JP2005330122A - Apparatus and method for heat treating fluoride crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus and a method for heat treating a fluoride crystal, which are effective for heat treatment for removing strain of a large diameter, heavy fluoride crystal and provided for obtaining a low strain, high accuracy large diameter fluoride crystal. <P>SOLUTION: The apparatus has such a constitution that a member for alleviating stress in the crystal is arranged on the bottom of the crystal between a crucible for accommodating the fluoride crystal and the fluoride crystal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention belongs to the technical field of a heat treatment apparatus for fluoride crystals, particularly a single crystal heat treatment method for calcium fluoride, barium fluoride, lithium fluoride, magnesium fluoride and the like used as an optical component.

  Excimer lasers are attracting attention as the only high-power lasers that oscillate in the ultraviolet region, and are expected to be applied in the electronics, chemical, and energy industries. Specifically, it is used for processing, chemical reaction, etc. of metals, resins, glass, ceramics, semiconductors and the like.

An apparatus that generates excimer laser light is known as an excimer laser oscillation apparatus. A laser gas such as Ar, Kr, Xe, F ₂ , or Cl ₂ filled in the chamber is excited by electron beam irradiation or discharge. Excited atoms combine with ground state atoms to produce molecules that exist only in the excited state. This molecule is called an excimer. Since excimer is unstable, it immediately emits ultraviolet light and falls to the ground state. This is called a bond-free transition, and an excimer laser oscillation device is a device that multiplies ultraviolet light obtained by this transition in an optical resonator composed of a pair of mirrors and extracts it as laser light.

Among excimer laser light, KrF laser, ArF laser, and F ₂ laser are light with a wavelength of 248 nm, or light in a wavelength region called vacuum ultraviolet region such as 193 nm and 157 nm, respectively. The one with high transmittance must be used. Fluorides such as calcium fluoride, magnesium fluoride, barium fluoride, and lithium fluoride are known as glass materials that can be used in such an optical system from the transmission wavelength range.

Hereinafter, a conventional fluoride crystal manufacturing method and heat treatment (annealing) method will be described by taking calcium fluoride called fluorite (CaF _{2 in} stoichiometric ratio) as an example.

  As a conventional method for producing calcium fluoride, for example, there are methods described in Patent Document 2 and Patent Document 3. To put it simply, when a high-purity powder raw material made by chemical synthesis is melted as it is, the loss of the high-purity raw material into the crystal growth furnace becomes severe due to the bulk specific gravity. This is called the “crucible descent method” in which a crucible containing raw materials is lowered in a furnace to produce crystals. Hereinafter, the knowledge obtained by the present inventor until reaching the present invention will be described.

  FIG. 2 is a conceptual diagram showing a method for producing calcium fluoride.

  First, in step S1, a powder raw material is prepared. In step S3, the solidified lump is pulverized with a stainless pulverizer. Thereafter, in step S4, the crushed lump is put in a crucible for crystal growth and melted, and then slowly cooled to grow crystals to produce a calcium fluoride block.

  Here, step S2 is called a refining step, and is a step performed to reduce the change in the bulk density before and after melting before the growth step of step S4. It is also a process of removing. Both processes require highly accurate temperature control, and the same furnace in which only the crucible is changed may be used as the furnace for melting and slow cooling.

In steps S2 and S4, a scavenger, which is a metal fluoride, is added in order to remove CaO produced by reaction of the raw material (CaF ₂ ) with moisture or the like and impurities originally present in the raw material. For example, a PbF ₂ scavenger reacts with CaO to form CaF _2, and itself is removed as PbO or the like during crystal melting. In addition to PbF ₂ , ZnF ₂ or PTFE (tetrafluoroethylene resin) is used as a scavenger. As a result, CaO as an impurity is removed, and calcium fluoride having excellent transmittance characteristics is obtained. When it is desired to make the crystal orientation of the crystal to be manufactured specific, a small crystal called a seed crystal is put in the crucible lower part in step S4, and a part of the crystal is melted and then grown from there to obtain a desired crystal.

  The calcium fluoride block thus obtained is cut to a desired thickness after heat treatment (annealing). Further, when it is necessary to remove strain with high accuracy, it is heat-treated (annealed) once again and processed into a desired lens shape or the like and used as an optical article.

When calcium fluoride is oxidized, the transmittance is deteriorated. Therefore, the heat treatment is performed in a vacuum atmosphere, an inert gas atmosphere, or a fluorine-based gas atmosphere using an airtight container such as a heat treatment furnace disclosed in Patent Document 1, for example. Done. The main structural material of the heat treatment equipment is carbon, which is usually low in reactivity at a calcium fluoride heat treatment temperature of 1300 ° C or less, and a container (hereinafter referred to as a crucible) containing calcium fluoride or a support rod is made of carbon. Consists of materials. In this conventional example, since the heater is installed outside the airtight container, the material is not limited. However, when the heater is installed inside the airtight container, the carbon material is used up to the heater and the heat insulating material. In such a heat treatment furnace, calcium fluoride is heated up to a heat treatment temperature not higher than the melting point together with the crucible, and then gradually cooled to obtain calcium fluoride having a small strain.
JP 2000-281492 A JP-A-4-349199 JP-A-4-349198

  However, when a large-diameter (200 mm or more diameter) fluoride crystal is heat-treated using a conventional heat-treatment apparatus or heat-treatment method, stress due to its own weight is concentrated on the contact part with the crucible and is also applied during the heat-treatment. Thus, there is a problem that the residual stress increases when the temperature is returned to room temperature.

  In addition, since the large-diameter fluoride crystal and the crucible are in contact with each other on a large surface, thermal stress is generated due to the temperature difference between the contact portion and the non-contact portion due to one piece, and the strain is partially increased. It has occurred.

  The present invention has been made in view of the above problems, and is particularly effective for strain removal heat treatment of a large-diameter and large-weight fluoride crystal, and a heat treatment for obtaining a low-distortion and high-precision large-diameter fluoride crystal. An object is to provide an apparatus and a heat treatment method.

  For this reason, in the present invention, first, a vacuum vessel including a crucible for storing a fluoride crystal therein is provided, and a fluoride crystal can be heated and cooled together with the crucible by a heater installed inside or outside the vacuum vessel. In a heat treatment apparatus for a fluoride crystal, a heat treatment apparatus including a member for relaxing stress in the crystal between the crucible for storing the fluoride crystal and the fluoride crystal is provided.

  Secondly, the member that relieves the stress in the crystal is a substance aggregate having a smaller contact area than the fluoride crystal.

  Third, the member that relieves the stress in the crystal is a fiber that does not react with the fluoride crystal at the heat treatment temperature.

  Fourth, the main fiber component is carbon, alumina, silicon carbide, silicon nitride, or boron nitride.

  Fifth, the fiber thickness is 2 mm or more and 20 mm or less.

  Sixth, the wire thickness constituting the fiber is 0.05 mm or less.

  Seventh, the member that relieves stress in the crystal is a powder that does not react with the fluoride crystal at the heat treatment temperature.

  Eighth, the powder main component is a refractory metal made of tungsten, tantalum, or molybdenum.

  Ninth, the powder main component is carbon, alumina, silicon carbide, silicon nitride, or boron nitride.

  Tenth, the average particle size of the powder is 0.5 mm or less.

  Eleventh, the crystal surface roughness of the heat treatment is 0.1 mm (100 μm) or less.

  Twelfth, the crystal surface flatness to be heat-treated is 0.05 mm (50 μm) or less.

  Thirteenth, the crystal thickness when heat treatment is performed is a thickness that approximates the product glass material thickness.

  According to the fluoride crystal heat treatment method performed using the heat treatment apparatus, it is possible to efficiently obtain a fluoride crystal with low distortion.

  In strain-removing heat treatment of fluoride crystals with large diameters (diameter 200 mm or more), the stress due to its own weight is prevented from being concentrated in the contact area with the crucible, and the increase in residual stress when returning to room temperature is suppressed. can do.

  In addition, since the large-diameter fluoride crystal and the crucible are in contact with each other on a large surface, a temperature difference between the contact part and the non-contact part is likely to occur, and this causes thermal stress, which is partially Although the strain often increases, a fluoride crystal suitable for an optical component having a uniform and small strain can be obtained by using the heat treatment apparatus and method of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described. FIG. 1 is a cross-sectional view of a calcium fluoride heat treatment apparatus using a member that relieves stress in a crystal that best represents the characteristics of the present invention. In this apparatus, a bell jar 1 for blocking the internal atmosphere from the external atmosphere is mounted on a gantry 2, in which a cylindrical heat insulating material 3 with a lid on the top, and three cylindrical heaters 4, The crucible 5 in which the calcium fluoride crystal to be heat-treated is installed is installed concentrically. Although not shown, this apparatus is provided with a sensor for measuring the temperature of the crucible 5 and a control device for controlling the energization amount to the heater 4 so that the crucible temperature becomes a predetermined temperature. The crucible 5 is not placed directly on the gantry 2 but is supported by the support rod 6 so as not to be affected by the external thermal environment as much as possible. Since the support bar 6 is sealed by the sealing mechanism 10, it can move up and down even when the external atmosphere is shut off. This mechanism can be used not only for setting / removing crystals but also for adjusting the relative positional relationship between the crucible 5 and the heater 4.

In this figure, the crucible 5 is depicted as an assembly having four internal spaces, but each can be disassembled when the crystals are installed, and the crystals that are normally subjected to heat treatment are installed only in the upper three stages. This is because the temperature distribution becomes large in the lowermost stage due to conduction heat transfer by the support rod 6. If a solid scavenger such as PbF ₂ or ZnF ₂ is used instead, the scavenger box 7 may be placed here. The upper by may be set to the empty when using a gas scavenger such as _{F 2 · NF 3 · CF 4} · CHF 3, depending on the heater balance under the upper, middle and filling a heat insulating material 3 It becomes possible to make the temperature inside the crucible inside the stage more uniform.

  A member 9 that relieves stress in the crystal is inserted between the crucible 5 and the calcium fluoride crystal 8 installed in the upper three stages.

  A vacuum pump 11 and a gas supply cylinder 12 for supplying an inert gas such as helium, neon, and argon are connected to the inside of the heat treatment apparatus isolated from the external atmosphere by the bell jar 1 and the gantry 2. .

Hereinafter, a heat treatment process in the case of using ZnF ₂ as a scavenger and argon as an inert gas will be described. After the calcium fluoride crystal is placed in the heat treatment apparatus as described above, the inside of the bell jar 1 is decompressed to about 10 ⁻³ to 10 ⁻⁵ Pa by the vacuum pump 11. By energizing the heater 4, the calcium fluoride crystal 8 is heated to 200 to 400 ° C. together with the crucible 5. This heat deaeration can remove moisture adhering to the crystal surface, the crucible, the heat insulating material surface and the like.

Thereafter, high-purity argon gas is introduced from the gas supply cylinder 12 so that the pressure is close to 1 atm. Argon gas is less deteriorated due to crystal oxidation and scavenger solid diffusion in a heat treatment process over a longer period in a minute flow state than in a sealed state. When the energization amount to the heater 4 is further increased and the temperature is raised, ZnF ₂ installed in the scavenger box 7 melts and gradually evaporates before reaching the maximum heat treatment temperature of 1000 to 1300 ° C. At this time, ZnF ₂ fluorinates the remaining H ₂ O with HF gas and CaO with CaF _2, and becomes ZnO itself, which acts as an antioxidant scavenger for the calcium fluoride crystal 8.

  After reaching the maximum heat treatment temperature, the temperature is maintained for 20 to 100 hours to sufficiently remove strain. Here, if there is stress concentration due to its own weight in the calcium fluoride crystal subject to strain removal heat treatment, dislocations within the crystal are arranged so as to relieve the stress at high temperatures, so the residual stress when returning to room temperature is the part where the stress is concentrated It will be bigger. Generally, the true contact area in contact between solids is very small, and only partial protrusions are in contact with each other. It can be improved if the flatness and roughness of the calcium fluoride crystal surface and the crucible surface to be installed are finished with high accuracy, but the degree of improvement is limited in general machine accuracy such as grinding and polishing. A fine flaw is generated in the crucible, and above all, it is difficult to accurately process the calcium fluoride crystal before the heat treatment. This is a big problem with a calcium fluoride crystal having a large diameter and a large weight. However, in the heat treatment apparatus of the present invention, a member 9 for relaxing stress in the crystal is placed between the crucible 5 and the calcium fluoride crystal 8. Since it is inserted, its own weight is supported by the entire bottom surface of the calcium fluoride crystal 8, so that residual stress based on the stress concentration as described above does not occur.

  Thereafter, the energization amount to the heater 4 is gradually decreased to cool to room temperature. Since strain is newly introduced even during this cooling, sufficient care must be taken. Although depending on the target residual stress amount, if the cooling temperature gradient is the same, generally the strain introduction amount is large at high temperatures and the strain introduction amount is small at low temperatures. Therefore, it is desirable from the viewpoint of crystal quality and production efficiency to decrease the slow cooling rate at high temperatures and increase the slow cooling rate at low temperatures. Here, slow cooling was performed at 0.5 ° C./hour from the maximum temperature to 900 ° C., 1 ° C./hour up to 600 ° C., and 2 ° C./hour below room temperature.

  Two calcium fluoride crystals having a diameter of φ220 and a thickness of 50 mm cut out from an ingot grown by the crucible descending method were put in a crucible for heat treatment and simultaneously heat treated by the above process. The maximum temperature is 1200 ° C. The residual stress in the crystal can be known by transmitting the laser and measuring the birefringence distribution. FIG. 3 shows the in-plane distribution of the birefringence of the crystal measured at a 2.5 mm pitch using a laser with a wavelength of 633 nm. The unit is nm / cm. The distribution shown in FIG. 3A is obtained by placing the crystal on a member that relieves stress in the crystal. The average + 2 × standard deviation birefringence value is 0.952 nm / cm, whereas the distribution shown in FIG. The crystal was placed directly on the crucible, and the average + 2 × standard deviation birefringence value was 2.79 nm / cm. Both show a larger birefringence value in the peripheral part than in the central part, but it is greatly improved in FIG. 3-a compared to FIG. 3-b. Since the present heat treatment apparatus has a structure in which the crucible 5 is supported by the support rod 6, the internal temperature of the apparatus tends to be slightly lower than the upper part. For this reason, since the stress of contraction is exerted on the lower surface of the calcium fluoride crystal 8 and the stress of expansion is exerted on the upper surface, the crystal has a slightly convex shape. As a result, the crystal supports its own weight only at the periphery, so that a large residual stress remains in the peripheral portion and exhibits a large birefringence.

  The member used to relieve the stress in the crystal used at this time was a carbon fiber having a diameter of 10 to 20 μm and a length of 1 to 10 mm. is there. The calcium fluoride crystal is finished to a flatness of 20 μm and a roughness of 30 (Ra) μm by surface rough grinding before heat treatment. This is because if the flatness is poor, the weight cannot be sufficiently dispersed, and if the roughness is not good, the contact area with the member that relieves stress in the crystal is reduced. By collecting fine fibers in this way and making them elastic and cotton-like, the weight of the large-diameter calcium fluoride can be supported on the entire bottom surface of the crystal and stress concentration can be avoided, reducing the stress in the crystal of the present invention. Compared with the case where the member to be used is not used, a glass material having a very good birefringence, that is, a small residual stress, can be obtained.

  Apart from this, according to the experiments of the present inventors, if the diameter of the fiber used as a member for relieving the stress in the crystal is too thick, the effect is not exhibited, and it is sufficient that the diameter is 0.05 mm (50 μm) or less. There is a stress dispersion effect. Further, if the thickness of the member for relieving the stress in the crystal is too thin, there is no effect because there is no elasticity, and if it is too thick, the effect as a heat insulating material becomes dominant and the temperature distribution in the crystal is deteriorated. Specifically, it is desirable that the isotherm in the crystal is almost horizontal, but if the effect as a heat insulating material becomes dominant, the isotherm runs vertically in combination with the fact that the heat treatment apparatus is side heater heating. In such a case, effective distortion removal cannot be performed. According to the experimental study by the present inventors, it is desirable that the thickness of the member that relieves stress in the crystal is 2 mm or more and 20 mm or less.

  If carbon fiber is collected and made into cotton, if the workability of crystal replacement is poor, make a bag with carbon fiber cloth made of the same carbon, and put the carbon fiber in it to the desired thickness It is better to make a member that relieves stress in the futon-like crystals. If this is used, not only the same effects as in the first embodiment can be obtained, but also workability such as fiber weighing and uniform laying can be greatly improved. Alumina, silicon carbide, silicon nitride, boron nitride, or the like can also be used as the fiber material of the member that relieves stress in the crystal. However, when it is brought into direct contact, it may adhere to calcium fluoride at a high temperature around 1300 ° C. In such a case, if it is made a member that relieves stress in the futon-like crystal with a carbon fiber cloth, it does not come into direct contact with the calcium fluoride crystal, so even if the fiber material is scattered by high-temperature heat treatment It does not contaminate calcium fluoride itself.

  Tungsten powder was used as a member 9 that relieves stress under the calcium fluoride crystal 8. The powder used was obtained by removing large particles with a 0.5 mm mesh sieve in advance. The calcium fluoride crystal 8 is heated and cooled through the crucible 5. Because the temperature range is high, the heat and cooling of the crucible itself is dominated by radiant heat transfer, but the calcium fluoride crystal transmits almost all the infrared light that is dominant in radiant heat transport. The heat transfer from the crucible is dominant in its own heat transfer. Therefore, the member 9 that relieves stress in the crystal is required to be made of a material having good thermal conductivity and stable at the heat treatment temperature, but tungsten used in this embodiment is preferable. The thermal conductivity is as large as 105 W / mK (1500 ° C value), and it is stable in the heat treatment temperature range with a melting point of 3407 ° C. A calcium fluoride crystal having a diameter of 220 mm and a thickness of 50 mm was placed on a tungsten powder having a thickness of 10 mm and then heat-treated, and the birefringence value of average + 2 × standard deviation was 0.656 nm / cm.

  Examples of powders that meet the above conditions include refractory metals such as tantalum (74 W / mK: 1500 ° C., melting point 2980 ° C.) and molybdenum (88 W / mK: 1500 ° C., melting point 2620 ° C.). However, since it is an expensive material, it is better to use it according to the productivity and the target value for distortion removal. As an inexpensive material, although performance is slightly inferior in terms of thermal conductivity, powders of carbon, alumina, silicon carbide, silicon nitride, boron nitride, etc. can be used. In any case, if large particles are mixed, the effect of stress dispersion is impaired. Therefore, it is necessary to remove the large particles with a sieve of 0.5 mm mesh or less and to keep the average particle size to 0.5 mm or less.

  A carbon fiber felt was used as a member 9 that relieves stress under the calcium fluoride crystal 8. Felts that are entangled by making use of their properties of shrinking wool are called felts, but carbon fibers can also be entangled mechanically with a needle to form a cloth. This is called needle punch felt and is widely used in industrial filters. Since it is already molded, it is inferior in elasticity to the cotton-like stress dispersion material shown in Example 1, but the effect as a stress dispersion material can be expected by appropriately selecting the material, thickness, etc. Can do. First, the wire thickness constituting the fiber is 0.05 mm or less. More desirably, it is 0.02 mm or less. Second, the thickness of the felt is 2 mm or more and 20 mm or less. More desirably, it is set to 5 mm or more and 20 mm or less. Thirdly, one having a bulk density of 0.05 g / cm 3 or more is selected. According to the study by the present inventors, the effect as a stress dispersion material can be obtained even with carbon fiber felt by selecting a material that meets the above three conditions. A calcium fluoride crystal having a diameter of 220 mm and a thickness of 50 mm was placed on a 15 mm-thick carbon fiber felt and heat-treated, and the average + 2 × standard deviation birefringence value was 1.05 nm / cm.

  For comparison, a cylindrical calcium fluoride crystal is manufactured using a multistage type crystal growth crucible disclosed in JP-A-10-279396, and the deposited material immediately after the crystal growth is easily removed with a gold brush or the like. The heat treatment was performed without using a member that relieved the stress in the crystal and relieved the stress. After the heat treatment, it was processed to a diameter of 225 mm and a thickness of 50 mm by grinding. The crystals were measured at a 2.5 mm pitch using a laser with a wavelength of 633 nm. The in-plane distribution of the birefringence is shown in FIG. The unit was nm / cm, and the birefringence value of average + 2 × standard deviation was 1.82 nm / cm. Multi-stage crystal growth crucibles have connecting holes, and crystal plane orientations are connected there. Immediately after crystal growth, the deposited material is attached to portions other than the center. Therefore, the calcium fluoride crystal before heat treatment in this example has a shape in which the central portion is convex. If placed directly in the crucible in this state, the weight of the crystal is supported only at the central portion, causing a large stress concentration. The birefringence distribution shown in FIG. 4 shows exactly this effect and shows that there is a large residual stress in the central portion. This shows the importance of managing the surface accuracy of the crystal bottom. According to the experimental study, if the crystal surface roughness is 0.1 mm or less and the crystal surface flatness is 0.05 mm or less, a very uniform birefringence component can be obtained using the stress relieving member of the present invention. That is, a calcium fluoride crystal having a small strain can be obtained.

  The maximum temperature for heat treatment is 1000 to 1300 ° C., but it is difficult to accurately measure material constants such as the longitudinal elastic modulus up to this temperature range. The present inventors have measured the linear expansion coefficient of calcium fluoride crystals up to 1300 ° C, the longitudinal stiffness tensor up to 800 ° C, the thermal diffusivity up to 1000 ° C in increments of 100 ° C, and extrapolated. A rough calculation is possible. For example, in the vicinity of 900 ° C., according to the thermal stress calculation result, the maximum thermal stress reaches about 125 kPa at the above-mentioned descent rate of 0.5 ° C./hour in the size of φ200 × 50 mm. On the other hand, since the density at 900 ° C. is 2.92 g / cm 3, the average pressure applied to the bottom surface is 1.43 kPa. At first glance, the stress due to its own weight seems to be much smaller than the thermal stress, but the actual real contact area is so small that it may be partially increased.

  If the crystal thickness is h, the thermal diffusivity is α, and the temperature drop rate is v, the thermal stress is

に比例し、密度をρとすると底面の圧力はρｈに比例する。比例定数をｋとすると各温度域で

When the density is ρ, the bottom pressure is proportional to ρh. If the proportionality constant is k,

を満たすような温度降下速度を降伏応力との兼ね合いから決めることが最も効率的である。逆に言うと温度降下時に導入される歪の目標値がある場合には、結晶厚さを薄くすることで短時間に熱処理を行うことが可能である。したがって熱処理する結晶厚さは製品硝材厚さに近似する厚さで熱処理を行うと効率が良い。一方でスカベンジャーによる表面侵食もあるので本発明者らの検討によれば製品硝材厚さに５〜20mm程度さらに望ましくは8〜12mm加えた厚さで熱処理を行い、その後表面を除去することが歪を除去するために最適であることを見出した。

It is most efficient to determine the temperature drop rate that satisfies the above condition from the balance with the yield stress. In other words, if there is a target value of strain to be introduced when the temperature drops, the heat treatment can be performed in a short time by reducing the crystal thickness. Therefore, it is more efficient if the heat treatment is performed with a crystal thickness to be heat-treated that is close to the product glass material thickness. On the other hand, since there is surface erosion due to scavengers, according to the study by the present inventors, it is distorted to heat-treat the product glass material with a thickness of about 5 to 20 mm, more preferably 8 to 12 mm, and then remove the surface. Found to be optimal for removing.

  As described above, as described in the six comparative examples, using a member that relieves stress in the crystal composed of an appropriate material, shape, and thickness, heat treatment of the calcium fluoride crystal enables high accuracy. A large-diameter glass material with little residual stress suitable for an optical material can be obtained. Also, it is necessary to define the flatness, roughness, and thickness on the large-diameter calcium fluoride crystal side to be heat-treated.

  In addition, although the said Example demonstrated calcium fluoride, it cannot be overemphasized that the same thing can be said also about single crystal heat processings, such as barium fluoride, lithium fluoride, and magnesium fluoride which are the same fluoride crystals.

Cross-sectional view of a heat treatment apparatus for calcium fluoride using a member that relieves stress in the crystal of the present invention Conceptual diagram showing a method for producing calcium fluoride Diagram showing calcium fluoride Diagram showing calcium fluoride

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Bell jar 2 Base 3 Heat insulating material 4 Heater 5 Crucible 6 Support rod 7 Scavenger box 8 Calcium fluoride crystal 9 Member which relieves stress in crystal
10 Sealing mechanism
11 Vacuum pump
12 Gas supply cylinder

Claims (13)

  In a fluoride crystal heat treatment apparatus comprising a vacuum vessel provided with a crucible containing fluoride crystals therein, and capable of heating and cooling the fluoride crystals together with the crucible by a heater installed inside or outside the vacuum vessel. A heat treatment apparatus characterized in that a member for relieving stress in a crystal is installed on the crystal bottom between a crucible containing the crystal and a fluoride crystal.   The true contact area is increased by supporting the material aggregate having a smaller contact area than the fluoride crystal and deforming the aggregate in accordance with the shape of the bottom surface of the fluoride crystal. Heat treatment equipment.   3. The heat treatment apparatus according to claim 1, wherein the member that relieves stress in the crystal is a fiber that does not react with the fluoride crystal at a heat treatment temperature.   The heat treatment apparatus according to claim 3, wherein the fiber main component is carbon, alumina, silicon carbide, silicon nitride, or boron nitride.   The heat treatment apparatus according to claim 3, wherein the fiber has a thickness of 2 mm or more and 20 mm or less.   The heat treatment apparatus according to claim 3, wherein a thickness of the strand constituting the fiber is 0.05 mm or less.   3. The heat treatment apparatus according to claim 1, wherein the member that relieves stress in the crystal is a powder that does not react with a fluoride crystal at a heat treatment temperature.   The heat treatment apparatus according to claim 7, wherein the powder main component is a refractory metal made of tungsten, tantalum, or molybdenum.   The heat treatment apparatus according to claim 7, wherein the powder main component is carbon, alumina, silicon carbide, silicon nitride, or boron nitride.   The heat treatment apparatus according to claim 7, wherein the powder has an average particle size of 0.5 mm or less.   11. The method for heat treatment of fluoride crystals performed using the heat treatment apparatus according to claim 1, wherein the crystal surface roughness to be heat-treated is 0.1 mm or less.   The method for heat-treating a fluoride crystal using the heat-treating apparatus according to claim 1, wherein the crystal surface flatness to be heat-treated is 0.05 mm or less.   The method for heat-treating a fluoride crystal using the heat-treating apparatus according to claim 1, wherein the heat-treating crystal has a thickness approximate to the product glass material thickness.

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