Silicon carbide epitaxial substrate, silicon carbide semiconductor device, and method for manufacturing silicon carbide semiconductor device

Abstract

This silicon carbide epitaxial substrate comprises a silicon carbide substrate and a silicon carbide epitaxial layer. The silicon carbide epitaxial layer is positioned on the silicon carbide substrate. The silicon carbide epitaxial layer includes a boundary region connected to the silicon carbide substrate and a first layer connected to the boundary region. When the nitrogen concentration in the silicon carbide epitaxial substrate is measured by the secondary ion mass spectrometry, the nitrogen concentration in the first layer is a first nitrogen concentration, the minimum value of the nitrogen concentration in the boundary region is a second nitrogen concentration, the first nitrogen concentration is higher than the second nitrogen concentration, and a value obtained by dividing a value obtained by subtracting the second nitrogen concentration from the first nitrogen concentration by the first nitrogen concentration is greater than 0 and less than 0.5.

Description

Silicon carbide epitaxial substrate, silicon carbide semiconductor device, and method for manufacturing silicon carbide semiconductor device

[0001] The present disclosure relates to a silicon carbide epitaxial substrate, a silicon carbide semiconductor device, and a method for manufacturing a silicon carbide semiconductor device. This application claims priority to Japanese Patent Application No. 2023-155605, filed September 21, 2023. The entire contents of this Japanese patent application are incorporated herein by reference.

[0002] Japanese Patent Laid-Open Publication No. 2017-145150 (Patent Document 1) describes a method for manufacturing a silicon carbide epitaxial substrate.

[0003] JP 2017-145150 A

[0004] A silicon carbide epitaxial substrate according to the present disclosure includes a silicon carbide substrate and a silicon carbide epitaxial layer. The silicon carbide epitaxial layer is on the silicon carbide substrate. The silicon carbide epitaxial layer includes a boundary region continuous with the silicon carbide substrate and a first layer continuous with the boundary region. When the nitrogen concentration in the silicon carbide epitaxial substrate is measured by secondary ion mass spectrometry, the nitrogen concentration in the first layer is a first nitrogen concentration, the minimum value of the nitrogen concentration in the boundary region is a second nitrogen concentration, the first nitrogen concentration is higher than the second nitrogen concentration, and the value obtained by subtracting the second nitrogen concentration from the first nitrogen concentration divided by the first nitrogen concentration is greater than 0 and less than 0.5.

[0005] FIG. 1 is a plan view schematically illustrating the configuration of a silicon carbide epitaxial substrate according to the first embodiment. FIG. 2 is a cross-sectional view schematically illustrating a section taken along line II-II in FIG. 1. FIG. 3 is a cross-sectional view schematically illustrating a nitrogen concentration profile of the silicon carbide epitaxial substrate according to the first embodiment, measured by secondary ion mass spectrometry. FIG. 4 is a cross-sectional view schematically illustrating a nitrogen concentration profile of the silicon carbide epitaxial substrate according to the second embodiment, measured by secondary ion mass spectrometry. FIG. 5 is a cross-sectional view schematically illustrating the configuration of an apparatus for manufacturing a silicon carbide epitaxial substrate according to the present embodiment. FIG. 6 is a cross-sectional view schematically illustrating the relationship between gas flow rate and time. FIG. 7 is a cross-sectional view schematically illustrating the configuration of a silicon carbide semiconductor device according to the present embodiment. FIG. 8 is a cross-sectional view schematically illustrating a step of forming a body region. FIG. 9 is a cross-sectional view schematically illustrating a step of forming a source region. FIG. 10 is a cross-sectional view schematically illustrating a step of forming a trench in a first main surface. FIG. 11 is a cross-sectional view schematically illustrating a step of forming a gate insulating film. FIG. 12 is a cross-sectional view schematically illustrating a step of forming a gate electrode and an interlayer insulating film. 13 and 14 are schematic diagrams showing the relationship between gas flow rates and time in a method for manufacturing a silicon carbide epitaxial substrate according to a comparative example and an example, respectively.

[0006] [Problem to be Solved by the Present Disclosure] An object of the present disclosure is to provide a silicon carbide epitaxial substrate, a silicon carbide semiconductor device, and a method for manufacturing a silicon carbide semiconductor device that are capable of suppressing degradation due to electrical conduction of the silicon carbide semiconductor device. [Advantages of the Present Disclosure] According to the present disclosure, it is possible to provide a silicon carbide epitaxial substrate, a silicon carbide semiconductor device, and a method for manufacturing a silicon carbide semiconductor device that are capable of suppressing degradation due to electrical conduction of the silicon carbide semiconductor device.

[0007] [Outline of Embodiments of the Present Disclosure] First, an outline of embodiments of the present disclosure will be described. In the crystallographic descriptions in this specification, individual orientations are represented by [ ], collective orientations by < >, individual planes by ( ), and collective planes by {}. A negative crystallographic index is usually represented by placing a "-" (bar) above the number, but in this specification, a negative crystallographic index is represented by placing a negative sign before the number.

[0008] (1) Silicon carbide epitaxial substrate 100 according to the present disclosure includes silicon carbide substrate 10 and silicon carbide epitaxial layer 20. Silicon carbide epitaxial layer 20 is on silicon carbide substrate 10. Silicon carbide epitaxial layer 20 includes boundary region 30 continuous with silicon carbide substrate 10 and first layer 11 continuous with boundary region 30. When the nitrogen concentration in silicon carbide epitaxial substrate 100 is measured by secondary ion mass spectrometry, the nitrogen concentration in first layer 11 is a first nitrogen concentration N1, the minimum value of the nitrogen concentration in boundary region 30 is a second nitrogen concentration N2, first nitrogen concentration N1 is higher than second nitrogen concentration N2, and the value obtained by subtracting second nitrogen concentration N2 from first nitrogen concentration N1 and dividing the value by first nitrogen concentration N1 is greater than 0 and less than 0.5. As a result, in silicon carbide semiconductor device 300 manufactured using silicon carbide epitaxial substrate 100, degradation due to electrical conduction can be suppressed.

[0009] (2) In silicon carbide epitaxial substrate 100 according to (1) above, boundary region 30 may include second layer 12 located between first layer 11 and silicon carbide substrate 10. The nitrogen concentration in second layer 12 may decrease toward silicon carbide substrate 10.

[0010] (3) In silicon carbide epitaxial substrate 100 according to (2) above, boundary region 30 may include third layer 13 located between second layer 12 and silicon carbide substrate 10. The nitrogen concentration in third layer 13 may increase toward silicon carbide substrate 10.

[0011] (4) Silicon carbide epitaxial substrate 100 according to the present disclosure includes silicon carbide substrate 10 and silicon carbide epitaxial layer 20. Silicon carbide epitaxial layer 20 is on silicon carbide substrate 10. Silicon carbide epitaxial layer 20 includes first layer 11, second layer 12 located between first layer 11 and silicon carbide substrate 10 and continuous with first layer 11, and third layer 13 continuous with both second layer 12 and silicon carbide substrate 10. When the nitrogen concentration in silicon carbide epitaxial substrate 100 is measured by secondary ion mass spectrometry, the nitrogen concentration in first layer 11 is a first nitrogen concentration N1, the nitrogen concentrations in each of second layer 12 and third layer 13 increase toward silicon carbide substrate 10, the rate of increase of the nitrogen concentration in third layer 13 is greater than the rate of increase of the nitrogen concentration in second layer 12, the nitrogen concentration at the boundary between second layer 12 and third layer 13 is a third nitrogen concentration N3, first nitrogen concentration N1 is lower than the third nitrogen concentration N3, and the value obtained by subtracting the third nitrogen concentration N3 from the first nitrogen concentration N1 divided by the first nitrogen concentration N1 is greater than −1.0 and less than 0. This makes it possible to suppress electrical degradation in silicon carbide semiconductor device 300 manufactured using silicon carbide epitaxial substrate 100.

[0012] (5) A silicon carbide semiconductor device 300 according to the present disclosure includes silicon carbide epitaxial substrate 100 according to any one of (1) to (4) above, and electrode 116 provided on silicon carbide epitaxial substrate 100. This can suppress electrical degradation of silicon carbide semiconductor device 300.

[0013] (6) A method for manufacturing silicon carbide semiconductor device 300 according to the present disclosure includes the following steps: Silicon carbide epitaxial substrate 100 according to any one of (1) to (4) above is prepared. Electrode 116 is formed on silicon carbide epitaxial substrate 100. This makes it possible to provide silicon carbide semiconductor device 300 capable of suppressing electrical degradation.

[0014] [Details of the embodiment of the present disclosure] Hereinafter, details of the embodiment of the present disclosure will be described. In the following description, the same or corresponding elements are denoted by the same reference numerals, and the same description thereof will not be repeated.

[0015] First Embodiment Fig. 1 is a plan view schematically showing the configuration of a silicon carbide epitaxial substrate 100 according to the first embodiment. Fig. 2 is a cross-sectional view schematically showing the configuration along line II-II in Fig. 1. As shown in Figs. 1 and 2, the silicon carbide epitaxial substrate 100 according to the present embodiment has a silicon carbide substrate 10 and a silicon carbide epitaxial layer 20. The silicon carbide epitaxial layer 20 is on the silicon carbide substrate 10. The silicon carbide epitaxial layer 20 is in contact with the silicon carbide substrate 10. The silicon carbide epitaxial layer 20 has a first main surface 1.

[0016] Silicon carbide epitaxial layer 20 forms the front surface (first main surface 1) of silicon carbide epitaxial substrate 100. Silicon carbide substrate 10 forms the back surface (second main surface 2) of silicon carbide epitaxial substrate 100. As shown in FIG. 1 , silicon carbide epitaxial substrate 100 has an outer peripheral edge 5. Outer peripheral edge 5 has, for example, an orientation flat 3 and an arc-shaped portion 4. Orientation flat 3 extends along first direction 101. As shown in FIG. 1 , orientation flat 3 is linear when viewed in a direction perpendicular to first main surface 1. Arc-shaped portion 4 is continuous with orientation flat 3. Arc-shaped portion 4 is arc-shaped when viewed in a direction perpendicular to first main surface 1.

[0017] 1 , when viewed in a direction perpendicular to the first main surface 1, the first main surface 1 extends along a first direction 101 and a second direction 102. When viewed in a direction perpendicular to the first main surface 1, the first direction 101 is a direction perpendicular to the second direction 102.

[0018] The first direction 101 is, for example, the <11-20> direction. The first direction 101 may be, for example, the [11-20] direction. The first direction 101 may be a direction obtained by projecting the <11-20> direction onto the first main surface 1. From another perspective, the first direction 101 may be, for example, a direction including a <11-20> direction component.

[0019] The second direction 102 is, for example, the <1-100> direction. The second direction 102 may be, for example, the [1-100] direction. The second direction 102 may be, for example, a direction obtained by projecting the <1-100> direction onto the first main surface 1. From another perspective, the second direction 102 may be, for example, a direction including a <1-100> direction component.

[0020] First main surface 1 may be a plane inclined with respect to the {0001} plane. When first main surface 1 is inclined with respect to the {0001} plane, the inclination angle (off angle) with respect to the {0001} plane is, for example, 2° or more and 6° or less. When first main surface 1 is inclined with respect to the {0001} plane, the inclination direction (off direction) of first main surface 1 is, for example, the <11-20> direction. From another perspective, first direction 101 may be the off direction of first main surface 1.

[0021] As shown in FIG. 1 , the maximum diameter W1 of the first main surface 1 is, for example, 100 mm (4 inches). The maximum diameter W1 may be 125 mm (5 inches) or more, or 150 mm (6 inches) or more. There is no particular upper limit to the maximum diameter W1. The maximum diameter W1 may be, for example, 200 mm (8 inches) or less. The maximum diameter W1 is the maximum distance between any two points on the outer circumferential edge 5.

[0022] In this specification, 4 inches refers to 100 mm or 101.6 mm (4 inches x 25.4 mm / inch). 6 inches refers to 150 mm or 152.4 mm (6 inches x 25.4 mm / inch). 8 inches refers to 200 mm or 203.2 mm (8 inches x 25.4 mm / inch).

[0023] 2 , silicon carbide substrate 10 has second main surface 2 and third main surface 15. Third main surface 15 is located opposite second main surface 2. Second main surface 2 is the back surface of silicon carbide epitaxial substrate 100. Second main surface 2 is spaced apart from silicon carbide epitaxial layer 20. Third main surface 15 is in contact with silicon carbide epitaxial layer 20.

[0024] The polytype of silicon carbide constituting silicon carbide substrate 10 is, for example, 4H. Similarly, the polytype of silicon carbide constituting silicon carbide epitaxial layer 20 is, for example, 4H. Third direction 103 is a direction from silicon carbide epitaxial layer 20 toward silicon carbide substrate 10. Third direction 103 is perpendicular to each of first direction 101 and second direction 102.

[0025] 2 , silicon carbide epitaxial layer 20 has boundary region 30, first layer 11, and drift layer 14. Boundary region 30 is on silicon carbide substrate 10. Boundary region 30 is continuous with silicon carbide substrate 10. First layer 11 is on boundary region 30. First layer 11 is continuous with boundary region 30. Drift layer 14 is on first layer 11. Drift layer 14 is continuous with first layer 11. Drift layer 14 constitutes first main surface 1. In the third direction, first layer 11 is located between boundary region 30 and drift layer 14.

[0026] Boundary region 30 includes, for example, second layer 12 and third layer 13. In third direction 103, second layer 12 is located between first layer 11 and silicon carbide substrate 10. More specifically, second layer 12 is located between first layer 11 and third layer 13. Second layer 12 is continuous with both first layer 11 and third layer 13. In third direction 103, third layer 13 is located between second layer 12 and silicon carbide substrate 10. Third layer 13 is continuous with both second layer 12 and silicon carbide substrate 10.

[0027] Silicon carbide substrate 10 contains nitrogen (N) as an n-type impurity. The conductivity type of silicon carbide substrate 10 is n-type. Silicon carbide epitaxial layer 20 contains nitrogen as an n-type impurity. The conductivity type of silicon carbide epitaxial layer 20 is n-type. Specifically, first layer 11, second layer 12, third layer 13, and drift layer 14 each contain nitrogen as an n-type impurity. The conductivity type of first layer 11, second layer 12, third layer 13, and drift layer 14 each is n-type.

[0028] Figure 3 is a schematic diagram showing a nitrogen concentration profile of silicon carbide epitaxial substrate 100 according to the first embodiment, measured by secondary ion mass spectrometry. The horizontal axis of Figure 3 represents position in third direction 103. The left side of the horizontal axis is the first main surface 1 side. The right side of the horizontal axis is the second main surface 2 side. The vertical axis of Figure 3 represents the nitrogen concentration on a logarithmic scale.

[0029] In third direction 103, first position P1 is a position of a part of first layer 11. In a direction perpendicular to first main surface 1, second position P2 is a position of the boundary between first layer 11 and second layer 12. First layer 11 is arranged between first position P1 and second position P2. In third direction 103, third position P3 is a position of the boundary between second layer 12 and third layer 13. Second layer 12 is arranged between second position P2 and third position P3. In third direction 103, fourth position P4 is a position of the boundary between third layer 13 and silicon carbide substrate 10. Third layer 13 is arranged between third position P3 and fourth position P4.

[0030] 3 , when the nitrogen concentration in silicon carbide epitaxial substrate 100 is measured by secondary ion mass spectrometry, the average value of the nitrogen concentration in first layer 11 is first nitrogen concentration N1. In third direction 103, the nitrogen concentration in first layer 11 is substantially constant. Specifically, the value obtained by dividing the difference between the maximum and minimum values ​​of the nitrogen concentration in first layer 11 by the average value of the nitrogen concentration in first layer 11 may be 10% or less.

[0031] The average value of the nitrogen concentration in silicon carbide substrate 10 is fourth nitrogen concentration N4. In third direction 103, the nitrogen concentration in silicon carbide substrate 10 is substantially constant. Specifically, the value obtained by dividing the difference between the maximum and minimum values ​​of the nitrogen concentration in silicon carbide substrate 10 by the average value of the nitrogen concentration in silicon carbide substrate 10 may be 10% or less.

[0032] 3 , the nitrogen concentration in second layer 12 may decrease toward silicon carbide substrate 10. The nitrogen concentration in second layer 12 decreases substantially monotonically toward silicon carbide substrate 10. The nitrogen concentration in third layer 13 may increase toward silicon carbide substrate 10. The nitrogen concentration in third layer 13 increases substantially monotonically toward silicon carbide substrate 10.

[0033] The absolute value of the rate of increase of the nitrogen concentration in the third layer 13 is greater than the absolute value of the rate of decrease of the nitrogen concentration in the second layer 12. The rate of decrease of the nitrogen concentration in the second layer 12 is the value obtained by subtracting the nitrogen concentration at the third position P3 (second nitrogen concentration N2) from the nitrogen concentration at the second position P2 (first nitrogen concentration N1), and dividing the result by the thickness of the second layer 12. Similarly, the rate of increase of the nitrogen concentration in the third layer 13 is the value obtained by subtracting the nitrogen concentration at the third position P3 (second nitrogen concentration N2) from the nitrogen concentration at the fourth position P4 (fourth nitrogen concentration N4), and dividing the result by the thickness of the third layer 13.

[0034] The minimum value of the nitrogen concentration in the boundary region 30 is the second nitrogen concentration N2. The first nitrogen concentration N1 is higher than the second nitrogen concentration N2. The first nitrogen concentration N1 is lower than the fourth nitrogen concentration N4. The value obtained by subtracting the second nitrogen concentration N2 from the first nitrogen concentration N1 and dividing the result by the first nitrogen concentration N1 is greater than 0 and less than 0.5.

[0035] The value obtained by subtracting the second nitrogen concentration N2 from the first nitrogen concentration N1 and dividing the result by the first nitrogen concentration N1 may be greater than 0.01, greater than 0.03, or greater than 0.05. The value obtained by subtracting the second nitrogen concentration N2 from the first nitrogen concentration N1 and dividing the result by the first nitrogen concentration N1 may be less than 0.2 or less than 0.1.

[0036] The first nitrogen concentration N1 is, for example, 1×10 16 cm -3 5x10 or more 18 cm -3 The first nitrogen concentration N1 is, for example, 1×10 17 cm -3 It can be more than 5 x 10 17 cm -3It may be 1×10 or more. 18 cm -3 The first nitrogen concentration N1 may be, for example, 3×10 18 cm -3 It may be 1×10 or less. 18 cm -3 It may be 5×10 or less. 17 cm -3 It may be the following:

[0037] The fourth nitrogen concentration N4 is, for example, 1×10 18 cm -3 1x10 or more 20 cm -3 The fourth nitrogen concentration N4 is, for example, 5×10 18 cm -3 1x10 or more 19 cm -3 Silicon carbide substrate 10 has a thickness of, for example, not less than 200 μm and not more than 500 μm.

[0038] The thickness of the first layer 11 is greater than the thickness of the second layer 12. The thickness of the first layer 11 is, for example, 0.3 μm or greater. The thickness of the first layer 11 may be three times or more, or five times or more, the thickness of the second layer 12. The thickness of the first layer 11 is greater than the thickness of the third layer 13. The thickness of the first layer 11 may be three times or more, or five times or more, the thickness of the third layer 13.

[0039] The thickness of the second layer 12 is, for example, 0.1 μm. The thickness of the second layer 12 may be, for example, 0.05 μm or more and 0.2 μm or less. The thickness of the third layer 13 is, for example, 0.1 μm. The thickness of the third layer 13 may be, for example, 0.05 μm or more and 0.2 μm or less. The thickness of the second layer 12 may be approximately the same as the thickness of the third layer 13. The value obtained by dividing the thickness of the second layer 12 by the thickness of the third layer 13 may be 0.6 or more and 1.4 or less, or 0.8 or more and 1.2 or less.

[0040] 2 , silicon carbide epitaxial layer 20 of silicon carbide epitaxial substrate 100 according to this embodiment has drift layer 14. The thickness of drift layer 14 is greater than the thickness of first layer 11. The nitrogen concentration of drift layer 14 may be substantially the same as or different from the nitrogen concentration of first layer 11. The nitrogen concentration of drift layer 14 may be lower than the nitrogen concentration of first layer 11. The nitrogen concentration of drift layer 14 may be 0.1 times or less the nitrogen concentration of first layer 11.

[0041] Next, a method for measuring the nitrogen concentration will be described. The nitrogen concentration in each of the above regions can be measured by SIMS (Secondary Ion Mass Spectrometry). For example, an IMS7f manufactured by Cameca can be used as the SIMS. For example, when the primary ions are Cs ＋ The measurement conditions are that the primary ions are ions and the primary ion energy is 15 keV. The measurement point may be any point located 5 mm or more inward from the outer periphery of the substrate toward the center of the epitaxial layer.

[0042] Second Embodiment Next, the configuration of silicon carbide epitaxial substrate 100 according to the second embodiment will be described. Silicon carbide epitaxial substrate 100 according to the second embodiment differs from silicon carbide epitaxial substrate 100 according to the first embodiment mainly in the nitrogen concentration profiles in each of second layer 12 and third layer 13, but is otherwise substantially the same as silicon carbide epitaxial substrate 100 according to the first embodiment. The following description will focus on the configuration that differs from silicon carbide epitaxial substrate 100 according to the first embodiment.

[0043] 4 is a schematic diagram showing a nitrogen concentration profile in silicon carbide epitaxial substrate 100 according to the second embodiment, measured by secondary ion mass spectrometry. The definitions of the horizontal and vertical axes in Fig. 4 are the same as those of the horizontal and vertical axes in Fig. 3.

[0044] 4 , when the nitrogen concentration in silicon carbide epitaxial substrate 100 is measured by secondary ion mass spectrometry, the nitrogen concentration in each of second layer 12 and third layer 13 increases toward silicon carbide substrate 10. The nitrogen concentration in each of second layer 12 and third layer 13 increases substantially monotonically toward silicon carbide substrate 10.

[0045] The third position P3 is the boundary position between the second layer 12 and the third layer 13. When the vertical axis represents the nitrogen concentration and the horizontal axis represents the position in the third direction 103, the third position P3 is an inflection point of the nitrogen concentration. From another perspective, the nitrogen concentration profile in the second layer 12 may be downward convex, and the nitrogen concentration profile in the third layer 13 may be upward convex.

[0046] The nitrogen concentration at the boundary between the second layer 12 and the third layer 13 is a third nitrogen concentration N3. The first nitrogen concentration N1 is lower than the third nitrogen concentration N3. The third nitrogen concentration N3 is lower than the fourth nitrogen concentration N4.

[0047] The rate of increase in the nitrogen concentration in the third layer 13 is greater than the rate of increase in the nitrogen concentration in the second layer 12. The rate of increase in the nitrogen concentration in the second layer 12 is the value obtained by subtracting the nitrogen concentration at the second position P2 (first nitrogen concentration N1) from the nitrogen concentration at the third position P3 (third nitrogen concentration N3), and dividing the result by the thickness of the second layer 12. Similarly, the rate of increase in the nitrogen concentration in the third layer 13 is the value obtained by subtracting the nitrogen concentration at the third position P3 (third nitrogen concentration N3) from the nitrogen concentration at the fourth position P4 (fourth nitrogen concentration N4), and dividing the result by the thickness of the third layer 13.

[0048] The value obtained by subtracting the third nitrogen concentration N3 from the first nitrogen concentration N1 and dividing the result by the first nitrogen concentration N1 is greater than −1.0 and less than 0. The value obtained by subtracting the third nitrogen concentration N3 from the first nitrogen concentration N1 and dividing the result by the first nitrogen concentration N1 may be less than −0.1 or less than −0.5.

[0049] (Apparatus for manufacturing silicon carbide epitaxial substrate) Figure 5 is a cross-sectional view schematically showing the configuration of an apparatus for manufacturing silicon carbide epitaxial substrate 100 according to this embodiment. As shown in Figure 5, the apparatus for manufacturing silicon carbide epitaxial substrate 100 is, for example, a hot-wall horizontal CVD (Chemical Vapor Deposition) apparatus. Apparatus 250 for manufacturing silicon carbide epitaxial substrate 100 mainly includes chamber 201, gas supply unit 235, control unit 245, heating element 203, quartz tube 204, a heat insulating material (not shown), and an induction heating coil (not shown).

[0050] The heating element 203 has, for example, a cylindrical shape and defines a chamber 201 therein. The heating element 203 is made of, for example, graphite. The heating element 203 is provided inside a quartz tube 204. A heat insulating material surrounds the outer periphery of the heating element 203. The induction heating coil is wound, for example, along the outer periphery of the quartz tube 204. The induction heating coil is configured so that an alternating current can be supplied to it from an external power source (not shown). This causes the heating element 203 to be induction heated. As a result, the chamber 201 is heated by the heating element 203.

[0051] Chamber 201 is formed by being surrounded by an inner wall surface 205 of heating element 203. Chamber 201 is provided with a susceptor 210 that holds silicon carbide substrate 10. Susceptor 210 is made of, for example, silicon carbide. Silicon carbide substrate 10 is placed on susceptor 210. Susceptor 210 is placed on stage 206. Stage 206 is rotatably supported by a rotation shaft 209. Rotation of stage 206 causes susceptor 210 to rotate.

[0052] Manufacturing apparatus 250 for silicon carbide epitaxial substrate 100 further includes gas inlet 207 and gas exhaust outlet 208. Gas exhaust outlet 208 is connected to an exhaust pump (not shown). Arrows in FIG. 5 indicate the flow of gas. Gas is introduced into chamber 201 from gas inlet 207 and exhausted from gas exhaust outlet 208. The pressure within chamber 201 is adjusted by balancing the amount of gas supplied and the amount of gas exhausted.

[0053] The gas supply unit 235 is configured to be able to supply a mixed gas containing a source gas, a dopant gas, and a carrier gas to the chamber 201. Specifically, the gas supply unit 235 includes, for example, a first gas supply unit 231, a second gas supply unit 232, a third gas supply unit 233, and a fourth gas supply unit 234.

[0054] The first gas supply unit 231 is configured to be able to supply a first gas containing, for example, carbon atoms. The first gas supply unit 231 is, for example, a gas cylinder filled with the first gas. The first gas is, for example, propane (C3H8) gas. The first gas may also be, for example, methane (CH4) gas, ethane (C2H6) gas, acetylene (C2H2) gas, etc.

[0055] The second gas supply unit 232 is configured to be able to supply a second gas containing, for example, silane gas. The second gas supply unit 232 is, for example, a gas cylinder filled with the second gas. The second gas is, for example, silane (SiH4) gas. The second gas may be a mixed gas of silane gas and a gas other than silane.

[0056] The third gas supply unit 233 is configured to be able to supply a third gas containing, for example, ammonia gas. The third gas supply unit 233 is, for example, a gas cylinder filled with the third gas. The third gas is a doping gas containing N (nitrogen atoms). Ammonia gas is more susceptible to thermal decomposition than nitrogen gas, which has a triple bond. The third gas may be nitrogen gas.

[0057] The fourth gas supply unit 234 is configured to be able to supply a fourth gas (carrier gas) such as hydrogen, for example. The fourth gas supply unit 234 is, for example, a gas cylinder filled with hydrogen.

[0058] The control unit 245 is configured to be able to control the flow rate of the mixed gas supplied from the gas supply unit 235 to the chamber 201. Specifically, the control unit 245 may include a first gas flow rate control unit 241, a second gas flow rate control unit 242, a third gas flow rate control unit 243, and a fourth gas flow rate control unit 244. Each control unit may be, for example, an MFC (Mass Flow Controller). The control unit 245 is disposed between the gas supply unit 235 and the gas inlet 207. In other words, the control unit 245 is disposed in a flow path connecting the gas supply unit 235 and the gas inlet 207.

[0059] (Method for Manufacturing Silicon Carbide Epitaxial Substrate) Next, a method for manufacturing silicon carbide epitaxial substrate 100 according to this embodiment will be described.

[0060] First, silicon carbide substrate 10 is prepared. A silicon carbide single crystal of polytype 4H is manufactured by, for example, sublimation. Next, silicon carbide substrate 10 is prepared by slicing the silicon carbide single crystal by, for example, a wire saw. Silicon carbide substrate 10 contains n-type impurities, for example, nitrogen. The conductivity type of silicon carbide substrate 10 is, for example, n-type. Next, mechanical polishing is performed on silicon carbide substrate 10. Next, chemical mechanical polishing is performed on silicon carbide substrate 10.

[0061] Next, silicon carbide epitaxial layer 20 is formed on silicon carbide substrate 10. Specifically, silicon carbide epitaxial layer 20 is formed by epitaxial growth on silicon carbide substrate 10, for example, by a chemical vapor deposition (CVD) method. In the epitaxial growth, for example, silane (SiH4) and propane (C3H8) are used as source gases, and hydrogen (H2) is used as a carrier gas. The temperature of the epitaxial growth is, for example, about 1400°C or higher and 1700°C or lower. In the epitaxial growth, for example, ammonia gas is used as a dopant gas. As a result, nitrogen atoms are introduced into silicon carbide epitaxial layer 20.

[0062] Specifically, first, a carrier gas is introduced into the chamber 201. The flow rate of the carrier gas is, for example, 100 slm. Next, ammonia gas is introduced into the chamber 201. After the ammonia gas is introduced into the chamber 201, silane and propane are introduced into the chamber 201.

[0063] 6 is a schematic diagram showing the relationship between gas flow rate and time. As shown in FIG. 6, at a first time point T1, the flow rate of ammonia gas is set to a first flow rate A1. After the first time point T1, the flow rate of ammonia gas is maintained at the first flow rate A1. The first flow rate A1 is, for example, 1.5 sccm.

[0064] Next, at a second time T2, silane and propane are introduced into the chamber 201. The second time T2 is later than the first time T1. From the second time T2 to a third time T3, the flow rates of the silane and propane increase. At the third time T3, the flow rates of the silane and propane reach their target flow rates. After the third time T3, the flow rate of silane is maintained at the second flow rate A2, and the flow rate of propane is maintained at the third flow rate A3. The second flow rate A2 is, for example, 46 sccm. The third flow rate A3 is, for example, 29 sccm. The pressure inside the chamber 201 is, for example, 2 kPa or more and 6 kPa or less.

[0065] 6 , between first time point T1 and second time point T2, neither silane nor propane is introduced into chamber 201. In other words, between first time point T1 and second time point T2, the flow rates of each of silane and propane are 0. Therefore, between first time point T1 and second time point T2, silicon carbide epitaxial layer 20 is not substantially formed on silicon carbide substrate 10.

[0066] After second point in time T2, silicon carbide epitaxial layer 20 is formed on silicon carbide substrate 10. Specifically, between second point in time T2 and third point in time T3, third layer 13 is formed on silicon carbide substrate 10, and second layer 12 is formed on third layer 13. After third point in time T3, first layer 11 is formed on second layer 12. Next, drift layer 14 is formed on first layer 11. In this manner, silicon carbide epitaxial substrate 100 having silicon carbide substrate 10 and silicon carbide epitaxial layer 20 is prepared (see FIG. 2 ).

[0067] (Silicon Carbide Semiconductor Device) Next, the configuration of a silicon carbide semiconductor device 300 according to this embodiment will be described. FIG. 7 is a cross-sectional schematic diagram showing the configuration of the silicon carbide semiconductor device 300 according to this embodiment. As shown in FIG. 7 , the silicon carbide semiconductor device 300 according to this embodiment mainly includes a silicon carbide epitaxial substrate 100, a gate insulating film 115, a gate electrode 127, a source electrode 116, a drain electrode 120, a source wiring 119, and an interlayer insulating film 126. The silicon carbide epitaxial substrate 100 includes a silicon carbide substrate 10, a silicon carbide epitaxial layer 20, a first main surface 1, and a second main surface 2. The silicon carbide epitaxial layer 20 includes a first layer 11, a second layer 12, a third layer 13, a drift layer 14, a body region 113, a source region 114, and a contact region 118.

[0068] The body region 113 is formed on the drift layer 14. The body region 113 is in contact with the drift layer 14. The body region 113 contains p-type impurities such as aluminum. The body region 113 has p-type conductivity. The source region 114 is formed on the body region 113. The source region 114 contains n-type impurities such as phosphorus. The source region 114 has n-type conductivity. The concentration of the n-type impurities contained in the source region 114 may be higher than the concentration of the p-type impurities contained in the body region 113.

[0069] The contact region 118 penetrates the source region 114 and the body region 113. The contact region 118 is in contact with each of the source region 114, the body region 113, and the third layer 13. The contact region 118 contains a p-type impurity such as aluminum. The concentration of the p-type impurity contained in the contact region 118 may be higher than the concentration of the n-type impurity contained in the source region 114.

[0070] A trench 106 is provided in the first main surface 1. The trench 106 is defined by a side surface 123 and a bottom surface 124. The side surface 123 is formed by the source region 114, the body region 113, and the drift layer 14. The bottom surface 124 is formed by the drift layer 14.

[0071] The gate insulating film 115 is in contact with the drift layer 14 at its bottom surface 124, and in contact with the drift layer 14, the body region 113, and the source region 114 at its side surfaces 123. The gate electrode 127 is disposed on the gate insulating film 115. The gate electrode 127 is in contact with the gate insulating film 115 inside the trench 106. The gate electrode 127 faces each of the side surfaces 123 and the bottom surface 124 of the trench 106.

[0072] The interlayer insulating film 126 covers the gate electrode 127. The interlayer insulating film 126 is in contact with the gate insulating film 115. The interlayer insulating film 126 is made of a material containing, for example, silicon dioxide. The source electrode 116 is in contact with each of the source region 114 and the contact region 118. The source electrode 116 is made of a material containing, for example, Ti, Al, and Si. The source wiring 119 is in contact with the source electrode 116. The source wiring 119 is electrically connected to the source electrode 116. The source wiring 119 covers the source electrode 116 and the interlayer insulating film 126. The drain electrode 120 is in contact with the silicon carbide substrate 10 at the second main surface 2.

[0073] In the above embodiment, the silicon carbide semiconductor device 300 is a MOSFET, but the silicon carbide semiconductor device 300 is not limited to a MOSFET. The silicon carbide semiconductor device 300 may be, for example, an IGBT (Insulated Gate Bipolar Transistor) or the like.

[0074] (Method for Manufacturing Silicon Carbide Semiconductor Device) Next, a method for manufacturing silicon carbide semiconductor device 300 according to this embodiment will be described.

[0075] First, a silicon carbide epitaxial substrate 100 according to this embodiment is prepared (see FIG. 1 ). Next, a step of forming a body region is performed. FIG. 8 is a schematic cross-sectional view showing the step of forming a body region. Specifically, p-type impurities such as aluminum are ion-implanted into drift layer 14 of silicon carbide epitaxial layer 20. This forms body region 113 having p-type conductivity. The thickness of body region 113 is, for example, 0.9 μm.

[0076] Next, a step of forming a source region is performed. FIG. 9 is a schematic cross-sectional view showing the step of forming the source region. Specifically, n-type impurities such as phosphorus are ion-implanted into the body region 113. This forms a source region 114 having n-type conductivity. The thickness of the source region 114 is, for example, 0.4 μm. The concentration of the n-type impurity contained in the source region 114 may be higher than the concentration of the p-type impurity contained in the body region 113.

[0077] Next, a p-type impurity such as aluminum is ion-implanted into the source region 114 to form a contact region 118. The contact region 118 is formed to penetrate the source region 114 and the body region 113 and to be in contact with the third layer 13. The concentration of the p-type impurity contained in the contact region 118 may be higher than the concentration of the n-type impurity contained in the source region 114.

[0078] Next, activation annealing is performed to activate the implanted impurities. The temperature of the activation annealing is preferably 1500° C. or higher and 1900° C. or lower, for example, about 1700° C. The activation annealing time is, for example, about 30 minutes. The atmosphere for the activation annealing is preferably an inert gas atmosphere, for example, an Ar atmosphere.

[0079] Next, a step of forming trenches in the first main surface 1 is performed. FIG. 10 is a cross-sectional schematic diagram showing the step of forming trenches in the first main surface 1. As shown in FIG. 10, a mask 117 having openings is formed on the first main surface 1 including the source region 114 and the contact region 118. Using the mask 117, the source region 114, the body region 113, and a portion of the drift layer 14 are removed by etching. As an etching method, for example, reactive ion etching, particularly inductively coupled plasma reactive ion etching, can be used. Specifically, for example, inductively coupled plasma reactive ion etching using SF or a mixed gas of SF and O as a reactive gas can be used. By etching, a recess is formed in the first main surface 1.

[0080] Next, thermal etching is performed on the recesses. Thermal etching can be performed, for example, by heating the first main surface 1 with the mask 117 formed thereon in an atmosphere containing a reactive gas having at least one type of halogen atom. The at least one type of halogen atom includes at least one of chlorine (Cl) atoms and fluorine (F) atoms. The atmosphere can include, for example, Cl2, BCl3, SF6, or CF4. For example, thermal etching is performed using a mixed gas of chlorine gas and oxygen gas as the reactive gas, with the heat treatment temperature set to, for example, 700°C or higher and 1000°C or lower. The reactive gas may contain a carrier gas in addition to the above-mentioned chlorine gas and oxygen gas. Examples of the carrier gas that can be used include nitrogen gas, argon gas, and helium gas.

[0081] 10 , trenches 106 are formed in the first main surface 1 by thermal etching. The trenches 106 are defined by side surfaces 123 and a bottom surface 124. The side surfaces 123 are formed by the source region 114, the body region 113, and the drift layer 14. The bottom surface 124 is formed by the drift layer 14. Next, the mask 117 is removed from the first main surface 1.

[0082] Next, a step of forming a gate insulating film is performed. FIG. 11 is a cross-sectional schematic view showing the step of forming a gate insulating film. Specifically, silicon carbide epitaxial substrate 100 having trench 106 formed in first main surface 1 is heated in an oxygen-containing atmosphere at a temperature of, for example, 1300° C. or higher and 1400° C. or lower. This forms gate insulating film 115 that is in contact with drift layer 14 at bottom surface 124, in contact with drift layer 14, body region 113, and source region 114 at side surface 123, and in contact with source region 114 and contact region 118 at first main surface 1.

[0083] Next, a step of forming a gate electrode is performed. FIG. 12 is a schematic cross-sectional view showing the step of forming a gate electrode and an interlayer insulating film. The gate electrode 127 is formed inside the trench 106 so as to be in contact with the gate insulating film 115. The gate electrode 127 is disposed inside the trench 106 and is formed on the gate insulating film 115 so as to face each of the side surface 123 and the bottom surface 124 of the trench 106. The gate electrode 127 is formed by, for example, a low pressure chemical vapor deposition (LPCVD) method.

[0084] Next, an interlayer insulating film 126 is formed. The interlayer insulating film 126 is formed so as to cover the gate electrode 127 and to be in contact with the gate insulating film 115. The interlayer insulating film 126 is formed by, for example, chemical vapor deposition. The interlayer insulating film 126 is made of, for example, a material containing silicon dioxide. Next, the interlayer insulating film 126 and part of the gate insulating film 115 are etched so as to form openings over the source region 114 and the contact region 118. As a result, the contact region 118 and the source region 114 are exposed from the gate insulating film 115.

[0085] Next, a step of forming a source electrode is performed. The source electrode 116 is formed so as to be in contact with each of the source region 114 and the contact region 118. The source electrode 116 is formed by, for example, a sputtering method. The source electrode 116 is made of, for example, a material containing Ti, Al, and Si.

[0086] Next, alloying annealing is performed. Specifically, the source electrode 116 in contact with each of the source region 114 and the contact region 118 is maintained at a temperature of, for example, 900° C. or higher and 1100° C. or lower for about 5 minutes. As a result, at least a portion of the source electrode 116 is silicided. This forms the source electrode 116 in ohmic contact with the source region 114. Preferably, the source electrode 116 forms ohmic contact with the contact region 118.

[0087] Next, the source wiring 119 is formed. The source wiring 119 is electrically connected to the source electrode 116. The source wiring 119 is formed so as to cover the source electrode 116 and the interlayer insulating film 126.

[0088] Next, a step of forming drain electrode 120 is carried out. First, silicon carbide substrate 10 is polished at second main surface 2. Next, drain electrode 120 is formed. Drain electrode 120 is formed so as to be in contact with silicon carbide substrate 10 at second main surface 2. In this manner, silicon carbide semiconductor device 300 according to the present embodiment is manufactured (see FIG. 7 ).

[0089] Next, the effects of silicon carbide epitaxial substrate 100, silicon carbide semiconductor device 300, and method for manufacturing silicon carbide semiconductor device 300 according to this embodiment will be described.

[0090] The inventors have conducted extensive research and experiments into ways to suppress degradation due to electrical conduction of silicon carbide semiconductor device 300, and as a result have obtained the following findings and discovered silicon carbide epitaxial substrate 100 according to the present embodiment.

[0091] In order to prevent basal plane dislocations in silicon carbide substrate 10 from being inherited by silicon carbide epitaxial layer 20, a buffer layer in which the nitrogen concentration gradually decreases may be formed in the early stage of growth of silicon carbide epitaxial layer 20. Specifically, the buffer layer is formed on silicon carbide substrate 10 by gradually introducing into chamber 201 the flow rates of source gases such as silane and propane and dopant gases such as nitrogen or ammonia over a certain period of time.

[0092] However, in addition to the propane gas, carbon is also supplied to the interior of chamber 201 from the carbon member within chamber 201. The amount of carbon supplied from the carbon member within chamber 201 is smaller than the amount of carbon supplied from the propane gas. Therefore, in the later stage of growth of silicon carbide epitaxial layer 20 when the flow rate of propane gas is high, the influence of carbon from the carbon member within chamber 201 is negligible, but in the early stage of growth of silicon carbide epitaxial layer 20 when the flow rate of propane gas is low, the influence of carbon gas desorbed from the carbon member within chamber 201 cannot be ignored.

[0093] Specifically, the ratio of carbon to silicon (C / Si ratio) is higher in the early stage of growth of silicon carbide epitaxial layer 20 compared to the later stage of growth of silicon carbide epitaxial layer 20. When the C / Si ratio is higher, nitrogen atoms are less likely to be incorporated into silicon carbide epitaxial layer 20. Therefore, in the early stage of growth of silicon carbide epitaxial layer 20, a region with an excessively low nitrogen concentration is formed. Since a region with an excessively low nitrogen concentration can cause electrical degradation of silicon carbide semiconductor device 300, it is necessary to suppress the occurrence of a region with an excessively low nitrogen concentration.

[0094] The silicon carbide epitaxial substrate 100 according to the present embodiment includes a silicon carbide substrate 10 and a silicon carbide epitaxial layer 20. The silicon carbide epitaxial layer 20 is on the silicon carbide substrate 10. The silicon carbide epitaxial layer 20 includes a boundary region 30 that is continuous with the silicon carbide substrate 10, and a first layer 11 that is continuous with the boundary region 30. When the nitrogen concentration in the silicon carbide epitaxial substrate 100 is measured by secondary ion mass spectrometry, the nitrogen concentration in the first layer 11 is a first nitrogen concentration N1, the minimum value of the nitrogen concentration in the boundary region 30 is a second nitrogen concentration N2, the first nitrogen concentration N1 is higher than the second nitrogen concentration N2, and the value obtained by subtracting the second nitrogen concentration N2 from the first nitrogen concentration N1 and dividing the value by the first nitrogen concentration N1 is greater than 0 and less than 0.5. As a result, in silicon carbide semiconductor device 300 manufactured using silicon carbide epitaxial substrate 100, degradation due to electrical conduction can be suppressed.

[0095] In silicon carbide epitaxial substrate 100 according to the present embodiment, the value obtained by subtracting first nitrogen concentration N1 from second nitrogen concentration N2 and dividing the result by second nitrogen concentration N2 may be smaller than 0.1. This makes it possible to further suppress electrical degradation in silicon carbide semiconductor device 300 manufactured using silicon carbide epitaxial substrate 100.

[0096] The silicon carbide epitaxial substrate 100 according to this embodiment includes a silicon carbide substrate 10 and a silicon carbide epitaxial layer 20. The silicon carbide epitaxial layer 20 is on the silicon carbide substrate 10. The silicon carbide epitaxial layer 20 includes a first layer 11, a second layer 12 located between the first layer 11 and the silicon carbide substrate 10 and continuous with the first layer 11, and a third layer 13 continuous with both the second layer 12 and the silicon carbide substrate 10. When the nitrogen concentration in silicon carbide epitaxial substrate 100 is measured by secondary ion mass spectrometry, the nitrogen concentration in first layer 11 is a first nitrogen concentration N1, the nitrogen concentrations in each of second layer 12 and third layer 13 increase toward silicon carbide substrate 10, the rate of increase of the nitrogen concentration in third layer 13 is greater than the rate of increase of the nitrogen concentration in second layer 12, the nitrogen concentration at the boundary between second layer 12 and third layer 13 is a third nitrogen concentration N3, first nitrogen concentration N1 is lower than the third nitrogen concentration N3, and the value obtained by subtracting the third nitrogen concentration N3 from the first nitrogen concentration N1 divided by the first nitrogen concentration N1 is greater than −1.0 and less than 0. This makes it possible to suppress electrical degradation in silicon carbide semiconductor device 300 manufactured using silicon carbide epitaxial substrate 100.

[0097] Silicon carbide semiconductor device 300 according to the present embodiment includes silicon carbide epitaxial substrate 100 described above and an electrode provided on silicon carbide epitaxial substrate 100. This makes it possible to suppress deterioration of silicon carbide semiconductor device 300 due to electrical conduction.

[0098] In the method for manufacturing silicon carbide semiconductor device 300 according to this embodiment, the above-described silicon carbide epitaxial substrate 100 is prepared. Electrodes are formed on silicon carbide epitaxial substrate 100. This makes it possible to provide silicon carbide semiconductor device 300 that is capable of suppressing electrical degradation.

[0099] (Sample Preparation) Silicon carbide epitaxial substrates 100 according to Samples 1 to 9 were prepared. Silicon carbide epitaxial substrates 100 according to Samples 1 to 3 are comparative examples. Silicon carbide epitaxial substrates 100 according to Samples 4 to 9 are examples.

[0100] 13 is a schematic diagram showing the relationship between gas flow rates and time in a comparative example of a method for manufacturing silicon carbide epitaxial substrate 100. As shown in FIG. 13 , in the comparative example of a method for manufacturing silicon carbide epitaxial substrate 100, the time when ammonia gas is introduced into chamber 201 is the same as the time when silane and propane are each introduced into chamber 201. The first flow rate was set to 1.5 sccm. The second flow rate A2 was set to 46 sccm. The third flow rate A3 was set to 29 sccm.

[0101] 14 is a schematic diagram showing the relationship between gas flow rates and time in the manufacturing method of silicon carbide epitaxial substrate 100 of the example. As shown in FIG. 14 , in the manufacturing method of silicon carbide epitaxial substrate 100 of the example, the point at which ammonia gas is introduced into chamber 201 precedes the point at which silane and propane are introduced into chamber 201. Approximately 30 seconds after the flow rate of ammonia gas became constant, the introduction of silane and propane into chamber 201 began. The first flow rate was set to 1.5 sccm. The second flow rate A2 was set to 46 sccm. The third flow rate A3 was set to 29 sccm.

[0102] (Evaluation Method) The nitrogen concentration was measured in the silicon carbide epitaxial substrate 100 of the comparative example and the silicon carbide epitaxial substrate 100 of the example. The nitrogen concentration was measured by SIMS. The SIMS used was an IMS7f manufactured by Cameca Corporation. The primary ions were Cs ＋ The primary ion energy was 15 keV, and the measurement point was the center of the epitaxial layer.

[0103] (Evaluation results)

[0104]

[0105] Table 1 shows the first nitrogen concentration N1, second nitrogen concentration N2, third nitrogen concentration N3, fourth nitrogen concentration N4, first change rate of nitrogen concentration, and second change rate of nitrogen concentration of the silicon carbide epitaxial substrates of Samples 1 to 9. The first change rate (%) of nitrogen concentration is calculated as (N1-N2) / N1. The second change rate (%) of nitrogen concentration is calculated as (N1-N3) / N1. When the gradient A of the nitrogen concentration in the boundary region formed by the second layer 12 and the third layer 13 is calculated every 0.05 μm using the following Equation 1, the gradient A is 5.0×10 18 cm -3 The nitrogen concentration at which the nitrogen concentration reached a maximum within the range of 0.05 μm or less was defined as the third nitrogen concentration N3. In Equation 1, x represents the thickness of the boundary region measured by SIMS, and y represents the measured value of the nitrogen concentration in the boundary region measured by SIMS.

[0106]

[0107] As shown in Table 1, in the silicon carbide epitaxial substrates of Samples 1 to 3, the first rate of change in nitrogen concentration was not less than 83.4% and not more than 98.6%. On the other hand, in the silicon carbide epitaxial substrates of Samples 4 to 6, the first rate of change in nitrogen concentration was not less than 12.3% and not more than 40.4%. In the silicon carbide epitaxial substrates of Samples 7 to 9, the second rate of change in nitrogen concentration was not less than −51.7% and not more than −41.7%. From the above results, it was confirmed that by using the manufacturing conditions shown in FIG. 14 , it is possible to manufacture silicon carbide epitaxial substrates in which the first rate of change in nitrogen concentration is greater than 0 and less than 0.5, or silicon carbide epitaxial substrates in which the second rate of change in nitrogen concentration is greater than −1.0 and less than 0.

[0108] The embodiments and examples disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined by the claims, not by the above-described embodiments and examples, and is intended to include meanings equivalent to the claims and all modifications within the scope thereof.

[0109] 1 First main surface, 2 Second main surface, 3 Orientation flat, 4 Arc-shaped portion, 5 Outer periphery, 10 Silicon carbide substrate, 11 First layer, 12 Second layer, 13 Third layer, 14 Drift layer, 15 Third main surface, 20 Silicon carbide epitaxial layer, 30 Boundary region, 100 Silicon carbide epitaxial substrate, 101 First direction, 102 Second direction, 103 Third direction, 106 Trench, 113 Body region, 114 Source region, 115 Gate insulating film, 116 Source electrode (electrode), 117 Mask, 118 Contact region, 119 Source wiring, 120 Drain electrode, 123 Side, 124 Bottom, 126 Interlayer insulating film, 127 Gate electrode, 201 Chamber, 203 Heating element, 204 Quartz tube, 205 Inner wall surface, 206 stage, 207 gas inlet, 208 gas exhaust port, 209 rotating shaft, 210 susceptor, 231 first gas supply unit, 232 second gas supply unit, 233 third gas supply unit, 234 fourth gas supply unit, 235 gas supply unit, 241 first gas flow rate control unit, 242 second gas flow rate control unit, 243 third gas flow rate control unit, 244 fourth gas flow rate control unit, 245 control unit, 250 manufacturing apparatus, 300 silicon carbide semiconductor device, A1 first flow rate, A2 second flow rate, A3 third flow rate, N1 first nitrogen concentration, N2 second nitrogen concentration, N3 third nitrogen concentration, N4 fourth nitrogen concentration, P1 first position, P2 second position, P3 third position, P4 fourth position, T1 first time point, T2 second time point, T3 third time point, W1 maximum diameter.

Claims

1. A silicon carbide epitaxial substrate comprising: a silicon carbide substrate; and a silicon carbide epitaxial layer on the silicon carbide substrate, wherein the silicon carbide epitaxial layer includes a boundary region continuous with the silicon carbide substrate, and a first layer continuous with the boundary region, and when a nitrogen concentration in the silicon carbide epitaxial substrate is measured by secondary ion mass spectrometry, the nitrogen concentration in the first layer is a first nitrogen concentration, a minimum value of the nitrogen concentration in the boundary region is a second nitrogen concentration, the first nitrogen concentration is higher than the second nitrogen concentration, and a value obtained by subtracting the second nitrogen concentration from the first nitrogen concentration and dividing the value by the first nitrogen concentration is greater than 0 and less than 0.

5.

2. The silicon carbide epitaxial substrate according to claim 1, wherein the boundary region includes a second layer located between the first layer and the silicon carbide substrate, and a nitrogen concentration in the second layer decreases toward the silicon carbide substrate.

3. The silicon carbide epitaxial substrate according to claim 2, wherein the boundary region includes a third layer located between the second layer and the silicon carbide substrate, and a nitrogen concentration in the third layer increases toward the silicon carbide substrate.

4. A silicon carbide epitaxial substrate comprising: a silicon carbide substrate; and a silicon carbide epitaxial layer on the silicon carbide substrate, the silicon carbide epitaxial layer including a first layer, a second layer located between the first layer and the silicon carbide substrate and connected to the first layer, and a third layer connected to each of the second layer and the silicon carbide substrate, wherein when a nitrogen concentration in the silicon carbide epitaxial substrate is measured by secondary ion mass spectrometry, the nitrogen concentration in the first layer is a first nitrogen concentration, the nitrogen concentrations in each of the second layer and the third layer increase toward the silicon carbide substrate, a rate of increase in the nitrogen concentration in the third layer is greater than a rate of increase in the nitrogen concentration in the second layer, the nitrogen concentration at the boundary between the second layer and the third layer is a third nitrogen concentration, and the first nitrogen concentration is lower than the third nitrogen concentration, a value obtained by subtracting the third nitrogen concentration from the first nitrogen concentration and dividing the value by the first nitrogen concentration is greater than −1.0 and smaller than 0, in a silicon carbide epitaxial substrate.

5. A silicon carbide semiconductor device comprising: a silicon carbide epitaxial substrate according to any one of claims 1 to 4; and an electrode provided on the silicon carbide epitaxial substrate.

6. A method for manufacturing a silicon carbide semiconductor device, comprising the steps of: preparing a silicon carbide epitaxial substrate according to any one of claims 1 to 4; and forming an electrode on the silicon carbide epitaxial substrate.

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