JP6702249B2 - Oxide single crystal manufacturing method and oxide single crystal pulling apparatus - Google Patents

Description

  The present invention relates to a method for producing an oxide single crystal by a pulling method and a single crystal pulling apparatus.

The pulling method is a method of growing a single crystal by heating and melting a raw material in a crucible to form a melt, bringing a seed crystal into contact with the melt, and pulling while rotating.
This method has an advantage that a single crystal does not come into contact with the crucible and mechanical strain due to the crucible does not occur, so that a crystal of high quality and large diameter can be easily produced.

Therefore, semiconductor silicon (Si), compound semiconductor gallium arsenide (GaAs), gallium phosphide (GaP), indium phosphide (InP), and oxide crystal yttrium aluminum garnet (Y ₃ Al ₅ O ₁₂ ; YAG), sapphire (Al ₂ O ₃ ), lithium niobate (LiNbO ₃ ; LN), lithium tantalate (LiTaO ₃ ; LT), bismuth germanate (Bi ₄ Ge ₃ O ₁₂ ; BGO), etc. It is manufactured by this method.

  Among them, lithium niobate (LN) and lithium tantalate (LT) are important oxide single crystal materials for electronic devices and optical devices, and in particular, surface acoustic wave (SAW) devices. Widely used in.

The production of oxide single crystals by the pulling method is usually performed using a single crystal pulling apparatus. FIG. 6 shows a configuration example of a conventional oxide single crystal pulling apparatus.
The single crystal pulling apparatus 90 is equipped with a crucible 1 for holding a melt 12 in which a raw material oxide is melted and a melt for melting the raw material oxide, a work coil 2a constituting a heating device for heating the raw material oxide, and a seed crystal 11 attached. It is composed of a lifting mechanism including a holding rod 3 to be held. The single crystal pulling apparatus is designed according to the type of single crystal to be grown.

As the heating device 2 of the oxide single crystal pulling device, a resistance heater is used when the crystal growth temperature is lower than about 1200° C., but high frequency induction heating is usually used at 1200° C. or higher. In this case, the crucible 1 made of a noble metal such as platinum (Pt) or iridium (Ir) is placed inside the work coil, and the crucible 1 itself serves as a heating element. This high frequency induction heating is also usually used in the crystal growth of lithium niobate (LN) and lithium tantalate (LT).
Further, in an oxide single crystal pulling apparatus, generally, a change in crystal weight is measured, and a heater output or a high frequency output is fed back to control the diameter of the single crystal.

  As a prior art related to the present invention, JP 2012-250874 A (Patent Document 1) can be mentioned.

JP2012-250874A

  Voids are one of the crystal defects often observed when growing oxide single crystals such as lithium niobate (LN) and lithium tantalate (LT) by the pulling method. A void is a void that is generated when bubbles generated in the melt are incorporated into the growing crystal. If a void is generated, that portion cannot be commercialized, and the manufacturing yield is reduced. Therefore, suppressing the generation of voids has become an important issue in the production of oxide single crystals.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing an oxide single crystal and an oxide single crystal pulling apparatus capable of suppressing the occurrence of voids.

The present inventor presumed that the cause of the void was caused by the solidification of the melt near the bottom of the crucible. That is, in the pulling of an oxide single crystal, the diameter control of the crystal is performed by adjusting the output of the heating device, that is, by controlling the heater output or high frequency output (while keeping the pulling rotation speed and pulling rate constant), the crystal is fixed. Since it grows at the liquid interface, such output control is performed on the solid-liquid interface. When forming the shoulder portion of the single crystal, output control is performed by gradually lowering the temperature of the melt at the solid-liquid interface, but at that time, the temperature near the bottom of the crucible, which is far from the solid-liquid interface, also decreases. Part of the melt may solidify. At this time, gas dissolved in the melt is taken into the solidified raw material, and bubbles are generated when the solidified raw material is melted again, and the bubbles are taken into the growing single crystal and a void is generated. Conceivable. Further, when the solidified raw material is melted again, the solidified raw material removes latent heat from the melt, so that the temperature at the bottom of the crucible is drastically lowered, which further causes voids.
Therefore, the present inventor has found that it is sufficient to prevent the melt from solidifying at the bottom of the crucible in order to suppress the generation of voids, and based on such findings, earnestly studied, leading to the present invention. It was

That is, the present invention provides the following oxide single crystal production method and oxide single crystal pulling apparatus.
[1] A method for producing an oxide single crystal for growing a lithium tantalate single crystal or a lithium niobate single crystal from a melt of a raw material oxide in a crucible by a pulling method, which comprises heating a melt in a crucible. The diameter of the single crystal to be grown is controlled by adjusting the output of the apparatus, and the temperature lowering rate of the crucible bottom during the growth of this single crystal is set to 6° C./hr or less, and/or during the growth of this single crystal. A method for producing an oxide single crystal, wherein the number of single crystals in which voids are generated in the obtained single crystal is 11% or less by setting the temperature decrease of the continuous crucible bottom to less than 12°C.
[2] The method for producing an oxide single crystal according to [1], wherein during the growth of the single crystal, the shoulder portion of the single crystal is formed.
[3] The method for producing an oxide single crystal according to [1] or [2], wherein the heating device is a high frequency induction heating device using a work coil.
[4] The method for producing an oxide single crystal according to [3], wherein the crucible bottom is arranged so that the bottom of the crucible is within ±35% of the total height of the work coil with reference to the height center of the work coil.
[5] A heat insulating structure is provided around the crucible, and the heat conductivity of the heat insulating structure on the bottom side of the crucible is smaller than the heat conductivity of the heat insulating structure on the outer peripheral surface side of the crucible. [1] to [4] The method for producing an oxide single crystal according to 1.
[6] The melt convection in the crucible is a natural convection in which the temperature of the bottom of the crucible at least during the formation of the shoulder portion of the single crystal fluctuates 5 times or more in a width of 0.5° C. or more and 1° C. or less per hour. The method for producing an oxide single crystal according to any one of [1] to [5].
[7] The method for producing an oxide single crystal according to any one of [1] to [6], wherein the crystal rotation speed during the growth of the single crystal is 10 rpm or less.
[ 8 ] A crucible to which a raw material oxide is supplied, a heat retaining structure provided around the crucible, and a high frequency induction heating device for inductively heating a crucible in which a work coil is arranged so as to surround an outer peripheral surface of the heat retaining structure. , A pulling mechanism for an oxide single crystal that holds a seed crystal and grows using the seed crystal, and adjusts the output of the heating device to melt the raw material oxide in the crucible to form a melt and grow the melt. an oxide oxide single crystal pulling apparatus of the output control unit and a single crystal or a lithium niobate single crystal manufacturing lithium tantalate Ru provided with controlling the diameter of the single crystal, the crucible, crucible bottom workcoils The height of the work coil is set to be within ±35% of the total height of the work coil, and the diameter of the single crystal to be grown is controlled by the pulling mechanism and the output control unit. The temperature drop rate of the crucible bottom is set to 6° C./hr or less, and/or the temperature decrease of the continuous crucible bottom during the growth of the single crystal is set to less than 12° C. A device for pulling an oxide single crystal, which is controlled to suppress generation .
[9] control to suppress generation of voids in the grown single crystal is, the thermal insulation structure, and shall such from which during and heat insulating material arranged on the crucible bottom of the outer peripheral surface and the work coil of the crucible, it is that smaller Kusuru than the thermal conductivity of the heat insulating material the thermal conductivity of the heat insulating material disposed on the crucible bottom is placed on a crucible outer peripheral surface (8) oxide single crystal pulling apparatus according.
[10] The control for suppressing the generation of voids in the single crystal to be grown is such that the region from the crucible bottom to the liquid surface involved in the single crystal growth of the melt in the crucible is based on the central portion in the height direction of the work coil. The oxide single crystal pulling apparatus according to [8] or [9], which is arranged so as to be within ±35% of the total height of the work coil.

  According to the present invention, it becomes possible to suppress the generation of voids during crystal growth of an oxide single crystal. As a result, it is expected that the production yield of oxide single crystals will be improved.

It is a cross-sectional schematic diagram which showed the structural example of the single crystal pulling apparatus which concerns on this invention. 4 is a temperature chart of the crucible bottom during crystal growth of Example 1. 4 is a temperature chart of the bottom of the crucible during crystal growth of Comparative Example 1. FIG. 9 is a diagram showing a relationship between a crucible bottom lowering temperature ΔT and a void generation rate in Example 4; It is a figure which shows the relationship between the crucible bottom temperature fall rate v of Example 4, and a void generation rate. It is a cross-sectional schematic diagram which showed an example of the conventional single crystal pulling apparatus.

Below, the manufacturing method of the oxide single crystal and the oxide single crystal pulling apparatus which concern on this invention are demonstrated.
[Method for producing oxide single crystal]
The method for producing an oxide single crystal according to the present invention is a method for producing an oxide single crystal in which a single crystal of an oxide is grown from a melt of a raw material oxide in a crucible by a pulling method, and the melt in a crucible is used. The output of the heating device for heating the single crystal is controlled to control the diameter of the single crystal to be grown, and the temperature lowering rate of the crucible bottom during the growth of the single crystal is set to 6° C./hr or less, and/or the single crystal. The temperature decrease of the continuous crucible bottom during the growing of is less than 12°C.

  Here, the pulling method is a method using an oxide single crystal pulling apparatus, and examples thereof include the Czochralski method (CZ method), the double crucible CZ method, and the continuous charge CZ method.

The production method of the present invention is performed by using oxide crystal yttrium aluminum garnet (Y ₃ Al ₅ O ₁₂ ; YAG), sapphire (Al ₂ O ₃ ), lithium niobate (LiNbO ₃ ; LN), lithium tantalate (LiTaO). ₃ ; LT), bismuth germanate (Bi ₄ Ge ₃ O ₁₂ ; BGO), etc., but can be used for the production of various single crystals, such as lithium tantalate (LT) single crystal and lithium niobate (LN) single crystal. Is preferably used in the production of Generally, crucibles such as iridium (Ir) and platinum-rhodium (Pt-Rh) are used in the production of LT crystals, and platinum (Pt) crucibles are used in the production of LN crystals. Further, in both cases, a high frequency induction heating device using a work coil is usually used as the heating device.

  The method for producing an oxide single crystal of the present invention is characterized in that the temperature lowering rate v of the crucible bottom during the growth of the single crystal is set to 6° C./hr or less. Further, it is characterized in that the temperature decrease ΔT of the continuous crucible bottom during single crystal growth is set to less than 12°C. Alternatively, the temperature decreasing rate v of the crucible bottom during single crystal growth is set to 6° C./hr or less, and the continuous temperature decrease ΔT of the crucible bottom is set to less than 12° C. By doing so, it is possible to prevent the melt from solidifying at the bottom of the crucible and to suppress the generation of voids in the crystal.

  The term "during single crystal growth" here refers to the period from the start of pulling the oxide single crystal by the pulling method to the end of the formation of the straight body portion. The temperature decrease rate v of the crucible bottom is obtained from the temperature decrease of the crucible bottom every one hour during crystal growth. Further, the temperature decrease ΔT of the crucible bottom is a continuous temperature decrease during the growth of the single crystal, and the temperature decrease of the crucible bottom between the minimum value and the continuous maximum value during the continuous temperature decrease period during the single crystal growth. Calculate from the difference. The continuous temperature decrease is a temperature change (temperature decrease) corresponding to the diameter control of the single crystal during the growth of the single crystal, and does not include a minute temperature change due to natural convection of the melt.

The temperature drop near the bottom of the crucible is particularly likely to occur when the diameter of the single crystal is increased to a predetermined size (when forming the shoulder portion). This is because the heater output or high frequency output of the heating device tends to decrease when the diameter is increased. Conventionally, at this time, the temperature of the bottom of the crucible is greatly lowered and the melt is solidified, so that it is considered that voids are generated.
Therefore, in this case, among the temperature fluctuations of the crucible bottom, the minimum value considered to be the time when the formation of the shoulder portion of the single crystal in the formation period of the shoulder portion of the single crystal and the time point when the formation of the shoulder portion was started The difference from the maximum (maximum) value is the temperature decrease ΔT of the crucible bottom.

  The temperature decrease rate v and the temperature decrease ΔT of the crucible bottom are preferably as small as possible, but in reality, the temperature decrease rate of 0.5° C./hr or more and the temperature decrease of 1° C. or more occur due to the diameter control of the crystal. .. Further, the temperature lowering rate v of the crucible bottom is more preferably 5° C./hr or less, and the temperature decrease ΔT of the crucible bottom is more preferably 9° C. or less.

  In the production method of the present invention, a method of decreasing the temperature lowering rate v of the crucible bottom during the single crystal growth to 6° C./hr or less, and a temperature decrease ΔT of the continuous crucible bottom during the single crystal growth of less than 12° C. The method is not limited.

  The heating device used in the present invention is preferably a high frequency induction heating device using a work coil. In high frequency induction heating, a metal crucible such as platinum (Pt) or iridium (Ir) itself serves as a heating element, so that the thermal response is fast and the diameter control of the oxide single crystal is easy.

  Usually, in the production of an oxide single crystal using a high frequency induction heating device, the solid-liquid interface of the grown crystal is located near the center of the work coil in the height direction. In high-frequency induction heating, the upper and lower ends of the work coil in the height direction tend to be less heated than the center of the work coil in the height direction, so the temperature of the crucible bottom away from the solid-liquid interface of the melt tends to decrease. End up. Therefore, as described above, in order to set the temperature decrease rate v of the crucible bottom per hour and the temperature decrease ΔT of the continuous crucible bottom per hour during the growth of the single crystal to the predetermined values, the crucible 1 is set as shown in FIG. It is preferable to arrange the crucible bottom 1a so as to be within ±35% (within ±0.35H) of the total height H of the work coil 2a with reference to the center portion CP in the height direction of the work coil 2a. It is more preferable to arrange it so as to be within ±25% (within ±0.25H) of the total height H of 2a, and within ±12.5% (within ±0.125H) of the total height H of the work coil 2a. It is more preferable to arrange so that

  Further, regarding the heat insulating structure provided around the crucible, it is also preferable to make the heat conductivity of the heat insulating structure on the bottom side of the crucible smaller than the heat conductivity of the heat insulating structure on the outer peripheral surface side of the crucible. For example, it can be realized by providing a material having a lower thermal conductivity than the heat insulating material used as the heat insulating structure on the outer peripheral surface side of the crucible inside the crucible table on which the crucible is placed.

  It is also effective to make natural convection dominant with respect to melt convection. In general, it is said that forced convection due to crystal rotation is more dominant in suppressing voids, but if the crystal rotation speed is increased to strengthen forced convection, it becomes difficult to increase the diameter. As a result, the output of the heating device further decreases, and as a result, the temperature of the crucible bottom also decreases.

  On the other hand, when natural convection is increased, the temperature at the bottom of the crucible is less likely to decrease. In this case, the temperature of the bottom of the crucible fluctuates finely, and it is preferable that the temperature of the bottom of the crucible fluctuates up and down 5 times or more in a width of 0.5° C. or more and 1° C. or less per hour at least when forming the shoulder portion. .. For example, the natural convection can be strengthened by adjusting the position of the after-heater or the like on the upper part of the crucible to lower the temperature near the upper end of the crucible so that heat can be easily released from above the melt. Alternatively, it is possible to make natural convection dominant by reducing the crystal rotation speed, preferably 10 rpm or less.

  According to the manufacturing method of the present invention, solidification of the melt in the vicinity of the crucible bottom during crystal growth of an oxide single crystal is suppressed, and generation of voids in the oxide single crystal can be suppressed.

[Oxide single crystal pulling device]
Next, the oxide single crystal pulling apparatus according to the present invention will be described.
FIG. 1 shows a structural example of an oxide single crystal pulling apparatus according to the present invention.
The oxide single crystal pulling apparatus 100 is one of the apparatuses for realizing the method for producing an oxide single crystal according to the present invention, and holds the melt 12 in which the raw material oxide is supplied and the raw material oxide is melted. High frequency induction heating for induction heating of the crucible 1, the heat insulating structure (crucible base 5, heat insulating material 6) provided around the crucible 1, and the work coil 2a surrounding the heat insulating material 6. Apparatus, a holding mechanism for holding a seed crystal 11, a pulling mechanism for an oxide single crystal 13 grown using the seed crystal 11, and a raw material oxidation inside the crucible 1 by adjusting the output of the heating apparatus. An object is melted to form a melt 12, and an output control unit 2b for controlling the diameter of the grown oxide single crystal 13 is provided.

  Here, the crucible 1 that holds the melt 12 in which the oxide raw material is melted is made of a noble metal such as platinum (Pt) or iridium (Ir), and is placed on the crucible base 5 that also serves as a heat insulating material that constitutes the heat insulating structure. The crucible bottom 1b is installed so as to contact the crucible base 5. A heat insulating material 6 that constitutes a heat insulating structure is provided on the outer peripheral surface 1a of the crucible. As the crucible table 5 and the heat insulating material 6, a heat resistant material such as zirconia or alumina is used. The crucible base 5 may be filled with a heat insulating material such as a heat resistant material such as zirconia or alumina. A thermocouple 4 is provided at the center of the crucible bottom 1b. The temperature change of the crucible bottom 1b can be measured by this thermocouple 4.

  Furthermore, a reflector 8, an after-heater 7 as a cylindrical heat-reflecting plate, and an after-heater lid 9 as a lid for the after-heater 7 are provided on the upper part of the crucible 1 which is a donut-shaped disc-shaped heat-reflecting plate.

  The crucible 1, the melt 12, the crucible stand 5, the heat insulating material 6, the reflector 8, the after heater 7, and the after heater lid 9 are installed in a cylindrical refractory container 10 made of quartz or alumina.

  Further, a heating device having a work coil 2a and an output control unit 2b provided around the refractory container 10 is provided. The crucible 1 can be induction-heated by the high-frequency induction heating of this heating device, and the raw material oxide in the crucible 1 can be melted to prepare the melt 12.

  The seed crystal 11 attached to the holding rod 3 is inserted into the crucible 1 through an opening provided in the center of the reflector 8 and the after-heater lid 9, and after being immersed in the melt 12, at a predetermined rotation speed and pulling speed. The seed crystal 11 is pulled up.

  The output of the heating device is adjusted by the output control unit 2b, and changes at any time during crystal growth. By varying the output of the heating device, the diameter of the oxide single crystal 13 is controlled to form the shoulder portion 13a and the straight body portion 13b. At this time, the pulling mechanism measures the change in the weight of the oxide single crystal 13 and feeds it back to the output control unit 2b. The output control unit 2b receives the result and adjusts the high frequency output to control the diameter of the oxide single crystal 13. Is done.

  Here, in the oxide single crystal pulling apparatus 100 of the present invention, the crucible 1 has a bottom 1b within ±35% of the total height H of the work coil 2a with reference to the height direction central portion CP of the work coil 2a. (That is, within ±0.35H). At this time, in the crucible 1, the region from the crucible bottom 1b to the solid-liquid interface (that is, the liquid surface) involved in the single crystal growth of the melt 12 is based on the height direction central portion CP of the work coil 2a. It is preferable to arrange it within ±35% of the total height.

  In growing an oxide single crystal by the pulling method, a high-frequency induction heating device is used because the crystal growing temperature is as high as 1200°C or higher. In this high-frequency induction heating device, the upper and lower end portions in the height direction of the work coil 2a tend to be more difficult to be heated than the center portion CP in the height direction, and the temperature at the position away from the solid-liquid interface (crucible bottom 1b) decreases. The melt 12 is likely to solidify near the crucible bottom 1b. As a result, voids are generated in the oxide single crystal.

  In the present invention, by devising the arrangement of the crucible 1 in the height direction with respect to the work coil 2a as described above, the temperature on the crucible bottom 1b side away from the solid-liquid interface of the melt 12 is prevented from decreasing and the crucible It is possible to suppress solidification of the melt 12 in the vicinity of the bottom 1b, and eventually to suppress generation of voids in the oxide single crystal.

  In addition, the heat insulating structure is composed of a crucible base (heat insulating material) 5 and a heat insulating material 6 arranged between the crucible outer peripheral surface 1a and the work coil 2a and on the crucible bottom 1b side, and is arranged on the crucible bottom 1b side. It is preferable that the thermal conductivity of the crucible base (heat insulating material) 5 is smaller than the thermal conductivity of the heat insulating material 6 arranged on the crucible outer peripheral surface 1a side. As a result, the solidification of the melt 12 in the vicinity of the crucible bottom 1b can be further suppressed, and consequently the generation of voids in the oxide single crystal can be suppressed.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[Example 1]
A lithium tantalate single crystal was produced using the oxide single crystal pulling apparatus 100 as shown in FIG.
(Oxide single crystal pulling device)
Here, the size of the work coil 2a is 280 mm in diameter (inner diameter) and H280 mm in height. The crucible 1 is an iridium crucible having an outer diameter of 150 mm, a height of 150 mm, a peripheral wall thickness of 2 mm, and a crucible bottom wall thickness of 2 mm. Further, as the reflector 8, an iridium donut plate having an outer diameter of 160 mm, an inner diameter of 120 mm and a thickness of 2 mm was used. The after-heater 7 installed above the reflector 8 has a cylindrical shape made of iridium and has an outer diameter of 150 mm and a height of 180 mm.
On the outer peripheral surface side of the crucible 1, hollow bubbles (hollow particles) made of zirconia were filled as a heat insulating material 6 between the outer peripheral surface of the crucible 1 and the refractory container 10. Moreover, hollow bubbles (hollow particles) made of zirconia were packed in the crucible table 5.

  The arrangement of the crucible 1 was set so that the position of the crucible bottom 1b was 70 mm below the center CP of the height H of the work coil 2a. This is because the crucible bottom 1b is located below the central portion CP in the height direction of the work coil 2a by a height corresponding to 25.0% of the total height H of the work coil 2a, that is, the center in the height direction of the work coil 2a. It is a position within ±25% of the total height H of the work coil 2a with reference to the portion CP.

(Preparation of oxide single crystal)
Lithium carbonate and tantalum pentoxide were weighed in a crucible 1 of the single crystal pulling apparatus 100 so that a congruent melting composition was obtained, and raw materials mixed, molded and calcined were put and melted.
Subsequently, the seed crystal 11 attached to the holding rod 3 was inserted into the melt 12 and pulled up while rotating at a rotation speed of 7 rpm. At that time, the high frequency output of the work coil 2a fluctuated from time to time, so that the shoulder portion 13a and the straight body portion 13b of the oxide single crystal 13 were formed.
The produced lithium tantalate single crystal had a diameter of 4 inches, and the straight barrel portion 13b had a length of 100 mm. When the produced lithium tantalate single crystal was visually observed, no void was observed.

  FIG. 2 shows a temperature chart of the bottom of the crucible during the growth of the single crystal of Example 1. In FIG. 2, the origin is temporarily set to 0 hour (hr), and time elapses from the right side to the left side in the figure. It is considered that the temperature of the crucible bottom decreases with the decrease of the high frequency output, and the point where the temperature becomes the minimum corresponds to the time when the formation of the shoulder portion of the single crystal is finished. In addition, the temperature chart shows that the temperature at the bottom of the crucible fluctuates finely and that natural convection is dominant.

  In FIG. 2, the temperature decrease rate v and the temperature decrease ΔT of the crucible bottom during the growth of the single crystal become maximum when the shoulder is formed, and the temperature decrease rate v at this time is 3.0° C./hr. The temperature decrease ΔT was 5.5°C. The temperature decrease rate v is the maximum slope per hour, and the temperature decrease ΔT is the minimum value considered to be the time when the shoulder formation is completed and the maximum (maximum) from the time when the shoulder formation is started. The difference from the value shall be taken (hereinafter the same). That is, in FIG. 2, the time point of the black circle which takes the maximum value of the crucible bottom temperature related to the temperature decrease ΔT shows the time point when the formation of the shoulder portion of the single crystal is started, and the time point of the black circle which takes the minimum value shows the formation of the shoulder portion. It shows the time when it is finished (the same applies to FIG. 3).

[Example 2]
The position of the crucible bottom 1b was set 20 mm below the center CP of the height H of the work coil 2a, and otherwise the lithium tantalate single crystal was produced in the same manner as in Example 1. This is because the crucible bottom 1b is located below the central portion CP in the height direction of the work coil 2a by a height corresponding to 7.14% of the total height H of the work coil 2a, that is, in the height center of the work coil 2a. It is a position within ±7.14% of the total height H of the work coil 2a with reference to the portion CP. When the produced lithium tantalate single crystal was visually observed, no void was observed.

  The temperature chart of the crucible bottom during the growth of the single crystal had the same tendency as in FIG. That is, the temperature decrease rate v and the temperature decrease ΔT of the crucible bottom during the growth of the single crystal become maximum when the shoulder portion of the single crystal is formed, and the temperature decrease rate v at this time is 2.0° C./hr. The temperature decrease ΔT was 4.0°C.

[Example 3]
A lithium blanket made of alumina was packed in the crucible base 5, and a lithium tantalate single crystal was produced in the same manner as in Example 1 except for the above. The thermal conductivity of the alumina blanket is lower than that of the zirconia hollow bubble. When the produced lithium tantalate single crystal was visually observed, no void was observed.

  The temperature chart of the crucible bottom during the growth of the single crystal had the same tendency as in FIG. That is, the temperature decrease rate v and the temperature decrease ΔT of the crucible bottom during the growth of the single crystal become maximum when the shoulder of the single crystal is formed, and the temperature decrease rate v at this time is 2.0° C./hr. The temperature decrease ΔT was 4.0°C.

[Comparative Example 1]
The position of the crucible bottom 1b was set 120 mm below the center portion CP of the height H of the work coil 2a, and otherwise the lithium tantalate single crystal was produced by the same method as in Example 1. This is because the crucible bottom 1b is located below the central portion CP in the height direction of the work coil 2a by a height corresponding to 42.9% of the total height H of the work coil 2a, that is, in the height direction center of the work coil 2a. The position is within ±42.9% of the total height H of the work coil 2a with reference to the portion CP. When the produced lithium tantalate single crystal was visually observed, voids were observed.

  FIG. 3 shows a temperature chart of the crucible bottom during the growth of the single crystal. As in FIG. 2, it is considered that the temperature of the bottom of the crucible decreases in accordance with the decrease in the high frequency output, and the minimum point corresponds to the time when the formation of the shoulder portion of the single crystal is finished. On the other hand, in the temperature chart of FIG. 3, unlike the case of FIG. 2, no minute fluctuation in the temperature of the crucible bottom is seen. Therefore, it can be seen that natural convection is not dominant.

  In FIG. 3, the temperature decrease rate v and the temperature decrease ΔT of the crucible bottom during the growth of the single crystal become maximum when the shoulder of the single crystal is formed, and the temperature decrease rate v at this time is 8.0° C./ The hr and the temperature decrease ΔT were 18.0°C.

[Comparative example 2]
A lithium tantalate single crystal was produced in the same manner as in Example 1 except that the inside of the crucible base 5 was made hollow. The thermal conductivity of air is higher than that of zirconia hollow bubbles, and it does not serve as a heat insulating material. When the produced lithium tantalate single crystal was visually observed, voids were observed.

  The temperature chart of the crucible bottom during the growth of the single crystal had the same tendency as in FIG. That is, the temperature decrease rate v and the temperature decrease ΔT of the crucible bottom during the growth of the single crystal become maximum when the shoulder portion of the single crystal is formed, and the temperature decrease rate v at this time is 7.0° C./hr. The temperature decrease ΔT was 12.0°C.

[Example 4]
In the oxide single crystal pulling apparatus 100 of Example 1, the height position of the crucible 1 with respect to the work coil 2a, the heat retaining structure, etc. were adjusted so that the magnitude of the temperature drop ΔT at the bottom of the crucible during single crystal growth was different. A lithium tantalate single crystal was prepared. At each temperature decrease ΔT, 100 lithium tantalate single crystals were prepared and the void generation rate was evaluated. The void generation rate here is obtained by visually determining the presence or absence of voids in the single crystal, and calculating the ratio (number ratio) of the samples in which voids have occurred in the sample having the temperature decrease ΔT.
FIG. 4 shows the evaluation result. If the temperature drop ΔT of the crucible bottom is less than 12°C, the void occurrence rate is relatively low, 11% or less, but if the temperature drop ΔT of the crucible bottom is 12°C or more, the void occurrence rate exceeds 20% and rapidly increases. I understand.

The samples were rearranged from the viewpoint of the temperature decrease rate v of the crucible bottom during the growth of the single crystal, and the relationship between the temperature decrease rate v of the crucible bottom and the void generation rate was evaluated.
The result is shown in FIG. When the temperature decrease rate v of the crucible bottom is 6°C/hr or less, the void generation rate is relatively low at 11% or less, but when the temperature decrease rate v of the crucible bottom exceeds 6°C/hr, the void generation rate increases to more than 20%. I understand.

  It should be noted that although the present invention has been described with reference to the above-described embodiments, the present invention is not limited to these embodiments, and those skilled in the art can think of other embodiments, additions, changes, deletions, and the like. The present invention can be modified within the scope of the present invention, and is included in the scope of the present invention as long as the effects of the present invention can be obtained in any aspect.

1 crucible 1a crucible outer peripheral surface 1b crucible bottom 2a work coil 2b output control unit 3 holding rod 4 thermocouple 5 crucible stand 6 heat insulating material 7 afterheater 8 reflector 9 afterheater lid 10 refractory container 11 seed crystal 12 melt 13 oxide Single crystal 13a Shoulder 13b Straight body 90, 100 Oxide single crystal pulling device CP Work coil central part H Work coil total height

Claims (10)

A method for producing an oxide single crystal for growing a lithium tantalate single crystal or a lithium niobate single crystal from a melt of a raw material oxide in a crucible by a pulling method, the output of a heating device for heating the melt in the crucible. To control the diameter of the single crystal to be grown, and to set the temperature lowering rate of the crucible bottom during the growth of this single crystal to 6° C./hr or less, and/or to the continuous crucible during the growth of this single crystal. A method for producing an oxide single crystal, wherein the number of void-generated single crystals in the obtained single crystal is 11% or less by setting the temperature decrease of the bottom to less than 12°C. The method for producing an oxide single crystal according to claim 1, wherein the growing of the single crystal is during the formation of the shoulder portion of the single crystal.   The method for producing an oxide single crystal according to claim 1, wherein the heating device is a high frequency induction heating device using a work coil.   The method for producing an oxide single crystal according to claim 3, wherein the crucible bottom is arranged such that the bottom of the crucible is within ±35% of the total height of the work coil with reference to the center in the height direction of the work coil.   A heat insulating structure is provided around the crucible, and the heat conductivity of the heat insulating structure on the bottom side of the crucible is smaller than the heat conductivity of the heat insulating structure on the outer peripheral surface side of the crucible. Method for producing a single crystal of a substance.   The melt convection in the crucible is a natural convection in which the temperature of the bottom of the crucible at least at the time of forming the shoulder portion of the single crystal fluctuates 5 times or more in a width of 0.5° C. or more and 1° C. or less per hour. 6. A method for producing an oxide single crystal according to any one of items 1 to 5. The method for producing an oxide single crystal according to claim 1, wherein a crystal rotation speed during the growth of the single crystal is 10 rpm or less. A crucible to which a raw material oxide is supplied, a heat retaining structure provided around the crucible, a high-frequency induction heating device for inductively heating a crucible by arranging a work coil so as to surround an outer peripheral surface of the heat retaining structure, and a seed crystal. And a pulling mechanism of an oxide single crystal grown by using the seed crystal and melting the raw material oxide in the crucible by adjusting the output of the heating device to form a melt, and growing the oxide single crystal. an oxide single crystal pulling apparatus of the output control unit and a single crystal or a lithium niobate single crystal manufacturing lithium tantalate Ru provided with controlling the diameter of the crystal, the height direction of the crucible, the crucible bottom work coil The work coil is arranged so as to be within ±35% of the total height of the work coil, the diameter of the single crystal to be grown is controlled by the pulling mechanism and the output control unit, and the bottom of the crucible is grown. The temperature decrease rate of 6° C./hr or less and/or the temperature decrease of the continuous crucible bottom during the growth of this single crystal is less than 12° C. to suppress the generation of voids in the grown single crystal. A device for pulling an oxide single crystal, which is characterized in that it is controlled . Control to suppress generation of voids in the single crystal to the fostering, the thermal insulation structure, and shall such a heat insulating material and between which is disposed the crucible bottom side of the outer peripheral surface and the work coil of the crucible, the crucible bottom thermal conductivity of heat insulating material disposed on the oxide single crystal pulling apparatus according to claim 8, wherein the small Kusuru than the thermal conductivity of the heat insulating material disposed on the crucible outer peripheral surface. Control to suppress the generation of voids in the single crystal to be grown, the crucible, the region from the crucible bottom to the liquid surface involved in the single crystal growth of the melt of the work coil with reference to the center in the height direction of the work coil. The oxide single crystal pulling apparatus according to claim 8 or 9, which is arranged so as to be within ±35% of the total height.

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