JP2008195575A - Method for manufacturing aluminum oxide single crystal and aluminum oxide single crystal obtained by using the method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently manufacturing an aluminum oxide single crystal having a high quality and suitable for an electronic component material or an optical component material by suppressing the occurrence of micro-bubbles. <P>SOLUTION: In the method for manufacturing the aluminum oxide single crystal by a melting/solidifying process, by which a raw material for the single crystal is charged into a crucible provided in a furnace of a single crystal manufacturing apparatus and is heated and melted, and a growing crystal is pulled from a melt of the raw material, when the raw material for the single crystal in heated and melted, the raw material for the single crystal is melted under conditions sufficient to remove the gas produced from the raw material for the single crystal by heating, in a nitrogen or inert gas atmosphere and at a furnace pressure of ≥0.1 MPa, then after introducing oxygen into the furnace, the melt of the raw material is further heated for at least 5 h under a mixed gas atmosphere comprising oxygen and nitrogen or an inert gas, thereafter pulling of a growing crystal is started, and the mixed gas atmosphere comprising oxygen and nitrogen or the inert gas is successively maintained even during crystal growth. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing an aluminum oxide single crystal and an aluminum oxide single crystal obtained by using this method. More specifically, the present invention relates to a high-quality aluminum oxide single crystal by suppressing generation of pits and microbubbles during single crystal growth. The present invention relates to a method for efficiently producing a crystal and an aluminum oxide single crystal obtained by using this method.

  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 the majority of oxide single crystal materials such as LN, LT, YAG and aluminum oxide are grown by melt solidification because their crystal characteristics and large diameter crystals can be obtained. In particular, the Czochralski method (Cz method) is most widely used because of its high technical perfection.
The oxide single crystal manufacturing method by the Czochralski method is as follows. First, the crucible is filled with an oxide raw material, and the crucible is heated by a high frequency induction heating method or a resistance heating method to melt the raw material, and then cut into a predetermined crystal orientation. The seed crystal is brought into contact with the surface of the raw material melt, and the seed crystal is pulled up at a predetermined speed while rotating the seed crystal at a predetermined rotation speed to grow a single crystal.

  When an aluminum oxide single crystal is grown by the Czochralski method, innumerable minute bubbles are likely to be generated in the crystal. These fine bubbles include a relatively large bubble that expresses a minute pit called a pit having a diameter of several μm when a wafer serving as an epitaxial growth substrate is polished, and a light scattering laser tomography method (see Non-Patent Document 1). ), There are relatively small bubbles called microbubbles which can be confirmed in a cloud shape 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.

The cause of the fine bubbles in the crystal is that when the aluminum oxide single crystal is grown, oxygen atoms (O) and oxygen molecules (O ₂ ) generated by decomposition of the raw material at high temperature are supersaturated in the melt, It is known that this is taken into the grown single crystal and becomes bubbles in the single crystal.
In order to avoid entrapment of bubbles, it has been proposed to grow a single crystal in a reducing atmosphere such as 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. However, when the wafer was cut out from the grown single crystal and polished and polished, a large number of pits were present on the surface of the wafer, so that the introduction of the bubbles could not 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 disclosed that supersaturation of the gas component generated near the interface can be suppressed by enhancing the convection of the melt, and the amount of the gas component taken into the crystal can be reduced (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 considered that the generation of pits and microbubbles can be suppressed by reducing the amount of minute bubbles taken into the crystal during growth.

And 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 reported that the convection of the melt can be strengthened and the effect of stirring 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. As a result of this change and the surface tension flow being induced, it can be considered that the flow in the same direction as the natural convection of the melt is remarkably accelerated and the effect of stirring is increased.

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, if the melt height in the crucible decreases to some extent due to crystal growth, the tip of the growth interface and the bottom of the crucible come into contact with each other, and crystal growth continues beyond that. Is impossible, and there is a problem that the crystals obtained cannot be made so large with respect to the amount of raw material filled in the crucible. In addition, as a result of remarkably accelerating the flow in the same direction as the natural convection of the melt, the single crystal growth rate in the melt is increased, and there is a problem that crystal defects are likely to occur in the obtained crystal.
As a method for suppressing excessive convection of the melt, Patent Document 2 proposes increasing the rotational speed of the grown crystal to, for example, 20 revolutions / minute or more, particularly 30 to 120 revolutions / minute. However, with such means, although the crystal yield can be increased, the generation of fine bubbles in the single crystal cannot be sufficiently suppressed.

  In order to solve such a problem, the present inventors in the production of an oxide single crystal by a high-frequency induction heating type Czochralski method, heating a raw material for a single crystal (hereinafter also simply referred to as a raw material) in a crucible and We found that if the pressure inside the furnace is reduced during melting and the gas adsorbed or contained in the raw material is forcibly removed, the amount of gas components taken into the single crystal can be suppressed, and the amount of pits and microbubbles generated can be reduced. Yes.

However, in the production of an oxide single crystal by the Czochralski method using a high frequency induction heating method, particularly when a high frequency oscillator having an oscillation frequency of 10 kHz or more is used, if the pressure in the furnace is lower than 0.01 MPa, When the temperature reaches 1000 ° C., a discharge phenomenon occurs, the power supply becomes unstable, and it becomes impossible to melt the raw material and further grow the crystal. Forcibly removing gas components by depressurization is the most effective method for suppressing pits and microbubbles, but it cannot be provided for an induction heating single crystal manufacturing apparatus having an oscillation frequency of 10 kHz or more.
Japanese Patent Laid-Open No. 04-132695 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 during single crystal growth in view of the above-mentioned problems of the prior art, and this method. It is to provide an aluminum oxide single crystal.

  As a result of intensive research to solve the above problems, the present inventors have melted the raw material under a specific pressure as an atmosphere of nitrogen or an inert gas during heating of the raw material, introduced oxygen simultaneously with the melting of the raw material, By maintaining the melt in a mixed atmosphere with oxygen and nitrogen or an inert gas for a certain period of time, an induction heating single crystal manufacturing apparatus that cannot forcibly eliminate gas components by decompression or an oscillation frequency of 10 kHz or more Even when an induction heating single crystal manufacturing device is used, the gas component adsorbed or contained in the raw material can be released from the melt, reducing the amount of pits and microbubbles generated, and further, the growth of the single crystal in the melt By controlling the speed, the phenomenon that the growth interface becomes significantly convex toward the melt side is suppressed, crystal defects are reduced, the solidification rate from the raw material is improved, and single crystals are efficiently produced. It found that kill, which resulted in the completion of the present invention.

  That is, according to the first invention of the present invention, an aluminum oxide single crystal is obtained by a melt solidification method in which a raw material for a single crystal is placed in a crucible in a furnace of a single crystal manufacturing apparatus 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 in the method for producing the first, it is sufficient to first remove the gas generated from the single crystal raw material by heating in a nitrogen or inert gas atmosphere at a furnace pressure of 0.1 MPa or more. The raw material for single crystal is melted under various conditions, oxygen is then introduced into the furnace, and the raw material melt is subsequently heated for 5 hours or more in a mixed gas atmosphere consisting of oxygen and nitrogen or an inert gas. There is provided a method for producing an aluminum oxide single crystal, characterized in that pulling is started and maintained in a mixed atmosphere of oxygen and nitrogen or an inert gas during crystal growth.

According to the second invention of the present invention, in the first invention, the single crystal manufacturing apparatus is a high frequency induction heating type apparatus having an oscillation frequency of 10 kHz or more. Is provided.
According to a third aspect of the present invention, there is provided the method for producing an aluminum oxide single crystal according to the first aspect, wherein the material of the crucible is iridium.
According to the fourth invention of the present invention, in the first invention, the single crystal raw material is heated and melted for 10 hours or more until the melting point reaches the melting point. Is provided.
Furthermore, according to the fifth aspect of the present invention, there is provided the method for producing an aluminum oxide single crystal according to the first aspect, wherein the oxygen concentration in the mixed atmosphere is 0.01 to 0.5% by volume. Provided.

On the other hand, according to the sixth invention of the present invention, there is provided an aluminum oxide single crystal obtained by the manufacturing method according to any one of the first to fifth inventions.
According to the seventh invention of the present invention, in the sixth invention, the scattered light intensity (irradiates the side surface of the aluminum oxide single crystal with laser light and emits it in the direction of 90 ° with respect to the incident laser light. The aluminum oxide single crystal is provided in which the scattered light is taken into a CCD camera and calculated by an image processing apparatus) is 130 or less.

According to the present invention, the generation of pits and microbubbles can be suppressed even in an induction heating single crystal manufacturing apparatus that cannot forcibly exclude gas components due to reduced pressure or in an induction heating single crystal manufacturing apparatus with an oscillation frequency of 10 kHz or more. Thus, a high quality aluminum oxide single crystal can be produced efficiently.
When the raw material is heated in the crucible, the gas component adsorbed or contained in the raw material is eliminated by making the atmosphere of only nitrogen or inert gas, but the gas component from the raw material into the melt is slightly removed. Incorporation occurs, but when the raw material is melted and mixed with oxygen and nitrogen or an inert gas and the melt is maintained for a certain period of time, residual gas components can be eliminated, and gas generated by decomposition of aluminum oxide Generation of components 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 aluminum oxide single crystal of the present invention and the production method thereof will be described in detail with reference to the drawings.

1. Method for Producing Aluminum Oxide Single Crystal The method for producing an aluminum oxide single crystal of the present invention comprises melting and solidifying a raw material for single crystal in a crucible in a furnace of a single crystal production apparatus, heating and melting, and pulling a growth crystal from the raw material melt. In the method of producing an aluminum oxide single crystal by the method, when the raw material for single crystal is heated and melted, it is first generated from the raw material for single crystal by heating in a nitrogen or inert gas atmosphere at a furnace pressure of 0.1 MPa or more. The raw material for single crystal is melted under conditions sufficient to remove the gas, oxygen is then introduced into the furnace, and the raw material melt is subsequently heated for 5 hours or longer in a mixed gas atmosphere consisting of oxygen and nitrogen or an inert gas. Thereafter, the growth crystal is started to be pulled up, and is maintained in a mixed atmosphere of oxygen and nitrogen or an inert gas during the crystal growth.

  In the present invention, when a raw material is put into a crucible in a furnace of a single crystal manufacturing apparatus and heated and melted by the melt solidification method, the grown crystal is pulled up from the raw material melt to produce an aluminum oxide single crystal. A means for monitoring the oxygen concentration in the furnace body of the apparatus is provided, and the amount of gas components released when the raw material is heated and melted is confirmed by the behavior of the oxygen concentration. Adjustment is made to be in the range of 01 to 0.5% by volume, and the work of maintaining the melt is performed.

  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 high-frequency induction heating method in which 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 a high-frequency coil having an oscillation frequency of 10 kHz or more as a heating device outside the furnace material Apparatus. The apparatus is provided with means for monitoring the oxygen concentration in the furnace body, means for supplying a mixed gas of oxygen and nitrogen or an inert gas into the furnace body, and means for reducing the pressure inside the furnace body as necessary.

  Since the melting point of alumina as a raw material is slightly higher than 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 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 raw material in the furnace body is heated and melted at normal pressure and grown by the Czochralski method, innumerable fine bubbles 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 adsorbed or contained in the raw material is not completely removed before melting of the raw material and remains in the melt, which is taken into the crystal Many of them are bubbles. Therefore, during heating and melting of the raw material in the crucible, the atmosphere in the furnace body is limited to nitrogen or an inert gas, and the gas adsorbed or contained in the raw material is excluded. Most gas components are released when melted, but some gas components are left behind in the melt. For this reason, the melt is maintained in a mixed atmosphere of oxygen and nitrogen or an inert gas simultaneously with melting of the raw material, and the remaining gas components in the melt are eliminated. In this step, if the time for maintaining the melt is short, the remaining gas components are not sufficiently removed. Therefore, it is necessary to leave the melt for 5 hours or longer in a mixed atmosphere.

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 many gas components are removed by leaving the melt after the raw material is melted. The This is more effective when the aluminum oxide sintered body has a specific surface area of 0.1 to 10 m ² / g.

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. In the crackle raw material, although the viscosity of the aluminum oxide melt is high and the surface tension is large, by leaving the melt after heating and melting, the gas components dissolved in the melt as microbubbles are almost removed. 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 raw material melting step in the present invention will be shown. The raw material melting 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, as described above, when the raw material is put in the crucible in the furnace and the raw material is heated, the atmosphere in the furnace is limited to nitrogen or an inert gas, and the gas generated from the raw material by heating is eliminated. While melting. Under a nitrogen or inert gas atmosphere, at a furnace pressure of 0.1 MPa or more, conditions are set sufficient to remove gas generated from the single crystal raw material by heating. The furnace pressure of 0.1 MPa or more means that the furnace is not further depressurized. It is desirable that the raw material is gradually heated over 10 hours or more, preferably 12 hours or more until reaching the melting point. The heating rate until the raw material reaches the melting point is not particularly limited, but it is possible to suppress gas incorporation into the melt by heating gradually over a long time without rapidly heating.

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, so that the oxygen concentration in the furnace gradually increases. When the raw material is melted, the oxygen concentration in the furnace body temporarily increases to 100 to 1000 ppm. However, since minute gas components in the melt remain, the oxygen concentration in the furnace body gradually decreases.
If the heating is continued at 2050 to 2150 ° C. after the raw material is melted, minute gas components in the melt are gradually discharged by natural convection of the melt, but it is a raw material when the oxygen concentration in the furnace body becomes 100 ppm or less. The decomposition of aluminum oxide causes the generation of new gas components. On the other hand, in an atmosphere where the oxygen concentration in the furnace body is 0.5% by volume or more, the natural convection of the melt is weakened, and not only the minute gas components in the melt are hardly discharged, but also the oxygen in the atmosphere and the crucible material Reaction with iridium, which is a cause of iridium inclusion. Therefore, if the oxygen concentration in the furnace body is adjusted to be in the range of 0.01 to 0.5% by volume and the melt is maintained for 5 hours or more, the remaining minute gas components can be eliminated. After elapse of 5 hours or more, preferably 8 hours or more, more preferably 10 hours or more after melting, crystal growth is started while rotating the seed crystal in contact with the melt surface. By maintaining this oxygen concentration even during single crystal growth, decomposition of aluminum oxide can be suppressed. If it is less than 5 hours, minute gas components remaining in the melt cannot be sufficiently removed.

  Single crystal growth is carried out by adjusting the rotation speed and pulling speed to form the neck and shoulders according to the usual method, except that the atmosphere in the furnace is adjusted as follows in the raw material melting process. Form. 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 weight of the crystal during growth, deriving the diameter, growth speed, 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 components contained in the melt, there are no bubbles that precipitate in the crystal during single crystal growth. Even during the growth of the single crystal, the growth rate of the single crystal in the melt is adjusted by adjusting the oxygen concentration to a range of 0.01 to 0.5% by volume, preferably 0.1 to 0.3% by volume. This reduces the defect that the crystal defects are reduced by controlling the crystal defects, and the growth interface is not significantly convex on the melt side, so that the crystal obtained with respect to the raw material cannot be made so large.

  In this way, a raw material is melted and allowed to stand in a specific oxygen concentration atmosphere, and then a single crystal is grown, thereby forcibly eliminating gas components regardless of the form of the raw material and by decompression. Even if it is an inexpensive induction heating single crystal manufacturing device that cannot be used or an induction heating single crystal manufacturing device with an oscillation frequency of 10 kHz or more, it can be adsorbed on the surface of the melt by a simple operation of leaving the melt for a certain period of time. Alternatively, the contained gas component can be easily removed. As a result, excess gas contained in the melt is reduced, so that 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. Can do.

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.

2. Aluminum oxide single crystal The aluminum oxide single crystal of the present invention is a single crystal containing two elements of aluminum and oxygen obtained by the above production method.

  Furthermore, it is a single crystal in which the scattered light intensity obtained by the light scattering laser tomography method is reduced to 130 or less. The scattered light intensity is measured by irradiating the side surface of the aluminum oxide single crystal processed into a cylindrical shape with laser light and capturing the scattered light emitted in the direction of 90 ° with respect to the incident laser light into the CCD camera. Calculate with the device.

  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]
Microscopic 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. That is, 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 the scattered light 3 emitted in the direction of 90 ° with respect to the incident direction of the irradiated laser beam 2 is converted into a CCD. The image was taken into the camera 4 and processed by the image processing apparatus 5 into 256 gradations from 0 to 255, and the intensity average of the 40 mm square of the crystal portion in the image was calculated as the scattered light intensity. As a result, if the scattered light intensity is 130 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]
Using a high-frequency induction heating type growth furnace with an oscillation frequency of 12 kHz and an output of 40 kW, 10 kg of 4N (99.99%) Al ₂ O ₃ raw material was charged into an 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.
Vacuuming was started before the raw material was heated, and when the pressure in the furnace was reduced to 20 Pa, the vacuuming was stopped and nitrogen gas was introduced. After the pressure in the furnace reached 0.1 MPa, only nitrogen gas was allowed to flow at a flow rate of 3 liters per minute to start heating the raw material. At this time, the oxygen concentration in the furnace was 0.1 ppm. The raw material was gradually heated over 12 hours until the raw material reached the melting point, and when the raw material reached 1000 ° C. or higher, the oxygen concentration started to increase. When the raw material melted, the oxygen concentration in the furnace rapidly increased from 200 ppm to 400 ppm. Thirty minutes after the melting of the raw material, when the oxygen concentration decreased to 300 ppm, oxygen gas was introduced into the furnace to prepare a mixed atmosphere of oxygen and nitrogen, and the oxygen concentration in the furnace was adjusted to 0.3% by volume.
Five hours after melting the raw material, 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 101 mm and a length of the straight body portion of 136 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 51 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 112), 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 carried out in the same manner as in Example 1 except that powdered aluminum oxide raw material was used as the raw material. As a result, a crystal having a diameter of 99 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 41 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 126), 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
This was carried out in the same manner as in Example 1 except that a powdered aluminum oxide raw material was used and the oxygen concentration in the furnace was adjusted to 0.1% by volume after melting the raw material.
As a result, a crystal having a diameter of 99 mm and a length of the straight body portion of 123 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 60 mm convex. Further, 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 59), 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, when the raw material was heated and melted, it was implemented except that oxygen was introduced into the furnace from the start of heating to create a mixed atmosphere of oxygen and nitrogen, and the oxygen concentration in the furnace was adjusted to 0.3% by volume. Performed as in Example 1.
As a result, the obtained crystal had a diameter of 119 mm, a straight body length of 120 mm, and a growth interface at the bottom of the crystal was 49 mm convex. 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 scattered light intensity was strong (scattered light intensity 140) 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 from the start of heating to the end of crystal growth except that a low oxygen concentration atmosphere containing only nitrogen was used without introducing oxygen into the furnace.
As a result, when a crystal having a diameter of 99 mm and a length of the straight body portion of 121 mm was obtained, the growth was stopped because the crystal bottom contacted the crucible bottom. When the growth interface at the bottom of the crystal was measured, it was as large as 73 mm convex. 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 scatter was observed (scattered light intensity 49), 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.

[Comparative Example 3]
For comparison, the same procedure as in Example 1 was performed except that the melt was allowed to stand after melting for 3 hours.
As a result, the obtained crystal had a diameter of 104 mm, a length of the straight body portion of 126 mm, and a growth interface at the bottom of the crystal having a convexity of 44 mm. When this crystal was irradiated with a laser having a wavelength of 532 nm and the scattered light inside the crystal was observed, the scattered light intensity was high (scattered light intensity 138), and it was found that there were many fine bubbles inside the crystal. Further, when this crystal was polished on a wafer, many fine dents were observed.

As described above, in the examples, until the raw material is heated and melted in the crucible, the atmosphere in the furnace body is only nitrogen or inert gas, and after the raw material is melted, the oxygen concentration is 0.01 to 0.5% by volume. By maintaining the melt in a mixed atmosphere with oxygen and nitrogen or an inert gas for a certain period of time, it is possible to eliminate the gas adsorbed or contained in the raw material, and continue to oxygen after the start of crystal growth. Further, the excess gas contained in the melt was reduced by carrying out in a mixed atmosphere with nitrogen or an inert gas. As a result, it was possible to suppress gas uptake into the single crystal and to suppress inclusion because it was not grown under a low oxygen concentration. 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, when it was performed in an atmosphere containing oxygen through raw material heating and crystal growth, the gas adsorbed or contained in the raw material could not be sufficiently eliminated. In addition, only nitrogen or inert gas should be used during heating, and even if the crystal growth is performed in a mixed atmosphere containing oxygen after melting the raw material, the gas in the melt should be sufficiently eliminated if the melt is left for a short time. I could not. When the atmosphere inside the furnace was heated from the raw material to crystal growth under a low oxygen concentration of only nitrogen or inert gas, iridium inclusions were generated.

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 (7)

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 into a crucible in a furnace of a single crystal production apparatus 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, first, in a nitrogen or inert gas atmosphere, at a furnace pressure of 0.1 MPa or higher, the single crystal raw material is heated under conditions sufficient to remove the gas generated from the single crystal raw material by heating. The raw material is melted, oxygen is then introduced into the furnace, the raw material melt is subsequently heated for 5 hours or longer in a mixed gas atmosphere consisting of oxygen and nitrogen or an inert gas, and then the growth crystal is pulled up. A method for producing an aluminum oxide single crystal, which is continuously maintained in a mixed atmosphere of oxygen and nitrogen or an inert gas during growth.   2. The method for producing an aluminum oxide single crystal according to claim 1, wherein the single crystal production apparatus is a high frequency induction heating type apparatus having an oscillation frequency of 10 kHz or more.   The method for producing an aluminum oxide single crystal according to claim 1, wherein the material of the crucible is iridium.   The method for producing an aluminum oxide single crystal according to claim 1, wherein the single crystal raw material is heated and melted for 10 hours or more until the melting point reaches the melting point.   The method for producing an aluminum oxide single crystal according to claim 1, wherein the oxygen concentration in the mixed atmosphere is 0.01 to 0.5% by volume.   The aluminum oxide single crystal obtained by the manufacturing method in any one of Claims 1-5.   Scattered light intensity (irradiated with laser light on the side surface of the aluminum oxide single crystal, and the scattered light emitted in the direction of 90 ° with respect to the incident laser light is taken into the CCD camera and calculated by the image processing apparatus) is 130. The aluminum oxide single crystal according to claim 6, wherein:

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