Method for manufacturing silicon carbide epitaxial substrate and method for manufacturing silicon carbide semiconductor device

Abstract

In the first step, a first buffer layer is formed on a first silicon carbide substrate by epitaxial growth using a first dopant gas that contains ammonia. In the second step, a first drift layer is formed on the first buffer layer by epitaxial growth using a second dopant gas. In the third step after the second step, a second buffer layer is formed on a second silicon carbide substrate by epitaxial growth using a third dopant gas that contains ammonia. In the fourth step, a second drift layer is formed on the second buffer layer by epitaxial growth using a fourth dopant gas. The value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas is 0.95 to 1.05 inclusive.

Description

Method for manufacturing silicon carbide epitaxial substrate and method for manufacturing silicon carbide semiconductor device

[0001] The present disclosure relates to a method for manufacturing a silicon carbide epitaxial substrate and a method for manufacturing a silicon carbide semiconductor device.

[0002] WO 2017 / 056691 (Patent Document 1) describes a method for forming a silicon carbide layer on a silicon carbide single crystal substrate using a mixed gas containing silane, ammonia, and hydrogen.

[0003] International Publication No. 2017 / 056691

[0004] A method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure includes a first step, a second step, a third step, and a fourth step. In the first step, a first buffer layer is formed by epitaxial growth on a first silicon carbide substrate using a first dopant gas containing ammonia. In the second step, a first drift layer is formed by epitaxial growth on the first buffer layer using a second dopant gas. After the second step, in a third step, a second buffer layer is formed by epitaxial growth on a second silicon carbide substrate using a third dopant gas containing ammonia. In the fourth step, a second drift layer is formed by epitaxial growth on the second buffer layer using a fourth dopant gas. A value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas is 0.95 or more and 1.05 or less.

[0005] FIG. 1 is a plan view schematic showing the configuration of a silicon carbide epitaxial substrate according to this embodiment. FIG. 2 is a cross-sectional view schematic taken along line II-II in FIG. 1 . FIG. 3 is a partial cross-sectional view schematic showing the configuration of an apparatus for manufacturing a silicon carbide epitaxial substrate according to this embodiment. FIG. 4 is a flow diagram generally showing a method for manufacturing a silicon carbide epitaxial substrate according to this embodiment. FIG. 5 is a cross-sectional view schematic showing the step of forming a first buffer layer. FIG. 6 is a cross-sectional view schematic showing the step of forming a first drift layer. FIG. 7 is a cross-sectional view schematic showing the step of forming a second buffer layer. FIG. 8 is a cross-sectional view schematic showing the step of forming a second drift layer. FIG. 9 is a cross-sectional view schematic showing the step of forming a body region. FIG. 10 is a cross-sectional view schematic showing the step of forming a source region. FIG. 11 is a cross-sectional view schematic showing the step of forming a trench in a first main surface. FIG. 12 is a cross-sectional view schematic showing the step of forming a gate insulating film. FIG. 13 is a cross-sectional view schematic showing the step of forming a gate electrode and an interlayer insulating film. FIG. 14 is a cross-sectional view schematically showing the configuration of a silicon carbide semiconductor device according to this embodiment.

[0006] [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.

[0007] (1) A method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure includes a first step, a second step, a third step, and a fourth step. In the first step, a first buffer layer is formed by epitaxial growth on a first silicon carbide substrate using a first dopant gas containing ammonia. In the second step, a first drift layer is formed by epitaxial growth on the first buffer layer using a second dopant gas. After the second step, in a third step, a second buffer layer is formed by epitaxial growth on a second silicon carbide substrate using a third dopant gas containing ammonia. In the fourth step, a second drift layer is formed by epitaxial growth on the second buffer layer using a fourth dopant gas. A value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas is 0.95 or more and 1.05 or less.

[0008] (2) In the method for manufacturing a silicon carbide epitaxial substrate according to (1) above, each of the second dopant gas and the fourth dopant gas may contain ammonia.

[0009] (3) In the method for manufacturing a silicon carbide epitaxial substrate according to (1) or (2) above, a value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas may be equal to or greater than 0.99 and equal to or less than 1.01.

[0010] (4) In the method for manufacturing a silicon carbide epitaxial substrate according to any one of (1) to (3) above, the value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas may be 1.00.

[0011] (5) In the method for manufacturing a silicon carbide epitaxial substrate according to any one of (1) to (4) above, each of the second dopant gas and the fourth dopant gas may be an ammonia gas.

[0012] (6) In the method for manufacturing a silicon carbide epitaxial substrate according to any one of (1) to (4) above, each of the second dopant gas and the fourth dopant gas may be nitrogen gas.

[0013] (7) In the method for manufacturing a silicon carbide epitaxial substrate according to any one of (1) to (6) above, the nitrogen concentration in each of the first buffer layer and the second buffer layer is 3×10 18 cm -3 It may be more than that.

[0014] (8) In the method for manufacturing a silicon carbide epitaxial substrate according to any one of (1) to (7) above, the first buffer layer and the second buffer layer may each have a thickness of 1 μm or more.

[0015] (9) In the method for manufacturing a silicon carbide epitaxial substrate according to any one of (1) to (8), the first silicon carbide substrate may have a main surface in contact with the first buffer layer. The main surface may be a plane inclined at an angle of 6° or less with respect to the (0001) plane or the (000-1) plane.

[0016] (10) A method for manufacturing a silicon carbide semiconductor device according to the present disclosure includes the steps of preparing a silicon carbide epitaxial substrate using the method for manufacturing a silicon carbide epitaxial substrate described in any one of (1) to (9) above, and forming an electrode on the silicon carbide epitaxial substrate.

[0017] [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.

[0018] Fig. 1 is a plan view schematic diagram showing the configuration of a silicon carbide epitaxial substrate 100 according to this embodiment. Fig. 2 is a cross-sectional view schematic diagram taken along line II-II in Fig. 1. As shown in Figs. 1 and 2, the silicon carbide epitaxial substrate 100 according to this 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.

[0019] Silicon carbide epitaxial layer 20 forms a front surface (first main surface 1 ) of silicon carbide epitaxial substrate 100. Silicon carbide substrate 10 forms a back surface (second main surface 2 ) of silicon carbide epitaxial substrate 100.

[0020] 1 , silicon carbide epitaxial substrate 100 has outer peripheral edge 5. Outer peripheral edge 5 is continuous with each of first main surface 1 and second main surface 2. Outer peripheral edge 5 has, for example, orientation flat 3 and arc-shaped portion 4.

[0021] 1 , when viewed in a direction from silicon carbide epitaxial layer 20 toward silicon carbide substrate 10, the direction from center A of first main surface 1 toward orientation flat 3 is defined as first direction 101. When viewed in a direction from silicon carbide epitaxial layer 20 toward silicon carbide substrate 10, the direction perpendicular to first direction 101 is defined as second direction 102. Orientation flat 3 extends along second direction 102. When viewed in a direction from silicon carbide epitaxial layer 20 toward silicon carbide substrate 10, first main surface 1 extends along each of first direction 101 and second direction 102.

[0022] 1 , orientation flat 3 is linear when viewed in a direction from silicon carbide epitaxial layer 20 toward silicon carbide substrate 10. Arc-shaped portion 4 is continuous with orientation flat 3. Arc-shaped portion 4 is also arc-shaped. A notch may be provided instead of orientation flat 3. In this case, first direction 101 is the direction from center A of first main surface 1 toward the notch.

[0023] The first direction 101 is, for example, the <1-100> direction. The first direction 101 may be, for example, the [1-100] direction. The first direction 101 may be, for example, a direction obtained by projecting the <1-100> direction onto the first main surface 1. From another perspective, the first direction 101 may be, for example, a direction including a <1-100> direction component.

[0024] The second direction 102 is, for example, the <11-20> direction. The second direction 102 may be, for example, the [11-20] direction. The second direction 102 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.

[0025] The first main surface 1 may be a surface inclined with respect to the {0001} plane. When the 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 the first main surface 1 is inclined with respect to the {0001} plane, the inclination direction (off direction) of the first main surface 1 is, for example, the <11-20> direction. From another perspective, the second direction 102 may be the off direction of the first main surface 1.

[0026] 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 150 mm (6 inches) or more, or 200 mm (8 inches) or more. There is no particular upper limit to the maximum diameter W1. The maximum diameter W1 may be, for example, 250 mm (10 inches) or less. The maximum diameter W1 is the maximum distance between any two points on the outer circumferential edge 5.

[0027] 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). 10 inches refers to 250 mm or 254 mm (10 inches x 25.4 mm / inch).

[0028] 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. As shown in Figure 2, 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.

[0029] 2 , silicon carbide epitaxial layer 20 has buffer layer 11 and drift layer 14. Buffer layer 11 is on silicon carbide substrate 10. Buffer layer 11 is continuous with silicon carbide substrate 10. Drift layer 14 is on buffer layer 11. Drift layer 14 is continuous with buffer layer 11. Drift layer 14 constitutes first main surface 1. In third direction 103, buffer layer 11 is located between silicon carbide substrate 10 and drift layer 14.

[0030] Silicon carbide substrate 10 has a third main surface 8. Third main surface 8 is opposite second main surface 2. Third main surface 8 is in contact with buffer layer 11. Third main surface 8 is, for example, the (0001) plane. Third main surface 8 may be, for example, a plane inclined at an angle of 6 degrees or less with respect to the (0001) plane or the (000-1) plane. The inclination angle with respect to the (0001) plane or the (000-1) plane may be 4 degrees or less, or may be 3 degrees or less.

[0031] The thickness of the buffer layer 11 is defined as a first thickness T1. The first thickness T1 is, for example, 1 μm or more. The first thickness T1 may be, for example, 2 μm or more, 3 μm or more, or 5 μm or more. The first thickness T1 may be 20 μm or less, or 10 μm or less.

[0032] The thickness of drift layer 14 is set to second thickness T2. Second thickness T2 may be greater than first thickness T1. Second thickness T2 is, for example, 5 μm or greater. Second thickness T2 may be, for example, 10 μm or greater, 30 μm or greater, or 50 μm or greater. Second thickness T2 may be 100 μm or less, or 80 μm or less.

[0033] 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, buffer layer 11 and drift layer 14 each contain nitrogen as an n-type impurity. The nitrogen concentration of buffer layer 11 is higher than the nitrogen concentration of drift layer 14. The conductivity type of buffer layer 11 and drift layer 14 is n-type.

[0034] The nitrogen concentration of the buffer layer 11 is, for example, 3×10 18 cm -3 The nitrogen concentration in the buffer layer 11 is, for example, 5×10 18 cm -3 It may be 7×10 or more. 18 cm -3 It may be 9×10 or more. 18 cm -3 The nitrogen concentration of the buffer layer 11 may be, for example, 5×10 19 cm -3 It may be 1×10 or less. 19 cm -3 It may be the following:

[0035] The nitrogen concentration in the drift layer 14 is, for example, 1×10 15 cm -3 The nitrogen concentration in the drift layer 14 is, for example, 3×10 15 cm -3 It may be 7×10 or more. 15 cm -3 It may be 1×10 or more. 16 cm -3 The nitrogen concentration in drift layer 14 may be, for example, 1×10 18 cm -3 It may be 1×10 or less. 17 cm -3 It may be the following:

[0036] Buffer layer 11 may be composed of a single layer, or may be composed of two or more layers with different nitrogen concentrations. Buffer layer 11 may have, for example, a first buffer region 7 and a second buffer region 6. First buffer region 7 is provided on a silicon carbide substrate. Second buffer region 6 is provided on first buffer region 7.

[0037] The nitrogen concentration in the first buffer region 7 may be higher than the nitrogen concentration in the second buffer region 6. The nitrogen concentration in the first buffer region 7 is, for example, 8×10 18 cm -3 The nitrogen concentration in the second buffer region 6 is, for example, 1×10 18 cm -3 The nitrogen concentration in the drift layer 14 is, for example, 7×10 15 cm -3 is.

[0038] The thickness of the first buffer region 7 may be greater than the thickness of the second buffer region 6. The thickness of the first buffer region 7 is, for example, 1 to 10 μm. The thickness of the second buffer region 6 is, for example, 1 to 10 μm. The thickness of the drift layer 14 is, for example, 10 μm. By sandwiching the second buffer region 6 between the first buffer region 7 and the drift layer 14, it is possible to suppress the occurrence of defects due to differences in the crystal lattice constants.

[0039] (Apparatus for manufacturing silicon carbide epitaxial substrate) Figure 3 is a partial cross-sectional schematic diagram showing the configuration of an apparatus for manufacturing silicon carbide epitaxial substrate 100 according to this embodiment. As shown in Figure 3, 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, heating element 203, quartz tube 204, holder 210, a heat insulator (not shown), and an induction heating coil (not shown).

[0040] 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 holder 210 is heated by the heating element 203.

[0041] Holder 210 is formed to be surrounded by inner wall surface 205 of heating element 203. Silicon carbide substrate 10 is placed on holder 210. Holder 210 may be made of silicon carbide, for example. Holder 210 is placed on stage 206. Stage 206 is rotatably supported by rotation shaft 209. Rotation of stage 206 causes holder 210 to rotate.

[0042] Manufacturing apparatus 250 for silicon carbide epitaxial substrate 100 further includes main piping section 207 and gas exhaust port 208. Main piping section 207 is a gas supply port. Main piping section 207 is connected to chamber 201. Gas exhaust port 208 is connected to an exhaust pump (not shown). Arrows in FIG. 3 indicate the flow of gas. Gas is introduced from main piping section 207 into chamber 201 and exhausted from gas exhaust port 208. The pressure inside chamber 201 is adjusted by balancing the amount of gas supplied and the amount of gas exhausted.

[0043] 3 , manufacturing apparatus 250 for silicon carbide epitaxial substrate 100 includes a first dopant gas supply unit 231, a second dopant gas supply unit 232, a carrier gas supply unit 234, a source gas supply unit 233, a first dopant gas pipe 251, a second dopant gas pipe 252, a source gas pipe 253, and a carrier gas pipe 254. Each of first dopant gas supply unit 231, second dopant gas supply unit 232, carrier gas supply unit 234, and source gas supply unit 233 is connected to chamber 201.

[0044] The first dopant gas supply unit 231 is connected to the main pipe unit 207. The first dopant gas pipe 251 connects the first dopant gas supply unit 231 and the main pipe unit 207. The second dopant gas supply unit 232 is connected to the main pipe unit 207. The second dopant gas pipe 252 connects the second dopant gas supply unit 232 and the main pipe unit 207.

[0045] The carrier gas supply unit 234 is connected to the main pipe unit 207. The carrier gas pipe 254 connects the carrier gas supply unit 234 and the main pipe unit 207. The source gas supply unit 233 is connected to the main pipe unit 207. The source gas pipe 253 connects the source gas supply unit 233 and the main pipe unit 207.

[0046] The first dopant gas supply unit 231 is an ammonia gas supply unit configured to be able to supply ammonia gas. The first dopant gas supply unit 231 may be filled with diluted ammonia gas. Specifically, the first dopant gas supply unit 231 may contain ammonia gas diluted with hydrogen gas. The concentration of ammonia gas in the first dopant gas supply unit 231 is, for example, 1%. The concentration of ammonia gas in the first dopant gas supply unit 231 may be, for example, not less than 0.5% and not more than 50%. The concentration of ammonia gas is expressed in volume percent.

[0047] The second dopant gas supply unit 232 is a nitrogen gas supply unit configured to supply nitrogen gas or an ammonia gas supply unit configured to supply ammonia gas. The second dopant gas supply unit 232 may be filled with diluted nitrogen gas or diluted ammonia gas. Specifically, the second dopant gas supply unit 232 may contain nitrogen gas diluted with hydrogen gas or ammonia gas diluted with hydrogen gas. When the second dopant gas supply unit 232 contains ammonia gas, the concentration of the ammonia gas in the second dopant gas supply unit 232 is, for example, 0.1%. The concentration of the ammonia gas in the first dopant gas supply unit 231 may be, for example, 0.01% or more, 0.05% or more, 1% or less, or 0.2% or less.

[0048] The concentration of ammonia gas in the first dopant gas supply unit 231 may be higher than the concentration of ammonia gas in the second dopant gas supply unit 232. Specifically, the concentration of ammonia gas in the first dopant gas supply unit 231 may be two or more times, five or more times, or ten or more times the concentration of ammonia gas in the second dopant gas supply unit 232. The concentration of ammonia gas in the first dopant gas supply unit 231 may be 100 or less times, 50 or less times, or 20 or less times the concentration of ammonia gas in the second dopant gas supply unit 232.

[0049] The carrier gas supply unit 234 is configured to be able to supply a carrier gas. The carrier gas is, for example, hydrogen gas. The carrier gas may be, for example, argon gas. The carrier gas may be, for example, a mixed gas of hydrogen gas and argon gas. The carrier gas supply unit 234 is, for example, a gas cylinder filled with hydrogen.

[0050] The raw material gas supply unit 233 is configured to be able to supply a raw material gas. The raw material gas is a gas that serves as a raw material for silicon carbide. Examples of the raw material gas include propane (C3H8) gas and silane (SiH4) gas. The raw material gas may be diluted with hydrogen. The raw material gas supply unit 233 may have a carbon-containing gas supply unit and a silicon-containing gas supply unit. Specifically, the raw material gas supply unit 233 has a gas cylinder capable of supplying a gas containing a compound containing carbon and hydrogen (e.g., propane gas) and a gas cylinder capable of supplying a gas containing a compound containing silicon and hydrogen (e.g., silane gas).

[0051] The connection between the first dopant gas pipe 251 and the main pipe section 207 is referred to as a first connection section 261. The connection between the second dopant gas pipe 252 and the main pipe section 207 is referred to as a second connection section 262. The connection between the source gas pipe 253 and the main pipe section 207 is referred to as a third connection section 263. The connection between the carrier gas pipe 254 and the main pipe section 207 is referred to as a fourth connection section 264.

[0052] Each of the first connecting portion 261 and the second connecting portion 262 is located closer to the chamber 201 than the third connecting portion 263. From another perspective, each of the first connecting portion 261 and the second connecting portion 262 may be located downstream of the third connecting portion 263 in the direction in which the gas flows in the main piping portion 207. The third connecting portion 263 may be located closer to the chamber 201 than the fourth connecting portion 264. The first connecting portion 261 may be located closer to the chamber 201 than the second connecting portion 262.

[0053] The manufacturing apparatus 250 for silicon carbide epitaxial substrate 100 has a first flow rate control unit 241, a second flow rate control unit 242, a third flow rate control unit 243, and a fourth flow rate control unit 244. The first flow rate control unit 241 controls the flow rate of gas flowing through the first dopant gas pipe 251. The second flow rate control unit 242 controls the flow rate of gas flowing through the second dopant gas pipe 252. The third flow rate control unit 243 controls the flow rate of gas flowing through the source gas pipe 253. The fourth flow rate control unit 244 controls the flow rate of gas flowing through the carrier gas pipe 254. Each control unit is, for example, an MFC (Mass Flow Controller).

[0054] In the process of forming the buffer layer 11, ammonia gas having the same concentration as the ammonia concentration in the first dopant gas pipe 251 is supplied to the main pipe section 207 using the first dopant gas pipe 251 and the first dopant gas supply section 231. In the process of forming the drift layer 14, nitrogen gas or low-concentration ammonia gas is supplied to the main pipe section 207 using the second dopant gas pipe 252 and the second dopant gas supply section 232.

[0055] The main piping section 207 may be a single main piping or may be multiple main piping. The main piping section 207 may have, for example, a first main piping (not shown), a second main piping (not shown), and a third main piping (not shown). The carrier gas supply section and the silane gas supply section may be connected to, for example, the first main piping. The carrier gas supply section and the propane gas supply section may be connected to, for example, the second main piping. The carrier gas supply section and the ammonia gas supply section may be connected to, for example, the third main piping.

[0056] The first dopant gas supply unit 231 is connected to the first dopant gas pipe 251, but can also be connected to the chamber 201 without going through the main pipe unit 207. In this case, the first dopant gas pipe 251 (the portion between the first flow rate control unit 241 and the chamber 201) may be connected to the carrier gas supply unit 234 via a flow rate control unit.

[0057] The second dopant gas supply unit 232 is connected to the second dopant gas pipe 252, but can also be connected to the chamber 201 without going through the main pipe unit 207. In this case, the second dopant gas pipe 252 (the portion between the second flow rate control unit 242 and the chamber 201) may be connected to the carrier gas supply unit 234 via a flow rate control unit.

[0058] The source gas supply unit 233 is connected to the source gas pipe 253, but can also be connected to the chamber 201 without going through the main pipe unit 207. In this case, the source gas supply unit 233 (the portion between the third flow rate control unit 243 and the chamber 201) may be connected to the carrier gas supply unit 234 via a flow rate control unit.

[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] 4 is a flow diagram schematically showing a method for manufacturing silicon carbide epitaxial substrate 100 according to this embodiment. As shown in Fig. 4, the method for manufacturing silicon carbide epitaxial substrate 100 according to this embodiment includes a first step (S11) of forming first buffer layer 11a, a second step (S12) of forming first drift layer 14a, a third step (S21) of forming second buffer layer 11b, and a fourth step (S22) of forming second drift layer 14b.

[0061] First, a plurality of silicon carbide substrates 10 are prepared. A silicon carbide single crystal of polytype 4H is manufactured by, for example, sublimation. Next, a plurality of silicon carbide substrates 10 are prepared by slicing the silicon carbide single crystal by, for example, a wire saw. Each of the plurality of silicon carbide substrates 10 contains an n-type impurity such as nitrogen. The conductivity type of each of the plurality of silicon carbide substrates 10 is, for example, n-type. Next, mechanical polishing is performed on each of the plurality of silicon carbide substrates 10. Next, chemical mechanical polishing is performed on each of the plurality of silicon carbide substrates 10. The plurality of silicon carbide substrates 10 includes a first silicon carbide substrate 10a and a second silicon carbide substrate 10b.

[0062] Next, first silicon carbide substrate 10a is placed in chamber 201 of manufacturing apparatus 250 for silicon carbide epitaxial substrate 100. Specifically, first silicon carbide substrate 10a is placed inside chamber 201 in a state where it is placed on holder 210.

[0063] Next, a first step (S11) of forming a first buffer layer 11a is performed. FIG. 5 is a cross-sectional schematic diagram showing the step of forming the first buffer layer 11a. As shown in FIG. 5, the first buffer layer 11a is formed on the first silicon carbide substrate 10a by epitaxial growth. 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. After the temperature of the holder 210 reaches, for example, about 1600°C, the source gas, dopant gas, and carrier gas are supplied to the chamber 201. Specifically, a mixed gas containing silane, propane, ammonia, and hydrogen is introduced into the chamber 201 through the main piping portion 207. In the chamber 201, the respective gases are thermally decomposed, and the first buffer layer 11a is formed on the first silicon carbide substrate 10a.

[0064] The flow rate of silane gas supplied to the chamber 201 is adjusted to, for example, 46 sccm. The flow rate of silane gas may be, for example, 30 sccm or more and 60 sccm or less. The flow rate of propane gas supplied to the chamber 201 is adjusted to, for example, 14 sccm. The flow rate of propane gas may be, for example, 10 sccm or more and 20 sccm or less. The flow rate of hydrogen gas supplied to the chamber 201 is adjusted to, for example, 120 slm. The flow rate of hydrogen gas may be, for example, 100 slm or more and 150 slm or less.

[0065] In forming the first buffer layer 11a, ammonia gas supplied from the first dopant gas supply unit 231 is used as the dopant gas. As described above, the first dopant gas supply unit 231 contains ammonia gas diluted with hydrogen gas. The concentration of the ammonia gas in the first dopant gas supply unit 231 is, for example, 1%. The flow rate of the ammonia gas supplied to the chamber 201 is controlled using the first flow rate control unit 241. For example, the flow rate of ammonia gas diluted with hydrogen gas to a concentration of 1% is, for example, 10 sccm or more and 200 sccm or less. Note that this flow rate, when converted to the flow rate of ammonia gas with a concentration of 100%, is, for example, 0.1 sccm or more and 2.0 sccm or less.

[0066] As described above, the first buffer layer 11 a is formed on the first silicon carbide substrate 10 a by epitaxial growth using the first dopant gas, which contains ammonia gas. The first dopant gas is supplied from the first dopant gas supply unit 231.

[0067] The thickness of the first buffer layer 11a is, for example, 1 μm or more. The thickness of the first buffer layer 11a may be, for example, 3 μm or more, or 5 μm or more. The thickness of the first buffer layer 11a may be, for example, 10 μm or less, or 8 μm or less.

[0068] The nitrogen concentration of the first buffer layer 11a is, for example, 3×10 18 cm -3 The nitrogen concentration in the first buffer layer 11a is, for example, 5×10 18 cm -3 It may be 7×10 or more. 18 cm -3 It may be 9×10 or more. 18 cm -3 The nitrogen concentration of the first buffer layer 11a may be, for example, 5×10 19 cm -3 It may be 1×10 or less. 19 cm -3 It may be the following:

[0069] Next, a second step (S12) of forming first drift layer 14 a is performed. Fig. 6 is a schematic cross-sectional view showing the step of forming first drift layer 14 a. As shown in Fig. 6, first drift layer 14 a is formed on first buffer layer 11 a by epitaxial growth.

[0070] The flow rate of silane gas supplied to the chamber 201 is adjusted to, for example, 115 sccm. The flow rate of silane gas may be, for example, 80 sccm or more and 150 sccm or less. The flow rate of propane gas supplied to the chamber 201 is adjusted to, for example, 37.5 sccm. The flow rate of propane gas may be, for example, 25 sccm or more and 50 sccm or less. The flow rate of hydrogen gas supplied to the chamber 201 is adjusted to, for example, 120 slm. The flow rate of hydrogen gas may be, for example, 100 slm or more and 150 slm or less.

[0071] In forming the first drift layer 14a, nitrogen gas or ammonia gas supplied from the second dopant gas supply unit 232 is used as the dopant gas. When the second dopant gas supply unit 232 contains nitrogen gas, the flow rate of the nitrogen gas supplied to the chamber 201 is controlled using the second flow rate control unit 242. For example, undiluted nitrogen gas with a concentration of 100% can be used. The nitrogen gas may also be diluted with hydrogen gas. The flow rate of the nitrogen gas, converted into the flow rate of nitrogen gas with a concentration of 100%, is, for example, 200 sccm or less.

[0072] When the second dopant gas supply unit 232 contains ammonia gas diluted with hydrogen gas, the concentration of the ammonia gas in the second dopant gas supply unit 232 is, for example, 0.1%. The flow rate of the ammonia gas supplied to the chamber 201 is controlled using the second flow rate control unit 242. For example, the flow rate of ammonia gas diluted with hydrogen gas to a concentration of 0.1% is, for example, 10 sccm to 200 sccm. Note that this flow rate, when converted to the flow rate of ammonia gas with a concentration of 100%, is, for example, 0.01 sccm to 0.2 sccm.

[0073] In the claims, the flow rate of the dopant gas refers to the flow rate when the concentration of the diluted dopant gas is converted into the flow rate of the dopant gas with a concentration of 100% when the dopant gas is diluted.

[0074] The concentration of ammonia gas in the first dopant gas supply unit 231 is higher than the concentration of ammonia gas in the second dopant gas supply unit 232. Specifically, the concentration of ammonia gas in the first dopant gas supply unit 231 may be two or more times, five or more times, or ten or more times the concentration of ammonia gas in the second dopant gas supply unit 232. The concentration of ammonia gas in the first dopant gas supply unit 231 may be 100 or less times, 50 or less times, or 20 or less times the concentration of ammonia gas in the second dopant gas supply unit 232.

[0075] As described above, the first drift layer 14a is formed on the first buffer layer 11a by epitaxial growth using the second dopant gas. The second dopant gas is nitrogen gas or ammonia gas. The second dopant gas is supplied from the second dopant gas supply unit 232. The thickness of the first drift layer 14a is, for example, 5 μm or more. The nitrogen concentration of the first drift layer 14a is, for example, 1×10 15 cm -3 1x10 or more 17 cm -3 In this manner, first silicon carbide epitaxial substrate 100a is manufactured. First silicon carbide epitaxial substrate 100a is removed from chamber 201.

[0076] Next, second silicon carbide substrate 10b is placed in chamber 201 of manufacturing apparatus 250 for silicon carbide epitaxial substrate 100. Specifically, second silicon carbide substrate 10b is placed inside chamber 201 in a state where it is placed on holder 210.

[0077] Next, a third step (S21) of forming a second buffer layer 11b is performed. FIG. 7 is a schematic cross-sectional view showing the step of forming the second buffer layer 11b. As shown in FIG. 7, the second buffer layer 11b is formed on the second silicon carbide substrate 10b by epitaxial growth. 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. After the temperature of the chamber 201 reaches, for example, about 1600°C, the source gas, dopant gas, and carrier gas are supplied to the chamber 201. Specifically, a mixed gas containing silane, propane, ammonia, and hydrogen is introduced into the chamber 201 through the main pipe 207. In the chamber 201, the respective gases are thermally decomposed, and the second buffer layer 11b is formed on the second silicon carbide substrate 10b.

[0078] The flow rates of the silane gas, propane gas, and hydrogen gas in the third step (S21) of forming the second buffer layer 11b are substantially the same as the flow rates of the silane gas, propane gas, and hydrogen gas in the first step (S11) of forming the first buffer layer 11a.

[0079] In forming the second buffer layer 11b, ammonia gas supplied from the first dopant gas supply unit 231 is used as the dopant gas. As described above, the first dopant gas supply unit 231 contains ammonia gas diluted with hydrogen gas. The concentration of the ammonia gas in the first dopant gas supply unit 231 is, for example, 1%. The flow rate of the ammonia gas supplied to the chamber 201 is controlled using the first flow rate control unit 241.

[0080] The flow rate of the third dopant gas in the third step (S21) of forming the second buffer layer 11b is substantially the same as the flow rate of the first dopant gas in the first step (S11) of forming the first buffer layer 11a.

[0081] As described above, the second buffer layer 11b is formed on the second silicon carbide substrate 10b by epitaxial growth using the third dopant gas. The third dopant gas includes ammonia gas. The third dopant gas is supplied from the first dopant gas supply unit 231. The concentration of the first dopant gas is substantially the same as the concentration of the third dopant gas. The nitrogen concentration of the second buffer layer 11b is substantially the same as the nitrogen concentration of the first buffer layer 11a. The nitrogen concentration of the second buffer layer 11b is, for example, 3×10 18 cm -3 That's all.

[0082] The thickness of the second buffer layer 11b is substantially the same as the thickness of the first buffer layer 11a, and is, for example, 1 μm or more.

[0083] Next, a fourth step (S22) of forming second drift layer 14 b is performed. Fig. 8 is a schematic cross-sectional view showing the step of forming second drift layer 14 b. As shown in Fig. 8, second drift layer 14 b is formed on second buffer layer 11 b by epitaxial growth.

[0084] The flow rates of silane gas, propane gas, and hydrogen gas in the fourth step (S22) of forming the second drift layer 14b are substantially the same as the flow rates of silane gas, propane gas, and hydrogen gas in the second step (S12) of forming the first drift layer 14a.

[0085] As described above, the second drift layer 14b is formed on the second buffer layer 11b by epitaxial growth using the fourth dopant gas. The fourth dopant gas is nitrogen gas or ammonia gas. The fourth dopant gas is supplied from the second dopant gas supply unit 232. The thickness of the second drift layer 14b is, for example, 5 μm or more. The nitrogen concentration of the second drift layer 14b is, for example, 1×10 15 cm -3 1x10 or more 17 cm -3 In this manner, second silicon carbide epitaxial substrate 100b is manufactured as follows.

[0086] The second dopant gas is the same as the fourth dopant gas. When the second dopant gas is nitrogen gas, the fourth dopant gas is nitrogen gas. When the second dopant gas is ammonia gas, the fourth dopant gas is ammonia gas. The concentration of the second dopant gas is the same as the concentration of the fourth dopant gas. From another perspective, the second dopant gas and the fourth dopant gas are each supplied from the same second dopant gas supply unit 232.

[0087] The value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas is 0.95 or more and 1.05 or less. In other words, the flow rate of the fourth dopant gas is controlled so that the value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas is 0.95 or more and 1.05 or less. The value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas may be, for example, 0.97 or more and 1.03 or less, 0.99 or more and 1.01 or less, 0.995 or more and 1.005 or less, or 1.00.

[0088] The first step (S11) to the fourth step (S22) may be repeatedly performed. The value obtained by dividing the flow rate of the second dopant gas in the second or subsequent second step (S12) by the flow rate of the fourth dopant gas in the second or subsequent fourth step (S22) may be 0.95 or more and 1.05 or less. The first first step (S11) and the second step (S12) may be performed after cleaning of the manufacturing apparatus 250, after maintenance of the manufacturing apparatus 250, after stopping and then restarting the operation of the manufacturing apparatus 250, or after any step in a series of epitaxial growth steps.

[0089] In another aspect, ammonia gas supplied from the first dopant gas supply unit 231 and ammonia gas supplied from the second dopant gas supply unit 232 may be used simultaneously in each of the first step of forming the first buffer layer 11a and the third step of forming the second buffer layer 11b, and ammonia gas supplied from the second dopant gas supply unit 232 may be used in each of the second step of forming the first drift layer 14a and the fourth step of forming the second drift layer 14b.

[0090] As described above, in the method for manufacturing silicon carbide epitaxial substrate 100 according to this embodiment, first silicon carbide epitaxial substrate 100a and second silicon carbide epitaxial substrate 100b are manufactured.

[0091] First silicon carbide epitaxial substrate 100a has first silicon carbide substrate 10a, first buffer layer 11a, and first drift layer 14a (see FIG. 6). First buffer layer 11a is formed on first silicon carbide substrate 10a. First drift layer 14a is formed on first buffer layer 11a. Similarly, second silicon carbide epitaxial substrate 100b has second silicon carbide substrate 10b, second buffer layer 11b, and second drift layer 14b (see FIG. 8). Second buffer layer 11b is formed on second silicon carbide substrate 10b. Second drift layer 14b is formed on second buffer layer 11b. First silicon carbide epitaxial substrate 100a and second silicon carbide epitaxial substrate 100b each have substantially the same structure as silicon carbide epitaxial substrate 100 according to this embodiment.

[0092] After the chamber maintenance, the carrier concentration of each of the first drift layer (first time) and the second drift layer (second time) was 1.00 × 10 16 cm -3 and 1.01 x 10 16 cm -3That is, the difference in carrier concentration between the first drift layer (first time) and the second drift layer (second time) was suppressed to within ±1%. The growth conditions of the epitaxial layer of the first drift layer and the gas flow conditions including the dopant gas were the same for the first drift layer (first time) and the second drift layer (second time). The conditions for the buffer layer, etc. were also the same.

[0093] Thereafter, the epitaxial layer growth was continued under the same growth conditions for the drift layer, buffer layer, and other epitaxial layers, as well as the same gas flow conditions, including the dopant gas. The carrier concentrations of the third drift layer (third time), fourth drift layer (fourth time), and fifth drift layer (fifth time) were each 1.02 × 10 16 cm -3 , 1.01 × 10 16 cm -3 and 1.02 x 10 16 cm -3 The carrier concentration was measured using a CV measurement device. The measurement point for the carrier concentration was the center of silicon carbide epitaxial substrate 100.

[0094] Furthermore, after the second step (S12), the third step (S21) can be performed without measuring the carrier concentration of the first drift layer (first time). After the second step, the third step can be performed without performing calibration. The same recipe can be used as the growth program for forming the first drift layer and the growth program for forming the second drift layer.

[0095] Next, a method for manufacturing a silicon carbide epitaxial substrate in which nitrogen is doped during silicon carbide epitaxial growth is disclosed, in which ammonia gas is passed through a pre-heating region 211 for thermal decomposition before reaching silicon carbide substrate 10 on which a silicon carbide epitaxial layer is formed.

[0096] Since the ammonia gas serving as the nitrogen source is thermally decomposed in preheating region 211 before reaching silicon carbide substrate 10, the in-plane distribution of nitrogen becomes uniform across the wide surface of silicon carbide substrate 10 where epitaxial growth is occurring. As a result, the in-plane concentration distribution of nitrogen doped into the growing silicon carbide epitaxial layer also becomes uniform.

[0097] In the preheating region 211, the gas passes through a region where the wall surfaces of the members in the chamber 201 are at 1300° C. or higher, so that the ammonia gas can be sufficiently thermally decomposed in advance without causing any significant disturbances in the gas flow.

[0098] Here, the preheating region 211 refers to a space for heating the flowing gas, and includes a long, thin tube heated from the outside, a space with a heat transfer coil installed inside, a large space with fins or the like formed on the inner wall surface, etc.

[0099] The upper limit of the temperature of the wall surface of the region is preferably 1350° C. or higher to ensure reliable pyrolysis even if the length of the chamber is somewhat short, and is preferably 1800° C. or lower from the viewpoint of thermal efficiency. For example, preheating region 211 can be the region from the end of chamber 201 to silicon carbide substrate 10 that is closest to the end of chamber 201. Preheating region 211 is preferably 50 mm or longer and 1000 mm or shorter.

[0100] Next, a silicon carbide semiconductor manufacturing apparatus is disclosed, which is a silicon carbide epitaxial substrate manufacturing apparatus that dopes nitrogen during silicon carbide epitaxial growth, and has a pre-heating region 211 for thermally decomposing ammonia gas before the ammonia gas reaches silicon carbide substrate 10 on which a silicon carbide epitaxial layer is formed.

[0101] Since the ammonia gas serving as the nitrogen source is thermally decomposed in pre-heating region 211 before reaching silicon carbide substrate 10, the in-plane distribution of nitrogen becomes uniform across the wide surface of silicon carbide substrate 10 where epitaxial growth is occurring, and as a result, the in-plane concentration distribution of nitrogen doped into the growing silicon carbide epitaxial layer also becomes uniform.

[0102] In the preheating region 211, the gas passes through a region where the wall surfaces of the members in the chamber 201 are at 1300° C. or higher, so that the ammonia gas can be sufficiently thermally decomposed in advance without causing any significant disturbances in the gas flow.

[0103] Here, the preheating region 211 refers to a space for heating the flowing gas, and includes a long, thin tube heated from the outside, a space with a heat transfer coil installed inside, a large space with fins or the like formed on the inner wall surface, etc.

[0104] The upper limit of the temperature of the wall surface of the region is preferably 1350° C. or higher to ensure reliable pyrolysis even if the length of the chamber is somewhat short, and is preferably 1800° C. or lower from the viewpoint of thermal efficiency. For example, preheating region 211 can be the region from the end of chamber 201 to silicon carbide substrate 10 that is closest to the end of chamber 201. Preheating region 211 is preferably 50 mm or longer and 1000 mm or shorter.

[0105] (Method for Manufacturing Silicon Carbide Semiconductor Device) Next, a method for manufacturing silicon carbide semiconductor device 300 according to this embodiment will be described.

[0106] First, silicon carbide epitaxial substrate 100 according to this embodiment is prepared. Specifically, either first silicon carbide epitaxial substrate 100a or second silicon carbide epitaxial substrate 100b is prepared (see FIG. 2 ). Next, a step of forming a body region is performed. FIG. 9 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. Body region 113 has a thickness of, for example, 0.9 μm.

[0107] Next, a step of forming a source region is performed. FIG. 10 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.

[0108] 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 drift layer 14. 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.

[0109] Next, activation annealing is performed to activate the implanted impurities. The temperature of the activation annealing is, for example, about 1700° C. The activation annealing time is, for example, about 30 minutes. The atmosphere of the activation annealing is, for example, an Ar atmosphere.

[0110] Next, a step of forming trenches in the first main surface 1 is performed. FIG. 11 is a cross-sectional schematic diagram showing the step of forming trenches in the first main surface 1. As shown in FIG. 11, 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 can be used using SF or a mixed gas of SF and O as a reactive gas. By etching, a recess is formed in the first main surface 1.

[0111] 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.

[0112] 11 , 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.

[0113] Next, a step of forming a gate insulating film is performed. FIG. 12 is a schematic cross-sectional 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.

[0114] Next, a step of forming a gate electrode is performed. FIG. 13 is a cross-sectional view showing a 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.

[0115] 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.

[0116] 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.

[0117] 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 900° C. or higher and 1100° C. or lower for about 5 minutes. This causes at least a portion of the source electrode 116 to be silicided. This forms the source electrode 116 in ohmic contact with the source region 114.

[0118] 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.

[0119] 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.

[0120] (Silicon Carbide Semiconductor Device) Next, the configuration of a silicon carbide semiconductor device 300 according to this embodiment will be described. FIG. 14 is a cross-sectional schematic diagram showing the configuration of the silicon carbide semiconductor device 300 according to this embodiment. As shown in FIG. 14 , 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 buffer layer 11, a drift layer 14, a body region 113, a source region 114, and a contact region 118.

[0121] 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. The body region 113 is a p-type region. The drift layer 14 is an n-type region. A body diode is formed between the p-type region and the n-type region.

[0122] 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 drift layer 14. 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.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] Next, the effects of the method for manufacturing silicon carbide epitaxial substrate 100 and the method for manufacturing silicon carbide semiconductor device 300 according to this embodiment will be described.

[0127] Nitrogen gas or ammonia gas is used to dope nitrogen into a silicon carbide epitaxial layer. Nitrogen gas is chemically very stable compared to ammonia gas and is therefore less likely to be decomposed by heat. Nitrogen gas that is not completely decomposed accumulates in the chamber over time. Therefore, the nitrogen concentration tends to be higher in the later stages of epitaxial growth than in the early stages.

[0128] Furthermore, in silicon carbide epitaxial substrate 100, the nitrogen concentration in buffer layer 11 is higher than the nitrogen concentration in drift layer 14. The nitrogen concentration in buffer layer 11 may be 10 or more times or 100 or more times the nitrogen concentration in drift layer 14. If a large amount of nitrogen gas is used as a dopant gas when forming buffer layer 11, a large amount of nitrogen gas will accumulate in the chamber.

[0129] As a result, when silicon carbide epitaxial substrates are manufactured continuously, the nitrogen concentration in the drift layer (second drift layer 14 b) of the silicon carbide epitaxial substrate manufactured later (second time) (second silicon carbide epitaxial substrate 100 b) tends to be higher than the nitrogen concentration in the drift layer (first drift layer 14 a) of the silicon carbide epitaxial substrate manufactured earlier (first time) (first silicon carbide epitaxial substrate 100 a).

[0130] When nitrogen gas is used as a dopant gas in order to reduce fluctuations in the nitrogen concentration in the drift layers of multiple silicon carbide epitaxial substrates, the flow rate of the nitrogen gas in the process of forming the subsequent (second) drift layer (second drift layer 14 b) needs to be set lower than the flow rate of the nitrogen gas in the process of forming the previous (first) drift layer (first drift layer 14 a).

[0131] According to the method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure, in a first step, a first buffer layer 11a is formed by epitaxial growth on a first silicon carbide substrate 10a using a first dopant gas containing ammonia. In a second step, a first drift layer 14a is formed by epitaxial growth on the first buffer layer 11a using a second dopant gas. In a third step, a second buffer layer 11b is formed by epitaxial growth on a second silicon carbide substrate 10b using a third dopant gas containing ammonia. In a fourth step, a second drift layer 14b is formed by epitaxial growth on the second buffer layer 11b using a fourth dopant gas. The flow rate of the second dopant gas divided by the flow rate of the fourth dopant gas is equal to or greater than 0.95 and equal to or less than 1.05.

[0132] According to the method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure, a dopant gas containing ammonia is used in each of the first step for forming the first buffer layer 11a and the third step for forming the second buffer layer 11b. From another perspective, because nitrogen gas is not used in each of the first and third steps, accumulation of unthermally decomposed nitrogen gas in the chamber is prevented. Therefore, when silicon carbide epitaxial substrates are successively formed, fluctuations in the nitrogen concentration in the drift layer can be reduced without substantially changing the flow rates of the second and fourth dopant gases. By checking the manufacturing equipment log, if the recipes for the second and fourth steps are the same, it can be determined that the flow rate of the second dopant gas is substantially the same as the flow rate of the fourth dopant gas.

[0133] In the method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure, each of the second dopant gas and the fourth dopant gas may contain ammonia. That is, since nitrogen gas is not used in each of the second and fourth steps, accumulation of nitrogen gas remaining without being thermally decomposed in the chamber is further prevented. Therefore, when silicon carbide epitaxial substrates are continuously formed, fluctuations in the nitrogen concentration in the drift layer can be further reduced.

[0134] In the method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure, the value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas may be equal to or greater than 0.99 and equal to or less than 1.01, thereby further reducing fluctuations in the nitrogen concentration in the drift layer when silicon carbide epitaxial substrates are continuously formed.

[0135] According to the method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure, each of the second dopant gas and the fourth dopant gas may be nitrogen gas. Ammonia gas can epitaxially grow a buffer layer having a high nitrogen concentration. On the other hand, when the nitrogen concentration in the drift layer is particularly low, the ammonia gas must be controlled at a low flow rate, which can be difficult to control. Therefore, when epitaxial growth is performed at a low nitrogen concentration, it is easier to control the concentration by introducing the dopant using nitrogen gas.

[0136] In a silicon carbide semiconductor device having a body diode, such as a MOSFET, when holes, which are minority carriers, move from the drift layer to the silicon carbide substrate, the energy generated when the holes and electrons recombine can cause basal plane dislocations to grow into stacking faults, resulting in an increase in forward resistance. By forming a buffer layer with a high nitrogen concentration between the silicon carbide substrate and the drift layer, it is possible to prevent holes, which are minority carriers, from moving from the drift layer to the silicon carbide substrate. As a result, it is possible to prevent an increase in forward resistance.

[0137] According to the method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure, the nitrogen concentration in each of the first buffer layer 11a and the second buffer layer 11b is 3×10 18 cm -3 This makes it possible to prevent an increase in forward resistance in a silicon carbide semiconductor device having a body diode.

[0138] According to the method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure, each of first buffer layer 11 a and second buffer layer 11 b may have a thickness of 1 μm or more. This further prevents holes, which are minority carriers, from migrating from the drift layer to the silicon carbide substrate. As a result, an increase in forward resistance can be further prevented.

[0139] In the method for manufacturing a silicon carbide semiconductor device according to the present disclosure, a silicon carbide epitaxial substrate is prepared using the method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure, thereby preventing an increase in forward resistance in a silicon carbide semiconductor device having a body diode.

[0140] The embodiments 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 is intended to include meanings equivalent to the claims and all modifications within the scope thereof.

[0141] REFERENCE SIGNS LIST 1 First main surface 2 Second main surface 3 Orientation flat 4 Arc-shaped portion 5 Outer periphery 6 Second buffer region 7 First buffer region 8 Third main surface 10 Silicon carbide substrate 10a First silicon carbide substrate 10b Second silicon carbide substrate 11 Buffer layer 11a First buffer layer 11b Second buffer layer 14 Drift layer 14a First drift layer 14b Second drift layer 20 Silicon carbide epitaxial layer 100 Silicon carbide epitaxial substrate 100a First silicon carbide epitaxial substrate 100b Second 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 117 Mask 118 Contact region 119 Source wiring 120 Drain electrode 123 Side surface 124 Bottom surface 126 Interlayer insulating film 127 Gate electrode 201 Chamber 203 Heating element 204 Quartz tube 205 Inner wall surface 206 Stage 207 Main pipe section 208 Gas exhaust port 209 Rotating shaft 210 Holder 211 Preheating region 231 First dopant gas supply section 232 Second dopant gas supply section 233 Source gas supply section 234 Carrier gas supply section 241 First flow rate control section 242 Second flow rate control section 243 Third flow rate control section 244 Fourth flow rate control section 250 Manufacturing apparatus 251 First dopant gas pipe 252 Second dopant gas pipe 253 Source gas pipe 254 Carrier gas pipe 261 First connecting portion 262 Second connecting portion 263 Third connecting portion 264 Fourth connecting portion 300 Silicon carbide semiconductor device A Center T1 First thickness T2 Second thickness W1 Maximum diameter

Claims

1. A method for manufacturing a silicon carbide epitaxial substrate, comprising: a first step of epitaxially growing a first buffer layer on a first silicon carbide substrate using a first dopant gas containing ammonia; a second step of epitaxially growing a first drift layer on the first buffer layer using a second dopant gas; a third step of epitaxially growing a second buffer layer on a second silicon carbide substrate using a third dopant gas containing ammonia after the second step; and a fourth step of epitaxially growing a second drift layer on the second buffer layer using a fourth dopant gas, wherein the flow rate of the second dopant gas divided by the flow rate of the fourth dopant gas is 0.95 or more and 1.05 or less.

2. The method for producing a silicon carbide epitaxial substrate according to claim 1, wherein each of the second dopant gas and the fourth dopant gas contains ammonia.

3. A method for producing a silicon carbide epitaxial substrate according to claim 1 or 2, wherein the value obtained by dividing the flow rate of the second dopant gas by the flow rate of the fourth dopant gas is 0.99 or more and 1.01 or less.

4. A method for manufacturing a silicon carbide epitaxial substrate according to any one of claims 1 to 3, wherein the flow rate of the second dopant gas divided by the flow rate of the fourth dopant gas is 1.

00.

5. The method for manufacturing a silicon carbide epitaxial substrate according to any one of claims 1 to 4, wherein each of the second dopant gas and the fourth dopant gas is ammonia gas.

6. The method for producing a silicon carbide epitaxial substrate according to any one of claims 1 to 4, wherein each of the second dopant gas and the fourth dopant gas is nitrogen gas.

7. The nitrogen concentration in each of the first buffer layer and the second buffer layer is 3×10 18 cm -3 The method for manufacturing a silicon carbide epitaxial substrate according to any one of claims 1 to 6, wherein:

8. A method for manufacturing a silicon carbide epitaxial substrate according to any one of claims 1 to 7, wherein the first buffer layer and the second buffer layer each have a thickness of 1 µm or more.

9. A method for manufacturing a silicon carbide epitaxial substrate according to any one of claims 1 to 8, wherein the first silicon carbide substrate has a main surface in contact with the first buffer layer, and the main surface is a plane inclined at an angle of 6 degrees or less with respect to the (0001) plane or the (000-1) plane.

10. A method for manufacturing a silicon carbide semiconductor device, comprising the steps of: preparing a silicon carbide epitaxial substrate using the method for manufacturing a silicon carbide epitaxial substrate according to any one of claims 1 to 9; and forming electrodes on the silicon carbide epitaxial substrate.

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