JP6035584B2 - Method for producing fluorite crystals - Google Patents

Description

The present invention relates to a method for producing a fluorite crystal (CaF ₂ , calcium fluoride crystal) that can be used as an optical lens, for example, a lens material used in semiconductor lithography or the like.

  Fluorite crystals have very small chromatic dispersion, lower refractive index and lower dispersion than general optical glass, and have special partial dispersion characteristics (abnormal partial dispersion, Abbe number: 95). Semiconductor lithography (including steppers and scanners) that are devices for transferring fine patterns onto wafers, window plates such as achromatic lenses (apochromats), infrared analyzers and excimer lasers, TV camera lenses and microscope lenses Widely used for lenses.

  In particular, for steppers (reduction projection type exposure devices) that are responsible for miniaturization in semiconductor lithography equipment, the wavelength of light sources has been shortened to improve resolution, and excimer lasers as high-power lasers that oscillate in the ultraviolet region are used as light sources. With the development of the stepper used for this, fluorite crystals have attracted attention as a lens material suitable for this. Fluorite crystals are characterized by high transmittance of light in a wavelength region called a vacuum ultraviolet region such as an ArF laser (wavelength: 193 nm) among excimer laser light.

  This type of fluorite crystal was obtained by mixing a synthesized powdered calcium fluoride raw material and a scavenger, placing them in a crucible, heating and melting, and then slowly cooling to grow calcium fluoride crystals. The crystal is placed in a growth crucible together with a seed crystal as necessary, and the raw material is melted by gradually raising the temperature to the melting point of fluorite (1370 ° C. to 1450 ° C.) while maintaining the inside of the furnace in a vacuum atmosphere. In general, the growth crucible is gradually lowered and crystallized from the lower part of the crucible to grow a fluorite crystal, which is generally produced by heat treatment.

  However, the conventional method for producing fluorite crystals has a problem in that impurities and oxides contained in the raw material powder remain in the crystal. A method for increasing the transmittance of (fluorite crystal) has been proposed.

  For example, in Patent Document 1, a mixture of calcium fluoride powder and a scavenger is melted, followed by slow cooling to crystal growth, and the last crystallized portion of the calcium fluoride crystal grown by the above process is removed. A method for producing a fluorite crystal is disclosed, comprising: a purification step including a step; and a growth step in which crystallized calcium fluoride and strontium fluoride are melted and then slowly cooled to grow a crystal. ing.

  Patent Document 2 discloses a fluorite crystal manufacturing method by Bridgeman's method in which a raw material is heated and melted to grow a crystal to obtain a single crystal ingot, and a polycrystal is heated and melted to grow a crystal to produce a single crystal ingot. The first step of obtaining the first step, the second step of performing the first step a plurality of times, the second step of cutting out the lower portion in the growth direction of the obtained single crystal ingots, and the lower portions of the single crystal ingots cut out in the second step are combined A manufacturing method including a third step of obtaining a single crystal ingot by crystal growth by heating and melting is disclosed.

  Further, Patent Document 3 is obtained by mixing calcium fluoride powder and a scavenger in a method for producing a synthesized fluorite crystal having an internal transmittance per 10 mm with respect to light having a wavelength of 135 nm of 70% or more. A step of melting the mixture followed by slow cooling and crystal growth, and a purification step including the step of removing the last crystallized portion of the calcium fluoride crystal-grown by the above-mentioned step, and the crystallized calcium fluoride There is disclosed a method for producing a fluorite crystal, which comprises a growth step in which a crystal is grown by slow cooling after melting, and the purification step is repeated a plurality of times.

Japanese Patent No. 3955782 (Japanese Patent Laid-Open No. 9-255328) JP-A-10-203899 JP 2003-238293 A

  In order to eliminate the residue of impurities and oxides and to increase the transmittance of fluorite crystals, as described above, as described above, impurities or the like are present in the last crystallized portion of calcium fluoride that has grown. Although this method was to remove this portion because it remained, there was a problem that the yield was lowered by the amount removed.

  Thus, the present invention proposes a new method for producing fluorite crystals that can increase the transmittance of fluorite crystals without removing a part of the crystal grown or solidified calcium fluoride. is there.

  In the present invention, a mixture obtained by mixing calcium fluoride powder and a scavenger is melted, then cooled and solidified to obtain a molten solidified body A, and the total amount of the obtained molten solidified body A is crushed and melted. A melt solidification step of obtaining a solidified solid body B by melting a mixture obtained by mixing the total amount of the melt solidified solid body and the scavenger, and then solidifying by cooling. Crushing step of crushing the entire amount of the molten solidified body B obtained in step 1 to make a molten solidified body crushed material, and crystal growth to obtain a fluorite crystal by cooling the crystal after cooling the molten solidified body crushed material The present invention proposes a method for producing a fluorite crystal comprising a step and a heat treatment step of heat-treating the fluorite crystal obtained in the crystal growth step.

As in the present invention, a mixture of calcium fluoride powder and scavenger is melted and solidified, and the total amount of the obtained molten solidified body is crushed, and then a scavenger is added again to melt and solidify repeatedly to melt and solidify. By performing the process, it has been found that the transmittance of the fluorite crystal can be increased without removing a part of the crystal grown or solidified calcium fluoride.
Therefore, according to the present invention, an optically excellent fluorite crystal can be industrially efficiently produced. Therefore, the fluorite crystal obtained by the production method of the present invention is, for example, a TV camera lens, a microscope lens, Infrared analysis window materials, lens materials such as lenses used in semiconductor lithography equipment, especially ArF (argon fluoride) excimer laser exposure equipment and F ₂ (fluorine) excimer laser exposure equipment that require high optical properties Or it can use suitably as lens materials for steppers, such as an exposure apparatus which used the laser of the vacuum ultraviolet wavelength range for the light source.

It is the graph which showed the relationship between a wavelength and an absorption coefficient, in order to demonstrate (gamma) value.

  Embodiments of the present invention will be described in detail below, but the scope of the present invention is not limited to the embodiments described below.

  The method for producing a fluorite crystal according to the present embodiment (hereinafter, this production method is referred to as “the present production method”) includes mixing raw materials, heating and melting the mixture, and cooling and solidifying the mixture. A is obtained, and the total amount of the obtained molten solidified product A is crushed to obtain a molten solidified product crushed product. The mixture obtained by mixing the total amount of the molten solidified product crushed product and the scavenger is melted and then cooled to solidify. A melt-solidified step of obtaining a melt-solidified body B, a crushing step of crushing the entire amount of the melt-solidified body B obtained in the melt-solidified step to obtain a melt-solidified product, and melting the melt-solidified product And a crystal growth step of cooling and crystal growth to obtain a fluorite crystal, and a heat treatment step of heat-treating the fluorite crystal obtained in the crystal growth step.

<Raw material>
Examples of the raw material include a mixture of powdered calcium fluoride raw material and a scavenger, that is, a reaction material that removes impurities (mainly oxygen) in the fluorite crystal. However, other materials can be added as appropriate.

  As a calcium fluoride raw material, a well-known calcium fluoride raw material can be used suitably. In order to produce a fluorite crystal used in the ultraviolet or vacuum ultraviolet region, it is preferable to use artificially synthesized high-purity calcium fluoride powder as a raw material. For example, powdery calcium fluoride raw material powder obtained by synthesizing calcium carbonate and hydrogen fluoride can be used.

Examples of the scavenger include zinc fluoride (ZnF ₂ ), lead fluoride (PbF ₂ ), bismuth fluoride (BiF ₃ ), sodium fluoride (NaF), and lithium fluoride (LiF).

The mixing ratio of the scavenger to 100 mol% of calcium fluoride (CaF ₂ ) in the calcium fluoride raw material is preferably 1 to 4 mol%. If it is 1 mol% or more, the scavenger effect, that is, the effect of removing impurities (mainly oxygen) in the fluorite crystal can be effectively exhibited, while if it is 4 mol% or less, the scavenger is contained in the fluorite crystal. Can be avoided. From this point of view, the content is more preferably 1.2 mol% or more, or 3 mol% or less, and even more preferably 1.5 mol% or more, or 2.5 mol% or less.

<Melt solidification process>
In this step, after the raw material mixture is heated and melted, it is cooled and solidified to obtain a molten solidified body A, and the total amount of the obtained molten solidified body A is crushed to obtain a molten solidified body crushed material. What is necessary is just to make it melt | dissolve the mixture which added the new scavenger again to the crushed material whole quantity, and to solidify by cooling, and to obtain the molten solidified body B here.

More specifically, for example, a calcium fluoride raw material and a scavenger are mixed and put in a crucible, heated and melted by heating in a vacuum atmosphere, and then slowly cooled to obtain a molten solidified body A, and the obtained melt The entire amount of the solidified body A is crushed to obtain a molten solidified body crushed material. Next, the mixture obtained by adding a new scavenger to the total amount of the melted and solidified product may be melted and then cooled and solidified to obtain a melted and solidified product B.
In the present invention, “vacuum” means an atmosphere of 10 ⁻³ Pa or less.

  Thus, by adding a scavenger and repeating the melt solidification, the γ value (absorption coefficient of 200 nm to 800 nm) of the melt solidified body as an intermediate for producing fluorite crystals can be obtained without removing a part of the melt solidified body. Integration value) can be significantly reduced, and the internal transmittance and laser durability of the product (fluorite crystal) can be increased.

  If necessary, the entire amount of the molten solidified body B is crushed to obtain a molten solidified body crushed material, and a mixture obtained by adding a new scavenger to the whole molten solidified body crushed material is melted, and then cooled and solidified. A series of processes for obtaining a melt-solidified body may be performed once or twice or more.

At this time, the mixing amount of the scavenger is the melting amount obtained by crushing the whole amount of the molten solidified body B with respect to 100 mol% of calcium fluoride in the molten solidified body crushed material obtained by crushing the whole amount of the molten solidified body A. It is preferable that the amount of the scavenger to be mixed is 1 to 4 mol% with respect to 100 mol% of calcium fluoride in the crushed solid product. If it is 1 mol% or more, the scavenger effect, that is, the effect of removing impurities (mainly oxygen) in the fluorite crystal can be effectively exhibited, while if it is 4 mol% or less, the scavenger is a fluorite crystal. It can be avoided to remain inside. From this point of view, the content is more preferably 1.2 mol% or more, or 3 mol% or less, and even more preferably 1.5 mol% or more, or 2.5 mol% or less.
Even in the case where the crystal growth of calcium fluoride is repeated by crushing, heating and melting and slow cooling, the mixing ratio of the scavenger is preferably in the above range.

  The melt-solidified body thus obtained, that is, the polycrystalline body, is an intermediate when producing a fluorite crystal (intermediate body for producing fluorite crystal), and as can be seen from the results of Examples described later, The γ value (the integrated value of the absorption coefficient of 200 nm to 800 nm) can be 70 or less, particularly 50 or less, and more preferably 1 or more, or 20 or less, and can be significantly reduced.

In the melt-solidification step, the atmosphere in the furnace is preferably initially set to a vacuum, and from the time when the dehydration reaction is completed, the furnace is preferably set to a vacuum atmosphere by a vacuum exhaust system.
The temperature profile in the melt-solidification step is preferably raised from room temperature to a temperature range higher than the melting point of calcium fluoride (1420 ° C.) over a period of about one day, and kept in this temperature range for several days.

In addition, the heat melting may be performed by putting the raw material mixture in a crucible and heating and melting by a heating means suitable for the melting temperature, equipment and the like.
Heating and melting is preferably performed in an oxygen-free atmosphere (including a vacuum atmosphere and an inert gas atmosphere).

  Means and level of crushing the molten solidified body are tools that do not contain impurities, and tools that can be removed in the subsequent process even if they enter, such as a plastic hammer (even if part of the hammer is mixed, it can be volatilized and removed later). It is preferably used or crushed by hand to a suitable size, for example, a few mm square to a fist size.

<Crushing process>
The means for crushing the melt-solidified body obtained in the melt-solidification step and the degree thereof are the same as described above.

<Crystal growth process>
The melted and solidified product obtained in the crushing step can be melted and then cooled and crystal grown to obtain fluorite crystals. At this time, a fluorite crystal may be grown using a seed crystal as necessary. Further, when melting the melted and solidified product, it may be mixed with a scavenger and melted as necessary.

  The crystal growth method in this case is not particularly limited. For example, the Bridgman-Stockbarger method (also referred to as “BS method”), Czochralski (also referred to as “CZ method”), zone melt method, improved methods thereof, and other methods A known crystal growth method such as a melt growth method can be appropriately employed.

  The BS method is a method in which raw materials are put in a crucible and melted, and a single crystal is grown from the bottom of the crucible while pulling down the crucible. The crystal growing apparatus is relatively inexpensive and has a feature that a single crystal having a large diameter can be grown relatively easily. On the other hand, it is difficult to control the crystal growth orientation, and excessive stress is applied during crystal growth or cooling, so that it is said that the stress distribution remains in the crystal and strain and dislocations are easily induced.

  On the other hand, the CZ method is a method in which a raw material is put in a crucible and melted, and a seed (seed crystal) is brought into contact with a molten liquid surface and grown (crystallized) while rotating a single crystal. The CZ method is said to facilitate the growth of the target crystal orientation because it is possible to identify and crystallize the crystal orientation.

More specifically, an example of the crystal growth method will be described. The seed crystal is placed at the bottom of the crucible, the molten solidified product obtained in the crushing step is filled into the crucible, and the crucible is placed in the crystal growth apparatus. Then, evacuation is performed until the degree of vacuum inside the crystal growth apparatus reaches about 1 × 10 ⁻³ to 10 ⁻⁴ Pa by a vacuum exhaust system, the crucible is heated by a heating apparatus, and the raw material filled in the crucible is melted.
After the raw material in the crucible is melted, when the crucible is gradually pulled down vertically at a speed of about 0.1 mm / hour to 3 mm / hour, the raw material that has become a melt in the crucible starts to solidify from the vicinity of the seed crystal, A single crystal is grown. When all the raw materials in the crucible are solidified, the lowering of the crucible is completed, and the crucible is cooled to about room temperature while being gradually cooled by a heating device, whereby an ingot-like fluorite crystal can be grown.

  The ingot-like fluorite crystal grown as described above is preferably cut out so that a surface having a predetermined size and a predetermined orientation appears as necessary, and subjected to a heat treatment step. For example, it can be cut into a disk shape having a diameter of about 200 mm and a thickness of about 40 mm and used for the heat treatment step.

<Heat treatment process>
Next, the fluorite crystal grown in the above process is put in a container, the container is placed in a heat treatment furnace, and the heat treatment furnace is heated uniformly to 900 ° C. to 1300 ° C. Remove the distortion.
A heating temperature of 1140 ° C. or higher is not preferable because it causes structural changes.
In the heat treatment step, it is preferable to return the temperature of the fluorite crystal to room temperature while maintaining the state in which the distortion of the crystal is eliminated by the heat treatment.

The atmosphere in the heat treatment, that is, the atmosphere in the annealing case may be a vacuum atmosphere or an inert gas atmosphere such as argon (Ar). Of these, an inert gas atmosphere such as argon is preferable, and an atmosphere obtained by mixing and injecting a fluorine-based gas into argon gas is preferable. An atmosphere in which an inert gas such as argon gas is mixed with fluorine gas by thermal decomposition of a scavenger (for example, PbF ₂ ) is also a preferred example.

The temperature profile in the heat treatment step is not particularly limited. Since the melting point of fluorite is about 1370 ° C. to 1410 ° C., the fluorite crystal does not melt and maintains a solid state, while giving sufficient energy to each atom constituting the fluorite crystal to each appropriate position. The temperature range is not particularly limited, and it may be heated to a temperature at which the anisotropy due to disorder of the crystal structure can be eliminated. As a guideline, it is preferable to raise the temperature to 1000 to 1350 ° C. in order to more effectively eliminate the anisotropy due to disorder of the crystal structure.
The rate of temperature rise is not particularly limited, but it is necessary to raise the temperature in the furnace so that the fluorite crystal does not break due to thermal shock, such as cracking. For example, it is 10 ° C./h to 300 ° C./h. It is preferable to raise the temperature.

  In the cooling step after the heat treatment, if it is rapidly cooled, strain is likely to remain in the crystal, and slip defects are introduced and dislocations and the like increase. Therefore, it is preferable to cool slowly over time. On the other hand, if too much time is spent, productivity is significantly impaired. From such a viewpoint, in the cooling step after the heat treatment, for example, it is preferable to cool to near room temperature at a cooling rate of 0.7 to 3.0 ° C./h.

<Molding process>
The fluorite crystal after the heat treatment may be appropriately cut and processed into a suitable shape as necessary. For example, what is necessary is just to process into the shape which makes a surface parallel to a (111) plane. As a more specific example, there is a method in which a fluorite crystal having a disc shape is cut into a shape having a surface parallel to the (111) plane, and the surface is surface ground to smooth the surface. Can do.

<Application>
Fluorite crystals obtained by this production method are, for example, achromatic lenses (apochromats), TV camera lenses, microscope lenses, window materials for infrared analysis, lenses used in semiconductor lithography (steppers, scanners), and other optical lenses. Can be used as In particular, since a fluorite crystal having high crystal homogeneity and excellent laser durability can be obtained macroscopically, a high-precision stepper, that is, an ultraviolet or vacuum ultraviolet wavelength region such as an ArF (Argon fluoride) excimer laser is used. It can be suitably used as a lens material for a stepper such as an exposure apparatus using a laser as a light source. Furthermore, since the fluorite crystal has excellent laser durability, it can be suitably used as an optical element such as a window material of a laser light source in the ultraviolet or vacuum ultraviolet wavelength region such as an ArF excimer laser or a resonator mirror. .

<Explanation of words>
In addition, the treatment temperature in the crystal growth step and the heat treatment step indicates the atmospheric temperature in the furnace unless otherwise specified.

In the present invention, when described as “X to Y” (X and Y are arbitrary numbers), unless otherwise specified, “X is preferably greater than X” or “preferably Y”. It also includes the meaning of “smaller”.
Further, when described as “X or more” (X is an arbitrary number), it means “preferably larger than X” unless otherwise specified, and described as “Y or less” (Y is an arbitrary number). In the case, unless otherwise specified, the meaning of “preferably smaller than Y” is also included.

Examples of the present invention and comparative examples will be described below. However, the present invention is not limited to the contents described below.
First, an evaluation method of the obtained fluorite crystal will be described.

<Measurement method of absorption coefficient and calculation method of γ value>
Here, a gamma ray (1.17 MeV, 1.33 MeV) emitted from a radioactive isotope ⁶⁰ Co is irradiated to a radiation source at a predetermined dose, and at that time, a color center induced in the crystal is measured by a spectrophotometer. And an induced color center absorption spectrum was obtained. It is known that the relationship between laser durability and γ-ray-induced color center absorption intensity has a negative correlation. That is, in the crystal with high laser durability, the γ-ray induced color center absorption intensity is small. From this correlation, the laser durability of the fluorite crystal can be evaluated.

  Specifically, optical polishing was performed so that both end faces of the molten solidified body became parallel planes, and the optical length (thickness of the molten solidified body) was set to 30 mm. Such a molten solidified body was held in a dark box, and γ-rays (1.33 MeV) from 60Co were irradiated in the atmosphere at a dose of 5.4 kGy to induce a color center in the molten solidified body. Next, immediately after irradiation, the absorption spectrum in the ultraviolet-visible wavelength region (200 nm to 800 nm) of this molten solidified body was measured using a self-recording spectrophotometer (U-4100, Hitachi High-Technologies).

As the “absorption coefficient”, the natural logarithm of the transmittance subjected to the reflection correction of the end face was taken according to Lambert · Beer's law, and a value (cm ⁻¹ ) normalized by the length was calculated.
Further, the “γ value” was calculated as a value (cm ⁻¹ · nm) obtained by integrating the absorption coefficient (cm ⁻¹ ) in a section from a wavelength of 200 nm to 800 nm in the obtained absorption spectrum.

<Laser durability evaluation method>
The fluorite crystal was irradiated with an ArF excimer laser, the transmittance before and after irradiation was measured, and the laser durability was examined using the rate of decrease in transmittance as an index.

The fluorite crystal was irradiated with 200,000 pulses of ArF excimer laser with an output of 30 mJ / cm ² , and the transmittance (internal transmittance) at 193.4 nm before and after irradiation was measured.
The internal transmittance at 193.4 nm was measured with a vacuum ultraviolet spectrophotometer.

Table 1 shows the values of the internal transmittance (%) per 1 cm thickness of the fluorite crystal at the calculated transmitted light wavelength of 193.4 nm before and after the laser irradiation.
The internal transmittance per 1 cm thickness of these fluorite crystals was calculated as follows.
Internal transmittance per 1 cm thickness (%) = exp {ln (z / R) × 1 / d × 1} × 100
here,
z: Measured value of transmittance at a wavelength of 193.4 nm (%)
R: Theoretical transmittance (92.256%) in consideration of reflection at the end face at a wavelength of 193.4 nm
d: Thickness of fluorite crystal (cm)
It is.

(Comparative Example 1)
Lead fluoride (PbF ₂ ) as a scavenger is added to the raw material calcium fluoride powder in an amount corresponding to 1.5 mol% when the calcium fluoride in the raw material calcium fluoride is 100 mol%. After putting the crucible into the furnace and making the furnace in a vacuum state (10 ⁻³ Pa or less), the temperature was raised to 1400 ° C. ± 20 ° C. over 6 hours to melt the raw material. After maintaining this temperature for one day, it was naturally cooled over several days, the molten solidified body A was taken out from the furnace, and the entire amount of the molten solidified body A was kneaded into a large size by hand.

Next, the total amount of molten and solidified crushed material obtained by crushing as described above, and lead fluoride (PbF ₂ ) in an amount corresponding to 2.0 mol% when calcium fluoride in the crushed material is 100 mol%. And the mixture is put in a crucible of a single crystal growth furnace, the inside of the furnace is evacuated and the crucible is heated, and the degree of vacuum is 10 ⁻³ Pa or less, and the temperature is 1400 ° C. ± 20 ° C. After maintaining the above, the crucible was lowered to grow fluorite crystals.
Next, after depressurizing the inside of the heat treatment furnace at room temperature to form a vacuum atmosphere, the atmosphere inside the furnace was quickly replaced with an Ar gas atmosphere, and then the maximum temperature of 1000 to 1000 hours in a heating apparatus with a heating time of 36 hours. The temperature was raised to 1200 ° C., and the temperature was maintained for 24 hours. Then, it cooled to room temperature over about 10 days.
The crystal obtained by heat treatment in this manner was cut out, and both ends of the (111) plane were optically polished to obtain a fluorite crystal.

Example 1
The total amount of the melt-solidified product A obtained in the same manner as in Comparative Example 1 was crushed by hand into a fist size, and the calcium fluoride in the crushed product of the melt-solidified product A was 100 mol%. In this case, lead fluoride (PbF ₂ ) in an amount corresponding to 1.5 mol% is added, the crucible containing these is put in the furnace, the inside of the furnace is evacuated, the crucible is heated, and the degree of vacuum is 10 ^−3. The temperature was set to 1400 ° C. ± 20 ° C. at Pa or lower, and this was maintained for 1 day or longer, and then the crucible was lowered to obtain a molten solidified body B.
This melt-solidified body B was crystal-grown, heat-treated and processed in the same manner as the melt-solidified body A of Comparative Example 1 to obtain fluorite crystals.

(Example 2)
When the total amount of the melt-solidified body B obtained in the same manner as in Example 1 is crushed by hand into a fist size, and the calcium fluoride in the crushed material of the obtained melt-solidified body B is 100 mol% An amount of lead fluoride (PbF ₂ ) corresponding to 1.5 mol% was added to the reactor, the crucible containing these was placed in the furnace, the inside of the furnace was evacuated and the crucible was heated, and the degree of vacuum was 10 ⁻³ Pa. Hereinafter, the temperature was set to 1400 ° C. ± 20 ° C., and this was maintained for one day or more, and then the crucible was lowered to obtain a molten solidified product C. This melt-solidified body C was crystal-grown, heat-treated and processed in the same manner as the melt-solidified body B of Example 1 to obtain fluorite crystals.

(Discussion)
As a result, as in Examples 1 and 2, the entire amount of the molten solidified body was crushed into a molten solidified body, and a new scavenger was again added to the total amount of the molten solidified body to melt again, followed by cooling. By repeating a series of processes to obtain a molten solidified body by solidification, the γ value (of the color center absorption coefficient) of the molten solidified body as an intermediate for producing fluorite crystals can be obtained without removing a part of the molten solidified body. It was found that the integrated value of the wavelength section 200 nm to 800 nm) can be significantly reduced. Therefore, by producing a fluorite crystal using this molten solidified body, the internal transmittance and laser durability at a wavelength of 193 nm can be obtained compared to the case of a fluorite crystal produced by a conventional method such as a comparative example. It turned out that it can raise.

(Comparative Example 2)
In the process until obtaining the molten solidified body A, the raw material calcium fluoride powder is fluorinated as a scavenger in an amount corresponding to 2.5 mol% when the calcium fluoride in the raw material calcium fluoride is 100 mol%. Except for the addition of lead (PbF ₂ ), a melt-solidified body A was obtained in the same manner as in Comparative Example 1, and crystal growth, heat treatment and processing were performed in the same manner as in the melt-solidified body A of Comparative Example 1 to obtain fluorite crystals. .

Example 3
The total amount of the molten solidified product A obtained in the same manner as in Comparative Example 2 was crushed by hand into a fist size, and the calcium fluoride in the crushed material of the molten solidified product A was 100 mol%. In this case, lead fluoride (PbF ₂ ) in an amount corresponding to 2.5 mol% is added, the crucible containing these is put in the furnace, the inside of the furnace is evacuated, the crucible is heated, and the degree of vacuum is 10 ^−3. The temperature was set to 1400 ° C. ± 20 ° C. at Pa or lower, and this was maintained for 1 day or longer, and then the crucible was lowered to obtain a molten solidified body B.
This melt-solidified body B was crystal-grown, heat-treated and processed in the same manner as the melt-solidified body A of Comparative Example 1 to obtain fluorite crystals.

Example 4
When the total amount of the melt-solidified body B obtained in the same manner as in Example 3 is crushed by hand into a large fist size, and the calcium fluoride in the crushed material of the obtained melt-solidified body B is 100 mol% An amount of lead fluoride (PbF ₂ ) corresponding to 2.5 mol% was added to the reactor, the crucible containing these was placed in the furnace, the inside of the furnace was evacuated, the crucible was heated, and the degree of vacuum was 10 ⁻³ Pa. Hereinafter, the temperature was set to 1400 ° C. ± 20 ° C., and this was maintained for one day or more, and then the crucible was lowered to obtain a molten solidified product C. This melt-solidified body C was crystal-grown, heat-treated and processed in the same manner as the melt-solidified body B of Example 3 to obtain fluorite crystals.

(Comparative Example 3)
In the process until obtaining the molten solidified product A, the raw material calcium fluoride powder is fluorinated as a scavenger in an amount corresponding to 4.0 mol% when the calcium fluoride in the raw material calcium fluoride is 100 mol%. Except for the addition of lead (PbF ₂ ), a melt-solidified body A was obtained in the same manner as in Comparative Example 1, and crystal growth, heat treatment and processing were performed in the same manner as in the melt-solidified body A of Comparative Example 1 to obtain fluorite crystals. .

(Example 5)
The total amount of the melt-solidified product A obtained in the same manner as in Comparative Example 3 was crushed by hand into a fist size, and the calcium fluoride in the crushed product of the melt-solidified product A was 100 mol%. In this case, lead fluoride (PbF ₂ ) in an amount corresponding to 4.0 mol% is added, the crucible containing these is put in the furnace, the inside of the furnace is evacuated, the crucible is heated, and the degree of vacuum is 10 ^−3. The temperature was set to 1400 ° C. ± 20 ° C. at Pa or lower, and this was maintained for 1 day or longer, and then the crucible was lowered to obtain a molten solidified body B.
This melt-solidified body B was crystal-grown, heat-treated and processed in the same manner as the melt-solidified body A of Comparative Example 1 to obtain fluorite crystals.

(Example 6)
When the total amount of the melt-solidified body B obtained in the same manner as in Example 5 is crushed by hand to a large fist size, and the calcium fluoride in the crushed material of the obtained melt-solidified body B is 100 mol% The amount of lead fluoride (PbF ₂ ) corresponding to 4.0 mol% was added to the crucible, the crucible containing these was put into the furnace, the inside of the furnace was evacuated, the crucible was heated, and the degree of vacuum was 10 ⁻³ Pa. Hereinafter, the temperature was set to 1400 ° C. ± 20 ° C., and this was maintained for one day or more, and then the crucible was lowered to obtain a molten solidified product C. This melt-solidified body C was crystal-grown, heat-treated and processed in the same manner as the melt-solidified body B of Example 5 to obtain fluorite crystals.

(Comparative Example 4)
In the process until obtaining the molten solidified body A, the raw material calcium fluoride powder is fluorinated as a scavenger in an amount corresponding to 5.0 mol% when the calcium fluoride in the raw material calcium fluoride is 100 mol%. Except for the addition of lead (PbF ₂ ), a melt-solidified body A was obtained in the same manner as in Comparative Example 1, and crystal growth, heat treatment and processing were performed in the same manner as in the melt-solidified body A of Comparative Example 1 to obtain fluorite crystals. .

(Example 7)
The total amount of the melt-solidified product A obtained in the same manner as in Comparative Example 4 was crushed by hand into a fist size, and the calcium fluoride in the crushed product of the melt-solidified product A was 100 mol%. In this case, lead fluoride (PbF ₂ ) in an amount corresponding to 5.0 mol% is added, the crucible containing these is put in the furnace, the inside of the furnace is evacuated, the crucible is heated, and the degree of vacuum is 10 ^−3. The temperature was set to 1400 ° C. ± 20 ° C. at Pa or lower, and this was maintained for 1 day or longer, and then the crucible was lowered to obtain a molten solidified body B.
This melt-solidified body B was crystal-grown, heat-treated and processed in the same manner as the melt-solidified body A of Comparative Example 1 to obtain fluorite crystals.

(Comparative Example 5)
A fluorite crystal was obtained in the same manner as in Comparative Example 1 except that lead fluoride (PbF ₂ ) as a scavenger was not added to the raw material calcium fluoride powder.

(Comparative Example 6)
The entire amount of the melt-solidified product A obtained in the same manner as in Comparative Example 5 was crushed by hand into a fist size, the crucible containing this was put into the furnace, the inside of the furnace was evacuated, and the crucible was heated. Then, the degree of vacuum was 10 ⁻³ Pa or less, the temperature was 1400 ° C. ± 20 ° C., and this was maintained for 1 day or longer, and then the crucible was lowered to obtain a molten solidified body B. This melt-solidified body B was crystal-grown, heat-treated and processed in the same manner as the melt-solidified body A of Comparative Example 1 to obtain fluorite crystals.

(Discussion)
From the above comparative examples and examples and the results of the tests conducted so far, the mixture of calcium fluoride powder and scavenger was melted and solidified, and the total amount of the obtained molten solidified material was crushed and again added with scavenger to melt and solidify. By repeating the melt-solidification step by repeating a series of treatments, the transmittance of the fluorite crystal can be increased without removing a portion of the crystal grown or solidified calcium fluoride. It has been found that the γ value (integral value of the absorption coefficient of 200 nm to 800 nm) of the melt-solidified product as an intermediate during the production can be reduced to 70 or less, particularly 50 or less, especially 1 to 20 or so.

Claims (4)

Melting a mixture of calcium powder and scavenger fluoride, followed by the raw material melting and solidification step of cooling and solidifying to obtain a melt solidified body A, the crushed the whole amount of the resulting melt solidified body A in the raw material melting and solidification step a melt solidified body crushed a, and the melt-solidified product of ruptured obtained by mixing the total amount and scavengers a mixture melting, followed by cooling solidifying to obtain a melt solidified body Ru melt solidifying step, the melting and solidification and crushing step to resulting melt solidified body melt solidified body crushed crushed the entire amount of the step, after melting the melt-solidified disrupted product obtained by crushing step, crystal growth and cooling a crystal growth step of obtaining a fluorite crystal by a heat treatment step of heat-treating the fluorite crystal obtained in the crystal growth step, the method for producing a fluorite crystal having a,
Wherein the melting and solidification process, firefly, which comprises mixing the 1～4Mol% of the amount of scavenger with respect to calcium fluoride 100 mol% of the melt solidified body crushed in A obtained crushed whole amount of the melt solidified body A A method for producing stone crystals. Melting a mixture of calcium powder and scavenger fluoride, followed by the raw material melting and solidification step of cooling and solidifying to obtain a melt solidified body A, the crushed the whole amount of the resulting melt solidified body A in the raw material melting and solidification step a melt solidified body crushed a, and the melt-solidified product of ruptured obtained by mixing the total amount and scavengers a mixture melting, followed by cooling solidifying to obtain a melt solidified body Ru melt solidifying step, The entire amount of the molten solidified body obtained in the melt solidification step is crushed to obtain a molten solidified body crushed material, and the mixture obtained by mixing the total amount of the molten solidified body crushed material and the scavenger is melted and then cooled to solidify. and re-melting and solidification to obtain a melt solidified body by, the re-melting and solidification process is performed one or more times, the crushing step to melt solidified body crushed crushed the entire amount of the finally obtained melt solidified body , in the crushing process After melting was melt solidified body crushed, includes a crystal growth step of cooling by crystal growth to obtain a fluorite crystal, and a heat treatment step of heat-treating the crystal growth step fluorite crystals obtained in In the method for producing fluorite crystals,
Wherein the melting and solidification process, or mixing 1～4Mol% of the amount of scavenger with respect to calcium fluoride 100 mol% in the melt-solidified product of ruptured A obtained crushed whole amount of the melt solidified body A, or,
In the remelting and solidifying step, scavengers in an amount of 1 to 4 mol% are mixed with 100 mol% of calcium fluoride in the molten and solidified product obtained by crushing the entire amount of the molten and solidified product. Crystal production method. Wherein the melt-solidified step, mixing 1～4Mol% of the amount of scavenger with respect to calcium fluoride 100 mol% in the melt-solidified product of ruptured A obtained crushed whole amount of the melt solidified body A, and,
The scavenger in an amount of 1 to 4 mol% is mixed with 100 mol% of calcium fluoride in the melted and solidified product obtained by crushing the entire amount of the melted and solidified product in the remelting and solidifying step. 2. A method for producing a fluorite crystal according to 2 . The method for producing a fluorite crystal according to claim 2 or 3 , wherein the remelting and solidification step is performed twice or more.

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