Method for manufacturing silicon carbide single crystal

Abstract

To provide a method for manufacturing a silicon carbide single crystal having high reliability.SOLUTION: A method for manufacturing a silicon carbide single crystal includes steps for: (a) preparing a seed crystal having a first main surface and a second main surface on its opposite side and composed of the silicon carbide single crystal; (b) measuring difference Δh between a highest point and a 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 arranged in the first main surface of the seed crystal, and calculating a value a=Δh / S (unit: mm-1) obtained bt dividing the difference Δh by an area S of the regions; (c) determining the seed crystal as acceptable, the seed crystal satisfying a_Max<1×10-4 when a maximum value is a_Max of the values a in the whole region of the first main surface; and (d) growing a single crystal containing silicon carbide on the surface of the seed crystal accepted in the step (c).SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[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 manufactured, for example, by using a solution method or a sublimation method. The solution method is a method for manufacturing 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, thereby growing a silicon carbide single crystal on the seed crystal while pulling up the shaft. The sublimation method is described in Patent Document 1 (JP Patent Publication 2011-111372). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2011-111372 A Summary of the Invention [Problem to be solved by the invention]

[0004] When stress is generated in a seed crystal, a silicon carbide single crystal grown using the seed crystal is produced in a stressed state, and there is a risk of internal cracks occurring. Therefore, there is a demand for growing a silicon carbide single crystal using a seed crystal with less internal stress, thereby producing a silicon carbide single crystal with high reliability. [Means for solving the problem]

[0005] A method for producing a silicon carbide single crystal in one embodiment 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 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 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 comprising silicon carbide on a surface of the seed crystal that has passed the step (c). Effect of the Invention

[0006] According to one embodiment, a highly reliable silicon carbide single crystal can be manufactured. [Brief description of the drawings]

[0007] [Figure 1] 1 is a flow diagram showing a method for producing a silicon carbide single crystal in an embodiment. [Diagram 2] 2 is a flow chart showing a method for producing a silicon carbide single crystal, continuing from FIG. 1 . [Diagram 3] FIG. 2 is a plan view showing an example in which the main surface of a wafer is divided into sites. [Figure 4] FIG. 2 is a plan view illustrating a three-point reference plane. [Diagram 5] 1 is a cross-sectional view illustrating the difference between the highest point and the lowest point on the surface of a wafer on a three-point reference plane. [Figure 6] FIG. 1 is a diagram showing a configuration of a single crystal manufacturing apparatus according to an embodiment. [Figure 7] FIG. 2 is a diagram showing a state in which a silicon carbide single crystal is grown on a lower surface of a seed crystal in an embodiment. [Figure 8] 1 is a table showing the relationship between the shape accuracy of a wafer (seed crystal) and cracks in a grown silicon carbide single crystal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0009] In the following, a method for producing a silicon carbide single crystal is described, in which a seed crystal having a small internal stress is selected to prevent cracking of the silicon carbide single crystal when the silicon carbide single crystal is grown by a solution method using a seed crystal made of silicon carbide. The wafer 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 the silicon carbide single crystal.

[0010] <Method for producing silicon carbide single crystal> A method for producing a silicon carbide single crystal will be described with reference to Fig. 1 and Fig. 2. Fig. 1 and Fig. 2 are a flow chart showing the method for producing a silicon carbide single crystal. In particular, Fig. 1 shows a flow chart for preparing and selecting a seed crystal to be used in growing the 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 ingot cut into thin films (wafers) in this manner are seed crystals.

[0013] Next, it is measured whether the seed crystal wafer satisfies SORI<30μm (step S3 in FIG. 1). This measurement is performed after the cutting process in step S2, without performing a polishing process on the wafer surface. SORI is one of the parameters that represents the shape of the wafer when it is 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<30μm (if step S3 is No), the wafer is likely to have excessively large internal stress and is unsuitable for use as a seed crystal, so it is rejected (step S8 in FIG. 1).

[0014] If the wafer satisfies SORI<30 μm (if step S3 is Yes), the first main surface of the wafer, which has a first main surface and a second main surface opposite to the first main surface, is divided into sites (areas) in a three-point reference plane. FIG. 3 shows three examples of the first main surface of the wafer 1 divided into sites. FIG. 3 shows plan views of the wafer 1 divided into a site ST1 having a length and width of 10×10 mm, a site ST2 having a length and width of 15×15 mm, and a site ST3 having a length and width of 20×20 mm, from the left. The site division is performed virtually and does not involve processing such as carving boundaries between sites into the wafer 1. FIG. 3 shows a wafer 1 having 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 area of ​​the first main surface of the wafer (seed crystal) 1, which is substantially circular. The wafer 1 is not completely circular, and has a portion (orientation flat) where a part of the outer periphery is linear. Instead of such a portion, a notch may be present. Point A is positioned, for example, at a point where three radii of the substantially circular wafer 1 overlap with three radii that are at an angle of 120 degrees to each other. Point A is positioned 3% inside the diameter of the wafer 1 from the edge of the wafer 1. Ideally, the surface of the wafer is flat, but in reality, the wafer has warping or unevenness, so that each of the three points A is not necessarily located at the same height as all the points on the first main surface.

[0016] After dividing the sites as described above, the difference Δh (unit: mm) between the highest point and the lowest point of the height of the first main surface in 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, 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 is measured as the difference Δh in each site ST. Note that although the wafer 1 is generally a flat disk shape, in FIG. 5 the wafer 1 is shown in an excessively distorted shape to make the positional relationship between the highest point H and the lowest point L easier to understand.

[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 representation of the local warpage in the wafer.

[0018] Next, let a_Max be the maximum value of a among all the sites on the wafer. 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 met (No in step S6), 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).

[0019] Wafer a_Max<1×10 -4 If this is satisfied (if step S6 is Yes), the wafer has small internal stress and passes the test 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 calculations in step S5, and the judgment in step S6 are performed after the cutting step in step S2, without performing any surface treatment such as polishing on the wafer surface. If surface treatment such as polishing is performed on the wafer, the wafer will deform due to the Twyman effect, etc., and the original shape accuracy of the wafer cannot be evaluated. For this reason, the shape of the wafer immediately after the cutting step is measured here.

[0020] Next, the surface of the seed crystal (wafer) that passed the steps S6 and S7 in Fig. 1 is subjected to surface treatment such as polishing and etching (step S9 in Fig. 2). By performing such polishing, damage such as pits that occurred on the surface of the seed crystal during cutting (step S2 in Fig. 1) is removed. In addition, scratches that occurred during the cutting or polishing can be removed by the above 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 in this embodiment.

[0022] 6, a single crystal manufacturing apparatus 100 has a container (thermal insulating member) 10. The internal space of this container 10 is filled with, for example, argon gas, and a pedestal 11 that can rotate in the horizontal direction is disposed therein. 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 is at a high temperature, and the graphite (C, carbon) constituting 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 a bottom that is, for example, circular in plan view, and is provided with a cylindrical portion on the periphery of the circular bottom. That is, the crucible 12, which is a container, is a cylindrical body with a bottom. The lid 15a is disposed on the upper 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. Also, 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 has passed the test in steps S6 and S7 in FIG. 1 and has been subjected to the surface treatment in 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 inside of the shaft 16 has a hollow structure, and a thermocouple 17 for measuring the temperature in the vicinity of the seed crystal 30 is inserted into the shaft 16. The temperature in the vicinity of the seed crystal 30 can also be measured by disposing a radiation thermometer having a measurement 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 shaft 16 having the fin structure 16a to move in the vertical direction. The control unit 50 is configured to control the shaft 16 so that the shaft 16 having the fin structure 16a moves in the vertical direction through the inside of the lid 15a and the cylindrical part of the crucible 12. The control unit 50 is configured to move the shaft 16 having the seed crystal 30 attached to the tip end thereof 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 move 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, so that a single crystal made of silicon carbide can be grown on the lower surface of the seed crystal 30.

[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 in the vertical direction and to be able to rotate either clockwise or counterclockwise. This allows the pedestal 11 attached to the crucible holding shaft 18 and the crucible 12 placed on the pedestal 11 to be moved in the vertical direction and rotated in the horizontal direction by the crucible holding shaft 18. The crucible holding shaft 18 has a hollow structure inside, and is configured to allow a thermocouple or a radiation thermometer to be inserted therein for temperature measurement.

[0028] 6 shows a state in which the lower 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] On the other hand, Fig. 7 shows a state in which silicon carbide single crystal 40 is grown on the lower surface 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 having the seed crystal 30 attached to the tip thereof downward. 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 in order, 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 located below the lid 15a.

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

[0035] Thereafter, when crystal growth is to be ended, control unit 50 stops the ascent of shaft 16. This ends 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 embodiment> In a growth method in which a silicon carbide single crystal is grown beneath 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 there is a risk of internal cracks occurring. When a silicon carbide single crystal in which such cracks have occurred is sliced ​​into cross sections and the cross sections are observed, 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, can be confirmed.

[0037] Since silicon carbide single crystals containing such cracks are unsuitable for use as substrates or the like, it is important to suppress the occurrence of cracks.

[0038] FIG. 8 shows, in the form of a table, the relationship between the shape accuracy of the wafer (seed crystal) immediately after cutting with a wire saw (step S2 in FIG. 1) and cracks in the grown silicon carbide single crystal. This table shows the results obtained from experiments and calculations performed by the present inventors. Many of the wafers in which cracks occurred have a large SORI, but No. 10 has 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 confirmed in the silicon carbide single crystal grown using wafer No. 10 as a seed crystal.

[0039] When the above value a was evaluated, all samples that did not develop cracks after growth had an 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 The value is greater than or equal to 1. From this, it can be seen that a_Max is small (1×10 -4 It can be seen that by selecting a wafer having a smaller value of 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 smaller value of a has a smaller internal stress. Therefore, when a silicon carbide single crystal is grown using the 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 and of the same shape, 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 when the site area is enlarged. For this reason, it is preferable to select the 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 making the area small.

[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 having a diameter of less than 100 nm and using them for growing a silicon carbide single crystal, it is possible to reduce the occurrence of cracks in the grown silicon carbide single crystal.

[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), and it is not necessary to polish the wafers that are not used, which reduces the manufacturing cost of the silicon carbide single crystal.

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

[0045] 1 wafer 1a First main surface 2 3-point reference plane 10 containers 11 Pedestal 12 Crucible 16 axes 14 Coil 15a Lid 16a Fin structure 17 Thermocouple 20 solution 30 seed crystals 40 Silicon carbide single crystal 50 Control section 100 Single crystal manufacturing equipment ST, ST1, ST2, ST3 sites

Claims

1. (a) preparing a seed crystal having a first main surface and an opposite second main surface and made of a silicon carbide single crystal; (b) In each of a plurality of regions arranged on the first main surface of the seed crystal, 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 is measured, and a value a=Δh / S (unit: mm －１ ) calculating (c) When the maximum value of the values ​​a in all the regions of the first main surface is a_Max, a_Max<1×10 －４ A step of determining that the seed crystals satisfying the above criteria are acceptable; (d) growing a single crystal comprising silicon carbide on a surface of the seed crystal that has passed the step (c); The present invention relates to a method for producing a silicon carbide single crystal.

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

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

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