Crystal growth apparatus

Abstract

To provide a crystal growth apparatus capable of preventing a raw material in the bottom of a crucible from solidifying and changing the state of a raw material melt into a state suitable for growing a crystal.SOLUTION: A crystal growth apparatus 1 for growing a single crystal C from a raw material melt M obtained by melting a raw material by induction heating includes: a bottomed cylindrical crucible 2 for storing the raw material melt M obtained by melting the raw material; an induction coil 5 for induction-heating the crucible 2; and an auxiliary heating element 10 provided below the crucible 2 and induction-heated by the induction coil 5. The auxiliary heating element 10 being an annular member has only an inclined surface inclined so as to separate from the crucible 2 as a surface positioned on the side of the crucible 2 directs to the outside from the inside. A decline of the temperature of the central part in the bottom of the crucible 2 can be prevented, so that a decline of the temperature of the raw material melt M accompanying the growth of the single crystal C can be prevented; therefore solidification of the raw material in the bottom of the crucible 2 accompanying the growth of the single crystal C can be prevented.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a crystal growth apparatus. [Background technology]

[0002] The Czochralski method is a widely used method for growing oxide single crystals, such as lithium tantalate and lithium niobate. In the Czochralski method, a crucible filled with raw material for the oxide single crystal is heated to a high temperature to melt the raw material. A seed crystal is then brought into contact with the surface of the raw material melt in the crucible from above, and the seed crystal is then raised while rotating, thereby growing an oxide single crystal in the same orientation as the seed crystal. The seed crystal is a rectangular single crystal (usually with a cross-sectional side measuring several millimeters) cut according to a certain crystal orientation and having the same composition as the raw material melt.

[0003] As an apparatus for growing single crystals by the Czochralski method, an apparatus that uses induction heating is known (see Patent Documents 1 and 2). Such an apparatus that uses induction heating uses a metal crucible around which an induction coil is arranged, and when a high-frequency current is passed through the induction coil, an eddy current can be generated in the crucible. The eddy current then heats the crucible, and the raw material inside the crucible can be melted by the heat of the crucible.

[0004] In recent years, the market for oxide single crystals as surface acoustic wave device materials has expanded, and to ensure production volume, there is a demand for growing single crystals with larger diameters.

[0005] When growing single crystals using the Czochralski method, in order to increase the diameter of the grown crystal (hereinafter sometimes referred to as the pulled crystal), a larger crucible must be used, and the amount of raw material melt contained in the crucible must also increase. When the diameter of the pulled single crystal is larger, the amount of heat required to heat the raw material melt increases compared to when the diameter of the pulled single crystal is smaller. Furthermore, in order to grow a high-quality single crystal, it is necessary to properly control the temperature gradient within the crucible in order to grow a uniform single crystal.

[0006] Here, since the induction coil is installed so as to surround the sidewall of the crucible, the heat generated at the bottom of the crucible is smaller than that at the sidewall. In particular, since the center of the bottom of the crucible is far from the induction coil, the temperature of the raw material melt at the center of the bottom of the crucible and its vicinity is likely to drop. As a result, even if the upper part of the raw material melt is at an appropriate temperature for growing a single crystal, the temperature near the center of the bottom of the crucible drops, and the raw material melt may begin to solidify. If the diameter of the crucible is increased, this phenomenon becomes more likely to occur because the distance between the induction coil and the center of the bottom of the crucible increases.

[0007] Furthermore, if the growth of a single crystal is continued after the raw material melt at the center of the bottom of the crucible has started to solidify, the solidified crystal near the bottom of the crucible may grow upward and fuse with the single crystal being grown. If such fusion occurs, further single crystal growth cannot be performed, and the single crystal growth may have to be stopped.

[0008] Therefore, in order to prevent the temperature at the bottom of the crucible from decreasing, the techniques disclosed in Patent Documents 1 and 2 have been developed.

[0009] Patent Document 1 discloses a single crystal pulling apparatus having a crucible whose side surfaces are formed by a cylindrical member and whose bottom surface is formed by a disk-shaped member. In the crucible of this single crystal pulling apparatus, the disk-shaped member protrudes radially outward from the outer circumferential surface of the cylindrical member, and the protruding portion forms a flange-shaped outer circumferential portion. It is described that since this flange-shaped outer circumferential portion is located closest to the heating coil, the disk-shaped member generates more heat and becomes hotter than the upper part or the cylindrical member, thereby creating a temperature gradient in which the lower part of the crucible is hotter than the upper part.

[0010] Patent Document 2 discloses a method for producing oxide single crystals, in which a crystal material in a crucible is heated by high-frequency waves to form a melt, from which a single crystal is obtained by a pulling method. Patent Document 2 discloses a crystal-growing furnace used in the method for producing oxide single crystals, which comprises a platinum-rhodium crucible placed on a refractory stand inside a refractory crucible, a heating coil placed around the refractory crucible, and an auxiliary heat source provided in the space above the refractory stand. The auxiliary heating element is disclosed to be smaller than the area of the bottom of the crucible and to have a predetermined length in the height direction of the crucible, and it is described that such an auxiliary heating element is well resistant to high-frequency waves and provides appropriate heating, thereby preventing crystallization. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-284854 [Patent Document 2] Japanese Patent Application Publication No. 54-162686 Summary of the Invention [Problem to be solved by the invention]

[0012] However, when using a crucible having the structure described in Patent Document 1, although the flange-shaped outer periphery can heat the bottom of the crucible to a high temperature, the flange-shaped outer periphery is closest to the heating coil and therefore experiences the highest temperature, and therefore deteriorates most rapidly. Because the flange outer periphery is formed integrally with the disc-shaped member that forms the bottom of the crucible, deterioration of the flange outer periphery requires the entire crucible to be replaced. Therefore, if deterioration of the flange outer periphery occurs frequently, the entire crucible must be replaced frequently, which reduces maintainability. Furthermore, as the crucible undergoes repeated single crystal growth, it gradually deforms due to volume changes caused by the melting and solidification of the single crystal material, and the outer periphery of the flange at the bottom of the crucible, which is integrally formed with the crucible, also deforms, making it difficult to maintain the initial temperature distribution characteristics.

[0013] On the other hand, in Patent Document 2, the auxiliary heating element is separated from the crucible, and even if the auxiliary heating element is damaged, only the auxiliary heating element needs to be replaced, preventing a decrease in maintainability. However, although Patent Document 2 describes auxiliary heating elements of various shapes, it does not describe at all how the shape of the auxiliary heating element affects the heating state of the crucible and the temperature of the raw material melt in the crucible, and does not describe what shape the auxiliary heating element should have in order to appropriately adjust the temperature of the raw material melt in the crucible. In particular, when the auxiliary heating element has a horizontal surface on the surface facing the crucible, the portion of this horizontal surface away from the heating coil contributes very little to heat generation, just like the center of the bottom of the crucible. Therefore, when the auxiliary heating element has such a horizontal surface, the presence of the horizontal surface can actually lower the temperature of the center of the bottom of the crucible. This problem occurs when the auxiliary heating element has a horizontal surface on the surface facing the crucible, but D2 does not take this problem into consideration at all, and does not disclose any method for preventing the temperature drop at the center of the bottom of the crucible.

[0014] In view of the above circumstances, the present invention has an object to provide a crystal growth apparatus that can prevent solidification of the raw material at the bottom of the crucible and can keep the raw material melt in a state suitable for crystal growth. [Means for solving the problem]

[0015] The crystal growth apparatus of the first invention is a crystal growth apparatus that grows a single crystal from a raw material melt obtained by melting raw materials by induction heating, and is equipped with a bottomed cylindrical crucible that contains the raw material melt, an induction coil that inductively heats the crucible, and a metallic auxiliary heating element that is provided below the crucible and inductively heated by the induction coil, and is characterized in that the auxiliary heating element is an annular member whose surface facing the crucible has only an inclined surface that inclines so that it moves away from the crucible as it moves from the inside to the outside. A crystal growth apparatus according to a second aspect of the present invention is the crystal growth apparatus according to the first aspect of the present invention, characterized in that the inclined surface of the auxiliary heating element has an inclination angle of 15 to 45 degrees with respect to the central axis of the through hole. A crystal growth apparatus according to a third aspect of the present invention is the crystal growth apparatus according to the first aspect of the present invention, characterized in that the outer diameter of the auxiliary heating element is 0 to 20 mm shorter than the outer diameter of the bottom surface of the crucible. A fourth aspect of the present invention is a crystal growth apparatus according to the first aspect of the present invention, characterized in that the difference between the inner diameter and the outer diameter of the auxiliary heating element is 15% to 40% of the outer diameter of the bottom surface of the crucible. A crystal growth apparatus according to a fifth aspect of the present invention is the crystal growth apparatus according to the first aspect of the present invention, characterized in that the auxiliary heating element is disposed so that the distance from its upper end to the bottom surface of the crucible is 5 to 20 mm. [Effects of the Invention]

[0016] According to the first aspect of the present invention, the bottom of the crucible can be heated appropriately, thereby preventing the raw material from solidifying at the bottom of the crucible during the growth of the single crystal. According to the second aspect of the present invention, the inclination angle of the surface of the auxiliary heating element is set to an appropriate angle, so that the flow of the raw material melt in the crucible can be made suitable for growing a single crystal. According to the third to fifth aspects of the present invention, the crucible can be effectively heated by the auxiliary heating element. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic explanatory diagram of a crystal growth apparatus 1 of the present embodiment. [Figure 2] 1A is a schematic explanatory diagram of the crucible 2, crucible stand 3, and auxiliary heating element 10 in the crystal growth apparatus 1 of this embodiment, and FIG. 1B is a view taken along the line BB in FIG. [Figure 3] 1A and 1B are schematic explanatory views of the auxiliary heating element 10 alone, in which (A) is a plan view, (B) is a side view, and (C) is a cross-sectional view taken along line CC of (A). [Figure 4] (A) is a graph showing the results of a numerical simulation of the temperature distribution in the center of the crucible, and (B) is a graph showing the results of a numerical simulation of the velocity distribution in the center of the crucible. [Figure 5] 10 shows the results of a numerical simulation of the effect of the inclination angle of the surface 10a of the auxiliary heating element 10 on the heat generation density. DETAILED DESCRIPTION OF THE INVENTION

[0018] The crystal growth apparatus of this embodiment is an apparatus for growing single crystals by the Czochralski method, and is characterized by its structure that allows the growth of single crystals even with large diameters.

[0019] The material from which a single crystal can be formed using the crystal growth apparatus of this embodiment is not particularly limited, but examples of materials from which a single crystal can be formed using the crystal growth apparatus of this embodiment include lithium niobate and lithium tantalate.

[0020] Furthermore, the size of the single crystal grown by the crystal growth apparatus of this embodiment is not particularly limited. For example, single crystals with diameters of 50 to 230 mm and lengths of 50 to 150 mm can be produced. In particular, the crystal growth apparatus of this embodiment is suitable for growing large single crystals that are difficult to grow using general crystal growth apparatuses that grow single crystals by the Czochralski method. For example, the crystal growth apparatus of this embodiment is suitable for growing single crystals with diameters of 200 to 230 mm and lengths of 80 to 150 mm.

[0021] <Crystal growth apparatus 1 of this embodiment> First, an outline of the crystal growth apparatus 1 of this embodiment will be described.

[0022] The crystal growth apparatus 1 of this embodiment is a crystal growth apparatus that grows a single crystal C using the Czochralski method. The Czochralski method is a method used to produce oxide single crystals in air or an inert gas atmosphere, in which a rectangular parallelepiped single crystal (hereinafter sometimes referred to as a seed crystal SC) cut along a certain crystal orientation is gradually pulled up while rotating from a state in which the tip of the seed crystal SC is immersed in a raw material melt M of the same composition as the raw material, thereby producing a single crystal C with a larger diameter while propagating the properties of the seed crystal SC. Note that a rectangular parallelepiped single crystal with a cross-sectional side of about several millimeters is generally used as the seed crystal SC.

[0023] In Fig. 1, reference numeral 1c denotes a vessel of the crystal growth apparatus 1 of this embodiment. This vessel 1c is a hollow vessel made of refractory material, and a crucible 2, which will be described later, is provided inside. A through-hole h is formed in the ceiling of this vessel 1c so that its central axis coincides with the central axis of the vessel 1c. This through-hole h is formed so that a pulling shaft 21, which will be described later, can be inserted into the vessel 1c.

[0024] As shown in Fig. 1, an induction coil 5 is disposed around the container 1c for induction heating a crucible 2, a reflector 1r, an after-heater 1d, and an auxiliary heating element 10 (see Fig. 2), which will be described later. A power source 31 is electrically connected to the induction coil 5. The power source 31 is provided to supply current to the induction coil 5, and the current supplied to the induction coil 5 is controlled by a control unit 30.

[0025] The periphery of the induction coil 5, i.e., the periphery of the container 1c, is covered by a chamber (not shown) to insulate it from the high heat of the crucible 2 and the induction coil 5. In the chamber, the container 1c is placed on a support stand (not shown).

[0026] As shown in FIG. 1, a refractory crucible stand 3 is provided inside the container 1c, and a crucible 2 is provided on the crucible stand 3. The crucible 2 is a cylindrical member with a bottom and an opening at the top, and is disposed so that its central axis 2a is substantially coaxial with the central axis of the container 1c. The crucible 2 is a container having a space therein capable of storing and holding a raw material, i.e., a raw material melt M in which a metal to be crystallized or the like is melted. The crucible 2 is made of a metallic material that can be induction-heated by an induction coil 5 and is resistant to damage by the high-temperature raw material melt M. For example, the crucible 2 is made of platinum, iridium, or the like. A heat insulating material 1f is provided in the space between the side wall 2a of the crucible 2 and the inner surface of the container 1c, and in the space between the side wall 2a of the crucible stand 3 and the inner surface of the container 1c.

[0027] As shown in Figures 1 and 2, a reflector 1r is provided at the upper end of the crucible 2. This reflector 1 is an annular plate material and is provided so as to reduce the size of the opening of the crucible 2. Specifically, the reflector 1 is formed so that its outer diameter is equal to or larger than the outer diameter at the upper end of the crucible 2 and its inner diameter is smaller than the inner diameter at the upper end of the crucible 2, and its center is positioned on the central axis 2a of the crucible 2. This reflector 1r is also formed from a metal material that can be induction heated by the induction coil 5.

[0028] A cylindrical after-heater 1d is provided on the reflector 1r. The after-heater 1d is formed so that its inner diameter is approximately the same as or shorter than the inner diameter of the reflector 1r, and is disposed so that its central axis is approximately coaxial with the central axis 2a of the crucible 2. The after-heater 1d is also made of a metal material that can be induction-heated by the induction coil 5. A heat insulating material 1g is provided in the space between the outer surface of the reflector 1r and the inner surface of the container 1c.

[0029] As shown in Fig. 2, an auxiliary heating element 10 is provided inside the crucible stand 3 on which the crucible 2 is placed. The auxiliary heating element 10 is made of a metal material that can be induction-heated by the induction coil 5. Details of the auxiliary heating element 10 will be described later.

[0030] The crystal growth apparatus 1 of this embodiment has a pulling shaft 21 that is inserted into the container 1c through the through-hole h of the container 1c. The pulling shaft 21 is movable (movable up and down) along the central axis of the container 1c by a pulling shaft driver 22 and rotatable about the central axis. The lower end of the pulling shaft 21 has a seed crystal holder 21a that holds the seed crystal SC. Therefore, by moving the pulling shaft 21 by the pulling shaft driver 22 while the seed crystal SC is held by the seed crystal holder 21a of the pulling shaft 21, the seed crystal SC can be immersed in the raw material melt M in the crucible 2, and the seed crystal SC can be pulled up from the state where it is immersed in the raw material melt M. The lifting shaft drive unit 22 is electrically connected to the control unit 30, and the control unit 30 controls its vertical movement and rotation.

[0031] The crystal growth apparatus 1 of this embodiment has the above-described configuration, and therefore, can grow a single crystal C in the following manner.

[0032] First, raw material is placed in the crucible 2, and a high-frequency current is supplied from the power supply 31 to the induction coil 5 by the control unit 30. Then, the crucible 2, reflector 1r, after-heater 1d, and auxiliary heating element 10 generate heat through induction heating, and the raw material in the crucible 2 melts to form raw material melt M.

[0033] When the raw material melt M is formed, the pulling shaft 21 holding the seed crystal SC in the seed crystal holding portion 21a is inserted into the container 1c through the through-hole h of the container 1c. Then, the tip of the seed crystal SC held by the pulling shaft 21 is immersed in the raw material melt M in the crucible 2.

[0034] Thereafter, the pulling shaft 21 is gradually raised while being rotated. Here, as the pulling shaft 21 is raised to grow the single crystal C, the raw material melt M in the crucible 2 decreases and the temperature of the raw material melt M also drops. Therefore, in order to properly grow the single crystal C, the control unit 30 grows the single crystal C while adjusting the current supplied from the power source 31 to the induction coil 5, the rate at which the pulling shaft 21 is raised, and the rotation speed, based on the weight of the single crystal being grown.

[0035] When a single crystal C of the desired size (diameter, length) is grown, the control unit 30 stops the movement and rotation of the pulling shaft 21 and stops the supply of current to the induction coil 5. Then, after the single crystal C, the crucible 2, the reflector 1r, the after-heater 1d, and the auxiliary heating element 10 have cooled to predetermined temperatures, the single crystal C can be separated from the raw material melt M to obtain a single crystal C of the desired size.

[0036] As the single crystal C is pulled, it moves away from the crucible 2, and the temperature of the single crystal C decreases toward the top (closer to the seed crystal SC). This causes a larger temperature distribution within the single crystal C (larger temperature difference between the top and bottom), which can lead to defects such as cracking of the single crystal C. However, since the after-heater 1d is installed above the crucible 2 and can heat the pulled single crystal C, an appropriate temperature distribution can be maintained within the single crystal C.

[0037] <Auxiliary heating element 10> Next, the auxiliary heating element 10 will be described in detail. As shown in FIG. 2, the crucible support 3 includes a main body 3a having a groove 3h recessed from its top surface, and a plate member 3b disposed to cover the groove 3h formed in the main body 3a. The groove 3h formed in the main body 3a is formed along a circle centered on the central axis 3c of the main body 3a, and its outer diameter W2 (i.e., the diameter of the outer surface) is formed to be smaller than the outer diameter of the crucible 2. More specifically, when the crucible 2 is placed on the crucible support 3 so that the central axis 3c of the main body 3a and the central axis 2a of the crucible 2 are coaxial, the groove 3h is formed in the main body 3a such that the outer surface of the groove 3h is located inward from the outer surface of the crucible 2. For example, if the outer diameter of the crucible 2 is 200 to 300 mm, the groove 3h is formed in the main body 3a such that the outer surface of the groove 3h is located approximately 0 to 20 mm inward from the outer surface of the crucible 2.

[0038] As shown in Fig. 2, the auxiliary heating element 10 is accommodated in the groove 3h of the main body 3a of the crucible base 3. The auxiliary heating element 10 is an annular member that can be accommodated in the groove 3h of the main body 3a. In other words, the auxiliary heating element 10 is an annular member with a through-hole 10h formed in the center that penetrates from the front to the back, and is formed so that its inner diameter D1 is equal to or slightly longer than the inner diameter W1 of the groove 3h (i.e., the diameter of the inner surface), and its outer diameter D2 is equal to or slightly shorter than the outer diameter W2 of the groove 3h (see Fig. 3).

[0039] The auxiliary heating element 10 has a front surface 10a and a back surface 10b formed as inclined surfaces, and is housed in the groove 3h in the main body 3a of the crucible base 3 so that the front surface 10a moves away from the crucible 2 from the inside to the outside. In other words, when the auxiliary heating element 10 is housed in the groove 3h in the main body 3a of the crucible base 3, the front surface 10a is formed in the shape of a truncated cone with a smaller diameter at the top than at the bottom.

[0040] Although the end surface 10t (see FIG. 3(C)) of the auxiliary heating element 10 may be a surface that is not inclined (a surface perpendicular to) the central axis of the crucible 2, this end surface 10t is not included in the "surface located on the crucible side" referred to in the claims. In other words, the "surface located on the crucible side" referred to in the claims means only the surface 10a in FIG. 3.

[0041] The crucible 2 is placed on the crucible stand 3 with the auxiliary heating element 10 accommodated in the groove 3h of the main body 3a, and the crucible 2 is positioned so that its central axis 2a is coaxial with the central axis of the crucible stand 3 (i.e., the central axis 3c of the main body 3a). This allows the auxiliary heating element 10 to be positioned below the bottom 2b of the crucible 2 and slightly inward from the side wall 2a of the crucible 2. Because the auxiliary heating element 10 is positioned in this position, when the auxiliary heating element 10 is induction heated by high frequency waves from the induction coil 5, the bottom 2b of the crucible 2 can be heated by the heat generated by the auxiliary heating element 10.

[0042] By providing the auxiliary heating element 10 as described above, the following effects can be obtained. When a single crystal is grown by the Czochralski method using the crystal growth apparatus 1 of this embodiment, the seed crystal SC is pulled upward to grow the single crystal C, increasing its length. As the length of the single crystal C increases, the single crystal C cools, and the temperature of the raw material melt M in the crucible 2 decreases. As the temperature of the raw material melt M decreases, solidification of the raw material melt M begins in the center of the bottom 2b of the crucible 2, which is far from the induction coil 5. However, by providing the auxiliary heating element 10 as described above, the bottom 2b of the crucible 2 can be heated by the auxiliary heating element 10, preventing a decrease in the temperature of the center of the bottom 2b of the crucible 2 and preventing a decrease in the temperature of the raw material melt M associated with the growth of the single crystal C. Therefore, solidification of the raw material melt M at the bottom 2b of the crucible 2 associated with the growth of the single crystal C can be prevented.

[0043] Moreover, because the auxiliary heating element 10 is annular and has a through-hole 10h formed in its center, the auxiliary heating element 10 does not directly heat the center of the bottom 2b of the crucible 2, but heats the peripheral portion of the bottom 2b of the crucible 2. This is preferable because even if the auxiliary heating element 10 is provided on the crucible base 3, the bottom 2b of the crucible 2 can be heated under conditions that do not deviate significantly from conventional process conditions.

[0044] In particular, if the surface 10a of the auxiliary heating element 10 has only an inclined surface as described above, the auxiliary heating element 10 can concentrate on heating the peripheral portion of the bottom 2b of the crucible 2 while ensuring a certain area for the surface 10a of the auxiliary heating element 10. In other words, by inclining the surface 10a of the auxiliary heating element 10, the amount of heat generated by the auxiliary heating element 10 when induction heated by the induction coil 5 can be increased, and the auxiliary heating element 10 can efficiently heat the peripheral portion of the bottom 2b of the crucible 2. The angle θ that the surface 10a of the auxiliary heating element 10 makes with respect to the central axis 10c is not particularly limited, but is preferably 15 to 45°, and more preferably 30 to 45°. If the angle θ is less than 15°, the auxiliary heating element 10 will not be able to sufficiently heat the bottom 2b of the crucible 2, and if the angle θ is greater than 45°, the efficiency of heating the peripheral portion of the bottom 2b of the crucible 2 will decrease.

[0045] Furthermore, when the surface 10a of the auxiliary heating element 10 is inclined, as the difference between the inner diameter D1 and the outer diameter D2 of the auxiliary heating element 10 increases, the distance (the distance in the direction of the central axis 2a of the crucible 2) from the portion of the auxiliary heating element 10 that is induction heated by the induction coil 5 to the bottom 2b of the crucible 2 increases. As a result, a larger amount of heat is consumed to heat the crucible support 3 out of the heat generated by the auxiliary heating element 10, and the efficiency with which the auxiliary heating element 10 heats the bottom 2b of the crucible 2 decreases. Therefore, it is desirable to set the difference between the inner diameter D1 and the outer diameter D2 of the auxiliary heating element 10 within an appropriate range depending on the diameter of the crucible 2. For example, if the outer diameter of the crucible 2 is 200 mm, the outer surface of the groove 3h is located approximately 0 to 20 mm inward from the side wall 2a of the crucible 2, and the angle θ that the surface 10a of the auxiliary heating element 10 makes with respect to the central axis 10c is 15 to 45°, then the difference between the inner diameter D1 and the outer diameter D2 of the auxiliary heating element 10 is preferably 15% to 40% of the outer diameter of the crucible 2, and more preferably 20% to 35%.

[0046] Furthermore, if the distance L (see FIG. 2A (see)) from the auxiliary heating element 10 to the bottom 2b of the crucible 2 is too close, heat from the crucible 2 will escape, and if it is too far, heating of the crucible 2 will be weak. Therefore, it is desirable to set the distance L from the upper end of the auxiliary heating element 10 to the bottom 2b of the crucible 2 within an appropriate range depending on the diameter of the crucible 2 and its position relative to the induction coil 5. For example, if the outer diameter of the crucible 2 is 200 mm and the outer surface of the groove 3h is located approximately 0 to 20 mm inward from the side wall 2a of the crucible 2, the distance L from the auxiliary heating element 10 to the bottom 2b of the crucible 2 is preferably 5 to 20 mm, and more preferably 10 to 15 mm.

[0047] Furthermore, because auxiliary heating element 10 is induction heated by induction coil 5, the amount of heat generated depends largely on its surface area due to the skin effect, but has little dependence on thickness, so there are no particular limitations on the thickness of auxiliary heating element 10. However, if auxiliary heating element 10 is too thin, it becomes difficult to handle. On the other hand, if auxiliary heating element 10 is made of heat-resistant metals such as platinum or iridium, the cost of manufacturing auxiliary heating element 10 increases as the thickness increases. Therefore, the thickness of auxiliary heating element 10 is preferably 0.5 mm or more and 3 mm or less, and more preferably 1 mm to 2 mm.

[0048] In the above example, the groove 3h is formed in the main body 3a so that when the crucible 2 is placed on the crucible support 3 so that the central axis 3c of the main body 3a and the central axis 2a of the crucible 2 are coaxial, the outer surface of the groove 3h is located inward of the side wall 2a of the crucible 2. That is, the outer edge of the auxiliary heating element 10 placed in the groove 3h is located inward of the side wall 2a of the crucible 2, but the relative position of the outer edge of the auxiliary heating element 10 and the side wall 2a of the crucible 2 is not particularly limited. However, if the distance between the induction coil 5 and the auxiliary heating element 10 is large, the amount of heat generated by the auxiliary heating element 10 decreases. However, if the distance between the induction coil 5 and the auxiliary heating element 10 is too small, the induced current in the crucible 2 decreases. Therefore, it is desirable to maintain the relative position of the outer edge of the auxiliary heating element 10 and the outer surface of the crucible 2 within an appropriate range. For example, if the outer diameter of the crucible 2 is 200 mm and the angle θ that the surface 10a of the auxiliary heating element 10 makes with respect to the central axis 3c is 15 to 45°, the outer edge of the auxiliary heating element 10 is located inward from the side wall 2a of the crucible 2, and the distance from the outer edge of the auxiliary heating element 10 to the side wall 2a of the crucible 2 is preferably 0 to 20 mm, more preferably 2.5 to 10 mm, and even more preferably 4 to 8 mm.

[0049] <About Crucible Stand 3> The groove 3h formed in the crucible base 3 is not particularly limited in its cross-sectional shape as long as it can accommodate the auxiliary heating element 10. For example, the groove 3h may be a rectangular groove or a groove with a wide, oblique cross-section. The corner formed by the inner surface of the groove 3h and the upper surface of the crucible base 3 may be chamfered to form an inclined surface 3e. In this case, the inclined surface 3e is formed so that when the auxiliary heating element 10 is accommodated in the groove 3h, the back surface 10b of the auxiliary heating element 10 comes into surface contact with the inclined surface 3e. This allows the auxiliary heating element 10 to be easily and stably placed in the groove 3h. When the inclined surface 3e is provided, the auxiliary heating element 10 is formed so that its inner diameter D1 is slightly shorter than the inner diameter W1 of the groove 3h (see FIG. 3).

[0050] <About induction coil 5> The induction coil 5 is not particularly limited in structure, as long as it can inductively heat the crucible 2, the after-heater 1d, the auxiliary heating element 10, etc. For example, a high-frequency induction heating device consisting of a high-frequency heating coil may be used as the induction coil 5. In this case, the power supply 31 is configured as a high-frequency power supply capable of supplying high-frequency current to the induction coil 5. Furthermore, the power supply 31 may supply power not only to the induction coil 5 but also to the entire crystal growth apparatus 1.

[0051] <About After-Heater 1d> The size of the after-heater 1d is not particularly limited, as long as its inner diameter is larger than the diameter of the single crystal C to be obtained and smaller than the outer diameter of the crucible 2. The total length of the after-heater 1d is also not particularly limited, but it is desirable to set it to, for example, longer than half the total length of the single crystal C to be obtained and shorter than twice the total length of the single crystal C.

[0052] <Regarding the control unit 30> The control unit 30 may not only control the power supply 31 and the pulling axis drive unit 22, but also control the operation of the entire crystal growth apparatus 1, including the crystal growth process performed by the crystal growth apparatus 1 of this embodiment. The configuration of the control unit 30 is not particularly limited. The control unit 30 may be composed of, for example, a microcomputer equipped with a CPU (Central Processing Unit), a central processing unit, and memories such as ROM (Read Only Memory) and RAM (Random Access Memory) and operated by a program, or may be composed of an electronic circuit such as an ASIC (Application Specific Integrated Circuit) developed for a specific application. [Example]

[0053] When the raw material melt is heated by the crystal growth apparatus of the present invention, the state of the raw material melt in the crucible was confirmed by numerical simulation.

[0054] In a crystal growth apparatus such as that shown in Figures 1 and 2, the temperature distribution, velocity distribution, and heat density of the raw material melt in the crucible at the final stage of pulling were confirmed by numerical simulation using a model equipped with an auxiliary heating element as shown in Figure 3. The numerical simulation was performed using the thermal fluid analysis software CFX by ANSYS.

[0055] The model was set under the following conditions: The crucible was made of platinum, had an outer diameter of 200 mm, and had a side wall and bottom thickness of 3 mm. The crucible base was made of zirconia and had an outer diameter of 270 mm, a height of 40 mm, an outer diameter of the groove of 200 mm, a groove width of 35 mm and a groove depth of 35 mm. The heating conditions for the crucible were such that the heater was controlled so that the temperature at which the melt, gas, and seeds came into contact was the melting point. The raw material melt was lithium niobate, and was contained in the crucible to a height of 190 mm. The auxiliary heating element was a platinum plate with a thickness of 1.5 mm, and the angle of the surface relative to the central axis was changed to +15, +30, +45, +60, +75°, and -45°. When the surface angle with respect to the central axis of the auxiliary heating element is negative, the distance between the bottom surface of the crucible and the surface of the auxiliary heating element becomes shorter as the surface of the auxiliary heating element moves from the inside to the outside. When the surface angle with respect to the central axis of the auxiliary heating element is positive, the distance between the bottom surface of the crucible and the surface of the auxiliary heating element becomes longer as the surface of the auxiliary heating element moves from the inside to the outside. A zirconia valve was placed around the crucible and crucible stand as a heat insulator to a thickness of 35 mm.

[0056] The comparative example was a numerical simulation performed under the same conditions except that no auxiliary heating element was provided.

[0057] The results are shown in Figures 4 and 5. In the simulation of FIG. 4, the inner diameter of the auxiliary heating element is set to be the same as the outer diameter of the crucible. In addition, in the simulation of Figure 5, the outer diameter of the auxiliary heating element was fixed at 198 mm, and the width of the auxiliary heating element (half the length of D2-D1 in Figure 3) was fixed at 15 mm, and the angle that the surface of the auxiliary heating element made with respect to the vertical direction was changed.

[0058] As shown in Figure 4(A), in the comparative example (without auxiliary heating element), the temperature of the raw material melt on the central axis of the crucible is at most about 1 K even at a position higher than the melting point of the raw material (the temperature shown by the dotted line in Figure 4(A)), and it can be confirmed that there is a high possibility that the raw material will solidify. On the other hand, when an auxiliary heating element was installed, the temperature of the raw material melt above the central axis of the crucible was higher by nearly 5 K overall. Although the temperature of the raw material melt near the bottom of the crucible (up to about 10 mm) was below the melting point, this range was narrow, and it was expected that even if the raw material solidified, it would remain within a narrow range.

[0059] It is desirable for the flow of the raw material melt along the central axis of the crucible to be downward, that is, toward the bottom of the crucible. However, as shown in Figure 4(B), when there is no auxiliary heating element or when the surface of the auxiliary heating element is at +45° to the vertical, the flow of the raw material melt along the central axis of the crucible is downward. On the other hand, when the surface of the auxiliary heating element is at -45° to the vertical, the flow of the raw material melt along the central axis of the crucible is upward. When the surface of the auxiliary heating element is at -45° to the vertical, it can be seen that although the temperature of the raw material melt increases, the flow of the raw material melt is worsened.

[0060] As shown in Figure 5, as the angle that the surface of the auxiliary heating element makes with the vertical direction increases, the inclination approaches a flat surface, and it can be seen that the heat density increases at angles of around 15 to 45 degrees. From these results, it can be seen that the surface of the auxiliary heating element should be inclined with respect to the vertical direction so that the distance between the bottom surface of the crucible and the surface of the auxiliary heating element increases from the inside to the outside, and that it is desirable to keep the inclination in the range of 15 to 45 degrees.

[0061] From the above results, it can be confirmed that using an auxiliary heating element that is inclined so that the distance between the bottom surface of the crucible and the surface of the auxiliary heating element increases from the inside to the outside in the vertical direction, as in the crystal growth apparatus of the present invention, is effective in preventing the raw material melt from solidifying. In particular, it has been confirmed that the surface of the auxiliary heating element should be inclined relative to the vertical direction so that the distance between the bottom surface of the crucible and the surface of the auxiliary heating element increases from the inside to the outside, and that the inclination should desirably be in the range of 15° to 45°. [Industrial Applicability]

[0062] The crystal growth apparatus of the present invention is suitable as a crystal growth apparatus for growing single crystals using the Czochralski method. [Explanation of symbols]

[0063] 1. Crystal growth equipment 2 Crucible 3 Crucible stand 5. Induction Coil 10 Auxiliary heating element 10a surface 10b back side 10h through hole SC seed crystal C single crystal M Raw material melt

Claims

1. A crystal growth apparatus for growing a single crystal from a raw material melt obtained by melting a raw material by induction heating, a cylindrical crucible with a bottom that contains a raw material melt; an induction coil for induction heating the crucible; an auxiliary heating element provided below the crucible and induction-heated by the induction coil; The auxiliary heating element is The annular member has only an inclined surface on the surface facing the crucible that is inclined from the inside to the outside so as to move away from the crucible. A crystal growth apparatus characterized by:

2. The inclined surface of the auxiliary heating element is The inclination angle of the surface is 15 to 45 degrees with respect to the central axis of the through hole.

2. The crystal growth apparatus according to claim 1.

3. The auxiliary heating element is The outer diameter is 0 to 20 mm shorter than the outer diameter of the bottom surface of the crucible.

2. The crystal growth apparatus according to claim 1.

4. The auxiliary heating element is The difference between the inner diameter and the outer diameter is 15% to 40% of the outer diameter of the bottom surface of the crucible.

2. The crystal growth apparatus according to claim 1.

5. The auxiliary heating element is The distance from the top end to the bottom surface of the crucible is set to 5 to 20 mm.

2. The crystal growth apparatus according to claim 1.

Images