Annealing crucible and annealing method for fluoride crystals

Abstract

This invention belongs to the field of fluoride crystal annealing technology, specifically relating to an annealing crucible and annealing method for fluoride crystals. The annealing crucible includes: an outer crucible; and an inner crucible disposed inside the outer crucible. The inner crucible has multiple through holes and is used to place the fluoride crystal and a protective material. The protective material covers the fluoride crystal to prevent direct contact between the fluoride crystal and the inner crucible and air. A gap layer is provided between the inner and outer crucibles to fill with fragmented crystal material. The annealing crucible can selectively allow at least a portion of the protective material to overflow into the gap layer along the through holes. The "double-layer crucible" structure of this invention, combining the outer crucible and the through-hole inner crucible, provides a buffer space for the expansion of the fluoride crystal during annealing, effectively reducing the axial and radial temperature gradients of the crystal, generating a more uniform annealing environment, and allowing the internal stress of the crystal to be released slowly.

Description

Technical Field

[0001] This invention belongs to the field of fluoride crystal annealing technology, specifically relating to an annealing crucible and annealing method for fluoride crystals. Background Technology

[0002] The optical projection imaging system is a core component of a lithography machine, and its performance determines the imaging performance of the machine. It is primarily designed and manufactured from a large number of ultraviolet-grade optical materials. Calcium fluoride (CaF2) crystal, due to its high transmittance in the deep ultraviolet band, constant average and local refractive indices, and stable physicochemical properties, is one of the indispensable core optical materials in the optical projection imaging system. The lithography imaging system places high demands on the performance indicators of optical materials, such as an internal transmittance of ≥99.7%@193nm and a stress birefringence ≤3nm / cm for CaF2 crystal.

[0003] Currently, the growth of large-size calcium fluoride crystals employs the crucible lowering method. This method uses a fully or semi-enclosed crucible, gradually cooling as the crucible slowly descends to achieve crystal growth. However, the unavoidable mechanical vibrations of this method cause temperature fluctuations at the solid-liquid interface and a large radial temperature gradient during crystal growth, leading to internal stress within the crystal. Therefore, to reduce internal stress, annealing in an annealing furnace is generally required after growth to further reduce crystal stress. The most common crystal annealing method is to directly place the ingot or a roughly machined sample of a certain size and shape into the crucible. A small number of large-size crystals are also annealed using crushed crystal material as filler, but this method cannot guarantee that the radial temperature gradient of the crystal is controlled within a small range, making it difficult to avoid the generation of new stress during secondary annealing. Simultaneously, during annealing, the calcium fluoride crystal undergoes oxidation reactions with residual air in the furnace cavity, as well as with foreign matter, water vapor, or oxygen adhering to or adsorbed on the filler material or crystal surface, affecting deep ultraviolet transmittance. Summary of the Invention

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

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] An annealing crucible for fluoride crystals, comprising:

[0007] Outer crucible;

[0008] An inner crucible is located inside the outer crucible. The inner crucible has multiple through holes. The inner crucible is used to place the fluoride crystal and the protective material. The protective material covers the fluoride crystal to prevent the fluoride crystal from directly contacting the inner crucible.

[0009] A gap layer is provided between the inner crucible and the outer crucible to fill the crushed crystal material.

[0010] The annealing crucible can selectively allow at least a portion of the protective material to overflow into the interstitial layer along the through-hole.

[0011] In one 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 disposed inside the outer crucible body.

[0012] In one embodiment of the present invention, the outer crucible lid is provided with a vent hole in the middle for discharging air and impurities from the crucible.

[0013] As one embodiment of the present invention, the inner crucible includes an inner crucible body and an inner crucible lid covering the top opening of the inner crucible body;

[0014] Both the inner crucible body and the inner crucible cover are provided with multiple through holes.

[0015] In one embodiment of the present invention, the fluoride crystal expands when heated. When the fluoride crystal in the annealing crucible is at an annealing temperature exceeding a set threshold, the fluoride crystal pushes the protective material to overflow from the through hole into the interstitial layer.

[0016] In one embodiment of the present invention, the protective material is a mixture of fluoride and oxygen scavenger, and the particle size of the protective material is smaller than the pore size of the through holes.

[0017] In one embodiment of the present invention, the particle size of the protective material is preferably 0.1 mm to 1 mm, and the pore size of the through hole is preferably 0.5 mm to 2 mm. This ensures that the protective material can overflow into the gap layer along the through hole.

[0018] In one embodiment of the present invention, the fluoride crystal fragments are multiple fluoride crystal fragments, and when the fluoride crystal fragments fill the interstitial layer, there are gaps between the multiple fluoride crystal fragments.

[0019] In one embodiment of the present invention, the fluoride crystal expands when heated. When the fluoride crystal in the annealing crucible is in a state where the annealing temperature exceeds a set threshold, the fluoride crystal pushes the protective material to overflow along the through hole into the gap between the fluoride crystal fragments.

[0020] In one embodiment of the present invention, the set threshold is preferably 900°C.

[0021] In one embodiment of the present invention, the fluoride crystal fragments are preferably made by crushing fluoride crystal waste to reduce costs and increase the recycling rate of waste.

[0022] In one 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 interior of the furnace. At the same time, the larger particle size results in a larger porosity between the fluoride crystal fragments, so that when the fluoride crystals expand during annealing, the mixture in the inner crucible can more easily overflow and fill the gaps.

[0023] In one embodiment of the present invention, the annealing crucible is preferably a graphite crucible, a molybdenum crucible, a quartz crucible, or an alumina crucible.

[0024] To achieve the above objectives, the present invention also adopts the following technical solution:

[0025] A method for annealing fluoride crystals using the above-mentioned annealing crucible includes the following steps:

[0026] Step S1: Place the fluoride crystal in the inner crucible;

[0027] Step S2, the fluoride crystals are coated with the protective material;

[0028] Step S3: Fill the gap layer between the inner crucible and the outer crucible with the crushed crystal material;

[0029] Step S4: Place the annealing crucible into a vacuum annealing furnace;

[0030] Step S5: Evacuate the vacuum annealing furnace;

[0031] Step S6: Fill the vacuum chamber of the vacuum annealing furnace with a fluorine-containing atmosphere gas;

[0032] Step S7: Heat the fluoride crystal to an annealing temperature that is higher than a set threshold but lower than its melting point;

[0033] Step S8: After maintaining the annealing temperature for a period of time, gradually reduce the annealing temperature to room temperature to complete the annealing.

[0034] In one embodiment of the present invention, step S5 is preferably performed by evacuating the vacuum annealing furnace to a base vacuum level ≥ 5 × 10⁻⁶. -4 Pa.

[0035] In one embodiment of the present invention, step S6 is preferably to fill the vacuum chamber of the vacuum annealing furnace with a fluorine-containing atmosphere gas until the vacuum chamber is filled with the fluorine-containing atmosphere gas.

[0036] In one embodiment of the present invention, steps S5 and S6 are repeated at least 3 times to better remove air and impurities from the vacuum chamber and purify the vacuum chamber.

[0037] In one embodiment of the present invention, the annealing temperature is preferably 900-1100°C.

[0038] In one embodiment of the present invention, the oxygen scavenger is preferably NH4F, NH4HF2, PbF2, or ZnF2. PbF2 is more preferred. Specifically, during the annealing process, fluoride crystals undergo oxidation reactions with residual air in the furnace cavity, as well as with foreign matter, water vapor, or oxygen adhering to or adsorbing on the surface of the filling material or crystals. In high-temperature environments, fluoride crystals are prone to hydrolysis, leading to the growth of refractory oxides. Therefore, adding NH4F, NH4HF2, PbF2, or ZnF2 as an oxygen scavenger can solve the above problems. For example, calcium fluoride crystals react with water in the following reaction: CaF2 + H2O → CaO + HF. Adding PbF2 as an oxygen scavenger can then cause the following reaction: PbF2 + CaO → PbO + CaF2, thereby removing the refractory CaO.

[0039] In one 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 residual air in the furnace cavity and impurities such as foreign matter, water vapor, or oxygen adhering to or adsorbing on the surface of the protective material or fluoride crystals, while also ensuring that the deoxidizer reacts completely and avoiding the waste of raw materials due to unreacted deoxidizer, which would also cause environmental pollution.

[0040] In one embodiment of the present invention, the fluorine-containing atmosphere is preferably a mixture 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 it can react with impurities in the crystal, thereby further removing impurities and preventing the fluoride crystal from contacting impurities such as water and oxygen.

[0041] In one embodiment of the present invention, the volume fraction of the carbon tetrafluoride in the mixed atmosphere is preferably 1% to 10%. This ensures that the carbon tetrafluoride reacts fully while avoiding waste and environmental pollution.

[0042] Compared with the prior art, the present invention has the following beneficial technical effects:

[0043] 1. The “double crucible” structure, which combines an outer crucible and an inner crucible with multiple through holes, can effectively reduce the axial and radial temperature gradients of the crystal during annealing, generate a more uniform annealing environment, and allow the internal stress of the crystal to be released slowly.

[0044] 2. The multiple through holes in the inner crucible allow the atmosphere inside the entire annealing crucible to circulate, promoting the filling of the crucible with a fluorine protective atmosphere;

[0045] 3. Multiple through-holes in the inner crucible provide buffer space for the expansion of fluoride crystals during annealing, effectively managing the pressure generated during crystal expansion and releasing stress within the crystal. Specifically, the protective material coating the fluoride crystals in the inner crucible is a mixture of small-particle fluoride and oxygen scavenger, while the interstitial layer between the inner and outer crucibles is filled with larger-particle fragments. Therefore, when the crystal expands during annealing, the smaller-particle protective material can overflow through the through-holes into the gaps between the fragments, effectively managing the pressure generated during crystal expansion and releasing stress within the crystal.

[0046] 4. A first layer of fluoride protection is provided by coating fluoride crystals with a mixture of fluoride and oxygen scavenger; a second layer of fluoride protection is provided by filling the annealing furnace with a mixture of inert gas and carbon tetrafluoride. Annealing the fluoride crystals in this dual fluoride protection environment can avoid or reduce the entry of trace oxygen into the crystal and the generation of defects such as fluorine vacancies. It can effectively eliminate adsorbed moisture in the raw materials or residual moisture in the annealing furnace, and can further fluorinate the raw materials to remove any oxide impurities that may be present in the raw materials, thereby ensuring the transmittance of the fluoride crystals.

[0047] 5. The annealing crucible and annealing method of the present invention can reduce the internal stress of fluoride crystals while ensuring the transmittance of fluoride crystals and avoid generating new stress in the crystals during secondary annealing. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying 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 effort.

[0049] Figure 1 This is a schematic diagram of the structure of an annealing crucible for fluoride crystals according to a specific embodiment of the present invention;

[0050] Figure 2 This is a cross-sectional view of an annealing crucible for fluoride crystals according to a specific embodiment of the present invention.

[0051] Figure 3This is a flowchart of an annealing method for fluoride crystals according to a specific embodiment of the present invention;

[0052] Figure 4 This is a stress birefringence diagram of calcium fluoride crystal before annealing in Embodiment 1 of the present invention;

[0053] Figure 5 This is a stress birefringence diagram of calcium fluoride crystal after annealing in Embodiment 1 of the present invention;

[0054] Figure 6 This is a comparison diagram of the transmittance changes of calcium fluoride crystals before and after annealing in Example 1 of the present invention.

[0055] Figure 7 This is a stress birefringence diagram of calcium fluoride crystal before annealing in Comparative Example 1 of this invention.

[0056] Figure 8 This is a stress birefringence diagram of calcium fluoride crystal after annealing in Comparative Example 1 of this invention.

[0057] Figure 9 This is a schematic diagram of the scattering particles in the calcium fluoride crystal before annealing in Comparative Example 2 of the present invention.

[0058] Figure 10 This is a schematic diagram of the scattering particles in the annealed calcium fluoride crystal of Comparative Example 2 of the present invention.

[0059] Figure 11 This is a comparison diagram of the transmittance changes of calcium fluoride crystals before and after annealing in Comparative Example 2 of this invention.

[0060] Explanation of reference numerals in the attached figures:

[0061] 100: Outer crucible; 110: Outer crucible body; 120: Outer crucible lid; 121: Vent hole;

[0062] 200: Inner crucible; 210: Inner crucible body; 220: Inner crucible lid; 230: Through hole;

[0063] 300: Fluoride crystals. Detailed Implementation

[0064] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Furthermore, it should be understood that the specific embodiments described herein are only for illustration and explanation of the present invention and are not intended to limit the present invention. In the present invention, unless otherwise stated, directional terms such as "up," "down," "left," "right," "front," and "back" generally refer to up, down, left, and right in the actual use or working state of the device, specifically the drawing directions in the accompanying drawings.

[0065] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of embodiments of the present invention. Furthermore, the descriptions of each embodiment in the following embodiments have their own emphasis; for parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0066] like Figure 1 and Figure 2 The diagram illustrates an annealing crucible for fluoride crystals according to a specific embodiment of the present invention. The crucible includes an outer crucible 100 and an inner crucible 200 disposed inside the outer crucible 100. The inner crucible 200 has multiple through holes 230. The inner crucible 200 is used to hold a protective material for the fluoride crystal 300 and the fluoride crystal 300 to be annealed. The protective material covers the fluoride crystal 300 to prevent direct contact between the fluoride crystal 300 and the inner wall of the inner crucible 200 and air. A gap layer 400 is provided between the inner crucible 200 and the outer crucible 100. The gap layer 400 is filled with fragmented crystal material to separate the inner crucible 200 from the outer crucible 100 and further prevent the fluoride crystal 300 from contacting air. The annealing crucible can selectively allow at least a portion of the protective material to overflow along the through holes 230 into the gap layer 400, thereby providing a buffer space for the expansion of the fluoride crystal 300.

[0067] 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 110. The outer crucible 100 is configured as a split structure, thereby facilitating the placement and removal of the inner crucible 200.

[0068] Preferably, the outer crucible cover 120 is provided with a vent hole 121 in the middle for venting air and impurities from the crucible.

[0069] 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 as a split structure, facilitating the loading and unloading of the fluoride crystals 300 and the filling of the protective material. Preferably, both the inner crucible body 210 and the inner crucible cover 220 are provided with multiple through holes 230. More preferably, the through holes 230 are uniformly distributed in the inner crucible 200, thereby facilitating the uniform overflow of the protective material into the interstitial layer 400.

[0070] In a preferred embodiment, the lower surface of the outer crucible cover 120 has an axially downward extending outer cover boss for engaging the top opening of the outer crucible body 110. The lower surface of the inner crucible cover 220 has an axially downward extending inner cover boss for engaging the top opening of the inner crucible body 210.

[0071] In a preferred embodiment, the particle size of the protective material is preferably smaller than the pore size of the through hole 230, so as to facilitate the passage of the protective material through the through hole 230. More preferably, the particle size of the protective material is preferably 0.1 mm to 1 mm, and the pore size of the through hole 230 is preferably 0.5 mm to 2 mm. This ensures that the protective material can smoothly overflow into the gap layer 400 along the through hole 230.

[0072] In a more preferred embodiment, the protective material is preferably a mixture of fluoride and oxygen scavenger, thereby providing a fluorine-protected environment for the fluoride crystals 300 to prevent the fluoride crystals 300 from undergoing an oxidation reaction with oxygen or impurities during the annealing process and growing refractory oxides.

[0073] In a preferred embodiment, the fluoride crystal fragments are multiple fluoride crystal fragments, and when the fluoride crystal fragments fill the interstitial layer 400, there are gaps between the multiple fluoride crystal fragments. More preferably, the fluoride crystal fragments are preferably made by crushing fluoride crystal waste to reduce costs and improve the recycling rate of waste.

[0074] In a preferred embodiment, the fluoride crystal 300 can expand when heated. When the annealing crucible is in a state where the annealing temperature exceeds a set threshold, the fluoride crystal 300 in the inner crucible 200 continues to expand, thereby pushing the protective material covering the fluoride crystal 300 to overflow along the through hole 230 into the gap between the plurality of fluoride crystal fragments in the gap layer 400, thereby providing a buffer space for the expansion of the fluoride crystal 300.

[0075] In one embodiment of the present invention, the set threshold is preferably 900°C.

[0076] In a preferred embodiment, the particle size of the fluoride crystal fragments is larger than the pore size of the through hole 230, thereby preventing the fluoride crystal fragments from entering the through hole 230. At the same time, the larger particle size results in a larger porosity between the fluoride crystal fragments, so that when the fluoride crystal 300 expands during annealing, the protective material covering it can more easily overflow into the gaps between the fluoride crystal fragments.

[0077] In a preferred embodiment, the through hole 230 may also be a frustum-shaped through hole with an inner opening diameter larger than the outer opening diameter, thereby ensuring that the protective material inside the inner crucible 200 overflows to the outside and prevents the crushed material on the outside from intruding to the inside.

[0078] In one specific embodiment, when annealing the fluoride crystal 300 using the above-mentioned annealing crucible, the fluoride crystal 300 is first placed in the inner crucible body 210; secondly, the fluoride crystal 300 is coated with a mixture of calcium fluoride and an oxygen scavenger, while the gap layer 400 is filled with fluoride crystal fragments; finally, the top openings of the inner crucible body 210 and the outer crucible body 110 are covered by the inner crucible cover 220 and the outer crucible cover 120, respectively, thereby completing the placement step of the fluoride crystal 300.

[0079] Preferably, the fluoride crystal 300 is placed in the exact center of the inner crucible 200, and the protective material fills 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. This ensures that the protective material around the fluoride crystal receives a uniform driving force.

[0080] In one specific embodiment, the width of the gap 400 is preferably 10-20 mm to ensure that the fluoride crystal 300 is heated uniformly.

[0081] like Figure 3 The diagram shows a flowchart of an annealing method for fluoride crystals according to a specific embodiment of the present invention. This annealing method utilizes the annealing crucible described in the above embodiment. The method specifically includes the following steps:

[0082] Step S1: Place the fluoride crystal in the middle position inside the inner crucible;

[0083] Step S2 involves coating the fluoride crystals with a mixture of fluoride powder and oxygen scavenger powder to prevent the fluoride crystals from directly contacting the inner crucible and air, while also preventing the fluoride crystals from undergoing a hydrolysis reaction and growing refractory oxides.

[0084] Step S3: Fill the gap layer between the inner and outer crucibles with the crushed crystal material;

[0085] Step S4: Place the annealing crucible into the vacuum annealing furnace;

[0086] Step S5: Evacuate the vacuum annealing furnace;

[0087] Step S6: Fill the vacuum chamber of the vacuum annealing furnace with a fluorine-containing atmosphere gas;

[0088] Step S7: Heat the fluoride crystal in a fluorine-containing atmosphere to an annealing temperature that is higher than a set threshold but lower than the melting point of the fluoride crystal;

[0089] Step S8: After maintaining the annealing temperature for a period of time, gradually cool to room temperature at a constant cooling rate;

[0090] Step S9: Remove the crystal and complete the annealing process.

[0091] In one specific embodiment, the oxygen scavenger in step S2 is preferably NH4F, NH4HF2, PbF2, or ZnF2. PbF2 is more preferred. Specifically, during the annealing process, fluoride crystals react with residual air in the furnace cavity, as well as foreign matter, water vapor, or oxygen adhering to or adsorbing on the surface of the filling material or crystals, causing oxidation. In high-temperature environments, fluoride crystals are prone to hydrolysis, growing refractory oxides. Therefore, adding NH4F, NH4HF2, PbF2, or ZnF2 as an oxygen scavenger can solve the above problems. For example, calcium fluoride crystals react with water as follows: CaF2 + H2O → CaO + HF. Adding PbF2 as an oxygen scavenger can cause the following reaction: PbF2 + CaO → PbO + CaF2, thereby removing the refractory CaO. More preferably, the mass fraction of the oxygen scavenger in the calcium fluoride and oxygen scavenger mixture is preferably 2%. This ensures that the deoxidizer can completely eliminate residual air in the furnace cavity and impurities such as foreign matter, water vapor, or oxygen adhering to or adsorbing on the surface of protective materials or fluoride crystals. It also ensures that the deoxidizer reacts completely and avoids the waste of raw materials due to unreacted deoxidizer, as well as environmental pollution.

[0092] In a preferred embodiment, the crystalline material in step S3 is a fluoride crystal fragment.

[0093] In one specific embodiment, the vacuum annealing furnace is preferably a high-vacuum annealing furnace.

[0094] In a preferred embodiment, steps S5 and S6 are repeated at least three times to better remove air and impurities from the vacuum chamber.

[0095] In one specific embodiment, step S5 further includes: evacuating the vacuum annealing furnace to a vacuum level ≥ 5 × 10⁻⁶. -4 Pa.

[0096] Further, step S6 is preferably to fill the vacuum chamber of the vacuum annealing furnace with a fluorine-containing atmosphere gas until the entire vacuum chamber is filled with the fluorine-containing atmosphere gas.

[0097] Furthermore, the fluorine-containing atmosphere gas in step S6 is preferably a mixture of high-purity argon and carbon tetrafluoride. The volume fraction of carbon tetrafluoride in the mixture is preferably 4%.

[0098] Example 1:

[0099] In Example 1, a calcium fluoride crystal with dimensions of φ150mm×50mm is annealed using the annealing crucible and annealing method of the present invention, including the following steps:

[0100] Step S10: Place a calcium fluoride crystal with dimensions of φ150mm×50mm in the middle position inside the inner crucible.

[0101] Step S20: Calcium fluoride crystals are coated with a mixture of calcium fluoride powder and PbF2 powder to avoid direct contact between the calcium fluoride crystals and the inner crucible and air, and to prevent the calcium fluoride crystals from undergoing hydrolysis to grow refractory calcium oxide.

[0102] Step S30: Fill the gap layer between the inner and outer crucibles with calcium fluoride crystal fragments;

[0103] Step S40: Place the annealing crucible into a high-vacuum annealing furnace;

[0104] Step S50: Evacuate the high-vacuum annealing furnace until its vacuum degree is ≥5×10⁻⁶. -4 Pa;

[0105] Step S60: A mixture of high-purity argon and carbon tetrafluoride is introduced into the vacuum chamber of the vacuum annealing furnace until the mixture fills the entire vacuum chamber.

[0106] Step S70, repeat steps S50 and S60 three times;

[0107] Step S80: The calcium fluoride crystals are heated to an annealing temperature of 1000°C under a mixed gas of high-purity argon and carbon tetrafluoride.

[0108] Step S90: After maintaining the annealing temperature at 1000℃ for 96 hours, gradually cool to room temperature at a cooling rate of 10℃ / h.

[0109] Step S100: Remove the crystal and complete the annealing process.

[0110] In step S20, the particle size of the mixture of calcium fluoride powder and PbF2 powder is 0.1 mm to 2 mm, and the content of PbF2 is 2%; in step S60, the volume fraction of carbon tetrafluoride in the mixture of high-purity argon and carbon tetrafluoride is 4%.

[0111] Reference Figure 4 The figure shows the stress birefringence diagram of the calcium fluoride crystal before annealing in Example 1. Figure 4 The table in the upper right corner shows the minimum, maximum, and average stress birefringence of calcium fluoride crystals before annealing: 0.06 nm / cm, 6.02 nm / cm, and 1.66 nm / cm, respectively. Figure 4 The table at the bottom right shows the percentage of each stage of stress birefringence in the calcium fluoride crystal before annealing. The ends of the annealed calcium fluoride crystal from Example 1 were polished and stress birefringence was tested, yielding... Figure 5 The stress birefringence diagram of the annealed calcium fluoride crystal is shown, in which... Figure 5 The table in the upper right corner shows the minimum, maximum, and average stress birefringence of the annealed calcium fluoride crystals as 0.013 nm / cm, 1.762 nm / cm, and 0.511 nm / cm, respectively. Figure 5 The table in the lower right corner shows the percentage of each stage of stress birefringence in the annealed calcium fluoride crystal. (Comparison) Figure 4 and Figure 5 After annealing, the stress birefringence of calcium fluoride crystals is significantly reduced.

[0112] Simultaneously, test samples with dimensions of φ25mm×10mm were processed using the annealed calcium fluoride crystals described above, and their transmittance was tested. Figure 6 As shown, comparing the transmittance of calcium fluoride crystals before and after annealing, the transmittance of calcium fluoride crystals remains almost unchanged.

[0113] Comparative Example 1:

[0114] In Comparative Example 1, a calcium fluoride crystal with the same dimensions of φ150mm×50mm was annealed using a single-layer graphite crucible and annealing method from the prior art, including the following steps:

[0115] Step S11: Place a calcium fluoride crystal with dimensions of φ150mm×50mm in the middle of a single-layer graphite crucible;

[0116] Step S21: Calcium fluoride crystals are coated with a mixture of calcium fluoride powder and PbF2 powder with a diameter of 0.1 mm to 2 mm to avoid direct contact between the calcium fluoride crystals and the inner wall of the single-layer graphite crucible and the air.

[0117] Step S31: Place the single-layer graphite crucible into a high-vacuum annealing furnace;

[0118] Step S41: Evacuate the high-vacuum annealing furnace until its vacuum degree is ≥5×10⁻⁶. -4 Pa;

[0119] Step S51: A mixture of high-purity argon and carbon tetrafluoride is introduced into the vacuum chamber of the vacuum annealing furnace until the mixture fills the entire vacuum chamber.

[0120] Step S61, repeat steps S50 and S60 three times;

[0121] Step S71: Heat the calcium fluoride crystals to an annealing temperature of 1000°C under a mixture of high-purity argon and carbon tetrafluoride.

[0122] Step S81: After maintaining the annealing temperature at 1000℃ for 96 hours, gradually cool to room temperature at a cooling rate of 10℃ / h.

[0123] Step S91: Remove the crystal and complete the annealing process;

[0124] In step S21, the content of PbF2 in the mixture of calcium fluoride powder and PbF2 powder is 2%.

[0125] Reference Figure 7 The figure shows the stress birefringence diagram of the calcium fluoride crystal before annealing in Comparative Example 1. Figure 7 The table in the upper right corner shows the minimum, maximum, and average stress birefringence of calcium fluoride crystals before annealing: 0.03 nm / cm, 5.43 nm / cm, and 1.59 nm / cm, respectively. Figure 7 The table in the lower right corner shows the percentage of each stage of stress birefringence in the calcium fluoride crystal before annealing. The ends of the annealed calcium fluoride crystal in Comparative Example 1 were polished and stress birefringence was tested, yielding... Figure 8 The stress birefringence diagram of the annealed calcium fluoride crystal is shown, in which... Figure 8 The table in the upper right corner shows the minimum, maximum, and average stress birefringence of the annealed calcium fluoride crystals as 0.07 nm / cm, 3.08 nm / cm, and 1.38 nm / cm, respectively. Figure 8 The table in the lower right corner shows the percentage of each stage of stress birefringence in the annealed calcium fluoride crystal. (Comparison) Figure 7 and Figure 8 The stress birefringence of the crystal is reduced after annealing, but the reduction is small. Compared with the existing single-layer crucible, the "double crucible" structure of the present invention, which combines the outer crucible and the inner crucible with multiple through holes, can effectively reduce the stress birefringence of the crystal after annealing.

[0126] Comparative Example 2:

[0127] In Comparative Example 2, calcium fluoride crystals with the same dimensions of φ150mm×50mm were annealed using the annealing crucible of the present invention, including the following steps:

[0128] Step S12: Place a calcium fluoride crystal with dimensions of φ150mm×50mm in the middle position inside the inner crucible.

[0129] Step S22: Calcium fluoride crystals are coated with calcium fluoride powder with a diameter of 0.1 mm to 2 mm to avoid direct contact between the calcium fluoride crystals and the inner wall of the single-layer graphite crucible and the air.

[0130] Step S32: Fill the gap layer between the inner and outer crucibles with calcium fluoride crystal fragments;

[0131] Step S33: Place the annealing crucible into a high-vacuum annealing furnace;

[0132] Step S50: Evacuate the high-vacuum annealing furnace until its vacuum degree is ≥5×10⁻⁶. -4 Pa

[0133] Step S42: Heat the calcium fluoride crystals to an annealing temperature of 1000°C;

[0134] Step S52: After maintaining the annealing temperature at 1000℃ for 96 hours, gradually cool to room temperature at a cooling rate of 10℃ / h.

[0135] Step S62: Remove the crystal and complete the annealing process.

[0136] The difference between the annealing method in Comparative Example 2 and the annealing method in Example 1 is that the protective material coating the calcium fluoride crystals does not contain PbF2; and the vacuum chamber of the vacuum annealing furnace is not filled with a mixture of high-purity argon and carbon tetrafluoride as a fluorine protective environment.

[0137] Furthermore, referring to Figure 9 No scattering particles were found in the calcium fluoride crystals before annealing, according to reference... Figure 10 A large number of scattering particles were found in the annealed calcium fluoride crystals in Comparative Example 2.

[0138] Furthermore, test samples with dimensions of φ25mm × 10mm were processed using annealed calcium fluoride crystals, and their transmittance was tested. Figure 11 As shown, the transmittance of calcium fluoride crystals decreased significantly after annealing. Meanwhile, compared with... Figure 9 and 10 The annealing method of the present invention adds PbF2 as an oxygen scavenger to the protective material and fills the annealing furnace cavity with a mixture of inert gas and carbon tetrafluoride as a fluorine protective environment. That is, the use of the double fluorination environment can effectively reduce the occurrence of scattering particles in the crystal and ensure the transmittance of calcium fluoride crystal.

[0139] Compared with Comparative 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 calcium fluoride crystals compared with the conventional annealing crucibles and annealing methods; 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 calcium fluoride crystal remains almost unchanged, the internal stress of fluoride crystals is reduced and new stresses are avoided in the crystal during secondary annealing.

[0140] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0141] 1. The “double crucible” structure, which combines an outer crucible and an inner crucible with multiple through holes, can effectively reduce the axial and radial temperature gradients of the crystal during annealing, generate a more uniform annealing environment, and allow the internal stress of the crystal to be released slowly.

[0142] 2. The multiple through holes in the inner crucible allow the atmosphere inside the entire annealing crucible to circulate, promoting the filling of the crucible with a fluorine protective atmosphere;

[0143] 3. Multiple through-holes in the inner crucible provide buffer space for the expansion of fluoride crystals during annealing, effectively managing the pressure generated during crystal expansion and releasing stress within the crystal. Specifically, the protective material coating the fluoride crystals in the inner crucible is a mixture of small-particle fluoride and oxygen scavenger, while the interstitial layer between the inner and outer crucibles is filled with larger-particle fragments. Therefore, when the crystal expands during annealing, the smaller-particle protective material can overflow through the through-holes into the gaps between the fragments, effectively managing the pressure generated during crystal expansion and releasing stress within the crystal.

[0144] 4. A first layer of fluoride protection is provided by coating fluoride crystals with a mixture of fluoride and oxygen scavenger; a second layer of fluoride protection is provided by filling the annealing furnace with a mixture of inert gas and carbon tetrafluoride. Annealing the fluoride crystals in this dual fluoride protection environment can avoid or reduce the entry of trace oxygen into the crystal and the generation of defects such as fluorine vacancies. It can effectively eliminate adsorbed moisture in the raw materials or residual moisture in the annealing furnace, and can further fluorinate the raw materials to remove any oxide impurities that may be present in the raw materials, thereby ensuring the transmittance of the fluoride crystals.

[0145] 5. The annealing crucible and annealing method of the present invention can reduce the internal stress of fluoride crystals while ensuring the transmittance of fluoride crystals and avoid generating new stress in the crystals during secondary annealing.

[0146] The above provides a detailed description of the present invention. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

[0147] Throughout this specification, the terms "an embodiment," "embodiment," or "specific embodiment" refer to a particular feature, structure, or characteristic described in connection with an embodiment that is included in at least one embodiment of the invention, but not necessarily in all embodiments. Therefore, the various representations of the phrases "in one embodiment," "in an embodiment," or "in a specific embodiment" in different places throughout the specification do not necessarily refer to the same embodiment. Furthermore, a particular feature, structure, or characteristic of any specific embodiment of the invention can be combined with one or more other embodiments in any suitable manner. It should be understood that other variations and modifications of the embodiments described and illustrated herein may be based on the teachings herein and will be considered part of the spirit and scope of the invention.

[0148] It should also be understood that one or more of the elements shown in the figures may be implemented in a more separate or more integrated manner, or may even be removed because they are inoperable in certain circumstances or provided because they may be useful for a particular application.

[0149] Furthermore, unless otherwise expressly stated, any arrows in the accompanying drawings should be considered illustrative only and not limiting. Additionally, unless otherwise stated, the term "or" as used herein is generally intended to mean "and / or". Where a term is anticipated to provide a separation or combination capability that is unclear, a combination of components or steps will also be considered as indicated.

Claims

1. An annealing crucible for fluoride crystals, characterized by, The annealing crucible comprises: an outer crucible; an inner crucible arranged inside the outer crucible, the inner crucible being provided with a plurality of through holes, the inner crucible being used for placing the fluoride crystal and fluoride and oxygen scavenger mixture, the fluoride and oxygen scavenger mixture being used for coating the fluoride crystal to prevent the fluoride crystal from directly contacting the inner crucible and air; a gap layer arranged between the inner crucible and the outer crucible and used for filling the crystal fragments, wherein the annealing crucible can selectively cause the fluoride and oxygen scavenger mixture to at least partially overflow along the through holes to the gap layer.

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 of claim 2, wherein A middle portion of the outer crucible cover is provided with a gas permeable hole.

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. Both the inner crucible body and the inner crucible cover are provided with a plurality of through holes.

5. The annealing crucible according to any one of claims 1 to 3, characterized in that The fluoride crystal expands when heated, and when the fluoride crystal in the annealing crucible is at an annealing temperature exceeding a set threshold, the fluoride crystal pushes the fluoride and oxygen scavenger mixture to overflow along the through holes to the gap layer.

6. The annealing crucible according to any one of claims 1 to 3, characterized in that The particle size of the fluoride and oxygen scavenger mixture is smaller than the pore size of the through holes.

7. The annealing crucible according to any one of claims 1 to 3, characterized in that The crystal fragments are a plurality of fluoride crystal fragments, and when filled in the gap layer, the plurality of fluoride crystal fragments have gaps therebetween.

8. A method of annealing a fluoride crystal using the annealing crucible according to any one of claims 1 to 7, characterized by, The method comprises the following steps: Step S1, placing the fluoride crystal in the inner crucible; Step S2, coating the fluoride crystal with the fluoride and oxygen scavenger mixture; Step S3, filling the crystal fragments 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, vacuumizing the vacuum annealing furnace; Step S6, filling a fluorine-containing atmosphere gas into a vacuum cavity of the vacuum annealing furnace; Step S7, heating the fluoride crystal to an annealing temperature higher than a set threshold and lower than its melting point; Step S8, maintaining the annealing temperature for a period of time, and then gradually reducing the annealing temperature to room temperature.

9. The method of annealing a fluoride crystal of claim 8, wherein, The oxygen scavenger is NH4F, NH4HF2, PbF2 or ZnF2.

10. The annealing method of claim 8 or 9, wherein the annealing is performed at a temperature of 500 to 700°C. 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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