JP7109197B2 - Silicon carbide single crystal manufacturing method, silicon carbide single crystal ingot - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a silicon carbide single crystal and a silicon carbide single crystal ingot used therefor.

Silicon carbide (SiC), which is a semiconductor material, has a larger bandgap than Si (silicon), which is widely used as a substrate for devices. Research is being conducted to fabricate devices and the like.

A SiC device is formed by chemical vapor deposition (CVD) or the like on a SiC single crystal substrate obtained by processing a silicon carbide bulk single crystal grown by a sublimation method or the like to form an active region of the device. It is produced using a SiC epitaxial wafer on which an epitaxial layer (film) is formed.

A sublimation method is widely known as one method for producing a bulk single crystal of silicon carbide. In the sublimation method, a seed crystal made of a silicon carbide single crystal is placed in a graphite crucible, and the crucible is heated to supply a sublimation gas sublimated from the raw material powder in the crucible to the seed crystal, thereby making the seed crystal larger. This is a method of growing a SiC single crystal. When the seed crystal is adhered to the graphite member and arranged, if air bubbles or foreign substances are present on the adhesion surface, the penetrating defect D may extend from the back surface of the seed crystal. (Patent Document 1, Patent Document 2). This is considered to be caused by re-sublimation from the rear surface of the seed crystal, and a through hole may be formed in the wafer like a micropipe. In Patent Document 1, after flattening the rear surface of the seed crystal and the rear surface of the crucible lid (graphite member), they are physically brought into close contact without adhesion and mechanically fixed to prevent the generation of air bubbles during adhesion. there is However, mechanical fixation is difficult to perform reproducibly. In addition, in Patent Document 2, in the process of bonding the seed crystals, it is said that heat treatment for drying and curing the adhesive is performed under specific conditions to prevent poor adhesion, but it is impossible to completely remove air bubbles and foreign matter during bonding. difficult.

Since silicon carbide single crystals are used as substrates for electronic devices, conductive silicon carbide single crystals doped with impurity atoms such as nitrogen are usually formed. In particular, when used as a substrate for a power device, it is required to have a low resistance and is doped with nitrogen at a high concentration. Usually, as a seed crystal for growing a conductive silicon carbide single crystal, a seed crystal made of a silicon carbide single crystal doped to the same degree as the single crystal to be obtained is used. Patent Literature 3 describes a manufacturing method in which the concentration ratio of the impurity additive element between the seed crystal and the grown crystal grown on the seed crystal is within 5 times.

Japanese Patent No. 4523733 Japanese Patent No. 4224755 JP 2010-95397 A

FIG. 3 is a microscopic image of a cross section around the back surface 50b side of silicon carbide seed crystal 50 during growth of silicon carbide single crystal 40 . From the image, it can be seen that many large penetrating defects D that can be visually confirmed extend from the rear surface of the seed crystal. The penetrating defect D extends from the rear surface of the seed crystal, and is considered to be caused by air bubbles or foreign matter generated when the seed crystal and the graphite member are adhered to each other. It is difficult to form a good epitaxial film on a wafer having such threading defects D.

With the current technology, it is possible to detect large voids by, for example, ultrasonic measurement, but it is difficult to detect small defects such as air bubbles and foreign matter. Further, as described above, since the seed crystal of the silicon carbide single crystal having low resistance is doped with nitrogen at a high concentration, it is opaque to visible light. It is difficult to judge the quality by directly looking at the bonded portion through the lens. Therefore, even if the seed crystal contains defects such as small air bubbles or foreign matter at the stage of bonding the seed crystal to the graphite member, the crystal can be grown as it is, and the occurrence of the through defect D caused by it can be suppressed. difficult to do.

The present invention has been made in view of such circumstances, and prevents the use of a seed crystal and a graphite member in a state in which there is poor adhesion between a seed crystal and a graphite member during the growth of a single crystal of silicon carbide by a sublimation method. It is an object of the present invention to provide a method for manufacturing a silicon carbide single crystal that can avoid the problem of extension of penetrating defects from the rear surface of the crystal. Another object of the present invention is to provide a silicon carbide single crystal ingot having a seed crystal used in this silicon carbide single crystal manufacturing method.

In order to solve the above problems, the present invention employs the following means.

(1) A method for producing a silicon carbide single crystal according to one aspect of the present invention is a method for producing a silicon carbide single crystal by a sublimation method, in which a graphite member in a crucible has a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ . A first interface inspection step of attaching a silicon carbide seed crystal suppressed to the following and inspecting the state of the interface between the graphite member and the silicon carbide seed crystal, supplying sublimation gas of silicon carbide raw material and nitrogen gas, and a single crystal growth step of growing a silicon carbide single crystal doped with nitrogen at a concentration of 10 ¹⁸ cm ⁻³ or higher.

(2) In the method for producing a silicon carbide single crystal according to (1) above, the silicon carbide seed crystal may have a thickness of 1 mm or more.

(3) In the first interface inspection step of the method for producing a silicon carbide single crystal according to either (1) or (2), bubbles are detected at the interface between the graphite member and the silicon carbide seed crystal. In this case, vacuum defoaming or pressure defoaming can be performed to remove the air bubbles.

(4) In the method for producing a silicon carbide single crystal according to any one of (1) to (3) above, in the first interface inspection step, foreign matter is detected at the interface between the graphite member and the silicon carbide seed crystal. is detected, the silicon carbide seed crystal can be temporarily removed from the graphite member, the foreign matter removed, and then attached again.

(5) In the method for producing a silicon carbide single crystal according to any one of (1) to (4) above, silicon carbide provided with a low-doped region having a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ or less. A single crystal ingot may be provided and a portion of the lightly doped region may be cut from the ingot and used as the silicon carbide seed crystal.

(6) In the method for producing a silicon carbide single crystal according to any one of (1) to (5) above, after the step of growing the single crystal, the silicon carbide seed crystal and the carbide grown thereon The method may further include a second interface inspection step of removing the silicon single crystal ingot from the graphite member and inspecting the state of the interface between the silicon carbide seed crystal and the silicon carbide single crystal from the silicon carbide seed crystal side. can.

(7) A silicon carbide single crystal ingot according to an aspect of the present invention is provided with a region having a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ or less and a region having a nitrogen concentration of 3×10 ¹⁸ cm ⁻³ or more. .

Since the seed crystal used for growing the silicon carbide single crystal in the present invention has a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ or less, one side of the seed crystal is transparent to the extent that the other side can be visually observed. is. Therefore, when the seed crystal is attached to the graphite member, the back side of the seed crystal is visually observed, and the state of the interface between the seed crystal and the graphite member, specifically, there is a penetrating defect (macro defect) from the back surface of the seed crystal. It is possible to inspect for the presence of air bubbles, foreign matter, etc., which are the factors that cause .

As a result of the inspection, if air bubbles are present, remove them by performing vacuum defoaming or pressure defoaming. As a result, it is possible to grow the silicon carbide single crystal on the seed crystal without air bubbles or foreign matter at the interface with the graphite member. In this way, the generation factor of the threading defect from the rear surface of the seed crystal is found and removed at an early stage of the manufacturing process, thereby causing the problem of basal plane dislocations caused by the threading defect extending from the rear surface of the seed crystal. can be avoided, and a high-quality SiC device can be obtained as a final product.

Further, since the silicon carbide single crystal ingot of the present invention is provided with a region having a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ or less, this portion is cut out and used as a seed crystal for growing silicon carbide single crystals. can do.

1 is a longitudinal sectional view of a silicon carbide single crystal manufacturing apparatus according to an embodiment of the present invention; FIG. (a) to (d) are diagrams showing a flow of manufacturing steps of a silicon carbide single crystal according to an embodiment of the present invention. 4 is a cross-sectional image of the periphery of the back surface side of the silicon carbide seed crystal during growth of the silicon carbide single crystal.

Hereinafter, the present invention will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, in order to make it easier to understand the features of the present invention, there are cases where the characteristic parts are enlarged for convenience, and the dimensional ratios of each component are different from the actual ones. There is In addition, the materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited to them, and can be implemented with appropriate modifications within the scope of the present invention. .

FIG. 1 is a vertical cross-sectional view of a silicon carbide single crystal manufacturing apparatus 100 according to an embodiment of the present invention, showing a state in which an ingot of silicon carbide single crystal 10 (silicon carbide single crystal ingot) is formed. ing. A silicon carbide single crystal manufacturing apparatus 100 includes at least a crucible 101 , a graphite member 102 in which a silicon carbide seed crystal (seed) 20 is arranged at one end side of the crucible 101 , and a coil surrounding the outer wall of the crucible 101 . 103 , and the raw material 30 is accommodated at the other end of the crucible 101 . It is preferable that silicon carbide single crystal manufacturing apparatus 100 further includes a tapered guide 104 whose diameter increases from graphite member 102 toward raw material 30 . Further, silicon carbide single crystal manufacturing apparatus 100 may have a heating element (not shown) that generates heat by induction heating of coil 103 between crucible 101 and coil 103 .

In silicon carbide single crystal manufacturing apparatus 100 , crucible 101 is heated by passing an alternating current through coil 103 , and sublimation gas (raw material gas) is generated from raw material 30 . , to the silicon carbide seed crystal 20 on the graphite member 102 . By supplying sublimation gas to silicon carbide seed crystal 20 , an ingot of silicon carbide single crystal 10 is crystal-grown on the surface of silicon carbide seed crystal 20 .

FIGS. 2(a) to 2(d) are diagrams showing the flow of manufacturing steps of a silicon carbide single crystal according to an embodiment of the present invention, in each step, the ingot peripheral portion 105 of the silicon carbide single crystal 10 ( 1) is enlarged. A method for manufacturing a silicon carbide single crystal will be described below with reference to FIGS. 2(a) to 2(d).

(Seed crystal preparation step)
First, a seed crystal for use in producing the silicon carbide single crystal of the present invention is prepared. As shown in FIG. 2A, silicon carbide seed crystal 20A is attached to graphite member 102 using a carbon adhesive or the like, and silicon carbide single crystal 10A is grown on the surface of silicon carbide seed crystal 20A by sublimation. . During the silicon carbide single crystal growth in this seed crystal preparation step, growth is performed without supplying nitrogen gas (no nitrogen doping). A normal silicon carbide single crystal is grown while adjusting the amount of nitrogen (N ₂ ) supplied into the crucible 101 in order to obtain a wafer having a desired resistance value. Further, in general, a seed crystal is obtained from a portion of the manufactured silicon carbide ingot and grown again, which is repeated. At this time, it was common to use an undoped seed crystal when manufacturing an undoped ingot, and to use a nitrogen-doped seed crystal when manufacturing a nitrogen-doped ingot. In the past, when an n-type silicon carbide wafer with a high nitrogen concentration was produced, it was common to use a seed crystal with a similarly high nitrogen concentration. In contrast, the present invention is different from the prior art in that an n-type silicon carbide ingot (and thus a silicon carbide wafer) with a high nitrogen concentration is manufactured using a seed crystal with a low nitrogen concentration.

The obtained ingot of silicon carbide single crystal 10A is removed from graphite member 102, and a portion 10a thereof is cut out. Cut portion 10a corresponds to silicon carbide seed crystal 20B used for manufacturing the silicon carbide single crystal of the present invention.

The thickness of the silicon carbide seed crystal is preferably 1 mm or more, more preferably 2 mm or more. If the seed crystal is less than 1 mm, it is likely to crack due to the difference in thermal expansion coefficient from graphite, and crystal defects may occur in the seed crystal when the temperature is raised during recrystallization. As for the upper limit, 10 mm or less is preferable because if the thickness is too large, the cost increases.

Silicon carbide seed crystal 20B is not doped with nitrogen and has a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ or less. It has transparency to the extent that it can be visually (transparently) seen. In particular, when the silicon carbide seed crystal has a thickness of 1 mm or more, it becomes opaque to visible light in the thickness direction at a nitrogen concentration of 3×10 ¹⁸ cm ⁻³ or more, which is the nitrogen concentration of substrates normally used for devices. Therefore, it is highly effective to increase the transmittance by suppressing the nitrogen concentration to 5×10 ¹⁷ cm ⁻³ or less.

Further, the nitrogen concentration of the seed crystal is preferably 5×10 ¹⁷ cm ⁻³ or less, more preferably 3×10 ¹⁷ cm ⁻³ or less, and further preferably 1×10 ¹⁷ cm ⁻³ or less. preferable. A lower nitrogen concentration increases the transparency and improves the accuracy of the inspection. Especially when the substrate is thick, a low nitrogen concentration is more effective.

In the seed crystal preparation step described above, a method of growing without supplying nitrogen gas was shown. A very small amount of nitrogen gas may be supplied within the range of a concentration of 5×10 ¹⁷ cm ⁻³ or less.

(First interface inspection step)
Next, using this silicon carbide seed crystal 20B, the silicon carbide single crystal of the present invention is manufactured. As shown in FIG. 2B, silicon carbide seed crystal 20B is attached to graphite member 102 using a carbon adhesive or the like.

Subsequently, utilizing the transparency of the silicon carbide seed crystal 20B, the contact surface (rear surface) 20b side with the graphite member from the single crystal growth surface 20a side is observed visually or using a predetermined observation means. , the state of the interface (first interface) between graphite member 102 and silicon carbide seed crystal 20B is inspected. The inspection of the state of the interface here refers to whether or not factors (bubbles, foreign matter, etc.) that cause penetrating defects extending from the back surface of the silicon carbide seed crystal 20B are present at the interface with the graphite member 102 or not. means inspection.

If the inspection of the interface state detects the existence of the penetrating defect generating factor P extending from the rear surface as shown in FIG. 2B, it is removed. For example, if air bubbles are present at the interface, they are removed by vacuum defoaming or pressure defoaming. Further, when foreign matter is present at the interface, silicon carbide seed crystal 20B is temporarily removed from graphite member 102, and foreign matter adhering to silicon carbide seed crystal 20B or graphite member 102 is removed by tweezers or the like. Reinstall on top.

After removing the penetrating defect generating factor P from the rear surface of the seed crystal, the state of the interface is inspected again. The removal work and the inspection work are repeated until the generation factor P of penetrating defects extending from the back surface, such as air bubbles or foreign matter, is no longer detected visually or by a predetermined observation means, or until the surface density is reduced to a predetermined criterion or less. It is preferable that no bubbles or foreign matter be visually observed.

(Single crystal growth step)
Next, through the first interface inspection step, the sublimation gas of the silicon carbide raw material and the nitrogen gas are supplied by the sublimation method in a state where the generation factor P of penetrating defects extending from the back surface to the interface with the graphite member 102 is reduced. Then, as shown in FIG. 2(c), a silicon carbide single crystal 10B is grown on the surface of the silicon carbide seed crystal 20B. The supply of nitrogen gas is adjusted so that the growing silicon carbide single crystal 10B is doped with nitrogen at a concentration of 3×10 ¹⁸ cm ⁻³ or more. As a result, the nitrogen concentration in silicon carbide single crystal 10B is six times or more the nitrogen concentration in silicon carbide seed crystal 20B.

The nitrogen concentration in the single crystal growth step is preferably 3×10 ¹⁸ cm ⁻³ or more, more preferably 5×10 ¹⁸ cm ⁻³ or more. When the silicon carbide single crystal has a high nitrogen concentration, the carrier concentration increases, and the resistance of the substrate when used as a device can be lowered.

Finally, as shown in FIG. 2D, the ingot of grown silicon carbide single crystal 10B is removed from graphite member 102 together with silicon carbide seed crystal 20B.

It should be noted that, during the growth of silicon carbide single crystal 10B, by temporarily stopping the supply of nitrogen gas or temporarily reducing the amount of supply of nitrogen gas, the ingot of silicon carbide single crystal 10B after growth has a nitrogen concentration of is less than 5×10 ¹⁷ cm ⁻³ will be provided. Regions other than the lightly doped region have a concentration necessary for device operation (usually 3×10 ¹⁸ cm ⁻³ or higher). The position of the lightly doped region can be adjusted by the timing of stopping the supply of nitrogen gas or the timing of reducing the amount of nitrogen gas supplied.

Since the portion of the low-doped region has transparency, it is cut from the ingot and used as a silicon carbide seed crystal for performing the first interface inspection step when growing another silicon carbide single crystal. be able to.

When the lightly doped region is provided as described above, the nitrogen concentration changes continuously at the boundary with another region (highly doped region), that is, the nitrogen concentration is gradually increased from the lightly doped region to the highly doped region. You may adjust the supply amount of nitrogen gas so that it may increase to . In this way, by reducing the change in nitrogen concentration between the lightly doped region and the highly doped region, it is possible to suppress the occurrence of defects such as dislocations due to the lattice constant difference between the two regions.

An ingot of silicon carbide single crystal 10B of the present invention can be obtained through the above single crystal growth step, and after this step, the interface inspection described below may be performed.

(Second interface inspection step)
In the state shown in FIG. 2(d), using the transparency of the silicon carbide seed crystal 20B, the single crystal growth surface 20a side from the back surface 20b side of the silicon carbide seed crystal 20B is observed visually or by a predetermined observation means. Observation is performed using the silicon carbide single crystal 10B to inspect the state of the growth interface (second interface) of the silicon carbide single crystal 10B. The inspection of the state of the growth interface here means inspection as to whether or not defects due to dense dislocations, heterogeneous polymorphs, or the like exist at the growth interface of silicon carbide single crystal 10B.

Since the first interface inspection step has been performed, dislocations and defects found in this step are at least not due to through defects extending from the back surface due to the state of the interface between the silicon carbide seed crystal 20B and the graphite member 10B. I know it. Therefore, the dislocations and defects are considered to be caused by the shape of the surface of the silicon carbide seed crystal 20B and the single crystal growth conditions in the crucible 101, such as sublimation gas distribution and temperature distribution. measures can be taken by focusing on

As described above, the nitrogen concentration of the seed crystal used for growing the silicon carbide single crystal in the present embodiment is suppressed to 5×10 ¹⁷ cm ⁻³ or less. It is as transparent as possible. Therefore, when the seed crystal is attached to the graphite member, the back side of the seed crystal is visually observed, and the state of the interface between the seed crystal and the graphite member, specifically, the cause of the penetrating defect extending from the back side of the seed crystal. It can be inspected for the presence of certain air bubbles, foreign objects, and the like.

As a result of the inspection, if air bubbles are present, remove them by performing vacuum defoaming or pressure defoaming. By removing it, it becomes possible to grow a silicon carbide single crystal on the seed crystal without air bubbles or foreign matter at the interface with the graphite member. In this way, the generation factor of the threading defect from the rear surface of the seed crystal is found and removed at an early stage of the manufacturing process, thereby causing the problem of basal plane dislocations caused by the threading defect extending from the rear surface of the seed crystal. can be avoided, and a high-quality SiC device can be obtained as a final product.

INDUSTRIAL APPLICABILITY The present invention can be utilized when growing SiC single crystals by the sublimation method, and provides a means that reduces killer defects that affect the characteristics of devices using SiC single crystals and greatly contributes to yield improvement. It is something to do.

DESCRIPTION OF SYMBOLS 100... Manufacturing apparatus of silicon carbide single crystal 101... Crucible 102... Graphite member 103... Coil 104... Taper guide 105... Peripheral parts of ingot 10, 10A, 10B, 40... Silicon carbide single crystal 10a Portions of silicon carbide single crystal 20, 20A, 20B, 50 Silicon carbide seed crystal 20a Single crystal growth surfaces of silicon carbide seed crystal 20b, 50b Silicon carbide Seed crystal back surface 30 Raw material D Threading defect extending from the seed crystal back surface P Threading defect generation factor extending from the seed crystal back surface

Claims (8)

A method for producing a silicon carbide single crystal by a sublimation method, comprising:
A first interface inspection step of attaching a silicon carbide seed crystal having a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ or less to a graphite member in a crucible and inspecting the state of the interface between the graphite member and the silicon carbide seed crystal. When,
and a single crystal growth step of supplying a sublimation gas of a silicon carbide raw material and a nitrogen gas to grow a silicon carbide single crystal doped with nitrogen at a concentration of 3×10 ¹⁸ cm ⁻³ or more. A method for producing a silicon carbide single crystal. 2. The method for producing a silicon carbide single crystal according to claim 1, wherein the silicon carbide seed crystal has a thickness of 1 mm or more. In the first interface inspection step, when air bubbles are detected at the interface between the graphite member and the silicon carbide seed crystal, vacuum defoaming or pressure defoaming is performed to remove the air bubbles. 3. The method for producing a silicon carbide single crystal according to claim 1. In the first interface inspection step, when foreign matter is found at the interface between the graphite member and the silicon carbide seed crystal, the silicon carbide seed crystal is temporarily removed from the graphite member, the foreign matter is removed, and 4. The method for producing a silicon carbide single crystal according to claim 1, wherein the silicon carbide single crystal is attached again. Preparing an ingot of a silicon carbide single crystal provided with a low-doped region having a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ or less, cutting out a portion of the low-doped region from the ingot and using it as the silicon carbide seed crystal. The method for producing a silicon carbide single crystal according to any one of claims 1 to 4, characterized by: After the single crystal growth step, the silicon carbide seed crystal and the silicon carbide single crystal ingot grown thereon are removed from the graphite member, and the silicon carbide seed crystal and the silicon carbide seed crystal are separated from the silicon carbide seed crystal side. 6. The method for producing a silicon carbide single crystal according to claim 1, further comprising a second interface inspection step of inspecting a state of an interface with the single crystal. A silicon carbide single crystal ingot,
The silicon carbide single crystal ingot includes a lightly doped region having a nitrogen concentration of 5×10 ¹⁷ cm ⁻³ or less, a highly doped region having a nitrogen concentration of 3×10 ¹⁸ cm ⁻³ or more, the lightly doped region and the highly doped region. an interface with the doped region;
the thickness of the highly doped region is greater than the thickness of the lightly doped region;
the thickness of the lightly doped region is 2 mm or more and 10 mm or less;
A silicon carbide single crystal ingot, wherein the lightly doped region is a seed crystal used in manufacturing the silicon carbide single crystal ingot. 8. The silicon carbide single crystal ingot according to claim 7, wherein said lightly doped region has no threading defects extending from a surface of said lightly doped region opposite said interface with said highly doped region to said interface.

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