JP2009102194A - Metal fluoride single crystalline body pulling apparatus and method of manufacturing metal fluoride single crystalline body using the same apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus of pulling a single crystal which is easy to manufacture a high grade metal fluoride single crystalline body small in crystal defect such as bubbles or negative crystal by decreasing a fallen material falling down into a raw material molten liquid. <P>SOLUTION: The single crystal pulling apparatus is provided with a gas flow passage 023 through which a gas flows out of a chamber in the direction of side part above a single crystal pulling chamber surrounded by a ceiling plate 019 and a heat insulating wall 010 arranged between a heater 009 and the chamber 008. As a result, it is extremely suppressed that the metal fluoride gas volatilized from the raw material molten liquid 004 during pulling the single crystal 018 is condensed and solidified on the lower surface of the ceiling plate 019, is peeled off by some reason and falls down into the raw material molten liquid 004. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a pulling apparatus used for manufacturing a metal fluoride single crystal used for an optical material and the like, and a method for manufacturing the metal fluoride single crystal using the apparatus.

Single crystal of metal fluoride such as calcium fluoride and barium fluoride has high transmittance over a wide wavelength band, low dispersion and excellent chemical stability. Demand is expanding as optical materials such as various devices using lasers, lenses for cameras, CVD devices, and window materials. In particular, a calcium fluoride single crystal is a light source window material, a light source lens, an ArF laser (193 nm) or an F ₂ laser (157 nm), which is being developed as a next-generation short wavelength light source in the photolithography technology. Expectation is expected as a projection system lens.

  Conventionally, such a single crystal of a metal fluoride is generally produced by the Bridgeman method (crucible descent method) or the Czochralski method (single crystal pulling method). Here, the Bridgman method is a method of growing a single crystal in a crucible by cooling the molten liquid of the single crystal production raw material in the crucible while gradually lowering the whole crucible. On the other hand, the Czochralski method is a method in which a seed crystal made of a target single crystal is brought into contact with the melt surface of the single crystal production raw material in the crucible, and then the seed crystal is gradually pulled up from the heating region of the crucible. In this method, a single crystal is grown under the seed crystal by cooling.

  In the Czochralski method, the produced single crystal can be grown (grown) without contacting the crucible wall, so there is a low possibility that it will be polycrystallized, and a large single crystal with little strain is efficiently produced. It is the outstanding method which can be manufactured (for example, refer patent documents 1-4).

  In the case of producing a metal fluoride single crystal by the Czochralski method, a single crystal pulling apparatus 100 as shown in FIG. 1 has been conventionally used (for example, Patent Documents 1 to 4).

  In the pulling apparatus shown in FIG. 1, the crucible for melting the raw material metal fluoride is a double-structure crucible composed of an outer crucible 101 and an inner crucible 102, and the inner crucible 102 has a wall (bottom wall and / or side wall). Is provided with a through-hole 103 through which the raw material metal fluoride melt 104 can flow in the inner crucible and the outer crucible through the wall. When the crystal is pulled up, the raw material melt in the crucible is reduced by an amount corresponding to the pulled crystal amount, that is, the raw material melt level in the crucible is lowered. In the illustrated embodiment, the inner crucible is fixed at a predetermined position (height), and the outer crucible is raised by an amount corresponding to the relative lowering of the raw material melt surface. As a result, the raw material melt in the inner crucible can be kept constant, and the position of the raw material melt surface (= crystal growth interface) can be prevented from changing.

  The outer crucible is raised and rotated by an outer crucible support shaft 105 that can move up and down and rotate.

  The crucible is heated by the heater 109. A heat insulating wall 110 is disposed between the crystal pulling furnace chamber 108 and the heater 109 so as to surround the heater 109, and more usually, the heat insulating wall is also provided below the crucible. By disposing the heat insulating wall 110, the chamber 108 is protected from the radiant heat of the heater 109, the heat is prevented from being dissipated to the outside, and the temperature around the crucible is easily maintained.

  The upper opening of the heat insulating wall 110 is usually covered with a ceiling plate 119 disposed so as to contact the upper end of the heat insulating wall, and a single crystal pulling chamber surrounded by these is formed. By installing the ceiling plate 119, (1) heat dissipation in the upper direction is suppressed, the heat retention of the single crystal pulling chamber surrounded by the heat insulating wall 110 and the ceiling plate 119 is improved, and the single crystal pulling chamber is (2) Prevents radiant heat from the melt and the single crystal being pulled directly from reaching the chamber 108, and (3) Dust falls from above to the melt. Mixing can be prevented.

  The ceiling plate 119 has an insertion hole 120 through which the single crystal pulling rod 115 is inserted. Further, if necessary, a window hole 122 for observing the state of the single crystal being pulled up or the state of the melt in the crucible may be drilled.

  On the center axis of the crucible, a seed crystal holder 117 that holds the seed crystal 116 and a crystal pulling shaft 115 that supports the holder so as to move up and down and rotate are arranged. In the Czochralski method, the seed crystal 116 held by the holder 117 is brought into contact with the sufficiently melted raw material in the crucible, and then the single crystal 118 having a predetermined diameter is gradually pulled up while being rotated. Grow.

JP 2004-182587 A JP 2004-182588 A JP 2005-029455 A JP 2006-347834 A

  However, according to the study by the present inventors, when a single crystal pulling apparatus having the structure as described above is used to produce a metal fluoride single crystal such as calcium fluoride, the crystals grow rapidly during the pulling. And the melt is solidified in the shape of a stalagmite at the bottom of the inner crucible, the stalagmite collides with the single crystal being pulled up, or bubbles are mixed into the pulled single crystal. It has become clear that troubles often occur.

  And when the present inventors investigated the cause of such trouble, one of the causes is that the metal fluoride volatilized from the melt is condensed and solidified on the lower surface of the ceiling board, and this solidified product is somehow It became clear that it was falling into the melt due to the factor.

  That is, when the present inventors manufactured a metal fluoride single crystal with a single crystal pulling apparatus having a conventional structure and observed the inside of the apparatus after the pulling was completed, a large amount of flaky solid matter was found in the apparatus. On the other hand, a white solid was adhered to the lower surface of the ceiling board, and a trace of part of the solid was peeled off was observed. These solids were metal fluoride. Therefore, the present inventors further conducted a model experiment (see Reference Example described later) about the effect of falling objects in the melt during crystal pulling. When there was a large fallen object, it was found that the formation of stalagmite-like solidified material and the inclusion of bubbles occurred as described above. From these results, the above conclusion has been reached.

  Further, as disclosed in Japanese Patent Application Laid-Open No. 2006-347792, the furnace pressure during crystal growth is often performed in a reduced pressure state for the purpose of making it difficult to produce a defect consisting of a negative crystal called a scattering center (SC). However, if the pressure in the furnace is reduced as described above, the volatilization of the metal fluoride is more likely to occur, and thus the amount of the agglomerated solidified material attached to the ceiling plate and the amount of the fall are further increased.

  Therefore, the present invention prevents the metal fluoride volatilized from the melt from condensing and solidifying on the lower surface of the ceiling plate and falling into the melt, thereby stably producing a high-quality metal fluoride single crystal. An object is to provide an apparatus that can be manufactured.

  The present inventors have conducted intensive studies in view of the above problems. As a result, the ceiling plate is directed to the side by directing the flow of gas containing components volatilized from the raw material melt by bringing the ceiling plate into a slightly lifted state without bringing the upper end of the heat insulating wall into contact with the ceiling plate. As a result of finding that the agglomerated solidified product of the metal fluoride adhering to the material is greatly reduced and further studying it, the present invention was completed.

That is, the present invention
A chamber constituting a crystal growth furnace;
A crucible disposed in the chamber and containing a melt of a raw material for producing a single crystal;
A melting heater disposed so as to surround the periphery of the crucible;
A single crystal pulling shaft which is provided with a seed crystal holding part at the tip, and which can move up and down so that the seed crystal held in the seed crystal holding part can come into contact with the melt of the single crystal production raw material housed in the crucible; ,
A heat insulating wall disposed in the chamber so as to surround a side periphery of the single crystal pulling region at least above the crucible;
A ceiling plate that covers the opening at the top of the heat insulation wall and has at least an insertion hole for inserting the single crystal pulling rod;
A single crystal pulling apparatus for metal fluoride comprising:
A gas flow part capable of flowing gas from the single crystal pulling chamber to the outside is provided on an upper side portion of the single crystal pulling chamber configured with the heat insulating wall as a side wall and the ceiling plate as a ceiling wall. This is a single crystal pulling apparatus for metal fluoride.

  When the metal fluoride single crystal is produced (pulled) using the pulling apparatus of the present invention, the metal fluoride volatilized from the raw material melt surface is less likely to aggregate and solidify on the ceiling plate. Therefore, it can suppress that the aggregation solidification thing of a metal fluoride peels and drops | omits from a ceiling board, and falls in a melt. Thereby, the factor of the disorder of the melt during the pulling of the metal fluoride single crystal can be reduced, and hence it becomes easy to stably produce a high performance metal fluoride single crystal.

  Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings.

  FIG. 2 is a schematic view showing a typical embodiment of the single crystal pulling apparatus of the present invention. This single crystal pulling apparatus is a conventionally known single crystal for metal fluoride, except that a gas flow part capable of flowing gas from the pulling chamber to the outside is provided on the upper side of the single crystal pulling chamber. It is the same as the pulling device.

  This single crystal pulling apparatus includes a double-structure crucible including an outer crucible 001 and an inner crucible 002 in a chamber 008 constituting a crystal growth furnace, and extends outside the chamber through the bottom wall of the chamber 008. The outer crucible support shaft 005 supports the outer crucible 001 so that it can move up and down and rotate.

  The wall portion (bottom wall and / or side wall) of the inner crucible 002 is provided with a through-hole 003 through which the raw material metal fluoride melt 004 can flow in the inner crucible and in the outer crucible. ing. When the crystal is pulled up, the raw material melt in the crucible is reduced by an amount corresponding to the pulled crystal amount. In the illustrated embodiment, the inner crucible 002 is fixed at a predetermined position (height), and the outer crucible is raised by an amount corresponding to the relative lowering of the raw material melt surface. Thereby, it is possible to keep the raw material melt in the inner crucible constant and not to change the position of the raw material melt surface (= crystal growth interface). Although the double-structure crucible is adopted in the above embodiment, the present invention may be applied to a single crystal pulling furnace having a single crucible structure, and the same effect can be obtained.

  The gap between the inner wall of the outer crucible 001 and the outer wall of the inner crucible 002 is provided with an opening shielding member 007 to prevent volatilization of the raw material melt from the raw material melt 004 and to prevent falling substances from entering from above. Measures are taken to prevent it. It is not impossible to provide a member for shielding the opening on the inner wall of the inner crucible 002 in the same manner. However, since it becomes easy to produce an extreme temperature difference between the upper part and the lower part of the member as a partition, it is often difficult to stably produce high-quality crystals. Furthermore, when the shielding member is provided on the inner wall side of the inner crucible in this way, there may be a problem that the metal fluoride condenses and solidifies on the lower surface of the shielding member and falls.

  A melting heater 009 is disposed so as to surround the outer crucible 001, and the crucible (the molten metal fluoride raw material melt) is heated by the heater.

  A heat insulating wall 010 is disposed between the melting heater 009 and the chamber 008 so as to surround the melting heater. Usually, a heat insulating wall is also provided below the crucible. By providing the heat insulating wall 010, the chamber is protected from being exposed to high temperatures, and the heat retention in the single crystal pulling chamber surrounded by the heat insulating wall 010 and a ceiling plate 019 described later is enhanced.

  A plurality of rod heaters are usually arranged as the melting heater 009. In order to make uniform the radiant heat reaching the crucible from the melting heater, an isolation wall 011 is provided around. In order to prevent the heat of the melting heater 109 from escaping upward, the upper end of the isolation wall 111 is made higher than the upper end of the heater, and the isolation wall and the thermal insulation wall are interposed between the upper end and the thermal insulation wall 110. A lid member 112 that closes the gap is horizontally mounted to close the gap.

  In FIG. 4, the above-described inner crucible 002 is fixed at a predetermined height by disposing a connecting member 113 on an annular plate on the lid material 112, and an inner crucible hanging rod 114 suspended from the connecting member. This is done by supporting the inner crucible 002.

  A crystal pulling shaft 015 that can move up and down is disposed on the center axis of the crucible, and a seed crystal holder 017 that holds a seed crystal 016 is attached to the tip thereof. In the Czochralski method, a seed crystal held in the holder is brought into contact with a sufficiently melted raw material in a crucible, and the single crystal 018 is grown by gradually pulling it up while rotating.

  The upper end opening of the heat insulating wall 010 surrounding the melting heater is covered with a ceiling plate 019. An insertion hole 020 for inserting the crystal pulling shaft 015 is formed at the center of the ceiling plate 019. Further, the ceiling plate 019 is also provided with a window hole 022 for confirming the state of the raw material melt or single crystal in the crystal pulling chamber from the observation window 021. In FIG. 1, the ceiling plate is single, but a plurality of sheets may be provided in multiple stages for the purpose of controlling heat dissipation.

  By providing this ceiling plate 019, it is possible to prevent the radiant heat from the raw material melt surface and the single crystal from directly reaching the chamber 008, and the ceiling plate 019 is used as a ceiling wall and the heat insulating wall 010 is used as a side wall. It is easy to improve the heat retention in the crystal pulling chamber and to impart an appropriate temperature gradient in the chamber. Furthermore, it has an effect of preventing dust from falling from the space above the ceiling plate 019 and mixing into the melt. In order to obtain these effects, the total area of all the openings such as the insertion hole 020 and the window hole 022 of the ceiling plate 019 is 20% or less of the opening area of the upper end of the heat insulating wall covered by the ceiling plate. It is preferably 10% or less.

  A feature of the present invention is that a gas circulation portion 023 capable of flowing gas from the room to the outside of the single crystal pulling chamber having the heat insulating wall 010 as a side wall and the ceiling plate 019 as a ceiling wall. Is in the point provided. In the present invention, the bottom wall of the single crystal pulling chamber is usually composed of a bottom heat insulating wall, but is not limited to this. For example, the bottom of the chamber may constitute the bottom wall of the single crystal pulling chamber. However, you may be comprised by the other member. Further, the side wall of the single crystal pulling chamber does not need to be constituted only by the heat insulating wall 010, and deviates from the purpose of the heat insulating wall arrangement for thermally protecting members such as a chamber arranged outside the heat insulating wall. As long as it is not, it may be made of a non-insulating member. The same applies to the ceiling plate 019.

  The metal fluoride volatilized from the melt reaches the ceiling plate 019 as an upward flow. A part of the metal fluoride gas flows out of the crystal pulling chamber from the single crystal pulling rod insertion hole 020 and the window hole 022, but a part is cooled by the ceiling plate 019 and is condensed and solidified on the lower surface of the ceiling plate. Usually, the pulling of the metal fluoride single crystal by the Czochralski method is performed for several tens of hours or more in order to obtain a long single crystal, and during the period, the aggregated solidified product is gradually applied to the lower surface of the ceiling plate. It accumulates in and increases in thickness. The agglomerated solid product having a thickness of a certain level or more is peeled off from the ceiling plate by its own weight under a slight stimulus, and in some cases falls into the melt.

  If there is such a fallen object, the temperature of the melt may partially decrease, or it may be due to physical disturbance (waves) on the melt surface due to the impact of the fall of the solidified product. The formation of stalagmite-like solidified material at the bottom of the inner crucible and the mixing of bubbles are extremely increased.

  If the ceiling plate 019 is not provided, or if an opening having an area far larger than the opening area of a general insertion hole or window hole is provided in the ceiling plate so that the gas can easily flow out upward, the bottom surface of the ceiling plate However, this method is not practical because the effect of providing the ceiling plate as described above cannot be obtained.

  Therefore, in the present invention, the ceiling plate 019 is kept provided, and a gas circulation part 023 capable of gas circulation is provided above the side part of the single crystal pulling chamber so that the metal fluoride gas can be supplied through the gas circulation part (gap). By flowing out of the room from the side, the retention of the metal fluoride gas on the lower surface of the ceiling plate is reduced, and the amount of coagulation and solidification of the metal fluoride on the lower surface of the ceiling plate is reduced.

  In addition, since at least a part of the single crystal pulling room and the outside is partitioned by a heat insulating material, there is a large temperature difference between the room and the outside. Therefore, most of the metal fluoride gas that has flowed out of the single crystal pulling chamber out of the gas circulation section 023 is less likely to aggregate and solidify outside, return to the room again, and aggregate and solidify on the ceiling plate 019.

  As can be understood from the above objects and effects, it is possible to obtain a higher effect by providing the gas circulation part at the upper side even at the side of the single crystal pulling chamber, and in the case of a crucible during crystal pulling (in the case of a double structure crucible) Inner crucible) When the distance from the upper end to the lower surface of the ceiling plate is 1, it is preferable that the position of the lower end of the opening as the gas circulation part is provided above the position where the distance from the top is ½. It is more preferable to provide it above the position that becomes 1/3, and it is even more preferable to provide it above the position that becomes 1/5. Moreover, it is preferable that the position of the upper end of the opening as the gas circulation part is also provided higher, and it is particularly preferable that the position corresponds to the uppermost end of the side part.

  Although the method for forming the gas circulation part is not particularly limited, as shown in FIG. 2, a method of slightly lifting (floating) the ceiling plate 019 without being in close contact with the upper end of the heat insulating wall 010 is simple. And it is preferable at the point which can make the position of a gas distribution part into the uppermost end of a side part. Furthermore, if formed in this way, voids are generated substantially over the entire circumference of the single crystal pulling chamber, so that the metal fluoride gas flows out of the chamber evenly. As a result, the metal fluoride gas stays in a portion far from the gas circulation portion, and therefore, the possibility that agglomerated solidified product is easily generated on the lower surface of the ceiling plate in the portion can be reduced.

  The method of floating the ceiling board in this way is not particularly limited. For example, as shown in FIG. 3, pin-like members 024 are fitted in several places on the ceiling board 019, and the lower surface of the ceiling board is placed. A small piece-shaped member between the bottom surface of the ceiling plate 019 and the upper end of the heat insulating wall 010 as shown in FIG. Examples include a method of inserting 025, and a method of suspending a suspension member 026 from a chamber or the like and holding a ceiling plate by the member, as shown in FIG. Of course, the ceiling board may be lifted (floated) by a method other than the above example. The method of floating the ceiling plate 019 can be determined appropriately according to the manufacturing cost and difficulty of the pulling device itself, the crystal pulling and preparation, the operability during post-processing, etc. Good.

  Further, as another method of providing the gas circulation part 023 above the side of the single crystal pulling chamber, a groove or a convex part 027 (see FIG. 6) is provided at the upper end of the heat insulating wall 010, or the upper part (preferably near the upper end). A method of forming a through-hole is also mentioned.

The gas circulation area when the gas circulation part is provided is not particularly limited. The larger the area, the easier the metal fluoride gas flows out to the side. However, if the area is too large, the effect of providing the ceiling plate and / or the heat insulating wall may be reduced. For these reasons, when the area of the opening at the top of the heat insulating wall is 100%, the total cross-sectional area of the narrowest part of the gas flow part is preferably 5 to 50%, preferably 10 to 45%. More preferably, it is considerable. For example, when the opening at the upper part of the heat insulating wall is a circle having a diameter of 1000 mm (opening area: 7850 cm ² ), the distance between the upper end of the heat insulating wall and the lower surface of the ceiling board is provided when the gap is provided by the method of floating the ceiling plate. Is preferably about 12.5 to 125 mm, and more preferably about 25 to 112.5 mm.

  Also, when the method of floating the ceiling plate is adopted, if the distance between the upper end of the heat insulation wall and the lower surface of the ceiling plate is too close, the resistance increases and the gas does not flow easily, but if the distance is too far away, the effect of providing the ceiling plate is reduced. May be. Therefore, in this method, the lower limit of the absolute value is preferably 3 mm or more, more preferably 5 mm or more, and particularly preferably 10 mm or more, regardless of the opening area. On the other hand, the upper limit is preferably 150 mm or less, more preferably 125 mm or less, and particularly preferably 100 mm or less.

  The material of the member constituting the single crystal pulling apparatus of the present invention as described above is the same as the material of a known Czochralski furnace used when producing a metal fluoride single crystal. For example, as a member installed in the chamber, graphite materials such as graphite, vitreous graphite, and silicon carbide-deposited graphite, and refractory metals such as platinum, platinum-rhodium alloy, and molybdenum can be used. It is particularly preferable that the material is made of a graphite-based material because it is stable at the melting temperature of the raw material metal fluoride, has little possibility of contaminating the metal fluoride, and is inexpensive.

  The method for producing a single crystal by using the single crystal pulling apparatus of the present invention may follow a known method. A typical manufacturing method is briefly described below.

  The metal fluoride single crystal that can be produced by the single crystal pulling apparatus of the present invention is not particularly limited as long as it is a single crystal that can be produced by the Czochralski method. Specific examples of the metal fluoride include lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, magnesium fluoride, calcium fluoride, barium fluoride, strontium fluoride, aluminum fluoride, and barium fluoride. Lithium, potassium magnesium fluoride, lithium aluminum fluoride, calcium strontium fluoride, potassium magnesium fluoride, lithium strontium fluoride, cesium calcium fluoride, lithium calcium aluminum fluoride, lithium strontium aluminum fluoride, lanthanoid fluorides, etc. Is mentioned. Among these, it is preferable to use for production of alkaline earth metal fluorides such as magnesium fluoride, calcium fluoride, and barium fluoride.

  As the metal fluoride used as a raw material, those having as few impurities as possible are preferable. A crucible in a single crystal pulling furnace using as a raw material metal fluoride, which is usually heated and melted in the presence of a scavenger such as zinc fluoride, lead fluoride, or carbon tetrafluoride to remove most of impurities such as oxides and moisture. Insert inside.

  The raw material metal fluoride charged in the crucible is preferably subjected to a heat treatment under reduced pressure prior to melting to further remove adsorbed moisture. After sufficiently heating to remove adsorbed moisture, the raw material metal fluoride is melted.

  During the heating and melting, it is preferable to charge the raw material into the outer crucible. In this case, it is preferable that the outer crucible 001 is pulled down and heated and melted in a semi-enclosed space formed by the outer wall of the inner crucible 002 and the closing member 007.

  After the raw material is sufficiently melted, the outer crucible 001 is raised, and the raw material melt 004 is caused to flow into the inner crucible 002 from the through hole 003. Thereafter, the seed crystal mounted on the seed crystal holder 017 at the tip of the crystal pulling shaft 015 is brought into contact with the surface of the raw material melt, and then gradually pulled to grow a single crystal body 018.

  The temperature at which the single crystal is pulled is determined in accordance with the target metal fluoride. For example, at the measurement temperature at the bottom of the outer crucible, in the case of calcium fluoride, 1440 ° C. or higher, preferably 1440-1520 ° C. In the case of barium fluoride, it is preferably carried out at a temperature of 1300 to 1400 ° C. Moreover, it is preferable that the temperature increase rate to this temperature is 10-500 degreeC / hour.

  In order to eliminate the influence of residual moisture, the removal of water and the pulling up by the heating are preferably performed in the presence of a scavenger. Scavengers include solid scavengers such as zinc fluoride, lead fluoride, and polytetrafluoroethylene that are charged together with the raw metal fluoride, as well as carbon tetrafluoride, carbon trifluoride, and hexafluoride introduced into the chamber as an atmosphere. A gas scavenger such as ethane fluoride is used. It is preferable to use a solid scavenger, and the amount used is preferably 0.005 to 5 parts by weight with respect to 100 parts by weight of the raw metal fluoride.

  The seed crystal 016 used for the pulling method is a single crystal of metal fluoride, and the growth surface of the seed crystal is the [111] plane, the [100] plane, etc. according to the crystal main growth plane of the as-grown single crystal to be produced. A contact surface with the raw material melt may be used. After bringing the seed crystal into contact with the raw material melt surface, the single crystal can be grown by gradually pulling it up at a speed of 0.1 to 20 mm / h. As the single crystal body 018 grows, the raw material melt in the inner crucible 002 is consumed. By raising the outer crucible 001, an amount of the raw material melt reaching the reduced amount is gradually passed through the through-hole 003. By supplying it to the inner crucible, the position of the melt surface, that is, the crystal growth surface can be made constant.

  During the growth of the single crystal, the seed crystal 016 is preferably rotated about the pulling shaft 015, and the rotation speed is preferably 5 to 30 times / minute. In addition to the rotation of the seed crystal, the outer crucible may be rotated at the same rotational speed in the direction opposite to the rotation direction of the seed crystal.

  The furnace pressure during pulling up of the single crystal may be under pressure, normal pressure, or reduced pressure. However, as described above, the pressure inside the furnace is generally reduced under reduced pressure because it is less likely to cause a negative crystal called a scattering center. Is preferred. When crystal growth is performed under reduced pressure, the furnace pressure is preferably 0.5 to 70 kPa, and more preferably 5 to 50 kPa. Thus, when crystal growth is performed under reduced pressure, the volatilization amount of the metal fluoride from the raw material melt increases. The production apparatus of the present invention is particularly useful for the production of a metal fluoride single crystal under such conditions that the amount of volatilization of the metal fluoride increases.

  After pulling up the single crystal 018 having a desired size in this way, the temperature is lowered to a temperature at which it can be taken out from the furnace. The temperature decreasing rate is preferably 0.01 to 3 ° C./min. In order to obtain an as-grown single crystal which is less likely to be cracked or chipped during the processing described below, it is set to 0.1 to 0.5 ° C./min. Is more preferable.

  After pulling up the single crystal in this manner, polishing / grinding or heat treatment can be performed as necessary to obtain final products such as lens blanks, lenses, window materials, and the like.

  Hereinafter, embodiments of the present invention will be described in more detail with specific experimental examples, but the present invention is not limited thereto.

Foam / SC observation method:
In a dark room, a halogen light source (MegaLight 100 manufactured by SCHOTT: lamp 12V100W maximum output) was brought into close contact with the surface of the obtained as-grown single crystal, and irradiated from each direction to determine the presence and position of light scattering points (defects) and their positions. It was observed visually. At that time, bubbles that do not change the appearance even when the irradiation angle of the light source is changed are bubbles, and those that change the appearance depending on the irradiation angle are SC.

  Reference example: A model experiment was conducted to investigate the effect of falling objects on the raw material melt.

  The single crystal pulling apparatus shown in FIG. 1 was used to pull up the calcium fluoride single crystal. While growing a single crystal body with a diameter of 165 mm in the straight body at a pulling speed of 4 mm / Hr, 0.1 g, 1 g and 7 g of calcium fluoride were respectively added to the raw material melt when the straight body length reached 35 mm, 50 mm and 80 mm. Let it fall inside. Immediately after 0.1 g of calcium fluoride was dropped, no abnormal growth behavior was observed, but when 1 g of calcium fluoride was dropped, a temporary increase of about +6 mm to the control diameter after a few minutes. Rapid growth occurred. Further, since 7 g of calcium fluoride crystal was dropped and a stalagmite-like crystal was formed on the bottom of the inner crucible 2 minutes later, the crystal growth was stopped at that time, and the cooled single crystal was observed after cooling.

  As a result, no abnormalities were observed at the site when 0.1 g of calcium fluoride was dropped, but at the site when 1 g of calcium fluoride was dropped, it was estimated that the crystal growth interface ( A large number of bubbles were found along the shape of the line.

  From this result, it has been clarified that if there is a relatively large fallen object in the raw material melt during crystal growth, the production of the single crystal is adversely affected. On the other hand, it was also found that a small amount would not be a big problem.

Example 1
A calcium fluoride single crystal was manufactured using a single crystal pulling apparatus in which a gap was provided between the upper end of the heat insulating wall and the lower surface of the ceiling plate as shown in FIG. The inner crucible has an inner diameter of 360 mm, the center depth is 127 mm, the outer crucible has an inner diameter of 500 mm, and the heat insulating wall has an inner diameter of 680 mm. The position of the upper end of the heat insulation wall was 335 mm higher than the position of the upper end of the inner crucible, and the ceiling plate was a pin-like member as shown in the schematic diagram of FIG. The ceiling plate used was a crystal pulling shaft insertion hole with a diameter of 140 mm (area 15394 mm ² ) and a 4898 mm ² viewing window at the center.

After the furnace is fully baked in the presence of zinc fluoride, 80 kg of raw material metal fluoride and 10 g of zinc fluoride as a scavenger are placed in the space formed by the outer wall of the inner crucible, the inner wall of the outer crucible and the shielding member. The evacuation was started. When the internal pressure reached 5 × 10 ⁻³ Pa or less, the heater was energized while evacuation was continued to start heating the raw material. The temperature was raised at about 50 ° C./Hr until the temperature at the bottom of the crucible reached 250 ° C., and this temperature was maintained for 24 hours. The degree of vacuum in the chamber at that time was 5 × 10 ⁻⁴ Pa. Thereafter, the temperature was raised again at about 50 ° C./Hr, and after reaching 600 ° C., the temperature was further maintained for 12 hours. Thereafter, the vacuum exhaust line was shut off, and high-purity argon was supplied into the chamber. The gas was not introduced until the internal pressure was reduced to 30 kPa (abs) and the pulling was completed and the temperature was lowered to around room temperature.

  After returning to 30 kPa, the temperature was raised to around 1500 ° C. and held for 3 hours to melt the raw material. In this state, the position of the outer crucible was raised and a part of the melt was caused to flow into the inner space of the inner crucible through the through hole, so that the melt of calcium fluoride raw material was also stored in the inner crucible.

  After the temperature of the melt was lowered until the temperature of the raw material melt surface level became substantially equal to the melting temperature of calcium fluoride, the seed crystal was brought into contact with the melt surface and pulled up at 4 mm / Hr for growth. A single crystal having a diameter of the straight body portion of 225 mm and a length of the straight body portion of 150 mm was grown, the crystal was separated from the melt, and the temperature was lowered to room temperature.

  During the above single crystal growth, observe the single crystal in the furnace from the viewing window about every hour, and if there is a change in the appearance of the shoulder of the single crystal (attachment of fallen objects or its increase), the time zone Was recorded.

  As a result, during the period from when the shoulder is formed to when the crystal is separated from the melt, the fallen object is hardly observed to adhere to the shoulder, and can be grown with a deviation of ± 1 mm or less with respect to the control diameter. did it. Moreover, although the bubble and SC were visually observed about the taken-out single crystal body, all hardly existed (FIG. 7).

Comparative Example 1
By adhering the upper end of the heat insulating wall and the lower surface of the ceiling plate as shown in FIG. 1, the calcium fluoride single crystal is pulled up in the same manner as in Example 1 except that an apparatus in which no gap is formed is used. A single crystal having substantially the same shape was obtained.

  As in Example 1, when the single crystal inside the furnace was observed from the observation window about every hour to observe the appearance of the shoulder of the single crystal, about 29.5 hours after the start of pulling (directly) It was observed twice that the aspect of the shoulder portion changed significantly after the trunk length of about 29 mm) and after about 46.5 hours (at the time of 97 mm). Further, it was confirmed that a rapid growth of about +11 mm with respect to the control diameter occurred after 23 hours and 40 minutes. This rapid growth almost coincides with the second time zone in which the appearance of the shoulder has changed, and is considered to be caused by falling objects.

  Further, as schematically shown in FIG. 7, when the bubbles and SC were visually observed with respect to the single crystal taken out, it was estimated that the crystal growth interface (at the site corresponding to the time zone when the appearance of the shoulder changed significantly). A large number of SCs and bubbles were mixed along the shape of the line.

Comparative Example 2
The calcium fluoride single crystal was pulled up in the same manner as in Comparative Example 1 except that the pressure (pressure in the furnace) for returning before raising the temperature to around 1500 ° C. was 60 kPa (abs). A crystal was obtained.

  As a result, although the temporary rapid growth did not occur during the growth, the appearance of the shoulder changed significantly after about 50.5 hours from the start of the pulling (when the straight body length was about 113 mm). Observed once. When the bubbles and SC were visually observed with respect to the single crystal taken out, the amount corresponding to the time zone in which the appearance of the shoulder portion changed significantly was not as much as in Comparative Example 1, but it was assumed that the crystal growth interface ( Along the shape of the (line), mixing of SC and foam was confirmed.

Example 2
In Example 1, the calcium fluoride single crystal was pulled up in the same manner as in Example 1 except that the ceiling plate was lifted 60 mm from the upper end of the heat insulating wall to obtain a single crystal having substantially the same shape.

  As a result, although the heat dissipation increased, the output slightly changed, but as in Example 1, almost no adhesion to the shoulder due to falling objects was observed during the growth, and the deviation of ± 1 mm or less with respect to the control diameter I was able to train with. Moreover, although the bubble and SC were visually observed about the taken-out single crystal body, all were hardly existing.

Example 3
In Example 1, the calcium fluoride single crystal was pulled up in the same manner as in Example 1 except that the following points were changed.
・ Inner crucible: inner diameter 250mm, center depth 114mm
・ Outer crucible: 360mm inner diameter
・ Insulation wall: inner diameter 550mm
・ Distance between the upper end of the heat insulating material and the upper end of the inner crucible: 430 mm
・ Distance between heat insulation wall top and ceiling board bottom: 20mm
・ Peeping hole in ceiling plate: area 7000mm ²
・ Raw metal fluoride charge: 30kg
・ Zinc fluoride charge: 3g
・ Crystal straight body: Diameter 130mm, length 320mm
As a result, as in Example 1, almost no adhesion of the fallen object to the shoulder was observed during the growth, and the growth could be performed with a deviation of ± 1 mm or less with respect to the control diameter. Moreover, although the bubble and SC were visually observed about the taken-out single crystal body, all were hardly existing.

Comparative Example 3
By adhering the upper end of the heat insulating wall and the lower surface of the ceiling plate as shown in FIG. 1, the calcium fluoride single crystal is pulled up in the same manner as in Example 3 except that an apparatus in which no gap is formed is used. A single crystal having substantially the same shape was obtained.

  As in Example 1, when the single crystal inside the furnace was observed from the observation window about every hour to observe the appearance of the shoulder of the single crystal, about 34 hours after the start of pulling (the length of the straight body) It was observed once that the appearance of the shoulder changed remarkably at about 78 mm). Immediately after observing the change, there was no temporary rapid growth, but 40 hours after the shoulder was formed (when the length of the straight body was about 100 mm), a temporary increase of about +6 mm with respect to the control diameter. Rapid growth occurred. Further, when the bubbles and SC were visually observed with respect to the taken out single crystal, the crystal growth interface (presumed to be at the site corresponding to the time zone in which the appearance of the shoulder changed significantly and the time zone in which temporary rapid growth occurred was estimated. A large number of SCs and bubbles were mixed along the shape of the line.

The schematic diagram which shows the structure of the conventional CZ method single crystal pulling furnace. The schematic diagram which shows the one aspect | mode of the single crystal pulling furnace of this invention. The schematic diagram which shows the example which lifted the ceiling board with the pin-shaped member and provided the gas flow path. The schematic diagram which shows the example which lifted the ceiling board with the small piece-like member, and provided the gas flow path. The schematic diagram which shows the example which lifted the ceiling board with the ceiling board suspension member, and provided the gas flow path. The schematic diagram which shows the example which provided the groove part in the heat insulation wall upper part, and provided the gas flow path. The schematic diagram which shows the quantity of the defect in the single crystal body manufactured in Example 1 and Comparative Example 1. FIG.

Explanation of symbols

001, 101: outer crucible 002, 102: inner crucible 003, 103: through-hole 004 in inner crucible wall, 104: raw material melt 005, 105: outer crucible support shaft 006, 106: cradle 007, 107: opening closed Member 008, 108: Chamber 009, 109: Melting heater 010, 110: Heat insulation wall 011, 111: Isolation wall 012, 112: Lid material 013, 113: Connection member 014, 114: Inner crucible hanging rod 015, 115: Crystal Pulling shafts 016, 116: Seed crystal 017, 117: Seed crystal holder 018, 118: Single crystal body 019, 119: Ceiling plate 020, 120: Crystal pulling shaft insertion hole 021, 121: Viewing window 022, 122: Window hole 023: Gas circulation part 024: Pin-like member 025: Small piece-like member 026: Ceiling board hanging member 027: Groove part of heat insulation wall upper end

Claims (4)

A chamber constituting a crystal growth furnace;
A crucible disposed in the chamber and containing a melt of a raw material for producing a single crystal;
A melting heater disposed so as to surround the periphery of the crucible;
A single crystal pulling shaft which is provided with a seed crystal holding part at the tip, and which can move up and down so that the seed crystal held in the seed crystal holding part can come into contact with the melt of the single crystal production raw material housed in the crucible; ,
A heat insulating wall disposed in the chamber so as to surround a side periphery of the single crystal pulling region at least above the crucible;
A ceiling plate that covers the opening at the top of the heat insulation wall and has at least an insertion hole for inserting the single crystal pulling rod;
A single crystal pulling apparatus for metal fluoride comprising:
A gas flow part capable of flowing gas from the single crystal pulling chamber to the outside is provided on an upper side portion of the single crystal pulling chamber configured with the heat insulating wall as a side wall and the ceiling plate as a ceiling wall. A single crystal pulling device for metal fluoride.   2. The single crystal pulling for metal fluoride according to claim 1, wherein the gas circulation part is a gap between the ceiling plate provided by lifting the ceiling plate so that the ceiling plate does not adhere to the upper end of the heat insulating wall and the upper end of the heat insulating wall. apparatus.   A method for producing a metal fluoride single crystal using the single crystal pulling apparatus for metal fluoride according to claim 1 or 2.   The method according to claim 3, wherein the pulling of the metal fluoride single crystal is performed under reduced pressure.

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