Patent Application
20170306522
The present invention relates to a single crystal production apparatus and a crucible employed therein. In particular, it relates to an apparatus for producing a SiC single crystal by a solution growth process and a crucible employed therein.
A solution growth process is an example of a method for producing a SiC single crystal. In the solution growth process, a seed crystal attached to the bottom edge of a seed shaft is brought into contact with a Si—C solution contained in a crucible. The portion of the Si—C solution in vicinity to the seed crystal is supercooled, whereby a SiC single crystal grows on the seed crystal.
The Si—C solution is a solution in which carbon (C) is dissolved in a melt of Si or a Si alloy. An example of a way of forming the Si—C solution is heating a graphite crucible containing Si by an induction heater. For example, a high-frequency coil is used as the induction heater. The crystal growth surface of the seed crystal attached to the seed shaft is brought into contact with the formed Si—C solution, whereby a SiC single crystal is grown.
It is preferred that the Si—C solution is stirred during the crystal growth so that the composition of the solution and the temperature distribution of the solution can be kept homogeneous. The heating by a high-frequency coil provides Lorentz force to the Si—C solution. Thereby, the Si—C solution is caused to flow and is stirred.
However, if the Si—C solution is not stirred adequately, it is hard to keep the composition of the solution and the temperature distribution of the solution homogeneous. In this case, SiC polycrystals are likely to be generated. If the SiC polycrystals stick to the crystal growth surface of the SiC single crystal, it will hinder the growth of the SiC single crystal.
Japanese Patent Application Publication No. 2005-179080 (Patent Literature 1) discloses a production method and a production apparatus that inhibit generation of polycrystals.
In the production method and the production apparatus disclosed in Patent Literature 1, a crucible containing a material solution is heated by a normal conductive coil. Patent Literature 1 teaches the following. The normal conductive coil provides Lorentz force to the melt. The Lorentz force makes the melt bulge like a dome. Consequently, it is possible to produce a SiC single bulk crystal stably without causing growth of polycrystals and without increasing the number of crystal defects.
Patent Literature 1: Japanese Patent Application Publication No. 2005-179080
The production method and the production apparatus disclosed in
Patent Literature 1, however, need an additional copper side wall having a slit because the melt bulges like a dome.
Recently, since SiC single crystals are usable for various purposes, large diameter SiC crystals are subjected to increasing demand. For production of a large diameter SiC crystal, a crucible with a larger diameter is required. In a case where a high-frequency coil is used as the induction heater, the high-frequency coil is typically disposed around the crucible. Accordingly, if the diameter of the crucible is increased, it is necessary to increase the diameter of the high-frequency coil.
The heating by an induction heater generates a magnetic flux inside the crucible. The magnetic flux generates Lorentz force and Joule heat in the Si—C solution by electromagnetic induction. The Lorentz force stirs the Si—C solution. The Joule heat heats the Si—C solution. The magnitudes of the Lorentz force and the Joule heat depend on the strength of the magnetic flux penetrating to the inside of the crucible. With regard to a high-frequency coil, as the diameter thereof is increasing, the magnetic flux in the center thereof becomes weaker. Accordingly, the stirring and the heating of the Si—C solution are likely to be inadequate. Inadequate stirring and heating of the Si—C solution cause generation of polycrystals, thereby hindering the growth of the SiC single crystal.
An object of the present invention is to provide a SiC single crystal production apparatus capable of easily stirring and heating a Si—C solution.
A SiC single crystal production apparatus according to an embodiment of the present invention comprises a crucible capable of containing a Si—C solution, a seed shaft, and an induction heater. The crucible is capable of containing a Si—C solution. The crucible includes a tubular portion and a bottom portion. The tubular portion includes a first outer peripheral surface and an inner peripheral surface. The bottom portion is disposed at a lower end of the tubular portion. The bottom portion defines an inner bottom surface of the crucible. The seed shaft includes a bottom edge which a seed crystal is attachable to. The induction heater is disposed around the tubular portion of the crucible. The induction heater heats the crucible and the Si—C solution. The first outer peripheral surface includes a first groove extending in a direction crossing a circumferential direction of the tubular portion.
The SIC single crystal production apparatus according to the present invention is capable of easily stirring and heating a Si—C solution.
A SiC single crystal production apparatus according to an embodiment of the present invention comprises a crucible capable of containing a Si—C solution, a seed shaft, and an induction heater. The crucible is capable of containing a Si—C solution. The crucible includes a tubular portion and a bottom portion. The tubular portion includes a first outer peripheral surface and an inner peripheral surface. The bottom portion is located at the lower end of the tubular portion. The bottom portion defines an inner bottom surface of the crucible. The seed shaft includes a bottom edge which a seed crystal is attachable to. The induction heater is disposed around the tubular portion of the crucible. The induction heater heats the crucible and the Si—C solution. The first outer peripheral surface includes a first groove extending in a direction crossing a circumferential direction of the tubular portion.
Thus, according to the embodiment, the crucible used for production of a SiC single crystal includes a first groove in the first outer peripheral surface of the tubular portion. The first groove extends in a direction crossing the circumferential direction of the tubular portion. In this case, the magnetic flux generated by the induction heater and directed in the axial direction of the induction heater easily penetrates to the inside of the crucible. This promotes stirring and heating of the Si—C solution.
It is preferred that the first groove extends in the axial direction of the tubular portion.
In this case, the current induced in the wall of the crucible by the magnetic flux does not cross the first groove. Therefore, the induced current flows deep in the wall of the crucible, and the magnetic flux penetrates more deeply into the inside of the crucible.
It is preferred that the lower end of the first groove is to be located below the liquid surface of the Si—C solution.
In this case, from a lateral view, the first groove partly overlaps the Si—C solution in the crucible. Therefore, the magnetic flux penetrates directly into the Si—C solution. Accordingly, the Si—C solution receives Lorentz force more easily, and stirring of the Si—C solution is promoted. Also, the current induced by the high-frequency coil becomes greater, and heating of the Si—C solution is promoted.
It is preferred that the groove in the outer peripheral surface of the tubular portion is to extend, from a lateral view, at least from the inner bottom surface to the liquid surface of the Si—C solution.
In this case, stirring and heating of the Si—C solution is further promoted.
The bottom surface of the crucible preferably includes a second outer peripheral surface and an outer bottom surface. The second outer peripheral surface links with the first outer peripheral surface of the tubular portion. The outer bottom surface is located at the lower end of the second outer peripheral surface. The inner bottom surface is concave. The second peripheral surface has a second groove. The second groove extends in a direction crossing the circumferential direction of the tubular portion, and the second groove increases in depth as it comes closer to the outer bottom surface.
In this case, the second groove extends almost to the inner bottom surface. This promotes stirring and heating of the portion of the Si—C solution near the concave inner bottom surface.
The crucible according to the present embodiment is employed in the above-described apparatus for producing a SiC single crystal.
A SiC single crystal production method according to an embodiment of the present invention comprises: a preparation step of preparing the above-described production apparatus; a formation step of heating and melting the material for Si—C solution in the crucible by the induction heater to form a Si—C solution; and a growth step of bringing the seed crystal into contact with the Si—C solution and growing a SiC single crystal on the seed crystal while heating and stirring the Si—C solution.
The SiC single crystal production apparatus according to the present embodiment and the crucible employed in the production apparatus will hereinafter be described.
As described above, when the magnetic flux generated by the high-frequency coil penetrates more deeply into the inside of the crucible, the Si—C solution is stirred and heated more. During a crystal growth, stirring and heating of the Si—C solution inhibits generation of SiC polycrystals. This will be described below.
When the composition of the Si—C solution during a crystal growth is homogeneous, it is easy to inhibit generation of SiC polycrystals. In order to make the composition and the temperature of the Si—C solution homogeneous, it is necessary to stir and heat the Si—C solution. Also, during production of a SiC single crystal by the solution growth process, it is important to supply carbon in the Si—C solution to the crystal growth surface of the SiC single crystal. Supplying carbon to the crystal growth surface of the SiC single crystal during a crystal growth promotes the growth of the SiC single crystal. Therefore, also from the viewpoint of the crystal growth speed of the SiC single crystal, it is necessary to stir the Si—C solution.
An example of a way of stirring the Si—C solution is electromagnetic stirring by use of a high-frequency coil. An alternating current flow along the high-frequency coil generates a magnetic flux inside the high-frequency coil. Because of the alternating current flow, the direction and the strength of the magnetic flux change, and the Si—C solution receives Lorentz force. The Si—C solution in the crucible is caused to flow by the Lorentz force, and is stirred. Accordingly, when the magnetic flux penetrates more deeply into the inside of the crucible, the Si—C solution receives greater Lorentz force, and the Si—C solution is stirred more.
The magnetic flux generates an induced current in the crucible and the Si—C solution. Thereby, Joule heat is generated in the crucible and the Si—C solution. Accordingly, when the magnetic flux penetrates more deeply into the inside of the crucible, greater Joule heat is generated in the Si—C solution, and the crucible and the Si—C solution are heated more.
The strength of magnetic flux in the center of the high-frequency coil is inversely proportional to the radius of the coil. In other words, the greater the radius of the coil, the weaker magnetic flux is generated in the coil. The weaker the magnetic flux, the weaker the Lorentz force, and the less the Joule heat.
As described above, in order to stir and heat the Si—C solution in the crucible, it is necessary to make the magnetic flux penetrate deeply into the inside of the crucible. However, the tubular portion of the crucible is thick, and the thickness hinders penetration of the magnetic flux. Therefore, it is difficult to stir and heat the Si—C solution in the crucible.
According to the present embodiment, in the outer surface of the tubular portion of the crucible used for production of a SiC single crystal, a groove extending in a direction crossing the circumferential direction of the tubular portion is made. The thickness of the tubular portion in the area where the groove is made is reduced. Accordingly, the magnetic flux generated by the high-frequency coil easily penetrates to the inside of the crucible, and the Si—C solution is stirred and heated easily.
Some embodiments of the present invention will hereinafter be described with reference to the drawings. In the drawings, the same parts or the counterparts are provided with the same reference symbols, and descriptions of these parts will not be repeated.
The chamber 2 houses the induction heater 3, the heat insulator 4 and the crucible 5. When a SiC single crystal is produced, the chamber 2 is cooled.
The heat insulator 4 is like a housing. The heat insulator 4 houses the crucible 5 and keeps the crucible 5 warm. The heat insulator 4 has a top lid and a bottom lid, each of which has a through hole in the center. The seed shaft 6 is inserted through the through hole made in the top lid. The rotation device 200 is inserted through the through hole made in the bottom lid.
The seed shaft 6 extends downward from above the chamber 2. The top edge of the seed shaft 6 is attached to the drive unit 9. The seed shaft 6 pierces into the chamber 2 and the heat insulator 4. During a crystal growth, the bottom edge of the seed shaft 6 is located inside the crucible 5. A seed crystal 8 is attachable to the bottom edge of the seed shaft 6, and when a SiC single crystal is produced, a seed crystal 8 is attached to the bottom edge. The seed crystal is preferably a SiC single crystal. The seed shaft 6 is movable up and down by the drive unit 9. The seed shaft 6 is also rotatable around the axis by the drive unit 9.
The rotation device 200 is attached to the outer bottom surface 52C of the crucible 5. The rotation device 200 pierces through the lower side of the heat insulator 4 and the lower side of the chamber 2. The rotation device 2 is capable of rotating the crucible 5 around the central axis of the crucible 5. The rotation device 200 is also capable of lifting and lowering the crucible 5.
The induction heater 3 is disposed around the crucible 5, and more specifically, is disposed around the heat insulator 4. The induction heater 3 is, for example, a high-frequency coil. In this case, the axis of the high-frequency coil is directed in the vertical direction of the production apparatus 1. It is preferred that the high-frequency coil is arranged coaxially with the seed shaft 6.
The crucible 5 contains a Si—C solution 7. The material of the crucible 5 preferably contains carbon. In this case, the crucible 5 serves as a supply source of carbon to the Si—C solution 7. The crucible 5 is made of, for example, graphite. The crucible 5 is heated by the induction heater 3. Accordingly, the crucible 5 serves as a heat source to heat the Si—C solution 7 during formation of the Si—C solution and growth of the SiC single crystal.
The Si—C solution 7 is the material of the SiC single crystal, and contains silicon (Si) and carbon (C). Si—C solution 7 may contain not only Si and C but also other metal elements. The Si—C solution 7 is produced by dissolving carbon (C) in a melt of Si or a mixture of Si and other metal elements (a Si alloy).
When a SiC single crystal is produced, the seed shaft 6 is lowered to bring the seed crystal 8 into contact with the Si—C solution 7. At the moment, the crucible 5 and the surround are kept at a crystal growth temperature. The crystal growth temperature depends on the composition of the Si—C solution. The crystal growth temperature is typically 1600 to 2000° C. The SiC single crystal is grown while the Si—C solution is maintained at the crystal growth temperature.
The outer peripheral surface 51A of the tubular portion 51 has a plurality of grooves 10. The grooves 10 extend in a direction crossing the circumferential direction of the tubular portion 51. In the case shown in
In the tubular portion 51, as described above, the portions where the grooves 10 are made are thinner than the portions where the grooves 10 are not made. Therefore, as compared with a case where no such grooves as the grooves 10 are made, an induced current flows deep in the wall of the crucible, and the magnetic flux generated by the high-frequency coil penetrates to the inside of the crucible easily. Accordingly, Si—C solution is likely to be stirred.
The direction of the magnetic flux generated by the high-frequency coil is the same as the axial direction of the coil. Accordingly, the direction of the magnetic flux is perpendicular to the circumferential direction of the tubular portion 51. Therefore, when the grooves 10 extend in a direction crossing the circumferential direction of the tubular portion 51, the magnetic flux crosses the grooves 10. Thus, in this case, the magnetic flux partly penetrates to the inside of the crucible through the thin portions of the tubular portion 51, and therefore, the magnetic flux penetrates to the inside of the crucible easily. Further, when the grooves 10 extend in the axial direction of the tubular portion 51 (that is, extend perpendicularly to the circumferential direction of the tubular portion 51) as shown in
When the magnetic flux penetrates to the inside of the crucible easily, the induced current generated in the Si—C solution 7 around the center of the crucible is great as compared with a case where no such grooves as the grooves 10 are made. Accordingly, the Joule heat generated in the Si—C solution 7 is great, which promotes heating of the Si—C solution 7.
The lower limit of the depth of the grooves 10 is preferably 10% of the thickness of the tubular portion 51. The upper limit of the depth of the grooves 10 is preferably 90% of the thickness of the tubular portion 51. More desirably, the lower limit of the depth of the grooves 10 is 30% of the thickness of the tubular portion 51, and the upper limit of the depth of the grooves 10 is 70% of the thickness of the tubular portion 51. The cross-sectional shape of each of the grooves 10 need not be rectangular, and may be semicircular, semi-elliptical or the like. The cross-sectional shape of the grooves 10 is not particularly limited as long as it helps partial thickness reduction of the tubular portion 51 and magnetic flux penetration to the inside of the crucible. In the case of
Preferably, the grooves 10 are circumferentially arranged along the outer peripheral surface 51 at uniform intervals as shown in
As seen in
In this case, from a lateral view, the grooves 10 overlap the Si—C solution 7. Therefore, the magnetic flux is likely to penetrate into the Si—C solution directly, which further promotes stirring and heating of the Si—C solution 7.
The inner bottom surface of the crucible may be concave. When the inner bottom surface is concave, it is preferred that the portion of the Si—C solution 7 near the inner bottom surface is stirred more.
The bottom portion 520 includes not a flat inner bottom surface as the inner bottom surface 52B of the bottom portion 52 but a concave inner bottom surface 520B. As shown in
In order to stir the Si—C solution 7 filled in the space defined by the concave inner bottom surface 520B, it is preferred that grooves extending almost to the inner bottom surface 520B are made. Therefore, the outer peripheral surface 52A of the bottom portion 520 has grooves 100. The grooves 100, as with the grooves 10, extend in a direction crossing the circumference direction of the tubular potion 51. The grooves 100 also increase in depth as they come from the upper part of the bottom portion 520 toward the outer bottom surface 52C. Specifically, the depth DB of the lower parts (near the outer bottom surface 52C) of the grooves 100 is greater than the depth DU of the upper parts of the grooves 100.
In this case, the grooves 100 are made to extend almost to the concave inner bottom surface 520B. Accordingly, the magnetic flux penetrates into the Si—C solution 7 filled in the space defined by the concave inner bottom surface 520B, which promotes stirring and heating of the Si—C solution 7.
As is the case with the first embodiment, when the grooves 100 extend in the axial direction of the tubular portion 51 (extend perpendicularly to the circumferential direction of the tubular portion 51), the magnetic flux penetrates more deeply into the inside of the crucible 50.
A production method according to an embodiment of the present invention comprises a preparation step, a formation step, and a growth step. In the preparation step, the production apparatus 1 is prepared, and a seed crystal 8 is attached to the seed shaft 6. In the formation step, a Si—C solution 7 is produced by the induction heater 3. In the growth step, the seed crystal 8 is brought into contact with the Si—C solution 7, whereby a SiC single crystal is grown. These steps will hereinafter be described.
With reference to
In the formation step, the material for Si—C solution 7 in the crucible 5 is heated, whereby a Si—C solution 7 is produced. The crucible 5 is placed on the rotation device 200 in the chamber 2. The crucible 5 contains material for Si—C solution 7. It is preferred that the crucible 5 and the rotation device 200 are coaxially arranged. The heat insulator 4 is disposed around the crucible 5. The induction heater 3 is disposed around the heat insulator 4.
Next, the chamber 2 is filled with an inert gas. The inert gas is, for example, helium, argon or the like. The pressure inside the chamber 2 is preferably the atmospheric pressure. If the pressure inside the chamber 2 is below the atmospheric pressure (reduced pressure) or if the inside of the chamber 2 is vacuum, the Si—C solution 7 in the crucible 5 vapors easily. Vaporization of the Si—C solution 7 leads to a great change in the level of the liquid surface of the Si—C solution 7, thereby resulting in an instable growth of the SiC single crystal. The induction heater 3 heats the crucible 5 and the material for Si—C solution 7 in the crucible 5. The material for Si—C solution is, for example, Si or a mixture of Si and other metal elements (a Si alloy). The heated material for Si—C solution 7 melts. For example, when the crucible 5 is graphite, carbon is dissolved out from the graphite crucible 5, whereby a Si—C solution 7 is produced.
After the production of the Si—C solution 7, the seed crystal 8 is dipped in the Si—C solution 7. Specifically, the seed shaft 6 is lowered to bring the seed crystal 8 attached to the bottom edge of the seed shaft 6 into contact with the Si—C solution 7. After the seed crystal 8 comes into contact with the Si—C solution 7, the induction heater 3 heats the crucible 5 and the Si—C solution 7 to maintain the crucible 5 and the Si—C solution 7 at a crystal growth temperature. The crystal growth temperature depends on the composition of the Si—C solution 7. The crystal growth temperature is typically 1600 to 2000 C.
Next, the portion of the Si—C solution 7 in vicinity to the seed crystal 8 is supercooled, whereby SiC is supersaturated. In order to supercool the portion of the Si—C solution, for example, the induction heater 3 is controlled to make the temperature of the portion of the Si—C solution 7 in vicinity to the seed crystal 8 lower than the temperature of the other portion. Alternatively, the portion in vicinity to the seed crystal 8 may be cooled by a coolant. Specifically, a coolant is circulated inside the seed shaft 6. The coolant is, for example, an inert gas such as helium, argon or the like.
Thermal flow of the Si—C solution in crucibles that differ from one another in the form of grooves was simulated.
The simulation was conducted on the assumption that a SiC single crystal production apparatus having the same structure as the production apparatus I shown in
In the thermal flow analysis, three crucibles (S1 to S3) that differ from one another in the form of grooves were used as computation models. The crucible S1 had no grooves. The crucible S2 had grooves in the outer peripheral surface of the tubular portion, and the grooves extended from the bottom edge to the top edge of the tubular portion as shown in
On the above conditions, a thermal flow analysis by simulation was performed. For the simulation, a general-purpose thermal flow analysis application (made by COMSOL, tradename: COMSOL-Multiphysics) was used.
As seen in
As seen in
As seen in
The absolute values of the maximum flow velocities of the Si—C solution in the crucibles S1 to S3 were calculated from the flow analysis results. That in the crucible S1 was 0.198 m/s, that in the crucible S2 was 0.215 m/s, and that in the crucible S3 was 0.268 m/s. These results show that the crucibles according to the embodiments provided great Lorentz force to the Si—C solution, as compare with the crucible S1 having no grooves. In other words, the crucibles according to the embodiments stirred the Si—C solution well, as compared with the crucible S1 having no grooves.
In Example 2, SiC single crystals were produced by use of crucibles (E1 and E2) that differ in the form of the grooves in the outer peripheral surface. Then, the quality of the produced SiC single crystals was evaluated.
The crucible E1 was made of graphite, and was in the shape of a cylinder having an inner diameter of 110 mm and an outer diameter of 130 mm. The inner bottom surface of the crucible E1 was semispherically concave. The seed crystal used for this example was a SiC single crystal. The seed crystal attached to the seed shaft had a diameter of 2 inches. The material for Si—C solution contained Si and Cr at an atom ratio of Si:Cr=6=4. The temperature around the SiC seed crystal was 1950° C. The crystal growth time was 10 hours.
The crucible E2 was a crucible having eight grooves on the outer peripheral surface of the tubular portion of the crucible E1. The grooves extended in the axial direction of the tubular portion from the bottom edge to the top edge of the tubular portion. The grooves were arranged at uniform intervals around the axis of the tubular portion. Each of the grooves had the following dimensions: a width of 6 mm, a depth of 4 mm, and a length of 155 mm. There were no other differences in structure between the crucible E1 and the crucible E2. The conditions of SiC single crystal production were the same as the conditions of SiC single crystal production by use of the crucible E1.
The crystal growth surface of each of the produced SiC single crystals was observed by an optical microscope.
The embodiments described above are merely examples, and the present invention is not restricted to the embodiments.
LIST OF REFERENCE SYMBOLS
3: induction heater
5, 50: crucible
51: tubular portion
51A: outer peripheral surface of tubular portion
52, 520: bottom portion
52A: outer peripheral surface of bottom portion
52B, 520B: inner bottom surface of bottom portion
52C: outer bottom surface of bottom portion
7: Si—C solution
10, 100: groove
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-213233 | Oct 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/005177 | 10/13/2015 | WO | 00 |
