Crystal growth apparatus and oxide single crystal growth method

Abstract

To provide a crystal growth apparatus that suppresses deformation in a crucible while maintaining thermal insulation property of the crucible, and prevents decline in single crystallization rate due to repeated growth and increase in cost for growth, and to provide an oxide single crystal growth method.SOLUTION: A crystal growth apparatus includes: a metallic crucible for storing and holding a crystal raw material; and a cylinder-molding thermal insulation material for covering a periphery of the crucible to suppress deformation of the crucible.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a crystal growth apparatus and a method for growing an oxide single crystal. [Background technology]

[0002] Lithium tantalate (LiTaO3; hereafter abbreviated as LT) single crystals and lithium niobate (LiNbO3) single crystals have traditionally been used as materials for surface acoustic wave elements (SAW filters) used primarily in mobile communication devices to remove noise from electrical signals.

[0003] Industrially, LT and LN single crystals are grown using the Czochralski (Cz) method. For example, LT single crystals are grown in an iridium (Ir) crucible in a high-frequency induction-heated growth furnace in a nitrogen-oxygen mixed gas atmosphere. The Cz method involves immersing a seed crystal in a raw material melt in a cylindrical crucible and then pulling the seed crystal upward while rotating it, thereby growing a single crystal with the same orientation as the seed crystal. The seed crystal rotation speed and pulling speed depend on the type of crystal being grown and the temperature environment during growth, and must be appropriately selected based on these conditions. After growth, the crystal is cooled at a predetermined rate in the growth furnace and then removed from the furnace. The removed single crystal undergoes annealing and poling processes, followed by slicing and polishing to form substrates with thicknesses of several hundred microns, which are used as SAW filter materials.

[0004] When growing LT single crystals using the Cz method in a high-frequency induction heating furnace, as shown in Patent Document 1, an Ir crucible is typically placed in a work coil that generates a high-frequency magnetic field. The Ir crucible is surrounded by a felt-like insulator such as zirconia bubble wrap or fiber flux, and an alumina container to maintain heat, creating an optimal temperature environment for crystal growth. Here, the optimal temperature environment for crystal growth refers to a growth atmosphere in which the crystal is pulled and a temperature distribution in the raw material melt that are axially symmetrical with respect to the pulling axis, and the temperature gradients in the growth atmosphere and the melt are maintained within appropriate ranges. When growing LN single crystals using a similar method, the structure is similar except that the crucible is made of platinum (Pt). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-165796 Summary of the Invention [Problem to be solved by the invention]

[0006] In the case of a high-frequency induction heating furnace, the high-frequency magnetic field generated by the work coil causes eddy currents to flow in the crucible wall, making the crucible itself a heating element for melting the raw materials and growing the crystal. Therefore, the work coil, the crucible, and the refractories surrounding the crucible are arranged axially symmetrically with respect to the pulling axis.

[0007] However, with the conventional method, repeated crystal growth caused the deformation of the Ir or Pt crucible, which was the heating element, to progress, leading to a deterioration in the axial symmetry of the temperature distribution and a tendency for the single crystal yield to decrease. This was due to the use of fluid heat-insulating materials such as zirconia bubbles around the crucible, prioritizing heat retention.

[0008] Therefore, the present invention aims to provide a crystal growth apparatus and a method for growing oxide single crystals that maintain the heat retention of the crucible while suppressing deformation of the crucible, and prevent a decrease in the single crystal yield and an increase in growth costs due to repeated growth. [Means for solving the problem]

[0009] In order to achieve the above object, a crystal growth apparatus according to one aspect of the present invention is a crystal growth apparatus using a high-frequency induction heating furnace and a Cz method, a metal crucible for storing and holding crystal raw material; A cylindrically shaped heat insulating material that covers the periphery of the crucible and suppresses deformation of the crucible (excluding the case where the cylindrically shaped heat insulating material has an opening on a side surface), The cylindrically shaped heat insulating material is made of a material that satisfies the conditions of a thermal conductivity of 0.5 to 2 W / m K, a thermal expansion coefficient of 1% or less, a bending strength of 8 MPa or more, and a compressive strength of 80 MPa or more, The ratio of the inner diameter R of the cylindrical heat insulating material to the outer diameter r of the crucible satisfies the relationship R / r=1.01±0.005. [Effects of the Invention]

[0010] According to the present invention, it is possible to suppress deformation of the crucible that occurs when crystal growth is repeated. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing an example of a crystal growth apparatus according to an embodiment of the present invention. [Figure 2] 1 is a diagram showing an example of a crucible, a crucible stand, and a molded heat insulating material of a crystal growth apparatus according to an embodiment of the present invention. FIG. [Figure 3] FIG. 10 is a diagram showing a modified example of a crucible when using a conventional crystal growth apparatus. [Figure 4] FIG. 1 is a diagram showing an example of a crucible, a crucible stand, and a molded heat insulating material when a conventional crystal growth apparatus is used. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0013] Fig. 1 is a cross-sectional view showing an example of a crystal growth apparatus according to an embodiment of the present invention. As shown in Fig. 1, the crystal growth apparatus according to this embodiment includes a crucible 10, a crucible stand 20, a molded insulating material 30, a refractory material 40, a reflector 50, an after-heater 60, an induction coil 70, a pulling shaft 80, a mounting table 90, and a chamber 100.

[0014] A seed crystal holder 81 is provided at the lower end of the pulling shaft 80, and holds a seed crystal 110. A raw material melt 120 is stored and held within the crucible .

[0015] In the crystal growth apparatus according to this embodiment, the crucible 10 is placed on a crucible stand 20. A cylindrical shaped insulating material 30 is provided around the crucible 10 and the crucible stand 20. The shaped insulating material 30 is made of metal and is provided to prevent heat generated by the crucible 10 as a heating element from leaking to the outside and to prevent the lower end of the crucible 10 from deforming and protruding outward due to repeated use. A refractory material 40 is also provided to surround the outside of the shaped insulating material 30. The refractory material 40 has a lower vessel 41 that surrounds the outside of the shaped insulating material 30 and an upper vessel 42 that surrounds the upper portion thereof. Details of the crucible 10, the shaped insulating material 30, and the refractory material 40 will be described later.

[0016] An after-heater 60 is installed above the crucible 10 via a reflector 50. A heat insulating material 30 is provided so as to surround the periphery of the crucible 10 and the reflector 50. Furthermore, an induction coil 70 is provided so as to surround the crucible 10, crucible stand 20, molded heat insulating material 30, refractory material 40, reflector 50, and after-heater 60. A chamber 100 is provided outside the induction coil 70, and covers the entire periphery of the refractory material 40 and induction coil 70.

[0017] A pulling shaft 80 is provided above the crucible 10. The pulling shaft 80 has a seed crystal holder 81 at its lower end and is configured to be movable up and down by a pulling shaft drive motor (not shown). A mounting table 90 is provided below the refractory material 40 and within the chamber 100, and supports the entire structure except for the chamber 100.

[0018] Although not shown, a control unit for controlling the operation of the entire crystal growth apparatus and a power supply for supplying power to the induction coil 70 and the entire crystal growth apparatus are provided outside the chamber 100.

[0019] Next, the individual components will be described.

[0020] The crucible 10 is a container for storing and holding crystal raw materials and growing single crystals. 。L Since oxide crystals such as T are grown in an oxygen-containing atmosphere, it is preferable that the material be a heat-resistant, oxygen-insensitive precious metal such as Pt (platinum), Rh (rhodium), or Ir (iridium), or an alloy thereof.

[0021] Crucible stand 20 is provided as a support stand that supports crucible 10 from below. Crucible stand 20 may be made of various materials as long as it has sufficient heat resistance to withstand the heat of induction coil 70 and durability to support crucible 10. Crucible stand 20 is made of a sintered refractory material such as alumina, zirconia, magnesia, or calcia.

[0022] The reflector 50 and the after-heater 60 are also required to have the same characteristics as the crucible 10, and therefore, like the crucible 10, they are preferably made of a noble metal such as Pt, Rh, or Ir, or an alloy thereof.

[0023] The induction coil 70 is a means for heating the crucible 10, the reflector 50, and the after-heater 60, and is disposed so as to surround the crucible 10, the reflector 50, and the after-heater 60. The induction coil 70 may take any form as long as it can inductively heat the crucible 10, the reflector 50, and the after-heater 60, but for example, it may be configured as a high-frequency induction heating device made up of a high-frequency heating coil.

[0024] The pulling shaft 80 is a means for holding the seed crystal 110, bringing the seed crystal 110 into contact with the surface of the raw material melt 120 held in the crucible 10, and pulling up a single crystal while rotating. The pulling shaft 80 has a seed crystal holder 81 at its lower end that holds the seed crystal 110, and is equipped with a pulling shaft drive motor (not shown). The pulling shaft drive motor is a rotation drive mechanism that rotates the crystal while pulling it up.

[0025] The chamber 100 serves to retain heat generated by the crucible 10 and the induction coil 70 of the after-heater 50 inside and prevent it from being released to the outside. The chamber 100 is made of a highly heat-resistant material. The refractory material 40 and the chamber 100 have openings 43, 101 in the ceiling surface, and are configured so that the pulling shaft 80 can be inserted.

[0026] The mounting table 90 is a support means for supporting the entire structure including the refractory material 40 .

[0027] Next, the molded heat insulating material 30 of the crystal growth apparatus according to this embodiment will be described.

[0028] Fig. 2 is a diagram showing an example of the crucible 10, crucible stand 20, and molded heat insulating material 30 of the crystal growth apparatus according to this embodiment. Fig. 3 is a diagram showing a modified example of the crucible 10 when a conventional crystal growth apparatus is used.

[0029] As shown in Figure 2, the present invention is characterized in that, instead of the bubble material placed around the Ir or Pt crucible 10, a cylindrical molded body with a thermal expansion coefficient of 1% or less and a bending strength of 8 MPa or more is used as an insulating material, in order to have the same thermal conductivity as the bubble material and the effect of suppressing crucible deformation that could not be expected with fluid bubble material.

[0030] Conventionally, in order to suppress heat conduction, spherical so-called bubble material 130 has been used as a heat insulating material around crucible 10. Although the heat insulating material made of bubble material 130 can reduce the amount of heat conduction, it does not have the function of holding crucible 10, and therefore does not have the effect of suppressing deformation of crucible 10 due to repeated use.

[0031] In other words, in general, when crystal growth is repeated, the side wall of an Ir or Pt crucible 10 is deformed so that it widens outward, i.e., the diameter increases. As shown in Figure 3, the location where such deformation occurs is the largest near the bottom of the crucible side wall. Therefore, the most significant reason for deformation of the crucible 10 is thought to be the pressure (the force of the raw material melt 120 trying to expand) exerted on the crucible side wall by the load of the raw material melt 120 inside the crucible at high temperatures.

[0032] FIG. 4 shows a conventional thermal insulation material. Conventionally, as shown in FIG. 4, heat retention was prioritized, and a ball-shaped thermal insulation material 131 was provided on the inner peripheral surface of the refractory material 40, and spherical bubble material 130 with a diameter of several millimeters was filled around the crucible inside. The thermal conductivity of the bubble material when filled was 0.6 to 0.9 W / m·K. However, because the bubble material 130 flows, it could not withstand the load applied to the crucible side wall. In contrast, the present invention is characterized by suppressing deformation of the crucible 10 by placing a cylindrically shaped thermal insulation material 30, which has a thermal conductivity similar to that when filled with bubble material, a low thermal expansion coefficient, and high strength, around the crucible instead of the bubble material 130.

[0033] Specifically, for example, zirconia or alumina is used for the shaped insulating material 30. That is, a shaped sintered body of zirconia or a shaped sintered body of alumina is used as the shaped insulating material 30. These are sintered bodies with high heat resistance, and the material itself is molded hard, so that it acts as a wall to prevent the bottom end of the crucible 10 from extending sideways, thereby preventing deformation of the crucible 10.

[0034] The molded insulating material 30 is formed, for example, in a cylindrical shape. Since the crucible stand 20 is provided below the crucible 10, the molded insulating material 30 is formed in a cylindrical shape so as to cover the outer peripheral surfaces of the crucible 10 and the crucible stand 20, which are formed in a cylindrical shape as a whole. Note that, although the crucible 10 and the crucible stand 20 are formed to have the same diameter in Figs. 1 and 2, the diameter of the crucible stand 20 may be smaller than that of the crucible 10. 。

[0035] In addition, when the diameter of the crucible stand 20 is smaller than that of the crucible 10, if the shaped insulating material 30 is cylindrical, a gap will be created between the crucible stand 20 and the shaped insulating material 30. However, instead of being cylindrical, the inner peripheral surface of the shaped insulating material 30 may be formed with a step so as to fit into this shape. 。

[0036] However, if the crucible 10 and the crucible stand 20 have the same diameter, it is easier to prevent the lower end of the crucible 10 from sagging and it is also easier to hold the molded insulation material 30, so it is more preferable to make the diameters of the crucible 10 and the crucible stand 20 the same and to configure the molded insulation material 30 in a cylindrical shape.

[0037] The pressure applied to the crucible sidewall varies depending on the weight of the raw materials in the crucible and the crucible size, but is generally less than 1 MPa for LN and LT crystal growth at current production levels. Therefore, by installing a shaped insulation material 30 with a thermal conductivity of 0.5 to 2 W / m·K, a bending strength of 8 MPa or more, and a compressive strength of 80 MPa or more around the crucible, deformation of the crucible 10 can be suppressed. In other words, using a shaped insulation material 30 with low thermal conductivity, high bending strength, and high compressive strength suppresses deformation of the crucible 10 due to deterioration over time. Here, since the bubble material 130 is a spherical insulating material, it is naturally unable to achieve high bending strength and high compressive strength. In the crystal growth apparatus according to this embodiment, by using sintered materials such as zirconia and alumina, which already have high bending strength and compressive strength, the bending strength and compressive strength are further increased, thereby suppressing deformation of the crucible 10. Although the bubble material 130 may have a lower thermal conductivity, this can be overcome by using a material with as low a thermal conductivity as possible. Even if the thermal conductivity is somewhat sacrificed, the effect of suppressing deformation of the crucible 10 is greater than the effect of growing a single crystal with uniform quality and reducing the cost spent on the crucible 10, so this is an overall advantage.

[0038] As described above, the metal crucible 10 made of platinum or iridium expands when heated. In contrast, the expansion coefficient of the shaped insulating material 30 made of zirconia or alumina is at least one order of magnitude smaller than that of platinum or iridium. Therefore, the inner diameter of the shaped insulating material 30 must be designed taking this difference in expansion coefficient into consideration. If the difference between the inner diameter of the shaped insulating material 30 and the outer diameter of the metal crucible 10 is small, stress is applied to the shaped insulating material 30 due to the expansion of the metal crucible 10 at high temperatures, causing cracks. Conversely, if the difference between the inner diameter of the shaped insulating material 30 and the outer diameter of the metal crucible 10 is large, the effect of suppressing deformation of the metal crucible 10 is not fully achieved.

[0039] Therefore, in order to prevent both deformation of the crucible 10 and cracking of the shaped insulating material 30, it is necessary to properly design the inner diameter of the shaped insulating material 30. In the present invention, the ratio of the inner diameter R of the cylindrical shaped insulating material 30 to the outer diameter r of the platinum or iridium metal crucible placed inside it is set to R / r = 1.01 ± 0.005. Setting R / r to 1.01 makes it possible to prevent both deformation of the crucible and cracking of the shaped insulating material.

[0040] In this way, by adjusting not only the material but also the shape of the shaped heat insulating material 30, it is possible to prevent damage to the shaped heat insulating material 30 and further enhance the effect of suppressing deformation of the crucible 10.

[0041] That is, according to the present invention, it is possible to suppress deformation of the crucible that occurs when crystal growth is repeated, and to reduce changes in the axial symmetry of the temperature environment and temperature gradient during growth.Furthermore, it is possible to stably obtain a high single crystallization rate over a long period of time, thereby improving productivity and reducing costs in growing LN and LT crystals.

[0042] [Example] Next, examples of the present invention will be specifically described with reference to comparative examples.

[0043] During the growth, the change in the melt surface position at the start of growth was monitored each time as a measure of deformation of the crucible. This was based on the fact that since the amount of raw material charged in the crucible was constant, if the crucible deformed and the internal volume changed, the melt surface position at the start of growth would change.

[0044] [Example 1] As shown in Figure 2, an Ir crucible with an outer diameter of 200 mm and a height of 200 mm was placed on a crucible stand with an outer diameter of 200 mm inside an alumina refractory, and a zirconia molded insulation material with an inner diameter of 202 mm was placed between the alumina container and the Ir crucible. 50 consecutive φ6-inch LT single crystals were grown using a high-frequency induction heating furnace. The zirconia molded insulation material used had a thermal conductivity of 0.8 W / m K, a thermal expansion coefficient of 0.08%, a bending strength of 8 MPa, and a compressive strength of 80 MPa.

[0045] As a result, 46 single crystals were successfully grown, with a single crystallization rate of 92%. The rate of change in the melt surface position at the start of growth was 0.04 mm / cycle, and the difference in the melt surface position between the first and 50th growth cycles was approximately 2 mm, which meant that deformation of the crucible was successfully suppressed.

[0046] Looking at the number of single crystals obtained in 50 growths for every 10 growths, we found that single crystals could be obtained stably even when the number of growths increased from 9 to 10, 10 to 9, and then to 8.

[0047] [Comparative Example 1] As shown in Figure 4, an Ir crucible with an outer diameter of φ200 mm and a height of 200 mm was placed on a crucible stand with an outer diameter of φ200 mm inside an alumina refractory, and zirconia bubbles with a diameter of approximately φ1 to 3 mm were filled between the alumina container and the Ir crucible. φ6-inch LT single crystals were grown 50 times in succession using a high-frequency induction heating furnace.

[0048] As a result, 31 single crystals were successfully grown, with a single crystallization rate of 62%. The rate of change in the melt surface position at the start of growth was 0.6 mm / cycle, and the difference in the melt surface position between the first and 50th growth cycles was approximately 30 mm. The shape of the crucible changed, and the internal volume increased significantly.

[0049] Looking at the number of single crystals obtained in 50 growths for every 10 growths, the single crystal yield decreased as the number of growths increased from 9, 8, 6, 5, and 3.

[0050] As described above, this example demonstrates that the crystal growth apparatus and oxide single crystal growth method according to this embodiment can suppress thermal deformation of the crucible and enable stable growth of single crystals.

[0051] The present invention can suppress the deformation of metal crucibles, which has been a problem in the Cz method using a high-frequency induction heating furnace, and maintain the axial symmetry of the temperature distribution in the crystal growth environment for a long period of time. This can stabilize and improve the single crystallization rate of LT and LN crystal growth, improving productivity and reducing costs.

[0052] Although the preferred embodiments and examples of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments and examples, and various modifications and substitutions can be made to the above-described embodiments and examples without departing from the scope of the present invention. [Explanation of symbols]

[0053] 10 Crucible 20 Crucible stand 30 Molded insulation material 40~42 Refractories 43, 101 aperture 50 Reflector 60 After-heater 70 Induction Coil 80 Pulling shaft 90 Mounting table 100 chambers 110 Seed Crystal 120 Raw material melt 130 Bubble material

Claims

1. A crystal growth apparatus using a Cz method with a high-frequency induction heating furnace, a metal crucible for storing and holding crystal raw material; A cylindrically shaped heat insulating material that covers the periphery of the crucible and suppresses deformation of the crucible (excluding the case where the cylindrically shaped heat insulating material has an opening on a side surface), The cylindrically shaped heat insulating material is made of a material that satisfies the following conditions: a thermal conductivity of 0.5 to 2 W / m K, a thermal expansion coefficient of 1% or less, a bending strength of 8 MPa or more, and a compressive strength of 80 MPa or more; A crystal growth apparatus in which the ratio of the inner diameter R of the cylindrical heat insulating material to the outer diameter r of the crucible satisfies the relationship R / r = 1.01 ± 0.

005.

2. 2. The crystal growth apparatus according to claim 1, wherein the cylindrical heat insulating material is made of zirconia or alumina.

3. A method for growing oxide single crystals by a Cz method using a high-frequency induction heating furnace, comprising the steps of: A cylindrically shaped heat insulating material (excluding cases where the cylindrically shaped heat insulating material has an opening on a side surface) made of a material that satisfies the conditions of a thermal conductivity of 0.5 to 2 W / m K, a thermal expansion coefficient of 1% or less, a bending strength of 8 MPa or more, and a compressive strength of 80 MPa or more is arranged around a metal crucible that stores and holds a crystal raw material, and a single crystal is grown in a state where deformation of the crucible is suppressed, A method for growing an oxide single crystal, wherein the ratio of the inner diameter R of the cylindrically shaped insulating material to the outer diameter r of the crucible satisfies the relationship R / r = 1.01 ± 0.

005.

4. 4. The method for growing an oxide single crystal according to claim 3, wherein the oxide single crystal is lithium tantalate or lithium niobate.

Images