Method and device for growing crystal

Abstract

In a first step S101, the raw material is directly induction heated using a high-frequency induction heating coil (103) while cooling the outer periphery of a raw material (151) to or below the melting point of the raw material by a cooling mechanism (102) to form a raw material melt (152) (raw material melting step). In a second step S102, a seed crystal (161) is brought into contact with the melt (seeding step). In a third step S103, a crystal (153) is grown from the seed crystal (crystal growth step). In the third step S103, the high-frequency induction heating coil and the seed crystal are gradually separated from each other while maintaining the relative distance between the high-frequency induction heating coil and the outer periphery of the raw material, and the crystal is grown in the melt.

Description

Crystal growth method and apparatus

[0001] The present invention relates to a crystal growth apparatus and a crystal growth method.

[0002] Oxide single crystals, such as sapphire (an optical material and substrate material), β-GaO (a semiconductor material), LYSO (a scintillator material), and LiTaO (a piezoelectric material), are widely used in industry as materials for electronic and optical devices. Among these, crucibles for holding high-temperature melts are used to produce functional oxide crystals with melting points exceeding 1600°C, and materials for the crucibles include iridium, platinum, molybdenum, and tungsten. Methods for producing these functional oxide crystals vary depending on the material properties, and known methods include the Czochralski method (hereinafter referred to as the CZ method), the Edge-Defined Film-fed Growth (EFG) method, and the vertical Bridgman method (Non-Patent Document 1).

[0003] Among these, the chiroporous method (hereinafter referred to as the KY method), a crystal growth method similar to the CZ method, involves growing crystals below the surface of a melt. This method is used to manufacture sapphire substrates for semiconductor film formation. The KY method includes a vacuum system, a heating system, a cooling system, a thermal insulation system, and a control system. A tungsten resistance heating element and a molybdenum crucible are primarily used for growing sapphire crystals using the KY method. Crystal growth begins in the melt from a seed crystal brought into contact with the molten raw material in the crucible from above. The crystal grows in the melt, and the growth rate is controlled by the output of the resistance heating element. The crystal grown in the melt is pulled out of the melt just before contacting the crucible. Currently, the KY method is one of the effective methods for growing large-diameter sapphire substrate crystals (Patent Document 1).

[0004] However, because the KY method for producing high-melting-point oxides uses a metal crucible, crystal growth must be performed under a high vacuum or inert gas atmosphere to prevent oxidation and contamination of the raw material melt by the crucible's composition. This makes it unsuitable for growing crystals of materials with high vapor pressures, such as gallium oxide. Furthermore, the diameter of the crystal depends on the inner diameter of the crucible. In other words, to increase the crystal diameter, a larger crucible is required. However, because the crucible materials that can be used at the melting points of gallium oxide and sapphire are limited to expensive noble metals such as iridium, it is difficult to reduce costs by increasing the size of the crucible.

[0005] As a crystal growth method that does not use the above-mentioned expensive crucible, research and development has been conducted on the skull melt method, in which an electromagnetic field is directly applied to the raw material to be grown using an induction coil to heat the raw material and grow a single crystal from the molten raw material. Patent Documents 2 and 3 disclose a cold crucible suitable for the skull melt method and a single crystal growth method using a cold crucible. Furthermore, Patent Document 4 discloses a cold crucible made of gallium oxide and a method for producing a gallium oxide single crystal using the skull melt method using a cold crucible, and Patent Document 5 discloses the configuration of an apparatus for producing a single crystal using the skull melt method. However, Patent Documents 2 to 5 do not disclose specific production conditions for the single crystal. Furthermore, Patent Documents 2 to 5 do not disclose specific production conditions for the single crystal. Furthermore, although research and development has been conducted on crystal growth using the skull melt method, there are still many challenges to industrial mass production and commercialization, and a method that enables industrially satisfactory crystal growth has been long awaited.

[0006] JP 2006-312571 A U.S. Patent Application Publication No. 4049384 JP 60-2876 A JP 2017-61396 A JP 2018-191426 A

[0007] Hideo Aida et al., Growth of β-Ga2O3 single crystals by the Edge-Defined, Film Fed Growth Method, Jpn. Journal of Applied Physics, Vol. 47, No. 11, 2008, pp. 8506-8509.

[0008] As mentioned above, the conventional KY method uses a metal crucible, and therefore requires crystal growth under a high vacuum or inert gas atmosphere to prevent oxidation and wear of the crucible and contamination of the raw material melt by the crucible's composition. This makes it unsuitable for growing crystals of materials with high vapor pressures, such as gallium oxide. Furthermore, since the crystal diameter depends on the inner diameter of the crucible, a larger crucible is required to increase the crystal diameter. However, crucible materials that can be used with high melting points, such as gallium oxide and sapphire, are limited to expensive noble metals such as iridium, making it difficult to reduce costs when using a larger crucible. Another issue with the skull melt method is that no method or apparatus has been developed that can achieve industrially satisfactory crystal growth.

[0009] An object of the present invention is to provide a manufacturing method and a manufacturing apparatus that can produce crystals industrially advantageously.

[0010] The crystal growth method of the present invention includes a raw material melting step in which the raw material is directly induction-heated using a high-frequency induction heating coil while cooling the outer periphery of the raw material to below the melting point of the raw material, thereby forming a molten raw material; a seeding step in which a seed crystal is brought into contact with the molten raw material; and a crystal growth step in which a crystal is grown from the seed crystal.In the crystal growth step, the high-frequency induction heating coil and the seed crystal are gradually separated while maintaining the relative distance between the high-frequency induction heating coil and the outer periphery of the raw material, thereby growing a crystal within the molten raw material.

[0011] In one configuration example of the above crystal growth method, the raw material melt is a solution.

[0012] In one configuration example of the above crystal growth method, the raw material comprises an oxide material.

[0013] In one configuration example of the above crystal growth method, the oxide material is gallium oxide, and the temperature of the gallium oxide raw material is continued to be adjusted by adjusting the output of the high-frequency power supply even after the molten gallium oxide becomes a solid state at a temperature below the melt temperature.

[0014] In one configuration example of the above crystal growth method, in the crystal growth step, the moving speed of the high-frequency induction heating coil is determined depending on the growth rate of the crystal.

[0015] In one configuration example of the above crystal growth method, in at least one of the raw material melting process, seeding process, and crystal growth process, the temperature of the raw material melt is controlled by controlling the output of the high-frequency power supply applied to the high-frequency induction heating coil, by controlling at least one of the oscillation frequency or input voltage of the high-frequency power supply.

[0016] In one configuration example of the above crystal growth method, the electrical resistivity ρ of the melt of the oxide material, the oscillation frequency f of the high-frequency power supply, the diameter D of the raw material melt, and the magnetic permeability μ of the melt of the oxide material satisfy the following formula (1):

[0017]

[0018] In one example of the crystal growth method, the location heated by the high-frequency induction heating coil is moved from one end of the raw material in contact with the seed crystal to the other end, thereby growing a crystal within the melt of the raw material.

[0019] The crystal growth apparatus of the present invention is a crystal growth apparatus for forming a molten material by directly inductively heating the material using a high-frequency induction heating coil while cooling the outer periphery of the material to below the melting point of the material, and growing a crystal in the molten material using a seed crystal, in which the high-frequency induction heating coil and the seed crystal are gradually separated from each other while maintaining a relative distance between the high-frequency induction heating coil and the outer periphery of the material.

[0020] In one example of the crystal growth apparatus, the output of the high frequency power supply applied to the high frequency induction heating coil is controlled by controlling at least one of the oscillation frequency and input voltage of the high frequency power supply to control the temperature of the raw material melt.

[0021] In one example of the configuration of the crystal growth apparatus, a mechanism for measuring the weight of the crystal is further provided, and the output of the high frequency power source is controlled in accordance with the change in weight per unit time.

[0022] In one example of the above crystal growth apparatus, the location heated by the high-frequency induction heating coil is moved from one end of the raw material in contact with the seed crystal to the other end, thereby growing a crystal within the melt of the raw material.

[0023] As described above, according to the present invention, large-diameter, long crystals can be obtained at low cost and with high quality.

[0024] FIG. 1 is a flowchart illustrating a crystal growth method according to a first embodiment of the present invention. FIG. 2A is a diagram illustrating the configuration of a crystal growth apparatus according to a first embodiment of the present invention. FIG. 2B is a diagram illustrating the configuration of a crystal growth apparatus according to a first embodiment of the present invention. FIG. 2C is a diagram illustrating the configuration of a crystal growth apparatus according to a first embodiment of the present invention. FIG. 2D is a diagram illustrating the configuration of a crystal growth apparatus according to a first embodiment of the present invention. FIG. 2E is a diagram illustrating the configuration of a crystal growth apparatus according to a first embodiment of the present invention. FIG. 3 is a flowchart illustrating a method for controlling the diameter of a raw material crystal in a crystal growth method according to an embodiment of the present invention. FIG. 4 is a diagram illustrating the configuration of a crystal diameter control mechanism in a crystal growth apparatus according to an embodiment of the present invention. FIG. 5 is a diagram illustrating the configuration of a high-frequency heating device used in a crystal growth apparatus according to an embodiment of the present invention. FIG. 6 is a schematic circuit diagram of an inverter unit (A) and a schematic diagram of high-frequency waves generated by a transistor (B). FIG. 7 is a diagram illustrating an outline of a part of the inverter unit circuit, including an LCR circuit incorporating a heating coil. Figure 8 is a conceptual diagram (A) for explaining the phase difference between the output current waveform and the reference waveform, and a waveform diagram (B) showing an example of the output current waveform and the output voltage waveform. Figure 9 is a graph showing the time changes of the drive frequency, phase difference, current, and voltage when frequency control based on phase difference is performed in the crystal growth process of the crystal growth method according to the embodiment. Figure 10 is a graph of the phase difference, drive frequency, current, and voltage when frequency control based on phase difference is performed in the melting process of the crystal growth method according to the embodiment. Figure 11 is a graph of the phase difference, drive frequency, current, and voltage when frequency control based on phase difference is performed in the crystal growth process of the crystal growth method according to the embodiment.

[0025] Preferred embodiments of the present invention will be described in more detail below with reference to the drawings, but the present invention is not limited to these specific examples.

[0026] First, a crystal growth method according to an embodiment of the present invention will be described with reference to Fig. 1. This crystal growth method makes it possible to obtain single crystals by crystal growth from raw materials such as zirconium oxide, titanium, titanium-aluminum alloy, iron oxide, magnesium oxide, calcium oxide, strontium oxide, yttrium oxide, chromium oxide, lanthanum chromium oxide, strontium titanate, cadmium oxide, scandium oxide, lutetium oxide, lanthanum nickel oxide, gallium oxide, aluminum oxide, magnesium-aluminum oxide, yttrium-aluminum garnet, gadolinium-aluminum gallium garnet, lanthanum-gadolinium silicon oxide, strontium iodide, cerium bromide, lithium calcium-aluminum fluoride, lithium tantalate, and lithium niobate.

[0027] This crystal growth method includes a raw material melting process in which the raw material is directly induction heated using a high-frequency induction heating coil while the outer periphery of the raw material is cooled below the melting point of the raw material, thereby forming a molten body of the raw material; a seeding process in which a seed crystal is brought into contact with the melt; and a crystal growth process in which a crystal is grown from the seed crystal.In the crystal growth process, the high-frequency induction heating coil and the seed crystal are gradually separated while maintaining the relative distance between the high-frequency induction heating coil and the outer periphery of the raw material, thereby growing a crystal within the melt.

[0028] The following description will be given with reference to FIG. 1 . First, in step S101, while the surrounding area of ​​a solid source material is cooled, the center of the source material at one end is heated by induction heating and melted (source material melting step). In this step, a metal that easily absorbs a magnetic field can be used as a starter for the initial heating. Using a metal element contained in the source material reduces the effects of contamination. When the source material becomes liquid due to heat generated by the starter, its resistivity drops significantly, improving its magnetic field absorption and forming a stable molten zone. In the case of gallium oxide, examples of metals used as starters include Ir and Ga metal. In addition, to prevent decomposition due to high-temperature heating, a solvent is added to the source material, such as alkali halides such as LiCl, LiF, and KCl, alkali metal-containing borides such as NaBO, KBO, and LiBO, PbO, KO, and NaOH. In other words, the melted source material functions as a solution.

[0029] The raw material body may be, for example, cylindrical. The raw material body may be, for example, a cylinder with a bottom diameter of 100 mm and a height of 100 mm. The raw material body is placed so that the other end (bottom surface) of the raw material body faces the ground and the axis of the cylinder is perpendicular to the ground.

[0030] Next, in the second step S102, a seed crystal is brought into contact with the molten material at the center of one end (top surface) of the raw material body (seeding step). By heating as described above, the central portion of one end of the raw material body becomes a molten material, and the seed crystal is brought into contact with this molten material at the top surface. In this step, the oscillation frequency of the high-frequency power supply is maintained within a range that is offset from the resonant frequency of the device containing the molten material, thereby maintaining the temperature gradient of the molten material as described above. This temperature gradient control is also maintained in subsequent steps described below. Here, the high-frequency power supply, together with the high-frequency induction heating coil, constitutes a heating mechanism for heating the raw material body by high-frequency induction heating. The heating mechanism also includes a mechanism (not shown) that applies the oscillation frequency of the high-frequency power supply to the high-frequency induction heating coil at a frequency that is offset from the resonant frequency of the device containing the molten material.

[0031] Next, in the third step S103, while cooling the periphery of the source material, the area heated by the induction heating coil is moved from one end of the source material to the other, and the area where the source material melts in the center of the source material is moved from one end to the other (crystal growth step). Here, the above-mentioned movement is performed from one end of the source material to the other end of the source material placed on the ground side. This movement causes a source material crystal to grow from the point where the seed crystal contacts in a direction from one end of the source material to the other end. In this crystal growth step, the high-frequency induction heating coil and the seed crystal are gradually separated relative to each other while maintaining the relative distance between the high-frequency induction heating coil and the outer periphery of the source material, and crystal growth is performed in the molten material. The area heated by the high-frequency induction heating coil is moved from one end of the source material contacting the seed crystal to the other end, and crystal growth is performed in the molten material.

[0032] Here, at least in the second step S102 and the third step S103, the heating of the center of the raw material by the induction heating coil is performed by setting the oscillation frequency of the high-frequency power supply within a range that is shifted from the resonant frequency of the device containing the melt, so that the temperature of the raw material is lower in the center of the raw material in a plane perpendicular to the direction from one end to the other end of the raw material (temperature gradient of the melt). In this way, the oscillation frequency of the high-frequency power supply is changed within a range that is shifted from the resonant frequency of the device containing the melt, thereby controlling the temperature gradient of the melt in the plane, thereby controlling the diameter of the raw material crystals. However, this is not necessarily required in this embodiment, and such control means can be used as appropriate, and other control means (for example, a means for changing the magnetic field output or a means for changing the position of the coil, etc.) can also be used suitably.

[0033] When the raw material is an oxide, the second step S102 and the third step S103 can be performed in an atmosphere with an oxygen concentration of 10% or more by volume, preferably 50% or more, and more preferably 80% or more.

[0034] Here, by changing the concentration of the dopant impurity added to the source body in the direction from one end to the other end of the source body, the impurity concentration can be made uniform throughout the crystal growth direction. In addition, in the third step S103, at least one of the seed crystal and the source body can be rotated around an axis facing the direction of movement as a rotation axis.

[0035] The dopant to be added may be silicon, germanium, tin, iron, lithium, magnesium, or chromium in the case of gallium oxide, or cerium, europium, ytterbium, neodymium, terbium, erbium, holmium, thulium, or praseodymium in the case of oxides or ionic crystals of optical materials.

[0036] After growing a crystal of the raw material from one end of the raw material body to a predetermined location in the direction toward the other end, heating is stopped, and the molten material inside the raw material body is slowly cooled and solidified. As a result, a single crystal of the raw material is formed inside the raw material body, growing from the seed crystal. The raw material surrounding the raw material body is then removed, and the single crystal of the raw material is extracted. Among oxide materials, gallium oxide has the property of maintaining sufficient electrical conductivity for heating even after the melt has solidified. Using this property, in the case of gallium oxide, higher quality crystals can be obtained by controlling the temperature so that the material is slowly cooled, for example, to room temperature, without stopping the control of the induction heating coil.

[0037] According to the above-described embodiment, the periphery of the source material, whose outer periphery is cooled below the melting point and remains solid, can be regarded as a crucible containing the melt. Therefore, there is no problem of impurity contamination from the crucible. Furthermore, since there is no need to use a crucible made of expensive materials, manufacturing costs are reduced. Furthermore, by lowering the upper end of the coil by 5 mm or more from the top surface of the solution, the temperature is maintained lower in the center of the source material in a plane perpendicular to the direction from one end of the source material to the other, making it possible to produce high-quality crystals.

[0038] Next, a crystal growth apparatus for carrying out the crystal growth method according to the above-described embodiment will be described with reference to Figures 2A to 2E. This crystal growth apparatus is an apparatus for growing oxide crystals, ionically bonded crystals, etc., and includes a holder 101, a cooling mechanism 102, and a high-frequency induction heating coil 103. It also includes a growth chamber 104 that houses the holder 101, the cooling mechanism 102, and the high-frequency induction heating coil 103.

[0039] As shown in FIG. 2A , the holder 101 is made of a solid raw material and holds a raw material body 151. The raw material body 151 is, for example, a cylinder. The raw material body 151 is placed on the holder 101 so that the other end (bottom surface) of the raw material body 151 faces the ground and the axis of the cylinder is perpendicular to the ground. A seed crystal 161 is placed on one end (top surface) of the raw material body 151. In addition, in the first embodiment, a holding rod 162 is provided to hold the seed crystal 161. The seed crystal 161 is composed of a crystal made of the same material as the raw material body 151, or an oxide crystal, an ionic crystal, or a covalent crystal.

[0040] Cooling mechanism 102 is disposed so as to cover the peripheral surface of source material 151 held on holding table 101, and cools source material 151. Cooling mechanism 102 is formed, for example, from copper piping through which a cooling medium flows, and this copper piping is disposed in a coiled, wavy, or bundled U-shaped piping. Furthermore, cooling mechanism 102 is disposed so as to completely cover source material 151 from the top to the bottom in the crystal growth direction, or is provided with a mechanism that moves simultaneously with high-frequency induction heating coil 103.

[0041] High-frequency induction heating coil 103 is disposed surrounding cooling mechanism 102. High-frequency induction heating coil 103 constitutes a heating mechanism for heating, by high-frequency induction heating, the portion of raw material 151 covered by cooling mechanism 102. The heating mechanism also includes a mechanism (not shown) for applying to high-frequency induction heating coil 103 an oscillation frequency of a high-frequency power supply that is shifted from the resonance frequency of the device containing the molten raw material.

[0042] 2B , when the region of raw material body 151 cooled by cooling mechanism 102 is heated to, for example, 1000° C. by a heating mechanism using high-frequency induction heating coil 103, the central portion of raw material body 151 becomes molten material 152, where the raw material is melted. Meanwhile, the peripheral portion of raw material body 151 around this region remains solid because it is cooled by cooling mechanism 102. In this state, the peripheral portion of raw material body 151, which has been cooled by cooling mechanism 102 and remains solid, serves as a crucible for containing molten material 152.

[0043] The growth chamber 104 includes an exhaust pipe 105 and a gas introduction pipe 106. An exhaust mechanism (not shown) that exhausts the interior of the growth chamber 104 is connected to the exhaust pipe 105. The gas introduction pipe 106 includes a gas introduction mechanism (not shown) that introduces a gas containing oxygen into the growth chamber 104. After the interior of the growth chamber 104 is exhausted by the exhaust mechanism, the interior of the growth chamber 104 can be adjusted to an oxygen concentration of 10% or more by volume by introducing oxygen gas, or a mixed gas of carbon dioxide gas and an inert gas such as argon gas or nitrogen gas, using the gas introduction mechanism.

[0044] The crystal growth apparatus also includes a moving mechanism (not shown) that moves the cooling mechanism 102, the high-frequency induction heating coil 103, and the source material 151 relative to one end of the source material 151 from one end to the other. First, as shown in FIG. 2C , the cooling mechanism 102 and the high-frequency induction heating coil 103 are disposed on one end (top side) of the source material 151, and the cooling mechanism 102 and the heating mechanism are operated, thereby carrying out the second step S102. In this state, as shown in FIGS. 2D and 2E , the moving mechanism is operated to move (lower) the cooling mechanism 102 and the high-frequency induction heating coil 103 to the other end (downward), thereby carrying out the third step S103.

[0045] In addition, a moving mechanism (not shown) for moving holding rod 162 and seed crystal 161 in a direction away from raw material body 151 may be provided. In addition, a rotation mechanism (not shown) for rotating seed crystal 161 and raw material body 151 in different directions, each of which rotates around an axis facing the direction of movement by the moving mechanism as a rotation axis, may also be provided.

[0046] Using this crystal growth apparatus, first, as shown in FIG. 2A, a source material 151 is fixed on a holding table 101. For example, a source material powder made by mixing and sintering YO, AlO, and CeO powders in a molar ratio of 3:5:0.015 is filled into a water-cooled hearth with an inner diameter (diameter) of 150 mm inside the cooling mechanism 102 to form a source material 151 with a diameter of 150 mm. Metallic aluminum is also placed inside the source material 151. The water-cooled hearth is fixed on the holding table 101.

[0047] A seed crystal 161 made of a single crystal of yttrium aluminum garnet (YAG) is held by a holding rod 162 at a distance from one end of the source material 151. A moving mechanism (not shown) is controlled to position the cooling mechanism 102 and the high-frequency induction heating coil 103 on one end (top) of the source material 151.

[0048] In the above-described state, the growth chamber 104 is sealed, and the cooling mechanism 102 and the high-frequency induction heating coil 103 are operated. As shown in FIG. 2B , the metallic aluminum in the source material 151 at one end thereof is heated and melted by induction heating using a high-frequency wave of 1 MHz. The molten metallic aluminum melted by this induction heating is used as a heat source to melt the raw material powder that constituted the source material 151, thereby obtaining a melt (melt) 152 having a diameter of 140 mm. In this example, the resonant frequency of the device including the melt 152 is 1 MHz. The melt of the above-described raw material may be a solution.

[0049] Next, while controlling the oscillation frequency of the high-frequency power supply applied to high-frequency induction heating coil 103 within a range of 0.8 to 1.2 MHz, holding rod 162 is lowered as shown in FIG. 2C , so that the lower end of seed crystal 161 comes into contact with the upper surface of melt 152 formed in the center of one end side (upper surface) of raw material body 151.

[0050] When an AC current flows through the molten mass 152, which is a conductor, as the frequency of the AC increases, the current concentrates on the surface of the molten mass 152 (current density increases) and becomes more difficult to flow (current density decreases) toward the center of the molten mass 152 (skin effect). By adjusting the frequency applied to the high-frequency induction heating coil 103 with respect to the resonant frequency of the molten mass 152, the heating efficiency of the region penetrated by the skin effect can be controlled, and the temperature gradient within the surface of the molten mass 152 can be optimized.

[0051] In the above-described control of the oscillation frequency, since the resonant frequency of the device including the melt 152 also changes with changes in the temperature and melted area of ​​the melt 152, the phase difference between the oscillation frequency of the induction heating power supply and the resonant frequency of the device including the melt 152 is measured in real time, and the oscillation frequency of the induction heating power supply is controlled so that the phase difference is constant. As a result, as shown in Fig. 2D, growth of the crystal 153 starts in the in-plane direction of the melt 152 from the center to the periphery, and in the vertical direction opposite to the seed crystal.

[0052] Here, the optimum frequency for growing the crystal 153 shown in FIG. 2D is determined and controlled so as to satisfy equation (1), where ρ is the electrical resistivity of the melt of the oxide material, f is the oscillation frequency of the high-frequency power supply, D is the diameter of the melt, and μ is the magnetic permeability of the melt of the oxide material.

[0053]

[0054] The electrical resistivity of the melt of most oxide materials is 10 -3 Since the resonant frequency is on the order of Ωm, for example, when the diameter D of the melt is 100 mm, the resonant frequency of the melt-containing device is desirably 200 kHz or higher. If the resonant frequency of the melt-containing device is too high, the center of the raw material remains unmelted, and if the resonant frequency of the melt-containing device is too low, the temperature of the center of the melt surface becomes too high, making it impossible to contact the seed crystal. As the melt diameter D increases, the resonant frequency of the melt-containing device required for heating and melting decreases.

[0055] Because the oscillator output during the crystal growth process is strongly affected by fluctuations in the volume of the melt, it is necessary to stabilize the oscillator output. Preferably, the phase difference between the oscillation frequency of the induction heating power supply and the resonant frequency of the device containing the melt is monitored, and the switching frequency of the gate voltage of the oscillation circuit of the induction heating power supply is controlled so that the phase difference remains constant.

[0056] As shown in Figure 2E, from a state in which the in-plane temperature gradient of the melt 152 is optimized, a movement mechanism (not shown) is controlled to move (lower) the cooling mechanism 102 and the high-frequency induction heating coil 103 toward the other end. In this way, the area to be heated by induction heating is moved from one end to the other end of the raw material body 151. As a result of this movement, as shown in Figures 2D and 2E, the other end of the melt 152, where the raw material in the center of the raw material body 151 is melted, gradually moves (lowers) from one end to the other end of the raw material body 151.

[0057] Meanwhile, one end of the source material 151 in contact with the seed crystal 161 gradually becomes unheated and the temperature drops. As the temperature drops, a crystal 153 starts to grow from one end of the source material 151 to the other end. According to this embodiment, a 100 mm diameter, yellow, transparent Ce-doped YAlO 12 Thus, crystal 153 was obtained. The growth rate of crystal 153 is determined by the compositions of seed crystal 161 and source material 151 and the temperature gradient due to induction heating, and can be calculated in advance. Therefore, the moving speeds of cooling mechanism 102 and high-frequency induction heating coil 103 from one end to the other are controlled to be equal to or lower than the calculated growth rate of crystal 153.

[0058] As described above, the crystal 153 is grown from the point where the seed crystal 161 contacted the crystal 153 until the length of the straight body reaches the set length, and then the crystal 153 is separated and slowly cooled, after which heating is stopped. After sufficient cooling, the crystal 153 is removed.

[0059] Next, we will explain how to control the diameter of the crystal in the crystal growth method and apparatus according to the embodiment of the present invention. In the third step S103, the oscillation frequency of the high-frequency power supply is changed within a range that is offset from the resonant frequency of the apparatus containing the melt, thereby controlling the temperature gradient of the melt within the plane and controlling the diameter of the raw material crystal.

[0060] More specifically, as shown in FIG. 3, the third step S103 includes a weight measurement step S301, a diameter calculation step S302, and a magnetic field output control step S303.

[0061] The weight measurement step S301 measures the weight of the grown crystal at a set cycle. The diameter calculation step S302 calculates the diameter from the crystal density and the seed crystal pull rate (crystal growth rate) based on the weight measured in the weight measurement step S301. The magnetic field output control step S303 controls the output and frequency of the magnetic field applied to the coil so as to eliminate the difference between the diameter calculated in the diameter calculation step S302 and a set reference value.

[0062] The crystal growth apparatus according to the embodiment of the present invention may further include a crystal diameter control mechanism for implementing the above-described crystal diameter control method. As shown in FIG. 4, the crystal diameter control mechanism includes a weight measurement unit 301, a diameter calculation unit 302, a magnetic field output control unit 303, and a high-frequency power supply 304.

[0063] The weight measuring unit 301 measures the weight of the raw material crystal grown using the seed crystal at a set period. The weight measuring unit 301 can be configured, for example, by a load cell. The load cell can be provided, for example, on a holding rod that holds the seed crystal.

[0064] A diameter calculation unit 302 calculates the diameter of the crystal grown during the set period from the crystal density and the distance the crystal is pulled up during the set period, based on the weight measured by the weight measurement unit 301. A magnetic field output control unit 303 controls the power and frequency of the current output from a high-frequency power supply 304 to a high-frequency induction heating coil for high-frequency induction heating, so as to eliminate the difference between the diameter calculated by the diameter calculation unit 302 and a set reference value.

[0065] In this way, an automatic diameter control system can be constructed that controls the diameter of the growing crystal by using the results of measuring the weight of the growing crystal and controlling the power and frequency of the current applied to the high-frequency induction heating coil by the high-frequency power supply 304.

[0066] The diameter calculation unit 302 and magnetic field output control unit 303 of the crystal diameter control mechanism according to the above-described embodiment can be implemented as a computer device equipped with a CPU (Central Processing Unit), a main memory device, an external memory device, a network connection device, etc., and the above-described functions (crystal diameter control method) can be realized by the CPU operating (executing) a program loaded in the main memory device. The above-described program is a program for causing a computer to execute the crystal diameter control method described in the above-described embodiment. The network connection device is connected to a network. Furthermore, each function can be distributed among multiple computer devices.

[0067] The materials grown by the crystal growth method according to the above-described embodiment are gallium oxide, sapphire, gadolinium aluminum gallium garnet, lithium tantalate, lithium niobate, yttrium oxide, and yttrium aluminum garnet. When the magnetic field frequency was controlled to 400 kHz to 5 MHz and the molten zone width was controlled to 50 mm to 150 mm, it was found that the materials had good crystal growth properties.

[0068] A high-frequency heating device that can be suitably used in the above-described embodiment will be described below with reference to the drawings.

[0069] <High-frequency heating device 1> The high-frequency heating device 1 will be described with reference to Fig. 5. Note that the high-frequency heating device 1 is not limited to the semiconductor type shown in Fig. 5, and a vacuum tube type can also be suitably used. The high-frequency heating device 1 includes at least a phase difference detection unit 2, a frequency control unit 3, an inverter unit 4, and an LCR circuit 5.

[0070] (Phase Difference Detector 2) The phase difference detector 2 detects the phase difference between the high-frequency voltage generated by the frequency controller 3 and the high-frequency current flowing through the LCR circuit 5. The phase difference here refers to the difference between the phase of the voltage of the reference waveform applied to the gate of the transistor 19 and the phase of the operating current, as shown in FIG. 5 . This phase difference also coincides with the difference between the phase of the operating voltage, which is the output of the transistor 19 switched by the reference waveform input to the gate of the transistor 19, and the phase of the operating current. Therefore, the phase difference detector 2 detects the difference between the phase of the voltage of the reference waveform applied to the gate of the transistor 19 and the phase of the operating current, or the difference between the phase of the operating voltage of the transistor 19 and the phase of the operating current. The phase difference detector 2 includes at least a phase comparator 12, a low-pass filter 13, and an A / D converter 14, and also includes a part of the computer 15.

[0071] [Phase Comparator 12] Phase comparator 12 detects the phase difference between two input signals. Specifically, phase comparator 12 converts the phase difference between the output current waveform of LCR circuit 5 and the reference waveform generated by arbitrary waveform generator 16 into a voltage and outputs it as an error signal.

[0072] [Low-pass filter 13] The low-pass filter 13 smoothes the error signal (pulse) corresponding to the phase difference output by the phase comparator 102 and outputs it as a DC voltage. The low-pass filter 13 outputs a DC voltage of, for example, 0 to 5 V. Note that this low-pass filter 13 is sometimes called a loop filter.

[0073] [A / D Converter 14] The A / D converter 14 converts the DC voltage output by the low-pass filter 103 into a digital signal.

[0074] [Computer 15] The computer 15 detects the difference in phase between the voltage of a reference waveform applied to the gate of transistor 19 (described later) and the phase of the operating current, or the difference in phase between the operating voltage and the operating current of transistor 19 (phase difference), compares it with a set threshold, and when the threshold is crossed (changing from a value above the threshold to a value below the threshold, or changing from a value below the threshold to a value equal to or above the threshold), increases or decreases the drive frequency by a preset change amount, updating the drive frequency, and inputs a control signal to the arbitrary waveform generator 16 to generate the updated drive frequency.

[0075] The computer 15 also compares the detected phase difference with a set threshold, and when the phase difference crosses the threshold, updates the drive frequency by a certain amount of frequency variation, and inputs a signal for generating the updated drive frequency to the arbitrary waveform generator 16. The certain amount of frequency variation can be a width determined in advance from the actual manufacturing conditions. The phase difference threshold here is the threshold when the drive frequency approaches the resonant frequency by a certain amount of frequency variation, and there are two thresholds: an upper threshold and a lower threshold.

[0076] In the feedback control, when the phase difference becomes larger than a set value, the set value of the phase difference is set as the upper limit threshold value of the phase difference.

[0077] Furthermore, when the phase difference becomes smaller than the set phase value, the drive frequency is moved away from the resonance frequency by a certain amount of frequency fluctuation. The set phase difference value at this time is the lower threshold of the phase difference. The upper and lower thresholds are collectively called the phase difference threshold. The frequency fluctuation amount refers to the amount by which the drive frequency is changed when the phase difference deviates from the set threshold.

[0078] (Frequency Control Unit 3) The frequency control unit 3 includes at least an arbitrary waveform generator 16 and a part of the computer 15. The frequency control unit 3 uses the computer 15 and the arbitrary waveform generator 16 to generate an arbitrarily set waveform (frequency pulse).

[0079] [Arbitrary Waveform Generator 16] The arbitrary waveform generator 16 generates a reference waveform. The arbitrary waveform generator 16 also generates a waveform with a drive frequency updated by a control signal input from the computer 15 and outputs it to the gate of the transistor 19. At the same time, it outputs this as a reference signal to the phase comparator 12. The arbitrary waveform generator 16 generates a frequency pulse based on the phase difference detected by the phase difference detection unit 2. The computer 15 controls the transistor 19 with the updated frequency pulse to generate a high-frequency voltage with an updated drive frequency to be applied to the LCR circuit 5.

[0080] (Inverter Unit 4) The inverter unit 4 generates a high-frequency voltage. The high-frequency voltage generated by the inverter unit 4 drives the LCR circuit 5. As a result, a high-frequency current flows from the LCR circuit 5 to the heating coil 51, heating the raw material. As shown in FIG. 5 , the inverter unit 4 includes at least a thyristor regulator 17, a step-up transformer 18, and a plurality of transistors 19.

[0081] [Thyristor Regulator 17] The thyristor regulator 17 controls the AC current output from the power supply.

[0082] [Step-up Transformer 18] The step-up transformer 18 increases the voltage output from the power supply.

[0083] [Transistor 19] The transistor 19 operates as a switching element. The transistor 19 applies a high-frequency voltage of a drive frequency to the LCR circuit 5. The transistor 19 is preferably made of SiC. The transistor preferably has a structure of a metal-oxide-semiconductor field-effect transistor (MOSFET), a metal-semiconductor field-effect transistor (MESFET), or an insulated gate bipolar transistor (IGBT).

[0084] Fig. 6A shows a schematic circuit diagram of the inverter unit, and Fig. 6B shows a general diagram of the high frequency generated by the transistors. As shown in Fig. 6A, the inverter unit 4 has a plurality of transistors 109 connected in parallel. The inverter unit 4 rectifies the three-phase current output from the thyristor regulator 17 using bridge diodes, stores charge in capacitors, and passes the current from the capacitors as direct current to the plurality of transistors 109 (AC / DC conversion). As a result, a voltage (gate voltage) corresponding to the charge stored in each transistor 19 is applied, and the transistors 19 function as switching elements.

[0085] The frequency control is performed by applying a high-frequency waveform generated by an arbitrary waveform generator 106 in response to a frequency control signal output from the computer 15 to the gate of the transistor 19 to control the gate voltage. th 6A, a rectangular wave that turns on and off is formed by combining the transistor 19A with the transistor 109D and by combining the transistor 19B with the transistor 19C.

[0086] As shown in FIG. 6B , the frequency of the output voltage Vout is controlled by controlling the gate voltage of each transistor 19. Through this frequency control, the transistor 19 outputs a square wave frequency (drive frequency) in the range of several hundred kHz to 10 MHz. The drive frequency output by the transistor 19 is preferably, for example, 20 MHz or less. This allows for melting of raw materials with higher melting points. The drive frequency output by the transistor 19 is more preferably 10 MHz or less, and even more preferably 6 MHz or less. Furthermore, the drive frequency output by the transistor 19 is preferably 100 kHz or more, and more preferably 300 kHz or more. In particular, when the heating target is gallium oxide, a frequency of 300 kHz or more and 400 kHz or less is preferred. The duty of the square wave can be adjusted, but here a 40% duty is generated.

[0087] The transistor 19 is preferably made of SiC. Using SiC as a semiconductor material allows for a high breakdown voltage and reduces switching loss and series resistance loss, thereby enabling higher output and precise frequency control, and enabling precise control of temperature distribution even with materials having high melting points. Furthermore, as described above, a wide range of frequencies can be achieved, making it possible to adjust the frequency depending on the size of the melt, the temperature dependence of the resistivity, and the resistivity difference between the solid and liquid.

[0088] (LCR circuit 5) The LCR circuit 5 is a circuit (LCR series circuit) in which a coil, a capacitor, and a resistor are connected in series. The LCR circuit 5 includes a heating coil 51. The heating coil 51 corresponds to the coil in the LCR circuit. The LCR circuit 5 removes harmonic components of the square wave and generates a fundamental sinusoidal wave current.

[0089] An induced current generated in the LCR circuit 5 by the high-frequency current generated by the high-frequency heating device 1 flows through the heating coil 51, generating a magnetic field around the heating coil 51. Inside the heating coil 51, raw materials placed in a basket (not shown) contain conductive material, and induction heating begins from the conductive material. Eddy currents flow through the raw materials in a direction that opposes changes in the magnetic field that change in response to changes in the high-frequency current, and Joule heat is generated due to the electrical resistance of the raw materials, heating the raw materials and controlling their temperature.

[0090] The combined impedance of the LCR series circuit is expressed by the following equation (2).

[0091]

[0092] In equation (2), ωL is also called inductive reactance XL, and 1 / ωC ​​is also called capacitive reactance XC. Here, since frequency f = ω / 2π, a change in frequency f causes changes in inductive reactance XL and capacitive reactance XC. This changes the phase difference (phase angle) θ between the AC voltage and AC current. For example, when inductive reactance XL is larger or smaller than capacitive reactance XC, the phase angle θ is expressed by the following equation (3). Furthermore, when inductive reactance XL and capacitive reactance XC are equal to each other, the phase angle θ is 0.

[0093]

[0094] As described above, when the frequency of the output voltage of the transistor is updated, the phase difference θ between the high-frequency voltage and the high-frequency current changes, which changes the high-frequency voltage and the high-frequency current applied to the heating coil 51. This changes the heating efficiency of the raw material, and the temperature distribution of the raw material.

[0095] When crystals are grown by directly heating the raw material by induction heating in the skull melt method, the convection state of the melt of the raw material, in other words, the temperature distribution of the melt, is extremely important.

[0096] When a magnetic field is applied to a conductive melt, an eddy current is generated in the melt, inducing a magnetic field that cancels out the applied magnetic field. Therefore, the penetration depth δ of the applied magnetic field is calculated by using the resistivity ρ of the melt, the relative magnetic permeability μ of the melt, and the frequency f, as follows: δ = 5.03 × (ρ / (μ × f)) 1 / 2 ...follows equation (4).

[0097] As shown in equation (4), the penetration depth δ of the magnetic field depends on the frequency f. Therefore, in order to optimally control the temperature distribution of the melt by changing the penetration depth δ of the magnetic field, it is important to design the entire system, as well as the resonant frequency and driving frequency band of the system, taking into consideration the sizes of the basket and heating coil to be used, etc.

[0098] For example, if the frequency is low and the magnetic field penetrates deep into the melt, the temperature of the raw material melt near the center of the melt where the seed crystal is placed will exceed the melting point, preventing crystal growth. On the other hand, if the frequency is increased and the penetration depth of the magnetic field is reduced, the temperature of the raw material melt near the seed crystal will fall below the melting point, allowing crystal growth to proceed.

[0099] [Current Sensor 7] The high-frequency heating device 1 can be equipped with a current sensor 7. The current sensor 7 measures the amplitude and frequency of the AC current output by the transistor 19. The current sensor 7 is preferably a Rogowski-type current sensor. Compared to an output current waveform measured by a wire-wound-type current sensor, an output current waveform measured by a Rogowski-type current sensor has less noise. Therefore, by using a Rogowski-type current sensor 7, the phase difference can be measured and controlled with high accuracy.

[0100] The output current waveform of the current sensor 7 is the waveform of the current flowing through the LCR circuit 5, and this is the current waveform after feedback. The rectangular wave output voltage V out , and a sine wave output current flowing through the heating coil is output.

[0101] The output current waveform after feedback is input to the phase comparator 12, which compares the output current waveform with the reference waveform described above, and outputs the phase difference between them.

[0102] The high-frequency heating device 1 can reduce output fluctuations by controlling the drive frequency of the LCR circuit 5. The suppression of output fluctuations by the high-frequency heating device 1 is extremely effective for crystal growth of high-melting-point compounds, which require precise control of temperature distribution.

[0103] (Basket) The high-frequency heating device 1 includes a basket (not shown). The basket is a coolable container (cold container) with a cylindrical space inside, and raw materials can be placed inside. For example, a cooling path (not shown) through which cooling water flows is arranged inside the basket, and the basket is cooled by the cooling water flowing through the cooling path. The basket is made of a material with high thermal conductivity. Examples of materials for the basket include copper, silver, aluminum, and iron. Because the basket is made of a material with high thermal conductivity, the raw materials placed inside it are cooled by the basket.

[0104] The raw material is heated by the heating coil 51 and melted, but the temperature is low near the water-cooled basket, so the molten liquid solidifies and sinters (remains solid). This sintered body (skull) holds the molten liquid. The temperature of the molten raw material is adjusted by controlling the frequency of the high-frequency voltage applied to the heating coil 51 located outside the basket.

[0105] (Raw Material) The raw material is a high-melting point compound, for example, a high-melting point oxide, such as gallium oxide (β-GaO), gadolinium aluminum gallium garnet (Gd(Al,Ga)O), or the like. 12 ), lithium tantalate (LiTaO), lithium niobate (LiNbO), yttrium oxide (YO), etc. In addition to the above, the high-melting-point oxide may also include high-melting-point oxides containing various elements.

[0106] As described above, oxides have a large resistivity ρ, so a high frequency f must be applied to heat oxide raw materials, but the high-frequency heating device 1 can output a frequency f on the order of several hundred kHz to 10 MHz, making it possible to heat oxide raw materials. Furthermore, because it can output a high frequency f, it is possible to increase the diameter of the melt inside the raw material, thereby enabling the diameter of the crystal to be increased.

[0107] Furthermore, since the high frequency heating device 1 is capable of adjusting the frequency, the temperature of the melt near the seed crystal can be set to a temperature close to the melting point, allowing crystal growth to proceed.

[0108] (Control Device) The high-frequency heating device 1 may be equipped with a control device (not shown). The control device controls various operations of the high-frequency heating device 1. For example, the control device controls the rod lifting speed, the basket cooling capacity, etc.

[0109] (Rotation Mechanism) The high-frequency heating device 1 preferably further includes a rotation mechanism (not shown) that rotates at least one of the rod and the raw material around the central axis of the rod in its extending direction as the rotation axis. The rotation mechanism rotates the rod relative to the raw material, thereby achieving the effect of reducing non-uniformity in the temperature distribution within the device and in the melt.

[0110] (Operation) Here, the operation of frequency control of the high-frequency heating device 1 will be described with reference to Figures 5, 7, and 8. Figure 7 is a diagram showing an outline of a circuit including part of the circuit of the inverter unit 4, the LCR circuit 5, and the heating coil 51. Figure 8(A) is a conceptual diagram for explaining the phase difference between the output current waveform and the reference waveform, and Figure 8(B) is a waveform diagram showing an example of the output current waveform and the output voltage waveform.

[0111] As shown in FIG. 7, the LCR circuit 5 receives a high-frequency voltage V out1 When the high-frequency rectangular wave is applied to the LCR circuit 5, an induced voltage V out2 As a result, a sine wave high frequency current I out2 This is because the harmonics of the square wave of the high frequency voltage are filtered by the LCR circuit 5, and only the sine wave of the fundamental square wave frequency remains. out2 is input to the current sensor 7, and the current sensor 7 outputs an output current waveform.

[0112] As shown in FIG. 8A, this output current waveform I out1and a high-frequency voltage waveform (reference waveform) generated by the arbitrary waveform generator 16 are input to the phase comparator 12, which outputs a voltage (output voltage waveform) corresponding to the phase difference θ between the output current waveform and the reference waveform. The voltage corresponding to this phase difference is passed through the low-pass filter 13, converted into a digital signal by the A / D converter 14, and input to the computer 15, which detects the phase difference between the high-frequency current (output current waveform) and the output voltage waveform. The output current waveform and output voltage waveform are shown, for example, in FIG. 8B.

[0113] Furthermore, the computer 15 compares the detected phase difference with a set threshold, and when the phase difference crosses the threshold, updates the drive frequency by a certain frequency fluctuation amount and inputs a signal for generating the updated drive frequency to the arbitrary waveform generator 16. The arbitrary waveform generator 16 generates a square wave of the updated frequency and controls the gate voltage of the transistor 19 in the inverter unit 4. The transistor 19 outputs a high-frequency voltage of the updated drive frequency and applies it to the LCR circuit 5.

[0114] As described above, feedback control is performed. That is, in at least one of the melting process and the crystal growth process, the driving frequency of the high-frequency voltage applied to the LCR circuit 5 is controlled based on the phase difference between the high-frequency voltage and the high-frequency current, or the phase difference between the gate voltage of the transistor that determines the frequency of the wave heating device and the high-frequency current. In this way, the temperature distribution of the raw material melt is controlled.

[0115] In this way, stable heating control is possible by controlling the frequency of the high frequency magnetic field through feedback so that the phase difference between the output current waveform and the output voltage waveform falls within a predetermined range.

[0116] Furthermore, by configuring a high-frequency heating device with a phase difference detection unit that detects the phase difference between the high-frequency voltage and the high-frequency current, a frequency control unit that compares the detected phase difference with a threshold value and feeds back the result to the heating frequency, an inverter unit that generates the frequency-controlled high-frequency voltage, and an LCR circuit that performs induction heating, it is possible to detect the phase difference between the high-frequency voltage and the high-frequency current and control the frequency.Furthermore, in the above-mentioned high-frequency heating device, stable feedback control can be achieved at low cost by generating high-frequency waves by controlling the gate voltage of the transistor in the inverter unit using software with a frequency control unit including a computer.

[0117] As a preferred embodiment of the present invention, an example will be described in which the drive frequency is controlled based on the phase difference between the high-frequency voltage applied to an LCR circuit and the high-frequency current flowing through the LCR circuit in the crystal growth process of a crystal growth method using gallium oxide as a raw material.

[0118] Figure 9 shows the time variations in temperature, drive frequency, phase difference between the high-frequency voltage and high-frequency current, high-frequency voltage applied to the LCR circuit, and high-frequency current flowing through the LCR circuit during the crystal growth process, in which the drive frequency is controlled based on the above phase difference.

[0119] By controlling the driving frequency, the voltage applied to the LCR circuit and the current flowing therethrough are controlled, enabling a stable crystal growth process.

[0120] Next, a process will be described in which the drive frequency is controlled based on the phase difference between the high frequency voltage applied to the LCR circuit and the high frequency current flowing through the LCR circuit in the melting step of the raw material in a crystal growth method using gallium oxide as the raw material.

[0121] Figure 10 shows an example of controlling the drive frequency when the lower threshold of the phase difference is set to 9.17 degrees. The lower threshold is set to appropriately prevent overcurrent from flowing through the transistor during this process, thereby preventing damage. The phase difference at this time varies between 9.15 and 9.33 degrees, and when the phase difference falls below the lower threshold of 9.17 degrees, the drive frequency is shifted to the higher frequency side by a frequency fluctuation of 30 Hz.

[0122] The high-frequency current flowing through the LCR circuit contains noise but has decreased from 15.3 A to 14.1 A, and the high-frequency voltage applied to the LCR circuit contains noise but has increased from 349 V to 350.5 V. Because the amount of fluctuation in these currents and voltages is smaller than the change in phase difference, the method of monitoring the phase difference and controlling the frequency allows for more precise control of the temperature distribution than current and voltage control.

[0123] Next, a process will be described in which the drive frequency is controlled based on the phase difference between the high frequency voltage applied to the LCR circuit and the high frequency current flowing through the LCR circuit in the crystal growth step of a crystal growth method using gallium oxide as a raw material.

[0124] In Figure 11, the lower threshold is set to 11.5 degrees and the upper threshold is set to 11.75 degrees. When the phase difference falls below the lower threshold, the drive frequency is increased, and when it rises above the upper threshold, the drive frequency is decreased to optimize the temperature. The frequency fluctuation in this case is 10 Hz. While the current and voltage contain a lot of noise, the phase difference exhibits a relatively stable change.

[0125] As described above, according to the embodiments of the present invention, it is possible to provide a manufacturing method and a manufacturing apparatus that can manufacture crystals industrially advantageously.

[0126] The inventors discovered that when forming a molten zone of a raw material solution using high-frequency induction heating with a high-frequency induction heating coil and producing crystals by longitudinal crystal growth in the molten zone, efficient and satisfactory crystal growth can be achieved by controlling the physical properties of the material, the width of the molten zone, and the oscillation frequency. They also discovered that crystallization can occur from the center of the melt surface by continuously moving the upper end of the high-frequency coil from the surface of the melt toward the interior of the melt during heating and melting.

[0127] That is, some or all of the embodiments of the present invention may be described as in the following supplementary notes, but are not limited to the following.

[0128] [Supplementary Note 1] This crystal growth method includes a raw material melting step in which a raw material is directly induction-heated using a high-frequency induction heating coil while cooling the outer periphery of the raw material below its melting point to form a melt of the raw material, a seeding step in which a seed crystal is brought into contact with the melt, and a crystal growth step in which a crystal is grown from the seed crystal. The crystal growth step is characterized in that the high-frequency induction heating coil and the seed crystal are gradually separated while maintaining a relative distance between the high-frequency induction heating coil and the outer periphery of the raw material. Here, "maintaining a relative distance" means that the center of the raw material is positioned approximately at the axial center of the holding rod 106, the high-frequency induction heating coil is positioned a constant distance from the approximate center of the holding rod, and the high-frequency heating coil moves in a direction parallel to the axial direction of the holding rod. Furthermore, "gradually separated" means that the high-frequency induction heating coil can be moved from one end above the raw material to the other end toward the ground, or the high-frequency induction heating coil can be fixed and the crystal, raw material melt, and raw material can be moved upward from the ground.

[0129] This method allows for simultaneous melting and solidification of the solid raw material in the direction of the high-frequency induction heating coil, making it easier to control the crystal growth direction and promote axial crystal growth. Furthermore, compared to the CZ method, the temperature gradient acting on the growing crystal is gentler, making it easier to obtain high-quality crystals with fewer internal strains and inclusions. While the pulling method achieves stable growth up to half the diameter of the crucible, this method allows for crystal growth up to a diameter roughly equal to the inner diameter of the crucible, making it possible to achieve larger diameters. Furthermore, the absence of a precious metal crucible reduces costs, and the crystal can be grown in a high-oxygen atmosphere, resulting in higher-quality crystals with fewer oxygen defects.

[0130] [Appendix 2] The crystal growth method according to appendix 1, wherein the melt is a solution.

[0131] Here, the term "solution" refers to a solution containing a solvent and a solute. Examples of the solvent include alkali halides such as LiCl, LiF, and KCl, alkali metal-containing borides such as NaBO2, K2B4O7, and LiBO2, and PbO, KO, and NaOH. The solute is a material obtained by removing the solvent from the raw material. By preparing a solution containing such a solvent and solute, the melting temperature of the raw material can be lowered. This makes it possible to suppress the decomposition of the solute due to temperature. Furthermore, it becomes possible to grow only crystals of the desired composition.

[0132] [Supplementary Note 3] A crystal growth method, characterized in that the melt according to Supplementary Note 1 or Supplementary Note 2 is an oxide material. Examples of oxide materials include gallium oxide, germanium oxide, iron oxide, magnesium oxide, calcium oxide, strontium oxide, yttrium oxide, chromium oxide, lanthanum chromium oxide, strontium titanate, cadmium oxide, scandium oxide, lutetium oxide, lanthanum nickel oxide, gallium oxide, aluminum oxide, magnesium aluminum oxide, yttrium aluminum garnet, gadolinium aluminum gallium garnet, lanthanum gadolinium silicon oxide, strontium iodide, cerium bromide, lithium calcium aluminum fluoride, lithium tantalate, and lithium niobate. This method allows crystal growth in a melt, resulting in a relatively small temperature gradient on the crystal, making it easy to obtain a high-quality crystal with a relatively low dislocation density.

[0133] [Appendix 4] A crystal growth method characterized in that the oxide material described in Appendix 3 is gallium oxide, and the temperature of the gallium oxide raw material is continued to be adjusted by adjusting the output of the high-frequency power source even after the molten gallium oxide has become a solid state at a temperature below the melt temperature.

[0134] Among oxide materials, gallium oxide has the property of maintaining sufficient electrical conductivity for heating even after the melt solidifies. By utilizing this property and slowly cooling the crystals through controlled induction heating, higher quality crystals can be obtained. In the present invention, gallium oxide crystal growth is carried out in the melt. After the crystals have grown to the required size, the movement of the high-frequency heating coil is stopped and the output of the high-frequency induction heating is gradually reduced. The melt around the crystal then gradually solidifies, and the slow cooling process begins. The solidification of the melt around the crystal is detected by fluctuations in the load cell output. It is known that rapid cooling of high-temperature crystals can cause distortion or cracks in the crystal, degrading the crystal's properties. Therefore, it is necessary to slowly cool the crystals grown under conditions that do not degrade the crystal's properties. As described above, gallium oxide has the property of maintaining sufficient electrical conductivity for heating even after the melt solidifies. By utilizing this property and slowly cooling the crystals through controlled induction heating, higher quality crystals can be obtained.

[0135] [Appendix 5] A crystal growth method characterized by determining the moving speed of a high-frequency induction heating coil according to the growth rate of the crystal described in Appendix 1. Here, the coil moving speed is controlled so as to optimize the crystal growth state. According to this method, by controlling the moving speed of the heating coil, it is possible to control the crystal growth rate in the axial direction of the growing crystal, and it becomes easy to adjust the axial length of the growing crystal by changing the moving distance of the coil.

[0136] [Appendix 6] The crystal growth method according to Appendix 5 is characterized in that in at least one of the raw material melting step, seeding step, and crystal growth step, the temperature of the raw material melt is controlled by controlling the output of the high-frequency power supply applied to the high-frequency induction heating coil and at least one of the oscillation frequencies of the high-frequency power supply. Here, controlling at least one of the oscillation frequency and input voltage of the high-frequency power supply is performed so that the temperature of the raw material melt is optimal for crystal growth. According to this method, if the crystal size is abnormally grown or insufficient, it is possible to control the crystal size by increasing or decreasing the oscillator output.

[0137] [Appendix 7] The crystal growth method according to Appendix 1, wherein the electrical resistivity ρ of the melt of the oxide material, the oscillation frequency f of the high-frequency power supply, the diameter D of the melt, and the magnetic permeability μ of the melt of the oxide material satisfy the following formula (1):

[0138]

[0139] According to this method, it is possible to selectively locally heat the outer periphery of the raw material melt by high frequency waves, regardless of the type of oxide, and it is possible to stably heat and melt the oxide raw material melt without using a crucible.

[0140] [Appendix 8] A crystal growth apparatus for forming a melt of the raw material by directly inductively heating the raw material using a high-frequency induction heating coil while cooling the outer periphery of the raw material below the melting point of the raw material, and growing a crystal in the melt using a seed crystal, characterized in that the high-frequency induction heating coil and the seed crystal are gradually separated from each other while maintaining a relative distance between the high-frequency induction heating coil and the outer periphery of the raw material.

[0141] With this configuration, the melting and solidification of the solid raw material proceed simultaneously in the direction of the heating coil movement, making it easier to control the crystal growth direction and promote crystal growth in the axial direction. Furthermore, the temperature gradient applied to the growing crystal is gentler than with the CZ method, making it easier to obtain high-quality crystals with fewer internal strains and inclusions. While the CZ method achieves stable growth up to half the diameter of the crucible, the present invention allows the crystal growth diameter to be approximately equal to the inner diameter of the crucible, thereby enabling larger diameters. Furthermore, the present invention provides a manufacturing apparatus that does not use a precious metal crucible, thereby reducing costs and enabling crystal growth in a high-oxygen atmosphere, thereby suppressing oxygen defects and producing higher-quality crystals.

[0142] [Appendix 9] The crystal growth apparatus according to Appendix 8 is characterized in that the output of the high frequency power supply applied to the high frequency induction heating coil is controlled by controlling at least one of the oscillation frequency and the input voltage of the high frequency power supply, thereby controlling the temperature of the molten raw material.

[0143] With this configuration, the temperature gradient of the melt can be optimally controlled by measuring the phase difference between the resonant frequency of the device and the oscillation frequency of the high-frequency power supply, controlling the oscillation frequency so that the fluctuation in the phase difference is within a certain value, and controlling the input voltage to the oscillator up and down.

[0144] [Appendix 10] The crystal growth apparatus according to appendix 9 further comprises a mechanism for measuring the weight of the crystal, and controls the output of the high frequency power supply in accordance with the change in weight per unit time. This configuration makes it possible to measure abnormal growth or insufficient growth of the growing crystal as a change in weight per unit time, thereby enabling stabilization of the crystal size during the growth process.

[0145] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0146] The manufacturing apparatus and manufacturing method of the present invention are useful for crystal growth and can be used to manufacture various types of crystals.

[0147] 1...high frequency heating device, 2...phase difference detection unit, 3...frequency control unit, 4...inverter unit, 5...LCR circuit, 7...current sensor, 12...phase comparator, 13...low pass filter, 14...A / D converter, 15...computer, 16...arbitrary waveform generator, 17...thyristor regulator, 18...step-up transformer, 19...transistor, 51...heating coil, 101...holding stand, 102...cooling mechanism (cold container: basket), 103...high frequency induction heating coil, 104...growth chamber, 105...exhaust piping, 106...gas introduction piping, 151...raw material, 152...melt, 153...seed crystal, 161...seed crystal, 162...holding rod

Claims

1. A crystal growth method comprising a raw material melting step in which a molten raw material is formed by directly inductively heating the raw material using a high-frequency induction heating coil while cooling the outer periphery of the raw material to below the melting point of the raw material, a seeding step in which a seed crystal is brought into contact with the molten raw material, and a crystal growth step in which a crystal is grown from the seed crystal, wherein the crystal growth step is performed by gradually separating the high-frequency induction heating coil and the seed crystal while maintaining the relative distance between the high-frequency induction heating coil and the outer periphery of the raw material.

2. A crystal growth method according to claim 1, wherein the melt of the raw material is a solution.

3. A crystal growth method according to claim 1, wherein the source material contains an oxide material.

4. A crystal growth method according to claim 3, wherein the oxide material is gallium oxide, and the temperature of the raw material is continued to be adjusted by adjusting the output of the high frequency power source even after the molten gallium oxide has become solid at a temperature below the melt temperature.

5. A crystal growth method according to claim 1, wherein the moving speed of the high frequency induction heating coil is determined in accordance with the growth speed of the crystal in the crystal growing step.

6. A crystal growth method according to claim 5, wherein the temperature of the molten raw material is controlled in at least one of the raw material melting step, the seeding step and the crystal growth step by controlling the output of the high frequency power supply applied to the high frequency induction heating coil, by controlling at least one of the oscillation frequency or input voltage of the high frequency power supply.

7. A crystal growth method according to claim 1, wherein the electrical resistivity ρ of the melt of the oxide material, the oscillation frequency f of the high frequency power supply, the diameter D of the melt of the raw material, and the magnetic permeability μ of the melt of the oxide material satisfy the following formula (1):

8. A crystal growth apparatus for forming a melt of a raw material by directly inductively heating the raw material using a high-frequency induction heating coil while cooling the outer periphery of the raw material to below the melting point of the raw material, and growing a crystal in the melt of the raw material using a seed crystal, wherein the high-frequency induction heating coil and the seed crystal are gradually spaced apart while maintaining a relative distance between the high-frequency induction heating coil and the outer periphery of the raw material.

9. A crystal growth apparatus according to claim 8, wherein the output of the high frequency power supply applied to the high frequency induction heating coil is controlled by controlling at least one of the oscillation frequency and input voltage of the high frequency power supply, thereby controlling the temperature of the molten raw material.

10. A crystal growth apparatus according to claim 9, further comprising a mechanism for measuring the weight of the crystal, and controlling the output of the high frequency power supply in accordance with the change in weight per unit time.

11. A crystal growth method according to claim 1, wherein the location heated by the high-frequency induction heating coil is moved from one end of the raw material in contact with the seed crystal to the other end, thereby growing the crystal within the molten raw material.

12. A crystal growth apparatus according to claim 8, wherein the location heated by the high frequency induction heating coil is moved from one end of the raw material in contact with the seed crystal to the other end, thereby growing the crystal within the molten raw material.

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