JPH11130594A - Production of fluoride crystal, fluoride crystal and optical part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a fluoride crystal excellent in optical characteristics capable of completely removing water content in a raw material and an oven before melting of the raw material by using a reactive gas or solid scavenger and decreasing dislocation density of crystal and improving transmission characteristics as a result and provide a method for producing the fluoride crystal and obtain an optical part. SOLUTION: This method for producing fluoride crystal comprises successively carrying out a step for charging a fluoride raw material into a growing furnace, a step for raising the fluoride raw material to a temperature lower by 50-200 deg.C than the melting point, a step for melting the fluoride raw material in an inert gas atmosphere, a step for growing crystal by immersing seed crystal in a melt and then pulling up the crystal. In the production method, heating is preferably carried out at 100-300 deg.C in a vacuum atmosphere or inert gas atmosphere before raising the temperature of the crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a fluoride crystal which is suitable for various optical elements, lenses, window materials, prisms and the like used particularly in a wide wavelength range from the vacuum ultraviolet region to the far infrared region. And optical components.

[0002]

2. Description of the Related Art Fluorite crystals such as fluorite have high transmittance in a wide wavelength range from the vacuum ultraviolet region to the far infrared region, and are widely used for various optical elements, lenses, window materials, prisms and the like.

[0003] Such a fluoride crystal is prepared by the Bridgman-Stockberger method (Stockbarger, J. Opt. Soc, A
m. 39, (1949)). In this method, a raw material is put in a crucible made of graphite or the like, heated and melted in a vacuum atmosphere, and a crystal is grown by moving the crucible in a furnace having a temperature gradient. It is characterized by advantages such as easy control of crystal shape and enlargement of the crystal diameter, and relatively low cost of the apparatus.

[0004]

However, in this method, since the crystal is grown inside the crucible, impurities are liable to be mixed into the melt from the crucible, dislocations and defects are likely to enter the crystal, and the growing crystal is It has drawbacks such as difficulty in control because it cannot be observed.

In order to overcome the above disadvantages, fluoride crystals are grown by the Czochralski method (pulling method) (Nassau, J. Appl. Phys. 32, (1961),
Cockay ne et al, Nature 203, (1964)), compared to crystals grown by the Bridgman-Stockberger method.
Dislocation density and transmission characteristics are not excellent at all (Leckebus
ch & Recker, J. Cyrst. Growth 13/14, (1972)).

It is considered that the cause is that the melted fluoride easily reacts with water, and even if a small amount of water remains in the furnace, it reacts immediately to form an oxide. When the oxides are taken into the pulled crystal,
The transmission characteristics are reduced and the dislocation density is greatly increased.

SUMMARY OF THE INVENTION The present invention has been made in view of the above technical problems, and controls the atmosphere in a growth furnace to completely remove the raw material and water in the furnace before melting the raw material. It is an object of the present invention to provide a fluoride crystal having excellent optical characteristics and a method for producing the same, which can reduce the dislocation density and improve the transmission characteristics.

[0008]

According to the present invention, there is provided a method for producing a fluoride crystal, comprising the steps of: loading a fluoride raw material into a growth furnace; dissolving the fluoride raw material in an inert gas atmosphere; The step of growing the crystal by dipping it in the melt and pulling it up is performed in order.

In the method for producing a fluoride crystal according to the present invention, a step of charging a fluoride raw material into a growth furnace together with a solid scavenger of fluoride is performed, and after the charging, the raw material is dissolved in an inert gas atmosphere or a vacuum atmosphere. And a step of dipping the seed crystal in the melt and pulling it up to grow the crystal.

[0010] The fluoride crystal of the present invention is characterized by being produced by the above-mentioned crystal production method.

An optical component according to the present invention is characterized in that the fluoride crystal is formed into a predetermined shape.

The reactive gas is methane tetrafluoride (CF ₄ ), methane trifluoride (CHF ₃ ), methane difluoride (CH ₂ F ₂ ), ethane hexafluoride (C ₂ F) ₆ ) and a fluorocarbon-based gas such as propane octafluoride (C ₃ F ₈ ). Note that the reactive gas may be diluted with an inert gas before use.

The fluoride used as the solid scavenger is preferably lead fluoride, zinc fluoride, cadmium fluoride, manganese fluoride, bismuth fluoride, sodium fluoride, and lithium fluoride.

The fluoride crystals may be calcium fluoride, barium fluoride or magnesium fluoride.

[0015]

FIG. 2 is a flowchart showing an example of a crystal manufacturing process. The following steps will be described.

(Purification Step) In the present invention, it is preferable to purify the fluoride raw material prior to the crystal growth step. In addition, when the fluoride raw material is a high-purity raw material containing no moisture or other impurities, the purification step is not necessarily required.

FIG. 3 shows a fluoride raw material and a solid scavenger.
Into the crucible of the refining furnace shown in The solid scavenger is added in an amount of 0.04 mol% or more and 0.1 mol% or less of the raw material.

In FIG. 3, reference numeral 301 denotes a chamber of a refining furnace, which is connected to a vacuum exhaust system. 302
Is a heat insulating material, 303 is a heater, 304 is a crucible, 305
Is a fluoride raw material, and 306 is a lowering shaft for lowering the crucible.

After evacuating the inside of the chamber 301 of the purification furnace, the heater 303 is energized to heat the fluoride raw material 305 and completely melt the fluoride raw material 305. Subsequently, the melted fluoride raw material 305 is gradually cooled to grow crystals (melting / growing).

Since the crystal obtained in this step may have a grain boundary, precise temperature control is not required. During slow cooling, the crucible 304 is lowered and the shaft 306 is lowered.
It is preferred that the pressure be lowered. By lowering, the effect of removing impurities is further improved.

[0021] Of the crystals thus obtained, particularly the upper part, that is, the part that has crystallized last with time is removed. Since impurities tend to collect in this portion, impurities that adversely affect the characteristics are removed by this removing operation.

If it is desired to obtain a crystal having higher purity, the crystal is put into the crucible 304 again, reacted, melted and crystallized.
A series of steps for removing the upper portion is repeated a plurality of times.

(Single Crystal Growth Step—Using Reactive Gas) FIG. 4 shows an example of a crystal growth furnace used for the pulling method.

The growth furnace is formed by a chamber 1 and has a crucible 4 disposed therein. The heater 3 is provided around the crucible 4, and the heat retaining cylinder 2 is further provided around the heater 3. The interior of the chamber 1 can be evacuated by a vacuum pump 5, and a reactive gas, an inert gas, or a mixed gas thereof can be introduced into the chamber 1 from a reactive gas source 6a and an inert gas source 6b. ing. These gases are exhausted from the exhaust port 12.
Reference numeral 10 denotes a pulling shaft for pulling up the seed crystal 9, and reference numeral 11 denotes a viewing window for observing the inside of the chamber 11 from outside.

The purified fluoride raw material is put into the crucible 4 of the growth furnace shown in FIG.

After the inside of the chamber 1 of the growth furnace is evacuated by the vacuum pump 5, the heater 3 is energized to heat the fluoride raw material, and the temperature around the crucible 4 is preferably 100 ° C.
The temperature is kept at 300 ° C. or lower and kept for several hours to remove water adsorbed on the surface of the fluoride raw material, the crucible 4 and the inner wall of the chamber 1 (dehydration).

If the temperature is lower than 100 ° C., the removal of water is insufficient. If the temperature is higher than 300 ° C., the raw material reacts with water to generate a large amount of oxides. Also, 1
50-250 degreeC is more preferable, and 170-220 degreeC is still more preferable.

The holding time is preferably 4 to 48 hours, more preferably 24 to 48 hours. In less than four hours,
Moisture removal is insufficient, and if it exceeds 48 hours, there is no particular problem, but it takes time.

After the dehydration step, a reactive gas is charged into the chamber 1 of the growth furnace at a pressure of 0.1 atm or more and 0.5 atm or less, and then an inert gas such as Ar is charged into the chamber 1 of the growth furnace. The internal pressure is adjusted to about 1 atm. One atmosphere is
If the pressure is less than 1 atm, a large amount of the raw material is vaporized when the raw material is dissolved.

As the reactive gas, methane tetrafluoride (C
F ₄ ), methane trifluoride (CHF ₃ ), methane difluoride (CH ₂ F ₂ ), ethane hexafluoride (C ₂ F ₆ ), and propane octafluoride (C ₃ F ₈ ) are desirable.

In the present invention, a reactive gas is used to remove impurities, particularly oxides, in the raw material. The reactive gas is HF or F ₂ , N ₂ as a hydrogen fluoride-based gas.
Examples include F ₃ , SF ₆ , XeF ₂ , BF ₃ and a fluorocarbon-based gas. In particular, a fluorocarbon-based gas can be preferably used because it is easily removed after reacting with a raw material.

After the introduction of the reactive gas, the temperature is slowly increased, preferably at 50 to 200 ° C. from the melting point, an inert gas is introduced at a flow rate of 50 cc / min to 500 cc / min, and the furnace pressure is reduced to about 1 atm. , And gradually replace the atmosphere in the furnace with an inert gas atmosphere.

Here, the reason why the temperature from the melting point to the temperature of 50 to 200 ° C. or less is set as the reactive gas atmosphere is that if the temperature is lower than 200 ° C. or less, the reaction of the oxide is insufficient, and If it exceeds, the reactive gas remains in the raw material, and the translucency of the crystal decreases. It is more preferably 75 to 175 ° C below the melting point, and still more preferably 100 to 100 ° C from the melting point.
150 ° C.

Heating is further continued to completely melt the fluoride raw material. Thereafter, the seed crystal 9 is immersed in the melt, and the seed crystal 9 is gradually pulled up by the pulling shaft 10 to grow a single crystal. Lifting speed is 0.1 ~ per hour
5.0 mm is preferred.

(Single Crystal Growth Step—When Using Group Scavenger) A purified fluoride raw material and a solid scavenger are charged into a crucible 4 of a growth furnace shown in FIG.

The addition amount of the solid scavenger is desirably 0.001 mol% or more and 0.01 mol% or less of the fluoride raw material. If it is less than 0.001 mol, it is difficult to remove oxides, and if it exceeds 0.01 mol, a large amount of solid scavenger is taken into the crystal. Further, the amount of the solid scavenger is more preferably 0.001 to 0.005 mol, and further preferably 0.001 to 0.003 mol.

The fluoride used as the solid scavenger is preferably lead fluoride, zinc fluoride, cadmium fluoride, manganese fluoride, bismuth fluoride, sodium fluoride, and lithium fluoride.

After evacuating the inside of the chamber 1 of the growth furnace by the vacuum pump 5, the heater 3 is energized to heat the fluoride raw material. In particular, the temperature around the crucible 4 is set to 100 ° C. or higher.
The temperature is kept at 00 ° C. or lower, and the temperature is maintained for several hours to remove water adsorbed on the surface of the raw material or the crucible (dehydration).

Thereafter, an inert gas such as Ar is filled to set the pressure in the chamber 1 of the growth furnace to about 1 atm.

Then, an inert gas is introduced at a flow rate of 50 cc / min or more and 500 cc / min or less, the pressure in the chamber 1 of the growth furnace is maintained at about 1 atm, the temperature is raised, and the raw material is completely melted. Thereafter, the seed crystal is immersed in the melt and gradually pulled up to grow a single crystal. The lifting speed is preferably 0.1 to 5.0 mm per hour.

(Annealing Step) Subsequently, the crystal-grown fluoride single crystal is heat-treated in an annealing furnace shown in FIG. In FIG. 5, reference numeral 501 denotes a chamber of an annealing furnace,
502 is a heat insulating material, 503 is a heater, 504 is a crucible,
505 is a fluoride crystal.

In this annealing step, the crucible 504 is heated to a temperature of 400 to 500 ° C. or less of the fluoride crystal melting point. The heating time is 20 hours or more, more preferably 20 to 3 hours.
0 hours.

In the case of a crystal having a strength against thermal shock such as magnesium fluoride, the annealing step may be omitted.

(Processing and assembling steps) Thereafter, the optical article is shaped into a required shape (convex lens, concave lens, disk shape, plate shape, etc.). If necessary, an antireflection film may be provided on the back surface of the fluoride crystal optical article. As the antireflection film, magnesium fluoride, aluminum oxide, or tantalum oxide is preferably used, and these can be formed by vapor deposition by resistance heating, electron beam vapor deposition, sputtering, or the like. Since the optical article obtained by the present invention hardly contains water, the adhesion of the antireflection film is excellent.

The lenses and window materials thus obtained are very useful for various laser optical systems. Further, by doping impurities during the growth process, it becomes possible to produce a functional optical element.

[0046]

The present invention will be described in more detail with reference to the following examples.

Example 1 Zinc fluoride was added as a scavenger to a synthetic calcium fluoride raw material at about 0.1 mol% with respect to calcium fluoride and mixed. Next, this mixture was placed in a crucible of a refining furnace shown in FIG.
After heating to 360 ° C. to melt the raw material, the crucible was lowered and gradually cooled to crystallize the raw material. The upper part of the crystallized calcium fluoride corresponding to the upper part of the crucible was removed by 1 mm in thickness.

Next, the crystal block was placed in a crucible for a single crystal pulling furnace shown in FIG. The furnace was evacuated to heat the crucible. Vacuum pressure of 6 × 10 ^-4 Torr,
After maintaining the temperature at 200 ° C. for 5 hours, methane tetrafluoride was injected into a 0.2 atmosphere furnace. Inject Ar further,
The total pressure was adjusted to 1 atm. Then, the temperature of the crucible was slowly increased to 800 ° C. From that point, Ar was passed through the furnace at a flow rate of 500 cc / min.
The atmosphere in the furnace was gradually replaced with an Ar atmosphere. The furnace pressure at that time was adjusted to 1 atm.

The heating was continued and the temperature was raised to 1380 ° C. to completely melt the raw materials. The crucible and the seed crystal were rotated, and the seed crystal was slowly lowered and brought into contact with the melt surface. Adjust the output of the heater while observing from the viewing window,
The seed crystal was raised at a speed of 5 mm / h to grow a single crystal.

Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace and 0.1% by weight of zinc fluoride were charged. Evacuate the furnace and raise the temperature of the crucible from room temperature to 900 ° C
At a rate of 100 ° C./h for 20 hours at 900 ° C.
Held. Then, the temperature was lowered at a rate of 6 ° C./h and cooled to room temperature.

The thus obtained calcium fluoride single crystal was cut and polished to obtain a disk having a thickness of 10 mm, and the transmission spectrum and the dislocation density were measured. A part is shown in Table 1 and FIG.

The dislocation density was measured by immersing a sample in diluted nitric acid, etching the surface, and counting the number of etch pits generated at that time.

The measurement results are shown in Table 1 and FIG.

Example 2 Zinc fluoride as a scavenger was added to a high purity synthetic calcium fluoride raw material at about 0.1 mol% with respect to calcium fluoride and mixed.
Next, the mixture was placed in a crucible of a refining furnace shown in FIG. 3 and heated to 1360 ° C. to dissolve the raw material, and then the crucible was lowered and gradually cooled to crystallize the raw material. The upper part of the crystallized calcium fluoride corresponding to the upper part of the crucible was removed by 1 mm in thickness.

Next, the above crystal block was put together with a scavenger in a crucible of a single crystal pulling furnace shown in FIG. The scavenger added zinc fluoride in an amount of 0.01 mol% based on the calcium fluoride crystal block. The furnace was evacuated to heat the crucible. After maintaining the vacuum degree at 6 × 10 ⁻⁴ Torr and the temperature at 200 ° C. for 5 hours, Ar was injected to adjust the whole pressure to 1 atm. The heating was continued and the temperature was raised to 1380 ° C. to completely melt the raw materials. The crucible and the seed crystal were rotated, and the seed crystal was slowly lowered and brought into contact with the melt surface.
The output of the heater was adjusted while observing through the viewing window, and the seed crystal was raised at a speed of 5 mm / h to grow a single crystal.

Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace and 0.1% by weight of zinc fluoride were put. Evacuate the furnace and raise the temperature of the crucible from room temperature to 900 ° C
At a rate of 100 ° C./h for 20 hours at 900 ° C.
Held. Then, the temperature was lowered at a rate of 6 ° C./h and cooled to room temperature.

The transmission spectrum and dislocation density of the calcium fluoride single crystal thus obtained were the same as in Example 1.

Comparative Example 1 A calcium fluoride single crystal was produced in the same manner as in Example 2 except that the Bridgman method was used as the growth method, and using the same raw materials as in Examples 1 and 2. It was found that the transmittance of the obtained crystal was worse than that of Examples 1 and 2, and the dislocation density was extremely large.

[0059]

[Table 1] Example 3 In this example, the heating temperature during dehydration was investigated by changing the temperature in various ways. Table 2 shows the results.

As a result, it was found that a crystal having a high transmittance and a low dislocation density can be produced by setting the heating temperature to a suitable temperature.

[0061]

[Table 2] Example 4 In this example, the effect of the amount of solid scavenger added was examined. Table 3 shows the results. As a result, it was found that a crystal having high transmittance and low dislocation density can be produced by setting the addition amount of the solid scavenger to a suitable addition amount.

[0062]

[Table 3] Example 5 In this example, the effect of temperature upon heating in a reactive gas atmosphere was examined. Table 4 shows the results. As a result, it was found that a crystal having a high transmittance and a low dislocation density can be produced by setting a suitable temperature.

[0063]

[Table 4]

[0064]

According to the present invention, it is possible to provide a fluoride crystal having excellent optical properties and low dislocation density. As a result, it is possible to provide stable and reliable laser optical components and various optical elements.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of Examples and Comparative Examples.

FIG. 2 is a flowchart showing steps up to the production of an optical component.

FIG. 3 is a sectional view showing a refining furnace for a fluoride raw material.

FIG. 4 is a sectional view showing a single crystal growth furnace.

FIG. 5 is a sectional view showing an annealing furnace.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Growth furnace chamber, 2 Heating cylinder, 3 Heater, 4 Crucible, 5 Vacuum pump, 6a Reactive gas source, 6b Inert gas source, 7 Raw material melt (fluoride raw material), 8 crystals, 9 seed crystals, 10 Pull-up shaft, 11 viewing window, 12 exhaust port, 13a, 13b valve, 301 refining furnace chamber, 302 heat insulator, 303 heater, 304 crucible, 305 fluoride raw material, 501 annealing furnace chamber, 502 heat insulator, 503 heater, 504 crucible, 505 fluoride crystal.

Claims (14)

[Claims] 1. A step of charging a fluoride raw material into a growth furnace, a step of dissolving the fluoride raw material in an inert gas atmosphere, and a step of growing a crystal by immersing a seed crystal in a melt and pulling it up. A method for producing a fluoride crystal, which is performed in order. 2. The method for producing a fluoride crystal according to claim 1, wherein, after the charging step, a step of heating the fluoride raw material in a reactive gas atmosphere is performed. 3. In the heating step, before heating to a temperature lower by 50 to 200 ° C. than a melting point of the fluoride raw material,
100 to 3 in a vacuum atmosphere or an inert gas atmosphere
The method for producing a fluoride crystal according to claim 1, wherein the heating is performed at a temperature of 00 ° C. 4. The method according to claim 2, wherein the reactive gas is a fluorocarbon-based gas. 5. The carbon fluoride gas includes methane tetrafluoride (CF ₄ ), methane trifluoride (CHF ₃ ), methane difluoride (CH ₂ F ₂ ), and ethane hexafluoride (C ₂ F _6). 5.) The method according to claim 4, wherein propane octafluoride (C ₃ F ₈ ) is used. 6. The crystal production method according to claim 1, wherein the fluoride raw material is calcium fluoride, barium fluoride or magnesium fluoride. 7. A step of charging a fluoride raw material together with a solid scavenger of fluoride into a growth furnace, a step of dissolving the fluoride raw material in an inert gas atmosphere or a vacuum atmosphere after the charging, and converting a seed crystal into a melt. A method of growing a crystal by dipping and then pulling up the crystal in order. 8. The method for producing a fluoride crystal according to claim 6, wherein heating is performed at a temperature of 100 to 300 ° C. in a vacuum atmosphere or an inert gas atmosphere before melting. 9. The fluoride used as a solid scavenger is one or more of lead fluoride, zinc fluoride, cadmium fluoride, manganese fluoride, bismuth fluoride, sodium fluoride, and lithium fluoride. The method for producing a crystal according to claim 7 or 8, wherein 10. The solid scavenger is charged with 0.1% of the raw material.
The method for producing a crystal according to any one of claims 7 to 9, wherein 0.001 mol% or less is added in an amount of 0.001 mol% or less. 11. The fluoride raw material is calcium fluoride,
The crystal production method according to any one of claims 7 to 10, wherein the crystal is barium fluoride or magnesium fluoride. 12. A fluoride crystal produced by the method for producing a crystal according to claim 1. 13. A fluoride crystal produced by the method for producing a crystal according to claim 7. Description: 14. An optical component formed by molding the fluoride crystal according to claim 12 into a predetermined shape.