a second diamond polishing slurry, wherein the diamond polishing powder concentration in the second diamond polishing slurry is 0.2 to 0.3 wt%, the diamond polishing powder particle size is 0.25μm, and the polishing disc rotation speed is 1000 to 1500 r / min.

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