Method for producing high-purity aluminum fluoride and method for producing fluoride single crystal

Abstract

To provide a method for producing high-purity aluminum fluoride suitable for use as an optical material.SOLUTION: A method for producing high-purity aluminum fluoride comprises a step of sublimating a raw aluminum fluoride in a first crucible under reduced pressure and condensing the sublimated raw aluminum fluoride in a second crucible.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing high-purity aluminum fluoride and a method for producing a fluoride single crystal. More specifically, the present invention relates to a method for producing high-purity aluminum fluoride powder purified through a gas phase state, and a method for producing a fluoride single crystal using the aluminum fluoride powder produced by the method as a raw material. [Background technology]

[0002] In the field of optical materials, calcium fluoride (CaF2) crystals, lithium fluoride (LiF) crystals, etc. are known to transmit a wide range of light from ultraviolet to infrared, and are expected to be used as optical materials for lenses, etc. in the wavelength range of 160 nm or less. Therefore, methods for purifying these fluoride single crystals individually are being researched.

[0003] On the other hand, although aluminum fluoride (AlF3) also has the property of transmitting ultraviolet light, the use of a single crystal of fluoride AlF3 alone as an optical material is not generally practiced, and therefore, sufficient research has not yet been conducted into methods for purifying AlF3 alone. For example, Patent Document 1 discloses a method for producing high-purity aluminum fluoride for use as a component of semiconductor manufacturing equipment. In this method, aluminum fluoride is heated and sublimated in a non-oxidizing atmosphere containing hydrogen chloride, and then re-aggregated in a low-temperature region. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-091832 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the above-mentioned Patent Document 1, the purpose is merely to remove impurity elements (metal impurities) such as iron (Fe), sodium (Na), calcium (Ca), nickel (Ni), and silicon (Si) from aluminum fluoride in consideration of its use as a component of semiconductor manufacturing equipment. Therefore, it cannot be said that oxygen (O) and hydrogen (H) are sufficiently removed in order to use aluminum fluoride purified according to this document as an optical material. Furthermore, because the method of this document uses hydrogen chloride, there is a risk that chlorine (Cl) and hydrogen (H) may be mixed into the reaggregated aluminum fluoride.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing high-purity aluminum fluoride suitable for use as an optical material. Another object of the present invention is to provide a method for producing a fluoride single crystal using the high-purity aluminum fluoride. [Means for solving the problem]

[0007] As a result of extensive research conducted by the inventors to solve the above-mentioned problems, they discovered that it is important to carry out the sublimation and coagulation of aluminum fluoride as a raw material under reduced pressure, which led to the completion of the present invention.

[0008] That is, the present invention includes, for example, the following aspects [1] to [5]. [1] A method for producing high-purity aluminum fluoride, comprising the step of sublimating raw aluminum fluoride in a first crucible under reduced pressure in a second crucible. [2] The above process is carried out under the conditions of a temperature of the first crucible of 910 to 1130°C, a temperature of the second crucible of 570 to 770°C, and a pressure of 5 × 10 -4 ~1×10 3 The method of producing a semiconductor device according to [1], wherein the semiconductor device is a semiconductor substrate. [3] The method according to [1] or [2], wherein the purity of the raw material aluminum fluoride is 99.9% or more. [4] A method for producing a fluoride single crystal, comprising a step of producing a fluoride single crystal from a raw material containing high-purity aluminum fluoride produced by the production method according to any one of [1] to [3]. [5] The method according to [4], wherein the fluoride single crystal is LiCaAlF6. [Effects of the Invention]

[0009] The present invention provides a method for producing high-purity aluminum fluoride suitable for use as an optical material. The present invention purifies aluminum fluoride powder raw material alone using a vapor phase method, and the present invention can sufficiently remove oxygen (O) and hydrogen (H) from the aluminum fluoride powder raw material. The present invention also provides a method for producing a fluoride single crystal using the high-purity aluminum fluoride. As described above, the present invention uses an aluminum fluoride powder raw material from which oxygen impurities and hydrogen impurities have been sufficiently removed, and therefore, the present invention can grow a high-quality fluoride single crystal. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an apparatus for producing high-purity aluminum fluoride. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an apparatus for producing a fluoride single crystal. [Figure 3] FIG. 3 shows a powder X-ray diffraction chart of the residue in the first crucible. [Figure 4] FIG. 4 shows a powder X-ray diffraction chart of the material collected in the second crucible. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the drawings are schematic, and for example, the relationship between thickness and planar dimensions may differ from the actual relationship. Furthermore, the embodiments shown below exemplify configurations for embodying the technical idea of ​​the present disclosure, and the technical idea of ​​the present disclosure is not limited to the materials, shapes, structures, etc. of the components described below.

[0012] <Method for producing high-purity aluminum fluoride> A method for producing high-purity aluminum fluoride includes a step of sublimating raw aluminum fluoride (AlF3) in a first crucible under reduced pressure in a second crucible. This production method can also be described as a method for purifying raw AlF3 by sublimation. The sublimation method is a technique in which raw AlF3 containing impurities is heated to a high temperature, and then the AlF3 is sublimated in a low-temperature area using the vapor pressure difference between the impurities and AlF3, thereby separating and removing the impurities from the AlF3. In the above manufacturing method, the AlF3 used as a raw material is called raw AlF3, and the obtained AlF3 is called high-purity AlF3.

[0013] FIG. 1 is a schematic cross-sectional view showing an apparatus for producing high-purity aluminum fluoride. FIG. 1 shows the structure of a sublimation furnace, which is the production apparatus. The sublimation furnace includes a first crucible 2 (sublimation crucible) into which the raw material AlF3 is introduced and sublimated, a second crucible 3 (sublimation crucible) for sublimating the sublimated raw material, a cylindrical conduit 4 connecting these two crucibles, a crucible stand 7 for enabling these crucibles to be raised and lowered, a sublimation mechanism including a first heater 5 for heating the first crucible 2 and a second heater 6 for heating the second crucible 3, and a heat-retaining mechanism including a carbon heat-retaining material 1 covering the entire sublimation mechanism. The sublimation mechanism and heat-retaining mechanism, which are the main components, are housed in a sealed container (not shown). The sealed container can be placed in a reduced-pressure environment (vacuum state), and the sublimation furnace is provided with a pressure-reducing mechanism (not shown) for this purpose. The first crucible 2, the conduit portion 4, and the second crucible 3 are integrated together and placed on a crucible stand 7 equipped with a movement mechanism.

[0014] In principle, it is also possible to place the first crucible 2 below the sublimation furnace and the second crucible 3 above the sublimation furnace (the upper side of the sublimation furnace is at a low temperature and the lower side is at a high temperature). However, in this case, there is a risk that the sublimed material may fall into the first crucible 2, so the crucible arrangement shown in Figure 1 is preferred.

[0015] The first crucible 2, the second crucible 3, the first heater 5, and the second heater 6 can also be referred to as the upper (high temperature part) crucible, the lower (low temperature part) crucible, the upper (high temperature part) heater, and the lower (low temperature part) heater, respectively.

[0016] The raw material AlF3 introduced into the first crucible 2 is sublimated in the first crucible 2, which is heated by the first heater 5. The sublimated AlF3 moves through the conduit 4 and coagulates in the second crucible 3, which is heated by the second heater 6 to a temperature lower than that of the first crucible 2. This allows high-purity AlF3 to be obtained in the second crucible 3. From the viewpoints of cost and impurity control, it is preferable that the material of the crucible and the conduit be carbon.

[0017] The purity of the raw material AlF3 is preferably 99.9% or higher, more preferably 99.99% or higher, from the viewpoint of obtaining high-purity AlF3 for use in optical materials, and is preferably less than 99.999% from the viewpoint of cost. The raw material AlF3 may be in any form, such as powder, polycrystalline mass, or single crystal, but since sublimation occurs from the solid surface, powder is preferred from the viewpoint of shortening the sublimation time. On the other hand, the purity of the obtained high-purity AlF3 is preferably 99.999% or more from the viewpoint of use as an optical material. The upper limit of the purity of high-purity AlF3 is not particularly limited, but it can be substantially 100%, and may be less than 100%. Purity here refers to the remainder when the weight percentage (%) of metal chemicals other than Al is subtracted from 100. For example, H2O and CO2 adsorbed to the target chemical (AlF3) are not taken into account when calculating purity.

[0018] The pressure inside the sealed vessel in which the sublimation and coagulation mechanism and the heat retention mechanism are installed, that is, the pressure inside the sublimation furnace when the above-mentioned sublimation and coagulation process is carried out, is set to 1×10 3 From the viewpoint of further improving the yield, the pressure is preferably 600 Pa or less, or more preferably 10 Pa or less. It is possible to sublimate AlF3 alone under 1 atmosphere. However, this requires high-temperature processing, which is not desirable in terms of manufacturing costs. Furthermore, sublimation at high temperatures is undesirable because it increases the likelihood of raw material dissipation from the carbon crucible, reducing the sublimation recovery rate. On the other hand, under reduced pressure, sufficient vapor pressure for sublimation can be achieved without high temperatures (for example, even at temperatures below 1130°C). This allows sublimation to be performed in an atmosphere free of oxygen and water. Furthermore, lowering the sublimation temperature suppresses the generation of environmental impurities, such as those generated by the manufacturing equipment itself, thereby preventing the incorporation of such impurities into AlF3 during sublimation. Lowering the sublimation temperature is also advantageous from the perspective of manufacturing costs. Although there is no particular lower limit for the pressure conditions, from the viewpoint of manufacturing costs, etc., it is set to 5×10 -4 Pa, can be 1 x 10 -3 It may be Pa.

[0019] A temperature gradient is created between the first crucible 2 and the second crucible 3 by independent first heater 5 and second heater 6. The heaters may be carbon resistance heaters. The power setting values ​​(SP, 0-100%) of these heaters are set by a PID programmer, and the heater current and voltage are feedback-controlled while being measured by a power converter. The temperature is measured by inserting thermocouples into the central axes of the first crucible 2 and the second crucible 3 (cylindrical chambers).

[0020] In order to sublimate AlF3, it is preferable to minimize the temperature gradient inside the first crucible 2. Furthermore, it is also preferable to minimize the temperature gradient from the first crucible 2 to the conduit part 4 so that the sublimated AlF3 does not solidify. It is preferable that the lowest part (the crucible support 7 side) inside the second crucible 3 has the lowest temperature. This makes it easy to prevent the sublimated AlF 3 from re-sublimating inside the second crucible 3.

[0021] The temperature of the first crucible 2 can be adjusted according to the pressure inside the sealed container in order to obtain a vapor pressure sufficient to sublimate AlF. From the viewpoint of sublimating AlF and improving the recovery rate of the sublimated AlF, the temperature of the first crucible 2 is preferably 910 to 1130°C, more preferably 1020 to 1130°C, and even more preferably 1050 to 1100°C. The temperature of the second crucible 3 can be adjusted according to the pressure inside the sealed container in consideration of sublimating AlF. The temperature of the second crucible 3 is preferably 570 to 770°C, more preferably 650 to 770°C, and even more preferably 695 to 750°C, from the viewpoint of sublimating AlF at a temperature lower than that of the first crucible 2 and improving the recovery rate of the sublimated AlF (for example, from the viewpoint of suppressing a decrease in the recovery rate due to sublimation in the conduit portion 4).

[0022] The time required for the sublimation and coagulation steps is not particularly limited as it varies depending on the amount of raw material AlF3, etc., but can be set to, for example, 24 to 72 hours (1 to 3 days). After the steps are completed, the entire apparatus is cooled to room temperature, and high-purity AlF3 is recovered from the second crucible 3.

[0023] <Method for producing fluoride single crystals> The method for producing a fluoride single crystal includes a step of producing a fluoride single crystal from a raw material containing high-purity aluminum fluoride produced by the method for producing high-purity aluminum fluoride described above. An example of a fluoride single crystal (composite fluoride single crystal) is LiCaAlF6 single crystal. Fluoride single crystals can be grown by a single crystal growth method from a melt, such as the Czochralski method.

[0024] 2 is a schematic cross-sectional view showing a manufacturing apparatus for a fluoride single crystal using the high-frequency induction heating Czochralski method. The main components of this manufacturing apparatus are a crucible 10 covered with a heat insulating material 8 and containing raw material 9, a high-frequency heating coil 11 provided outside the heat insulating material 8 for heating the raw material 9 in the crucible 10, and a pulling shaft 13 provided above the crucible 10 so as to be movable up and down, holding a seed crystal 12 at its tip and rotating in the direction of the arrow. These main components are housed in a sealed container (not shown). The gas atmosphere inside the sealed container can be adjusted, and it is also possible to create a reduced-pressure environment (vacuum state).

[0025] A method for producing a fluoride single crystal will now be described. First, the raw material 9 is charged into the crucible 10, and then a high frequency current is passed through the high frequency heating coil 11 to generate heat, and the raw material 9 in the crucible 10 is heated to a temperature equal to or higher than its melting point and melted. Next, the pulling shaft 13 is lowered to bring the seed crystal 12 into contact with the center of the molten raw material melt. The high-frequency power is then adjusted to gradually solidify the raw material melt around the seed crystal 12, while simultaneously rotating and pulling the seed crystal 12. This operation is performed continuously. A single crystal is produced in this manner. At the beginning of pulling, the rotation speed and pulling rate of the seed crystal 12 are controlled so that the outer diameter of the grown single crystal gradually increases, forming a shoulder directly below the seed crystal 12. Once the outer diameter of the grown single crystal reaches the desired diameter, the rotation speed and pulling rate of the seed crystal are controlled to form a body. Once the crystal has grown to the desired size, it is separated from the melt and slowly cooled to room temperature.

[0026] The raw materials include high-purity AlF3, and if growing a LiCaAlF6 single crystal, for example, they may contain LiF powder purified as another fluoride, CaF2 powder of the grade used in semiconductor manufacturing equipment, etc. When growing a single crystal, each raw material powder is melted once and then solidified before use.

[0027] The atmosphere for growing a fluoride single crystal can be adjusted depending on the type of fluoride single crystal desired. For example, to grow a LiCaAlF6 single crystal, an inert atmosphere or an atmosphere containing a fluoride material can be used.

[0028] The obtained fluoride single crystal can be suitably used as an optical material such as a lens. [Example]

[0029] The present invention will be described below with reference to examples, but the present invention is not limited to the contents of these examples.

[0030] <Production of high-purity aluminum fluoride> Example 1 High-purity aluminum fluoride was produced using the sublimation furnace shown in Figure 1. The first crucible, the conduit tube, and the second crucible were all made of carbon. First, raw material AlF3 powder with a purity of 99.99% was filled into the first crucible. Then, a vacuum atmosphere (1 to 10 Pa) was created inside the sealed container using a pressure reduction mechanism, and the first crucible was heated to 1030°C (±10°C) and the second crucible to 730°C (±10°C) using the first and second carbon resistance heaters. After maintaining this state for 12 hours, they were cooled to room temperature, and a precipitate was confirmed in the second crucible. As described below, this precipitate was AlF3. In this way, high-purity AlF3 was produced.

[0031] Table 1 shows the set temperature of the first crucible, the set temperature of the second crucible, the pressure inside the furnace during sublimation, and the recovery rate (weight of recovered sublimed material / filled amount of raw material AlF3 powder x 100).

[0032] (Other Examples and Reference Examples) High-purity aluminum fluoride was produced in the same manner as in Example 1, except that the production conditions were changed as shown in Table 1. Note that Reference Example 1 was carried out under 1 atmosphere without creating a vacuum atmosphere inside the sealed container.

[0033] Figure 3 shows the powder X-ray diffraction chart of the residue (a small amount of white powder remaining at the bottom) in the first crucible. From Figure 3, it is speculated that Al2O3 based on oxygen impurities in the raw material AlF3 and Al2OC based on Al2O3 and the crucible material remained in the first crucible. Also, Figure 4 shows the powder X-ray diffraction chart of the recovered material in the second crucible. Figure 4 shows that purified AlF3 (high-purity AlF3) was separated in the second crucible. These facts indicate that by sublimating and condensing the raw material AlF3 under reduced pressure, the separation of the oxide Al2O3 impurities contained in the raw material AlF3 was achieved. Here, it is speculated that in addition to the chemical reaction (Equation 1), the chemical reaction (Equation 2) also occurred. High-temperature part: Al2O3(s) + AlF3(g)↑ ⇒ High-temperature part: Al2O3(s) + Low-temperature part: AlF3(s)↓ ···(Equation 1) High-temperature part: Al2O3(s) + 3C(s / g) ⇒ High-temperature part: Al2OC(s) + 2CO(g)↑ System outside ···(Equation 2)

[0034]

Table 1

[0035] <Manufacture of LiCaAlF6 single crystal> (Example I) Using the manufacturing apparatus shown in Figure 2, the LiCaAlF6 single crystal was manufactured. LiF powder with a purity of 99.99%, CaF2 powder with a purity of 99.99%, and the high-purity AlF3 powder obtained in Example 1 were weighed and mixed so that Li:Ca:Al = 1.05:1:1.05 (atomic ratio). Then, the mixed powder was melted in a melting furnace in an atmosphere containing fluoride materials to prepare a molten raw material. The carbon crucible filled with this molten raw material was placed in a Czochralski furnace, the surrounding area was kept warm with carbon furnace material, and an induction coil was placed around that. A high-frequency current was passed through the coil to perform induction heating. After the raw material was melted, a seed crystal was attached and a single crystal was grown at a pulling rate of 0.5 mm / h. After pulling the crystal the specified distance, it was separated from the melt and cooled to room temperature. The resulting crystal had a straight body diameter of 50 mm and a length of 35 mm.

[0036] (Comparative example I) A LiCaAlF6 single crystal was produced in the same manner as in Example I, except that the raw AlF3 powder used in producing high-purity AlF3 with a purity of 99.99% was used as the AlF3 powder. The obtained crystal had a straight body diameter of 50 mm and a length of 30 mm.

[0037] The crystals obtained in Example I and Comparative Example I were cut into 10 mm thick pieces, both surfaces were mirror-polished, and the inside of the crystal was observed while irradiating the polished surface with white light. As a result, it was confirmed that the amount of scattering material inside the crystal was reduced in Example I compared to Comparative Example I. [Explanation of symbols]

[0038] 1...heat insulating material, 2...first crucible, 3...second crucible, 4...conduit portion, 5...first heater, 6...second heater, 7...crucible stand, 8...heat insulating material, 9...raw material, 10...crucible, 11...high frequency heating coil, 12...seed crystal, 13...pulling shaft.

Claims

1. A method for producing high-purity aluminum fluoride, comprising the step of sublimating raw aluminum fluoride in a first crucible under reduced pressure in a second crucible.

2. The process is carried out under the following conditions: the temperature of the first crucible is 910 to 1130°C, the temperature of the second crucible is 570 to 770°C, and the pressure condition is 5 × 10 －４ ~1 x 10 ３ The process according to claim 1, wherein the process is carried out as follows:

3. 2. The method according to claim 1, wherein the purity of the raw material aluminum fluoride is 99.9% or more.

4. A method for producing a fluoride single crystal, comprising the step of producing a fluoride single crystal from a raw material containing high-purity aluminum fluoride produced by the production method according to any one of claims 1 to 3.

5. The fluoride single crystal is LiCaAlF ６ The method according to claim 4, wherein

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