JP4735334B2 - Method for producing aluminum oxide single crystal - Google Patents

Description

  The present invention relates to a method for producing an aluminum oxide single crystal, and more particularly to a method for efficiently producing a high quality aluminum oxide single crystal by suppressing generation of pits and microbubbles.

  Aluminum oxide single crystals are widely used as an epi growth substrate for producing blue LEDs and white LEDs. These LEDs are expected to spread in the lighting field from the viewpoint of energy saving, and are attracting attention from various fields.

  There are various methods for growing oxide single crystals, but most of the oxide single crystal materials such as LN, LT, YAG and aluminum oxide are obtained by melt solidification because their crystal characteristics and large crystal diameters are obtained. It is nurtured. In particular, the Czochralski method (Cz method), which is one of the melt solidification methods, is most widely used because of its versatility and high technical perfection.

In order to produce an oxide single crystal by the Czochralski method, first, an oxide raw material is filled in a crucible, and the raw material is melted by heating the crucible by a high frequency induction heating method or a resistance heating method. After the raw material is melted, the seed crystal cut in a predetermined crystal orientation is brought into contact with the surface of the raw material melt, and the single crystal is grown by pulling upward at a predetermined speed while rotating the seed crystal at a predetermined rotation speed.
However, when an aluminum oxide single crystal is grown by the Czochralski method, innumerable minute bubbles are likely to be generated in the crystal. The microbubbles include a relatively large bubble that causes pits (small pits with a diameter of several μm) and a light scattering laser tomography method (see Non-Patent Document 1). ), There is a so-called microbubble that can be confirmed like a cloud when irradiated with laser light. Among these, although the influence of microbubbles has not been determined yet, it is said that the LED characteristics are adversely affected together with the pits.

Until now, when growing aluminum oxide single crystals, oxygen atoms (O) and oxygen molecules (O ₂ ) generated by decomposition of the raw material at high temperature exist in the supersaturated state in the melt. It is known that it is taken in and becomes a bubble in the single crystal. In order to avoid this, it has been proposed to grow a single crystal in a reducing atmosphere using hydrogen gas or carbon monoxide gas (see Patent Document 1).
As a result, oxygen atoms (O) and oxygen molecules (O ₂ ) present in the melt are removed by reaction with hydrogen gas or carbon monoxide gas, so the amount of bubbles taken into the grown single crystal is certain. To decrease. However, when a wafer is cut out from the grown single crystal and polished and polished, a large number of pits exist on the surface of the wafer, and the amount of bubbles taken in cannot be sufficiently suppressed.

In addition, gas components that are in equilibrium with the melt tend to be discharged from the melt at the solid-liquid interface where crystallization occurs, and in the melt near the interface, gas components become supersaturated and bubbles are generated. Cheap. However, it is said that by enhancing the convection of the melt, it is possible to suppress the supersaturation of the gas component that occurs in the vicinity of the interface and to reduce the amount of gas component taken into the crystal (see Non-Patent Document 2).
Therefore, first, the gas component contained in the raw material is removed as much as possible before melting, the supersaturated gas component present in the melt is reduced, and after melting, the convection is strengthened and the effect of stirring is increased. It is thought that the generation of pits and microbubbles can be suppressed by reducing the amount of minute bubbles taken into the crystal during growth.

By the way, although it is a manufacturing method of the aluminum oxide single crystal containing titanium, when the single crystal is grown in a low oxygen concentration atmosphere, it is said that the convection of the melt can be strengthened and the stirring effect can be increased (patent document) 2). Here, when an aluminum oxide single crystal containing titanium is grown in a low oxygen concentration atmosphere where the oxygen partial pressure is 10 ⁻² to 10 ⁻⁷ atm, the melt is reduced on the surface of the melt, and accordingly the surface tension is increased. There is an explanation that the flow in the same direction as the natural convection of the melt is remarkably promoted as a result of the occurrence of the surface tension flow and the surface tension flow. It can be considered that the effect of stirring is increased by the convection of the melt being promoted.

  However, when an aluminum oxide single crystal is grown in a low oxygen concentration atmosphere, the growth interface tends to be significantly convex on the melt side. When crystal growth is performed in such a situation, the tip of the growth interface and the bottom of the crucible come into contact with each other when the melt height in the crucible is reduced to some extent by crystal growth. For this reason, it is impossible to continue the crystal growth beyond that, and there arises a problem that the crystal obtained cannot be made so large with respect to the amount of the raw material filled in the crucible. In addition, as a result of remarkably promoting the flow in the same direction as the natural convection of the melt, the single crystal growth rate in the melt is increased, and crystal defects are likely to occur in the obtained crystal.

In order to solve such problems, in Patent Document 2, excessive convection of the melt is suppressed by increasing the number of rotations of the growth crystal to, for example, 20 rotations / minute or more, particularly 30 to 120 rotations / minute. Has proposed. However, with such means, although the crystal yield can be increased, the generation of fine bubbles in the single crystal cannot be sufficiently suppressed.
JP 04-132695 A JP 09-278592 A Applied Physics Vol.55 No.6 1986 P542-569 Proceedings of the 28th National Conference on Crystal Growth, 22Pa2 1997 P15

  An object of the present invention is to provide a method for efficiently producing a high-quality aluminum oxide single crystal by suppressing the generation of pits and microbubbles in view of the above-mentioned problems of the prior art.

  The inventors of the present invention have made extensive studies in order to solve the above-mentioned conventional problems, and as a result of examining in detail the generation mechanism of the gas component that causes bubbles contained in the aluminum oxide single crystal, the gas component is oxidized. It also occurs when aluminum is decomposed, but not only that, but the aluminum oxide powder, which is widely used as a raw material, originally has an adsorbed or encapsulated gas component, which remains in the melt and is taken into the crystals. And found out that it would become a pit or microbubble. Then, when a single crystal raw material (hereinafter also simply referred to as a raw material) is heated and melted in a crucible, the pressure inside the furnace is reduced, and the gas adsorbed or contained in the raw material is forcibly removed to give a gas component to the single crystal. In addition, the amount of pits and microbubbles generated can be reduced, and the growth rate of single crystals in the melt can be controlled to suppress the phenomenon that the growth interface becomes significantly convex toward the melt side. Thus, the inventors have found that single crystals can be efficiently produced by reducing crystal defects, improving the solidification rate from the raw materials, and have completed the present invention.

That is, according to the first invention of the present invention, in a method for producing an aluminum oxide single crystal by a melt solidification method in which a raw material for a single crystal is placed in a crucible in a furnace body and heated and melted, and a grown crystal is pulled up from the raw material melt When the single crystal raw material is heated and melted, it is melted while removing the gas generated from the single crystal raw material by heating while maintaining the furnace body at 0.01 MPa or less. Provided is a method for producing an aluminum oxide single crystal characterized by maintaining a reduced pressure of ˜0.1 MPa and pulling the grown crystal so that the oxygen concentration in the mixed atmosphere is 0.01 to 0.5%. Is done.

According to a second aspect of the present invention, there is provided a method for producing an aluminum oxide single crystal according to the first aspect, wherein the single crystal raw material is gradually heated and melted over 10 hours or more. Is done.
Furthermore, according to the third invention of the present invention, the aluminum oxide single crystal according to the first or second invention is characterized in that after the raw material for single crystal is heated, pressure reduction is started after the temperature reaches 1000 ° C. A manufacturing method is provided.

According to a fourth aspect of the present invention, there is provided the method for producing a single crystal according to the first aspect, wherein the raw material melt is maintained in a mixed atmosphere of oxygen and nitrogen or an inert gas. The
Furthermore, according to a fifth aspect of the present invention, there is provided the method for producing a single crystal according to the first aspect, wherein the material of the crucible is iridium.

According to the present invention, the furnace body is depressurized while the raw material is heated and melted in the crucible, and the gas adsorbed or contained in the raw material is forcibly excluded. Uptake is suppressed, and generation of minute bubbles in the melt can be suppressed. In addition, by controlling the growth rate of the single crystal in the melt, the phenomenon that the growth interface becomes significantly convex toward the melt side is suppressed, crystal defects are reduced, and the degree of convexity is further reduced. A solid crystal can be improved and a single crystal can be produced efficiently.
The single crystal thus obtained has reduced pits, microbubbles, crystal defects, and the like due to minute bubbles, and further has no inclusion (inclusion) from the crucible material. If a single crystal is used, electronic component materials and optical component materials having excellent characteristics can be provided.

  Hereinafter, the manufacturing method of the aluminum oxide single crystal of this invention is demonstrated in detail using drawing.

The method for producing an aluminum oxide single crystal of the present invention is a method for producing an aluminum oxide single crystal by a melt solidification method in which a raw material for a single crystal is put into a crucible in a furnace body and heated and melted, and a grown crystal is pulled up from the raw material melt When the single crystal raw material is heated and melted, it is melted while removing the gas generated from the single crystal raw material by heating while maintaining the furnace body at 0.01 MPa or less. The growth crystal is pulled up so that the pressure is reduced to 0.1 MPa and the oxygen concentration in the mixed atmosphere is 0.01 to 0.5% .

  That is, in order to grow an aluminum oxide single crystal in an apparatus when a single crystal raw material is put into a crucible in a furnace body and melted by heating and melted by pulling the growth crystal from the raw material melt. A means for reducing the pressure inside the furnace and a means for monitoring the degree of pressure reduction, and reducing the pressure inside the furnace to 0.01 MPa or less while heating the raw material used for growing the single crystal to the melting point as well as at room temperature I do.

  In the present invention, in order to grow an aluminum oxide single crystal, a conventional oxide single crystal growing apparatus based on the Czochralski method can be used. For example, a crucible formed of a noble metal, a furnace material formed of alumina or the like as a heat insulating material around the crucible, and an apparatus in which a high-frequency coil as a heating device is disposed outside the furnace material. The apparatus is provided with means for decompressing the furnace body, means for monitoring the degree of decompression, and means for supplying a mixed gas of oxygen and nitrogen or an inert gas into the furnace body.

  Since the melting point of alumina, which is a single crystal raw material, is slightly over 2000 ° C., it is preferable to use an iridium-made crucible. As the heat insulating material, a heat insulating material such as foamed zirconia may be filled. Above the crucible, a pulling device for pulling up the single crystal from the material melt is provided, and the furnace material is shielded by a shielding plate.

First, the single crystal raw material described above is put into a crucible, and then the crucible is heated by a high frequency coil to melt the raw material to obtain a raw material melt.
When the aluminum oxide single crystal raw material in the furnace body is heated and melted at normal pressure and grown by the Czochralski method, innumerable microbubbles are likely to be generated in the crystal. The gas that causes aluminum oxide bubbles is also generated by the decomposition of aluminum oxide, but the gas component adsorbed or contained in the raw material is not completely removed before the raw material is melted, and remains in the melt, which becomes crystals. Many are taken into bubbles. Therefore, the furnace body is depressurized while the single crystal raw material is heated and melted in the crucible, and the gas adsorbed or contained in the raw material is forcibly removed. In this step, if the decompression operation is completed before the raw material is melted, the removal of gas components becomes insufficient, and therefore, the process is performed until the raw material is substantially melted.

In the present invention, an ordinary aluminum oxide powder is used as a raw material for a single crystal. The aluminum oxide powder is aluminum oxide substantially composed of two elements of Al and O. In addition to Al and O, Ti, Cr, Si, Ca, Mg, and the like may be included in accordance with the type of target aluminum oxide single crystal. Among these, Si, Ca, Mg and the like can be inevitably contained as components of the sintering aid, but the content is desirably as small as possible. The diameter and density of aluminum oxide are not particularly limited, but for handling, for example, the diameter is preferably 10 mm or less, preferably 5 mm or less, and the density is 5 g / cm ³ or less, preferably 3 g / cm. What is ³ or less is good.

Since normal aluminum oxide powder has a large specific surface area of about 5 to 10 m ² / g, many gas components are adsorbed or encapsulated, but it is completely removed before melting of the raw material and remains in the melt. No pits or microvalves are formed because they are not taken into the crystal. The same applies to an aluminum oxide sintered body having a specific surface area of 0.1 to 10 m ² / g.

  If it is an aluminum oxide sintered compact, the commercial item for semiconductor manufacture can be used, However, It can also manufacture by the method as shown next. For example, α alumina particles are added as seeds to a sol or gel of an α alumina precursor that is converted to α alumina when baked, and after the sol is gelled, the gel of the α alumina precursor to which the seed crystal is added is 900 to Sinter at a temperature of 1350 ° C. and grind the resulting sintered product.

The crackle raw material is obtained by pulverizing an aluminum oxide single crystal raw material produced by the Bernoulli method to a diameter of 20 mm or less. However, the specific surface area is as small as less than 0.1 m ² / g and the adsorbed gas is small. However, since the aluminum oxide powder is dissolved and the single crystal produced from the obtained melt is pulverized, innumerable bubbles are often included therein. With crackle raw materials, despite the high viscosity and high surface tension of the aluminum oxide melt, it is easy to escape from the melt without melting into the melt by forming a reduced pressure during heating and melting. go. Since the crackle raw material has a high density, there is an advantage that the number of raw material inputs before the growth can be reduced as compared with other raw material forms such as aluminum oxide powder.

  Next, a preferred embodiment of the decompression step in the present invention will be shown. The depressurization step includes a raw material heating and melting stage and a melt single crystal growing stage. In the heating and melting stage of the raw material, it is desirable that the raw material is gradually heated over 10 hours or more, preferably 12 hours or more until the melting point is reached. The heating rate until the raw material reaches the melting point is not particularly limited, but it is possible to suppress the incorporation of bubbles into the single crystal by heating gradually over a long time without rapidly heating.

  Vacuuming may be performed while the raw material placed in the crucible is at room temperature, but it is preferable to start evacuation after the raw material reaches 1000 ° C. or higher after heating. This is because when the raw material is less than 1000 ° C., the generation of gas is extremely small. The evacuation must be continued until the pressure in the furnace becomes 0.01 MPa or less.

When the temperature of the raw material in the furnace becomes 1000 ° C. or higher, the gas component adsorbed on the raw material is discharged, and the degree of vacuum in the furnace gradually decreases. If the degree of vacuum decreases and exceeds 0.02 MPa, there is a concern about adverse effects on microbubbles. Therefore, evacuation must be performed again until the pressure in the furnace becomes 0.01 MPa or less, preferably 0.005 MPa or less. The pressure in the furnace may temporarily exceed 0.01 MPa, but if it exceeds 60 min, the generation of bubbles cannot be sufficiently suppressed.
In order to monitor the degree of decompression, it is desirable to measure the pressure at intervals of 10 to 60 min, preferably at intervals of 20 to 50 min. This decompression operation is repeated until the raw material is melted. Deterioration of the degree of vacuum in the furnace due to the evacuation stop is usually 0.001 to 0.002 MPa until the raw material is melted, but the degree of deterioration increases to 0.005 to 0.01 MPa when the raw material is melted. Sometimes.

  Thereafter, in the single crystal growth stage of the raw material melt, the above depressurization operation is continuously performed every 10 to 60 minutes. When the decompression operation is continued for 1 to 5 hours, the degree of vacuum is almost the same as that before melting, and the degree of vacuum is 0.005 to 0.007 MPa.

When the raw material is sufficiently melted, the seed crystal is brought into contact with the melt surface to start crystal growth. At this time, oxygen and nitrogen or an inert gas are supplied into the furnace body to form a mixed gas atmosphere thereof. A mixed gas such as oxygen and nitrogen is introduced until the pressure reaches 0.01 to 0.1 MPa. If the growth is carried out while maintaining a reduced pressure, the raw material is decomposed and high quality crystals cannot be obtained.
Here, the oxygen concentration is adjusted to be 0.01 to 0.5%, preferably 0.1 to 0.3%. If the oxygen concentration is less than 0.01%, crystal defects are likely to occur in the crystal obtained for the reasons described later, and if it exceeds 0.5%, microbubbles are likely to be generated. Further, the progress of the oxidation of the crucible material not only promotes the deterioration of the crucible, but also the metal oxide of the crucible material is scattered in the furnace and easily mixed into the melt. As a result, the crucible material is taken into the crystal and becomes inclusion. This oxygen concentration is maintained even during single crystal growth. After elapse of 3 hours or more, especially 5 hours or more from the melting of the raw material, the seed crystal is brought into contact with the obtained melt, and the seed crystal is pulled up while being rotated by a pulling device.

  Single crystal growth is carried out by adjusting the rotation speed and pulling speed to form the neck and shoulder, except for reducing the furnace atmosphere and mixing with oxygen and nitrogen or inert gas. A straight body part is formed. At this time, it is preferable to measure the temperature of the melt surface in the vicinity of the interface between the single crystal and the raw material melt using a radiation thermometer or the like. The crystal shape is adjusted by measuring the crystal weight during growth, deriving the diameter, growth rate, and the like by calculation, and adjusting the rotation speed and pulling speed. Also, the melt temperature can be controlled by feeding back the change in crystal weight to the power applied to the high frequency induction coil.

  As a result of the reduction of excess gas contained in the melt, there are no bubbles that precipitate in the crystal during single crystal growth. During the growth of the single crystal, the growth rate of the single crystal in the melt is controlled by adjusting the oxygen concentration to a range of 0.01 to 0.5%, preferably 0.1 to 0.3%. This eliminates the problem that crystal defects are reduced, the growth interface is not significantly convex on the melt side, and crystals obtained with respect to the raw material cannot be made so large.

By the way, as described in Patent Document 2, when an aluminum oxide single crystal is grown in a low oxygen concentration atmosphere, the growth interface tends to be remarkably convex on the melt side. When single crystal growth is performed in such a situation, if the height of the raw material melt in the crucible is reduced to some extent by crystal growth, the tip of the growth interface and the bottom of the crucible come into contact with each other. For this reason, it is impossible to continue the crystal growth beyond that, and there arises a problem that the crystal obtained cannot be so large with respect to the amount of the raw material filled in the crucible. In addition, as a result of remarkably promoting the flow in the same direction as the natural convection of the melt, the single crystal growth rate in the melt is increased, and crystal defects are likely to occur in the obtained crystal.
As described in Patent Document 2, for example, such a problem may be achieved by adjusting the crystal growth rate by increasing the number of rotations of the crystal during the growth, but is disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-231958 by the present applicant. If the convection of the melt is adjusted using a growth furnace having a heater at the bottom of the crucible, the problem can be solved without greatly increasing the rotational speed.

  In this way, by growing the single crystal by heating and melting the raw material under specific decompression conditions, the gas adsorbed or encapsulated on the surface can be easily excluded regardless of the form of the raw material. The excess gas contained in the melt can be reduced, the number of minute bubbles taken into the crystal during single crystal growth can be reduced, and the number of pits and microvalves in the resulting single crystal can be reduced. be able to.

  The degree of minute bubbles taken into the grown single crystal can be observed by irradiating the crystal with laser light and observing the scattered light according to the light scattering laser tomography method disclosed in Non-Patent Document 1. FIG. 1 shows an optical system for measuring light scattering. The aluminum oxide single crystal 1 processed into a cylindrical shape is irradiated with a laser beam 2 having a wavelength of 532 nm and an output of 500 mW, and scattered light 3 emitted in a direction of 90 ° with respect to the incident direction of the irradiated laser beam 2 is emitted from the CCD camera 4. The image processing apparatus 5 processes the intensity to 256 gradations from 0 to 255, and calculates the average intensity of the 40 mm square of the crystal portion in the image as the scattered light intensity. At this time, the polarization direction of the laser light is linearly polarized light that is 90 ° with respect to the direction of the CCD camera.

  Next, the grown single crystal is cut to obtain a wafer having a diameter of about 3 inches, for example, and polished to check the occurrence of pits. When any raw material having a different non-surface area is used, the average number of pits generated is tens or less.

  Although the scattered light intensity is considered to indicate the amount of microbubbles taken into the grown single crystal, according to the present invention, the amount of microbubbles and the number of pits hardly change. This means that there is no great difference in the amount of the gas component taken into the crystal depending on the raw material form being grown. Aluminum oxide powder, which is a powder material with a large specific surface area, has a large amount of gas adsorbed to the material, but by reducing the pressure inside the furnace during melting, innumerable voids are formed due to grain growth during heating due to melting. Neither is it confined to the gas component.

The aluminum oxide single crystal obtained by the above production method is a single crystal containing two elements of aluminum and oxygen, and the scattered light intensity obtained by the light scattering laser tomography method is reduced to 140 or less.
By slicing a wafer from this single crystal and polishing it, an epicrystal substrate can be obtained. Since there are very few microbubbles in the single crystal, the number of pits and microbubbles is reduced, and electronic component materials and optical component materials having excellent characteristics can be provided.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[Measurement of minute bubbles]
The minute bubbles taken into the grown single crystal were observed according to the light scattering laser tomography method. Specifically, as shown in FIG. 1, the side surface of the cylindrically processed aluminum oxide single crystal is irradiated with laser light, and the scattered light emitted from the end surface of the aluminum oxide single crystal is taken into the CCD camera, and an image is obtained. The scattered light intensity was calculated by the processing apparatus. As a result, if the scattered light intensity is 140 or less, it is determined that the amount of microbubbles in the grown single crystal is small.

[Evaluation of pits]
Fifty wafers were sliced from the grown single crystal and polished to measure how many pits there were. The smaller the number of pits, the better the single crystal is grown.

[Example 1]
A growth furnace having a heater at the bottom of the crucible disclosed in JP-A-2005-231958 was used, and 10 kg of 4N (99.99%) Al ₂ O ₃ raw material was charged into the iridium crucible. The apparatus includes means for reducing the pressure inside the furnace, means for monitoring the degree of pressure reduction, and means for supplying a mixed gas of oxygen and nitrogen or an inert gas. The Al ₂ O ₃ raw material is called crackle, which is obtained by grinding an aluminum oxide single crystal grown by Bernoulli method to a size of about 20 mm square.
This raw material was gradually heated over 12 hours until reaching the melting point. Vacuuming was started when the raw material was 1000 ° C. or higher, and the vacuuming was temporarily stopped when the pressure in the furnace was reduced to 0.005 MPa. Above this temperature, the gas component adsorbed on the raw material was discharged, so the degree of vacuum in the furnace gradually decreased and became 0.006 MPa after 30 minutes. Therefore, after 30 minutes, evacuation was performed again until the pressure in the furnace reached 0.005 MPa. This depressurization operation was repeated once every 30 min until the raw material melted. The degree of vacuum in the furnace due to the evacuation stop was such that it deteriorated to 0.006 MPa in 30 minutes after the stop until the raw material melted, but it deteriorated to 0.008 MPa when the raw material melted. As a result of continuing the decompression operation every 30 minutes after melting, the degree of deterioration was improved, and after 3 hours, the degree of vacuum was almost the same as before melting, and the degree of vacuum after 30 minutes was 0.006 MPa. It was.
After performing the above-described pressure reduction operation for another 3 hours, oxygen and nitrogen were introduced into the furnace until the pressure reached 0.1 MPa. Here, it was made to flow at a flow rate of 3 liters per minute while adjusting the oxygen concentration to be 0.3%. Thereafter, the aluminum oxide single crystal cut in the a-axis direction was used as a seed crystal, and the seed crystal was lowered to near the melt. The seed crystal is gradually lowered while rotating at a speed of 2 revolutions per minute, and the seed crystal is raised at a pulling speed of 2 mm / h while gradually lowering the temperature by bringing the tip of the seed crystal into contact with the melt. Crystal growth was performed.
As a result, a crystal having a diameter of 102 mm and a length of the straight body portion of 118 mm, in which no bubbles were observed visually, was obtained. Further, when the growth interface at the bottom of the crystal was measured, it was 22 mm convex. Furthermore, when this crystal was irradiated with a laser having a wavelength of 532 nm and the scattered light inside the crystal was observed, it was found that there was almost no scattering (scattered light intensity 85), and there were few fine bubbles inside the crystal. Further, when this crystal was made into a wafer and polished, no minute depressions could be confirmed.

[Example 2]
This was performed in the same manner as in Example 1 except that the evacuation was started after the crystal raw material was charged into the crucible and before the raw material was heated and melted. That is, evacuation was started while the raw material was at room temperature. As a result, a crystal having a diameter of 102 mm and a length of the straight body portion of 118 mm, in which bubbles were not observed visually, was obtained. Further, when the growth interface at the bottom of the crystal was measured, it was 22 mm convex. Furthermore, when this crystal was irradiated with a laser having a wavelength of 532 nm and the scattered light inside the crystal was observed, it was found that there was almost no scattering (scattered light intensity 84) and there were few fine bubbles inside the crystal. Further, when this crystal was polished on a wafer and fine polishing was not confirmed.

Example 3
After performing the decompression operation to heat and melt the raw material, oxygen and nitrogen were introduced into the furnace, and the same procedure as in Example 1 was performed except that the oxygen concentration was adjusted to 0.01%.
As a result, a crystal having a diameter of 102 mm and a length of the straight body portion of 118 mm, in which bubbles were not observed visually, was obtained. The growth interface at the bottom of the crystal was measured and found to be 48 mm convex. Furthermore, when this crystal was irradiated with a laser having a wavelength of 532 nm and the scattered light inside the crystal was observed, it was found that there was almost no scattering (scattered light intensity 68), and there were few fine bubbles inside the crystal. Further, when this crystal was polished on a wafer and fine polishing was not confirmed.

[Comparative Example 1]
For comparison, the same procedure as in Example 1 was performed except that no decompression operation was performed when the raw material was heated and melted.
As a result, the obtained crystal had a diameter of 98 mm, a length of the straight body part of 121 mm, and a growth interface of the crystal bottom part having a convexity of 21 mm. When this crystal was irradiated with a laser having a wavelength of 532 nm and the scattered light inside the crystal was observed, it was found that the intensity of the scattered light was strong (scattered light intensity 124) and that there were many fine bubbles inside the crystal. Further, when this crystal was polished on a wafer, many fine dents were observed.

[Comparative Example 2]
For comparison, the same procedure as in Example 1 was performed except that no decompression operation was performed when the raw material was heated and melted. Further, after melting the raw material, oxygen and inert gas were not introduced, and crystals were grown at a low oxygen partial pressure.
10 kg of 4N (99.99%) Al ₂ O ₃ raw material is introduced into an iridium crucible as a starting material, and nitrogen is flowed into the furnace at a flow rate of 3 liters per minute. After heating for 6 hours at a temperature slightly higher than the melting point, an aluminum oxide single crystal cut in the a-axis direction was used as a seed crystal, and the seed crystal was lowered to near the melt. The seed crystal is gradually lowered while rotating at a speed of 2 revolutions per minute, and the seed crystal is lowered at a pulling speed of 2 mm / h while gradually lowering the temperature by bringing the tip of the seed crystal into contact with the melt. The crystal was grown by raising.
As a result, when a crystal having a diameter of 100 mm and a length of the straight body portion of 115 mm was obtained, the growth was stopped because the crystal bottom contacted the crucible bottom. The growth interface at the bottom of the crystal was measured and found to be as large as 88 mm. The crystal was irradiated with a laser having a wavelength of 532 nm, and the scattered light inside the crystal was observed. Although very little scattering was observed (scattered light intensity 45), a relatively large scatterer was observed, and it was found that there might be inclusions (inclusions) although there are few fine bubbles inside the crystal. It was.
Further, when this crystal was polished and polished on a wafer, minute dents could not be confirmed, but several protruding foreign matters having a size of several μm were observed on the wafer. When this was analyzed by EPMA, it was iridium. It has been reported that when grown under a low oxygen partial pressure, iridium and GGG generate iridium inclusions, but aluminum oxide is also observed in the same manner.

As described above, in the embodiment, when the raw material is heated and melted in the crucible, the furnace body is depressurized to forcibly exclude the gas adsorbed or contained in the raw material and start crystal growth. If necessary, a mixed atmosphere of oxygen and nitrogen or an inert gas with an oxygen concentration of 0.01 to 0.5% is used, and this oxygen concentration is also used during crystal growth. As a result, the gas uptake into the single crystal could be suppressed, and inclusion segregation could be suppressed because the growth was not performed at a low oxygen partial pressure. Furthermore, the phenomenon that the growth interface becomes extremely convex toward the melt side is suppressed to reduce crystal defects, and the degree of solidification from the raw material can be increased by reducing the degree of convexity, thereby efficiently producing a single crystal. We were able to.
On the other hand, in the comparative example, the furnace body was not depressurized while the raw material was heated and melted in the crucible, so that the gas adsorbed or contained in the raw material could not be forcibly excluded. Moreover, since the crystal was grown at a low oxygen partial pressure, segregation of inclusions could not be suppressed.

It is explanatory drawing which shows the means to investigate the micro bubble (micro valve | bulb) in a crystal | crystallization using the optical system which irradiates a laser beam to the grown single crystal and measures light scattering.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Grown single crystal 2 Laser light 3 Scattered light 4 CCD camera 5 Image processing device

Claims (5)

In a method for producing an aluminum oxide single crystal by a melt solidification method in which a raw material for a single crystal is put in a crucible in a furnace and heated and melted, and a growth crystal is pulled up from the raw material melt,
When the single crystal raw material is heated and melted, it is melted while removing the gas generated from the single crystal raw material by heating while maintaining the furnace body at 0.01 MPa or less. A method for producing an aluminum oxide single crystal, wherein the growth crystal is pulled up so as to maintain a reduced pressure of 0.1 MPa and an oxygen concentration in a mixed atmosphere is 0.01 to 0.5% .   The method for producing an aluminum oxide single crystal according to claim 1, wherein the raw material for single crystal is gradually heated and melted over 10 hours or more.   3. The method for producing an aluminum oxide single crystal according to claim 1, wherein after the raw material for single crystal is heated, the pressure reduction is started after the temperature reaches 1000 ° C. 3.   The method for producing an aluminum oxide single crystal according to claim 1, wherein the raw material melt is maintained in a mixed atmosphere of oxygen and nitrogen or an inert gas.   The method for producing an aluminum oxide single crystal according to claim 1, wherein the material of the crucible is iridium.

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