Closed growth method of high-purity laser-damage-resistant fluoride crystal

Abstract

The invention discloses a closed growth method of a high-purity laser-damage-resistant fluoride crystal, and relates to the technical field of crystal growth. The preparation method comprises the following steps: treating fluoride powder with an HF solution, and performing F2 high-temperature roasting purification; growing crystals in a Bridgman furnace under the assistance of a uniform magnetic field with proper intensity; and finally, processing the crystal by using CF4 and O2 mixed gas plasma to eliminate crystal surface damage and form a fluorine-rich passivation layer. Heat convection in the melt can be inhibited through assistance of the magnetic field, the crystallization process is kept stable, the impurity content and dislocation density of the crystal can be effectively reduced, the laser damage resistance of the crystal is improved, and the method is suitable for the high-end optical field.

Description

Technical Field

[0001] This application relates to the field of crystal growth technology, and in particular to a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals. Background Technology

[0002] Fluoride crystals are an important class of optical functional materials. For example, calcium fluoride crystals, with their excellent light transmittance in the deep ultraviolet to mid-infrared bands, low optical absorption coefficient, and good mechanical stability, have irreplaceable application value in high-end optical fields such as deep ultraviolet lithography, high-energy laser systems, ultraviolet detectors, and inertial confinement fusion. In the field of deep ultraviolet lithography, calcium fluoride crystals are the core material for fabricating lithography objectives, and their optical uniformity and resistance to laser damage directly determine the lithography resolution and equipment lifespan. In high-energy laser devices, fluoride crystals are often used as key optical components such as laser windows and lenses, which need to withstand long-term irradiation by high-energy lasers, and their laser damage threshold becomes a key indicator restricting system performance.

[0003] However, with the rapid development of technologies in the aforementioned fields, higher requirements have been placed on the performance of fluoride crystals. Chinese Patent Publication No. CN116716659A discloses a method for growing calcium fluoride crystals and the calcium fluoride crystals themselves. This method uses a closed crystal furnace to prepare calcium fluoride crystals under an atmosphere of argon, carbon tetrafluoride, and helium. This method effectively maintains the shape of the solid-liquid interface during crystal growth, while improving the thermal conductivity of the calcium fluoride crystals, avoiding crucible crystallization and the formation of shoulder bubbles, and obtaining large-size calcium fluoride crystals with low dislocation density and high transmittance.

[0004] However, fluoride crystals prepared by existing technologies still suffer from insufficient purity and high levels of oxygen and metallic impurities, making it difficult to meet the requirements of high-energy laser systems. Therefore, how to prepare fluoride crystals with high purity, low impurity content, high laser damage threshold, and excellent optical properties has become a key technical problem that urgently needs to be solved in the field of optical materials. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this application provides a closed-loop growth method for high-purity laser-damage-resistant fluoride crystals. The fluoride powder is purified by HF washing and F2 calcination to remove impurities such as oxygen and metal ions. Then, a uniform magnetic field is used to suppress melt turbulence and reduce the dislocation density of the crystal. Finally, plasma technology is used to eliminate surface damage and form a fluorine-rich passivation layer, thereby improving the surface's laser resistance.

[0006] To achieve the above objectives, this application provides the following technical solution:

[0007] This application provides a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals, comprising the following steps:

[0008] Preliminary impurity removal treatment was performed on fluoride powder with a purity of 99.99% using HF solution;

[0009] The fluoride powder, after preliminary impurity removal, was placed in a reaction boat and calcined in an F2 atmosphere for further impurity removal.

[0010] The deeply purified fluoride powder was placed into a crucible in a sealed Bridgman furnace with a uniform magnetic field, and the furnace was evacuated to 10°C. -4 Below Pa, a mixture of Ar and CF4 gas is introduced, and the pressure is maintained at 0.05 MPa;

[0011] The temperature is increased to 1270-1460℃ at 75-80℃ / h and held at the temperature for 9-10 hours to homogenize the fluoride melt, during which a uniform magnetic field is turned on.

[0012] The crucible is lowered at a rate of 0.8–1.2 mm / h, with an axial temperature gradient of 5–20 °C / cm and a radial temperature gradient of ≤1 °C / cm.

[0013] After the fluoride crystals have grown, they are annealed in situ at 800–950°C for 80–100 hours, and then cooled to room temperature at a rate of 5–10°C / h.

[0014] After the fluoride crystal is grown, it is cut and polished, then placed in a vacuum plasma treatment chamber. A mixture of CF4 and O2 gas is introduced, and the crystal is treated for 10 to 15 minutes under the conditions of radio frequency power of 300 to 500 W and pressure of 5 to 7 Pa to obtain the high-purity laser-damage resistant fluoride crystal.

[0015] Furthermore, the fluoride crystals include any one of calcium fluoride, barium fluoride, and magnesium fluoride.

[0016] Furthermore, the fluoride crystals also include any one of strontium fluoride, beryllium fluoride, and cerium fluoride.

[0017] Furthermore, the concentration of the HF solution is 3-5%.

[0018] Further, the preliminary impurity removal step is as follows: add fluoride powder with a purity of 99.99% to a 3-5% HF solution, stir at a constant temperature of 70-80°C for 4-5 hours, filter, and wash with ultrapure water until the pH range is between 6.5 and 8.5.

[0019] Furthermore, the reaction boat includes any one of a boron nitride reaction boat, a zirconium oxide reaction boat, and a silicon carbide reaction boat.

[0020] Furthermore, the conditions for deep impurity removal are: calcination temperature of 600-700℃, holding time of 7-9 hours, and F2 flow rate of 50-100 sccm.

[0021] Furthermore, the uniform magnetic field is any one or both of longitudinal and transverse uniform magnetic fields; the strength of the uniform magnetic field is 0.1 to 0.4 T.

[0022] Furthermore, the crucible includes either a graphite crucible with a nanoscale BN coating sprayed on its inner wall or a silicon carbide crucible.

[0023] Furthermore, in the Ar and CF4 mixture, the volume ratio of Ar to CF4 is 100:(0.05~0.12).

[0024] Furthermore, in the CF4 and O2 mixture, the volume ratio of CF4 to O2 is (8.5–9.5):(0.5–1.5).

[0025] The beneficial effects of this application are:

[0026] This application uses a uniform magnetic field to assist crystal growth. During crystal growth, temperature differences exist in different regions in the horizontal or vertical directions, which easily lead to thermal convection, causing instability at the melt-solid interface and other positions, affecting the crystal growth quality. After using a magnetic field, when thermal convection passes through the longitudinal uniform magnetic field, a Lorentz force opposite to the direction of thermal convection is generated to suppress thermal convection, thereby stabilizing the solid-liquid interface, reducing the generation of defects in the crystal, and improving the optical uniformity of the crystal. When thermal convection passes through the transverse uniform magnetic field, the generated Lorentz force can suppress the heat flow pairs between the inside and surface of the melt, reducing the amount of F-rich material inside the melt. - F-depleted zone on melt surface - The transfer of the region, thereby reducing F - The volatilization of the stoichiometry ensures the stability of the stoichiometry within the crystal.

[0027] The fluoride crystal wafers after cutting and polishing are subjected to plasma treatment and a mixture of CF4 and O2 is introduced. Under the action of radio frequency electric field, CF4 and O2 are ionized into plasma state, which includes free radicals, ions and high-energy electrons. Under the bombardment of a large number of particles, contaminants on the surface of the fluoride crystal are stripped off. At the same time, fluorine free radicals fill the fluoride ion vacancies on the crystal surface, compensating for the surface fluorine loss caused by polishing. Furthermore, the excess fluoride ions further react to form a fluorine-rich passivation layer, which ultimately significantly improves the chemical stability of the crystal surface and the laser damage threshold. Attached Figure Description

[0028] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0029] Figure 1 A flowchart of a closed-loop growth method for high-purity laser-damage-resistant fluoride crystals provided for this application;

[0030] Figure 2 An appearance diagram of a high-purity laser-damage resistant fluoride crystal provided for this application. Detailed Implementation

[0031] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0032] Many specific details are set forth in the following description in order to provide a full understanding of this application. However, this application may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0033] This application provides a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals, the preparation process of which is described in [link to method]. Figure 1 Specifically, it includes the following steps:

[0034] Add fluoride powder with a purity of 99.99% to HF solution, stir at a constant temperature of 70-80℃ for 4-5 hours, filter, and wash with ultrapure water until neutral, with a pH range of 6.5-8.5.

[0035] It should be noted that in this step, the concentration of the HF solution is 3-5%. HF can effectively remove oxide impurities and surface moisture from the fluoride powder.

[0036] The fluoride powder after preliminary impurity removal was placed in a reaction boat and calcined at high temperature in an F2 atmosphere. The calcination temperature was 600-700℃, and the holding time was 7-9 hours. The F2 flow rate was 50-100 sccm.

[0037] It should be noted that the reaction boat used in this step includes any one of boron nitride reaction boat, zirconium oxide reaction boat, and silicon carbide reaction boat, and the purity of the above reaction boat should reach 99.9% or higher; during the calcination process, F2 reacts with impurities to generate volatile fluorides to remove impurities from the raw materials.

[0038] The purified fluoride powder was placed into a crucible in a sealed Bridgman furnace with a uniform magnetic field, and the furnace was evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 gas is introduced, and the pressure is maintained at 0.05 MPa;

[0039] It should be noted that the crucible mentioned in this step includes either a graphite crucible with a nano-scale BN coating sprayed on its inner wall or a silicon carbide crucible. The coating thickness of the graphite crucible is 5-10 μm, and the purity of the crucible should reach 99.9% or higher. In the Ar and CF4 mixed gas, the volume ratio of Ar to CF4 is 100:(0.05-0.12), and the purity of Ar is 5N-6N. Introducing CF4 can provide active fluorine atoms during crystal growth, compensate for fluorine loss in the melt, and reduce fluorine vacancy defects.

[0040] The temperature is increased to 1270-1460℃ at 75-80℃ / h and held at the temperature for 9-10 hours to homogenize the melt, during which a uniform magnetic field is turned on.

[0041] It should be noted that the uniform magnetic field can be any one or both of the longitudinal and transverse uniform magnetic fields, with a magnetic field strength of 0.1 to 0.4 T. The magnetic field strength is required to effectively suppress the thermal convection of the fluoride melt without inhibiting the normal migration of ions in the melt. Under the premise of ensuring normal diffusion of the solute, it can effectively suppress more than 90% of the convection.

[0042] Then the crucible is lowered at a rate of 0.8 to 1.2 mm / h, with an axial temperature gradient of 5 to 20 °C / cm and a radial temperature gradient of ≤1 °C / cm.

[0043] After the fluoride crystals have grown, they are annealed in situ at 800–950°C for 80–100 hours, and then cooled to room temperature at a rate of 5–10°C / h.

[0044] It should be noted that this step can eliminate internal stress in the crystal and prevent crystal cracking.

[0045] After the fluoride crystal is grown, it is cut and polished, then placed in a vacuum plasma treatment chamber. A mixture of CF4 and O2 gas is introduced, and the crystal is treated for 10 to 15 minutes under the conditions of radio frequency power of 300 to 500 W and pressure of 5 to 7 Pa to obtain the high-purity laser-damage resistant fluoride crystal.

[0046] It should be noted that in this step, the volume ratio of CF4 to O2 in the CF4-O2 mixture is (8.5-9.5):(0.5-1.5), and the purity of CF4 is 5N-6N. Plasma treatment of the crystal surface eliminates damage to the crystal surface, while CF4 provides fluorine radicals to fill fluorine ion vacancies on the crystal surface and further forms a fluorine-rich passivation layer, improving the crystal's resistance to laser damage.

[0047] The following detailed description, using specific examples, will further illustrate this point.

[0048] Example 1

[0049] like Figure 1As shown, a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0050] 1. Add 1 kg of 99.99% pure CaF2 powder to a 3% HF solution, stir at 75°C for 5 hours, filter, and wash with ultrapure water until the pH value is 7.

[0051] 2. Place the CaF2 powder after preliminary impurity removal in a boron nitride reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 700℃ and the holding time is 8 hours. The F2 flow rate is 100 sccm.

[0052] 3. The purified CaF2 powder is placed into a crucible in a sealed Bridgman furnace with a uniform longitudinal magnetic field. The crucible is a 99.9% pure graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.1 is introduced, and the pressure is maintained at 0.05 MPa;

[0053] 4. Increase the temperature to 1450℃ at 80℃ / h and keep it at that temperature for 10 hours to homogenize the melt. During this period, turn on the longitudinal uniform magnetic field with a magnetic field strength of 0.1T.

[0054] 5. Then lower the crucible at a rate of 0.8 mm / h, setting the axial temperature gradient to 5℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0055] 6. After the calcium fluoride crystals have grown, they are annealed in situ at 900℃ for 80 hours, and then cooled to room temperature at a rate of 5℃ / h.

[0056] 7. After the calcium fluoride crystal has been grown, it is cut and polished, and then placed in a vacuum plasma treatment chamber. A mixed gas with a volume ratio of CF4 to O2 of 9:1 is introduced and treated for 10 minutes under the conditions of 500W radio frequency power and 5Pa pressure to obtain the high-purity laser-damage fluoride crystal described in Example 1.

[0057] Example 2

[0058] like Figure 1 As shown, a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0059] 1. Add 1 kg of 99.99% pure CaF2 powder to a 5% HF solution, stir at 75°C for 5 hours, filter, and wash with ultrapure water until the pH value is 6.5;

[0060] 2. Place the CaF2 powder after preliminary impurity removal in a zirconia reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 600℃ and the holding time is 7 hours. The F2 flow rate is 50 sccm.

[0061] 3. The purified CaF2 powder is placed into a crucible in a closed Bridgman furnace equipped with both longitudinal and transverse uniform magnetic fields. The crucible is a 99.9% pure silicon carbide crucible with a nano-scale BN coating sprayed on its inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.05 is introduced, and the pressure is maintained at 0.05 MPa;

[0062] 4. Heat the melt to 1460℃ at a rate of 75℃ / h and keep it at that temperature for 9 hours to homogenize the melt. During this period, a uniform magnetic field with a longitudinal and transverse strength of 0.1T is activated.

[0063] 5. Then lower the crucible at a rate of 1 mm / h, setting the axial temperature gradient to 10℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0064] 6. After the calcium fluoride crystals have grown, they are annealed in situ at 950℃ for 100 hours, and then cooled to room temperature at a rate of 8℃ / h.

[0065] 7. After the calcium fluoride crystal has been grown, it is cut and polished, and then placed in a vacuum plasma treatment chamber. A mixed gas with a volume ratio of CF4 to O2 of 8.5:0.5 is introduced and treated for 12 minutes under the conditions of radio frequency power of 400W and pressure of 6Pa to obtain the high-purity laser-damage fluoride crystal described in Example 2.

[0066] Example 3

[0067] like Figure 1 As shown, a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0068] 1. Add 1 kg of 99.99% pure CaF2 powder to a 3% HF solution, stir at 75°C for 4.5 hours, filter, and wash with ultrapure water until the pH value is 8.5;

[0069] 2. Place the pre-purified CaF2 powder in a silicon carbide reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 650℃ and the holding time is 9 hours. The F2 flow rate is 100 sccm.

[0070] 3. The purified CaF2 powder is placed into a crucible in a sealed Bridgman furnace with a transverse uniform magnetic field. The crucible is a 99.9% pure graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.12 is introduced, and the pressure is maintained at 0.05 MPa;

[0071] 4. Increase the temperature to 1450℃ at 80℃ / h and keep it at that temperature for 10 hours to homogenize the melt. During this period, turn on the transverse uniform magnetic field with a magnetic field strength of 0.2T.

[0072] 5. Then lower the crucible at a rate of 1.2 mm / h, setting the axial temperature gradient to 5℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0073] 6. After the calcium fluoride crystals have grown, they are annealed in situ at 800℃ for 90 hours, and then cooled to room temperature at a rate of 10℃ / h.

[0074] 7. After the calcium fluoride crystal has been grown, it is cut and polished, and then placed in a vacuum plasma treatment chamber. A mixed gas with a volume ratio of CF4 to O2 of 9.5:1.5 is introduced and treated for 15 minutes under the conditions of radio frequency power of 300W and pressure of 7Pa to obtain the high-purity laser-damage fluoride crystal described in Example 3.

[0075] Example 4

[0076] like Figure 1 As shown, a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0077] 1. Add 1 kg of 99.99% pure CaF2 powder to a 3% HF solution, stir at 75°C for 4 hours, filter, and wash with ultrapure water until the pH value is 7.

[0078] 2. Place the CaF2 powder after preliminary impurity removal in a boron nitride reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 700℃ and the holding time is 8 hours. The F2 flow rate is 75 sccm.

[0079] 3. The purified CaF2 powder is placed into a crucible in a sealed Bridgman furnace with a uniform longitudinal magnetic field. The crucible is a 99.9% pure graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.1 is introduced, and the pressure is maintained at 0.05 MPa;

[0080] 4. Heat the melt to 1270℃ at a rate of 78℃ / h and hold the temperature for 9.5 hours to homogenize the melt. During this period, a longitudinal uniform magnetic field with a strength of 0.3T is activated.

[0081] 5. Then lower the crucible at a rate of 0.8 mm / h, setting the axial temperature gradient to 20℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0082] 6. After the calcium fluoride crystals have grown, they are annealed in situ at 900℃ for 80 hours, and then cooled to room temperature at a rate of 5℃ / h.

[0083] 7. After the calcium fluoride crystal has been grown, it is cut and polished, and then placed in a vacuum plasma treatment chamber. A mixed gas with a volume ratio of CF4 to O2 of 9:1 is introduced and treated for 10 minutes under the conditions of 500W radio frequency power and 5Pa pressure to obtain the high-purity laser-damage fluoride crystal described in Example 4.

[0084] Example 5

[0085] like Figure 1 As shown, a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0086] 1. Add 1 kg of 99.99% pure MgF2 powder to 4% HF solution, stir at 70℃ for 5 hours, filter and wash with ultrapure water until the pH value is 7.

[0087] 2. Place the pre-purified MgF2 powder in a boron nitride reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 700℃ and the holding time is 8 hours. The F2 flow rate is 100 sccm.

[0088] 3. The purified MgF2 powder is placed into a crucible in a sealed Bridgman furnace with a uniform longitudinal magnetic field. The crucible is a 99.9% pure graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.1 is introduced, and the pressure is maintained at 0.05 MPa;

[0089] 4. Heat the melt to 1270℃ at a rate of 75℃ / h and keep it at that temperature for 10 hours to homogenize the melt. During this period, a longitudinal uniform magnetic field with a strength of 0.4T is activated.

[0090] 5. Then lower the crucible at a rate of 0.8 mm / h, setting the axial temperature gradient to 5℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0091] 6. After the magnesium fluoride crystals have grown, they are annealed in situ at 900℃ for 80 hours, and then cooled to room temperature at a rate of 5℃ / h.

[0092] 7. After the magnesium fluoride crystal has been grown, it is cut and polished, and then placed in a vacuum plasma treatment chamber. A mixed gas with a CF4 to O2 volume ratio of 9:1 is introduced and treated for 10 minutes under the conditions of 500W radio frequency power and 5Pa pressure to obtain the high-purity laser-damage fluoride crystal described in Example 5.

[0093] Example 6

[0094] like Figure 1 As shown, a closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0095] 1. Add 1 kg of 99.99% pure BaF2 powder to a 5% HF solution, stir at 80°C for 5 hours, filter, and wash with ultrapure water until the pH value is 7.

[0096] 2. Place the pre-purified BaF2 powder in a boron nitride reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 700℃ and the holding time is 8 hours. The F2 flow rate is 100 sccm.

[0097] 3. The purified BaF2 powder is placed into a crucible in a sealed Bridgman furnace with a uniform longitudinal magnetic field. The crucible is a 99.9% pure graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.1 is introduced, and the pressure is maintained at 0.05 MPa;

[0098] 4. Heat the melt to 1370℃ at a rate of 75℃ / h and keep it at that temperature for 10 hours to homogenize it. During this period, a longitudinal uniform magnetic field with a strength of 0.15T is activated.

[0099] 5. Then lower the crucible at a rate of 0.8 mm / h, setting the axial temperature gradient to 5℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0100] 6. After the barium fluoride crystal growth is completed, it is annealed in situ at 900℃ for 80 hours, and then cooled to room temperature at a rate of 5℃ / h.

[0101] 7. After the barium fluoride crystal has been grown, it is cut and polished, then placed in a vacuum plasma treatment chamber. A mixed gas with a CF4 to O2 volume ratio of 9:1 is introduced and treated for 10 minutes under the conditions of 500W radio frequency power and 6Pa pressure to obtain the high-purity laser-damage fluoride crystal described in Example 6.

[0102] Comparative Example 1

[0103] A closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0104] 1. Place 1 kg of 99.99% pure CaF2 powder in a boron nitride reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 700℃ and the holding time is 8 hours. The F2 flow rate is 100 sccm.

[0105] 2. The purified CaF2 powder is placed into a crucible in a sealed Bridgman furnace with a uniform longitudinal magnetic field. The crucible is a 99.9% pure graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.1 is introduced, and the pressure is maintained at 0.05 MPa;

[0106] 3. Increase the temperature to 1450℃ at 80℃ / h and keep it at that temperature for 10 hours to homogenize the melt. During this period, turn on the longitudinal uniform magnetic field with a magnetic field strength of 0.1T.

[0107] 4. Then lower the crucible at a rate of 0.8 mm / h, setting the axial temperature gradient to 5℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0108] 5. After the calcium fluoride crystals have grown, they are annealed in situ at 900℃ for 80 hours, and then cooled to room temperature at a rate of 5℃ / h.

[0109] 6. After the calcium fluoride crystal has been grown, it is cut and polished, and then placed in a vacuum plasma treatment chamber. A mixed gas with a volume ratio of CF4 to O2 of 9:1 is introduced and treated for 10 minutes under the conditions of 500W radio frequency power and 5Pa pressure to obtain the high-purity laser-damage fluoride crystal described in Comparative Example 1.

[0110] Comparative Example 2

[0111] A closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0112] 1. Add 1 kg of 99.99% pure CaF2 powder to a 3% HF solution, stir at 75°C for 5 hours, filter, and wash with ultrapure water until neutral.

[0113] 2. The purified CaF2 powder is placed into a crucible in a sealed Bridgman furnace with a uniform longitudinal magnetic field. The crucible is a 99.9% pure graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.1 is introduced, and the pressure is maintained at 0.05 MPa;

[0114] 3. Increase the temperature to 1450℃ at 80℃ / h and keep it at that temperature for 10 hours to homogenize the melt. During this period, turn on the longitudinal uniform magnetic field with a magnetic field strength of 0.1T.

[0115] 4. Then lower the crucible at a rate of 0.8 mm / h, setting the axial temperature gradient to 5℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0116] 5. After the calcium fluoride crystals have grown, they are annealed in situ at 900℃ for 80 hours, and then cooled to room temperature at a rate of 5℃ / h.

[0117] 6. After the calcium fluoride crystal has been grown, it is cut and polished, and then placed in a vacuum plasma treatment chamber. A mixed gas with a volume ratio of CF4 to O2 of 9:1 is introduced and treated for 10 minutes under the conditions of 500W radio frequency power and 5Pa pressure to obtain the high-purity laser-damage fluoride crystal described in Comparative Example 2.

[0118] Comparative Example 3

[0119] A closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0120] 1. Add 1 kg of 99.99% pure CaF2 powder to a 3% HF solution, stir at 75°C for 5 hours, filter, and wash with ultrapure water until neutral.

[0121] 2. Place the CaF2 powder after preliminary impurity removal in a boron nitride reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 700℃ and the holding time is 8 hours. The F2 flow rate is 100 sccm.

[0122] 3. The purified CaF2 powder is placed into a crucible in a sealed Bridgman furnace. The crucible is a high-purity graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.1 is introduced, and the pressure is maintained at 0.05 MPa;

[0123] 4. Increase the temperature to 1450℃ at a rate of 80℃ / h and hold the temperature for 10 hours to homogenize the melt;

[0124] 5. Then lower the crucible at a rate of 0.8 mm / h, setting the axial temperature gradient to 5℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0125] 6. After the calcium fluoride crystals have grown, they are annealed in situ at 900℃ for 80 hours, and then cooled to room temperature at a rate of 5℃ / h.

[0126] 7. After the calcium fluoride crystal has been grown, it is cut and polished, and then placed in a vacuum plasma treatment chamber. A mixed gas with a volume ratio of CF4 to O2 of 9:1 is introduced and treated for 10 minutes under the conditions of 500W radio frequency power and 5Pa pressure to obtain the high-purity laser-damage fluoride crystal described in Comparative Example 3.

[0127] Comparative Example 4

[0128] A closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals includes the following steps:

[0129] 1. Add 1 kg of 99.99% pure CaF2 powder to a 3% HF solution, stir at 75°C for 5 hours, filter, and wash with ultrapure water until neutral.

[0130] 2. Place the CaF2 powder after preliminary impurity removal in a boron nitride reaction boat and calcine it at high temperature in an F2 atmosphere. The calcine temperature is 700℃ and the holding time is 8 hours. The F2 flow rate is 100 sccm.

[0131] 3. The purified CaF2 powder is placed into a crucible in a sealed Bridgman furnace with a uniform longitudinal magnetic field. The crucible is a high-purity graphite crucible with a nano-scale BN coating sprayed on the inner wall. The furnace is then evacuated to 10°C. -4 Pa, a mixture of Ar and CF4 in a volume ratio of 100:0.1 is introduced, and the pressure is maintained at 0.05 MPa;

[0132] 4. Increase the temperature to 1450℃ at 80℃ / h and keep it at that temperature for 10 hours to homogenize the melt. During this period, turn on the longitudinal uniform magnetic field with a magnetic field strength of 0.1T.

[0133] 5. Then lower the crucible at a rate of 0.8 mm / h, setting the axial temperature gradient to 5℃ / cm and the radial temperature gradient to ≤1℃ / cm;

[0134] 6. After the calcium fluoride crystal growth is completed, it is annealed in situ at 900℃ for 80 hours, and then cooled to room temperature at 5℃ / h to obtain the high-purity laser-damage resistant fluoride crystal described in Comparative Example 4.

[0135] Inductively coupled plasma mass spectrometry was used to test the metal impurity content of Examples 1-6 and Comparative Examples 1-4 to reflect the purity of the fluoride crystals.

[0136] The laser damage threshold of Examples 1-6 and Comparative Examples 1-4 was tested using an excimer laser at 193 nm to demonstrate the laser damage resistance of the fluoride crystals.

[0137] Stress birefringence tests were performed on Examples 1-6 and Comparative Examples 1-4 using a polarizing interferometer to reflect the surface roughness of the fluoride crystals.

[0138] The ultraviolet light transmittance of Examples 1-6 and Comparative Examples 1-4 in the 193nm band was measured using an ultraviolet spectrophotometer.

[0139] The test results are shown in Table 1.

[0140] Table 1. Test results of fluoride crystals prepared in Examples 1-6 and Comparative Examples 1-4

[0141]

[0142]

[0143] As shown in Table 1, the performance of Examples 1-6 is superior to that of Comparative Examples 1-4. This is mainly because in Examples 1-6, the fluoride powder was washed with HF solution, which initially removed the metal cations and oxygen impurities present in the fluoride powder. Then, high-temperature calcination in an F2 environment further removed impurities. These two steps improved the purity of the fluoride powder, thereby enhancing the relevant properties of the fluoride crystal. Compared with Examples 1-6, Comparative Example 3 lacked the effect of an axial uniform magnetic field. During crystal growth, the Lorentz force generated by the magnetic field can effectively suppress turbulence in the melt. The presence of turbulence in the melt leads to a high dislocation density in the grown crystal, affecting its transmission performance. Compared with Examples 1-6, Comparative Example 4 lacked the plasma treatment step, resulting in many defects on the crystal surface, which affects the relevant properties of the crystal.

[0144] The above results are sufficient to prove that this application can make up for the deficiencies of the prior art and solve the problem of low laser damage threshold of fluoride crystals.

[0145] It should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the spirit and scope of the technical solutions of this application, and all such modifications and substitutions should be covered within the scope of the claims of this application.

Claims

1. A closed-loop growth method for high-purity, laser-damage-resistant fluoride crystals, characterized in that, Includes the following steps: Preliminary impurity removal treatment was performed on fluoride powder with a purity of 99.99% using HF solution; The fluoride powder, after preliminary impurity removal, was placed in a reaction boat and calcined under an F2 atmosphere for further impurity removal. The deeply purified fluoride powder was placed into a crucible in a sealed Bridgman furnace with a uniform magnetic field, and the furnace was evacuated to 10°C. -4 Below Pa, a mixture of Ar and CF4 gas is introduced, and the pressure is maintained at 0.05 MPa; The temperature is increased to 1270-1460℃ at 75-80℃ / h and held at the temperature for 9-10 hours to homogenize the fluoride melt, during which a uniform magnetic field is turned on. The crucible is lowered at a rate of 0.8–1.2 mm / h, with an axial temperature gradient of 5–20 °C / cm and a radial temperature gradient of ≤1 °C / cm. After the fluoride crystals have grown, they are annealed in situ at 800–950°C for 80–100 hours, and then cooled to room temperature at a rate of 5–10°C / h. After the fluoride crystal is grown, it is cut and polished, then placed in a vacuum plasma treatment chamber. A mixture of CF4 and O2 gas is introduced, and the crystal is treated for 10 to 15 minutes under the conditions of radio frequency power of 300 to 500 W and pressure of 5 to 7 Pa to obtain the high-purity laser-damage resistant fluoride crystal.

2. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, The fluoride crystals include any one of calcium fluoride, barium fluoride, and magnesium fluoride.

3. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, The concentration of the HF solution is 3-5%.

4. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, The preliminary impurity removal step is as follows: add fluoride powder with a purity of 99.99% to a 3-5% HF solution, stir at a constant temperature of 70-80°C for 4-5 hours, filter, and wash with ultrapure water until the pH range is between 6.5 and 8.

5.

5. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, The reaction boat includes any one of boron nitride reaction boat, zirconium oxide reaction boat, and silicon carbide reaction boat.

6. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, The conditions for deep impurity removal are: calcination temperature of 600-700℃, holding time of 7-9 hours, and F2 flow rate of 50-100 sccm.

7. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, The uniform magnetic field is any one or both of longitudinal and transverse uniform magnetic fields; the strength of the uniform magnetic field is 0.1 to 0.4 T.

8. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, The crucible includes either a graphite crucible with a nanoscale BN coating sprayed on its inner wall or a silicon carbide crucible.

9. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, In the Ar and CF4 mixture, the volume ratio of Ar to CF4 is 100:(0.05 to 0.12).

10. The closed-loop growth method for high-purity laser-damage-resistant fluoride crystals according to claim 1, characterized in that, In the CF4 and O2 mixture, the volume ratio of CF4 to O2 is (8.5-9.5):(0.5-1.5).

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