Sic composite substrate, method for manufacturing sic composite substrate, and semiconductor device

Abstract

This SiC composite substrate includes an SiC substrate and a polycrystalline SiC layer laminated on the top surface of the SiC substrate. The polycrystalline SiC layer is formed by alternately laminating a high concentration layer containing an impurity at a first doping amount and a low concentration layer containing an impurity at a second doping amount less than the first doping amount. The impurity of the high concentration layer and the impurity of the low concentration layer may both be nitrogen.

Description

SiC composite substrate, method for manufacturing SiC composite substrate, and semiconductor device

[0001] The present disclosure relates to a SiC composite substrate, a method for manufacturing a SiC composite substrate, and a semiconductor device.

[0002] Conventionally, SiC-based devices such as Schottky barrier diodes (SBDs), MOSFETs, and IGBTs have been used for power control applications. Single-crystal SiC substrates on which such SiC-based devices are formed are typically manufactured by a sublimation recrystallization method known as the modified Lely method. However, this method suffers from the drawback of low efficiency in crystal growth and wafer processing, resulting in high manufacturing costs. To reduce manufacturing costs, SiC composite substrates have been manufactured by growing a polycrystalline growth layer on a single-crystal SiC substrate using chemical vapor deposition (CVD).

[0003] In such SiC composite substrates, warping may occur during crystal growth or after grinding of the substrate. For SiC freestanding films formed by CVD, there is a technology to suppress warping by providing predetermined layers on both sides (see Patent Documents 1 and 2).

[0004] Japanese Patent Application Laid-Open No. 2001-158666 Japanese Patent Application Laid-Open No. 2003-113472

[0005] However, the inventors of the present disclosure have discovered that doping a SiC composite substrate with impurities such as nitrogen (N) to reduce the resistivity of the substrate changes the grain size of the crystal grains, thereby reducing the effect of suppressing warpage. Furthermore, suppressing warpage is also desired in SiC composite substrates in which a polycrystalline SiC layer is stacked on a single crystal substrate such as 4H—SiC.

[0006] The present disclosure has been proposed in view of the above-described circumstances, and aims to provide a SiC composite substrate in which a polycrystalline SiC layer is crystal-grown on a SiC substrate, and which does not warp, and a method for manufacturing the same.

[0007] In order to solve the above-mentioned problems, the present disclosure provides a SiC composite substrate including a SiC substrate and a polycrystalline SiC layer laminated on a top surface of the SiC substrate, wherein the polycrystalline SiC layer is formed by alternately laminating high-concentration layers containing impurities at a first doping level and low-concentration layers containing impurities at a second doping level lower than the first doping level.

[0008] The method for manufacturing a SiC composite substrate of the present disclosure includes providing a SiC substrate and forming a polycrystalline SiC layer on a top surface of the SiC substrate by alternately stacking high-concentration layers of polycrystalline SiC containing a first impurity at a first doping amount and low-concentration layers of polycrystalline SiC containing the first impurity at a second doping amount that is lower than the first doping amount.

[0009] FIG. 1 is a cross-sectional view of a SiC composite substrate according to the first embodiment. FIG. 2A is a view illustrating a polycrystalline SiC layer in the SiC composite substrate according to the first embodiment. FIG. 2B is a view illustrating a polycrystalline SiC layer in the SiC composite substrate according to the first embodiment. FIG. 3A is a view illustrating a case where the top surface of the SiC composite substrate according to the first embodiment has been ground. FIG. 3B is a view illustrating a case where the bottom surface of the SiC composite substrate according to the first embodiment has been ground. FIG. 4 is a cross-sectional view of a SiC composite substrate according to a modification of the first embodiment. FIG. 5A is a view illustrating a polycrystalline SiC layer in the SiC composite substrate according to the modification of the first embodiment. FIG. 5B is a view illustrating a polycrystalline SiC layer in the SiC composite substrate according to the modification of the first embodiment. FIG. 6A is a process diagram of a method for manufacturing a SiC composite substrate according to the first embodiment. FIG. 6B is a process diagram of a method for manufacturing a SiC composite substrate according to the first embodiment. FIG. 6C is a process diagram of a method for manufacturing a SiC composite substrate according to the first embodiment. FIG. 7 is a view illustrating flow rates of precursor gases in the method for manufacturing a SiC composite substrate according to the first embodiment. FIG. 8 is a cross-sectional view of a Schottky barrier diode to which the SiC composite substrate according to the first embodiment is applied. FIG. 9 is a cross-sectional view of a trench FET to which the SiC composite substrate according to the first embodiment is applied. FIG. 10 is a cross-sectional view of a planar FET to which the SiC composite substrate according to the first embodiment is applied. FIG. 11 is a cross-sectional view of an IGBT to which the SiC composite substrate according to the first embodiment is applied. FIG. 12 is a cross-sectional view of a SiC composite substrate according to a second embodiment. FIG. 13A is a diagram illustrating a polycrystalline SiC layer in the SiC composite substrate according to the second embodiment. FIG. 13B is a diagram illustrating a polycrystalline SiC layer in the SiC composite substrate according to the second embodiment. FIG. 14 is a diagram illustrating flow rates of precursor gases in the method for manufacturing a SiC composite substrate according to the second embodiment. FIG. 15 is a cross-sectional view of a SiC composite substrate according to a third embodiment. FIG. 16 is a diagram illustrating a polycrystalline SiC layer in the SiC composite substrate according to the third embodiment. FIG. 17 is a diagram illustrating flow rates of precursor gases in the method for manufacturing a SiC composite substrate according to the third embodiment.

[0010] [Detailed Description] Hereinafter, embodiments of a SiC composite substrate and a manufacturing method thereof will be described in detail with reference to the drawings. The embodiments are comprehensive or specific examples, and the numerical values, shapes, materials, components, component installation positions, and connection forms are merely examples and are not intended to limit the scope of the present disclosure. Furthermore, among the components in the embodiments, components that are not recited in the independent claims that represent the highest concepts are described as optional components. Furthermore, the dimensional proportions in the drawings are exaggerated for the sake of explanation and may differ from the actual proportions. Furthermore, the embodiments and their modified examples may include similar components, and similar components are assigned common reference numerals, and redundant description will be omitted.

[0011] [First Embodiment] (SiC Composite Substrate) FIG. 1 is a cross-sectional view of a SiC composite substrate 1 according to a first embodiment of the present disclosure. The SiC composite substrate 1 of FIG. 1 is configured by laminating a polycrystalline SiC layer 15 on a top surface 11a of a SiC substrate 11. The SiC substrate 11 may be configured of single-crystal SiC or polycrystalline SiC. The SiC substrate 11 may be configured of a hexagonal crystal system such as 4H or 6H, or a cubic crystal system such as 3C. Furthermore, the top surface 11a of the single-crystal SiC substrate 11 is assumed to be the (0001) plane or the (000-1) plane, but may be other planes. The SiC substrate 11 may be formed by a sublimation method or an epitaxial method.

[0012] The polycrystalline SiC layer 15 is formed by alternately stacking high-concentration layers 12 containing an impurity at a first doping amount and low-concentration layers 13 containing an impurity at a second doping amount lower than the first doping amount. In the SiC composite substrate 1 according to the first embodiment, the impurity in both the high-concentration layers 12 and the low-concentration layers 13 is nitrogen (N). The first doping amount of nitrogen (N) in the high-concentration layers 12 is, for example, 1.0E20 atom / cm. ３ The second doping amount of nitrogen (N) in the low concentration layer 13 is, for example, 1.0E20 atom / cm ３ However, the optimum values ​​of the first doping amount and the second doping amount vary depending on the film formation conditions, and therefore the doping amounts shown are merely examples.

[0013] The polycrystalline SiC layer 15 is composed of two to one hundred high-concentration layers 12 and low-concentration layers 13 in total. The high-concentration layers 12 of the polycrystalline SiC layer 15 are in contact with the SiC substrate 11. The thicknesses of the high-concentration layers 12 and the low-concentration layers 13 are set so that the total thickness of the high-concentration layers 12 and the total thickness of the low-concentration layers 13 are each 250 μm or less. For example, if the polycrystalline SiC layer 15 is composed of two layers, one high-concentration layer 12 and one low-concentration layer 13, the thicknesses of the high-concentration layers 12 and the low-concentration layers 13 are each 250 μm or less. For example, if the polycrystalline SiC layer 15 is composed of 100 layers, one high-concentration layer 12 and one low-concentration layer 13, the thicknesses of each of the high-concentration layers 12 and the low-concentration layers 13 are each 5 μm or less. When the polycrystalline SiC layer 15 includes a plurality of high-concentration layers 12 and low-concentration layers 13, it is preferable that the plurality of high-concentration layers 12 have the same thickness, and the plurality of low-concentration layers 13 have the same thickness. However, the thicknesses of the plurality of high-concentration layers 12 may be different, and the thicknesses of the plurality of low-concentration layers 13 may be different.

[0014] In the SiC composite substrate 1, the SiC substrate 11 is configured as an n-type semiconductor doped with n-type impurities such as nitrogen (N), phosphorus (P), and arsenic (As), but may also be configured as a p-type semiconductor doped with p-type impurities such as boron (B) and aluminum (Al). The polycrystalline SiC layer 15 may have the same conductivity type as the SiC substrate 11, or the SiC substrate 11 may be a p-type semiconductor and the polycrystalline SiC layer 15 an n-type semiconductor. The polycrystalline SiC layer 15 may be formed by chemical vapor deposition (CVD).

[0015] 2A and 2B are diagrams illustrating the polycrystalline SiC layer 15 in the SiC composite substrate 1 according to the first embodiment. When the polycrystalline SiC layer 15 is formed by CVD, increasing the nitrogen (N) doping amount reduces the grain size of the SiC crystal grains, while decreasing the nitrogen (N) doping amount increases the grain size of the SiC crystal grains. As shown in FIG. 2A , the SiC crystal grains have a smaller grain size in the lowermost high-concentration layer 12 of the polycrystalline SiC layer 15 stacked on the top surface 11a of the SiC substrate 11. As the grain size decreases, the SiC crystal grains have more grain boundaries and aggregate in a direction that reduces the grain boundaries. This causes the layer to contract, generating tensile stress 12a. Therefore, the lowermost high-concentration layer 12 of the polycrystalline SiC layer 15 applies tensile stress to the top surface 11a of the SiC substrate 11, causing the SiC composite substrate 1 to warp so that the polycrystalline SiC layer 15 side is recessed.

[0016] 2B , in the SiC composite substrate 1 according to the first embodiment, the polycrystalline SiC layer 15 is formed by alternately stacking high-concentration layers 12 and low-concentration layers 13. As described above, tensile stress 12a is generated within the high-concentration layer 12. In the low-concentration layer 13, the SiC crystal grains increase in size, causing the SiC crystal grains to push against each other, creating a repulsive force between the SiC crystal grains, which acts in an expanding direction as the layer, generating compressive stress 13a.

[0017] In the polycrystalline SiC layer 15, high-concentration layers 12 in which tensile stress 12a occurs and low-concentration layers 13 in which compressive stress 13a occurs are alternately stacked, so that the tensile stress 12a and the compressive stress 13a cancel each other out. As a result, the stress applied from the polycrystalline SiC layer 15 to the SiC substrate 11 is reduced, and warping of the SiC composite substrate 1 is suppressed.

[0018] Here, the number of high-concentration layers 12 and low-concentration layers 13 included in the polycrystalline SiC layer 15 may be controlled depending on the magnitude relationship between the tensile stress 12a generated in the high-concentration layers 12 and the compressive stress 13a generated in the low-concentration layers 13. Similarly, the thicknesses of the high-concentration layers 12 and low-concentration layers 13 included in the polycrystalline SiC layer 15 may be controlled depending on the magnitude relationship between the tensile stress 12a generated in the high-concentration layers 12 and the compressive stress 13a generated in the low-concentration layers 13. Both the number and thickness of the high-concentration layers 12 and low-concentration layers 13 may be controlled.

[0019] 3A and 3B are diagrams illustrating the case where the SiC composite substrate 1 according to the first embodiment is ground. FIG. 3A is a diagram illustrating the case where the top surface of the SiC composite substrate 1 is ground. As shown in FIG. 3A , even when the upper part of the polycrystalline SiC layer 15 is ground from the top surface of the SiC composite substrate 1, the remaining polycrystalline SiC layer 15 is composed of alternating high-concentration layers 12 and low-concentration layers 13. In the remaining polycrystalline SiC layer 15, the tensile stress 12a generated in the high-concentration layer 12 and the compressive stress 13a generated in the low-concentration layer 13 cancel each other out. Therefore, the stress exerted by the polycrystalline SiC layer 15 on the SiC substrate 11 is reduced, and warpage in the SiC composite substrate 1 is suppressed.

[0020] 3B is a diagram illustrating the case where the bottom surface of the SiC composite substrate 1 according to the first embodiment is ground. As shown in FIG. 3B , when the bottom surface 11 b of the SiC substrate 11, which corresponds to the bottom surface of the SiC composite substrate 1, is ground to a predetermined height, the polycrystalline SiC layer 15 stacked on the top surface 11 a of the SiC substrate 11 remains unchanged. Even after grinding, the stress exerted by the polycrystalline SiC layer 15 on the SiC substrate 11 is reduced in the same manner as before grinding, and warpage in the SiC composite substrate 1 is suppressed.

[0021] As described above, in the SiC composite substrate 1 according to the first embodiment, the polycrystalline SiC layer 15 is configured by alternately stacking the high-concentration layers 12 in which the tensile stress 12a occurs within the layer and the low-concentration layers 13 in which the compressive stress 13a occurs within the layer. With the SiC composite substrate 1, the tensile stress 12a and the compressive stress 13a cancel each other out, and it is possible to provide a SiC composite substrate 1 in which the occurrence of warping is suppressed.

[0022] Fig. 4 is a cross-sectional view showing a SiC composite substrate 2 as a modification of the first embodiment. The SiC composite substrate 2 of the modification differs in that, while the high-concentration layer 12 is in contact with the top surface 11a of the SiC substrate 11 in the SiC composite substrate shown in Fig. 1, a low-concentration layer 13 is in contact with the top surface 11a of the SiC substrate 11. Since the other configurations are the same as those of the SiC composite substrate 1 shown in Fig. 1, corresponding components are designated by common reference numerals to clarify the correspondence.

[0023] 5A and 5B are diagrams illustrating a polycrystalline SiC layer 15 in a SiC composite substrate 2 according to a modification of the first embodiment. As shown in Fig. 5A, in the lowest low-concentration layer 13 of the polycrystalline SiC layer 15 stacked on the top surface 11a of the SiC substrate 11, large SiC crystal grains push against each other, causing repulsive forces between the SiC crystal grains, resulting in compressive stress 13a within the layer. As a result, compressive stress is applied to the top surface 11a of the SiC substrate 11 by the lowest low-concentration layer 13 of the polycrystalline SiC layer 15, causing the SiC composite substrate 2 to warp so that the bottom surface 11b of the SiC substrate 11 is recessed.

[0024] As shown in FIG. 5B , in the modified SiC composite substrate 2, the polycrystalline SiC layer 15 is formed by alternately stacking low-concentration layers 13 and high-concentration layers 12. As described above, compressive stress 13a occurs within the low-concentration layer 13. In the high-concentration layer 12, small crystal grains tend to aggregate, creating attractive forces between the SiC crystal grains, generating tensile stress 12a within the layer. The polycrystalline SiC layer 15 is formed by alternately stacking the low-concentration layers 13, in which compressive stress 13a occurs, and the high-concentration layers 12, in which tensile stress 12a occurs, so that the compressive stress 13a and the tensile stress 12a cancel each other out. The SiC composite substrate 2 reduces the stress applied to the SiC substrate 11 from the polycrystalline SiC layer 15, thereby suppressing warpage in the SiC composite substrate 2.

[0025] In this manner, even in the modified SiC composite substrate 2, the polycrystalline SiC layer 15 is formed by alternately stacking low-concentration layers 13 in which compressive stress 13a occurs and high-concentration layers 12 in which tensile stress 12a occurs. With the SiC composite substrate 2, the compressive stress 13a and the tensile stress 12a cancel each other out, and it is possible to provide a SiC composite substrate 2 in which warping is suppressed.

[0026] (Manufacturing Method of SiC Composite Substrate) Next, a manufacturing method of the SiC composite substrate 1 shown in FIG. 1 will be described. First, as shown in FIG. 6A, a SiC substrate 11 is provided. The SiC substrate 11 may be fabricated by sublimation or formed by epitaxial growth on a seed crystal. The SiC substrate 11 is doped with n-type impurities and configured as an n-type semiconductor.

[0027] As shown in FIG. 6B, a lowermost high-concentration layer 12 of a polycrystalline SiC layer 15 is formed on the top surface 11a of the SiC substrate 11. The polycrystalline SiC layer 15 is made of, for example, silicon tetrachloride (SiCl ４ ) and methane (CH ４ ) as a precursor gas, and nitrogen gas (N ２ The polycrystalline SiC layer 15 can be grown by adding nitrogen gas (N) at a temperature of 1600° C. or less and a pressure of 300 hPa or less. Under these conditions, a polycrystalline SiC layer 15 having a crystal polytype of 3C can be grown. ２ ) and methane (CH ４ By controlling the ratio of the flow rates of the nitrogen (N) and carbon (C), the doping amount of nitrogen (N) can be controlled in proportion to the logarithm of the ratio of nitrogen (N) to carbon (C). The doping amount of nitrogen (N) in the high concentration layer 12 can be controlled to, for example, 1.0E20 atom / cm ３ The above can be used to form the film.

[0028] 6C, a low concentration layer 13 is formed in the high concentration layer 12 formed in the process shown in FIG. 6B. The low concentration layer 13 is formed under the film formation conditions described for forming the high concentration layer 12, except that the doping amount of nitrogen (N) is set to, for example, 1.0E20 atom / cm. ３ It can be formed by making it less than.

[0029] FIG. 7 shows a method for manufacturing the SiC composite substrate 1 shown in FIGS. 6A to 6C, in which silicon tetrachloride (SiCl ) is used as a precursor gas as an example. ４ ) and methane (CH ４ ) and nitrogen gas (N ２ 1 is a graph showing the change in flow rate of each precursor gas over time when CVD is performed using a gas mixture containing 100% fluorine-containing gas and 100% fluorine-containing gas.

[0030] At time t0, the flow rates of all three precursor gases are zero. At time t1, the flow of the three precursor gases begins. ４ ) and methane (CH ４ The flow rate of the gas 2 is kept constant from time t1 onwards during the film formation by CVD.

[0031] Nitrogen gas (N ２ The flow rate of nitrogen gas (N) is relatively increased between times t1 and t2. Between times t1 and t2, the ratio of nitrogen (N) to carbon (C) becomes relatively large, and the doping amount of nitrogen (N) increases, allowing the growth of the high concentration layer 12. ２ The flow rate of nitrogen gas (N) is relatively reduced between times t2 and t3. Between times t2 and t3, the ratio of nitrogen (N) to carbon (C) is relatively reduced, and the doping amount of nitrogen (N) is reduced, allowing the low concentration layer 13 to grow. Similarly, the flow rate of nitrogen gas (N ２ The flow rate of the SiO 2 ) is relatively high from time t3 to t4 and relatively low from time t4 to t5. The high-concentration layer 12 can be grown from time t3 to t4, and the low-concentration layer 13 can be grown from time t4 to t5. By controlling the flow rate in the same way after time t5, the high-concentration layer 12 and the low-concentration layer 13 can be grown alternately.

[0032] In FIG. 7, nitrogen gas (N ２ The high-concentration layer 12 and the low-concentration layer 13 are alternately grown by increasing or decreasing the flow rate of the nitrogen gas (N) at regular intervals to a relatively high flow rate and a relatively low flow rate. Alternatively, the high-concentration layer 12 and the low-concentration layer 13 can be alternately grown by alternately turning the nitrogen inlet pipe on and off.

[0033] By controlling the flow rate of the precursor gas as described above with reference to Fig. 7, the process of forming the high-concentration layer 12 shown in Fig. 6B and the process of forming the low-concentration layer 13 shown in Fig. 6C are repeated following the process of Fig. 6C so that the high-concentration layer 12 and the low-concentration layer 13 are alternately stacked. The process of forming the high-concentration layer 12 and the low-concentration layer 13 is continued until the polycrystalline SiC layer 15 composed of the high-concentration layer 12 and the low-concentration layer 13 reaches a predetermined total number. By this series of processes, the SiC composite substrate 1 shown in Fig. 1 is formed. When performing CVD, nitrogen gas (N ２ By controlling the flow rate of the gas, the high concentration layer 12 and the low concentration layer 13 can be deposited continuously, so that film formation can be performed while maintaining the machine time and growth rate.

[0034] In FIG. 7, nitrogen gas (N ２ When the flow rate of nitrogen gas (N) is relatively reduced between times t1 and t2, the amount of nitrogen (N) doped is reduced between times t1 and t2, and thus a low concentration layer 13 can be grown on the top surface 11a of the SiC substrate 11. ２ When the flow rate of nitrogen gas (N) is relatively increased between times t2 and t3, the amount of nitrogen (N) doped increases between times t2 and t3, and the high concentration layer 12 can be grown on the low concentration layer 13. ２ ) is relatively reduced to grow the low concentration layer 13, whereby the modified SiC composite substrate 2 shown in FIG. 4 can be manufactured.

[0035] Also, in Figure 7, methane (CH ４ The flow rate of methane (CH ４ ) may be increased or decreased.

[0036] In addition, in FIG. 7, silicon tetrachloride (SiCl ) is used as a precursor gas of silicon (Si). ４ ) was used as the precursor gas for carbon (C), and methane (CH ４ ) was used as a precursor gas for nitrogen (N), and nitrogen gas (N ２However, the precursor gas for silicon (Si) is, for example, monosilane (SiH ４ ), and the precursor gas for carbon (C) can be, for example, propane (C ３ H ８ ) or ethylene (C ２ H ４ ), and the precursor gas for nitrogen (N) can be ammonia (NH ３ ) may be used. Furthermore, the precursor gas of silicon (Si) and the precursor gas of carbon (C) may be dimethylsilane (C ２ H ８ It may be a single gas, such as Si).

[0037] Here, as an example of the flow rate ratio of the precursor gas of carbon (C) and the precursor gas of nitrogen (N), ３ H ８ ) was used as a precursor gas for nitrogen (N), and nitrogen gas (N ２ ) or ammonia (NH ３ The flow rate ratio when propane (C ３ H ８ ) flow rate of nitrogen gas (N ２ ) or ammonia (NH ３ The doping amount of nitrogen (N) added to polycrystalline SiC layer 15 may be controlled by setting the ratio of the flow rates of 1.0E-4 to 1.0E1, for example.

[0038] In the manufacturing method of the SiC composite substrate 1 according to the first embodiment, the polycrystalline SiC layer 15 is formed by alternately stacking high-concentration layers 12 in which tensile stress 12a occurs within the layers and low-concentration layers 13 in which compressive stress 13a occurs within the layers. These tensile stresses 12a and compressive stresses 13a cancel each other out, thereby reducing the stress exerted from the polycrystalline SiC layer 15 on the SiC substrate 11. The manufacturing method of the SiC composite substrate 1 can provide a manufacturing method of the SiC composite substrate 1 in which the occurrence of warpage is suppressed.

[0039] (Semiconductor Device) Next, a semiconductor device according to a first embodiment will be described. Here, a case where a single-crystal SiC substrate is used as the SiC substrate 11 will be described. The semiconductor device according to the first embodiment uses a SiC composite substrate 1 as shown in FIG. 1 , and is configured such that the single-crystal SiC substrate 11 of the SiC composite substrate 1 serves as a drift layer and the polycrystalline SiC layer 15 serves as a substrate layer. Note that, since the SiC composite substrate 1 is used upside down in the semiconductor device, the top and bottom surfaces of the single-crystal SiC substrate 11 and the polycrystalline SiC layer 15 of the SiC composite substrate 1 are located below and above the semiconductor device, respectively.

[0040] Below, we will explain examples of electronic devices using the SiC composite substrate 1. As examples, we will explain a Schottky barrier diode (SBD), a trench metal oxide semiconductor field effect transistor (MOSFET), a planar MOSFET, and an insulated-gate bipolar transistor (IGBT).

[0041] 8 is a cross-sectional view of an SBD 20. The SBD 20 is fabricated using the SiC composite substrate 1 shown in FIG. 1. In the SBD 20, the single-crystal SiC substrate 11 and the polycrystalline SiC layer 15 are stacked in the SiC composite substrate 1, and the single-crystal SiC substrate 11 serves as a drift layer, while the polycrystalline SiC layer 15 serves as a substrate layer. The top surface of the polycrystalline SiC layer 15 is covered with a cathode electrode 21, which is connected to a cathode terminal K.

[0042] The bottom surface 11b of the single-crystal SiC substrate 11 is provided with a contact hole 23 that exposes a part of the single-crystal SiC substrate 11 as a body region 22, and a field insulating film 25 is formed in a field region 24 that surrounds the body region 22. The field insulating film 25 is made of silicon oxide (SiO ２), but may be made of other insulating materials such as silicon nitride (SiN). An anode electrode 26 is formed on the field insulating film 25 and is connected to an anode terminal A.

[0043] A p-type junction termination extension (JTE) structure 27 is formed in the vicinity of the bottom surface 11b (surface layer portion) of the single-crystal SiC substrate 11 so as to be in contact with the anode electrode 26. The JTE structure 27 is formed along the contour of the contact hole 23 in the field insulating film 25 so as to straddle the inside and outside of the contact hole 23.

[0044] 9 is a cross-sectional view of a trench MOSFET 30. The trench MOSFET 30 is fabricated using the SiC composite substrate 1 shown in FIG. 1. In the trench MOSFET 30, the single-crystal SiC substrate 11 and the polycrystalline SiC layer 15 are stacked in the SiC composite substrate 1, and the single-crystal SiC substrate 11 serves as a drift layer, while the polycrystalline SiC layer 15 serves as a substrate layer. The top surface of the polycrystalline SiC layer 15 is covered with a drain electrode 31, which is connected to a drain terminal D.

[0045] A p-type body region 32 is formed on the bottom surface 11b of the single crystal SiC substrate 11. The portion of the single crystal SiC substrate 11 on the polycrystalline SiC layer 15 side of the body region 32 is a drain region 33(11) that remains in the state of the single crystal SiC substrate 11. A gate trench 34 is formed in the single crystal SiC substrate 11. The gate trench 34 penetrates the body region 32 from the bottom surface 11b of the single crystal SiC substrate 11, and its deepest portion reaches the drain region 33(11).

[0046] A gate insulating film 35 is formed on the inner surface of the gate trench 34 and on the bottom surface 11b of the single-crystal SiC substrate 11 so as to cover the entire inner surface of the gate trench 34. The inside of the gate insulating film 35 is filled with, for example, polysilicon, thereby embedding a gate electrode 36 in the gate trench 34. A gate terminal G is connected to the gate electrode 36.

[0047] The surface layer of the body region 32 is formed with a highly doped n-type impurity dopant (nPt) that forms part of the side surface of the gate trench 34. + The single-crystal SiC substrate 11 has a highly doped p-type source region 37 formed therein. The p-type source region 37 is connected to the body region 32 and extends from the bottom surface 11b of the single-crystal SiC substrate 11 through the source region 37. + A mold body contact region 38 is formed.

[0048] On the single crystal SiC substrate 11, silicon oxide (SiO ２ An interlayer insulating film 41 made of a material selected from the group consisting of a silicon nitride film, ...

[0049] By applying a predetermined voltage (a voltage equal to or greater than the gate threshold voltage) to the gate electrode 36 while a predetermined potential difference is generated between the source electrode 43 and the drain electrode 31 (between the source and drain), a channel can be formed in the body region 32 near the interface with the gate insulating film 35 due to the electric field from the gate electrode 36. This allows a current to flow between the source electrode 43 and the drain electrode 31, turning on the trench MOSFET 30.

[0050] 10 is a cross-sectional view of a planar MOSFET 50. The planar MOSFET 50 is fabricated using the SiC composite substrate 1 shown in FIG. 1. In the planar MOSFET 50, in the SiC composite substrate 1 in which a single-crystal SiC substrate 11 and a polycrystalline SiC layer 15 are stacked, the single-crystal SiC substrate 11 serves as a drift layer, and the polycrystalline SiC layer 15 serves as a substrate layer. The top surface of the polycrystalline SiC layer 15 is covered with a drain electrode 51, which is connected to a drain terminal D.

[0051] A p-type body region 52 is formed in a well shape on the bottom surface 11b of the single-crystal SiC substrate 11. In the single-crystal SiC substrate 11, the portion on the polycrystalline SiC layer 15 side of the body region 52 is a drain region 53 (11) that is maintained in the state of the single-crystal SiC substrate 11. A surface layer of the body region 52 is formed by a highly doped n + A source region 54 of a type is formed at a distance from the periphery of the body region 52. Inside the source region 54, a heavily doped p + A body contact region 55 is formed on the source region 54. The body contact region 55 penetrates the source region 54 in the depth direction and is connected to the body region 52.

[0052] A gate insulating film 56 is formed on the bottom surface 11b of the single-crystal SiC substrate 11. The gate insulating film 56 covers a portion of the body region 52 surrounding the source region 54 (the peripheral portion of the body region 52) and the outer periphery of the source region 54. A gate electrode 57 made of, for example, polysilicon is formed on the gate insulating film 56. The gate electrode 57 faces the peripheral portion of the body region 52 with the gate insulating film 56 interposed therebetween. A gate terminal G is connected to the gate electrode 57.

[0053] On the single crystal SiC substrate 11, silicon oxide (SiO ２ An interlayer insulating film 58 made of a material such as a silicon dioxide film (SiO 2 ) is formed on the body contact region 55. A source electrode 62 is connected to the source region 54 and the body contact region 55 through a contact hole 61 formed in the interlayer insulating film 58. A source terminal S is connected to the source electrode 62.

[0054] By applying a predetermined voltage (a voltage equal to or greater than the gate threshold voltage) to the gate electrode 57 while a predetermined potential difference is generated between the source electrode 62 and the drain electrode 51 (between the source and drain), a channel can be formed in the body region 52 near the interface with the gate insulating film 56 due to the electric field from the gate electrode 57. This allows a current to flow between the source electrode 62 and the drain electrode 51, turning the planar MOSFET 50 on.

[0055] 11 is a cross-sectional view of an IGBT 70. The IGBT 70 is fabricated using the SiC composite substrate 1 shown in FIG. 1. In the IGBT 70, in the SiC composite substrate 1 in which a single-crystal SiC substrate 11 and a polycrystalline SiC layer 15 are stacked, the single-crystal SiC substrate 11 serves as a drift layer, and the polycrystalline SiC layer 15 serves as a substrate layer. The top surface of the polycrystalline SiC layer 15 is covered with a collector electrode 71, which is connected to a collector terminal C.

[0056] A p-type body region 72 is formed in a well shape on the bottom surface 11b of the single-crystal SiC substrate 11. In the single-crystal SiC substrate 11, the portion on the polycrystalline SiC layer 15 side of the body region 72 is a drain region 73 (11) that is maintained in the state of the single-crystal SiC substrate 11. A surface layer of the body region 72 is formed with a highly doped n + A type emitter region 74 is formed at a distance from the periphery of the body region 72. Inside the emitter region 74, a heavily doped p + A molded body contact region 75 is formed on the emitter region 74. The body contact region 75 penetrates the emitter region 74 in the depth direction and is connected to the body region 72.

[0057] A gate insulating film 76 is formed on the bottom surface 11b of the single-crystal SiC substrate 11. The gate insulating film 76 covers a portion of the body region 72 surrounding the emitter region 74 (the peripheral portion of the body region 72) and the outer periphery of the emitter region 74. A gate electrode 77 made of, for example, polysilicon is formed on the gate insulating film 76. The gate electrode 77 faces the peripheral portion of the body region 72 with the gate insulating film 76 interposed therebetween. A gate terminal G is connected to the gate electrode 77.

[0058] On the single crystal SiC substrate 11, silicon oxide (SiO ２ An interlayer insulating film 78 made of a material selected from the group consisting of a silicon nitride film, ...

[0059] By applying a predetermined voltage (a voltage equal to or greater than the gate threshold voltage) to the gate electrode 77 while a predetermined potential difference is generated between the emitter electrode 82 and the collector electrode 71 (between the emitter and collector), a channel can be formed in the body region 72 near the interface with the gate insulating film 76 due to the electric field from the gate electrode 77. This allows a current to flow between the emitter electrode 82 and the collector electrode 71, turning the IGBT 70 on.

[0060] The semiconductor device according to the first embodiment uses the SiC composite substrate 1 according to the first embodiment, which is configured to prevent warpage. Therefore, the occurrence of warpage in the semiconductor device is suppressed, and the quality of the semiconductor device is ensured.

[0061] Second Embodiment (SiC Composite Substrate) Fig. 12 is a cross-sectional view of a SiC composite substrate 3 according to a second embodiment of the present disclosure. The SiC composite substrate 3 in Fig. 12 differs from the SiC composite substrate 1 according to the first embodiment shown in Fig. 1 in that a polycrystalline SiC layer 18, different from the polycrystalline SiC layer 15, is stacked on the top surface 11a of the SiC substrate 11.

[0062] The polycrystalline SiC layer 18 is formed by alternately stacking high-concentration layers 16 containing an impurity at a first doping amount and low-concentration layers 17 containing an impurity at a second doping amount lower than the first doping amount. In the SiC composite substrate 3 according to the second embodiment, the impurity in both the high-concentration layers 16 and the low-concentration layers 17 is phosphorus (P). The first doping amount of the impurity in the high-concentration layers 16 is, for example, 1.0E18 atom / cm ３ More than 5.0E19 atoms / cm ３ The first doping amount of the impurity in the high concentration layer 16 is, for example, 5.0E19 atom / cm ３ The reason for setting the upper limit will be described later. The second doping amount of the impurity in the low concentration layer 17 is, for example, 1.0E18 atom / cm ３ However, the optimum values ​​of the first doping amount and the second doping amount vary depending on the film formation conditions, and therefore the doping amounts shown are merely examples.

[0063] The polycrystalline SiC layer 18 is composed of a total of 2 to 100 high-concentration layers 16 and low-concentration layers 17. The high-concentration layers 16 of the polycrystalline SiC layer 18 are in contact with the SiC substrate 11. The thicknesses of the high-concentration layers 16 and the low-concentration layers 17 are set so that the total thickness of the high-concentration layers and the total thickness of the low-concentration layers are each 250 μm or less. When the polycrystalline SiC layer 18 includes multiple high-concentration layers 16 and multiple low-concentration layers 17, it is preferable that the multiple high-concentration layers 16 have the same thickness and the multiple low-concentration layers 17 have the same thickness. However, the thicknesses of the multiple high-concentration layers 16 and the multiple low-concentration layers 17 may be different.

[0064] In the SiC composite substrate 3, the SiC substrate 11 is configured as an n-type semiconductor doped with n-type impurities such as nitrogen (N), phosphorus (P), and arsenic (As), but may also be configured as a p-type semiconductor doped with p-type impurities such as boron (B) and aluminum (Al). The polycrystalline SiC layer 18 may have the same conductivity type as the SiC substrate 11, or the SiC substrate 11 may be a p-type semiconductor and the polycrystalline SiC layer 18 an n-type semiconductor. The polycrystalline SiC layer 18 may be formed by chemical vapor deposition (CVD).

[0065] 13A and 13B are diagrams illustrating the polycrystalline SiC layer 18 in the SiC composite substrate 3 according to the second embodiment. When the polycrystalline SiC layer 18 is formed by CVD, the doping amount of phosphorus (P) is set to, for example, 5.0E19 atom / cm ３ When the doping amount of phosphorus (P) is increased within the following range, the grain size of the SiC crystal grains increases, and when the doping amount of phosphorus (P) is decreased, the grain size of the SiC crystal grains decreases. ３ If the amount is increased, the grain size of the SiC crystal grains becomes small, and the grain size of the SiC crystal grains cannot be uniquely controlled by the doping amount of phosphorus (P), so it is preferable to set an upper limit to the doping amount of phosphorus (P).

[0066] 13A , the SiC crystal grains in the lowermost high-concentration layer 16 of the polycrystalline SiC layer 18 stacked on the top surface 11 a of the SiC substrate 11 have larger grain sizes. As the grain sizes increase, the SiC crystal grains push against each other, creating repulsive forces between them, which act in the direction of expansion of the layer, generating compressive stress 16 a. As a result, the lowermost high-concentration layer 16 of the polycrystalline SiC layer 18 applies compressive stress to the top surface 11 a of the SiC substrate 11, causing the SiC composite substrate 3 to warp so that the bottom surface 11 b of the SiC substrate 11 is recessed.

[0067] 13B , in the SiC composite substrate 3 according to the second embodiment, the polycrystalline SiC layer 18 is formed by alternately stacking high-concentration layers 16 and low-concentration layers 17. As described above, compressive stress 16a occurs within the high-concentration layer 16. In the low-concentration layer 17, the SiC crystal grains have many grain boundaries due to their small grain size, and the SiC crystal grains aggregate in a direction that reduces the grain boundaries, causing the layer to contract and generating tensile stress 17a.

[0068] In the polycrystalline SiC layer 18, high-concentration layers 12 in which compressive stress 16a occurs and low-concentration layers 17 in which tensile stress 17a occurs are alternately stacked, so that the compressive stress 16a and the tensile stress 17a cancel each other out. The SiC composite substrate 3 reduces the stress applied to the SiC substrate 11 from the polycrystalline SiC layer 18, and can provide a SiC composite substrate in which warping is suppressed.

[0069] Here, the number of high-concentration layers 16 and low-concentration layers 17 included in polycrystalline SiC layer 18 may be controlled depending on the magnitude relationship between compressive stress 16a generated in high-concentration layers 16 and tensile stress 17a generated in low-concentration layers 17. Similarly, the thicknesses of high-concentration layers 16 and low-concentration layers 17 included in polycrystalline SiC layer 18 may be controlled depending on the magnitude relationship between compressive stress 16a generated in high-concentration layers 16 and tensile stress 17a generated in low-concentration layers 17. Both the number and thickness of high-concentration layers 16 and low-concentration layers 17 may be controlled.

[0070] (Method for manufacturing SiC composite substrate) Next, a method for manufacturing a SiC composite substrate 3 according to the second embodiment shown in Figure 12 will be described. The method for manufacturing the SiC composite substrate 3 differs from the method for manufacturing the SiC composite substrate 1 according to the first embodiment shown in Figures 6A to 6C in the following respects. In the method for manufacturing the SiC composite substrate 3, a high-concentration layer 16 different from the high-concentration layer 12 is formed, a low-concentration layer 17 different from the low-concentration layer 13 is formed, and a polycrystalline SiC layer 18 different from the polycrystalline SiC layer 15 is formed. The cross-sectional views during each step of the manufacturing method are omitted because the high-concentration layer, low-concentration layer, and polycrystalline SiC layer formed in the cross-sectional views shown in Figures 6A to 6C are replaced as described above.

[0071] First, as in Fig. 6A, a SiC substrate 11 is provided. The SiC substrate 11 may be fabricated by sublimation or may be formed by epitaxial growth on a seed crystal. The SiC substrate 11 is doped with n-type impurities and configured as an n-type semiconductor.

[0072] 6B, a high-concentration layer 16 is formed as the lowest layer of a polycrystalline SiC layer 18 on the top surface 11a of the SiC substrate 11, and a low-concentration layer 17 is formed in the high-concentration layer 16 as in FIG. 6C. The polycrystalline SiC layer 18 is formed by, for example, monosilane (SiH ４ ) and methane (CH ４ ) as a precursor gas, and phosphine (PH ３ By performing CVD while adding phosphine (PH), a polycrystalline SiC layer with a 3C polytype can be grown. ３ ) and methane (CH ４ By controlling the ratio of the flow rates of carbon (C) and phosphorus (P), the doping amount of phosphorus (P) can be controlled in proportion to the logarithm of the ratio of phosphorus (P) to carbon (C).

[0073] FIG. 14 shows a method for manufacturing a SiC composite substrate 3, in which monosilane (SiH ４ ) and methane (CH ４ ) and phosphine (PH ３ 1 is a graph showing the change in flow rate of each precursor gas over time when CVD is performed using a gas mixture containing 100% fluorine-containing gas and 100% fluorine-containing gas.

[0074] At time t0, the flow rates of all three precursor gases are zero. At time t1, the flow of the three precursor gases begins. ４ ) and methane (CH ４ The flow rate of the gas 2 is kept constant from time t1 onwards during the film formation by CVD.

[0075] Phosphine (PH ３ The flow rate of phosphine (PH) is relatively increased between times t1 and t2. Between times t1 and t2, the ratio of phosphorus (P) to carbon (C) becomes relatively large, and the doping amount of phosphorus (P) increases, allowing the high concentration layer 16 to grow. ３ The flow rate of phosphine (PH) is relatively reduced between times t2 and t3. Between times t2 and t3, the ratio of phosphorus (P) to carbon (C) is relatively reduced, and the doping amount of phosphorus (P) is reduced, allowing the low concentration layer 17 to grow. Similarly, the flow rate of phosphine (PH ３ The flow rate of the hydrogen ions 14 is set relatively high between times t3 and t4, and relatively low between times t4 and t5. The high-concentration layer 16 can be grown between times t3 and t4, and the low-concentration layer 17 can be grown between times t4 and t5. By controlling the flow rate in the same way after time t5, the high-concentration layer 16 and the low-concentration layer 17 can be grown alternately.

[0076] In FIG. 14, phosphine (PH ３ ) is increased or decreased to a relatively high flow rate and a relatively low flow rate at regular intervals, thereby alternately growing the high-concentration layer 16 and the low-concentration layer 17. Alternatively, the high-concentration layer 16 and the low-concentration layer 17 can also be grown alternately by alternately turning the phosphine inlet pipe on and off.

[0077] As described above, by controlling the flow rate of the precursor gas as explained in Fig. 14, the process of forming the high concentration layer 16 and the low concentration layer 17 is continued until the polycrystalline SiC layer 18 composed of the high concentration layer 16 and the low concentration layer 17 reaches a predetermined total number. By this series of processes, the SiC composite substrate 3 shown in Fig. 12 is formed. When performing CVD, phosphine (PH ３By controlling the flow rate of the gas, the high concentration layer 16 and the low concentration layer 17 can be deposited continuously, so that film formation can be performed while maintaining the machine time and growth rate.

[0078] In the method for manufacturing a SiC composite substrate 3 according to the second embodiment, high-concentration layers 16 in which compressive stress 16a occurs and low-concentration layers 17 in which tensile stress 17a occurs are alternately stacked to form a polycrystalline SiC layer 18. According to the method for manufacturing a SiC composite substrate 3, the compressive stress 16a and the tensile stress 17a cancel each other out, so that the stress exerted from the polycrystalline SiC layer 18 on the SiC substrate 11 is reduced, and a method for manufacturing a SiC composite substrate 3 in which warpage is suppressed can be provided.

[0079] (Semiconductor Device) Next, a semiconductor device according to a second embodiment will be described. Here, a case where a single-crystal SiC substrate is used as the SiC substrate 11 will be described. The semiconductor device according to the second embodiment uses a SiC composite substrate 3 as shown in FIG. 12 , and is configured such that the single-crystal SiC substrate 11 of the SiC composite substrate 3 serves as a drift layer and the polycrystalline SiC layer 18 serves as a substrate layer. Note that, since the SiC composite substrate 3 is used upside down in the semiconductor device, the top and bottom surfaces of the single-crystal SiC substrate 11 and the polycrystalline SiC layer 18 of the SiC composite substrate 3 are located below and above the semiconductor device, respectively.

[0080] As an electronic device using the SiC composite substrate 3, a similar type of semiconductor device can be configured by using a SiC composite substrate 3 different from the SiC composite substrate 1 for the semiconductor device according to the first embodiment. For example, a Schottky barrier diode (SBD) shown in FIG. 8, a trench MOSFET shown in FIG. 9, a planar MOSFET shown in FIG. 10, and an insulated gate bipolar transistor (IGBT) shown in FIG. 11 can be configured using a SiC composite substrate 3 different from the SiC composite substrate 1.

[0081] The semiconductor device according to the second embodiment uses the SiC composite substrate 3 according to the second embodiment, which is configured to prevent warpage. Therefore, the occurrence of warpage in the semiconductor device is suppressed, and the quality of the semiconductor device is ensured.

[0082] [Third Embodiment] (SiC Composite Substrate) Fig. 15 is a cross-sectional view of a SiC composite substrate 4 according to a third embodiment of the present disclosure. The SiC composite substrate 4 in Fig. 15 differs from the SiC composite substrate 1 according to the first embodiment shown in Fig. 1 in that a polycrystalline SiC layer 19, different from the polycrystalline SiC layer 15, is stacked on the top surface 11a of the SiC substrate 11.

[0083] The polycrystalline SiC layer 19 is formed by alternately stacking high-concentration layers 12 containing a first impurity at a first doping amount and low-concentration layers 14 containing the first impurity at a second doping amount lower than the first doping amount and the second impurity at a third doping amount. In the SiC composite substrate 4 according to the third embodiment, the first impurity in the high-concentration layer is nitrogen (N), the first impurity in the low-concentration layer is nitrogen (N), and the second impurity in the low-concentration layer is phosphorus (P). The first doping amount of nitrogen (N) in the high-concentration layer is, for example, 1.0E20 atom / cm ３ The second doping amount of nitrogen (N) in the low concentration layer is, for example, 1.0E20 atom / cm ３ The third doping amount of phosphorus (P) in the low concentration layer is, for example, 5.0E18 atom / cm ３ However, the optimum values ​​of the first doping amount, the second doping amount, and the third doping amount vary depending on the film formation conditions, and therefore the doping amounts shown are merely examples.

[0084] The polycrystalline SiC layer 19 is composed of a total of 2 to 100 high-concentration layers 12 and low-concentration layers 14. The high-concentration layers 12 of the polycrystalline SiC layer 19 are in contact with the SiC substrate 11. The thicknesses of the high-concentration layers 12 and the low-concentration layers 14 are set so that the total thickness of the high-concentration layers 12 and the total thickness of the low-concentration layers 14 are each 250 μm or less. When the polycrystalline SiC layer 19 includes multiple high-concentration layers 12 and multiple low-concentration layers 14, it is preferable that the multiple high-concentration layers 12 have the same thickness and the multiple low-concentration layers 14 have the same thickness. However, the thicknesses of the multiple high-concentration layers 12 and the multiple low-concentration layers 14 may be different.

[0085] In the SiC composite substrate 4, the SiC substrate 11 is configured as an n-type semiconductor doped with n-type impurities such as nitrogen (N), phosphorus (P), and arsenic (As), but may also be configured as a p-type semiconductor doped with p-type impurities such as boron (B) and aluminum (Al). The polycrystalline SiC layer 19 may have the same conductivity type as the SiC substrate 11, or the SiC substrate 11 may be a p-type semiconductor and the polycrystalline SiC layer 19 may be an n-type semiconductor. The polycrystalline SiC layer 19 may be formed by chemical vapor deposition (CVD).

[0086] 16 is a diagram illustrating the polycrystalline SiC layer 19 in the SiC composite substrate 4 according to the third embodiment. When the polycrystalline SiC layer 19 is formed by CVD, increasing the amount of nitrogen (N) doping reduces the grain size of the SiC crystal grains, whereas decreasing the amount of nitrogen (N) doping increases the grain size of the SiC crystal grains. ３ Increasing the doping amount of phosphorus (P) within the following range increases the grain size of the SiC crystal grains, while decreasing the doping amount of phosphorus (P) decreases the grain size of the SiC crystal grains. As shown in Figure 16, in the lowermost high-concentration layer 12 of the polycrystalline SiC layer 19 stacked on the top surface 11a of the SiC substrate 11, the doping amount of nitrogen (N) is increased and the doping amount of phosphorus (P) is set to zero. In the high-concentration layer 12, the SiC crystal grains have many grain boundaries due to their smaller grain size, and the SiC crystal grains aggregate in a direction that reduces the grain boundaries, causing the layer to contract and generating tensile stress 12a.

[0087] As shown in FIG. 16 , in the SiC composite substrate 4 according to the third embodiment, the polycrystalline SiC layer 19 is formed by alternately stacking high-concentration layers 12 and low-concentration layers 14. As described above, tensile stress 12a is generated within the high-concentration layer 12. In the low-concentration layer 14, when the nitrogen (N) doping amount is reduced and the phosphorus (P) doping amount is increased, the grain size of the SiC crystal grains increases due to the combined effects of these two factors. As the grain size of the SiC crystal grains increases, the SiC crystal grains push against each other, creating repulsive forces between them, which act in the direction of expansion of the layer, generating compressive stress 14a. By doping the low-concentration layer 14, which has a reduced nitrogen (N) doping amount, with phosphorus (P), an n-type additive like nitrogen (N), compressive stress 14a can be generated while reducing the resistivity of the low-concentration layer 14.

[0088] Here, an example will be given of the relationship between the second doping amount of nitrogen (N) and the third doping amount of phosphorus (P) in the low concentration layer 14. The low concentration layer 14 of the polycrystalline SiC layer 19 may be grown by setting the ratio of the third doping amount of phosphorus (P) to the sum of the second doping amount of nitrogen (N) and the third doping amount of phosphorus (P) to a range greater than 0 and equal to or less than 1.

[0089] The polycrystalline SiC layer 19 is configured by alternately stacking high-concentration layers 12, in which tensile stress 12a occurs, and low-concentration layers 14, in which compressive stress 14a occurs, so that the tensile stress 12a and compressive stress 14a cancel each other out. The SiC composite substrate 4 reduces the stress applied to the SiC substrate 11 from the polycrystalline SiC layer 19, thereby providing a SiC composite substrate with reduced warping. Furthermore, the SiC composite substrate 4 can reduce the resistivity of the entire polycrystalline SiC layer 19 by doping the low-concentration layers 14 with phosphorus (P).

[0090] Here, the number of high-concentration layers 12 and low-concentration layers 14 included in the polycrystalline SiC layer 19 may be controlled depending on the magnitude relationship between the tensile stress 12a generated in the high-concentration layers 12 and the compressive stress 14a generated in the low-concentration layers 14. Similarly, the thicknesses of the high-concentration layers 12 and low-concentration layers 14 included in the polycrystalline SiC layer 19 may be controlled depending on the magnitude relationship between the tensile stress 12a generated in the high-concentration layers 12 and the compressive stress 14a generated in the low-concentration layers 14. Both the number and thickness of the high-concentration layers 12 and low-concentration layers 14 may be controlled. Furthermore, in the polycrystalline SiC layer 19, the doping amount of phosphorus (P) in the high-concentration layers 12 is set to zero, but the doping amount may be relatively smaller than that in the low-concentration layers 14.

[0091] (Method for manufacturing SiC composite substrate) Next, a method for manufacturing a SiC composite substrate 4 according to the third embodiment shown in Fig. 15 will be described. The method for manufacturing the SiC composite substrate 4 differs from the method for manufacturing the SiC composite substrate 1 according to the first embodiment shown in Figs. 6A to 6C in the following respects. In the method for manufacturing the SiC composite substrate 4, a low-concentration layer 14 different from the low-concentration layer 13 is formed, and a polycrystalline SiC layer 19 different from the polycrystalline SiC layer 15 is formed. The cross-sectional views during each step of the manufacturing method are omitted because the high-concentration layer, low-concentration layer, and polycrystalline SiC layer formed in the cross-sectional views shown in Figs. 6A to 6C are replaced as described above.

[0092] First, as in Fig. 6A, a SiC substrate 11 is provided. The SiC substrate 11 may be fabricated by sublimation or may be formed by epitaxial growth on a seed crystal. The SiC substrate 11 is doped with n-type impurities and configured as an n-type semiconductor.

[0093] 6B, a high-concentration layer 12 is formed as the lowest layer of a polycrystalline SiC layer 19 on the top surface 11a of the SiC substrate 11, and a low-concentration layer 14 is formed in the high-concentration layer 12 as in FIG. 6C. The polycrystalline SiC layer 19 is formed by, for example, monosilane (SiH ４ ) and methane (CH ４ ) as a precursor gas, and nitrogen gas (N ２ ) and phosphine (PH ３) is added to the polycrystalline SiC layer 19. Under these conditions, the polycrystalline SiC layer 19 having a crystal polytype of 3C can be grown. ２ ) and phosphine (PH ３ ) and methane (CH ４ By controlling the flow rate ratio of the nitrogen (N) and carbon (C), the doping amount of nitrogen (N) can be controlled in proportion to the logarithm of the ratio of nitrogen (N) to carbon (C). Also, the doping amount of phosphorus (P) can be controlled in proportion to the logarithm of the ratio of phosphorus (P) to carbon (C).

[0094] FIG. 17 shows a method for manufacturing a SiC composite substrate 4, in which monosilane (SiH ４ ) and methane (CH ４ ) and nitrogen gas (N ２ ) and phosphine (PH ３ 1 is a graph showing the change in flow rate of each precursor gas over time when CVD is performed using a gas mixture containing 100% fluorine-containing gas and 100% fluorine-containing gas.

[0095] At time t0, the flow rates of all four precursor gases are zero. At time t1, the flow of the four precursor gases begins. ４ ) and methane (CH ４ The flow rate of the gas 2 is kept constant from time t1 onwards during the film formation by CVD.

[0096] Nitrogen gas (N ２ The flow rate of phosphine (PH ３ The flow rate of nitrogen gas (N) is set to zero between time t1 and time t2. Between time t1 and time t2, the ratio of nitrogen (N) to carbon (C) becomes relatively large, and the doping amount of nitrogen (N) increases, allowing the growth of the high concentration layer 12. ２ The flow rate of phosphine (PH ３The flow rate of nitrogen (N) is relatively increased between times t2 and t3. Between times t2 and t3, the ratio of nitrogen (N) to carbon (C) is relatively decreased, and the doping amount of nitrogen (N) is reduced, allowing the low-concentration layer 17 to grow. On the other hand, between times t2 and t3, the ratio of phosphorus (P) to carbon (C) is relatively increased, and the doping amount of phosphorus (P) is increased, allowing the low-concentration layer 17 to grow with reduced resistivity.

[0097] Similarly, nitrogen gas (N ２ The flow rate of phosphine (PH ３ The flow rate of the SiO 2 ) is set to zero between times t3 and t4, and is set to a relatively high value between times t4 and t5. The high-concentration layer 12 can be grown between times t3 and t4, and the low-concentration layer 17 can be grown between times t4 and t5. By controlling the flow rate in the same way after time t5, the high-concentration layer 16 and the low-concentration layer 17 can be grown alternately.

[0098] In FIG. 17, nitrogen gas (N ２ ) and phosphine (PH ３ The high concentration layer 12 and the low concentration layer 14 are grown alternately by increasing or decreasing the flow rate of phosphine (PH) to a relatively high flow rate and a relatively low flow rate at regular intervals. Alternatively, the high concentration layer 12 and the low concentration layer 14 can be grown alternately by alternately turning on and off the nitrogen inlet pipe and the phosphine inlet pipe. ３ ) was set to zero when growing the high concentration layer 12, but may be set to a relatively small flow rate.

[0099] As described above, by controlling the flow rate of the precursor gas as explained in FIG. 17, the process of forming the high concentration layer 12 and the low concentration layer 14 is continued until the polycrystalline SiC layer 19 composed of the high concentration layer 12 and the low concentration layer 14 reaches a predetermined total number. By this series of processes, the SiC composite substrate 4 shown in FIG. 15 is formed. When performing CVD, nitrogen gas (N ２ ) and phosphine (PH ３By controlling the flow rate of the gas, the high concentration layer 12 and the low concentration layer 14 can be deposited continuously, so that film formation can be performed while maintaining the machine time and growth rate.

[0100] In the method for manufacturing a SiC composite substrate 4 according to the third embodiment, a polycrystalline SiC layer 19 is formed by alternately stacking high-concentration layers 12 in which tensile stress 12a occurs within the layers and low-concentration layers 14 in which compressive stress 14a occurs within the layers. According to the method for manufacturing a SiC composite substrate 4, the tensile stress 12a and the compressive stress 14a cancel each other out, thereby reducing the stress applied from the polycrystalline SiC layer 19 to the SiC substrate 11 and suppressing the occurrence of warpage. Furthermore, according to the method for manufacturing a SiC composite substrate 4, the resistivity of the entire polycrystalline SiC layer 19 can be reduced by doping the low-concentration layers 14 with phosphorus (P).

[0101] (Semiconductor Device) Next, a semiconductor device according to a third embodiment will be described. Here, a case where a single-crystal SiC substrate is used as the SiC substrate 11 will be described. The semiconductor device according to the third embodiment uses a SiC composite substrate 4 as shown in FIG. 15 , and is configured such that the single-crystal SiC substrate 11 of the SiC composite substrate 4 serves as a drift layer and the polycrystalline SiC layer 19 serves as a substrate layer. Note that, since the SiC composite substrate 4 is used upside down in the semiconductor device, the top and bottom surfaces of the single-crystal SiC substrate 11 and the polycrystalline SiC layer 19 of the SiC composite substrate 4 are located below and above the semiconductor device, respectively.

[0102] As an electronic device using a SiC composite substrate 4, a similar semiconductor device can be configured using a SiC composite substrate 4 different from the SiC composite substrate 1 in the semiconductor device according to the first embodiment. In the examples of the Schottky barrier diode (SBD) in FIG. 8, the trench MOSFET in FIG. 9, the planar MOSFET in FIG. 10, and the insulated gate bipolar transistor (IGBT) in FIG. 11, they can be configured using a SiC composite substrate 4 different from the SiC composite substrate 1.

[0103] The semiconductor device according to the third embodiment uses the SiC composite substrate 4 according to the third embodiment, which is configured to prevent warpage. This suppresses warpage in the semiconductor device, ensuring the quality of the semiconductor device. Furthermore, in the semiconductor device according to the third embodiment, the resistivity of the entire polycrystalline SiC layer 19 can be reduced by doping the low-concentration layer 14 with phosphorus (P). This allows for the provision of a semiconductor device with a low resistivity of the substrate layer and a small voltage drop.

[0104] Although the present disclosure has been described in detail above, it will be apparent to those skilled in the art that the present disclosure is not limited to the embodiments described herein. One or more elements of one embodiment can be combined with one or more elements of another embodiment. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure, as defined by the claims. Therefore, the description of the present disclosure is intended to be illustrative and explanatory, and is not intended to be limiting of the present disclosure.

[0105] For example, the SiC composite substrate according to the first to third embodiments of the present disclosure may be used as a dummy wafer. Dummy wafers are generally produced by forming a SiC film on graphite and then separating the graphite substrate by heat treatment or the like. The SiC composite substrate according to the present disclosure reduces the stress applied from the polycrystalline SiC layer to the SiC substrate 11, thereby providing a dummy wafer that is less likely to warp when the graphite substrate is separated.

[0106] (Supplementary Note 1) The SiC composite substrates 1, 3 include a SiC substrate 11 and polycrystalline SiC layers 15, 18 stacked on the top surface 11a of the SiC substrate 11. The polycrystalline SiC layers 15, 18 are formed by alternately stacking high-concentration layers 12, 16 containing a first doping amount of impurity and low-concentration layers 13, 17 containing a second doping amount of impurity lower than the first doping amount. In the polycrystalline SiC layers 15, 18, the tensile stress 12a and compressive stress 16a generated in the high-concentration layers 12, 16 can be offset by the compressive stress 13a and tensile stress 17a generated in the low-concentration layers 13, 17. The stress applied to the SiC substrate 11 from the polycrystalline SiC layers 15, 18 is reduced, thereby providing SiC composite substrates 1, 3 in which warpage is suppressed.

[0107] (Supplementary Note 2) In the SiC composite substrates 1 and 3 described in Supplementary Note 1, the SiC substrate 11 may include single crystal SiC.

[0108] (Supplementary Note 3) In the SiC composite substrate 1, 3 described in Supplementary Note 1 or 2, the high concentration layers 12, 16 of the polycrystalline SiC layers 15, 18 may be in contact with the top surface 11a of the SiC substrate 11.

[0109] (Supplementary Note 4) In the SiC composite substrate 1, 3 described in Supplementary Note 1 or 2, the low concentration layers 13, 17 of the polycrystalline SiC layers 15, 18 may be in contact with the top surface 11a of the SiC substrate 11.

[0110] (Supplementary Note 5) In the SiC composite substrate 1 according to any one of Supplementary Notes 1 to 4, the impurities in the high concentration layer 12 and the low concentration layer 13 may both be nitrogen (N).

[0111] (Appendix 6) In the SiC composite substrate 1 described in Appendix 5, the doping amount of impurities in the high concentration layer 12 is 1.0E20 atom / cm ３ It may be more than that.

[0112] (Supplementary Note 7) In the SiC composite substrate 1 described in Supplementary Note 5 or 6, the doping amount of the impurity in the low concentration layer 13 is 1.0E20 atom / cm ３ It may be less than.

[0113] (Supplementary Note 8) In the SiC composite substrate 1 according to any one of Supplementary Notes 5 to 7, the grain size of the crystal grains in the high concentration layer 12 of the polycrystalline SiC layer 15 is smaller than the grain size of the crystal grains in the low concentration layer 13 .

[0114] (Supplementary Note 9) In the SiC composite substrate 3 according to any one of Supplementary Notes 1 to 4, the impurities in the high concentration layer 16 and the low concentration layer 17 may both be phosphorus (P).

[0115] (Supplementary Note 10) In the SiC composite substrate 3 described in Supplementary Note 9, the grain size of the crystal grains in the high concentration layer 16 of the polycrystalline SiC layer 18 is larger than the grain size of the crystal grains in the low concentration layer 17 .

[0116] (Supplementary Note 11) The SiC composite substrate 4 includes a SiC substrate 11 and a polycrystalline SiC layer 19 laminated on the top surface 11a of the SiC substrate 11. The polycrystalline SiC layer 19 is formed by alternately laminating a high-concentration layer 12 of polycrystalline SiC containing a first impurity at a first doping amount and a low-concentration layer 14 containing the first impurity at a second doping amount lower than the first doping amount and a second impurity at a third doping amount. In the polycrystalline SiC layer 19, the tensile stress 12a generated in the high-concentration layer 12 and the compressive stress 14a generated in the low-concentration layer 14 can be offset. The stress applied from the polycrystalline SiC layer 19 to the SiC substrate 11 is reduced, thereby providing a SiC composite substrate 4 with reduced warpage. Furthermore, the resistivity of the entire polycrystalline SiC layer 19 can be reduced.

[0117] (Supplementary Note 12) In the SiC composite substrate 4 described in Supplementary Note 11, the first impurity may be nitrogen (N), and the second impurity may be phosphorus (P).

[0118] (Appendix 13) In the SiC composite substrate 4 described in Appendix 12, the ratio of the third doping amount of phosphorus (P) to the sum of the second doping amount of nitrogen (N) and the third doping amount of phosphorus (P) in the low concentration layer 14 may be in the range of greater than 0 and less than or equal to 1.

[0119] (Supplementary Note 14) In the SiC composite substrate 1, 3, or 4 according to any one of Supplementary Notes 1 to 13, the total thickness of the high-concentration layers 12, 16 and the total thickness of the low-concentration layers 13, 17, 14 may each be 250 μm or less.

[0120] (Supplementary Note 15) In the SiC composite substrate 1, 3, 4 according to any one of Supplementary Notes 1 to 14, the polycrystalline SiC layers 15, 18, 19 may include high concentration layers 12, 16 and low concentration layers 13, 17, 14 in a total number ranging from 2 to 100.

[0121] (Supplementary Note 16) In the SiC composite substrate 1, 3, 4 described in any one of Supplementary Notes 1 to 15, the polycrystalline SiC layer 15, 18, 19 includes a plurality of high-concentration layers 12, 16 and low-concentration layers 13, 17, 14, and the plurality of high-concentration layers 12, 16 may have the same thickness. The plurality of low-concentration layers 13, 17, 14 may have the same thickness.

[0122] (Supplementary Note 17) In the SiC composite substrates 1, 3, and 4 according to any one of Supplementary Notes 1 to 16, the SiC substrate 11 and the polycrystalline SiC layers 15, 18, and 19 may be of the same conductivity type.

[0123] (Supplementary Note 18) In the SiC composite substrates 1, 3, and 4 according to any one of Supplementary Notes 1 to 16, the SiC substrate 11 may constitute a p-type semiconductor, and the polycrystalline SiC layers 15, 18, and 19 may constitute an n-type semiconductor.

[0124] (Supplementary Note 19) The manufacturing method of the SiC composite substrate 1, 3 includes the steps of providing a SiC substrate 11 and forming a polycrystalline SiC layer 15, 18 on the top surface 11a of the SiC substrate 11. The step of forming the polycrystalline SiC layer 15, 18 alternately stacks high-concentration polycrystalline SiC layers 12, 16 containing a first impurity at a first doping amount and low-concentration polycrystalline SiC layers 13, 17 containing the first impurity at a second doping amount lower than the first doping amount. In the polycrystalline SiC layers 15, 18, the tensile stress 12a and compressive stress 16a generated in the high-concentration layers 12, 16 can be offset by the compressive stress 13a and tensile stress 17a generated in the low-concentration layers 13, 17. This reduces the stress applied to the SiC substrate 11 from the polycrystalline SiC layer 15, 18, thereby providing a manufacturing method of the SiC composite substrate 1, 3 in which warpage is suppressed.

[0125] (Supplementary Note 20) In the method for manufacturing SiC composite substrates 1 and 3 described in Supplementary Note 19, the step of forming polycrystalline SiC layers 15 and 18 may be performed such that high concentration layers 12 and 16 of polycrystalline SiC are in contact with top surface 11 a of SiC substrate 11.

[0126] (Supplementary Note 21) In the method for manufacturing SiC composite substrates 1 and 3 described in Supplementary Note 19, the step of forming polycrystalline SiC layers 15 and 18 may be performed such that low concentration layers 13 and 17 of polycrystalline SiC are in contact with top surface 11 a of SiC substrate 11.

[0127] (Supplementary Note 22) In the method for manufacturing the SiC composite substrate 1, 3 according to any one of Supplementary Notes 19 to 21, the step of forming the polycrystalline SiC layers 15, 18 may employ chemical vapor deposition using a precursor gas of silicon (Si), a precursor gas of carbon (C), and a precursor gas of the first impurity. The high-concentration polycrystalline SiC layers 12, 16 and the low-concentration polycrystalline SiC layers 13, 17 may be alternately stacked by repeatedly increasing and decreasing the ratio of the flow rates of the precursor gas of carbon (C) and the precursor gas of the first impurity while keeping the flow rate of the precursor gas of silicon (Si) constant.

[0128] (Supplementary Note 23) In the method for manufacturing SiC composite substrate 1 according to Supplementary Notes 19 to 21, the first impurity may be nitrogen (N).

[0129] (Supplementary Note 24) In the method for manufacturing SiC composite substrate 1 according to Supplementary Note 22, the first impurity may be nitrogen (N).

[0130] (Appendix 25) In the method for producing the SiC composite substrate 1 according to Appendix 24, in the step of forming the polycrystalline SiC layer 15, the precursor gas of silicon (Si) is silicon tetrachloride (SiCl ４ ) or monosilane (SiH ４ The precursor gas of carbon (C) may be methane (CH ４ ), propane (C ３ H ８ ), ethylene (C ２ H ４ ), and the precursor gas of nitrogen (N) may be nitrogen gas (N ２ ) or ammonia (NH ３ ) may also be used.

[0131] (Appendix 26) In the method for manufacturing SiC composite substrate 1 described in Appendix 24 or 25, in the step of forming polycrystalline SiC layer 15, the ratio of the flow rate of the nitrogen (N) precursor gas to the flow rate of the carbon (C) precursor gas may be in the range of 1.0E-4 or more and 1.0E1 or less.

[0132] (Appendix 27) In the method for manufacturing SiC composite substrate 1 described in any one of Appendices 24 to 26, in the step of forming polycrystalline SiC layer 15, high concentration layers 12 and low concentration layers 13 of polycrystalline SiC may be alternately stacked by increasing or decreasing the flow rate of nitrogen (N) precursor gas.

[0133] (Appendix 28) In the method for manufacturing SiC composite substrate 1 described in any one of Appendices 24 to 26, in the step of forming polycrystalline SiC layer 15, high concentration layers 12 and low concentration layers 13 of polycrystalline SiC may be alternately stacked by on / off control of an inlet pipe for a nitrogen (N) precursor gas.

[0134] (Supplementary Note 29) In the method for manufacturing SiC composite substrate 3 according to any one of Supplementary Notes 19 to 21, the first impurity may be phosphorus (P).

[0135] (Supplementary Note 30) In the method for manufacturing SiC composite substrate 3 according to Supplementary Note 22, the first impurity may be phosphorus (P).

[0136] (Supplementary Note 31) In the method for producing the SiC composite substrate 3 according to Supplementary Note 30, in the step of forming the polycrystalline SiC layer 18, the precursor gas of silicon (Si) is silicon tetrachloride (SiCl ４ ) or monosilane (SiH ４ The precursor gas of carbon (C) may be methane (CH ４ ), propane (C ３ H ８ ), ethylene (C ２ H ４ ), and the precursor gas for phosphorus (P) may be phosphine (PH ３ ) may also be used.

[0137] (Supplementary Note 32) A method for manufacturing a SiC composite substrate 4 includes providing a SiC substrate 11 and forming a polycrystalline SiC layer 19 on the top surface 11a of the SiC substrate 11. The step of forming the polycrystalline SiC layer 19 alternately stacks a high-concentration layer 12 of polycrystalline SiC containing a first impurity at a first doping amount and a low-concentration layer 14 of polycrystalline SiC containing the first impurity at a second doping amount lower than the first doping amount and a second impurity at a third doping amount. In the polycrystalline SiC layer 19, the tensile stress 12a generated in the high-concentration layer 12 and the compressive stress 14a generated in the low-concentration layer 14 can be offset. This method reduces the stress applied to the SiC substrate 11 from the polycrystalline SiC layer 19, thereby providing a method for manufacturing a SiC composite substrate 4 in which warpage is suppressed. Furthermore, the resistivity of the entire polycrystalline SiC layer 19 can be reduced.

[0138] (Appendix 33) In the method for manufacturing SiC composite substrate 4 described in Appendix 32, the step of forming polycrystalline SiC layer 19 uses chemical vapor deposition using a precursor gas of silicon (Si), a precursor gas of carbon (C), a precursor gas of a first impurity, and a precursor gas of a second impurity. In the step of forming polycrystalline SiC layer 19, the flow rate of the precursor gas of silicon (Si) is kept constant, and the ratio of the flow rates of the precursor gas of carbon (C), the precursor gas of the first impurity, and the precursor gas of the second impurity is repeatedly increased and decreased, thereby alternately stacking high-concentration layers 12 and low-concentration layers 14 of polycrystalline SiC.

[0139] (Supplementary Note 34) In the method for manufacturing SiC composite substrate 4 according to Supplementary Note 32, the first impurity may be nitrogen (N), and the second impurity may be phosphorus (P).

[0140] (Appendix 35) In the method for manufacturing SiC composite substrate 4 according to appendix 33, the first impurity may be nitrogen (N), and the second impurity may be phosphorus (P).

[0141] (Appendix 36) In the method for manufacturing SiC composite substrate 4 described in Appendix 35, when the flow rate of the nitrogen (N) precursor gas is increased, the flow rate of the phosphorus (P) precursor gas may be reduced or set to zero, and when the flow rate of the nitrogen (N) precursor gas is reduced, the flow rate of the phosphorus (P) precursor gas may be increased.

[0142] (Appendix 37) In the method for manufacturing the SiC composite substrate 4 according to appendix 35 or 36, in the step of forming the polycrystalline SiC layer 19, the precursor gas of silicon (Si) is silicon tetrachloride (SiCl ４ ) or monosilane (SiH ４ The precursor gas of carbon (C) may be methane (CH ４ ), propane (C ３ H ８ ), ethylene (C ２ H ４ ), and the precursor gas of nitrogen (N) may be nitrogen gas (N ２ ) or ammonia (NH ３ The precursor gas of phosphorus (P) may be phosphine (PH ３ ) may also be used.

[0143] (Appendix 38) In a semiconductor device using the SiC composite substrate 1, 3, or 4 according to any one of Appendices 1 to 17, the single-crystal SiC substrate 11 and the polycrystalline SiC layer 15, 18, or 19 of the SiC composite substrate 1, 3, or 4 may serve as a drift layer and a substrate layer, respectively. This suppresses the occurrence of warpage in the SiC composite substrate 1, 3, or 4 constituting the semiconductor device, thereby ensuring the quality of the semiconductor device.

[0144] (Supplementary Note 39) The semiconductor device according to Supplementary Note 38 may constitute at least one of a Schottky barrier diode, a planar MOSFET, a trench MOSFET, and an IGBT.

[0145] 1, 2, 3, 4 SiC composite substrate 11 SiC substrate 12, 16 High concentration layer 13, 14, 17 Low concentration layer 15, 18, 19 Polycrystalline SiC layer

Claims

1. A SiC composite substrate comprising: a SiC substrate; and a polycrystalline SiC layer laminated on the top surface of the SiC substrate, the polycrystalline SiC layer being formed by alternating high-concentration layers containing a first doping amount of impurity and low-concentration layers containing a second doping amount of impurity that is lower than the first doping amount.

2. The SiC composite substrate of claim 1, wherein the SiC substrate comprises single crystal SiC.

3. The SiC composite substrate according to claim 1 or 2, wherein the high concentration layer of the polycrystalline SiC layer contacts the top surface of the SiC substrate.

4. The SiC composite substrate according to claim 1 or 2, wherein the low concentration layer of the polycrystalline SiC layer contacts the top surface of the SiC substrate.

5. The SiC composite substrate according to any one of claims 1 to 4, wherein the impurities in the high concentration layer and the low concentration layer are both nitrogen.

6. A SiC composite substrate comprising: a SiC substrate; and a polycrystalline SiC layer laminated on the top surface of the SiC substrate, the polycrystalline SiC layer being formed by alternating high-concentration layers of polycrystalline SiC containing a first impurity at a first doping amount and low-concentration layers containing the first impurity at a second doping amount lower than the first doping amount and a second impurity at a third doping amount.

7. The SiC composite substrate according to claim 6, wherein the first impurity is nitrogen and the second impurity is phosphorus.

8. A SiC composite substrate according to any one of claims 1 to 7, wherein the total thickness of the high concentration layers and the total thickness of the low concentration layers are each 250 µm or less.

9. A SiC composite substrate according to any one of claims 1 to 8, wherein the polycrystalline SiC layer includes a total of 2 to 100 high concentration layers and low concentration layers.

10. A SiC composite substrate according to any one of claims 1 to 9, wherein the SiC substrate and the polycrystalline SiC layer are of the same conductivity type.

11. A method for manufacturing a SiC composite substrate, comprising: providing a SiC substrate; and forming a polycrystalline SiC layer on a top surface of the SiC substrate by alternately stacking high-concentration layers of polycrystalline SiC containing a first impurity in a first doping amount and low-concentration layers of polycrystalline SiC containing the first impurity in a second doping amount lower than the first doping amount.

12. The method for manufacturing a SiC composite substrate according to claim 11, wherein the step of forming the polycrystalline SiC layer is performed so that the high concentration layer of the polycrystalline SiC is in contact with the top surface of the SiC substrate.

13. The method for manufacturing a SiC composite substrate according to claim 11, wherein the step of forming the polycrystalline SiC layer is performed so that the low concentration layer of the polycrystalline SiC is in contact with the top surface of the SiC substrate.

14. A method for manufacturing a SiC composite substrate according to any one of claims 11 to 13, wherein the step of forming the polycrystalline SiC layer uses a chemical vapor deposition method using a silicon precursor gas, a carbon precursor gas, and a precursor gas of the first impurity, and alternately stacks the high concentration layers and the low concentration layers of the polycrystalline SiC by repeatedly increasing and decreasing the ratio of the flow rates of the carbon precursor gas and the precursor gas of the first impurity while keeping the flow rate of the silicon precursor gas constant.

15. A method for producing a SiC composite substrate according to any one of claims 11 to 14, wherein the first impurity is nitrogen.

16. A method for manufacturing a SiC composite substrate, comprising: providing a SiC substrate; and forming a polycrystalline SiC layer on a top surface of the SiC substrate by alternately stacking high-concentration layers of polycrystalline SiC containing a first impurity at a first doping amount and low-concentration layers of polycrystalline SiC containing the first impurity at a second doping amount lower than the first doping amount and a second impurity at a third doping amount.

17. A method for producing a SiC composite substrate as described in claim 16, wherein the step of forming the polycrystalline SiC layer uses a chemical vapor deposition method using a silicon precursor gas, a carbon precursor gas, a precursor gas of the first impurity, and a precursor gas of the second impurity, and the high concentration layers and the low concentration layers of the polycrystalline SiC are alternately stacked by repeatedly increasing and decreasing the ratio of the flow rates of the carbon precursor gas, the precursor gas of the first impurity, and the precursor gas of the second impurity while keeping the flow rate of the silicon precursor gas constant.

18. A method for producing a SiC composite substrate according to claim 16 or 17, wherein the first impurity is nitrogen and the second impurity is phosphorus.

19. A semiconductor device using a SiC composite substrate according to any one of claims 1 to 10, wherein the SiC substrate and the polycrystalline SiC layer of the SiC composite substrate serve as a drift layer and a substrate layer, respectively.

20. The semiconductor device according to claim 19, which constitutes at least one of a Schottky barrier diode, a planar MOSFET, a trench MOSFET, and an IGBT.

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