JP2016033102A - Sapphire single crystal and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high quality sapphire single crystal less in pits or particles by suppressing capture of air bubbles and contamination of a crucible material during growth.SOLUTION: The sapphire single crystal is grown by a vertical temperature gradient freezing method or a vertical Bridgman method using a crucible 6 made of tungsten. The method for manufacturing the sapphire single crystal comprises: the growth step of increasing a temperature in a furnace 1 to a constant temperature in a range of 2040-2060°C, adjusting a temperature gradient in a growth axis direction in a region of 2 cm in a vertical direction across a solid-liquid interface to be 5.0-6.0°C/cm and crystallizing a raw material melt 9 while controlling a growth rate in a range of 0.8-1.3 mm/hr; and the cooling step of lowering the temperature in the furnace 1 to a constant temperature in a range of 1900-2000°C, adjusting the temperature gradient in the growth axis direction of the sapphire single crystal at 1.9°C/cm or less toward the upper surface of the sapphire single crystal from the bottom of the crucible 6 and lowering the temperature in the furnace 1 at 10-30°C/hr to cool the sapphire single crystal to a room temperature.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a sapphire single crystal by a vertical temperature gradient solidification method (VGF method), and a sapphire single crystal obtained by this production method.

  In recent years, white LEDs (light emitting diodes) have been widely used as lighting devices due to demands for energy saving and space saving. This white LED is configured by combining a blue LED in which a GaN-based semiconductor is formed on a sapphire single crystal substrate and a phosphor. For this reason, with the increase in demand for white LEDs, the demand for sapphire single crystal substrates has also increased rapidly. Moreover, since the cost reduction is required for the spread of white LEDs in general lighting applications, the sapphire single crystal substrate is also required to be reduced in price.

  Generally, a sapphire single crystal substrate is manufactured by cutting out a disk-shaped wafer from a sapphire single crystal ingot grown from a sapphire raw material melt (alumina melt). As a method for growing a sapphire single crystal, a pulling method in which a single crystal is pulled from a melt and solidified, represented by the Czochralski method (Cz method; rotational pulling method) and a ribbon-like crystal growth method (EFG method); There is a unidirectional solidification method in which a sapphire raw material melt is solidified in a crucible, such as a vertical temperature gradient solidification method (VGF method) and a vertical Bridgman method (VB method).

  Since the unidirectional solidification method does not require the grown sapphire single crystal to be pulled up, not only can the crystal growing apparatus be miniaturized and simplified, but also the degree of freedom in growing the sapphire single crystal to be grown is high, and the c-axis It is superior to the pulling method in that it can easily obtain a substrate in the direction and can also handle the production of a large-diameter sapphire single crystal exceeding 6 inches (152.4 mm) in diameter. Yes. For this reason, the practical use of the manufacturing method of the sapphire single crystal by the unidirectional solidification method is advancing now.

  In growing a sapphire single crystal by the unidirectional solidification method, it is necessary to take out the sapphire single crystal from the crucible after the growth. However, depending on the relationship between the linear expansion coefficient of the crucible material and sapphire, the sapphire single crystal is clamped by thermal contraction during cooling, and it is necessary to destroy the crucible when the sapphire single crystal is taken out. There is a problem such as. In addition, because the melting point of sapphire is as high as about 2040 ° C, crucible materials for growing sapphire single crystals are limited to expensive noble metals with excellent high-temperature durability, reducing running costs. There is also the problem of wanting.

  In order to solve these problems, Japanese Patent Laid-Open No. 2011-42650 discloses tungsten (W), molybdenum (Mo), or molybdenum tungsten (MoW) as a crucible material used for growing a sapphire single crystal by a unidirectional solidification method. It has been proposed to use alloys. Since these materials have high-temperature durability and a linear expansion coefficient smaller than that of sapphire, the grown sapphire single crystal can be taken out without destroying the crucible. Further, since these materials are less expensive than noble metals, the running cost can be reduced.

  By the way, in the unidirectional solidification method in which the temperature gradient is provided in the vertical direction and the crystal is grown in the crucible, there is a problem that a large amount of bubbles are easily taken into the obtained sapphire single crystal. The cause of this is that in the unidirectional solidification method, a temperature gradient is formed at the lower side of the crucible and at a higher temperature on the upper side of the crucible, so that thermal convection of the raw material melt hardly occurs, and the gas dissolved in the raw material melt Difficult to be discharged to the outside, and unlike the pulling method, the solid-liquid interface during crystal growth is almost stationary, so once bubbles are attached to the solid-liquid interface, they are likely to be taken into the crystal as they are Conceivable.

  Further, in the unidirectional solidification method, the raw material melt is in contact with the inner peripheral surface of the crucible in a high temperature environment, so that the constituent components of the crucible are taken into the sapphire single crystal as impurities. There is also. Actually, when components of a sapphire single crystal grown using a crucible made of tungsten, molybdenum, or molybdenum tungsten alloy are analyzed, the components of the crucible are detected.

  In this way, when a sapphire single crystal containing a large amount of bubbles and impurities is processed into a wafer shape, pits (micro holes) and particles (micro holes and protrusions) appear on the processed surface, which deteriorates the yield of the wafer. Become.

JP 2011-42560 A

  An object of this invention is to provide the high quality sapphire single crystal with few bubbles and impurities. Moreover, an object of this invention is to provide the manufacturing method which makes it possible to obtain such a sapphire single crystal easily in industrial scale production.

  The present invention is a method for producing a sapphire single crystal in which a seed crystal and a raw material are housed in a tungsten crucible, and crystal growth is performed by a vertical temperature gradient solidification method or a vertical Bridgman method. After raising the temperature to a constant temperature in the range of 2060 ° C., at this temperature, the temperature gradient in the growth axis direction in the region of 2 cm in the vertical direction across the solid-liquid interface is 5.0 ° C./cm to 6.0 ° C./cm. The growth step of obtaining a sapphire single crystal by crystallizing the raw material melt while controlling the growth rate within the range of 0.8 mm / hr to 1.3 mm / hr while maintaining the temperature gradient. After the growth step, the furnace temperature is lowered to a constant temperature in the range of 1900 ° C. to 2000 ° C., and the temperature gradient in the growth axis direction of the sapphire single crystal is changed from the crucible bottom to the top of the sapphire single crystal. A cooling step of adjusting the temperature in the furnace to 10 ° C./hr to 30 ° C./hr to cool the sapphire single crystal to room temperature while maintaining the temperature gradient. It is characterized by providing.

  In the cooling step, it is preferable that the temperature in the furnace is lowered over 3 hours and the temperature gradient is adjusted.

  Moreover, the sapphire single crystal of the present invention is obtained by the above production method, and has an impurity concentration of 5 mass ppm or less.

When the sapphire single crystal is processed into a disk shape, the number of pits and particles per unit area on the surface is preferably 30 / cm ² or less.

  According to the present invention, a high-quality sapphire single crystal with few bubbles and impurities can be easily obtained in an industrial scale production process. For this reason, the industrial significance of the present invention is extremely large.

FIG. 1 is a schematic cross-sectional view of an example of an apparatus for manufacturing a sapphire single crystal by a vertical temperature gradient solidification method to which the present invention is applied. FIG. 2 is a diagram showing a particle distribution of a wafer obtained by processing a sapphire single crystal grown using a tungsten crucible according to the present invention. FIG. 3 is a diagram showing the particle distribution of a wafer obtained by processing a sapphire single crystal grown using a crucible made of molybdenum tungsten alloy.

  In view of the above-described problems, the present inventors have conducted extensive research. As a result, in the vertical temperature gradient solidification method (VGF method) and vertical Bridgman method (VB method) using a crucible made of tungsten, particularly in the vertical temperature gradient solidification method, by appropriately controlling the growth conditions and cooling conditions, The inventor has obtained knowledge that it is possible to effectively suppress the incorporation of bubbles and the incorporation of impurities into the sapphire single crystal. The present invention has been completed based on this finding.

1. Hereinafter, a method for producing a sapphire single crystal of the present invention will be described in detail with reference to FIG. In addition, although the manufacturing method of the sapphire single crystal of the present invention can be applied regardless of the growth axis direction and the size of the sapphire single crystal to be obtained, the present invention can be preferably applied hereinafter. A case where a large sapphire single crystal having a diameter of 150 mm to 250 mm and a total length of 150 mm to 800 mm is grown using the vertical temperature gradient solidification method as a growth axis will be described as an example.

(1) Vertical temperature gradient solidification method In this invention, it is preferable to employ | adopt the vertical temperature gradient solidification method (henceforth "VGF method") among unidirectional solidification methods as a manufacturing method of a sapphire single crystal.

  In the vertical Bridgman method, it is necessary to largely move the crucible vertically in the chamber of the growing apparatus. Therefore, it is necessary not only to incorporate a crucible moving mechanism into the growth apparatus, but also to increase the overall height of the chamber, so that the growth apparatus has to be large and expensive. is there. On the other hand, the VGF method forms a constant temperature distribution in the furnace by adjusting the output of the heater divided into a plurality of parts without moving the crucible, and geometrically distributes the temperature distribution. Such a problem does not occur because the crystal is grown by moving the crystal.

[Manufacturing equipment (growing furnace)]
The production method of the present invention can grow a sapphire single crystal by the VGF method as shown in FIG. 1, for example, and in the growth step and the cooling step, the raw material melt or the growth axis direction of the sapphire single crystal It implements using the sapphire single crystal growth apparatus (growth furnace) 1 which can control a temperature gradient.

  The growth furnace 1 includes a cylindrical chamber 2 and a heat insulating material 3 disposed inside the chamber 2, as in the conventional growth furnace. The size of the growth furnace 1 is appropriately selected according to the size of the sapphire single crystal to be obtained. However, when growing the sapphire single crystal having the dimensions described above, the diameter is generally 600 mm to 1000 mm, The height is about 800 mm to 2000 mm. Further, the growth furnace 1 is provided with an opening (not shown), and an inert gas, preferably argon gas, is supplied / exhausted into the chamber 2 through this opening. Sometimes, the chamber 2 is filled with an inert gas.

  A cylindrical heater 4 having an inner diameter of 250 mm to 350 mm and a height of 450 mm to 1200 mm is disposed inside the heat insulating material 3, and when the sapphire single crystal is grown, a hot zone 5 is formed by the cylindrical heater 4. Become. Moreover, the cylindrical heater 4 is comprised from the upper stage heater 4a, the middle stage heater 4b, and the lower stage heater 4c, and the temperature of the growth axis direction in a growth process and a cooling process is adjusted by adjusting the input electric power to these heaters 4a-4c. It is possible to control the gradient.

  The crucible 6 is placed on a support shaft 7 disposed coaxially with the cylindrical heater 4. The support shaft 7 may be configured to be movable in the vertical direction, if necessary, and may be configured to rotate so that the crucible 6 is rotated in the hot zone 5 when growing the sapphire single crystal. Also good. In addition, the crucible 6 is formed in a cup shape, and the single crystal 8 and the raw material 9 are accommodated in a stacked state in this order.

  In addition, the growth furnace 1 of the present invention is provided with temperature measuring means (not shown) for measuring the temperature of the raw material melt in the crucible 6 and the bottom of the crucible 6. Such temperature measuring means is not particularly limited, but it is preferable to use a non-contact type thermometer such as a radiation thermometer.

[crucible]
In the method for producing a sapphire single crystal of the present invention, it is necessary to use a tungsten product as the crucible 6. Tungsten has a melting point of 3400 ° C. and is higher than the melting point of molybdenum (2600 ° C.). Therefore, under the growth conditions described later, the crucible material is effectively suppressed from being mixed as an impurity in the growth crystal. It becomes possible to do.

  Tungsten has a property that the linear expansion coefficient in the range from room temperature to 2000 ° C. is smaller than that of sapphire. For this reason, the crucible made of tungsten has little dimensional change in the cooling process (shrinkage ratio of the inner diameter dimension and the outer diameter dimension), and the sapphire single crystal can be easily taken out after cooling.

(2) Growing process In the growing process, first, the crucible 6 in which the seed crystal 8 and the raw material 9 are stored is placed in the hot zone. In this state, power is applied to the cylindrical heater 4 to heat the seed crystal and the raw material 9 to the melting point of alumina (about 2040 ° C.) or higher, and melt the raw material 9 and the portion of the seed crystal 8 that contacts the raw material 9. To do. Thereafter, the electric power supplied to the upper heater 4a, middle heater 4b, and lower heater 4c constituting the cylindrical heater 4 is adjusted, and a region of 2 cm in the vertical direction across the solid-liquid interface (hereinafter referred to as “the vicinity of the solid-liquid interface”). The temperature gradient in the growth axis direction is controlled in the range of 5.0 ° C./cm to 6.0 ° C./cm. Subsequently, while maintaining the temperature gradient in the growth axis direction in the vicinity of the solid-liquid interface, the temperature of the hot zone 5 (in the furnace is adjusted so that the growth rate of the grown crystal becomes 0.8 mm / hr to 1.3 mm / hr. Temperature) is gradually decreased to grow a sapphire single crystal.

  In the growth step of the present invention, the growth axis is preferably the c-axis in order to efficiently cut out the substrate in the c-axis direction from the grown sapphire single crystal. However, in the present invention, the growth axis of the sapphire single crystal is not particularly limited, and the present invention can also be applied when the a-axis or the r-axis is used as the growth axis of the sapphire single crystal.

[In-furnace temperature]
In the initial stage of the growth process, in order to melt the raw material 9 and the portion of the seed crystal 8 that comes into contact with the raw material 9, the temperature inside the furnace is heated to the melting point of the raw material 9 (alumina) by the cylindrical heater 4. It will be necessary. Specifically, the temperature in the furnace is raised to 2040 ° C. or higher, preferably 2050 ° C. or higher. The upper limit of the furnace temperature is not particularly limited, but if the temperature is excessively high, not only the energy consumption increases, but the load on the growth furnace 1 also increases. For this reason, the upper limit of the furnace temperature is about 2060 ° C.

[Temperature gradient in the growth axis direction near the solid-liquid interface]
The temperature inside the furnace is raised to the above-described range, and the portion of the raw material 9 and the seed crystal 8 that contacts the raw material 9 is melted, and then the power ratio of the heaters 4a to 4c is adjusted, thereby near the solid-liquid interface The temperature gradient in the growth axis direction is 5.0 ° C./cm to 6.0 ° C./cm, preferably 5.2 ° C./cm to 6 in the upward direction of the growth axis (from the bottom side to the top side of the crucible). It is controlled to 0.0 ° C./cm, more preferably 5.2 ° C./cm to 5.8 ° C./cm, and still more preferably 5.6 ° C./cm.

Here, the temperature gradient in the growth axis direction in the vicinity of the solid-liquid interface is the temperature gradient (solid phase) in the region of 2 cm in the vertical direction across the interface between the sapphire single crystal (solid phase) and the raw material melt (liquid phase). The temperature difference (T ₁ −T _s ) between the temperature T _s (° C.) and the liquid phase temperature T _l (° C.) is converted into a temperature difference per unit length. This temperature gradient can be controlled by adjusting the power ratio of the heaters 4a to 4c and the setting position of the crucible, and adjusting the state of heat conduction from the upper part to the lower part of the raw material melt. For example, lowering the lower end of the crucible below the lower end of the lower heater and exposing it to a lower temperature region can increase heat dissipation and control the temperature gradient.

  If the temperature gradient in the growth axis direction in the vicinity of the solid-liquid interface is less than 5.0 ° C./cm, the growth rate must be slowed in order to prevent the generation of bubbles, resulting in a problem that the growth cycle becomes long. On the other hand, in order to form a temperature gradient exceeding 6.0 ° C./cm in the growth axis direction in the vicinity of the solid-liquid interface, the temperature difference in the growth axis direction of the raw material melt must be increased. Problem arises that a heavy load is applied.

  In the prior art, when a sapphire single crystal is grown by a unidirectional solidification method, the bottom of the crucible is at an arbitrary temperature (about 1500 ° C.) at the temperature rising stage from the viewpoint of heat resistance of the thermocouple and simplicity of temperature measurement. In general, the temperature of the raw material melt is measured to adjust the temperature gradient.

  In contrast, in the present invention, the temperature gradient in the growth axis direction in the vicinity of the solid-liquid interface is adjusted from the initial stage of growth of the sapphire single crystal to the stage of completion of the growth. As a result, the temperature gradient during the growth of the sapphire single crystal can be controlled with high accuracy, and the generation of bubbles can be prevented while suppressing the residual stress.

  However, when assuming production on an industrial scale, it is practically impossible to directly measure the temperature gradient in the growth axis direction near the solid-liquid interface at this stage. In this regard, the present inventors calculated the temperature gradient in the growth axis direction in the crucible measured by measuring the temperature of the bottom of the crucible and the surface of the raw material melt using a non-contact type thermometer. It has been confirmed that the temperature gradient in the growth axis direction near the solid-liquid interface in the growth stage of the sapphire single crystal is substantially the same as the temperature gradient in the growth axis direction in the crucible. For this reason, in the present invention, the temperature gradient in the growth axis direction in the vicinity of the solid-liquid interface described above is determined by the growth in the crucible obtained from the surface temperature of the raw material melt and the temperature at the bottom of the crucible using a non-contact thermometer An axial temperature gradient will be substituted.

[Growth speed]
In the present invention, the growth rate of the sapphire single crystal during growth is 0.8 mm / hr to 1.3 mm / hr, preferably 0.8 to 1.2 mm / hr, more preferably 1.0 mm / hr to 1.mm. It is controlled in the range of 2 mm / hr. The growth rate of the sapphire single crystal is a value obtained by the following formula (A), where L is the total length of the finally obtained sapphire single crystal, L _{0 is} the total length of the seed crystal, and t is the growth time. means.
Safia single crystal growth rate = (L−L ₀ ) / t (A)

  If the growth rate of the sapphire single crystal is within such a range, the movement of the solid-liquid interface during growth can be sufficiently slowed down. Can be effectively suppressed from being taken into the grown crystal. If the growth rate is less than 0.8 mm / hr, the entrapment of bubbles can be suppressed, but the productivity is greatly deteriorated. On the other hand, if the growth rate exceeds 1.3 mm / hr, the number of bubbles taken into the grown crystal increases, making it impossible to obtain a high-quality sapphire single crystal.

(2) Cooling step In the cooling step, the furnace temperature is lowered to a constant temperature in the range of 1900 ° C to 2000 ° C, and the temperature gradient in the growth axis direction of the grown sapphire single crystal is changed from the bottom of the crucible to the upper surface of the sapphire single crystal. After adjusting the electric power supplied to the heaters 4a to 4c so as to become 1.9 ° C./cm or less toward the temperature, the temperature in the furnace is lowered at 10 ° C./hr to 30 ° C./hr while maintaining this temperature gradient. The sapphire single crystal is cooled to room temperature. By controlling the cooling conditions of the sapphire single crystal in this way, it is possible to diffuse the microbubbles taken into the sapphire single crystal into the crystal and release it outside in the growth process.

[In-furnace temperature]
After the growth step, the furnace temperature needs to be lowered to a constant temperature in the range of 1900 ° C to 2000 ° C, preferably 1900 ° C to 1950 ° C, more preferably 1900 ° C to 1920 ° C.

  The furnace temperature at this stage is required to be lower than the melting point of sapphire, but if it is lower than 1900 ° C., the microbubbles taken into the sapphire single crystal are difficult to diffuse and discharged to the outside. In addition, the thermal strain generated inside the sapphire single crystal cannot be suppressed, and cracks are likely to occur after cooling. On the other hand, when the furnace temperature at this stage exceeds 2000 ° C., the temperature gradient in the growth axis direction of the sapphire single crystal is adjusted to 1.9 ° C./cm or less from the bottom of the crucible toward the upper surface of the sapphire single crystal. It becomes difficult to control the heaters 4a to 4c.

[Temperature gradient in the growth axis direction of sapphire single crystal]
In the cooling step, the temperature in the furnace is lowered as described above, and the temperature gradient in the growth axis direction of the sapphire single crystal obtained by the growth step is 1.9 ° C./from the bottom of the crucible toward the upper surface of the sapphire single crystal. It is necessary to adjust the temperature gradient to not more than cm, preferably not more than 1.8 ° C./cm, and maintain this temperature gradient throughout the cooling process. The temperature gradient in the growth axis direction of the sapphire single crystal can be obtained by measuring the temperature of the upper surface of the grown sapphire single crystal and the temperature of the bottom of the crucible using a non-contact thermometer.

  When the temperature gradient in the growth axis direction of the sapphire single crystal exceeds 1.8 ° C./cm, the residual stress is not sufficiently relaxed depending on other cooling conditions, and the cooling rate is 30 ° C./hr or less. However, the residual stress generated at the time of growth cannot be sufficiently eliminated, and there is a risk of causing cracks. In particular, when the temperature gradient exceeds 1.9 ° C./cm, this tendency becomes remarkable. The lower limit of the temperature gradient is not particularly limited, but is preferably 0.2 ° C./cm or more, and preferably 0.4 ° C./cm or more from the viewpoint of securing the productivity of the sapphire single crystal. It is more preferable.

[Adjustment of temperature gradient in the growth axis direction of sapphire single crystal]
In the cooling step, the temperature in the furnace is preferably lowered to a constant temperature in the range of 1900 ° C. to 2000 ° C. over 3 hours or more, more preferably 5 hours or more, and the temperature gradient in the growth axis direction of the sapphire single crystal is 1 It is preferable to adjust to 9 ° C./cm or less. Thereby, generation | occurrence | production of the crack resulting from the thermal shock accompanying adjustment of a temperature gradient can be prevented effectively, without preventing the spreading | diffusion of the bubble in a sapphire single crystal.

  In addition, the means for adjusting the temperature gradient in the growth axis direction of the sapphire single crystal is not particularly limited, and may be performed by adjusting the heaters 4a to 4c. You may carry out by arrange | positioning in an inner soaking zone.

[Cooling rate]
In the cooling step, the temperature in the furnace is maintained at a cooling rate of 10 ° C./hr to 30 ° C./hr, preferably 20 ° C./hr to 30 ° C./hr, more preferably 30 ° C./hr while maintaining the temperature gradient described above. It is necessary to cool the sapphire and the single crystal to room temperature.

  When the cooling rate is less than 10 ° C./hr, sufficient productivity cannot be ensured. On the other hand, when the cooling rate exceeds 30 ° C./hr, the microbubbles taken into the sapphire single crystal are difficult to diffuse and are not sufficiently released outside the crystal. In addition, the residual stress cannot be sufficiently eliminated, and cracks may occur.

2. Sapphire Single Crystal The sapphire single crystal of the present invention is obtained by the production method described above, and is characterized by extremely few impurities and bubbles in the crystal (see FIGS. 2 and 3).

  Specifically, in the sapphire single crystal of the present invention, the impurity concentration caused by the crucible material or the like can be preferably 5 ppm by mass or less, more preferably 2 ppm by mass or less, and even more preferably 1 ppm by mass or less. . The impurity concentration of the sapphire single crystal can be measured by, for example, ICP-MS (inductively coupled plasma mass spectrometry).

Further, when the sapphire single crystal of the present invention is processed into a disc-shaped wafer, the number of pits and particles present on the surface per unit area (hereinafter referred to as “defect density”) is preferably 30 / cm ² or less, more preferably 22 pieces / cm ² or less. The defect density is the number of pits and particles per wafer after measuring the number of pits and particles present on the surface of all wafers processed into a disk shape using a wafer surface inspection device ( The average number can be calculated by dividing the average number by the surface area of the wafer.

Example 1
[Growth process]
A seed crystal 8 of a c-axis sapphire single crystal having an outer diameter of 150 mm, a height of 50 mm, and a mass of 3.6 kg is stored in the bottom of a tungsten crucible 6 having an inner diameter of 155 mm, a depth of 400 mm, and a thickness of 10 mm. After that, a total of 11.4 kg of raw material 9 consisting of a sapphire granule raw material and a sapphire single crystal end material was stored in the upper part. The crucible 6 was placed on the support shaft 7 of the growth furnace 1 and the chamber 2 was sealed. Then, nitrogen gas was introduced into the chamber 2 at a flow rate of 4 L / min. Thereafter, a total of 46 kW of electric power is applied to the heaters 4 a to 4 c, the temperature of the hot zone 5 in the chamber 2 (in-furnace temperature) is raised to 2040 ° C. The part in contact with was melted. Subsequently, using a non-contact type thermometer (manufactured by LumaSense Technologies, IMPAC ISR 12-L0), the temperature of the surface of the raw material melt 9 and the bottom of the crucible 6 is measured, and the temperature gradient in the growth axis direction near the solid-liquid interface Asked. The power ratio of the heaters 4a to 4c is set so that the temperature gradient becomes 5.6 ° C./cm from the bottom of the crucible 6 toward the surface of the raw material melt 9; the upper heater 4a: the middle heater 4b: the lower heater 4c. = 6: 3: 2 After standing in this state for about 16 hours and confirming that the furnace temperature and the temperature of the raw material melt 9 were stable, the growth rate was 1.2 mm / hr while maintaining the temperature gradient described above. The outputs of the heaters 4a to 4c were gradually reduced to grow sapphire single crystals.

[Cooling process]
In the furnace, the power ratio of the heaters 4a to 4c is changed so that the upper heater 4a: the middle heater 4b: the lower heater 4c = 1: 1: 2.5 over 3 hours after the completion of the growing process. While the temperature is lowered to 1900 ° C., the temperature gradient in the growth axis direction of the sapphire single crystal is 1.8 ° C./cm from the bottom of the crucible 6 toward the upper surface of the sapphire single crystal while maintaining this temperature. Adjusted. Thereafter, while maintaining this temperature gradient, the outputs of the heaters 4a to 4c are gradually reduced so that the cooling rate of the furnace temperature becomes 30 ° C./hr, and cooled to room temperature. The diameter is 153 mm and the total length is 200 mm. A sapphire single crystal having a mass of 15.0 kg was obtained.

[Evaluation]
The obtained sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of the wafer using a wafer surface inspection apparatus (Candela, manufactured by KLA-Tencor), an average of 1500 pieces per wafer (defect density: 8.5 pieces / cm ² ). It was confirmed that pits and particles were present. The result is shown in FIG. In FIG. 2, the colored portion represents defects such as pits and particles. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

In Examples 1 to 8 and Comparative Examples 1 to 6, based on these results, those having a defect density of 30 / cm ² or less and an impurity concentration of 5 mass ppm or less are referred to as “good (◯)”. evaluated. A defect density exceeding 30 / cm ² or an impurity concentration exceeding 5 ppm by mass was evaluated as “defective (×)”.

(Example 2)
The diameter was 153 mm and the total length was the same as in Example 1 except that the raw material stored in the tungsten crucible 6 was 11.3 kg and the cooling rate was 10 ° C./hr in the cooling step. A sapphire single crystal with a mass of 14.9 kg.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it was confirmed that an average of 1300 (7.4 / cm ² ) pits and particles existed per wafer. It was done. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Example 3)
In the growth step, a sapphire single crystal having a diameter of 153 mm, a total length of 200 mm, and a mass of 15.0 kg was obtained in the same manner as in Example 1 except that the growth rate of the sapphire single crystal was 1.0 ° C./hr. It was.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it was confirmed that 1200 (6.8 / cm ² ) pits and particles exist on average per wafer. It was done. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

Example 4
Except that the raw material stored in the tungsten crucible 6 was 11.5 kg and that the temperature in the furnace was lowered to 2000 ° C. and the temperature gradient was adjusted at this temperature in the cooling process. In the same manner as in Example 1, a sapphire single crystal having a diameter of 153 mm, a total length of 200 mm, and a mass of 15.1 kg was obtained.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that an average of 1500 (8.5 / cm ² ) pits and particles exist per wafer. confirmed. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Example 5)
In the growth step, the diameter was 153 mm, the total length was 200 mm, and the mass was 15.0 kg, except that the temperature gradient in the growth axis direction near the solid-liquid interface was 5.0 ° C./cm. A sapphire single crystal was obtained.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of the wafer using a wafer surface inspection apparatus, it is found that 2000 (11.3 / cm ² ) pits and particles exist on average per wafer. confirmed. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Example 6)
In the growth step, the diameter is 153 mm, the total length is 200 mm, and the mass is 15.0 kg, except that the temperature gradient in the growth axis direction near the solid-liquid interface is 6.0 ° C./cm. A sapphire single crystal was obtained.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that an average of 800 (4.5 / cm ² ) pits and particles exist per wafer. confirmed. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Example 7)
In the growth step, a sapphire single crystal having a diameter of 153 mm, a total length of 200 mm, and a mass of 15.0 kg was obtained in the same manner as in Example 1 except that the growth rate of the sapphire single crystal was 0.8 mm / hr. .

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that on average, 1100 (6.2 / cm ² ) pits and particles exist per one wafer. confirmed. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Example 8)
In the growth step, a sapphire single crystal having a diameter of 153 mm, a total length of 200 mm, and a mass of 15.0 kg was obtained in the same manner as in Example 1 except that the growth rate of the sapphire single crystal was 1.3 mm / hr. .

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that 3600 (20.4 / cm ² ) pits and particles exist on average per one wafer. confirmed. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Comparative Example 1)
Except that the raw material stored in the tungsten crucible 6 was 11.5 kg and that the growth rate of the sapphire single crystal was set to 1.5 ° C./hr in the growth step, the same as in Example 1. A sapphire single crystal having a diameter of 153 mm, a total length of 200 mm, and a mass of 15.1 kg was obtained.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that there are on average 10,000 (56.6 / cm ² ) pits and particles per one wafer. confirmed. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Comparative Example 2)
The diameter was 153 mm and the total length was the same as in Example 1 except that the raw material stored in the tungsten crucible 6 was 11.5 kg and that the cooling rate was 35 ° C./hr in the cooling step. A sapphire single crystal having a mass of 200 mm and a mass of 15.1 kg.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that there are on average 10,000 (56.6 / cm ² ) pits and particles per one wafer. confirmed. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Comparative Example 3)
In the cooling step, the temperature in the furnace was lowered to 1850 ° C., and the temperature gradient was adjusted at this temperature, as in Example 1, with a diameter of 153 mm, a total length of 200 mm, and a mass of 15.0 kg. A sapphire single crystal was obtained.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that an average of 30,000 (169.9 / cm ² ) pits and particles exist per one wafer. confirmed. Moreover, as a result of analyzing impurities contained in this wafer using ICP-MS, 1 mass ppm of tungsten was detected.

(Comparative Example 4)
In a crucible 6 made of a tungsten-molybdenum alloy (mass ratio, W: Mo = 1: 1) having a diameter of 155 mm and a depth of 400 mm, the diameter is 150 mm, the height is 50 mm, and the mass is 3.6 kg. In the same manner as in Example 1, except that the seed crystal 8 of the axial sapphire single crystal was accommodated and then a total of 11.3 kg of the raw material 9 composed of the sapphire granule raw material and the sapphire single crystal end material was accommodated in the upper part. Was 153 mm, the total length was 195 mm, and the mass was 14.9 kg.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that an average of 41,000 (232.1 / cm ² ) pits and particles exist per one wafer. confirmed. The result is shown in FIG. In FIG. 3, the colored portions represent pits and particles. Moreover, as a result of analyzing the impurity contained in this wafer using ICP-MS, 3 mass ppm tungsten and 10 mass ppm molybdenum were detected.

(Comparative Example 5)
In a crucible 6 made of a tungsten-molybdenum alloy (mass ratio, W: Mo = 7: 3) having a diameter of 155 mm and a depth of 400 mm, the diameter is 150 mm, the height is 50 mm, and the mass is 3.6 kg. In the same manner as in Example 1, except that the seed crystal 8 of the axial sapphire single crystal was accommodated and then a total of 11.3 kg of the raw material 9 composed of the sapphire granule raw material and the sapphire single crystal end material was accommodated in the upper part. Was 153 mm, the total length was 196 mm, and the mass was 14.9 kg.

This sapphire single crystal was processed into a wafer having an outer diameter of 150 mm and a thickness of 1 mm. As a result of observing the surface of this wafer using a wafer surface inspection apparatus, it is found that 34,000 (192.5 / cm ² ) pits and particles exist on average per one wafer. confirmed. Moreover, as a result of analyzing the impurity contained in this wafer using ICP-MS, 1 mass ppm tungsten and 6 mass ppm molybdenum were detected.

(Comparative Example 6)
In the cooling step, the diameter is 153 mm as in Example 1 except that the temperature gradient in the growth axis direction of the sapphire single crystal is 2.0 ° C./cm from the bottom of the crucible toward the top surface of the sapphire single crystal. A sapphire single crystal having a total length of 200 mm and a mass of 15.0 kg was obtained.

This wafer had many cracks and could not be processed into a wafer. For this reason, in this comparative example, the wafer surface inspection and the impurity concentration measurement were not performed.

1 Sapphire single crystal growth equipment (growing furnace)
2 Chamber 3 Heat insulating material 4 Cylindrical heater 4a Upper heater 4b Middle heater 4c Lower heater 5 Hot zone 6 Crucible 7 Support shaft 8 Seed crystal 9 Raw material (raw material melt)

Claims (4)

A method for producing a sapphire single crystal in which a seed crystal and a raw material are stored in a crucible made of tungsten, and crystal growth is performed by a vertical temperature gradient solidification method or a vertical Bridgman method,
After raising the furnace temperature to a constant temperature in the range of 2040 ° C. to 2060 ° C., at this temperature, the temperature gradient in the growth axis direction in the region of 2 cm in the vertical direction across the solid-liquid interface is 5.0 ° C./cm While adjusting the temperature to 6.0 ° C./cm and maintaining the temperature gradient, the raw material melt is crystallized while controlling the growth rate within the range of 0.8 mm / hr to 1.3 mm / hr, A growth process for obtaining a sapphire single crystal;
After the growth step, the temperature in the furnace is lowered to a constant temperature in the range of 1900 ° C. to 2000 ° C., and the temperature gradient in the growth axis direction of the sapphire single crystal is 1 from the bottom of the crucible toward the upper surface of the sapphire single crystal. A cooling step of adjusting the temperature to 9 ° C./cm or less and lowering the furnace temperature at 10 ° C./hr to 30 ° C./hr while maintaining the temperature gradient, and cooling the sapphire single crystal to room temperature;
A method for producing a sapphire single crystal.   The manufacturing method of the sapphire single crystal of Claim 1 which adjusts the said temperature gradient while lowering | hanging the said furnace temperature over 3 hours or more in the said cooling process.   A sapphire single crystal obtained by the production method according to claim 1 or 2 and having an impurity concentration of 5 mass ppm or less. The sapphire single crystal according to claim 3, wherein the number of pits and particles per unit area existing on the surface is 30 pieces / cm ² or less when processed into a disk shape.