JP2018193277A - Crystal growth apparatus - Google Patents

Abstract

To provide a crystal growth apparatus low in cost, having a high nondefective rate and capable of corresponding to a long-sized crystal to be grown.SOLUTION: The crystal growth apparatus comprises: a metal crucible capable of holding a raw material melt and having a cylindrical side surface; a metal annular reflector arranged at the upper end so as to cover a peripheral edge portion of the crucible; a metal cylindrical after-heater provided on the reflector; and a heating induction coil including a lower coil part arranged around the crucible, an upper coil part separated from the lower coil part and arranged above the lower coil part and a roughly wound coil part provided between the upper coil part and the lower coil part and connecting the upper coil part and lower coil part.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a crystal growth apparatus.

  As a method for producing a single crystal, the Czochralski method is widely used in which a crucible filled with a raw material is heated and melted, then a seed crystal is brought into contact with the surface of the raw material melt, and the crystal is grown while raising the seed crystal. .

  In crystal growth by the Czochralski method, a high-frequency induction heating method and a resistance heating method are used as means for heating the crucible. The heating method is selected in consideration of the melting point of the crystal to be grown and the atmosphere in which the crystal is grown.

  The oxide single crystal to be grown has a high melting point, and when it is necessary to grow in an oxidizing atmosphere to suppress decomposition of the raw material melt, a high-frequency induction heating method is used because there is no suitable resistance heating element. There are many cases.

  In the high frequency induction heating method, an induction coil is arranged around the crucible, and a high frequency current is supplied to the induction coil. By passing a high frequency current through the induction coil, an eddy current is generated in the crucible, whereby the crucible generates heat and the raw material in the crucible melts.

  When the seed crystal is brought into contact with the surface of the raw material melt and crystal growth is performed while raising the seed crystal, the upper part of the grown crystal is separated from the heat generating crucible as the crystal grows. Further, since the heat inside the crystal escapes along the pulling shaft, the upper part of the crystal is cooled. Therefore, when the heating element is only the crucible, the temperature difference in the growing crystal becomes large, and thermal stress due to the temperature difference in the crystal is generated. Then, cracks and crystal defects due to thermal stress are generated inside the crystal. Therefore, in order to reduce the thermal stress in the crystal, an effort has been made to keep the upper portion of the crucible (the growing single crystal) warm.

  For example, Patent Document 1 discloses an oxide single crystal in which a cylindrical after heater, which is a heating element other than a crucible, is arranged above a crucible in order to suppress cracking due to thermal stress caused by a temperature difference in the crystal. A manufacturing method is disclosed.

  Patent Document 2 discloses a method for producing an oxide single crystal that keeps the upper part of the crucible by arranging a doughnut-shaped reflector.

JP-A-7-187880 JP 7-33586 A JP 2014-125404 A

  By the way, in recent years, the market for oxide single crystals, particularly lithium tantalate, is expanding as a surface acoustic wave device material, and it is required to lengthen the single crystals in order to secure production. However, along with the lengthening of the crystal, polycrystallization that occurs in the bending and straight body of the crystal, or cracks due to thermal strain during cooling, solidification of the raw material of the crucible bottom, etc. are likely to occur. This is a cause of lowering the yield rate of crystals. These reductions in the yield rate are attributed to the temperature distribution in the furnace during crystal growth.

  Therefore, in Patent Document 3, a first induction coil arranged around the crucible and a second derivative arranged around the after heater are arranged, and individual high frequency power supplies are prepared for the respective derivatives. A crystal growth apparatus is disclosed in which high-frequency currents to be supplied are set to have opposite phases. In the crystal growth apparatus described in Patent Document 3, the after-heater can be individually heated, so that an appropriate temperature distribution can be set. However, two power supply facilities are required, the equipment cost is high, and a wide installation location is required. Further, it is necessary to appropriately control the magnetic field caused by the first high-frequency current and the magnetic field caused by the second high-frequency current by reversing the phases of the first high-frequency current and the second high-frequency current. Sophisticated control is required. For the reasons described above, the crystal growing apparatus described in Patent Document 3 has a problem that it requires complicated and high-performance equipment, resulting in high costs.

  In view of the above circumstances, an object of the present invention is to provide a crystal growth apparatus that is low in cost, has a high yield rate, and can cope with an increase in the length of a grown crystal.

In order to achieve the above object, a crystal growth apparatus according to one embodiment of the present invention includes a metal crucible having a cylindrical side surface capable of holding a raw material melt,
A metal annular reflector disposed at the upper end so as to cover the periphery of the crucible;
A metal cylindrical after-heater provided on the reflector;
A lower coil portion disposed around the crucible, an upper coil portion spaced apart from the lower coil portion and disposed above the lower coil portion, and between the upper coil portion and the lower coil portion And a heating induction coil having a coarsely wound coil portion that connects the upper coil portion and the lower coil portion.

  ADVANTAGE OF THE INVENTION According to this invention, the crystal growth apparatus which can respond to the lengthening of the growth crystal | crystallization which is low-cost and has a high yield rate can be provided.

It is the composition outline figure showing an example of the crystal growth device concerning the embodiment of the present invention. It is the simulation result which calculated the temperature distribution in the crystal under growth using the crystal growth device provided with metal reflectors. It is the figure which showed the calculation result of the stress which generate | occur | produces on the growth crystal | crystallization surface by the induction coil of the crystal growth apparatus which concerns on this embodiment, and the conventional induction coil.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

A Czochralski crystal growth apparatus according to an embodiment of the present invention includes lithium niobate LiNbO ₃ (hereinafter LN), lithium tantalate LiTaO ₃ (hereinafter LT), and yttrium aluminum grown in an oxidizing atmosphere or an inert gas atmosphere. This is a crystal growth apparatus used for manufacturing an oxide single crystal such as garnet Y ₃ Al ₅ O ₁₂ (hereinafter referred to as YAG). In the Czochralski method, the tip of a single crystal called a seed crystal cut out according to a certain crystal orientation is brought into contact with the raw material melt and gradually pulled up while rotating, thereby propagating the properties of the seed crystal with a large diameter. This is a method for producing a single crystal.

  FIG. 1 is a schematic configuration diagram illustrating an example of a crystal growth apparatus according to an embodiment of the present invention. As shown in FIG. 1, the crystal growing apparatus according to the present embodiment includes a crucible 10, a reflector 20, an after heater 30, a crucible base 40, a lifting shaft 50, an induction coil 60, and a refractory 70. And a heat insulating material 80 and a chamber 90. The heating means is an induction coil 60 that heats the crucible 10 and the after heater 30. The reflector 20 is also heated although its calorific value is smaller than that of the crucible 10 and after heater. A power source 100 is provided to supply high frequency power to the induction coil 60. When the single crystal is manufactured, the raw material melt 110 is stored and held in the crucible 10, and the seed crystal 120 is held at the lower end of the pulling shaft 50.

  The crucible 10 is a container for storing and holding the raw material melt 110 and has a cylindrical side surface and a circular bottom surface. The crucible 10 is made of a metal material. Although the crucible 10 may be comprised with various metal materials, for example, it may be comprised with iridium and platinum excellent in heat resistance.

  The reflector 20 is heat reflection means for reflecting the heat in the heated crucible 10 and returning it to the crucible 10. The reflector 20 is provided at the upper end of the side surface of the crucible 10 so as to cover the peripheral edge of the upper end of the side surface of the crucible 10. Provided. Therefore, the reflector 20 has an annular shape. The reflector 20 is also made of a metal material. The reflector 20 may be made of various metal materials, but may be made of iridium, platinum or the like having excellent heat resistance, for example, like the crucible 10.

  The after heater 30 is a heating means for heating the single crystal pulled up from the crucible 10. The after heater 30 is provided on the reflector 20 and has a cylindrical shape. The after heater 30 is also made of a metal material. The after-heater 30 may be made of various metal materials, but may be made of iridium, platinum or the like having excellent heat resistance, as with the crucible 10, for example.

  The crucible base 40 is a support base for supporting the crucible 10 and is disposed below the crucible 10.

  The pulling shaft 50 is a single crystal pulling means for pulling the single crystal by holding the seed crystal 120 at the lower end and bringing the seed crystal 120 into contact with the raw material melt 110 held by the crucible 10. The pulling shaft 50 is connected to a motor or the like (not shown) and configured to be able to pull up the single crystal while rotating.

  An induction coil 60 is disposed outside the side surface of the refractory 70. The induction coil 60 has an upper coil part 61, a lower coil part 62, and a coarsely wound coil part 63. A lower coil portion 62 surrounds the periphery of the crucible in the horizontal direction. An upper coil portion 61 surrounds the upper portion of the after heater 30. Furthermore, between the upper coil portion 61 and the lower coil portion 62, a coarsely wound coil portion 63 that connects them is provided.

  The refractory 70 is a means for covering the hot zone of the crucible 10 and the after heater 30 from the outside.

  The heat insulating material 80 is a means for covering the periphery of the crucible 10 and suppressing the release of heat to the outside.

  The chamber 90 is a means for covering the whole including the crucible 10, the after heater 30, the induction coil 60, the refractory 70, the crucible base 40, and the like, and maintains the crystal growth atmosphere and prevents heat from leaking to the outside. To play a role.

  The power source 100 is a power supply source for supplying high frequency power to the induction coil 60 and performing high frequency induction heating.

  In the manufacture of the single crystal, the crucible 10 filled with the single crystal raw material is heated by high frequency induction heating to obtain the raw material melt 110. Thereafter, the seed crystal 120 connected to the pulling shaft 50 is brought into contact with the surface of the raw material melt to produce a single crystal. The crystal growth method is not particularly limited, and a known technique can be used.

  Here, various problems occur when the grown crystal is lengthened. As such problems, there are a phenomenon in which the grown crystal is bent, cracks due to polycrystallization, and cracks during cooling.

  One possible cause of the bending of the grown crystal is that the exhaust heat (temperature difference) inside the crystal is small. In the growing part during crystal growth, solidification latent heat is generated due to the phase change from the melt to the solid. In order not to adversely affect the crystal growth, it is important to quickly and surely release the latent heat of solidification. The latent heat of solidification is transferred through the grown crystal and exhausted. It is considered that when the temperature distribution in the furnace is asymmetric, crystal growth is promoted in a portion where the amount of exhaust heat is large, and the crystal shape is bent or twisted. When the growth crystal is lengthened, the amount of decrease in the raw material melt surface accompanying crystallization increases and the amount of exposure on the side surface of the crucible increases. When the crucible 10 that is a heating element is exposed, the temperature difference in the crystal is reduced by heating from the side surface of the grown crystal, and the bending phenomenon becomes remarkable.

  In order to suppress the bending phenomenon, it is considered effective to increase the temperature gradient in the crystal (in the growth furnace). By increasing the temperature gradient in the growth furnace, it is considered that natural convection in the raw material melt 110 is also promoted and an effect of suppressing the bending phenomenon can be obtained.

  On the other hand, in order to suppress polycrystallization and cracking during cooling, it is considered effective to reduce the temperature gradient in the crystal (in the growth furnace). It is considered that polycrystallization occurs due to the introduction and accumulation of dislocations due to the stress caused by the temperature difference in the crystal being applied to the crystal. Similarly, in the process of cooling the crystal, strain is generated due to the temperature difference within the crystal (in the growth furnace). Therefore, it is considered that reducing the temperature difference is effective for suppressing cracking during cooling.

  As described above, the problem of conflicting temperature gradients in the crystal (in the growth furnace) arises due to the lengthening of the crystal. In order to solve these problems, it is necessary to form an optimum temperature gradient by the shape of the induction coil.

  In the induction coil conventionally used, the crucible, the reflector, and the after heater have a heat generation distribution that is uniform or biased to one side. The temperature gradient is too small when heat is generated evenly when the crystal is elongated. On the other hand, in the uneven heat generation distribution, the temperature gradient is too large.

  Therefore, in order to lengthen the crystal, in this embodiment, the configuration of the induction coil 60 has been greatly devised. It is considered that the occurrence of cracking during polycrystallization or cooling is caused by the temperature difference in the vertical direction of the crystal because the cooling of the upper part of the crystal is promoted by heat radiation from the seed shaft connected to the seed crystal 120.

  Moreover, the crystal | crystallization outer peripheral part under the growth of the reflector vicinity is heated by the heat_generation | fever of the reflector 20. FIG. On the other hand, the temperature of the crystal central portion decreases due to heat radiation from the seed shaft. Therefore, it is considered that stress due to the temperature difference in the crystal diameter direction occurs in the vicinity of the reflector and polycrystallization occurs.

  FIG. 2 is a simulation result of calculating the temperature distribution in the crystal being grown when the crystal growth apparatus including the metal reflector 20 is used. At this time, the dielectric coils are evenly arranged from the crucible to the upper end of the after heater. In FIG. 2, regions A, B, and C are shown in descending order of temperature. As shown in FIG. 2, a region below the reflector 20 is a region A where the temperature is highest. And below that, temperature falls in order and becomes area B and area C. In this case, the area near the lower part of the reflector 20 is the area A, and the temperature is too high. Therefore, this point needs to be corrected, but it is shown that the heat reflectance is high. A configuration in which the reflector 20 is not heated by changing the material of the reflector 20 to one that does not flow an induced current, for example, made of ceramics is also conceivable. However, the reflector 20 has an object of increasing the radial temperature gradient of the surface of the melt raw material 110 and improving the crystal diameter controllability by reflecting radiation from the surface of the melt raw material 110. In the case of ceramic or the like, such heat reflectance cannot be shown. Therefore, the reflector 20 is desirably made of metal having a high reflectance. However, if the reflector 20 is simply provided, as shown in FIG. 2, the temperature in the vicinity of the lower portion of the reflector 20 becomes too high, and the single crystal may be cracked.

  Therefore, in the present embodiment, in order to solve these problems, the induction coil 60 is configured as follows. First, in order to heat the upper end part of the after heater 30, the upper coil part 61 was provided. As shown in the region A in FIG. 2, the upper end portion of the after heater 30 generates a large amount of heat due to the high frequency induction heating due to the edge effect. Therefore, in FIG. 1, the center position of the coil length in the vertical direction (or height direction, vertical direction) of the upper coil portion 61 is arranged within 30 mm from the top and bottom of the upper end of the after heater. If the center position in the vertical direction of the upper coil portion 61 is set higher than 30 mm above the upper end of the after-heater than this range, the heat generated by the after-heater 30 becomes weak, and the heat dissipation from the seed shaft cannot be suppressed. The occurrence of cracking during polycrystallization and cooling increases. Moreover, when the center position in the up-down direction of the upper coil portion 61 is disposed below 30 mm beyond the upper end of the after heater, the temperature gradient in the furnace becomes small, and a crystal bending phenomenon occurs.

  Moreover, the upper end part of the lower coil part 62 for heating the crucible 10 is arrange | positioned in the range within 30 mm of the upper and lower sides of a crucible upper end part. Here, since the reflector 20 which covers the peripheral part of the crucible 10 is provided at the upper end of the crucible 10, it may be regarded as a range within 30 mm above and below the reflector 20. If the upper end of the lower coil is disposed above 30 mm from the upper end of the crucible, the temperature gradient at the upper part of the melt is reduced and a bending phenomenon occurs. Also, if the upper end of the lower coil part 62 is disposed below the upper end of the crucible by more than 30 mm, the temperature of the upper part of the crucible is lowered and the temperature difference in the grown crystal is increased. There is a risk of cracking.

  A coarsely wound coil portion 63 is provided between the upper coil portion 61 and the lower coil portion 62 to connect the upper and lower coil portions 61 and 62 to each other. The coarsely wound coil portion 63 is changed from a half turn to one turn depending on the diameter of the induction coil 60. When the number of turns of the coarse winding portion 63 is large, the heat generation of the crucible 10 or the after heater 30 by the coarse winding coil portion 63 becomes large, and the temperature gradient becomes small. Therefore, a bending phenomenon occurs. In addition, when the upper and lower coil portions 61 and 62 are connected to each other by sudden bending, the current concentrates on this portion, and the life of the induction coil 60 is shortened due to coil deterioration. By connecting the upper and lower coil portions 61 and 62 with the coarsely wound coil portion 63, the magnetic field generated by the coil becomes weak on the crucible. Therefore, it is possible to reduce the heat generation of the reflector 20 installed on the crucible, and the influence of the heating of the crystal outer periphery via the reflector 20 is reduced, and the temperature difference in the crystal diameter direction can be reduced. . Thereby, it is possible to suppress polycrystallization.

  The upper coil portion 61 and the lower coil portion 62 are preferably configured such that the lower coil portion 62 has a larger number of turns and a smaller winding pitch than the upper coil portion 61. In the growth of a single crystal, it is most important to efficiently heat the crucible 10, and although heating by the after heater 30 is important, it becomes auxiliary heating until it gets tired of heating the crucible 10. is there. That is, because the heating in the crucible 10 requires the strongest heating, the lower coil portion 62 should be configured to be heated at the highest temperature.

  FIG. 3 is a diagram showing a calculation result of stress generated on the surface of the grown crystal by the induction coil 60 of the crystal growth apparatus according to the present embodiment and the conventional induction coil in which the coarsely wound coil portion 63 does not exist.

  FIG. 3A shows a case where only the conventional lower coil section 62 is provided. That is, an induction coil having 9 turns is disposed only around the crucible, and the upper end of the induction coil is at the same position as the upper end of the crucible 10. That is, the upper coil portion 61 does not exist on the outer peripheral portion of the after heater 30. In this case, since the calorific value of the after-heater 30 is small and the temperature difference in the crystal is large, the stress on the crystal outer peripheral surface becomes large. Therefore, there are concerns about the occurrence of polycrystallization and cracking.

  FIG. 3B shows a configuration in which the number of turns is 14 in order to extend the configuration of the induction coil shown in FIG. In this case, the upper end of the induction coil was set at the same position as the upper end of the after heater. In this case, since the after heater generated heat, the temperature difference in the vertical direction of the crystal was reduced, and the stress on the crystal surface was reduced.

  FIG. 3C shows a case where the induction coil 60 provided in the crystal growth apparatus according to the present embodiment is used, and the lower coil portion 62 is provided around the crucible with the same configuration as that shown in FIG. In order to generate heat at the upper end portion of the after heater, the upper coil portion 61 having two turns is disposed at the upper end portion of the after heater, and the upper and lower coils are connected by a half-turn coarse winding coil portion 63. In this configuration, by suppressing the heat generation of the reflector 20 on the crucible in addition to the heat generation of the after-heater 30, it is possible to reduce the stress on the crystal surface compared to the configuration of FIG. . As described above, it was confirmed that the stress on the crystal surface was reduced by using the induction coil 60 according to the present embodiment.

Thus, according to the crystal growing apparatus according to the present embodiment, the metal reflector 20 is provided at the upper end of the crucible 10, and the induction coil 60 having the upper coil portion 61, the lower coil portion 62, and the coarsely wound coil portion 63 is provided. By using the high frequency induction heating, the stress on the crystal surface can be reduced and the occurrence of polycrystallization and cracking can be suppressed.
[Example]

Hereinafter, examples in which single crystal growth was performed using the crystal growth apparatus according to the present embodiment will be specifically described with reference to comparative examples. In the following description, a method for growing a lithium tantalate single crystal will be described as an example. For ease of understanding, elements corresponding to the constituent elements described so far are denoted by the same reference numerals and description thereof is omitted.
[Example 1]

  An iridium crucible 10 is filled with a raw material of lithium tantalate. The high-frequency induction coil 60 is composed of three parts: an upper coil part 61, a coarsely wound coil part 63, and a lower coil part 62. In addition, the lithium tantalate raw material of the crucible 10 is melted by keeping the temperature with the refractory 70 made of zirconia and alumina, and the iridium pulling shaft 50 is rotated at 1 to 20 rpm at a speed of 1 to 5 mm / h. By pulling up vertically, a single crystal was continuously obtained from the seed crystal 120.

Here, the distance between the upper end portion of the lower coil portion 62 and the upper end of the iridium crucible 10 was set to 0 mm. The distance between the center position of the coil length in the vertical direction of the upper coil portion 61 and the upper end portion of the after heater 30 was set to 0 mm. Moreover, in order to fix in the coil position of the upper coil part 61 and the lower coil part 62, the upper and lower coil parts 61 and 62 were connected by the semi-coiled rough winding coil part 63. FIG. The grown crystal was a single crystal and no cracks were generated.
[Example 2]

The same operation as in Example 1 was performed except that the center position of the upper coil portion 61 in the vertical direction was arranged at a position 30 mm above the upper end portion of the after heater 30. The grown crystal was a single crystal and no cracks were generated.
[Example 3]

The same operation as in Example 1 was performed except that the center position of the upper coil portion 61 in the vertical direction was arranged at a position 30 mm below the upper end portion of the after heater 30. The grown crystal was a single crystal and no cracks were generated.
[Example 4]

The same operation as in Example 1 was performed except that the upper end portion of the lower coil portion 62 was disposed at a position 30 mm above the upper end of the iridium crucible 10. The grown crystal was a single crystal and no cracks were generated.
[Example 5]

The same operation as in Example 1 was performed except that the upper end portion of the lower coil portion 62 was disposed at a position 30 mm below the upper end of the iridium crucible 10. The grown crystal was a single crystal and no cracks were generated.
[Comparative Example 1]

The same operation as in Example 1 was performed except that the center position of the upper coil portion 61 in the vertical direction was arranged at a position 40 mm above the upper end portion of the after heater 30. The grown crystal was polycrystalline and cracked.
[Comparative Example 2]

The same operation as in Example 1 was performed except that the center position of the upper coil portion 61 in the vertical direction was arranged at a position 40 mm below the upper end of the after heater 30. The grown crystal was a single crystal and no cracking occurred, but bending occurred.
[Comparative Example 3]

The same operation as in Example 1 was performed except that the upper end portion of the lower coil portion 62 was disposed at a position 40 mm above the upper end of the iridium crucible 10. The grown crystal was a single crystal and no cracking occurred, but bending occurred.
[Comparative Example 4]

  The same operation as in Example 1 was performed except that the upper end portion of the lower coil portion 62 was disposed at a position 40 mm below the upper end of the iridium crucible 10. The grown crystal was polycrystalline and cracked.

  As described above, according to the crystal growing apparatus according to the present example, it was shown that bending and cracking of the single crystal can be prevented by appropriately setting the positions of the upper coil portion 61 and the lower coil portion 62.

  The preferred embodiments and examples of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments and examples, and the above-described embodiments and examples can be performed without departing from the scope of the present invention. Various modifications and substitutions can be made to the embodiments.

10 crucible 20 reflector 30 after heater 40 crucible base 50 lifting shaft 60 induction coil 61 upper coil part 62 lower coil part 63 coarse winding coil part 63
70 Refractory 80 Heat insulation 90 Chamber 100 Power supply 110 Raw material melt 120 Seed crystal

Claims (6)

A metal crucible having a cylindrical side surface capable of holding a raw material melt;
A metal annular reflector disposed at the upper end so as to cover the periphery of the crucible;
A metal cylindrical after-heater provided on the reflector;
A lower coil portion disposed around the crucible, an upper coil portion spaced apart from the lower coil portion and disposed above the lower coil portion, and between the upper coil portion and the lower coil portion A crystal growth apparatus comprising: a heating induction coil that is provided and has a coarsely wound coil part that connects the upper coil part and the lower coil part.   The crystal growth apparatus according to claim 1, wherein the center position of the coil length in the vertical direction of the upper coil portion of the induction coil is within a range of 30 mm above and below the upper end portion of the after heater.   The crystal growth apparatus according to claim 1 or 2, wherein an upper end portion of the lower coil portion of the induction coil is within a range of 30 mm above and below the upper end of the crucible.   The winding number of the said lower coil part is more than the winding number of the said upper coil part, and the winding number of the said upper coil part is larger than the winding number of the said coarse winding coil part. The crystal growth apparatus described.   The crystal growing apparatus according to any one of claims 1 to 4, wherein the number of turns of the coarsely wound coil portion is 1 or less.   The crystal growth apparatus according to claim 1, wherein the crucible and the reflector are made of iridium or platinum.