JP2015027921A - Sapphire single crystal core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sapphire core that produces a sapphire single crystal substrate of a large diameter, in which a similar condition is applied to a substrate obtained from any location of the sapphire core and an irregular warp is not formed in subjecting a nitride-based compound to epitaxial growth.SOLUTION: A sapphire core has a length of 200 mm or more, a diameter of 150 mm or more, and a ratio of stress birefringence of an upper end surface and that of a lower end surface in the range between 1:1.2 and 1:0.8, preferably having concentric circular distributions of both the stress birefringence of the upper end surface and that of the lower end surface. The sapphire core is obtained by growing a c-axis sapphire single crystal having a body diameter of 150 mm or more by a Chokralsky method in a furnace having an insulation wall of a temperature gradient in the height direction up to 5 cm from the interface of sapphire melt in the range between -5°C/cm and -0.5°C/cm and cutting and processing the sapphire single crystal in the horizontal direction.

Description

  The present invention relates to a sapphire single crystal core for manufacturing a sapphire substrate used as an epitaxial growth substrate or an optical material.

  Sapphire (aluminum oxide) single crystals are widely used as nitride compound semiconductors, silicon epitaxial growth substrates, high-strength window materials, and the like. In recent years, demand for LEDs as LED televisions, LED lighting, and the like has been increasing from the viewpoint of energy saving, and thus demand for sapphire substrates for nitride-based compound semiconductor epitaxial growth has been increasing.

  Known methods for producing a sapphire single crystal include Bernoulli method, EFG (Edge-defined Film-fed Growth) method, Czochralski method, kiloporous method, HEM (Heat Exchange Method) method and the like.

  At present, the most common method for growing a sapphire single crystal which is a material for a large substrate having a diameter of 150 mm (which is generally referred to as a 6-inch substrate by those skilled in the art) is the kiloporous method. The kiloporous method is a kind of melt growth method. While the seed crystal in contact with the raw material melt surface is not pulled up or pulled up at an extremely slow speed of 1 mm / hour or less, the heater output is gradually lowered to remove the crucible. By cooling, this is a method of growing a single crystal under the surface of the raw material melt, and a large-diameter single crystal having excellent crystal characteristics can be obtained relatively easily.

  However, the kiloporous method is known to be greatly affected by the growth rate that varies depending on the crystal orientation. In the case of a sapphire single crystal, it is difficult to grow a crystal in the c-axis (Miller index <0001>) direction, In general, a seed crystal with the a-axis (<11-20>) facing the lower surface is brought into contact with a melt and crystal is grown in the a-axis direction (for example, Patent Document 1).

  In order to obtain the c-axis sapphire single crystal substrate from the sapphire single crystal having the a-axis as the growth direction, the c-axis which is the side direction of the single crystal using a core drill or the like. It is necessary to cut out a cylindrical body (core) having a desired diameter from the direction and cut it into a disk shape with a multi-wire saw or the like.

  The c-axis sapphire single crystal core obtained by cutting out from the side c-axis direction of the sapphire single crystal grown in the a-axis direction using the kiloporous method has a stress difference between both end portions and the center portion. This is because the stress of the crystal differs between the central portion and the outer peripheral portion, and the core obtained by cutting out from the side surface of the crystal inherits the stress distribution of the crystal. In addition, since the end face of the core has a plane perpendicular to the crystal growth direction, the stress distribution in the core plane direction may have irregular fluctuations along the crystal growth direction. Are known. On the other hand, according to the Czochralski method, it is relatively easy to grow a sapphire single crystal in the c-axis direction (for example, Patent Document 2). However, since the stress distribution of the crystal grown along the c-axis changes along the growth direction, there is a difference in stress between the upper part and the lower part of the crystal. Therefore, a stress difference is generated in the c-axis sapphire core produced from the c-axis sapphire single crystal in the core length direction.

JP 2004-83407 A JP 2010-143781 A

  When a substrate is made from a core having an irregular stress distribution in the core surface direction or a core having a stress difference in the core length direction, the resulting substrate also reflects the stress birefringence from the core, and the substrate The surface has an irregular stress birefringence distribution, and the stress birefringence becomes inhomogeneous depending on the cut-out position from the core.

  Such irregular and inhomogeneous stress distribution is particularly large when the substrate size is 6 inches or more, and the degree of warpage differs depending on the substrate during epitaxial growth of the nitride compound, and the substrate becomes wavy or wavy. This may cause irregular warpage such as occurrence. Due to the occurrence of such warpage, if a gap is formed between the heating body (susceptor) and a part of the substrate in the MOCVD apparatus, the substrate is not heated uniformly, so the film thickness and film composition of the nitride compound are not uniform. Resulting in a reduction in LED yield.

  Therefore, as a result of intensive studies in view of the above points, the inventors have determined that the temperature gradient in the height direction from the sapphire melt interface to 5 cm is −5 ° C./cm to −0.5 ° C./cm in the Czochralski method. The sapphire single crystal core having a diameter of 150 mm or more is obtained by growing the c-axis sapphire single crystal in a growth furnace configured with a heat insulating wall so as to fit in the range of It has been found that a c-axis sapphire single crystal core with little difference between stress birefringence at the upper end surface and stress birefringence at the lower end surface can be obtained with high productivity. Furthermore, the c-axis sapphire single crystal was cut horizontally to form a core, and the in-plane stress distribution was found to be a regular concentric circle, and the present invention was completed.

  That is, the present invention is a sapphire single crystal core having a length of 200 mm or more and a diameter of 150 mm or more, and the ratio of the stress birefringence of the upper end surface to the stress birefringence of the lower end surface is from 1: 1.2 to 1: 0. It is a sapphire core in the range of 8.

  Another invention is the above-described sapphire single crystal core in which the distribution of stress birefringence on the upper end surface and the lower end surface is concentric.

  According to the present invention, the large-scale c-axis sapphire substrate obtained by processing the c-axis sapphire single crystal core has the same degree of stress birefringence for each substrate regardless of where the substrate is cut out, and concentric stress distribution. Have By using the substrate, the warpage of the substrate during the epitaxial growth of the nitride-based compound can be made to the same and regular saddle type (dome type or reverse dome type).

  Then, by making the warpage to be the same and regular bowl shape, the heating body of the MOCVD apparatus is made to the same bowl shape, or the gas composition is changed concentrically with respect to the substrate so that the substrate has a large diameter. Even in this case, a uniform film can be formed, and the yield of LEDs can be dramatically improved.

The schematic diagram which shows the sapphire single-crystal core of this invention. The schematic diagram which shows the structure of the Czochralski method single crystal pulling furnace in an Example. The schematic diagram which shows the structure of an annealing furnace. An example of the processing process of a sapphire single crystal body. The stress birefringence distribution of the core obtained in the Example. The schematic diagram which shows the structure of the Czochralski method single crystal pulling furnace in a comparative example.

  The present invention relates to a sapphire core having a length of 200 mm or more and a diameter of 150 mm or more. As is well known to those skilled in the art, the sapphire core is a cylindrical member as shown in FIG. 1 and usually has an orientation flat (orientation flat) on a part of its side surface in order to align the orientation of the substrate after cutting (slicing). A notch called is provided. In the present invention, the diameter of 150 mm or more means that the diameter determined on the assumption that sapphire exists also in this notch is 150 mm or more. From another viewpoint, in consideration of the existence of this orientation flat, for example, if the diameter of the inscribed circle is 140 mm or more, the core itself is a core of 6 inches (150 mm) or more. The upper limit of the diameter is not particularly defined, but a diameter of 170 mm or less is preferable in consideration of the generation accuracy and usefulness of cracks and lineage of crystals in the production by the Czochralski method.

  The sapphire core of the present invention has two planes parallel to both ends (the upper end surface and the lower end surface). The c-axis of the sapphire core is preferably in the range of 90 ± 1 ° with respect to the plane.

  The sapphire core of the present invention has a length (height) in the direction perpendicular to the plane of 200 mm or more, more preferably 250 mm or more. The upper limit is not particularly defined, but it is preferably 500 mm or less in consideration of crystal cracking and cracking generation accuracy, usefulness, etc. in the production by the Czochralski method.

  The greatest feature of the sapphire core of the present invention is that the ratio between the stress birefringence at the upper end surface and the stress birefringence at the lower end surface is in the range of 1: 1.2 to 1: 0.8. Here, the stress birefringence of the upper and lower end surfaces is obtained by cutting out a sample having a thickness of 2 mm from the upper end and the lower end, irradiating the sample with a wavelength of 633 nm, and irradiating a laser beam of 633 nm for a range of 140 mm in diameter every 1 to 4 mm square. The stress birefringence distribution is measured from the phase difference of the obtained transmitted light (see FIG. 5), and the root mean square may be obtained. A device for obtaining the phase difference is commercially available as a birefringence / stress measuring device or the like.

  As described above, the stress birefringence of the upper end surface and the lower end surface is the same or approximate, so that the physical properties such as the warp of the substrate cut out from the sapphire core become the same even if cut out from any location, and the epitaxial growth It is possible to make the same conditions when performing the above. The ratio is more preferably 1: 1.1 to 1: 0.9.

  In the sapphire core of the present invention, the stress birefringence distribution on the upper end surface and the lower end surface is preferably concentric. As a result, the substrate cut out from the sapphire core also has a concentric stress birefringence distribution. As described above, the substrate having such physical properties can be warped in a regular saddle shape.

  Although the absolute value of stress birefringence is not particularly limited, it is preferable that the stress birefringence of the upper end surface and the lower end surface is in the range of 100 to 200 nm / cm, respectively.

  In addition to air bubbles that can be confirmed with room light such as fluorescent lamps, by irradiating the inside of the crystal with a high-intensity halogen lamp in a dark room, there are no microscopic bubbles that can be visually confirmed, or there are bubbles The length in the vertical direction is preferably 3% or less of the total length.

  Although the sapphire core of the present invention is a single crystal, it also has no subgrains that can be confirmed by X-ray topography, that is, a true single crystal or close to it. The measurement conditions of the X-ray topograph for confirmation of the subgrain are shown in Table 1 below.

  In the present invention, in a grayscale image photographed under the above-described conditions and represented by shades of 256 gradations of lightness 0 (black) to 255 (white), the surface having a boundary where the lightness differs by 16 or more (and to it) When the accompanying grain boundary is not recognized, it is assumed that there are no subgrains that can be confirmed by X-ray topography. Note that the determination can be made simply by the presence or absence of striae that can be observed by crossed Nicols or the like in a dark room.

  The above-described sapphire core of the present invention is heat-treated by a Czochralski method on an as-grown single crystal having a diameter of 150 mm or more and a length of 200 mm or more manufactured with the c-axis direction as the crystal growth direction (vertical direction). It can be obtained by performing processing such as cutting and grinding.

  As described above, in the conventional kiloporous method, the growth rate varies greatly depending on the crystal orientation. Therefore, in the sapphire single crystal, the crystal growth is performed with the a axis in the vertical direction from the viewpoint of productivity, that is, the growth rate. I have to do it. This is the cause of the stress birefringence in the core length direction of the finally obtained c-axis sapphire single crystal core being inhomogeneous and having an in-plane irregular stress birefringence distribution.

  On the other hand, the Czochralski method, which performs crystallization under a strong temperature gradient compared to the kiloporous method, has a small difference in growth rate depending on the crystal orientation, and crystal growth at almost the same growth rate in any crystal orientation. It can be performed. Therefore, a long c-axis sapphire core can be efficiently obtained by growing a sapphire single crystal in the vertical direction of the c-axis and cutting off the shoulder and tail.

  However, since the stress distribution of the sapphire single crystal grown on the c-axis changes along the growth direction, there is a difference in stress birefringence between the upper and lower portions of the crystal in the conventionally known method. Therefore, a stress difference occurs in the core length direction also in the c-axis sapphire core manufactured from the c-axis sapphire single crystal. When a substrate is manufactured from such a core, the in-plane stress distribution is concentric, but the degree of warpage varies depending on the substrate. As a result, the film thickness and film composition at the time of epitaxial growth are non-uniform, which causes a problem of reducing the yield of LEDs.

  According to the study by the present inventors, in order to obtain the sapphire core of the present invention, the conventional Czochralski method for obtaining a sapphire single crystal having the c-axis and a-axis growth directions is simply applied. When the sapphire single crystal is grown by the Czochralski method, the temperature gradient from the sapphire melt interface in the pulling direction is in the range of -5 ° C / cm to -0.5 ° C / cm. It is necessary. As a result, the sapphire single crystal can suppress a change in stress distribution in the growth direction, and a c-axis sapphire core having no difference in stress distribution between the upper end and the lower end of the core can be obtained with high productivity.

  FIG. 2 is an example (schematic diagram) of a crystal growth apparatus used when the sapphire core of the present invention is manufactured by the above-mentioned Czochralski method.

  This single crystal pulling apparatus includes a chamber 1 that constitutes a crystal growth furnace, and a single crystal pulling rod 2 that can be moved up and down by a drive mechanism (not shown) through an opening on the upper wall of the chamber. It is suspended. A seed crystal body 4 is attached to the tip of the single crystal pulling rod via a holder 3, and the seed crystal body is disposed on the central axis of the crucible 5. Further, a load cell 6 for measuring the crystal weight is provided at the upper end of the single crystal pulling apparatus.

  As the crucible 5, a known crucible can be used as a crucible used in the Czochralski method. In general, an opening viewed from the top is circular, has a cylindrical body, and has a flat bottom surface, a bowl shape, or an inverted conical shape. In addition, the material of the crucible is suitable for a material which can withstand the melting point of aluminum oxide as a raw material melt and has low reactivity with aluminum oxide, and iridium, molybdenum, tungsten, rhenium or alloys thereof are generally used. Used. In particular, it is preferable to use iridium or tungsten having excellent heat resistance.

  A heat insulating wall 7a is installed around the crucible so as to surround the bottom and outer periphery of the crucible. Further, a heat insulating wall 7b surrounding the side periphery of the single crystal pulling area above the crucible is provided. The heat insulating walls 7a and 7b can be used without limitation on known heat insulating materials or structures for heat insulation. A material, an alumina-based material, a carbon-based material, a reflective material in which metal plates such as tungsten and molybdenum are laminated, and the like can be used particularly suitably. Since the heat insulating wall used here is used in an environment where the temperature difference between the inner surface and the outer surface is very large, the material is likely to be significantly deformed and cracked by repeated heating and cooling. When the temperature gradient in the crystal growth region changes every moment due to such deformation and cracking of the heat insulating wall, stable crystal production becomes difficult. Therefore, the entire heat insulating wall is not composed of a single piece of material, but is composed of a combination of heat insulating materials divided into several parts. The change can be suitably reduced.

  The opening at the upper end of the heat insulating wall surrounding the single crystal pulling area includes an insertion hole for the single crystal pulling rod, an opening for imaging a single crystal body, a melt, and the like by an imaging device described later, and a two-color thermometer. As a result, the opening for measuring the temperature of the sapphire melt is at least closed by the perforated ceiling plate 8. Thereby, since the single crystal pulling area is accommodated in the single crystal pulling chamber formed by the heat insulating walls 7a and 7b and the ceiling plate 8, the heat retention is greatly improved. The ceiling board should just be formed with the well-known heat insulating material or the structure for heat insulation similarly to the heat insulation wall. Further, the ceiling plate is not necessarily flat and may have any shape as long as the upper end opening of the enclosure of the heat insulating wall is closed except for a perforated portion described later. For example, the shape other than the plate shape may be a truncated cone shape, an inverted truncated cone shape, a shade shape, an inverted shade shape, a dome shape, an inverted dome shape, or the like.

  The temperature gradient from the sapphire melt interface can be controlled by the construction method of the heat insulating walls 7a and 7b and the ceiling plate 8. As described above, in order to suppress the change in the stress distribution in the growth direction, the temperature gradient from the sapphire melt interface in the pulling direction to the upper 5 cm must be in the range of −5 ° C./cm to −0.5 ° C./cm. There is. By reducing the temperature gradient in the height direction, a crystal with no stress difference between the top and bottom of the crystal can be obtained, but the Czochralski method requires a certain temperature gradient in the height direction in principle, If the temperature gradient is too low, the diameter cannot be controlled and crystal growth becomes difficult. In this case, if the temperature gradient is lower than −0.5 ° C./cm, the diameter cannot be controlled and crystal growth becomes difficult. Preferably, it is in the range of -3 ° C / cm to -1 ° C / cm.

  The method of constructing the heat insulating wall for lowering the temperature gradient is not limited. For example, for each of the heat insulating walls 7a and 7b and the ceiling plate 8, the heat insulating material is thickened or thinned in whole or in part, the number of heat insulating materials is increased or decreased, and heat insulating materials having different heat insulating performance are used. Various methods can be taken. Moreover, you may combine these several methods.

  A high-frequency coil 9 is installed around the outer periphery of the heat insulating wall and approximately the height of the crucible. At that time, the temperature gradient may be controlled by changing the position of the high-frequency coil with respect to the crucible. A high frequency power source (not shown) is connected to the high frequency coil. The high-frequency power source is connected to a control device composed of a general computer, and the output is appropriately adjusted. In addition to analyzing the weight change of the load cell and adjusting the output of the high frequency power supply, the control device also controls the rotation speed of the crystal pulling shaft and crucible, the pulling speed, and the valve operation for gas inflow and outflow. It is common to do.

  A two-color thermometer 10 can be used to measure the temperature of the sapphire melt. In the illustrated embodiment, the two-color thermometer 10 is installed so that the sapphire melt is looked down vertically from the upper wall of the chamber. In general, thermocouples are used to measure the temperature in the furnace. However, when growing a sapphire single crystal, the furnace contains many atmospheres containing oxygen at 2000 ° C or higher, and the thermocouple is severely degraded. Have difficulty. Therefore, it is preferable to use a two-color thermometer capable of non-contact temperature measurement because it is optical.

  A window 11 is provided on the upper wall of the chamber for temperature measurement with a two-color thermometer. That is, the temperature of the sapphire melt is measured through the window. The material of the window material may be a material that transmits the visible light region, but a material such as quartz or calcium fluoride that absorbs less in the infrared region is preferably used so that it is difficult to be heated by radiation from the high temperature part. .

  The upper wall of the chamber is preferably provided with a window 12 for observing a seed crystal or a single crystal (not shown) with an imaging device 13 such as a camera. That is, the inside of the furnace is observed by optical means through the window. The material of the window material may be a material that transmits the visible light region, but a material such as quartz or calcium fluoride that absorbs less in the infrared region is preferably used so that it is difficult to be heated by radiation from the high temperature part. . The type of the imaging device for obtaining data by optical means may be a photosensitive camera using a silver salt reaction or the like, or an electronic camera using a photosensitive element such as a CCD or CMOS. It is preferable to use an electronic camera capable of using an image obtained by shooting the image.

  As a raw material for producing a sapphire single crystal core for a sapphire substrate for semiconductors, aluminum oxide (alumina) having a purity of 4N (99.99%) or more is usually used. Since impurities are mixed into or between the lattices of the sapphire single crystal and become the starting point of crystal defects, when raw materials with low purity are used, subgrains tend to occur and the crystals tend to be colored. The cause of coloration of the crystal is a color center (color center) caused by crystal defects formed by impurities, which indirectly indicates the number of crystal defects. In particular, since chromium as an impurity significantly affects the coloring, it is preferable to use a raw material having a chromium content of less than 100 ppm. Further, a material having a bulk density as high as possible is suitable because it can fill a crucible with a large amount of raw materials and suppress scattering of the raw materials in the furnace. The bulk density of a preferable raw material is 1.0 g / ml or more, more preferably 2.0 g / ml or more. As raw materials having such properties, those obtained by granulating aluminum oxide powder with a roller press or the like, and crushed sapphire (crackle, crush sapphire, etc.) are known.

  The raw material is charged into the crucible installed in the crystal growth furnace, and heated to obtain a raw material melt. The rate of temperature increase until the raw material reaches a molten state is not particularly limited, but is preferably 50 to 200 ° C./hour.

  A seed crystal mounted on a seed crystal holder at the tip of the crystal pulling shaft is brought into contact with the surface of the raw material melt and then gradually pulled to grow a single crystal. The temperature of the raw material melt at the portion where the seed crystal contacts when the single crystal pulling is performed is inevitably a temperature slightly lower than the melting point in order for the crystal to grow stably without causing abnormal growth ( It is known that it becomes a supercooling temperature). In the case of a sapphire single crystal, it is preferably carried out at a temperature of 2000 to 2050 ° C.

  The seed crystal used for pulling is a sapphire single crystal, and the tip vertical direction in contact with the raw material melt surface needs to be the c-axis.

  The shape of the tip that contacts the raw material melt when the c-axis is the tip tip vertical direction of the seed crystal is not particularly limited and may be constituted by an unspecified surface, but is preferably a plane of the c-plane. . Further, the side surface of the seed crystal is not particularly limited, and an arbitrary shape can be selected, but a cylindrical shape or a quadrangular prism shape is preferable.

  Moreover, it is common to have an enlarged part and / or a constricted part and / or a through-hole for holding with a holder above the seed crystal.

  Since the quality of the single crystal to be grown largely depends on the quality of the seed crystal, special attention must be paid to its selection. As the seed crystal, one having as few crystal imperfections as possible, called crystal defects and dislocations, is desirable. The quality of the crystal structure can be evaluated by using a method such as etch pit density measurement, AFM, or X-ray topography on the front end surface of the seed crystal or its vicinity. In addition, since the number of crystal defects tends to increase as the residual stress increases, it is also effective to select a crystal having a low degree of stress, such as crossed Nicol observation or stress birefringence.

  After contacting the seed crystal with the raw material melt, the shoulder (expanded portion) is formed by controlling the number of revolutions of the seed crystal and / or crucible, the pulling speed, the output of the high frequency coil, etc. After the diameter is expanded, the crystal is pulled up so as to maintain the crystal diameter.

  The size of the single crystal body to be produced is determined depending on the size of the single crystal body to be produced by exceeding the diameter of 150 mm by expanding the diameter. Generally, the larger the crystal diameter in the growth of the Czochralski method, There is a tendency to generate subgrains and minute bubbles. Therefore, for example, from the viewpoint of mass-producing a 6-inch class substrate, the thickness is preferably 150 to 170 mm.

  The pulling can usually be performed at a speed of 0.1 to 20 mm / hour, but if the pulling speed is too low, the amount of crystal growth per unit time is reduced and the productivity is deteriorated, and if the pulling speed is too high, it is grown. Environmental fluctuations are large and subgrains and minute bubbles are likely to occur. Considering the compatibility between productivity and crystal quality, the pulling speed is preferably 0.5 to 10 mm / hour, more preferably 1 to 5 mm / hour.

  During the growth of the single crystal, the seed crystal is preferably rotated at a speed of 0.1 to 30 revolutions / minute about the pulling axis. In addition to the rotation of the seed crystal, the crucible may be rotated at the same rotational speed in the opposite direction or the same direction as the rotation direction of the seed crystal.

  The furnace pressure during the pulling of the single crystal may be any of under pressure, normal pressure, and reduced pressure, but it is convenient to carry out under normal pressure. The atmosphere is preferably an inert gas such as nitrogen or argon, or an atmosphere containing oxygen in an amount of 0 to 10% by volume in the inert gas.

  The length of the straight body portion of the single crystal is preferably 200 mm or more, and more preferably 250 mm or more so that it can be efficiently processed with a multi-wire saw. When the length of the straight body is less than 200 mm, in order to cut efficiently with a multi-wire saw, an additional process is performed in which a plurality of cores are joined together with a precise orientation and cut with a multi-wire saw after the total length is 200 mm or more. This is not preferable because it reduces manufacturing efficiency and increases manufacturing cost. On the other hand, if the length exceeds 500 mm, the temperature environment change in the hot zone in the furnace being grown tends to be too large, and stable growth tends to be difficult.

  Thus, after pulling up the sapphire single crystal having the desired straight body diameter and length and having the c-axis in the vertical direction, the single crystal is separated from the raw material melt. The method of separating the single crystal from the raw material melt is not particularly limited, such as a method of separating by increasing the heater output (increasing the temperature of the raw material melt), a method of separating by increasing the crystal pulling speed, a method of separating by lowering the crucible, etc. Any method may be adopted. In order to reduce the temperature fluctuation (heat shock) at the moment when the single crystal is separated from the raw material melt, the crystal diameter is gradually decreased by gradually increasing the heater output or gradually increasing the crystal pulling speed. It is effective to perform tail processing.

  The single crystal separated from the raw material melt is cooled to a temperature at which it can be taken out from the furnace. A faster cooling rate can increase the productivity of the growth process, but if it is too fast, the stress strain remaining inside the single crystal will increase, causing crushing and cracking during cooling and later processing, There is a possibility that an abnormal warpage may occur in the substrate obtained. On the other hand, if the cooling rate is too slow, the time for occupying the crystal growth furnace becomes long, and the productivity of the growth process decreases. Considering these, the cooling rate is preferably 10 to 200 ° C./hour.

  The sapphire single crystal produced in this way can be subjected to heat treatment (ingot annealing) as necessary. The purpose of the heat treatment includes prevention of cracking during cutting, reduction of crystal stress, improvement of crystal defects and coloring, and the like. FIG. 3 is an example (schematic diagram) of an annealing apparatus used for the heat treatment of the present invention.

  In this annealing apparatus, a container 16 for storing a single crystal body 15 is installed inside a chamber 14, and a heating body 17 is installed so as to surround the container. The container 16 and the heating body 17 that store the single crystal body are stored in a heat insulating region constituted by a heat insulating wall 18 that surrounds the ceiling, the bottom, and the outer periphery.

  The material of the container 16 for storing the single crystal can be used without particular limitation as long as it can withstand the temperature and atmosphere during the heat treatment. Specifically, metal materials such as iridium, molybdenum, tungsten, rhenium or a mixture thereof, zirconia and hafnia materials including those stabilized by adding yttrium, calcium, magnesium, etc., or alumina, boron nitride It can be selected from materials such as aluminum nitride, and heat insulating materials such as carbon heat insulating materials.

  The method and jig for installing the single crystal 15 in the container 16 are not particularly limited, and any known method can be adopted. As an example, as shown in FIG. 3, aluminum oxide powder is spread on the bottom of the container, and the shoulder or tail of the single crystal body is buried therein, so that the container can be stably installed.

  As the heating body 17 for heating the heat retaining zone to an arbitrary temperature, a heating body by a known heating method can be adopted. Specifically, by using a resistance heating method using carbon, tungsten or the like as a heating body, it is possible to stably heat up to around 2000 ° C.

  As a material for the heat insulating wall 18 constituting the heat retaining region, a known heat insulating material that can withstand the temperature during heat treatment and has no reactivity or corrosiveness to the atmosphere can be arbitrarily selected and used. Specifically, zirconia-based and hafnia-based materials including those stabilized by adding yttrium, calcium, magnesium, etc., or heat-insulating materials such as alumina-based materials and carbon heat insulating materials can be suitably used. However, oxide heat insulators such as zirconia, hafnia, and alumina may react and become brittle or release metal impurities in an atmosphere containing hydrogen, and carbon heat insulators react and become brittle in an atmosphere containing oxygen. Be careful because it may burn or burn.

  In the heat treatment of the sapphire single crystal, the maximum temperature and atmosphere, the holding time at the maximum temperature, the temperature rising / cooling rate, and the like are appropriately adjusted according to the purpose. Specifically, for the purpose of preventing cracking during cutting and reducing stress in the crystal, the maximum temperature is 1400 to 2000 ° C. and the maximum temperature is maintained under vacuum exhaust or any gas atmosphere. It is preferable that the heating rate is 20 to 200 ° C./hour and the cooling rate is 1 to 50 ° C./hour for 6 to 48 hours. In addition, when the purpose is to improve crystal defects and coloring, in an arbitrary gas atmosphere containing 1 to 10% oxygen (oxidizing atmosphere), or in an arbitrary gas atmosphere containing 1 to 10% hydrogen under vacuum exhaust ( In a reducing atmosphere, the crystal quality can be suitably improved by carrying out the heat treatment at a maximum temperature of 1400 to 1850 ° C. and an arbitrary maximum temperature holding time and a temperature raising / cooling rate.

  A well-known process can be used for the process of processing a sapphire single crystal into a sapphire core. FIG. 4 is an example of a process for processing a sapphire single crystal.

  Specifically, as shown in FIG. 4A, the straight body part to be a product is left and the shoulder part and the tail part are cut. Thereafter, as shown in FIG. 4 (b), cylindrical grinding is performed in order to remove the irregularities on the side surface of the straight body part and form a cylindrical shape with a constant diameter. Furthermore, as shown in FIG.4 (c), the flat part called orientation flat is formed in the specific direction of a straight body part side surface, and it is set as a sapphire core.

  The cutting means shown in the above (a) is not particularly limited, and cutting means using a cutting blade, high-pressure water, laser or the like can be exemplified. Among them, a cutting blade such as an inner peripheral blade, an outer peripheral blade, a band saw or a wire saw, and an endless cutting blade such as a band saw or a wire saw can be used preferably.

  The c-axis sapphire core having a diameter of 150 mm (excluding the orientation flat portion) or more and a length of 200 mm or more obtained as described above is a multi-chip for cutting a general sapphire core of 300 mm or more without requiring additional steps such as joining. It can be cut with a wire saw, and a c-plane sapphire substrate can be produced efficiently.

  The sapphire core having a diameter of 150 mm (excluding the orientation flat portion) or more obtained as described above has a ratio of stress birefringence of the upper end surface to stress birefringence of the lower end surface of 1: 1.2 to 1: 0.8. It is in the range and has uniform distortion characteristics in a concentric circle shape. Therefore, since the substrate obtained by processing the core has the same warpage and a regular saddle shape, a uniform film can be formed even if the substrate has a large diameter, which is extremely suitable for epitaxial growth. Can be used for

  Hereinafter, embodiments of the present invention will be described in more detail with specific experimental examples, but the present invention is not limited thereto.

Example First, in order to make the temperature gradient in the height direction from the sapphire melt interface to 5 cm within a range of −5 ° C./cm to −0.5 ° C./cm, the heat insulating wall in the furnace as shown in FIG. Configured. Next, 50 kg of high-purity alumina (AKX-5 manufactured by Sumitomo Chemical Co., Ltd.) having a purity of 4N (99.99%) was charged as a starting material into an iridium crucible. The crucible containing the raw material is placed in a high-frequency induction heating type Czochralski type crystal pulling furnace, and the inside of the furnace is evacuated to 100 Pa or less, and then nitrogen gas containing 0.5 vol% oxygen is introduced to atmospheric pressure. did. After reaching atmospheric pressure, the gas having the same composition as above was introduced into the furnace at 2.0 L / min, and evacuation was performed so that the furnace pressure was maintained at atmospheric pressure.

  The heating of the crucible was started and gradually heated over 9 hours until reaching the temperature at which the aluminum oxide raw material in the crucible melted. After adjusting the heater output appropriately with reference to the state of convection on the surface of the raw material melt (spoke pattern; visible through the camera 13) and the temperature of the sapphire melt measured with a two-color thermometer, a square columnar shape with the c-face at the tip The seed crystal of the sapphire single crystal was gradually lowered while rotating at a speed of 1 revolution / minute, and the tip of the seed crystal was brought into contact with the raw material melt. The heater output was further finely adjusted so that the seed crystal did not melt and the crystal did not grow on the surface of the raw material melt, and then the seed crystal was started to be pulled at a pulling rate of 1.5 mm / hour.

  Crystal growth was performed while appropriately controlling the heater output so that the crystal diameter estimated from the observation of the crystal by the camera and the change in the weight of the load cell became the desired diameter. After the diameter expansion to 165 mm is completed, the pulling speed is increased to 3 mm / hour while maintaining the diameter of 160 to 170 mm, and when the length of the straight body reaches 170 to 180 mm, 2 mm / Continued to pull up in time. After the length of the straight body part reached 300 mm, the heater output was gradually increased to perform tail treatment, and then the single crystal was separated from the raw material melt at a pulling rate of 10 mm / min.

  The separated single crystal was cooled to room temperature over 30 hours. As a result, a sapphire single crystal having a diameter of 160 mm and a length of the straight body portion of 300 mm, the vertical direction of which coincides with the c-axis direction, was obtained. When this single crystal was observed with a high-intensity halogen lamp in a dark room, no bubbles or the like were observed inside the crystal, and no striae or the like was observed even in crossed Nicol observation.

  The above single crystal was placed in the heating furnace for ingot annealing shown in FIG. 3 and heated to 1600 ° C. over 20 hours while flowing argon gas at a rate of 3 L / min. Then, after hold | maintaining at 1600 degreeC for 24 hours, it cooled to room temperature over 35 hours.

  After the ingot annealing, the upper part of the crystal (shoulder part) and the lower part of the crystal (tail part) were cut with a band saw, and the upper and lower cut surfaces of the straight body of the single crystal body were adjusted to a c-plane with a surface grinder. Then, after making it cylindrical shape with a diameter of 150 mm with a cylindrical grinding device, an orientation flat was formed on a predetermined side surface, and a sapphire single crystal core without a bubble having an axial direction of c axis with a diameter of 150 mm and a length of 300 mm was obtained.

  A wafer-like sample having a diameter of 150 mm and a thickness of 2 mm was cut out immediately from the upper and lower ends of the sapphire single crystal core, and the sample was subjected to surface grinding, lapping, and polishing. The obtained sample was subjected to measurement of stress birefringence distribution in a range of 140 mm in diameter by using EXICOR (light source wavelength: 633 nm) manufactured by HINDS. As a result, a concentric stress distribution as shown in FIG. 5 was shown. Furthermore, as shown in Table 2, it was confirmed that the ratio of stress birefringence at the upper end surface and stress birefringence at the lower end surface was in the range of 1: 1.2 to 1: 0.8.

Comparative Example The heat insulating wall in the furnace was configured as shown in FIG. 6 so that the temperature gradient in the height direction from the sapphire melt interface to 5 cm was higher than −5 ° C./cm. Specifically, the heat insulating walls of the outermost layers 7a and 7b were removed from the configuration of the heat insulating wall of the example, and the thickness of the ceiling plate 8 was halved. A sapphire single crystal having a c-axis in the vertical direction and having a diameter of 160 mm and a length of the straight body portion of 300 mm was obtained in the same manner as in Example except that the configuration of the heat insulating wall was different.

  When the above single crystal was annealed and cut in the same manner as in the examples, a sapphire single crystal core having an axial direction of c axis and a diameter of 150 mm and a length of 300 mm could be obtained. Further, as in the example, a wafer-like sample having a diameter of 150 mm and a thickness of 2 mm was obtained from the immediate vicinity of the upper and lower ends of the sapphire single crystal core, and the stress birefringence distribution was measured for a range of 140 mm in diameter. Thus, the ratio of the stress birefringence at the upper end surface to the stress birefringence at the lower end surface was 3.4.

1: chamber 2: single crystal pulling rod 3: seed crystal holder 4: seed crystal body 5: crucible 6: load cell 7a, 7b: heat insulating wall 8: ceiling plate 9: high frequency coil 10: two-color thermometer 11: window 12: Window 13: Camera 14: Chamber 15: Single crystal 16: Container 17: Heating body 18: Thermal insulation wall

Claims (3)

  A sapphire core having a length of 200 mm or more and a diameter of 150 mm or more, wherein the ratio of stress birefringence at the upper end surface to stress birefringence at the lower end surface is in the range of 1: 1.2 to 1: 0.8. .   2. The sapphire core according to claim 1, wherein the distribution of stress birefringence on the upper end surface and the lower end surface is concentric.   The sapphire core according to claim 1 or 2, wherein stress birefringence of the upper end surface and the lower end surface is in a range of 100 to 200 nm / cm, respectively.

Images