JP2016013949A - Method of manufacturing single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a single crystal of high quality by suppressing thermal stress applied from a pedestal to a seed crystal substrate during crystal growth.SOLUTION: A seed crystal substrate 1 has, on a reverse side PB, a first direction SX of a first coefficient of thermal expansion and a second direction of a second coefficient of thermal expansion larger than the first coefficient of thermal expansion. An intermediate member 31 includes one graphite plate 3 forming a bearing surface PS. The intermediate member 31 has, on the bearing surface PS, a third direction of a third coefficient of thermal expansion and a fourth direction of a fourth coefficient of thermal expansion larger than the third coefficient of thermal expansion. A process of arranging the intermediate member 31 on the seed crystal substrate 1 is so performed that the third direction CX is close to a direction SX between the direction SX and a direction SY, and the fourth direction CY is close to a direction SY between the direction SX and direction SY.

Description

  The present invention relates to a method for producing a single crystal, and more particularly to a method for producing a single crystal using a seed crystal substrate.

  Silicon carbide (SiC) is known as a semiconductor material having excellent thermal and chemical properties, and also having excellent electrical characteristics because the band gap is larger than that of silicon (Si). . In particular, polytype 4H-type SiC has high practicality as a semiconductor substrate for power devices because of its high electron mobility and saturated electron velocity. SiC substrates having a diameter of about 150 mm (6 inches) are commercially available. However, these generally have a high crystal defect density, and it is desired to reduce the crystal defect density for use as a semiconductor device. Therefore, it is required to improve the quality of the SiC single crystal from which the SiC substrate is cut out.

  As a method for obtaining a single crystal as a semiconductor, especially a SiC single crystal, a sublimation recrystallization method is widely used. In the sublimation recrystallization method, the seed crystal and the raw material are placed facing each other in the crucible, the inside of the crucible is heated to about 2300 ° C., which is the sublimation point of the raw material, and the gas sublimated from the raw material is regenerated on the seed crystal. This is a method of growing a crystal by crystallization. As a method for heating the crucible, a method using induction heating by high frequency is common. For this reason, graphite is generally used as the crucible material from the viewpoint of conductivity and heat resistance. As the shape of the crucible, a cylindrical shape is used in order to suppress temperature non-uniformity due to the skin effect due to high-frequency induction heating and to facilitate processing. Furthermore, the crucible is covered with a heat insulating material in order to efficiently heat by suppressing thermal radiation from the crucible. This heat insulating material is not electrically conductive and needs to withstand a high temperature of about 2400 ° C., and is made of felt-like graphite.

  In order to obtain a high quality crystal by the sublimation recrystallization method, it is necessary to precisely control the temperature of the seed crystal and the grown crystal in the crystal growth. For that purpose, it is important to efficiently remove the heat generated by the recrystallization of the sublimation gas from the seed crystal and the grown crystal. Therefore, in order to ensure heat conduction between the seed crystal and the pedestal, it is desirable that the seed crystal and the pedestal are in close contact with each other, and the seed crystal is generally attached to the graphite pedestal using an adhesive. Has been done. However, when an adhesive is used, due to the difference in the thermal expansion coefficient between the single crystal and the pedestal, thermal stress is applied to the seed crystal and the crystal during high temperature processing such as crystal growth, resulting in crystal defects or crystal It may be damaged. Further, not only the sublimation recrystallization method but generally the quality of a single crystal can be greatly influenced by the mounting state of a seed crystal used for the growth.

  As a method of relieving the thermal stress applied to the seed crystal and the grown crystal, the pedestal is constituted by a multilayer structure of three or more graphite members, and the average value of the thermal expansion coefficients of the respective graphite members is brought close to the thermal expansion coefficient of the seed crystal. Thus, there is known a method for suppressing the generation of thermal stress in a SiC single crystal due to the difference in thermal expansion coefficient between SiC and graphite (see, for example, Patent Document 1).

JP 2009-120419 A

  By configuring the pedestal as a multilayer structure as shown in Patent Document 1, the thermal expansion coefficient of the pedestal can be effectively brought close to the thermal expansion coefficient of the seed crystal substrate. However, the coefficient of thermal expansion of the SiC single crystal differs depending on the crystal orientation, and the technique of the above-mentioned patent document does not consider this point. For example, when a seed crystal having a {1100} plane is used, since the thermal expansion coefficients of the <0001> direction and the <1120> direction are different on the bonding surface of the seed crystal, unless the pedestal is designed in consideration of the direction. Inevitably, there is a mismatch in thermal expansion coefficient between the pedestal and the seed crystal substrate. As a result, the seed crystal and the grown crystal are subjected to a considerable amount of thermal stress from the pedestal.

  The present invention has been made to solve the above-described problems, and its purpose is to obtain a higher quality single crystal by suppressing thermal stress applied to the seed crystal substrate from the pedestal during crystal growth. It is to provide a method for producing a single crystal.

  The method for producing a single crystal of the present invention is a method for producing a single crystal by performing crystal growth at a crystal growth temperature higher than room temperature, and includes the following steps.

  A step of preparing a seed crystal substrate having a growth surface and a back surface opposite to the growth surface is performed. The seed crystal substrate has a first direction having a first thermal expansion coefficient at the crystal growth temperature and a second direction having a second thermal expansion coefficient larger than the first thermal expansion coefficient at the crystal growth temperature on the back surface. And have.

  A step of disposing an intermediate member having a support surface for supporting the seed crystal substrate and an attachment surface opposite to the support surface on the back surface of the seed crystal substrate is performed. The intermediate member includes at least one graphite plate. The at least one graphite plate includes one graphite plate that forms a support surface. The step of arranging the intermediate member includes a step of bonding the support surface to the back surface of the seed crystal substrate. The intermediate member has a third direction having a third thermal expansion coefficient at the crystal growth temperature and a fourth direction having a fourth thermal expansion coefficient larger than the third thermal expansion coefficient at the crystal growth temperature on the support surface. And have. The step of disposing the intermediate member includes the third direction being closer to the first direction out of the first direction and the second direction, and the fourth direction being the first out of the first direction and the second direction. It is performed so as to be closer to the direction of 2.

  A step of attaching the attachment surface of the intermediate member to the pedestal is performed. A step of performing crystal growth at the crystal growth temperature is performed on the growth surface of the seed crystal substrate supported by the pedestal via the intermediate member.

  According to the present invention, the intermediate member is attached to the seed crystal substrate so as to face the direction in consideration of both the anisotropy of the thermal expansion coefficient of the intermediate member and the anisotropy of the thermal expansion coefficient of the seed crystal substrate. The intermediate member thus attached suppresses thermal stress applied to the seed crystal substrate from the pedestal during crystal growth. Thereby, a high quality single crystal can be obtained.

It is a perspective view which shows roughly the structure of the seed crystal substrate prepared in the manufacturing method of the single crystal in Embodiment 1 of this invention. It is sectional drawing which shows roughly the grinding | polishing process of the growth surface of a seed crystal substrate in the manufacturing method of the single crystal in Embodiment 1 of this invention. It is sectional drawing which shows roughly the grinding | polishing process of the back surface of a seed crystal substrate in the manufacturing method of the single crystal in Embodiment 1 of this invention. It is a perspective view which shows roughly the structure of the one graphite board prepared as an intermediate member in the manufacturing method of the single crystal in Embodiment 1 of this invention. It is sectional drawing which shows schematically the structure of the graphite block from which one graphite board is cut out in the manufacturing method of the single crystal in Embodiment 1 of this invention. It is sectional drawing which shows roughly the process of adhere | attaching the support surface of one graphite substrate on the back surface of a seed crystal substrate in the manufacturing method of the single crystal in Embodiment 1 of this invention. It is sectional drawing which shows roughly the baking process of the adhesive agent hardening layer in the manufacturing method of the single crystal in Embodiment 1 of this invention. FIG. 8 is a cross-sectional view schematically showing a seed crystal substrate and one graphite substrate as an intermediate member bonded to each other by an adhesive layer formed by the step of FIG. 7. It is sectional drawing which shows roughly the process of attaching the base and the attachment surface of an intermediate member with an adhesive hardening layer in the manufacturing method of the single crystal in Embodiment 1 of this invention. FIG. 10 is a cross-sectional view schematically showing a state where the adhesive cured layer of FIG. 9 is an adhesive layer. It is sectional drawing which shows schematically the mode of the crystal growth in the manufacturing method of the single crystal in Embodiment 1 of this invention. It is sectional drawing which shows roughly 1 process of the manufacturing method of the single crystal in Embodiment 2 of this invention. It is sectional drawing which shows roughly 1 process of the manufacturing method of the single crystal in Embodiment 2 of this invention. It is sectional drawing which shows roughly 1 process of the manufacturing method of the single crystal in Embodiment 2 of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

  According to the crystallographic notation, [] indicates the direction individually, <> indicates the equivalent direction comprehensively, () indicates the surface individually, and {} indicates the equivalent surface comprehensively. In order to indicate the negative component of the index, a minus sign in front of the number is used in place of the overline on the number due to restrictions on notation in the application procedure.

<Embodiment 1>
(Seed crystal substrate)
Referring to FIG. 1, seed crystal substrate 1 having a growth surface PG and a back surface PB opposite to growth surface PG is prepared. The seed crystal substrate 1 has, for example, a disk shape with a diameter of about 75 mm (3 inches). The seed crystal substrate 1 is made of SiC. The seed crystal substrate 1 has a hexagonal crystal structure, and thus has a crystal direction defined therein by the <11-20> direction, the <1100> direction, the <0001> direction, and the like. The polytype of the seed crystal substrate 1 is 4H.

  The growth surface PG and the back surface PB have {11-20} surfaces. In other words, the growth surface PG and the back surface PB include the <1100> direction and the <0001> direction in the present embodiment.

  The seed crystal substrate 1 has a direction SX (first direction) and a direction SY (second direction) on the back surface PB. In the present embodiment, the directions SX, SY, and SZ correspond to the <0001> direction, the <1100> direction, and the <11-20> direction that are orthogonal to each other. Therefore, the directions SX, SY and SZ form a three-dimensional orthogonal coordinate system.

In the seed crystal substrate 1, the direction SX has a thermal expansion coefficient α _SX (first thermal expansion coefficient) at the crystal growth temperature, and the direction SY has a thermal expansion coefficient α _SY larger than the thermal expansion coefficient α _SX at the crystal growth temperature. (Second thermal expansion coefficient). Here, the crystal growth temperature is a temperature approximately corresponding to a temperature used for crystal growth performed using the seed crystal substrate 1 as described later, and is approximately 2000 ° C. in the present embodiment.

  Referring to FIG. 2, seed crystal substrate 1 is bonded onto ceramic polishing plate 2 so that growth surface PG of seed crystal substrate 1 is exposed. The growth surface PG of the seed crystal substrate 1 supported by the polishing plate 2 is polished using diamond abrasive grains. Next, the damaged layer existing on the growth surface PG is removed by CMP (Chemical Mechanical Polishing). Thereafter, the seed crystal substrate 1 is removed from the polishing plate 2. Next, the seed crystal substrate 1 is cleaned.

  Referring to FIG. 3, seed crystal substrate 1 is affixed on ceramic polishing plate 2 so that back surface PB of seed crystal substrate 1 is exposed. The back surface PB of the seed crystal substrate 1 supported by the polishing plate 2 is roughened by a polishing process or the like. Thereby, the adhesiveness of the back surface PB in the case where the adhesive is applied is improved by increasing the surface area of the back surface PB. Thereafter, the seed crystal substrate 1 is removed from the polishing plate 2. Next, the seed crystal substrate 1 is cleaned.

(About graphite plates as intermediate members)
Referring to FIG. 4, graphite plate 3 (one graphite plate) is prepared as intermediate member 31 in the present embodiment. That is, in the present embodiment, the intermediate member 31 is composed of a single graphite plate 3, and the graphite plate 3 forms a support surface PS of the intermediate member 31 and a mounting surface PF opposite to the support surface PS. . The graphite plate 3 preferably has a thickness of 0.2 mm or more and 1.5 mm or less, for example, a thickness of about 1 mm. The intermediate member 31 has a direction CX (third direction) having a thermal expansion coefficient α _CX (third thermal expansion coefficient) at the crystal growth temperature and a larger thermal expansion coefficient α _CX at the crystal growth temperature on the support surface PS. And a direction CY (fourth direction) having a thermal expansion coefficient α _CY (fourth thermal expansion coefficient). The directions CX, CY and CZ correspond to the respective axes of the three-dimensional orthogonal coordinate system. Therefore, in the present embodiment, the angle between directions SX and SY (FIG. 1) and the angle between directions CX and CY (FIG. 4) are both 90 ° and are equal to each other.

In relation to the thermal expansion coefficients α _SX and α _SY of the seed crystal substrate 1, the thermal expansion coefficient α _CX has a difference of 0.5% or less with respect to the thermal expansion coefficient α _SX , and the thermal expansion coefficient α _CY preferably has a difference of 0.5% or less with respect to the thermal expansion coefficient α _SY . In this case, if there is an anisotropy of about 1% or more between the thermal expansion coefficient α _SX and the thermal expansion coefficient α _SY , the thermal expansion coefficient α _CX exceeds 0.5% with respect to the thermal expansion coefficient α _SY . And the coefficient of thermal expansion α _CY has a difference of more than 0.5% with respect to the coefficient of thermal expansion α _SX .

  Referring to FIG. 5, graphite plate 3 is formed by being cut out from graphite block 3L. The shape of the graphite block 3L is, for example, a rectangle of about 300 mm × 600 mm × 1000 mm. Although the graphite block is generally marketed as an isotropic member, it has a certain degree of anisotropy in terms of thermal expansion coefficient of less than 10%. Therefore, the thermal expansion coefficient of the graphite block 3L at the measurement temperature (one temperature) is measured in at least two directions. The measurement temperature is preferably 400 ° C. or higher. Based on the result, the graphite plate 3 is cut out from the graphite block 3L so that the graphite plate 3 has a thermal expansion coefficient as close as possible to that of the seed crystal substrate 1 having anisotropy.

  In order to accurately know the coefficient of thermal expansion at the crystal growth temperature, it is preferable to perform measurement using the crystal growth temperature as the measurement temperature. However, when the crystal growth temperature is high, measurement at a high temperature is required, which may increase the time and cost required for the measurement. For this reason, the measurement may be performed at a lower temperature, particularly a temperature lower than 2000 ° C., and the thermal expansion coefficient at a crystal growth temperature of 2000 ° C. or higher may be estimated using the result.

  In the present embodiment, it is desirable to measure the thermal expansion coefficient at 2000 ° C. (corresponding to the crystal growth temperature) in three directions X, Y and Z orthogonal to each other of the graphite block 3L. A thermal expansion coefficient at 2000 ° C. can be estimated by extrapolating the measurement results up to about 0 ° C. Based on this result, directions CX, CY and CZ are determined, and the graphite plate 3 is cut out from the graphite block 3L. Each of the directions CX, CY, and CZ may be the same as any of the directions X, Y, and Z, or may be a direction synthesized from two or three of the directions X, Y, and Z. If the relationship between the coordinate system based on the directions X, Y and Z and the coordinate system based on the directions CX, CY and CZ is determined, the direction CX of the graphite plate 3 is determined based on the measurement result of the thermal expansion coefficient of the graphite block 3L. And the coefficient of thermal expansion in each of CY and CY can be calculated or approximated. The directions CX and CY are such values that the thermal expansion coefficients at the crystal growth temperatures in the directions CX and CY of the graphite plate 3 are as close as possible to the thermal expansion coefficients of the <0001> direction and the <1100> direction of the SiC single crystal, respectively. Is defined to have a value within 0.5% of the difference.

(About bonding process)
Referring to FIG. 6, graphite plate 3 as intermediate member 31 is arranged on back surface PB of seed crystal substrate 1. The orientation of the graphite plate 3 on the back surface PB of the seed crystal substrate 1 is selected so that the thermal expansion anisotropy of the seed crystal substrate 1 and the thermal expansion anisotropy of the graphite plate 3 are as close as possible. Therefore, in the graphite plate 3, the direction CX (FIG. 4) is closer to the direction SX among the direction SX and the direction SY (FIG. 1), and the direction CY (FIG. 4) is the direction SX and the direction SY (FIG. 1). Arranged closer to the direction SY. Preferably, the graphite plate 3 is arranged so that the directions CX and CY correspond to the directions SX and SY, respectively.

  Specifically, the support surface PS of the graphite plate 3 is bonded to the back surface PB of the seed crystal substrate 1. First, an adhesive is applied to at least one of the back surface PB and the support surface PS. The adhesive is preferably a carbon adhesive or a SiC adhesive. Next, the seed crystal substrate 1 and the graphite plate 3 are stacked on each other. Next, the cured adhesive layer 4Ap is formed by curing the applied adhesive. The adhesive can be cured by heating the adhesive to a predetermined curing temperature.

  Referring to FIG. 7, the remaining organic solvent component is completely removed by firing adhesive cured layer 4 </ b> Ap. Specifically, the seed crystal substrate 1 and the graphite plate 3 bonded by the adhesive hardened layer 4Ap are stored in the firing graphite crucible 5. The periphery of the firing graphite crucible 5 is covered with a heat insulating material (not shown) in order to prevent heat radiation and efficiently heat. Then, the graphite crucible 5 for firing is heated by induction heating.

  Further, referring to FIG. 8, adhesive layer 4A is obtained from adhesive-cured layer 4Ap by the above baking. That is, the seed crystal substrate 1 and the graphite plate 3 as the intermediate member 31 bonded to each other by the adhesive layer 4A are obtained.

  Next, referring to FIG. 9, the attachment surface PF of the intermediate member 31 is attached to the graphite pedestal 6. Specifically, first, the mounting surface PF is bonded to the pedestal 6 by the adhesive cured layer 4Bp similar to the above-described adhesive cured layer 4Ap (FIG. 6).

  Further, referring to FIG. 10, adhesive layer 4 </ b> B is obtained from adhesive cured layer 4 </ b> Bp by the same method as the above-described firing (FIG. 7). That is, the graphite plate 3 as the intermediate member 31 and the base 6 bonded to each other by the adhesive layer 4B are obtained.

  As described above, the seed crystal substrate 1 is attached to the pedestal 6 through the graphite plate 3 as the intermediate member 31 by the above-described two bonding and firing steps. Although the order of the two bonding and firing steps can be changed, it is preferable to perform the steps of the seed crystal substrate 1 and the graphite plate 3 first as described above. When the seed crystal substrate 1 and the graphite plate 3 are bonded later, the organic solvent component of the adhesive cured layer 4Ap (FIG. 6) that bonds the seed crystal substrate 1 and the graphite plate 3 is removed in the subsequent firing step. This is because there is a possibility that this is not performed sufficiently. Due to the organic solvent component remaining between the seed crystal substrate 1 and the graphite plate 3, deterioration of heat conduction between the seed crystal substrate 1 and the pedestal 6 due to poor adhesion of the seed crystal substrate 1, or the seed crystal Due to the sublimation of SiC from the back surface PB of the substrate 1, the quality of the grown crystal may be deteriorated. In addition, after the seed crystal substrate 1, the graphite plate 3 and the pedestal 6 are bonded by the adhesive hardened layers 4Ap and 4Bp, the adhesive hardened layers 4Ap and 4Bp can be baked in one step at a time. In order to bond the substrate 1 and the graphite plate 3 with good adhesion, it is desirable to perform a baking step after each bonding step.

(About crystal growth)
Referring to FIG. 11, single crystal 50 is formed by performing crystal growth on growth surface PG of seed crystal substrate 1 supported by pedestal 6 via intermediate member 31. For this purpose, a crucible set 10 having a crystal growth crucible 11 and a lid 12 is prepared. The raw material 20 is stored in the crystal growth crucible 11. A pedestal 6 is attached to the lid 12. By attaching the lid 12 to the crystal growth crucible 11, the crystal growth crucible 11 is sealed, and the growth surface PG of the seed crystal substrate 1 is opposed to the raw material 20. Thermal insulation materials (not shown) are arranged around the crucible set 10, and then these are arranged in a crystal growth furnace. The inside of the furnace is evacuated and filled with argon as an inert gas. The temperature in the direction in which the seed crystal substrate 1 and the raw material 20 face each other by keeping the temperature in the vicinity of the lid 12 (upper part in the drawing) of the crystal growth crucible 11 lower than the temperature in the vicinity of the raw material 20 (lower part in the drawing). While providing a gradient, the crystal growth crucible 11 is heated to a temperature of about 2000 ° C. or higher, which is the crystal growth temperature. For example, the upper and lower temperatures are 2100 ° C. and 2200 ° C., respectively. Next, crystal growth is started by changing the pressure in the furnace while maintaining this crystal growth temperature.

(Example)
By the above method, a SiC ingot having a diameter of 75 mm and a height of 20 mm was obtained as the single crystal 50. A SiC substrate was obtained by slicing the obtained SiC ingot. The crystal quality of the SiC substrate was evaluated by observing the surface after etching the SiC substrate with molten KOH. It was confirmed that the etch pit density was very low at 1200 pieces / cm ² . Therefore, it was confirmed that a very high quality single crystal can be obtained by using this method.

On the other hand, when the graphite plate 3 was not disposed between the seed crystal substrate 1 and the pedestal 6, the etch pit density was as high as several thousand pieces / cm ² . From this, it was found that when the seed crystal substrate 1 was attached to the pedestal 6 without using the graphite plate 3, a high-quality single crystal could not be obtained stably.

(effect)
According to the method for manufacturing a single crystal of the present embodiment, the direction of the thermal expansion coefficient of the intermediate member 31 and the anisotropy of the thermal expansion coefficient of the seed crystal substrate 1 are both taken into consideration. An intermediate member 31 is attached to the seed crystal substrate 1. The intermediate member 31 attached in this way suppresses thermal stress applied from the pedestal 6 to the seed crystal substrate 1 during crystal growth. Thereby, it is possible to suppress the seed crystal substrate 1 or the single crystal 50 from being damaged or causing crystal defects in the single crystal 50. Therefore, a high quality single crystal can be obtained.

  When the directions CX and CY (FIG. 4) are orthogonal to each other, the intermediate member 31 is made to correspond to the difference in thermal expansion coefficient between the directions SX and SY (FIG. 1) orthogonal to each other on the back surface PB of the seed crystal substrate 1. Can be arranged.

Preferably, the thermal expansion coefficient α _CX has a difference of 0.5% or less with respect to the thermal expansion coefficient α _SX , and the thermal expansion coefficient α _CY has a difference of 0.5% or less with respect to the thermal expansion coefficient α _SY . Have Thereby, the absolute value of the thermal expansion coefficient can be made substantially the same between the seed crystal substrate 1 and the intermediate member 31. In the case of the seed crystal substrate 1 described above, the difference between the thermal expansion coefficients α _SX and α _SY is about 1%. Therefore, if an intermediate member having an isotropic thermal expansion coefficient α _SI is used, if α _SI is an intermediate value between α _SX and α _SY , the difference in thermal expansion coefficient in each direction is about 0.5% ( If the error is taken into account, the difference in thermal expansion coefficient in one direction is suppressed to less than 0.5%, so that the difference in thermal expansion coefficient in the other direction is 0.5%. Will inevitably be exceeded. On the other hand, according to the present embodiment, by considering the anisotropy of the thermal expansion coefficient between the directions SX and SY of the seed crystal substrate 1, the difference in thermal expansion coefficient in each direction is 0.5%. By suppressing to the following, the thermal expansion characteristics of the seed crystal substrate 1 and the thermal expansion characteristics of the graphite plate 3 as the intermediate member 31 can be effectively equalized in consideration of the anisotropy.

  The anisotropy of the thermal expansion coefficient is optimized by measuring the thermal expansion coefficient of the graphite block 3L in at least two, preferably three, directions X, Y and Z (FIG. 5), and A graphite plate 3 whose anisotropy direction is known can be prepared.

  In measuring the thermal expansion coefficient, the measurement temperature is preferably 400 ° C. or higher. Thereby, the anisotropy of the thermal expansion coefficient at the time of crystal growth performed at a high temperature can be more appropriately considered as compared with the case where measurement is performed at about room temperature.

  From the thermal expansion coefficient at a measurement temperature lower than 2000 ° C. (for example, about 400 ° C.), the thermal expansion coefficient at a crystal growth temperature of 2000 ° C. or higher may be estimated. This makes it possible to more easily measure the thermal expansion coefficient while properly taking into account the anisotropy of the thermal expansion coefficient during crystal growth.

  When the thickness of the graphite plate 3 is 0.2 mm or more, the thermal stress can be more sufficiently suppressed. If the thickness is less than 0.2 mm, the graphite plate 3 is likely to be deformed by the thermal expansion of the pedestal 6 bonded to the graphite plate 3, so that the thermal stress relaxation effect tends to be insufficient.

  Moreover, when the thickness of the graphite plate 3 is 1.5 mm or less, it is easy to maintain sufficient heat conduction between the seed crystal substrate and the pedestal 6. When the thickness exceeds 1.5 mm, the heat conduction between the pedestal 6 and the seed crystal substrate 1 is reduced, so that heat generated by recrystallization on the seed crystal substrate 1 is efficiently removed through the pedestal 6. It becomes difficult to do. As a result, there is a possibility that crystal defects increase due to an increase in the thermal gradient in the crystal.

  When the seed crystal substrate 1 is made of SiC, in general, thermal stress applied from the pedestal 6 to the seed crystal substrate 1 during crystal growth tends to be particularly problematic. According to the present embodiment, such thermal stress can be effectively suppressed.

  In the present embodiment, as shown in FIG. 1, the <0001> direction, that is, the c-axis, is included in the in-plane direction of the back surface PB. In such a case, the difference in thermal expansion coefficient between the c-axis direction and the direction perpendicular to the c-axis direction is particularly large on the back surface PB. According to the present embodiment, such thermal stress can be effectively suppressed. This can be described more generally as follows. The seed crystal substrate 1 has a hexagonal crystal structure having a <11-20> direction, a <1100> direction, and a <0001> direction, and the <11-20> direction is a back surface PB of the seed crystal substrate 1. The angle formed between the <0001> direction and the back surface PB of the seed crystal substrate 1 is smaller than at least one of the angle and the angle formed between the <1100> direction and the back surface PB of the seed crystal substrate 1. In this case, a direction close to the <0001> direction (so-called c-axis) and a direction perpendicular thereto are positioned on the back surface PB of the seed crystal substrate 1. In such a case, the anisotropy of the thermal expansion coefficient on the back surface PB of the seed crystal substrate 1 tends to be particularly large. According to the present embodiment, such thermal stress can be effectively suppressed.

  When the adhesive layers 4A and 4B (FIG. 11) are formed of a carbon adhesive or a SiC adhesive, strong adhesion that can sufficiently withstand thermal stress can be easily performed.

<Embodiment 2>
(Preparation method for single crystal growth)
In the first embodiment, the intermediate member 31 between the seed crystal substrate 1 and the pedestal 6 has the single graphite plate 3. On the other hand, in the present embodiment, the intermediate member between seed crystal substrate 1 and pedestal 6 has a layer structure of a plurality of graphite plates. Hereinafter, the manufacturing method of the single crystal of this Embodiment is demonstrated concretely. The seed crystal substrate 1 preparation method (FIGS. 1 to 3) and the crystal growth method itself (FIG. 11) are the same as those in the first embodiment.

  Referring to FIG. 12, a graphite plate 3A (one graphite plate) is prepared. The support surface PS of the graphite plate 3A is bonded to the back surface PB of the seed crystal substrate 1 by the adhesive layer 4A by the same method as in FIGS.

  Referring to FIG. 13, a graphite plate 3B having a mounting surface PF is prepared. By the same method as described above, the surface opposite to the mounting surface PF of the graphite plate 3B is bonded onto the graphite plate 3A by the adhesive layer 4M. As a result, the intermediate member 32 having the support surface PS and the attachment surface PF and including the graphite plates 3A and 3B is disposed on the seed crystal substrate 1.

  The graphite plate 3B may have a thermal expansion characteristic different from that of the graphite plate 3A. The graphite plate 3B may have the same thermal expansion characteristic as that of the graphite plate 3A. In this case, the anisotropic direction of the graphite plate 3A and the anisotropic direction of the graphite plate 3B Both are laminated so that is different. As a result, the thermal expansion characteristics of the intermediate member 32 can be different from any of the thermal expansion characteristics of the graphite plates 3A and 3B. The thermal expansion characteristics of the intermediate member 32 can be predicted by cutting each of the graphite plates 3A and 3B from the graphite block whose thermal expansion coefficient is measured including its anisotropy. In the present embodiment, each of the graphite plates 3A and 3B is prepared and laminated so that the laminate of the graphite plates 3A and 3B has the same thermal expansion characteristics as the graphite plate 3 of the first embodiment. The relative orientation of time is determined.

  Referring to FIG. 14, next, as in the first embodiment, the attachment surface PF of the intermediate member 32 is bonded to the pedestal 6 by the adhesive layer 4B. Note that the order of the three bonding steps (including the firing step as in the first embodiment) can be changed, but as in the first embodiment, the steps of the seed crystal substrate 1 and the graphite plate 3A are performed first. Is preferable.

  Since the configuration other than the above is substantially the same as the configuration of the first embodiment described above, the same or corresponding elements are denoted by the same reference numerals, and description thereof is not repeated.

(Example)
The crystal defects of the ingot (single crystal) obtained using the above method were evaluated by the same method as in the first embodiment. As a result, it was confirmed that the etch pit density was as low as 1355 / cm ² . Therefore, it was confirmed that a very high quality single crystal can be obtained by using this method.

(effect)
According to the present embodiment, substantially the same effect as in the first embodiment can be obtained. Furthermore, the thermal expansion coefficient of the intermediate member 32 can be adjusted by the combination of the graphite plates 3A and 3B.

  In the present embodiment, intermediate member 32 includes two graphite plates 3A and 3B, but the number of graphite plates is not limited to this. However, this number is preferably 3 or less, so that the heat conduction between the seed crystal substrate 1 and the pedestal 6 can be sufficiently maintained.

  Further, for the same reason as described in the first embodiment, when the graphite plates 3A and 3B have a total thickness of 0.2 mm or more, the thermal stress can be more sufficiently suppressed, and 1.5 mm or less. When the total thickness is provided, the heat conduction between the seed crystal substrate 1 and the pedestal 6 can be sufficiently maintained.

<Appendix>
In each of the above embodiments, the {11-20} plane is used as the growth surface of the seed crystal substrate (see FIG. 1), but an off angle of about several degrees may be provided with respect to the {11-20} plane. Alternatively, a plane orientation other than the {11-20} plane may be used. The first and second directions (FIG. 1: directions SX and SY) with respect to the seed crystal substrate can be appropriately determined according to the crystal structure and crystal orientation used, and do not necessarily need to be orthogonal to each other. The same applies to the third and fourth directions (FIG. 4: directions CX and CY) related to the intermediate member. The polytype of SiC is not limited to 4H. Further, the crystal structure of SiC is not limited to the hexagonal system, and may be a cubic crystal, for example. The material of the seed crystal substrate is not limited to SiC. The crystal growth method is not limited to the sublimation recrystallization method, and may be a high temperature CVD (Chemical Vapor Deposition) method, for example.

  It should be noted that the present invention can be freely combined with each other within the scope of the invention, and each embodiment can be appropriately modified or omitted.

  1 seed crystal, 2 polishing plate, 3, 3A graphite plate (one graphite plate), 3B graphite plate, 3L graphite block, 4A, 4B, 4M adhesive layer, 4Ap, 4Bp adhesive hardened layer, graphite crucible for firing, 6 base, 10 crucible set, 11 crystal growth crucible, 12 lid, 20 raw material, 31, 32 intermediate member, PB back surface, PG growth surface, PF mounting surface, PS support surface.

Claims (11)

A method for producing a single crystal by performing crystal growth at a crystal growth temperature higher than room temperature,
Providing a seed crystal substrate having a growth surface and a back surface opposite to the growth surface, the seed crystal substrate having a first direction having a first thermal expansion coefficient at the crystal growth temperature on the back surface; And a second direction having a second coefficient of thermal expansion greater than the first coefficient of thermal expansion at the crystal growth temperature, and further supporting the seed crystal substrate on the back surface of the seed crystal substrate. Disposing an intermediate member having a support surface and a mounting surface opposite to the support surface, the intermediate member including at least one graphite plate, and the at least one graphite plate includes the support surface. The step of arranging the intermediate member includes a step of adhering the support surface to the back surface of the seed crystal substrate, and the intermediate member is formed on the support surface at the crystal growth temperature. A third direction having a third coefficient of thermal expansion and a fourth direction having a fourth coefficient of thermal expansion greater than the third coefficient of thermal expansion at the crystal growth temperature; and In the arranging step, the third direction is closer to the first direction out of the first direction and the second direction, and the fourth direction is the first direction and the second direction. A step of being closer to the second direction among the directions, and further attaching the mounting surface of the intermediate member to the pedestal;
Performing crystal growth at the crystal growth temperature on the growth surface of the seed crystal substrate supported by the pedestal via the intermediate member,
A method for producing a single crystal.   2. The method for producing a single crystal according to claim 1, wherein the first direction and the second direction are orthogonal to each other, and the third direction and the fourth direction are orthogonal to each other.   The third thermal expansion coefficient has a difference of 0.5% or less with respect to the first thermal expansion coefficient, and the fourth thermal expansion coefficient with respect to the second thermal expansion coefficient is 0.00. The manufacturing method of the single crystal of Claim 1 or 2 which has a difference of 5% or less. Measuring the coefficient of thermal expansion of the graphite block at one temperature in at least two directions;
Further cutting out the one graphite plate from the graphite block,
The manufacturing method of the single crystal of any one of Claim 1 to 3.   The method for producing a single crystal according to claim 4, wherein in the step of measuring the thermal expansion coefficient, the one temperature is 400 ° C. or higher. In the step of measuring the thermal expansion coefficient, the one temperature is less than 2000 ° C.,
Before the step of cutting out the graphite plate, further comprising a step of estimating a thermal expansion coefficient at 2000 ° C. or higher based on the thermal expansion coefficient at the one temperature.
The method for producing a single crystal according to claim 4.   The method for producing a single crystal according to any one of claims 1 to 6, wherein the at least one graphite plate has a total thickness of 0.2 mm or more and 1.5 mm or less.   The method for producing a single crystal according to claim 1, wherein the at least one graphite plate includes a plurality of graphite plates.   The method for producing a single crystal according to claim 8, wherein the plurality of graphite plates are three or less graphite plates.   The method for producing a single crystal according to claim 1, wherein the seed crystal substrate is made of silicon carbide.   The seed crystal substrate has a hexagonal crystal structure having a <11-20> direction, a <1100> direction, and a <0001> direction, and the <11-20> direction corresponds to the back surface of the seed crystal substrate. The angle formed between the <0001> direction and the back surface of the seed crystal substrate is smaller than at least one of the angle formed and the angle formed between the <1100> direction and the back surface of the seed crystal substrate. The method for producing a single crystal according to any one of 1 to 10.

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