Annealing crucible and annealing method for fluoride crystals

Abstract

The invention belongs to the technical field of fluoride crystal annealing, and particularly relates to an annealing crucible and an annealing method for fluoride crystals. The inner crucible is arranged inside the outer crucible, a plurality of through holes are formed in the inner crucible, the inner crucible is used for placing the fluoride crystal and a protective material, and the protective material wraps the fluoride crystal to prevent the fluoride crystal from being in direct contact with the inner crucible and air; a gap layer is arranged between the inner crucible and the outer crucible and is used for filling broken crystal materials, and the annealing crucible can selectively enable at least part of the protective materials to overflow towards the gap layer along the through holes. According to the double-layer crucible structure formed by combining the outer crucible and the inner crucible with the through hole, a buffer space can be provided for expansion of fluoride crystals in the annealing process, the axial and radial temperature gradients of the crystals are effectively reduced, a more uniform annealing environment is generated, and the internal stress of the crystals is slowly released.

Description

Technical Field

[0001] The invention belongs to the technical field of fluoride crystal annealing, and particularly relates to an annealing crucible and an annealing method for fluoride crystals. Background Art

[0002] The optical projection imaging system is the core component of the lithography machine. Its performance determines the imaging performance of the lithography machine. It is mainly designed and manufactured with a large number of ultraviolet-grade optical materials. Calcium fluoride (CaF2) crystal is one of the indispensable core optical materials in the optical projection imaging system because of its high transmittance in the deep ultraviolet band, constant average refractive index and local refractive index, and stable physical and chemical properties. The lithography machine imaging system has high requirements for the performance indicators of optical materials, such as the transmittance of CaF2 crystal ≥99.7%@193nm, stress birefringence ≤3nm / cm, etc.

[0003] At present, the growth of large-sized calcium fluoride crystals adopts the crucible descent method, which uses a fully enclosed or semi-enclosed crucible, and gradually cools down as the crucible slowly descends to achieve crystal growth. However, the inevitable mechanical vibration of this method will cause temperature fluctuations at the solid-liquid interface and a large radial temperature gradient during crystal growth, which in turn causes stress inside the crystal. Therefore, in order to reduce the stress inside the crystal, after the growth is completed, it is generally necessary to anneal in an annealing furnace to further reduce the crystal stress. At present, the most common crystal annealing method is to place the ingot or a sample with a certain size and shape after rough processing directly into the crucible. There are also a small number of large-sized crystal annealings using broken crystal materials, but this method cannot ensure that the radial temperature gradient of the crystal is controlled within a small range, so it is difficult to avoid the generation of new stress in the secondary annealing. At the same time, during the annealing process, the calcium fluoride crystals react with the residual air in the furnace chamber and the impurities such as foreign matter, water vapor or oxygen attached or adsorbed on the filling raw materials or crystal surface to react with oxidation, affecting the transmittance of deep ultraviolet light. Summary of the invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide an annealing crucible and an annealing method for fluoride crystals. The present invention can eliminate the thermal stress generated during the crystal growth process without affecting the crystal transmittance, and can avoid the crystal from generating new stress during secondary annealing, thereby reducing the crystal cracking rate during the processing and solving the problem of high stress in fluoride crystals.

[0005] To achieve the above object, the present invention adopts the following technical solution: A fluoride crystal annealing crucible, comprising: outer crucible; An inner crucible disposed inside the outer crucible, the inner crucible being provided with a plurality of through holes, the inner crucible being used to place the fluoride crystals and a protective material, the protective material covering the fluoride crystals to prevent the fluoride crystals from directly contacting the inner crucible; A gap layer is provided between the inner crucible and the outer crucible for filling broken crystals, wherein The annealing crucible can selectively allow the protective material to at least partially overflow toward the gap layer along the through hole.

[0006] As an embodiment of the present invention, the outer crucible includes an outer crucible body and an outer crucible cover covering the top opening of the outer crucible body, and the inner crucible is arranged inside the outer crucible body.

[0007] As an embodiment of the present invention, a vent hole for discharging air and impurities in the crucible is provided in the middle of the outer crucible cover.

[0008] As an embodiment of the present invention, the inner crucible includes an inner crucible body and an inner crucible cover covering the top opening of the inner crucible body; The inner crucible body and the inner crucible cover are both provided with a plurality of through holes.

[0009] As an embodiment of the present invention, the fluoride crystals expand when heated. When the fluoride crystals in the annealing crucible are at an annealing temperature exceeding a set threshold, the fluoride crystals push the protective material to overflow along the through hole toward the gap layer.

[0010] As an embodiment of the present invention, the protective material is a mixture of fluoride and deoxidizer, and the particle size of the protective material is smaller than the aperture of the through hole.

[0011] As an embodiment of the present invention, the particle size of the protective material is preferably 0.1 mm to 1 mm, and the aperture of the through hole is preferably 0.5 to 2 mm, so as to ensure that the protective material can overflow from the through hole to the gap layer.

[0012] As an embodiment of the present invention, the crushed crystal material is a plurality of fluoride crystal fragments. When the fluoride crystal fragments are filled in the gap layer, there are gaps between the plurality of fluoride crystal fragments.

[0013] As an embodiment of the present invention, the fluoride crystals expand when heated. When the annealing temperature of the fluoride crystals in the annealing crucible exceeds a set threshold, the fluoride crystals push the protective material to overflow along the through hole into the gap between the fluoride crystal fragments.

[0014] As an embodiment of the present invention, the set threshold is preferably 900°C.

[0015] As an embodiment of the present invention, the fluoride crystal fragments are preferably formed by breaking up fluoride crystal waste materials to reduce costs and increase the recycling rate of waste materials.

[0016] As an embodiment of the present invention, the particle size of the fluoride crystal fragments is larger than the aperture of the through hole to avoid direct contact between the fluoride crystals and the air and the inside of the furnace body. At the same time, the larger the particle size, the larger the porosity between the fluoride crystal fragments, so that when the fluoride crystals expand during annealing, the mixture in the inner crucible can overflow more easily and fill the gap.

[0017] As an embodiment of the present invention, the annealing crucible is preferably a graphite crucible, a molybdenum crucible, a quartz crucible or an alumina crucible.

[0018] To achieve the above object, the present invention also adopts the following technical solution: A method for annealing fluoride crystals using the above annealing crucible comprises the following steps: Step S1, placing the fluoride crystal in the inner crucible; Step S2, coating the fluoride crystal with the protective material; Step S3, filling the broken crystal material into the gap layer between the inner crucible and the outer crucible; Step S4, placing the annealing crucible into a vacuum annealing furnace; Step S5, evacuating the vacuum annealing furnace; Step S6, filling the vacuum chamber of the vacuum annealing furnace with fluorine-containing atmosphere gas; Step S7, heating the fluoride crystal to an annealing temperature higher than a set threshold and lower than its melting point; Step S8, after maintaining the annealing temperature for a period of time, gradually lowering the annealing temperature to room temperature to complete the annealing.

[0019] As an embodiment of the present invention, the step S5 is preferably to evacuate the vacuum annealing furnace to a background vacuum degree of ≥5×10 -4 Pa.

[0020] As an embodiment of the present invention, the step S6 is preferably to fill the vacuum chamber of the vacuum annealing furnace with fluorine-containing atmosphere gas until the vacuum chamber is filled with the fluorine-containing atmosphere gas.

[0021] As an embodiment of the present invention, the step S5 and the step S6 are repeated in a cycle for at least 3 times to better discharge the air and impurities in the vacuum chamber and purify the vacuum chamber.

[0022] As an embodiment of the present invention, the annealing temperature is preferably 900-1100°C.

[0023] As an embodiment of the present invention, the deoxidizer is preferably NH4F, NH4HF2, PbF2 or ZnF2. More preferably, it is PbF2. Specifically, since the fluoride crystals undergo oxidation reactions with the residual air in the furnace chamber and impurities such as foreign matter, water vapor or oxygen attached or adsorbed on the surface of the filling raw materials or crystals during the annealing process, the fluoride crystals are very likely to undergo hydrolysis reactions in a high temperature environment to grow refractory oxides, so adding NH4F, NH4HF2, PbF2 or ZnF2 as deoxidizers can solve the above problems. For example, calcium fluoride crystals react with water as follows: CaF2+H2O→CaO+HF. At this time, adding PbF2 as a deoxidizer can cause the following reaction: PbF2+CaO→PbO+CaF2, thereby removing refractory CaO.

[0024] As an embodiment of the present invention, the mass fraction of the deoxidizer in the calcium fluoride and deoxidizer mixture is preferably 1% to 5%. This ensures that the deoxidizer can completely eliminate the residual air in the furnace chamber and foreign matter, water vapor or oxygen and other impurities attached or adsorbed on the surface of the protective material or fluoride crystals, and can also ensure that the deoxidizer reacts completely, and avoids the waste of raw materials and environmental pollution caused by the unreacted excess deoxidizer.

[0025] As an embodiment of the present invention, the fluorine-containing atmosphere gas is preferably a mixed atmosphere of argon and carbon tetrafluoride. Specifically, since carbon tetrafluoride has a fluorination effect, it is easy to react with impurities such as water and oxygen in the vacuum furnace, and can react with impurities in the crystal, thereby further removing impurities and preventing the fluoride crystal from contacting with impurities such as water and oxygen.

[0026] As an embodiment of the present invention, the volume fraction of the carbon tetrafluoride in the mixed atmosphere is preferably 1% to 10%, thereby ensuring that the carbon tetrafluoride reacts fully while avoiding waste and environmental pollution.

[0027] Compared with the prior art, the present invention has the following beneficial technical effects: 1. The "double crucible" structure, which is composed of an outer crucible and an inner crucible with multiple through holes, can effectively reduce the axial and radial temperature gradients of the crystal during the annealing process, produce a more uniform annealing environment, and slowly release the internal stress of the crystal; 2. The multiple through holes provided in the inner crucible can enable the atmosphere inside the entire annealing crucible to circulate, promoting the interior of the crucible to be filled with fluorine protective atmosphere; 3. The multiple through holes of the inner crucible provide a buffer space for the expansion of the fluoride crystals during the annealing process, effectively solving the pressure generated during the crystal expansion process and effectively releasing the stress in the crystal. That is, since the protective material covering the fluoride crystals in the inner crucible is a mixture of fluoride and deoxidizer with smaller particle size, and the gap layer between the inner crucible and the outer crucible is filled with broken crystals with larger particle size, when the crystal expands during the annealing process, the protective material with smaller particle size can overflow into the gap between the broken crystals through the through holes, thereby effectively solving the pressure generated during the crystal expansion process and effectively releasing the stress in the crystal; 4. Fluoride crystals are coated with a mixture of fluoride and deoxidizer to provide a first fluorine protection environment; a mixture of inert gas and carbon tetrafluoride is filled into the annealing furnace chamber to provide a second fluorine protection environment. Annealing fluoride crystals in a double fluorine protection environment can avoid or reduce the entry of trace oxygen into the crystal and the generation of defects such as fluorine vacancies, and can effectively eliminate moisture adsorbed in the raw materials or residual in the annealing furnace. At the same time, the raw materials can be further fluorinated to remove oxide impurities that may exist in the raw materials, thereby ensuring the transmittance of the fluoride crystals; 5. The annealing crucible and annealing method of the present invention can reduce the internal stress of the fluoride crystal while ensuring the transmittance of the fluoride crystal and avoid the crystal from generating new stress during secondary annealing. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 It is a schematic structural diagram of a fluoride crystal annealing crucible according to a specific embodiment of the present invention; Figure 2 is a cross-sectional view of a crucible for annealing fluoride crystals according to a specific embodiment of the present invention; Figure 3 A flow chart of a method for annealing a fluoride crystal according to a specific embodiment of the present invention; Figure 4 This is a stress birefringence diagram of the calcium fluoride crystal before annealing according to Example 1 of the present invention; Figure 5 This is a stress birefringence diagram of the calcium fluoride crystal after annealing according to Example 1 of the present invention; Figure 6 This is a comparison diagram of the transmittance change of the calcium fluoride crystal before and after annealing according to Example 1 of the present invention; Figure 7This is a stress birefringence diagram of the calcium fluoride crystal before annealing in Comparative Example 1 of the present invention; Figure 8 This is a stress birefringence diagram of the calcium fluoride crystal after annealing in Comparative Example 1 of the present invention; Fig. 9 is a schematic diagram of scattering particles in the calcium fluoride crystal before annealing in Comparative Example 2 of the present invention; Fig.10 It is a schematic diagram of scattering particles in the calcium fluoride crystal after annealing in Comparative Example 2 of the present invention; Fig.11 This is a comparison chart of the transmittance changes of the calcium fluoride crystal before and after annealing in Comparative Example 2 of the present invention.

[0030] Description of reference numerals: 100: outer crucible, 110: outer crucible body, 120: outer crucible cover, 121: air vent; 200: inner crucible, 210: inner crucible body, 220: inner crucible cover, 230: through hole; 300: Fluoride crystals. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those skilled in the art without creative work belong to the scope of protection of the present invention. In addition, it should be understood that the specific implementation methods described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention. In the present invention, unless otherwise specified, the directional words used, such as "up", "down", "left", "right", "front", and "back", generally refer to the up, down, left, and right of the device in actual use or working state, specifically the drawing direction in the accompanying drawings.

[0032] It should be noted that the description order of the following embodiments is not intended to limit the preferred order of the embodiments of the present invention. In addition, in the following embodiments, the description of each embodiment has its own emphasis, and for parts not described in detail in one embodiment, reference can be made to the relevant description of other embodiments.

[0033] like Figure 1 and Figure 2As shown, a crucible for annealing fluoride crystals according to a specific embodiment of the present invention is shown, comprising: an outer crucible 100, an inner crucible 200 disposed inside the outer crucible 100, a plurality of through holes 230 being provided on the inner crucible 200, the inner crucible 200 being used to place a protective material for the fluoride crystals 300 and the fluoride crystals 300 to be annealed, the protective material being used to coat the fluoride crystals 300 to prevent the fluoride crystals 300 from directly contacting the inner wall of the inner crucible 200 and the air, and a gap layer 400 being provided between the inner crucible 200 and the outer crucible 100. The gap layer 400 is used to fill broken crystal materials to separate the inner crucible 200 from the outer crucible 100 and further prevent the fluoride crystals 300 from contacting the air. The annealing crucible can selectively allow the protective material to at least partially overflow toward the gap layer 400 along the through holes 230, thereby providing a buffer space for the fluoride crystals 300 to expand.

[0034] Continue to refer to Figure 1 and Figure 2 In a preferred embodiment, the outer crucible 100 includes an outer crucible body 110 and an outer crucible cover 120 covering the top opening of the outer crucible body 110. The inner crucible 200 is disposed inside the outer crucible body inner portion 110. The outer crucible 100 is configured as a split structure, so that the inner crucible 200 can be easily placed and taken out.

[0035] Preferably, a vent hole 121 for discharging air and impurities in the crucible is provided in the middle of the outer crucible cover 120 .

[0036] In a more preferred embodiment, the inner crucible 200 includes an inner crucible body 210 and an inner crucible cover 220 covering the top opening of the inner crucible body 210. The inner crucible 200 is configured in the form of a split structure, so as to facilitate the placement and removal of the fluoride crystals 300 and the filling of the protective material. Preferably, the inner crucible body 210 and the inner crucible cover 220 are both provided with a plurality of through holes 230. More preferably, the through holes 230 are uniformly arranged in the inner crucible 200, so as to facilitate the uniform overflow of the protective material into the gap layer 400.

[0037] In a preferred embodiment, the lower surface of the outer crucible cover 120 has an outer cover boss extending axially downward, and the outer cover boss is used to engage the top opening of the outer crucible body 110. The lower surface of the inner crucible cover 220 has an inner cover boss extending axially downward, and the inner cover boss is used to engage the top opening of the inner crucible body 210.

[0038] In a preferred embodiment, the particle size of the protective material is preferably smaller than the aperture of the through hole 230, so that the protective material can pass through the through hole 230. More preferably, the particle size of the protective material is preferably 0.1 mm to 1 mm, and the aperture of the through hole 230 is preferably 0.5 to 2 mm. This ensures that the protective material can smoothly overflow from the through hole 230 to the gap layer 400.

[0039] In a more preferred embodiment, the protective material is preferably a mixture of fluoride and deoxidizer, thereby providing a fluorine protective environment for the fluoride crystal 300 to prevent the fluoride crystal 300 from undergoing an oxidation reaction with oxygen or impurities during the annealing process to grow refractory oxides.

[0040] In a preferred embodiment, the broken crystal material is a plurality of fluoride crystal fragments, and when the fluoride crystal fragments are filled in the gap layer 400, there are gaps between the plurality of fluoride crystal fragments. More preferably, the fluoride crystal fragments are preferably formed by breaking up fluoride crystal waste materials to reduce costs and increase the reuse rate of waste materials.

[0041] In a preferred embodiment, the fluoride crystals 300 can expand after being heated. When the annealing temperature of the annealing crucible exceeds a set threshold, the fluoride crystals 300 in the inner crucible 200 continue to expand, thereby pushing the protective material covering the fluoride crystals 300 to overflow along the through holes 230 into the gaps between the multiple fluoride crystal fragments in the gap layer 400, thereby providing a buffer space for the expansion of the fluoride crystals 300.

[0042] As an embodiment of the present invention, the set threshold is preferably 900°C.

[0043] In a preferred embodiment, the particle size of the fluoride crystal fragments is larger than the aperture of the through hole 230, thereby preventing the fluoride crystal fragments from invading the through hole 230. At the same time, the larger the particle size, the larger the porosity between the fluoride crystal fragments, so that when the fluoride crystal 300 is annealed and expands, the protective material covering it can more easily overflow into the gaps between the fluoride crystal fragments.

[0044] In a preferred embodiment, the through hole 230 may also be a truncated cone-shaped through hole with an inner opening diameter larger than an outer opening diameter, thereby ensuring that the protective material inside the inner crucible 200 overflows to the outside and preventing the broken crystal material outside from invading the inside.

[0045] In a specific embodiment, when the annealing crucible described above is used to anneal the fluoride crystals 300, the fluoride crystals 300 are first placed in the inner crucible body 210; secondly, the fluoride crystals 300 are coated with a mixture of calcium fluoride and deoxidizer, and the gap layer 400 is filled with fluoride crystal fragments; finally, the inner crucible cover 220 and the outer crucible cover 120 are used to cover the top openings of the inner crucible body 210 and the outer crucible body 110, respectively, thereby completing the step of placing the fluoride crystals 300.

[0046] Preferably, the fluoride crystal 300 is placed in the middle of the inner crucible 200, and the protective material is filled in the gap between the fluoride crystal 300 and the inner crucible 200. Preferably, the protective material fills the gap to support the fluoride crystal 300 in the center of the inner crucible 200. Ensure that the driving force on the protective material around the fluoride crystal is uniform.

[0047] In a specific embodiment, the width of the gap 400 is preferably 10-20 mm to ensure that the fluoride crystal 300 is heated evenly.

[0048] like Figure 3 FIG. 1 is a flow chart of a method for annealing a fluoride crystal according to a specific embodiment of the present invention. The method for annealing a fluoride crystal is implemented using the annealing crucible in the above embodiment. The method specifically includes the following steps: Step S1, placing a fluoride crystal in the middle of the inner crucible; Step S2, coating the fluoride crystals with a mixture of fluoride powder and deoxidizer powder to avoid direct contact between the fluoride crystals and the inner crucible and air, and to prevent the fluoride crystals from undergoing a hydrolysis reaction to grow refractory oxides; Step S3, filling the broken crystal material into the gap layer between the inner crucible and the outer crucible; Step S4, placing the annealing crucible into a vacuum annealing furnace; Step S5, evacuating the vacuum annealing furnace; Step S6, filling the vacuum chamber of the vacuum annealing furnace with fluorine-containing atmosphere gas; Step S7, heating the fluoride crystal in a fluorine-containing atmosphere gas to an annealing temperature higher than a set threshold and lower than a melting point of the fluoride crystal; Step S8, maintaining the annealing temperature for a period of time, and then gradually cooling to room temperature at a constant cooling rate; Step S9, taking out the crystal and completing annealing.

[0049] In a specific embodiment, the deoxidizer in step S2 is preferably NH4F, NH4HF2, PbF2 or ZnF2. More preferably, it is PbF2. Specifically, since the fluoride crystals undergo oxidation reactions with the residual air in the furnace chamber and impurities such as foreign matter, water vapor or oxygen attached or adsorbed on the surface of the filling raw materials or crystals during the annealing process, the fluoride crystals are very likely to undergo hydrolysis reactions in a high-temperature environment to grow refractory oxides, so adding NH4F, NH4HF2, PbF2 or ZnF2 as deoxidizers can solve the above problems. For example, calcium fluoride crystals and water will undergo the following reaction CaF2+H2O→CaO+HF. At this time, adding PbF2 as a deoxidizer can produce the following reaction PbF2+CaO→PbO+CaF2, thereby removing refractory CaO. More preferably, the mass fraction of the deoxidizer in the calcium fluoride and deoxidizer mixture is preferably 2%. This ensures that the deoxidizer can completely eliminate the residual air in the furnace chamber and impurities such as foreign matter, water vapor or oxygen attached or adsorbed on the surface of the protective material or fluoride crystals, and can also ensure that the deoxidizer reacts completely, avoiding the waste of raw materials and environmental pollution caused by the failure of excess deoxidizer to react.

[0050] In a preferred embodiment, the crushed crystal material in step S3 is fluoride crystal fragments.

[0051] In a specific embodiment, the vacuum annealing furnace is preferably a high vacuum annealing furnace.

[0052] In a preferred embodiment, the steps S5 and S6 are cycled at least three times to better exhaust the air and impurities in the vacuum chamber.

[0053] In a specific embodiment, the step S5 further comprises: evacuating the vacuum annealing furnace to a vacuum degree of ≥5×10 -4 Pa.

[0054] Furthermore, the step S6 is preferably to fill the vacuum chamber of the vacuum annealing furnace with fluorine-containing atmosphere gas until the fluorine-containing atmosphere gas fills the entire vacuum chamber.

[0055] Furthermore, the fluorine-containing atmosphere gas in step S6 is preferably a mixed gas of high-purity argon and carbon tetrafluoride, wherein the volume fraction of carbon tetrafluoride in the mixed gas is preferably 4%.

[0056] Embodiment 1: In Example 1, the annealing crucible and annealing method of the present invention are used to anneal calcium fluoride crystals with a size of φ150 mm×50 mm, comprising the following steps: Step S10, placing a calcium fluoride crystal with a size of φ150 mm×50 mm in the middle of the inner crucible; Step S20, coating the calcium fluoride crystals with a mixture of calcium fluoride powder and PbF2 powder to prevent the calcium fluoride crystals from directly contacting the inner crucible and the air, and to prevent the calcium fluoride crystals from undergoing a hydrolysis reaction to grow refractory calcium oxide; Step S30, filling calcium fluoride crystal fragments into the gap layer between the inner crucible and the outer crucible; Step S40, placing the annealing crucible into a high vacuum annealing furnace; Step S50, evacuate the high vacuum annealing furnace to a vacuum degree of ≥5×10 -4 Pa; Step S60, filling a mixed gas of high-purity argon and carbon tetrafluoride into the vacuum chamber of the vacuum annealing furnace until the mixed gas fills the entire vacuum chamber; Step S70, looping steps S50 and S60 three times; Step S80, heating the calcium fluoride crystal to an annealing temperature of 1000° C. in a mixed gas of high-purity argon and carbon tetrafluoride; Step S90, maintaining the annealing temperature at 1000°C for 96 hours, and then gradually cooling to room temperature at a cooling rate of 10°C / h; Step S100, taking out the crystal and completing annealing, The particle size of the mixture of calcium fluoride powder and PbF2 powder in step S20 is 0.1 mm to 2 mm, and the content of PbF2 therein is 2%; the volume fraction of carbon tetrafluoride in the mixed gas of high-purity argon and carbon tetrafluoride in step S60 is 4%.

[0057] Reference Figure 4 , is the stress birefringence diagram of the calcium fluoride crystal before annealing in Example 1, where Figure 4 The table on the upper right shows that the minimum, maximum, and average values ​​of stress birefringence of calcium fluoride crystal before annealing are 0.06 nm / cm, 6.02 nm / cm, and 1.66 nm / cm, respectively. Figure 4 The table on the lower right shows the proportion of each stage of stress birefringence in the calcium fluoride crystal before annealing. The two ends of the calcium fluoride crystal after annealing in Example 1 were polished and tested for stress birefringence, and the obtained Figure 5 The stress birefringence diagram of the annealed calcium fluoride crystal is shown in FIG. Figure 5 The table on the upper right shows that the minimum, maximum, and average values ​​of the stress birefringence of the annealed calcium fluoride crystal are 0.013 nm / cm, 1.762 nm / cm, and 0.511 nm / cm, respectively. Figure 5 The table on the lower right shows the proportion of each stage of stress birefringence in the annealed calcium fluoride crystal. Figure 4 and Figure 5 , the stress birefringence of calcium fluoride crystal is significantly reduced after annealing.

[0058] At the same time, the above annealed calcium fluoride crystal was used to process a test sample with a size of φ25mm×10mm and its transmittance was tested, such as Figure 6 As shown, by comparing the transmittance of calcium fluoride crystal before and after annealing, the transmittance of calcium fluoride crystal remains almost unchanged.

[0059] Comparative Example 1: In Comparative Example 1, a single-layer graphite crucible and an annealing method in the prior art are used to anneal calcium fluoride crystals having the same size of φ150 mm×50 mm, including the following steps: Step S11, placing a calcium fluoride crystal with a size of φ150 mm×50 mm in the middle of a single-layer graphite crucible; Step S21, coating the calcium fluoride crystals with a mixture of calcium fluoride powder and PbF2 powder with a diameter of 0.1 mm to 2 mm to prevent the calcium fluoride crystals from directly contacting the inner wall of the single-layer graphite crucible and the air; Step S31, placing the single-layer graphite crucible into a high vacuum annealing furnace; Step S41, evacuate the high vacuum annealing furnace to a vacuum degree of ≥5×10 -4 Pa; Step S51, filling a mixed gas of high-purity argon and carbon tetrafluoride into the vacuum chamber of the vacuum annealing furnace until the mixed gas fills the entire vacuum chamber; Step S61, looping steps S50 and S60 three times; Step S71, heating the calcium fluoride crystal to an annealing temperature of 1000° C. in a mixed gas of high-purity argon and carbon tetrafluoride; Step S81, maintaining the annealing temperature at 1000°C for 96 hours, and then gradually cooling to room temperature at a cooling rate of 10°C / h; Step S91, taking out the crystal and completing annealing; Wherein, the content of PbF2 in the mixture of calcium fluoride powder and PbF2 powder in step S21 is 2%.

[0060] Reference Figure 7 , is the stress birefringence diagram of the calcium fluoride crystal before annealing in Comparative Example 1, where Figure 7 The table on the upper right shows that the minimum, maximum, and average values ​​of stress birefringence of calcium fluoride crystal before annealing are 0.03 nm / cm, 5.43 nm / cm, and 1.59 nm / cm, respectively. Figure 7 The table on the lower right shows the proportion of each stage of stress birefringence in the calcium fluoride crystal before annealing. The two ends of the calcium fluoride crystal after annealing in Comparative Example 1 were polished and tested for stress birefringence, and the obtained Figure 8The stress birefringence diagram of the annealed calcium fluoride crystal is shown in FIG. Figure 8 The table on the upper right shows that the minimum, maximum and average values ​​of stress birefringence of annealed calcium fluoride crystal are 0.07 nm / cm, 3.08 nm / cm and 1.38 nm / cm respectively. Figure 8 The table on the lower right shows the proportion of each stage of stress birefringence in the annealed calcium fluoride crystal. Figure 7 and Figure 8 The stress birefringence of the crystal after annealing is reduced but the reduction is small. Compared with the existing single-layer crucible, the "double crucible" structure of the outer crucible and the inner crucible with multiple through holes of the present invention can effectively reduce the stress birefringence of the crystal after annealing.

[0061] Comparative Example 2: In Comparative Example 2, the annealing crucible of the present invention is used to anneal calcium fluoride crystals with the same size of φ150 mm×50 mm, comprising the following steps: Step S12, placing a calcium fluoride crystal with a size of φ150 mm×50 mm in the middle of the inner crucible; Step S22, coating the calcium fluoride crystals with calcium fluoride powder having a diameter of 0.1 mm to 2 mm to prevent the calcium fluoride crystals from directly contacting the inner wall of the single-layer graphite crucible and the air; Step S32, filling calcium fluoride crystal fragments into the gap layer between the inner crucible and the outer crucible; Step S33, placing the annealing crucible into a high vacuum annealing furnace; Step S50, evacuate the high vacuum annealing furnace to a vacuum degree of ≥5×10 -4 Pa Step S42, heating the calcium fluoride crystal to an annealing temperature of 1000° C.; Step S52, maintaining the annealing temperature at 1000°C for 96 hours, and then gradually cooling to room temperature at a cooling rate of 10°C / h; Step S62, taking out the crystal and completing annealing.

[0062] The difference between the annealing method of comparative example 2 and the annealing method of embodiment 1 is that there is no PbF2 in the protective material coating the calcium fluoride crystals; and the vacuum chamber of the vacuum annealing furnace is not filled with a mixed gas of high-purity argon and carbon tetrafluoride as a fluorine protective environment.

[0063] Further, refer to Fig. 9 No scattering particles were found in the calcium fluoride crystal before annealing. Fig.10 A large number of scattered particles were found in the calcium fluoride crystals after annealing in Comparative Example 2.

[0064] Furthermore, the annealed calcium fluoride crystal was used to process a test sample with a size of φ25mm×10mm, and its transmittance was tested. Fig.11 As shown, the transmittance of calcium fluoride crystals is significantly reduced after annealing. Fig. 9 and 10 According to the annealing method of the present invention, PbF2 is added to the protective material as a deoxidizer and a mixed gas of inert gas and carbon tetrafluoride is filled into the annealing furnace chamber as a fluorine protective environment. That is, the use of a double fluorination environment can effectively reduce the appearance of scattering particles in the crystal and ensure the transmittance of the calcium fluoride crystal.

[0065] By comparing Example 1, Comparative Example 1 and Comparative Example 2, the double-layer annealing crucible and annealing method of the present invention can better reduce the internal stress of the calcium fluoride crystal compared with the previous annealing crucible and annealing method; by adding fluorine to protect the environment, the generation of scattering particles in the crystal can be effectively reduced; while ensuring that the transmittance of the calcium fluoride crystal remains almost unchanged, the internal stress of the fluoride crystal is reduced and the crystal is prevented from generating new stress during secondary annealing.

[0066] Compared with the prior art, the present invention has the following beneficial effects: 1. The "double crucible" structure, which is composed of an outer crucible and an inner crucible with multiple through holes, can effectively reduce the axial and radial temperature gradients of the crystal during the annealing process, produce a more uniform annealing environment, and slowly release the internal stress of the crystal; 2. The multiple through holes provided in the inner crucible can enable the atmosphere inside the entire annealing crucible to circulate, promoting the interior of the crucible to be filled with fluorine protective atmosphere; 3. The multiple through holes of the inner crucible provide a buffer space for the expansion of the fluoride crystals during the annealing process, effectively solving the pressure generated during the crystal expansion process and effectively releasing the stress in the crystal. That is, since the protective material covering the fluoride crystals in the inner crucible is a mixture of fluoride and deoxidizer with smaller particle size, and the gap layer between the inner crucible and the outer crucible is filled with broken crystals with larger particle size, when the crystal expands during the annealing process, the protective material with smaller particle size can overflow into the gap between the broken crystals through the through holes, thereby effectively solving the pressure generated during the crystal expansion process and effectively releasing the stress in the crystal; 4. Fluoride crystals are coated with a mixture of fluoride and deoxidizer to provide a first fluorine protection environment; a mixture of inert gas and carbon tetrafluoride is filled into the annealing furnace chamber to provide a second fluorine protection environment. Annealing fluoride crystals in a double fluorine protection environment can avoid or reduce the entry of trace oxygen into the crystal and the generation of defects such as fluorine vacancies, and can effectively eliminate moisture adsorbed in the raw materials or residual in the annealing furnace. At the same time, the raw materials can be further fluorinated to remove oxide impurities that may exist in the raw materials, thereby ensuring the transmittance of the fluoride crystals; 5. The annealing crucible and annealing method of the present invention can reduce the internal stress of the fluoride crystal while ensuring the transmittance of the fluoride crystal and avoid the crystal from generating new stress during secondary annealing.

[0067] The scheme of the present invention is introduced in detail above. Specific examples are used in this article to illustrate the principle and implementation mode of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

[0068] References throughout this specification to "one embodiment," "an embodiment," or "a specific embodiment" mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, and not necessarily in all embodiments. Thus, various appearances of the phrases "in one embodiment," "in an embodiment," or "in a specific embodiment" in different places throughout this specification do not necessarily refer to the same embodiment. In addition, the particular features, structures, or characteristics of any specific embodiment of the invention may be combined with one or more other embodiments in any suitable manner. It should be understood that other variations and modifications of the application embodiments described and illustrated herein may be possible in accordance with the teachings herein and are to be considered part of the spirit and scope of the invention.

[0069] It should also be understood that one or more of the elements shown in the figures may also be implemented in a more separate or more integrated manner, or even removed because they are inoperable in certain circumstances or provided because they may be useful depending on the application.

[0070] In addition, unless otherwise explicitly indicated, any marking arrows in the drawings should be regarded as exemplary only and not limiting. In addition, unless otherwise indicated, the term "or" used herein is generally intended to mean "and / or". In the case where the term is not clear because it is anticipated that the ability to separate or combine is provided, the combination of components or steps will also be regarded as indicated.

Claims

1. An annealing crucible for fluoride crystals, characterized in that: include: outer crucible; An inner crucible disposed inside the outer crucible, the inner crucible being provided with a plurality of through holes, the inner crucible being used to place the fluoride crystals and the protective material, the protective material covering the fluoride crystals to prevent the fluoride crystals from directly contacting the inner crucible and the air; A gap layer is provided between the inner crucible and the outer crucible for filling broken crystals, wherein The annealing crucible can selectively allow the protective material to at least partially overflow toward the gap layer along the through hole.

2. The annealing crucible according to claim 1, characterized in that: The outer crucible comprises an outer crucible body and an outer crucible cover covering a top opening of the outer crucible body, and the inner crucible is arranged inside the outer crucible body.

3. The annealing crucible for fluoride crystals according to claim 2, characterized in that: A vent hole is arranged in the middle of the outer crucible cover.

4. The annealing crucible according to any one of claims 1 to 3, characterized in that: The inner crucible comprises an inner crucible body and an inner crucible cover covering a top opening of the inner crucible body; The inner crucible body and the inner crucible cover are both provided with a plurality of through holes.

5. The annealing crucible according to any one of claims 1 to 4, characterized in that: The fluoride crystals expand when heated. When the fluoride crystals in the annealing crucible are at an annealing temperature exceeding a set threshold, the fluoride crystals push the protective material to overflow along the through hole toward the gap layer.

6. The annealing crucible according to any one of claims 1 to 5, characterized in that: The particle size of the protective material is smaller than the aperture of the through hole, and the protective material is a mixture of fluoride and deoxidizer.

7. The annealing crucible according to any one of claims 1 to 6, characterized in that: The broken crystal material is a plurality of fluoride crystal fragments. When the broken crystal material is filled in the gap layer, there are gaps between the plurality of fluoride crystal fragments.

8. A method for annealing a fluoride crystal in an annealing crucible according to any one of claims 1 to 7, characterized in that: The following steps are involved: Step S1, placing the fluoride crystal in the inner crucible; Step S2, coating the fluoride crystal with the protective material; Step S3, filling the broken crystal material into the gap layer between the inner crucible and the outer crucible; Step S4, placing the annealing crucible into a vacuum annealing furnace; Step S5, evacuating the vacuum annealing furnace; Step S6, filling the vacuum chamber of the vacuum annealing furnace with fluorine-containing atmosphere gas; Step S7, heating the fluoride crystal to an annealing temperature higher than a set threshold and lower than its melting point; Step S8, after maintaining the annealing temperature for a period of time, gradually lowering the annealing temperature to room temperature.

9. The annealing method of fluoride crystal according to claim 8, characterized in that: The protective material is a mixture of fluoride and deoxidizer, and the deoxidizer is NH4F, NH4HF2, PbF2 or ZnF2.

10. The annealing method of fluoride crystal according to claim 8 or 9, characterized in that: The fluorine-containing atmosphere gas is a mixed atmosphere of argon and carbon tetrafluoride, and the volume fraction of carbon tetrafluoride in the mixed atmosphere is 1% to 10%.

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