JP4380283B2 - Crucible and method for growing calcium fluoride single crystal using crucible - Google Patents

Description

The present invention relates to a crucible for growing a single crystal and a method for growing a calcium fluoride (CaF ₂ ) single crystal using the crucible.

  Conventionally, a vertical Bridgman (hereinafter abbreviated as VB) method is known as a method for producing a single crystal of calcium fluoride. The VB method is a method in which a crucible containing a calcium fluoride raw material is moved vertically in a furnace of a crystal growth apparatus having a temperature gradient around the melting point of the calcium fluoride raw material. First, the calcium fluoride raw material is melted by raising the crucible, and then the crucible is gradually lowered (ie, cooled) to gradually crystallize from the lower part to the upper part to grow a single crystal of calcium fluoride. It is a method to make it. There is also a method of melting the calcium fluoride raw material only by controlling the temperature of the heater of the crystal growth apparatus without raising the temperature of the crucible.

An example of the crucible used in such a VB method is the crucible disclosed in Patent Document 1. The crucible of Patent Document 1 is composed of a raw material container and a seed crystal container arranged below the raw material container. The crystal orientation can be made constant by growing the single crystal using the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion as a seed. That is, when all the seed crystals in the seed crystal accommodating portion are melted, the orientation control cannot be performed at all. On the other hand, it has been found that the desired result cannot be obtained even when only the seed crystal in the vicinity of the upper end of the seed crystal accommodating portion is melted. From such a background, it is empirically known that when the seed crystal up to the vicinity of the middle part of the seed crystal accommodating part is melted, the subsequent growth of the single crystal is suitably performed. Therefore, conventionally, the temperature of the heater of the crystal growth apparatus is controlled, and adjustment is performed so as to melt the seed crystal up to the vicinity of the intermediate portion of the seed crystal accommodating portion.
Japanese Patent Laid-Open No. 10-265296

  However, when the crucible cooling is controlled by the difference in material components, the degree of vacuum in the crystal growth furnace, or the cooling water, the seed crystal in the seed crystal accommodating portion is always brought to the desired position depending on the temperature of the cooling water. It was difficult to melt. For this reason, trial and error had to be repeated in order to melt the seed crystal in the seed crystal accommodating portion to a desired position and obtain a desired single crystal.

  This invention is made | formed in view of the said situation, and it aims at providing the crucible which can determine easily the boundary position of the fusion | melting part of a seed crystal, and an unmelted part.

  In order to achieve the above object, a crucible according to the present invention is a crucible for growing a single crystal, a bottomed seed crystal containing portion that extends in the vertical direction that contains a seed crystal, and a raw material that contains a single crystal raw material A storage unit, which is disposed above the seed crystal storage unit and includes a raw material storage unit communicating with the seed crystal storage unit, and a temperature detecting means for detecting an internal temperature of the seed crystal storage unit. Yes.

  In addition, as a method for growing a calcium fluoride single crystal using the above-described crucible, a step of preparing a crucible, a step of storing a seed crystal of calcium fluoride in a seed crystal storage unit, and a calcium fluoride in a raw material storage unit And a crucible placed in a furnace of a crystal growth apparatus in which the inside is heated with a predetermined temperature gradient in the vertical direction, and the crucible is heated and the raw material received in the raw material storage part And gradually melting the seed crystal accommodated in the seed crystal accommodating portion from above to below, detecting the internal temperature of the seed crystal accommodating portion by the temperature detecting means during heating of the crucible, and by the temperature detecting means Based on the detected internal temperature of the seed crystal accommodating part, the boundary position between the melted part and the unmelted part of the seed crystal accommodated in the seed crystal accommodating part is a predetermined height from the bottom end of the seed crystal accommodating part. When it is determined that it is between the first position and the second position above the first position by a predetermined height, the heating is terminated, the cooling is started, and the single crystal is grown. Is included.

  According to this, since the internal temperature of the seed crystal accommodating part can be detected by the temperature detecting means, based on the detected internal temperature, the melted part of the seed crystal accommodated in the seed crystal accommodating part and the unmelted part The boundary position with the part can be easily determined.

  As the temperature detecting means, there is a thermocouple, and it is preferable that the thermocouple is disposed in the vicinity of the side surface of the seed crystal accommodating portion. Thereby, the temperature of the seed crystal in the vicinity where the thermocouple is arranged can be easily measured.

  Further, it is preferable that there are a plurality of thermocouples and they are arranged separately from each other in the vertical direction. Thereby, the temperature gradient of the seed crystal accommodating portion can be understood based on the arrangement of each thermocouple and the temperature measured from each thermocouple.

  Here, one of the two thermocouples is disposed at an upper position from the height position of the bottom end of the seed crystal accommodating part by a height corresponding to 25 to 50% of the depth of the seed crystal accommodating part, It is effective that the other is disposed at a position above the height position of the bottom end of the seed crystal accommodating portion by a height corresponding to 60 to 80% of the depth of the seed crystal accommodating portion. In this case, when the seed crystal is melted within a range sandwiched between the two thermocouples, the subsequent growth of the single crystal is suitably performed. And whether it is in such a state can be easily judged by whether the melting | fusing point of a seed crystal exists between the temperature measured from two thermocouples.

  In the growing method of the calcium fluoride single crystal, the first position is a position above the bottom end of the seed crystal accommodating part by a height corresponding to 25% of the depth of the seed crystal accommodating part, and the second position However, it is effective that the position is higher than the bottom end of the seed crystal accommodating part by a height corresponding to 80% of the depth of the seed crystal accommodating part. When the boundary position between the melted portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion comes within the range between the first position and the second position, the heating is finished, and the cooling is started to simply This is because when the crystal is grown, a single crystal of calcium fluoride is preferably grown along the crystal plane of the seed crystal of the unmelted portion.

  According to the crucible including the temperature detecting means as described above, the boundary position between the molten portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion can be easily determined. Crystals can be grown more reliably.

  Hereinafter, an embodiment of a crucible for growing a calcium fluoride single crystal according to the present invention will be described with reference to the drawings. In the drawings to be referred to, FIG. 1 is a schematic view showing a schematic structure of a crystal growth apparatus provided with a crucible according to the present embodiment, and FIG. 2 is a cross-sectional view showing the structure of the crucible according to the present embodiment shown in FIG. .

  As shown in FIG. 1, the crucible 1 according to this embodiment is disposed inside the main heater 2AA and the sub heater 2AB in a vacuum VB furnace 2 as a crystal growth apparatus 100 by the VB method. The crucible 1 is melted at a very low speed through the shaft 2B to melt the calcium fluoride raw material M, and then lowered at a very low speed through the shaft 2B to cool the raw material M. This is for growing the seed crystal S made of a single crystal of calcium fluoride into a single crystal along, for example, the (111) oriented crystal plane.

The inside of the vacuum VB furnace 2 is depressurized to 10 ⁻⁴ Pa or less by the vacuum pump 2C, and the upper part in the vacuum VB furnace 2 is heated to, for example, about 1600 ° C. by the main heater 2AA. The lower part in the vacuum VB furnace 2 is heated by the auxiliary heater 2AB. In the shaft 2B of the vacuum VB furnace 2, cooling water passages 2D and 2E communicating with a water jacket 1G described later are formed.

  Here, as shown in FIG. 2, the crucible 1 of the present embodiment includes a crucible body 1A and a lid member 1B that covers an opening of the crucible body 1A. The crucible body 1A is made of a heat-resistant material that can increase the smoothness of the inner surface and reduce the wettability with the calcium fluoride raw material M, that is, a high-purity carbon material. The lid member 1B is also made of a heat-resistant high-purity carbon material.

  The crucible body 1A is formed with a large-diameter raw material container 1D in which a calcium fluoride raw material M (see FIG. 1) is accommodated. Further, a small-diameter seed crystal accommodating portion 1E that accommodates, for example, a cylindrical seed crystal S (see FIG. 1) is formed as a straight circular hole from the center of the crucible body 1A to the vicinity of the bottom. A cone surface 1F that forms the bottom of the raw material container 1D is formed between the raw material main part 1D and the seed crystal container 1E. Further, the boundary surface between the raw material main portion 1D and the seed crystal accommodating portion 1E is the upper end surface of the seed crystal S inserted into the seed crystal accommodating portion 1E, and is determined by the length of the seed crystal S.

  At the bottom of the crucible main body 1A, a water jacket 1G is formed around the lower portion of the seed crystal housing portion 1E to forcibly cool the lower portion of the seed crystal S (see FIG. 1) housed in the seed crystal housing portion 1E to prevent melting. Is formed. The water jacket 1G communicates with the cooling water passages 2D and 2E (see FIG. 1) in the shaft 2B to form a cooling water circulation passage.

  Here, a concave curved surface 1J is formed at the boundary between the wall surface 1H of the raw material storage portion 1D and the cone surface 1F, and the wall surface 1H of the raw material storage portion 1D and the cone surface 1F are smoothly continuous via the concave curved surface 1J. is doing. Further, a convex curved surface 1L is formed between the cone surface 1F and the wall surface 1K of the seed crystal housing portion 1E, and the cone surface 1F and the wall surface 1K of the seed crystal housing portion 1E are smoothly connected via the convex curved surface 1L. is doing.

  The wall surface 1H of the raw material container 1D is formed straight, and the inner diameter between the wall surfaces 1H is set to 250 mm, for example. Further, the inner diameter of the seed crystal accommodating portion 1E is set to 20 mm, for example.

  Here, in the crucible 1 of the present embodiment, the thermocouple 1CA and the thermocouple 1CB are vertically positioned in the vicinity of the side surface of the seed crystal housing portion 1E as temperature detecting means for measuring the internal temperature of the seed crystal housing portion. Are separated from each other. In this case, the high-purity carbon material, which is the material of the crucible body 1A, has a porous structure, so that it can be easily processed, and a hole for inserting a thermocouple that does not affect the temperature in the crucible is suitably formed. Can do. And since a high purity carbon material does not react with the metal normally used for a thermocouple, a thermocouple can be installed in the crucible 1 suitably.

  At this time, the thermocouples 1CA and 1CB are preferably thermocouples using a platinum rhodium alloy. This is because such a thermocouple is suitable for high-precision measurement at a high temperature and can suitably measure the internal temperature of the seed crystal accommodating portion. Examples of the thermocouple using platinum rhodium include JIS-B type (+ leg: platinum rhodium alloy containing 30% rhodium,-leg: platinum rhodium alloy containing 6% rhodium).

  With such two thermocouples, the temperature gradient of the seed crystal accommodating portion 1E is derived based on the positions where the thermocouples 1CA and 1CB are installed and the temperatures measured from the thermocouples 1CA and 1CB. By determining the position indicating the melting point of the seed crystal S, the boundary position between the molten portion and the unmelted portion of the seed crystal S in the seed crystal housing portion 1E can be easily determined.

  Furthermore, the thermocouple 1CA is arranged at a position higher than the height of the bottom end of the seed crystal accommodating portion 1E by a height corresponding to 25 to 50% of the depth of the crystal accommodating portion, and the thermocouple 1CB is It is preferable that the height corresponding to 60 to 80% of the depth of the seed crystal accommodating part 1E is arranged at a position above the height of the bottom end of the seed crystal accommodating part 1E. In this case, when the seed crystal is melted within a range sandwiched between the two locations of the seed crystal accommodating portion 1E, the temperature of which is measured by the thermocouple 1CA and the thermocouple 1CB, the subsequent growth of the single crystal is suitably performed. . Whether or not such a state exists is whether or not the melting point of the seed crystal S exists between the temperatures measured from the thermocouple 1CA and the thermocouple 1CB without deriving the temperature gradient of the seed crystal container 1E. It can be easily judged by just

  Next, a method for growing a calcium fluoride single crystal using the crucible 1 will be described.

  First, the crucible 1 is prepared, the lid member 1B is removed, and the seed crystal S made of calcium fluoride is accommodated in the seed crystal accommodating portion 1E of the crucible 1. Thereafter, the raw material M of calcium fluoride is housed in the raw material container 1D, and the raw material container 1D of the crucible body 1A is closed with the lid member 1B. At this time, the crucible 1 is lowered by the shaft 2B.

Next, the inside of the vacuum VB furnace 2 is depressurized to 10 ⁻⁴ Pa or less, and the main heater 2AA is heated to around 1600 ° C. Then, the crucible 1 is raised at a slow speed of about 10 mm / h by the shaft 2B and held at the raised position for about 10 hours.

  At this time, the internal temperature of the seed crystal accommodating portion 1E is measured by the thermocouple 1CA and the thermocouple 1CB, and based on this internal temperature, the boundary position between the molten portion and the unmelted portion of the seed crystal S is the seed crystal accommodating portion 1E. When it is determined that it is within a predetermined height range from the bottom end of the crucible, the heating of the crucible 1 is terminated. The range of the predetermined height refers to the first position above the bottom end of the seed crystal accommodating part 1E by a height corresponding to 25% of the depth of the seed crystal accommodating part 1E, and the seed crystal accommodating part 1E. It is preferable that the height is equivalent to 80% of the depth and the range is sandwiched between the bottom end of the seed crystal accommodating portion 1E and the second position above. Further, the first position is above the bottom end of the seed crystal accommodating part 1E by a height corresponding to 40% of the depth of the seed crystal accommodating part 1E, and the second position is 70 of the depth of the seed crystal accommodating part 1E. It is more preferable that the height corresponding to% is above the bottom end of the seed crystal accommodating portion 1E, and the first position is the height corresponding to 50% of the depth of the seed crystal accommodating portion 1E. It is particularly preferable that the second position is above the bottom end of the seed crystal accommodating portion 1E by a height corresponding to 60% of the depth of the seed crystal accommodating portion 1E. This is because a single crystal of calcium fluoride is suitably grown along the crystal plane of the seed crystal of the unmelted portion.

  After the heating of the crucible 1 is finished, the crucible 1 is subsequently lowered at a very low speed of about 1.0 mm / h by the shaft 2B and held at the lowered position in the vacuum VB furnace 2 for about 5 hours. As a result, the molten calcium fluoride raw material M is cooled and grown into a single crystal along, for example, the (111) -oriented crystal surface of the seed crystal S, and the desired calcium fluoride single crystal is obtained.

  As mentioned above, although preferred embodiment of this invention was described in detail, it cannot be overemphasized that this invention is not limited to the said embodiment. The crucible 1 can be used not only for growing calcium fluoride single crystals but also for growing other single crystals.

  For example, in this embodiment, the raw material M of calcium fluoride is melted by raising the crucible 1 at a very low speed via the shaft 2B, but in the state where the crucible 1 is raised by the shaft 2B in advance, Even when the crucible 1 is heated by controlling the heater 2AA, the same action as in the present embodiment can be obtained.

  Further, if a thermocouple is further arranged in the vicinity of the side surface of the raw material container 1D, the internal temperature in the raw material container 1D can be measured. For example, the temperature of the main heater 2AA and the sub heater 2AB is adjusted. Easy to do. Moreover, if the thermometer is arrange | positioned to the water jacket 1G, the temperature control of a cooling water can be performed easily and the temperature which cools the lower part of the seed crystal S can be kept more constant.

  Further, when the boundary surface between the raw material storage portion 1D and the seed crystal storage portion 1E is below the convex curved surface 1L, a position near the surface sandwiched between the convex curved surface 1L and the wall surface 1K of the seed crystal storage portion 1E; Even if a thermocouple is disposed in the vicinity of the wall surface 1K of the seed crystal housing part 1E, the temperature gradient of the seed crystal housing part 1E can be derived.

  In the present embodiment, two thermocouples are arranged in the crucible 1, but the first position above the bottom end of the seed crystal accommodating part 1E by a height corresponding to 25% of the depth of the seed crystal accommodating part 1E. And the vicinity of the side surface of the seed crystal accommodating portion 1E within a range sandwiched between the bottom end of the seed crystal accommodating portion 1E and the second position above the height corresponding to 80% of the depth of the seed crystal accommodating portion 1E Only one thermocouple may be arranged at the position. In this case, when the temperature of the thermocouple reaches the melting point of the seed crystal S, the boundary position between the melted portion and the unmelted portion of the seed crystal S in the seed crystal housing portion 1E has reached the temperature measurement position of the thermocouple. Therefore, when heating of the crucible 1 is completed at this time, a single crystal of calcium fluoride is preferably grown along the crystal face of the seed crystal in the unmelted portion. In addition, the more preferable position of the 1st position and the 2nd position, The especially preferable position is the same as that of this embodiment.

  Examples of the present invention will be described below. The present invention is not limited to these examples.

Example 1
The crucible is made of high-purity carbon and has an outer diameter of 175 mm × height of 325 mm (including an inner diameter of φ10 mm × height of 100 mm), 50 mm above the bottom of the single crystal container, and from the bottom of the single crystal container A sample was prepared by attaching thermocouples (JIS-B type) each having a diameter of 3.5 mm and a length of 80 mm at three locations 95 mm above and near the raw material container.

FIG. 3 shows the relationship between the position of the crucible and the temperature of the thermocouple when the inside of the vacuum VB furnace is depressurized to 10 ⁻⁴ Pa or less and the crucible is heated by the main heater and the sub heater while the crucible is empty. . The crucible position 12 was initially in the position of FIG. 1 and the crucible was lowered over time. The temperature 10 of the main heater was maintained at 1500 ° C., the temperature 11 of the sub heater was raised to around 950 ° C., and then the temperature was adjusted. In this case, the temperature 13 of the thermocouple 50 mm above the bottom end of the single crystal housing portion, the temperature 14 of the thermocouple 95 mm above the bottom end of the single crystal housing portion, and the temperature of the thermocouple near the raw material housing portion It can be seen that the temperature 15, that is, three temperatures of the crucible, have a correlation with the temperature 10 of the main heater, the temperature 11 of the auxiliary heater, and the position 12 of the crucible.

  Now, a high-purity calcium fluoride single crystal (manufactured by Hitachi Chemical Co., Ltd., diameter φ10 mm × length 100 mm, lengthwise crystal orientation (111)) is accommodated in the above-mentioned crucible as a seed crystal in the seed crystal accommodating portion. Furthermore, high-purity calcium fluoride powder (manufactured by Stella Chemifa Co., Ltd.) was accommodated in the raw material container as a raw material for calcium fluoride. Moreover, high purity zinc fluoride powder (manufactured by Stella Chemifa Co., Ltd.) was mixed as an additive to the raw material of calcium fluoride. At this time, the position of the crucible was set to the lowest position.

Thereafter, the inside of the vacuum VB furnace was depressurized to 10 ⁻⁴ Pa or less, the main heater was heated to around 1500 ° C., the position of the crucible was raised at a speed of 10 mm / h, and held at the raised position for about 10 hours. At this time, the temperature of each part is measured with a thermocouple, and the boundary position between the molten portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion is a position 50 mm above the bottom end of the seed crystal accommodating portion. The main heater was stopped when it came, and the crucible position was lowered at a speed of 0.7 mm / h or less and held at the lowered position in the vacuum VB furnace for 5 hours. Thereafter, the inside of the vacuum VB furnace was cooled at a rate of 50 to 100 ° C./h. When the temperature in the crucible became 50 ° C. or lower, nitrogen was placed in a vacuum VB furnace until atmospheric pressure was reached, and the crystals in the crucible were taken out.

(Examples 2 to 6)
When the heating of the main heater and the sub heater is stopped, the boundary position between the molten portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion is set to a position 80 mm above the bottom end of the seed crystal accommodating portion. Except for the above, a calcium fluoride crystal of Example 2 was obtained in the same manner as Example 1. Further, when the heating of the main heater and the sub heater is stopped, the boundary position between the molten portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion is a position 70 mm above the bottom end of the seed crystal accommodating portion. Except that, calcium fluoride crystals of Example 3 were obtained in the same manner as Example 1. In addition, when the heating of the main heater and the sub heater is stopped, the boundary position between the melted portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion is a position 60 mm above the bottom end of the seed crystal accommodating portion. Except that, calcium fluoride crystals of Example 4 were obtained in the same manner as Example 1. In addition, when the heating of the main heater and the sub heater is stopped, the boundary position between the molten portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion is a position 40 mm above the bottom end of the seed crystal accommodating portion. Except that, calcium fluoride crystals of Example 5 were obtained in the same manner as Example 1. In addition, when the heating of the main heater and the sub heater is stopped, the boundary position between the molten portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion is a position 25 mm above the bottom end of the seed crystal accommodating portion. Except that, calcium fluoride crystals of Example 6 were obtained in the same manner as Example 1.

(Comparative Examples 1 and 2)
When the heating of the main heater and the sub heater is stopped, the boundary position between the molten portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion is set to a position 85 mm above the bottom end of the seed crystal accommodating portion. Except for the above, a calcium fluoride crystal of Comparative Example 1 was obtained in the same manner as Example 1. Further, when the heating of the main heater and the sub heater is stopped, the boundary position between the molten portion and the unmelted portion of the seed crystal accommodated in the seed crystal accommodating portion is a position 20 mm above the bottom end of the seed crystal accommodating portion. Except that, calcium fluoride crystals of Comparative Example 2 were obtained in the same manner as Example 1.

  In each of the calcium fluoride crystals of Examples 1 to 6 and Comparative Examples 1 and 2, the crystals were sandwiched between two deflection films shifted by 90 °, and the light transmission state was observed visually when illuminated with an illuminator. After that, it is judged that single light is not seen through light transmission, and polycrystal is one with a transmission part, and is further processed into a disk shape (φ150mm x thickness 100mm) with a cutting machine and polished with a mirror polisher In the same manner, it was reconfirmed whether it was single crystal or polycrystal. Thereafter, the crystal orientation was confirmed by the X-ray-Laue method. Each method is judged as defective when the obtained crystal is polycrystalline, and when the volume of the single crystal portion, that is, the portion having the same crystal orientation (111) as the seed crystal is less than 50% by volume. The number of defects relative to the number of tests (10) was evaluated. The implementation conditions and results of Examples 1 to 6 and Comparative Examples 1 and 2 are collectively shown in FIG.

  As is clear from FIG. 4, Examples 1 and 4 have particularly good yields of calcium fluoride single crystals, and Examples 3 and 5 have the next best yields of calcium fluoride single crystals. In Examples 2 and 6, it was found that the yield of the calcium fluoride single crystal was the next best. Moreover, in Comparative Example 1 and Comparative Example 2, the crystal state of calcium fluoride became polycrystalline, and a single crystal could not be obtained.

It is a mimetic diagram showing a schematic structure of a vacuum VB furnace provided with a crucible concerning an embodiment of the present invention. It is sectional drawing which shows the structure of the crucible which concerns on one Embodiment shown in FIG. FIG. 3 is a relationship diagram of a crucible position, a heater temperature, and a thermocouple temperature in a vacuum VB furnace equipped with a crucible according to an embodiment of the present invention. It is a test result of the crystal state and yield concerning Examples 1-6 of the present invention and comparative examples 1 and 2.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Crucible, 1A ... Crucible body, 1CA, 1CB ... Thermocouple (temperature detection means), 1D ... Raw material container, 1E ... Seed crystal container, 1K ... Wall surface of seed crystal container, 2 ... Vacuum VB furnace (crystal (Reactor of growth apparatus), 100 ... crystal growth apparatus, M ... raw material of single crystal of calcium fluoride, S ... seed crystal of calcium fluoride.

Claims (4)

In a crucible for growing single crystals,
A bottomed seed crystal accommodating portion extending in the vertical direction for accommodating the seed crystal;
A raw material storage unit for storing a single crystal raw material, the raw material storage unit disposed above the seed crystal storage unit and communicating with the seed crystal storage unit;
Temperature detecting means for detecting the internal temperature of the seed crystal storage unit,
The temperature detecting means is a thermocouple, and the thermocouple is disposed in the vicinity of the wall surface of the seed crystal accommodating portion;
A plurality of the thermocouples;
A plurality of the thermocouples are vertically separated from each other in order to derive a temperature gradient of the seed crystal accommodating part and to determine a boundary position between the molten part and the unmelted part of the seed crystal accommodated in the seed crystal accommodating part. A crucible characterized by being arranged in the form of One of the two thermocouples is arranged at an upper position from the height position of the bottom end of the seed crystal accommodating part by a height corresponding to 25 to 50% of the depth of the seed crystal accommodating part, claim the other, characterized in that disposed above the position from the height position of the bottom end of the seed by a height equivalent to 60-80% of the depth of the crystal housing part the seed crystal accommodation section The crucible according to 1 . Preparing a crucible according to claim 1 or 2 ,
Storing a seed crystal of calcium fluoride in the seed crystal storage unit;
Containing the calcium fluoride single crystal raw material in the raw material containing portion;
The crucible is disposed in a furnace of a crystal growth apparatus in which the inside is heated in a vertical direction with a predetermined temperature gradient, and the crucible is heated and accommodated in the raw material and seed crystal accommodating portion accommodated in the raw material accommodating portion. Gradually melting the formed seed crystal from above to below;
Detecting the internal temperature of the seed crystal accommodating portion with a plurality of the thermocouples during heating of the crucible;
Based on the internal temperature of the seed crystal accommodating part detected by the thermocouple, a temperature gradient of the seed crystal accommodating part is derived, and a melting portion and an unmelted part of the seed crystal accommodated in the seed crystal accommodating part are obtained. When it is determined that the boundary position is between a first position that is a predetermined height above the bottom end of the seed crystal container and a second position that is a predetermined height above the first position. And a step of growing the single crystal by terminating heating and starting cooling to grow a single crystal. The first position is a position above the bottom end of the seed crystal accommodating part by a height corresponding to 25% of the depth of the seed crystal accommodating part, and the second position is the position of the seed crystal accommodating part. 4. The method for growing a calcium fluoride single crystal according to claim 3 , wherein the height is equivalent to 80% of the depth, and the position is located above the bottom end of the seed crystal housing portion.

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