Method for producing silicon carbide single crystal

Abstract

Realized is a method for producing a highly reliable silicon carbide single crystal. To accomplish the foregoing, used is a method for producing a silicon carbide single crystal comprising: (a) a step in which a seed crystal that has a first principal surface and a second principal surface on the side opposite thereof and comprises a silicon carbide single crystal is prepared; (b) a step in which, for each of a plurality of regions arranged side-by-side on the first principal surface of the seed crystal, a height difference Δh between the highest point and the lowest point of the first principal surface in a direction perpendicular to a three-point reference plane of the first principal surface is measured, and a value a = Δh / s (units: mm−1) is calculated by dividing the difference Δh by the area S of the region; (c) a step in which, the maximum value of the values a for all of the regions of the first principal surface being a_Max, the seed crystal that satisfies a_Max < 1 × 10−4 is determined to pass examination; and (d) a step in which a single crystal that includes silicon carbide is grown on the surface of the seed crystal that has passed examination in step (c).

Description

Method for producing silicon carbide single crystal

[0001] The present invention relates to a technique that is effective when applied to a method for producing a silicon carbide single crystal.

[0002] In recent years, silicon carbide has been attracting attention as a material used in power semiconductor elements that require a relatively high breakdown voltage. Single crystals made of silicon carbide (hereinafter referred to as silicon carbide single crystals) are produced, for example, by using a solution method or a sublimation method. The solution method is a method of producing silicon carbide single crystals by bringing a seed crystal attached to the tip of a shaft into contact with a solution containing carbon and silicon contained in a crucible, causing the silicon carbide single crystal to grow on the seed crystal while the shaft is pulled up. The sublimation method is described in Patent Document 1 (JP 2011-111372 A).

[0003] JP 2011-111372 A

[0004] When stress occurs in a seed crystal, the silicon carbide single crystal grown using the seed crystal is produced in a stressed state, which may result in internal cracks. Therefore, there is a need to grow a silicon carbide single crystal using a seed crystal with little internal stress, thereby producing a highly reliable silicon carbide single crystal.

[0005] In one embodiment, a method for producing a silicon carbide single crystal includes the steps of: (a) preparing a seed crystal having a first main surface and a second main surface opposite to the first main surface, the seed crystal being made of a silicon carbide single crystal; (b) measuring a difference Δh in height between the highest point and the lowest point of the first main surface in a direction perpendicular to a three-point reference plane of the first main surface in each of a plurality of regions aligned on the first main surface of the seed crystal; and dividing the difference Δh by an area S of the region to obtain a value a=Δh / S (unit: mm －１ (c) calculating a_Max<1×10, where a_Max is the maximum value of the values ​​a in all the regions of the first main surface; －４ (d) growing a single crystal containing silicon carbide on the surface of the seed crystal that passed the step (c).

[0006] According to one embodiment, a highly reliable silicon carbide single crystal can be manufactured.

[0007] 1 is a flow chart showing a method for manufacturing a silicon carbide single crystal in an embodiment. FIG. 2 is a flow chart showing a method for manufacturing a silicon carbide single crystal following FIG. 1. FIG. 3 is a plan view showing an example of dividing the main surface of a wafer into sites. FIG. 4 is a plan view illustrating a three-point reference plane. FIG. 5 is a cross-sectional view illustrating the difference between the highest point and the lowest point on the surface of the wafer in the three-point reference plane. FIG. 6 is a diagram showing the configuration of a single crystal manufacturing apparatus in an embodiment. FIG. 7 is a diagram showing a state in which a silicon carbide single crystal has been grown on the lower surface of a seed crystal in an embodiment. FIG. 8 is a table showing the relationship between the shape accuracy of a wafer (seed crystal) and cracks in the silicon carbide single crystal after growth.

[0008] In all the drawings for explaining the embodiments, the same components are generally designated by the same reference numerals, and repeated explanations thereof will be omitted. In addition, hatching may be used even in plan views to make the drawings easier to understand.

[0009] The following describes a method for producing a silicon carbide single crystal, in which a seed crystal made of silicon carbide is grown by a solution method, and cracks in the silicon carbide single crystal are prevented by selecting a seed crystal with low internal stress. The wafer referred to in this application does not refer to a wafer used as a substrate for a semiconductor device, but refers to a seed crystal used for growing a silicon carbide single crystal.

[0010] <Method for Manufacturing Silicon Carbide Single Crystal> A method for manufacturing a silicon carbide single crystal will be described with reference to Figures 1 and 2. Figures 1 and 2 show a flow chart illustrating a method for manufacturing a silicon carbide single crystal. In particular, Figure 1 shows a flow chart for preparing and selecting a seed crystal to be used in growing a silicon carbide single crystal.

[0011] In the process of producing a silicon carbide single crystal, first, a seed crystal ingot (bulk single crystal) made of silicon carbide is prepared (step S1 in FIG. 1).

[0012] Next, the ingot is cut using a wire saw to obtain a plurality of wafers (step S2 in FIG. 1). The thin films (wafers) cut out from the ingot in this manner are seed crystals.

[0013] Next, it is measured whether the seed crystal wafer satisfies SORI<30 μm (step S3 in FIG. 1 ). This measurement is performed after the cutting process in step S2, without polishing the wafer surface. SORI is one of the parameters that represents the shape of the wafer when held without vacuum suction, and is the sum of the distances between the highest and lowest points on the wafer surface, based on the least-squares plane of the wafer. In other words, this distance is the distance in the direction perpendicular to the least-squares plane. SORI is always a positive value. The least-squares plane is a plane calculated by the least-squares method from all data on the wafer surface. If the wafer does not satisfy SORI<30 μm (if step S3 is No), the wafer is likely to have excessively large internal stress and is therefore unsuitable for use as a seed crystal, and is therefore rejected (step S8 in FIG. 1 ).

[0014] If the wafer satisfies the SORI < 30 μm requirement (if step S3 is Yes), the first main surface of the wafer, which has a first main surface and a second main surface opposite the first main surface, is divided into sites (regions) in a three-point reference plane. Figure 3 shows three examples of the first main surface of a wafer 1 divided into sites. Figure 3 shows plan views of the wafer 1 divided into, from left to right, site ST1, which is 10 × 10 mm in length and width, site ST2, which is 15 × 15 mm, and site ST3, which is 20 × 20 mm. The site division is performed virtually and does not involve processing such as carving boundaries between sites into the wafer 1. Figure 3 shows a wafer 1 with a diameter of 150 mm as an example.

[0015] As shown in FIG. 4 , the three-point reference plane is a plane passing through three points A located within the measurement region of the substantially circular first main surface of the wafer (seed crystal) 1. The wafer 1 is not perfectly circular, and in this case, it has a portion (orientation flat) where part of the periphery is linear. Instead of such a portion, it may have a notch. Point A is positioned, for example, at a point where three radii of the substantially circular wafer 1 overlap, where the three radii are at an angle of 120 degrees to each other. Point A is positioned 3% inward of the diameter of the wafer 1 from the edge of the wafer 1. Ideally, the surface of a wafer is flat, but in reality, it has warpage or unevenness, so each of the three points A is not necessarily located at the same height as all points on the first main surface.

[0016] After dividing the wafer into sites, the difference Δh (unit: mm) between the highest and lowest points of the height of the first main surface within each site is measured using the three-point reference plane as a reference (step S4 in FIG. 1). That is, as shown in FIG. 5, within each site ST, the difference Δh is measured as the distance between the highest point H and the lowest point L of the first main surface 1a in the direction perpendicular to the three-point reference plane 2. Note that although the wafer 1 is generally a flat disk, in FIG. 5 the wafer 1 is shown in an excessively distorted shape to make it easier to understand the positional relationship between the highest point H and the lowest point L.

[0017] Next, for each site, the area S of the site (unit: mm ２ ) divided by a = Δh / S (unit: mm －１ ) is calculated (step S5 in FIG. 1). The value a evaluated here is a numerical value of the local warpage in the wafer.

[0018] Next, when the maximum value of the values ​​a at all sites on the wafer is a_Max, a_Max<1×10 －４ It is determined whether the wafer satisfies the formula a_Max<1×10 (step S6 in FIG. 1). －４ If the above condition is not satisfied (if step S6 is No), the wafer may have excessively large internal stress and is therefore unsuitable for use as a seed crystal, and is therefore rejected (step S8 in FIG. 1).

[0019] Wafer a_Max<1×10 －４ If the above condition is satisfied (if step S6 is Yes), the wafer has small internal stress and is acceptable for use as a seed crystal (step S7 in FIG. 1). This completes the selection of the seed crystal. The measurements in steps S3 and S4, the calculation in step S5, and the judgment in step S6 are performed after the cutting process in step S2 without performing surface treatment such as polishing on the wafer surface. If the wafer is subjected to surface treatment such as polishing, the wafer will deform due to the Twyman effect, making it impossible to evaluate the original shape accuracy of the wafer. For this reason, the shape of the wafer immediately after the cutting process is measured here.

[0020] Next, the surface of the seed crystal (wafer) that passed steps S6 and S7 in Fig. 1 is subjected to surface treatments such as polishing and etching (step S9 in Fig. 2). This polishing removes damage such as pits that occurred on the surface of the seed crystal during cutting (step S2 in Fig. 1). Furthermore, scratches that occurred during cutting or polishing can be removed by the etching.

[0021] Next, a silicon carbide single crystal is grown using a single crystal manufacturing apparatus (steps S10 to S12 in FIG. 2). FIG. 6 shows the configuration of a single crystal manufacturing apparatus 100 according to this embodiment.

[0022] 6, a single crystal manufacturing apparatus 100 has a container (heat insulating member) 10. The internal space of this container 10 is filled with, for example, argon gas, and a pedestal 11 that can rotate horizontally is disposed on the pedestal 11. A crucible 12 is disposed on the pedestal 11.

[0023] The crucible 12 is made of, for example, graphite and contains a high-temperature solution 20 containing silicon (Si) inside. A coil 14 through which a high-frequency current flows is provided on the outer periphery of the container 10 of the single crystal manufacturing apparatus 100, and the crucible 12 is heated by induction heating based on the high-frequency current flowing through the coil 14. As a result, the solution 20 contained in the crucible 12 reaches a high temperature, and the graphite (C, carbon) that constitutes the crucible 12 melts, so that the solution 20 contains carbon and silicon. A lid 15a is placed on the crucible 12. The lid 15a is configured to be able to come into contact with the crucible 12.

[0024] The crucible 12 has, for example, a bottom that is circular in plan view and has a cylindrical portion on the periphery of the circular bottom. In other words, the crucible 12, which is a container, is a cylindrical body with a bottom. The lid 15a is placed on the top surface of the cylindrical portion. The lid 15a has a disk-like (donut-like) structure with a hole in the center. Here, the outer diameter of the crucible 12 and the outer diameter of the lid 15a are the same. Furthermore, the inner diameter of the lid 15a is smaller than the inner diameter of the cylindrical portion of the crucible 12.

[0025] Next, the single crystal manufacturing apparatus 100 is provided with a shaft 16 that can move in the vertical direction, and a seed crystal 30 made of silicon carbide is attached to the tip (lower end) of this shaft 16. The seed crystal 30 held at the tip of the shaft 16 is a seed crystal (wafer) that passed steps S6 and S7 in FIG. 1 and has been subjected to the surface treatment of step S9 in FIG. 2. The shaft 16 also has a fin structure 16a attached to it so as to intersect with the extension direction of the shaft 16. Furthermore, the interior of the shaft 16 has a hollow structure, and a thermocouple 17 is inserted into the shaft 16 to measure the temperature near the seed crystal 30. The temperature near the seed crystal 30 can also be measured by disposing a radiation thermometer with a measuring diameter smaller than the inner diameter of the shaft 16 at the upper end of the shaft 16, instead of inserting the thermocouple 17.

[0026] The single crystal manufacturing apparatus 100 has a control unit 50 that controls the movement of the shaft 16 having the fin structure 16 a in the vertical direction. The control unit 50 is configured to control the shaft 16 having the fin structure 16 a so that the shaft 16 moves in the vertical direction passing through the inside of the lid 15 a and the cylindrical portion of the crucible 12. The control unit 50 is configured to move the shaft 16, having the seed crystal 30 attached to its tip, in the downward direction, so that the lower surface of the seed crystal 30 comes into contact with the surface of the solution 20 contained in the crucible 12, and to grow a single crystal made of silicon carbide on the lower surface of the seed crystal 30 by moving the shaft 16 in the upward direction after the lower surface of the seed crystal 30 comes into contact with the surface of the solution 20.

[0027] The pedestal 11 is connected to a crucible-holding shaft 18 that holds the pedestal 11. The crucible-holding shaft 18 is configured to be movable up and down and to rotate either clockwise or counterclockwise. As a result, the pedestal 11 attached to the crucible-holding shaft 18 and the crucible 12 placed on the pedestal 11 can be moved up and down and rotated horizontally by the crucible-holding shaft 18. The crucible-holding shaft 18 has a hollow interior, allowing a thermocouple, a radiation thermometer, or the like to be inserted therein for temperature measurement.

[0028] 6 shows a state in which the bottom surface of the seed crystal 30 is in contact with the surface of the solution 20. In this case, as shown in FIG. 6, the fin structure 16a attached to the shaft 16 is located below the lid 15a.

[0029] 7 shows a state in which silicon carbide single crystal 40 has been grown on the underside of a seed crystal. In this case, as shown in FIG. 7, fin structure 16a attached to shaft 16 is located above lid 15a.

[0030] The single crystal manufacturing apparatus 100 is configured as described above.

[0031] Next, the operation of the single crystal manufacturing apparatus 100 will be described.

[0032] First, as shown in FIG. 6, the seed crystal 30 is held at the tip (lower end) of the shaft 16 (step S10 in FIG. 2).

[0033] Next, the control unit 50 moves (lowers) the shaft 16, to which the seed crystal 30 is attached at its tip. As a result, the seed crystal 30 attached to the shaft 16 passes through the inside of the lid 15a provided on the crucible lid, and then contacts the surface of the solution 20 containing carbon and silicon contained in the crucible 12 (step S11 in FIG. 2). At this time, the fin structure 16a attached to the shaft 16 is positioned below the lid 15a.

[0034] Next, as shown in Fig. 7 , control unit 50 slowly moves (raises) shaft 16 upward, thereby growing silicon carbide single crystal 40 on the underside of seed crystal 30 being pulled up (step S12 in Fig. 2 ). At this time, fin structure 16a attached to shaft 16 passes inside lid 15a. Because seed crystal 30 is integrated with silicon carbide single crystal 40, seed crystal 30 is not illustrated in Fig. 7 to distinguish it from silicon carbide single crystal 40.

[0035] Thereafter, when crystal growth is to be terminated, control unit 50 stops the lifting of shaft 16. This terminates the growth of silicon carbide single crystal 40. At this time, fin structure 16a attached to shaft 16 is finally positioned above lid 15a. By operating single crystal manufacturing apparatus 100 in the above manner, silicon carbide single crystal 40 can be manufactured.

[0036] <Effects of the Embodiments> In a growth method in which a silicon carbide single crystal is grown below a seed crystal, if stress is generated within the seed crystal, the silicon carbide single crystal grown using the seed crystal is manufactured in a stressed state, and therefore there is a risk of cracks occurring inside. When a silicon carbide single crystal in which such cracks have occurred is sliced ​​and the cross section is observed, it is possible to confirm, for example, a mode in which multiple linear cracks are lined up, or a mode in which ring-shaped cracks have occurred around the center of the circle that is the cross section.

[0037] Since such cracked silicon carbide single crystals are not suitable for use as substrates or the like, it is important to suppress the occurrence of cracks.

[0038] Here, Figure 8 shows, in a table, the relationship between the shape accuracy of the wafer (seed crystal) immediately after cutting with a wire saw (step S2 in Figure 1) and cracks in the grown silicon carbide single crystal. This table shows the results obtained from experiments and calculations conducted by the present inventors. Many of the wafers in which cracks occurred had large SORIs, but No. 10 had a small SORI. For this reason, it would seem that wafer No. 10 has low internal stress and is suitable for use as a seed crystal. However, linear cracks were observed in the silicon carbide single crystal grown using wafer No. 10 as a seed crystal.

[0039] Therefore, when the value a was evaluated, all of the samples in which no cracks occurred after growth had a_Max of 1×10 －４ In wafer No. 10, a_Max was less than 1×10, similar to other wafers in which cracks were confirmed in the grown silicon carbide single crystal. －４ From this, it can be seen that the value of a_Max is small (1 × 10 －４ It can be seen that by selecting a wafer having a small value of a (less than a) as a seed crystal, it is possible to suppress the occurrence of cracks in the grown silicon carbide single crystal. In other words, a wafer having a small value of a has a small internal stress. Therefore, when a silicon carbide single crystal is grown using such a wafer as a seed crystal, the stress generated within the silicon carbide single crystal can be reduced.

[0040] As shown in Figure 1, when the sites for evaluating the value a are square, there is an area on the periphery where the value a is not evaluated. This area where the value a is not evaluated tends to increase as the site area is enlarged. For this reason, it is preferable to select sites for measuring the value a so that the value a of the entire wafer can be evaluated as much as possible, for example by reducing the area.

[0041] In the method for producing a silicon carbide single crystal according to the present embodiment, a_Max is 1×10 －４ By selecting seed crystals of less than this size and using them for growing silicon carbide single crystals, it is possible to reduce the occurrence of cracks in the grown silicon carbide single crystals.

[0042] When the seed crystal wafer is bonded to a jig, the wafer is forcibly flattened (deformed), so the SORI and the difference Δh must be measured without bonding it to a jig.

[0043] Furthermore, in this embodiment, the seed crystal can be selected at the time of cutting with the wire saw (step S2 in FIG. 1 ), eliminating the need to polish unused wafers, thereby reducing the manufacturing cost of silicon carbide single crystals.

[0044] The invention made by the present inventors has been specifically described above based on the embodiments thereof, but it goes without saying that the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the invention.

[0045] REFERENCE SIGNS LIST 1 wafer 1a first main surface 2 three-point reference plane 10 container 11 pedestal 12 crucible 16 shaft 14 coil 15a lid 16a fin structure 17 thermocouple 20 solution 30 seed crystal 40 silicon carbide single crystal 50 control unit 100 single crystal manufacturing apparatus ST, ST1, ST2, ST3 site

Claims

1. (a) preparing a seed crystal having a first main surface and a second main surface opposite to the first main surface and made of a silicon carbide single crystal; (b) measuring the difference in height Δh between the highest point and the lowest point of the first main surface in a direction perpendicular to a three-point reference plane of the first main surface in each of a plurality of regions aligned on the first main surface of the seed crystal, and dividing the difference Δh by the area S of the region to obtain a value a=Δh / S (unit: mm －１ (c) calculating a_Max<1×10, where a_Max is the maximum value of the values ​​a in all the regions of the first main surface; －４ (d) growing a single crystal comprising silicon carbide on a surface of the seed crystal that has passed the step (c).

2. The method for producing a silicon carbide single crystal according to claim 1, further comprising: (c1) polishing the seed crystal that has passed the step (c) prior to the step (d).

3. A method for producing a silicon carbide single crystal as defined in claim 1, wherein the step (d) comprises: (d1) a step of moving an axis having the seed crystal that passed the step (c) attached to its tip downward, thereby bringing the bottom surface of the seed crystal into contact with a solution contained in a crucible; and (d2) a step of growing the single crystal on the bottom surface of the seed crystal by moving the axis upward, wherein the solution contains carbon and silicon.

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