JP6695182B2 - Method for producing seed crystal for growing silicon carbide single crystal and method for producing silicon carbide single crystal ingot - Google Patents

Description

  This invention provides a method for producing a seed crystal used when growing a silicon carbide single crystal on a seed crystal by the sublimation recrystallization method, and a silicon carbide single crystal on the growth surface of the obtained seed crystal by the sublimation recrystallization method. The present invention relates to a method for producing a grown silicon carbide single crystal ingot.

  Silicon carbide (hereinafter referred to as SiC) is a wide band gap semiconductor having a wide band gap of 2.2 to 3.3 eV, and due to its excellent physical and chemical characteristics, it has been researched and developed as an environment resistant semiconductor material. Is being done. Particularly in recent years, attention has been paid to materials for short-wavelength optical devices from blue to ultraviolet, high-frequency electronic devices, high-voltage / high-power electronic devices, and the like, and research and development for producing devices (semiconductor elements) using SiC have become popular. There is.

  In order to put SiC devices into practical use, it is indispensable to produce large-diameter SiC single crystals, and most of them are bulk SiC produced by a sublimation recrystallization method using a seed crystal (called an improved Rayleigh method). A method of growing a single crystal has been adopted (see Non-Patent Document 1). That is, a SiC sublimation raw material is accommodated in the crucible, a seed crystal made of a SiC single crystal is attached to the lid of the crucible, and the raw material is sublimated to grow the SiC single crystal on the seed crystal by recrystallization. .. Then, after obtaining a substantially cylindrical SiC bulk single crystal (hereinafter referred to as a SiC single crystal ingot), generally, a SiC single crystal substrate is manufactured by cutting out to a thickness of about 300 to 600 μm, and power is consumed. It is used for the manufacture of SiC devices in the electronics field and the like.

By the way, in the SiC single crystal, in addition to hollow hole-shaped defects called micropipes penetrating in the growth direction, crystal defects such as dislocation defects and stacking faults exist. Since these crystal defects deteriorate device performance, the reduction thereof is an important issue in application of SiC devices. Among these, the dislocation defects include threading edge threading dislocations, basal plane dislocations, and threading screw dislocations. For example, in a commercially available SiC single crystal substrate, threading screw dislocations are 8 × 10 ^{2 to} 3 × 10 ³ (dislocations / cm ² ), and threading edge dislocations are 5 × 10 ³ to 2 × 10 ⁴ (dislocations / cm ² ). ² ) and basal plane dislocations are reported to be present in the range of 2 × 10 ³ to 2 × 10 ⁴ (dislocations / cm ² ) (see Non-Patent Document 2).

  In recent years, researches and investigations on crystal defects of SiC and device performance have been advanced, and it has been reported that threading screw dislocation defects cause a leak current of a device and shorten the life of a gate oxide film. 3 and 4), a SiC single crystal ingot with a reduced threading screw dislocation density is required to manufacture a high-performance SiC device.

Yu. M. Tairov and V. F. Tsvetkov, Journal of Crystal Growth, vol. 52 (1981) pp.146-150 Noboru Otani, Proceedings of 17th Lecture Meeting on SiC and Related Wide Gap Semiconductors Research Group, 2008, p8 Q. Wahab et al. Appl. Phys. Lett 76 (2000) 2725 Denso Technical Review Vol. 16 (2011) 90

  In the production of a SiC single crystal ingot by the sublimation recrystallization method using a seed crystal, the dislocation density of threading screw dislocations on the growing SiC single crystal side tends to increase at the interface between the seed crystal and the SiC single crystal to be grown.

  Therefore, an object of the present invention is to reduce the dislocation density by suppressing threading screw dislocations generated near the interface between the seed crystal and the grown SiC single crystal, so that the SiC single crystal having a small threading screw dislocation density from the initial stage of growth. It is intended to provide a method for producing a seed crystal capable of producing an ingot.

  The present inventors diligently studied means for suppressing the influence of the increase in screw threading dislocations at the interface between the seed crystal and the grown SiC single crystal in the growth of the SiC single crystal using the sublimation recrystallization method. As a result, when a silicon carbide single crystal ingot was cut, a cut surface was mechanically polished, and a certain amount of the surface was removed by chemical mechanical polishing (CMP), a seed crystal was used, sublimation recrystallization was performed. The present invention was found to be able to suppress the generation of threading screw dislocations by growing a SiC single crystal on the seed crystal by the method.

That is, the gist of the present invention is as follows.
(1) A method for producing a seed crystal used when growing a silicon carbide single crystal on a seed crystal by a sublimation recrystallization method, which comprises cutting a silicon carbide single crystal ingot, mechanically polishing a cut surface, A method for producing a seed crystal for growing a silicon carbide single crystal, which comprises removing at least a depth of 2 μm from the surface of the polishing surface by chemical mechanical polishing.
(2) A method for producing a silicon carbide single crystal ingot, which comprises growing a silicon carbide single crystal by a sublimation recrystallization method on a growth surface of a seed crystal produced by the method described in (1).

  According to the seed crystal for growing a SiC single crystal obtained by the present invention, when the SiC single crystal is grown by the sublimation recrystallization method, an increase in threading screw dislocation in the vicinity of the interface between the seed crystal and the growing SiC single crystal is suppressed. It can be suppressed to reduce the density of threading screw dislocations that adversely affect the device. Further, by using this seed crystal, an SiC single crystal having a low dislocation density can be grown from the initial stage of growth. Therefore, the obtained SiC single crystal ingot allows the SiC substrate to be cut out even from the initial growth region, which is industrially very useful in that the yield of the SiC substrate production can be improved.

FIG. 1 is a schematic sectional view showing a single crystal manufacturing apparatus for manufacturing a SiC single crystal ingot. FIG. 2A and FIG. 2B show how an evaluation SiC single crystal substrate used for evaluating the threading dislocation density in the vicinity of the seed crystal interface of the SiC single crystal ingots obtained in Examples and Comparative Examples is cut out. It is a schematic explanatory view showing. FIG. 3 is a graph showing how the threading dislocation density changes with the growth height of the SiC single crystal ingots obtained in Examples and Comparative Examples.

Hereinafter, the present invention will be described in more detail.
As described above, the object of the present invention is to suppress threading screw dislocations generated in the vicinity of the interface between the seed crystal and the grown SiC single crystal when the SiC single crystal is grown by the sublimation recrystallization method, Is to reduce the dislocation density of Si so that a SiC single crystal ingot with a small threading screw dislocation density can be obtained from the initial stage of growth.

  Generally, for a seed crystal for growing a SiC single crystal, a previously obtained SiC single crystal ingot is cut, and a cut surface is mechanically polished to produce a seed crystal. Therefore, it is considered that a damaged layer is formed on the surface of the seed crystal by such a processing step. When a seed crystal having a damage layer on the surface is heated to a high temperature, sublimation selectively occurs from a portion of the damage layer where the strain is large, and irregularities called latent scratches are formed on the surface. When crystal growth starts in this uneven portion, threading screw dislocations are likely to occur due to mismatch with surrounding growth steps.

  Here, the chemical mechanical polishing does not newly generate a damaged layer on the seed crystal, and the damaged layer generated by the mechanical polishing can be removed. Therefore, the present inventors further chemically damage the growth surface side of the seed crystal by chemical mechanical polishing before growing the SiC single crystal by the sublimation recrystallization method, so that the damage layer that causes latent scratches is formed in advance. The present invention has been accomplished based on the idea of removing.

  By the way, as described in, for example, JP-A-2014-210687 (paragraph 0043) and JP-A-2014-024703 (paragraph 0052), chemical mechanical polishing may be performed when a seed crystal is manufactured. .. However, although these are subjected to chemical mechanical polishing for the purpose of removing the surface damage layer, the depth of the surface damage layer is confirmed by a surface roughness observation device such as an AFM (atomic force microscope). The thickness removed by chemical mechanical polishing is about 0.5 μm, and in that case, it is not possible to completely remove the deeper damaged layer that causes latent scratches.

  Therefore, as a result of conducting various experiments and examining the depth to which the growth surface side of the seed crystal should be removed by chemical mechanical polishing, the effect of the present invention is obtained unless the surface after the mechanical polishing surface is removed to a depth of at least 2 μm. It was found that the reduction of threading screw dislocation density, which is, does not occur. From this, it is considered that the damage layer that causes a latent scratch exists up to a depth of less than 2 μm from the surface of the seed crystal. There is no upper limit to the depth to be removed by chemical mechanical polishing, but chemical mechanical polishing takes time, so removing more than 2 μm on the growth surface side of the seed crystal is necessary from the viewpoint of industrial production efficiency. Not preferable. The amount removed by chemical mechanical polishing can be grasped by comparing the thicknesses before and after polishing.

  Known methods can be adopted for the chemical mechanical polishing in the present invention. For example, a slurry containing ultrafine suspended particles such as colloidal silica may be used and processed by using a polishing pad. Further, regarding the back surface of the seed crystal opposite to the growth surface on which the SiC single crystal is grown, even if the damaged layer remains, it does not affect the generation of threading screw dislocation density during crystal growth, so chemical mechanical polishing is essential. However, even if the back surface of the seed crystal is further chemically mechanically polished to improve the smoothness, for example, for the purpose of increasing the adhesion when mounting the seed crystal on the inner surface of the graphite upper lid that forms the graphite crucible. Good.

  Further, the cutting of the SiC single crystal ingot at the time of producing the seed crystal can be performed in the same manner as a known method by using a wire saw or the like, and the mechanical polishing of the cut surface also includes diamond particles or the like. A known method can be performed by using a polishing liquid. The method for producing a seed crystal in the present invention is not limited to the diameter of the obtained seed crystal.

  Hereinafter, the present invention will be described more specifically based on Examples and the like. The present invention is not limited to the examples below.

(Example 1)
FIG. 1 is an apparatus for manufacturing a SiC single crystal ingot according to an embodiment of the present invention, showing an example of a single crystal growth (manufacturing) apparatus by an improved Rayleigh method (sublimation recrystallization method). Crystal growth is performed by sublimating the SiC sublimation raw material 1 loaded in the graphite crucible body 4 forming the graphite crucible by induction heating, and the SiC single crystal substrate arranged on the graphite upper lid 3 also forming the graphite crucible. It is performed by recrystallizing it on the seed crystal 2 consisting of.

  In Example 1, a SiC single crystal ingot previously obtained by the sublimation recrystallization method was cut with a wire saw and processed into a disk-shaped thick plate having a thickness of about 1.1 mm. Next, the cut surface was mechanically polished by using a polishing liquid containing diamond particles. In this mechanical polishing, processing was performed so that one surface of the cut surface with the wire saw was polished to a depth of about 50 μm. Next, one surface corresponding to the crystal growth surface of the obtained disk-shaped thin plate was further subjected to chemical mechanical polishing to remove it to a depth of 2 μm. At that time, a slurry containing ultrafine suspended particles such as colloidal silica and a polishing pad were used for processing. In this way, a seed crystal 2 having a thickness of 1.0 mm, a diameter of 51 mm and a (000-1) plane having an off angle of 4 ° as a crystal growth surface was produced.

  Using the seed crystal 2 obtained as described above, a SiC single crystal was grown by the single crystal growth apparatus shown in FIG. 1 above to manufacture a SiC single crystal ingot. Here, in the single crystal growth apparatus, the graphite crucible main body 4 and the graphite upper lid 3 forming the graphite crucible are coated with the graphite felt 5 for heat shield, and the graphite support inside the double quartz tube 6 is supported. It is installed on the rod 7. After the inside of the double quartz tube 6 is evacuated by the vacuum evacuation device 8, high-purity Ar gas and nitrogen gas are introduced through the pipe 9 while controlling the Ar: nitrogen = 15: 1 by the mass flow controller 10. Then, the pressure in the quartz tube (growth atmosphere pressure) was adjusted by the vacuum exhaust device 8 to 80 kPa. Under this pressure, an electric current was passed through the work coil 11 to raise the temperature, and the temperature of the seed crystal 2 attached to the graphite upper lid 3 was raised to 2200 ° C. Then, the growth atmosphere pressure was reduced to 1.3 kPa over 30 minutes to sublimate the SiC sublimation raw material 1 loaded in the graphite crucible body 4, while the (000-1) plane of the seed crystal 2 was changed to a crystal growth plane. The crystal growth was performed for 30 hours.

  By the above process, a SiC single crystal ingot having a height of 8 mm and a diameter of 51 mm was obtained. Regarding the obtained SiC single crystal ingot, first, at a position of a growth height of 2 mm, which is represented by the height from the crystal growth surface of the seed crystal, parallel to the seed crystal surface (that is, an off angle of 4 °). The evaluation substrate A was cut out so that The obtained evaluation substrate A was immersed in molten KOH at 520 ° C. for 5 minutes so that the entire surface of the substrate was immersed in molten KOH etching, and the surface of the etched evaluation substrate A was observed with an optical microscope (magnification: 80 times). And the dislocation density was measured. Here, according to the method described in J. Takahashi et al., Journal of Crystal Growth, 135, (1994), 61-70, the shell-shaped pits are basal plane dislocations, and the small hexagonal pits are threading edge dislocations. Using the medium- and large-sized hexagonal pits as threading screw dislocations, dislocation defects due to etch pit shapes were classified and the dislocation density of threading screw units was calculated. As a result, the dislocations were distributed almost uniformly in the remaining central circle area excluding the ring-shaped area (the area having a width of 2.55 mm from the outer circumference) that occupies a portion corresponding to 5% in the inner diameter direction from the outer circumference of the evaluation substrate A. It was confirmed that the density was dispersed. Similarly, when the seed crystal obtained in Example 1 was also confirmed, the dislocation density was uniformly distributed in the plane.

  Next, as shown in FIG. 2A, the direction in which the SiC single crystal ingot obtained above is inclined 4 ° in the direction opposite to the off direction of the c-axis of the seed crystal 2 becomes the c-axis of the crystal. Thus, a substrate for evaluation B having an off angle on the (0001) plane in the direction opposite to the seed crystal was obtained by cutting out (the diagonal thick solid line in the drawing indicates the cut out surface). At that time, the evaluation substrate B was cut out so as to pass through the diametrical center of the seed crystal, and the surface of the evaluation substrate B contained a crystal region made of the seed crystal and a crystal region made of the grown SiC single crystal. As shown in FIG. 2B, the angle between the side a on the surface of the evaluation substrate B and the side b on the diameter direction of the seed crystal is 8 °, and the height h is The positional relationship is such that a right triangle is formed. By using this cutting method, it is possible to continuously observe the change in dislocation density with growth in the crystal growth direction. The interface 17 between the seed crystal 2 and the grown SiC single crystal 16 can also be directly observed. Furthermore, since the change in the growth direction is substantially magnified to 1 / sin 8 ° (about 7 times), more detailed and highly accurate dislocation density measurement can be performed.

  The obtained evaluation substrate B was immersed in molten KOH at 520 ° C. for 5 minutes so that the entire surface of the substrate was immersed in molten KOH etching as in the case of the above-described evaluation substrate A. The measurement point on the a side (= h / sin8 °) shown in FIG. 2 (b) is mainly 2 mm (change in height h in the growth direction) so that the surface follows the change in height of the grown SiC single crystal. Was observed with an optical microscope (magnification: 80 times) for each dislocation density. The results are shown in Table 1. The growth height (mm) in Table 1 represents the height from the crystal growth surface of the seed crystal 2.

(Comparative Example 1)
In Comparative Example 1, a seed crystal was manufactured in the same manner as in Example 1 except that the crystal growth surface of the seed crystal was not subjected to chemical mechanical polishing. That is, a SiC single crystal ingot previously obtained by the sublimation recrystallization method was cut in the same manner as in Example 1 to be processed into a disk-shaped thick plate having a thickness of about 1.1 mm, and then mechanically polished in the same manner as in Example 1. Then, it was processed into a disk-shaped thin plate having a diameter of 51 mm and a thickness of 1.0 mm to obtain a seed crystal 2 according to Comparative Example 1. This seed crystal 2 has a (000-1) plane having an off angle of 4 ° as a crystal growth plane as in Example 1. Then, using the seed crystal 2 according to Comparative Example 1, a SiC single crystal ingot was manufactured under the same conditions as in Example 1.

  The obtained SiC single crystal ingot was cut out in the same manner as in Example 1 so that the (0001) plane had an off angle in the direction opposite to the seed crystal to obtain a substrate for evaluation B, and KOH was obtained in the same manner as in Example 1. Etching was performed and the dislocation density was measured with an optical microscope. The results are shown in Table 1.

Here, with respect to the results shown in Table 1, FIG. 3 is a graph showing the relationship between the growth height and the threading screw dislocation density at each measurement point in Example 1 and Comparative Example 1. As can be seen from the graph of FIG. 3, the SiC single crystal ingot of Comparative Example 1 manufactured by the conventional method has a threading screw dislocation density of about 9000 / cm ² at a growth height of 0.6 mm, whereas The SiC single crystal ingot of Example 1 manufactured using the manufacturing method of the invention has a low value of about 2000 pieces / cm ² . Further, in the growth height of 1.9 mm, the threading screw dislocation density of Comparative Example 1 is about 1000 / cm ² , whereas the SiC single crystal ingot of Example 1 manufactured by the manufacturing method of the present invention is It is reduced to about 200 pieces / cm ² . In particular, in the early stage of growth near the interface with the seed crystal, the threading dislocation density was high in the past, which was unsuitable for cutting out a SiC single crystal substrate, whereas according to the present invention, the growth height is about 2 mm. From this, it is understood that it is possible to cut out a SiC single crystal substrate having a quality higher than that of the seed crystal.

1: Sublimation raw material, 2: Seed crystal, 3: Graphite upper lid, 4: Graphite crucible body, 5: Graphite felt, 6: Double quartz tube, 7: Graphite support rod, 8: Vacuum exhaust device, 9: Piping, 10: Mass flow controller, 11: Work coil.

Claims (2)

A method for producing a seed crystal used when growing a silicon carbide single crystal on a seed crystal by a sublimation recrystallization method, comprising cutting a silicon carbide single crystal ingot and mechanically polishing the cut surface, A seed crystal is produced by removing at least a depth of 2 μm from the surface by chemical mechanical polishing , wherein the growth height of a silicon carbide single crystal grown using the seed crystal (from the crystal growth surface of the seed crystal is A method for producing a seed crystal for growing a silicon carbide single crystal , wherein the threading screw dislocation density at a height of 1.9 mm or more is not more than the threading screw dislocation density of the seed crystal .   A method for producing a silicon carbide single crystal ingot, which comprises growing a silicon carbide single crystal on a growth surface of a seed crystal produced by the method according to claim 1 by a sublimation recrystallization method.

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