Impurity removal process for high-transmittance calcium fluoride crystals

Abstract

The invention discloses a high-transmittance calcium fluoride crystal impurity removal process, and belongs to the technical field of crystal impurity removal. The method comprises the following steps: mixing a calcium fluoride raw material with a modified deoxidant in an argon atmosphere by adopting a Bridgman-Stockbarger method, regulating and controlling the melting process of the calcium fluoride raw material through a gradient heating method, and finally obtaining the calcium fluoride crystal with high transmittance through descending crystallization and annealing. The modified deoxidant is of a three-layer core-shell structure of calcium, calcium fluoride and yttrium fluoride, calcium fluoride is subjected to in-situ conversion on the surfaces of metal calcium particles, then the calcium fluoride layer is etched through hydrofluoric acid, yttrium ions are adsorbed, finally, yttrium fluoride is generated on the surface of calcium fluoride in situ, and calcium, calcium fluoride and calcium fluoride core-shell particles are coated; according to the calcium fluoride high-temperature deoxygenization method, the calcium metal inside can be protected at low temperature, release of the calcium metal can be achieved at high temperature, deep deoxygenization is achieved in the calcium fluoride high-temperature melt, the optical performance of the crystal can be effectively improved, and the calcium fluoride crystal with high transmittance is obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of crystal impurity removal, and more particularly to a high-transmittance calcium fluoride crystal impurity removal process. BACKGROUND

[0002] Calcium fluoride crystal has extremely high ultraviolet light transmittance, high laser damage threshold and low refractive index, and is a key material for realizing deep ultraviolet lithography. With the semiconductor industry's continuous pursuit of high-precision and high-resolution lithography technology, high-quality calcium fluoride crystal and its growth have become the focus of attention. Calcium fluoride crystal is limited by intrinsic defects, impurities and other crystal structure defects during the growth process, affecting the quality of the crystal and reducing the optical performance of the crystal, thereby affecting the actual use of the crystal. It should be noted that the presence of oxide impurities in the crystal will destroy the optical performance of the grown crystal. In addition to forming special optical absorption optical active centers, it can also cause hydrolysis at a relatively low concentration above 600 ℃. The hydrolysis product is calcium oxide. As an external phase, calcium oxide can cause the melt to supercool before the growth interface, which can disturb the normal growth of the calcium fluoride melt, polycrystalline growth and the formation of light scattering centers.

[0003] Calcium fluoride crystal growth usually adopts the Czochralski method and the Bridgman method. The calcium fluoride crystal growth method and calcium fluoride crystal disclosed in the Chinese patent with the authorization announcement number CN116716659B controls the vacuum degree of the calcium fluoride crystal growth in the range of 20-80 kpa, and uses a mixed gas of argon, carbon tetrafluoride and helium with a volume ratio of 100: (0.016-0.1): (0.02-0.2) as the growth atmosphere, which can reduce the structural defects existing in the production of large-size calcium fluoride by the Czochralski method, and further improve the quality of the calcium fluoride crystal. However, it does not mention the removal of oxygen-containing impurities in the calcium fluoride crystal raw material. The growth method of large-size calcium fluoride crystal and calcium fluoride crystal disclosed in the Chinese patent with the authorization announcement number CN118407130B can fully overflow the bubbles in the calcium fluoride melt by adjusting the top baffle of the crucible, the temperature of the crystal furnace and using argon as the convection gas, which can significantly reduce the bubble content in the calcium fluoride crystal and improve the quality of the calcium fluoride crystal. However, this scheme does not involve the removal of calcium fluoride crystal.

[0004] Therefore, it is very important to develop a high-transmittance calcium fluoride crystal impurity removal process. SUMMARY

[0005] The purpose of the present application is to provide a high-transmittance calcium fluoride crystal impurity removal process, which uses a down-drawing crucible method to prepare calcium fluoride crystals; a gradient heating method is used to melt the calcium fluoride raw material, which can reduce the bubble content in the calcium fluoride crystals and reduce the defects of the calcium fluoride crystals; by adding a modified oxygen scavenger to the calcium fluoride crystals, the purpose of deep oxygen removal is achieved, and the problems of unstable oxygen removal effect and introduction of many impurities of traditional oxygen scavengers are overcome, and finally high-transmittance calcium fluoride crystals are prepared.

[0006] To achieve the above purpose, the present application provides a high-transmittance calcium fluoride crystal impurity removal process, characterized in that the process comprises

[0007] Step S1, mix calcium fluoride with a modified oxygen scavenger, introduce argon into the crucible, and heat to a first temperature and keep warm;

[0008] Step S2, heat to a second temperature and keep warm, and open the crucible cover;

[0009] Step S3, continuously heat to a third temperature and keep warm, continue to heat to a fourth temperature, and keep warm until the melt is clear;

[0010] Step S4, cool to the third temperature, lower the crucible, start crystallization, anneal after crystallization, and cool to room temperature.

[0011] Further, in step S1, the mass ratio of calcium fluoride to modified oxygen scavenger is 100: (0.5-1); the crucible cover has a through hole with a diameter of 1-1.2 mm; the first temperature is 190-210 ℃, and the heating rate is 8-12 ℃ / h; the time for keeping warm is 18-30 h; in step S2, the second temperature is 1150-1250 ℃, and the heating rate is 18-22 ℃ / h; the time for keeping warm is 8-10 h.

[0012] Further, in step S3, the third temperature is 1350-1380 ℃, and the heating rate is 20-25 ℃ / h; the time for keeping warm is 6-10 h; the fourth temperature is 1400-1420 ℃, and the heating rate is 20-25 ℃ / h; in step S4, the speed of lowering the crucible is 0.8-1.2 mm / h, and the height of lowering the crucible is 180-240 mm; the annealing temperature is 600-700 ℃, and the annealing time is 96-120 h; the cooling rate is 3-5 ℃ / h.

[0013] Further, in step S1, the preparation method of the modified oxygen scavenger comprises:

[0014] Step A1, dispersing the metal calcium particles in anhydrous ethanol, adding ammonium fluoride ethanol solution, reacting at room temperature, centrifuging, washing with ethanol, drying in argon, to obtain calcium@calcium fluoride core-shell particles;

[0015] Step A2, dispersing the calcium@calcium fluoride core-shell particles in anhydrous ethanol, adding hydrofluoric acid anhydrous ethanol solution dropwise, reacting, centrifuging, washing with ammonium fluoride anhydrous ethanol solution and anhydrous ethanol in turn, drying in argon, to obtain etched calcium@calcium fluoride core-shell particles;

[0016] Step A3, dispersing the etched calcium@calcium fluoride core-shell particles in yttrium nitrate anhydrous ethanol solution, swelling at room temperature, adding ammonium fluoride anhydrous ethanol solution, reacting, aging, filtering, drying in argon, to obtain calcium@calcium fluoride@yttrium fluoride core-shell particles, which are used as modified oxygen scavengers.

[0017] Further, in step A1, the mass-volume ratio of the metal calcium particles and anhydrous ethanol is 1g: (5-10) mL; the concentration of the ammonium fluoride ethanol solution is 0.3-0.6 mol / L; the volume ratio of anhydrous ethanol and water in the ethanol solution is (99-99.5):(0.5-1); the mass-volume ratio of the metal calcium particles and the ammonium fluoride ethanol solution is 1g: (87.5-175) mL; the stirring speed of the reaction is 100-200 rpm; the reaction time is 20-40 min; the drying temperature is 40-60℃

[0018] Further, in step A2, the mass-volume ratio of the calcium@calcium fluoride core-shell particles and anhydrous ethanol is 1g: (10-20) mL; the concentration of the hydrofluoric acid anhydrous ethanol solution is 0.01-0.03 mol / L, and the addition speed is 0.1-0.5 mL / min; the mass-volume ratio of the calcium@calcium fluoride core-shell particles and the hydrofluoric acid anhydrous ethanol solution is 1g: (1-3) mL; the stirring speed of the reaction is 100-200 rpm, and the reaction time is 5-10 min; the concentration of the ammonium fluoride anhydrous ethanol solution is 0.01-0.02 mol / L; the drying temperature is 40-60℃;

[0019] Further, in the step A3, the mass-volume ratio of the etched calcium@calcium fluoride core-shell particles and the yttrium nitrate anhydrous ethanol solution is 1g:(5-10)mL; the concentration of the yttrium nitrate anhydrous ethanol solution is 0.05-0.1mol / L; the room temperature swelling time is 20-40min; the concentration of the ammonium fluoride anhydrous ethanol solution is 0.15-0.3mol / L, and the adding speed of the ammonium fluoride anhydrous ethanol solution is 0.1-0.3mL / min; the volume ratio of the yttrium nitrate anhydrous ethanol solution and the ammonium fluoride anhydrous ethanol solution is 1:(1.1-1.15); the reaction time is 45-90min; the aging temperature is 30-40℃, and the aging time is 1.5-3h; and the drying temperature is 60-80℃.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] The present application prepares high-transmittance calcium fluoride crystals. The gradient temperature rising method is used as the calcium fluoride melting strategy, and through four-stage temperature control design, the bubble content in the calcium fluoride crystals is reduced, the defects of the calcium fluoride crystals are reduced, and the quality of the calcium fluoride crystals is improved. The present application also constructs a modified oxygen scavenger with a calcium@calcium fluoride@yttrium fluoride core-shell structure, takes metal calcium as the core, generates a calcium fluoride layer to coat the metal calcium in situ in the ethanol system, avoids the reaction of the metal calcium with the fluorination agent in the subsequent liquid phase synthesis of yttrium fluoride, and causes the failure of the high-efficiency oxygen scavenging core; then the calcium fluoride layer is etched by hydrofluoric acid to form a rough surface, which is beneficial to the adsorption of yttrium ions, and then yttrium fluoride is generated in situ on the surface of the calcium fluoride, thereby protecting the internal oxygen scavenging core, and finally forming a modified oxygen scavenger with metal calcium as the high-efficiency oxygen scavenging core, calcium fluoride as the protective transition layer, and yttrium fluoride as the shell, which can be added to the calcium fluoride raw material for use. The yttrium fluoride with stable chemical properties as the shell layer can effectively protect the internal high-efficiency oxygen scavenging core from being eroded by volatile impurities in the raw material and avoid being consumed prematurely; in the subsequent high-temperature melting stage, the yttrium fluoride shell layer is loose and melts first, yttrium ions compete with calcium oxide for oxygen ions to generate yttrium oxide precipitates, and the residual yttrium fluoride can also be incorporated into the calcium fluoride lattice to inhibit the formation of color centers and improve the optical energy of the final product; after further heating, the calcium fluoride layer is co-melted with the raw material, the internal metal calcium is released, and the oxygen scavenging effect is further exerted to deeply capture the oxygen-containing impurities in the calcium fluoride melt, thereby realizing high-efficiency oxygen scavenging; and finally, high-transmittance calcium fluoride crystals are obtained through the crucible lowering method. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a flowchart for the preparation method of the modified oxygen scavenger.

[0023] Figure 2 It is a temperature control curve diagram for the temperature rising of the calcium fluoride raw material. DETAILED DESCRIPTION

[0024] The technical solutions of the present application will be described in detail below through specific examples. It should be understood that the examples of the present application and the specific features in the examples are detailed descriptions of the technical solutions of the present application, and are not limitations of the technical solutions of the present application. In the case of no conflict, the technical features in the examples of the present application and the examples can be combined with each other.

[0025] The term "and / or", only describes the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " generally represents that the associated objects before and after it are in an "or" relationship.

[0026] Example 1

[0027] As shown in Figure 1 A method for preparing a modified oxygen scavenger includes:

[0028] Step A1, dispersing metal calcium particles in anhydrous ethanol, the mass-volume ratio of the metal calcium particles and the anhydrous ethanol being 1g:7mL; adding a 0.5mol / L ammonium fluoride ethanol solution, wherein the volume ratio of ethanol and water is 99.3:0.7; the mass-volume ratio of the metal calcium particles and the ammonium fluoride ethanol solution being 1g:150mL; stirring at room temperature at a speed of 150rpm for 30min; centrifuging to collect the precipitate, washing the precipitate with anhydrous ethanol, and drying the precipitate in argon at a temperature of 50°C to obtain calcium@calcium fluoride core-shell particles;

[0029] Step A2, dispersing the calcium@calcium fluoride core-shell particles in anhydrous ethanol to form a calcium@calcium fluoride core-shell particle dispersion, the mass-volume ratio of the calcium@calcium fluoride core-shell particles and the anhydrous ethanol being 1g:15mL; adding a 0.02mol / L hydrofluoric acid anhydrous ethanol solution to the calcium@calcium fluoride core-shell particle dispersion at a speed of 0.3mL / min, the mass-volume ratio of the calcium@calcium fluoride core-shell particles and the hydrofluoric acid anhydrous ethanol solution being 1g:2mL; stirring at a speed of 150rpm for 7min, collecting the precipitate after centrifugation, and sequentially washing the precipitate with a 0.015mol / L ammonium fluoride anhydrous ethanol solution and anhydrous ethanol, and drying the precipitate in argon at a temperature of 50°C to obtain etched calcium@calcium fluoride core-shell particles;

[0030] Step A3, the etched calcium@calcium fluoride core-shell particles were dispersed in a yttrium nitrate anhydrous ethanol solution with a concentration of 0.07 mol / L to obtain a reaction solution, the mass-volume ratio of the etched calcium@calcium fluoride core-shell particles and the yttrium nitrate anhydrous ethanol solution was 1 g:7 mL; the reaction solution was swelled at room temperature for 30 min, then a 0.2 mol / L ammonium fluoride anhydrous ethanol solution was added to the reaction solution at a speed of 0.2 mL / min, the volume ratio of the yttrium nitrate anhydrous ethanol solution and the ammonium fluoride anhydrous ethanol solution was 1:1.12; then the reaction was carried out at room temperature for 60 min and aged at 35℃ for 2 h, the solid was filtered out and dried in argon, the drying temperature was 70℃, to obtain calcium@calcium fluoride@yttrium fluoride core-shell particles, which were used as a modified oxygen scavenger.

[0031] Example 2

[0032] As shown in Figure 1 , a modified oxygen scavenger, the preparation method thereof comprises:

[0033] Step A1, the metal calcium particles were dispersed in anhydrous ethanol, the mass-volume ratio of the metal calcium particles and the anhydrous ethanol was 1 g:10 mL; a 0.3 mol / L ammonium fluoride ethanol solution was added, wherein the volume ratio of ethanol and water was 99.5:0.5; the mass-volume ratio of the metal calcium particles and the ammonium fluoride ethanol solution was 1 g:175 mL; the reaction was stirred at room temperature at a speed of 200 rpm for 20 min; the precipitate was collected by centrifugation, the precipitate was washed with anhydrous ethanol, and the precipitate was dried in argon at a temperature of 40℃, to obtain calcium@calcium fluoride core-shell particles;

[0034] Step A2, the calcium@calcium fluoride core-shell particles were dispersed in anhydrous ethanol to form a calcium@calcium fluoride core-shell particle dispersion, the mass-volume ratio of the calcium@calcium fluoride core-shell particles and the anhydrous ethanol was 1 g:20 mL; a 0.01 mol / L hydrofluoric acid anhydrous ethanol solution was added to the calcium@calcium fluoride core-shell particle dispersion at a speed of 0.5 mL / min, the mass-volume ratio of the calcium@calcium fluoride core-shell particles and the hydrofluoric acid anhydrous ethanol solution was 1 g:3 mL; the reaction was stirred at a speed of 200 rpm for 5 min, the precipitate was collected after centrifugation, the precipitate was washed with a 0.01 mol / L ammonium fluoride anhydrous ethanol solution and anhydrous ethanol in sequence, and the precipitate was dried in argon at a temperature of 40℃, to obtain etched calcium@calcium fluoride core-shell particles;

[0035] Step A3, the etched calcium@calcium fluoride core-shell particles were dispersed in a yttrium nitrate anhydrous ethanol solution with a concentration of 0.05 mol / L to obtain a reaction solution, the mass-volume ratio of the etched calcium@calcium fluoride core-shell particles and the yttrium nitrate anhydrous ethanol solution was 1 g:10 mL; the reaction solution was swelled at room temperature for 40 min, then the yttrium nitrate anhydrous ethanol solution was added to the reaction solution at a speed of 0.3 mL / min, the concentration of the yttrium nitrate anhydrous ethanol solution was 0.15 mol / L, the volume ratio of the yttrium nitrate anhydrous ethanol solution and the ammonium fluoride anhydrous ethanol solution was 1:1.15; then the reaction was carried out at room temperature for 90 min and aging was carried out at 30℃ for 3 h, the solid was filtered out and dried in argon, the drying temperature was 60℃, to obtain calcium@calcium fluoride@yttrium fluoride core-shell particles, which were used as modified oxygen scavengers.

[0036] Example 3

[0037] As shown in Figure 1 , a modified oxygen scavenger, the preparation method thereof comprises:

[0038] Step A1, the metal calcium particles were dispersed in anhydrous ethanol, the mass-volume ratio of the metal calcium particles and the anhydrous ethanol was 1 g:5 mL; 0.6 mol / L of the ammonium fluoride ethanol solution was added, wherein the volume ratio of the ethanol and water was 99:1; the mass-volume ratio of the metal calcium particles and the ammonium fluoride ethanol solution was 1 g:87.5 mL; the reaction was stirred at room temperature at a speed of 100 rpm for 40 min; the precipitate was collected by centrifugation, the precipitate was washed with anhydrous ethanol, and the precipitate was dried in argon at a temperature of 60℃, to obtain calcium@calcium fluoride core-shell particles;

[0039] Step A2, the calcium@calcium fluoride core-shell particles were dispersed in anhydrous ethanol to form a calcium@calcium fluoride core-shell particle dispersion, the mass-volume ratio of the calcium@calcium fluoride core-shell particles and the anhydrous ethanol was 1 g:10 mL; 0.03 mol / L of the hydrofluoric acid anhydrous ethanol solution was added to the calcium@calcium fluoride core-shell particle dispersion at a speed of 0.1 mL / min, the mass-volume ratio of the calcium@calcium fluoride core-shell particles and the hydrofluoric acid anhydrous ethanol solution was 1 g:1 mL; the reaction was stirred at a speed of 100 rpm for 10 min, the precipitate was collected after centrifugation, the precipitate was washed with 0.02 mol / L of the ammonium fluoride anhydrous ethanol solution and anhydrous ethanol in sequence, and the precipitate was dried in argon at a temperature of 60℃, to obtain etched calcium@calcium fluoride core-shell particles;

[0040] Step A3, the etched calcium@calcium fluoride core-shell particles were dispersed in yttrium nitrate anhydrous ethanol solution with a concentration of 0.1 mol / L to obtain a reaction solution, the mass-volume ratio of the etched calcium@calcium fluoride core-shell particles and the yttrium nitrate anhydrous ethanol solution was 1 g:4 mL; the reaction solution was swelled at room temperature for 20 min, then the yttrium nitrate anhydrous ethanol solution was added into the reaction solution at a speed of 0.1 mL / min, the concentration of the yttrium nitrate anhydrous ethanol solution was 0.2 mol / L, the volume ratio of the yttrium nitrate anhydrous ethanol solution and the ammonium fluoride anhydrous ethanol solution was 1:1.1; then the reaction was carried out at room temperature for 45 min and aging at 40℃ for 1.5 h, the solid was filtered out and dried in argon, the drying temperature was 80℃, to obtain calcium@calcium fluoride@yttrium fluoride core-shell particles, which were used as modified oxygen scavenger.

[0041] Example 4

[0042] A high-transmittance calcium fluoride crystal impurity removal process, the steps of which include:

[0043] The calcium fluoride and the modified oxygen scavenger prepared in Example 1 were mixed in an argon atmosphere according to a mass ratio of 100:0.75, placed in a crucible, and covered with a crucible cover having a small hole with a diameter of 1.1 mm. The crystal furnace was vacuumed and argon was introduced, and the crucible was placed in the furnace. The temperature was increased to 200℃ at a speed of 10℃ / h and kept for 24 h. Then the temperature in the furnace was increased to 1200℃ at a speed of 20℃ / h and kept for 9 h, and the crucible cover at the top was opened. The temperature was continuously increased to 1365℃ at a speed of 22℃ / h and kept for 8 h, and then continuously increased to 1410℃ at a speed of 22℃ / h and kept until the melt in the crucible was clear. After cooling to 1365℃, the crucible was lowered at a speed of 1.0 mm / h, and the melt in the crucible began to crystallize. The furnace was cooled to 650℃ after the crucible was lowered to 200 mm, and annealed for 108 h. The temperature was decreased to room temperature at a speed of 4℃ / h, to obtain high-transmittance calcium fluoride crystals. The temperature control curve of the heating process is shown in Figure 2 .

[0044] Example 5

[0045] A high-transmittance calcium fluoride crystal impurity removal process, the steps of which include:

[0046] Calcium fluoride and the modified oxygen scavenger prepared in Example 2 were mixed in an argon atmosphere at a mass ratio of 100:1. The mixture was placed in a crucible, and the crucible lid, which had a small hole with a diameter of 1.2 mm, was closed. The crystal furnace was evacuated and argon gas was introduced. The crucible was placed inside, and the temperature was raised to 190 °C at a rate of 8 °C / h and held for 30 h. Then, the furnace temperature was raised to 1150 °C at a rate of 18 °C / h and held for 10 h. The crucible lid was then opened. The temperature was then raised to 1350 °C at a rate of 20 °C / h and held for 10 h. Subsequently, the temperature was raised to 1400 °C at a rate of 20 °C / h and held until the melt in the crucible became clear. After cooling to 1350℃, the crucible was lowered at a rate of 1.2 mm / h, during which the melt inside the crucible began to crystallize. Upon reaching a depth of 240 mm, the crucible was further cooled to 600℃ and annealed for 120 h. Finally, it was cooled to room temperature at a rate of 5℃ / h to obtain high-transmittance calcium fluoride crystals. The temperature control curve during the heating process is shown in the figure. Figure 2 As shown.

[0047] Example 6

[0048] A process for removing impurities from high-transmittance calcium fluoride crystals, comprising the following steps:

[0049] Calcium fluoride and the modified oxygen scavenger prepared in Example 3 were mixed in an argon atmosphere at a mass ratio of 100:0.5, placed in a crucible, and covered with a crucible lid with a small hole of 1 mm in diameter. The crystal furnace was evacuated and argon gas was introduced. The crucible was placed inside and the temperature was raised to 210 °C at a rate of 10 °C / h and held for 18 h. Then, the furnace temperature was raised to 1250 °C at a rate of 22 °C / h and held for 8 h. The crucible lid was then opened. The temperature was then raised to 1380 °C at a rate of 25 °C / h and held for 6 h. Subsequently, the temperature was raised to 1420 °C at a rate of 25 °C / h and held until the melt in the crucible became clear. After cooling to 1380℃, the crucible was lowered at a rate of 0.8 mm / h, during which the melt inside the crucible began to crystallize. Upon reaching a depth of 180 mm, the crucible was further cooled to 700℃ and annealed for 96 h. Finally, it was cooled to room temperature at a rate of 3℃ / h to obtain high-transmittance calcium fluoride crystals. The temperature control curve during the heating process is shown in the figure. Figure 2 As shown.

[0050] Comparative Example 1

[0051] A high-transmittance calcium fluoride crystal impurity removal process differs from Example 4 in that no modified deoxidizer is added, while the other operating steps and process parameters are exactly the same as in Example 4.

[0052] Comparative Example 2

[0053] A high-transmittance calcium fluoride crystal impurity removal process differs from Example 4 in that the modified deoxidizer used is yttrium fluoride, while the other operating steps and process parameters are exactly the same as in Example 4.

[0054] Comparative Example 3

[0055] A high-transmittance calcium fluoride crystal impurity removal process differs from Example 4 in that the modified deoxidizer used is metallic calcium particles, while the other operating steps and process parameters are exactly the same as in Example 4.

[0056] Comparative Example 4

[0057] A high-transmittance calcium fluoride crystal impurity removal process differs from Example 4 in that it does not involve holding at the first and second temperatures, but instead directly raises the furnace temperature to the third temperature and holds it thereafter. The other operating steps and process parameters are exactly the same as in Example 4.

[0058] Performance testing:

[0059] Internal transmittance: A deep ultraviolet double-beam automatic spectrophotometer with a wavelength range of not less than 150-400 nm, a bandwidth of not more than 0.17 nm, a wavelength accuracy of ±0.1 nm, and a photometer accuracy of ±0.003 Å was used. For each embodiment and comparative example, two identical calcium fluoride crystals were used as samples. One sample was polished on one side, and the other on both sides. The thickness d of the double-sided polished sample was measured in cm using a micrometer. The surface roughness of the sample was not greater than 1 nm, and the parallelism between the two sides of the double-sided polished sample was not greater than 1′. Internal transmittance was tested under nitrogen protection, and the transmittance T of the double-sided polished sample was measured at 0°C. ℷ Subsequently, the rough surface of the single-sided polished sample was coated with black, and the single-sided residual reflectance R of the single-sided polished sample was measured at an incident angle of no more than 10°. ℷ The internal permeability T of each embodiment and comparative example is calculated using the following formula. i :

[0060]

[0061] Table 1. Internal transmittance of each embodiment and comparative example at a wavelength of 193 nm.

[0062] Internal transmission (%) Example 4 94.7 Example 5 93.8 Example 6 94.2 Comparative Example 1 81.4 Comparative Example 2 86.5 Comparative Example 3 91.3 Comparative Example 4 89.1

[0063] As shown in Table 1, the internal transmittance of each embodiment is higher than 93.5%, demonstrating good crystal transmittance.

[0064] In Comparative Example 1, no oxygen scavenger was added. During the melting of calcium fluoride, residual moisture and other oxygen-containing impurities in the calcium fluoride raw material could not be removed by the oxygen scavenger. Instead, they reacted with calcium ions in the melt, and the resulting oxygen impurities were completely retained in the crystal, leading to the formation of numerous color centers. This further resulted in strong intrinsic absorption, manifested as extremely low internal transmittance. In Comparative Example 2, the oxygen scavenger was yttrium fluoride. Yttrium fluoride itself is a stable compound and does not possess reducing properties, so it cannot remove oxygen impurities. However, the introduction of yttrium ions can effectively inhibit the formation of color centers, thereby alleviating some of the absorption caused by oxygen impurities in the calcium fluoride crystal. During the crystallization process, the yttrium fluoride crystal is compatible with the calcium fluoride matrix and does not introduce scattering. In Comparative Example 3, the oxygen scavenger was metallic calcium particles. Metallic calcium has high reactivity, therefore requiring a strict argon atmosphere during operation. Furthermore, during the melting of the calcium fluoride raw material, the metallic calcium melts into a liquid at approximately 850 °C, reacting with some impurities in the calcium fluoride and being prematurely consumed. This means that when the calcium fluoride melts into a molten state, the oxygen impurities inside the raw material cannot be removed. However, compared to the other comparative examples, its oxygen impurity removal efficiency is better. Therefore, the internal permeability of Comparative Example 3 is higher than the other comparative examples, but still lower than the examples. In Comparative Example 4, the calcium fluoride melting process involved a one-step heating without gradient heating, causing the volatile impurities in the raw material to rapidly vaporize, forming a small vapor environment. Although the calcium fluoride raw material contained an oxygen scavenger, some calcium fluoride still hydrolyzed in the vapor environment to form calcium oxide, creating color centers. This resulted in its internal permeability being lower than Comparative Example 3, but higher than Comparative Examples 1 and 2.

[0065] In summary, this invention employs a gradient heating method as the melting strategy for calcium fluoride, combined with a modified oxygen scavenger constructing a calcium@calcium fluoride@yttrium fluoride core-shell structure, to prepare high-transmittance calcium fluoride crystals. The modified oxygen scavenger uses metallic calcium particles as the core, converting the calcium on the particle surface into calcium fluoride in an ethanol system, achieving in-situ coating of calcium fluoride to obtain calcium@calcium fluoride core-shell particles. Subsequently, a low-concentration hydrofluoric acid is used to etch the calcium fluoride layer, giving it a micron-level rough surface. The etched calcium@calcium fluoride core-shell structure is then dispersed in anhydrous yttrium nitrate ethanol solution. The rough surface of the core-shell structure allows for the embedding of yttrium ions. Anhydrous ammonium fluoride ethanol solution is then added as a fluorinating agent, heterogeneously nucleating with yttrium ions on the surface of the calcium@calcium fluoride core-shell particles to generate yttrium fluoride, which coats the surface of the calcium@calcium fluoride core-shell particles, forming a three-layer core-shell structure of calcium@calcium fluoride@yttrium fluoride. This structure is then added to the calcium fluoride raw material as a modified oxygen scavenger. The modified oxygen scavenger prepared by this method does not introduce impurities affecting transmittance into the calcium fluoride melt. Calcium fluoride and the raw material are homologous, and yttrium fluoride can integrate into the calcium fluoride lattice, suppressing color centers. Molten yttrium fluoride can also capture oxygen-containing impurities in the raw material, competitively binding oxygen ions to generate trappable yttrium oxide precipitates, without affecting the transmittance of the final product. Metallic calcium can combine with fluorine in yttrium fluoride to generate calcium fluoride homologous to the raw material. Therefore, this modified oxygen scavenger does not introduce impurities affecting light transmittance into the calcium fluoride crystal. The melting point of metallic calcium is much lower than that of calcium fluoride and yttrium fluoride. Therefore, when heated, the metallic calcium core should melt into a liquid first. However, due to the mass transfer obstruction of the double shell, the melting temperature of metallic calcium rises significantly. Simultaneously, calcium fluoride hinders the outflow of the metallic calcium melt, preventing it from reacting with the yttrium fluoride shell to generate yttrium fluoride-doped calcium fluoride impurities. Therefore, in practical applications, the yttrium fluoride shell is melted first. The yttrium ions within it combine with trace amounts of oxygen ions in the melt to form a high-melting-point, low-solubility yttrium oxide precipitate. This prevents oxygen ions from reacting with calcium ions in calcium fluoride to form calcium oxide impurities. The resulting yttrium oxide particles are uniform in size, facilitating subsequent collection. Unreacted yttrium fluoride can also be incorporated into the calcium fluoride lattice, suppressing color center formation and further enhancing the optical properties of the calcium fluoride crystal. Next, calcium fluoride melts, including the calcium fluoride in the core-shell structure and the calcium fluoride particles used as raw materials. Simultaneously, as the calcium fluoride shell melts, the molten metallic calcium core within it is also released. This metallic calcium core combines with fluorine in the yttrium fluoride to form calcium fluoride, which fills the raw material, further increasing its transmittance. Finally, a high-transmittance calcium fluoride crystal is obtained using the crucible lowering method.

[0066] The embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A high-transmittance calcium fluoride crystal impurity removal process, characterized by, Comprising Step S1, the calcium fluoride and modified oxygen scavenger are mixed, argon is introduced into the crucible, and the temperature is raised to a first temperature, and the temperature is kept constant; Step S2, the temperature is raised to a second temperature, and the temperature is kept constant, and the crucible cover is opened; Step S3, continuously raise the temperature to a third temperature, keep the temperature constant, continue to raise the temperature to a fourth temperature, and keep the temperature constant until the melt is clear; Step S4, the temperature is lowered to the third temperature, the crucible is lowered, the crystallization is started, and the annealing is carried out after the crystallization is completed, and the temperature is lowered to room temperature.

2. The high transmission calcium fluoride crystal impurity removal process of claim 1, wherein, In the step S1, the mass ratio of the calcium fluoride to the modified oxygen scavenger is 100:(0.5-1); the crucible cover has a through hole with a diameter of 1-1.2 mm; the first temperature is 190-210 ℃, and the heating rate is 8-12 ℃ / h; the temperature keeping time is 18-30 h.

3. The high transmission calcium fluoride crystal impurity removal process of claim 1, wherein, In the step S2, the second temperature is 1150-1250 ℃, the heating rate is 18-22 ℃ / h, and the temperature keeping time is 8-10 h.

4. The high transmission calcium fluoride crystal process of claim 1, wherein, In the step S3, the third temperature is 1350-1380 ℃, the heating rate is 20-25 ℃ / h, the temperature keeping time is 6-10 h, and the fourth temperature is 1400-1420 ℃, the heating rate is 20-25 ℃ / h.

5. The high transmission calcium fluoride crystal process of claim 1, wherein, In the step S4, the speed of the crucible lowering is 0.8-1.2 mm / h, the height of the crucible lowering is 180-240 mm, the annealing temperature is 600-700 ℃, the annealing time is 96-120 h, and the cooling speed is 3-5 ℃ / h.

6. The high transmission calcium fluoride crystal process of claim 1, wherein, The preparation method of the modified oxygen scavenger in the step S1 comprises: Step A1, dispersing metal calcium particles in anhydrous ethanol, adding ammonium fluoride ethanol solution, reacting at room temperature, centrifuging, washing with anhydrous ethanol, and drying in argon to obtain calcium@calcium fluoride core-shell particles; Step A2, dispersing the calcium@calcium fluoride core-shell particles in anhydrous ethanol, adding hydrofluoric acid anhydrous ethanol solution, reacting, centrifuging, washing with ammonium fluoride anhydrous ethanol solution and anhydrous ethanol in turn, and drying in argon to obtain etched calcium@calcium fluoride core-shell particles; Step A3, dispersing the etched calcium@calcium fluoride core-shell particles in yttrium nitrate anhydrous ethanol solution, swelling at room temperature, adding ammonium fluoride anhydrous ethanol solution, reacting, aging, filtering, and drying in argon to obtain calcium@calcium fluoride@yttrium fluoride core-shell particles, which are used as modified oxygen scavengers.

7. The high transmission calcium fluoride crystal process of claim 6, wherein, In step A1, the mass-volume ratio of the metal calcium particles to anhydrous ethanol is 1g:(5-10)mL; the concentration of the ammonium fluoride ethanol solution is 0.3-0.6mol / L; the volume ratio of anhydrous ethanol to water in the ethanol solution is (99-99.5):(0.5-1); the mass-volume ratio of the metal calcium particles to the ammonium fluoride ethanol solution is 1g:(87.5-175)mL; the stirring speed of the reaction is 100-200rpm; the reaction time is 20-40min; and the drying temperature is 40-60℃.

8. The high transmission calcium fluoride crystal process of claim 6, wherein, The mass-volume ratio of the calcium@calcium fluoride core-shell particles and anhydrous ethanol in the step A2 is 1 g: (10-20) mL; the concentration of the hydrofluoric acid anhydrous ethanol solution is 0.01-0.03 mol / L, and the adding speed is 0.1-0.5 mL / min; the mass-volume ratio of the calcium@calcium fluoride core-shell particles and the hydrofluoric acid anhydrous ethanol solution is 1 g: (1-3) mL; the stirring speed of the reaction is 100-200 rpm, and the reaction time is 5-10 min; the concentration of the ammonium fluoride anhydrous ethanol solution is 0.01-0.02 mol / L; and the drying temperature is 40-60 °C.

9. The high transmission calcium fluoride crystal process of claim 6, wherein, In the step A3, the mass-volume ratio of the etched calcium@calcium fluoride core-shell particles and yttrium nitrate anhydrous ethanol solution is 1 g: (5-10) mL; the concentration of the yttrium nitrate anhydrous ethanol solution is 0.05-0.1 mol / L; the room temperature swelling time is 20-40 min; the concentration of the ammonium fluoride anhydrous ethanol solution is 0.15-0.3 mol / L, and the adding speed of the ammonium fluoride anhydrous ethanol solution is 0.1-0.3 mL / min; the volume ratio of the yttrium nitrate anhydrous ethanol solution and the ammonium fluoride anhydrous ethanol solution is 1: (1.1-1.15); the reaction time is 45-90 min; the aging temperature is 30-40 °C, and the aging time is 1.5-3 h; and the drying temperature is 60-80 °C.

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