JP4461735B2 - Crucible and method for growing single crystal using crucible - Google Patents

Description

  The present invention relates to a crucible for growing a single crystal by melting and cooling an optical component material such as calcium fluoride and a method for producing a single crystal using the crucible.

  Conventionally, a crucible for growing a single crystal by melting and cooling calcium fluoride has been known to grow calcium fluoride into a single crystal along the crystal plane of the seed crystal. (For example, refer to Patent Document 1).

  In this type of crucible, a large-diameter raw material containing portion into which a calcium fluoride raw material is introduced and a small-diameter seed crystal containing portion into which a calcium fluoride seed crystal is contained are continuously connected via a tapered cone surface. Is formed.

When growing a calcium fluoride single crystal using the above crucible, a crucible containing a seed crystal in the seed crystal storage part and a calcium fluoride raw material in the raw material storage part is placed in the crystal growth furnace, A single crystal is grown by evacuating and forming a temperature gradient, raising and lowering the crucible to melt and cool part of the seed crystal and the raw material. Here, when the calcium fluoride raw material is melted, it is difficult to control the temperature so that only a part of the seed crystal melts, and the entire seed crystal may melt. However, when all of the seed crystal is melted, it becomes difficult to obtain a single crystal having a target crystal orientation. That is, the yield of the target single crystal is greatly reduced. For this reason, the crucible is connected to the cooling rod through the support rod, and when the raw material is melted, the crucible is cooled through the support rod by the cooling rod, thereby cooling the bottom of the seed crystal. ing.
Japanese Patent Laid-Open No. 10-265296

  However, the conventional method for growing a single crystal described above has the following problems.

  That is, since the seed crystal accommodating part of the crucible used in the method for growing a single crystal is generally formed by a drill with a sharp tip, the bottom surface of the seed crystal accommodating part has a conical shape. On the other hand, the bottom surface of the seed crystal is generally a flat surface. For this reason, when the seed crystal is accommodated in the seed crystal accommodating portion, a gap is formed between the bottom surface of the seed crystal and the bottom surface of the seed crystal accommodating portion. As a result, when the raw material is melted, the bottom of the seed crystal is not sufficiently cooled, so that the entire seed crystal is melted, making it difficult to obtain a single crystal having a target crystal orientation.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a crucible capable of sufficiently cooling the bottom of a seed crystal when a raw material is melted and a method for growing a single crystal using the crucible. To do.

  In order to solve the above problems, the present invention is a crucible for growing a single crystal along a crystal plane of a seed crystal of an optical component material by melting and cooling the optical component material, It has a raw material accommodating part accommodated as a raw material and a seed crystal accommodating part for accommodating a seed crystal, and the bottom part of the seed crystal accommodating part has a shape that matches the end part of the seed crystal.

  According to this crucible, when the seed crystal is accommodated in the seed crystal accommodating part, the bottom part of the seed crystal accommodating part has a shape that matches the end part of the seed crystal. The space | gap which arises between the edge part surfaces of a seed crystal can be made small enough. For this reason, when the optical component material is further accommodated as a raw material in the raw material accommodating portion of the crucible and the raw material is melted while cooling the bottom portion of the seed crystal through the crucible, the bottom portion of the seed crystal is sufficiently cooled. Therefore, the whole seed crystal is sufficiently prevented from being melted.

  Specifically, in the above crucible, the end of the seed crystal has an end surface and a side surface continuous to the end surface, and the bottom of the seed crystal housing portion matches the bottom surface and the side surface of the seed crystal continuous to the bottom surface. And the end surface is a flat surface, and the bottom surface is a flat surface.

  The present invention also provides a method for growing a single crystal of an optical component material using the above-described crucible, wherein the end of the crucible seed crystal accommodating portion has a shape that matches the bottom of the seed crystal accommodating portion as a seed crystal. A seed crystal containing step of containing a seed crystal having a portion, a raw material containing step of containing a material for an optical component as a raw material in a raw material containing portion, and melting and cooling the raw material in the crucible on the crystal surface of the seed crystal And a growing step of growing a single crystal of the optical component material.

  According to this method for growing a single crystal, when the seed crystal is accommodated in the seed crystal accommodating portion of the crucible, the bottom of the seed crystal accommodating portion has a shape that matches the end of the seed crystal. The space | gap produced between the surface which comprises a bottom part, and the edge part surface of a seed crystal can be made small enough. For this reason, when the optical component material is further accommodated as a raw material in the raw material accommodating portion of the crucible and the raw material is melted while cooling the bottom portion of the seed crystal through the crucible, the bottom portion of the seed crystal is sufficiently cooled. Therefore, the whole seed crystal is sufficiently prevented from being melted.

  According to the crucible and the method for growing a single crystal using the crucible according to the present invention, when the seed crystal is accommodated in the seed crystal accommodating portion and the raw material is accommodated in the raw material accommodating portion, the bottom portion of the seed crystal is cooled via the crucible. However, when the raw material is melted, the bottom of the seed crystal is sufficiently cooled. Therefore, the whole seed crystal is sufficiently prevented from being melted.

  Hereinafter, embodiments of a crucible according to the present invention will be described with reference to the drawings. In the present embodiment, a crucible suitable for use as a seed crystal having a cylindrical and flat end surface will be described. The description will be made assuming that the seed crystal is made of calcium fluoride. In the drawings to be referred to, FIG. 1 is a schematic view showing a schematic structure of a vacuum VB furnace equipped with a crucible according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view showing the structure of the crucible according to the embodiment shown in FIG. FIG. 3 and FIG. 3 are enlarged views showing the bottom part of the seed crystal accommodating part.

As shown in FIG. 1, a crucible 1 according to the present embodiment is disposed inside a heater 2A in a vacuum VB furnace 2 as a single crystal growth apparatus by a vertical Bridgman (hereinafter abbreviated as VB) method, By moving up and down at a very low speed through the shaft 2B, the raw material M of calcium fluoride (CaF ₂ ) is melted and cooled, and this is seeded with a single crystal of calcium fluoride (CaF ₂ ). For example, S is grown into a single crystal along a crystal plane of (1,1,1) orientation.

The inside of the vacuum VB furnace 2 is depressurized to 10 ⁻⁴ Pa or less by a vacuum pump 2C, and heated to, for example, about 1400 to 1500 ° C. by a heater 2A. In order to prevent the entire seed crystal S from being melted by the heating of the heater 2A, the shaft 2B of the vacuum VB furnace 2 is configured to constitute a cooling water circulation path.

  That is, the shaft 2B is formed of a double tube in which the upper end of the inner tube 2B1 is retracted from the upper end of the outer tube 2B2, and a cap-like heat transfer member 2D is fitted and fixed to the upper end portion. And this heat-transfer member 2D is comprised so that the lower part of the seed crystal S may be forcedly cooled by connecting to the center part of the bottom member 1C of the crucible 1 mentioned later.

  Here, as shown in FIG. 2, the crucible 1 of this embodiment includes a crucible body 1A, a lid member 1B that covers the opening of the crucible body 1A, and a bottom member 1C that is fixed to the lower portion of the crucible body 1B. It is prepared for. The crucible body 1A is made of high-purity carbon material (C) as a material that has heat resistance and can improve the smoothness of the inner surface, and the inner surface is coated with glossy glassy carbon (GC). ing.

The crucible body 1A is formed with a raw material storage portion 1D in which a raw material M of calcium fluoride (CaF ₂ ) (see FIG. 1) is stored. The raw material container 1D has a cylindrical wall surface 1H, a concave curved surface 1J formed continuously on the bottom member 1C side of the wall surface 1H, and a tapered shape formed continuously on the bottom member 1C side of the concave curved surface 1J ( A funnel-shaped cone surface 1F. Accordingly, the cone surface 1F constitutes the bottom of the raw material container 1D.

  In addition, a seed crystal accommodating portion 1E that accommodates a cylindrical seed crystal S (see FIG. 1) is formed at the center of the crucible body 1A from the bottom member 1C. The seed crystal accommodating portion 1E has a shape that matches the seed crystal S. In particular, the bottom of the seed crystal housing portion 1E has a shape that matches the end of the seed crystal S.

  The bottom part of the seed crystal housing part 1E is shaped to coincide with the end part of the seed crystal S in this way when the seed crystal S is housed in the seed crystal housing part 1E. This is to sufficiently reduce the gap between the surface to be processed and the end surface of the seed crystal S.

  Specifically, the end surface of the seed crystal S has a flat end surface S1 and a cylindrical side surface S2 that is continuous with the end surface S1 and perpendicular to the end surface S1, and constitutes the bottom of the seed crystal housing portion 1E. The surface has a flat bottom surface 1N and a cylindrical wall surface 1K that is continuous with the bottom surface 1N and is perpendicular to the bottom surface 1N. The diameter of the wall surface 1K substantially matches the diameter of the seed crystal S. In addition, the wall surface 1K of the seed crystal storage unit has a smaller diameter than the wall surface 1H of the raw material storage unit 1D.

  A convex curved surface 1L is formed at the boundary portion between the cone surface 1F and the wall surface 1K of the seed crystal accommodating portion 1E, and the cone surface 1F and the wall surface 1K of the seed crystal accommodating portion 1E are smoothed via the convex curved surface 1L. It is continuous.

  On the other hand, the lid member 1B and the bottom member 1C are also made of a heat-resistant high-purity carbon material. A connecting cylinder portion 1C1 for fitting and fixing a heat transfer member 2D (see FIG. 1) fixed to the upper end portion of the shaft 2B of the vacuum VB furnace 2 projects from the center portion of the bottom surface of the bottom member 1C. ing.

In the crucible 1, if the cone angle θ of the cone surface 1F is too small, residual stress and strain are generated in the calcium fluoride (CaF ₂ ) crystal grown in the raw material container 1D. Polycrystal (heterogeneous phase) is likely to occur. On the other hand, if the cone angle θ of the cone surface 1F is too large, the growth of a single crystal of calcium fluoride (CaF ₂ ) is likely to be hindered. Therefore, the cone angle θ of the cone surface 1F is preferably 95 ° to 150 °, and more preferably 120 ° to 130 ° in these ranges.

Further, when the concave curved surface 1J and the convex curved surface 1L are angular because the radius of curvature is too small, when the calcium fluoride (CaF ₂ ) melted in the raw material container 1D is crystallized by cooling, the concave concave curved surface is formed. Polycrystals (heterogeneous phases) are likely to occur with 1J and the convex curved surface 1L as nuclei. In addition, when calcium fluoride (CaF ₂ ) shrinks by cooling, these angular concave curved surfaces 1
Calcium fluoride (CaF ₂ ) adheres to J and the convex curved surface 1L, and residual stress and distortion are generated in the crystal, and polycrystal (heterogeneous phase) is likely to be generated due to this.

  Therefore, the curvature radii of the concave curved surface 1J and the convex curved surface 1L are set to a large curvature radius of 1/10 or more of the inner diameter (for example, 250 mm) between the wall surfaces 1H of the raw material container 1D. For example, the radius of curvature of the concave curved surface 1J is set to about 60 mm, and the radius of curvature of the convex curved surface 1L is set to about 50 mm.

Furthermore, when the surface roughness of the wall surface 1H, the cone surface 1F, etc. of the raw material container 1D is rough, the wall surface 1 is used when calcium fluoride (CaF ₂ ) melted in the raw material container 1D is crystallized by cooling.
Polycrystals (heterogeneous phases) are likely to occur with minute irregularities such as H and cone surface 1F as nuclei. In addition, when calcium fluoride (CaF ₂ ) contracts due to cooling, calcium fluoride (CaF ₂ ) adheres to the wall surface 1H and the cone surface 1F, resulting in residual stress and distortion in the crystal. Therefore, polycrystal (heterogeneous phase) is easily generated.

  Therefore, in the crucible 1, the inner surface of the crucible extending from the wall surface 1H of the raw material container 1D of the crucible body 1A to the wall surface 1K of the seed crystal container 1E through the concave curved surface 1J, the cone surface 1F, and the convex curved surface 1L is, for example, the maximum height. The surface roughness according to the method is finished to Rmax 3.2 s or less. That is, the inner surface of the crucible body 1A made of the high-purity carbon material (C) is finished to about Rmax 6.4 s, for example, and the surface is coated with glassy carbon (GC) and finished to about Rmax 3.2 s. .

  And the crucible inner surface comprised of glassy carbon (GC) having a surface roughness of Rmax 3.2 s or less in this way has a contact angle with water droplets of at least 100 ° or less, for example 90 °.

  Next, a method for growing a single crystal of an optical component material 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 (seed crystal accommodating step). Here, as the seed crystal S, one having a cylindrical shape and a flat end surface and having a diameter substantially equal to the diameter of the wall surface 1K of the seed crystal accommodating portion 1E is used. When such a seed crystal S is accommodated in the seed crystal accommodating portion 1E, at least the bottom portion of the seed crystal accommodating portion 1E matches the end portion of the seed crystal S. For this reason, the space | gap which arises between the side surface S2 of the seed crystal S and the wall surface 1K of the seed crystal accommodating part 1E, and between the end surface S1 of the seed crystal S and the bottom face 1N of the seed crystal accommodating part 1E is made small enough. be able to.

  After the seed crystal S is accommodated in the seed crystal accommodating portion 1E, the raw material M of calcium fluoride is accommodated in the raw material accommodating portion 1D (raw material accommodating step).

  Subsequently, the raw material container 1D of the crucible body 1A is closed with the lid member 1B.

Next, the inside of the vacuum VB furnace 2 is depressurized to 10 ⁻⁴ Pa or less, and the heater 2A is heated to around 1400 to 1500 ° 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, if the entire seed crystal S is melted, it becomes difficult to obtain a single crystal having a target crystal orientation. Therefore, the heat transfer member 2D is made of cooling water circulating in the shaft 2B from the inner tube 2B1 to the outer tube 2B2. Then, the lower part of the seed crystal S is forcibly cooled. At this time, if gaps are formed between the side surface S2 of the seed crystal S and the wall surface 1K of the seed crystal housing portion 1E, and between the bottom surface 1N of the seed crystal housing portion 1E and the bottom surface S1 of the seed crystal S, Since the gap has a lower thermal conductivity than the carbon constituting the bottom member 1C, the bottom of the seed crystal housing portion 1E is not sufficiently cooled. However, as described above, in the crucible 1, the seed crystal S The gaps between the side surface S2 and the wall surface 1K of the seed crystal accommodating portion 1E and between the bottom surface 1N of the seed crystal accommodating portion 1E and the bottom surface S1 of the seed crystal S are sufficiently small. Therefore, the bottom of the seed crystal S is sufficiently cooled, and the entire seed crystal S is sufficiently prevented from melting.

After melting the raw material M of calcium fluoride, the crucible 1 is lowered at a very low speed of, for example, about 1.0 mm / h, which is 1.5 mm / h or less by the shaft 2B, and lowered in the vacuum VB furnace 2 for about 5 hours. Hold in position. As a result, the molten raw material M of calcium fluoride (CaF ₂ ) is cooled and grown into a single crystal along, for example, the (1,1,1) -oriented crystal plane of the seed crystal S (growth step).

Thereafter, the molten calcium fluoride (CaF ₂ ) in the crucible 1 is controlled at 70 ° C./h or less by controlling on / off of the heater 2A of the vacuum VB furnace 2 in order to prevent quenching (cracking due to thermal shock). For example, it is cooled at a cooling rate of about 30 ° C./h.

Here, in the crucible 1, the inner surface of the crucible extending from the wall surface 1H of the raw material container 1D of the crucible body 1A to the wall surface 1K of the seed crystal container 1E through the concave curved surface 1J, the cone surface 1F, and the convex curved surface 1L is about Rmax 3.2 s, for example. It has a smooth surface. For this reason, when the calcium fluoride (CaF ₂ ) melted in the raw material container 1D is crystallized along the (1,1,1) -oriented crystal plane of the seed crystal S by cooling, Is prevented from being generated on the inner surface of the crucible.

Further, since calcium fluoride (CaF ₂ ) is easily separated from the inner surface of the crucible when shrinking by cooling, the occurrence of residual stress and distortion in the calcium fluoride (CaF ₂ ) crystal is suppressed. As a result, a single crystal of calcium fluoride (CaF ₂ ) is easily grown.

Moreover, since the molten calcium fluoride (CaF ₂ ) in the crucible 1 is cooled at a cooling rate of 70 ° C./h or less, for example, about 30 ° C./h, quenching (cracking due to thermal shock) is prevented and good. Grown into a single crystal.

In addition, in order to cool the molten calcium fluoride (CaF ₂ ) and grow it into a single crystal, the speed at which the crucible 1 is lowered at a very low speed, that is, the growth speed is 1.5 mm / h or less, for example 1.0 mm / h. Therefore, the crystal orientation of the grown single crystal is stable as shown in FIG. It was found that when the growth rate was 1.5 mm / h or more and 2 mm / h, the crystal orientation was dispersed and not stable as shown in FIG.

  The present invention is not limited to the embodiment described above. For example, in the above embodiment, the wall surface 1K of the seed crystal housing portion 1E of the crucible 1 is cylindrical, but the shape of the wall surface 1K may be prismatic when housing a prismatic seed crystal. .

  Moreover, although the bottom surface of the seed crystal accommodating part 1E is a flat surface, the bottom surface of the seed crystal accommodating part of this invention is not restricted to a flat surface. When the end surface S1 of the seed crystal S is conical, the bottom surface 1N of the seed crystal housing portion 1E is also conical. In short, the bottom surface 1N of the seed crystal accommodating portion 1E has a sufficient gap between the end surface S1 of the seed crystal S and the bottom surface 1N of the seed crystal accommodating portion 1E when the seed crystal S is accommodated in the seed crystal accommodating portion 1E. Any shape that can be reduced may be used.

  Further, in the above embodiment, the case where the seed crystal is calcium fluoride is described. However, the crucible of the present invention and the method for growing a single crystal using the crucible are other opticals other than calcium fluoride. The present invention can also be applied to component materials (for example, barium fluoride and magnesium fluoride).

  Hereinafter, the content of the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

Example 1
First, a crucible 1 shown in FIG. 1 was prepared. The crucible body 1A, the lid member 1B, and the bottom member 1C were all made of high purity carbon (high purity carbon made by Nippon Carbon). The wall surface 1K of the seed crystal accommodating part 1E was cylindrical, and its inner diameter was 20 mm. The bottom surface of the seed crystal accommodating portion 1E was a flat surface perpendicular to the wall surface 1K. The wall surface of the raw material container was also cylindrical, and the inner diameter was 250 mm. The radius of curvature of the concave curved surface 1J was 60 mm, and the radius of curvature of the convex curved surface was 50 mm. Further, the inner surface of the raw material container 1D and the inner surface of the seed crystal container are coated with glass-like carbon (Nisshinbo glass-like carbon coat) having an impregnation layer thickness of 1.0 mm, and the contact angle with water droplets is 90 °. I did it.

  In this crucible 1, the lid member 1B was removed, and a cylindrical seed crystal S having a diameter of 10 mm and a length of 10 cm was accommodated in the seed crystal accommodating portion 1E of the crucible 1. Here, the material of the seed crystal S used was calcium fluoride, and the shape of the seed crystal S was flat at the end face.

  Subsequently, the raw material M of calcium fluoride was accommodated in the raw material accommodating part 1D. Subsequently, the raw material container 1D of the crucible body 1A was closed with the lid member 1B.

Next, the inside of the vacuum VB furnace 2 is depressurized to 10 ⁻⁴ Pa or less, the heater 2A is heated to around 1400 to 1500 ° C., and the crucible 1 is raised at a slow speed of about 10 mm / h by the shaft 2B, for about 10 hours. Held in the raised position. At that time, the lower part of the seed crystal S was forcibly cooled through the heat transfer member 2D with cooling water circulating in the shaft 2B from the inner tube 2B1 to the outer tube 2B2.

After melting the raw material M of calcium fluoride, the crucible 1 was 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. In this way, the molten calcium fluoride (CaF ₂ ) raw material M was cooled and grown into a single crystal along the (1,1,1) -oriented crystal plane of the seed crystal S.

Thereafter, the molten calcium fluoride (CaF ₂ ) in the crucible 1 was cooled at a cooling rate of about 30 ° C./h by controlling the heater 2 A of the vacuum VB furnace 2 on and off.

(Comparative Example 1)
A crucible similar to that in Example 1 was prepared except that the bottom surface of the seed crystal accommodating portion 1E was conical, and using this crucible, the calcium fluoride was grown into a single crystal in the same manner as in Example 1.

(Evaluation of crystal quality)
About the calcium fluoride single crystal obtained in Example 1 and Comparative Example 1, two sheets of Polar Film (color: gray, area 15 inches × 8.5 inches, thickness 0.29 mm) manufactured by Edmund Industrial Optics were used. Set the film surfaces parallel to each other, place crystals between the films, place a light source on the outside of one film, observe the crystals from the outside of the other film, and adjust the angle and position of the crystals. The portion that was not a single crystal was measured as a polycrystal. Finally, the volume of the polycrystalline portion was calculated, and the ratio of the total crystal volume to the polycrystalline volume was defined as the occurrence rate of the polycrystalline body. As a result, in the single crystal of Example 1, the rate of occurrence of polycrystals was 30% or less, whereas in the single crystal of Comparative Example 1, the rate of occurrence of polycrystals exceeded 30%. From this, it was found that the single crystal of Example 1 was superior in crystal quality to the single crystal of Comparative Example 1. From this, it was proved that a single crystal with good crystal quality can be obtained by eliminating the gap between the bottom surface of the seed crystal accommodating part and the bottom surface of the seed crystal accommodating part.

It is a mimetic diagram showing a schematic structure of a vacuum VB furnace provided with a crucible concerning one embodiment of the present invention. It is sectional drawing which shows the structure of the crucible which concerns on one Embodiment shown in FIG. It is an enlarged view which shows the bottom part of the seed crystal accommodating part of FIG. It is a figure which shows the distribution condition of the crystal orientation in the crystal | crystallization obtained when the growth speed | rate which lowers the crucible shown in FIG. 1 in the vacuum VB furnace 2 at very low speed was 1.0 mm / h. It is a figure which shows the distribution condition of the crystal orientation in the crystal | crystallization obtained when the growth speed | rate which lowers the crucible shown in FIG. 1 in the vacuum VB furnace 2 at very low speed was 2.0 mm / h.

Explanation of symbols

1 crucible 1A crucible body 1B lid member 1C bottom member 1D raw material container 1E seed crystal container 1F cone surface 1H raw material container wall 1J concave curved surface 1K seed crystal container wall 1L convex curved surface 1N bottom surface 2 vacuum VB furnace 2A heater 2B the end face of the seed crystal S1 seed crystal of the material S calcium fluoride (CaF ₂₎ of the shaft 2C vacuum pump 2D heat transfer member M calcium fluoride (CaF ₂₎

Claims (3)

A crucible for growing a single crystal along a crystal plane of a seed crystal of an optical component material by melting and cooling the optical component material,
A raw material container for accommodating the optical component material as a raw material;
Is said seed seed crystal accommodation section you accommodate crystals, and a seed crystal accommodation section in which the seed crystal accommodated is adapted to be cooled from below,
A crucible characterized in that a bottom portion of the seed crystal accommodating portion has a shape matching an end portion of the seed crystal. The end of the seed crystal has an end surface and a side surface continuous with the end surface,
The bottom of the seed crystal accommodating part is configured by a bottom surface and a wall surface that is continuous with the bottom surface and matches a side surface of the seed crystal,
The crucible according to claim 1, wherein the end surface is a flat surface and the bottom surface is a flat surface. A method for growing a single crystal using the crucible according to claim 1 or 2 to grow a single crystal of an optical component material,
A seed crystal accommodating step of accommodating, in the seed crystal accommodating portion of the crucible, as the seed crystal, a seed crystal having an end portion having a shape matching the bottom of the seed crystal accommodating portion;
A raw material storage step of storing the optical component material as a raw material in the raw material storage portion;
A growth step of growing a single crystal of the optical component material along the crystal plane of the seed crystal by melting and cooling the raw material in the crucible;
A method for growing a single crystal, comprising:

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